PbTe nanowires coupled to a superconductor have recently been proposed as a potential Majorana platform. The hallmark of the one-dimensional nature of ballistic nanowires is their quantized conductance. Here, we report the observation of conductance plateaus at multiples of the quantized value $2e^2/h$ in PbTe nanowires at finite magnetic fields. The quantized plateaus, as a function of source-drain bias and magnetic field, allow for the extraction of the Landé $g$ factor, sub-band spacing, and effective mass. The coefficient of 2 in the plateau conductance indicates the presence of valley degeneracy arising from the crystal orientation of the nanowires, which are grown on a (001) substrate. Occasionally, this degeneracy can be lifted by a gate voltage that breaks the mirror symmetry. Our results demonstrate the one dimensionality of PbTe nanowires and fulfill one of the necessary conditions for the realization of Majorana zero modes.

We studied the growth of an indium triple-atomic-layer film and the two-dimensional free-electron-like electronic states on Si(111) by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and angle-resolved photoelectron spectroscopy (ARPES). By depositing In on the In/Si(111)-$\sqrt{7}\times \sqrt{3}$-rect surface below 100 K, followed by brief postannealing up to 140 K, we successfully obtained well-crystalline films exhibiting sharp superstructure LEED spots. We revealed an $(11\times 11)$ superlattice of the triple-layer structure, while both LEED and STM showed a $(5.5\times 5.5)$ pseudoperiodicity. This pseudoperiodicity was attributed to the moiré interference between the Si(111)-($11\times 11$) lattice ($a$ = 3.84 Å) and the In $(13\times 13)$ hexagonal lattice, which has a lattice constant of 3.25 Å, with the ratio very close to 13/11. ARPES measurements unveiled two free-electron-like states with Fermi wave vectors of 1.32 and 1.46 ${\AA }^{-1}$. We also observed replica Fermi surfaces, which are associated with the reciprocal lattice vectors of both the $(1\times 1)$ Si(111) and the In hexagonal layers. This further confirms the hexagonal atomic arrangement of the In triple-layer structure.

Using the exact Bethe ansatz solution, a one-dimensional correlated electron system coupled to the lattice is considered. Electrons belonging to two orbital bands interact via the exchange coupling. It is shown that the coupling to the lattice strains can produce a phase transition to the state in which the degeneracy in the orbital filling is removed. In such a state the formation of superconducting correlations is enhanced. The influence of the external magnetic field on the transition is studied.

The thermodynamics of several reactions involving atomic and molecular hydrogen with group-III acceptors is investigated. The results provide a first-principles-level account of thermally- and carrier-activated processes involving these species. Acceptor-hydrogen pairing is revisited as well. We present a refined physicochemical picture of long-range migration, compensation effects, and short-range reactions, leading to fully passivated $\equiv \text{Si-H}\cdots X\equiv$ structures, where $X$ is a group-III acceptor element. The formation and dissociation of acceptor-H and acceptor-${\mathrm{H}}_2$ complexes is considered in the context of light- and elevated-temperature-induced degradation (LeTID) of silicon-based solar cells. Besides explaining observed trends and answering several fundamental questions regarding the properties of acceptor-hydrogen pairing, we find that the ${\mathrm{BH}}_2$ complex is a by-product along the reaction of ${\mathrm{H}}_2$ molecules with boron toward the formation of BH pairs (along with subtraction of free holes). The calculated changes in Helmholtz free energies upon the considered defect reactions, as well as activation barriers for ${\mathrm{BH}}_2$ formation/dissociation (close to $\sim 1$ eV), are compatible with the experimentally determined activation energies of degradation/recovery rates of Si:B-based cells during LeTID. Dihydrogenated acceptors heavier than boron are anticipated to be effective-mass-like shallow donors, and therefore they are unlikely to show similar nonradiative recombination activity.

