It is a common wisdom that the emergence of plasticity in crystalline solids is accompanied by the collapse of the softest vibrational mode. However, the recent advance in the multi-principal element alloys (MPEAs) complicates this scenario with essential local chemical inhomogeneity rendered by formation of the chemical short-range order (CSRO) via strong enthalpic interaction between specific atom pairs. Here we develop a set of computational metrics to characterize the features of phonons and the instability pathway in a prototypical ternary CoCrNi MPEA by atomistic simulations. There exists strong anharmonicity in MPEA quantified by a large Grüneisen parameter, which is modulated by the degree of CSRO. Both localized and extended vibrational modes exist at either low- or high-frequency vibrations, whereas extremely high-frequency modes prefer local vibration. Introduction of CSRO increases the phonon density of states at high frequency and reduces the degree of anharmonicity. Therefore, CSRO enhances elastic stability. Upon loading, phonon instability occurs via annihilation of the softest mode, akin to the scenario seen in conventional crystals. However, softening of several low-frequency modes occurs simultaneously in MPEA, and it is the cooperative softening of them that leads to the onset of plasticity. Amid phonon instability, essential variations in the Grüeisen parameter and participation ratio are observed, signifying the correlation between vibrational and configurational space in such compositional complex alloys. The results of unusual phonon features could give insight into how the structure, elasticity, and plasticity interact in generic multi-component high-entropy alloys.

The addition of solute provides an effective means to alter the stability of phases in metallic materials. This is particularly true for the metastable tω phase in Ti- and Zr-based alloys; however, the underlying mechanism remains inconclusive. In the present work, we show that the $\omega \to \beta$ phase-transformation process can be hindered to stabilize the ω phase over a wide temperature and pressure range. This is demonstrated by molecular dynamics simulations of Zr-Nb alloys. In particular, we show that Nb dopants lead to the formation of a defect structure in the Zr-ω phase, i.e., there is a corotation of six atoms in the {0002} plane by 30 ° along the $c$ axis of the ω lattice. We refer to this as a local interlayer twist (LIT). The LITs are energetically preferred within Nb-rich regions and can interlock ω lattices, thus retarding the $\omega \to \beta$ phase transformation. Furthermore, our density-functional theory calculations suggest that this mechanism should work in Zr- and Ti-based alloys with small solute atoms. The findings enrich our understanding of complex phase-transformation kinetics in alloys.

When the coupled electron-nuclear dynamics are subjected to strong Floquet driving, there is a strong breakdown of the Born-Oppenheimer approximation. In this paper, we derive a Fokker-Planck equation to describe nonadiabatic molecular dynamics with electronic friction for Floquet-driven systems. We first provide a new derivation of the Floquet quantum-classical Liouville equation (QCLE) for driven electron-nuclear dynamics. We then transform the Floquet QCLE into a Fokker-Planck equation with explicit forms of frictional force and random force. We recast the electronic friction in terms of Floquet Green's functions such that we can evaluate the electronic friction explicitly. We show that the Floquet electronic friction tensor exhibits antisymmetric terms even at the equilibrium for a real-valued Hamiltonian, suggesting that there is a Lorentz-like force in Floquet-driven non-Born Oppenheimer dynamics even without any spin-orbit couplings.

The remarkable and fascinating properties of two-dimensional (2D) materials have raised them to the rank of most promising candidates for technological applications. In particular, the possibility of long-range ferromagnetic order in 2D materials is generating a growing excitement. Here, we demonstrate that flat bands (FBs) can pave the way for room-temperature ferromagnetism in 2D compounds. Indeed, the magnetic exchanges between localized spins are largely dominated by the FB-FB contribution. This contribution is ferromagnetic and scales linearly with the local coupling, thus leading to crossover temperatures (ferromagnetic phase/paramagnetic phase) higher by an order of magnitude than those currently reported in experiments. High crossover temperatures could be reached in micrometer-sized FB systems.

