${\mathrm{Ba}}_2\mathrm{Y}\mathrm{Re}{\mathrm{O}}_6$ is a double perovskite material where the ${\mathrm{Re}}^{5+}$ ($d^2$) ions occupy a frustrated face-centered cubic lattice. Despite strong antiferromagnetic interactions between the Re ions, as indicated by a large negative Weiss constant $\theta =-616$ K, spin freezing in ${\mathrm{Ba}}_2\mathrm{Y}\mathrm{Re}{\mathrm{O}}_6$ only occurs at 45 K. Since no long-range order of magnetic dipoles has previously been found in either muon spin rotation or neutron diffraction experiments below this temperature, it has been assumed that the low-temperature state is a spin glass. In stark contrast with these findings, we here show that ${\mathrm{Ba}}_2\mathrm{Y}\mathrm{Re}{\mathrm{O}}_6$ does in fact order with a strongly reduced dipole moment ${\mu }_0=0.29–0.42{\mu }_B$ via a polarized neutron diffraction experiment on a powder sample. Using the symmetries of the two likeliest magnetic structures and the properties of the $5d^2$ configuration in the presence of strong spin-orbit coupling and strong crystal fields, we use a recently derived single-ion wave function for ${\mathrm{Re}}^{5+}$ to predict a large quadrupolar moment based on the experimental ${\mu }_o$.

We investigate the effects of two electronic bands at the negative electronic compressibility (NEC) in a two-dimensional electron gas (2DEG). We use a simple homogeneous model with Coulombic interactions and first-order multiband coupling to examine the role of effective mass and relative permittivity in relation to the critical carrier density, where compressibility turns negative. We demonstrate that the population of a second band, along with the presence of interband coupling, can dramatically change the crossover carrier density. Given the difficulty in determining and confirming multiband electronic systems, this model provides a potential method for identifying multiband electronic systems using precise bulk electronic properties measurements. To help illustrate this method, we apply our results to the observed NEC in the 2DEG at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ (LAO/STO) interface and determine that, for the known parameters of LAO/STO, the system is likely a realization of a two-band 2DEG. Furthermore, we provide general limits on the interband coupling with respect to the electronic band population.

The effective $g$ factor of holes is measured in modulation-doped ZnSe/(Zn,Mg)(S,Se) quantum wells and from surface-state $p$-doped CdTe/(Cd,Mg)Te quantum wells by time-resolved pump-probe Kerr rotation. The measurements are performed at a temperature of 1.7 K and in magnetic fields up to 5 T applied in the Voigt geometry with orientation perpendicular to the quantum-well growth axis. The absolute value of the in-plane hole $g$ factor increases with growing magnetic field in both studied heterostructures. A theoretical model is developed that considers the influence of magnetic field and interface mixing of heavy-hole and light-hole states on the $g$ factor. The model results are in good agreement with the experimental data.

We study the high-temperature phase diagram of the chiral magnetic insulator ${\mathrm{Cu}}_2\mathrm{O}\mathrm{Se}{\mathrm{O}}_3$ by measuring the spin-Hall magnetoresistance (SMR) in a thin Pt electrode. We find distinct changes in the phase and amplitude of the SMR signal at critical lines separating different magnetic phases of bulk ${\mathrm{Cu}}_2\mathrm{O}\mathrm{Se}{\mathrm{O}}_3$. The skyrmion lattice state appears as a strong dip in the SMR phase. A strong enhancement of the SMR amplitude is observed in the conical spiral state, which we explain by an additional symmetry-allowed contribution to the SMR present in noncollinear magnets. We demonstrate that the SMR can be used as an all-electrical probe of chiral surface twists and skyrmions in magnetic insulators.

