We design a neural network Ansatz for variationally finding the ground-state wave function of the homogeneous electron gas, a fundamental model in the physics of extended systems of interacting fermions. We study the spin-polarized and paramagnetic phases with 7, 14, and 19 electrons over a broad range of densities from $r_s=1$ to $r_s=100$, obtaining similar or higher accuracy compared to a state-of-the-art iterative backflow baseline even in the challenging regime of very strong correlation. Our work extends previous applications of neural network Ansätze to molecular systems with methods for handling periodic boundary conditions, and makes two notable changes to improve performance: splitting the pairwise streams by spin alignment and generating backflow coordinates for the orbitals from the network. We illustrate the advantage of our high-quality wave functions in computing the reduced single-particle density matrix. This contribution establishes neural network models as flexible and high-precision Ansätze for periodic electronic systems, an important step towards applications to crystalline solids.

We present measurements and simulations of semiconductor-superconductor heterostructure devices that are consistent with the observation of topological superconductivity and Majorana zero modes. The devices are fabricated from high-mobility two-dimensional electron gases in which quasi-one-dimensional wires are defined by electrostatic gates. These devices enable measurements of local and nonlocal transport properties and have been optimized via extensive simulations to ensure robustness against nonuniformity and disorder. Our main result is that several devices, fabricated according to the design's engineering specifications, have passed the topological gap protocol defined in Pikulin et al. (arXiv:2103.12217). This protocol is a stringent test composed of a sequence of three-terminal local and nonlocal transport measurements performed while varying the magnetic field, semiconductor electron density, and junction transparencies. Passing the protocol indicates a high probability of detection of a topological phase hosting Majorana zero modes as determined by large-scale disorder simulations. Our experimental results are consistent with a quantum phase transition into a topological superconducting phase that extends over several hundred millitesla in magnetic field and several millivolts in gate voltage, corresponding to approximately one hundred microelectronvolts in Zeeman energy and chemical potential in the semiconducting wire. These regions feature a closing and reopening of the bulk gap, with simultaneous zero-bias conductance peaks at both ends of the devices that withstand changes in the junction transparencies. The extracted maximum topological gaps in our devices are $20\text{–}60\mu \mathrm{eV}$. This demonstration is a prerequisite for experiments involving fusion and braiding of Majorana zero modes.

We present the magnetic orders and excitations of the distorted kagome intermetallic magnet TmAgGe. Using neutron single crystal diffraction we identify the propagation vectors $\mathbf{k}$ = ($\frac{1}{2}$ 0 0) and $\mathbf{k}$ = (0 0 0) and determine the magnetic structures of the zero-field and magnetic field-induced phases for $H$ along the $a$ and $[-110]$ crystal directions. We determine the experimental magnetic field- temperature ($H, T$)-phase diagram and reproduce it by Monte Carlo simulations of an effective spin exchange Hamiltonian for one distorted kagome layer. Our model includes a strong axial single-ion anisotropy and significantly smaller exchange couplings, which span up to the third-nearest neighbors within the layer. Single crystal inelastic neutron scattering (INS) measurements reveal an almost flat, only weakly dispersive mode around 7 meV that we use alongside bulk magnetization data to deduce the crystal-electric field (CEF) scheme for the ${\mathrm{Tm}}^{3+}$ ions. Random phase approximation (RPA) calculations based on the determined CEF wave functions of the two lowest quasidoublets enable an estimation of the interlayer coupling that is compatible with the experimental INS spectra. No evidence for low-energy spin waves associated to the magnetic order was found, which is consistent with the strongly Ising nature of the ground state.

To obtain a local description with a small basis size from density functional theory codes often a transformation to a local orthonormal Wannier function basis is desirable. In order to do so while enforcing the constraints of the space group symmetry the symmetry conserving maximally projected Wannier functions (SCMPWF) approach has been implemented in the full potential-local orbital code, FPLO. SCMPWFs are obtained from the initial step of the maximally localized Wannier function algorithm, projecting a subset of wave functions onto a set of suitably chosen local trial functions with prescribed symmetry properties with subsequent orthonormalization. The particular nature of the local orbitals in FPLO makes them an ideal set of projectors since they are constructed to be a small basis adapted to the occupied sector and the lowest-lying unoccupied states. While in many cases projection onto the FPLO basis orbitals is sufficient, the option is there to choose particular local linear combinations as projectors, in order to treat cases of bond centered Wannier functions. This choice turns out to lead to very localized Wannier functions, which obey the space group symmetry of the crystal by construction. Furthermore we discuss the interplay of the Berry connection and position operator and especially its possible approximation, symmetries, and the optimal choice of Bloch sum phase gauge in cases where the basis is not explicitly known. We also introduce various features, which are accessible via the FPLO implementation of SCMPWFs, discuss and compare performance and provide example applications.

