This paper presents a first-principles study of the Debye-Waller factor and the Debye temperature for amorphous silicon ($a$-Si) from lattice-dynamical calculations and direct molecular-dynamics simulations using density-functional theory (DFT). The effects of temperature and structural disorder on the intensity of the diffraction maxima and the vibrational mean-square displacement (MSD) of Si atoms are studied in the harmonic approximation, with particular emphasis on the bond-length disorder, the presence of coordination defects, and microvoids in $a$-Si networks. It has been observed that the MSDs associated with tetrahedrally bonded Si atoms are considerably lower than their dangling-bond counterparts—originating from isolated and vacancy-induced clustered defects—and those on the surface of microvoids, leading to an asymmetric non-Gaussian tail in the distribution of atomic displacements. An examination of the effect of anharmonicity on the MSD at high temperatures using direct ab initio molecular-dynamics simulations (without the harmonic approximation) suggests that the vibrational motion in $a$-Si is practically unaffected by anharmonic effects at temperatures below 400 K, as far as the present DFT calculations are concerned. The Debye temperature of $a$-Si is found to be in the range of 488–541 K from specific-heat and MSD calculations using first-principles lattice-dynamical calculations in the harmonic approximation, which matches closely with the experimental value of 487–528 K obtained from specific-heat measurements of $a$-Si at low temperatures.

Time-periodic (Floquet) drive can give rise to novel symmetry breaking and topological phases of matter. Recently, we showed that a quintessential Floquet topological phase known as the anomalous Floquet-Anderson insulator is stable to noise on the timing of its Floquet drive. Here, we perturb the anomalous Floquet-Anderson insulator at a single incommensurate frequency, resulting in a quasiperiodic two-tone drive. Our numerics indicate that a robust topological phase survives at weak noise with topological pumping that is more stable than the case of white noise. Within the topological phase, we show that particles move subdiffusively, which is directly responsible for stabilizing topological transport. Surprisingly, we discover that when quasiperiodic noise is sufficiently strong to kill topology, the system appears to exhibit diffusive dynamics, suggesting that the correlated structure of the quasiperiodic noise becomes irrelevant.

Understanding magnetic damping behavior at finite temperatures is crucial for magnetization reversal, especially in heat-assisted magnetic recording (HAMR) media. In this paper, we calculate the intrinsic magnetic damping of $L{1}_0\text{−}\mathrm{FePt}$, which is the prospective HAMR media, based on the Kamberský torque correlation model and the modified frozen thermal lattice disorder approach. Using the temperature-dependent scattering rate, the magnetic damping showed nonmonotonic behavior and slightly increased with increasing temperature, indicating that the lattice vibration enhances the interband transition around the Fermi level. Comparison of our results with the previous theoretical and experimental works clarified that, because the intrinsic damping of $L{1}_0\text{−}\mathrm{FePt}$ was always enhanced at high temperature, the reduction of the damping around the Curie temperature in the recent experiment emphasizes the importance of extrinsic contributions of damping for HAMR application.

We develop a semianalytical description for the Berezinskii-Kosterlitz-Thouless–like phase transition in nonequilibrium Bose-Einstein condensates. Our theoretical analysis is based on a noisy generalized Gross-Pitaevskii equation. Above a critical strength of the noise, spontaneous vortex-antivortex pairs are generated. We provide a semianalytical determination of the transition point based on a linearized Bogoliubov analysis, to which some nonlinear corrections are added. We present two different approaches that are in agreement with our numerical calculations in a wide range of system parameters. We find that for small losses and not too small energy relaxation the critical point approaches that of the equilibrium Berezinskii-Kosterlitz-Thouless transition. Furthermore, we find that losses tend to stabilize the ordered phase: keeping the other parameters constant and increasing the losses leads to a higher critical noise strength for the spontaneous generation of vortex-antivortex pairs. Our theoretical analysis is relevant for experiments on microcavity polaritons.

