Table of contents

Thermal transport across the interfaces between few-layer graphene sheets and soft materials exhibits intriguing anomalies when interpreted using the classical Kapitza model, e.g. the conductance of the same interface differs greatly for different modes of interfacial thermal transport. Using atomistic simulations, we show that such thermal transport follows a nonlocal flux-temperature drop constitutive law and is characterized jointly by a quasi-local conductance and a nonlocal conductance instead of the classical Kapitza conductance. The nonlocal model enables rationalization of many anomalies of the thermal transport across embedded few-layer graphene sheets and should be used in studies of interfacial thermal transport involving few-layer graphene sheets or other ultra-thin layered materials.

The interaction between polymers and biological membranes has recently gained significant interest in several research areas. On the biomedical side, dendrimers, linear polyelectrolytes, and neutral copolymers find application as drug and gene delivery agents, as biocidal agents, and as platforms for biological sensors. On the environmental side, plastic debris is often disposed of in the oceans and gets degraded into small particles; therefore concern is raising about the interaction of small plastic particles with living organisms. From both perspectives, it is crucial to understand the processes driving the interaction between polymers and cell membranes. In recent times progress in computer technology and simulation methods has allowed computational predictions on the molecular mechanism of interaction between polymeric materials and lipid membranes. Here we review the computational studies on the interaction between lipid membranes and different classes of polymers: dendrimers, linear charged polymers, polyethylene glycol (PEG) and its derivatives, polystyrene, and some generic models of polymer chains. We conclude by discussing some of the technical challenges in this area and future developments.

Top-down approaches based on etching techniques have almost reached their limits in terms of dimension. Therefore, novel assembly strategies and types of nanomaterials are required to allow technological advances. Self-assembly processes independent of external energy sources and unlimited in dimensional scaling have become a very promising approach. Here, we highlight recent developments in self-assembled DNA-polymer, silk-polymer and silk-DNA hybrids as promising materials with biotic and abiotic moieties for constructing complex hierarchical materials in 'bottom-up' approaches. DNA block copolymers assemble into nanostructures typically exposing a DNA corona which allows functionalization, labeling and higher levels of organization due to its specific addressable recognition properties. In contrast, self-assembly of natural silk proteins as well as their recombinant variants yields mechanically stable β-sheet rich nanostructures. The combination of silk with abiotic polymers gains hybrid materials with new functionalities. Together, the precision of DNA hybridization and robustness of silk fibrillar structures combine in novel conjugates enable processing of higher-order structures with nanoscale architecture and programmable functions.

Large-scale domain dynamics in proteins are found when flexible linkers or hinges connect domains. The related conformational changes are often related to the function of the protein, for example by arranging the active center after substrate binding or allowing transport and release of products. The adaptation of a specific active structure is referred to as 'induced fit' and is challenged by models such as 'conformational sampling'. Newer models about protein function include some flexibility within the protein structure or even internal dynamics of the protein. As larger domains contribute to the configurational changes, the timescale of the involved motions is slowed down. The role of slow domain dynamics is being increasingly recognized as essential to understanding the function of proteins. Neutron spin echo spectroscopy (NSE) is a technique that is able to access the related timescales from 0.1 up to several hundred nanoseconds and simultaneously covers the length scale relevant for protein domain movements of several nanometers distance between domains. Here we focus on these large-scale domain fluctuations and show how the structure and dynamics of proteins can be assessed by small-angle neutron scattering and NSE.

