We develop a systematic theory for excitons subject to Fermi-Hubbard physics in moiré twisted transition metal dichalcogenides (TMDs). Specifically, we consider excitons from two moiré bands with a Mott-insulating valence band sustaining ${120}^{\circ }$ spin order. These “Mott-moiré excitons,” which are achievable in twisted TMD heterobilayers, are bound states of a magnetic polaron in the valence band and a free electron in the conduction band. We find significantly narrower exciton bandwidths in the presence of Hubbard physics, serving as a potential experimental signature of strong correlations. We also demonstrate the high tunability of Mott-moiré excitons through the dependence of their binding energies, diameters, and bandwidths on the moiré period. In addition, we study bound states between charges outside of the strongly correlated moiré band and find that these as well exhibit signatures of spin correlation. Our work provides guidelines for future exploration of strongly correlated excitons in triangular Hubbard systems such as twisted TMD heterobilayers.

Single-photon sources with near-unity efficiency and indistinguishability play a major role in the development of quantum technologies. However, on-demand excitation of the emitter imposes substantial limitations to the source performance. Here, we show that coherent two-color pumping allows for population inversion arbitrarily close to unity in bulk quantum dots thanks to a decoupling effect between the emitter and its phonon bath. Driving a micropillar single-photon source with this scheme, we calculate very high photon emission into the cavity mode (0.95 photons per pulse) together with excellent indistinguishability (0.975) in a realistic configuration, thereby removing the limitations imposed by the excitation scheme on single-photon source engineering.

We investigate the stochastic dynamics of a thermal machine realized by a fast-driven Otto cycle. By employing a stochastic approach, we find that system coherences strongly affect fluctuations depending on the thermodynamic current. Specifically, we observe an increment in the system instabilities when considering the heat exchanged with the cold bath. On the contrary, the cycle precision improves when the system couples with the hot bath, where thermodynamic fluctuations reduce below the classical thermodynamic uncertainty relation bound. Violation of the classical bound holds even when a dephasing source couples with the system. We also find that coherence suppression not only restores the cycle cooling but also enhances the convergence of fluctuation relations by increasing the entropy production of the reversed process. An additional analysis unveiled that the stochastic sampling required to ensure good statistics increases for the cooling cycle while downsizes for the other protocols. Despite the simplicity of our model, our results provide further insight into thermodynamic relations at the stochastic level.

We report on the magnetization, specific heat, and electrical resistivity for a newly discovered heavy-fermion (HF) compound, ${\mathrm{YbV}}_6{\mathrm{Sn}}_6$, which is crystallized in a hexagonal ${\mathrm{HfFe}}_6{\mathrm{Ge}}_6$-type structure, highlighted by the stacking of the triangular ytterbium sublattice and kagome vanadium sublattice. Above 2 K, ${\mathrm{YbV}}_6{\mathrm{Sn}}_6$ shows typical HF properties due to the Kondo effect on the Kramers doublet of ${\mathrm{Yb}}^{3+}$ ions in the crystalline electric field. A remarkable magnetic ordering occurs at $T_{\mathrm{N}}=0.40$ K in zero field, while a weak external field suppresses the ordering and induces non-Fermi-liquid behavior. In strong magnetic field, the compound shows a heavy Fermi-liquid state. ${\mathrm{YbV}}_6{\mathrm{Sn}}_6$ is represented as one of the few examples of Yb-based HF compounds hosting a triangular Kondo lattice on which a magnetic field induces quantum criticality near zero temperature.

Orbital angular momentum (OAM) and torque transfer play central roles in a wide range of magnetic textures and devices including skyrmions and spin-torque electronics. Analogous topological structures are now also being explored in ferroelectrics, including polarization vortex arrays in ferroelectric/dielectric superlattices. Unlike magnetic toroidal order, electric toroidal order does not couple directly to linear external fields. Instead, we find that the presence of an electric toroidal moment in a ferrorotational phase transfers measurable torque and OAM to a localized electron beam in the ballistic limit. We record these torque transfers from a high-energy electron beam using a momentum-resolved detector. This approach provides a high-sensitivity method to detect polarization fields and their more complex order parameters and topologies. In addition to toroidal order, we also demonstrate high-precision measurements of vorticity and chirality for polar vortexlike phases.

