Table of contents

This editorial serves as the preface to a special issue of New Journal of Physics, which collects together solicited papers on a common subject, x-ray beams with high coherence. We summarize the issue's content, and explain why there is so much current interest both in the sources themselves and in the applications to the study of the structure of matter and its fluctuations (both spontaneous and driven). As this collection demonstrates, the field brings together accelerator physics in the design of new sources, particle physics in the design of detectors, and chemical and materials scientists who make use of the coherent beams produced.

Focus on X-ray Beams with High Coherence Contents

Femtosecond pulse x-ray imaging with a large field of viewB Pfau, C M Günther, S Schaffert, R Mitzner, B Siemer, S Roling, H Zacharias, O Kutz, I Rudolph, R Treusch and S Eisebitt

The FERMI@Elettra free-electron-laser source for coherent x-ray physics: photon properties, beam transport system and applicationsE Allaria, C Callegari, D Cocco, W M Fawley, M Kiskinova, C Masciovecchio and F Parmigiani

Beyond simple exponential correlation functions and equilibrium dynamics in x-ray photon correlation spectroscopyAnders Madsen, Robert L Leheny, Hongyu Guo, Michael Sprung and Orsolya Czakkel

The Coherent X-ray Imaging (CXI) instrument at the Linac Coherent Light Source (LCLS) Sébastien Boutet and Garth J Williams

Dynamics and rheology under continuous shear flow studied by x-ray photon correlation spectroscopy Andrei Fluerasu, Pawel Kwasniewski, Chiara Caronna, Fanny Destremaut, Jean-Baptiste Salmon and Anders Madsen

Exploration of crystal strains using coherent x-ray diffraction Wonsuk Cha, Sanghoon Song, Nak Cheon Jeong, Ross Harder, Kyung Byung Yoon, Ian K Robinson and Hyunjung Kim

Coherence properties of the European XFEL G Geloni, E Saldin, L Samoylova, E Schneidmiller, H Sinn, Th Tschentscher and M Yurkov

Fresnel coherent diffractive imaging: treatment and analysis of data G J Williams, H M Quiney, A G Peele and K A Nugent

Imaging of complex density in silver nanocubes by coherent x-ray diffraction R Harder, M Liang, Y Sun, Y Xia and I K Robinson

Methodology for studying strain inhomogeneities in polycrystalline thin films during in situ thermal loading using coherent x-ray diffraction N Vaxelaire, H Proudhon, S Labat, C Kirchlechner, J Keckes, V Jacques, S Ravy, S Forest and O Thomas

Ptychographic coherent diffractive imaging of weakly scattering specimens Martin Dierolf, Pierre Thibault, Andreas Menzel, Cameron M Kewish, Konstantins Jefimovs, Ilme Schlichting, Konstanze von König, Oliver Bunk and Franz Pfeiffer

Dose requirements for resolving a given feature in an object by coherent x-ray diffraction imaging Andreas Schropp and Christian G Schroer

FLASH: new opportunities for (time-resolved) coherent imaging of nanostructures R Treusch and J Feldhaus

Structure of a single particle from scattering by many particles randomly oriented about an axis: toward structure solution without crystallization? D K Saldin, V L Shneerson, M R Howells, S Marchesini, H N Chapman, M Bogan, D Shapiro, R A Kirian, U Weierstall, K E Schmidt and J C H Spence

Analysis of strain and stacking faults in single nanowires using Bragg coherent diffraction imaging V Favre-Nicolin, F Mastropietro, J Eymery, D Camacho, Y M Niquet, B M Borg, M E Messing, L-E Wernersson, R E Algra, E P A M Bakkers, T H Metzger, R Harder and I K Robinson

Coherent science at the SwissFEL x-ray laser B D Patterson, R Abela, H-H Braun, U Flechsig, R Ganter, Y Kim, E Kirk, A Oppelt, M Pedrozzi, S Reiche, L Rivkin, Th Schmidt, B Schmitt, V N Strocov, S Tsujino and A F Wrulich

Energy recovery linac (ERL) coherent hard x-ray sources Donald H Bilderback, Joel D Brock, Darren S Dale, Kenneth D Finkelstein, Mark A Pfeifer and Sol M Gruner

Statistical and coherence properties of radiation from x-ray free-electron lasers E L Saldin, E A Schneidmiller and M V Yurkov

Microscopic return point memory in Co/Pd multilayer films K A Seu, R Su, S Roy, D Parks, E Shipton, E E Fullerton and S D Kevan

Holographic and diffractive x-ray imaging using waveguides as quasi-point sources K Giewekemeyer, H Neubauer, S Kalbfleisch, S P Krüger and T Salditt

Mapping the conformations of biological assemblies P Schwander, R Fung, G N Phillips Jr and A Ourmazd

Imaging the displacement field within epitaxial nanostructures by coherent diffraction: a feasibility study Ana Diaz, Virginie Chamard, Cristian Mocuta, Rogerio Magalhães-Paniago, Julian Stangl, Dina Carbone, Till H Metzger and Günther Bauer

The potential for two-dimensional crystallography of membrane proteins at future x-ray free-electron laser sources Cameron M Kewish, Pierre Thibault, Oliver Bunk and Franz Pfeiffer

Coherence properties of hard x-ray synchrotron sources and x-ray free-electron lasers I A Vartanyants and A Singer

Coherent imaging of biological samples with femtosecond pulses at the free-electron laser FLASH A P Mancuso, Th Gorniak, F Staier, O M Yefanov, R Barth, C Christophis, B Reime, J Gulden, A Singer, M E Pettit, Th Nisius, Th Wilhein, C Gutt, G Grübel, N Guerassimova, R Treusch, J Feldhaus, S Eisebitt, E Weckert, M Grunze, A Rosenhahn and I A Vartanyants

The Linac Coherent Light Source (LCLS) became the first ever operational hard x-ray free-electron laser in 2009. It will operate as a user facility capable of delivering unique research opportunities in multiple fields of science. The LCLS and the LCLS Ultrafast Science Instruments (LUSI) construction projects are developing instruments designed to make full use of the capabilities afforded by the LCLS beam. One such instrument is being designed to utilize the LCLS coherent beam to image with high resolution any sub-micron object. This instrument is called the Coherent X-ray Imaging (CXI) instrument. This instrument will provide a flexible optical system capable of tailoring key beam parameters for the users. A suite of shot-to-shot diagnostics will also be provided to characterize the beam on every pulse. The provided instrumentation will include multi-purpose sample environments, sample delivery and a custom detector capable of collecting two-dimensional (2D) data at 120 Hz. In this paper, the LCLS will be briefly introduced, as well as the technique of coherent x-ray diffractive imaging (CXDI). A few examples of scientific opportunities arising from use of the CXI instrument will be described. Finally, the conceptual layout of the instrument will be presented, together with a description of the key requirements for the overall system and of specific devices required.

