When doped with a high density of mobile charge carriers, monolayer transition-metal dichalcogenide (TMD) semiconductors can host new types of composite many-particle exciton states that do not exist in conventional semiconductors. Such multiparticle bound states arise when a photoexcited electron-hole pair couples not to just a single Fermi sea that is quantum-mechanically distinguishable (as in the case of conventional charged excitons or trions), but rather couples simultaneously to multiple Fermi seas, each having distinct spin and valley quantum numbers. Composite six-particle “hexciton” states were recently identified in electron-doped ${\mathrm{WSe}}_2$ monolayers, but under suitable conditions they should also form in all other members of the monolayer TMD family. Here we present spectroscopic evidence demonstrating the emergence of many-body hexcitons in charge-tunable ${\mathrm{WS}}_2$ monolayers (at the A-exciton) and ${\mathrm{MoSe}}_2$ monolayers (at the B-exciton). The roles of distinguishability and carrier screening on the stability of hexcitons are discussed.

The efficient generation of spin currents and spin torques via spin-orbit coupling is an important goal of spintronics research. One crucial metric for spin current generation is the spin Hall angle, which is the ratio of the spin Hall current to the transversely flowing charge current. A typical approach to measure the spin Hall angle in nonmagnetic materials is to generate spin currents via spin pumping in an adjacent ferromagnetic layer and measure the transverse voltage from the inverse spin Hall effect in the nonmagnetic layer. However, given that the spin Hall effect also occurs in ferromagnets, single ferromagnetic layers could generate a self-induced transverse voltage during spin pumping as well. Here we show that manganite-based ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ (LSMO) films deposited by pulsed laser deposition exhibit a significant self-induced inverse spin Hall voltage while undergoing spin pumping. A spin pumping voltage of $1.86\mu \mathrm{V}$ is observed in the LSMO (12 nm) film. Using density functional theory and the Kubo formalism, we calculate the intrinsic spin current conductivities of these films and show that they are in reasonable agreement with the experimental measurements.

The topological Hall effect (THE), an electrical transport signature of systems with chiral spin textures like skyrmions, has been observed recently in topological insulator (TI)-based magnetic heterostructures. The strong spin-orbit coupling and the broken spatial inversion symmetry in such heterostructures could lead to a sizable interfacial Dzyaloshinsky-Moriya interaction, favorable for skyrmion formation and pronounced THE. However, the intriguing interplay between the topological surface state (TSS) and THE is yet to be fully understood. In this work, we report an unprecedentedly large THE signal of ∼10 Ω (∼4 µΩ cm at 2 K) with an electrically reversible sign in a top-gated 4 nm TI ${\left({\mathrm{Bi}}_{0.3}{\mathrm{Sb}}_{0.7}\right)}_2{\mathrm{Te}}_3$ (BST) grown on a ferrimagnetic insulator (FI) europium iron garnet ($\mathrm{Eu}\mathit{IG}$). The dependence of THE on temperature, external magnetic field angle, and gate bias was investigated and is consistent with the prediction of a skyrmion-driven THE, amenable to elucidate the origin of THE that occurred in TI-based heterostructures. Moreover, a sign change in THE was discovered as the Fermi level was tuned electrically from the upper (electron-doped region) to the lower parts (hole-doped region) of the gapped BST Dirac cone and vice versa. We show that the exploitation of the TSS features has led to a sign-reversal of THE repeatedly in a TI/FI top-gate stack. This discovery may impact technological applications in ultralow power skyrmion-based spintronics.

Chiral molecules cannot be superposed with their own mirror image. This yields two enantiomers of opposite handedness that are made out of the same building blocks, but interact differently with their environment. Hence, chiral sensing is of utmost importance for biology, chemistry, and life sciences. However, the impact of the handedness and chirality is very weak in most sensing schemes. Nevertheless, it has been demonstrated recently that chirality may result in strong coupling between resonant states with high quality factors. This is achieved by spectrally overlapping two quasibound states in the continuum in a periodic array of nanostructures. We demonstrate that this requires neither quasibound states in the continuum nor periodic arrays, which is exemplified for three achiral systems: a sphere with equal permittivity and permeability, a single core-shell structure, and a dielectric metasurface. For such achiral systems, we have shown previously that isolated resonant states exhibit a quadratic energy shift in the Pasteur parameter. However, for quasidegenerate states, we observe, using the rigorous resonant-state expansion and full-wave simulations, a linear energy shift and linear splitting in the presence of a chiral medium or molecule. Thus, the splitting is more sensitive to low concentrations of chiral molecules, which paves the way for novel chiral sensing schemes.

