A systematic, computational study of the Gilbert damping in $\mathrm{Co}|\mathrm{Ni}, \mathrm{Co}|\mathrm{Pd}$, and $\mathrm{Co}|\mathrm{Pt}$ multilayers is carried out using first-principles scattering calculations. The damping we find shows little temperature dependence and agrees well with experimental findings only when the interface damping enhancement is taken into account. In the absence of this extrinsic enhancement, the intrinsic damping of bulklike $\mathrm{Co}|\mathrm{Ni}$ multilayers increases linearly with increasing ratio of Ni to Co layers. Though spin-orbit coupling is the common origin of magnetocrystalline anisotropy and magnetization damping, the latter is not sensitive to the interface roughness that affects magnetic anisotropy strongly.

We present a comprehensive theory of light-controlled multiband superconductivity, and apply it to predict distinctive signatures of light-driven superconducting (SC) states in terahertz multidimensional coherent spectroscopy (THz-MDCS) experiments. We first derive gauge-invariant Maxwell-Bloch equations for multiband BCS superconductors with spatial fluctuations. We consider driving electromagnetic fields determined self-consistently by Maxwell's equations. By calculating the THz-MDCS spectra measured experimentally in the clean SC limit, we identify unique signatures of finite-momentum Cooper-pairing states that live longer than the laser pulse. They are controlled by a pair of THz laser pulses with well-defined relative phase (pulse pair). The pseudospin oscillators that describe the properties of these SC states are parametrically driven by both finite-momentum Cooper pairing and by time oscillations of the order-parameter relative phase. We show that such strong parametric driving leads to drastic changes in the THz-MDCS spectral shape from the predictions of third-order nonlinear susceptibility calculations. These spectral changes strongly depend on the interband-to-intraband interaction ratio and on the collective modes of the light-driven state. For negligible interband interaction, the spectra show a transition with increasing field, from traditional pump-probe, four-wave-mixing, and third-harmonic generation peaks determined by the laser frequency to sidebands determined by the excitations of the driven system. These sidebands emerge from difference-frequency Raman processes in the nonequilibrium SC state. For interband couplings weaker than the intraband pairing, we show that the Leggett phase collective mode leads to harmonic sidebands around the traditional pump-probe peaks. Additional Higgs collective mode peaks result from light-induced inversion-symmetry breaking in a thin-film geometry. For strong interband coupling, we find a transition from a nonequilibrium finite Cooper-pair momentum state characterized by hybrid-Higgs amplitude mode peaks in THz-MDCS spectra to a driven state identified experimentally by the emergence of Floquet-type sidebands at bi-Higgs frequencies. Those dominant bi-Higgs-frequency satellites are manifestations of a new order parameter relative phase collective mode that characterizes the nonequilibrium SC state. The predicted interaction- and field-dependent transitions in the spectral profile allow us to propose THz-MDCS experiments for quantum tomography of light-driven superconductivity.

${\mathrm{CuCr}}_2{\mathrm{O}}_4$, which belongs to the family of ${AB}_2{\mathrm{O}}_4$ spinels, was recently reported to be a type-I multiferroic material. The observations of type-I multiferroicity, i.e., where ferroelectricity (FE) arises from a structural origin, is rare in the ${AB}_2{\mathrm{O}}_4$ spinel family, thus this result is significant. The role of both Jahn-Teller (JT) active ${\mathrm{Cu}}^{2+}$ and Cr sublattice magnetic ordering was suggested in the earlier study, and a more careful investigation to elucidate the origin of the observed FE is required. With the aim to elucidate the origin of the observed FE, we carried out a detailed investigation of the structural, electrical, and magnetic properties of a ${\mathrm{Cu}}_{1-x}{\mathrm{Cd}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ $(0\le x\le 1)$ series, that enables an $x$-dependent controlled tuning of the ${\mathrm{Cu}}^{2+}$ driven structural and Cr-sublattice driven magnetic transition temperatures. We track and investigate the correlation of the observed pyrocurrent peaks with the temperature-dependent structural and magnetic transitions in this series for various $x$ members. From careful investigations, we rule out any intrinsic origin and establish a space-charge or extrinsic origin for the observed switchable pyrocurrent peaks that point to an absence of ferroelectricity.

