We report on small-cluster exact-diagonalization calculations which prove the formation of electron-hole pairs (excitons) as a prerequisite for spontaneous interlayer phase coherence in double-layer systems described by the extended Falicov-Kimball model. Evaluating the anomalous Green's function and momentum distribution function of the pairs, and thereby analyzing the dependence of the exciton binding energy, condensation amplitude, and coherence length on the Coulomb interaction strength, we demonstrate a crossover between a BCS-like electron-hole pairing transition and a Bose-Einstein condensation of tightly bound preformed excitons. We furthermore show that a mass imbalance between electrons and holes tends to suppress the condensation of excitons.

Rashba nanowires in a magnetic field exhibit a helical regime when the spin-orbit momentum is close to the Fermi momentum, $k_F\approx k_{SO}$. We show that this regime is characterized by a strongly anisotropic electron-spin susceptibility, with an exponentially suppressed signal along one direction in spin space, and that there are no low-frequency spin fluctuations along this direction. Since the spin response in the gapless regime $k_F\neg \approx k_{SO}$ has a power-law behavior in all three directions, spin measurements provide a signature of the helical regime that complements spin-insensitive conductance measurements.

In this paper, we have explored computationally the solidification process of a nickel cluster consisting of 2057 atoms. This process has the characteristic features of the first-order phase transition occurring in a finite system. The focus of our research is placed on the elucidation of correlated dynamics of a large ensemble of particles in the course of the nanoscale liquid-solid phase transition through the computation and analysis of the results of molecular dynamics (MD) simulations with the corresponding theoretical model. This problem is of significant interest and importance because the controlled dynamics of systems on the nanoscale is one of the central topics in the development of modern nanotechnologies. MD simulations in large molecular systems are rather computer power demanding. Therefore, in order to advance with MD simulations, we have used modern computational methods based on the graphics processing units (GPUs). The advantages of the use of GPUs for MD simulations in comparison with the CPUs are demonstrated and benchmarked. The reported speedup reaches factors greater than 400. This work opens a path towards exploration with the use of MD of a larger number of scientific problems inaccessible earlier with the CPU-based computational technology.

Cooling of a gas of trapped paraexcitons in Cu${}_2$O to 100 mK, close to the lattice temperature, is reported. We show that the activation of a transverse acoustic scattering mechanism by the applied stress is responsible for the production of ultracold excitons. This low temperature is indispensable to achieving a stable and pure Bose-Einstein condensate of three-dimensional excitons in a strongly inelastic environment.

We investigate the light emission of optically excited tris(8-hydroxyquinolinato) aluminum (Alq${}_3$) films by temperature-dependent, time-integrated, as well as time-resolved photoluminescence (PL) at various photon densities. The Alq${}_3$ films are deposited on Si (001) substrate using organic molecular beam deposition. At high excitation densities, the PL efficiency decreases when the temperature is reduced from 170 to 15 K. At low laser intensities, the PL efficiency is nearly temperature independent. The observed PL quenching at high-excitation densities is assigned to singlet-singlet annihilation revealing a low-temperature bimolecular quenching coefficient that is more than two orders of magnitude higher than previously reported at room temperature. The observed strong bimolecular interaction at low temperature is attributed to an enhanced local (microscopic) density of captured excitons in extended traps. The reduction of the exciton annihilation with increasing temperature is assigned to a thermally activated occupation of nonquenchable localized exciton states. Above 190 K, the PL efficiency starts to decrease independently from the excitation level which is ascribed to a thermally activated detrapping of excitons and subsequent migration to nonradiative centers outside the traps. A coupled-rate equation model, including bimolecular quenching, thermally activated occupation of nonquenchable states, and detrapping of excitons at higher temperatures, supports these interpretations.

One-dimensional electrons with a linearized dispersion relation are equivalent to a collection of harmonic plasmon modes, which represent long wavelength density oscillations. An immediate consequence of this Luttinger model of one-dimensional electron systems is the absence of inelastic scattering processes responsible for the relaxation of nonequilibrium states. In a generic nonlinear Luttinger liquid, plasmons may decay and thus acquire a finite lifetime. We show that equilibration of plasmons is hierarchical and has profound implications for the dynamics after a thermal quench. We also develop a thermal transport theory and compute thermal conductance of the nonlinear Luttinger liquid by treating the collision integral of plasmons in a manifestly nonperturbative way.

