Valley chiral kagome networks can arise in various situations like, for example, in double-aligned graphene-hexagonal boron nitride and periodically strained graphene. Here, we construct a phenomenological scattering model based on the symmetries of the network to investigate the energy spectrum and magnetotransport in this system. Additionally, we consider the effects of a finite Rashba spin-orbit coupling on the transport properties of the kagome network. We identify conditions where the interplay of the Rashba spin-orbit coupling and the geometry of the lattice results in a reduction of the periodicity of the magnetoconductance and characteristic sharp resonances. Moreover, we find a finite spin polarization of the conductance, which could be exploited in spintronic devices.

Two recent electronic transport experiments from Columbia University and Harvard University have reported record high mobility and low channel densities in flux-grown transition metal dichalcogenide (TMD) ${\mathrm{WSe}}_2$ monolayers [J. Pack et al., arXiv:2310.19782 [cond-mat.mes-hall] (2023); A. Y. Joe et al., Phys. Rev. Lett. 132, 056303 (2024)]. A two-dimensional (2D) metal-insulator transition (MIT) is demonstrated in the Columbia sample at low densities, a regime where the formation of a Wigner crystal (WC) is theoretically anticipated in the absence of disorder. We employ the finite-temperature Boltzmann theory to understand the low-temperature transport properties of monolayer TMDs, taking into account realistic disorder scattering. We analyze the experimental results, focusing on the 2D MIT behavior and the influence of temperature and density on mobility and resistivity in the metallic phase. We provide a discussion of the nontrivial carrier density dependence of our transport results. Our analysis elucidates the linear temperature-dependent resistivity observed in the metallic phase, attributing it to Friedel oscillations associated with screened charged impurities. Furthermore, we explore whether Coulomb disorder could lead to the MIT through either a quantum Anderson localization transition or a classical percolation transition in the long-range disorder potential landscape. Our theoretical estimates of the disorder-induced MIT critical densities, although smaller, are within a factor of approximately 2 of the experimental critical density. We examine the exceptionally high melting temperature $\sim 10\mathrm{K}$ of WCs observed experimentally in the ${\mathrm{MoSe}}_2$ systems at low density, which is strongly enhanced by the disorder-induced localization effect, since the pristine melting temperature is an order of magnitude smaller. This suggests that the observed 2D low-density MIT behavior is likely a result of the complex interplay between disorder effects and interaction-driven WC physics, offering a comprehensive understanding of the low-temperature transport phenomena in TMD monolayers.

We investigate the detection of quartets in hybrid multiterminal Josephson junctions. Using simple models of quantum dots coupled to superconducting leads, we find that quartets are ubiquitous in quantum coherent structures and show how to rigorously extract their contribution to the current-phase relation (CPR). We also demonstrate that quartets are closely related to the hybridization of Andreev bound states (ABSs) in these systems and propose a method to identify quartets directly in ABS spectra. We illustrate our method by analyzing the spectroscopic measurements of the ABS spectrum in a three-terminal Josephson junction realized in an InAs/Al heterostructure. Our analysis strongly suggests the existence of quartets in the studied hybrid system.

We investigate the impact of spin torque and spin pumping on the surface magnon polariton dispersion in a antiferromagnetic insulator-semiconductor heterostructure. In the bilayer system, the surface magnon polaritons conventionally couple to the plasma oscillations in the semiconductor via electromagnetic fields. Additionally, magnons in the antiferromagnetic insulator layer may interact with the semiconductor layer via spin torques and their reciprocal phenomena of spin pumping. Due to the spin-to-charge conversion from the spin-Hall and inverse spin-Hall effects in the semiconductor layer with a strong spin-orbit coupling, this can couple the magnons to the plasmons in the semiconductor layer. Our research reveals that modifications in the mode frequency and the hybridization gap induced by these phenomena depend on the thickness of the antiferromagnetic layer. In thick layers, the spin-pumping contribution to the frequency shift and damping is inversely proportional to the wavelength, while in thin layers it is inversely proportional to the thickness. Furthermore, hybridization of the surface magnon polariton and dispersive magnons in the antiferromagnet is shown to depend on both the thickness and wavelength of the modes.