Core excitons in solids have garnered increasing interest, yet their behavior and decay mechanisms are not fully understood. Here, we use attosecond extreme ultraviolet (XUV) transient absorption spectroscopy, performed with a broadband 25–45-eV sub-fs XUV pump pulse and a 500–1000-nm sub-5-fs near-infrared (NIR) supercontinuum probe pulse to monitor the excitation, dynamics, and decay of core excitons in $\mathrm{Ca}{\mathrm{F}}_2$ at the ${\mathrm{Ca}}^{2+}$ $M_{2,3}$ edge. The XUV pulses are used to excite core excitons in $\mathrm{Ca}{\mathrm{F}}_2$ based around the ${\mathrm{Ca}}^{2+}$ and the polarization of the medium is subsequently perturbed by the time-delayed NIR pulses to measure the spectral changes and decays. A number of features are identified in the transient absorption spectrum, which suggest transfer between excitonic states, Stark shifts, and the emergence of light-induced states. We find that various core excitons identified exhibit coherence lifetimes spanning 3–7 fs. Furthermore, a NIR-intensity-dependent analysis finds a negative correlation with the coherence lifetime of various identified excitonic features, supporting a phonon-mediated mechanism as responsible for the core-exciton decoherence. We present a computational band structure projection analysis strategy to estimate the orbital structure of the core excitons and determine which core-excitonic transitions should be allowed by selection rules with the probe beam. This strategy is found to successfully describe the observed spectroscopic data. The outlined joint spectroscopic and computational investigation of core excitons is a powerful technique that explains the complex behavior of core excitons in solid-state materials.

We present a detailed study of the local atomic and magnetic structure of the type-I multiferroic perovskite system $(\mathrm{Sr},\mathrm{Ba})(\mathrm{Mn},\mathrm{Ti}){\mathrm{O}}_3$ using x-ray and neutron pair distribution function (PDF) analysis, polarized neutron scattering, and muon spin relaxation ($\mu \mathrm{SR}$) techniques. The atomic PDF analysis reveals widespread nanoscale tetragonal distortions of the crystal structure even in the paraelectric phase with average cubic symmetry, corresponding to incipient ferroelectricity in the local structure. Magnetic PDF analysis, polarized neutron scattering, and $\mu \mathrm{SR}$ likewise confirm the presence of short-range antiferromagnetic correlations in the paramagnetic state, which grow in magnitude as the temperature approaches the magnetic transition. We show that these short-range magnetic correlations coincide with a reduction of the tetragonal (i.e., ferroelectric) distortion in the average structure, suggesting that short-range magnetism can play an important role in magnetoelectric and/or magnetostructural phenomena even without genuine long-range magnetic order. The reduction of the tetragonal distortion scales linearly with the local magnetic order parameter, pointing to spontaneous linear magnetoelectric coupling in this system. These findings provide greater insight into the multiferroic properties of $(\mathrm{Sr},\mathrm{Ba})(\mathrm{Mn},\mathrm{Ti}){\mathrm{O}}_3$ and demonstrate the importance of investigating the local atomic and magnetic structure to gain a deeper understanding of the intertwined degrees of freedom in multiferroics.

The origin of the intraband term of the magnetocrystalline anisotropy (MCA) energy, which has been shown to be present for layered systems without the inversion symmetry and be of similar magnitude as the interband term [M. Cinal, Phys. Rev. B 105, 104403 (2022)], is investigated by considering a layered magnetic system perturbed by the spin-orbit coupling (SOC) at zero temperature. It is revealed that the second-order intraband contributions to the system band energy (the sum of the energies of occupied electron states), which are determined by the squares of the first-order corrections to electron energies at the Fermi surface (FS), actually result from small shifts of the FS sheets due to the SOC, the sheets being curves in the two-dimensional Brillouin zone. Thus, the intraband contributions to the band energy of the perturbed system, and, consequently, to the MCA energy defined as the difference of the band energies for two magnetization directions, come from the narrow stripelike regions of the Brillouin zone between the perturbed and unperturbed FS sheets for consecutive energy bands. In addition, the representation of the MCA energy as half the difference of the SOC energies for two magnetization directions, often used for spatial decomposition of this energy, is reexamined. It is shown that such an alternative representation, previously derived from the standard second-order perturbation-theory (PT) formula for the MCA energy which does not contain intraband terms [V. Antropov, L. Ke, and D. Åberg, Solid State Commun. 194, 35 (2014)], is also valid in the absence of the inversion symmetry since the PT expansion of this representation includes, in fact, the intraband terms, and exactly reproduces the present extended second-order PT formula for the MCA energy. The theoretical findings are illustrated with the exemplary results for Co-based layered systems with and without the inversion symmetry: the Co film, the Co/Cu and Cu/Pd bilayers, as well as the Pd/Co/Pd trilayer.

