We analyze the saturation value of the bipartite entanglement and number entropy starting from a random product state deep in the many-body localized (MBL) phase. By studying the probability distributions of these entropies we find that the growth of the saturation value of the entanglement entropy stems from a significant reshuffling of the weight in the probability distributions from the bulk to the exponential tails. In contrast, the probability distributions of the saturation value of the number entropy are converged with system size, and exhibit a sharp cutoff for values of the number entropy which correspond to one particle fluctuating across the boundary between the two halves of the system. Our results therefore rule out slow particle transport deep in the MBL phase and confirm that the slow entanglement entropy production stems uniquely from configurational entanglement.

In this work, we study magnon-magnon coupling in synthetic antiferromagnets (SyAFs) using microwave spectroscopy at room temperature. Two distinct spin-wave modes are clearly observed and are hybridized at degeneracy points. We provide a phenomenological model that captures the coupling phenomena and experimentally demonstrate that the coupling strength is controlled by the out-of-plane tilt angle as well as the interlayer exchange field. We numerically show that a spin-current mediated damping in SyAFs plays a role in influencing the coupling strength.

Second-order topological superconductors (SOTSs) host localized Majorana fermions and provide a new platform for topological quantum computation. We propose a feasible way to realize networks based on SOTSs which allow one to nucleate and braid Majorana bound states (MBSs) in an all-electrical manner without fine-tuning. The proposed setups are scalable in a straightforward way and can accommodate any even number of MBSs. Moreover, the MBSs in the networks allow defining qubits whose states can be initialized and read out by measuring Josephson currents flowing between SOTS islands. Our proposal can be implemented in monolayers of $\text{FeTe}{}_{1-x}{\text{Se}}_x$, monolayers of $1T^{\prime }\text{−}{\mathrm{WTe}}_2$, and inverted Hg(Cd)Te quantum wells in proximity to conventional superconductors.

For an overdamped particle moving over a two-dimensional periodic substrate under combined dc and ac drives, a series of steps can appear in the velocity force curves that are known as Shapiro steps. Here we show that for skyrmions driven over a two-dimensional periodic obstacle array with a dc drive and an ac drive that is either parallel or perpendicular to the dc drive, the system exhibits numerous transverse and longitudinal synchronization dynamics due to the Magnus force. These phenomena originate in interactions between two different types of phase-locking effects: Shapiro steps and directional locking. In some cases, the skyrmion Hall angle is constant but longitudinal Shapiro steps appear, while in other regimes the skyrmion Hall angle can either increase or decrease with increasing dc drive during the phase locking as the skyrmion locks to different symmetry directions of the obstacle lattice. For a transverse ac drive, we find that strong Hall angle overshoots can occur in certain locked phases where the skyrmion is moving at an angle that is considerably larger than the intrinsic Hall angle. For the strongest Magnus force, the phase-locking effects are reduced and there are larger regions of disordered dynamics. We show that the skyrmion Hall angle can be controlled by fixing the dc drive and changing the amplitude of the ac drive.

We have calculated the scattering (reflection and transmission) coefficients of linear exchange spin waves normally incident upon a helimagnetic layer sandwiched between two semi-infinite ferromagnetic media. Our calculations show that, despite the helimagnetic order induced in the layer by the Dzyaloshinskii-Moriya interaction (DMI), the scattering is reciprocal and insensitive to the presence of the helimagnetic order in the layer. This comes as a result of the disappearance of the DMI from the boundary conditions in the considered geometry under the small-amplitude approximation and from the specific form of the nonreciprocity of the spin-wave dispersion relation in the helimagnetic material. We show that the helimagnetic layer's interfaces act as a system of two semicrossed polarizers for the circularly polarized spin waves incident from the ferromagnetic media. This results from the ellipticity of the magnetic precession induced by the easy-plane anisotropy in the helimagnetic layer. Our calculations also reveal the importance of evanescent solutions to correctly describe the spin-wave scattering in samples with elliptical precession. Our findings will aid development of magnonic devices containing helimagnetic constituents.

We develop a versatile first-principles many-body scheme for multiorbital lattice susceptibilities including full frequency-dependent local vertex effects and investigate the different instabilities in the metallic phase of the quasi-two-dimensional compound ${\mathrm{LiVS}}_2$. Application of such advanced correlated electronic structure methods for the $t_{2g}$ subspace reveals a highly entangled spin-orbital hexagonal ordering bringing about an inherently intersite order parameter for the trimerization transition. The importance of such nontrivial ordering towards the formation of intriguing insulating phases at low temperatures is discussed.