The linear cross-entropy (LXE) has been recently proposed as a scalable probe of the measurement-driven phase transition between volume- and area-law-entangled phases of pure-state trajectories in certain monitored quantum circuits. Here, we demonstrate that the LXE can distinguish distinct area-law-entangled phases of monitored circuits with symmetries, and extract universal behavior at the critical points separating these phases. We focus on (1+1)-dimensional monitored circuits with an onsite ${\mathbb{Z}}_2$ symmetry. For an appropriate choice of initial states, the LXE distinguishes the area-law-entangled spin-glass and paramagnetic phases of the monitored trajectories. At the critical point, described by two-dimensional percolation, the LXE exhibits universal behavior which depends sensitively on boundary conditions, and the choice of initial state. With open boundary conditions, we show that the LXE relates to crossing probabilities in critical percolation, and is thus given by a known universal function of the aspect ratio of the dynamics, which quantitatively agrees with numerical studies of the LXE at criticality. The LXE probes correlations of other operators in percolation with periodic boundary conditions. We show that the LXE is sensitive to the richer phase diagram of the circuit model in the presence of symmetric unitary gates. Last, we consider the effect of noise during the circuit evolution, and propose potential solutions to counter it.

Electronic nematic order has been reported in a rich landscape of materials, encompassing not only a range of intertwined correlated and topological phenomena but also different underlying lattice symmetries. Motivated by these findings, we investigate the behavior of electronic nematicity as the spherical symmetry of three-dimensional (3D) space is systematically lowered by the lattice environment. We consider all 32 crystallographic point groups as well as four major classes of quasicrystalline point groups, given the recent observations of electronic phases of interest in quasicrystalline materials and artificial twisted quasicrystals. Valuable insights are gained by establishing a mapping between the five-component charge-quadrupolar nematic order parameter of the electronic fluid and the 3D tensorial order parameter of nematic liquid crystals. We find that a uniaxial nematic state is only generically realized in polyhedral point groups (icosahedral and cubic), with the nematic director pointing along different sets of rotational symmetry axes. Interestingly, icosahedral point groups are the only ones in which the five nematic order parameter components transform as the same irreducible representation, making them the closest analog of 3D isotropic nematics. In axial point groups, one of the nematic components is always condensed, whereas the other four components decompose into an in-plane and an out-of-plane nematic doublet, resulting in biaxial nematic ground states. Because these two nematic doublets behave as $Z_q$ clock order parameters, this allows us to identify the types of crystals and quasicrystals that can host interesting electronic nematic phenomena enabled by the critical properties of the $q\ge 4$ clock model, such as emergent continuous nematic fluctuations in 3D, critical phases with quasi-long-range nematic order in two dimensions (2D), and Ashkin-Teller nematicity in 2D.

We present a comparative analysis of the validity of Eliashberg theory for the cases of fermions interacting with an Einstein phonon and with soft nematic fluctuations near an Ising-nematic/Ising-ferromagnetic quantum-critical point (QCP) in two spatial dimensions. In both cases, Eliashberg theory is obtained by neglecting vertex corrections. For the phonon case, the reasoning to neglect vertex corrections is the Migdal “fast electron/slow boson” argument because the phonon velocity is much smaller than the Fermi velocity, $v_F$. The same argument allows one to compute the fermionic self-energy within Eliashberg theory perturbatively rather than self-consistently. For the nematic case, the velocity of a collective boson is comparable to $v_F$ and this argument does not work. Nonetheless, we argue that while two-loop vertex corrections near a nematic QCP are not small parametrically, they are small numerically. At the same time, perturbative calculation of the fermionic self-energy can be rigorously justified when the fermion-boson coupling is small compared to the Fermi energy by effectively invoking the fast electron/slow boson argument, this time because bosons are Landau overdamped. Furthermore, we argue that for the electron-phonon case Eliashberg theory breaks down at some distance from where the dressed Debye frequency would vanish, while for the nematic case it holds all the way to a QCP. From this perspective, Eliashberg theory for the nematic case actually works better than for the electron-phonon case.

We report the observation of the terahertz-induced ratchet effect in graphene-based two-dimensional (2D) metamaterials. The metamaterial consists of a graphite gate patterned with an array of triangular antidots placed under a graphene monolayer. We show that the ratchet current appears due to the noncentrosymmetry of the periodic structure unit cell. The ratchet current is generated owing to the combined action of a spatially periodic in-plane electrostatic potential and a periodically modulated radiation electric field caused by near-field diffraction. The magnitude and direction of the ratchet current are shown to be controlled by voltages applied to both back and patterned gates, which change the lateral asymmetry, carrier type, and density. The phenomenological and microscopic theories of ratchet effects in graphene-based 2D metamaterials are developed. The experimental data are discussed in the light of the theory based on the solution of the Boltzmann kinetic equation and the calculated electrostatic potential profile. The theory describes well all the experimental results and shows that the observed ratchet current consists of the Seebeck thermoratchet contribution as well as the linear ratchet one, which is sensitive to the orientation of the radiation electric field vector with respect to the triangles.

