We predict the occurrence of odd-frequency equal-spin triplet correlations at normal metal-superconductor junction. This result is significant because equal-spin triplet correlations are associated with the presence of dissipation-less pure spin current. Inserting a spin-flipper at the interface of a normal metal-superconductor junction excites equal-spin triplet correlations. The existence of odd-frequency equal-spin triplet correlations in the absence of odd-frequency mixed-spin triplet correlations is the main conclusion of this work. It tallies well with the measured local magnetization density of states and spin-polarized local density of states at the interface. The importance of spin-flip scattering to the obtained results is manifest when we compare our normal metal-spin flipper-superconductor junction to other hybrid junctions where either only spin mixing or both spin mixing and spin-flip scattering are present.

Hyperferroelectricity is an intriguing phenomenon in that electric polarization persists under the open-circuit boundary condition (OCBC). Using first-principles density functional calculations, we investigate the hyperferroelectric properties of ${\mathrm{LiNbO}}_3$, a technologically important ferroelectric solid. We find that (i) the longitudinal-optic ${\mathrm{A}}_{2\mathrm{u}}({\mathrm{LO}}_1)$ phonon is soft with imaginary frequency $96i{\mathrm{cm}}^{-1}$ when centrosymmetric ${\mathrm{LiNbO}}_3$ is under OCBC, and this soft LO phonon is shown to differ drastically from the corresponding transverse-optic ${\mathrm{A}}_{2\mathrm{u}}({\mathrm{TO}}_1)$ phonon. (ii) The ${\mathrm{A}}_{2\mathrm{u}}({\mathrm{LO}}_1)$ phonon is able to yield, under OCBC, a free-energy minimum with well depth of $-9$ meV at a nonzero polarization of 0.023 C/${\mathrm{m}}^2$, thereby capable of producing hyperferroelectricity. (iii) The origin that ${\mathrm{A}}_{2\mathrm{u}}({\mathrm{LO}}_1)$ is capable of inducing HyFE is determined, and it stems from the extraordinarily small mode effective charge of this soft LO phonon. This finding also provides a key guiding principle for searching and designing new hyperferroelectrics. (iv) Despite that ${\mathrm{A}}_{2\mathrm{u}}({\mathrm{LO}}_1)$ is able to induce HyFE, we find that the ground state of ${\mathrm{LiNbO}}_3$ under OCBC is nevertheless not polar, revealing that the existence of a soft LO phonon does not guarantee HyFE. (v) Interestingly, we further show that ${\mathrm{LiNbO}}_3$ under OCBC may exhibit unusual tristable polarization states, with two potential wells of depth $-23$ meV and $-1.9$ meV, respectively. Besides providing insightful knowledge for understanding HyFE in general, these results reveal that there is rich and interesting hyperferroelectric physics in ${\mathrm{LiNbO}}_3$ under OCBC, in particular.

Magnetic interlayer coupling is a key ingredient in designing magnetic multilayers with functionalities that reach out to the realm of applications. In epitaxial ferromagnetic (FM) oxide multilayers, the magnetic interlayer coupling is, however, less studied and its prediction is often a challenging task. Ultrathin FM ${\mathrm{SrRuO}}_3$ epitaxial films with perpendicular magnetic anisotropy, interfaced with suitable oxides, may be susceptible of forming skyrmions. Hence, a strong FM interlayer coupling would be beneficial to achieve uniform switching behavior of a ${\mathrm{SrRuO}}_3$-based multilayer. Previous studies reported that the coupling of two ${\mathrm{SrRuO}}_3$ layers separated by a non-FM oxide spacer is at best weakly FM and the two FM layers switch at markedly different fields. Here we study the magnetic interlayer coupling between two FM ${\mathrm{SrRuO}}_3$ layers separated by ultrathin ${\mathrm{LaNiO}}_3$ in epitaxial heterostructures grown on ${\mathrm{SrTiO}}_3$(100) single crystals. We found that FM ${\mathrm{SrRuO}}_3$ layers separated by 2 monolayers (MLs) thick ${\mathrm{LaNiO}}_3$ show weak FM interlayer coupling of about $106\mu \mathrm{J}/m^2$ at 10 K. The coupling becomes strongly FM for four MLs thick (about 1.6 nm) ${\mathrm{LaNiO}}_3$ spacers and the two ${\mathrm{SrRuO}}_3$ layers reverse their magnetization at a common value of the perpendicular magnetic field. This is likely due to a transition of the ${\mathrm{LaNiO}}_3$ spacer from insulating to metallic, as its thickness increases.