X-ray photon correlation spectroscopy (XPCS) has emerged as a unique technique allowing the measurement of dynamics of materials on mesoscopic lengthscales. One of the most common problems associated with the use of bright x-ray beams is beam-induced radiation damage, and this is likely to become an even more limiting factor at future synchrotron and free-electron laser sources. Flowing the sample during data acquisition is one of the simplest methods allowing the radiation damage to be limited. In addition to distributing the dose over many different scatterers, the method also enables new functionalities such as time-resolved studies. Here, we further develop a recently proposed experimental technique that combines XPCS and continuously flowing samples. More specifically, we use a model colloidal suspension to show how the macroscopic advective response to flow and the microscopic dissipative dynamics (diffusion) can be quantified from the x-ray data. Our results show very good quantitative agreement with a Poisseuille-flow hydrodynamical model combined with Brownian mechanics. The method has many potential applications, e.g. in the study of dynamics of glasses and gels under continuous shear/flow, protein aggregation processes and the interplay between dynamics and rheology in complex fluids.

We measured coherent x-ray diffraction (CXD) on zeolite microcrystals in order to gain information on internal density distribution and to learn more about the strain developed during the synthesis and attachment process on the substrate. From the distortion and asymmetry of the diffraction pattern on the (020) Bragg peak, the strain field distribution is estimated. We inverted the diffraction patterns from a less strained crystal to obtain the three-dimensional image of the shape and internal strain fields using the error reduction and hybrid input–output phase retrieval algorithms. We also show a few examples of characteristic distortion modes relevant to CXD of zeolites.

The European x-ray free-electron laser (XFEL) provides x-ray self-amplified spontaneous emission (SASE) FEL radiation in the wavelength range from 0.1 to 3 nm using three undulator systems. The SASE mode of operation at the European XFEL defines specific behavior of longitudinal and transverse coherence properties. In this paper, we describe the evolution of the temporal and transverse correlation functions along the undulator length, and we extract the corresponding evolution of coherence time and degree of transverse coherence as typical figures of merit. Generation of coherent radiation inside the FEL undulators is followed by beam transport to the experiments. During transport, the total number of coherent modes is preserved, but the wavefront can be disturbed, and we analyze the conditions under which this occurs. It is emphasized that the development of experimental observables for the degree of coherence and wavefront properties will be important for experiments using coherent x-ray radiation.

Fresnel coherent diffractive imaging (FCDI) is a relatively recent addition to the suite of imaging tools available at third generation x-ray sources. It shares the strengths of other coherent diffractive techniques: resolution limits that are independent of focusing optics, single-plane measurement and high dose efficiency. The more challenging experimental geometry and detailed reconstruction algorithms of FCDI provide enhanced numerical stability and convergence properties to the iterative algorithms commonly used. Experimentally, a diverging beam is utilized, which facilitates sample alignment and allows the imaging of extended samples. We describe the underlying physics and assumptions that give rise to the FCDI iterative reconstruction algorithms, as well as their implications for the design of a successful FCDI experiment.

When using coherent x-rays to perform lensless imaging, it is the complex wave field exiting the sample or, in the case of the Bragg geometry, the deformed electron density distribution of a crystal, that is being sought. For most samples, to some extent, the image will be complex, containing both an amplitude and phase variation across the sample. We have developed versions of the hybrid input–output (HIO) and error reduction (ER) algorithms that are very robust for the inversion to complex objects from three-dimensional (3D) coherent x-ray diffraction (CXD) data measured around a Bragg spot of a small crystal. The development and behavior of these algorithms will be discussed in the context of inverting a 3D CXD pattern measured around a (111) Bragg spot of a silver nanocube.

Coherent x-ray diffraction is used to investigate the mechanical properties of a single grain within a polycrystalline thin film in situ during a thermal cycle. Both the experimental approach and finite element simulation are described. Coherent diffraction from a single grain has been monitored in situ at different temperatures. This experiment offers unique perspectives for the study of the mechanical properties of nano-objects.

Applying iterative phase retrieval schemes to ptychographic data, i.e. diffraction patterns collected with a localized illumination probe from overlapping regions of a specimen, has enabled the investigation of extended specimens previously inaccessible by other coherent x-ray diffractive imaging methods. While the technique had initially been limited by the requirement of precise knowledge of the illumination function, recent algorithmic developments allow now the simultaneous reconstruction of both the probe and the object. However, these new approaches suffer from an inherent ambiguity, which affects especially the case of weakly scattering specimens. We present new schemes to circumvent this problem and introduce new tools for obtaining information about the scattering behaviour of weak phase objects already during data collection. The new techniques are experimentally demonstrated for a data set taken on Magnetospirillum gryphiswaldense.

We address the question of what dose is required to image an object by coherent x-ray diffraction imaging (CXDI) and to resolve a certain sub-unit or feature of that object. We show that a necessary condition for being able to resolve the detail is that the feature can be imaged by itself. The quality of the reconstruction of the feature is nearly independent of the surrounding, whether it is embedded in a larger object or not. This allows one to easily estimate the dose requirements for identifying atoms and clusters in larger objects. We illustrate the result by a numerical example and give an estimate for the dose required to resolve single atoms of different elemental species in CXDI experiments at free-electron laser and synchrotron radiation sources.

FLASH (free electron laser in Hamburg) is a unique, ultra-brilliant soft x-ray source providing highly coherent femtosecond pulses, currently in a wavelength range of 6.8–47 nm. Up to several 10¹² coherent photons within a 10–70 fs pulse allow the study of dynamical changes in nanometer-sized structures. This is a big step towards the ultimate goal of observing femtosecond dynamics on the atomic length scale, for example 'watching bio-machines at work'. In this review, the properties of FLASH are summarized with a focus on coherence, and the potential of FLASH for structural investigations is illustrated with an overview of the recently performed coherent imaging experiments.

In this paper is demonstrated a complete algorithm for determining the electron density of an individual particle from diffraction patterns of many particles, randomly oriented about a single axis. The algorithm operates on angular correlations among the measured intensity distributions. We also demonstrate the ability to recover the angular correlation functions of a single particle from measured diffraction patterns.

Coherent diffraction imaging (CDI) on Bragg reflections is a promising technique for the study of three-dimensional (3D) composition and strain fields in nanostructures, which can be recovered directly from the coherent diffraction data recorded on single objects. In this paper, we report results obtained for single homogeneous and heterogeneous nanowires with a diameter smaller than 100 nm, for which we used CDI to retrieve information about deformation and faults existing in these wires. We also discuss the influence of stacking faults, which can create artefacts during the reconstruction of the nanowire shape and deformation.

The Paul Scherrer Institute is planning the construction of a hard-x-ray free-electron laser, the SwissFEL, by 2016, which will produce intense, ultrashort pulses of transversely coherent radiation in the wavelength range 0.1–7 nm, with future extensions to cover the range 0.08–30 nm. Special design considerations include (a) a compact construction, compatible with the status of a national facility, (b) a uniform 100 Hz repetition rate, well suited to sample manipulations and detector readout, (c) flexible wavelength tuning by the electron beam energy and undulator gaps, (d) soft x-rays at approximately 1 nm wavelength, with circular polarization and Fourier-transform-limited pulses, (e) hard x-rays of pulse duration 5–20 fs and (f) an independent source of high-energy, half-cycle terahertz pump pulses. The science case for the Swiss FEL project, which emphasizes the dynamics of condensed matter systems and the damage-free imaging of nanostructures, includes novel considerations that make optimal use of these features.

