In strange metals, a strong and anomalous scattering effect exists and increases linearly with temperature. In FeSe, we observed that the temperature dependence of resistivity exhibits non-Fermi-liquid behavior in two regions below and above a critical pressure, $p_{\text{c}}\sim 2$ GPa. As pressure increases, a transition from quadratic to nonsaturating magnetoresistance is observed, with a distinct crossover between these two behaviors indicated by $B^{*}$ in the derivative analysis. After subtracting the quadratic term from the magnetoresistance, the residual magnetoresistance clearly exhibits an $H$-linear behavior. Additionally, two segments of $H$-linear magnetoresistance appear with increasing pressure, each arising from distinct origins. Notably, the two $H$-linear magnetoresistances exist and develop within the strange metal states. These results suggest that $H$-linear magnetoresistance is in strong correlation with the strange metal state, which may affect superconductivity in FeSe under pressure. Our study provides valuable insights into the strange metal state and clues for underlying unconventional superconductivity.

Starting from microscopic Gor'kov theory we study the effect of disorder on the superconducting phase transition in proximity coupled superconductor-altermagnet (S/AM) bilayers. Potential scattering on point impurities is shown to result in a strong enhancement of the superconducting critical temperature ($T_c$) due to the averaging of the momentum-dependent exchange field induced by the altermagnetic layer. In the presence of magnetic impurities the $T_c$ value may even become a nonmonotonic function of impurity concentration due to the interplay between nonmagnetic and magnetic scattering mechanisms. The experimental observation of such peculiar effects of disorder on superconductivity can facilitate the detection of altermagnetic ordering and the use of S/AM structures in superconducting spintronics.

We generalize a recently proposed theory for atomic resolution phonon spectroscopy in scanning transmission electron microscopy, which explicitly includes the dependence of the spectrum on probe position, to apply to thick specimens, where probe channeling is important. We also take into account anisotropy due to atoms of the same species exhibiting different vibrational modes. This approach considers the contribution from individual atoms to the spectrum. Simulations to explore the dependence of the spectrum, in particular of its shape, on the probe position and thickness are carried out for the perovskite ${\mathrm{SrTiO}}_3$, which has recently been experimentally investigated, and computational efficiency is discussed.

The Dzyaloshinskii-Moriya interaction (DMI) has explained successfully the weak ferromagnetism in some centrosymmetric antiferromagnets. However, in recent years, it was generally claimed that the DMI is not effective in such systems. We reconciled these views by separating the conventional antiferromagnets from altermagnets. Altermagnets are collinear magnets having zero magnetization preserved by crystal symmetries in the nonrelativistic limit. The spin-up and spin-down sublattices are connected by rotation (proper or improper and symmorphic or nonsymmorphic). Consequently, the system shows even-parity wave spin order in the k-space lifting the Kramer's degeneracy in the nonrelativistic band structure leading to unconventional magnetism. The staggered DMI is one of the mechanisms which can create weak ferromagnetism or weak ferrimagnetism in centrosymmetric and noncentrosymmetric altermagnets while it is not effective in conventional antiferromagnets. Once the spin-orbit coupling is included in an altermagnetic system (where the time-reversal symmetry is broken) with staggered DMI, the components of spin moments of the two sublattices along the Néel vector are antiparallel but the other two spin components orthogonal to the Néel vector can be null, parallel or antiparallel. In cases where we have different bands showing parallel and antiparallel spin components at the same time, the magnetic order results in weak ferrimagnetism. The altermagnetic compounds can host weak ferromagnetism, weak ferrimagnetism or zero magnetization. Restricted to the altermagnet with two atoms and staggered DMI, the Hall vector is orthogonal to the Néel vector in the case of weak ferromagnetism and weak ferrimagnetism with a magnetic component proportional to the DMI. When we keep the crystal symmetry, the NiAs phase and the wurtzite phase have the same weak ferromagnetism. We find a sign change of the magnetization, and possibly of the anomalous Hall effect, as a function of the band filling and Néel vector. We describe the dependence of the weak ferromagnetism on the charge doping.

