Quantum transport studies of spin-dependent phenomena in solids commonly employ the Kubo or Keldysh formulas for the nonequilibrium density operator in the steady-state linear-response regime. Its trace with operators of interest, such as the spin density, spin current density, and so on, gives expectation values of experimentally accessible observables. As is well known, for local observables, these formulas require summing over the manifolds of both Fermi-surface and Fermi-sea states. However, the significantly different results yielded by the two formulations when applied to the same system have ignited vigorous debates. Here, we revisit this problem using an infinite-size graphene system with proximity-induced spin-orbit and magnetic exchange effects as a test bed. By considering such system as being composed of central active region in between two semi-infinite leads, in the spirit of the Landauer setup for quantum transport, we prove the numerically exact equivalence of the Kubo and Keldysh approaches via the computation of spin Hall current density and spin-orbit torque in both clean and disordered limits. The key to reconciling the two approaches is the numerical frameworks we put forward for (i) evaluation of the Kubo(-Bastin) formula for a system attached to semi-infinite leads, which ensures a continuous energy spectrum and evades the need for commonly used phenomenological broadening otherwise responsible for ambiguities, and (ii) proper evaluation of the Fermi-sea term in the Keldysh approach, which must include the voltage drop across the central active region even if it is disorder free.

Intriguing analogies between the nickelates and the cuprates provide a promising avenue for unraveling the microscopic mechanisms underlying high-$T_c$ superconductivity. While electron correlation effects in the nickelates have been extensively studied, the role of electron-phonon coupling (EPC) remains highly controversial. Here, by taking pristine ${\mathrm{LaNiO}}_2$ as an exemplar nickelate, we present an in-depth study of EPC for both the nonmagnetic (NM) and the $C$-type antiferromagnetic ($C$-AFM) phases using advanced density functional theory methods without invoking $U$ or other free parameters. The weak EPC strength $\lambda$ in the NM phase is found to be greatly enhanced ($\sim 4\times$) due to the presence of magnetism in the $C$-AFM phase. This enhancement arises from strong interactions between the flat $\mathrm{Ni}\text{−}3d{z}^2$ bands and the low-frequency phonon modes associated with Ni and La vibrations, rather than solely from the high-frequency oxygen breathing modes. The resulting phonon softening is shown to yield a distinctive kink in the electronic structure around 15 meV, which would provide an experimentally testable signature of our predictions. Our study highlights the critical role of local magnetic moments and EPC in the nickelate.

This study explores the potential of thorium-doped boron-carbon clathrates to possess beneficial superconducting and mechanical properties as a function of pressure. Using first-principles density functional theory calculations, we assess the dynamic stability, electronic structure, and superconductivity of ${\mathrm{ThB}}_3{\mathrm{C}}_3$ and ${\mathrm{ThB}}_4{\mathrm{C}}_2$, considering the electron-doped superconducting nature of ${\mathrm{ThB}}_3{\mathrm{C}}_3$ and the unexpected metallic behavior of ${\mathrm{ThB}}_4{\mathrm{C}}_2$, despite it being isoelectronic with diamond. ${\mathrm{ThB}}_3{\mathrm{C}}_3$ adopts a cubic Pm$\overline{3}n$ structure, dynamically stable between 70 and 200 GPa, while ${\mathrm{ThB}}_4{\mathrm{C}}_2$ adopts a tetragonal I4/mmm structure, remaining dynamically stable from ambient pressure up to 200 GPa. Convex-hull analysis reveals their thermodynamic stability under high pressure: ${\mathrm{ThB}}_4{\mathrm{C}}_2$ is stable at both 100 and 200 GPa, whereas ${\mathrm{ThB}}_3{\mathrm{C}}_3$ remains metastable across the pressure range considered. Our work is the first to demonstrate that this family of compounds can be metalized via electron doping, as opposed to the hole doping that has been studied to date. ${\mathrm{ThB}}_3{\mathrm{C}}_3$ is predicted to exhibit a superconducting transition temperature $\left(T_c\right)$ of 56 K at 70 GPa, driven by strong electron-phonon coupling (EPC), while ${\mathrm{ThB}}_4{\mathrm{C}}_2$, despite its lower $T_c$ of 0.7 K, is found to maintain superconductivity at ambient pressure. These results emphasize the potential of thorium-doped boron-carbon clathrates as promising candidate materials with tunable superconducting, electronic, and mechanical properties, paving the way for future experimental synthesis and practical applications.

