Here, we report the tunning study of superconductivity and charge density wave in $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$ through the V kagome net doping. Combining electrical transport and magnetic susceptibility measurements, we reveal the opposite effect of Ta and Mo doping. Ta doping promotes the superconductivity but suppresses charge density wave, which may be explained by the lift of van Hove singularity towards $E_{\mathrm{F}}$. On the contrary, Mo doping suppresses the superconductivity quickly and leads to a continuous increase of charge density wave temperature up to 100.7 K. Our result suggests that electron doping can also effectively tune charge density wave state in $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$.

We analyze the effects of the long-range Coulomb interaction on the distribution of Berry curvature among the bands near charge neutrality of twisted bilayer graphene (TBG) closely aligned with hexagonal boron nitride (hBN). Due to the suppressed dispersion of the narrow bands, the band structure is strongly renormalized by electron-electron interactions, and thus, the associated topological properties of the bands are sensitive to filling. Using a Hartree formalism, we calculate the linear and nonlinear Hall conductivities, and find that for certain fillings, the remote bands contribute substantially to the Hall currents while the contribution from the central bands is suppressed. In particular, we find that these currents are generically substantial near regions of energies where the bands are highly entangled with each other, often featuring doping-induced band inversions. Our results demonstrate that topological transport in TBG/hBN is substantially modified by electron-electron interactions, which offer a simple explanation to recent experimental results.

Most Mott systems display a low-temperature phase coexistence region around the metal-insulator transition. The domain walls separating the respective phases have very recently been observed displaying unusual properties both in simulations and in experiments. First, they often cover a significant volume fraction, thus cannot be neglected. Second, they resemble neither a typical metal nor a standard insulator, displaying unfamiliar temperature dependence of (local) transport properties. Here we take a closer look at such domain wall matter by examining an appropriate unstable solution of the Hubbard model. We show that transport in this regime is dominated by the emergence of “resilient quasiparticles” displaying strong non-Fermi liquid features, reflecting the quantum-critical fluctuations in the vicinity of the Mott point.

We investigate local and nonlocal signatures of hybridization between a quantum dot (QD) state and an extended Andreev bound state (ABS) in a gate-defined InAs nanowire (NW) with multiple side probes. When a QD in one of the side probes was hybridized with an ABS in the NW, a characteristic spectroscopic pattern was observed both locally, i.e., in the probe with the QD, and nonlocally, in the tunnel conductance of a remote probe. Nonlocal signatures of hybridization reveal the extended nature of the ABS.

In the band theory for non-Hermitian systems, the energy eigenvalues, which are complex, can exhibit nontrivial topology which is not present in Hermitian systems. In one dimension, it was recently noted theoretically and demonstrated experimentally that the eigenvalue topology is classified by the braid group. The classification of eigenvalue topology in higher dimensions, however, remained an open question. Here, we give a complete description of eigenvalue topology in two and three dimensional systems, including the gapped and gapless cases. We reduce the topological classification problem to a purely computational problem in algebraic topology. In two dimensions, the Brillouin zone torus is punctured by exceptional points, and each nontrivial loop in the punctured torus acquires a braid group invariant. These braids satisfy the constraint that the composite of the braids around the exceptional points is equal to the commutator of the braids on the fundamental cycles of the torus. In three dimensions, there are exceptional knots and links, and the classification depends on how they are embedded in the Brillouin zone three-torus. When the exceptional link is contained in a contractible ball, the classification can be expressed in terms of the knot group of the link. Our results provide a comprehensive understanding of non-Hermitian eigenvalue topology in higher dimensional systems, and should be important for the further explorations of topologically robust open quantum and classical systems.