Recently, systems with strong coupling between electronic and vibrational degrees of freedom have attracted a great deal of attention. In this work, we consider the transient dynamics of the system consisting of strongly coupled vibrons and excitons driven by an external monochromatic field. We show that, under coherent pumping, polarization of excitons exhibits complex quantum dynamics which can be divided into three stages. In the first stage, exciton oscillations at its eigenfrequency relax due to the transition of excitation to a set of shifted Fock states of vibrons. We demonstrate that these shifted Fock states play the role of an effective reservoir for the excited exciton state. The time of relaxation to this reservoir depends on exciton-vibron coupling. In the second stage, excitation transits from the reservoir of the vibronic shifted states to electronic degrees of freedom. As a result, revival of oscillations at exciton eigenfrequency appears. Thus, the dynamics of molecular polarization exhibit collapses and revivals. At the final stage, these collapses and revivals dissipate and polarization exhibits the Rayleigh response at the frequency of the external field. Discovered collapses and revivals manifest in the radiation spectrum as a multiple splitting of the spectral line near the exciton transition frequency.

We study the influence of nonlocal damping on the magnon properties of Fe, Co, Ni, and ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$ ($x=30\%,50\%$) alloys. The Gilbert damping parameter is typically considered as a local scalar both in experiment and in theoretical modeling. However, recent works have revealed that Gilbert damping is a nonlocal quantity that allows for energy dissipation between atomic sites. With the Gilbert damping parameters calculated from a state-of-the-art real-space electronic structure method, magnon lifetimes are evaluated from spin dynamics and linear response, where a good agreement is found between these two methods. It is found that nonlocal damping affects the magnon lifetimes in different ways depending on the system. Specifically, we find that in Fe, Co, and Ni, the nonlocal damping decreases the magnon lifetimes, while in ${\mathrm{Fe}}_{70}{\mathrm{Co}}_{30}$ and ${\mathrm{Fe}}_{50}{\mathrm{Co}}_{50}$ an opposite, nonlocal damping effect is observed, and our data show that it is much stronger in the former.

Spatiotemporal quenches are efficient at preparing ground states of critical Hamiltonians that have emergent low-energy descriptions with Lorentz invariance [Agarwal, Bhatt, and Sondhi, Phys. Rev. Lett. 120, 210604 (2018)]. The critical transverse field Ising model with nearest-neighbor interactions, for instance, maps to free fermions with a relativistic low energy dispersion. However, spin models realized in artificial quantum simulators based on neutral Rydberg atoms, or trapped ions, generically exhibit long-range power-law decay of interactions with $J\left(r\right)\sim 1/r^{\alpha }$ for a wide range of $\alpha$. In this paper, we study the fate of spatiotemporal quenches in these models with a fixed velocity $v$ for the propagation of the quench front, using the numerical time-dependent variational principle. For $\alpha \gtrsim 3$, where the critical theory is suggested to have a dynamical critical exponent $z=1$, our simulations show that optimal cooling is achieved when the front velocity $v$ approaches $c$, the effective speed of excitations in the critical model. The energy density is inhomogeneously distributed in space, with prominent hot regions populated by excitations copropagating with the quench front, and cold regions populated by counterpropagating excitations. Lowering $\alpha$ largely blurs the boundaries between these regions. For $\alpha <3$, we find that the Doppler cooling effect disappears, as expected from renormalization group results for the critical model, which suggest a dispersion $\omega \sim q^z$ with $z<1$. Instead, we show that excitations are controlled by two relevant length scales whose ratio is related to that of the front velocity to a threshold velocity that ultimately determines the adiabaticity of the quench.

We investigate the physical properties of high-quality single crystals CeGaGe and PrGaGe using magnetization, heat capacity, and magnetotransport measurements. Gallium-indium binary flux was used to grow these single crystals that crystallize in a body-centered tetragonal structure. Magnetic susceptibility data reveal a magnetic phase transition around 6.0 and 19.4 K in CeGaGe and PrGaGe, respectively, which is further confirmed by heat capacity and electrical resistivity data. A number of additional anomalies have been observed below the ordering temperature in the magnetic susceptibility data, indicating a complex magnetic structure. The magnetic measurements also reveal a strong magnetocrystalline anisotropy in both compounds. Our detailed analysis of the crystalline electric field (CEF) effect as observed in magnetic susceptibility and heat capacity data suggests that the $J=5/2$ multiplet of CeGaGe splits into three doublets, while the $J=4$ degenerate ground state of PrGaGe splits into five singlets and two doublets. The estimated energy levels from the CEF analysis are consistent with the magnetic entropy.

