Multiorbital quantum impurity models with general interaction and hybridization terms appear in a wide range of applications, including embedding, quantum transport, and nanoscience. However, most quantum impurity solvers are restricted to a few impurity orbitals, discretized baths, diagonal hybridizations, or density-density interactions. Here, we generalize the inchworm quantum Monte Carlo method to the interaction expansion, and we explore its application to typical single- and multiorbital problems encountered in investigations of impurity and lattice models. Our implementation generically outperforms bare and bold-line quantum Monte Carlo algorithms in the interaction expansion. For the systems studied here, our implementation remains inferior to the more specialized hybridization expansion and auxiliary field algorithms. The problem of convergence to unphysical fixed points, which hampers so-called bold-line methods, is not encountered in inchworm Monte Carlo.

In bilayers of semiconducting transition metal dichalcogenides, the twist angle between layers can be used to introduce a highly regular periodic potential modulation on a length scale that is large compared to the unit cell. In such structures, correlated states can emerge, in which excitons in the heterostructure are strongly localized to the potential minima due to exciton-exciton interactions. We explore the transition between Mott and extended exciton phases in terms of a moiré-Bose-Hubbard Hamiltonian. Hopping and on-site interaction parameters are obtained from a Wannier representation of the interlayer-exciton wave functions, and a nonlocal Rytova-Keldysh model is used to attribute for the dielectric screening of excitons in the two-dimensional material. For sufficiently small exciton concentrations and substrate screening our model predicts the emergence of Mott-insulating states, establishing twisted transition metal dichalcogenide heterostructures as possible quantum simulators for bosonic many-body systems.

Given a critical quantum spin chain, we show how universal information about its quantum critical point can be extracted from wavefunction overlaps. More specifically, we consider overlap between low-energy eigenstates of the spin chain Hamiltonian with different boundary conditions, namely periodic boundary conditions and open boundary conditions. We show that such overlaps decay polynomially with the system size, where the exponent only depends on the central charge. Furthermore, the bulk-to-boundary operator product expansion (OPE) coefficients can be extracted from the overlaps involving excited states. We illustrate the proposal with the Ising model and the three-state Potts model.

It is suggested that many-body quantum chaos appears as the spontaneous symmetry breaking of unitarity in interacting quantum many-body systems. It has been shown that many-body level statistics, probed by the spectral form factor (SFF) defined as $K(\eta ,t)=\langle |\mathrm{Tr}exp(-\eta H+itH){|}^2\rangle$, is dominated by a diffuson-type mode in a field theory analysis. The key finding of this Letter is that the “unitary” $\eta =0$ case is different from the $\eta \to 0^{\pm }$ limit, with the latter leading to a finite mass of these modes due to interactions. This mass suppresses a rapid exponential ramp in the SFF, which is responsible for the fast emergence of Poisson statistics in the noninteracting case, and gives rise to a nontrivial random matrix structure of many-body levels. The interaction-induced mass in the SFF shares similarities with the dephasing rate in the theory of weak localization and the Lyapunov exponent of the out-of-time-ordered correlators.

Time reversal symmetry breaking Weyl semimetals are unique among Weyl materials in allowing the minimal number of Weyl points, thus offering the clearest signatures of the associated physics. Here we present neutron diffraction, density-functional theory, and transport measurement results which indicate that ${\mathrm{EuCd}}_2{\mathrm{As}}_2$, under ambient field, strain, and pressure, is such a material with a single pair of Weyl points. Our work reveals a magnetic structure (magnetic space group $C{2}^{\prime }/m^{\prime }$) with Eu moments pointing along the [210] direction in plane and canted $\sim$ ${30}^{\circ }$ out of plane. Density functional theory calculations using this structure show that the observed canting drastically alters the relevant electronic bands, relative to the in-plane order, leading to a single set of well-defined Weyl points. Furthermore, we find the canting angle can tune the distance of the Weyl points above the Fermi level, with the smallest distance at low canting angles. Finally, transport measurements of the anomalous Hall effect and longitudinal magnetoresistance exhibit properties indicative of a chiral anomaly, thus supporting the neutron scattering and DFT results suggesting ${\mathrm{EuCd}}_2{\mathrm{As}}_2$ is close to the ideal situation of the Weyl hydrogen atom.

Flat-band systems are a promising platform for realizing exotic collective ground states with spontaneously broken symmetry because the electron-electron interactions dominate over the kinetic energy. A collective ground state of particular interest is the chased-after exciton condensate (EC). However, in flat-band systems other collective ground states can compete with an EC phase, and the conventional treatment of the effect of thermal and quantum fluctuations predicts the EC phase should be unstable. Here, using double-twisted bilayer graphene (TBLG) heterostructures as an example, we show that, for realistic interaction strengths, the EC phase is favored with respect to other TBLG's phases—orbital magnetism and superconductivity—when the TBLGs have opposite doping, and that the quantum metric of the Bloch wave functions stabilizes the EC, reversing the conclusion that would be drawn from the conventional approach in which quantum metric contributions are neglected. Our results suggest that the quantum metric plays a critical role in determining the stability of exciton condensates in double layers formed by systems with flat bands.