In this work, we study effects of magnon interactions in the excitation spectrum of ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ in the quantum paramagnetic phase in transverse field, where the $1/S$ spin-wave theory exhibits unphysical divergences at the critical field. We propose a self-consistent Hartree-Fock approach that eliminates such unphysical singularities while preserving the integrity of the singular threshold phenomena of magnon decay and spectrum renormalization that are present in both theory and experiment. With the microscopic parameters adopted from previous studies, this method yields a close quantitative agreement with the available experimental data for ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ in the relevant regime. Insights into the general structure of the spin-anisotropic model of ${\mathrm{CoNb}}_2{\mathrm{O}}_6$ and related zigzag chain materials are also provided and a discussion of the effects of additional longitudinal field on the spectrum is given.

Quantum spin liquids (QSLs) are theoretical states of matter with long-range entanglement and exotic quasiparticles. However, they generally elude quantitative theory, rendering their underlying phases mysterious and hampering efforts to identify experimental QSL states. Here we study triangular-lattice resonating-valence-bond QSL candidate materials ${\mathrm{KYbSe}}_2$ and ${\mathrm{NaYbSe}}_2$. We measure the magnon modes in their 1/3 plateau phase, where quantitative theory is tractable, using inelastic neutron scattering and fit them using nonlinear spin wave theory. We also fit the ${\mathrm{KYbSe}}_2$ heat capacity using high-temperature series expansion. Both ${\mathrm{KYbSe}}_2$ fits yield the same magnetic Hamiltonian to within uncertainty, confirming previous estimates and showing the Heisenberg ratio $J_2/J_1$ to be an accurate model for these materials. Most importantly, comparing ${\mathrm{KYbSe}}_2$ and ${\mathrm{NaYbSe}}_2$ shows that the smaller $A$-site ${\mathrm{Na}}^{+}$ ion has a larger $J_2/J_1$ ratio. However, hydrostatic pressure applied to ${\mathrm{KYbSe}}_2$ increases the ordering temperature (a result consistent with density functional theory calculations), indicating that pressure decreases $J_2/J_1$. These results show how the periodic table and hydrostatic pressure can tune the $A{\mathrm{YbSe}}_2$ materials in a controlled way.

Square-lattice systems offer a direct route for realizing two-dimensional (2D) quantum magnetism with frustration induced by competing interactions. In this work, the square lattice materials ${\mathrm{YbBi}}_2{\mathrm{IO}}_4$ and ${\mathrm{YbBi}}_2{\mathrm{ClO}}_4$ were investigated using a combination of magnetization and specific-heat measurements on polycrystalline samples. Specific-heat measurements provide evidence for long-range magnetic order below $T_{\mathrm{N}}$ = 0.21 K (0.25 K) for ${\mathrm{YbBi}}_2{\mathrm{IO}}_4$ $\left({\mathrm{YbBi}}_2{\mathrm{ClO}}_4\right)$. On the other hand, a rather broad maximum is found in the temperature-dependent magnetic susceptibility, located at $T_{\max }$ = 0.33 K (0.38 K) in ${\mathrm{YbBi}}_2{\mathrm{IO}}_4$ $\left({\mathrm{YbBi}}_2{\mathrm{ClO}}_4\right)$, consistent with the quasi-2D magnetism expected for the large separation between the magnetic layers. Estimation of the magnetic entropy supports the expected Kramers' doublet ground state for ${\mathrm{Yb}}^{3+}$ and the observed paramagnetic behavior is consistent with a well-isolated doublet. Roughly two-thirds of the entropy is consumed above $T_{\mathrm{N}}$, due to a combination of the quasi-2D behavior and magnetic frustration. The impact of frustration is examined from the viewpoint of a simplified $J_1-J_2$ square lattice model, which is frustrated for antiferromagnetic interactions. Specifically, a high-temperature series expansion analysis of the temperature-dependent specific-heat and magnetization data yields $J_2$/$J_1$ = 0.30 (=0.23) for ${\mathrm{YbBi}}_2{\mathrm{IO}}_4$ $\left({\mathrm{YbBi}}_2{\mathrm{ClO}}_4\right)$. This simplified analysis suggests strong frustration that should promote significant quantum fluctuations in these compounds, and thus motivates future work on the static and dynamic magnetic properties of these materials.

