We investigate the lattice coupling to the spin and charge orders in the striped nickelate, ${\mathrm{La}}_{1.75}{\mathrm{Sr}}_{0.25}{\mathrm{NiO}}_4$, using time-resolved resonant x-ray scattering. Lattice-driven dynamics of both spin and charge orders are observed when the pump photon energy is tuned to that of an $E_u$ bond- stretching phonon. We present a likely scenario for the behavior of the spin and charge order parameters and its implications using a Ginzburg-Landau theory.

We report a postgrowth aging mechanism of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$(111) films with scanning tunneling microscopy in combination with density functional theory calculation. It is found that a monolayered structure with a squared lattice symmetry gradually aggregates from the surface steps. Theoretical calculations indicate that the van der Waals (vdW) gap not only acts as a natural reservoir for self-intercalated Bi and Te atoms, but also provides them easy diffusion pathways. Once hopping out of the gap, these defective atoms prefer to develop into a two-dimensional BiTe superstructure on the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$(111) surface driven by positive energy gain. Considering the common nature of weak bonding between vdW layers, we expect such unusual diffusion and aggregation of the intercalated atoms may be of general importance for most kinds of vdW layered materials.

Intrinsic thermal resistivity critically depends on features of phonon dispersions dictated by harmonic interatomic forces and masses. Here we present the effects of functional group mass variance on vibrational properties and thermal conductivity ($\kappa$) of functionalized graphene from first-principles calculations. We use graphane, a buckled graphene backbone with covalently bonded hydrogen atoms on both sides, as the base material and vary the mass of the hydrogen atoms to simulate the effect of mass variance from other functional groups. We find nonmonotonic behavior of $\kappa$ with increasing mass of the functional group and an unusual crossover from acoustic-dominated to optic-dominated thermal transport behavior. We connect this crossover to changes in the phonon dispersion with varying mass which suppress acoustic phonon velocities, but also give unusually high velocity optic modes. Further, we show that out-of-plane acoustic vibrations contribute significantly more to thermal transport than in-plane acoustic modes despite breaking of a reflection-symmetry-based scattering selection rule responsible for their large contributions in graphene. This work demonstrates the potential for manipulation and engineering of thermal transport properties in two-dimensional materials toward targeted applications.

The exchange-correlation hole is a central concept in density functional theory. It not only provides justification for an exchange-correlation energy functional but also serves as a local ingredient for nonlocal range-separated density functionals. However, due to the nonlocal nature, modeling the conventional exact exchange hole presents a great challenge to density functional theory. In this work, we propose a semilocal exchange hole underlying the Tao-Perdew-Staroverov-Scuseria (TPSS) meta-generalized gradient approximation functional. Our model is distinct from previous ones not only at small separation between an electron and the hole around the electron but also in the way it interpolates between rapidly varying and slowly varying densities. Here the interpolation is determined by the wave-vector analysis on the infinite-barrier model for a jellium surface. Numerical tests show that our exchange-hole model mimics the conventional exact one quite well for atoms. As a simple application, we apply the hole model to construct a TPSS-based range-separated functional. We find that this range-separated functional can substantially improve the band gaps and barrier heights of TPSS, without losing much accuracy for atomization energies.

It is long known that the best single-site coherent potential approximation falls short of describing Anderson localization. Here, we study a binary alloy disorder [or equivalently, a spinless Falicov-Kimball (FK)] model and construct a dominantly analytic cluster extension that treats intracluster $(1/d,d=$ spatial dimension) correlations exactly. We find that, in general, the irreducible two-particle vertex exhibits clear nonanalyticities before the band splitting transition of the Hubbard type occurs, signaling onset of an unusual type of localization at strong coupling. Using time-dependent response to a sudden local quench as a diagnostic, we find that the long-time wave-function overlap changes from a power-law to an anomalous form at strong coupling, lending additional support to this idea. Our results also imply such “strong” localization in the equivalent FK model, the simplest interacting fermion system.

We report that the interlayer states common to the compounds $Ae\mathrm{AlSi}$ ($Ae=\mathrm{Ca}$, Sr, Ba) arise from F-center-like electrons arrayed in periodic cavities. The SrPtSb-type intermetallic phases exhibit electrons localized to columns of the trigonal bipyramidal $A{e}_3\mathrm{A}{\mathrm{l}}_2$ cages running perpendicular to the honeycomb layers. Ab initio calculations in combination with hard/soft x-ray photoemission spectroscopic measurements reveal that these features correspond to the anionic electrons that hybridize with apical $\mathrm{Al}3p_{\mathrm{z}}$ orbitals from the honeycomb layers above and below. Extra bands with a significant dispersion along the $k_{\mathrm{z}}$ direction therefore contribute to the Fermi level in contrast to the apparent two-dimensional connectivity of the bonding in the compounds, and completely account for the presence of interlayer states. Our study demonstrates how the cage centers may serve as electronically important crystallographic sites, and extend the anionic electron concept into honeycomb lattice compounds.

