The magnetic hyperfine field and electric field gradients at a radioactive impurity probe of ${}^{111}\mathrm{Cd}(\leftarrow {}^{111m}\mathrm{Cd})$ occupying the La/Ca $A$ site in a perovskite manganaite ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ ($T_C\sim 250\mathrm{K}$) were measured by means of time-differential perturbed angular correlation (TDPAC) spectroscopy. In the paramagnetic-insulator phase at room temperature, the ${}^{111}\mathrm{Cd}(\leftarrow {}^{111m}\mathrm{Cd})$ probes are distributed in two different environments: distorted and less distorted sites; whereas in the ferromagnetic-metal phase below $T_C$, the oscillatory structure of the distorted component vanishes from the TDPAC function. The vanishing of the oscillation is ascribable to dynamic motion of polarons dragged by conduction electrons induced by the double exchange interaction. In liquid helium, the dynamic motion freezes, leaving averaged local distortion. The dynamic motion and freezing of local structures associated with the colossal-magnetoresistance phase transition to the ferromagnetic phase is discussed on the basis of temperature-dependent electromagnetic fields at the probe nuclei of the $A$ site ions.

Machine learning methods are being actively considered as a new tool for describing many-body physics. However, so far, their capabilities have been demonstrated only in previously studied models, such as the Ising model. Here, we consider a simple problem demonstrating that neural networks can be successfully used to give new insights into statistical physics. Specifically, we consider a three-dimensional (3D) lattice dimer model which consists of sites forming a lattice and bonds connecting the neighboring sites in such a way that every bond can be either empty or filled with a dimer and the total number of dimers ending at one site is fixed to be 1. Dimer configurations can be viewed as equivalent if they are connected through a series of local flips, i.e., simultaneous “rotation” of a pair of parallel neighboring dimers. It turns out that the whole set of dimer configurations on a given 3D lattice can be split into distinct topological classes, such that dimer configurations belonging to different classes are not equivalent. In this paper we identify these classes by using neural networks. More specifically, we train the neural networks to distinguish dimer configurations from two known topological classes, and afterward, we test them on dimer configurations from unknown topological classes. We demonstrate that a 3D lattice dimer model on a bipartite lattice can be described by an integer topological invariant (Hopf number), whereas a lattice dimer model on a nonbipartite lattice is described by $Z_2$ invariant. Thus, we demonstrate that neural networks can be successfully used to identify new topological phases in condensed-matter systems, whose existence can later be verified by other (e.g., analytical) techniques.

We present a computational technique to calculate time- and momentum-resolved nonequilibrium spectral density of correlated systems using a tunneling approach akin to scanning tunneling spectroscopy. The important difference is that our probe is extended, basically a copy of the sample, allowing one to extract the momentum information of the excitations. We illustrate the method by measuring the spectrum of a Mott-insulating extended Hubbard chain after a sudden quench with the aid of time-dependent density-matrix renormalization-group calculations. We demonstrate that the system realizes a nonthermal state that is an admixture of spin and charge density wave states, with corresponding signatures that are recognizable as in-gap subbands. In particular, we identify a band of excitons and one of stable antibound states at high energies that gains enhanced visibility after the pump. We do not appreciate noticeable relaxation within the time scales considered, which is attributed to the lack of decay channels due to spin-charge separation. These ideas can be readily applied to study transient dynamics and spectral signatures of correlation-driven nonequilibrium processes.

When considering transition-metal dichalcogenides (TMDCs) in van der Waals heterostructures for periodic ab initio calculations, usually, lattice mismatch is present, and the TMDC needs to be strained. In this study we provide a systematic assessment of biaxial strain effects on the orbital, spin-orbit, and optical properties of the monolayer TMDCs using ab initio calculations. We complement our analysis with a minimal tight-binding Hamiltonian that captures the low-energy bands of the TMDCs around the $K$ and $K^{\prime }$ valleys. We find characteristic trends of the orbital and spin-orbit parameters as a function of the biaxial strain. Specifically, the orbital gap decreases linearly, while the valence (conduction) band spin splitting increases (decreases) nonlinearly in magnitude when the lattice constant increases. Furthermore, employing the Bethe-Salpeter equation and the extracted parameters, we show the evolution of several exciton peaks, with biaxial strain, on different dielectric surroundings, which are particularly useful for interpreting experiments studying strain-tunable optical spectra of TMDCs.