Melting of a real solid involves three stages: first comes surface melting, then the melting of grain boundaries, and finally the melting of the bulk. The first and second stages are often called “premelting.” While surface melting and bulk melting have been extensively studied, melting of grain boundaries was not. We developed an optical method which allowed us to study all three stages of melting in pure materials. Using Sn crystals and polycrystals, we were able to detect melting of the grain boundaries inside the solid around 5 K below the bulk melting temperature, and thus lead to a better understanding of how a real solid melts.

Electronic Raman scattering couples to the charge excitations in the system, including the plasmons. However, the plasmon response has a spectral weight of $\sim q^2$, where $q$, the momentum transferred by light, is small. In this paper, we show that in inversion-symmetry broken systems where Rashba type spin-orbit coupling affects the states at the Fermi energy (which is a known low-energy effect) as well as the transition elements to other states (a high-energy effect), there is an additional coupling of the plasmons to the Raman vertex, even at zero momentum transfer, that results in a spectral weight that is proportional to the spin-orbit coupling. The high-energy effect is due to the breaking of SU(2) spin invariance in the spin-flip transitions to the intermediate state. We present a theory for this coupling near the resonant regime of Raman scattering and show that in giant Rashba systems it can dominate over the conventional $q^2$ weighted coupling. We also provide experimental support along with a symmetry based justification for this spin-mediated coupling by identifying a prominent $c$-axis plasmon peak in the fully symmetric $A_1$ channel of the resonant Raman spectrum of the giant Rashba material BiTeI. This new coupling could lead to ways of manipulating coherent charge excitations in inversion-broken systems. This process is also relevant for spectroscopic studies in ultrafast spectroscopies, certain driven Floquet systems and topologically nontrivial phases of matter where strong inversion-breaking spin-orbit coupling plays a role.

A thorough description of the physics of a magnetic compound requires the validation of its microscopic spin Hamiltonian. Here, from the analysis of muon-spin rotation spectra recorded in the magnetically ordered state at low temperature in zero and finite magnetic fields, we determine the minimal Hamiltonian for the chiral binary intermetallic magnet MnSi, consistent with its high-temperature nonsymmorphic cubic space group $P{2}_{1}3$. The model provides constraints for the orientation of the Moriya vector characterizing the microscopic Dzyaloshinskii-Moriya interaction, with respect to the Mn nearest-neighbor bonds. Small twist and canting of the magnetic structure are revealed. Our result indicates that, within experimental uncertainties, the magnetoelastic coupling is not strong enough to lower the paramagnetic crystal symmetry in the magnetically ordered state. Additional implications from our work are discussed and complementary studies are suggested.

We use polarization-resolved Raman spectroscopy to demonstrate that for a three-dimensional giant Rashba system the bulk plasmon collective mode can directly couple to the Raman response even in the long-wavelength $\mathbf{q}\to \mathit{0}$ limit. Although conventional theory predicts the plasmon spectral weight to be suppressed as the square of its quasimomentum and thus negligibly weak in the Raman spectra, we observe a sharp in-gap plasmon mode in the Raman spectrum of BiTeI below the Rashba continuum. This coupling, in a polar system with spin-orbit coupling, occurs without assistance from phonons when the incoming photon excitation is resonant with Rashba-split intermediate states. We discuss the distinctive features of BiTeI's giant Rashba system band structure that enable the direct observation of plasmons in Raman scattering.

