Table of contents

In an array of coupled cavities where the cavities are doped with an atomic V-system, and the two excited levels couple to cavity photons of different polarizations, we show how to construct various spin models employed in characterizing phenomena in condensed matter physics, such as the spin-(1/2) Ising, XX, Heisenberg, and XXZ models. The ability to construct networks of arbitrary geometry also allows for the simulation of topological effects. By tuning the number of excitations present, the dimension of the spin to be simulated can be controlled, and mixtures of different spin types produced. The facility of single-site addressing, the use of only the natural hopping photon dynamics without external fields, and the recent experimental advances towards strong coupling, makes the prospect of using these arrays as efficient quantum simulators promising.

The Galilean constitutive equations for the electrodynamics of moving media are derived for the first time. They explain all the historic and modern experiments which were interpreted so far in a relativistic framework assuming the constant light celerity principle. Here, we show the latter to be sufficient but not necessary.

In this paper, we investigate the fermion tunneling through the event horizon of a Vaidya black hole which is non-stationary. We further take into account the particle's self-gravitation in the dynamical background space-time, and calculate the tunneling probability. The result shows that the tunneling probability is related not only to the change of Bekenstein-Hawking entropy but also to the integral of the changing horizon, which is different from the case of stationary black holes.

In this paper, we investigate two versions of equations which describe the motion of relativistic membranes in the Minkowski space and prove the equivalence between them. Moreover, we give another geometric explanation about the equivalence by Noether's second theorem. Applying the same argument, we also give the equivalence between two versions of equations governing the motion of relativistic strings.

We explore the diffusion process in the non-Markovian spatio-temporal noise. There is a non-trivial short-memory regime, i.e., the Markovian limit characterized by a scaling relation between the spatial and temporal correlation lengths. In this regime, a Fokker-Planck equation is derived by expanding the trajectory around the systematic motion and the non-Markovian nature amounts to the systematic reduction of the potential. For a system with the potential barrier, this fact leads to the renormalization of both the barrier height and collisional prefactor in the Kramers escape rate, with the resultant rate showing a maximum at some scaling limit.

We have investigated the validity of the fluctuation-dissipation theorem (FDT) and the applicability of the concept of effective temperature in a number of non-equilibrium soft glassy materials. Using a combination of passive and active microrheology to measure displacement fluctuations and the mechanical response function of probe particles embedded in the materials, we have directly tested the validity of the FDT. Our results show no violation of the FDT over several decades in frequency (1–10⁴ Hz) for hard-sphere colloidal glasses and colloidal glasses and gels of Laponite. We further extended the bandwidth of our measurements to lower frequencies (down to 0.1 Hz) using video microscopy to measure the displacement fluctuations, again without finding any deviations from the FDT.

We solve the Ricci-flat equations of extended general relativity to obtain an interesting class of cosmological models. The solutions are analogous to the 4D ones of Bianchi type-I of Kasner type and have significant implications for astrophysics.

The Generalized Uncertainty Principle (GUP), motivated by current alternatives of quantum gravity, produces significant modifications to the Hawking radiation and the final stage of black hole evaporation. We show that incorporation of the GUP into the quantum tunneling process (based on the null-geodesic method) causes correlations between the tunneling probability of different modes in the black hole radiation spectrum. In this manner, the quantum information becomes encrypted in the Hawking radiation, and information can be recovered as non-thermal GUP correlations between tunneling probabilities of different modes.

We show by means of two pulse electron spin echo experiments on Fe³⁺ transition metal ions in natural ZnO that their spins have a spin coherence time limited at T=6 K first by instantaneous diffusion (≈150 μs), then by nuclear spectral diffusion (≈450 μs) and ultimately by spin-lattice processes (≈1.4 ms). We predict a spin coherence time in the millisecond range for such a spin qubit in isotopically and chemically purified ZnO. The spin state of a single Fe³⁺ spin qubit could be readout by optical methods and it could be coherently manipulated using pulsed electron spin resonance (ESR) methods. A free carrier electrically gated between two nearby Fe³⁺ ions could efficiently couple two Fe³⁺ spin qubits. All those elements suggest the high potential of the Fe³⁺ spin qubits in ZnO for the implementation of a scalable ESR quantum computer.

