We have grown single crystals of mixed-valence pseudobrookite titanate ${\mathrm{MgTi}}_2{\mathrm{O}}_5-{\mathrm{Ti}}_3{\mathrm{O}}_5$ systems (${\mathrm{Mg}}_{1-x}{\mathrm{Ti}}_{2+x}{\mathrm{O}}_5$) and have systematically investigated their physical properties as a novel canonical mixed-valence system which shows a successive change from the nonmagnetic band insulating phase (${\mathrm{MgTi}}_2{\mathrm{O}}_5$) to the charge-ordered spin-singlet dimer phase (${\mathrm{Ti}}_3{\mathrm{O}}_5$). In comparison with ${\mathrm{Al}}_{1-x}{\mathrm{Ti}}_{2+x}{\mathrm{O}}_5$ [R. Takahama et al., Phys. Rev. Mater. 4, 074401 (2020)], we have clarified that, in the lower $x$ region of ${\mathrm{Mg}}_{1-x}{\mathrm{Ti}}_{2+x}{\mathrm{O}}_5$, the physical properties are greatly affected by the change of electron density with a variation of $x$ and we have found that the doped electrons enhance the optical conductivity along a Ti-Ti ladder direction. However, we have also clarified that, in the higher $x$ region, almost all ${\mathrm{Ti}}^{3+}$ ions produced by an increase in $x$ contribute not to an increase in itinerant electrons but to a development of ${\mathrm{Ti}}^{3+}-{\mathrm{Ti}}^{3+}$ dimer correlation. These observations indicate a crossover from the conductance along the ladder direction to a nontrivial conductance under the background of the dimer correlation.

Motivated by growing interest in atomically thin van der Waals magnetic materials, we present an ab initio theoretical study of the dependence of their magnetic properties on the electron/hole density $\rho$ induced via the electrical field effect. By focusing on the case of monolayer ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (a prototypical 2D Ising ferromagnet) and employing a hybrid functional, we first study the dependence of the gap and effective mass on the carrier concentration $\rho$. We then investigate the robustness of magnetism by studying the dependencies of the exchange couplings and magneto crystalline anisotropy energy (MAE) on $\rho$. In agreement with experimental results, we find that magnetism displays a bipolar electrically-tunable character, which is, however, much more robust for hole ($\rho >0$) rather than electron ($\rho <0$) doping. Indeed, the MAE vanishes for an electron density $\rho \approx -7.5\times {10}^{13}\mathrm{e}\times {\mathrm{cm}}^{-2}$, signaling the failure of a localized description based on a Heisenberg-type anisotropic spin Hamiltonian. This is in agreement with the rapid increase of the coupling between fourth-neighbor atoms with increasing electron density.

We show that steady-state expectation values predicted by the universal Lindblad equation (ULE) are accurate up to bounded corrections that scale linearly with the effective system-bath coupling $\mathrm{\Gamma }$ (second order in the microscopic coupling). We also identify a near-identity, quasilocal “memory-dressing” transformation, used during the derivation of the ULE, whose inverse can be applied to achieve relative deviations of observables that generically scale to zero with $\mathrm{\Gamma }$, even for nonequilibrium currents whose steady-state values themselves scale to zero with $\mathrm{\Gamma }$. This result provides a solution to recently identified limitations on the accuracy of Lindblad equations, which highlighted a potential for significant relative errors in currents of conserved quantities. The transformation we identify allows for high-fidelity computation of currents in the weak-coupling regime, ensuring thermodynamic consistency and local conservation laws, while retaining the stability and physicality of a Lindblad-form master equation.

