We characterize, by means of large-scale fermion quantum Monte Carlo simulations, metallic and deconfined quantum phase transitions in a bilayer honeycomb model in terms of their quantum critical and finite-temperature properties. The model features three distinct phases at zero temperature as a function of interaction strength. At weak interaction, a fully symmetric Dirac semimetal state is realized. At intermediate and strong interaction, respectively, two long-range-ordered phases, each breaking different symmetries are stabilized. The ordered phases feature partial and full gap openings in the fermion spectrum, respectively. We clarify the symmetries of the different zero-temperature phases and the symmetry-breaking patterns across the two quantum phase transitions between them. The first transition between the disordered and long-range-ordered semimetallic phases has previously been argued to be described by the $(2+1)$-dimensional Gross-Neveu-SO(3) field theory. By performing simulations with an improved symmetric Trotter decomposition, we further substantiate this claim by computing the critical exponents $1/\nu$, ${\eta }_{\varphi }$, and ${\eta }_{\psi }$, which turn out to be consistent with the field-theoretical expectation within numerical and analytical uncertainties. The second transition between the two long-range-ordered phases has previously been proposed as a possible instance of a metallic deconfined quantum critical point. We further develop this scenario by analyzing the spectral functions in the single-particle, particle-hole, and particle-particle channels. Our results indicate gapless excitations with a unique velocity, supporting the emergence of Lorentz symmetry at criticality. We also compute the finite-temperature phase boundaries of the ordered states above the fully gapped state at large interaction. The phase boundary vanishes smoothly in the vicinity of the putative metallic deconfined quantum critical point, in agreement with the expectation for a continuous or weakly first-order transition.

We study the finite size scaling of the bulk polarization in a quasiperiodic (Aubry-André) model using the geometric analog of the Binder cumulant. As a proof of concept, we show that the geometric Binder cumulant method described here can reproduce the known literature values for the flat and raised cosine distributions, which are the two distributions that occur in the delocalized phase. For the Aubry-André model at half-filling, the phase transition point is accurately reproduced. Not only is the correct size scaling exponent of the variance obtained in the extended and the localized phases, but the geometric Binder cumulant undergoes a sign change at the phase transition. We also calculate the state resolved Binder cumulant as a function of disorder strength to gain insight into the mechanism of the localization transition.

We use a convolutional restricted Boltzmann machine (CRBM) neural network to construct a variational wave function (WF) for the Hubbard model on a square lattice and study it using the variational Monte Carlo (VMC) method. In the wave function, the CRBM acts as a correlation factor to a mean-field BCS state. The number of variational parameters in the WF does not grow automatically with the lattice size and it is computationally much more efficient compared with other neural-network-based WFs. We find that, in the intermediate- to strong-coupling regime of the model at half filling, the wave function outperforms even the highly accurate long-range backflow-Jastrow correlated wave function. Using the WF, we study the ground state of the half filled model as a function of onsite Coulomb repulsion $U$. We consider two cases for the next-nearest-neighbor hopping parameter, e.g., $t^{\prime }=0$ as well as a frustrated model case with $t^{\prime }\ne 0$. By examining several quantities, e.g., double occupancy, charge gap, momentum distribution, and spin-spin correlations, we find that the weekly correlated phase in both cases is paramagnetic metallic (PM). As $U$ is increased, the system undergoes a first-order Mott transition to an insulating state at a critical $U_c$, the value of which depends on $t^{\prime }$. The Mott state in both cases is spin gapped with long-range antiferromagnetic (AFM) order. Remarkably, the AFM order emerges spontaneously from the wave function which does not have any explicitly broken symmetry in it. Apart from some quantitative differences in the results for the two values of $t^{\prime }$, we find some interesting qualitative differences in the way the Mott transition takes place in the two cases.

Two-dimensional semi-Dirac models describe a family of novel materials that have anisotropic dispersion with a relativistic-type spectrum along one of the spatial directions and nonrelativistic along another. In the present paper we perform a detailed analysis of the collapse of Landau levels for such models in a perpendicular magnetic field and in-plane electric field by using a semiclassical approach. The anisotropic nature of semi-Dirac dispersion manifests itself in the possibility of a Landau level collapse for only one of the electric field directions. In addition, the topological transition with merging Dirac cones has its own distinct features in a Landau level collapse.

