Superconductor research has traditionally depended on experiments and theoretical approaches. However, the rapid advancement of data-driven methods and machine learning (ML) has opened avenues for accelerating superconductor discovery. Here, we integrate ML with density functional theory calculations to efficiently screen conventional B-C-N based superconductors and identify potential high-$T_{\mathrm{C}}$ candidates among $R_3{\mathrm{Ni}}_2{\mathrm{O}}_7$-type bilayer nickelates. We identify 13 binary and ternary B-C-N based superconductors with $T_{\mathrm{C}}\ge 10$ K, including 3 with $T_{\mathrm{C}}\ge 25$ K, two structural forms of ${\mathrm{B}}_2\mathrm{CN}$ ($T_{\mathrm{C}}=47.6$ and 43.7 K) and ${\mathrm{TiNbN}}_2$ $(T_{\mathrm{C}}=25.3 \mathrm{K})$. These B-C-N based compounds share a common feature of strong σ bonds, which is key to achieving relatively high $T_{\mathrm{C}}$. Moreover, we propose ${\mathrm{Tb}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ $(T_{\mathrm{C}}=61.6 \mathrm{K})$ and ${\mathrm{Ac}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ $(T_{\mathrm{C}}=70.3 \mathrm{K})$ as potential high-$T_{\mathrm{C}}$ nickelate superconductors under high pressure. Their electronic structures closely resemble those of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, especially in the hole-type band dominated by Ni $3d_z^2$ orbital character. We also analyze feature importance in the ML results for both conventional and high-$T_{\mathrm{C}}$ superconductors. These results advance the search for new superconductors and enhance the fundamental understanding of superconducting mechanisms.

The discovery of superconducting transitions in pressurized ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ and ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ has highlighted the pivotal role of density wave (DW) orders in nickelate superconductors. To gain a comprehensive understanding of the superconducting state, it is essential to elucidate the nature of the DW order. In this study, we utilized ${}^{139}\mathrm{La}$ nuclear quadrupole resonance (NQR) to investigate the charge-density wave (CDW) and spin-density wave (SDW) orders in both single-crystal and polycrystalline ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. Near $T_{\mathrm{DW}}\approx 133\mathrm{K}$, an abrupt change in both the linewidth and frequency of the La(2) site in the single-crystal sample provides compelling evidence for a first-order-like phase transition. The pronounced broadening of the NQR lines indicates the incommensurate nature of the DW order. Furthermore, the spin-lattice relaxation rate divided by temperature 1/$T_{1}T$ exhibits a strong enhancement at $T_{\mathrm{DW}}$, indicating the strong spin fluctuations above the first-order DW transition. These observations suggest an intricate interplay between incommensurate CDW and SDW orders. Our findings offer critical insights into the microscopic mechanisms of the DW state in ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ and establish an essential framework for exploring the interplay between DW and superconducting phases in nickelate superconductors.

High-temperature ferromagnetism and large anomalous Hall conductivity (AHC) in two-dimensional (2D) magnets are highly desirable for ultrathin 2D spintronic devices, as they endow these devices with superior magnetic stability and efficient spin-current conversion capabilities, thereby driving advancements in low-power and high-density spintronic technologies. Here, by first-principles calculations, we study the electronic and magnetic properties of ${\mathrm{Fe}}_4{\mathrm{GaTe}}_2$ which is an energetically and dynamically stable van der Waals ferromagnet with an in-plane magnetic anisotropy. Notably, we show that the Curie temperatures of monolayer, bilayer, triple-layer, quadruple-layer, and bulk ${\mathrm{Fe}}_4{\mathrm{GaTe}}_2$ are all above room temperature. By reorienting their spins from in-plane to out-of-plane, monolayer, few-layer, and bulk ${\mathrm{Fe}}_4{\mathrm{GaTe}}_2$ exhibit a large AHC range from 427–540 S/cm at the Fermi level, which is related to the $M_y$ mirror symmetry breaking. Furthermore, monolayer ${\mathrm{Fe}}_4{\mathrm{GaTe}}_2$ display perpendicular magnetic anisotropy under electron doping and retains the high Curie temperature and large AHC. Our findings suggest that van der Waals room-temperature ferromagnet ${\mathrm{Fe}}_4{\mathrm{GaTe}}_2$ holds promising applications in practical spintronic devices.

