The realization of magnetic skyrmions in nanomaterials offers significant potential for both fundamental research and practical applications. Unfortunately, while Janus structures offer a promising approach for achieving a Dzyaloshinskii-Moriya interaction (DMI) in two-dimensional (2D) materials, the relatively small DMI in these systems poses challenges for the effective induction of skyrmions. In this paper, we explore the magnetic properties of $5d$ transition metal atoms adsorption on a Janus MoSSe monolayer ($5d$-TM@MoSSe) through first-principles calculations. In particular, the positive exchange coefficient ($J$), large DMI($d_{\parallel }$), and perpendicular magnetic anisotropy (PMA) energy of 7.35 meV in Re@MoSSe are superior to those observed in the TM@MoSSe systems. Applying biaxial tensile strain can significantly enhance these parameters while also reversing the magnetic state and the chiral of $d_{\parallel }$ in Re@MoSSe. At 8% tensile strain, $d_{\parallel }$ reaches its maximum value of 4.324 meV; this is because the change of $d$ orbital distribution around Fermi level induced by tensile strain plays a key role in determining a DMI. The value is more than 60 times larger than that of zero strain and is superior to that of state-of-the-art heavy metal/ferromagnetic heterostructures. Furthermore, micromagnetic simulations reveal that antiferromagnetic (AFM) skyrmions in the Re@MoSSe monolayer under 8% tensile strain is highly stable, independent of external stray fields. These findings provide valuable insights into the study of AFM skyrmions and the DMI in 2D materials.

The bulk superconductivity of single-crystal ${\mathrm{SrSn}}_3$, a kagome metal featuring a two-dimensional tin kagome network in each layer, is investigated. ${\mathrm{SrSn}}_3$ single crystals, which crystallize in a centrosymmetric hexagonal structure, are synthesized and characterized. Superconducting properties are evaluated through resistivity, magnetic susceptibility, and specific heat measurements, confirming anisotropic type-II superconductivity with a critical temperature ($T_c$) of 5.5 K. The anomalous temperature dependence of $H_{c2}$, derived from electrical resistivity measurements, suggests the presence of two-gap superconductivity. Furthermore, specific heat analysis shows that a two-gap model provides a significantly better fit than a single-gap model. Band structure calculations reveal a topological state in ${\mathrm{SrSn}}_3$, indicating its potential as a platform for exploring the interplay between topology and superconductivity.

We study the thermoelectrical transport transverse to conventional and topological Josephson junctions with a central quantum dot (QD). For that purpose, we derive an effective resonant tunneling model where the QD is renormalized with an induced superconducting gap. By applying the Keldysh Green's function technique, we compute the local density of states as well as the transmission functions. In the latter case, we observe that the Andreev bound states forming on the QD are inverted if the junction has $p$-wave symmetry, meaning that electron and hole orbitals switch roles. We calculate the thermoelectric transport coefficients both analytically and numerically and show how the induced gaps and the band inversion are reflected in the electrical and heat conductance as well as the Seebeck coefficient, the latter experiencing a sign change in the topological case.

In this study, we present a systematic comparison of various approaches within the constrained random-phase approximation (cRPA) for calculating the Coulomb interaction parameter $U$. While defining the correlated space is straightforward for disentangled bands, the situation is more complex for entangled bands, where different projection schemes from hybridized bands to the target space can yield varying sizes of interaction parameters. We systematically evaluated different methods for calculating the polarizability functions within the correlated space. Furthermore, we analyze how different definitions of the correlated space, often constructed through Wannierization from Kohn-Sham orbitals, defines the orbital localization and play a crucial role in determining the interaction parameter. To illustrate these effects, we consider two sets of representative correlated $d$-orbital oxides: ${\mathrm{LiMO}}_2$ $(M=\mathrm{V}-\mathrm{Ni})$ as examples of isolated $d$-electron systems and ${\mathrm{SrMO}}_3$ $(M=\mathrm{Mn}, \mathrm{Fe}, \text{and} \mathrm{Co})$ as cases of entangled $d$-electron systems. Through this systematic comparison, we provide a detailed analysis of different cRPA methodologies for computing the Hubbard parameters.