In recent years there have been a number of proposals to utilize the specificity of DNA-based interactions for potential applications in nanoscience. One interesting direction is the self-assembly of micro- and nanoparticle clusters using DNA scaffolds. In this letter we consider a DNA scaffold method to self-assemble clusters of "colored" particles. Stable clusters of identical microspheres have recently been produced by an entirely different method. Our DNA-based approach self-assembles clusters with additional degrees of freedom associated with particle permutation. We demonstrate that in the non-equilibrium regime of irreversible binding the self-assembly process is experimentally feasible. These color degrees of freedom may allow for more diverse intercluster interactions essential for hierarchical self-assembly of larger structures.

We assume a constant current density in a homogeneous one-component plasma of infinite extent and calculate the resulting magnetic energy per particle. Our starting point is the conserved approximately relativistic (Darwin) energy for a system of electromagnetically interacting particles that arises from the neglect of radiation. For the idealized model of a homogeneous one-component plasma the energy only depends on the particle canonical momenta and the vector potential. The vector potential is then calculated in terms of the canonical momenta using recent theoretical advances and the plasma Hamiltonian is obtained. The result can be understood either as due to the energy lowering caused by the attraction of parallel currents or, alternatively, as due to the inductive inertia associated with the flow of net current.

We propose a novel scheme to manipulate the wave function of a trapped Bose-Einstein condensate (BEC) by coupling its internal degrees of freedom to external optical vortex modes. The atom-light coupling induces an effective magnetic field on the center-of-mass motion, which drives the trapped BEC to vortex or non-vortex states determined by the effective magnetic flux. With a suitable external trap, the wave function of the optical mode can be fully written into the BEC and stored there. Based on this model, we also propose new techniques to generate static spin accumulation and circulating spin current in cold atomic systems.

We show how continuous-variable systems can allow the direct communication of messages with an acceptable degree of privacy. This is possible by combining a suitable phase-space encoding of the plain message with real-time checks of the quantum communication channel. The resulting protocol works properly when a small amount of noise affects the quantum channel. If this noise is non-tolerable, the protocol stops leaving a limited amount of information to a potential eavesdropper.

We calculate the correction to the nuclear fission barrier produced by the atomic electrons. The result presented in analytical form is convenient to use in future nuclear calculations. The atomic electrons have a small stabilizing effect on nuclei, increasing the lifetime in the nuclear fission channel. This effect may be used to study the fission process.

Turbulent rotating convection in a cylinder is investigated both numerically and experimentally at Rayleigh number Ra=10⁹ and Prandtl number σ=6.4. In this letter we discuss two topics: the breakdown under rotation of the domain-filling large-scale circulation (LSC) typical for confined convection, and the convective heat transfer through the fluid layer, expressed by the Nusselt number. The presence of the LSC is addressed for several rotation rates. For Rossby numbers Ro≲1.2 no LSC is found (the Rossby number indicates relative importance of buoyancy over rotation, hence small Ro indicates strong rotation). For larger Rossby numbers a precession of the LSC in anticyclonic direction (counter to the background rotation) is observed. It is shown that the heat transfer has a maximal value close to Ro=0.18 being about 15% larger than in the non-rotating case Ro=∞. Since the LSC is no longer present at this Rossby value we conclude that the peak heat transfer is independent of the LSC.

A numerical molecular dynamics experiment measuring the two-time correlation function of the transversal velocity field in the homogeneous cooling state of a granular gas modeled as an ensemble of inelastic hard particles is reported. By measuring the decay rate and the amplitude of the correlations, the accuracy of the Landau-Langevin equation of fluctuating hydrodynamics is checked. The results indicate that although a Langevin approach can be valid, the fluctuation-dissipation relation must be modified, since the viscosity parameter appearing in it differs from the usual hydrodynamic shear viscosity.