The unique resonance and locking phenomena in the superconductor-ferromagnet-superconductor ${\phi }_0$ Josephson junction under external electromagnetic radiation are demonstrated when not just the electric but also the magnetic component of external radiation is taken into account. Due to the coupling of superconductivity and magnetism in this system, the magnetic moment precession of the ferromagnetic layer caused by the magnetic component of external radiation can lock the Josephson oscillations, which results in the appearance of a particular type of step in the current-voltage characteristics, completely different from the well-known Shapiro steps. We call these steps the Buzdin steps in the case when the system is driven only by the magnetic component and the chimera steps in the case when both magnetic and electric components are present. Unlike the Shapiro steps where the magnetic moment remains constant along the step, here it changes though the system is locked. The spin-orbit coupling substantially contributes to the amplitude, i.e., the size of these steps. Dramatic changes in their amplitudes are also observed at frequencies near the ferromagnetic resonance. Combinations of the Josephson and Kittel ferromagnetic resonances together with different types of locking pronounced in dynamics and current-voltage characteristics make the physics of this system very interesting and open up a series of new applications.

We report energy renormalizations from electron-phonon and electron-magnon interactions in spin minority surface resonances on Ni(111). The different interactions are identified, disentangled, and quantified from the characteristic signatures they provide to the complex self-energy and the largely different binding energies at which they occur. The observed electron-magnon interactions exhibit a strong dependence on momentum and the electron energy band position in the bulk Brillouin zone. In contrast, electron-phonon interactions observed from the same bands appear to be relatively momentum and symmetry independent. Additionally, a moderately strong $(\lambda >0.5)$ electron-phonon interaction is distinguished from a near-parabolic spin majority band not crossing the Fermi level.

Two-dimensional time-reversal-invariant topological superconductors host helical Majorana fermions at their boundary. We study the fate of these edge states under the combined influence of strong interactions and disorder, using the effective one-dimensional (1D) lattice model for the edge introduced by Jones and Metlitski [Phys. Rev. B 104, 245130 (2021)]. We specifically develop a strong-disorder renormalization-group analysis of the lattice model and identify a regime in which time-reversal is broken spontaneously, creating random magnetic domains; Majorana fermions localize to domain walls and form an infinite-randomness fixed point, identical to that appearing in the random transverse-field Ising model. While this infinite-randomness fixed point describes a fine-tuned critical point in a purely 1D system, in our edge context there is no obvious time-reversal-preserving perturbation that destabilizes the fixed point. Our analysis thus suggests that the infinite-randomness fixed point emerges as a stable phase on the edge of two-dimensional topological superconductors when strong disorder and interactions are present.

Cobalt is used in Li-ion batteries, but it is expensive and could be replaced by nickel to deliver better performance at a lower cost. With this motivation, we discuss how the character of redox orbitals of ${\mathrm{LiNiO}}_2$ can be ascertained through x-ray Compton scattering measurements combined with parallel first-principles simulations. Our analysis reveals the nature of hole states in Li-doped NiO resulting from the hybridization of $\text{O}2p$ and $\text{Ni}3d$ orbitals. Our study also gives insight into the ferromagnetic ground state and provides a pathway toward the rational design of next-generation battery materials.

