State-of-the-art approaches to extract transport coefficients of many-body quantum systems broadly fall into two categories: (i) they target the linear-response regime in terms of equilibrium correlation functions of the closed system; or (ii) they consider an open-system situation typically modeled by a Lindblad equation, where a nonequilibrium steady state emerges from driving the system at its boundaries. While quantitative agreement between (i) and (ii) has been found for selected model and parameter choices, also disagreement has been pointed out in the literature. Studying magnetization transport in the spin-$1/2$ XXZ chain, we here demonstrate that at weak driving, the nonequilibrium steady state in an open system, including its buildup in time, can remarkably be constructed just on the basis of correlation functions in the closed system. We numerically illustrate this direct correspondence of closed-system and open-system dynamics, and show that it allows the treatment of comparatively large open systems, usually only accessible to matrix product state simulations. We also point out potential pitfalls when extracting transport coefficients from nonequilibrium steady states in finite systems.

We study the triangular plaquette model (TPM), also known as the Newman-Moore model, in the presence of a transverse magnetic field on a lattice with periodic boundaries in both spatial dimensions. We consider specifically the approach to the ground state phase transition of this quantum TPM (QTPM), or quantum Newman-Moore model, as a function of the system size and type of boundary conditions. Using methods based on cellular automata, we obtain a characterization of the minimum energy configurations of the TPM for numerically accessible tori sizes. For the QTPM, we use these cycle patterns to obtain the symmetries of the model which, we argue, indicate the nature of its quantum phase transition: we always identify it as a first-order phase transition, with the addition of spontaneous symmetry breaking for system sizes which have degenerate classical ground states. For sizes accessible to numerics, we corroborate our findings with exact diagonalization, matrix product states, and quantum Monte Carlo simulations.

The high-pressure (HP) behavior of the tetragonal phase of ${\mathrm{Y}}_{1.8}{\mathrm{Eu}}_{0.2}{\mathrm{Ge}}_2{\mathrm{O}}_7$ has been studied using x-ray diffraction, x-ray absorption spectroscopy, Raman, and photoluminescence spectroscopy techniques. Our studies show pressure-induced amorphization in ${\mathrm{Y}}_{1.8}{\mathrm{Eu}}_{0.2}{\mathrm{Ge}}_2{\mathrm{O}}_7$ accompanied by partial transformation to a crystalline phase above $\sim 14.0$ GPa. Simultaneous high-pressure–high-temperature (HP-HT) experiments confirm the pyrochlore structure of the high-pressure crystalline phase. These structural transformations have been explained in terms of the coordination change at both of the cation sites as observed with x-ray absorption near-edge structure analysis of the Y edge, and extended x-ray absorption fine-structure analysis of the Ge edge. These inferences are also complemented by Raman spectroscopy, where the observed broadening and softening of $\mathrm{Ge}{\mathrm{O}}_4$ tetrahedra stretching Raman modes above 14 GPa are suggestive of distortion and coordination change at Ge sites. High-pressure photoluminescence studies show a gradual evolution to a relatively more symmetric environment, and the emergence of two different site symmetries for ${\mathrm{Eu}}^{3+}$ ions with pressure. At 13.4 GPa, two new emission lines appear at 579.3 and 585.2 nm, which correspond to ${}^{5}D_0\to {}^{7}F_0$ and ${}^{5}D_0\to {}^{7}F_1$ transitions in the transformed phases of the compound. After pressure cycling up to $\sim 24$ GPa, the system undergoes complete amorphization upon pressure release in the case of ambient temperature HP studies, whereas the pyrochlore phase is retained upon pressure release in the case of HP-HT studies. These studies demonstrate the quenchability of higher coordinated crystalline and amorphous phases at ambient conditions in this system depending on the pressure cycling pathway, which may be useful in the study of exotic magnetic phases in this system.

We investigate the $S=1$ antiferromagnetic quantum spin chain with the exchange and single-ion anisotropies in a magnetic field, using numerical exact diagonalization of finite-size clusters, level spectroscopy analysis, and the density matrix renormalization group (DMRG). We find that a translational symmetry broken magnetization plateau possibly appears at the half of the saturation magnetization, when the anisotropies compete with each other. The level spectroscopy analysis gives the phase diagram at half the saturation magnetization. The DMRG calculation presents the magnetization curves for some typical parameters and clarifies the spin structure in the plateau phase.

A recent experiment [X. Ding et al., Nature (London) 615, 50 (2023)] indicates that superconductivity in nickelates is restricted to a narrow window of hydrogen concentration, $0.22<x<0.28$ in ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2{\mathrm{H}}_x$. This reported necessity of hydrogen suggests that it plays a crucial role for superconductivity, as it does in the vast field of hydride superconductors. Using density-functional theory and its extensions, we explore the effect of topotactic hydrogen on the electronic structure and phonon-mediated superconductivity in nickelate superconductors. Our calculations show that the electron-phonon coupling in hydrogen-intercalated nickelates is not strong enough to drive the electron pairing, and thus cannot explain the reported superconductivity.