Ultrastrong light-matter coupling opens exciting possibilities to generate squeezed quantum states and entanglement. We propose achieving this regime in superconducting hybrid nanostructures with ferromagnetic interlayers. Strong confinement of the electromagnetic field between superconducting plates results in the existence of magnon-polariton (MP) modes with ultrastrong magnon-photon coupling, ultrahigh cooperativity, and colossal group velocities. These modes provide a numerically accurate explanation of recent experiments and have intriguing quantum properties. The MP quantum vacuum consists of the squeezed magnon and photon states with the degree of squeezing controlled in wide limits by the external magnetic field. The ground-state population of virtual photons and magnons is vast and can be used for generating correlated magnon and photon pairs. MP excitations contain bipartite entanglement between magnons and photons. Our results indicate that superconducting/ferromagnet nanostructures are very promising for quantum magnonics.

We report inelastic neutron scattering measurements of magnetic excitations in ${\mathrm{YbMnSb}}_2$, a low-carrier-density Dirac semimetal in which the antiferromagnetic Mn layers are interleaved with Sb layers that host Dirac fermions. We observe a measurable broadening of spin waves, which is consistent with substantial spin-fermion coupling. The spin-wave damping $\gamma$ in ${\mathrm{YbMnSb}}_2$ is roughly twice larger compared to that in a sister material, ${\mathrm{YbMnBi}}_2$, where an indication of a small damping consistent with a theoretical analysis of the spin-fermion coupling was reported. The interplane interaction between the Mn layers in ${\mathrm{YbMnSb}}_2$ is also much stronger, suggesting that the interaction mechanism is rooted in the same spin-fermion coupling. Our results establish the systematics of spin-fermion interactions in layered magnetic Dirac materials.

Electronic-structure calculations, the cluster-variation method of statistical mechanics, and the phase-field method were combined in attempted first-principles calculations of phase equilibria and microstructural evolution associated with the disorder-$\mathrm{L}{1}_0$ transition of the Fe-Pt system. The calculated disorder-$\mathrm{L}{1}_0$ transition temperature was within $\sim 10$ K difference from the experimental value, and the locus of spinodal ordering temperature is placed in the phase diagram. The calculated microstructure demonstrates preferential growth of the ordered domain along the <100> direction and, in the later period, an anisotropic morphology of an antiphase domain structure develops. We offered an interpretation from the atomistic point of view for this morphology. We therefore achieved consistent first-principles multiscale calculations of phase equilibria and microstructural evolution, bridging microscopic to mesoscopic scales without any adjusting parameters.

We report a systematic investigation of the finite-size effect on magnetostructural transition in CrN based on a series of nanoparticle samples with various sizes prepared by a high-pressure method, leading to the discovery of anomalous critical behaviors near transition, especially for spin correlation length. Such critical anomalies are presumably attributed to the strong spin-lattice coupling in this nitride, because the spin correlation of fluctuation can be greatly affected by the concurrent structural distortion. Using the conventional Gibbs-Thomson theorem and size-dependent effect, the scaling relationship between transition temperature and spin correlation length is deduced by involving a term of the transition-induced volume change that rationalizes the role that the lattice degree of freedom played during magnetic ordering. A phenomenological model is also established for understanding the kinetics and dynamics of magnetic fluctuation at transition in a finite-size system. In addition, our results also show that CrN nanoparticles possess a compressed surface shell, which can be described by Laplace pressure, obeying a unique $\frac{1}{d^{1.8}}$ finite-size scaling law.

We investigated the influence of two structural phase transitions in methylammonium lead tri-bromide ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$ $\left({\mathrm{MAPbBr}}_3\right)$ single crystals on the activity of polar phonons using time-domain THz transmission and far infrared (IR) reflectance spectra. The fitting using the Lorentz model of damped harmonic oscillators also enables us to determine all the polar phonon parameters, IR response functions, and their temperature dependence. The fit of the far IR spectra provides a nearly perfect quantitative description of the measured reflectance at room temperature using three phonons. On cooling, three IR active phonons of the cubic and tetragonal structures split into 15 separate bands, signaling the transformation to the orthorhombic phase below 150 K. This result is compared with the factor group analysis of the polar phonons in all three crystallographic phases. The absence of a soft optical phonon supports the previously published order-disorder character of the structural phase transitions.