Using Lanczos exact diagonalization (ED), we study the spin-1/2 $J_1\text{−}{J}_2$ Heisenberg model with the four-spin ring-exchange interaction $J_r$ on a triangular lattice. We mainly use the level spectroscopic technique of two 36-site tori to investigate the ground-state phase diagram, and further characterize phases by spin, dimer, and chiral correlation functions. The ground state has rich phases including several magnetic-ordered phases like the zigzag phase and tetrahedral phase, as well as several novel nonmagnetic phases, some of which exhibit valence bond solid behavior in their dimer correlation functions. However, we do not find direct evidence of a quantum spin liquid phase with spinon Fermi surface in this model. Our results can give a better understanding of the ground-state properties of the triangular Heisenberg model with ring-exchange interaction and help to understand the relevant triangular materials.

A self-interaction-corrected $\mathrm{GW\Gamma }$ method considering first-order three-point vertex corrections is developed and applied to simulations of the first ionization potentials of 96 isolated atoms and molecules in the GW100 benchmark set. By considering self-interaction corrections for the $\mathrm{GW\Gamma }$ one-electron self-energy terms, the accuracy of the $\mathrm{GW\Gamma }$ method is improved significantly in comparison to the self-interaction-included $\mathrm{GW\Gamma }$ method, for which the errors are approximately 2 times larger than that of the GW method. The remaining errors are found to be equivalent to that of the GW method. We investigated the first-order vertex corrections and self-interaction corrections in the GW and $\mathrm{GW\Gamma }$ terms in detail and found that although the contributions of the self-interaction are less than $\pm$0.5 eV for the GW term, significant contributions of the self-interaction (about $-$4 eV maximum ) are detected for the $\mathrm{GW\Gamma }$ terms. The self-interactions are strongly correlated with the detected simulation errors. Moreover, achieving self-interaction-free $\mathrm{GW\Gamma }$ is significantly important for evaluating the renormalized $Z$ factor because self-interactions cause numerical instability.

We propose an efficient analytical representation of the frequency-dependent $GW$ self-energy $\mathrm{\Sigma }$ via a multipole approximation ($\text{MPA-}\mathrm{\Sigma }$). The MPA self-energy model is interpolated from a small set of numerical evaluations of $\mathrm{\Sigma }$ in the complex frequency plane, similar to the MPA interpolation developed for the screened Coulomb interaction ($\text{MPA-}W$) [D. A. Leon et al., Phys. Rev. B 104, 115157 (2021)]. Crucially, $\text{MPA-}\mathrm{\Sigma }$ enables a multipole representation for the interacting Green's function $G$ ($\text{MPA-}G$) and, in turn, access to all the spectral properties, including quasiparticle energies (QP) and renormalization factors beyond the linearized QP equation. We validate the $\text{MPA-}\mathrm{\Sigma }$ and $\text{MPA-}G$ approaches for a diverse set of systems: bulk Si, Na, and Cu, monolayer ${\mathrm{MoS}}_2$, the NaCl ion pair, and the ${\mathrm{F}}_2$ molecule. We show that, just as for $\text{MPA-}W$, an appropriate choice of frequency sampling in $\text{MPA-}\mathrm{\Sigma }$ is critical to guarantee computational efficiency and high accuracy. Moreover, the combined $\text{MPA-}W$ and $\text{MPA-}\mathrm{\Sigma }$ scheme considerably reduces the cost of full-frequency self-energy calculations, especially for spectral band structures over a wide energy range.