It is shown that for Boussinesq flows in which rotation and stratification are equally strong, the forward cascade of potential enstrophy constrains the spectral distribution of horizontal kinetic energy and potential energy. Horizontal kinetic energy is suppressed in the small–aspect-ratio wave modes, and potential energy in suppressed in the large–aspect-ratio wave modes. Scaling estimates based on phenomenological arguments yield scaling of k_h^{- 3} and k_z^{- 3}, respectively, of the two spectra. High-resolution numerical simulations of the Boussinesq equations in the relevant parameter regimes show spectral scaling exponent closer to - 4, and hence even stronger suppression than is predicted by dimensional estimates.

The history of the linear perturbations in a "laser imprinting" configuration in inertial confinement fusion is described. The time-dependent mean flow is provided by a self-similar solution of gas dynamics equations with nonlinear heat conduction for semi-infinite slabs of perfect gases. The analysis is conducted with the Kovásznay modes, namely the vorticity, acoustic and entropy modes. Exact propagation equations for these three basic modes are derived. Both the similarity solutions and their linear perturbations are numerically computed with an adaptive multidomain Chebyshev method. In particular, the dynamics of the shock wave is detailed. Compressibility effects and possible implications to inertial confinement fusion experiments are emphasized.

Results from scanning electron microscopy, Fourier transform infrared spectroscopy and the measurement of thermally stimulated current show that a high density of the physical defects and the chemical defects are introduced into the surface of the silicone rubber plates after they are treated by corona discharge plasma. These defects behave electrically as shallow electron traps, leading to the formation of a uniform discharge in air at higher pressure when the corona-modified silicone rubber is used in dielectric barrier discharge.

Thermodynamic perturbation theory for central-force associating potentials and Monte Carlo simulations are used to study the phase behaviour of the dipolar Yukawa hard-sphere fluid over a wide range of the particle dipole moment, μ. Liquid-vapour coexistence is found to exist for values of μ far in excess of a "threshold" value found in earlier simulation studies. The predictions of the present theory are found to be in reasonably good agreement with computer simulation results, all the way up to the highest dipole moment studied.

We report X-ray diffraction measurements of the charge-ordered organic conductor θ-(BEDT-TTF)₂CsZn(SCN)₄ at various temperatures with external currents. The two kinds of diffuse scattering observed at q₁=(2/3 k 1/3) and q₂=(0 k 1/2) exhibit remarkable difference in the interplane correlation along the b-axis, the temperature dependence, and the electric-current dependence. These observations suggest that the two diffuse scatterings come from spatially different charge-ordered domains. The external current melts the q₂-type domains, which can control the competition of the two orders.

We apply a constant-pressure ab initio technique to investigate the high-pressure phase transformation of 6H-SiC and show that it transforms into a rocksalt structure. This phase change proceeds in two stages: 6H-SiC is first compressed along the c-direction and then it undergoes a shear deformation on the a-b planes. This transformation mechanism is quite similar to that of the wurtzite-to-rocksalt observed in 2H-SiC but there is no metastable phase identified along this path. The 6H-to-RS phase transition is also analyzed from the energy volume calculations. The computed transition parameters agree well with the experimental data.

In biomimetic and biological systems, interactions between surfaces are often mediated by adhesive molecules, nanoparticles, or colloids dispersed in the surrounding solution. We present here a general, statistical-mechanical model for two surfaces that interact via adhesive particles. The effective, particle-mediated interaction potential of the surfaces is obtained by integrating over the particles' degrees of freedom in the partition function. Interestingly, the effective adhesion energy of the surfaces exhibits a maximum at intermediate particle concentrations, and is considerably smaller both at low and high concentrations. The effective adhesion energy corresponds to a minimum in the interaction potential at surface separations slightly larger than the particle diameter, while a secondary minimum at surface contact reflects depletion interactions. Our results can be generalized to surfaces with specific receptors for solute particles, and have direct implications for the adhesion of biomembranes and for phase transitions in colloidal systems.