This work presents a theoretical demonstration of the Aharonov-Bohm (AB) effect in monolayer phosphorene nanorings (PNRs). Atomistic quantum transport simulations of PNRs are employed to investigate the impact of multiple modulation sources on the sample conductance. In the presence of a perpendicular magnetic field, we find that the conductance of both armchair and zigzag PNRs oscillate periodically in a low-energy window as a manifestation of the AB effect. Our numerical results reveal a giant magnetoresistance (MR) in zigzag PNRs (with a maximum magnitude approaching 2000%). It is attributed to the AB-effect-induced destructive interference phase over a wide energy range below the bottom of the second subband. We also demonstrate that PNR conductance is highly anisotropic, offering an additional way to modulate MR. The giant MR in PNRs is maintained at room temperature in the presence of the thermal broadening effect.

We study the influence of the proximity-induced pairing on an electronic version of the Dicke effect in a heterostructure, comprising three quantum dots vertically coupled between the metallic and superconducting leads. We discuss a feasible experimental procedure for detecting the narrow/broad (subradiant/superradiant) contributions by means of the subgap Andreev spectroscopy. In the Kondo regime and for small energy level detuning the Dicke effect is manifested in the differential conductance.

We investigate the optical transition selection rules for excitonic Rydberg series formed in massive Dirac cones. The entanglement of the exciton envelop function with the pseudospin texture leads to anomalous selection rules for one-photon generation of excitons, where $d$ orbitals can be excited with the opposite helicity selection rule from the $s$ orbitals in a given valley. The trigonal warping effects in realistic hexagonal lattices further renders more excited states bright, where $p$ orbitals can also be accessed by one-photon excitation with the opposite valley selection rules to the $s$ orbitals. The one-photon generation of exciton in the various states and the intraexcitonic transition between these states are both dictated by the discrete in-plane rotational symmetry of the lattices, and our results show that in hexagonal 2D materials the symmetry allowed transitions are enabled when trigonal warping effects are included in the massive Dirac fermion model. In monolayer transition metal dichalcogenides where excitons can be generated by visible light and intraexcitonic transitions can be induced by infrared light, we give the strength of these optical transitions, estimated using modified hydrogenlike envelope functions combined with the optical transition matrix elements between the Bloch states calculated at various $k$ points.

This work explores the Zn vacancy in ZnO using hybrid density functional theory calculations. The Zn vacancy is predicted to be an exceedingly deep polaronic acceptor that can bind a localized hole on each of the four nearest-neighbor O ions. The hole localization is accompanied by a distinct outward relaxation of the O ions, which leads to lower symmetry and reduced formation energy. Notably, we find that initial symmetry breaking is required to capture this effect. We present a simple model to rationalize our findings with regard to the approximately equidistant thermodynamic charge-state transition levels. Furthermore, by employing a one-dimensional configuration coordinate model with parameters obtained from the hybrid density functional theory calculations, luminescence line shapes were calculated. The results show that the isolated Zn vacancy is unlikely to be the origin of the commonly observed luminescence in the visible part of the emission spectrum from $n$-type material, but rather the luminescence in the infrared region.

We consider in detail a simple model supporting a single floppy mode that is often used to heuristically describe instances of negative thermal expansion. A key result is that the translational kinetic energy of the dilating bond network scales extensively with system size and results in dynamical properties which differ qualitatively from considerations built upon harmonic models. We develop an analogy between the dynamics of this model and a modified mechanical pendulum to elucidate the connection between the new results and the familiar harmonic limit. We then propose an appropriate Schrödinger equation for this system and study numerically the quantum mechanical solutions. Marked differences from conventional phonon dynamics and thermodynamics are seen in both classical and quantum limits, in particular a strong twofold enhancement of the (negative) coefficient of thermal expansion. We contextualize the results against real material parameters and discuss related empirical observations.

In this paper we present a thorough study of transport, spectral, and wave-function properties at the Anderson localization critical point in spatial dimensions $d=3$, 4, 5, 6. Our aim is to analyze the dimensional dependence and to assess the role of the $d\to \infty$ limit provided by Bethe lattices and treelike structures. Our results strongly suggest that the upper critical dimension of Anderson localization is infinite. Furthermore, we find that $d_U=\infty$ is a much better starting point compared to $d_L=2$ to describe even three-dimensional systems. We find that critical properties and finite-size scaling behavior approach by increasing $d$ those found for Bethe lattices: the critical state becomes an insulator characterized by Poisson statistics and corrections to the thermodynamics limit become logarithmic in the number $N$ of lattice sites. In the conclusion, we present physical consequences of our results, propose connections with the nonergodic delocalized phase suggested for the Anderson model on infinite-dimensional lattices, and discuss perspectives for future research studies.

Phase transitions in isotropic quantum antiferromagnets are associated with the condensation of bosonic triplet excitations. In three-dimensional quantum antiferromagnets, such as ${\mathrm{TlCuCl}}_3$, condensation can be either pressure or magnetic field induced. The corresponding magnetic order obeys universal scaling with thermal critical exponent $\varphi$. Employing a relativistic quantum field theory, the present work predicts the emergence of multiple (three) universalities under combined pressure and field tuning. Changes of universality are signaled by changes of the critical exponent $\varphi$. Explicitly, we predict the existence of two new exponents $\varphi =1$ and $1/2$ as well as recovering the known exponent $\varphi =3/2$. We also predict logarithmic corrections to the power law scaling.