Chiral $d$-wave superconductivity has recently been proposed in a wide range of materials based on both experiment and theoretical studies. Chiral superconductors host a finite Chern number set by the winding of the superconducting order parameter and associated topologically protected chiral edge modes. However, the chiral edge currents and orbital angular momentum (OAM) generated by the edge modes are not topologically protected and another, more robust, experimental probe is therefore needed to facilitate experimental verification of chiral $d$-wave superconductors. We have recently shown the appearance of quadruply quantized coreless vortices (CVs) in chiral $d$-wave superconductors, consisting of a closed domain wall decorated with eight fractional vortices, and generating a smoking-gun signature of the Chern number, chirality, and the superconducting pairing symmetry [P. Holmvall and A. M. Black-Schaffer, Phys. Rev. B 108, L100506 (2023)]. Specifically, the CV spontaneously breaks axial symmetry for parallel chirality and vorticity, with a signature appearing directly in the local density of states (LDOS) measurable with scanning tunneling spectroscopy (STS). In this paper, we first demonstrate a strong tunability of the CV size and shape directly reflected in the LDOS and then show that the LDOS signature is robust in the presence of regular Abrikosov vortices, strong confinement, system and normal-state anisotropy, different Fermi surfaces (FSs), nondegenerate order parameters, and even nonmagnetic impurities. In conclusion, our paper establishes CVs as a tunable and robust signature of chiral $d$-wave superconductivity.

We relate the probability distribution of the work done on a statistical system under a sudden quench to the Lanczos coefficients corresponding to evolution under the postquench Hamiltonian. Using the general relation between the moments and the cumulants of the probability distribution, we show that the Lanczos coefficients can be identified with physical quantities associated with the distribution, e.g., the average work done on the system and its variance, as well as the higher order cumulants. In a sense this gives an interpretation of the Lanczos coefficients in terms of experimentally measurable quantities. Consequently, our approach provides a way towards understanding spread complexity, a quantity that measures the spread of an initial state with time in the Krylov basis generated by the postquench Hamiltonian, from a thermodynamical perspective. We illustrate these relations with two examples. The first one involves quench done on a harmonic chain with periodic boundary conditions and with nearest neighbor interactions. As a second example, we consider mass quench in a free bosonic field theory in $d$ spatial dimensions in the limit of large system size. In both cases, we find out the time evolution of the spread complexity after the quench and relate the Lanczos coefficients with the cumulants of the distribution of the work done on the system.

Topological gapless phonons have been extensively studied both in crystal symmetry and materials realization due to their spinless properties. Based on degeneracy and geometry, they can be categorized into the topological nodal point, line, and surface as well as other complex topological phonons. However, the coexistence of more than two types of topological phonons in a single material is rarely reported due to strict symmetry constraints. By means of symmetry analysis, $k·p$ model, and first-principles calculations, we found that the quadratic Dirac point (QDP), type-III nodal ring (NR), and nodal surface (NS) can simultaneously emerge under the symmetry protection of space group $P{6}_3/mmc$. We also predicted a series of candidate materials for experimental realizations. As a representative, we performed a thorough investigation of ${\mathrm{BaNa}}_2$ in the C14 Laves phase, analyzing its bulk phonon dispersion relationships, topological invariants, and topological surface states. We unveiled the underlying mechanism for the coexistence of QDPs, NRs, and NSs. This study not only highlights the significance of crystal symmetry in realizing novel topological phonons but also lays the foundation for the analysis of topological materials with hybrid topological characteristics.

Hard x-ray angle-resolved photoemission spectroscopy reveals the momentum-resolved band structure in an epitaxial ${\mathrm{Mn}}_2\mathrm{Au}$(001) film capped by a 2-nm-thick ferromagnetic permalloy layer. By magnetizing the permalloy capping layer, the exceptionally strong exchange bias aligns the Néel vector in the ${\mathrm{Mn}}_2\mathrm{Au}$(001) film accordingly. Uncompensated interface Mn magnetic moments in ${\mathrm{Mn}}_2\mathrm{Au}$ were identified as the origin of the exchange bias using x-ray magnetic circular dichroism in combination with photoelectron emission microscopy. Using time-of-flight momentum microscopy, we measure the asymmetry of the band structure, $E\left(k\right)\ne E(-k)$, in ${\mathrm{Mn}}_2\mathrm{Au}$ resulting from the homogeneous orientation of the Néel vector. Comparison with theory shows that the Néel vector, determined by the magnetic moment of the top Mn layer, is antiparallel to the permalloy magnetization. The experimental results demonstrate that hard x-ray photoemission spectroscopy can measure the band structure of epitaxial layers beneath a metallic capping layer and corroborate the asymmetric band structure in ${\mathrm{Mn}}_2\mathrm{Au}$ that was previously inferred only indirectly.