A straightforward way to determine the electric field gradient and principal axes frame (EFG-PAF) on single-crystal samples with spin-$\frac{3}{2}$ nuclei is demonstrated. Nuclear quadrupole resonance (NQR) spectroscopy is used to determine the Rabi frequency for ${}^{35}\mathrm{Cl}$ in a single crystal of potassium chlorate $\left({\mathrm{KClO}}_3\right)$ by comparing the NQR signal for powder and single-crystal samples. By exploiting the geometrical dependence of the Rabi frequency with respect to the excitation direction, EFG-PAF is readily determined. Furthermore, relaxation times $T_1$ and $T_2^{*}$ were measured at multiple temperatures ranging from 17 to $200\mathrm{K}$, extending the results of previous works to colder temperatures where new relaxation mechanisms become dominant. The experiments were performed in a cryogen-free cryostat, which posed distinct challenges compared to a conventional cryogenic cooling setup. The successful operation of the NQR probe within a cryogen-free cryostat has the potential to make the technique more accessible and widen applications.

For density functional theory (DFT) based adiabatic spin dynamics, we investigate the relationship between the effective fields driving the dynamics of the atomic magnetic moments and the constraining fields used to enforce a specific noncollinear magnetic configuration. We revisit a derivation and a numerical comparison by Streib et al. [Phys. Rev. B 102, 214407 (2020)] suggesting that the effective field from the energy gradient with respect to the magnetic moment direction and the constraining fields differ for systems with mean-field terms in the Hamiltonian, which they extended to include DFT. We find that a derivation focused on the DFT case clarifies what quantities to extract from the computational software to best represent the two fields. In particular, the definition of the magnetic moment magnitude must be consistent when making the comparison. This reestablishes the relationship of the two fields as having equal amplitude but opposite directions, in agreement with several earlier works. We also show that the constraining fields can be rescaled to reflect the correct field acting on the full, physically meaningful magnetic moment, rather than on the reduced moment used for numerical stability. Our numerical investigation is extended beyond the original case of an iron dimer to supercells of body-centered cubic iron in both ferromagnetic and disordered magnetic backgrounds, which further supports our conclusions.

We study the phase diagram of a bilayer of antiparallel dipolar excitons with a 1:2 density ratio between the layers, as a function of temperature and density. Using quantum Monte Carlo simulations, we show that such a system supports the formation of trimers, namely, three-exciton bound states consisting of a single dipole in one layer and two dipoles in the second layer. At sufficiently low temperatures and densities, these trimers condense into a trimer superfluid phase. Increasing the excitonic density induces a quantum phase transition into a phase in which condensates of independent dipoles exist in both layers, in parallel to the trimers. We also study the thermal transitions out of these phases and find that while the normal state is reached directly from the trimer superfluid, the thermal disordering of the two-superfluid phase involves an intermediate state which is either a trimer superfluid or a single excitonic condensate in the denser layer. A potential experimental realization using transition-metal dichalcogenide heterostructures is discussed.

In indirect band gap materials, phonon-assisted processes are key mechanisms for photoluminescence (PL). Using a first-principles many-body approach, we systematically investigate the phonon-assisted PL in bilayer ${\mathrm{MoS}}_2$ and its dependence on temperature and external tensile strain. The effects of phonons are accounted for using a supercell approach: we identify the phonon momenta that are important to PL, construct supercells that are commensurate with these phonons, and examine the changes in the optical absorption after explicit displacements of atoms along each phonon mode. The PL intensity is then obtained via the van Roosbroeck-Shockley relationship from the optical absorption spectra. This approach enables us to investigate phonon-absorption and phonon-emission processes separately and how each process depends on temperature. Our results reveal that optical phonons associated with out-of-plane vibrations of S atoms and in-plane vibrations of Mo atoms contribute most to the indirect PL for unstrained bilayer ${\mathrm{MoS}}_2$. Additionally, we also discuss how the PL spectra and the phonon contributions evolve with strain. In particular, we show that at high strain, additional phonon channels become available due to the modulation of the electronic band structure.

One-dimensional zigzag chains offer a striking platform for hosting Dirac fermions protected by glide reflection symmetry, along with nearly flat electronic bands. Though rich in theoretical promise, these lattices are mostly found as structural fragments rather than in fully formed bulk crystals. In this study, we investigate the electronic structure of ferromagnetic ${\mathrm{FePd}}_2{\mathrm{Te}}_2$, with angle-resolved photoemission spectroscopy and density functional theory. We find out that the Fe zigzag chains contribute to Dirac-like dispersions and flat orbitals near $E_F$. With the presence of strong uniaxial ferromagnetism, both the Dirac fermions and the near-flat band are intrinsically spin polarized. Additionally, the presence of Weyl points near $E_F$ is predicted. ${\mathrm{FePd}}_2{\mathrm{Te}}_2$ could be the first quasi-1D system for investigating the Dirac point and flat bands, as well as emergent topological phenomena.