We revisit the issue of plasmon damping due to electron-electron interaction. The plasmon linewidth can be related to the imaginary part of the charge susceptibility or, equivalently, to the real part of the optical conductivity, $\mathrm{Re}\sigma (q,\omega )$. Approaching the problem first via a standard semiclassical Boltzmann equation, we show that $\mathrm{Re}\sigma (q,\omega )$ of a two-dimensional (2D) electron gas scales as $q^2{T}^2/{\omega }^4$ for $\omega \ll T$, which agrees with the results of Principi et al. [Phys. Rev. B 88, 195405 (2013)] and Sharma et al. [Phys. Rev. B 104, 045142 (2021)] but disagrees with that of Mishchenko et al. [Phys. Rev. B 69, 195302 (2004)], according to which $\mathrm{Re}\sigma (q,\omega )\propto q^2{T}^2/{\omega }^2$. To resolve this disagreement, we rederive $\mathrm{Re}\sigma (q,\omega )$ using the original method of Mishchenko et al. for an arbitrary ratio $\omega /T$ and show that while the last term is, indeed, present, it is subleading to the $q^2{T}^2/{\omega }^4$ term. We give a physical interpretation of both leading and subleading contributions in terms of the shear and bulk viscosities of an electron liquid, respectively. We also calculate $\mathrm{Re}\sigma (q,\omega )$ for a three-dimensional electron gas and doped monolayer graphene. We find that, all other parameters being equal, finite temperature has the strongest effect on the plasmon linewidth in graphene, where it scales as $T^{4}lnT$ for $\omega \ll T$.

Magnomechanical devices, in which magnetic excitations couple to mechanical vibrations, have been discussed as efficient and broadband microwave signal transducers in the classical and quantum limit. We experimentally investigate the resonant magnetoelastic coupling between the ferromagnetic resonance modes in metallic ${\mathrm{Co}}_{25}{\mathrm{Fe}}_{75}$ thin films, featuring ultralow magnetic damping as well as sizable magnetostriction, and standing transverse elastic phonon modes in sapphire, silicon, and gadolinium gallium garnet at cryogenic temperatures. For all substrates, we observe a coherent interaction between the acoustic and magnetic modes. We identify the phonon modes as transverse shear waves propagating with slightly different velocities ($\mathrm{\Delta }v/v\simeq {10}^{-5}$); i.e., all investigated substrates show potential for phononic birefringence as well as phonon-mediated angular momentum transport. Our magnon-phonon hybrid systems operate in a coupling regime analogous to the Purcell enhanced damping in cavity magnonics.

Moiré materials are artificial crystals formed at van der Waals heterojunctions that have emerged as a highly tunable platform that is able to realize much of the rich quantum physics of electrons in atomic scale solids, and in several cases even new quantum phases of matter. Here we use finite-size exact diagonalization methods to explore the physics of single-band itinerant electron ferromagnetism in semiconductor moiré materials. We predict where ferromagnetism is likely to occur in triangular-lattice moiré systems, and where it is likely to yield the highest Curie temperatures.

We analyze the spectrum of spatially uniform, single-particle fluctuation modes in the linear electromagnetic response of a semiconductor laser. We show that if the decay rate of the interband polarization, ${\gamma }_p$, and the relaxation rate of the occupation distribution, ${\gamma }_f$, are different, a gapless regime exists in which the order parameter ${\mathrm{\Delta }}^{\left(0\right)}\left(k\right)$ (linear in the coherent photon field amplitude and the interband polarization) is finite but there is no gap in the real part of the single-particle fluctuation spectrum. As the laser is an open, pumped, and dissipative system, this regime may be considered a nonequilibrium analog of gapless superconductivity. We analyze the fluctuation spectrum in both the photon laser limit, where the interactions among the charged particles are ignored, and the more general model with interacting particles. In the photon laser model, the order parameter is reduced to a momentum-independent quantity, which we denote by $\mathrm{\Delta }$. We find that, immediately above the lasing threshold, the real part of the fluctuation spectrum remains gapless when $0<\left|\mathrm{\Delta }\right|<\sqrt{2/27}|{\gamma }_f-{\gamma }_p|$ and becomes gapped when $\left|\mathrm{\Delta }\right|$ exceeds the upper bound of this range. Viewed as a complex function of $\left|\mathrm{\Delta }\right|$ and the electron-hole energy, the eigenvalue set displays some interesting exceptional point (EP) structure around the gapless-gapped transition. The transition point is a third-order EP, where three eigenvalues (and eigenvectors) coincide. Switching on the particle interactions in the full model modifies the spectrum of the photon laser model and, in particular, leads to a more elaborate EP structure. However, the overall spectral behavior of the continuous (noncollective) modes of the full model can be understood on the basis of the relevant results of the photon laser model.