In this work, the nonlinear optical response, and in particular, the high harmonic generation of semiconductors, is addressed by using the Wannier gauge. One of the main problems in the time evolution of the Semiconductor Bloch equations resides in the fact that the dipole couplings between different bands can diverge and have a random phase along the reciprocal space, and this leads to numerical instability. To address this problem, we propose the use of the maximally localized Wannier functions that provide a framework to map ab initio calculations to an effective tight-binding Hamiltonian with great accuracy. We show that working in the Wannier gauge, the basis set in which the Bloch functions are constructed directly from the Wannier functions, the dipole couplings become smooth along the reciprocal space, thus avoiding the problem of random phases. High harmonic generation is computed for a two-dimensional monolayer of hexagonal boron nitride as a numerical demonstration.

Transition metal dichalcogenides (TMDs) in the $1T$ polymorph are subject to a rich variety of periodic lattice distortions, often referred to as charge-density waves (CDWs) when not too strong. We study from first principles the fermiology and phonon dispersion of three representative single-layer transition metal disulfides with different occupation of the $t_{2g}$ subshell: ${\mathrm{TaS}}_2\left(t_{2g}^1\right),{\mathrm{WS}}_2\left(t_{2g}^2\right)$, and ${\mathrm{ReS}}_2\left(t_{2g}^3\right)$ across a broad range of doping levels. While strong electron-phonon interactions are at the heart of these instabilities, we argue that away from half-filling of the $t_{2g}$ subshell, the doping dependence of the calculated CDW wave vector can be explained from simple fermiology arguments, so that a weak-coupling nesting picture is a useful starting point for understanding. On the other hand, when the $t_{2g}$ subshell is closer to half-filling, we show that nesting is irrelevant, while a real-space strong-coupling picture of bonding Wannier functions is more appropriate and simple bond-counting arguments apply. Our study thus provides a unifying picture of lattice distortions in $1T$ TMDs that bridges the two regimes, while the crossover between these regimes can be attained by tuning the filling of the $t_{2g}$ orbitals.

We analyze the time-dependent formation of the spectral function of an Anderson impurity model in the Kondo regime within a numerically exact real-time quantum Monte Carlo framework. At steady state, splitting of the Kondo peak occurs with nontrivial dependence on voltage and temperature, and with little effect on the location or intensity of high-energy features. Examining the transient development of the Kondo peak after a quench from an initially uncorrelated state reveals a two-stage process where the initial formation of a single central Kondo peak is followed by splitting. We analyze the time dependence of splitting in detail and demonstrate a strong dependence of its characteristic timescale on the voltage. We expect both the steady state and the transient phenomenon to be experimentally observable.

The coherent spin dynamics of resident electrons and holes in an isotopically purified ZnSe/(Zn,Mg)Se single quantum well is investigated in different regimes, requiring corresponding adaption of the applied time-resolved pump-probe Kerr rotation technique. The purification of the Zn and Se atom species in the crystal to the isotopes with zero nuclear spin is expected to lead to an extension of the spin dephasing times of resident carriers, due to the suppression of their interaction with the nuclear spins. Indeed, we find no indication of carrier-nuclear interaction in this sample and link the observed carrier spin relaxation to the spin-orbit interaction. Theoretical considerations support the experimental results.

Mesoscopic polarization structures such as polar nanoregions and polarization domain walls are the key factors connecting microscopic fundamental polarization structures with macroscopic practical dielectric properties. A snapshot observation of the domain in the relaxor ferroelectrics $\mathrm{Pb}\left[{\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right)}_{1\text{−}x}\mathrm{T}{\mathrm{i}}_x\right]{\mathrm{O}}_3$ just in the vicinity of morphotropic phase boundary region was performed by use of 7-ps single shot soft x-ray laser pulse. A self-assembled evolution of the oblique polarization domain was observed in $\mathrm{Pb}\left[{\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right)}_{72.2}\mathrm{T}{\mathrm{i}}_{27.8}\right]{\mathrm{O}}_3$ under the sample temperature decreased with the thermal equilibrium condition. Based on energetic discussion, antiphase shift of domain wall pairs keeping flat boundaries was proposed for the dielectric response. A sharp enhancement in the dielectric response at the vicinity of morphotropic phase boundary region reported previously was recognized as evidence for the hierarchic nature of the present oblique polarization domain wall.