The recent achievement of room-temperature superconductivity at near-ambient pressure in nitrogen-doped lutetium hydride further boosts enthusiasm for the pursuit of high-temperature superconductors. Transition-metal phosphides (TMPs) have attracted substantial attention due to their fascinating properties encompassing superconductivity. Nevertheless, the superconducting scandium-based phosphides with high scandium concentration are not well comprehended. Towards this end, our work focuses on the rational design of scandium-rich phosphides via the first-principles swarm structure calculations under pressure. Strikingly, several metallic phases, viz., ScP, ${\mathrm{Sc}}_2\mathrm{P}$, and ${\mathrm{Sc}}_3\mathrm{P}$, are unambiguously uncovered wherein P atoms exhibit captivating configurations from ladder, linear chain, and eventually to isolated atom. Further electron-phonon coupling simulations elucidate that $P6/mmm {\mathrm{Sc}}_2\mathrm{P}$, isostructural to $\mathrm{Mg}{\mathrm{B}}_2$, possesses a remarkable superconducting transition temperature, $T_c$, of $\sim 20$ K at 100 GPa, mainly deriving from the large acoustic phonon softening in the low-frequency region. Tetragonal $I4/mmm {\mathrm{Sc}}_3\mathrm{P}$ is revealed to host a high phonon-mediated superconductivity of 20.5 K at 140 GPa, which is chiefly attributed to the significant coupling between the low-frequency softened acoustic and optical phonon modes associated with the Sc-dominated vibrations and Sc $3d$ electronic states around the Fermi energy. This study paves the way for discovering distinguished superconductors in transition metal-based phosphide systems.

Recent studies on disorder-induced many-body localization (MBL) in non-Hermitian quantum systems have attracted great interest. However, the non-Hermitian disorder-free MBL still needs to be clarified. We consider a one-dimensional interacting Stark model with nonreciprocal hoppings having time-reversal symmetry, the properties of which are boundary dependent. Under periodic boundary conditions (PBCs), such a model exhibits three types of phase transitions: the real-complex transition of eigenenergies, the topological phase transition, and the non-Hermitian Stark MBL transition. The real-complex and topological phase transitions occur at the same point in the thermodynamic limit but do not coincide with the non-Hermitian Stark MBL transition, which is quite different from the non-Hermitian disordered cases. By the level statistics, the system transitions from the Ginibre ensemble (GE) to the Gaussian orthogonal ensemble (GOE) to the Possion ensemble with the increase of the linear tilt potential's strength. The real-complex transition of the eigenvalues is accompanied by the GE-to-GOE transition in the ergodic regime. Moreover, the second transition of the level statistics corresponds to the occurrence of non-Hermitian Stark MBL. We demonstrate that the non-Hermitian Stark MBL is robust and shares many similarities with disorder-induced MBL, which several existing characteristic quantities of the spectral statistics and eigenstate properties can confirm. The dynamical evolutions of the entanglement entropy and the density imbalance can distinguish the real-complex and Stark MBL transitions. Finally, we find that our system under open boundary conditions lacks a real-complex transition, and the transition of non-Hermitian Stark MBL is the same as that under PBCs.

We study a fermionic two-band model with the interband transition resonantly coupled to a cavity. This model was recently proposed to explain cavity-enhanced charge transport, but a thorough characterization of the closed system, in particular localization of various excitations, is lacking. In this paper, using exact diagonalization, we characterize the system by its spectrum under various filling factors and variable disorder. As in the Dicke model, the effective light-matter coupling scales with the square root of the system size. However, there is an additional factor that decreases with increasing doping density. The transition from the weak-coupling regime to the strong-coupling regime occurs when the effective light-matter coupling is larger than the electronic bandwidth. Here, the formation of exciton-polaritons is accompanied by the formation of bound excitons. Photon spectral functions exhibit significant weights on the in-gap states between the polaritons, even without disorder. Finally, while the localization of electron-hole excitations in a disordered system is lifted by strong coupling, the same is not true for free charges, which remain localized at strong and even ultrastrong coupling. Based on this finding, we discuss scenarios for the observed enhanced charge transport.