We investigate the decoherence of ${\mathrm{Er}}^{3+}$ in ${\mathrm{Y}}_2\mathrm{Si}{\mathrm{O}}_5$ at low magnetic fields using the photon-echo technique. We reproduce accurately a variety of the decay curves with a unique coherence time by considering the so-called superhyperfine modulation induced by a large number of neighboring spins. There is no need to invoke any characteristic time of the spin fluctuations to reproduce very different decay curves. The number of involved nuclei increases when the magnetic is lowered. The experiment is compared with a model associating 100 surrounding ions with their exact positions in the crystal frame. We also derive an approximate spherical model (angular averaging) to interpret the main feature the observed decay curves close to zero field.

${\mathrm{Sr}}_2\mathrm{Ir}{\mathrm{O}}_4$ is an archetypal spin-orbit-coupled Mott insulator and has been extensively studied in part because of a wide range of predicted states. Limited experimental characterization of these states thus far brings to light the extraordinary susceptibility of the physical properties to the lattice, particularly, the Ir-O-Ir bond angle. Here, we report a microscopic rotation of the $\mathrm{Ir}{\mathrm{O}}_6$ octahedra below 50 K measured by single crystal neutron diffraction. This sharp lattice anomaly provides keys to understanding the anomalous low-temperature physics and a direct confirmation of a crucial role that the Ir-O-Ir bond angle plays in determining the ground state. Indeed, as also demonstrated in this study, applied electric current readily weakens the antiferromagnetic order via the straightening of the Ir-O-Ir bond angle, highlighting that even slight change in the local structure can disproportionately affect the physical properties in the spin-orbit-coupled system.

We report the magnetotransport properties and the electronic structure of $\mathrm{Na}{\mathrm{Cd}}_4{\mathrm{As}}_3$ single crystal. The electric resistivity of $\mathrm{Na}{\mathrm{Cd}}_4{\mathrm{As}}_3$ exhibits a metallic behavior. Clear Shubnikov–de Haas (SdH) oscillations have been observed at low temperatures and high magnetic fields. A single fundamental frequency and small cyclotron effective mass are obtained from the analysis of SdH oscillations, which originate from an ellipsoidal Fermi surface. The Hall resistivity reveals the multiband nature at low temperature and the electron carriers dominate the transport properties. First-principles calculations show that the electronic structure of $\mathrm{Na}{\mathrm{Cd}}_4{\mathrm{As}}_3$ undergoes a topological phase transition from topological crystalline insulator (TCI) to topological insulator (TI) induced by the structural phase transition. The breaking of mirror symmetry is considered to play a key role in the topological phase transition. Our findings show that $\mathrm{Na}{\mathrm{Cd}}_4{\mathrm{As}}_3$ is a suitable platform to study the effect of mirror symmetry on the band topology of TCI.

The electronic structures of ${30}^{\circ }$ twisted double bilayer graphene, which loses the translational symmetry due to the incommensurate twist angle, are studied by means of the tight-binding approximation. We demonstrate the interlayer decoupling across the ${30}^{\circ }$ twisted interface in the vicinity of the Fermi level from various electronic properties, including the density of states, effective band structure, optical conductivity, and Landau-level spectrum. However, at $Q$ points, the interlayer coupling results in the appearance of new Van Hove singularities in the density of states, new peaks in the optical conductivity and, importantly, the 12-fold-symmetry-like electronic states. The k-space tight-binding method is adopted to explain this phenomenon. The electronic states at $Q$ points show charge distribution patterns more complex than the ${30}^{\circ }$ twisted bilayer graphene due to the symmetry decrease. These phenomena also appear in the graphene monolayer on the AB-stacked graphene bilayer with a ${30}^{\circ }$ twist angle.

We report on two-photon absorption (TPA) and second harmonic generation (SHG) spectroscopy of para- and orthoexcitons in ${\mathrm{Cu}}_2\mathrm{O}$ subject to a strong magnetic field up to $10\mathrm{T}$. The magnetic field splits the orthoexciton into its three quasispin components $M=0,\pm 1$ and activates the symmetry and spin forbidden paraexciton by an admixture from the $M=0$ component of the orthoexciton. For the excitation of the paraexciton we suggest an alternative mechanism of TPA without an external perturbation. It involves instead of the electric dipole-electric dipole the electric quadrupole-magnetic dipole excitation process. By application of group theory we derive for both mechanisms of TPA and SHG polarization selection rules for each of the four resonances (one para- and three orthoexcitons) and present experimental results for different crystalline orientations which agree perfectly with the derivations from group theory. High spectral resolution of the used SHG technique allows us to refine the exchange splitting between the para- and orthoexciton of $\varepsilon =12.120\mathrm{meV}$ and the $g$ values of the upmost valence band $g_v=-0.72\pm 0.03$ and the lowest conduction band $g_c=2.38\pm 0.08$.