When cooled or compressed sufficiently rapidly, a liquid vitrifies into a glassy amorphous state. Vitrification in a dense liquid is associated with jamming of the particles. For hard spheres, the density and degree of order in the final structure depend on the compression rate: simple intuition suggests, and previous computer simulation demonstrates, that slower compression results in states that are both denser and more ordered. In this work, we use the Lubachevsky-Stillinger algorithm to generate a sequence of structurally arrested hard-sphere states by varying the compression rate. We find that while the degree of order, as measured by both bond-orientation and translation order parameters, increases monotonically with decreasing compression rate, the density of the arrested state first increases, then decreases, then increases again, as the compression rate decreases, showing a minimum at an intermediate compression rate. Examination of the distribution of the local order parameters and the distribution of the root-mean-square fluctuation of the particle positions, as well as direct visual inspection of the arrested structures, reveal that they are structurally heterogeneous, consisting of disordered, amorphous regions and locally ordered crystal-like domains. In particular, the low-density arrested states correspond with many interconnected small crystal clusters that form a polycrystalline network interspersed in an amorphous background, suggesting that jamming by the domains may be an important mechanism for these states.

Polymer nanoparticles are used for the targeted delivery of drugs, where their mechanical response to hydrodynamic flows can influence their performance. In this study, computer simulations elucidate the deformation of a polymer nanoparticle in a fluid under shear. It is found that hydrodynamic stresses can significantly deform polymer nanoparticles, with the particle becoming both stretched and squeezed (potentially expressing, or wringing out, an encapsulated fluid), and that the porous nature of polymer networks can allow fluid flow through the nanoparticle (potentially facilitating the release of an encapsulated drug). The use of computer simulations, therefore, which can capture the interactions between the mechanics of a polymer nanoparticle and the fluid dynamics of blood flow will play a significant role in developing and optimising polymer nanoparticle drug delivery systems.

Using local density approximation (LDA) calculations we predict GdFeO₃-like rotation of TiO₆ octahedra at the n-type interface between LaAlO₃ and SrTiO₃. This results in a narrowing of the Ti d bandwidth by 1/3 so that LDA+U calculations predict an antiferromagnetic, charge- and spin-ordered ground state for very modest values of U. Recent experimental evidence for magnetic interface ordering may be understood in terms of the close proximity of an antiferromagnetic insulating ground state to a ferromagnetic metallic excited state.

We consider the electronic current through a one-dimensional conductor in the ballistic transport regime and show that the quantum oscillations of a weakly pinned single-scattering target results in a temperature- and bias-voltage independent excess current at large bias voltages. This is a genuine quantum effect on transport that derives from an exponential reduction of electron backscattering in the elastic channel due to quantum delocalisation of the scatterer and from a suppression of low-energy electron backscattering in the inelastic channels caused by the Pauli exclusion principle. We show that both the mass of the target and the frequency of its quantum vibrations can be measured by studying the differential conductance and the excess current. We apply our analysis to the particular case of a weakly pinned C₆₀ molecule encapsulated by a single-wall carbon nanotube and find that the discussed phenomena are experimentally observable.

We report thickness-dependent magnetic and transport properties at the boundary separating A- and C-type antiferromagnetic phases in films of the overdoped manganite Nd_0.37Sr_0.63MnO₃. The thin films are prepared by DC magnetron sputtering on single-crystal LaAlO₃ substrates. All films, 5–120 nm in thickness, show a paramagnetic-ferromagnetic (PM-FM) transition. The small value of magnetization (⩽0.45 μ_B/Mn) is suggestive of the weak nature of the FM correlations that is different from the prototype PM-FM transition due to double exchange (DE). At lower film thicknesses, such as 5 and 25 nm, zero-field resistivity shows an insulator-like behavior. An insulator-metal (IM) transition is observed at higher film thicknesses (⩾60 nm). The magnetic-field independence of the observed IM transition (T_IM∼125 K) distinguishes it from the same observed in prototype manganites (x∼0.2–0.4) and explained by DE. For the thinnest films, a CMR effect as large as MR∼85% is observed in the low-temperature regime. The observed phenomena can be explained in terms of orbital-fluctuation–induced phase separation.