Emergent phenomena share the fascinating property of not being obvious consequences of the design of the system in which they appear. This characteristic is no less relevant when attempting to simulate such phenomena, given that the outcome is not always a foregone conclusion. The present survey focuses on several simple model systems that exhibit surprisingly rich emergent behavior, all studied by molecular dynamics (MD) simulation. The examples are taken from the disparate fields of fluid dynamics, granular matter and supramolecular self-assembly. In studies of fluids modeled at the detailed microscopic level using discrete particles, the simulations demonstrate that complex hydrodynamic phenomena in rotating and convecting fluids—the Taylor–Couette and Rayleigh–Bénard instabilities—can not only be observed within the limited length and time scales accessible to MD, but even allow quantitative agreement to be achieved. Simulation of highly counter-intuitive segregation phenomena in granular mixtures, again using MD methods, but now augmented by forces producing damping and friction, leads to results that resemble experimentally observed axial and radial segregation in the case of a rotating cylinder and to a novel form of horizontal segregation in a vertically vibrated layer. Finally, when modeling self-assembly processes analogous to the formation of the polyhedral shells that package spherical viruses, simulation of suitably shaped particles reveals the ability to produce complete, error-free assembly and leads to the important general observation that reversible growth steps contribute to the high yield. While there are limitations to the MD approach, both computational and conceptual, the results offer a tantalizing hint of the kinds of phenomena that can be explored and what might be discovered when sufficient resources are brought to bear on a problem.

We examine fluctuation-induced (pseudo-Casimir) interactions in nematic liquid-crystalline films confined between two surfaces, where one of the surfaces imposes a strong homeotropic anchoring (ensuring a uniform mean director profile), while the other one is assumed to be a chemically disordered substrate exhibiting an annealed distribution of anchoring energies. We employ a saddle-point approximation to evaluate the free energy of interaction mediated between the two surfaces and investigate how the interaction force is influenced by the presence of disordered surface anchoring energy. It is shown that the disorder results in a renormalization of the effective surface anchoring parameter in a way that it leads to quantitative and qualitative changes (including a change of sign at intermediate inter-surface separations) in the pseudo-Casimir interaction force when compared with the interaction force in the absence of disorder.

We develop a simple and effective description of the dynamics of dense hard sphere colloids in the aging regime deep in the glassy phase. Our description complements the many efforts to understand the onset of jamming in low density colloids, whose dynamics is still time-homogeneous. Based on a small set of principles, our model provides emergent dynamic heterogeneity, reproduces the known results for dense hard sphere colloids and makes detailed, experimentally-testable predictions for canonical observables in glassy dynamics. In particular, we reproduce the shape of the intermediate scattering function and particle mean-square displacements for jammed colloidal systems, and we predict a growth for the peak of the χ₄ mobility correlation function that is logarithmic in waiting-time. At the same time, our model suggests a novel unified description for the irreversible aging dynamics of structural and quenched glasses based on the dynamical properties of growing clusters of highly correlated degrees of freedom.

Motivated by a recent experiment (Pecker et al 2013 Nat. Phys.9 576), we study the stability, with respect to thermal effects, of Friedel and Wigner density fluctuations for two electrons trapped in a one-dimensional quantum dot. Diagonalizing the system exactly, the finite-temperature average electron density is computed. While the weak and strong interaction regimes display a Friedel oscillation or a Wigner molecule state at zero temperature, which as expected smear and melt as the temperature increases, a peculiar thermal enhancement of Wigner correlations in the intermediate interaction regime is found. We demonstrate that this effect is due to the presence of two different characteristic temperature scales: T_F, dictating the smearing of Friedel oscillations, and T_W, smoothing Wigner oscillations. In the early Wigner molecule regime, for intermediate interactions, T_F < T_W leading to the enhancement of the visibility of Wigner oscillations. These results complement those obtained within the Luttinger liquid picture, valid for larger numbers of particles.

Using first-principles calculations, we show manifestations of the quantum size effect in the dielectric function ε₂ of free-standing Al(1 1 1) ultrathin films of 1 monolayer to 20 monolayers, taking into account size dependent contributions from both interband and intraband electronic transitions. Overall the in-plane components (interband transition) of ε₂ increase with film thickness at all frequencies, converging towards a constant value. However, the out-of-plane components of ε₂ show a more complex behavior, and, only at frequencies less than 0.75 eV, increase with film thickness without convergence. This suggests that ultrathin films can possibly be used for low-loss plasmonics devices in the visible and ultraviolet range. Our findings may shed light on searching for low-loss plasmonic materials via quantum size effect.