We report our investigation on the electron-phonon coupling in graphite using scanning tunneling microscopy/spectroscopy (STM/STS), in combination with angle-resolved photoemission spectroscopy (ARPES). The exhibition of a gaplike feature near the Fermi level both in the STS and ARPES spectra confirms its intrinsic origination because of the electron-phonon coupling in graphite, where the phonon modes at $\pm 65$ and $\pm 160$ meV are highly resolvable in the clean samples. Under our controlled experiment using W, Pt/Ir, and Ag tips, we find that the presence of hydrogen at the tip apex or in the tip-substrate gap is responsible for smearing out the gaps in the STS spectra by providing additional tunneling channels. Due to its small adsorption energy of hydrogen for Ag among these tip materials, our results suggest that the use of Ag tips can be more suitable for resolving the intrinsic phonon-induced property in carbon-based materials.

The planar Hall effect (PHE), the appearance of an in-plane transverse voltage in the presence of coplanar electric ($\mathbf{E}$) and magnetic ($\mathbf{B}$) fields, occurs in regular Weyl semimetals (WSMs) as one of the fundamental manifestations of a chiral anomaly. A major issue, therefore, is whether there are alternate routes to the PHE, without invoking a chiral anomaly. We demonstrate that PHE exists in an inhomogeneous Weyl semimetal (IWSM) even in the absence of the aforesaid anomaly. Using the semiclassical Boltzmann transport theory, we show that PHE appears in an IWSM due to the strain-induced chiral gauge potential, which couples to the Weyl fermions of opposite chirality with opposite sign. Our study shows a resultant phase shift in the current associated with opposite chirality Weyl nodes, which, remarkably, leads to a finite chirality-dependent planar Hall effect (CPHE) in the IWSMs. Interestingly, we show that a small tilt in the Weyl node can generate a pure CPHE even in the absence of an applied magnetic field. The CPHE has important implications in “chiralitytronics.” We also discuss the experimental feasibility of these effects of strain in type-I IWSMs.

We demonstrate a scheme to dramatically enhance both the second- and third-harmonic generation (SHG, THG) in a graphene-insulator-graphene metasurface. The key underlying feature of our approach is the existence of a double-resonance phenomenon, namely, the metasurface is designed to possess fundamental plasmon resonances at both the fundamental frequency and the higher harmonic. This dual resonant field enhancement, combined with a favorable spatial overlap of the optical near fields, lead to the increase of the THG and SHG by $\sim {10}^9$ and $\sim {10}^6$, respectively. We also demonstrate that by tuning the Fermi energy of the graphene gratings the dual-resonance property can be locked in over a remarkably broad spectral range of $\sim 20\mathrm{THz}$, which is more than three orders of magnitude larger than the spectral tunability achievable in metal-based plasmonic systems. Importantly, the enhanced nonlinear frequency generation process can be readily switched in the same system between the second and third harmonic. This type of graphene metasurface could open up new avenues towards the development of novel ultracompact and multifrequency active photonic nanodevices.

The driven-dissipative many-body problem remains one of the most challenging unsolved problems in quantum mechanics. The advent of quantum computers may provide a unique platform for efficiently simulating such driven-dissipative systems. But, there are many choices for how one can engineer the reservoir. One can simply employ ancilla qubits to act as a reservoir and then digitally simulate them via algorithmic cooling. A more attractive approach, which allows one to simulate an infinite reservoir, is to integrate out the bath degrees of freedom and describe the driven-dissipative system via a master equation, that can also be simulated on a quantum computer. In this work, we consider the particular case of noninteracting electrons on a lattice driven by an electric field and coupled to a fermionic thermostat. Then, we provide two different quantum circuits: the first one reconstructs the full dynamics of the system using Trotter steps, while the second one dissipatively prepares the final nonequilibrium steady state in a single step. We run both circuits on the IBM quantum experience. For circuit (i), we achieved up to five Trotter steps. When partial resets become available on quantum computers, we expect that the maximum simulation time can be significantly increased. The methods developed here suggest generalizations that can be applied to simulating interacting driven-dissipative systems.