The competition between kinematic, relativistic, and Coulomb interactions has spurred intense experimental and theoretical investigations in iridium-based oxides. Their electronic structure is mostly understood in terms of the spin-orbital coupled effective $J$ state ($J_{\mathrm{eff}}$). However, the role of the Ir charge state in shaping the strength of effective spin-orbit coupling and defining the stability of the $J_{\mathrm{eff}}$ ground state has not been thoroughly explored. We argue here that the iridium-ruthenium triple perovskites, ${\mathrm{Ba}}_3{M\text{RuIrO}}_9$ ($M=\mathrm{Li}$, Mg, and In), are of particular interest in this regard. Using ab initio theory, we show here that the nominal charge states of Ir can be tuned from $+6$ ($5d^3$) to $+4$ ($5d^5$) by choosing nonmagnetic $M$ ions as $\mathrm{Li}(+1), \mathrm{Mg}(+2)$, and $\mathrm{In}(+3)$, while the Ru ions always remain in nominal $+5$ ($4d^3$) charge state. This variation modulates the influence of the spin-orbit coupling (SOC), which is found to be negligible in ${\mathrm{Ba}}_3{\mathrm{LiRuIrO}}_9$, moderate in ${\mathrm{Ba}}_3{\mathrm{MgRuIrO}}_9$, and determining in ${\mathrm{Ba}}_3{\mathrm{InRuIrO}}_9$. Our analysis classifies ${\mathrm{Ba}}_3{\mathrm{LiRuIrO}}_9$ as a correlation-driven insulator arising from Hubbard interactions and exchange-split $t_{2g}$ states, ${\mathrm{Ba}}_3{\mathrm{MgRuIrO}}_9$ as a SOC and correlation-driven insulator that does not conform to the commonly expected $J=0$ ground state and ${\mathrm{Ba}}_3{\mathrm{InRuIrO}}_9$ as $J_{\mathrm{eff}}=1/2$ Mott-Hubbard insulator. In the data reported here, the correlational electronic structure theory results in sizable magnetic moments of both Ru and Ir atoms in these systems and atomistic spin-dynamics simulations capture the experimental Néel temperature for ${\mathrm{Ba}}_3{\mathrm{LiRuIrO}}_9$ and ${\mathrm{Ba}}_3{\mathrm{MgRuIrO}}_9$ and provide evidence for a phase transition for ${\mathrm{Ba}}_3{\mathrm{InRuIrO}}_9$ when T $\to$ 0 K, to a multivalley magnetic state with strong magnetic frustration. The theory identifies that strong SOC in ${\mathrm{Ba}}_3{\mathrm{InRuIrO}}_9$ induces bond-dependent magnetic couplings with significant Dzyaloshinskii-Moriya interaction, strong symmetric anisotropic exchange, and finite in-plane single-ion anisotropy. The realization of such strong anisotropic interactions helps to stabilize a particularly complex energy landscape of ${\mathrm{Ba}}_3{\mathrm{InRuIrO}}_9$, which opens up for exotic magnetic quantum phases such as quantum spin liquid. Thus, by comparing the electronic structure and magnetism of isostructural iridates with different Ir-charge states, we provided a theoretical framework to demonstrate the structural and electronic conditions that drive the deviations from conventional magnetic ordering, facilitating the emergence of exotic quantum phases.

We theoretically consider disorder and temperature effects on the integer quantum Hall effect (IQHE) using a variety of distinct and complementary analytical and numerical techniques. In particular, we address simple, physical, and experimentally relevant questions: How does disorder and/or temperature affect the IQHE plateau width? Does the plateau width increase or decrease with disorder and/or temperature? What happens to the peak in the longitudinal conductance with increasing disorder/temperature? Does the longitudinal conductance obey any universal scaling property? Is there “floating” with increasing disorder and/or decreasing magnetic field? Can disorder destroy the IQHE? Is there an IQHE to localization transition? What is the Landau level dependence of the plateau width? Our detailed theory provides answers to these and other related experimentally relevant questions. We discuss our results in the context of existing experimental results and suggest future experiments arising from our work. A key finding is that disorder and temperature are intrinsically connected in affecting IQHE, and there is an intricate interplay between them leading to nonmonotonicity in how the IQHE plateau width behaves as a function of increasing disorder. Both must be considered on an equal footing in understanding IQHE experiments.

Two-dimensional van der Waals materials offer a highly tunable platform for engineering electronic band structures and interactions. By employing techniques such as twisting, gating, or applying pressure, these systems enable precise control over the electronic excitation spectrum. In moiré bilayer graphene, the tunability facilitates the transition from a symmetric Dirac semimetal phase through a quantum critical point into an interaction-induced long-range ordered phase with a finite band gap. At charge neutrality, the ordered state proposed to emerge from twist-angle tuning is the Kramers intervalley-coherent insulator. In this case, the transition falls into the quantum universality class of the relativistic Gross-Neveu-XY model in $2+1$ dimensions. Here, we refine estimates for the critical exponents characterizing this universality class using an expansion around the lower critical space-time dimension of two. We compute the order-parameter anomalous dimension ${\eta }_{\phi }$ and the correlation-length exponent $\nu$ at one-loop order, and the fermion anomalous dimension ${\eta }_{\psi }$ at two-loop order. Combining these results with previous findings from the expansion around the upper critical dimension, we obtain improved estimates for the universal exponents in $2+1$ dimensions via Padé interpolation. For $N_{\text{f}}=4$ four-component Dirac fermions, relevant to moiré bilayer graphene, we estimate $1/\nu =0.916\left(5\right), {\eta }_{\phi }=0.926\left(13\right)$, and ${\eta }_{\psi }=0.0404\left(13\right)$. For $N_{\text{f}}=2$, potentially relevant to recent tetralayer ${\mathrm{WSe}}_2$ experiments, the Gross-Neveu-XY fixed point may be unstable owing to a fixed-point collision at $N_{\text{f,c}}$, with $N_{\text{f,c}}=1+\sqrt{2}+\mathcal{O}\left(\epsilon \right)$ in the expansion around the lower critical dimension.

