We present a robust but still efficient simulation approach for high-resolution scanning tunneling microscopy (STM) with a flexible tip apex showing sharp submolecular features. The approach takes into account the electronic structure of the sample and tip as well as relaxation of the tip apex. We validate our model by achieving good agreement with various experimental images which allows us to explain the origin of several observed features. Namely, we have found that the high-resolution STM mechanism consists of standard STM imaging, convolving electronic states of the sample and the tip apex orbital structure, with the contrast heavily distorted by the relaxation of the flexible apex caused by interaction with the substrate.

We analyze the effects of an applied magnetic field on the phase diagram of a weakly correlated electron system with imperfect nesting. The Hamiltonian under study describes two bands: electron and hole ones. Both bands have spherical Fermi surfaces, whose radii are slightly mismatched due to doping. These types of models are often used in the analysis of magnetic states in chromium and its alloys, superconducting iron pnictides, AA-type bilayer graphene, borides, etc. At zero magnetic field, the uniform ground state of the system turns out to be unstable against electronic phase separation. The applied magnetic field affects the phase diagram in several ways. In particular, the Zeeman term stabilizes new antiferromagnetic phases. It also significantly shifts the boundaries of inhomogeneous (phase-separated) states. At sufficiently high fields, the Landau quantization gives rise to oscillations of the order parameters and of the Néel temperature as a function of the magnetic field.

One of the main obstacles that prevents practical applications of antiferromagnets is the difficulty of manipulating the magnetic order parameter. Recently, following the theoretical prediction [J. Železný et al., Phys. Rev. Lett. 113, 157201 (2014)], the electrical switching of magnetic moments in an antiferromagnet was demonstrated [P. Wadley et al., Science 351, 587 (2016)]. The switching is due to the so-called spin-orbit torque, which has been extensively studied in ferromagnets. In this phenomena a nonequilibrium spin-polarization exchange coupled to the ordered local moments is induced by current, hence exerting a torque on the order parameter. Here we give a general systematic analysis of the symmetry of the spin-orbit torque in locally and globally noncentrosymmetric crystals. We study when the symmetry allows for a nonzero torque, when is the torque effective, and its dependence on the applied current direction and orientation of magnetic moments. For comparison, we consider both antiferromagnetic and ferromagnetic orders. In two representative model crystals we perform microscopic calculations of the spin-orbit torque to illustrate its symmetry properties and to highlight conditions under which the spin-orbit torque can be efficient for manipulating antiferromagnetic moments.

Magnetic excitations are investigated for a hexagonal polar magnet $\mathrm{F}{\mathrm{e}}_2\mathrm{M}{\mathrm{o}}_3{\mathrm{O}}_8$ by terahertz spectroscopy. We observed magnon modes including an electric-field active magnon, electromagnon, in the collinear antiferromagnetic phase with spins parallel to the $c$ axis. We unravel the nature of these excitations by investigating the correlation between the evolution of the mode profile and the magnetic transition from antiferromagnetic to ferrimagnetic order induced by a magnetic field or Zn doping. We propose that the observed electromagnon mode involves the electric polarization oscillating within the $c$ plane induced by the collective precession of the spins through the same mechanism as producing the linear magnetoelectric effect.

With infrared (IR) and terahertz (THz) ellipsometry we investigated the anisotropy of the IR-active phonon modes in ${\text{SrTiO}}_3$ (110) single crystals in the tetragonal state below the so-called antiferrodistortive transition at $T^{*}=105$ K. In particular, we show that the anisotropy of the oscillator strength of the so-called $R$ mode, which becomes weakly IR active below $T^{*}$, is a valuable indicator of the orientation of the structural domains. Our results reveal that a monodomain state with the tetragonal axis ($c$ axis) parallel to the [001] direction can be created by applying a moderate uniaxial stress of about 2.3 MPa along the [1-10] direction (with a simple mechanical clamp). The resulting splitting of the IR-active phonon modes is reported.