Recently, magnetic multilayers composed of repetitive heavy metal/ferromagnetic bilayers have been extensively studied due to their large Dyzaloshinskii-Moriya interaction (DMI) and perpendicular magnetic anisotropy, which are considered key parameters to stabilize skyrmions at room temperature. However, the limitations of characterizing the ultrathin interfacial properties hinder the understanding of the microscopic origin of DMI. Here we report a bulklike enhancement of DMI strength ($D$) through the number of stacking layers ($N$) in ${\left[\mathrm{Co}/\mathrm{Tb}\right]}_N$ multilayers without introducing heavy metals and observe the value $1.38\mathrm{mJ}/{\mathrm{m}}^2$ of $D$ at $N=8$. Using a phenomenological model, we attribute this bulklike behavior of $D$ mainly to the increase in net interfacial DMI (iDMI) with $N$. The increase of iDMI is strongly correlated with the growth of the gradient in crystallographic asymmetry for adjacent interfaces during the stacking process. This structural phase transition concerning iDMI is demonstrated by the energy dispersive x-ray spectroscopy measurements and the $N$ dependence of the longitudinal resistivity. We suggest that the asymmetrical distribution of Tb atoms breaks interfacial symmetry, leading to enhanced deviations in the crystallographic qualities for the top Tb/Co and bottom Co/Tb interfaces, resulting in an increase of the iDMI as $N$ grows. Our findings suggest the potential of rare-earth transition-metal based magnetic multilayers for skyrmion applications due to their tunable magnetic properties regarding magnetic chiral textures.

Heating effects in Floquet-engineered systems are detrimental to the control of physical properties. In this paper, we show that the heating of periodically driven strongly correlated systems can be suppressed by multicolor driving, i.e., by applying auxiliary excitations which interfere with the absorption processes from the main drive. We focus on the Mott insulating single-band Hubbard model and study the effects of multicolor driving with nonequilibrium dynamical mean-field theory. The main excitation is a periodic electric field with frequency $\mathrm{\Omega }$ smaller than the Mott gap, while for the auxiliary excitations, we consider additional electric fields and/or hopping modulations with a higher harmonic of $\mathrm{\Omega }$. To suppress the three-photon absorption of the main excitation, which is a parity-odd process, we consider auxiliary electric-field excitations and a combination of electric-field excitations and hopping modulations. On the other hand, to suppress the two-photon absorption, which is a parity-even process, we consider hopping modulations. The conditions for an efficient suppression of heating are well captured by the Floquet effective Hamiltonian derived with the high-frequency expansion in a rotating frame. As an application, we focus on the exchange couplings of the spins (pseudospins) in the repulsive (attractive) model and demonstrate that the suppression of heating allows us to realize and clearly observe a significant Floquet-induced change of the low energy physics.

Quantum entanglement is a particularly useful characterization of topological orders which lack conventional order parameters. In this work, we study the entanglement in topologically ordered states between two arbitrary spatial regions, using two distinct mixed-state entanglement measures: the so-called computable cross-norm or realignment (CCNR) negativity, and the more well-known partial-transpose (PT) negativity. We first generally compute the entanglement measures: We obtain general expressions both in $(2+1)\mathrm{D}$ Chern-Simons field theories under certain simplifying conditions, and in the Pauli stabilizer formalism that applies to lattice models in all dimensions. While the field-theoretic results are expected to be topological and universal, the lattice results contain nontopological/nonuniversal terms as well. This raises the important problem of continuum-lattice comparison which is crucial for practical applications. When the two spatial regions and the remaining subsystem do not have triple intersection, we solve the problem by proposing a general strategy for extracting the topological and universal terms in both entanglement measures. Examples in the $(2+1)\mathrm{D}$ ${\mathbb{Z}}_2$ toric code model are also presented. In the presence of trisection points, however, our result suggests that the subleading piece in the PT negativity is not topological and depends on the local geometry of the trisections, which is in harmonics with a technical subtlety in the field-theoretic calculation.