Multiferroic behavior in the linear-chain spin $S=1/2$ compound ${\mathrm{CuCrO}}_4$ was proposed to appear due to competing nearest- and next-nearest-neighbor exchange interactions along the chain. Here, we report on our study of the long-range magnetic ordering using powder neutron diffraction and muon-spin rotation measurements. Consistently, both methods find incommensurate long-range antiferromagnetic ordering below 8.5(3) K. We determined the magnetic structure from neutron powder diffraction patterns based on the propagation vector $\vec{\tau }=(0,0,0.546\left(1\right))$. At 1.9 K, the magnetic moment of ${\mathrm{Cu}}^{2+}$ was refined to 0.48(2) ${\mu }_{\mathrm{B}}$. The Cu moments form a helicoidal spiral with an easy plane coinciding with the equatorial planes of the Jahn-Teller elongated ${\mathrm{CuO}}_6$ octahedra. Low-temperature high magnetic field measurements of the magnetization and the dielectric polarization show the multiferroic phase to extend up to $\sim 25$ T, after which a new, yet unknown phase appears. Full saturation of the magnetic moment is expected to occur at fields much beyond 60 T.

The coherence length of two Majorana zero-energy modes in a $p$-wave topological superconductor is inversely proportional to the superconducting order parameter. We studied the finite size effect of the Majorana zero modes at vortices in a topological insulator/superconductor heterostructure in the presence of a vortex and found that the coherence length of the two zero-energy modes at the terminals of a vortex line is independent of the superconducting order parameter and determined by the intrinsic properties of the topological insulator. This anomalous property illustrates that the superconducting topological insulator is topologically distinct, contrary to a $p$-wave topological superconductor.

By employing ab initio Migdal-Eliashberg calculations, we predict a fourfold enhancement of the superconducting critical temperature of $\mathrm{Mg}{\mathrm{B}}_2$ when proximity-coupled to the topological crystal ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. We support this result with calculations using the general Leavens scaling method. We show that this effect is a result of dynamic resonant antishielding of Cooper pairs by plasmon-polarons of Dirac electrons in the topological crystal. Our calculations show that such superconductivity enhancement varies strongly with Coulomb coupling between plasmon- polarons and Cooper pairs, with a pronounced maximum of $T_{\mathrm{c}}$ at a critical value of the coupling parameter. This feature is universal, and so can occur in other superconductor-topological crystal combinations, including with nonphonon mediated superconductors. We discuss methods to experimentally optimize the key coupling parameter.

Disorder has appeared as one of the main mechanisms to induce topologically trivial zero-energy states in superconductor-semiconductor systems, thereby challenging the detection of topological superconductivity and Majorana bound states. Here, we demonstrate that, for disorder in any part of the system, the formation of disorder-induced trivial zero-energy states can, to a large extent, be mitigated by keeping the coupling between the semiconductor and superconductor weak. The only exception is strong disorder in the semiconductor, where instead the strong-coupling regime is somewhat more robust against disorder. Furthermore, we find that the topological phase in this weak-coupling regime is robust against disorder, with a large and well-defined topological gap which is highly beneficial for topological protection. Our work shows the advantages and disadvantages of weak and strong couplings under disorder, important for designing superconductor-semiconductor hybrid structures.

We study the electronic structure of electron-doped ${\mathrm{Pr}}_{1.3-x}{\mathrm{La}}_{0.7}{\mathrm{Ce}}_x{\mathrm{CuO}}_4$ (PLCCO; $T_c=27\mathrm{K}, x=0.1$) and hole-doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212; $T_c=90\mathrm{K}$) cuprate superconductors using x-ray absorption spectroscopy and resonant photoemission spectroscopy (Res-PES). From Res-PES across the O $K$-edge and Cu $L$-edge, we identify the O $2p$ and Cu $3d$ partial density of states (PDOS) and their correlation satellites, which originate in two-hole Auger final states. Using the Cini-Sawatzky method, analysis of the experimental O $2p$ PDOS shows an oxygen on-site Coulomb energy for PLCCO to be $U_p$ = $3.3\pm 0.5\mathrm{eV}$, and for Bi2212, $U_p=5.6\pm 0.5\mathrm{eV}$, while the copper on-site Coulomb correlation energy is $U_d$ = $6.5\pm 0.5\mathrm{eV}$ for Bi2212. The expression for the Heisenberg exchange interaction $J$ in terms of the electronic parameters $U_d, U_p$, charge-transfer energy $\mathrm{\Delta }$, and Cu-O hopping $t_{pd}$ obtained from a simple ${\mathrm{Cu}}_2\mathrm{O}$ cluster model is used to carry out an optimization analysis consistent with $J$ known from scattering experiments. The analysis also provides the effective one-band on-site Coulomb correlation energy $\tilde{U}$ and the effective hopping $\tilde{t}$. PLCCO and Bi2212 are shown to exhibit very similar values of $\tilde{U}/\tilde{t}\sim 9–10$, confirming the strongly correlated nature of the singlet ground state in the effective one-band model for both materials.