A pressure-induced phase transition of FeOCl is discovered to occur at $P_c=15\pm 1$ GPa. It is preceded by extremely anisotropic lattice compression, which is explained by a gradual collapse of the van der Waals gap between the chlorine atoms on the borders of the slabs of this layered compound. Single-crystal x-ray diffraction in a diamond anvil cell is used to show that the high-pressure phase can be described as a fourfold superstructure with monoclinic lattice distortion, described by a $4a\times 1b\times 2c$ supercell with space group $B{2}_1/m$ ($b$ unique). The high-pressure crystal structures have been used to uncover the mechanism of the phase transition, as the formation of alternating regions of increased packing density of chlorine atoms within a single layer and regions of interpenetrating layers. Raman and Mössbauer spectroscopies indicate that the phase transition is not related to electronic effects or magnetic order.

We investigate dimensional reduction, the property that the critical behavior of a system in the presence of quenched disorder in dimension $d$ is the same as that of its pure counterpart in $d-2$, and its breakdown in the case of the random-field Ising model in which both the interactions and the correlations of the disorder are long ranged, i.e., power-law decaying. To some extent the power-law exponents play the role of spatial dimension in a short-range model, which allows us to probe the theoretically predicted existence of a nontrivial critical value separating a region where dimensional reduction holds from one where it is broken, while still considering the physical dimension $d=3$. By extending our recently developed approach based on a nonperturbative functional renormalization group combined with a supersymmetric formalism, we find that such a critical value indeed exists, provided one chooses a specific relation between the decay exponents of the interactions and of the disorder correlations. This transition from dimensional reduction to its breakdown should therefore be observable in simulations and numerical analyses, if not experimentally.

Accurate prediction of local properties of transition-metal nanoalloys from the electronic structure is a challenge for building new materials with novel properties in a controlled way. To this aim, developing unified descriptions of local electronic states as a function of a minimal set of parameters is the way to disentangle structural and chemical effects. This is achieved here within $sp-d$ tight-binding calculations using a self-consistent procedure taking into account both the changes in the structural environment (coordination effect) and in the chemical one (alloying effect). From these calculations, trends in the distributions of energy electronic states are obtained through band shifts and widths allowing one to study in a systematic way ordering tendency and local properties in nanoalloys in a wide range of sizes and structural complexity.

Closed quantum systems with quenched randomness exhibit many-body localized regimes wherein they do not equilibrate, even though prepared with macroscopic amounts of energy above their ground states. We show that such localized systems can order, in that individual many-body eigenstates can break symmetries or display topological order in the infinite-volume limit. Indeed, isolated localized quantum systems can order even at energy densities where the corresponding thermally equilibrated system is disordered, i.e., localization protects order. In addition, localized systems can move between ordered and disordered localized phases via nonthermodynamic transitions in the properties of the many-body eigenstates. We give evidence that such transitions may proceed via localized critical points. We note that localization provides protection against decoherence that may allow experimental manipulation of macroscopic quantum states. We also identify a “spectral transition” involving a sharp change in the spectral statistics of the many-body Hamiltonian.

We investigate the effects of geometric and material nonlinearities introduced by deformation on the linear dynamic response of two-dimensional phononic crystals. Our analysis not only shows that deformation can be effectively used to tune the band gaps and the directionality of the propagating waves, but also reveals how geometric and material nonlinearities contribute to the tunable response of phononic crystals. Our numerical study provides a better understanding of the tunable response of phononic crystals and opens avenues for the design of systems with optimized properties and enhanced tunability.

We calculate fluctuation corrections to the longitudinal conductivity of disordered superconductors subject to an external magnetic field. We derive analytic expressions that are valid in the entire metallic part of the temperature–magnetic field phase diagram as long as the effect of the magnetic field on the spin degrees of freedom of the electrons may be neglected. Our calculations are based on a kinetic equation approach. For the special case of superconducting films and wires in parallel magnetic fields, we perform a detailed comparison with results that were previously obtained with diagrammatic perturbation theory in the imaginary-time formalism. As an application, we study the fluctuation conductivity of films in tilted magnetic fields with a special focus on the low-temperature regime. We present a detailed discussion of the phenomenon of the nonmonotonic magnetoresistance and find that it displays a pronounced dependence on the tilting angle.

The high-$T_c$ alkali doped iron selenide superconductors K${}_x$Fe${}_{2-y}$Se${}_2$ have been recently shown to be intrinsically phase separated into Fe vacancy ordered block antiferromagnetic regions and superconducting regions at low temperatures. In this work, we use a microscopic five orbital Hubbard model to obtain the electronic low-energy states near the interfaces between block antiferromagnets and superconductors. It is found that abundant low-energy in-gap bound states exist near such interfaces irrespective of whether the superconductor has $d$- or $s$-wave pairing symmetry. By contrast, it is shown how scattering from (110) boundaries can provide a natural means to distinguish between these two leading pairing instabilities of the K${}_x$Fe${}_{2-y}$Se${}_2$ materials.