Molecular dynamics simulations can explore the characteristics and evolution of microstructures in alloys outside of experiments, with reliability and accuracy guaranteed by the interatomic potentials employed. Machine learning potential (MLP) is widely used for its accuracy close to first-principles calculations. When developing an MLP, the construction of the training dataset is crucial, determining the accuracy and generalization of the MLP. In this work, a Monte-Carlo-like (MCL) strategy is proposed to construct training datasets for developing MLPs of alloys, which is characterized by the efficient consideration of element distributions in alloys. As an example, a training dataset for the equimolar NbTiZrHf alloy is constructed based on the MCL strategy, and the corresponding MLP is developed subsequently. By comparing with two traditional strategies, it is found that the training dataset constructed based on the MCL strategy has greater dispersion, and the corresponding MLP has better prediction performance. In addition, a hybrid molecular statics and Monte Carlo simulation with the MCL-based MLP is performed to optimize the element distribution of the equimolar NbTiZrHf alloy, and segregation and short-range ordered structures are observed in the final configuration, which is consistent with the experimental results reported in the literature. The MCL strategy proposed in this work can provide a fast solution for considering the element distribution when constructing training datasets for developing MLPs of alloys.

Molecular multiferroics are receiving a large amount of interest, in part due to the ease of synthesis and their potential for tuneability. An important compound within this field is ammonium pentachloroaquaferrate(III), ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{FeCl}}_5·{\mathrm{H}}_2\mathrm{O}$, which has received a lot of attention due to it having both magnetic and ferroelectric transitions at 7.3 and 6.9 K, respectively. The magnetic structure is complex, with many different exchange pathways; however, to date there has been no consideration of magnetic interactions above the critical temperature. Our paper utilizes muon spin spectroscopy, inelastic neutron scattering and quasielastic neutron scattering (QENS) to show that, as one might expect for these complex magnetic systems, there is indeed evidence for short-range order above 7.3 K and this persists up to $\sim 15$ K. We also highlight the sensitivity of the QENS technique to the magnetic transition and any localized fluctuations.

We report an observation of a giant magneto-optical (MO) effect quadratic in magnetization (Cotton-Mouton effect) in a 50 nm thick layer of yttrium-iron garnet (YIG). By a combined theoretical and experimental approach, we managed to quantify both linear and quadratic MO effects in the studied material. We show that the quadratic MO signal in the thin YIG film can exceed the linear MO response, reaching values of 450 μrad that are comparable with Heusler alloys or ferromagnetic semiconductors. This relative enhancement of the quadratic MO response relative to the linear MO response is attributed to antiferromagnetic coupling of two Fe sublattices. Furthermore, we demonstrate that a proper choice of experimental conditions, particularly with respect to the used wavelength, is crucial for optimization of the quadratic MO effect, which can be used very efficiently for magnetometry measurement.

We state and prove two theorems about the ground state of magnetic systems described by very general Heisenberg-type models and discuss their implications for magnetic neutron scattering. The first theorem states that two models cannot have the same correlator without sharing the corresponding ground states. According to the second theorem, an $N$-qubit wave function cannot reproduce the correlators of a given system unless it represents a true ground state of that system. We discuss the implications for neutron scattering inverse problems. We argue that the first theorem provides a framework to understand neutron-based Hamiltonian learning. Furthermore, we propose a variational approach to quantum magnets based on the second theorem in which a representation of the wave function (held, for instance, in a neural network or in the qubit register of a quantum processor) is optimized to fit experimental neutron scattering data directly, without the involvement of a model Hamiltonian.

A Fermi surface coupled to a scalar field can be described in a $1/N$ expansion by choosing the fermion-scalar Yukawa coupling to be random in the $N$-dimensional flavor space, but invariant under translations. We compute the conductivity of such a theory in two spatial dimensions for a critical scalar. We find a Drude contribution, and verify that the proposed $1/{\omega }^{2/3}$ contribution to the optical conductivity at frequency $\omega$ has vanishing coefficient for a convex Fermi surface. We also describe the influence of impurity scattering of the fermions, and find that while the self-energy resembles a marginal Fermi liquid, the resistivity and optical conductivity behave like a Fermi liquid.