We determine the optical response of topological insulator thin films in the presence of a quantizing, external magnetic field. We explicitly take into account hybridization between the states of top and bottom surface. The interplay between hybridization and Zeeman energies gives rise to topological and normal insulator phases and phase transitions between them. The optical response in the two phases and at the phase transition point is investigated. We show that the difference in magneto-optical response can be used to distinguish the topological phase from the normal phase of the system. Further, the optical response also allows us to determine the gap generated by hybridization between top and bottom surface states of topological insulator thin films.

A high-pressure synchrotron radiation diffraction study has been carried out on Ni_1−xCu_xCr₂O₄ solid solutions. Observed pressure-controlled phase transitions, along with data previously collected for temperature-induced phase transitions, are analyzed in the framework of the unified phenomenological model that results in mapping of the generic phase diagram for the whole family of Ni_1−xCu_xCr₂O₄ solid solutions.

Resonant ultrasound spectroscopy has been used to measure the bulk modulus (K), shear modulus (G) and acoustic dissipation of polycrystalline perovskite samples across the CaTiO₃–SrTiO₃ solid solution in the temperature range ∼10–1350 K. A remarkable pattern of up to ∼25% softening of G as a function of both temperature and composition is due to coupling of shear strain with order parameters for the Pm$\overline 3 m \leftrightarrow I$4/mcm, I4/mcm ↔ Pnma and I4/mcm ↔ Pbcm transitions. Anomalies in K associated with the phase transitions are small, consistent with only weak coupling of octahedral tilting order parameter(s) with volume strain. A change from tricritical character for the Pm$\overline 3 m \leftrightarrow I$ 4/mcm transition towards second order character at Sr-rich compositions appears to be due to changing properties of the soft optic mode rather than to changes in magnitude of strain/order parameter coupling coefficients. Precursor softening of G ahead of the Pm$\overline 3 m \leftrightarrow I$ 4/mcm transition, due to fluctuations or clustering, occurs over a temperature interval of up to ∼200 K, and also changes character at the most Sr-rich compositions. The tetragonal structure with Sr-rich compositions is characterized by additional softening with falling temperature which is most likely related to the proximity of a ferroelectric instability. The I4/mcm ↔ Pnma transition is accompanied by stiffening, which is attributed to the effects of strong coupling between order parameters for M-point and R-point tilting. The pattern of attenuation at RUS frequencies in the tetragonal phase can be understood in terms of the mobility of twin walls which become pinned below ∼500 K, and the loss mechanism most likely involves local bowing of the walls by lateral motion of ledges rather than the advance and retraction of needle tips. Twin wall mobility is suppressed in the orthorhombic structure.

The layered antiferromagnetic ACrX₂ -type compounds are currently highlighted as prominent material candidates for low- and intermediate-temperature thermoelectric (TE) applications. A key to attain the enhanced TE characteristics is to apply high-temperature sintering which presumably introduces some cation disorder. Here we present spin unrestricted density functional theory analysis of electronic band structures and TE properties of Cu and Cr disordered CuCrX₂(X = S, Se) phases. A narrow band gap semiconductor to metal transition is observed on 8.3% Cr-site disorder for both the compounds, X = S and Se. The large p-type Seebeck coefficient realized in the metallic state for the Cr-disordered phases is the factor that makes these phases promising TE materials. These theoretical findings for the Cr-disordered phases are well in line with reported experimental data for electronic transport properties. Contrarily, the results revealed for the Cu-disordered phases do not agree with the experimental data. Hence the results of our theoretical analysis strongly point towards the Cr rather than the Cu disorder picture to explain the TE electronic transport characteristics of the high-temperature sintered phases of CuCrX₂(X = S, Se).