We study a chaotic particle-conserving kinetically constrained model, with a single parameter which allows us to break reflection symmetry. Through extensive numerical simulations we find that the domain wall state shows a variety of dynamical behaviors from localization all the way to ballistic transport, depending on the value of the reflection breaking parameter. Surprisingly, such anomalous behavior is not mirrored in infinite-temperature dynamics, which appear to scale diffusively, in line with expectations for generic interacting models. However, studying the particle density gradient, we show that the lack of reflection symmetry affects infinite-temperature dynamics, resulting in an asymmetric dynamical structure factor. This is in disagreement with normal diffusion and suggests that the model may also exhibit anomalous dynamics at infinite temperature in the thermodynamic limit. Finally, we observe low-entangled eigenstates in the spectrum of the model, a telltale sign of quantum many-body scars.

Finding distinct signatures of a quantum spin liquid (QSL) is an ongoing quest in condensed matter physics, invariably complicated by the presence of disorder in real materials. In this regard the two-dimensional kagome system ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6\left[{\left({\mathrm{Cl}}_x{\mathrm{Br}}_{(1-x)}\right)}_{3-y}{\left(\mathrm{OH}\right)}_y\right]$ $\left(\mathrm{YCOB}\text{−}\mathrm{Cl}\right)$, where the vast mismatch in size of Y and Cu avoids substitutional disorder, otherwise present in kagome materials, has emerged as a favorable candidate. In crystals of this system, with $x<0.4$ and no long-range order, we report an unusual field-dependent magnetization $M\left(B\right)$, where $M/B$ changes linearly with $\left|B\right|$, the absolute value of the field, in contrast to the expected quadratic behavior. Model calculations with a distribution of ferromagnetic (FM) clusters faithfully capture the observed features. YCOB-Cl exhibits a field-enhanced $T^2$ heat capacity expected in certain spin-liquid models. However, a concomitant linear $T$ behavior in the spin susceptibility has been lacking. Our work suggests that FM clusters, by dominating the contribution to the susceptibility, mask the true underlying behavior.

Acoustic phonons in piezoelectric materials strongly couple to electrons through a macroscopic electric field. We show that this coupling leads to a momentum-dependent divergence of the Fan-Migdal electron linewidth. We then develop a self-consistent theory for calculating electron linewidths, which not only removes this piezoelectric divergence but also considerably modifies the linewidth in nonpiezoelectric, polar materials. Our predictions await immediate experimental confirmation, and this self-consistent method should be broadly used in interpreting various experiments on the electronic properties of real materials.

Single-crystalline FeSi samples with a conducting surface state were studied under high pressure and magnetic field by means of electrical resistance measurements to explore how the bulk semiconducting state and the surface state are tuned by the application of pressure. We found that the energy gap associated with the semiconducting bulk phase begins to close abruptly at a critical pressure of ∼10 GPa and the bulk material becomes metallic with no obvious sign of any emergent phases or non-Fermi liquid behavior in temperature dependent electrical resistance in the neighborhood of the critical pressure above 3 K. Moreover, the metallic phase appears to remain at near-ambient pressure upon release of the pressure. Interestingly, the hysteresis in the electrical resistance vs magnetic field curve associated with the magnetically ordered conducting surface state decreases with pressure and vanishes at the critical pressure, while the slope of the electrical resistance vs magnetic field curve, which has a negative value for pressure below the critical pressure, decreases in magnitude with pressure and changes sign at the critical pressure. Thus the conducting surface state and the corresponding two-dimensional magnetic order collapse at the critical pressure where the energy gap of the bulk material starts to close abruptly, revealing the connection between the conducting surface state and the semiconducting bulk state in FeSi.

We study the transition between Néel and columnar valence-bond solid ordering in two-dimensional $S=3/2$ square lattice quantum antiferromagnets with SO(3) symmetry. According to the deconfined criticality scenario, this transition can be direct and continuous like the well-studied $S=1/2$ case. To study the global phase diagram, we work with four multi-spin couplings with full rotational symmetry, that are free of the sign-problem of quantum Monte Carlo. Exploring the phase diagram with quantum Monte Carlo simulations, we find that the phase transition between Néel and valence-bond solid is strongly first-order in the parts of the phase diagram that we have accessed.