Whether or not anomalies in the thermal conductivity in insulating cuprates can be attributed to antiferromagnetic order and magnons in a 2D Mott insulator remains an intriguing open question. To shed light on this issue, we investigate the thermal conductivity $\kappa$ and its relationship with the specific heat $c_v$ in the half-filled 2D single-band Hubbard model, using the numerically exact determinant quantum Monte Carlo algorithm and maximum entropy analytic continuation. At low temperatures where the charge degrees of freedom are gapped-out and $c_v$ exhibits a clear magnon peak, we observe that thermal conductivity $\kappa$ also tends to form a peak at similar temperatures. Reducing temperature further produces a sharp upturn in $\kappa$, associated with an increasing mean-free path. We identify this as the high-temperature side of the anomalous peak in insulating cuprates, where the mean-free path eventually is cut off by other scattering effects, including phonons, disorder, and physical size. Different scattering effects in our model are identified and analyzed in the thermal diffusivity.

Understanding and controlling quantum transport in low-dimensional systems is pivotal for heat management at the nanoscale. One promising strategy to obtain the desired transport properties is to engineer particular spectral structures. In this work we are interested in quasiperiodic disorder—incommensurate with the underlying periodicity of the lattice—which induces fractality in the energy spectrum. A well known example is the Fibonacci model which, despite being noninteracting, yields anomalous diffusion with a continuously varying dynamical exponent smoothly crossing over from superdiffusive to subdiffusive regime as a function of potential strength. We study the finite-temperature electric and heat transport of this model in linear response in the absence and in the presence of dephasing noise due to inelastic scattering. The dephasing causes both thermal and electric transport to become diffusive, thereby making thermal and electrical conductivities finite in the thermodynamic limit. Thus, in the subdiffusive regime it leads to enhancement of transport. We find that the thermal and electric conductivities have multiple peaks as a function of dephasing strength. Remarkably we observe that the thermal and electrical conductivities are not proportional to each other, a clear violation of the Wiedemann-Franz law, and the position of their maxima can differ. We argue that this feature can be utilized to enhance performance of quantum thermal machines. In particular, we show that by tuning the strength of the dephasing noise we can enhance the performance of the device in regimes where it acts as an autonomous refrigerator.

We report the magnetism and ESR of the $S_{\text{eff}}=1/2$ compound ${\mathrm{BaCo}}_2{\left({\mathrm{SeO}}_3\right)}_3·3{\mathrm{H}}_2\mathrm{O}$ with dimer-chain structure. The magnetic susceptibility and heat capacity show that the compound is antiferromagnetically ordered at $T_{\text{N}}=7.6$ K. The magnetization $M\left(H\right)$ curve at 2 K presents a spin-flop transition within 2.6–3.0 T when the magnetic field is applied along the $ab$ plane ($H\perp c$), pointing to the easy axis-like anisotropy. In high magnetic field, the magnetization increases nearly linearly until saturation at about 22 T, indicating a classical-like behavior. The magnetization process can be described with the intradimer exchange $J_1=-20.0$ K and interdimer exchanges $J_2=-14.6$ K and $J_3=-6.2$ K. Thus, ${\mathrm{BaCo}}_2{\left({\mathrm{SeO}}_3\right)}_3·3{\mathrm{H}}_2\mathrm{O}$ is approximately a three-dimensional antiferromagnet, although structurally it contains dimer chains.

Besides the charge and spin of electrons, the valley index is considered as another degree of freedom, which can be used as an information carrier in advanced electronic devices. The key for realizing applications of valleytronic devices is inducing and regulating valley polarization. Ferrovalley materials have ferromagnetism and valley features and exhibit interesting properties, e.g., anomalous valley Hall effect (AVHE). Also, the valley properties can be tuned efficiently by an electric field. Here, using first-principles calculations, we find that the $A$-type antiferromagnetic (AFM) $\mathrm{V}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{I}}_3$ heterostructure (HS) can realize perpendicular magnetic anisotropy and spontaneous valley polarization for realizing AVHE, while the magnetic anisotropy of ${\mathrm{VSe}}_2$ is in plane. Furthermore, we build a multiferroic HS ${\mathrm{In}}_2{\mathrm{Se}}_3/\mathrm{V}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{I}}_3$ by capping a ferroelectric (FE) layer (${\mathrm{In}}_2{\mathrm{Se}}_3$) on the $\mathrm{V}{\mathrm{Se}}_2/\mathrm{Cr}{\mathrm{I}}_3$ HS, to realize electric-field-controlled valley states. The half metal-to-semiconductor transition is achieved by switching the FE polarization from down to up, which is accompanied by turning off and on valley properties in this multiferroic HS. Our finding provides an alternative way for achieving and modulating spontaneous valley polarization and will be useful for spintronic and valleytronic devices based on the two-dimensional $A$-type AFM van der Waals HS.

