The spin-orbit interaction is popular for spintronic applications since, through the mechanism of spin-dependent asymmetric scattering, spin currents are generated from charge currents (spin Hall effect) or charge currents are generated from spin currents (inverse spin Hall effect). The discovery of spin, a century ago, relied on a magnetic field gradient to separate opposite spins; this mechanism has received scant attention as a means for generating spin and charge currents in semiconductors. Through the derivation of a set of coupled spin-charge drift-diffusion equations, our paper shows that magnetic field gradients can be used to generate charge currents from nonequilibrium spin polarization in confined solid state systems. We predict, in GaAs, a longitudinal “Stern-Gerlach” voltage. Nonintuitively, we find the spin diffusion length is reduced by the magnetic gradient, which has ramifications for interpreting spin transport experiments. This suppression is understood by invoking the idea of cocurrent and countercurrent exchange, which is a concept found in fields as disparate as animal physiology and thermal engineering.

The emerging field of topological acoustics provides exciting possibilities for controlling sound propagation with extraordinary robustness. Conventional topological acoustic waveguides built from topological edge states, which arise solely from the nontrivial topology in the momentum space, usually have restricted shapes due to their crystalline structures. Here, we show that, using an acoustic topological system with both nontrivial topologies in the real and momentum spaces, free-form acoustic topological waveguides can be constructed. These topological waveguides support disclination-localized states, caused by the interplay between topological lattice defects and the valley-Hall topology. As demonstrated in our simulations and experiments, such disclination waveguides can take arbitrary shapes and form open arcs within the lattice, which are not possible for previous crystalline acoustic topological waveguides. Furthermore, by increasing the number of topological lattice defects, we can realize more than one topological waveguide in one lattice. They join at the topological lattice defect and form a topological waveguide network, enabling more complicated functionalities such as beam splitting. Our results point to a promising direction for free-form acoustic topological devices with the advantages of embedded forms, easy cascading, and high robustness.

We study entanglement transitions in Clifford (stabilizer) random tensor networks (RTNs) and monitored quantum circuits, by introducing an exact mapping onto a (replica) statistical mechanics model. For RTNs and monitored quantum circuits with random Haar unitary gates, entanglement properties can be computed using statistical mechanics models whose fundamental degrees of freedom (“spins”) are permutations, because all operators commuting with the action of the unitaries on a tensor product Hilbert space are (linear combinations of) permutations of the tensor factors (“Schur-Weyl duality”). When the unitary gates are restricted to the smaller group of Clifford unitaries, the set of all operators commuting with this action, called the commutant, will be larger, and no longer form a group. We use the recent full characterization of this commutant by Gross et al. [Commun. Math. Phys. 385, 1325 (2021)] to construct statistical mechanics models for both Clifford RTNs and monitored quantum circuits, for on-site Hilbert-space dimensions which are powers of a prime number $p$. The elements of the commutant form the spin degrees of freedom of these statistical mechanics models, and we show that the Boltzmann weights are invariant under a symmetry group involving orthogonal matrices with entries in the finite number field ${\mathbf{F}}_p$ (“Galois field”) with $p$ elements. This implies that the symmetry group and consequently all universal properties of entanglement transitions in Clifford circuits and RTNs will, respectively, in general depend on and only on the prime $p$. We show that Clifford monitored circuits with on-site Hilbert-space dimension $d=p^M$ are described by percolation in the limits $d\to \infty$ at (a) $p=$ fixed but $M\to \infty$, and at (b) $M=1$ but $p\to \infty$. In the limit (a) we calculate the effective central charge, and in the limit (b) we derive the following universal minimal cut entanglement entropy $S_A=(\sqrt{3}/\pi )lnpln{L}_A$ for $d=p$ large at the transition. We verify those predictions numerically, and present extensive numerical results for critical exponents at the transition in monitored Clifford circuits for prime number on-site Hilbert-space dimension $d=p$ for a variety of different values of $p$, finding that projective and forced measurement schemes yield the same critical exponents and that they approach percolation values at large $p$. We clearly establish multifractal scaling of the purity, reflected in a continuous spectrum of critical exponents, while the typical exponent is the prefactor of the logarithm in the entanglement entropy. As a technical result, we generalize the notion of the Weingarten function, previously known for averages involving the Haar measure, to averages over the Clifford group.