Two-dimensional (2D) materials have emerged as a fertile ground displaying various fascinating properties, such as nontrivial band topology and exotic correlated states, which can be feasibly tuned by available means (e.g., electrostatic gating, stacking, sliding, and twist). Due to the scarcity of ever-experimentally synthesized 2D materials, large-scale target searches on 2D materials have been hindered. Here we focus our attention on all linear band crossings (LBCs) at high-symmetry points, lines, and planes that can be realized in the 80 layer groups (LGs) with (without) time-reversal symmetry using single-valued and double-valued representations (corresponding to spinless and spinful bands, respectively), and list all corresponding $k·p$ models expanded up to the first order of $\vec{q}=(q_x,q_y)$ (measured from the LBC). The relations of symmetry-related LBCs are explicitly provided, useful in studies on valley degrees of freedom, which have been overlooked in previous classifications on band crossings by space and layer groups to our knowledge. The results of 17 LGs, as wallpaper groups, can be applied to surfaces and interfaces of 3D materials or 2D materials grown on substrates. By exhaustive tabulation on all LBCs in the 80 LGs, we highlight LGs hosting LBCs that are enforced to exist necessarily. These LGs are then applied to categorize the 6351 2D structures in the 2D materials database (2DMatPedia) to 1707 (3035) materials necessarily hosting coexisting spinful and spinless LBCs (only necessarily hosting spinful LBCs). We also perform first-principles calculations on 66 selected 2D materials to identify the LBCs at high-symmetry points quantitatively in electronic and phononic bands. We take ${\mathrm{Zr}}_2{\mathrm{HBr}}_2$ crystallized in LG 15 to demonstrate the coexisting electronic Dirac nodal point and phononic Weyl nodal line emanating from a Weyl LBC. The group-theoretical results are expected to guide searches and designs for materials realization for a target LBC in 2D materials, coexisting LBCs carried by different types of excitations, and can also be applied to study the evolution of topological band crossings by external perturbations in the future.

The intrinsic low thermal conductivity of $A^{\mathrm{IV}}{B}^{\mathrm{VI}}$ thermoelectric materials has been widely accepted as being closely related to specific chemical bonding or electronic states, for example, resonant bonding, the lone-pair effect, and metavalent bonding. These concepts have different characteristics of localized or delocalized electronic state mechanisms; i.e., resonant bonding corresponds to localized electronic states, the lone-pair effect is correlated with delocalized $ns$ electronic states, and metavalent bonding is characterized by the competition between localized and delocalized electronic states. It seems that those concepts are contradictory in describing $A^{\mathrm{IV}}{B}^{\mathrm{VI}}$ materials such as GeTe, SnTe, and PbTe simultaneously. Meanwhile, the direct connection between electrons, lattice vibration, and low thermal conductivity is still unclear. Herein, differently from most of the existing works, we focus on how electronic states couple with lattice vibration in the concept of the pseudo-Jahn-Teller effect. Then we propose a general theoretical interpretation (orbital-lattice coupling) to describe the intimate relationship between electronic states and ultralow lattice thermal conductivity in thermoelectric materials or any other strong anharmonic systems. Taking the classical thermoelectric materials (GeTe, SnTe, and PbTe) and the typical ionic crystal NaCl, all with high-symmetry rocksalt structure, as examples, we reveal that the electronic states of $A^{\mathrm{IV}}{B}^{\mathrm{VI}}$ materials tend to spontaneously break their lattice symmetry to avoid degeneracy. Afterwards, the dynamic charge transfer and electronic orbital overlapping under atomic distortion lower the total energy, effectively. The coupled electronic orbitals are therefore linked to lattice instability. Our results build a direct bridge between electrons and lattice, thus providing an important insight into the combination of novel electronic properties and inherent low thermal conductivity, which is general in understanding thermoelectric properties.

Hole densities in group-V (P, As, and Sb) doped CdTe typically fall below ${10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$ although sufficient group-V dopants are incorporated. Previous theoretical studies suggested that the formation of AX centers compensates the acceptors, thereby limiting $p$-type doping. However, recent calculations including spin-orbit coupling effects suggest that AX centers are unstable and thus cannot hinder $p$-type doping. Therefore, the origin of the hole density pinning issue in CdTe remains elusive. Our first-principles calculations, incorporating spin-orbit coupling, coupled with detailed balance simulations, reveal that hole doping in CdTe remains significantly limited despite the instability of the AX centers. This limitation stems from the self-compensation driven by the native vacancies and the band-edge excitations induced by free carriers. Additionally, we find that As is the most favorable dopant among group-V dopants due to its relatively low formation energy and shallow transition level. Our understanding of the hole-limiting mechanism is important for improving the dopability of CdTe solar cells. Moreover, our analysis of the band-edge excitations is critical for describing the defect properties in semiconductors.