We present the magnetic and thermal properties of the bosonic-superfluid phase in a spin-dimer network using both quasistatic and rapidly changing pulsed magnetic fields. The entropy derived from a heat-capacity study reveals that the pulsed-field measurements are strongly adiabatic in nature and are responsible for the onset of a significant magnetocaloric effect (MCE). In contrast to previous predictions we show that the MCE is not just confined to the critical regions, but occurs for all fields greater than zero at sufficiently low temperatures. We explain the MCE using a model of the thermal occupation of exchange-coupled dimer spin states and highlight that failure to take this effect into account inevitably leads to incorrect interpretations of experimental results. In addition, the heat capacity in our material is suggestive of an extraordinary contribution from zero-point fluctuations and appears to indicate universal behavior with different critical exponents at the two field-induced critical points. The data at the upper critical point, combined with the layered structure of the system, are consistent with a two-dimensional nature of spin excitations in the system.

The magnetic structure of the five-leg ladder compound ${\mathrm{La}}_8{\mathrm{Cu}}_7{\mathrm{O}}_{19}$ has been determined using single crystalline neutron diffraction. The material orders antiferromagnetically with the propagation vector $q=(\frac{1}{2},\frac{1}{2},0)$ below $T_N=102.5\left(3\right)\mathrm{K}$. Above this temperature the magnetic susceptibility corresponds indeed to that of a five-leg $S=1/2$ spin ladder, assuming isotropic couplings along the rungs and the legs $[J/k_B=J^{\prime }/k_B=194\left(3\right)\mathrm{K}]$. Crystallographically, the Cu magnetic moments divide into two subsystems, depending on the environment. Moments situated at centers of octahedra built up by oxygens orient along the $b$ axis and couple to neighboring moments antiferromagnetically, whereas those creating a complicated ribbon structure possess all three Cartesian components. All Cu moments are found to have magnitudes between 0.69 and $1.32{\mu }_B$. Thus, it is seen that the five-leg ladder ${\mathrm{La}}_8{\mathrm{Cu}}_7{\mathrm{O}}_{19}$ exhibits long-range antiferromagnetic ordering and possesses a rather complicated magnetic structure which has a noncollinear character.

We present a spectrum-splitting approach to conduct all-electron Kohn-Sham density functional theory (DFT) calculations by employing Fermi-operator expansion of the Kohn-Sham Hamiltonian. The proposed approach splits the subspace containing the occupied eigenspace into a core subspace, spanned by the core eigenfunctions, and its complement, the valence subspace, and thereby enables an efficient computation of the Fermi-operator expansion by reducing the expansion to the valence-subspace projected Kohn-Sham Hamiltonian. The key ideas used in our approach are as follows: (i) employ Chebyshev filtering to compute a subspace containing the occupied states followed by a localization procedure to generate nonorthogonal localized functions spanning the Chebyshev-filtered subspace; (ii) compute the Kohn-Sham Hamiltonian projected onto the valence subspace; (iii) employ Fermi-operator expansion in terms of the valence-subspace projected Hamiltonian to compute the density matrix, electron density, and band energy. We demonstrate the accuracy and performance of the method on benchmark materials systems involving silicon nanoclusters up to 1330 electrons, a single gold atom, and a six-atom gold nanocluster. The benchmark studies on silicon nanoclusters revealed a staggering fivefold reduction in the Fermi-operator expansion polynomial degree by using the spectrum-splitting approach for accuracies in the ground-state energies of $\sim {10}^{-4}\text{Ha/atom}$ with respect to reference calculations. Further, numerical investigations on gold suggest that spectrum splitting is indispensable to achieve meaningful accuracies, while employing Fermi-operator expansion.