Energy recovery linacs (ERLs) have the potential to be superb coherent hard x-ray sources. ERLs are described with reference to a 5 GeV ERL design being studied at Cornell University. The properties of this ERL, and the x-ray beams that may be produced, are described and spectral curves are calculated and compared to other existing and future x-ray sources. It is shown that ERL and x-ray free electron laser (X-FEL) sources are complementary in terms of the experiments they may optimally serve. ERLs will be especially advantageous in a variety of coherent and nanobeam experiments where the sample must be repetitively probed and in cases where the samples are unique and the requisite scattering information cannot be obtained with a single X-FEL pulse. ERL strengths are elaborated relating to the very high coherent flux, inherently round beams, flexibility and quasi-continuous time structure of the sources. Examples are given where these x-ray characteristics will facilitate advancement of important 'big challenge' areas of science.

We describe the statistical and coherence properties of radiation from x-ray free-electron lasers (XFEL). It is shown that the XFEL radiation before saturation is described with Gaussian statistics. Particularly important is the case of the optimized XFEL studied in detail. Applying similarity techniques to the results of numerical simulations allowed us to find universal scaling relations for the main characteristics of an XFEL operating in the saturation regime: efficiency, coherence time and degree of transverse coherence. We find that with an appropriate normalization of these quantities, they are functions of only the ratio of the geometrical emittance of the electron beam to the radiation wavelength. Statistical and coherence properties of the higher harmonics of the radiation are highlighted as well.

We report soft x-ray speckle metrology measurements of microscopic return point and complementary point memory in Co/Pd magnetic films having perpendicular anisotropy. We observe that the domains assemble into a common labyrinth phase with a period that varies by nearly a factor of two between initial reversal and fields near saturation. Unlike previous studies of similar systems, the ability of the film to reproduce its domain structure after magnetic cycling through saturation varies from loop to loop, from position to position on the sample, and with the part of the speckle pattern used in the metrology measurements. We report the distribution of memory as a function of field and discuss these results in terms of the reversal process.

We report on lensless nanoscale imaging using x-ray waveguides as ultra-small sources for quasi-point-like illumination. We first give a brief account of the basic optical setup, an overview of the progress in waveguide fabrication and characterization, as well as the basics of image formation. We then compare one-step holographic and iterative ptychographic reconstruction, both for simulated and experimental data collected on samples illuminated by waveguided beams. We demonstrate that scanning the sample with partial overlap can substantially improve reconstruction quality in holographic imaging, and that divergent beams make efficient use of the limited dynamic range of current detectors, regardless of the reconstruction scheme. Among different experimental settings presented, smallest source dimensions of 29 nm (horizontal) ×17 nm have been achieved, using multi-modal interference effects. These values have been determined by ptychographic reconstruction of a Ta test structure at 17.5 keV and have been corroborated by simulations of field propagation inside the waveguide.

Mapping conformational heterogeneity of macromolecules presents a formidable challenge to x-ray crystallography and cryo-electron microscopy, which often presume its absence. This has severely limited our knowledge of the conformations assumed by biological systems and their role in biological function, even though they are known to be important. We propose a new approach to determining to high resolution the three-dimensional conformations of biological entities such as molecules, macromolecular assemblies and ultimately cells with existing and emerging experimental techniques. This approach may also enable one to circumvent current limits due to radiation damage and solution purification.

We investigate the feasibility of applying coherent diffraction imaging to highly strained epitaxial nanocrystals using finite-element simulations of SiGe islands as input in standard phase retrieval algorithms. We discuss the specific problems arising from both epitaxial and highly strained systems and we propose different methods to overcome these difficulties. Finally, we describe a coherent microdiffraction experimental setup using extremely focused x-ray beams to perform experiments on individual nanostructures.

Ultrashort pulses from x-ray free-electron laser (XFEL) sources promise to assist in obtaining the structures of membrane proteins at high resolution. We have reconstructed the electron density distribution of a two-dimensional (2D) aquaporin crystal from simulated XFEL data using ptychography, a diffractive imaging technique based on multiple exposures. Increasing the number of exposures compensates for Poisson noise, indicating that the achievable resolution is limited by the reproducibility of the crystals. This technique should therefore be applicable at all future ultrashort-pulsed hard x-ray sources.

A general theoretical approach based on the results of statistical optics is used for the analysis of the transverse coherence properties of third generation hard x-ray synchrotron sources and x-ray free-electron lasers (XFEL). Correlation properties of the wavefields are calculated at different distances from an equivalent Gaussian Schell-model source. This model is used to describe coherence properties of the 5 m undulator source at the synchrotron storage ring PETRA III. In the case of XFEL sources the decomposition of the statistical fields into a sum of independently propagating transverse modes is used for the analysis of the coherence properties of these new sources. A detailed calculation is performed for the parameters of the SASE1 undulator at the European XFEL. It is demonstrated that only a few modes contribute significantly to the total radiation field of that source.

Coherent x-ray imaging represents a new window to imaging non-crystalline, biological specimens at unprecedented resolutions. The advent of free-electron lasers (FEL) allows extremely high flux densities to be delivered to a specimen resulting in stronger scattered signal from these samples to be measured. In the best case scenario, the diffraction pattern is measured before the sample is destroyed by these intense pulses, as the processes involved in radiation damage may be substantially slower than the pulse duration. In this case, the scattered signal can be interpreted and reconstructed to yield a faithful image of the sample at a resolution beyond the conventional radiation damage limit. We employ coherent x-ray diffraction imaging (CXDI) using the free-electron LASer in Hamburg (FLASH) in a non-destructive regime to compare images of a biological sample reconstructed using different, single, femtosecond pulses of FEL radiation. Furthermore, for the first time, we demonstrate CXDI, in-line holography and Fourier transform holography (FTH) of the same unicellular marine organism using an FEL and present diffraction data collected using the third harmonic of FLASH, reaching into the water window. We provide quantitative results for the resolution of the CXDI images as a function of pulse intensity, and compare this with the resolutions achieved with in-line holography and FTH.

An improved generator for continuous unitary transformations is introduced to describe systems with unstable quasi-particles. Its general properties are derived and discussed. To illustrate this approach we investigate the asymmetric antiferromagnetic spin-1/2 Heisenberg ladder, which allows for spontaneous triplon decay. We present results for the low-energy spectrum and the momentum resolved spectral density of this system. In particular, we show the resonance behavior of the decaying triplon explicitly.

For a long time phase arrays have been used in a variety of wave transmission applications because of their simplicity and versatility. Conventionally there is a trade-off between the compactness of a phase array and its directivity. In this paper we demonstrate how by embedding a normal phase array within a superlens (made of negative refractive index material) we can overcome this constraint and create compact phase arrays with a virtual extent much larger than the physical size of the array. In this paper we also briefly discuss the apparent unphysical field divergences in superlenses and how to resolve this issue.