${\mathrm{La}}_{1.67}{\mathrm{Sr}}_{0.33}{\mathrm{NiO}}_4$ develops charge and spin stripe orders at temperatures of roughly 200 K, with modulation wave vectors that are temperature independent. Various probes of spin and charge response have provided independent evidence for some sort of change below $\sim 50$ K. In combination with a new set of neutron scattering measurements, we propose a unified interpretation of all of these observations in terms of a freezing of Ni-centered charges stripes, together with a glassy ordering of the spin stripes that shows up in neutron scattering as a slight rotation of the average spin direction.

Dirac states hosted by Sb/Bi square nets are known to exist in the layered antiferromagnetic ${\mathrm{AMnX}}_2$ (A = CaSr/Ba/Eu/Yb, X=Sb/Bi) material family with the space group to be $P4/nmm$ or $I4/mmm$. In this paper, we present a comprehensive study of quantum transport behaviors, angle-resolved photoemission spectroscopy (ARPES), and first-principles calculations on ${\mathrm{SrZnSb}}_2$, a nonmagnetic analog to ${\mathrm{AMnX}}_2$, which crystalizes in the $pnma$ space group with distorted square nets. From the quantum oscillation measurements up to 35 T, three major frequencies including $F_1=103\mathrm{T}, F_2=127\mathrm{T}$, and $F_3=160\mathrm{T}$ are identified. The effective masses of the quasiparticles associated with these frequencies are extracted, namely, $m_1^{*}=0.1 {\mathrm{m}}_e, m_2^{*}=0.1 {\mathrm{m}}_e$, and $m_3^{*}=0.09 {\mathrm{m}}_e$, where ${\mathrm{m}}_e$ is the free electron mass. From the three band Lifshitz-Kosevich fit, the Berry phases accumulated along the cyclotron orbit of the quasiparticles are $0.06\pi , 1.2\pi$, and $0.74\pi$ for $F_1, F_2$, and $F_3$, respectively. Combined with the ARPES data and the first-principles calculations, we reveal that $F_2$ and $F_3$ are associated with the two nontrivial Fermi pockets at the Brillouin zone edge while $F_1$ is associated with the trivial Fermi pocket at the zone center. In addition, the first-principles calculations further suggest the existence of a Dirac nodal line in the band structure of ${\mathrm{SrZnSb}}_2$.

We demonstrate the mechanism of spin current generation in ultrathin ferromagnetic film by voltage-induced interface magnetoelectric effect and provide a rigorous theoretical and numerical description of the phenomenon. Surprisingly, for MgO-Cu-Co-MgO systems the spin-dependent screening in thin (less than 20 nm) Co film produces spin accumulation 7 times higher than the accumulation induced by the bulk effect of spin-dependent conductivity in thick Co films. An experimental approach to validate our numerical predictions is proposed. The demonstrated effect opens routes to design highly miniaturized, voltage-controlled spintronic or magnonic devices, while the developed model is a useful tool to study spin currents driven by surface screening.