We present a computationally efficient approach to perform large-scale all-electron density functional theory calculations by enriching the classical finite element basis with compactly supported atom-centered numerical basis functions that are constructed from the solution of the Kohn-Sham (KS) problem for single atoms. We term these numerical basis functions as enrichment functions, and the resultant basis as the enriched finite element basis. The compact support for the enrichment functions is obtained by using smooth cutoff functions, which enhances the conditioning and maintains the locality of the enriched finite element basis. The integrals involved in the evaluation of the discrete KS Hamiltonian and overlap matrix in the enriched finite element basis are computed using an adaptive quadrature grid that is constructed based on the characteristics of enrichment functions. Further, we propose an efficient scheme to invert the overlap matrix by using a blockwise matrix inversion in conjunction with special reduced-order quadrature rules, which is required to transform the discrete Kohn-Sham problem to a standard eigenvalue problem. Finally, we solve the resulting standard eigenvalue problem, in each self-consistent field iteration, by using a Chebyshev polynomial based filtering technique to compute the relevant eigenspectrum. We demonstrate the accuracy, efficiency, and parallel scalability of the proposed method on semiconducting and heavy-metallic systems of various sizes, with the largest system containing 8694 electrons. We obtain accuracies in the ground-state energies that are $\sim 1$ mHa with reference ground-state energies employing classical finite element as well as Gaussian basis sets. Using the proposed formulation based on enriched finite element basis, for accuracies commensurate with chemical accuracy, we observe a staggering $50–300$-fold reduction in the overall computational time when compared to classical finite element basis. Further, we find a significant outperformance by the enriched finite element basis when compared to the Gaussian basis for the modest system sizes where we obtained convergence with Gaussian basis. We also observe good parallel scalability of the numerical implementation up to 384 processors for a representative benchmark system comprising of 280-atom silicon nanocluster.

The influence of electron-phonon interactions on the dynamics of a quantum dot coupled to a photonic cavity mode is investigated by using a nonequilibrium Green's function approach. Within a polaron frame, the self-consistent-Born approximation is used to treat the phonon-assisted scattering processes between the quantum dot polaron and the cavity. Two-time correlators of the quantum dot-cavity system are calculated by solving the Kadanoff–Baym equations, giving access to photon spectra and photon indistinguishability. The non-Markovian nature of the interaction with the phonon bath is shown to be very accurately described by our method in various regimes of cavity quantum electrodynamics (cavity-QED). The indistinguishability of the emitted photons emitted at zero temperature are found to be in very good agreement with a previously reported exact diagonalization approach [Phys. Rev. B 87, 081308 (2013)]. Besides, our method enables the calculations of photon indistinguishability at finite temperatures and for strong electron-phonon interactions. More generally, our method opens new avenues in the study of open quantum system dynamics coupled to non-Markovian environments.

We report the theoretical investigation of the disorder effects on the bulk states of inverted InAs/GaSb quantum wells. As disorder sources we consider the interface roughness and donors/acceptors supplied by intentional doping. We use a $\mathit{k}·\mathit{p}$ approach combined with a numerical diagonalization of the disordered Hamiltonian to get a full insight of the disordered eigenenergies and eigenfunctions of the electronic system. While interface roughness slightly perturbs the carrier motion, we show that dopants strongly bind and localize the bulk states of the structure. Moreover, both types of scatterers strengthen the intrinsic hybridization between holes and electrons in the structure.

Criticality in models of correlated electrons emerges in proximity of a low-temperature singularity in a two-particle Green function. Such singularities are generally related to a symmetry breaking of the one-particle self-energy. A consistent description demands that the symmetry breaking in the self-energy emerges at the critical point of the respective two-particle function. This cannot easily be achieved in models of correlated electrons, since there are two ways connecting one- and two-electron functions that cannot be made fully equivalent in approximations. We present a general construction of diagrammatic two-particle approximations consistent with the one-particle functions so that both produce qualitatively the same quantum critical behavior in thermodynamically equivalent descriptions. The general scheme is applied on the single-impurity Anderson model to derive qualitatively the same Kondo critical scale from the spectral function and the magnetic susceptibility.

Oxide-embedded bulk iron is investigated in terms of first principles calculations and data mining. Twenty-nine oxides are embedded into a vacancy site of iron where first principles calculations are performed and the resulting calculations are stored as a data set. A prediction of the dissolution energy of oxides within iron and the bulk modulus of oxide-embedded iron is performed using machine learning. In particular, support vector machine (SVM) and linear regression (LR) are implemented where descriptors for determining the dissolution energy and bulk modulus are revealed. With trained SVM and LR, the prediction of the dissolution energy for different oxides in iron and the inverse problem—deriving the corresponding descriptor variables from a desired bulk modulus—are achieved. The physical origin behind the chosen descriptors is also revealed where manipulating each individual descriptor within a multidimensional space allows for the prediction of the dissolution energy and bulk modulus. Thus, predictions of physical phenomena are, in principle, achievable if the appropriate descriptors are determined.