We show that topological characterization and classification in $D$-dimensional systems, which are thermodynamically large in only $D-\delta$ dimensions and finite in size in $\delta$ dimensions, is fundamentally different from that of systems thermodynamically large in all $D$ dimensions: As $(D-\delta )$-dimensional topological boundary states permeate into a system's $D$-dimensional bulk with decreasing system size, they hybridize to create novel topological phases characterized by a set of $\delta +1$ topological invariants, ranging from the $D$-dimensional topological invariant to the $(D-\delta )$-dimensional topological invariant. The system exhibits topological response signatures and bulk-boundary correspondences governed by combinations of these topological invariants taking nontrivial values, with lower-dimensional topological invariants characterizing fragmentation of the underlying topological phase of the system thermodynamically large in all $D$ dimensions. We demonstrate this physics for the paradigmatic Chern insulator phase, but show its requirements for realization are satisfied by a much broader set of topological systems.

The orbital, which represents the shape of the electron cloud, very often strongly influences the manifestation of various exotic phenomena, e.g., magnetism, metal-insulator transition, colossal magnetoresistance, unconventional superconductivity, etc. in solid-state systems. The observation of the antiferromagnetism in $RE{\mathrm{TiO}}_3$ ($RE=\text{rare-earth}$) series has been puzzling since the celebrated Kugel-Khomskii model of spin-orbital superexchange predicts ferromagnetism in an orbitally degenerate $d^1$ system. Further, the existence of the orbitally ordered vs. orbital liquid phase in both antiferromagnetic and paramagnetic phase have been unsettled issues thus far. To address these longstanding questions, we investigate single crystalline film of ${\mathrm{PrTiO}}_3$. Our synchrotron x-ray diffraction measurements confirm the retention of bulklike orthorhombic ($D_{2h}$) symmetry in the thin film geometry. We observe similar x-ray linear dichroism signal in both paramagnetic and antiferromagnetic phase, which can be accounted by ferro-orbital ordering (FOO). While the presence of $D_{2h}$ crystal field does not guarantee lifting of orbital degeneracy always, we find it to be strong enough in these rare-earth titanates, leading to the FOO state. Thus, our work demonstrates the orthorhombic distortion is the driving force for the orbital ordering of antiferromagnetic $RE{\mathrm{TiO}}_3$.

Valley polarization, the preferential occupation of electrons in certain valleys of the energy band structure, has recently emerged as a promising concept in the field of valleytronics, which aims to exploit the valley degree of freedom of electrons for information processing and storage. In two-dimensional (2D) $AB{X}_3$ ($A$ and $B$ represent two different metals, and $X$ represents ligand) hexagonal kagomé (HK) lattice, we proposed a general four-band tight-binding model to explain the phase transition from quantum anomalous Hall effect to anomalous valley Hall effect by substituting one metal $A$ with a different metal $B$ from HK lattice $A_2{X}_3$. To confirm the result of the model, we performed density functional theory calculations to study the electronic structure of 2D HK lattice $\mathrm{NbV}{\left({\mathrm{C}}_6{\mathrm{H}}_4\right)}_3, \mathrm{TaNb}{\left({\mathrm{C}}_6{\mathrm{H}}_4\right)}_3$, and ${\mathrm{TaNbO}}_3$, which exhibit large spontaneous valley polarization. The valley polarization emerged by effect of spin orbit coupling and the polarization could be tuned by spin direction. Our results reveal the mechanism of valley physics in $AB{X}_3$ HK lattice and shed light on future prediction of vallytronics exploration and may have potential applications in the field of valleytronics.

We investigate experimentally both the amplitude and phase channels of the collective modes in the quasi-1D charge-density-wave (CDW) system, ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$, by combining (i) optical impulsive-Raman pump-probe and (ii) terahertz (THz) time-domain spectroscopy, with high resolution and a detailed analysis of the full complex-valued spectra in both cases. This allows an unequivocal assignment of the observed bands to CDW modes across the THz range up to $9\mathrm{THz}$. We revise and extend a time-dependent Ginzburg-Landau model to account for the observed temperature dependence of the modes, where the combination of both amplitude and phase modes allows one to robustly determine the bare-phonon frequencies and electron-phonon (e-ph) coupling parameters. While the e-ph coupling is indeed strongest for the lowest-energy phonon, dropping sharply for the next higher frequency phonons, it grows back significantly for the higher-energy phonons, demonstrating their important role in driving the CDW formation. We also include a reassessment of our previous analysis of the lowest-lying phase modes. Assuming weaker electronic damping for the phase channel results in a qualitative picture more consistent with quantum-mechanical treatments of the collective modes, with a strongly coupled amplitudon and phason being the lowest frequency modes.