$A$-site ordered perovskites, ${\mathrm{CaCu}}_3{\mathrm{B}}_4{\mathrm{O}}_{12}$, which are derivatives of conventional ${AB\text{O}}_3$ perovskites, exhibit varying electronic and magnetic properties. With the objective of examining the role of Cu in this work, we have studied ${\mathrm{CaCu}}_3{\mathrm{Ti}}_4{\mathrm{O}}_{12}$ and ${\mathrm{CaCu}}_3{\mathrm{Zr}}_4{\mathrm{O}}_{12}$ and presented the cause of the crystallization of $A$-site ordered perovskite from conventional ${AB\text{O}}_3$ perovskite and the underlying mechanism leading to the stabilization of nontrivial and experimentally established $G$-type antiferromagnetic ($G$-AFM) ordering in these systems. The first-principles electronic structure calculations supplemented with phonon studies show that the formation of $A$-site ordered perovskite is driven by Jahn-Teller distortion of the ${\mathrm{CuO}}_{12}$ icosahedron. The crystal orbital Hamiltonian population analysis and magnetic exchange interactions estimated using spin dimer analysis infer that the nearest and next-nearest-neighbor interactions ($J_1$ and $J_2$) are direct and weakly ferromagnetic, whereas the third-neighbor interaction ($J_3$) is unusually strong and antiferromagnetic driven by an indirect superexchange mechanism. The structural geometry reveals that stabilization of $G$-AFM requires $J_1<2J_2, J_1<2J_3$. The experimental and theoretical values of Néel temperature agree well for $U\approx$ 7 eV, highlighting the role of strong correlation. The magnetic ordering is found to be robust against pressure and strain.

Orbital-free density functional theory and its time-dependent extension are efficient ab initio methods for calculating the electronic structure and dynamics of large systems. Through the calculation of the optical spectra of selected clusters, we reach three important conclusions: (1) The quality of the spectra is strongly affected by the quality of the corresponding ground-state electron density; (2) the adiabatic part of the electronic response to external perturbations can be safely evaluated at the semilocal level; and (3) the nonadiabatic, current-dependent part of the time-dependent Pauli potential is key to recover correct spectral envelopes.

Double quantum dots are one of the promising two-state quantum systems for realizing qubits. In the quest of successfully manipulating and reading information in qubit systems, it is of prime interest to control the charge response of the system to a gate voltage, as filled in by the dynamical charge susceptibility. We theoretically study this quantity for a nonequilibrium double quantum dot by using the functional integral approach and derive its general analytical expression. One highlights the existence of two lines of maxima as a function of the dot level energies, each of them being split under the action of a bias voltage. In the low frequency limit, we derive the capacitance and the charge relaxation resistance of the equivalent quantum RC-circuit with a notable difference in the range of variation for $R$ depending on whether the system is connected in series or in parallel. By incorporating an additional triplet state in order to describe the situation of a double quantum dot with spin, we obtain results for the resonator phase response which are in qualitative agreement with recent experimental observations in spin qubit systems. These results show the wealth of information brought by the knowledge of dynamical charge susceptibility in double quantum dots with potential applications for spin qubits.

We study the interplay of strong correlations and coherent driving by considering the strong coupling Kondo model driven by a time-periodic bias voltage. Combining a recent nonequilibrium renormalization group method with Floquet theory, we find that by the coherent dressing of the driving field side replicas of the Kondo resonance emerge in the conductance, which are not completely washed out by the decoherence induced by the driving. We show that to accurately capture the interplay of driving and strong correlations one needs to go beyond simple phenomenological pictures, which underestimate decoherence, or adiabatic approximations, highlighting the relevance of non-Markovian memory effects. Within our method the differential conductance shows good quantitative agreement with experimental data in the full crossover regime from weak to strong driving. We analyze memory effects in detail based on the response to short voltage pulses.

Arsenic telluride, ${\mathrm{As}}_2{\mathrm{Te}}_3$, is a layered van der Waals (vdW) semiconducting material usually known for its thermoelectric properties. It is composed of layers stacked together via weak vdW interactions, which can consequently be exfoliated into thin two-dimensional layers. Here, we studied the electronic properties of the $\alpha$ phase of ${\mathrm{As}}_2{\mathrm{Te}}_3$ by using angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT). In addition to the spectroscopic signature of $\alpha -{\mathrm{As}}_2{\mathrm{Te}}_3$, we were able to isolate anisotropic 2D electronic states, decoupled from the $\alpha -{\mathrm{As}}_2{\mathrm{Te}}_3$ electronic structure, that we propose to ascribe to single layer (SL) $\beta -\mathrm{tellurene}$. Our findings are supported by theoretical investigations using DFT, which reproduce the main ARPES experimental features. Our work thereby proposes $\alpha -{\mathrm{As}}_2{\mathrm{Te}}_3$ (100) surface as an interesting platform for the experimental exploration of the electronic band structure of SL $\beta -\mathrm{tellurene}$, which has been difficult to experimentally access otherwise.