We study the intrinsic, disorder-induced decoherence of an isolated quantum system under its own dynamics. Specifically, we investigate the characteristic time scale (i.e., the decoherence time) associated with an interacting many-body system losing the memory of its initial state. To characterize the erasure of the initial state memory, we define a time scale, the intrinsic decoherence time, by thresholding the gradual decay of the disorder-averaged return probability. We demonstrate the system-size independence of the intrinsic decoherence time in different models, and we study its dependence on the disorder strength. We find that the intrinsic decoherence time increases monotonically as the disorder strength increases in accordance with the relaxation of locally measurable quantities. We investigate several interacting spin (e.g., Ising and Heisenberg) and fermion (e.g., Anderson and Aubry-André) models to obtain the intrinsic decoherence time as a function of disorder and interaction strength.

Lead chalcogenides such as PbS, PbSe, and PbTe are of interest for their exceptional thermoelectric properties and strongly anharmonic lattice dynamics. Although PbTe has received the most attention, PbSe has a lower thermal conductivity and a nonlinear temperature dependence of thermal resistivity despite being stiffer, trends that prior first-principles calculations have not fully reproduced. Here, we use ab initio calculations that explicitly account for strong anharmonicity and a computationally efficient stochastic phase-space sampling scheme to identify the origin of this low thermal conductivity as an anomalously large anharmonic interaction, exceeding in strength that in PbTe, between the transverse optic and longitudinal acoustic branches. The strong anharmonicity is reflected in the striking observation of an intrinsic localized mode that forms in the acoustic frequencies. Our work shows the deep insights into thermal phonons that can be obtained from ab initio calculations that do not rely on perturbations from the ground-state phonon dispersion.

In ferroelectric materials the maximum electrostrain effect usually occurs at a phase boundary (often referred to as a “morphotropic phase boundary”) between two or more different phases due to lattice instability at such compositions. As a result it is not expected that the electrostrain maximum can appear in a single ferroelectric phase regime which is away from lattice instability. In this Rapid Communication we report an unexpected finding that the electrostrain maximum occurs in a single rhombohedral phase region of the $(1-x\%)\mathrm{Ba}\left(\mathrm{T}{\mathrm{i}}_{0.8}\mathrm{H}{\mathrm{f}}_{0.2}\right){\mathrm{O}}_3-x\%\left(\mathrm{B}{\mathrm{a}}_{0.7}\mathrm{C}{\mathrm{a}}_{0.3}\right)\mathrm{Ti}{\mathrm{O}}_3$ system. The composition showing the maximum electrostrain corresponds to an “invisible boundary” within the single ferroelectric phase, which is a vertical line starting from the quadruple point of the system. At the invisible boundary other anomalies, such as maximum spontaneous polarization also appear. The origin of electrostrain enhancement at the invisible boundary is considered to correlate with an easy polarization extension inherited from the quadruple point. The electrostrain enhancement effect at the invisible boundary has the advantage of having better temperature stability as compared with that at a phase boundary and thus may provide a way for designing high-electrostrain materials with improved temperature stability.