Understanding the numerous crucial properties of Dirac crystals, such as their thermal conductivity, necessitates the use of models that consider the interaction between Dirac electrons and persistent acoustic phonons in which the oscillation frequency $\omega$ depends on the phonon wave vector $q$ and is therefore dispersive. It is commonly assumed that the exceptionally high thermal conductivity of two-dimensional (2D) Dirac crystals is due to the near ideality of their phonon quantum gasses with undesired limitations originating from phenomena such as electron-phonon ($e$-ph) interactions. Electrons transferred to Dirac crystals from metal nanoparticles through doping have been shown to affect and limit the thermal conductivity of Dirac crystals due to $e$-ph interactions at distances up to several microns from the nanoparticle. Notably, the $e$-ph thermal conductivity is directly linked to the phonon scattering rate, demonstrating a proportional relationship. Customarily, when calculating the phonon scattering rate, it is common to overlook phonons with short-dispersive wavelengths since in metals $q$ is significantly smaller than the Fermi surface dimensions. However, this approach proves insufficient for analyzing 2D Dirac crystals. Furthermore, the in-plane phonon scattering rate is calculated up to the first order of magnitude consisting of two electrons and one phonon, i.e., three-particle interaction. In these calculations, only processes involving the decay of an electron and phonon, leading to the creation of a new electron (EP-E*), are considered. However, processes that involve the decay of an electron and the creation of a new electron and phonon (E-E*P*) are not taken into consideration. In this paper, we present an accurate expression for the phonon scattering rate and the $e$-ph thermal conductivity in 2D Dirac crystals for in-plane phonons considering phonons with short-dispersive wavelengths. We further demonstrate that even at room temperature, when calculating the phonon scattering rate and $e$-ph thermal conductivity, in the case of first-order $e$-ph interactions, the E-E*P* process assumes significance. In the end, we show the importance of incorporating second-order $e$-ph interactions, particularly the (EP-E*P*) interaction involving the decay of an electron and phonon and the creation of a new pair for in-plane phonons, when determining the phonon scattering rate and $e$-ph thermal conductivity at high temperatures and low Fermi energies. This four-particle interaction process proves significant in accurately characterizing these properties.

We propose a physical principle for implementation of controllable interactions of excitons in two-dimensional semiconductors. The key ingredients are tightly bound biexcitons and in-plane anisotropy of the host structure due to, e.g., a uniaxial strain. We show that anisotropy-induced splitting of the radiative exciton doublet couples the biexciton state to continua of exciton scattering states. As a result, two-body elastic scattering of excitons may be resonantly amplified when energetically tuned close to the biexciton by applying a transverse magnetic field or tuning the coupling with the microcavity photon mode. At the resonance excitonic fields may undergo quantum reaction of fusion. The fusion and a reverse reaction of biexciton disintegration may be used for efficient generation of squeezed light. For “bright” excitons in the absence of a microcavity, we predict giant molecules (Feshbach dimers) which can be obtained from a biexciton via rapid adiabatic sweeping of the magnetic field across the resonance. The molecules possess nontrivial entanglement properties. Our proposal holds promise for the strongly correlated photonics and quantum chemistry of light.

In a theoretical study, we investigate the spin dynamics of interacting nitrogen-vacancy (NV) centers and quadrupolar $I=3/2$ nuclear spins, specifically ${}^{11}\mathrm{B}$ spins in hexagonal boron nitride (h-BN) nanosheets located near the microdiamond surface. We demonstrate the possibility of obtaining external spin-polarization by magnetic-field sweeps across the level anticrossings around zero field. To achieve this, we leverage crystal strains to establish a polarization transfer mechanism that remains robust against variations in NV orientation, crystal strain inhomogeneity, and electron-nuclear effective couplings. These results pave the way for hyperpolarization of spins in nanomaterials near the diamond surface without experiencing polarization loss to intrinsic nuclear spin-1/2 species, such as ${}^{13}\mathrm{C}$ and ${}^1\mathrm{H}$ nuclear spins in diamond. The ${}^{11}\mathrm{B}$ spins in h-BN nanosheets, with their extended relaxation time and large surface area, present a promising alternative for relayed nuclear polarization to the liquid phase and for the development of quantum simulators based on surface nuclear spins.

Single ferromagnetic material generates transverse spin currents via in-plane charge current application and can exert spin-orbit torques (SOTs) on its magnetization, namely, self-induced SOTs, when the inversion symmetry in the bulk and/or interface is broken. In this Letter, we experimentally demonstrate that introducing a vertical gradient of magnetization within a ferromagnet is an effective way to generate self-induced SOTs. Self-induced SOTs in Ni-Fe alloys with a magnetization gradient are studied using spin-torque ferromagnetic resonance. It is found that the gradient direction of magnetization in the Ni-Fe layer can control not only the magnitude of the SOTs but even the sign of the torques. We propose a model to analyze self-induced SOTs and reveal that the asymmetric SOT profiles within a ferromagnet due to spatial variation of the magnetic moment play a dominant role in generating efficient self-induced SOTs. Our findings provide an alternative perspective on understanding the mechanism of generating self-induced SOTs and how to enhance their torque efficiency without the use of heavy metal elements.

