Layered ruthenium oxide ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ is a Mott insulator and its frozen state can be melted by various stimuli such as temperature variation, chemical substitution, pressure, and current application. In this study, we have investigated the variation of the electronic structure of ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$ with temperature and electric current by means of hard x-ray photoemission spectroscopy to gain valuable insights into the origin of the insulator-metal transition (IMT). We have observed the reduction in the insulating gap as well as the suppression of the spectral weight for the lower Hubbard band with increasing temperature and electric current density. The variation of the insulating gap well accounts for the characteristic nonlinear conductivity. Our spectroscopic results demonstrate the close relationship between the octahedral distortion of ${\mathrm{RuO}}_6$ and the IMT in ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$. Furthermore, to clarify the origin of the observed current-induced phenomena, the temperature increase of the sample due to electric current was evaluated on the basis of the Joule self-heating model. The simulated results revealed that the heating effects have a significant influence on the gap suppression and the spectral changes under current. However, this cannot explain all of the observed spectral changes under current, implying the possibility of an intrinsic current-induced effect.

Inspired by recent advances in critical quantum heat engines, we investigate heat transport near the critical point of a Lipkin-Meshkov-Glick model coupled to two bosonic reservoirs. In the steady-state and thermodynamic limit, we analyze the thermodynamic precision of the model by examining the scaling of the relative fluctuation and the correspondence between the thermodynamic uncertainty relation ratio and the purity of the reduced density matrix of the central system. Furthermore, we observe a pronounced minimum in the thermodynamic uncertainty relation ratio at the critical point. These results are also verified using Redfield equation, polaron-transformed master equation, and analytical results from the quantum jump theory. Our findings contribute to the understanding of critical thermodynamic precision and provide insights into quantum heat control in nonequilibrium phase transition systems.

Lanthanide nitride materials have recently been shown to act as catalysts for mild condition ammonia synthesis through reaction mechanisms differing from existing industrial catalysts. However, an in-depth study of the electronic structure of GdN, the prototypical lanthanide nitride, has not yet been completed. We report on an extensive in situ x-ray photoelectron spectroscopy and near-edge x-ray absorption study of the surface of epitaxial gadolinium (Gd) pre and post exposure to molecular nitrogen (${\mathrm{N}}_2$), compared with a near-stoichiometric gadolinium nitride (GdN) thin film. In this study, the nitrogen $1s$ core level is used as a proxy to probe the facile breaking of molecular nitrogen (${\mathrm{N}}_2$) by the pure gadolinium (Gd) surface at ambient temperature. We also investigate the evolution of Gd $3d, 4d$, and $4p$ core levels with nitrogen exposure compared with Gd $3^{+}$ atomic multiplet calculations. We find that the exchange splitting of the Gd $4s$ and $5s$ levels suggests that $4f$ electrons are not significantly involved in bonding with nitrogen, which is well aligned with ongoing computational assumptions in the literature. This confirms that Gd-N bonding is predominately ionic, with the valence and conduction bands being of N $2p$ and $\mathrm{Ln} 5d$ and $6s$ character, respectively. The valence band suggests that the formation of GdN by exposure of a Gd surface to ${\mathrm{N}}_2$ does not result in complete nitridation, at least for the dose used and depth studied. Indeed, the surface of epitaxial Gd, post exposure to ${\mathrm{N}}_2$, suggests the coexistence of both pure Gd and GdN, as confirmed by density of states calculations.

We study the Seebeck and Nernst effects in the complex honeycomb (CHC) model, where $m$ atoms are attached to each edge of the hexagon, with $m=0$ corresponding to graphene. When $m=0$ and $m=2$, the energy band structure exhibits a sombrerolike structure near the Fermi energy ($E_F$), and the transmission coefficient plateaus are $1,3,5,\cdots$. The peak height of the Seebeck coefficient satisfies the relation $ln2/n(n=\pm 1,\pm 2,)$. In contrast, for $m=1$ and $m=3$, the energy band structure forms a Dirac cone near the Fermi energy, with a flat band at $E_F=0$, while the transmission coefficient plateaus are $0,1,2,\cdots$ and the system exhibits insulation. Here, the peak height of the Seebeck coefficient follows the relation $ln2/(n\pm 1/2)$. We find that larger Seebeck coefficients and Nernst coefficients are obtained when $m$ is odd. At the same time, the Seebeck coefficient exhibits two additional positive and two additional negative peaks. Due to the flat band, the peak near $E_F=0$ has an opposite sign compared to the other peaks. Unlike the lattice structure when $m$ is even, the Nernst coefficient for odd $m$ does not exhibit a peak at the Fermi energy due to the presence of the band gap.