A clean interface between two Weyl semimetals features a universal, field-linear tunnel magnetoconductance of $(e^2/h)N_{\mathrm{ho}}$ per magnetic flux quantum, where $N_{\mathrm{ho}}$ is the number of chirality-preserving topological interface Fermi arcs. In this work we show that the linearity of the magnetoconductance is robust with respect to interface disorder. The slope of the magnetoconductance changes at a characteristic field strength $B_{\mathrm{arc}}$—the field strength for which the time taken to traverse the Fermi arc due to the Lorentz force is equal to the mean inter-arc scattering time. For fields much larger than $B_{\mathrm{arc}}$, the magnetoconductance is unaffected by disorder. For fields much smaller than $B_{\mathrm{arc}}$, the slope is no longer determined by $N_{\mathrm{ho}}$ but by the simple fraction $N_{\mathrm{L}}{N}_{\mathrm{R}}/(N_{\mathrm{L}}+N_{\mathrm{R}})$, where $N_{\mathrm{L}}$ and $N_{\mathrm{R}}$ are the numbers of Weyl-node pairs in the left and right Weyl semimetal, respectively. We also consider the effect of spatially correlated disorder potentials, where we find that $B_{\mathrm{arc}}$ decreases exponentially with increasing correlation length. Our results provide a possible explanation for the recently observed robustness of the negative linear magnetoresistance in grained Weyl semimetals.

Understanding which minimal effective model captures the essential physics of cuprates is a key step towards unraveling the mechanism behind high-$T_c$ superconductivity. Recent measurements of the dynamical spin structure factor (DSF) in cuprate ladder compounds have indicated the presence of an additional, attractive term in the single-band Hubbard model, potentially originating from electron-phonon interactions. Here, we demonstrate that similar DSF features can also be captured by $t\text{−}J$ descriptions with a smaller attractive term. Motivated by this observation, we systematically investigate the strength and origin of different contributions to the single-band Hamiltonians by downfolding either from the three-band Emery model or the electron-phonon coupled Hubbard-Holstein model. For one-dimensional systems, we find that the extended versions of both single-band descriptions can reproduce the experimentally observed DSF signatures. Finally, we extend our analysis to two dimensions by comparing two-hole correlation functions for the different single-band models. Our results provide new insights into the long-standing question of which single-band Hamiltonian can capture the essential physics of cuprates.

Dislocations in perovskites are believed to disrupt their ferroelectric properties. In this study, a novel molecular dynamics approach utilizing a modified embedded-atom method with charge equilibration interatomic potential is employed to provide qualitative new insights into how dislocations in the bulk can modulate the ferroelectric behavior of perovskites. Results show that stress gradients induced by various dislocation configurations/densities influence the polarization distribution pattern and the domain switching process. This modulation affects the shape of the hysteresis loop. The dynamics of the intrinsic charge-lattice coupling mechanism reveal that changes in polarization patterns are more pronounced in regions under larger compressive or tensile stress. Specific stress thresholds are identified that mark significant changes in polarization behavior under different loading conditions, offering a significant step towards understanding how the heterogeneous stress/strain state of a material influences its ferroelectric properties. Additionally, we qualitatively uncovered the effect of dislocation density on the energy characteristics, offering material design guidelines for specific electronic applications. Lower dislocation densities widen the hysteresis response and enhance the polarization properties, desirable for random-access memory applications, that require long-term data preservation. In contrast, higher dislocation densities result in narrower hysteresis loops and sharper switching responses, optimizing the conditions for high-speed computing and power-sensitive logic devices.

Here, we demonstrate that quantum and classical descriptions generally yield different results for the spontaneous emission in nanophotonic cavities. Starting from the quantized single-mode field in a general context of dispersive and lossy cavities, we derive the expression for emission-rate enhancement as well as key relevant parameters such as mode volume and quality factor. For general nanophotonic cavities, this ratio of the quantum to the classical description is typically below unity and varies with the material dispersion properties, scattering-to-absorption ratio, and morphology of the cavity. Notably, the two descriptions converge for lossless, nondispersive dielectric cavities and for noble-metal plasmonic cavities with sufficiently low scattering losses.

Quantized transport not only exists in gapped topological states but also in metallic states. Recently, Kane proposed a quantized nonlinear conductance in ballistic metals whose value is determined by the Euler characteristic of the Fermi sea [Kane et al., Phys. Rev. Lett. 128, 076801 (2022)]. In this paper, we consider two-dimensional noninteracting fermionic systems whose Fermi surface has nonvanishing Berry curvature. We find that the Berry curvature at the Fermi surface does not affect the quantized nonlinear transport for translationally invariant systems. When spatial inhomogeneity is introduced, such quantization breaks down due to the combined effect of Berry curvature and the gradient of the local potential. Such a breakdown of quantization can be observed in trapped ultracold atoms with topological bands.