This paper presents an alternative isotropic model of magnetization curves in ferromagnetic materials. It is based on the well-known Jiles-Atherton model, but significantly modifies its assumptions to be consistent with fundamental physical principles. As a result, the number of model parameters has been reduced to four. The differential model couples the volume energy density of the sample with the external magnetic field and includes the process of rigid and flexible overcoming of the pinning sites in the law of energy density conservation. The magnetic moment magnitude is connected with coherent regions called magnetic clusters, which are smaller than magnetic domains. The commonly used concept of anhysteretic magnetization, based on the Langevin function, is modified by a parameter $\beta$, which represents the average mutual interaction among magnetic clusters. The model was successfully tested on two isotropic materials: amorphous ${\mathrm{Fe}}_{77.5}{\mathrm{Si}}_{7.5}{\mathrm{B}}_{15}$ alloy and powdered magnetite. The main advantages compared with previous models are the simpler model equation and a much more convenient numerical and fitting procedure.

We report on magnetic orderings of nanospins in self-assemblies of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles (NPs), occurring at various stages of the magnetization process throughout the superparamagnetic (SPM)-blocking transition. Essentially driven by magnetic dipole couplings and by Zeeman interaction with a magnetic field applied out-of-plane, these magnetic orderings include a mix of long-range parallel and antiparallel alignments of nanospins, with the antiparallel correlation being the strongest near the coercive point below the blocking temperature. The magnetic ordering is probed via x-ray resonant magnetic scattering (XRMS), with the x-ray energy tuned to the $\mathrm{Fe}\text{−}{L}_3$ edge and using circular polarized light. By exploiting dichroic effects, a magnetic scattering signal is isolated from the charge scattering signal. We measured the nanospin ordering for two different sizes of NPs, 5 and 11 nm, with blocking temperatures $T_B$ of 28 and 170 K, respectively. At 300 K, while the magnetometry data essentially show SPM and absence of hysteresis for both particle sizes, the XRMS data reveal the presence of nonzero (up to 9%) antiparallel ordering when the applied field is released to zero for the 11 nm NPs. These antiparallel correlations are drastically amplified when the NPs are cooled down below $T_B$ and reach up to 12% for the 5 nm NPs and 48% for the 11 nm NPs, near the coercive point. The data suggest that the particle size affects the prevalence of the antiparallel correlations over the parallel correlations by a factor $\sim 1.6$ to 3.8 higher when the NP size increases from 5 to 11 nm.

We obtain the quantum phase diagram of the ionic Hubbard model including electron-hole symmetric density-dependent hopping. The boundaries of the phases are determined by crossings of excited levels with particular discrete symmetries, which coincide with jumps of charge and spin Berry phases with a topological meaning. Reducing the magnitude of the hopping terms that do not change the total number of singly occupied sites with respect to the other one, the region of the phase diagram occupied by the fully gapped spontaneously dimerized insulator (which separates the band insulating and Mott insulating phases) is enlarged, particularly for small values of the alternating on-site energy. This result might be relevant for experiments in cold atoms in which topological charge pumping is observed when alternation in the hopping is included.

We study a microscopic model for four spinless fermions on the square lattice which exhibits a quartet bound state in the strong coupling regime. The four-particle quantum states are analyzed using symmetry arguments and by introducing a zoo of relevant lattice animals. These considerations, as well as variational and exact diagonalization calculations, demonstrate the existence of a narrow quartet band at small hopping and a first-order transition to delocalized fermions at a critical hopping parameter, in qualitative contrast to, e.g., the BCS-BEC crossover in the attractive Hubbard model. In the case of pure attraction, an intermediate phase is found, in which a more extended and presumably more mobile hybrid quartet dominates the ground state. We comment on the relevance of the spin degree of freedom and on the reasons why electron quartetting is rarely observed in real materials.

Variational and diffusion quantum Monte Carlo (VMC and DMC) methods with Slater-Jastrow-backflow trial wave functions are used to study the spin-polarized three-dimensional uniform electron fluid. We report ground state VMC and DMC energies in the density range $0.5\le r_{\text{s}}\le 20$. Finite-size errors are corrected using canonical-ensemble twist-averaged boundary conditions and extrapolation of the twist-averaged energy per particle calculated at three system sizes $(N=113, 259, \mathrm{and} 387)$ to the thermodynamic limit of infinite system size. The DMC energies in the thermodynamic limit are used to parametrize a local spin density approximation correlation function for inhomogeneous electron systems.