We develop an approach for characterizing nonlocal quantum correlations in spin systems with exactly or nearly degenerate ground states. Starting with linearly independent degenerate eigenfunctions calculated with exact diagonalization we generate a finite set of their random linear combinations with Haar measure, which guarantees that these combinations are uniformly distributed in the space spanned by the initial eigenstates. Estimating the von Neumann entropy of the random wave functions helps to reveal previously unknown features of the quantum correlations in the phases with degeneracy of the ground state. For instance, spin spiral phase of the quantum magnet with Dzyaloshinskii-Moriya interaction is characterized by the enhancement of the entanglement entropy, which can be qualitatively explained by the changes in behavior of two- and three-spin correlation functions. To establish the connection between our theoretical findings and real experiments we elaborate on the problem of estimating observables on the basis of the single-shot measurements of numerous degenerate eigenstates. By the example of solving a simple Ising model it is shown that digital quantum simulations performed with quantum computers can provide accurate description of the entanglement of the degenerate systems even in the presence of noise. We also discuss the ground-state properties of degenerate fermionic and bosonic models, which can be useful for theoretical analysis of the experiments with ultracold atoms.

We investigate the dynamical lattice behavior of ultracold bosonic atoms loaded into an optical lattice with two-body dissipation. In the Mott insulating limit, the resulting system can emulate a variety of spin-1/2 non-Hermitian Heisenberg models such as the non-Hermitian Ising, XX, and XXZ models. These different spin models are implemented by tuning the parameter spaces of the hopping amplitude, the loss rates of atoms, and on-site interaction of strength. Moreover, utilizing the improved non-Hermitian density-matrix renormalization group (NH-DMRG) algorithm, we present the correlation functions, Bell nonlocality $\left(\mathcal{B}\right)$, and quantum Fisher information (QFI) of the non-Hermitian spin systems, where the QFI inversion and order-of-magnitude reduction demonstrate a high sensitivity to the non-Hermitian system parameters. In addition, there exists a parameter space where non-Hermitian and Hermitian systems exhibit the same phase transition behavior as indicated by QFI and $\mathcal{B}$. Our results open a new avenue toward the quantum simulation and observation of non-Hermitian many-body systems.

The supersolid phase of a dipolar Bose-Einstein condensate has an intriguing excitation spectrum displaying a band structure. Here, the dressing of an impurity in a one-dimensional dipolar supersolid with the excitations of the supersolid is studied. The ground-state energy of the supersolid polaron is calculated using a variational path integral approach, which obtained accurate results for other polaron systems within the Bogoliubov and Fröhlich approximations. A divergence is observed at the superfluid-supersolid phase transition. The polaron radius is also computed, showing that, as a function of impurity-atom interactions, the polaron can become localized to a single droplet, behaving like a small solid-state polaron.

We present the observation of photon number splitting (PNS) in a superconducting transmon qubit with different microwave photon sources. Such PNS is not limited to the lower energy levels and can also be observed in the higher energy levels of a superconducting transmon qubit. The sideband transition, Autler-Townes splitting, and the absence of PNS are demonstrated under certain conditions. Furthermore, we investigate the PNS induced by the nonclassical microwave photons emitted from a dual-mode Josephson parametric amplifier, which can operate in either the single mode or dual mode. The evolution from the dual-mode squeezing to the self-oscillation light emission is demonstrated. Our results provide a useful reference for future investigations, such as the exploration of more nonlinear effects in strongly driven circuit QED systems, the manipulation of quantum states, and the characterization of the quantum microwave source by utilizing PNS.