The non-Hermitian skin effect refers to the accumulation of eigenstates near the boundary in open boundary lattice models, which can be systematically characterized using the non-Bloch band theory. Here, we apply the non-Bloch band theory to investigate the stochastic reaction-diffusion process by mapping it to a non-Hermitian Kitaev chain. We exactly obtain the open boundary spectrum and the generalized Brillouin zone and identify a robust zero mode arising from the non-Bloch topology. Notably, distinct from its Hermitian counterpart in the quantum context, the zero mode supports anomalous dynamical crossover in the Markov process. We quantitatively demonstrate the intriguing dynamical effects through the spectral decomposition of the Hamiltonian on the non-Bloch eigenstates, and we confirm our findings by conducting stochastic simulations with high accuracy. Our study highlights the significant role and the general role of non-Bloch topology in nonequilibrium dynamics.

Molten salts could play an important role in energy storage, in the form of liquid batteries, and heat storage for solar and nuclear power. However, their widespread application is hindered by a limited understanding of the mechanisms by which they corrode metallic containers. This knowledge gap necessitates atomic-scale studies on salt-metal interactions. Molecular dynamics simulations are well suited for such research but require interatomic potential capable of accurately modeling both ionic and neutral states of salt and metal elements. Herein, we developed a moment tensor potential (MTP) with this capability, employing a small-cell training approach. The proposed MTP is compact: It is described by 449 parameters fitted on 609 configurations; 30% of these are one- or two-atom configurations. Extensive testing of our MTP points to a high-fidelity description of the structural and transport properties of solid/liquid Na, gaseous Cl, and crystalline/molten NaCl. Furthermore, we applied this MTP to calculate the standard reduction potential and solubility limit of Na in molten NaCl, achieving results that closely align with experimental and ab initio simulation data. This approach offers a robust framework for exploring the electrochemical and physical properties of molten salts across various compositions and solutes.

In this paper, the interplay between non-Hermiticity, quasidisorder, and repulsive interaction is studied for hard-core bosons confined in a one-dimensional optical lattice, where non-Hermiticity is induced by the nonreciprocal hoppings and the on-site gain and loss breaking the time-reversal symmetry. Although the energy spectra of the static system are fully complex, with the evolution of the initial state, the real part of the expectation value of the Hamiltonian under the time-evolved wave function changes stably. By means of the entanglement entropy and its dynamical evolution, as well as the inverse participation ratio, the many-body localization (MBL) is found to play the key role in the stability of the dynamical behavior of the real part of the expectation value, independent of whether the spectrum of the static Hamiltonian is real or complex. In the delocalization phase, the dynamical evolution of the real part of the expectation value is unstable. Meanwhile, the nearest-neighbor level spacings statistics shows the MBL transition accompanied by the transition from the Ginibre distribution to the complex Poisson distribution, different from the one in the time-reversal invariant system. In addition, the dynamical stability of the real part of the energy and the MBL transition can be characterized by the winding number, indicating that the MBL transition and the topological transition occur simultaneously, and the realization of the Hamiltonian is discussed.

We investigate the ultrafast carrier and phonon dynamics in the ferrimagnetic semiconductor ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ using time-resolved optical pump-probe spectroscopy. Our results reveal that the electron-phonon thermalization process with a subpicosecond timescale is prolonged by the hot-phonon bottleneck effect. We identify the subsequent relaxation processes associated with two nonradiative recombination mechanisms, i.e., phonon-assisted electron-hole recombination and defect-related Shockley-Read-Hall recombination. Temperature-dependent measurements indicate that all three relaxation components show large variation around 175 and 78 K, which is related to the initiation of spin fluctuation and ferrimagnetic order in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$. In addition, two pronounced coherent optical phonons are observed, in which the phonon with a frequency of 3.7 THz is attributed to the $A_{1g}$ mode of Te precipitates. Applying the strain pulse propagation model to the coherent acoustic phonons yields a penetration depth of 506 nm and a sound speed of 2.42 km/s in ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$. Our results develop understanding of the nonequilibrium properties of the ferrimagnetic semiconductor ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ and also shed light on its potential applications in optoelectronic and spintronic devices.