In two dimensions, magnetic higher-order topological insulators (HOTIs) are characterized by excess boundary charge and a compensating bulk “filling anomaly.” At the same time, without additional noncrystalline symmetries, the boundaries of two-dimensional HOTIs are gapped and featureless at low energies, while the bulk of the system is predicted to have a topological response to the insertion of lattice (particularly disclination) defects. Until recently, a precise connection between these effects has remained elusive. In this work, we point the direction towards a unifying field-theoretic description for the bulk and boundary response of magnetic HOTIs. By focusing on the low-energy description of the gapped boundary of a two-dimensional magnetic HOTI with no time-reversing symmetries, we show that the boundary charge and filling anomaly arise from the gravitational “Gromov-Jensen-Abanov” (GJA) response action [A. Gromov et al., Phys. Rev. Lett. 116, 126802 (2016)] in the context of the quantum Hall effect. As in quantum Hall systems, the GJA action cancels apparent anomalies associated with the bulk response to disclinations, allowing us to derive a concrete connection between the bulk and boundary theories of HOTIs. We show how our results elucidate the connection between higher-order topology and geometric response in band insulators, and point towards a route to understanding interacting higher-order topological phases beyond the simple cases considered here.

X-ray diffraction and energy dispersive x-ray spectroscopic analyses showed a natural galena (PbS) crystal from Freiberg in Saxony (Germany) to be a single phase specimen [rock salt (NaCl) structure type, space group $Fm\overline{3}m, a=5.932\left(1\right)$ Å] with stoichiometric composition and an enhanced dislocation density ($\delta \approx {10}^{11}{\mathrm{cm}}^{-2}$). The latter parameter leads to an increase of the electrical resistivity in the high-temperature regime, as well as to the appearance of phonon resonance with a characteristic frequency ${\omega }_{\mathrm{PR}}=3.8\left(1\right)$ THz. Being in the same range (i.e., 3–5.5 THz) with the sulfur optical modes of highest group velocities, it results in a drastic reduction (by $\sim$ 75%) of thermal conductivity ($\kappa$) at lower temperatures (i.e., $<100$ K), as well as in the appearance of a characteristic minimum in $\kappa$ at $T\approx$ 30 K. Furthermore, the studied galena is characterized by phonon-drag behavior and by temperature dependent switch of the charge carrier scattering mechanism regime (i.e., scattering on dislocations for $T<100$ K, on acoustic phonons for 100 K $<T<170$ K and on both acoustic and optical phonons for 170 K $<T<300$ K). The combined theoretical calculation and optical spectroscopic study confirm this mineral to be a direct gap degenerate semiconductor. The possible origins of the second-order Raman spectrum are discussed.

The anisotropy of nanoscale friction is analyzed for different samples using friction force microscopy under ultrahigh-vacuum conditions with varying scan directions. For both $\mathrm{Mo}{\mathrm{S}}_2$ and highly oriented pyrolytic graphite surfaces, our experiments reveal a sixfold symmetry of the friction vector plots in accordance with solutions of the Langevin equation, where the atomic force microscope tip is treated as a point mass elastically driven on the hexagonal surface lattices. In addition, the effect of temperature is analyzed both by experiment and by simulations. In both cases we find that increased temperature not only lowers the absolute friction values but also decreases the friction anisotropy. In contrast, experiments on NaCl cannot be explained on a similar basis. While the square symmetry is clearly revealed in the first measurements, it progressively deteriorates with repeated scanning, which is possibly due to abrasive wear occurring at the interface.

We present a theoretical study of the Berezinskii-Kosterlitz-Thouless transition of a two-dimensional superfluid in the presence of an externally imposed density modulation along a single axis. The subject is investigated in the context of the ${\left|\psi \right|}^4$ classical field theory, by means of analytical and numerical techniques. We show that, as the amplitude of the modulation increases, the physics of the system approaches that of the anisotropic $x\text{−}y$ model, with a suppressed superfluid transition temperature and an anisotropic response, but with no dimensional crossover.