We used first-principles calculations to study the doping effects in bilayer graphene, focusing on Au substitute doping in the upper layer of graphene. We found that Au doping in the upper layer maintains the lattice structure of the lower graphene layer. Our study on binding energy shows that the Au-doped bilayer structure is stable with Au atom tightly confined in a small region between the upper and lower layers. Charge density analysis indicates that charge is transferred from the Au donor to the carbon atoms in the lower layer, increasing the carrier density in the lower graphene.

A transport model for quantum cascade lasers based on density matrix formalism that incorporates the laser optical field is confronted with experiment. For a typical mid-infrared laser, very good agreement is found for both the current–voltage and current–optical power characteristics. Forcing thermal distribution with a unique temperature in all subbands was found to lead to an overestimate of electron heating in the injector. The model can then be used further to optimize and design new structures.

The full Klein–Nishina cross-section of the inverse Compton scattering interactions of electrons implies a significant reduction of the electron energy loss rate compared with the Thomson limit when the electron energy exceeds the critical Klein–Nishina energy E_K=γ_Km_ec²=0.27m²_ec⁴/(k_BT), where T denotes the temperature of the photon graybody distribution. As a consequence, the total radiative energy loss rate of single electrons exhibits sudden drops in the overall -dependence when the electron energy reaches the critical Klein–Nishina energy. The strength of the drop is proportional to the energy density of the photon radiation field. The diffuse galactic optical photon fields from stars of spectral type B and G-K lead to critical Klein–Nishina energies of 40 and 161 GeV, respectively. Associated with the drop in the loss rate are sudden increases (Klein–Nishina steps) in the equilibrium spectrum of cosmic-ray electrons. Because the radiative loss rate of electrons is the main ingredient in any transport model of high-energy cosmic-ray electrons, Klein–Nishina steps will modify any calculated electron equilibrium spectrum irrespective of the electron sources and the spatial transport mode. To delineate most clearly the consequences of the Klein–Nishina decreases in the radiative loss rate, we chose as an illustrative example the simplest realistic model for cosmic-ray electron dynamics in the galaxy, consisting of the competition of radiative losses and secondary production by inelastic hadron–hadron collisions. We demonstrate that the spectral structure in the FERMI and HESS data is well described and even the excess measured by ATIC might be explained by Klein–Nishina steps.

In this paper, We study the Zeeman spin-splitting in hole quantum wires oriented along the [011] and crystallographic axes of a high mobility undoped (100)-oriented AlGaAs/GaAs heterostructure. Our data show that the spin-splitting can be switched 'on' (finite g^*) or 'off' (zero g^*) by rotating the field from a parallel to a perpendicular orientation with respect to the wire, and the properties of the wire are identical for the two orientations with respect to the crystallographic axes. We also find that the g-factor in the parallel orientation decreases as the wire is narrowed. This is in contrast to electron quantum wires, where the g-factor is enhanced by exchange effects as the wire is narrowed. This is evidence for a k-dependent Zeeman splitting that arises from the spin- nature of holes.

The certificate of success for a number of important quantum information processing protocols, such as entanglement distillation, is based on the difference in the entanglement content of the system quantum states before and after the protocol. In such cases, effective bounds need to be placed on the entanglement of non-local states consistent with statistics obtained from local measurements. In this paper, we study numerically the ability of a hybrid homodyne detector that combines phase sensitivity and photon-number resolution to set accurate bounds on the entanglement content of two-mode quadrature squeezed states without the need for full state tomography. We show that it is possible to set tight lower bounds on the entanglement of a family of two-mode degaussified states using only a few measurements. This presents a significant improvement over the resource requirements for the experimental demonstration of continuous-variable entanglement distillation, which traditionally relies on full quantum state tomography.

Ultracold Fermi gases trapped in honeycomb optical lattices provide an intriguing scenario, where relativistic quantum electrodynamics (QED) can be tested. Here, we generalize this system to non-Abelian QED, where massless Dirac fermions interact with effective non-Abelian gauge fields. We show how in this setup a variety of topological phase transitions occur, which arise due to massless fermion pair production events, as well as pair annihilation events of two kinds: spontaneous and strongly interacting induced. Moreover, such phase transitions can be controlled and characterized in optical lattice experiments.

The interaction between a fixed solid spherical particle and stationary turbulence with zero mean flow is investigated numerically. The object diameter, D, lies in the inertial range (D≈0.6L≈0.9λ≈8η, where L, λ and η, respectively, denote the integral scale, the Taylor microscale and the Kolmogorov length) and the particle Reynolds number is close to 20. It is found that the turbulence statistics at different distances from the solid/fluid interface are modified by the presence of the object in a region that extends more than 10 times further than the viscous layer. This estimate is confirmed by the analysis of the correlation between the force and torque on the particle and the force and torque on spherical surfaces surrounding the particle, although the torque decorrelates somewhat faster with increasing distance from the object surface. The angular slip velocity of the particle, a quantity of crucial importance for the modeling of the turbulent transport of large objects, is also characterized.

We analyze transport data from a quantum point contact (QPC), fabricated on a modulation doped Si/SiGe heterostructure, to extract experimental estimates for the valley splitting. The experimental data are fit to a form derived from a valley coupling theory that takes into account the fact that the quantum well is grown on a miscut substrate. The results of the fitting analysis are compared to the results obtained by fitting to a simple phenomenological form; both methods indicate that electrostatic confinement and magnetic confinement enhance the valley splitting by reducing the lateral spatial extent of the electronic wavefunction. Consequently, the valley splitting can be much larger than the spin splitting for small magnetic fields. We observe different valley splittings for the two lowest orbital modes of the QPC, supporting the notion that when steps are present at the quantum well interface the spatial extent of the wavefunction plays a dominant role in determining the valley splitting.

We investigate spin effects in transport across fully interacting, finite-size graphene armchair nanoribbons (ACNs) contacted to collinearly spin-polarized leads. In such systems, the presence of short-range Coulomb interaction between bulk states and states localized at the ribbon ends leads to novel spin-dependent phenomena. Specifically, the total spin of the low-energy many-body states is conserved during tunneling but that of the bulk and end states is not. As a consequence, in the single-electron regime, dominated by Coulomb blockade phenomena, we find pronounced negative differential conductance features for ACNs contacted to parallel polarized leads. These features are, however, absent in an anti-parallel contact configuration, which in turn leads, within a certain gate and bias voltage region, to a negative tunneling magneto-resistance. Moreover, we analyze the changes in the transport characteristics under the influence of an external magnetic field.

Angular momentum intelligent states are defined to satisfy . They also share with angular momentum coherent states a number of features. In this paper we describe and illustrate the squeezing properties of angular momentum intelligent states. We analyze three classes of state: never squeezed, always squeezed and sometimes squeezed. Our conclusions are applicable for a broad range of definitions of the standard quantum limit for angular momentum systems.