We propose a tridiagonalization approach for non-Hermitian random matrices and Hamiltonians using singular value decomposition (SVD). This technique leverages the real and non-negative nature of singular values, bypassing the complex eigenvalues typically found in non-Hermitian systems. We analyze the tridiagonal elements, namely the Lanczos coefficients and the associated Krylov (spread) complexity, appropriately defined through the SVD, across several examples, including Ginibre ensembles and the non-Hermitian Sachdev-Ye-Kitaev model. We demonstrate that in chaotic cases, the complexity exhibits a distinct peak due to the repulsion between singular values, a feature absent in integrable cases. Using our approach, we analytically compute the Krylov complexity for two-dimensional non-Hermitian random matrices within a subset of non-Hermitian symmetry classes, including time-reversal, time-reversal${}^{†}$, chiral, and sublattice symmetry.

Non-Hermitian phenomena, such as exceptional points, non-Hermitian skin effects, and topologically nontrivial phases have attracted continued attention. In this work, we reveal how interactions and nonreciprocal hopping could collectively influence the behavior of two interacting bosons on quasiperiodic lattices. Focusing on the Bose-Hubbard model with Aubry-André-Harper quasiperiodic modulations and hopping asymmetry, we discover that interactions could enlarge the localization transition point of the noninteracting system into an intermediate mobility edge phase, in which localized doublons formed by bosonic pairs can coexist with delocalized states. Under the open boundary condition, the bosonic doublons could further show non-Hermitian skin effects, realizing doublon condensation at the edges, and their direction of skin localization can be flexibly tuned by the hopping parameters. A framework is developed to characterize the spectral, localization, and topological transitions accompanying these phenomena. Our work advances the understanding of localization and topological phases in non-Hermitian systems, particularly in relation to multiparticle interactions.

Recent advancements in laser technology have spurred growing interest in nonlinear and nonequilibrium phenomena. Here we investigate the geometric aspects of quantum tunneling and the nonreciprocal response, particularly focusing on the shift vector, in noncentrosymmetric insulators under a strong DC electric field. In insulators under a strong electric field, electrons undergoing Bloch oscillations interfere with each other by passing through different paths via Landau-Zener tunneling. We found that the interference effect due to multitunneling causes the oscillating nonreciprocal response that is significantly amplified with increasing electric field intensity. We also clarified the role of the shift vector in the interference conditions through an analysis of the nonequilibrium steady state. These results will contribute significantly to advancing a systematic understanding of quantum geometric effects in the nonperturbative regime.

We present the study of low-frequency dynamics of ${\mathrm{LiCu}}_3{\mathrm{O}}_3$—an antiferromagnet on a strongly depleted square lattice. The crystal structure contains two types of square planes with different ${\mathrm{Li}}^{+}\to {\mathrm{Cu}}^{2+}$ substitution rates (20% and 40%). Recent nuclear magnetic resonance (NMR), magnetization measurements and neutron scattering experiments revealed the occurrence of antiferromagnetic order at $T_{c1}=123\mathrm{K}$ and the change of the magnetic state at $T_{c2}\approx 30\mathrm{K}$. Electron spin resonance (ESR) intensity measurements confirmed that at $T_{c1}=123\mathrm{K}$ a magnetic phase transition of only a part of the sample occurs, while the rest of the sample remains paramagnetic. Supposedly, the order occurs in the magnetic planes with higher concentrations of magnetic ions. The ratio of ESR intensity values before and after the transition is in agreement with this suggestion. In the studied frequency and temperature range $(18–140\mathrm{GHz},$ $30–300\mathrm{K})$ only one electron paramagnetic resonance (EPR) branch was observed. At temperatures $T<30\mathrm{K},$ the ESR branch acquires a gap which gradually increases with temperature decrease. The behavior of the observed gap in the magnetic resonance spectra indicates that this gap is, in fact, a pseudogap of exchange nature associated with finite-sized clusters of magnetic ions.