The optical control of crystal structures is a promising route to change physical properties including the topological nature of a targeting material. Time-resolved x-ray diffraction measurements using an x-ray free-electron laser are performed to study the ultrafast lattice dynamics of ${\mathrm{VTe}}_2$, which is known to possess a unique charge-density-wave (CDW) ordering coupled to the topological surface states as a first-order phase transition. A significant oscillation of the CDW amplitude mode is observed at a superlattice reflection as well as Bragg reflections. The frequency of the oscillation is independent of the fluence of the pumping laser, which is prominent in the CDW ordering of the first-order phase transition. Furthermore, the timescale of the photoinduced $1T^{\prime \prime }$ to $1T$ phase transition is not equal to the half period of the CDW amplitude mode, which is typically the case for Peierls insulators.

The electrical detection of spin and orbital angular momentum usually requires the utilization of a ferromagnetic metal (FM). When an angular momentum is generated in a light metal (LM), for example, through the orbital Hall effect, a bilayer system that consists of the LM/FM is commonly used to study the effect. In this work, by studying the magnetoresistance in the zirconium/nickel bilayer, a typical LM/FM bilayer structure, we observe a negative angular dependent magnetoresistance (ADMR) in the $\beta$ scan direction. Through analysis, we exclude the contributions from the bulk effects and their combinations, such as the spin Hall effect and orbital Hall effect in zirconium, and the anomalous Hall effect in nickel. Instead, we attribute the negative ADMR to the interfacial spin-orbit coupling, which is caused by the imbalanced spin transmission and reflection at the interface. Our observation highlights the nontrivial contribution from the spin transport at the interface, which has often been overlooked in the study of the bulk effects in LM/FM bilayers.

Controlling the magnetic order of antiferromagnets is challenging due to their vanishing net magnetization. For this reason, the study of local spin textures in antiferromagnets is restricted by the difficulty in nucleating such states. Here, using atomistic simulations we demonstrate a method for nucleating localized spin textures in the grains of the thin-film antiferromagnet $\gamma \text{−}{\mathrm{IrMn}}_3$. Utilizing the exchange bias coupling between a ferromagnet and an antiferromagnet, we set the spin texture in the latter from a predefined spin texture in the former by means of a thermal cycling procedure. The local textures set in the antiferromagnetic grains are shown to be stable against field perturbations. We also discuss how various material parameters affect the efficiency of the setting and the characteristics of these set textures. The setting of antiferromagnetic spin textures provides a potential route to antiferromagnetic spintronic devices with noncollinear spin states such as skyrmions, bubbles, and domain walls.

Using scanning tunneling microscopy, we investigate the superconductivity and vortex properties in topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin films grown on the iron-based superconductor ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$. The proximity-induced superconductivity weakens in the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ film when the thickness of the film increases. Unlike the elongated shape of vortex cores observed in the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ film with 2-quintuple-layer (QL) thickness, the isolated vortex cores exhibit a star shape with six rays in the 1-QL film, and the rays are along the crystalline axes of the film. This is consistent with the sixfold rotational symmetry of the film lattice, and the proximity-induced superconductivity is still topologically trivial in the 1-QL film. At a high magnetic field, when the direction between the two nearest-neighboring vortices deviates from that of any crystalline axes, two cores connect each other by a pair of adjacent rays, forming a new type of electronic structure of vortex cores. On the 3-QL film, the vortex cores elongate along one of the crystalline axes of the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ film, similar to the results obtained on 2-QL films. The elongated vortex cores indicate a twofold symmetry of the superconducting gap induced by topological superconductivity with odd parity. This observation confirms possible topological superconductivity in heterostructures with a thickness of more than 2 QLs. Our results provide rich information for the vortex cores and vortex-bound states on the heterostructures consisting of the topological insulator and the iron-based superconductor.

The occupied electron energy bands of monolayer ${\mathrm{MoS}}_2$ are composed from out-of-plane $d$ orbitals at the Brillouin zone (BZ) center and from in-plane $d$ orbitals at the BZ corner. If a dopant would interact in an orbital selective manner with the ${\mathrm{MoS}}_2$ bands, it could provide a tuning knob to modulate the ${\mathrm{MoS}}_2$ energy bands according to the electron wave vector. Here we directly show by angle-resolved photoemission spectroscopy (ARPES) that Fe doping of epitaxial ${\mathrm{MoS}}_2$ is orbital selective. That is, Fe doping causes a larger energy upshift of the valence band at the BZ center compared to the BZ corner. The optical properties of Fe-doped ${\mathrm{MoS}}_2$ are investigated by ultrahigh vacuum photoluminescence and reveal a loss of photoluminescence upon Fe doping. The sample morphology is investigated by scanning tunneling microscopy and shows a two-dimensional core-shell structure with Fe chemisorbed along the edge of ${\mathrm{MoS}}_2$ islands. This structural determination is consistent with core-level spectroscopy measurements. Realistic ab initio calculations and tight-binding calculations of the electronic band structure fully explain ARPES and photoluminescence experiments and highlight that dopants with complex $d$-orbital structure interact with ${\mathrm{MoS}}_2$ in an orbital-sensitive manner. Our approach opens opportunities in band-structure engineering of two-dimensional materials.