The $f$ electrons near the surface of strongly correlated materials show unique properties due to the different environments from those in bulk. Here, we used the density functional theory (DFT) and DFT+dynamical mean field theory method to study the surface electronic structures of the P- and Ce-terminated layer structure of $\mathrm{Ce}{\mathrm{Co}}_2{\mathrm{P}}_2$. First, we find that the surface states of the paramagnetic P-terminated slab exhibit heavy-fermion behavior. Second, the flat bands of surface $\mathrm{Ce}\text{−}4f_{5/2}$ in the P-terminated slab was closer to the Fermi level ($E_F$) than that of $\mathrm{Ce}\text{−}4f_{5/2}$ in bulk and hybridized with $\mathrm{Co}\text{−}3d$ conductive bands at low temperature. However, the surface Ce of the Ce-terminated slab was closer to the atomic state because it is in contact with the vacuum, and its strength of hybridization is lower than that of the P-terminated slab, leading to a weaker Kondo resonance peak near $E_F$. Third, the relaxation of the surface adjusts the dispersion of the bands and enhances the Kondo resonance for the P-terminated slab. Finally, after considering the antiferromagnetic order of Co, we found that the band structure was closer to the experiment results. These results show that $\mathrm{Ce}{\mathrm{Co}}_2{\mathrm{P}}_2$ is an ideal material for studying interlayer coupling between the two-dimensional Kondo lattice and magnetic layers.

We consider the dynamics and stationary regime of a capacitively shunted transmon-type qubit in front of a mirror. The qubit is affected by probe and dressing signals. By varying the parameters of these signals and then analyzing the probe signal (reflected by the “atom plus mirror” system), it is possible to explore the system dynamics, which can be described by the Bloch equation. The obtained time-dependent occupation probabilities are related to the experimentally measured reflection coefficient. The study of this type of dynamics opens up new horizons for better understanding the “qubit plus mirror” circuit properties and the underlying physical processes, such as Landau-Zener-Stückelberg-Majorana transitions.

An asymmetric Bose-Einstein condensation (BEC) dome was observed in a recent experiment on the quantum dimer magnet ${\mathrm{Yb}}_2{\mathrm{Si}}_2{\mathrm{O}}_7\mathrm{e}$ [G. Hester et al., Phys. Rev. Lett. 123, 027201 (2019)], which is modeled by a “breathing” honeycomb lattice Heisenberg model with possible anisotropies. We report a remarkable agreement between key experimental features and predictions from numerical simulations of the magnetic model. Both critical fields, as well as critical temperatures of the BEC dome, can be accurately captured, as well as the occurrence of two regimes inside the BEC phase. Furthermore, we investigate the role of anisotropies in the exchange coupling and the $g$ tensor. While we confirm a previous proposal that anisotropy can induce a zero-temperature phase transition at magnetic fields smaller than the fully polarizing field strength, we find that this effect becomes negligible at temperatures above the anisotropy scale. Instead, the two regimes inside the BEC dome are found to be due to a nonlinear magnetization behavior of the isotropic breathing honeycomb Heisenberg antiferromagnet. Our analysis is performed by combining the density matrix renormalization group (DMRG) method with the finite-temperature techniques of minimally entangled typical thermal states (METTS) and quantum Monte Carlo (QMC).

We perform density functional theory and density functional theory plus dynamical mean-field theory calculations to study the electronic structures and topological properties of iron-based superconductors ${\mathrm{BaTh}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\left({\mathrm{N}}_{1-x}{\mathrm{O}}_x\right)}_2$ ($x=0.1$, 0.2, and 0.3). Taking into account electronic correlation corrections, we find that at small oxygen dopings ($x=0.1$ and 0.2), the ${\mathrm{BaTh}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\left({\mathrm{N}}_{1-x}{\mathrm{O}}_x\right)}_2$ superconductors have nontrivial band topology and Dirac-cone type topological surface states around the Fermi level on the (001) surface, which indicates that they can harbor Majorana zero modes on the (001) surface. Increasing the oxygen doping level, the ${\mathrm{BaTh}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\left({\mathrm{N}}_{1-x}{\mathrm{O}}_x\right)}_2$ superconductor undergoes a topological phase transition between $x=0.2$ and 0.3, and has trivial band topology and no topological surface states on the (001) surface at $x=0.3$. The coexistence of high-temperature superconductivity, topological surface states, and topological phase transition in ${\mathrm{BaTh}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\left({\mathrm{N}}_{1-x}{\mathrm{O}}_x\right)}_2$ superconductors makes them potential platforms to study topological superconductivity, Majorana zero modes and effects induced by a topological phase transition.