The microscopic origin of laser-induced ultrafast demagnetization remains an open question, to which the nonthermal electronic distribution plays a vital role at the initial stage. Herein, we investigate the connection between the nonthermal electronic distribution and the ultrafast spin dynamics, as well as the electronic structure evolution in ferromagnetic ${\mathrm{FeCl}}_2$ monolayer using real-time time-dependent density functional theory with self-consistent Hubbard $U$ correction. Our simulations reveal that femtosecond laser pulses induce ultrafast magnetic moment transfer from Fe to Cl atoms. More importantly, through a comprehensive analysis of orbital-resolved electronic structure, we elucidate the microscopic origin of this transfer, attributing it to specific intra-atomic and inter-atomic charge transfer pathways driven by nonthermal excitations. The extent of demagnetization of Fe atoms exhibits a nonmonotonic dependence on the laser photon energy, reaching a maximum at resonant excitation. In addition, the dynamical evolution of the band structure was studied based on the eigenstates of the instantaneous Hamiltonian. Under resonant excitation, the bandgap reduction reaches up to $41\%$ within tens of fs. These findings provide fundamental insights into ultrafast spin control and suggest a strategy to optically engineer the magnetism in two-dimensional magnetic materials.

We investigate the superconducting (SC) properties of experimentally realized $\gamma$-BiPd by solving the anisotropic Migdal-Eliashberg equations in conjunction with ab initio relativistic calculations of the electron and phonon band structures as well as electron-phonon coupling (EPC) matrix elements. Our study reveals that $\gamma$-BiPd possesses a complex Fermi surface (FS), consisting of two electron pockets and one hole pocket, each characterized by distinct atomic orbitals. Our key finding is that the superconductivity in $\gamma$-BiPd is primarily orbital selective, arising from Bi $p$ orbitals, and distributed anisotropically on the FS, although contribution from Pd $d$ orbitals, particularly on the hole pocket, is also discernible. While our results show an anisotropic nature of the k-dependent SC gap ${\mathrm{\Delta }}_{\mathbf{k}}$ and EPC strength ${\lambda }_{\mathbf{k}}$ across the FS, calculated superconducting quasiparticle density of states $N_S$ spectra exhibit a U-shaped gap and ${\mathrm{\Delta }}_{\mathbf{k}}$ distribution forms a single peak, being consistent with the spin-singlet $s$-wave superconductivity observed in this material. The calculated $T_c$ is $\sim 2.0\mathrm{K}$, agreeing in order of magnitude with the experimental value of 3.3 K in $\gamma$-BiPd thin films. The predicted EPC-enhanced Sommerfeld coefficient ${\gamma }_n$ of $0.141{\mathrm{mJ}/\mathrm{K}}^2{\mathrm{cm}}^3$ is similar to the experimental ${\gamma }_n$ value $\left(0.119{\mathrm{mJ}/\mathrm{K}}^2{\mathrm{cm}}^3\right)$ of the isoelectronic and isostructural $\mathrm{Bi}({\mathrm{Pd}}_{0.5}{\mathrm{Pt}}_{0.5)}$ alloy.