In this paper, we theoretically investigate the optical orientation and alignment of excitons in quantum dots with weak electron-hole exchange interaction and long exciton radiative lifetimes. This particular regime is realized in semiconductor heterosystems where excitons are indirect in the $\mathit{r}$ or $\mathit{k}$ space. The main role in the fine structure of excitonic levels in these systems is played by the hyperfine interaction of the electron in the confined exciton and fluctuations of the Overhauser field. Along with it, the effects of nonradiative recombination and exchange interaction are considered. We start with the model of vanishing exchange interaction and nonradiative exciton recombination and then take them into consideration in addition to the strong Overhauser field. In the nano-objects under study, the polarization properties of the resonant photoluminescence are shown to vary with the external magnetic field in a completely different way as compared with the behavior of conventional quantum dot structures.

We measured high-field magnetization, up to 56 T, in ${\mathrm{CeAl}}_2$ crystallized in ${\mathrm{MgCu}}_2$-type cubic structure, using a pulse magnet. We observed the conventional metamagnetic transition from antiferromagnetism to field-induced ferromagnetism near 5 T. Magnetic phase diagrams were determined for the $\langle 100\rangle , \langle 110\rangle$, and $\langle 111\rangle$ axes using high quality single crystals, as identified by the detection of the de Haas-van Alphen oscillation. As reported in previous research with polycrystalline samples for mapping the magnetic phase diagram of ${\mathrm{CeAl}}_2$, two phase boundaries were observed within the antiferromagnetic phase. This structure of the magnetic phase diagram is similar to that of centrosymmetric skyrmion system such as ${\mathrm{GdRu}}_2{\mathrm{Si}}_2$, in which the Ruderman–Kittel–Kasuya–Yosida interaction plays an important role. Our measurements using single crystals revealed that phase boundaries within the antiferromagnetic phase varied depending on magnetic field direction. For magnetization along $\langle 100\rangle$, a weak, broad shoulder was observed that reached to approximately 15 K, far higher than the 4 K Néel temperature. This single-crystal study resolves the detailed magnetic phase diagrams to three symmetry axes. Mapping the orientation-dependent magnetic phase diagrams supports the magnetic structure determination for each phase and each symmetry axis. The newly mapped orientation-dependent magnetic phase diagrams provide insights that clarify underlying magnetic correlation and plausible skyrmion phase in heavy fermion compound ${\mathrm{CeAl}}_2$.

Using computational and experimental techniques, we examine the nature of the $+2$ oxidation of titanium (Ti)-doped CdSe. Through stoichiometry and confirmed through magnetization measurements, the weakly doped ${\mathrm{Cd}}_{1-x}{\mathrm{Ti}}_x\mathrm{Se}$ $(x=0.00043)$ crystal shows the presence of a robust spin-1 magnetic state of Ti, which is indicative of a $+2$ oxidation state. Given the obscure nature of the $\mathrm{Ti} +2$ state, we investigate the electronic and magnetic states using density functional theory. Using a generalized gradient approximation with an onsite potential, we determine the electronic structure and magnetic properties for a supercell of CdSe with an ultralow concentration of Ti. We find that, in order to reproduce the magnetic moment of spin-1, an onsite potential of 4–6 eV must be included in the calculation. Furthermore, the electronic structure and density of states show the presence of a $\mathrm{Ti}\text{−}d$ impurity band above the Fermi level and a weakly metallic state for a $U=0 \mathrm{eV}$. However, the evolution of the electronic properties as a function of the Hubbard $U$ shows that the $\mathrm{Ti}\text{−}d$ drop below the Fermi around 4 eV with the onset of a semiconducting state. The impurity then mixes with the lower valence bands and produces the $+2$ state for the Ti atom.

Gapless helical edge modes are a hallmark of the quantum spin Hall effect. Protected by time-reversal symmetry, each edge contributes a quantized zero-temperature conductance quantum $G_0\equiv e^2/h$. However, the experimentally observed conductance in ${\mathrm{WTe}}_2$ decreases below $G_0$ per edge already at edge lengths around 100 nm, even in the absence of explicit time-reversal breaking due to an external field or magnetic impurities. In this work, we show how a time-reversal breaking excitonic condensate with a spin-spiral order that can form in ${\mathrm{WTe}}_2$ leads to the breakdown of conductance quantization. We perform Hartree-Fock calculations to compare time-reversal breaking and preserving excitonic insulators. Using these mean-field models we demonstrate via quantum transport simulations that weak nonmagnetic disorder reproduces the edge length scaling of resistance observed in the experiments. We complement this by analysis in the Luttinger liquid picture, shedding additional light on the mechanism behind the quantization breakdown.