The origin of the nontrivial power-law relationships between atomic packing density and the related structural properties is considered to be a key puzzle in understanding the nature of glasses. Here, we report the direct link between the medium-range structural evolution and elastic heterogeneity by systematically investigating the packing-density-dependent properties of various glasses based on extensive large-scale molecular dynamics simulations. It is shown that the power exponent of the peaks corresponding to the medium-range orders on the pair correlation function converges to the exponent of the first diffraction peak rather than the Euclidean dimension. This exponent can be regarded as an indicator of heterogeneous mechanical properties. The global power-law relationship results from intrinsic mechanical heterogeneity, with the nontrivial power-law response being a local feature. Our finding provides a different perspective on order in disordered materials and sheds some light on the structural-property relationship in glasses.

In two-dimensional superconductors, Yu-Shiba-Rusinov bound states, induced by the magnetic impurities, extend over long distances giving rise to a long-range hopping model supporting a large number of topological phases with distinct Chern numbers. Here, we study how the electron-electron interactions affect, on a mean-field level, the selection of the realized Chern numbers and the magnitudes of the topological energy gaps in this model. We find that, in the case of an individual choice of the model parameters, the interactions can enhance or reduce the topological gap as well as cause topological phase transitions because of the complex interplay of superconductivity, magnetism, and the large spatial extent of the Yu-Shiba-Rusinov states. By sampling a large number of realizations of Yu-Shiba-Rusinov lattice models with different model parameters, we show that, statistically, the interactions have no effect on the realized Chern numbers and typical magnitudes of the topological gaps. However, the interactions substantially increase the likelihood of the largest topological gaps in the tails of the energy gap distribution in comparison to the noninteracting case.

We consider the spin-$\frac{1}{2}$ Heisenberg chain (XXX model) weakly perturbed away from integrability by an isotropic next-to-nearest neighbor exchange interaction. Recently, it was conjectured that this model possesses an infinite tower of quasiconserved integrals of motion (charges) [D. Kurlov et al., Phys. Rev. B 105, 104302 (2022)]. In this work we first test this conjecture by investigating how the norm of the adiabatic gauge potential (AGP) scales with the system size, which is known to be a remarkably accurate measure of chaos. We find that for the perturbed XXX chain the behavior of the AGP norm corresponds to neither an integrable nor a chaotic regime, which supports the conjectured quasi-integrability of the model. We then prove the conjecture and explicitly construct the infinite set of quasiconserved charges. Our proof relies on the fact that the XXX chain perturbed by next-to-nearest exchange interaction can be viewed as a truncation of an integrable long-range deformation of the Heisenberg spin chain.

Moiré superlattices made with van der Waals layers are an excellent platform for exploring a wide range of exotic and important physical phenomena. In this study, we used first-principles calculations and atomistic spin dynamics simulations to design a two-dimensional van der Waals $\mathrm{Mn}{\mathrm{S}}_2/\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ multiferroic moiré heterosuperlattice with tunable skyrmions through magnetoelectric coupling. The inherent lattice mismatch between the two monolayers of $\mathrm{Mn}{\mathrm{S}}_2$ and $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ creates incommensurate moiré patterns, along with modulated magnetic anisotropy and emerging magnetic skyrmions in $\mathrm{Mn}{\mathrm{S}}_2$. The magnetic skyrmion in $\mathrm{Mn}{\mathrm{S}}_2$ is strongly influenced by magnetoelectric coupling and can be tuned by the ferroelectric polarization of $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$. Furthermore, these magnetic skyrmions can be controlled by a pulsed current to move or freeze within the moiré period under different ferroelectric polarization states of the $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{S}}_6$ layer. Our work showcases a two-dimensional van der Waals moiré heterosuperlattice with magnetoelectrically tuned magnetic skyrmions, which establishes a foundation for designing future spintronic devices.

There are few inelastic neutron scattering (INS) reports on the superconducting single crystals of the FeAs-1111 system, even though it was first discovered in 2008, due to the extreme difficulty in large single-crystal growth. In this paper, we have studied the low-energy spin excitations in optimally electron-doped ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$ single crystals with $T_{\mathrm{c}}=21$ K by INS. The resonance energy of the superconducting spin resonant mode with $E_{\mathrm{r}}=12$ meV amounts to $6.6k_{\mathrm{B}}{T}_{\mathrm{c}}$, which constitutes a large $E_{\mathrm{r}}/k_{\mathrm{B}}{T}_{\mathrm{c}}$ ratio among iron-based superconductors. The large ratio implies a strong coupling between conduction electrons and magnetic excitations in ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$. The resonance possesses a magnonlike upward dispersion along the transverse direction due to the anisotropy of the spin-spin correlation length within the $ab$ plane in the normal state, which points to a spin fluctuation mediated sign-reversed $s\pm$ wave pairing in ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$.