We considered the effect of optical pumping at the $f-f$ transitions frequency on $d-f$ superexchange in the rare-earth antiferromagnetic ferroborate ${\mathrm{Nd}}_{0.5}{\mathrm{Gd}}_{0.5}{\mathrm{Fe}}_3{\left({\mathrm{BO}}_3\right)}_4$ between ${\mathrm{Nd}}^{3+}$ ions in the excited states of the ${}^4{G}_{5/2}, {}^2{G}_{7/2}$, and ${\mathrm{Fe}}^{3+}$ in the ground state ${}^6{A}_1$. In the ferroborate, $d-f$ superexchange is directly related to the strong magnetoelectric coupling observed in the ground state of the material. We show that under optical pumping at the frequency of $f-f$ transitions ${}^4{I}_{9/2}\leftrightarrow ({}^4{G}_{5/2}+{}^2{G}_{7/2})$ in the ${\mathrm{Nd}}^{3+}$ ion, the nature of the $d\text{−}f$ interaction changes to FM. The phenomena can be explained by the occupation of the excited ${\mathrm{Nd}}^{3+}$ “spin pure” states with spin $1/2$, or their mixing to the optically excited states with spin $3/2$ due to the spin-orbit interaction. Near optically excited ions ${\mathrm{Nd}}^{3+}$, magnetic frustrations change from FM ordering under the $d\text{−}f$ AFM interaction to AFM ordering under the FM interaction in the basal hexagonal plane.

Scaling up trapped-ion quantum computers requires new trap materials to be explored. Here we present experiments with a surface ion trap made from the high-temperature superconductor YBCO, a promising material for future trap designs. We show that voltage noise from superconducting electrode leads is negligible within the sensitivity $S_V=9\times {10}^{-20}{\mathrm{V}}^2/\mathrm{Hz}$ of our setup, and for lead dimensions typical for advanced trap designs. Furthermore, we investigate the frequency and temperature dependence of electric-field noise above a YBCO surface. We find a $1/f$ spectral dependence of the noise and a nontrivial temperature dependence, with a plateau in the noise stretching over roughly $60\mathrm{K}$. The onset of the plateau coincides with the superconducting transition, indicating a connection between the dominant noise and the YBCO trap material. We exclude the YBCO bulk as origin of the noise and suggest further experiments to decide between the two remaining options explaining the observed temperature dependence: noise screening within the superconducting phase, or surface noise activated by the YBCO bulk through some unknown mechanism.

Restricted Boltzmann machines (RBMs) and deep Boltzmann machines (DBMs) are important models in machine learning, and recently found numerous applications in quantum many-body physics. We show that there are fundamental connections between them and tensor networks. In particular, we demonstrate that any RBM and DBM can be exactly represented as a two-dimensional tensor network. This representation gives characterizations of the expressive power of RBMs and DBMs using entanglement structures of the tensor networks, and also provides an efficient tensor network contraction algorithm for the computing partition function of RBMs and DBMs. Using numerical experiments, we show that the proposed algorithm is more accurate than the state-of-the-art machine learning methods in estimating the partition function of RBMs and DBMs, and have potential applications in training DBMs for general machine learning tasks.

A numerical method is presented for reproducing fermionic quantum gas microscope experiments in equilibrium. By employing nested componentwise direct sampling of fermion pseudodensity matrices, as they arise naturally in determinantal quantum Monte Carlo (QMC) simulations, a stream of pseudosnapshots of occupation numbers on large systems can be produced. There is a sign problem even when the conventional determinantal QMC algorithm can be made sign-problem free, and every pseudosnapshot comes with a sign and a reweighting factor. Nonetheless, this “sampling sign problem” turns out to be weak and manageable in a large, relevant parameter regime. The method allows one to compute distribution functions of arbitrary quantities defined in occupation number space and, from a practical point of view, facilitates the computation of complicated conditional correlation functions. While the projective measurements in quantum gas microscope experiments achieve direct sampling of occupation number states from the density matrix, the presented numerical method requires a Markov chain as an intermediate step and thus achieves only indirect sampling, but the full distribution of pseudosnapshots after (signed) reweighting is identical to the distribution of snapshots from projective measurements.

We study theoretically optomechanical damping and amplification spectra for vibrations interacting with excitonic polaritons in a zero-dimensional microcavity. We demonstrate that the spectra strongly depend on the ratio of the exciton-phonon and the photon-phonon coupling constants. The interference between these couplings enables a situation when optomechanical gain exists either only for a lower polaritonic resonance or only for an upper polaritonic resonance. Our results provide insight into the optomechanical interactions in various multimode systems, where several resonant oscillators, such as photons, plasmons, or excitons, are coupled to the same vibration mode.