The martensitic transformation is a fundamental physical phenomenon at the origin of important industrial applications. However, the underlying microscopic mechanism, which is of critical importance to explain the outstanding mechanical properties of martensitic materials, is still not fully understood. This is because for most martensitic materials the transformation is a fast process that makes in situ studies extremely challenging. Noble solids krypton and xenon undergo a progressive pressure-induced face-centered cubic (fcc) to hexagonal close-packed (hcp) martensitic transition with a very wide coexistence domain. Here, we took advantage of this unique feature to study the detailed transformation progress at the atomic level by employing in situ x-ray diffraction and absorption spectroscopy. We evidenced a four-stage pathway and suggest that the lattice mismatch between the fcc and hcp forms plays a key role in the generation of strain. We also determined precisely the effect of the transformation on the compression behavior of these materials.

We derive and solve quantum kinetic equations describing the accumulation of magnetoelastic bosons in an overpopulated magnon gas realized in a thin film of the magnetic insulator yttrium iron garnet. We show that in the presence of a magnon condensate, there is a non-equilibrium steady state in which incoherent magnetoelastic bosons accumulate in a narrow region in momentum space for energies slightly below the bottom of the magnon spectrum. The results of our calculations agree quite well with Brillouin light scattering measurements of the stationary non-equilibrium state of magnons and magnetoelastic bosons in yttrium iron garnet.

$\mathrm{Ba}{\mathrm{Ni}}_2{\mathrm{As}}_2$ is a superconductor chemically related to the Fe-based superconductors, with a complex and poorly understood structural phase transition. We show, based on first-principles calculations, that, in fact, there are two distinct competing structures. These structures are different from the electronic, transport, and bonding points of view but are close in energy. These arise due to complex As bonding patterns and drive distortions of the Ni layers. This is supported by photoemission experiments. This is very distinct from views of the distortion as being primarily driven by electrons at the Fermi surface as in a classical charge density wave, or by correlated electron physics associated with the Ni $d$ electrons and the Ni $d$ orbitals. The structural distortion, although connected with As does lead to an interplay of electronic and structural behavior including induced anisotropic electronic transport. The local bonding nature of the instabilities and the competition between distortions is discussed in the context of the complex behavior observed in $\mathrm{Ba}{\mathrm{Ni}}_2{\mathrm{As}}_2$ samples.

We study frustration-free Hamiltonians of fractional quantum Hall (FQH) states from the point of view of the matrix product state (MPS) representation of their ground and excited states. There is a wealth of solvable models relating to FQH physics, which, however, is mostly derived and analyzed from the vantage point of first-quantized “analytic clustering properties.” In contrast, one obtains long-ranged frustration-free lattice models when these Hamiltonians are studied in an orbital basis, which is the natural basis for the MPS representation of FQH states. The connection between MPS-like states and frustration-free parent Hamiltonians is the central guiding principle in the construction of solvable lattice models, but thus far, only for short-range Hamiltonians and MPSs of finite bond dimension. The situation in the FQH context is fundamentally different. Here we expose the direct link between the infinite-bond-dimension MPS structure of Laughlin–conformal field theory (CFT) states and their parent Hamiltonians. While focusing on the Laughlin state, generalizations to other CFT-MPSs will become transparent.

We present de Haas-van Alphen (dHvA) measurements on an Eu-based valence-fluctuating system. ${\mathrm{EuIr}}_2{\mathrm{Si}}_2$ exhibits a temperature-dependent, noninteger europium valence with ${\mathrm{Eu}}^{2.8+}$ at low temperatures. The comparison of experimental results from our magnetic-torque experiments in fields up to 32 T and density functional theory band-structure calculations with localized $4f$ electrons shows that the best agreement is reached for a Fermi surface based on a valence of ${\mathrm{Eu}}^{2.8+}$. The calculated quantum-oscillation frequencies for ${\mathrm{Eu}}^{3+}$ instead cannot explain all the experimentally observed frequencies. The effective masses, derived from the temperature dependence of the dHvA oscillation amplitudes, show not only a significant enhancement with masses up to $11m_e$ ($m_e$ being the free electron mass), but also a magnetic-field dependence of the heaviest mass. We attribute the formation of these heavy masses to strong correlation effects resulting from valence fluctuations of $4f$ electrons being strongly hybridized with conduction electrons. The increase of the heavy masses with magnetic field likely results from the onset of the expected field-induced valence crossover that enhances these valence fluctuations but does not alter the Fermi-surface topology in the field range studied.