The Halperin-Lee-Read Fermi sea of composite fermions at half-filled lowest Landau level is the realization of a fascinating metallic phase that is a strongly correlated “non-Fermi liquid” from the electrons' perspective. Remarkably, experiments have found that, as the width of the quantum well is increased, this state makes a transition into a fractional quantum Hall state, the origin of which has remained an important puzzle since its discovery more than three decades ago. We perform detailed and accurate quantitative calculations using a systematic variational framework for the pairing of composite fermions that closely mimics the Bardeen-Cooper-Schrieffer theory of superconductivity. Our calculations show that, (i) as the quantum-well width is increased, the single-component composite-fermion Fermi sea occupying the lowest symmetric subband of the quantum well undergoes an instability into a single-component $p$-wave paired state of composite fermions; (ii) the theoretical phase diagram in the quantum-well width–electron-density plane is in excellent agreement with experiments; (iii) a sufficient amount of asymmetry in the charge distribution of the quantum well destroys the fractional quantum Hall effect, as observed experimentally; and (iv) the two-component 331 state is energetically less favorable than the single-component paired state. Evidence for fractional quantum Hall effect has been seen in wide quantum wells also at quarter-filled lowest Landau level; here our calculations indicate an $f$-wave paired state of composite fermions. We further investigate bosons in the lowest Landau level at filling factor equal to one and show that a $p$-wave pairing instability of composite fermions, which are bosons carrying a single vortex, occurs for the short range as well as the Coulomb interaction, in agreement with exact diagonalization studies. The general consistency of the composite-fermion Bardeen-Cooper-Schrieffer approach with experiments lends support to the notion of composite-fermion pairing as the primary mechanism of fractional quantum Hall effects at even-denominator filling factors. Various experimental implications are mentioned.

Magnetic skyrmions and antiskyrmions are topologically protected quasiparticles exhibiting a whirling spin texture in real space. Antiskyrmions offer some advantages over skyrmions as they are expected to have higher stability and can be electrically driven with no transverse motion. However, unlike the widely investigated skyrmions, antiskyrmions are rarely observed due to the required anisotropic Dzyaloshinskii-Moriya interaction (DMI). Here, we propose to exploit the recently demonstrated van der Waals (vdW) assembly of two-dimensional (2D) materials that breaks inversion symmetry and creates conditions for anisotropic DMI. Using a 2D vdW magnet ${\mathrm{CrI}}_3$ as an example, we demonstrate, based on density-functional theory (DFT) calculations, that this strategy is a promising platform to realize antiskyrmions. Polar layer stacking of two centrosymmetric magnetic monolayers of ${\mathrm{CrI}}_3$ efficiently lowers the symmetry, resulting in anisotropic DMI that supports antiskyrmions. The DMI is reversible by switching the ferroelectric polarization inherited from the polar layer stacking, offering the control of antiskyrmions by an electric field. Furthermore, we find that the magnetocrystalline anisotropy and DMI of ${\mathrm{CrI}}_3$ can be efficiently modulated by Mn doping, creating a possibility to control the size of antiskyrmions. Using atomistic spin dynamics simulations with the parameters obtained from our DFT calculations, we predict the formation of antiskyrmions in a ${\mathrm{Cr}}_{0.88}{\mathrm{Mn}}_{0.12}{\mathrm{I}}_3$ bilayer and switching their spin texture with polarization reversal. Our results open a direction to generate and control magnetic antiskyrmions in 2D vdW magnetic systems.