We report the results of ac/dc magnetization study of the critical behavior in a weak ferromagnet BaIrO₃. The critical temperature has been determined to be T_c=182.8 K by the ac susceptibility measurement. We have analyzed our dc magnetization data near T_c with the help of the modified Arrott plot method. The critical exponents have been determined to be β=0.82±0.03, γ=1.03±0.03, and δ=2.20±0.01, which roughly obeys δ=γ/β+1. Using these values of T_c, β, and γ, we find that each magnetization-field-temperature (M-H-T) behavior below and above T_c obeys a scaling relation, following a single equation of state in which M/|ε|^β is uniquely related to H/|ε|^(γ+β) where ε≡(T-T_c)/T_c. These findings demonstrate that the ferromagnetic transition in BaIrO₃ belongs to a new universality class of the second-order phase transition.

Polycrystalline Cr-doped ZnO films are prepared by the co-sputtering method. Diamagnetism is observed in the conductive samples deposited in pure Ar. However, ferromagnetism is found in films with the same Cr dopant prepared under different oxygen partial pressures. The magnetization shows a strong dependence on the Cr concentration and, especially, on oxygen pressure. It is found that native point defects, which can be adjusted by the oxygen partial pressure during deposition, play a crucial role in the observed magnetic behaviors. The obtained ferromagnetism can be described by the dopant-donor/acceptor hybridization model, which associates exchange interaction with shallow-bound carriers. These results may help to understand the wide range of experimentally determined magnetic moments and its changes with different metal types and concentrations prepared by different groups and methods.

Carrier density dependence of electron spin relaxation in an intrinsic GaAs quantum well is investigated at room temperature using time-resolved circularly polarized pump-probe spectroscopy. It is revealed that the spin relaxation time first increases with density in the relatively low-density regime where the linear D'yakonov-Perel' spin-orbit coupling terms are dominant, and then tends to decrease when the density is large and the cubic D'yakonov-Perel' spin-orbit coupling terms become important. These features are in good agreement with theoretical predictions on density dependence of spin relaxation by Lüet al. (Phys. Rev. B, 73 (2006) 125314). A fully microscopic calculation based on numerically solving the kinetic spin Bloch equations with both the D'yakonov-Perel' and the Bir-Aronov-Pikus mechanisms included, reproduces the density dependence of spin relaxation very well.

We study the effect of weak and dilute disorder on the order parameter equation and transition temperature of a Pomeranchuk-type Fermi-surface instability using replica mean-field theory. We consider the example of a phase transition to a d_{x²- y²} type Fermi surface distortion, and show that, in the regime where such a transition is second order, the transition temperature is reduced by disorder in essentially the same way as that for a d-wave superconductor. We argue that observing this disorder dependence of metal-to-metal transition is a useful indicator of a finite angular momentum Fermi surface distortion.

The Euler equation of the optimal reversal trajectory for the fastest magnetization reversal is obtained for an arbitrary nano-magnetic structure. The Euler equation is useful in designing a magnetic field pulse and/or a polarized electric current pulse in magnetization reversal for two reasons. 1) It is straightforward to obtain the solution of the Euler equation, at least numerically, for a given magnetic nano-structure characterized by its magnetic anisotropy energy. 2) After obtaining the optimal reversal trajectory for a given magnetic nano-structure, finding a proper field/current pulse is an algebraic problem instead of the original non-linear differential equation.