Re-based superconductors provide a rich platform for the study of unconventional superconductivity. We have investigated the superconducting properties of ${\mathrm{Re}}_8\mathrm{NbTa}$, a new noncentrosymmetric cubic ($\alpha$-Mn structure) rhenium-based ternary superconductor using transport, magnetization, specific-heat, and muon-spin rotation/relaxation ($\mu \mathrm{SR}$) measurements. Specific-heat and transverse field $\mu \mathrm{SR}$ measurements suggest moderately coupled fully gapped superconductivity, well described by BCS theory. However, our zero-field $\mu \mathrm{SR}$ measurements reveal a small internal field onsetting around the superconducting $T_c$, indicating the unconventional superconducting ground state.

In situ variation of the electron density via a metallic gate can control the disorder potentials in two-dimensional electron gases (2DEGs). This also influences the negative magnetoresistance at low magnetic fields, which is commonly observed in ultrahigh mobility 2DEGs. We investigate the temperature-dependent giant negative magnetoresistance (GNMR) as a function of the electron density for several temperatures and currents. Thereby, we find that the GNMR behavior depends decisively on the electron density. This observation is attributed to a changed disorder potential with electron density. In the case of higher electron densities, a nonlinear current dependency of the GNMR is observed, which could be described within the hydrodynamic regime.

We show that all lowest Landau-level projected and unprojected chiral parton type fractional quantum Hall ground and edge-state trial wave functions, which take the form of products of integer quantum Hall wave functions, can be expressed as conformal field theory (CFT) correlation functions, where we can associate a chiral algebra to each parton state such that the CFT defined by the algebra is the “smallest” such CFT that can generate the corresponding ground and edge-state trial wave functions (assuming that the corresponding chiral algebra does indeed define a physically “sensible” CFT). A field-theoretic generalization of Laughlin's plasma analogy, known as generalized screening, is formulated for these states. If this holds, along with an additional assumption, we argue that the inner products of edge-state trial wave functions, for parton states where the “densest” trial wave function is unique, can be expressed as matrix elements of an exponentiated local action operator of the CFT, generalizing the result of Dubail et al. [Phys. Rev. B 85, 115321 (2012)], which implies the equality between edge-state and entanglement level counting to state counting in the corresponding CFT. We numerically test this result in the case of the unprojected $\nu =2/5$ composite fermion state and the bosonic $\nu =1{\varphi }_2^2$ parton state. We discuss how Read's arguments [Phys. Rev. B 79, 045308 (2009)] still apply, implying that conformal blocks of the CFT defined by the corresponding chiral algebra are valid quasihole trial wave functions, with the adiabatic braiding statistics given by the monodromy of these functions, assuming the existence of a quasiparticle trapping Hamiltonian. Generalizations of these constructions are discussed, with particular attention given to simple current constructions. It is shown that all chiral composite fermion wave functions can be expressed as CFT correlation functions without explicit symmetrization or antisymmetrization and that the ground, edge, and certain quasihole trial wave functions of the ${\varphi }_n^m$ parton states can be expressed as the conformal blocks of the $\text{U}\left(1\right)\otimes SU{\left(n\right)}_m$ WZW models. Finally, we discuss the relation of the ${\varphi }_2^k$ series with the Read-Rezayi series, where several examples of quasihole braiding statistics calculations are given.

The cubic $\beta$-Mn-type alloy ${\mathrm{Co}}_8{\mathrm{Zn}}_8{\mathrm{Mn}}_4$ is a chiral helimagnet that exhibits a peculiar temperature-dependent behavior in the spiral pitch, which decreases from 130 nm at room temperature to 70 nm below 20 K. Notably, this shortening is also accompanied by a structural transition of the metastable skyrmion texture, transforming from a hexagonal lattice to a square lattice of elongated skyrmions. The underlying mechanism of these transformations remains unknown, with interactions potentially involved including the temperature-dependent Dzyaloshinskii-Moriya interaction, magnetocrystalline anisotropy, and exchange anisotropy. Here, x-ray resonant magnetic small-angle scattering in vectorial magnetic fields was employed to investigate the temperature dependence of the anisotropic properties of the helical phase in ${\mathrm{Co}}_8{\mathrm{Zn}}_8{\mathrm{Mn}}_4$. Our results reveal quantitatively that the magnitude of the anisotropic exchange interaction increases by a factor of 4 on cooling from room temperature to 20 K, leading to a 5% variation in the helical pitch within the (001) plane at 20 K. While the anisotropic exchange interaction contributes to the shortening of the spiral pitch, its magnitude is insufficient to explain the variation in the spiral periodicity from room to low temperatures. Finally, we demonstrate that magnetocrystalline and exchange anisotropies compete, with each favoring different orientations of the helical vector in the ground state.