We critically analyze the recently reported observation of integer (IQAHE) and fractional (FQAHE) quantum anomalous Hall effects at zero applied magnetic field in pentalayer graphene. Our quantitative activation and variable range hopping transport analysis of the experimental data reveals that the observed IQAHE and FQAHE at different fillings all have similar excitation gaps of the order of 5–10 $\mathrm{K}$. In addition, we also find that the observed FQAHE manifests a large hidden background contact series resistance $>10$ $\mathrm{k}\mathrm{\Omega }$ of unknown origin whereas this contact resistance is much smaller $\sim 500$ $\mathrm{\Omega }$ in the observed IQAHE. Both of these findings are surprising as well as inconsistent with the well-established phenomenology of the corresponding high-field integer and fractional quantum Hall effects in two-dimensional semiconductor systems.

We study the performances of an imperfect quantum many-body Otto engine based on free fermion systems. Starting from the thermodynamic definitions of heat and work along ideal isothermal, adiabatic, and isochoric transformations, we generalize these expressions in the case when the hypotheses of ideality are relaxed (i.e., nonperfect thermalization with the external baths, as well as nonperfect quantum adiabaticity in the unitary dynamic protocols). These results are used to evaluate the work and the power delivered by an imperfect quantum many-body heat engine in a finite time, whose working substance is constituted by a quantum Ising chain in a transverse field. We discuss the emerging optimal working points as functions of the various model parameters.

Tracking room-temperature superconductors represents a significant challenge and opportunity, while the discovery of binary lanthanum hydrides provides a crucial platform for exploring ternary hydrides with exceptional properties. The theoretically studied La-Ca-H system is also one of the attractive candidates for room-temperature superconductivity. In this work, we have successfully synthesized two La-Ca hydrides by doping Ca elements into the La-H system. The temperature dependence of the electrical resistance demonstrates the superconductivity of $Fm\overline{3}m\text{−}\left(\mathrm{La},\mathrm{Ca}\right){\mathrm{H}}_{10}$ and $P{6}_3/mmc\text{−}\left(\mathrm{La},\mathrm{Ca}\right){\mathrm{H}}_{10}$, with superconducting critical temperature $\left(T_c\right)$ of 247 K (173 GPa) and 230 K (167 GPa), respectively. Present results show that doping with 10% or 15% Ca has only a negligible effect on the $T_c$ of ${\mathrm{LaH}}_{10}$, which is consistent with Anderson's theory. Although the high $T_c$ phase of La-Ca-H maintains a highly symmetric fcc structure, the doping of Ca may lead to premature H-cage distortion during the decompression process, thus affecting the structural stability of the system. Therefore, under the doping levels and synthesis conditions of this work, we think that Ca may not be an ideal dopant. Our study provides some hints on the key factors affecting superconductivity and informs the future search for room-temperature superconductors in more diverse hydride materials.

We report on the optical spectroscopy of the spin of two Co atoms in a quantum dot and interacting with a single exciton. Spectra of quantum dots containing two Co atoms are controlled by the carrier-Co exchange interaction and by the strain at the location of the magnetic atoms. A wide range of spectra can be obtained depending on the relative coupling of each atom to the confined exciton. We obtained a comprehensive interpretation of the experimental data with a spin Hamiltonian model. We show that the two Co atoms spins can be orientated by the injection of spin polarized carriers at zero magnetic field. This induces a correlation between the two spins that is observed in the intensity distribution of the emission spectra. The optical absorption in the phonon sideband of quantum dots doped with two Co reveals resonant absorptions which strongly depend on a transverse magnetic field. We show that these characteristic absorptions result from an interplay between the mixing of Co spin states induced by the presence of in-plane strain anisotropy at the magnetic atoms location and the transverse field.