Selenium in ambient conditions exhibits two crystalline allotropic forms. The thermodynamically stable phase is the pseudo-1D trigonal form with ${}^1{/}_{\infty }$[Se] chains. A metastable phase composed of $\left[{\mathrm{Se}}_8\right]$ rings also coexists. Both are studied herein by means of density functional theory. The recent SCAN functional offers a fair description of both phases. The Birch-Murnaghan equation of states is fitted onto ab initio results to obtain the energy-volume and pressure-energy relationships. Phonon properties (bands and DOS) are computed and a pressure-temperature phase diagram is derived. Intrinsic and extrinsic defect formation enthalpies are also computed using SCAN functional and HSE06-GD3 for the band shifts. The low $p$-type conductivity of the trigonal phase can be attributed to Se self-interstitials while the very low conductivity of the metastable phase is related to very deep native defects. Antimony, and bromine extrinsic defects are tested as potential dopants. Of those three, only antimony in the trigonal phase seems to possibly have a positive impact on the conductivity. Finally, the configurational entropy linked to defect creation is computed. Our calculations clearly show that the usual assumption that defect entropy is negligible compared to the enthalpy seems relevant.

Locally resonant acoustic materials were proposed to control sound using structures with feature size orders of magnitude smaller than the acoustic wavelength in air. We analytically derive the effective, frequency-dependent mass densities of resonant oscillators consisting of a heavy mass within a light shell, embedded in foam, in two dimensions, using a rigid core-shell approximation. The effective mass is expressible in closed form by elementary functions, enabling a mapping of the low-frequency physics to a simple one-dimensional model involving a point mass harmonically coupled within a box. The acoustic band structure of a two-dimensional square lattice of these oscillators is evaluated in the effective, frequency-dependent mass density model and compared with the exact solution. For the out-of-plane oscillation within this two-dimensional phononic crystal, the nonlinear eigenvalue equation resulting from the frequency-dependent density is solved with the Cutting Surface Method to yield the acoustic band structure. This agrees with the exact band structure obtained by finite element method calculations within $2\%$ numerical error. For the in-plane oscillations involving internal translations or rotations within the core-shell resonators, the one-dimensional mass-in-a-box model recaptures the exact acoustic modes at only certain high-symmetry points of the two-dimensional Brillouin zone. The description of more complex resonances within acoustic metamaterials requires a fundamental generalization of current effective mass models.

We study a one-dimensional $p$-wave superconductor subject to non-Hermitian quasiperiodic potentials. Although non-Hermiticity exists, the Majorana zero mode is still robust against the disorder perturbation. The analytic topological phase boundary is verified by calculating the energy gap closing point and the topological invariant. Furthermore, we investigate the localized properties of this model, quantitatively revealing that the topological phase transition is accompanied by the Anderson localization phase transition, and a wide critical phase emerges with amplitude increments of the non-Hermitian quasiperiodic potentials. Finally, we numerically uncover a unconventional real-complex transition of the energy spectrum, which is different from the conventional $\mathcal{PT}$ symmetric transition.

Quantum heat transfer through a generic superconducting setup consisting of a tunable transmon qubit placed between resonators that are terminated by thermal reservoirs is explored. Two types of architectures are considered: a sequential setting and a beam-splitter setting. Applying the numerical exact hierarchical equation of motion (HEOM) approach, steady-state properties are revealed, and experimentally relevant parameter sets are identified. Benchmark results are compared with predictions based on approximate treatments to demonstrate their failure in broad ranges of parameter space. These findings may allow improved future designs for heat control in superconducting devices.

The Dirac semimetal ${\mathrm{PdTe}}_2$ becomes superconducting at a temperature $T_c=1.6$ K. Thermodynamic and muon spin rotation experiments support type-I superconductivity, which is unusual for a binary compound. A key property of a type-I superconductor is the intermediate state, which presents a coexistence of superconducting and normal domains at magnetic fields lower than the thermodynamic critical field $H_c$. We present scanning SQUID microscopy studies of ${\mathrm{PdTe}}_2$ revealing coexisting superconducting and normal domains of tubular and laminar shape as the magnetic field is more and more increased, thus confirming type-I superconductivity in ${\mathrm{PdTe}}_2$. Values for the domain wall width in the intermediate state have been derived. The field amplitudes measured at the surface indicate bending of the domain walls separating the normal and superconducting domains.