Chiral exact flat bands (FBs) at charge neutrality have attracted much recent interest, presenting an intriguing condensed-matter system to realize exotic many-body phenomena, as specifically shown in magic-angle twisted bilayer graphene for superconductivity and triangulene-based superatomic graphene for exciton condensation. Yet, no generic physical model to realize such FBs has been developed. Here we present a mathematical theorem called bipartite double cover (BDC) theorem and prove that the BDC of line-graph (LG) lattices hosts at least two chiral exact flat bands of opposite chirality, i.e., yin-yang FBs, centered-around/at charge neutrality ($E=0$) akin to the chiral limit of twisted bilayer graphene. We illustrate this theorem by mapping it exactly onto tight-binding lattice models of the BDC of LGs of hexagonal lattice for strong topological and of triangular lattice for fragile topological FBs, respectively. Moreover, we use the orbital design principle to realize such exotic yin-yang FBs in non-BDC lattices to instigate their real material discovery. This paper not only enables the search for exact chiral FBs at zero energy beyond moiré heterostructures but also opens the door to discovering quantum semiconductors featured with FB-enabled strongly correlated carriers.

We study a four-orbital tight-binding model for ZrSiS from the square sublattice generated by the Si atoms. After studying three other alternatives, we endow this model with a new effective spin-orbit coupling (SOC) consistent with ab initio dispersions around the Fermi energy $E_F$ in four systematic steps: (1) We calculate the electronic dispersion of bulk ZrSiS using an implementation of density-functional theory based on numeric atomic orbitals [Soler et al., J. Phys.: Condens. Matter 14, 2745 (2002)] in which on-site and off-site SOC can be distinguished. As a result, we determine that local SOC-induced band gaps around $E_F$ are predominantly created by the on-site contribution. (2) Gradually reducing the atomic basis set size, we then create an electronic band structure with 16 orbitals per unit cell which retains the qualitative features of the dispersion around $E_F$, including SOC-induced band gaps. (3) Zr is the heaviest element on this compound, and it makes a non-negligible contribution to the electronic dispersion around $E_F$; we show that it provides the strongest contribution to the SOC-induced band gap. (4) Using the Löwdin partitioning approach, we project the effect of SOC onto the four-orbital Hamiltonian. In this way, we facilitate an effective SOC interaction that was explicitly informed by ab initio input.

Despite the significant developments in quantum anomalous Hall (QAH) insulators study in recent years, it remains an outstanding challenge to tune between different topological phases in the same material. In this work, an ultrathin van der Waals (vdW) heterostructure based on $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Se}}_4$ and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ was proposed by using model Hamiltonian and density-functional theory simulations, which was proved to be an excellent tunable QAH platform. Its band gap closes and reopens as hydrostatic pressure increases, with a topological phase transition around the critical pressure of 2.5 GPa. Further analyses reveal the main reason is the enhancement of interlayer interactions and the crystal-field splitting as the interlayer distance decreases. Our work provides clear physical insights and suggests a strategy for experimental realization and control of the QAH effect in real materials.

The strain dependence of thermal conductivity (κ) in monolayer graphene, with reports of enhancement, suppression, or even divergence, has been highly controversial. To address this open question, we have systematically investigated the effects of tensile strain on the κ of graphene using the exact solution of the Peierls-Boltzmann transport equation based on the first-principles interatomic force constants combined with machine learning assisted molecular dynamics simulations. In contrast to previous studies, we find that the κ in the strained graphene is convergent after considering four-phonon scattering, which is dominant for the long-wavelength flexural phonons because of its much weaker frequency dependence (${\tau }_4^{-1}\propto {\omega }^{\beta }$ with β < 2) compared to the three-phonon scattering case (${\tau }_3^{-1}\propto {\omega }^{\beta }$ with β > 2). Furthermore, κ exhibits nonmonotonic variations with increasing strain up to 8% due to the competition between phonon lifetime, group velocity, and heat capacity of acoustic phonons. Our results deepen the fundamental understanding of thermal transport in strained graphene and offer insights for tuning the thermal properties of two-dimensional materials through strain engineering.

Recent research has shown that monolayer black phosphorus (BP) can be metallic while sustaining spontaneous polarization. Using density-functional calculations and mean-field theory, we perform a systematic study on the origin of the metallic ferroelectricity in monolayer BP which is composed of only one element with a strong covalent bond. We find that, in contrast to ionic compounds, metallic monolayer BP exhibits lattice deformation between adjacent cells due to the covalent bond. This benefits the survival of spontaneous polarization despite the fact that the long-range Coulomb interactions between dipoles are screened by itinerant electrons. Through the orbital selective external potential method, we reveal that the strength of $s{p}^3$ hybridization of monolayer BP significantly impacts such lattice distortion and therefore the ferroelectricity. These findings significantly enrich our understanding of metallic polarization and provide insights to help discover novel ferroelectric materials.