We develop the theory of an Andreev junction, which provides a method to probe the intrinsic topology of the Fermi sea of a two-dimensional electron gas (2DEG). An Andreev junction is a Josephson $\pi$ junction proximitizing a ballistic 2DEG, and exhibits low-energy Andreev bound states that propagate along the junction. It has been shown that measuring the nonlocal Landauer conductance due to these Andreev modes in a narrow linear junction leads to a topological Andreev rectification (TAR) effect characterized by a quantized conductance that is sensitive to the Euler characteristic ${\chi }_F$ of the 2DEG Fermi sea. Here we expand on that analysis and consider more realistic device geometries that go beyond the narrow linear junction and fully adiabatic limits considered earlier. Wider junctions exhibit additional Andreev modes that contribute to the transport and degrade the quantization of the conductance. Nonetheless, we show that an appropriately defined rectified conductance remains robustly quantized provided large momentum scattering is suppressed. We verify and demonstrate these predictions by performing extensive numerical simulations of realistic device geometries. We introduce a simple model system that demonstrates the robustness of the rectified conductance for wide linear junctions as well as point contacts, even when the nonlocal conductance is not quantized. Motivated by recent experimental advances, we model devices in specific materials, including InAs quantum wells, as well as monolayer and bilayer graphene. These studies indicate that for sufficiently ballistic samples observation of the TAR effect should be within experimental reach.

We report crystal structure and magnetic behavior of the triangular antiferromagnet ${\mathrm{KYbO}}_2$, the A-site substituted version of the quantum spin liquid candidate ${\mathrm{NaYbO}}_2$. The replacement of Na by K introduces an anisotropic tensile strain with 1.6% in-plane and 12.1% out-of-plane lattice expansion. Compared to ${\mathrm{NaYbO}}_2$, both Curie-Weiss temperature and saturation field are reduced by about 20% as the result of the increased Yb-O-Yb angles, whereas the $g$ tensor of ${\mathrm{Yb}}^{3+}$ becomes isotropic with $g=3.08\left(3\right)$. Field-dependent magnetization shows the plateau at $\frac{1}{2}$ of the saturated value and suggests the formation of the up-up-up-down field-induced order in ${\mathrm{KYbO}}_2$ in contrast to the more common $\frac{1}{3}$ plateau with the up-up-down order that has been reported in the isostructural ${\mathrm{Yb}}^{3+}$ selenides.

Recent experiments have established that semiconductor-based moiré materials can host incompressible states at a series of fractional moiré-miniband fillings. These states have been identified as generalized Wigner crystals in which electrons localize on a subset of the available triangular-lattice moiré superlattice sites. In this article, we use momentum-space exact diagonalization to investigate the many-body ground-state evolution at rational fillings from the weak-hopping classical-lattice gas limit, in which only spin degrees-of-freedom are active at low energies, to the strong-hopping metallic regime where the Wigner crystals melt. We specifically address the nature of the magnetic ground states of the generalized Wigner crystals at fillings $\nu =1/3$ and $\nu =2/3$.

A thorough understanding of electrical and thermal transport properties of group-III nitride semiconductors is essential for their electronic and thermoelectric applications. Despite extensive previous studies, these transport properties were typically calculated without considering the nonequilibrium coupling effect between electrons and phonons, which can be particularly strong in group-III nitride semiconductors due to the high electric fields and high heat currents in devices based on them. In this work, we systematically examine the phonon drag effect, namely the momentum exchange between nonequilibrium phonons and electrons, and its impact on charge mobility and Seebeck coefficient in GaN and AlN by solving the fully coupled electron and phonon Boltzmann transport equations with ab initio scattering parameters. We find that, even at room temperature, the phonon drag effect can significantly enhance mobility and Seebeck coefficient in GaN and AlN, especially at higher carrier concentrations. Furthermore, we show that the phonon drag contribution to mobility and Seebeck coefficient scale differently with the carrier concentration and we highlight a surprisingly important contribution to the mobility enhancement from the polar optical phonons. We attribute both findings to the distinct mechanisms the phonon drag affects mobility and Seebeck coefficient. Our study advances the understanding of the strong phonon drag effect on carrier transport in wide bandgap GaN and AlN and gives new insights into the nature of coupled electron-phonon transport in polar semiconductors.