Exact solutions for non-Hermitian quantum many-body systems are rare but may provide valuable insights into the interplay between Hermitian and non-Hermitian components. We report our investigation of a non-Hermitian variant of a $p$-wave Kitaev chain by introducing staggered imbalanced pair creation and annihilation terms. We find that there exist fixed lines in the phase diagram, at which the ground state remains unchanged in the presence of a non-Hermitian term under the periodic boundary condition for a finite system. This allows the constancy of the topological index in the process of varying the balance strength at arbitrary rate, exhibiting the robustness of the topology for the non-Hermitian Kitaev chain under time-dependent perturbations. The underlying mechanism is investigated through the equivalent quantum spin system obtained by the Jordan-Wigner transformation for infinite chain. In addition, the exact solution shows that a resonant non-Hermitian impurity can induce a pair of zero modes in the corresponding Majorana lattice, which asymptotically approach the edge modes in the thermodynamic limit, manifesting the bulk-boundary correspondence. Numerical simulation is performed for the quench dynamics for the systems with slight deviation from the fixed lines to show the stability region in time. This work reveals the interplay between the pair creation and annihilation pairing processes.

Chiral nanomaterials exhibit distinctive circular dichroism spectra, making them highly attractive for potential applications in the chemical and pharmaceutical industries. Moreover, chiral materials can induce a variety of exotic electronic and spin phenomena, including chiral-induced spin selectivity, which has garnered increasing attention in spintronics and nanomaterial science. In this work, we theoretically predicted that a one-dimensional helical niobium triselenide $\left({\mathrm{NbSe}}_3\right)$ nanowire possesses remarkable orbital and spin polarization, due to the helical structure and inherent spin-orbit coupling. We further demonstrated that the device based on this helical nanowire exhibits a notable spin-polarized electronic tunneling conductance. Additionally, we determined the k-dependent circular dichroism spectra by examining the effects of electronic angular momentum polarization on the response of circularly polarized lights. Our findings can not only interpret the spin polarization in this one-dimensional chiral ${\mathrm{NbSe}}_3$ chain but also propose applications in spin-modulating junctions and circular-polarized light detectors.

The interplay between superconductivity (SC) and spin/charge density wave (DW) in organic metals shows many similarities to high-$T_c$ superconductors. It also contains many puzzles, for example, the anisotropic SC onset observed and the severalfold increase of the upper critical field $H_{c2}$ in the coexistence region, as well as the microscopic origin of SC/DW phase separation there. In this paper, by the direct calculation of the Landau expansion for DW free energy, we argue that the phase transition between DW and metallic/SC phase in organic superconductors goes by first order at low-enough temperature, which explains the spatial segregation of DW and SC at large length scale, consistent with experimental observations. This first-order phase transition is not directly related to SC and happens even above the SC transition temperature.

Valleytronics has become an emerging field for both theoretical and experimental research. Based on first-principles calculations, we demonstrate that the spin valley coupling and valley splitting properties of the $\mathrm{Mo}/{\mathrm{WSi}}_2{\mathrm{P}}_4$ monolayers can be modulated by using the two-dimensional ferromagnetic ${\mathrm{CrI}}_3$ semiconductor via the magnetic proximity effect. We show that the ${\mathrm{CrI}}_3/{\mathrm{MoSi}}_2{\mathrm{P}}_4\left({\mathrm{CrI}}_3/{\mathrm{WSi}}_2{\mathrm{P}}_4\right)$ heterostructures are thermally stable and exhibit type-II band alignment at room temperature, leading to their possible applications for optoelectronic devices at ambient conditions. Moreover, we estimate the Curie temperature of the ${\mathrm{CrI}}_3/{\mathrm{MoSi}}_2{\mathrm{P}}_4\left({\mathrm{CrI}}_3/{\mathrm{WSi}}_2{\mathrm{P}}_4\right)$ heterostructures and find that both valley splitting and valley-contrasting transport properties are expected to be enhanced below Curie temperature. Meanwhile, the valley polarization can be tuned by changing the magnetization direction and layer spacing. Most importantly, due to the influence of the Si-P sublayer, the valley-related properties of the ${\mathrm{CrI}}_3/{\mathrm{MoSi}}_2{\mathrm{P}}_4\left({\mathrm{CrI}}_3/{\mathrm{WSi}}_2{\mathrm{P}}_4\right)$ heterostructures are predicted to be quite robust. The calculated Berry curvature and circular polarization indicate that the ${\mathrm{CrI}}_3/{\mathrm{MoSi}}_2{\mathrm{P}}_4\left({\mathrm{CrI}}_3/{\mathrm{WSi}}_2{\mathrm{P}}_4\right)$ heterostructures can achieve the valley/spin Hall effect. Our research provides a comprehensive understanding of valley splitting in ${MA}_2{Z}_4$-based heterostructures.