Two-dimensional (2D) second-order topological insulators (SOTIs), characterized by zero-dimensional (0D) corner states, have garnered significant interest. However, such topological states have been primarily realized in nonmagnetic (NM), ferromagnetic (FM), and antiferromagnetic (AFM) systems. 2D ferrimagnetic (FiM) SOTIs, particularly those that simultaneously exhibit ultrastable corner states, are still lacking. Here, based on first-principles calculations and theoretical analysis, we reveal such SOTI state in a 2D metal-organic framework (MOF) material, $\mathrm{Cr}{\left(\mathrm{pyz}\right)}_2$ (pyz = pyrazine). This material exhibits FiM ground state with an easy axis aligned along the [001] direction. It hosts a nontrivial real Chern number in the spin-up channel, enabled by $PT$ symmetry, with 0D corner states observable in the disk. In contrast, the spin-down channel exhibits a trivial gapped bulk state. Notably, the topological corner states in monolayer $\mathrm{Cr}{\left(\mathrm{pyz}\right)}_2$ show high robustness, even if the symmetries are broken by introducing defects, the corner states persist. We also considered other external perturbations, including uniaxial/biaxial strain, ligand rotation, and electric fields; the corner states still remain stable. Even more, the energy positions of the corner states are also nearly unchanged. When spin-orbit coupling (SOC) is considered, the material maintains its corner states. This work identifies an ultrastable FiM SOTI state in the MOF system and provides an ideal platform for future experimental investigations and applications in spintronic devices.

Phonon-assisted photoluminescence (PL) in molybdenum-based two-dimensional dichalcogenides is typically weak due to the dormant phonon coupling with optically inactive momentum-dark (intervalley) excitons, unlike in tungsten-based dichalcogenides where such processes are more prominent. Despite this inefficiency, we revisit excitons in ${\mathrm{MoS}}_2$ using rigorous finite-momentum Bethe-Salpeter equation calculations to identify ways to enhance phonon-assisted recombination channels. Our ab initio results, complemented by group-theoretic analyses, reveal that while unstrained ${\mathrm{MoS}}_2$ exhibits no phonon-assisted PL emissions at cryogenic temperatures due to forbidden $A^{\prime \prime }$ phonon modes, biaxial strain opens a pathway to significantly intensify this emission by activating hole-phonon $A^{\prime }$-mediated scattering channels. By calculating allowed exciton-phonon matrix elements and scattering rates, we demonstrate how strain redistributes oscillator strengths toward radiative recombination. These findings provide a promising route to improve the PL emission efficiency in various metal dichalcogenide monolayers through strain engineering and offer valuable insights for further exploration of exciton-phonon dynamics, including time-resolved spectroscopic studies.

Using a formalism based on the non-Abelian Berry connection, we explore quantum geometric signatures of Wannier-Stark spectra in two-dimensional superlattices. The Stark energy can be written as intraband Berry phases, while Zener tunneling is given by interband Berry connections. We suggest that the gaps induced by interband hybridization can be probed by THz optical absorption and emission spectroscopy. This is especially relevant to modern moiré materials wherein mini-bands are often spectrally entangled, leading to strong interband hybridization in the Wannier-Stark regime. Furthermore, owing to their large superlattice constants, both the low-field and high-field regimes can be accessed in these materials using presently available technology. Importantly, even at moderate electric fields, we find that stimulated emission can dominate absorption, raising the possibility of lasing at practically relevant parameter regimes.

Dual topological insulators exhibit two distinct topological surface states protected by different symmetries. BiTe, a dual topological insulator with a superlattice structure, possesses weak and crystalline topological states at different facets. Using scanning tunneling spectroscopy, we detected a Dirac point within the band gap at the top surface, which stems from topological crystalline states and is protected by mirror symmetry. Moreover, we observed increased differential conductance only at the step edges that contain bismuth bilayers, corresponding to one-dimensional conduction channels of weak topological surface states. Our experimental confirmation of the coexistence of two different topological states makes BiTe an exceptional candidate with the possibility of tuning the topological nature by controlled symmetry breaking, which would selectively destroy certain surface states. These bismuth bilayers with 1D channels may have exotic properties, including forming Majorana modes if one can induce superconductivity into this, which is claimed to be important for fault-tolerant quantum computing.