Vacancy-ordered halide double perovskites hosting $4d/5d$ transition metals have emerged as a distinct platform for investigating unconventional magnetism arising out of the interplay of strong atomic spin-orbit coupling (SOC) and Coulomb interactions. Focusing on the $d^2$ system ${\mathrm{Cs}}_2{\mathrm{WCl}}_6$, our ab initio electronic structure calculation reveals very narrow electronic bands, fulfilling the necessary condition to realize exotic orders. Using this input, we solve the many-body spin-orbit coupled single-site problem by exact diagonalization and show that the multiplet structure of ${\mathrm{Cs}}_2{\mathrm{WCl}}_6$ hosts ground non-Kramers doublets on W, with vanishing dipole moment and a small gap to an excited magnetic triplet. Our work provides the rationale for the observed strong deviation from the classic Kotani behavior in ${\mathrm{Cs}}_2{\mathrm{WCl}}_6$ for the measured temperature dependence of the magnetic moment. The non-Kramers doublets on W exhibit nonzero quadrupolar and octupolar moments, and our calculated two-site exchange supports the dominance of intersite octupolar exchange over quadrupolar interactions. We predict ferro-octupolar order with $T_c\sim 5\mathrm{K}$, which may get somewhat suppressed by quantum fluctuations and disorder; this could be tested in future low-temperature experiments.

Magnetic hybrid-improper ferroelectrics show potential for multiferroicity that is mediated by nonpolar structural distortions that give rise to both ferroelectricity and a net-magnetic spin canting. Here, we report on a pronounced magnetoelectric response of the ferroelectric state to the intrinsic magnetic ordering and to external magnetic fields in $\mathrm{Ca}{}_3{\mathrm{Mn}}_{1.9}{\mathrm{Ti}}_{0.1}\mathrm{O}{}_7$, a layered perovskite-type system. We observe a sixfold increase in the temperature-dependent response of the polarization-induced optical second-harmonic-generation (SHG) signal to the magnetic order that we explain as a magnetically triggered interlayer coupling of the polar order. Furthermore, an applied magnetic field up to 3 T along the direction of the net magnetization enhances the polarization-induced SHG signal by about 30%. We interpret this as magnetic-field-induced spin canting, which possibly affects the Mn-O bonds and hence the polarization. Optical SHG is of particular advantage to this study as it provides access to the electric polarization with spatial resolution of its domains, unobstructed by the leakage currents that impede contact-based measurements. Our investigations advance the understanding and potential of a highly promising new category of multiferroic materials for magnetoelectric cross control and functionalization.

Most semiconductors and insulators exhibit indirect band gaps, but no theory is currently available to calculate light absorption and emission spectra of these systems over a wide spectral range with predictive accuracy. The standard textbook theory of indirect absorption becomes ill-defined and yields infinite absorption strength when a photon can promote both direct and phonon-assisted transitions. As a result, state-of-the-art ab initio methods for calculating optical spectra of solids are unable to describe direct and phonon-assisted transitions on the same footing. Here, we develop a rigorous first-principles approach that overcomes this limitation by including electron-phonon correlations via many-body quasidegenerate perturbation theory. Our present formalism enables accurate calculations of the optical spectra of materials with direct, indirect, and quasidirect band gaps, and reduces to the standard theories of direct-only absorption and indirect-only absorption in the appropriate limits. We demonstrate this methodology by investigating the optical absorption spectra of silicon, germanium, gallium arsenide, and diamond. In all cases, we obtain spectra in excellent agreement with experiments. As a more ambitious test, we investigate the temperature-dependent photoluminescence of germanium, and we obtain quantitative agreement with experiments.

Ensemble density functional theory (EDFT) is a promising alternative to time-dependent density functional theory for computing electronic excitation energies. Using coordinate scaling, we prove several fundamental exact conditions in EDFT and illustrate them on the exact singlet bi-ensemble of the Hubbard dimer. Several approximations violate these conditions, and some ground-state conditions from quantum chemistry do not generalize to EDFT. The strong correlation limit is derived for the dimer, revealing weight-dependent derivative discontinuities in EDFT.