We develop a theory of anomalies of fermionic topological phases of matter in ($2+1$)D with a general fermionic symmetry group $G_f$. In general, $G_f$ can be a nontrivial central extension of the bosonic symmetry group $G_b$ by fermion parity ${(-1)}^F$. We encounter four layers of obstructions to gauging the $G_f$ symmetry, which we dub the anomaly cascade: (i) An ${\mathcal{H}}^1(G_b,{\mathbb{Z}}_{\mathbf{T}})$ obstruction to extending the symmetry permutations on the anyons to the fermion parity gauged theory, (ii) An ${\mathcal{H}}^2(G_b,kerr)$ obstruction to extending the $G_b$ group structure of the symmetry permutations to the fermion parity gauged theory, where $r$ is a map that restricts symmetries of the fermion parity gauged theory to the anyon theory, (iii) An ${\mathcal{H}}^3(G_b,{\mathbb{Z}}_2)$ obstruction to extending the symmetry fractionalization class to the fermion parity gauged theory, and (iv) the well-known ${\mathcal{H}}^4(G_b,U(1\left[\left)\right)\right]$ obstruction to developing a consistent theory of $G_b$ symmetry defects for the fermion parity gauged theory. We describe how the ${\mathcal{H}}^2$ obstruction can be canceled by anomaly inflow from a bulk ($3+1$)D symmetry-protected topological state (SPT) and also its relation to the Arf invariant of spin structures on a torus. If any anomaly in the above sequence is nontrivial, the subsequent ones become relative anomalies. A number of conjectures regarding symmetry actions on super-modular categories, guided by general expectations of anomalies in physics, are also presented.

Calculation of topological invariants for crystalline systems is well understood in reciprocal space, allowing for the topological classification of a wide spectrum of materials. In this work, we present a technique based on the entanglement spectrum, which can be used to identify the hidden topology of systems without translational invariance. By training a neural network to distinguish between trivial and topological phases using the entanglement spectrum obtained from crystalline or weakly disordered phases, we can predict the topological phase diagram for generic disordered systems. This approach becomes particularly useful for gapless systems, while providing a computational speed-up compared to the commonly used Wilson loop technique for gapful situations. Our methodology is illustrated in two-dimensional models based on the Wilson-Dirac lattice Hamiltonian.

We report detailed valence fluctuation phenomena in the unique quantum-critical compound $\beta \text{−}{\mathrm{YbAlB}}_4$ under pressures and at low temperatures. We directly observed the drastic change in the pressure dependence of the mean Yb valence at about 3 GPa with no pressure-induced structural transition. Below 3 GPa, the anomaly in the temperature dependences of the mean Yb valence was observed at about 55 K under ambient pressure, corresponding to a previously observed reduced Kondo coherence temperature, and this temperature rapidly decreases to 22 K at 2.2 GPa, in contrast to the monotonous decrease of the high Kondo temperature ($\sim 250\mathrm{K}$ at ambient pressure). The present results reveal that this reduced Kondo coherence temperature is responsible for the unconventional quantum critical behaviors in $\beta \text{−}{\mathrm{YbAlB}}_4$. Above 3 GPa, it was found that the temperature dependence of the mean Yb valence exhibits a discontinuous change at low temperature (10.5 and 9 K under 4.8 and 5.5 GPa, respectively).

We study a series of dynamical protocols which involve periodically driving a quantum dot coupled to a putative nanowire hosting Majorana zero modes (MZMs) to (i) reduce the hybridization between MZMs, (ii) improve the coherence of the Majorana qubit with respect to $1/f$ dephasing noise and quasiparticle poisoning, and (iii) provide a definitive test to differentiate Andreev bound states (ABSs) from MZMs. The protocols are based on the notion of draiding, exchanging a pair of Majoranas twice, repeatedly, at high frequency [I. Martin and K. Agarwal, PRX Quantum 1, 020324 (2020)]. In this process, the exchanged Majorana operators acquire a robust minus sign such that terms in the Hamiltonian, linear in either operator, vanish on average. The four protocols proposed implement draiding by coupling quantum dot(s) to the end(s) of the nanowire. They are treated using Floquet theory and numerical simulations. The hybridization energy and decoherence rate are shown to be reduced by several orders of magnitude, in accordance with theoretical expectations, when the protocols are implemented on nanowires described by experimentally relevant parameters. The tunneling conductance computed in this Floquet setting reveals zero-bias peaks (ZBPs) that become more centered at zero voltage bias. When these protocols are implemented on nanowires supporting ABSs that mimic MZMs in ZBP measurements, the qubit coherence deteriorates, in stark contrast to the case where the nanowire supports MZMs and the coherence drastically improves, thus serving as a dynamical test to distinguish MZMs from trivial bound states.