The magnetic excitations in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ are studied by polarized and unpolarized neutron scattering experiments as a function of temperature. At the scattering vector of the Fermi-surface nesting with a half-integer out-of-plane component, there is no evidence for the appearance of a resonance excitation in the superconducting phase. The body of existing data indicates weakening of the scattered intensity in the nesting spectrum to occur at very low energies. The nesting signal persists up to 290 K but is strongly reduced. In contrast, a quasiferromagnetic contribution maintains its strength and still exhibits a finite width in momentum space.

Charge orders (COs) and entangled topology denote the fundamentals of correlated electron systems at low temperatures, and their profound revelation at atomic scale awaits experimental advances. Here, we report such a spatially resolved investigation using cryogenic atomic-scale electron microscopy and spectroscopy at 100 K, with the CO insulator of $\mathrm{Hg}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$ below 240 K as an exemplification. By the cryogenic technique, we can resolve the charge and lattice characteristics of the insulating CO phase at atomic resolution and identify the order parameters (OPs) at play as charge-density-modulated (∼31.4 Å, modulation length) and antiphase (∼10.5 Å) OPs. Both OPs are largely unaddressed forms of COs in correlated oxides and display the emergent topology of intertwining mesoscopic stripes of 10–40 nm in width and ≥500 nm in length, distinctly different from the atomically narrow CO stripes conventional to correlated systems and depicting the unique electronic contour of the phase. This preliminary demonstration of the cryogenic unveiling of the OPs and topological landscape paves the way to disentangling the multifarious COs and hidden topology in condensed matters and, meanwhile, calls for further developments in the atomic-scale cryogenic spectral probing at an enhanced speed.

Recently, the square planar ${\mathrm{MoS}}_2$ monolayer, which exhibits the Cairo pentagonal tiling (termed as $1\mathrm{P}\text{−}{\mathrm{MoS}}_2$), was identified as an intrinsic quantum anomalous Hall (QAH) insulator. However, there is a paucity of theoretical work concerning a suitable substrate to support its nontrivial electron transport properties, which is the prerequisite for practical applications. Here. we demonstrate that CuI(001) serves an excellent substrate candidate for epitaxial growth of the $1\mathrm{P}\text{−}{\mathrm{MoS}}_2$ sheet by showing that the intrinsic ferromagnetism and the QAH state of $1\mathrm{P}\text{−}{\mathrm{MoS}}_2$ remain unaltered in the $1\mathrm{P}\text{−}{\mathrm{MoS}}_2$/CuI(001) system. Further analyses of the strain effect on $1\mathrm{P}\text{−}{\mathrm{MoS}}_2$ reveal that the QAH is robust within a strain range from $-$ 2% to 2%. Our findings will inspire the experimental realization of QAH effects in two-dimensional (2D) pentagon-based materials.

We explicitly construct an integrable and strongly interacting dissipative quantum circuit via a trotterization of the Hubbard model with imaginary interaction strength. To prove integrability, we build an inhomogeneous transfer matrix, from which conserved superoperator charges can be derived, in particular, the circuit's dynamical generator. After showing the trace preservation and complete positivity of local maps, we reinterpret them as the Kraus representation of the local dynamics of free fermions with single-site dephasing. The integrability of the map is broken by adding interactions to the local coherent dynamics or by removing the dephasing. In particular, even circuits built from convex combinations of local free-fermion unitaries are nonintegrable. Moreover, the construction allows us to explicitly build circuits belonging to different non-Hermitian symmetry classes, which are characterized by the behavior under transposition instead of complex conjugation. We confirm all our analytical results by using complex spacing ratios to examine the spectral statistics of the dissipative circuits.