Recent experiments observed fractional Chern insulators (FCI) in twisted bilayer ${\mathrm{MoTe}}_2$ at zero magnetic field, yet even the single-particle model of this material is controversial, leading to unreliable predictions of the experimental phase diagram as discussed in [Yu et al., Phys. Rev. B 109, 045147 (2024)]. In this light, we revisit the single-particle model of twisted bilayer ${\mathrm{MoTe}}_2$. Utilizing large-scale density functional theory, we calculate the band structure of twisted AA-stacked bilayer ${\mathrm{MoTe}}_2$ at various twist angles relevant to experiment. We find that a band inversion occurs near $4.{41}^{\circ }$ between the second and third bands in one valley. Our ab initio band structure is in qualitative agreement with [Wang et al., Phys. Rev. Lett. 132, 036501 (2024)], but shows important differences in the remote bands and in the $\mathrm{\Gamma }$ valley. We incorporate two higher harmonic terms into the continuum model to capture the highest three valence bands per valley. We confirm that the two highest valence bands per valley have opposite Chern numbers with $\left|C\right|=1$ for experimentally relevant angles, and also use our model to predict a variety of Chern states in the remote bands accessible by displacement field. We also perform DFT calculations and build models for the AB-stacking configuration. Our paper serves as a foundation for accurate determination of the correlated phases in twisted bilayer ${\mathrm{MoTe}}_2$.

The experimental discovery of fractional Chern insulators (FCIs) in rhombohedral pentalayer graphene twisted on hexagonal boron nitride (hBN) has preceded theoretical prediction. Supported by large-scale first-principles relaxation calculations at the experimental twist angle of $0.{77}^{\circ }$, we obtain an accurate continuum model of $n=3,4,5,6,7$ layer rhombohedral graphene-hBN moiré systems. Focusing on the pentalayer case, we analytically explain the robust $\left|C\right|=0,5$ Chern numbers seen in the low-energy single-particle bands and their flattening with displacement field, making use of a minimal two-flavor continuum Hamiltonian derived from the full model. We then predict nonzero valley Chern numbers at the $\nu =-4,0$ insulators observed in experiment. Our analysis makes clear the importance of displacement field and the moiré potential in producing localized “heavy fermion” charge density in the top valence band, in addition to the nearly free conduction band. Lastly, we study doubly aligned devices as additional platforms for moiré FCIs with higher Chern number bands.

We investigate the magnetic structure and magnetoelectric(ME) effect in the high-field phase of the antiferromagnet ${\mathrm{LiFePO}}_4$ above the critical field of $31\mathrm{T}$. A neutron diffraction study in pulsed magnetic fields reveals the propagation vector to be $\mathbf{q}=0$ for the high-field magnetic structure. Pulsed-field electric polarization measurements show that, at the critical field, the low-field off-diagonal ME coupling ${\alpha }_{ab}$ is partially suppressed, and the diagonal element ${\alpha }_{bb}$ emerges. These results are consistent with a spin-flop transition where the spin direction changes from primarily being along the easy $b$ axis below the transition to being along $a$ above. The persistence of off-diagonal ME tensor elements above the critical field suggests a lowering of the magnetic point-group symmetry and hence a more complex magnetic structure in the high-field phase. In addition, neutron diffraction measurements in low magnetic fields show no observable field-induced spin canting, which indicates a negligible Dzyaloshinskii-Moriya interaction. The observed spin-flop field supports the Hamiltonian recently deduced from inelastic neutron studies and indicates that the system is less frustrated and with a larger single-ion anisotropy than originally thought. Our results demonstrate the effectiveness of combining pulsed-field neutron diffraction and electric polarization measurements to elucidate the magnetic structures and symmetries at the highest attainable field strengths.

Controlling zero-bias anomalies in magnetic atoms provides a promising strategy to engineer tunable quantum many-body excitations. Here we show how two different quantum impurities featuring spinaron and Kondo excitations can be controlled via quantum confinement engineering by using circular quantum corrals on a Ag(111) surface. In corrals built from both Ag and Co adatoms, the width of the zero-bias anomaly in the central Co adatom oscillates as a function of corral radius with a period of half of the Ag(111) surface state wavelength. Parameters extracted for Co/Ag(111) show only small differences in the extracted spinaron zero-bias anomaly between corral walls built from Ag or Co adatoms. In quantum corrals occupied by metal-free phthalocyanine, a $S=1/2$ Kondo system, we observe notable changes in the zero-bias anomaly line shape as a function of corral radius. Our results offer insight into many-body Kondo and spinaron resonances in which the electronic density is controlled by confinement engineering.