It is commonly accepted that the Kondo effect is associated with a substantial probability of spin flip owing to the antiferromagnetic exchange interaction. An accurate experimental determination of the Kondo spin flip probability is of fundamental interest. In this work, we determine this quantity by using nonlocal spin valves with nonmagnetic Cu channels that contain dilute magnetic Fe impurities. The measured spin flip probability is considerably lower than previously assumed and decreases from 0.14 ± 0.07 to 0.032 ± 0.007 as the Kondo resistivity increases from 0.61 nΩ cm to 8.7 nΩ cm. The findings are attributed to a cancellation effect from randomly oriented impurity spins within the spatial extent of the Kondo screening cloud.

A thorough understanding of the correlation between electronic and mechanical degrees of freedom is crucial to the development of quantum devices in a nanoelectromechanical system (NEMS). In this work, we first establish a fully quantum mechanical approach for transport through a NEMS device, which is valid for arbitrary bias voltages, temperatures, and electromechanical couplings. We find anomalous current–electric field characteristics at a low bias from where the current decreases with a rising electric field. We reveal that this unique transport feature arises from a combined effect of mechanical motion and Coulomb blockade. At a large oscillation amplitude, the rapid increase of backward tunneling events, which is identified in both electron-counting statistics and the phase space of charge-resolved Wigner functions, suppresses the forward current due to prohibition of double occupation. In the opposite limit of strong damping, the oscillator dissipates its energy to the environment and relaxes to the ground state rapidly. Electrons then transport via the lowest vibrational state such that the net current and its corresponding noise have a vanishing dependence on the electric field.

We calculated the interaction energies of hydrogen-benzene and hydrogen-hydrogen complexes using dispersion-inclusive exchange-correlation functionals in density functional theory and compared them with results from the coupled cluster method with single, double, and perturbative triple excitations and those from the adiabatic-connection fluctuation-dissipation theorem within the random phase approximation. For both complexes, the recent generations of nonlocal van der Waals density functionals show overall improvement over semilocal exchange-correlation functionals with dispersion corrections, yielding good agreement with the coupled cluster method. We discuss their accuracy and recommend suitable exchange-correlation functionals based on different evaluation metrics.

The formation of charge density wave order in Dirac fermion systems via electron-phonon coupling represents a significant topic in condensed matter physics. In this work, we investigate this phenomenon within the Holstein model on the honeycomb lattice, with a specific focus on the effect of disorder. While the interplay between electron-electron interactions and disorder has long been a central theme in the field, recent attention has increasingly turned to the combined influence of disorder and electron-phonon coupling. Using determinant quantum Monte Carlo simulations, we concentrate on the phase transitions of charge density wave order on the honeycomb lattice. Disorder is introduced through the random hopping of electrons in the system, which can localize electrons via the Anderson effect. Our primary result is that disorder suppresses the charge density wave phase, and the interplay between disorder and electron-phonon interactions extends the phase area. We also determine the transition temperature ${\beta }_c$ to the ordered phase as a function of the electron-phonon coupling. Additionally, we observed a suppression of electron kinetic energy and dc conductivity under disorder, highlighting the role of Anderson localization in the degradation of electronic transport. These findings offer significant theoretical insight into the stability and critical phenomena of correlated phases in disordered two-dimensional systems.

We develop a theory based on nonequilibrium Green's functions to describe indirect magnetic interactions in systems subjected to external fields. Using this framework, we investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in Dirac semimetals under the influence of both electric and magnetic fields. We show that these fields induce the Berry connection, the orbital magnetic moment, and the Berry curvature dipole in the nonequilibrium Green's function, driving the system out of equilibrium. In the nonequilibrium regime, four distinct types of RKKY interactions emerge: Heisenberg, Dzyaloshinskii-Moriya, Ising, and spin-frustrated interactions, all of which are tunable by the electric field. Notably, the electric field not only renormalizes the Heisenberg term but also introduces new components into the Ising and Dzyaloshinskii-Moriya interactions. The spin-frustrated interaction, arising from the spin-momentum locking of itinerant electrons, is unique to the nonequilibrium regime and allows for the manipulation of local magnetic moments to align at arbitrary relative angles under electric field control. Our findings provide a method for electrically engineering RKKY interactions and a technique for detecting the chiral anomaly.

We investigate persistent charge and spin currents in a ferromagnetic Hatano-Nelson ring with anti-Hermitian intradimer hopping, where nonreciprocal hopping generates a synthetic magnetic flux and drives a non-Hermitian Aharonov-Bohm effect. The system supports both real and imaginary persistent currents, with ferromagnetic spin splitting enabling all three spin-current components, dictated by the orientation of magnetic moments. The currents are computed using the current operator method within a biorthogonal basis. In parallel, the complex band structure is analyzed to uncover the spectral characteristics. We emphasize how the currents evolve across different topological regimes, and how they are influenced by chemical potential, ferromagnetic ordering, finite size, and disorder. Strikingly, disorder can even amplify spin currents, opening powerful new routes for manipulating spin transport in non-Hermitian systems.