Nonlinear responses give rise to various exciting phenomena, which are forbidden in linear responses. Among them, one of the most fascinating phenomena is the recently observed giant spontaneous Hall effect in ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pd}}_3$. This material is a promising candidate for a Weyl-Kondo semimetal, and this experiment implies that strong correlation effects can enhance the nonlinear Hall effect. However, most theoretical studies on nonlinear responses have been limited to free systems, and the connection between nonlinear responses and strong correlation effects is poorly understood. Motivated by these experiments and recent theoretical advances to analyze strong correlation effects on the nonlinear response, we study a periodic Anderson model describing ${\mathrm{Ce}}_3{\mathrm{Bi}}_4{\mathrm{Pd}}_3$ using the dynamical mean-field theory. We calculate the nonlinear longitudinal conductivity and the nonlinear Hall conductivity using the Kubo formula extended to the nonlinear response regime and clarify their temperature dependencies. We numerically show that strong correlations can enhance nonlinear conductivities, and we conclude that the magnitude of the experimentally observed giant nonlinear Hall effect can be explained by strong correlation effects.

Here, we report the effects of enhanced magnetic fields resulting from type-II superconducting NbTiN slabs adjacent to narrow Hall bar devices fabricated from epitaxial graphene. Observed changes in the magnetoresistances were found to have minimal contributions from device inhomogeneities, magnet hysteresis, electron density variations along the devices, and transient phenomena. We hypothesize that Abrikosov vortices, present in type-II superconductors, contribute to these observations. By determining the London penetration depth, coupled with elements of Ginzburg-Landau theory, one can approximate an upper bound on the effect that vortex densities at low fields $\left(<1\mathrm{T}\right)$ have on the reported observations. These analyses offer insights into device fabrication and how to utilize the Meissner effect for any low-field and low-temperature applications using superconductors.

We present the results of an experiment on measuring the gradient of the Casimir force between an Au-coated hollow glass microsphere and graphene-coated fused silica plate by means of a modified atomic force microscope cantilever-based technique operated in the dynamic regime. These measurements were performed in high vacuum at room temperature. The energy gap and the concentration of impurities in the graphene sample used have been measured utilizing scanning tunneling spectroscopy and Raman spectroscopy, respectively. The measurement results for the gradients of the Casimir force are found to be in a very good agreement with theory using the polarization tensor of graphene at nonzero temperature depending on the energy gap and chemical potential with no fitting parameters. The theoretical predictions of the same theory at zero temperature are experimentally excluded over the measurement region from 250 to 517 nm. We have also investigated a dependence of the thermal correction to the Casimir force gradient on the values of the energy gap, chemical potential, and on the presence of a substrate supporting the graphene sheet. It is shown that the observed thermal effect is consistent in size with that arising for pristine graphene sheets if the impact of real conditions such as nonzero values of the energy gap, chemical potential, and the presence of a substrate is included. Implications of the obtained results to the resolution of the long-standing problems in Casimir physics are discussed. In addition to the paper published previously [M. Liu et al., Phys. Rev. Lett. 126, 206802 (2021)], we present measurement results for the energy gap of the graphene sample, double the experimental data for the Casimir force, and perform a more complete theoretical analysis.

In sharp contrast to the previous view that interlayer shear enhances bending stiffness of van der Waals material, here we report a decreasing bending stiffness and natural frequency in double-layered ${\mathrm{MoS}}_2$ (${\mathrm{DLMoS}}_2$) nanoresonators with special stacking orders. Further theoretical analysis shows that the decreasing frequency is mainly due to the negative interlayer shear modulus in ${\mathrm{DLMoS}}_2$ nanoresonators with some stacking orders and twisting angles. Detailed analysis of the molecular structure reveals the essential roles of the interfacial potential in the interlayer shear modulus. The present study provides valuable physical insights for understanding the mechanical behaviors of double-layered van der Waals material resonators.

We present a model compound with a spin-1/2 frustrated square lattice, in which three ferromagnetic (F) interactions and one antiferromagnetic (AF) compet. Considering the effective spin-1 formed by the dominant F dimer, this square lattice can be mapped to a spin-1 spatially anisotropic triangular lattice. The magnetization curve exhibits gapped behavior indicative of a dominant one-dimensional (1D) AF correlation. In the field-induced gapless phase, the specific heat and magnetic susceptibility show a phase transition to an ordered state with 2D characteristics. These results indicate that the spin-1 Haldane state is extended to the 2D system. We demonstrate that the gapped ground state observed in the present spin-1/2 frustrated square lattice originates from the one-dimensionalization caused by frustration.