We show that two-dimensional gapped many-body quantum states are constrained by an infinite-dimensional symmetry which renders them transparent to weak disorder. This prevents disorder-induced localization when interactions are strong enough to open a gap. Using purely algebraic methods we derive all possible quantum states near the superconductor-to-insulator transition and we compute the meson spectrum of superinsulators.

Bound electron-hole pairs in semiconductors known as excitons can form a coherent state at low temperatures akin to a BCS condensate. The resulting phase is known as the excitonic insulator and has superfluid properties. Here we theoretically study the excitonic insulator in a pair of recently proposed two-dimensional candidate materials with nontrivial band topology. Contrary to previous works, we include interaction channels that violate the individual electron and hole number conservations. These are on equal footing with the number-conserving processes due to the substantial overlap of Wannier orbitals of different bands, which cannot be exponentially localized due to the nontrivial Chern numbers of the latter. Their inclusion is crucial to determine the symmetry of the electron-hole pairing and, by performing mean-field calculations at nonzero temperatures, we find that the order parameter in these systems is a chiral $d$ wave. We discuss the nontrivial topology of this unconventional state and discuss some properties of the associated Berezinskii-Kosterlitz-Thouless transition. In particular, we argue that here it becomes a smooth crossover and estimate the associated temperature to lie between 50 and $75\mathrm{K}$ on realistic substrates, which is over an order of magnitude larger than in the number-conserving approximation where $s$-wave pairing is favored. Our results highlight the interplay between topology at the single-particle level and long-range interactions, motivating further research in systems where both phenomena coexist.

Inspired by the recent experimental report on the pressure-induced superconductor-insulator transition in cuprate superconductors as well as the superconductivity of the Ruddlesden-Popper-phase ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under high pressure, we systematically investigated the single-orbital bilayer Hubbard model in the regime of large interlayer hybridization to mimic the pressure effects. We map out the phase diagram of interlayer hybridization versus density in the regime of intermediate to strong hybridization. In particular, we found that sufficiently strong hybridization can destroy the $s^{\pm }$-wave pairing and induces its transition to correlated metallic, pseudogap, and Fermi liquid phases depending on the doping regime. The phase diagram hosted by the bilayer model implies its role as a versatile platform to explore the pressure effects on the two-dimensional to three-dimensional crossover physics of Hubbard-type models.

At extreme pressures of approximately 500 GPa—conditions characteristic of the deep interiors of super-Earths—the combination of NaCl-type MgO and postperovskite-type ${\mathrm{MgSiO}}_3$ (PPv) has been reported to produce a postpost-perovskite (PPPv) phase of ${\mathrm{Mg}}_2{\mathrm{SiO}}_4$ with an $I\overline{4}2d$ symmetry. This PPPv silicate is proposed as the primary mantle silicate in these massive rocky exoplanets. Understanding the fundamental properties of PPPv, particularly in solid solutions with ${\mathrm{Fe}}_2{\mathrm{SiO}}_4$, is crucial for insight into the interior dynamics and compositions of such planets. In this study, we conduct an ab initio investigation of the properties of $\mathrm{F}{\mathrm{e}}^{2+}$-bearing PPPv at pressures ranging 400 GPa–1 TPa. Given the localized nature of $3d$ electrons in iron, we employ the $\mathrm{LDA}+U_{\mathrm{SC}}$ method alongside conventional density functional theory functionals to probe the electronic structure of this system. The dependence of the Hubbard parameter U on volume and spin state is carefully evaluated. Furthermore, we systematically explore the effects of pressure, temperature, and structural variations on the spin state of iron in $\mathrm{F}{\mathrm{e}}^{2+}$-bearing PPPv, providing valuable data to improve mantle modeling for super-Earth-type exoplanets.

Latent symmetries, which materialize after performing isospectral reductions, have recently been shown to be instrumental in revealing novel topological phases in one-dimensional systems, among many other applications. In this work, we explore how to construct a family of seemingly complicated two-dimensional models that result in energy-dependent Haldane models upon performing an isospectral reduction. In these models, we find energy-dependent latent Semenoff masses without introducing a staggered on-site potential. In addition, energy-dependent latent Haldane masses also emerge in decorated lattices with nearest-neighbor complex hoppings. Using the Haldane model's properties, we then predict the location of the topological gaps in the aforementioned family of models and construct phase diagrams to determine where the topological phases lie in parameter space. This idea yielded, for instance, useful insights in the case of a modified version of $\alpha$-graphyne and hexagonal plaquettes with additional decorations, where the gap-closing energies can be calculated using the ISR to predict topological phase transitions.