The physical properties of Sr${}_{14-x}$Ca${}_x$Cu${}_{24}$O${}_{41}$ are determined by the hole distribution between the edge-shared CuO${}_2$ chain and the corner-shared Cu${}_2$O${}_3$ two-leg ladder, but inconsistent results on the hole distribution were obtained in various experimental works in recent decades. In this work we reinvestigate the hole distribution by soft x-ray absorption spectroscopy at the Cu $L_3$ edge. By comparing with the pure ladder and the pure chain systems, we can unambiguously distinguish between the hole distributions in the chain and the ladder. We have found that there are 5.3 holes in the edge-shared chain and 0.7 holes in the corner-shared ladder on average for Sr${}_{14}$Cu${}_{24}$O${}_{41}$. Upon Ca substitution, the holes gradually transfer from the edge-shared chain to the corner-shared ladder, and there are up to 1.21 holes in the corner-shared ladder for Sr${}_{1.8}$Ca${}_{12.2}$Cu${}_{24}$O${}_{41}$.

The two-dimensional electron gas at the LaTiO${}_3$/SrTiO${}_3$ or LaAlO${}_3$/SrTiO${}_3$ oxide interfaces becomes superconducting when the carrier density is tuned by gating. The measured resistance and superfluid density reveal an inhomogeneous superconductivity resulting from percolation of filamentary structures of superconducting “puddles” with randomly distributed critical temperatures, embedded in a nonsuperconducting matrix. Following the evidence that superconductivity is related to the appearance of high-mobility carriers, we model intrapuddle superconductivity by a multiband system within a weak coupling BCS scheme. The microscopic parameters, extracted by fitting the transport data with a percolative model, yield a consistent description of the dependence of the average intrapuddle critical temperature and superfluid density on the carrier density.

Sn${}_{1-x}$In${}_x$Te is a possible candidate for topological superconductivity. Previous work has shown that substitution of In for Sn in the topological crystalline insulator SnTe results in superconductivity, with the transition temperature, $T_c$, growing with In concentration. We have performed a systematic investigation of Sn${}_{1-x}$In${}_x$Te for a broad range of $x$, synthesizing single crystals (by a modified floating-zone method) as well as polycrystalline samples. The samples have been characterized by x-ray diffraction, resistivity, and magnetization. For the single crystals, the maximum $T_c$ is obtained at $x=0.45$ with a value of 4.5 K, as determined by the onset of diamagnetism.

Longitudinal piezoelectric coefficient of a twinned ferroelectric perovskite material with an array of partially compensated head-to-head and tail-to-tail 90-degree domain walls has been studied by phase-field simulations in the framework of the Ginzburg-Landau-Devonshire model of BaTiO${}_3$ ferroelectric. In particular, it is shown that the magnitude of the build-in extrinsic charge at the domain wall and the nanoscale domain size can both promote rotation of the static polarization vector within the body of adjacent domains. This polarization rotation drives the domain closer to an orthorhombic state, and the proximity to this ferroelectric-ferroelectric phase transition is directly responsible for the enhancement of the properties. Our simulations and the theory also suggest that the same system with nominally overcompensated charged walls may show a negative effective longitudinal piezoelectric coefficient. The obtained results can be used for quantitative estimates of piezoelectric properties of domain-engineered crystals.

An extensive set of first-principles density functional theory calculations have been performed to study the behavior of He, C, and N solutes in austenite, dilute Fe-Cr-Ni austenitic alloys, and Ni in order to investigate their influence on the microstructural evolution of austenitic steel alloys under irradiation. The results show that austenite behaves much like other face-centered cubic metals and like Ni in particular. Strong similarities were also observed between austenite and ferrite. We find that interstitial He is most stable in the tetrahedral site and migrates with a low barrier energy of between 0.1 and 0.2 eV. It binds strongly into clusters as well as overcoordinated lattice defects and forms highly stable He-vacancy (V${}_m$He${}_n$) clusters. Interstitial He clusters of sufficient size were shown to be unstable to self-interstitial emission and VHe${}_n$ cluster formation. The binding of additional He and V to existing V${}_m$He${}_n$ clusters increases with cluster size, leading to unbounded growth and He bubble formation. Clusters with $n/m$ around 1.3 were found to be most stable with a dissociation energy of 2.8 eV for He and V release. Substitutional He migrates via the dissociative mechanism in a thermal vacancy population but can migrate via the vacancy mechanism in irradiated environments as a stable V${}_2$He complex. Both C and N are most stable octahedrally and exhibit migration energies in the range from 1.3 to 1.6 eV. Interactions between pairs of these solutes are either repulsive or negligible. A vacancy can stably bind up to two C or N atoms with binding energies per solute atom up to 0.4 eV for C and up to 0.6 eV for N. Calculations in Ni, however, show that this may not result in vacancy trapping as VC and VN complexes can migrate cooperatively with barrier energies comparable to the isolated vacancy. This should also lead to enhanced C and N mobility in irradiated materials and may result in solute segregation to defect sinks. Binding to larger vacancy clusters is most stable near their surface and increases with cluster size. A binding energy of 0.1 eV was observed for both C and N to a [001] self-interstitial dumbbell and is likely to increase with cluster size. On this basis, we would expect that, once mobile, Cottrell atmospheres of C and N will develop around dislocations and grain boundaries in austenitic steel alloys.