We present a comprehensive study of the structural, magnetic, and thermodynamic properties, as well as the adiabatic demagnetization refrigeration (ADR) performance of ${\mathrm{NaGdP}}_2{\mathrm{O}}_7$. Although ${\mathrm{NaGdP}}_2{\mathrm{O}}_7$ exhibits antiferromagnetic ordering at a Néel temperature of $T_{\mathrm{N}}=570\mathrm{mK}$ in zero field, ADR experiments achieved a minimum temperature of 220 mK starting from $T=2\mathrm{K}$ under an applied magnetic field of ${\mu }_{0}H=5\mathrm{T}$. The warm-up time back to $T=2\mathrm{K}$ exceeds 60 hours with 9 hours below $T=400\mathrm{mK}$ underscoring the potential of ${\mathrm{NaGdP}}_2{\mathrm{O}}_7$ as an efficient precooling stage in double-stage ADR systems. We show that the spin lattice of ${\mathrm{NaGdP}}_2{\mathrm{O}}_7$ can be seen as a network of ferromagnetic spin chains with antiferromagnetic interchain couplings and also investigate the influence of antiferromagnetic ordering on the magnetic entropy. We find that the temperature dependence of the entropy plays a more dominant role than its magnetic field dependence in the magnetically ordered state.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is known to induce nonreciprocity in spin wave propagation within the plane of magnetic thin films. This nonreciprocity can be utilized to design spin wave devices such as signal isolators and also provides an important means to quantitatively measure the magnitude of the DMI. Analytical expressions have been developed to estimate the effects of DMI on spin waves by assuming the magnetization dynamics are uniform throughout a film thickness. Here, we relax this assumption and use a so-called atomic layer method to calculate spin wave dispersion relations for not just the uniform thickness mode, but also for the perpendicular standing spin waves (PSSWs). Coupled magnetic equations of motion are written for each atomic plane within a thin film made of one or more materials. The method is tested against known analytic and numerical theories, and is then used to calculate the PSSW frequencies and mode profiles for single and bilayer magnetic films with interfacial DMI applied to the outer interfaces. These calculations show that the PSSW mode frequencies depend nonlinearly on the interfacial DMI strength and on the inverse thicknesses of the magnetic films. We develop an analytic estimate for the lowest two PSSW frequencies to explain this nonlinear dependence. Our method has the advantage that it can be used to calculate spin wave frequencies and depth profiles in any thin-film magnetic stack with atomic-scale variations in magnetic parameters, such as multiple DMI interfaces, or gradient properties.

Electrides usually have topological properties and high electrical conductivity, so searching for superconducting electrides over a wide range of temperature and pressure is a hot topic in condensed matter physics. Based on high-throughput structure search and the first-principles calculations, we predict that $2H\text{−}\mathrm{V}{\mathrm{N}}_2$ is a thermodynamically and dynamically stable electride with the superconducting transition temperature ($T_c$) of 51 K at ambient pressure. From the calculations of the electronic localization function, it is found that the excess unpaired electrons of the V atoms are likely to exist in the lattice vacancies as interstitial electrons, which makes $2H\text{−}\mathrm{V}{\mathrm{N}}_2$ an electride. On the other hand, the existence of irreducible representation at the vacancies also confirms that $2H\text{−}\mathrm{V}{\mathrm{N}}_2$ is an electride. Moreover, the flat bands, primarily originating from the V-$d$ orbitals, and the nesting of the Fermi surface induce a high density of states. This enhances the electron-phonon coupling (EPC) strength between the V-$d$ orbitals and the phonon modes associated with the vibrations of N atoms. The calculated EPC strength of $2H\text{−}\mathrm{V}{\mathrm{N}}_2$ electride at ambient pressure is 1.06, and the superconducting gap vanishes at 51 K. We find that $2H\text{−}\mathrm{V}{\mathrm{N}}_2$ electride has a $T_c$ of 51 K at ambient pressure, which gives a practicable method to connect the superconducting properties and the specificities of electrides in unitary material.

We present a comprehensive theoretical study of interacting and disordered topological phases of coupled Kitaev wires, which may support further realistic applications of Majorana fermions. We develop a variety of analytical, mathematical, and numerical methods for one- and two-coupled wires, associated with a topological marker accessible from real-space correlation functions on the wire(s). We verify the stability of the topological superconducting phase and quantify disorder effects close to the quantum phase transitions, e.g., through two-point correlation functions or using a renormalization group analysis of disorder. We show that the double critical Ising (DCI) phase—a fractional Majorana liquid characterized by a pair of half central charges and topological numbers—is stabilized by strong interactions against disorder, which respects the inversion symmetry between the wires (i.e., parity conservation on each wire). In the presence of an interwire hopping term, the DCI phase turns into a protected topological phase with a bulk gap. We study the localization physics developing along the critical line for weaker interactions.