We proposed and studied the tunneling optoresistance effect and magneto-optoresistance effect based on planar heterostructures formed by two different types of two-dimensional transition metal dichalcognides (TMDCs). We find that for either intrinsic or modulated planar heterostructures, there exist several ideal line-type resonant peaks in transmission spectra. By tuning the helicity of the off-resonant circularly polarized light irradiated on two modulated regions, the tunneling optoresistance effect can occur, which resembles the tunneling magnetoresistance effect realized by tuning the magnetic exchange field. Compared to cases where only the magnetic exchange field or off-resonant circularly polarized light is introduced, spin- and valley-resolved resonances are suppressed greatly and line-type resonant peaks even disappear, and considerable spin- and valley-resolved high/low resistance states can be achieved through simultaneously switching the direction of the magnetic exchange field and the helicity of the light. We find the magnetic-optical modulation parameters play a great role in the quantum transport process. When the intensity of the magnetic exchange field or the optically induced band gap increases, the tunneling optoresistance effect or the magneto-optoresistance effect can be considerably enhanced. Our results suggest that the tunneling optoresistance effect and magneto-optoresistance effect in TMDC planar heterostructures have a promising potential for applications in magnetic-optical memory and spintronic and valleytronic devices.

We study the atomic collapse phenomenon in $K$ and $K^{\prime }$ valleys of gapped graphene. Bound states induced by Coulomb impurity in the gap turn into atomic collapse resonances as the charge increases beyond the supercritical charge $Z_c$. $Z_c$ increases sublinearly with the band gap $\mathrm{\Delta }$. The atomic collapse resonances result in peaks in the local density of states (LDOS) at the same energies in $K$ and $K^{\prime }$ valleys, but the strong (weak) LDOS peaks in the $K$ valley are weak (strong) LDOS peaks in the $K^{\prime }$ valley reminiscent of pseudospin polarization phenomenon. From a spatial LDOS analysis of the atomic collapse resonance states, we assign specific atomic orbitals to the atomic collapse resonances. Remarkably, the two $p$ atomic orbital atomic collapse states are no longer degenerate and split into two, having lobes in different directions in the graphene plane.

In strongly coupled magnon-phonon systems, magnetic disorder plays a crucial role in shaping phonon properties and thermal transport behavior. However, conventional approaches typically rely on nonmagnetic (NM) approximations, neglecting the effects of magnetic ordering. To address the existing limitations, we propose an integrated computational framework that synergistically merges the special quasirandom structure (SQS) approach, first-principles calculations, and Boltzmann transport theory. This framework allows for a systematic exploration of the role of magnetic disorder in phonon thermal transport within monolayer CrOCl, with a particular focus on its impact on lattice thermal conductivity. Our results reveal that the transition from the ferromagnetic (FM) to paramagnetic (PM) phase strengthens harmonic interactions, leading to optical phonon hardening. Furthermore, this phase transition brings about marked alterations in lattice thermal conductivity. Through an in-depth analysis of relative errors in third-order interatomic force constants (IFC3), we attribute these changes to the complex impact of magnetic ordering on higher-order interactions, which, in turn, determine the strength of phonon-phonon scattering. This study establishes a robust framework for exploring thermal transport in magnetic materials and highlights the critical role of magnetic disorder in governing phonon-mediated heat conduction.

Perovskite-type $\mathrm{EuTi}{\mathrm{O}}_3$, which is quantum paraelectric and $G$-type antiferromagnetic (AFM) in the bulk form, has attracted great attention due to its strong spin-lattice coupling and strain-induced multiferroic in thin film form. In this study, significant magnetodielectric (MD) coupling is demonstrated in ${\mathrm{K}}_2\mathrm{Ni}{\mathrm{F}}_4$-type ferromagnetic (FM) layered perovskite $\mathrm{E}{\mathrm{u}}_2\mathrm{Ti}{\mathrm{O}}_4$, which features alternating rocksalt (RS)-type EuO layers and perovskite (Pv)-type $\mathrm{EuTi}{\mathrm{O}}_3$ layers. We find a dielectric anomaly as a local minimum of permittivity near the ferromagnetic transition temperature ($T_{\mathrm{C}}=9.5\mathrm{K}$) in the temperature-dependent permittivity under zero magnetic field. Remarkably, an applied magnetic field induces a substantial increase in permittivity at $T_{\mathrm{C}}$ (approximately 22% at 3 T), which is three times larger than that observed in bulk $\mathrm{EuTi}{\mathrm{O}}_3$. Our experimental and first-principles studies reveal that the insertion of EuO layers in $\mathrm{E}{\mathrm{u}}_2\mathrm{Ti}{\mathrm{O}}_4$ enhances the Eu-$f$/Ti-$d$ orbitals hybridization through pseudostrain in the $\mathrm{EuTi}{\mathrm{O}}_3$ layers, as well as modifies the magnetic exchange paths that favor ferromagnetism over antiferromagnetism. The enhanced hybridization highlights the role in the large MD coupling played by AFM superexchange interactions via the Ti $3d$ state, which competes with indirect FM interactions via the Eu $5d$ state. Our findings demonstrate the potential of layered structure to control MD coupling, paving the way for new strategies in designing MD materials.