The atomic-level structures of liquids and glasses are amorphous, lacking long-range order. We characterize the atomic structures by integrating radial distribution functions (RDF) from molecular dynamics (MD) simulations for several metallic liquids and glasses: $\mathrm{C}{\mathrm{u}}_{46}\mathrm{Z}{\mathrm{r}}_{54}, \mathrm{N}{\mathrm{i}}_{80}\mathrm{A}{\mathrm{l}}_{20}, \mathrm{N}{\mathrm{i}}_{33.3}\mathrm{Z}{\mathrm{r}}_{66.7}$, and $\mathrm{P}{\mathrm{d}}_{82}\mathrm{S}{\mathrm{i}}_{18}$. Resulting cumulative coordination numbers (CN) show that metallic liquids have a dimension of $\mathrm{d}=2.55\pm 0.06$ from the center atom to the first coordination shell and metallic glasses have $\mathrm{d}=2.71\pm 0.04$, both less than 3. Between the first and second coordination shells, both phases crossover to a dimension of $\mathrm{d}=3$, as for a crystal. Observations from discrete atom center-of-mass position counting are corroborated by continuously counting Cu glass- and liquid-phase atoms on an artificial grid, which accounts for the occupied atomic volume. Results from Cu grid analysis show short-range $\mathrm{d}=2.65$ for Cu liquid and $\mathrm{d}=2.76$ for Cu glass. Cu grid structures crossover to $\mathrm{d}=3$ at $\xi \sim 8\AA$ ($\sim 3$ atomic diameters). We study the evolution of local structural dimensions during quenching and discuss its correlation with the glass transition phenomenon.

The formation and temperature evolution of polar nanoregions (PNRs) in relaxor ferroelectrics is an intriguing issue that is still under debate. Therefore, we present an approach to estimate the volume fraction of PNRs by the example of the relaxor ferroelectric, $(1-x)\left(\mathrm{B}{\mathrm{i}}_{0.5}\mathrm{N}{\mathrm{a}}_{0.5}\right)\mathrm{Ti}{\mathrm{O}}_3-x\mathrm{BaTi}{\mathrm{O}}_3$ (BNT-$x\mathrm{BT}$). A detailed analysis of the Young's modulus, which is highly sensitive to small structural distortions, at temperatures $25{}^{\circ }\mathrm{C}<T<800{}^{\circ }\mathrm{C}$ for both poled and unpoled samples, is correlated to the temperature evolution of PNRs by utilizing a composite model. The extracted volume fraction of the PNRs and the increasing Young's modulus above the formerly suggested Burns temperature indicate that the formation of the PNRs does not occur at a defined temperature but rather in a broad temperature range starting around $\sim 720{}^{\circ }\mathrm{C}$.

The electron spin resonance spectrum of the $\mathit{XXZ}$ spin chain with finite length shows a double-peak structure at high temperatures around the electron paramagnetic resonance (EPR) frequency. This fact has been pointed out by direct numerical methods [S. El Shawish, O. Cépas, and S. Miyashita, Phys. Rev. B 81, 224421 (2010); H. Ikeuchi, H. De Raedt, S. Bertaina, and S. Miyashita, Phys. Rev. B 92, 214431 (2015)]. The question of whether the double-peak structure survives in the thermodynamics is of particular interest. We study the size dependence of the line shape, including the even-odd effect. It is found that the peaks forming the double-peak structure are assigned to individual resonances, each of which is specified by the magnetizations of the resonating states $(M,M-1)$. To understand dependences, we decompose the spectrum into contributions from transitions specified by the magnetization, and we characterize the structure of the spectrum by individual contributions. We analyze the size dependence of each contribution individually by extending the moment method introduced by M. Brockman et al. to each component, and we find that the mean of each peak approaches the paramagnetic resonance point with $1/N$ (where $N$ is the length of the chain), which indicates that the separation of the peaks of the double-peak structure also vanishes inversely with the system size. We also study the temperature dependence of the structure. At low temperatures, the spectrum has a single peak with a finite width at a position with a finite shift from the frequency of EPR, as pointed out by the analysis of field-theoretical works [M. Oshikawa and I. Affleck, Phys. Rev. Lett. 82, 5136 (1999)]. The study of the temperature dependence of the spectrum shows how the high-temperature spectrum changes to the low-temperature one with a drastic broadening of the spectrum.

Here we consider an interplay between the topology of the magnetization texture (which is a topological soliton, or skyrmion) in a planar magnetic nanoelement and the topology of the element itself (its connectivity). We establish the existence of a set of constraints, coupling these topologies, which are specific for multiply connected elements and are absent in the simply connected case. As an example, a specific constraint is derived for a case of a planar ring magnet, relating the angular positions of magnetic vortices and antivortices inside. We analyze the recent experimental data on the vortex magnetic domain walls in the ring to validate our findings.