The actinide cubic Laves compounds NpAl${}_2$, NpOs${}_2$, NpFe${}_2$, and PuFe${}_2$ have been examined by x-ray magnetic circular dichroism (XMCD) at the actinide $M_{4,5}$ absorption edges and Os $L_{2,3}$ absorption edges. They have the interesting feature that the $An-An$ spacing is close to the so-called Hill limit so that substantial hybridization between the 5$f$ states on neighboring atoms is expected to occur. The XMCD experiments performed at the $M_{4,5}$ absorption edges of Np and Pu allow us to determine the spectroscopic branching ratio, which gives information on the coupling scheme in these materials. In all materials, the intermediate coupling scheme is found appropriate. Comparison with the magnetization data for NpOs${}_2$ and neutron results for PuFe${}_2$ allows a determination of the individual orbital and spin magnetic moments and the magnetic-dipole contribution $m$${}_{md}$. The resulting orbital and spin magnetic moments are in good agreement with earlier values determined by neutron diffraction, and the values of $m$${}_{md}$ are non-negligible, and close to those predicted for intermediate coupling. There is a comparatively large induced moment on the Os atom in NpOs${}_2$ such that the Os contribution to the total moment per formula unit is $\sim$30$\%$ of the total. The spin and orbital moments at the Os site are parallel, in contrast to the antiparallel configuration that we find for Os impurities in a 3$d$ ferromagnetic transition-metal alloy. Calculations using the LDA+$U$ technique are reported. The ab initio computed XMCD spectra show good agreement with experimental spectra for small values (0–1 eV) of the Hubbard $U$ parameter, which demonstrates that the 5$f$ electrons in these compounds are relatively delocalized. The calculations confirm the sign and magnitude of the experimentally determined induced magnetic moments on the Os site in NpOs${}_2$. A posteriori, by comparison of the theoretical and measured XMCD spectra for a given material, we can determine the most appropriate LSDA+$U$ variant and, more importantly, the applicable value of the Hubbard $U$ parameter.

We present neutron scattering measurements of the phonon-roton (P-R) mode of superfluid ${}^4$He confined in 47 Å MCM-41 at $T=0.5$ K at wave vectors, $Q$, beyond the roton wave vector ($Q_R=1.92$ Å${}^{-1}$). Measurements beyond the roton require access to high wave vectors (up to $Q=4$ Å${}^{-1}$) with excellent energy resolution and high statistical precision. Only one previous measurement in porous media (in aerogel) with low statistical precision has been reported. At $T=0.5$ K, we find that the P-R mode in MCM-41 extends out to wave vector $Q\simeq 3.6$ Å${}^{-1}$, with the same energy and zero width (within precision) as observed in bulk superfluid ${}^4$He. Layer modes in the roton region are also observed. Specifically, the P-R mode energy, ${\omega }_Q$, increases with $Q$ for $Q>Q_R$ and reaches a plateau at a maximum energy ${\omega }_Q=2\Delta$ where $\Delta$ is the roton energy, $\Delta =0.74$ $\pm$ 0.01 meV in MCM-41. This upper limit means the P-R mode decays to two rotons if its energy exceeds 2$\Delta$. It also means that the P-R mode does not decay to two-layers modes. If the P-R could decay to two-layer modes, ${\omega }_Q$ would plateau at a lower energy, ${\omega }_Q=2{\Delta }_L$, where ${\Delta }_L=0.60$ meV is the energy of the roton-like minimum of the layer mode. Rather the P-R mode and the layer modes observed in porous media appear to be quite different modes with little interaction between them.