The hydrodynamic transport of local conserved densities furnishes an effective coarse-grained description of the dynamics of a many-body quantum system. However, the full quantum dynamics contains much more structure beyond the simplified hydrodynamic description. Here we show that systems with the same hydrodynamics can nevertheless belong to distinct dynamical universality classes, as revealed by new classes of experimental observables accessible in synthetic quantum systems, which can, for instance, measure simultaneous site-resolved snapshots of all of the particles in a system. Specifically, we study the full counting statistics of spin transport, whose first moment is related to linear-response transport, but the higher moments go beyond. We present an analytic theory of the full counting statistics of spin transport in various integrable and nonintegrable anisotropic one-dimensional spin models, including the XXZ spin chain. We find that spin transport, while diffusive on average, is governed by a distinct non-Gaussian dynamical universality class in the models considered. We consider a setup in which the left and right half of the chain are initially created at different magnetization densities, and consider the probability distribution of the magnetization transferred between the two half-chains. We derive a closed-form expression for the probability distribution of the magnetization transfer, in terms of random walks on the half-line. We show that this distribution strongly violates the large-deviation form expected for diffusive chaotic systems, and explain the physical origin of this violation. We discuss the crossovers that occur as the initial state is brought closer to global equilibrium. Our predictions can directly be tested in experiments using quantum gas microscopes or superconducting qubit arrays.

Inspired by the discovery of superconductivity in moiré materials with isolated narrow bandwidth electronic bands, here we analyze critically the question of what is the maximum attainable $T_c$ in interacting flat-band systems. We focus specifically on the low-energy effective theory, where the density-density interactions are projected to the set of partially filled flat bands. The resulting problem is inherently nonperturbative, where the standard mean-field approximation is not applicable. Here we develop further our recent Schrieffer-Wolf transformation based approach [PNAS 120, e2217816120 (2023)] to compute the effective electromagnetic response and the superconducting phase-stiffness in terms of “projected” gauge transformations and extend the formalism to compute the stiffness for excitonic superfluids. Importantly, our method requires neither any “Wannierization” for the narrow bands of interest, regardless of their (non)topological character, nor any knowledge of an underlying pairing symmetry, and can be set up directly in momentum-space. We use this formalism to derive upper bounds on the phase stiffness for sign-problem-free models, where their values are known independently from numerically exact quantum Monte Carlo computations. We also illustrate the analytical structure of these bounds for the superconducting and excitonic phase-stiffness for perfectly flat bands with Landau-level-like wave functions.

We investigate the effect that spatially modulated continuous conserved quantities can have on quantum ground states. We do so by introducing a family of one-dimensional local quantum rotor and bosonic models which conserve finite Fourier momenta of the particle number, but not the particle number itself. These correspond to generalizations of the standard Bose-Hubbard model and relate to the physics of Bose surfaces. First, we show that, while having an infinite-dimensional local Hilbert space, such systems feature a nontrivial Hilbert-space fragmentation for momenta incommensurate with the lattice. This is linked to the nature of the conserved quantities having a dense spectrum and provides the first such example. We then characterize the zero-temperature phase diagram for both commensurate and incommensurate momenta. In both cases, analytical and numerical calculations predict a phase transition between a gapped (Mott insulating) and quasi-long-range-order phase; the latter is characterized by a two-species Luttinger liquid in the infrared but dressed by oscillatory contributions when computing microscopic expectation values. Following a rigorous Villain formulation of the corresponding rotor model, we derive a dual description, from where we estimate the robustness of this phase using renormalization-group arguments, where the driving perturbation has ultralocal correlations in space but power-law correlations in time. We support this conclusion using an equivalent representation of the system as a two-dimensional vortex gas with modulated Coulomb interactions within a fixed symmetry sector. We conjecture that a Berezinskii-Kosterlitz-Thouless-type transition is driven by the unbinding of vortices along the temporal direction.