The behavior of a single porous particle with a diameter of 250 μm levitating in a radiofrequency (RF) plasma under pulsed argon ion beam bombardment was investigated. The motion of the particle under the action of the ion beam was observed to be an oscillatory motion. The Fourier-analyzed motion is dominated by the excitation frequency of the pulsed ion beam and odd higher harmonics, which peak near the resonance frequency. The appearance of even harmonics is explained by a variation of the particles's charge depending on its position in the plasma sheath. The Fourier analysis also allows a discussion of neutral and ion forces. The particle's charge was derived and compared with theoretical estimates based on the orbital motion-limited (OML) model using also a numerical simulation of the RF discharge. The derived particle's charge is about 7–15 times larger than predicted by the theoretical models. This difference is attributed to the porous structure of the particle.

We show that any pair of real symmetric tensors and can be realized as the effective electric permittivity and effective magnetic permeability of a metamaterial at a given fixed frequency. The construction starts with two extremely low-loss metamaterials, with arbitrarily small microstructure, whose existence is ensured by the work of Bouchitté and Bourel and Bouchitté and Schweizer: one having, at the given frequency, a permittivity tensor with exactly one negative eigenvalue, and a positive permeability tensor; and the other having a positive permittivity tensor, and a permeability tensor having exactly one negative eigenvalue. To achieve the desired effective properties, these materials are laminated together in a hierarchical multiple rank laminate structure, with widely separated length scales, and varying directions of lamination, but with the largest length scale still much shorter than the wavelengths and attenuation lengths in the macroscopic effective medium.

Measurements on entangled quantum states can produce outcomes that are nonlocally correlated. But according to Tsirelson's theorem, there is a quantitative limit on quantum nonlocality. It is interesting to explore what would happen if Tsirelson's bound were violated. To this end, we consider a model that allows arbitrary nonlocal correlations, colloquially referred to as 'box world'. We show that while box world allows more highly entangled states than quantum theory, measurements in box world are rather limited. As a consequence there is no entanglement swapping, teleportation or dense coding.

We report magnetic domain wall (DW) resistance in epitaxial films of FePd. When equal numbers of Fe and Pd atoms are present, this material forms an ordered structure with alternating crystal planes of Fe and Pd. We prepared films enriched with Pd to varying degrees, gradually degrading this structure. As might be expected, this increased the electrical resistivity of the films by introducing extra defects that can scatter electrons. However, unexpectedly, the additional resistance arising from the ∼10 nm thick DWs rose as a proportion of the overall resistivity, roughly doubling when halving the degree of chemical ordering—as determined from x-ray diffraction measurements—within the films. These data can be used to infer a rise in the spin polarization of the current flowing in the layers when extra Pd atoms are introduced. On the other hand, a separate measurement of spin polarization using a superconducting point contact technique that is insensitive to electron scattering revealed no changes as extra Pd was introduced. We conclude that Pd atoms scatter electrons of one spin far more strongly than the other, suggesting a possible means of producing highly spin-polarized currents for use in spintronic devices.

We present a quantum Monte Carlo study of the one-body density matrix (OBDM) and the momentum distribution of one-dimensional (1D) dipolar bosons, with dipole moments polarized perpendicular to the direction of confinement. We observe that the long-range nature of the dipole interaction has dramatic effects on the off-diagonal correlations: although the dipoles never crystallize, the system goes from a quasi-condensate regime at low interactions to a regime in which quasi-condensation is discarded in favor of quasi-solidity. For all strengths of the dipolar interaction we considered, the OBDM shows an oscillatory behavior coexisting with an overall algebraic decay, while the momentum distribution shows sharp kinks at the wavevectors of the oscillations, Q=±2πn (where n is the atom density), beyond which it is strongly suppressed. This momentum filtering effect introduces a characteristic scale in the momentum distribution, which can be arbitrarily squeezed by lowering the atom density. This shows that 1D dipolar Bose gases, realized e.g. by trapped dipolar molecules, show strong signatures of the dipolar interaction in time-of-flight measurements.

In this paper, we report the design, fabrication and preliminary testing of a 150 zone ion trap array built in a 'surface-electrode' geometry microfabricated on a single substrate. We demonstrate the transport of atomic ions between the legs of a 'Y'-type junction and measure the in-situ heating rates for the ions. The trap design demonstrates the use of a basic component design library that can be quickly assembled to form structures optimized for a particular experiment.

We demonstrate that excitons in semiconductor alloys are subject to competing localization effects due to disorder (random potential fluctuations) and shallow point defects (impurities). The relative importance of these effects varies with alloy chemical composition, impurity activation energy as well as temperature. We evaluate this effect quantitatively for Mg_xZn_1−xO:Al (0⩽x⩽0.058) and find that exciton localization at low (2 K) and high (300 K) temperatures is dominated by shallow donor impurities and alloy disorder, respectively.

In this paper, we describe a refined matrix product representation for many-body states that are invariant under SU(2) transformations and use it to extend the time-evolving block decimation (TEBD) algorithm to the simulation of time evolution in the presence of an SU(2) symmetry. The resulting algorithm, when tested in a critical quantum spin chain, proved to be more efficient than the standard TEBD.

Two parallel dielectric plates separated by vacuum interact through zero-point charge fluctuations and experience friction when the plates are in relative motion and the vacuum is sheared. Even at the absolute zero of temperature, residual quantum fluctuations remain because the zero-point energy gives rise to 'quantum friction'. In a recent paper, the reality of these fluctuations is questioned and the existence of quantum friction is called into question. Here we refute this assertion.

The problem of quantum dice rolling (DR)—a generalization of the problem of quantum coin flipping (CF) to more than two outcomes and parties—is studied in both its weak and strong variants. We prove by construction that quantum mechanics allows for (i) weak N-sided DR admitting arbitrarily small bias for any N and (ii) two-party strong N-sided DR saturating Kitaev's bound for any N. To derive (ii) we also prove by construction that quantum mechanics allows for (iii) strong imbalanced CF saturating Kitaev's bound for any degree of imbalance. Furthermore, as a corollary of (ii) we introduce a family of optimal 2m-party strong n^m-sided DR protocols for any pair m and n.

This paper presents a new approach for analysing the structural properties of time series from complex systems. Starting from the concept of recurrences in phase space, the recurrence matrix of a time series is interpreted as the adjacency matrix of an associated complex network, which links different points in time if the considered states are closely neighboured in phase space. In comparison with similar network-based techniques the new approach has important conceptual advantages, and can be considered as a unifying framework for transforming time series into complex networks that also includes other existing methods as special cases. It has been demonstrated here that there are fundamental relationships between many topological properties of recurrence networks and different nontrivial statistical properties of the phase space density of the underlying dynamical system. Hence, this novel interpretation of the recurrence matrix yields new quantitative characteristics (such as average path length, clustering coefficient, or centrality measures of the recurrence network) related to the dynamical complexity of a time series, most of which are not yet provided by other existing methods of nonlinear time series analysis.