Time-domain thermoreflectance (TDTR) characterization of FeRh throughout its first-order antiferromagnetic (AF) to ferromagnetic (FM) transition shows that the transient reflectance $\mathrm{\Delta }R\left(t\right)/R$ strongly depends on the magnetic order of the sample. Using TDTR, which uses optical pulses to induce small temperature excursions, we have found that $\mathrm{\Delta }R\left(t\right)/R$ of the AF phase exhibits a large negative response, while the response of the FM phase is positive. This magnetic phase sensitivity has allowed us to study the transient response of both the AF and FM phases to the pump-pulse excitation and the mixed phase of the material. These results are significant since the ultrafast properties of antiferromagnetic materials and mixed antiferromagnetic and ferromagnetic materials are difficult to detect using other conventional techniques. We have found that the AF phase exhibits a strong subpicosecond decaying signal not observed in the FM phase. The magnetic phase dependence of the sign of $\mathrm{\Delta }R\left(t\right)/R$ is qualitatively explained using the results of ab initio density functional theory calculations. Using the two-temperature model, we found that the change in the thermalization time across the transition is caused by differences in both the electronic heat capacity and the electron-phonon coupling factor of the AF and FM phases. The electron-phonon coupling constant in the AF phase is also determined using the two-temperature model conducted using the ntmpy code package. For the FM phase, we provide boundaries for the magnitude of the electron-phonon coupling factor for the FM phase. These results indicate that TDTR can be used to study the transient properties of magnetic materials that are otherwise challenging to probe.

Over the past decade, it has been proposed theoretically and confirmed experimentally that long-range spin-triplet (LRT) superconductivity can be generated in superconductor/ferromagnet hybrids either by the presence of spin textures (ST-LRT) or owing to spin-orbit coupling (SOC-LRT). Nevertheless, there has been no theoretical or experimental investigation to date suggesting that both contributions could simultaneously exist within an experimental system. To disentangle these contributions, we present a comprehensive study of superconducting quasiparticle interference effects taking place inside a ferromagnetic layer interfacing a superconductor, through the investigation of above-gap conductance anomalies (CAs) related to MacMillan-Rowell resonances. The bias dependence of the CAs has been studied under a wide range of in-plane (IP) and out-of-plane (OOP) magnetic fields in two types of epitaxial, V/MgO/Fe-based ferromagnet-superconductor junctions with interfacial spin-orbit coupling. We observe an anisotropy of the CA amplitude under small IP and OOP magnetic fields while remaining weakly affected by high fields, and implement micromagnetic simulations to help us distinguish between the ST-LRT and SOC-LRT contributions. Our findings suggest that further exploration of Fabry-Pérot-type interference effects in electron transport could yield valuable insights into the hybridization between superconductors and ferromagnets induced by spin-orbit coupling and spin textures.

Bloch wave functions of electrons have properties called quantum geometry, which has recently attracted much attention as the origin of intriguing physical phenomena. In this paper, we introduce the notion of the quantum-geometric pair potentials (QGPP) based on the generalized band representation and thereby clarify how the quantum geometry of electrons is transferred to the Cooper pairs they form. QGPP quantifies the deviation of multiband superconductors from an assembly of single-band superconductors and has a direct connection to the quantum-geometric corrections to thermodynamic coefficients. We also discuss their potential ability to emulate exotic pair potentials and engineer intriguing superconducting phenomena including topological superconductivity.

Density-functional theory models of δ-Pu accurately predict crystal structure, phonon density of states, and unit cell volumes but require magnetic degrees of freedom which have never been experimentally verified. Some models invoke an on-site cancellation of spin and orbital moments, engendering a near-zero bulk magnetization, undetectable by most probes. Here, we employ x-ray magnetic circular dichroism at the Pu $M_{4,5}$ edges to directly probe spin and orbital moments using the magneto-optical sum rules. The data show no dichroism within experimental error, constraining polarized moments at 6 T and 3 K to ${\mu }_{L,S}<0.1{\mu }_B$. These experiments point to the absence of even unconventional spin-orbit compensated order in δ-Pu.

We prove that multilayer superconductivity with interlayer pairing may naturally decompose into a series of weakly coupled bilayer and trilayer superconducting blocks in order to minimize its total free energy. Our work is motivated by the recent proposal of interlayer pairing induced by the interlayer superexchange interaction of nearly half-filled $d_{z^2}$ orbitals in the bilayer and trilayer nickelate superconductors. We explore general properties of interlayer pairing superconductivity and perform systematic Ginzburg-Landau analyses of an effective multilayer model. For real materials, our results imply strong superconducting order parameter modulation and a short coherence length along the $z$ axis (perpendicular to the layers). This reveals a unique feature of multilayer superconductivity with interlayer pairing and provides a basic framework for future experimental and theoretical investigations.