We propose a non-Hermitian model for Cooper pair splitters in which the process of electron tunneling into electrodes is characterized by non-Hermitian terms. We find that across a broad range of parameters, the energy levels consistently remain real, and coalescing states are always present. The Coulomb repulsion between electrons in a quantum dot affects the order of the coalescing states. This gives rise to two distinct dynamic behaviors: (i) when the initial state is an empty state, the final state supports a nonzero electron-escaping rate; (ii) the electron-escaping rate is zero for a single-electron initial state. In the former case, our exact solutions reveal that the average electron-escaping rate vanishes along a set of hyperbolic curves in the plane of the chemical potentials of the two quantum dots. The stability of the results in the presence of disordered perturbation is also investigated. Our findings pave the way for investigating Cooper pair splitters within the framework of non-Hermitian quantum mechanics.

We theoretically investigated the effects of high-frequency circularly polarized light (CPL) on the electronic properties of ${\beta }_{12}$ borophene system via Floquet engineering. The Floquet indirect electron hoppings enable the phase of the Dirac and triplet fermions in borophene modulated by light irradiation. First, we built the low-energy effective two-band model and three-band model to describe the photon-assisted electron behaviors analytically. We found that the gapped Dirac fermions harbor valley-dependent feature as well as the topological insulating phase transition between quantum anomalous Hall (QAH) and quantum valley Hall. Then we employ strong CPL to eliminate the inherent metallic phase in bulk of ${\beta }_{12}$ borophene nanoribbon, which becomes topological QAH insulator. Furthermore, the corresponding local current distributions are plotted in real space with the chiral edge state indicating QAH phase. The results show that the CPL-induced chiral edge states are topologically protected and robust in spite of the choice of parameter models and different edge configurations of the nanoribbons. Apart from developing the analysis of gapped Dirac and triplet fermions in borophene, our results with the semiconducting band and topological insulating phase induced by CPL may pave the way for the realization of ${\beta }_{12}$-borophene-based valleytronic and optoelectronic devices.

We report a large electron-mass enhancement and metal-to-nonmetal transition upon Pr doping in perovskite-type $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{P}{\mathrm{r}}_x\mathrm{Cu}{\mathrm{O}}_3$. With increasing the Pr content $x$ around 0.6, the $\mathrm{LaCu}{\mathrm{O}}_3$-type three-dimensional structure with trivalent Cu ions changes to a quasi-one-dimensional structure with nearly divalent Cu ions, which accompanies significant changes in the electronic properties. Based on the resistivity, optical conductivity, specific heat measurements, and first-principles calculations, we discuss the formation of a nearly localized nonmetallic state stabilized by the hybridization between Cu $3d$, O $2p$, and Pr $4f$ orbitals in the quasi-one-dimensional lattice. The present perovskite-type cuprates offer a unique opportunity to explore novel quantum phases of correlated electrons in a low-dimensional lattice where the spin/charge/orbital degrees of freedom of $A$- and $B$-site ions are entangled.

Treating the trion problem as an effective two-body system with exciton and electron components, we identify component exchange as the reason leading to trion formation. This mechanism can be visualized as a hole that toggles back and forth between two electrons. The coined term component exchange is meant to inform that the repeated change of the electron component of the exciton originates from an interaction that resembles the exchange interaction. We develop a Bethe-Salpeter equation for trions, showing that a bound trion state emerges if the electron and electron of the exciton are distinguishable particles (e.g., having opposite spins or residing in different valleys of the Brillouin zone). Similar to numerical techniques that treat the trion as a three-body problem, the trion Bethe-Salpeter equation yields similar binding energy without using parameters beyond effective masses and dielectric constants.