Describing the experimental signatures of quantum spin ice has been the focus of many theoretical efforts, as definitive experimental verification of this candidate quantum spin liquid is yet to be achieved. Gapped excitations known as visons have largely eluded those efforts. We provide a theoretical framework, which captures their dynamics and predicts new signatures in the magnetic response. We achieve this by studying the ring-exchange Hamiltonian of quantum spin ice in the large-$s$ approximation, taking into account the compact nature of the emergent U(1) gauge theory. We find the stationary solutions of the action—the instantons—which correspond to visons tunneling between lattice sites. By integrating out the instantons, we calculate the effective vison Hamiltonian, including their mass. We show that in the ground state virtual vison pairs simply renormalize the speed of light and give rise to an inelastic continuum of excitations. At low temperatures, however, thermally activated visons form a Debye plasma and introduce a mass gap in the photon spectrum, equal to the plasma frequency, which we calculate as a function of temperature. We demonstrate that this dynamical mass gap should be visible in energy-resolved neutron scattering spectra but not in the energy-integrated ones. We also show that it leads to the vanishing of the susceptibility of an isolated system, through a mechanism analogous to the Meissner effect, but that it does not lead to confinement of static spinons.

We report the single-crystal growth of the noncentrosymmetric cubic heavy-fermion paramagnet ${\mathrm{UPt}}_5$ and comparative Fermi surface studies of de Haas-van Alphen (dHvA) experiments and theoretical band calculations. We also present the results of single-crystal x-ray diffraction, electrical resistivity, heat capacity, and magnetization measurements using single crystals of ${\mathrm{UPt}}_5$. The dHvA experiments reveal nearly spherical split hole Fermi surfaces and largely split electron Fermi surfaces, which are in agreement with the results of a band-structure calculation. The largely split electron Fermi surfaces may be understood through the antisymmetric spin-orbit interaction derived from U-$5f$ and Pt-$5d$ electrons. We explain the observed unexpected dHvA frequencies quasiquantitatively based on orbital crossing using the coupled network analysis in the split hole Fermi surface pair. Furthermore, the temperature dependence of the dHvA amplitude reveals enhanced cyclotron effective masses of up to $29m_0$ and a peculiar anisotropic effective mass in the paramagnetic cubic heavy-fermion ${\mathrm{UPt}}_5$.

Circular dichroism (CD) is generally observed in the optically active chiral molecules that originate from macroscopic electric and magnetic dipoles, which is usually quite small. In solid states, the so-called valley CD may arise microscopically from interband transitions between two chiral electronic valley bands of nonzero Berry curvatures at a given $k$ point. However, generally, two sets of $K$ and $K^{\prime }$ valleys coexist in the Brillouin zone with opposite chiral selectivities, so that the net CD is zero for the whole material. Here, we demonstrate a giant CD originating from photoexcitation between two chiral Chern flat bands of opposite Chern numbers, namely, the enantiomorphic flat Chern bands. The dissymmetry factor $g$ of such flat CD can reach the theoretical maximum value of 2 with the optimal spin-orbit coupling strength. Based on first-principles calculations, we identify that the Li intercalated bilayer $\pi$-conjugated nickel-bis(dithiolene) hosts a set of yin-yang kagome bands with an estimated large $g=0.74$ under magnetic field. Furthermore, based on the flat-CD mechanism, we propose two flatband devices of topological photodetectors and circularly polarized lasers.

Here we study the static and dynamic transport properties of a low-energy two-band model proposed previously in Martino et al. [PRL 122, 217402 (2019)], with an anisotropic in-plane linear momentum dependence and a parabolic out-of-plane dispersion. The model is extended to include a negative band gap, which leads to the emergence of a Weyl semimetal (WSM) state, as opposed to the gapped semimetal (GSM) state when the band gap is positive. We calculate and compare the zero- and finite-frequency transport properties of the GSM and WSM cases. The DC properties that are calculated for the GSM and WSM cases are Drude spectral weight, mobility, and resistivity. We determine their dependence on the Fermi energy and crystal direction. The in- and out-of-plane optical conductivities are calculated in the limit of the vanishing interband relaxation rate for both semimetals. The main common features are an ${\omega }^{1/2}$ in-plane and ${\omega }^{3/2}$ out-of-plane frequency dependence of the optical conductivity. We seek particular features related to the charge transport that could unambiguously point to one ground state over the other, based on the comparison with the experiment. Differences between the WSM and GSM are in principle possible only at extremely low carrier concentrations and at low temperatures.