Circuit quantum electrodynamics (cQED) systems represent an important platform to study light-matter interaction at the nanoscale. However, an all-electrical scheme for photon statistics engineering in cQED, has so far not been established. Here, we propose a generation scheme of arbitrary photon statistics, based on current-driven joint interference effect in a three-body setup with one biased double quantum dot capacitively coupled to two microwave cavities. Antibunched, bunched, superthermal, and coherent photon emissions can be achieved and regulated by tuning inelastic electron tunneling processes. Generation of quantum correlation between cavities, indicated by the violation of classical Cauchy-Schwarz inequality, and quantum to classical transition can be further observed. Our scheme, proposed in a three-body cQED, can be extended toward many-body systems, as verified in a four-body setup.

The nature of correlated states in twisted bilayer graphene (TBG) at the magic angle has received intense attention in recent years. We present a numerical study of an interacting Bistritzer-MacDonald (IBM) model of TBG using a suite of methods in quantum chemistry, including Hartree-Fock, coupled cluster singles, doubles (CCSD), and perturbative triples [CCSD(T)], as well as a quantum chemistry formulation of the density matrix renormalization group method (DMRG). Our treatment of TBG is agnostic to gauge choices, and hence we present a new gauge-invariant formulation to detect the spontaneous symmetry breaking in interacting models. To benchmark our approach, we focus on a simplified spinless, valleyless IBM model. At integer filling ($\nu =0$), all numerical methods agree in terms of energy and $C_{2z}\mathcal{T}$ symmetry breaking. Additionally, as part of our benchmarking, we explore the impact of different schemes for removing “double-counting” in the IBM model. Our results at integer filling suggest that cross validation of different IBM models may be needed for future studies of the TBG system. After benchmarking our approach at integer filling, we perform the first systematic study of the IBM model near integer filling (for $\left|\nu \right|<0.2$). In this regime, we find that the ground state can be in a metallic and $C_{2z}\mathcal{T}$ symmetry breaking phase. The ground state appears to have low entropy, and therefore can be relatively well approximated by a single Slater determinant. Furthermore, we observe many low entropy states with energies very close to the ground-state energy in the near integer filling regime.

Spin-electronic devices are poised to become part of mainstream microelectronic technology. Downsizing them led to the field of molecular spintronics. Here, we provide proof-of-concept data that allow expanding this area from its traditional focus on single-molecule magnets to molecules in which spin centers are antiferromagnetically (AFM) coupled to result in a singlet ground state. In this context, and in contrast to all previous work on molecular spintronics, we develop a detection scheme of the spin state of the molecule that does not rely on a magnetic moment. Instead, we use quantum dot devices consisting of an isolated, contacted single-wall carbon nanotube covalently bound to a limited number of molecular AFMs, for which we chose representative coordination complexes incorporating four Mn(II) or Co(II) ions. Time-dependent quantum transport measurements along the functionalized nanotube show steplike transitions between several distinct current levels that we attribute to transitions between different AFM states of individual molecular complexes grafted on the nanotube. A statistical analysis of the switching events using factorial cumulants indicates that the cobalt complexes switch independently from each other, whereas a coherent superposition of the AFM spin states of the molecules along the nanotube is observed for the manganese complexes. The long coherence time of the superposition state (several seconds at 100 mK) is made possible by the absence of spin and orbital momentum in the relevant states of the manganese complex, while the cobalt complex includes a significant orbital momentum contribution due to the pseudo-octahedral coordination environment of the $d^7$ metal centers.