Fractional Chern insulators (FCI) were proposed theoretically about a decade ago. These exotic states of matter are fractional quantum Hall states realized when a nearly flat Chern band is partially filled, even in the absence of an external magnetic field. Recently, exciting experimental signatures of such states have been reported in twisted ${\mathrm{MoTe}}_2$ bilayer systems. Motivated by these experimental and theoretical progresses, in this paper, we develop a projective construction for the composite fermion states (either the Jain's sequence or the composite Fermi liquid) in a partially filled Chern band with Chern number $C=\pm 1$, which is capable of capturing the microscopics, e.g., symmetry fractionalization patterns and magnetoroton excitations. On the mean-field level, the ground states' and excited states' composite fermion wave functions are found self-consistently in an enlarged Hilbert space. Beyond the mean field, these wave functions can be projected back to the physical Hilbert space to construct the electronic wave functions, allowing direct comparison with FCI states from exact diagonalization on finite lattices. We find that the projected electronic wave function corresponds to the combinatorial hyperdeterminant of a tensor. When applied to the traditional Galilean invariant Landau level context, the present construction exactly reproduces Jain's composite fermion wave functions. We apply this projective construction to the twisted bilayer ${\mathrm{MoTe}}_2$ system. Experimentally relevant properties are computed, such as the magnetoroton band structures and quantum numbers.

Magnetic properties of ${\mathrm{Cu}}_2\left({\mathrm{MoO}}_4\right)\left({\mathrm{SeO}}_3\right)$, an S = $\frac{1}{2}$ centrosymmetric antiferromagnet (AFM), were investigated using superconducting quantum interference device magnetometry, neutron diffraction, and magnetoelectric (ME) measurements. The magnetic susceptibility measurements indicate a broad peak at ∼50 K, followed by a phase transition into AFM order at $T_{\mathrm{N}}=23.6\left(1\right)$ K. Above $T_{\mathrm{N}}$, a fit to the Curie-Weiss law gives a Curie-Weiss temperature ${\mathrm{\Theta }}_{\mathrm{CW}}=-68\left(1\right)$ K, suggesting the dominant AFM coupling. Neutron powder diffraction reveals that the $\mathrm{C}{\mathrm{u}}^{2+}$ spins are aligned AFM along the $c$ axis with weak noncollinearity under the magnetic space group of P$2_1^{\prime }/c$. The ME response indicates that a nondiagonal component of a ME tensor is active, supporting the simultaneous spatial and time reversal symmetry breaking under P$2_1^{\prime }/c$.

Within optical quantum information processing, single-photon sources based on a two-level system in a semiconductor material allow for on-demand generation of single photons. To initiate the spontaneous emission process, it is necessary to efficiently populate the excited state. However, reconciling the requirement for on-demand excitation with both high efficiency and high photon indistinguishability remains a challenge due to the presence of charge noise and phonon-induced decoherence in the solid-state environment. Here, we reconstruct the phonon spectral density experienced by ${\mathrm{WSe}}_2$ quantum emitters in the emission process, and we use this information to theoretically analyze the performance of the resonant, phonon-assisted, and Swing-UP of the quantum EmitteR population (SUPER) excitation schemes. Under resonant excitation, we obtain an exciton preparation fidelity limited to 0.80 by the strong phonon coupling, which improves to 0.96 for the SUPER scheme (or 0.89, depending on the type of emitter considered). Under near-resonant phonon-assisted excitation, our theory predicts near-unity excitation fidelity up to 0.976 (0.997). Additionally, we demonstrate that, assuming the suppression of the phonon sidebands, residual dephasing mechanisms such as charge and spin fluctuations are the dominating decoherence mechanisms undermining the photon indistinguishability.

Understanding how and to what magnitude solid-state qubits couple to metallic wires is crucial to the design of quantum systems such as quantum computers. Here, we investigate the coupling between a multilevel system, or qudit, and a superconducting (SC) resonator's electromagnetic field, focusing on the interaction involving both the transition and diagonal dipole moments of the qudit. Specifically, we explore the effective dynamical (time-dependent) longitudinal coupling that arises when a solid-state qudit is adiabatically modulated at small gate frequencies and amplitudes, in addition to a static dispersive interaction with the SC resonator. We derive Hamiltonians describing the longitudinal multilevel interactions in a general dispersive regime, encompassing both dynamical longitudinal and dispersive interactions. These Hamiltonians smoothly transition between their adiabatic values, where the couplings of the $n\mathrm{th}$ level are proportional to the level's energy curvature concerning a qudit gate voltage, and the substantially larger dispersive values, which occur due to a resonant form factor. We provide several examples illustrating the transition from adiabatic to dispersive coupling in different qubit systems, including the charge ($1e$ double quantum dot) qubit, the transmon, the double-quantum-dot singlet-triplet qubit, and the triple-quantum-dot exchange-only qubit. In some of these qubits, higher-energy levels play a critical role, particularly when their qubit's dipole moment is minimal or zero. For an experimentally relevant scenario involving a spin-charge qubit with magnetic field gradient coupled capacitively to a SC resonator, we showcase the potential of the dynamical longitudinal coupling. It enables close-to-quantum-limited quantum nondemolition measurements and remote geometric phase gates, demonstrating its practical utility in quantum information processing.