The Casimir interaction and torque are related phenomena originating from the exchange of electromagnetic excitations between objects. While the Casimir force exists between all types of objects, the material or geometrical anisotropy drives the emergence of the Casimir torque. Here both phenomena are studied theoretically between dielectric films with immersed parallel single-wall carbon nanotubes in the dilute limit with their chirality and collective electronic and optical response properties taken into account. It is found that the Casimir interaction is dominated by thermal fluctuations at submicron separations, while the torque is primarily determined by quantum mechanical effects. This peculiar quantum vs thermal separation is attributed to the strong influence of the reduced dimensionality and inherent anisotropy of the materials. Our study suggests that nanostructured anisotropic materials can serve as novel platforms to uncover new functionalities in ubiquitous Casimir phenomena.

In this study, we investigate the bulk electronic structure of Pb-substituted ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ single crystals, using two types of hard x-ray spectroscopy. High-energy-resolution fluorescence-detected x-ray absorption spectroscopy revealed a spectral change at low temperatures. Using density functional theory (DFT) simulations, we find that the temperature-induced change originates from a structural phase transition, similar to the pressure-induced transition in ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$. This finding suggests that the mechanism of bulk superconductivity induced by Pb substitution is the same as that under high pressure. Furthermore, a low-valence state with a mixture of divalent and trivalent Bi ions is discovered using hard x-ray photoemission spectroscopy with the aid of DFT calculations.

Majorana zero modes in superconductor-nanowire hybrid structures are a promising candidate for topologically protected qubits with the potential to be used in scalable structures. Currently, disorder in such Majorana wires is a major challenge, as it can destroy the topological phase and thus reduce the yield in the fabrication of Majorana devices. We study machine learning optimization of a gate array in proximity to a grounded Majorana wire, which allows us to reliably compensate even strong disorder. We propose a metric for optimization which can be implemented based on measurements of the nonlocal conductance through the wire.

The accurate prediction of electronic and optical properties in chalcopyrite semiconductors has been a persistent challenge for density-functional-theory (DFT) -based approaches. Addressing this issue, we demonstrate that very accurate results can be obtained using a nonempirical screened dielectric-dependent hybrid (DDH) functional. This approach showcases its impressive capability to accurately determine band gaps, optical bowing parameters, and optical-absorption spectra for chalcopyrite systems. What sets the screened DDH functional apart is its adeptness in capturing the many-body physics associated with highly localized $d$ electrons. Notably, the accuracy is comparable to the many-body perturbation-based methods (such as $G_0{W}_0$ or its various approximations for band gaps and the Bethe-Salpeter equation on the top of the $G_0{W}_0$ or its various approximations for optical spectra) with less computational cost, ensuring a more accessible application across various research domains. The present results show the predictive power of the screened DDH functional, pointing toward promising applications where computational efficiency and predictive accuracy are crucial considerations. Overall, the screened DDH functional offers a compelling balance between cost-effectiveness and precision, making it a valuable tool for future endeavors in exploring chalcopyrite semiconductors and beyond.

Excitons are neutral excitations that are composed of electrons and holes bound together by their attractive Coulomb interaction. The electron and the hole forming the exciton also interact with the underlying atomic lattice, and this interaction can lead to a trapping potential that favors exciton localization. The quasiparticle thus formed by the exciton and the surrounding lattice distortion is called excitonic polaron. Excitonic polarons have long been thought to exist in a variety of materials, and are often invoked to explain the Stokes shift between the optical absorption edge and the photoluminescence peak. However, quantitative ab initio calculations of these effects are exceedingly rare. In this manuscript, we present a theory of excitonic polarons that is amenable to first-principles calculations. We first apply this theory to model Hamiltonians for Wannier excitons experiencing Fröhlich or Holstein electron-phonon couplings. We find that, in the case of Fröhlich interactions, excitonic polarons only form when there is a significant difference between electron and hole effective masses. Then, we apply this theory to calculating excitonic polarons in lithium fluoride ab initio. The key advantage of the present approach is that it does not require supercells, therefore it can be used to study a variety of materials hosting either small or large excitonic polarons. This work constitutes the first step toward a complete ab initio many-body theory of excitonic polarons in real materials.