The recently emerging superconductivity in infinite-layer nickelates with the isostructural and isoelectronic characteristics of cuprates provides a new platform to explore the pairing mechanism of high-temperature superconductors. In this work, we studied the upper critical field $\left(H_{\mathrm{c}2}\right)$ of a high-quality ${\mathrm{La}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ thin film with superconducting transition temperature, $T_{\mathrm{c}}^{\mathrm{onset}}=18.8$ K, using high magnetic field up to 56 T. A very large $H_{\mathrm{c}2}, \sim 40$ T for $H\parallel c$ and $\sim 52$ T for $H\perp c$, was confirmed, which suggests that infinite-layer nickelates also have great application potential. The anisotropy of $H_{\mathrm{c}2}$ monotonically decreases from $\sim 10(\gamma =H_{\mathrm{c}2}^{ab}/H_{\mathrm{c}2}^c)$ near $T_{\mathrm{c}}$ to $\sim 1.5$ at 2 K. Angle dependence of $H_{\mathrm{c}2}$ confirms the crossover of superconductivity from two-dimensional to three-dimensional as the temperature decreases. We discussed that the interstitial orbital effect causes the weakening of anisotropy. The observed abnormal upturning of $H_{\mathrm{c}2}$ at low temperatures is found to be a universal behavior independent of film quality and rare-earth elements. Therefore, it should not be the Fulde-Ferrell-Larkin-Ovchinnikov state due to the fact that it is in the dirty limit and insensitive to disorder.

Iron-bearing oxides undergo a series of pressure-induced electronic, spin, and structural transitions that can cause seismic anomalies and dynamic instabilities in Earth's mantle and outer core. We employ x-ray diffraction and x-ray emission spectroscopy along with density-functional $\mathrm{theory}+\mathrm{dynamical}$ mean-field theory calculations to characterize the electronic structure and spin states, and crystal-structural properties of wüstite (${\mathrm{Fe}}_{1-x}\mathrm{O}$)–a basic oxide component of Earth's interior–at high pressure-temperature conditions up to 140 GPa and 2100 K. We find that FeO exhibits complex polymorphism under pressure, with abnormal compression behavior associated with electron-spin and crystallographic phase transitions, and resulting in a substantial change of bulk modulus. Our results reveal the existence of a high-pressure phase characterized by a metallic high-spin state of iron at the pressure-temperature conditions near to those of Earth's core-mantle boundary. The presence of high-spin metallic iron near the mantle can significantly influence the geophysical and geochemical properties of Earth's deep interior.

Ab initio Green-Kubo (aiGK) simulations of heat transport in solids allow for assessing lattice thermal conductivity in anharmonic or complex materials from first principles. In this work, we present a detailed account of their practical application and evaluation with an emphasis on noise reduction and finite-size corrections in semiconductors and insulators. To account for such corrections, we propose strategies in which all necessary numerical parameters are chosen based on the dynamical properties displayed during molecular dynamics simulations in order to minimize manual intervention. This paves the way for applying the aiGK method in semiautomated and high-throughput frameworks. The proposed strategies are presented and demonstrated for computing the lattice thermal conductivity at room temperature in the mildly anharmonic periclase MgO, and for the strongly anharmonic marshite CuI.

Using ultrashort laser pulses, it has become possible to probe the dynamics of long-range order in solids on microscopic timescales. In the conventional description of symmetry-broken phases within time-dependent Ginzburg-Landau theory, the order parameter evolves coherently, with small fluctuations along an average trajectory. Recent experiments, however, indicate that some systems can support a different scenario, named ultrafast inhomogeneous disordering, where the average order parameter is no longer representative of the state on the atomic scale. Here we theoretically show that ultrafast disordering can occur in a minimal, yet paradigmatic, model for a Peierls instability if atomic scale inhomogeneities of both the electronic structure and the charge density wave order parameter are taken into account. The latter is achieved using a nonequilibrium generalization of statistical dynamical mean-field theory coupled to stochastic differential equations for the order parameter.

We have investigated charge tunable devices based on ${\mathrm{WSe}}_2$ and ${\mathrm{WS}}_2$ monolayers encapsulated in hexagonal boron nitride. In addition to the well-known radiative recombination of neutral and charged excitons, stationary photoluminescence measurements highlight a weaker-intensity optical transition that appears when the monolayer is electrostatically doped with electrons. We have carried out a detailed characterization of this photoluminescence line based on its doping dependence, its variation as a function of the optical excitation power, its polarization characteristics, and its $g$ factor measured under longitudinal magnetic field. We interpret this luminescence peak as an impurity-assisted radiative recombination of the intervalley negatively charged exciton (triplet trion). In addition to the standard direct recombination, the triplet trion has a second recombination channel triggered by elastic scattering off the short-range impurity potential. The energy difference between the emitted photons from these two possible recombination processes of the same triplet trion is simply the single-particle conduction band spin-orbit splitting ${\mathrm{\Delta }}_c$. We measure ${\mathrm{\Delta }}_c=12\pm 0.5$ meV for the ${\mathrm{WSe}}_2$ monolayer and ${\mathrm{\Delta }}_c=12\pm 1$ meV for the ${\mathrm{WS}}_2$ monolayer. These results demonstrate the importance of second-order exciton recombination processes in transition-metal dichalcogenide structures.