The influence of a conducting layer on the magnetic flux penetration in a superconducting Nb film is studied by magneto-optical imaging. The metallic layer partially covering the superconductor provides an additional velocity-dependent damping mechanism for the flux motion that helps to protect the superconducting state when thermomagnetic instabilities develop. If the flux advances with a velocity slower than $w=2/{\mu }_0\sigma t$, where $\sigma$ is the cap layer conductivity and $t$ is its thickness, the flux penetration remains unaffected, whereas for incoming flux moving faster than $w$, the metallic layer becomes an active screening shield. When the metallic layer is replaced by a perfect conductor, it is expected that the flux braking effect will occur for all flux velocities. We investigate this effect by studying Nb samples with a thickness step. Some of the observed features, namely the deflection of the flux trajectories at the border of the thick center, as well as the favored flux penetration at the indentation, are reproduced by time-dependent Ginzburg-Landau simulations.

Using intrinsic multiple Andreev reflections effect spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors ${\mathrm{Sm}}_{1-x}{\mathrm{Th}}_x\mathrm{OFeAs}$ with various thorium doping and critical temperatures $T_C=21–54$ K. We observe a scaling between both superconducting gaps and $T_C$. The determined BCS ratio for the large gap $2{\mathrm{\Delta }}_L/k_B{T}_C=5.0–5.7$ and its eigen-BCS ratio (in a hypothetical case of zero interband coupling) $2{\mathrm{\Delta }}_L/k_B{T}_C^L=4.1–4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2{\mathrm{\Delta }}_S/k_B{T}_C=1.2–1.6$, remain nearly constant within all the $T_C$ range studied. The temperature dependences ${\mathrm{\Delta }}_{L,S}\left(T\right)$ agree well with a two-band BCS-like Moskalenko and Suhl model. We prove intraband coupling to be stronger than interband coupling, whereas Coulomb repulsion constants ${\mu }^{*}$ are finite in Sm-based oxypnictides.

Charged domain walls (CDWs), which possess metallic-type conductivity and can be created and controlled in the bulk of wide-band-gap ferroelectrics, attract nowadays a strong research interest. The most advanced method for production of stable CDWs involves weak super-band-gap illumination. Here, we investigate theoretically the impact of this illumination on the major wall properties including the energy and the spatial profiles of the polarization, of the electrostatic potential, and of the compensating charge carriers. The key material parameters determining the effect of light are the zero-field polarization strength, the dielectric permittivity, and the trap concentration. The main predictions are substantial reduction of the wall energies and decrease of the electric wall potential under light. These features facilitate creation of dense CDWs patterns and accessibility of the metallic-type wall conductivity.

Positron annihilation spectroscopy characterization results for neutron-irradiated $3C$-SiC are described here, with a specific focus on explaining the size and character of vacancy clusters as a complement to the current understanding of the neutron irradiation response of $3C$-SiC. Positron annihilation lifetime spectroscopy was used to capture the irradiation temperature and dose dependence of vacancy defects in $3C$-SiC following neutron irradiation from 0.01 to 31 dpa in the temperature range from $380{}^{\circ }\mathrm{C}$ to $790{}^{\circ }\mathrm{C}$. The neutral and negatively charged vacancy clusters were identified and quantified. The results suggest that the vacancy defects that were measured by positron annihilation spectroscopy technique contribute very little to the transient swelling of SiC. In addition, coincidence Doppler broadening measurement was used to investigate the chemical identity surrounding the positron trapping sites. It was found that silicon vacancy-related defects dominate in the studied materials and the production of the antisite defect ${\mathrm{C}}_{\mathrm{Si}}$ may result in an increase in the probability of positron annihilation with silicon core electrons.

The temperature dependence of the high-frequency shear modulus measured in the kHz range is compared with the mean-squared displacement measured in the nanosecond range for the two van der Waals bonded glass-forming liquids cumene and 5-polyphenyl ether. This provides an experimental test for the assumption connecting two versions of the shoving model for the non-Arrhenius temperature dependence of the relaxation time in glass formers. The two versions of the model are also tested directly and both are shown to work well for these liquids.

Ab initio molecular dynamics calculations are used to investigate the vibrational properties of the high-temperature $\delta$ and $\epsilon$ phases of plutonium. We combine the local-density approximation (LDA)$+U$ for strong electron correlations and the temperature-dependent effective potential method in order to calculate the phonon spectra of the two phases, as well as their dependence on temperature. Our results show that the $\epsilon$ phase can only be stabilized when temperature and correlations are simultaneously accounted for. We are also able to quantify the degree of anharmonicity of the two phases. While the $\delta$ phase is fairly harmonic up to 1000 K, we find that the $\epsilon$ phase is strongly anharmonic, which explains why this structure dominates the phase diagram at high temperature.