We re-examine the effect of long-range Coulomb interactions on the collective amplitude and phase modes in the incommensurate charge-density-wave ground state of quasi-one-dimensional conductors.Using an effective action approach we show that the longitudinal acoustic phonon protects the gapless linear dispersion of the lowest phase mode in the presence of long-range Coulomb interactions. Moreover, in Gaussian approximation, amplitude fluctuations are not affected by long-range Coulomb interactions. We also calculate the collective mode dispersions at finite temperatures and compare our results with the measured energies of amplitude and phase modes in ${\mathrm{K}}_{0.3}{\mathrm{MoO}}_3$. With the exception of the lowest phase mode, the temperature dependence of the measured mode energies can be quantitatively described within a multiphonon Fröhlich model for generic electron-phonon interactions neglecting long-range Coulomb interactions.

While the high-temperature thermal transport in crystalline materials has been recently carefully addressed, it is much less explored for amorphous materials. Most of the existing studies have focused on the low-/mid-temperature range and have generally found the increasing trend of thermal conductivity with temperature and converging to a constant value, mainly due to the temperature dependence of heat capacity. In this work, we investigate the temperature-dependent thermal conductivity of amorphous ${\mathrm{HfO}}_2$ with three different methods, including molecular dynamics, the Allen-Feldman theory, and the quasiharmonic Green-Kubo method, with the forces extracted from a machine-learning potential parametrized from first principles. While the Allen-Feldman theory and the quasiharmonic Green-Kubo method show the same temperature dependence trend as the previous expectation even at high temperatures, molecular dynamics simulations show a clear decreasing trend of thermal conductivity at high temperatures. By comparing the results from these approaches, we identify that two anharmonic effects, i.e., thermal expansion and vibrational mode softening, are the mechanisms of the decreased thermal conductivity of amorphous ${\mathrm{HfO}}_2$ at high temperatures.

Metal-halide perovskites exhibit excellent optoelectronic properties, making them suitable for various applications. In this work, we propose a novel approach, employing the message-passing neural networks (MPNNs), to model the potential energy surface (PES) of metal-halide perovskites. The MPNN model can learn the PES of perovskites with an error of less than 1 meV per atom, which is comparable to ab initio calculations. With the accurate and fast prediction of the MPNN model, we perform large-scale molecular dynamics simulations for ${\mathrm{CsPbBr}}_3$ and ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$, successfully identifying their temperature-dependent octahedron tilting patterns, ion displacement, and structural phases that are crucial to obtaining their electronic structure correctly. We also observe significant Ag migration and vacancy defects in ${\mathrm{Cs}}_2{\mathrm{AgBiBr}}_6$, which are further investigated using ab initio calculations, offering insights into its ambient stability. This work provides a comprehensive understanding of the structural phase transition in metal-halide perovskites, as well as information regarding ion migration.

We investigate the crucial role played by a global symmetry in the purification timescales and the phase transitions of monitored free fermionic systems separating a mixed and a pure phase. Concretely, we study Majorana and Dirac circuits with ${\mathbb{Z}}_2$ and U(1) symmetries, respectively. In the first case, we demonstrate the mixed phase of $L$ sites has a purification timescale that scales as ${\tau }_P\sim LlnL$. At $1\ll t\ll {\tau }_P$ the system attains a finite residual entropy, that we use to unveil the critical properties of the purification transition. In contrast, free fermions with U(1) manifest a sublinear purification timescale at any measurement rate and an apparent Berezinskii-Kosterlitz-Thouless criticality. We find the mixed phase is characterized by ${\tau }_P\sim L^{\alpha \left(p\right)}$, with a continuously varying exponent $\alpha \left(p\right)<1$.

We study the critical behavior at the ordinary surface universality class of the three-dimensional $\mathrm{O}\left(N\right)$ model, bounded by a two-dimensional surface. Using high-precision Monte Carlo simulations of an improved lattice model, where the leading bulk scaling correction is suppressed, and finite-size scaling analysis of the fourth cumulant of the surface magnetization, we obtain precise estimates of the scaling dimension of the surface field operator for $N=2,3,4$. We also determine the fixed-point values of two renormalization-group invariant observables, which characterize the finite-size scaling behavior at the ordinary transition.