We investigate the concept of entropy in probabilistic theories more general than quantum mechanics, with particular reference to the notion of information causality (IC) recently proposed by Pawlowski et al (2009 arXiv:0905.2292). We consider two entropic quantities, which we term measurement and mixing entropy. In the context of classical and quantum theory, these coincide, being given by the Shannon and von Neumann entropies, respectively; in general, however, they are very different. In particular, while measurement entropy is easily seen to be concave, mixing entropy need not be. In fact, as we show, mixing entropy is not concave whenever the state space is a non-simplicial polytope. Thus, the condition that measurement and mixing entropies coincide is a strong constraint on possible theories. We call theories with this property monoentropic.

Measurement entropy is subadditive, but not in general strongly subadditive. Equivalently, if we define the mutual information between two systems A and B by the usual formula I(A: B)=H(A)+H(B)-H(AB), where H denotes the measurement entropy and AB is a non-signaling composite of A and B, then it can happen that I(A:BC)<I(A:B). This is relevant to IC in the sense of Pawlowski et al: we show that any monoentropic non-signaling theory in which measurement entropy is strongly subadditive, and also satisfies a version of the Holevo bound, is informationally causal, and on the other hand we observe that Popescu–Rohrlich boxes, which violate IC, also violate strong subadditivity. We also explore the interplay between measurement and mixing entropy and various natural conditions on theories that arise in quantum axiomatics.

Information plays an important role in our understanding of the physical world. Hence we propose an entropic measure of information for any physical theory that admits systems, states and measurements. In the quantum and classical worlds, our measure reduces to the von Neumann and Shannon entropies, respectively. It can even be used in a quantum or classical setting where we are only allowed to perform a limited set of operations. In a world that admits superstrong correlations in the form of non-local boxes, our measure can be used to analyze protocols such as superstrong random access encodings and the violation of 'information causality'. However, we also show that in such a world no entropic measure can exhibit all the properties we commonly accept in a quantum setting. For example, there exists no 'reasonable' measure of conditional entropy that is subadditive. Finally, we prove a coding theorem for some theories that is analogous to the quantum and classical settings, providing us with an appealing operational interpretation.

The temperature-dependent properties of a triangular flux-line lattice (FLL) in the low-flux density regime were investigated by evaluating the imaged flux-line (FL) size and the lattice regularity observed in real space utilizing magnetic force microscopy (MFM). At low temperatures, pinning by randomly distributed point defects in the anisotropic type-II superconductor Bi₂Sr₂CaCu₂O_8+δ results in curved FLs and lateral disorder within the FLL (Bragg glass). Above 30 K, depinning of pancake vortices (PVs) leads to straightening of FLs and a better-ordered lattice. Evaluation of the temperature-dependent imaged FL size allows us to determine the stiffness of the potential, in which FLs in the lattice are caged due to mutual repulsion between them. At 54.1 K, far below melting temperatures reported so far, thermal fluctuations plus the lateral force exerted by the scanning tip facilitate decoupling of PVs near the surface and the image contrast exhibit a liquid-like behavior. Our analysis demonstrates the ability of MFM to obtain three-dimensional information on the arrangement of PVs.

When an electrolyte solution is pressurized into a molecular-sized nanopore, oppositely charged ions are strongly inclined to aggregate, which effectively reduces the ion solubility to zero. Inside the restrictive confinement, a unique quasi-periodic structure is formed where the paired ion couples are periodically separated by a number of water molecules. As the anion size or ion concentration varies, the geometrical characteristics of the confined ion structure would change considerably, leading to a significant variation in the transport pressure. Both experimental and simulation results indicate that, contradictory to the prediction of conventional theory, infiltration pressure decreases as the anions become larger.

In this paper, we first present a simple measure for multiqubit entanglement based on the strategy of bipartite cuts and the measure of negativity. Then, we establish generalized monogamy inequalities and associated partition-dependent residual entanglement (PRE) accounting for arbitrary partitions of a multiqubit system. By virtue of the defined quantities, we investigate the entanglement dynamics of a system of N qubits, either in the Greenberger–Horne–Zeilinger (GHZ)-type state or in the W state, interacting with N independent reservoirs in both Markovian and non-Markovian regimes. We observe entanglement revivals of qubits at instantaneous points of disappearance or after a finite interval of abrupt vanishing due to the memory effect of non-Markovian reservoirs. We also follow the whole entanglement evolution in terms of the PRE to demonstrate the process of transition between the bipartite entanglement of all possible bipartitions and the multipartite entanglement. In particular, we show that the change in time of entanglement formats differs qualitatively for the GHZ-type and W states.

We propose an experimental scheme to prove the photon commutation relation, [a, a^†]=1. The scheme exploits interaction between a single-mode cavity field and three two-level atoms, two of which are prepared in an entangled state. The success of the scheme is subject to observation of the atoms in predetermined states after the interaction. We show, in particular, that a reasonably high success probability can be obtained with our scheme by preparing the cavity field in a superposition of two Fock states and choosing the interaction times appropriately.

Manipulation of atomic and molecular beams is essential to atom optics applications including atom lasers, atom lithography, atom interferometry and neutral atom microscopy. The manipulation of charge-neutral beams of limited polarizability, spin or excitation states remains problematic, but may be overcome by the development of novel diffractive or reflective optical elements. In this paper, we present the first experimental demonstration of atom focusing using an ellipsoidal mirror. The ellipsoidal mirror enables stigmatic off-axis focusing for the first time and we demonstrate focusing of a beam of neutral, ground-state helium atoms down to an approximately circular spot, (26.8±0.5) μm×(31.4±0.8) μm in size. The spot area is two orders of magnitude smaller than previous reflective focusing of atomic beams and is a critical milestone towards the construction of a high-intensity scanning helium microscope.

The sheet plasmon in epitaxially grown graphene layers on SiC(0001) and the influence of surface roughness have been investigated in detail by means of low-energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS). We show that the existence of steps or grain boundaries in this epitaxial system is a source of strong damping, while the dispersion is rather insensitive to defects. To the first order, the lifetime of the plasmons was found to be proportional to the average terrace length and to the plasmon wavelength. A possible reason for this surprisingly efficient plasmon damping may be the close coincidence of phase (and group) velocities of the plasmons (almost linear dispersion) with the Fermi velocity of the electrons. Therefore, uncorrelated defects like steps only have to act as a momentum source to effectively couple plasmons to the electron–hole continuum.

Dice tossing is commonly believed to be random. However, throwing a fair cube is a dissipative process that is well described by deterministic classical mechanics. In Nagler and Richter (2008 Phys. Rev. E 78 036207; featured in 2008 Nature455 434), we proposed a simplified model to analyze the origin of the pseudorandomness: a barbell with two masses at its tips with only two final outcomes. In order to keep things simple, we focused on the symmetrical case of equal masses. Here, we complete the picture by considering the general asymmetric case of unequal masses. We show how, depending on the initial conditions, dissipation during bounces, and mass asymmetry, the degree of unpredictability varies. Our analysis reveals, for the simplest possible non-trivial dice throwing model, the effect of dice loading. A surprising consequence of dynamical resonances is that an experienced player may benefit sometimes more from an unloaded than from a loaded barbell. In addition, we investigate the influence of loading on the symmetry breaking process causing one mass to come to rest earlier than the others.