We demonstrate that a correlated equilibrium $f$-electron system with time-reversal symmetry can exhibit a ${\mathbb{Z}}_2$ non-Hermitian skin effect of quasiparticles. In particular, we analyze a two-dimensional periodic Anderson model with spin-orbit coupling by combining the dynamical mean-field theory (DMFT) and the numerical renormalization group. We prove the existence of the ${\mathbb{Z}}_2$ skin effect by explicitly calculating the topological invariant and show that spin-orbit interaction is essential to this effect. Our DMFT analysis demonstrates that the ${\mathbb{Z}}_2$ skin effect of quasiparticles is reflected on the pseudospectrum. Furthermore, we analyze temperature effects on this skin effect using the generalized Brillouin zone technique, which clarifies that the skin modes are strongly localized above the Kondo temperature.

Magnetoresistance of the double-atomic-layer $\text{rect-}\sqrt{7}\times \sqrt{3}\text{-In}/\mathrm{Si}\left(111\right)$ phase was measured in situ using the four-point-probe technique in the temperature range from 2.5 to 33 K and magnetic fields from $-8$ to $+8$ T. The $\text{rect-}\sqrt{7}\times \sqrt{3}\text{-In}/\mathrm{Si}\left(111\right)$ was found to demonstrate a classical quadratic behavior of magnetoresistance at fields lower than $\sim 0.2$ T, and a large positive linear magnetoresistance at fields up to 8 T in the low-temperature range of up to $\sim 10$ K. In contrast, the results obtained on the parent single- and double-atomic In layers formed on the ${\mathrm{NiSi}}_2/\text{Si}\left(111\right)$ substrate show much lower values of magnetoresistance. In view of the density functional theory calculation results, which reveal the presence of tiny Fermi pockets in the $\text{rect-}\sqrt{7}\times \sqrt{3}\text{-In/Si}\left(111\right)$ electronic band structure, we attribute the observed large linear magnetoresistance at the low-temperature range to a quantum mechanical origin.

Electrons (or holes) confined in two-dimensional (2D) semiconductor layers have served as model systems for studying disorder and interaction effects for almost 50 years. In particular, strong disorder drives the metallic 2D carriers into a strongly localized Anderson insulator (AI) at low densities whereas pristine 2D electrons in the presence of no (or little) disorder should solidify into a Wigner crystal (WC) at very low carrier densities in order to optimize their Coulomb potential energy. The ability to tune the carrier density continuously in a fixed sample allows the 2D semiconductor system to go from a high-density metallic Fermi liquid to a low-density disorder dominated Anderson insulator, or, if the sample is particularly clean, to a Coulomb interaction dominated low-density quantum Wigner crystal. Since the disorder in 2D semiconductors is mostly Coulomb disorder arising from random unintentional quenched charged impurities in the environment, the applicable physics is complex as the carriers interact with each other as well as with the random charged impurities through the same long-range Coulomb coupling. In addition, the Wigner crystallization occurs at such low carrier densities, that in most situations the relevant carrier density is comparable to the background charged impurity density even in ultraclean samples. By critically theoretically analyzing the experimental transport data in depth using a realistic transport theory to calculate the low-temperature 2D resistivity as a function of carrier density in 11 different experimental samples covering nine different materials, we establish, utilizing the Ioffe-Regel-Mott (IRM) criterion for strong localization, a direct connection between the critical localization density for the 2D metal-insulator transition (MIT) and the sample mobility deep into the metallic state, which for particularly clean samples could lead to a localization density low enough to make the transition appear to be a Wigner crystallization. We believe that the insulating phase is always an effective Coulomb disorder-induced strongly localized AI, which may have short-range WC-like correlations at very low carrier densities. Our theoretically calculated disorder-driven critical MIT density agrees well with experimental findings in all 2D samples, even for the ultraclean samples where the critical density approaches the WC transition density. In particular, the extrapolated critical density for the 2D MIT seems to vanish when the high-density mobility goes to infinity, indicating that transport probes a disorder-localized insulating ground state independent of how low the carrier density might be.