The exploration of the superconductivity in two-dimensional materials has garnered significant attention due to their promising low-power applications and fundamental scientific interest. Here, we report some stable non–van der Waals ${\mathrm{Nb}}_x{\mathrm{N}}_{x+1}$ ($x=1$–4) monolayers derived from the NbN bulk exfoliated along the [001] direction, as identified through first-principles calculations. Among these monolayers, ${\mathrm{Nb}}_2{\mathrm{N}}_3$, which crystallizes in the $P\overline{6}m2$ symmetry, stands out with an exceptional superconducting transition temperature of 77.8 K, setting a new high-$T_c$ benchmark for two-dimensional transition-metal nitrides and binary compounds. Our detailed analysis reveals that the strong superconductivity in ${\mathrm{Nb}}_2{\mathrm{N}}_3$ is driven by phonon modes dominated by N vibrations, with significant electron-phonon coupling contributions from N-$p$ and Nb-$d$ electronic states. Using the anisotropic Migdal-Eliashberg framework, we further determine the two-gap nature of the superconductivity in the ${\mathrm{Nb}}_2{\mathrm{N}}_3$ monolayer, characterized by pronounced electron-phonon coupling and anisotropic energy gaps. These results advance our understanding of superconductivity in two-dimensional transition-metal nitride and highlight their potential for nanoscale superconducting applications.

We investigate the electromagnetic wave absorption process in a coherently coupled two-component Bose-Einstein condensate model in different dimensionality at zero temperature. As the analog of phonon in the solid state physics, the elementary excitation of the Bose-Einstein condensate is described by Bogoliubov quasiparticle or bogolon for short. Due to the small magnitude of the sound velocity of the bogolon, the absorption process is prohibited by the conservation of energy and momentum. To surmount this depression, the additional degree of freedom must be considered inside of the simple Bose gas model. In this article, we develop a microscopical theory for electromagnetic wave absorption by a two-component Bose-Einstein condensate and investigate the absorption rate dependence in different dimensions. Our calculation shows the possibility of manipulating the absorption property by tuning the parameters of the condensates.

The orbital Hall effect (OHE) has several potential advantages over the spin Hall effect (SHE), the latter being well known for its many applications in spintronics. Like the spin Hall effect, the OHE occurs in nonmagnetic materials without stringent symmetry requirements, but unlike the SHE it does not rely on relatively weak spin-orbit interaction. In two-dimensional (2D) materials these advantages risk being nullified by the difficulty of turning the orbital moment away from the out-of-plane direction. Multilayered 2D materials offer a way out of this difficulty because the fluctuating in-plane component of the orbital moment, due to motion of electrons between the layers, can latch to a magnetic field. To describe this effect we have derived a semiphenomenological equation of motion for the density of orbital magnetic moment in stacked 2D materials subjected to a magnetic field. Unlike the equations of motion for the spin, these equations produce a strongly anisotropic dynamics, which is governed by an inverse effective mass tensor for which we provide a fully microscopic expression. As a first application, we combine our equation of motion with phenomenological drift-diffusion equations to formulate a theory of orbital Hanle magnetoresistance in multilayered 2D materials. This theoretical framework also offers a tool for exploring the microscopic theory of the orbital Hall effect, which remains an active area of debate.

We study transport in spin chains employing the Thouless approach based on the level sensitivity to the boundary conditions, $R$. Although spin transport in the integrable easy-axis XXZ model is diffusive, the corresponding $R$ is much closer to ballistic chains than to chaotic diffusive systems. In the case of the grand canonical ensemble, this observation can be rigorously justified, while in the case of the canonical ensemble it can be demonstrated by numerical calculations. Integrability-breaking perturbation (IBP) strongly reduces $R$, which reveals a pronounced minimum at the crossover from anomalous diffusive to normal dissipative transport. This minimum coincides with the onset of the universality of the random matrix theory. Results for various IBPs suggest a discontinuous jump of the spin conductivity in the thermodynamic limit, and moreover that its value is proportional to the strength of the IBP.