Nickelate superconductors are outstanding materials with intriguing analogies with the cuprates. These analogies suggest that their superconducting mechanism may be unconventional, although this fundamental question is currently under debate. Here, we scrutinize the role played by electronic correlations in enhancing the electron-phonon coupling in the infinite-layer nickelates and the extent to which this may promote superconductivity. Specifically, we use ab initio many-body perturbation theory to perform state-of-the-art $GW$ and Eliashberg-theory calculations. We find that the electron-phonon coupling is effectively enhanced compared to pure density-functional-theory calculations. This enhancement may lead to low-$T_c$ superconductivity in the parent compounds already. However, it remains marginal in the sense that it cannot explain the record $T_c\mathrm{s}$ obtained with doping. This circumstance implies that conventional superconductivity is preempted by another pairing mechanism in the infinite-layer nickelates.

Strong exciton-photon coupling offers an effective path for polariton-mediated long-distance coherent energy transfer (ET) between excitonic states. Here, we demonstrate strong coupling between excitons in ${\mathrm{WS}}_2$ monolayers, ${\mathrm{MoS}}_2$ bilayer, and photons in a tunable optical microcavity at room temperature. Full quantum dynamics simulations based on experimental parameters show that the demonstrated system provides an efficient and adjustable platform for ultrafast polariton-assisted ET between the excitons in two-dimensional materials when the separation between them exceeds $1\mathrm{\mu }\mathrm{m}$.

Topological semimetals are intriguing targets for exploring unconventional physical properties of massless fermions. Among them, nodal-line or nodal-ring semimetals have attracted attention for their unique one-dimensional band contact in momentum space and resulting nontrivial quantum phenomena. By field angular resolved magnetotransport measurements and theoretical calculations, we show that pressurized black phosphorus (BP) is an ideal nodal-ring semimetal with weak spin-orbit coupling, which has a sole and carrier density-tunable nodal ring isolated from other trivial bands. We also revealed that the large magnetoresistance effect and its field-angular dependence in semimetallic BP are due to highly anisotropic relaxation time. Our results establish pressurized BP as an elemental model material for exploring nontrivial quantum properties unique to the topological nodal ring.

We present an electron spin resonance (ESR) and nuclear magnetic resonance (NMR) study of rare-earth titanates derived from the spin-1/2 Mott insulator ${\mathrm{YTiO}}_3$. Measurements of single-crystalline samples of (Y, Ca, La)${\mathrm{TiO}}_3$ in a wide range of isovalent substitution (La) and hole doping (Ca) reveal several unusual features in the paramagnetic state of these materials. ESR of the unpaired Ti electron shows broad resonance lines at all temperatures and substitution/doping levels, which we find to be caused by short electronic spin-lattice relaxation times. We model the relaxation as an Orbach process that involves a low-lying electronic excited state, which enables the determination of the excited-state gap from the temperature dependence of the ESR linewidths. We ascribe the small gap to Jahn-Teller splitting of the two lower Ti $t_{2g}$ orbitals. The value of the gap closely follows the Curie temperature $T_C$. ${}^{89}\mathrm{Y}$ NMR demonstrates a clear discrepancy between the static and dynamic local magnetic susceptibilities, with deviations from Curie-Weiss behavior at temperatures far above $T_C$, consistent with the electronic gap detected with ESR. No significant changes are observed by NMR close to $T_C$, but suppression of fluctuations is detected in the NMR spin-lattice relaxation time at temperatures of about $3\times T_C$. Additionally, the nuclear spin-spin relaxation rate shows an unusual peak in dependence on temperature for all samples. These results provide insight into the interplay between orbital and spin degrees of freedom in rare-earth titanates and indicate that full orbital degeneracy lifting is associated with ferromagnetic order.

Spin waves hold promise for expanding the magnonics research field to include quantum information processing and classical information devices. Parametric pumping is considered a key technique to achieve this important advancement. Recently, a single-crystal iron has shown potential as a spin-wave excitation medium; however, parametric pumping in single-crystal iron films has not been investigated. In this study, we explored computationally and experimentally the characteristics of parametrically pumped spin waves in single-crystal iron thin films by respectively using large-scale micromagnetic simulations and a high-precision spectrum analyzer. The results demonstrate unconventional parametric pumping attributed to the competition between the anisotropic and excitation fields, emerging at external magnetic field and low power levels that would be insufficient to induce parametric pumping in isotropic materials. Systematic research on parametric pumping in iron could pave the way for low-energy spin-wave devices, enhancing quantum information device technology.