We studied the physical properties of two Kondo-lattice compounds, ${\mathrm{CeRu}}_2{\mathrm{As}}_2$ and ${\mathrm{CeIr}}_2{\mathrm{As}}_2$, by a combination of electric transport, magnetic, and thermodynamic measurements. They are of ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type and ${\mathrm{CaBe}}_2{\mathrm{Ge}}_2$-type crystalline structures, respectively. ${\mathrm{CeRu}}_2{\mathrm{As}}_2$ shows localized long-range antiferromagnetic ordering below $T_N=4.3\mathrm{K}$, with a moderate electronic Sommerfeld coefficient ${\gamma }_0=35\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$. A field-induced metamagnetic transition is observed near 2 T below $T_N$. Magnetic susceptibility measurements on aligned ${\mathrm{CeRu}}_2{\mathrm{As}}_2$ powders suggest that it has an easy axis and that the cerium moments align uniaxially along $c$ axis. In contrast, ${\mathrm{CeIr}}_2{\mathrm{As}}_2$ is a magnetically nonordered heavy-fermion metal with enhanced ${\gamma }_0>300\mathrm{mJ}/\mathrm{mol}{\mathrm{K}}^2$. The initial onset Kondo temperatures of the two compounds are, respectively, 6 and 30 K. We discuss the role of the crystal structure to the strength of Kondo coupling. This paper provides two dense Kondo-lattice materials for further investigations on electronic correlation, quantum criticality, and heavy-electron effects.

Quasicrystalline surfaces are potential templates for highly ordered but aperiodic molecular self-assemblies. Here, we report on the adsorption of ${\mathrm{C}}_{60}$ molecules on a two-dimensional oxide quasicrystal (OQC) and its $\sigma$-phase approximant at room temperature and $150\mathrm{K}$. The molecular films have been characterized by means of scanning tunneling microscopy and low-energy electron diffraction. A weak interaction of the molecules with the dodecagonal oxide template is found, resulting in a low desorption temperature of $510\mathrm{K}$. For low temperatures specific adsorption sites within the tiling of the OQC and the $\sigma$-phase approximant have been identified as preferred nucleation sites. The narrow spacing between these sites introduces repulsive interaction between ${\mathrm{C}}_{60}$ molecules and restricts the epitaxial growth to small patches. At room temperature, ${\mathrm{C}}_{60}$ nucleates in hcp islands of almost arbitrary rotational orientation on the OQC. A slight orientational preference for hexagonal islands with a ${15}^{\circ }$ rotation against the OQC tiling is discussed as a consequence of the preferential adsorption on the quadratic tiles.

Metallic phases of ${\mathrm{Al}}_2\mathrm{H}$ and AlH at megabar pressures are predicted to be possible by using ab initio density functional calculations. The ${\mathrm{Al}}_2\mathrm{H}$ phase is stabilized above 155 GPa, where several candidate structures are quite competitive; the structural properties suggest that ${\mathrm{Al}}_2\mathrm{H}$ has the phase where Al atoms form an hcp structure and H atoms occupy the octahedral sites in a random manner. The AlH phase is stable above 175 GPa, where the structure takes $R\overline{3}m$ symmetry. Superconducting transition temperature ($T_c$) of the ${\mathrm{Al}}_2\mathrm{H}$ phase is estimated to be of the order of 1 K. In contrast, $T_c$ of the $R\overline{3}m$ AlH reaches 58 K at 180 GPa. The electronic structures around the Fermi energy in the $R\overline{3}m$ AlH are insensitive to pressure compared with those in the well-known $Pm\overline{3}n$ phase of ${\mathrm{AlH}}_3$. Accordingly, while theoretical $T_c$ of the $Pm\overline{3}n{\mathrm{AlH}}_3$ rapidly decreases with compression and becomes almost zero above $\sim 200$ GPa, that of the $R\overline{3}m$ AlH remains to be 21 K even at 335 GPa. This means that although superconductivity was not observed experimentally in ${\mathrm{AlH}}_3$, it might be achieved in AlH.

Surface physics dominated by bulk properties has been one of the central interests in modern condensed matter physics, from electric polarization to bulk-boundary correspondence of topological insulators and superconductors. Here we extend theory of electric polarization to chirality polarizations, that is, surface charges corresponding to local antisymmetries characterized as a bulk property. Using the notion of chirality polarizations, we prove the recently proposed spectral bulk-boundary correspondence, a generalization of bulk-boundary correspondence in one-dimensional chiral symmetric systems into complex frequencies. We show a physically transparent proof via Wannier functions and a formal proof by considering the adiabatic change of surface chirality charges, highlighting the similarities and the differences between electric polarization and chirality polarizations.