Different to conventional ferromagnetic resonance methods, we use the phase-shift of spin-wave propagation to investigate spin-wave in conducting ferromagnetic thin films. Spin-wave wave vector (or wave number) k, a key parameter in the study of spin-wave dispersion and propagation, is extracted from the ratio of the phase-shift to the propagation distance and the ratio of the intercepts to the slopes of the plot of frequency square (f²) vs. the bias field (H). The wave vectors calculated by both methods are in good agreement. The in-plane anisotropy field H_k and saturation magnetization M_S can also be extracted from the phase-shift map.

We synthesized the samples Ba_1-xM_xFe₂As₂ (M=La and K) with a ThCr₂Si₂-type structure. These samples were systematically characterized by resistivity, thermoelectric power (TEP) and Hall coefficient (R_H). BaFe₂As₂ shows an anomaly in resistivity at about 140 K. The substitution of La for Ba leads to a shift of the anomaly to low temperature, but no superconducting transition is observed. Potassium doping leads to the suppression of the anomaly in resistivity and induces superconductivity at 38 K. The Hall coefficient and TEP measurements indicate that the TEP is negative for BaFe₂As₂ and La-doped BaFe₂As₂, indicating an n-type carrier; while potassium doping leads to a change of the sign in R_H and TEP. It definitely indicates a p-type carrier in the superconducting Ba_1-xK_xFe₂As₂ with double FeAs layers, being in contrast to the case of LnO_1-xF_xFeAs with a single FeAs layer.

We analyze daily log-returns data for a set of 1200 stocks, taken from US stock markets, over a period of 2481 trading days (January 1996–November 2005). We estimate the degree of non-stationarity in daily market volatility employing a polynomial fit, used as a detrending function. We find that the autocorrelation function of absolute detrended log-returns departs strongly from the corresponding original data autocorrelation function, while the observed leverage effect depends only weakly on trends. Such effect is shown to occur when both skewness and long-time memory are simultaneously present. A fractional derivative random walk model is discussed yielding a quantitative agreement with the empirical results.

The level of assortative mixing of nodes in real-world networks gives important insights about the networks design and functionality, and has been analyzed in detail. However, this network-level measure conveys insufficient information about the local-level structure and motifs present in networks. We introduce a measure of local assortativeness that quantifies the level of assortative mixing for individual nodes in the context of the overall network. We show that such a measure, together with the resultant local assortativeness distributions for the network, is useful in analyzing network's robustness against targeted attacks. We also study local assortativeness in real-world networks, identifying different phases of network growth, showing that biological and social networks display markedly different local assortativeness distributions to technological networks, and discussing the implications to network design.

Optical microscopy and multi-particle tracking are used to study the short-time self-diffusion of weakly charged silica spheres at a water-air interface. The measured short-time self-diffusion coefficient D^S_S has the form, D^S_S/D₀=α(1−βn), where n is the area fraction occupied by the particles and D₀ is the Stokes-Einstein diffusion coefficient of individual particles in the bulk fluid. The obtained values of α and β differ from those obtained for bulk suspensions, indicating that hydrodynamic interactions between the interfacial particles have interesting new features when compared with their three-dimensional counterpart.

The most costly and annoying characteristic of the e-mail communication system is the large number of unsolicited commercial e-mails, known as spams, that are continuously received. Via the investigation of the statistical properties of the spam delivering intertimes, we show that spams delivered to a given recipient are time correlated: if the intertime between two consecutive spams is small (large), then the next spam will most probably arrive after a small (large) intertime. Spam temporal correlations are reproduced by a numerical model based on the random superposition of spam sequences, each one described by the Omori law. This and other experimental findings suggest that statistical approaches may be used to infer how spammers operate.

We investigate the electrical current and flow (number of parallel paths) between two sets of n sources and n sinks in complex networks. We derive analytical formulas for the average current and flow as a function of n. We show that for small n, increasing n improves the total transport in the network, while for large n bottlenecks begin to form. For the case of flow, this leads to an optimal n^* above which the transport is less efficient. For current, the typical decrease in the length of the connecting paths for large n compensates for the effect of the bottlenecks. We also derive an expression for the average flow as a function of n under the common limitation that transport takes place between specific pairs of sources and sinks.