We show that a gapped spin-$S$ chain with antiferromagnetic (AFM) order exhibits, in the thermodynamic limit, exponentially localized fractional $\pm \frac{S}{2}$ edge modes when the system possesses U(1) symmetry. We show this for integrable and nonintegrable spin chains, both analytically and numerically. Through exact analytical solutions, we show that an AFM spin-$\frac{1}{2}$ chain with explicitly broken ${\mathbb{Z}}_2$ symmetry and an integrable AFM spin-1 chain with spontaneously broken ${\mathbb{Z}}_2$ symmetry have $\pm \frac{1}{4}$ and $\pm \frac{1}{2}$ fractionalized edge modes, respectively. Furthermore, employing the density matrix renormalization group technique, we extend this analysis to generic $XXZ\text{−}S$ chains with $S\le 3$ and demonstrate that these fractional spins are robust quantum observables, substantiated by the observation of variance of the associated fractional spin operators that is consistent with a vanishing functional form in the thermodynamic limit. Moreover, we find that the edge modes are robust to a disorder that couples to the Néel order parameter.

The symmetry requirements for realizing unconventional compensated magnets with spin-polarized bands such as altermagnets have recently been uncovered. The most recent addition to this family of magnets is parity-odd or $p$-wave magnets. We demonstrate that $p$-wave magnets are perfectly compatible with superconductivity due to the spin polarization of their electron bands and that they induce unexpected spin transport phenomena. We first show that $p$-wave magnetism can coexist with conventional superconductivity regardless of the magnitude of the spin splitting. We then predict that $p$-wave magnets induce a charge-to-spin conversion, which can be strongly enhanced by the presence of superconductivity providing a way to probe the coexistence in experiments. Our results open an avenue for material combinations with a synergetic relation between spintronics and superconductivity.

Nonequilibrium heat transport in quantum systems coupled to wide-band embeddings provides a striking example of the limitations of the generalized Kadanoff-Baym ansatz (GKBA), while solving the full two-time Kadanoff-Baym equations remains computationally prohibitive. To address this challenge, we propose an iterated solution to the reconstruction problem, resulting in a time-linear evolution scheme involving 14 correlators for systems with finite bandwidth embeddings. This approach eliminates artifacts related to the GKBA and resolves convergence issues that arise in the wide-band limit. Furthermore, it enables the calculation of energy- and time-resolved currents, facilitating the modeling of heat flows in quantum systems and energy- and time-resolved photoemission experiments, all at significantly reduced computational cost.

In this work, we propose and study a double Kondo lattice model which hosts robust superconductivity. The system consists of two identical Kondo lattice models in two separate layers, with Kondo coupling $J_K$ within each layer, while the localized spin moments are coupled together via an interlayer on-site antiferromagnetic spin coupling $J_{\perp }$. We consider the strong $J_{\perp }$ limit, wherein the local moments tend to form rung singlets and are thus gapped. However, the Kondo coupling $J_K$ transmits the interlayer entanglement between the local moments to the itinerant electrons. Consequently, the itinerant electrons experience a strong interlayer antiferromangetic spin coupling and form strong interlayer pairing, which is confirmed through numerical simulation in one dimensional system. Experimentally, the $J_K\to -\infty$ limits of the model describes the recently found bilayer nickelate ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, while the $J_K>0$ side can be realized in tetralayer optical lattice of cold atoms. Two extreme limits, $J_K\to -\infty$ and $J_K\to +\infty$ limit are shown to be simplified to a bilayer type II t-J model and a bilayer one-orbital t-J model, respectively. Thus, our double Kondo lattice model offers a unified framework for nickelate superconductor and tetralayer optical lattice quantum simulator upon changing the sign of $J_K$. We highlight both the qualitative similarity and the quantitative difference in the two sides of $J_K$. Finally, we discuss the possibility of a symmetric Kondo breakdown transition in the model with a symmetric pseudogap metal corresponding to the usual heavy Fermi liquid.

Spontaneous intervalley coherence is suspected in several different graphene multilayer systems, but is difficult to confirm because of a paucity of convenient experimental signatures. Here, we suggest that magnetoconductance features associated with quantum corrections to Drude conductivity can serve as a smoking gun for intervalley coherence that does not break time-reversal symmetry. In this class of ordered multilayers quantum transport corrections can produce weak localization or weak antilocalization, depending on whether the valley order belongs to the orthogonal or symplectic symmetry class. Our analysis motivates low-temperature weak-field magnetoresistance measurements in graphene multilayers in which time-reversal invariant intervalley coherent order is conjectured.