The optical absorption properties of LaFeO₃ (LFO) have been calculated using density functional theory and experimentally measured from several high quality epitaxial films using variable angle spectroscopic ellipsometry. We have analyzed the calculated absorption spectrum using different Tauc models and find the model based on a direct-forbidden transition gives the best agreement with the ab initio band gap energies and band dispersions. We have applied this model to the experimental data and determine the band gap of epitaxial LFO to be ∼2.34 eV, with a slight dependence on strain state. This approach has also been used to analyze the higher indirect transition at ∼3.4 eV. Temperature dependent ellipsometry measurements further confirm our theoretical analysis of the nature of the transitions. This works helps to provide a general approach for accurate determination of band gaps and transition energies in complex oxide materials.

MAX phases are a large family of layered ceramics with many potential structural applications. A set of first-principles calculations was performed for M₂AlC and M₂AlN (M = Sc, Ti, Cr, Zr, Nb, Mo, Hf, or Ta) MAX phases as well as for hypothetical M₂AlB to investigate trends in their electronic structures, formation energies, and various mechanical properties. Analysis of the calculated data is used to extend the idea that the elastic properties of MAX phases can be controlled according to the valence electron concentration. The valence electron concentration can be tuned through the various combinations of transition metal and nonmetal elements.

Scanning tunneling microscopy (STM) has revealed that the magnitude of the pseudo-gap in under-doped cuprates varies spatially and is correlated with disorder. The loop-current order, characterized by the anapole vector Ω, discovered in under-doped cuprates occurs in the same region of the temperature and doping as the pseudo gap observed in STM and ARPES experiments. Since translational symmetry remains unchanged in the pure limit, no gap occurs at the chemical potential. On the other hand for disorder coupling linearly to the different possible orientations of Ω, there can only be a finite temperature dependent static correlation length for the loop-current state at any temperature. This leads to formation of domains of the ordered state with different orientation and magnitude of Ω in each. For the characteristic size of the domains much larger than the Fermi-vectors $k_{\rm F}^{-1}$, the boundary of the domains leads to forward scattering of the Fermions. Such forward scattering is shown to push states near the chemical potential to energies both above and below it leading to a pseudo-gap with an angular dependence which is maximum in the $\pm|\hat{x} \pm \hat{y}| =0$ directions because the single-particle energies are degenerate in these directions for all domains. The magnitude of the average gap systematically increases with the square of the average loop order parameter measured by polarized neutron scattering. This result is tested. A unique result of the gap due to forward scattering is the lack of a bump in the density of states at the 'edge' of the pseudo-gap so that the depletion of states near the chemical potential is recovered only in integration up to the edge of the band. This is also in agreement with a variety of experiments. Some predictions for further experiments are provided. Due to the finite correlation length, low frequency excitations are expected at long wavelength at all temperatures in the 'ordered' phase. Such fluctuations motionally average over the shifts in frequencies of local probes such as NMR and muon resonance expected for a truly static order.

Random inductor–capacitor (LC) networks can exhibit percolative superconductor-insulator transitions (SITs). We use a simple and efficient algorithm to compute the dynamical conductivity σ(ω, p) of one type of LC network on large (4000 × 4000) square lattices, where δ = p − p_c is the tuning parameter for the SIT. We confirm that the conductivity obeys a scaling form, so that the characteristic frequency scales as Ω ∝ |δ|^νz with νz ≈ 1.91, the superfluid stiffness scales as ϒ ∝ |δ|^t with t ≈ 1.3, and the electric susceptibility scales as χ_E ∝ |δ|^−s with s = 2νz − t ≈ 2.52. In the insulating state, the low-frequency dissipative conductivity is exponentially small, whereas in the superconductor, it is linear in frequency. The sign of Im σ(ω) at small ω changes across the SIT. Most importantly, we find that right at the SIT Re σ(ω) ∝ ω^t/νz−1 ∝ ω^−0.32, so that the conductivity diverges in the DC limit, in contrast with most other classical and quantum models of SITs.