We explore the electronic structure of paramagnetic CrSBr by comparative first-principles calculations and angle-resolved photoemission spectroscopy. We theoretically approximate the paramagnetic phase using a supercell hosting spin configurations with broken long-range order and applying quasiparticle self-consistent $GW$ theory, without and with the inclusion of excitonic vertex corrections to the screened Coulomb interaction ($\mathrm{QS}GW$ and $\mathrm{QS}G\widehat{W}$, respectively). Comparing the quasiparticle band-structure calculations to angle-resolved photoemission data collected at 200 K results in excellent agreement. This allows us to qualitatively explain the significant broadening of some bands as arising from the broken magnetic long-range order and/or electronic dispersion perpendicular to the quasi-two-dimensional layers of the crystal structure. The experimental band gap at 200 K is found to be at least 1.51 eV at 200 K. At lower temperature, no photoemission data can be collected as a result of charging effects, pointing towards a significantly larger gap, which is consistent with the calculated band gap of approximately 2.1 eV.

We develop a framework for classifying locality preserving unitaries (LPUs) with internal, unitary symmetries in $d$ dimensions, based on $(d-1)$ dimensional “flux insertion operators” which are easily computed from the unitary. Using this framework, we obtain formulas for topological invariants of LPUs that prepare, or entangle, symmetry protected topological phases (SPTs). These formulas serve as edge invariants for Floquet topological phases in $(d+1)$ dimensions that “pump” $d$-dimensional SPTs. For 1D SPT entanglers and certain higher dimensional SPT entanglers, our formulas are completely closed-form.

TlBiSSe is a rare realization of a three-dimensional semimetal with a conically dispersing band that has an optical response which is well isolated from other contributions in a broad range of photon energies. We report optical and magneto-optical spectroscopy on this material. When the compound is chemically tuned into a state of the lowest carrier concentration, we find a nearly linear frequency dependence of the optical conductivity below 0.5 eV. Landau level spectroscopy allows us to describe the system with a massive Dirac model, giving a gap $2\mathrm{\Delta }=32$ meV and an in-plane velocity parameter $v_{xy}=4.0\times {10}^5$ m/s.

A quantum spin liquid with a spinon Fermi surface is an exotic insulator that hosts neutral Fermi surfaces inside the insulating gap. In an external magnetic field, it has been pointed out that the neutral Fermi surfaces are Landau quantized to form Landau levels due to the coupling to the induced emergent gauge magnetic field. In this paper, we calculate the electronic density of states (as observed in tunneling experiments) of the quantum spin liquid in an orbital magnetic field. We find that the Landau levels from the neutral Fermi surfaces give rise to a set of steps emerging at the upper and lower Hubbard band edges. Each of the Hubbard band edge steps further develop into a band edge resonance peak when a weak gauge binding arising from the gauge field fluctuations is taken into account. Importantly, each Hubbard band edge step and its resulting resonance peak in the weak gauge binding are found to have a corresponding Landau level from the neutral Fermi surfaces, so the Hubbard band edge steps and the band edge resonance peaks provide signatures to the unique feature that characterizes the Landau quantization of the in-gap neutral Fermi surfaces in the spin liquid. We further consider the strong gauge-binding regime where the band edge resonance peaks move into the Mott gap and develop into true in-gap bound states. In the strong gauge binding regime, we solve the Landau-level spectrum of the in-gap bound states in an orbital magnetic field. For the in-gap bound state with a Mexican-hat-like band dispersion, we find that the envelop energy to have a state excited from the bound state Landau levels decreases quadratically with the magnetic field. The quadratic decrease behavior of the envelop energy is consistent with the intuition that applying magnetic field localizes the states and energetically promotes the in-gap bound states formation. Finally, we discuss the connection of our results to the electronic density of states spectra measured in the layered 1T-${\mathrm{TaS}}_2$. We point out that a quantum spin liquid with a quasibound state in the upper Hubbard band can give the density of states spectra similar to the one measured in the experiment.