We study the dynamics of quantum matter interacting with time-energy entangled photons. We consider the stimulation of a collective mode of a two-dimensional material by means of one of the two partners of a time-energy entangled pair of photons. Using an exactly solvable model, we analyze the out-of-equilibrium properties of both light and matter degrees of freedom, and show how entanglement in the incident photons deeply modifies relevant timescales of the light-matter interaction process. We find that entanglement strongly suppresses the delay between the transmission and absorption events, which become synchronous in the limit of strongly entangled wave packets. By comparing numerical simulations with analytic modeling, we trace back this behavior to the representation of entangled wave packets in terms of a superposition of multiple train pulses containing an increasing number of ultrashort non-entangled packets. As a result, we show that the entangled driving allows the creation of a matter excitation on a timescale shorter than the temporal width of the pulse. Eventually, by analyzing temporal correlations of the excited matter degrees of freedom, we show that driving with entangled photons imprints characteristic temporal correlations of time-energy entangled modes in the matter degree of freedom.

The physical properties of the newly discovered Sr carbonate ${\mathrm{Sr}}_2{\mathrm{CO}}_4$ are of great importance for understanding deep carbon storage in Earth's interior. We investigated the melting curve, $P-T$ phase diagram, elastic properties, and thermodynamic behavior of the lower-mantle candidate mineral ${\mathrm{Sr}}_2{\mathrm{CO}}_4$ under high-temperature and high-pressure conditions using a deep learning potential model. The excellent agreement between our ab initio molecular dynamics results and high-precision structural optimizations validates the reliability of the trained potential. The accuracy of the deep learning potential was further verified by calculating the lattice constants and high-temperature and high-pressure equations of state of the ${\mathrm{Sr}}_2{\mathrm{CO}}_4$ Pnma structure using first-principles calculations, molecular dynamics simulations, and the quasiharmonic approximation. Zero-pressure melting temperatures of 1630, 1500, and 1508 K were obtained from the single-phase, void, and two-phase methods, respectively, and the melting transition was further confirmed through radial distribution functions and mean-square displacements. Elastic properties derived via the stress-strain approach reveal pronounced temperature-induced softening of the elastic constants and elastic moduli. Seismological parameters calculated from the elastic moduli indicate that ${\mathrm{Sr}}_2{\mathrm{CO}}_4$ Pnma remains a high-density and low-velocity carbonate throughout mantle conditions, suggesting potential relevance to deep carbon storage. Thermodynamic quantities, including heat capacity and entropy, are additionally evaluated within the quasiharmonic approximation. These results establish key constraints on the melting behavior, elasticity, and thermodynamics of ${\mathrm{Sr}}_2{\mathrm{CO}}_4$ under extreme conditions, providing insights into the high-pressure stability of alkaline-earth orthocarbonates within Earth's interior.

In magnetic topological insulators, a phase transition between a quantum anomalous Hall (QAH) phase and an Anderson localization phase can be triggered by the rotation of an applied magnetic field. Without the scattering paths along magnetic domains, this phase transition is governed by scattering induced by nonmagnetic disorder. We show that the QAH phase is strikingly robust in the presence of spatially uncorrelated disorder. The robustness is attributed to the quantum confinement induced by the short correlation length of the disorder. The critical behavior near the phase transition suggests a picture distinct from quantum percolation. This provides new insights into the robustness of the QAH effect in magnetic topological insulators with atomic defects, impurities, and dopants.

Among the orthochromites comprising a magnetic rare-earth constituent, ${\mathrm{CeCrO}}_3$ exhibits the highest Néel temperature of $\sim 260\mathrm{K}$. Using high-resolution neutron powder diffraction, we redetermined the low-temperature antiferromagnetic Cr and Ce magnetic moment configuration in ${\mathrm{CeCrO}}_3$. The Cr moments order with a weakly canted $G$-type antiferromagnetic structure (${\mathrm{\Gamma }}_2$) where the moments essentially align along the $b$ axis ($Pnma$ setting of the space group No. 62). Very weak Ce magnetic coherent scattering appears below $\sim 22\mathrm{K}$ due to a $C_x$-type ordering of the Ce moments (${\mu }_{\mathrm{Ce}}\approx 0.06{\mu }_{\mathrm{B}}$). From the temperature dependence of the lattice parameters, we analyze the volume magnetoelastic effects which cause a contraction of the cell volume on ordering of the Cr moments. Below $\sim 150\mathrm{K}$ a subtle but continuous adjustment of the $a$ and $c$ lattice parameters takes place which is attributed to a spin reorientation of the Cr magnetic moments. The temperature dependence of the Cr magnetic order parameter indicates a gap in the magnon spectrum. We reanalyze the specific heat capacity of ${\mathrm{CeCrO}}_3$ which at low temperatures exhibits additional contributions due to thermal excitation within the crystal-field split levels of the ${\mathrm{Ce}}^{3+}{}^{2}F_{5/2}$ Hund's rule ground state. A model calculation of the magnetization including magnetic polarization of the Ce moments is consistent with a spin reorientation of the Cr magnetic moments.