We theoretically study spin transport in two- or three-dimensional Rashba systems dynamically distorted by surface acoustic waves. The spin currents in the linear response to lattice distortion dynamics are calculated on the basis of a microscopic theory combined with local coordinate transformations. As a result, we find a mechanism of direct spin-current generation from lattice distortion not associated with a charge current or spin accumulation. Moreover, the in-plane helicity currents are generated by shear surface acoustic waves via the present mechanism. The generated helicity currents are not parallel to the vorticity of the lattice, and cannot be created with the conventional methods. Thus, our findings offer an alternative functionality of the conventional Rashba systems in the field of spintronics.

High-energy-resolution fluorescence-detected (HERFD) near-edge x-ray-absorption fine-structure measurements performed at the Pb and W $L_3$ absorption edges have been used to study the pressure dependence of the local atomic structure of $\mathrm{PbW}{\mathrm{O}}_4$ from 1 bar up to 22 GPa. Comparison of measured spectra with density-functional theory structural calculations and simulations based on solution of the Bethe-Salpeter equation establish an unambiguous $\mathrm{PbW}{\mathrm{O}}_4\text{−}\mathrm{I}$ to $\mathrm{PbW}{\mathrm{O}}_4\text{−}\mathrm{III}$ phase transition around 7 GPa. No evidence of either the Raspite or Fergusonite structures is found. The smaller core-hole lifetime broadening afforded by HERFD allows unique experimental determination of crystal-field splitting and its sensitivity to the oxygen cage geometry surrounding the Pb and W atoms thus giving definitive identification of the high-pressure phase of $\mathrm{PbW}{\mathrm{O}}_4$.

We modeled, both analytically and numerically, the magnitude and phase of spin waves propagating in thin magnetic films and scattered from mesoscale chiral magnonic resonators. Our calculations reveal a remarkably strong chiral scattering of propagating spin waves from magnon dark modes hosted by the resonator, exceeding in strength the scattering from its quasiuniform mode. We formulate conditions for the waveguide-resonator system to be used as an efficient spin-wave diode and as a phase shifter. Both these applications are found to be feasible when using the available ferromagnetic materials for the resonators.

Reaching strong coupling between a single molecule and a plasmonic nanocavity (PNC) at ambient conditions is important for many nanophotonic applications. Many efforts have been devoted towards such a goal. Here, we develop a semi-analytical method which can efficiently calculate the extinction spectrum of a general coupled system of a single molecule and a PNC. The field enhancement and the enhancement efficiency of a PNC are found to be crucial to obtain a clear large Rabi splitting. The bound for the coupling strength can be obtained correspondingly. Specifically, we find that a plasmonic raindrop dimer with a gap size of 1.5 nm can achieve strong coupling with a single common molecule of a transition dipole moment of ∼3.8 Debye. The raindrop dimer shows a better performance than the other common experimentally feasible plasmonic nanocavities. The case of a special nonlinear coupling where an additional peak appears at the Rabi splitting dip is also discussed. Our results pave the way for single-emitter quantum optics at ambient conditions.

By connecting Hund's physics with flat band physics, we establish an exact result for studying ferromagnetism in a multiorbital system. We consider a two-layer model consisting of a $p_x, p_y$-orbital honeycomb lattice layer and an $f$-orbital triangular lattice layer with sites aligned with the centers of the honeycomb plaquettes. The system features a flat band that admits a percolation representation for an appropriate chemical potential difference between the two layers. In this representation, the ground-state space is spanned by maximum-spin clusters of localized single-particle states, and averaging over the ground states yields a correlated percolation problem with weights due to the spin degeneracy of the clusters. A paramagnetic-ferromagnetic transition occurs as the band approaches half filling and the ground states become dominated by states with a large maximum-spin cluster, as shown by Monte Carlo simulation.

Electrical control of magnetism is of great interest for low-energy-consumption spintronic applications. Here, via first-principles calculations, we propose a van der Waals (vdW) multiferroic heterostructure composed of a magnetic ${\mathrm{CrI}}_3$ bilayer and a ferroelectric α-${\mathrm{In}}_2{\mathrm{Se}}_3$ monolayer substrate. Interestingly, the interlayer magnetism of bilayer ${\mathrm{CrI}}_3$ is switchable between the ferromagnetic and antiferromagnetic coupling by nonvolatile control of the ferroelectric polarization direction of α-${\mathrm{In}}_2{\mathrm{Se}}_3$. The interlayer magnetic coupling of ${\mathrm{CrI}}_3$ bilayer originates from the direct interaction of adjacent I atoms between ${\mathrm{CrI}}_3$ monolayers, which can be tuned by the polarization of ${\mathrm{In}}_2{\mathrm{Se}}_3$, explaining the electrical control of interlayer magnetic phase transition. Our work demonstrates a multiferroelectric material platform by artificially stacking two dimensional vdW layers, providing an effective method for achieving nonvolatile electrical control of atomic-thin vdW ferromagnets.