Large-scale gravitational phenomena are famously difficult to observe, making parallels in condensed matter physics a valuable resource. Here we show how spin nematic phases, found in magnets and cold atoms, can provide an analog to linearized gravity. In particular, we show that the Goldstone modes of these systems are massless spin-2 bosons, in one-to-one correspondence with quantized gravitational waves in flat spacetime. We identify a spin-1 model supporting these excitations and, using simulation, outline a procedure for their observation in a ${}^{23}\mathrm{Na}$ spinor condensate.

We demonstrate detection and measurement of electron paramagnetic spin resonances (EPR) of iron defects in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ utilizing generalized ellipsometry at frequencies between 110 and 170 GHz. The experiments are performed on an Fe-doped single crystal in a free-beam configuration in reflection at ${45}^{\circ }$ and magnetic fields between 3 and 7 T. In contrast with low-field, low-frequency EPR measurements, we observe all five transitions of the $s=5/2$ high-spin state ${\mathrm{Fe}}^{3+}$ simultaneously. We confirm that ferric ${\mathrm{Fe}}^{3+}$ is predominantly found at octahedrally coordinated Ga sites. We obtain the full set of fourth-order monoclinic zero-field splitting parameters for both octahedrally and tetrahedrally coordinated sites by employing measurements at multiple sample azimuth rotations. The capability of high-field EPR allows us to demonstrate that simplified second-order orthorhombic spin Hamiltonians are insufficient, and fourth-order terms as well as consideration of the monoclinic symmetry are needed. These findings are supported by computational approaches based on density-functional theory for second-order and on ligand-field theory for fourth-order parameters of the spin Hamiltonian. Terahertz ellipsometry is a way to measure spin resonances in a cavity-free setup. Its possibility of varying the probe frequency arbitrarily without otherwise changing the experimental setup offers unique means of truly disentangling different components of highly anisotropic spin Hamiltonians.

The search for elusive Nagaoka-type ferromagnetism in the Hubbard model has recently enjoyed renewed attention with the advent of a variety of experimental platforms enabling its realization, including moiré materials, quantum dots, and ultracold atoms in optical lattices. Here, we demonstrate a universal mechanism for Nagaoka ferromagnetism (that applies to both bipartite and nonbipartite lattices) based on the formation of ferromagnetic polarons consisting of a dopant dressed with polarized spins. Using large-scale density-matrix renormalization group calculations, we present a comprehensive study of the ferromagnetic polaron in an electron-doped Hubbard model, establishing various polaronic properties such as its size and energetics. Moreover, we systematically probe the internal structure of the magnetic state—through the use of pinning fields and three-point spin-charge-spin correlation functions—for both the single-polaron limit and the high-density regime of interacting polarons. Our results highlight the crucial role of mobile polarons in the birth of global ferromagnetic order from local ferromagnetism and provide a unified framework to understand the development and demise of the Nagaoka-type ferromagnetic state across dopings.

Electrons in condensed matter may transition into a variety of broken-symmetry phase states due to electron-electron interactions. Applying diverse mean-field approximations to the interaction term is arguably the simplest way to identify the phase states theoretically possible in a given setting. Here, we explore electron-electron attraction in a two-band system comprising symmetric conduction and valence bands touching each other at a single point. We assume a mean-field pairing between the electrons having opposite spins, momenta, and in contrast to the conventional superconducting pairing, residing in opposite bands, i.e., having opposite energies. We show that electrons transition into a correlated ground state if and only if the bands are flat enough, i.e., the transition is impossible in the case of conventional parabolic bands. Although this state is not superconducting in the usual sense and does not exhibit a gap in its excitation spectrum, it is nevertheless immune to elastic scattering caused by any kind of disorder and is therefore expected to exhibit high electric conductivity at low temperature, mimicking the behavior of a real superconductor. Having in mind the recent experimental realizations of flat-band electronic systems in twisted multilayers, we foresee an exciting opportunity for observing a class of highly conductive materials.