PdTe has recently been reported to be a type-II Dirac semimetal while a bulk nodal and surface nodeless superconductivity (SC) has been claimed to coexist. In this work, we applied the point-contact spectroscopy (PCS) method to systematically study the superconducting gap in PdTe single crystals with a SC transition temperature $T_c=4.3\mathrm{K}$. The obtained differential conductance curves show a common deviation from a single-gap superconducting behavior and can be better fitted by a two-gap Blonder-Tinkham-Klapwijk model, suggesting the larger gap ${\mathrm{\Delta }}_L$ with $2{\mathrm{\Delta }}_L=3.7k_B{T}_c$ and the smaller gap ${\mathrm{\Delta }}_S$ yielding $2{\mathrm{\Delta }}_S=1.1-2.2k_B{T}_c$ with a weak interband scattering. The variations of conductance spectra among different contacts are proposed to be caused by the anisotropy of Fermi surface topology associated with different gaps.

We study a two-orbital attractive Hubbard model with a repulsive Hund's exchange coupling $J$ as an idealized model for a two-band superconductor. This framework is motivated by systems where strong isotropic electron-phonon coupling drives the on-site Hubbard repulsion $U$ to negative while leaving the exchange term unaffected. We solve the model at zero temperature and half filling using dynamical mean-field theory, focusing on the intraorbital singlet superconducting phase and discarding other possible instabilities, such as interorbital pairing and charge-density wave ordering. Already at $J=0$, the two-orbital model features a superconductor-insulator transition as $\left|U\right|$ grows, in contrast to the single-orbital case, which remains superconducting for any $U<0$. We find that a finite $J$ strengthens the effect of the attractive $U$, both in the normal state and, even more significantly, in the superconducting state. However, this pushes the system towards an effectively stronger coupling and hence to a faster transition to the insulating state. Similar to the Mott transition in the repulsive model, the superconductor-insulator transition here is marked by a vanishing quasiparticle weight $Z$. This leads to a scenario that recalls strongly correlated superconductivity close to a Mott transition, where pairing is enhanced but phase coherence is rapidly lost, even though the present model is dominated by attractive interactions.

We demonstrate that in an ideal Weyl semimetal, in which the Fermi energy coincides with the band-touching nodes, weak direct internodal scattering is irrelevant and, as a result, the chiral charge is (almost) exactly conserved. This leads to an experimentally observable effect: in an applied magnetic field, the charge transport along the field becomes purely ballistic, with the conductance given by $e^2/h$ per magnetic flux quantum through the sample cross section. This is the strongest experimental manifestation of nontrivial topology in Weyl and Dirac semimetals.

An accurate understanding of the ground state electronic properties of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, a high-temperature superconductor under pressure, is key for unveiling the origin of its superconductivity. In this paper, we conduct a theoretical study of the electronic structure of the bilayer polymorph of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ using the hybrid functional approach, which is well suited to tackle the nonlocal correlation effects arising in this system from the molecular orbital splitting of the Ni $3d_{3z^2-r^2}$ states inside Ni-Ni dimers. Our calculations reveal that bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ is a strongly correlated magnetic system with robust Ni spin moments. Spin moments on individual Ni sites take on unusually small values because of the electron delocalization over molecular orbitals involving multiple Ni and O sites. We further find that the magnetism of bilayer ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ is intimately linked with charge distribution between different Ni and O orbitals. Two distinct regimes are identified in this regard. In one, molecular orbital physics drives the Ni $3d_{x^2-y^2}$ band towards half-filling, which is a well-established condition for unconventional high-temperature superconductivity upon hole doping in cuprates. In the other, the Ni $3d_{x^2-y^2}$ band is quarter-filled favoring spin- and charge-density wave states and Ni-O bond-disproportionation, which is consistent with several recent experimental claims. It is possible that superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ occurs as a result of a pressure-induced transition between these two competing regimes. Since none of the low energy phases discovered in this study are metallic, nonstoichiometry would be required for superconductivity to occur.