The real-time contour formalism for Green's functions provides time-dependent information of quantum many-body systems. In practice, the long-time simulation of systems with a wide range of energy scales is challenging due to both the storage requirements of the discretized Green's function and the computational cost of solving the Dyson equation. In this paper, we apply a real-time discretization based on a piecewise high-order orthogonal-polynomial expansion to address these issues. We present a superconvergent algorithm for solving the real-time equilibrium Dyson equation using the Legendre spectral method and the recursive algorithm for Legendre convolution. We show that the compact high-order discretization in combination with our Dyson solver enables long-time simulations using far fewer discretization points than needed in conventional multistep methods. As a proof of concept, we compute the molecular spectral functions of ${\mathrm{H}}_2$, LiH, ${\mathrm{He}}_2$, and ${\mathrm{C}}_6{\mathrm{H}}_4{\mathrm{O}}_2$ using self-consistent second-order perturbation theory and compare the results with standard quantum chemistry methods as well as the auxiliary second-order Green's function perturbation theory method.

Infinite projected entangled-pair states (iPEPS) have been introduced to accurately describe many-body wave functions on two-dimensional lattices. In this context, two aspects are crucial: the systematic improvement of the Ansatz by the optimization of its building blocks, i.e., tensors characterized by bond dimension $D$, and the extrapolation scheme to reach the “thermodynamic” limit $D\to \infty$. Recent advances in variational optimization and scaling based on correlation lengths demonstrated the ability of iPEPS to capture phases with spontaneously broken continuous symmetry such as the antiferromagnetic (Néel) one with high fidelity, in addition to valence-bond solids which are already well described by finite-$D$ iPEPS. In contrast, systems in the vicinity of continuous quantum phase transitions still present a challenge for iPEPS, especially when non-Abelian symmetries are involved. Here, we consider the iPEPS Ansatz to describe the continuous transition between the (gapless) antiferromagnet and the (gapped) paramagnet that exists in the $S=1/2$ Heisenberg model on coupled two-leg ladders. In particular, we show how accurate iPEPS results can be obtained down to a narrow interval around criticality and analyze the scaling of the order parameter in the Néel phase in a spatially anisotropic situation.

Fermion systems with flat bands can boost superconductivity by enhancing the density of states at the Fermi level. We use quasiexact numerical methods to show that repulsive interactions between spinless fermions in a one-dimensional (1D) flat-band system, the Creutz ladder, give a finite pairing energy that increases with repulsion, though charge quasiorder remains dominant. Adding an attractive component shifts the balance in favor of superconductivity and the interplay of two flat bands further yields a remarkable enhancement of superconductivity, well outside of known paradigms for 1D fermions.

Gapping of Dirac surface states through time reversal symmetry breaking may realize the axion insulator state in condensed matter. Despite tremendous efforts, only a few material systems fall into this category of intrinsic magnetic topological insulators. Recent theoretical calculations proposed the antiferromagnetic (AFM) ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ to be a topologically nontrivial magnetic insulator with gapped surface states. Here we use scanning tunneling microscopy and spectroscopy complemented with density-functional theory calculations and modeling to probe the surface electronic states in ${\mathrm{EuIn}}_2{\mathrm{As}}_2$. We find a spin-orbit induced bulk gap of $\sim 120\mathrm{meV}$ located only a few meV above the Fermi energy, within which topological surface states reside. Temperature-dependent measurements provide evidence of the partial gapping ($\sim 40\mathrm{meV}$) of the surface states at low temperatures below the AFM order, which decreases with increasing temperature but remains finite above $T_{\mathrm{N}}$.