We present a study on the intersublevel spacings of electrons and holes in a single layer of InAs self-assembled quantum dots. We use Fourier transform infrared transmission spectroscopy via a density chopping scheme for direct experimental observation of the intersublevel spacings of electrons without any external magnetic field. Epitaxial, complementary-doped and semi-transparent electrostatic gates are grown within the ultra high vacuum conditions of molecular beam epitaxy to voltage-tune the device, while a two dimensional electron gas (2DEG) serves as a back contact. Spacings of the hole sublevels are indirectly calculated from the photoluminescence spectrum by using a simple model given by Warburton et al [1]. Additionally, we observe that the intersubband resonances of the 2DEG are enhanced due to the quantum dot layer on top of the device.

We give an analytical and experimental demonstration of a classical analogue of the electromagnetic induced absorption (EIA) in a simple photonic device consisting of two stubs of lengths d₁ and d₂ grafted at the same site along a waveguide. By detuning the lengths of the two stubs (i.e. δ = d₂ − d₁) we show that: (i) the amplitudes of the electromagnetic waves in the two stubs can be written following the two resonators model where each stub plays the role of a radiative resonator with low Q factor. The destructive interference between the waves in the two stubs may give rise to a sharp resonance peak with high Q factor in the transmission as well as in the absorption. (ii) The transmission coefficient around the resonance induced by the stubs can be written following a Fano-like form. In particular, we give an explicit expression of the position, width and Fano parameter of the resonances as a function of δ. (iii) By taking into account the loss in the waveguides, we show that at the transmission resonance, the transmission (reflection) increases (decreases) as a function of δ. Whereas the absorption goes through a maximum around 0.5 for a threshold value δ_th which depends on the attenuation in the system and then falls to zero. (iv) We give a comparison between the phase of the determinant of the scattering matrix, the so-called Friedel phase and the phase of the transmission amplitude. (v) The effect of the boundary conditions at the end of the resonators on the EIA resonance is also discussed. The analytical results are obtained by means of the Green's function method, whereas the experiments are carried out using coaxial cables in the radio-frequency regime. These results should have important consequences for designing integrated devices such as narrow-frequency optical or microwave filters and high-speed switches.

Influence of disorder, antisite defects, martensite transition and compositional variation on the magnetic properties and electronic structure of Mn₂NiGa and Mn_1+xNi_2−xGa magnetic shape memory alloys have been studied by using full potential spin-polarized scalar relativistic Korringa–Kohn–Rostocker (FP-SPRKKR) method. Mn₂NiGa is ferrimagnetic and its total spin moment increases when disorder in the occupancy of Mn_Ni (Mn atom in Ni position) is considered. The moment further increases when Mn–Ga antisite defect [1] is included in the calculation. A reasonable estimate of T_C for Mn₂NiGa is obtained from the exchange parameters for the disordered structure. Disorder influences the electronic structure of Mn₂NiGa through overall broadening of the density of states and a decrease in the exchange splitting. Inclusion of antisite defects marginally broaden the minority spin partial DOS (PDOS), while the majority spin PDOS is hardly affected. For Mn_1+xNi_2−xGa where 1 ⩾ x ⩾ 0, as x decreases, Mn_Mn moment increases while Mn_Ni moment decreases in both austenite and martensite phases. For x ⩾ 0.25, the total moment of the martensite phase is smaller compared to the austenite phase, which indicates possible occurrence of inverse magnetocaloric effect. We find that the redistribution of Ni 3d- Mn_Ni 3d minority spin electron states close to the Fermi level is primarily responsible for the stability of the martensite phase in Mn–Ni–Ga.

The compounds A₂[FeCl₅(H₂O)] with A = K, Rb, Cs are identified as new linear magnetoelectric (non-multiferroic) materials. We present a detailed investigation of their linear magnetoelectric properties through measurements of pyroelectric currents, dielectric constants and magnetization. The anisotropy of the linear magnetoelectric effect of the K-based and Rb-based compound is consistent with the magnetic point group m'm'm', already reported in literature. A symmetry analysis of the magnetoelectric effect of the Cs-based compound allows us to determine the magnetic point group mmm' and to develop a model for its magnetic structure. In addition, magnetic-field versus temperature phase diagrams are derived and compared to the closely related multiferroic (NH₄)₂[FeCl₅(H₂O)].