We report the synthesis, structure, and physical properties of a new quaternary nitride ${\mathrm{LaCr}}_2{\mathrm{Ge}}_2\mathrm{N}$. The compound crystallizes in the ${\mathrm{CeCr}}_2{\mathrm{Si}}_2\mathrm{C}$-type structure ($P4$/mmm), featuring distinctive ${\mathrm{Cr}}_2\mathrm{N}$ square sheets within ${\mathrm{Cr}}_2{\mathrm{Ge}}_2\mathrm{N}$ block layers. Physical characterizations reveal enhanced electron correlations evidenced by a Sommerfeld coefficient substantially larger than band calculations and pressure-induced deviation from Fermi-liquid behavior. Magnetic measurements show short-range antiferromagnetic correlations developing around 460 K, followed by long-range magnetic ordering at 14 K. Additionally, subtle anomalies at 378 K suggest possible electronic ordering. First-principles calculations reveal nearly flat Cr-$3d$ bands near the Fermi level and predict a striped antiferromagnetic ground state. This work demonstrates how electron count variation in the ${\mathrm{CeCr}}_2{\mathrm{Si}}_2\mathrm{C}$-type structure family leads to magnetic ordering in ${\mathrm{LaCr}}_2{\mathrm{Ge}}_2\mathrm{N}$, contrasting with the paramagnetic behavior of ${\mathrm{LnCr}}_2{\mathrm{Si}}_2\mathrm{C}$ compounds.

The superconducting diode effect is a nonreciprocal transport phenomenon in which the critical current depends on the current direction. This effect is typically realized in superconductors with broken spatial-inversion and time-reversal symmetry. To break the time-reversal symmetry, external magnetic fields are commonly used. Here, we demonstrate a sign-controllable superconducting diode effect under zero external magnetic field in a van der Waals superconductor, trigonal ${\mathrm{PtBi}}_2$. The sign of the zero-field superconducting diode effect is controlled by the poling magnetic field, which is a large magnetic field applied prior to measurements. This result indicates that trapped magnetic flux is responsible for breaking the time-reversal symmetry. Our findings highlight the crucial role of trapped magnetic flux in generating the superconducting diode effect and provide a general pathway for realizing a zero-field superconducting diode effect.

Topology in photonics comes in two distinct flavors: global and local. Global topology considers invariants that are obtained by integrating over the energy band, whereas local topology considers defects, typically vortices, in the far-field emission. These topologies are described by a wide range of models built in both real and momentum space, which are connected only by computationally expensive numerical methods that lack physical intuition. Here we propose a general framework based on a real-space Hamiltonian capable of describing electric field as a vector in both near and far fields, allowing us to bridge between topological defects in the far-field and global topological invariants. The proposed Hamiltonian is constructed from the symmetry-representations of the lattice, is deformable to both atomic localized-mode (tight-binding) and photonic delocalized-mode (long-range) limits, and allows for independent control over the energies of eigenmodes of different symmetries at high-symmetry points of the Brillouin zone. This symmetry-based approach enables the design of structures with almost arbitrary topological properties and is not limited to photonic systems, but could apply to any system with engineered real-space couplings.