We find that, in the mesoscopic regime, modification of the material's surface can induce an extensive change of the material's magnetic moment. In other words, perturbation of order-$N^2$ atoms on the surface of a three-dimensional solid can change the magnetic moment proportionally to $N^3$. When the solid's surface is perturbed, it triggers two changes in the magnetization. One arises from variations of the electron wave function and energy, while the other arises from a modification in the kinetic angular momentum operator. In the macroscopic regime of our model, these two bulk effects cancel each other, resulting in no impact of the surface perturbation on the magnetization, consistent with prior work. In the mesoscopic regime, we find a departure from this behavior, as the cancellation of two terms is not complete.

We use strong-coupling Eliashberg theory to study the competition of separate superconducting orders at low temperatures. Specifically, we study magnon-mediated superconductivity in a trilayer heterostructure with a thin normal metal between two antiferromagnetic insulators. Spin-triplet $p$-wave, spin-triplet $f$-wave, and spin-singlet $d$-wave superconducting gaps have been predicted to occur close to the critical temperature for the superconducting instability. The gap symmetry with the largest critical temperature depends on parameters in the model. We confirm that the same gap symmetries appear at any temperature below the critical temperature. Furthermore, we show that the temperature can affect the competition between the different superconducting orders. In addition, we consider time-reversal-symmetry-breaking, complex linear combinations of candidate pairings, such as chiral $p$-, $f$-, and $d$-wave gaps, as well as $p_x+i{f}_y$-wave gaps. We find indications that some of these time-reversal-symmetry-breaking, nodeless gaps offer a greater condensation energy than the time-reversal symmetric gaps. This indicates that superconducting states with spontaneously broken time-reversal symmetry and nontrivial topology may be preferred in this system.

Identifying the nature of domain structures within crystals is crucial for researching $A_2^{\mathrm{III}}{B}_3^{\mathrm{VI}}$ compounds aimed at microelectronics applications. This paper provides a credible and convincing set of microscopic data on the domain structure formed in the high-temperature phase of an $\mathrm{I}{\mathrm{n}}_2\mathrm{T}{\mathrm{e}}_3$ crystal (zinc-blende type structure), resolving a long-standing debate regarding the domain boundaries and structural vacancies in this crystal family. The crystal exhibited a finely subdivided matrix comprising parallelogram-shaped domains, each bounded by {111} planes. The domain boundaries were identified as a type of 180° domain wall, across which the crystal structure underwent a unit-cell inversion between parallel and antiparallel states. The domain interiors comprised a series of regionally varied structural units according to a statistically random, but a systematic spatial distribution pattern of structural vacancies induced along the [001] type direction. Consequently, the local structure gradually reversed between the two opposite corners defined along the longitudinal axis of each domain.

In this paper, we investigate periodically driven open quantum systems within the framework of Floquet-Lindblad master equations. Specifically, we discuss Lindblad master equations in the presence of a coherent, time-periodic driving and establish their general spectral features. We also clarify the notions of transient and nondecaying solutions from this spectral perspective, and then prove that any physical system described by a Floquet-Lindblad equation must have at least one physical nonequilibrium steady state (NESS), corresponding to an eigenoperator of the Floquet-Lindblad evolution superoperator ${\mathcal{U}}_F$ with unit eigenvalue. Since the Floquet-Lindblad formalism encapsulates the entire information regarding the NESS, it in principle enables us to obtain nonlinear effects to all orders at once. The Floquet-Lindblad formalism thus provides a powerful tool for studying driven-dissipative solid-state systems, which we illustrate by deriving the nonlinear optical response of a simple two-band model of an insulating solid and comparing it with prior results established through Keldysh techniques.

The existence of one-dimensional strings of local excitations represents an intriguing aspect of the properties of strongly correlated topological quantum matter. Here, we study the impact of these one-dimensional string phases (1D SPs) on magnetization dynamics in Santa Fe ice lattice using micromagnetic simulations. The presence of 1D SPs results in the creation of parallel configurational anisotropic doublet or triplet pairs of spin wave (SW) modes, attributed to distinct stray field distributions. These modes induce the formation of an anticrossing gap in both reciprocal and nonreciprocal SW dispersion that can be experimentally measured.