The most striking feature of quantum mechanics is the existence of superposition states, where an object appears to be in different situations at the same time. The existence of such states has been previously tested with small objects, such as atoms, ions, electrons and photons (Zoller et al 2005 Eur. Phys. J. D 36 203–28), and even with molecules (Arndt et al 1999 Nature401 680–2). More recently, it has been shown that it is possible to create superpositions of collections of photons (Deléglise et al 2008 Nature455 510–14), atoms (Hammerer et al 2008 arXiv:0807.3358) or Cooper pairs (Friedman et al 2000 Nature406 43–6). Very recent progress in optomechanical systems may soon allow us to create superpositions of even larger objects, such as micro-sized mirrors or cantilevers (Marshall et al 2003 Phys. Rev. Lett.91 130401; Kippenberg and Vahala 2008 Science321 1172–6; Marquardt and Girvin 2009 Physics2 40; Favero and Karrai 2009 Nature Photon.3 201–5), and thus to test quantum mechanical phenomena at larger scales. Here we propose a method to cool down and create quantum superpositions of the motion of sub-wavelength, arbitrarily shaped dielectric objects trapped inside a high-finesse cavity at a very low pressure. Our method is ideally suited for the smallest living organisms, such as viruses, which survive under low-vacuum pressures (Rothschild and Mancinelli 2001 Nature406 1092–101) and optically behave as dielectric objects (Ashkin and Dziedzic 1987 Science235 1517–20). This opens up the possibility of testing the quantum nature of living organisms by creating quantum superposition states in very much the same spirit as the original Schrödinger's cat 'gedanken' paradigm (Schrödinger 1935 Naturwissenschaften23 807–12, 823–8, 844–9). We anticipate that our paper will be a starting point for experimentally addressing fundamental questions, such as the role of life and consciousness in quantum mechanics.

The Pierre Auger Observatory is an international facility dedicated to the full-sky study of the highest-energy cosmic rays. The southern site of the Auger Observatory was completed in June 2008. Data collected since January 2004 have yielded important information on the energy spectrum, the primary particle composition, the fluxes of photons and neutrinos and on the anisotropic distribution of the arrival directions of the most energetic particles. On this basis, the scientific motivation for the northern Auger Observatory site in Colorado, USA, is discussed. The overall layout, the key components and the expected performance of this 20 000 km² hybrid observatory comprised of an array of 4400 surface detectors and 39 fluorescence telescopes are described.

In this work, we have investigated the spectral function of graphene on a monolayer of intercalated gold on Ru(0001) using angle-resolved photoemission spectroscopy (ARPES). The intercalation leads to a decoupling of the graphene film, as documented by emergence of the characteristic linear π-bands near the Fermi level. However, a band gap at the band crossing is observed. We relate this gap opening to the broken symmetry of the two carbon sublattices, induced by the special lattice mismatch of the graphene and the intercalated gold monolayer.

We investigate a Bose gas with finite-range interaction using a scheme to eliminate unphysical processes in the T-matrix approximation. In this way the corrected T-matrix becomes suitable to calculate properties below the critical temperature. For attractive interaction, an Evans–Rashid transition occurs between a quasi-ideal Bose gas and a Bardeen–Cooper–Schrieffer-like phase with a gap dispersion. The gap decreases with increasing density and vanishes at a critical density where the single-particle dispersion becomes linear for small momenta, indicating Bose–Einstein condensation. The investigation of the pressure shows, however, that the mentioned quantum phase transitions might be inaccessible due to a preceding first-order transition.

We consider the possibility of adding noise to a quantum circuit to make it efficiently simulatable classically. In previous works, this approach has been used to derive upper bounds to fault tolerance thresholds—usually by identifying a privileged resource, such as an entangling gate or a non-Clifford operation, and then deriving the noise levels required to make it 'unprivileged'. In this work, we consider extensions of this approach where noise is added to Clifford gates too and then 'commuted' around until it concentrates on attacking the non-Clifford resource. While commuting noise around is not always straightforward, we find that easy instances can be identified in popular fault tolerance proposals, thereby enabling sharper upper bounds to be derived in these cases. For instance we find that if we take Knill's (2005 Nature434 39) fault tolerance proposal together with the ability to prepare any possible state in the XY plane of the Bloch sphere, then not more than 3.69% error-per-gate noise is sufficient to make it classical, and 13.71% of Knill's γ noise model is sufficient. These bounds have been derived without noise being added to the decoding parts of the circuits. Introducing such noise in a toy example suggests that the present approach can be optimized further to yield tighter bounds.

The dynamic behavior of the single-crystal Al under [001] uniaxial strain is simulated by classic molecular dynamics. The fcc–hcp structural transition is successfully observed when the loading pressure reaches about 90 GPa, and the reverse transition is also found with hysteresis. The mechanism and morphology evolution of both the forward and backward transitions are analyzed in detail. It is found in the process of the structural transition that the (010)_fcc or (100)_fcc planes transit into (0001)_hcp planes, and the twins of the hcp phase along the (112)-plane appear, whose boundaries finally become along the (110)-plane. Besides, we find the twinning (along the (110)_fcc planes) in the hcp phase prior to the back transition (hcp–fcc). Our simulations show the coexistence of fcc and hcp phases over a wide range of pressures, and finally, the phase transition is evaluated by using the radial distribution functions.

In Regge calculus, space–time is usually approximated by a triangulation with flat simplices. We present a formulation using simplices with constant sectional curvature adjusted to the presence of a cosmological constant. As we will show, such a formulation allows us to replace the length variables by three- or four-dimensional dihedral angles as basic variables. Moreover, we will introduce a first-order formulation, which, in contrast to using flat simplices, does not require any constraints. These considerations could be useful for the construction of quantum gravity models with a cosmological constant.

We propose a method to probe dispersive atom–surface interactions by measuring via two-photon Bragg spectroscopy the dynamic structure factor of a Bose–Einstein condensate above corrugated surfaces. This method takes advantage of the condensate coherence to reveal the spatial Fourier components of the lateral Casimir–Polder interaction energy.