Negative circular polarization of photoluminescence of large InP/(In,Ga)P quantum dots was studied subject to a magnitude and direction of a magnetic field. In a field tilted relative to the average optically oriented electron spin, a deviation of the dependences from the isotropic case is observed. Using spin-flip Raman scattering it was determined that the electron $g$ factor is isotropic $g_z=g_x=1.52\pm 0.05$, where $z$ is the structure growth axis. It is shown that the peculiarities of the behavior of optical orientation in a magnetic field are caused by the anisotropy of the electron spin relaxation time: the relaxation time of the spin component directed along the QD growth axis appears to be large relative to the relaxation time in the plane (${{\tau }_s}_z\gg {{\tau }_s}_x$). This case is not typical for an electron in QDs with the direct band gap in momentum space, since the dominant spin relaxation mechanism is usually due to the isotropic hyperfine interaction with the nuclear spins. The observed stabilization of the electron spin along the QDs growth axis is characteristic for the exciton-type spin dynamics determined by the isotropic electron-hole exchange interaction.

The multiple crystalline terminations in magnetic Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ display distinct topological and trivial surface states, which have successfully been distinguished experimentally. However, a model of pure terminations is known to be inadequate because these surfaces exhibit a high degree of spatial heterogeneity and point disorder. Here, we perform a spectromicroscopy study of the surface chemistry and surface electronic structure using photoemission measurements in combination with first-principles calculations of core levels. We identify an intermediate region with properties distinct from both the sulfur and tin terminations, and demonstrate that the spectral features in this region can be associated with a disordered termination with a varying density of surface tin vacancies. This work establishes heuristics for identifying variable surface disorder using photoemission, an important prerequisite to experimentally establishing the behavior of momentum-space topological surface features subject to variable surface disorder on a single cleave.

We use the Kubo response functions to calculate the electrical and thermal conductivity and Seebeck coefficient at low temperatures and frequencies in the quantum-critical region for fermions on a lattice. The theory uses scattering of the fermions with the previously derived collective fluctuations caused by topological defects of the quantum XY model coupled to fermions. As in the marginal Fermi-liquid phenomenology, the fluctuations are a scale invariant function $\mathcal{F}(\omega /T)$, but unlike it, they have both a momentum dependence and a momentum-dependent coupling to fermions. They have the unusual form of being products of $\mathcal{F}(\omega /T)$ and a function of the momentum transfer. This gives that there is no vertex correction to the single-particle self-energy over a range of momenta and frequencies near the Fermi-surface, unlike Migdal's theorem. The imaginary part of the single-particle self-energy then continues to be linear in $\max (\omega ,T)$ with a weak momentum dependence for a range of momentum near the Fermi surface, as in the phenomenology. This is used to solve the vertex equation in the Kubo formula for the transport properties at low temperatures with multiplicative corrections of $O(T/E_F)$. The microscopic model is applicable to the fluctuations of the loop-current order in cuprates as well as to a class of quasi-two-dimensional heavy-fermion and other metallic antiferromagnets, and was proposed recently also for the possible loop-current order in Moiré twisted bilayer graphene and bilayer ${\mathrm{WSe}}_2$. All these metals have a linear-in-temperature electrical resistivity in the quantum-critical region of their phase diagrams, often termed “Planckian” resistivity. The solution of the Kubo equation for transport shows that vertex renormalizations to the external fields, besides those caused by Aslamazov-Larkin (A-L) processes, are absent. A-L appears as an Umklapp scattering matrix, which gives a temperature-independent multiplicative factor for the electrical resistivity but does not affect the thermal conductivity. We also show that the mass renormalization, which gives a logarithmic enhancement of the marginal Fermi-liquid specific heat does not appear in the electrical resistivity and, more remarkably, in the thermal conductivity. On the other hand the mass renormalization $\propto ln{\omega }_c/T$ (${\omega }_c$ is the upper cutoff of the fluctuations), appears in the Seebeck coefficient. We also discuss in detail the conservation laws that play a crucial role in all transport properties. We calculate exactly, the numerical coefficients of the transport properties for a circular Fermi surface. The leading temperature dependence is shown to remain the same for a general Fermi surface, but it is too messy to calculate the numerical coefficient.