In heavy fermions the relaxation dynamics of photoexcited carriers has been found to be governed by the low energy indirect gap ${\mathrm{E}}_g$ resulting from hybridization between localized moments and conduction band electrons. Here, carrier relaxation dynamics in a prototype Kondo insulator $\mathrm{Yb}{\mathrm{B}}_{12}$ is studied over a large range of temperatures and over three orders of magnitude. We utilize the intrinsic nonlinearity of dynamics to quantitatively determine microscopic parameters, such as electron-hole recombination rate. The extracted value reveals that hybridization is accompanied by a strong charge transfer from localized $4f$ levels. The results imply the presence of a hybridization gap up to temperatures of the order of ${\mathrm{E}}_g/{\mathrm{k}}_B\approx 200$ K, which is extremely robust against electronic excitation. Finally, below 20 K the data reveal changes in the low energy electronic structure, attributed to short-range antiferromagnetic correlations between the localized levels.

Many charge density wave (CDW) systems exhibit $q\left(T\right)$ electron-hole modulations continuously varying with $T$ and saturating upon cooling at an incommensurate value even if the maximum occurring in the electron-hole Lindhard response does not exhibit such a thermal shift. Using a simple RPA argument we show that the experimental $q\left(T\right)$ can be understood if the electron-phonon coupling (EPC) $g\left(q\right)$, necessary to set coupled electronic and structural modulations, is momentum dependent. In this analysis, the sense of variation of $q\left(T\right)$ depends upon the sign of $\frac{\partial g\left(q\right)}{\partial q}$ and its amplitude of thermal variation is controlled by the electron-hole coherence length (or CDW rigidity) in the modulation direction. This model quantitatively accounts for the thermal dependence of $q\left(T\right)$ in the one-dimensional (1D) CDW system ${\mathrm{K}}_{0.3}\mathrm{Mo}{\mathrm{O}}_3$ (blue bronze) both in its CDW ground state and in its pretransitional CDW fluctuation regime. We suggest that such a general analysis can be extended to account for the $q\left(T\right)$ dependence observed in other 1D and 2D CDW systems such as the transition metal di- and trichalcogenides as well as the lanthanide and rare-earth tritellurides. Using a detailed analysis of the low frequency phonon spectrum of the blue bronze, we then propose a new scenario for the $q$ dependent EPC, where $g\left(q\right)$ is due to a momentum-dependent hybridization between the critical phonon branch bearing the Kohn anomaly and other low-lying phonon branches. This allows obtaining a sign of $\frac{\partial g\left(q\right)}{\partial q}$ in agreement with that deduced from the analysis of $q\left(T\right)$. Finally, we propose that similar hybridization effects could also be relevant for other 1D and 2D CDW systems exhibiting a thermally dependent modulation.

We develop a framework to evaluate the time-dependent resonant inelastic x-ray scattering (RIXS) signal with the use of nonequilibrium dynamical mean-field theory simulations. The approach is based on the solution of a time-dependent impurity model which explicitly incorporates the probe pulse. It avoids the need to compute four-point correlation functions and can in principle be combined with different impurity solvers. This opens a path to study time-resolved RIXS processes in multiorbital systems. The approach is exemplified with a study of the RIXS signal of a melting Mott antiferromagnet.

Graphene-based van der Waals heterostructures have received tremendous interest from both fundamental and experimental studies because they can enhance the properties and expand the possibility of applications of both graphene and two-dimensional materials. Motivated by the successful synthesis of the graphene/$\mathrm{Bi}{\mathrm{I}}_3$ heterostructure [Chang et al., Adv. Funct. Mater. 28, 1800179 (2018)]., here, we systematically investigate the electronic structure and interfacial characteristics of this material using first-principles simulations. We find that the structure of the graphene/$\mathrm{Bi}{\mathrm{I}}_3$ heterostructure is mainly characterized by weak van der Waals interactions, which keeps the heterostructure feasible. In the ground state, the graphene/$\mathrm{Bi}{\mathrm{I}}_3$ heterostructure forms the $n$-type Schottky contact with a barrier of 0.53 eV. The barriers of the Schottky contact can be adjusted by various factors, including interlayer coupling and electric gating. Both the interlayer coupling and electric gating lead to the transformation from the $n$-type Schottky contact to the $p$-type one or to the $n$-type Ohmic contact. These findings demonstrate that graphene/$\mathrm{Bi}{\mathrm{I}}_3$ can be considered a promising building block for high-performance photoresponsive optoelectronic devices.