Self-excitation is a mechanism that is ubiquitous for electromechanical power devices such as electrical generators. This is conventionally achieved by making use of the magnetic field component in electrical generators (Nedic and Lipo 2000 IEEE/IAS Conf. Records (Rome, Italy) vol 1 pp 51–6), a good and widely visible example of which is the wind turbine farm (Muljadi et al 2005 J. Sol. Energy Eng.127 581–7). In other words, a static force, such as the wind acting on rotor blades, can generate a resonant excitation at a certain mechanical frequency. For nanomechanical systems (Craighead 2000 Science290 1532–5; Roukes 2001 Phys. World14 25–31; Cleland 2003 Foundations of Nanomechanics (Berlin: Springer); Ayari et al 2007 Nano Lett.7 2252–7; Koenig et al 2008 Nat. Nanotechnol.3 482–4) such a self-excitation (SE) mechanism is also highly desirable, because it can generate mechanical oscillations at radio frequencies by simply applying a dc bias voltage. This is of great importance for low-power signal communication devices and detectors, as well as for mechanical computing elements. For a particular nanomechanical system—the single electron shuttle—this effect was predicted some time ago by Gorelik et al (Phys. Rev. Lett.80 4526–9). Here, we use a nanoelectromechanical single electron transistor (NEMSET) to demonstrate self-excitation for both the soft and hard regimes, respectively. The ability to use self-excitation in nanomechanical systems may enable the detection of quantum mechanical backaction effects (Naik et al 2006 Nature443 193–6) in direct tunneling, macroscopic quantum tunneling (Savelev et al 2006 New J. Phys.8 105–15) and rectification (Pistolesi and Fazio 2005 Phys. Rev. Lett.94 036806–4). All these effects have so far been overshadowed by the large driving voltages that had to be applied.

We present a scheme that produces a strong U(1)-like gauge field on cold atoms confined in a two-dimensional square optical lattice. Our proposal relies on two essential features, a long-lived metastable excited state that exists for alkaline-earth or ytterbium atoms and an optical superlattice. As in the proposal by Jaksch and Zoller (2003 New J. Phys.5 56), laser-assisted tunneling between adjacent sites creates an effective magnetic field. In the tight-binding approximation, atomic motion is described by the Harper Hamiltonian, with a flux across each lattice plaquette that can realistically take any value between 0 and π. We show how one can take advantage of the superlattice to ensure that each plaquette acquires the same phase, thus simulating a uniform magnetic field. We discuss the observable consequences of the artificial gauge field on non-interacting bosonic and fermionic gases. We also outline how the scheme can be generalized to non-Abelian gauge fields.

We study magnetohydrodynamics in a von Kármán flow driven by the rotation of impellers made of material with varying electrical conductivity and magnetic permeability. Gallium is the working fluid and magnetic Reynolds numbers of order unity are achieved. We find that specific induction effects arise when the impellers' electric and magnetic characteristics differ from that of the fluid. Implications with regard to the VKS dynamo are discussed.

A crystallographic and magnetic phase diagram of SmFeAsO_1−xF_x is determined as a function of x in terms of temperature based on electrical transport and magnetization, synchrotron powder x-ray diffraction, ⁵⁷Fe Mössbauer spectra (MS), and ¹⁴⁹Sm nuclear resonant forward scattering (NRFS) measurements. MS revealed that the magnetic moments of Fe were aligned antiferromagnetically at ∼144 K (T_N(Fe)). The magnetic moment of Fe (M_Fe) is estimated to be 0.34 μ_B/Fe at 4.2 K for undoped SmFeAsO; M_Fe is quenched in superconducting F-doped SmFeAsO. ¹⁴⁹Sm NRFS spectra revealed that the magnetic moments of Sm start to order antiferromagnetically at 5.6 K (undoped) and 4.4 K (T_N(Sm)) (x=0.069). Results clearly indicate that the antiferromagnetic (AF) Sm sublattice coexists with the superconducting phase in SmFeAsO_1−xF_x below T_N(Sm), while the AF Fe sublattice does not coexist with the superconducting phase.

The non-classical rotational inertia (NCRI) in solid helium was detected by a drop in the resonant period of a torsional oscillator. This non-classical response was interpreted as the first possible evidence of supersolidity. A number of subsequent experiments, however, reported unexpected phenomena within the supersolid context. Experimental and theoretical work have drawn attention to the role of disorder in solid helium to explain the inconsistency. We have investigated the non-classical response of solid ⁴He confined in porous gold set to torsional oscillation. When solid helium is grown rapidly, nearly 7% of the solid helium appears to be decoupled from the oscillation below about 200 mK. Dissipation appears at temperatures where the decoupling shows maximum variation. In contrast, the decoupling is substantially reduced in slowly grown solid helium. The dynamic response of solid helium was also studied by imposing a sudden increase in the amplitude of oscillation. Extended relaxation in the resonant period shift, suggesting the emergence of the pinning of low-energy excitations, was observed below the onset temperature of the non-classical response. The motion of a dislocation or a glassy solid is restricted in the entangled narrow pores and is not likely responsible for the period shift and long relaxation.

In this work, we investigate, using state-of-the-art high-resolution photoemission spectroscopy, the spectral evolution of an antiferromagnetic insulator, La_0.2Sr_0.8MnO₃, exhibiting linear specific heat. Experimental spectral functions exhibited Fermi liquid-like energy dependence at all the temperatures studied. Room temperature spectra possess finite density of states at the Fermi level that vanishes, generating a soft gap at about 265 K (the magnetic transition temperature). High-resolution spectra reveal a hard gap in the magnetically ordered phase (C-type antiferromagnet). These results indicate the signature of an amorphous phase, presumably magnetic in nature, coexisting with the long-range ordered phase in these materials.

Due to an error in the publication process, an incomplete version of the article by R Bindu et al 2010 New J. Phys.12 033003 was published on 5 March 2010. This version of the article has now been formally withdrawn by the journal and superseded by a new version published on 15 March 2010 with the article reference R Bindu et alNew J. Phys.12 033026.

We investigate the spectral evolution of an antiferromagnetic insulator, La_0.2Sr_0.8MnO₃, exhibiting linear specific heat, using state-of-the-art high resolution photoemission spectroscopy. Experimental spectral functions exhibit Fermi liquid-like energy dependence at all the temperatures studied. Room temperature spectra possess finite density of states at the Fermi level, which vanishes, generating a soft gap, at about 260 K (the magnetic transition temperature). High-resolution spectra reveal a hard gap in the magnetically ordered phase (C-type antiferromagnet). These results indicate an amorphous phase coexisting with the long-range ordered phase in these materials.

We propose a scheme of 'R-type' photoassociative adiabatic passage (PAP) to create polar molecular condensates from two different species of ultracold atoms. Due to the presence of a quasi-coherent population trapping state in the scheme, it is possible to associate atoms into molecules with a low-power photoassociation (PA) laser. One remarkable advantage of our scheme is that a tunable atom–molecule coupling strength can be achieved by using a time-dependent PA field, which exhibits larger flexibility than using a tunable magnetic field. In addition, our results show that the PA intensity required in the 'R-type' PAP could be greatly reduced compared to that in a conventional 'Λ-type' one.

In this paper, we discuss our recent Cu K-edge resonant inelastic x-ray scattering (RIXS) results for CuB₂O₄, a model system consisting of electronically isolated CuO₄ plaquettes, which exhibits anomalies in the incident photon energy dependence that are not captured by electron-only models. We show that these anomalous features can be qualitatively described by extending the electron-only considerations to include the lattice degrees of freedom. Our findings have important general implications for the interpretation of K-edge RIXS results for electronically higher-dimensional transition metal oxides.