Asymmetric two-junction SQUIDs with different current-phase relations in the two Josephson junctions, involving higher Josephson harmonics, demonstrate a flux-tunable Josephson diode effect (asymmetry between currents flowing in the opposite directions, which can be tuned by the magnetic flux through the interferometer loop). We theoretically investigate influence of junction capacitance and thermal fluctuations on performance of such Josephson diodes. Our main focus is on the “minimal model” with one junction in the SQUID loop possessing the sinusoidal current-phase relation and the other one featuring additional second harmonic. Capacitance generally weakens the diode effect in the resistive branch (R state) of the current-voltage characteristic (CVC) both in the absence and in the presence of external ac irradiation. At the same time, it leads to qualitatively new features of the Josephson diode effect such as asymmetry of the retrapping currents (which are a manifestation of hysteretic CVC). In particular, the limiting case of the single-sided hysteresis becomes accessible. In its turn, thermal fluctuations are known to lead to nonzero average voltages at any finite current, even below the critical value. We demonstrate that in the diode regime, the fluctuation-induced voltage can become strongly (exponentially) asymmetric. In addition, we find asymmetry of the switching currents arising both due to thermal activation and due to Josephson plasma resonances in the presence of ac irradiation.

Motivated by recent experimental observations of Kondo resonances in cobalt atoms on single layer $1T\text{−}{\mathrm{TaSe}}_2$, we theoretically investigate the effect of coupling a U(1) quantum spin liquid with a spinon Fermi surface to a lattice of Anderson impurities. Within the slave-rotor formalism, we find that above a critical coupling strength between the spin liquid and impurity lattice, the spinons hybridize to form heavy quasiparticles near the Fermi level, realizing a spinon Kondo lattice phase analogous to heavy fermion materials. Using the Bethe-Salpeter equation and accounting for emergent gauge fluctuations, we compute the spectral density and density of states, revealing the formation of spinon-chargon bound states in the spinon Kondo lattice phase. We characterize the thermodynamic and spectroscopic signatures of this phase, demonstrating specific heat and neutron scattering responses distinct from a pure quantum spin liquid. Our findings establish the spinon Kondo lattice as a framework to study the rich physics of spin liquids.

Based on the recently developed resummation-based quantum Monte Carlo method for the $\text{SU}\left(N\right)$ spin and loop-gas models, we developed an algorithm, dubbed ResumEE, to compute the entanglement entropy (EE) with greatly enhanced efficiency. Our ResumEE exponentially speeds up the computation of the exponentially small value of the $\langle e^{-S^{\left(2\right)}}\rangle$, where $S^{\left(2\right)}$ is the second-order Rényi EE, such that the $S^{\left(2\right)}$ for a generic 2D quantum $\text{SU}\left(N\right)$ spin models can be readily computed with high accuracy. We benchmark our algorithm with the previously proposed estimators of $S^{\left(2\right)}$ on 1D and 2D $\text{SU}\left(2\right)$ Heisenberg spin systems to reveal its superior performance and then use it to detect the entanglement scaling data of the Néel-to-VBS transition on 2D $\text{SU}\left(N\right)$ Heisenberg model with continuously varying $N$. Our ResumEE algorithm is efficient for precisely evaluating the entanglement entropy of $\text{SU}\left(N\right)$ spin models with continuous $N$ and reliable access to the conformal field theory data for the highly entangled quantum matter.

In muon spin relaxation/rotation/resonance $\left(\mu \mathrm{SR}\right)$ technology, positive muons serve as local spin probes for detecting electron-muon or nucleus-muon spin interactions. Given that a muon's mass is approximately one-ninth that of a proton, its zero-point energy significantly influences $\mu \mathrm{SR}$ experiments. The straightforward application of the Born-Oppenheimer approximation fails to adequately address the muon's behavior from an ab initio perspective. Despite previous efforts to correct for the quantum effects of the muon, existing approximations often fall short, yielding unsatisfactory results. This paper addresses these limitations by focusing on the many-body system of quantized muons in materials and directly solving the muon-included many-body equation using quantum Monte Carlo techniques. We further simulate the hyperfine couplings of the muon, demonstrating that this rigorous approach can produce accurate results and explicit error margins. This method proves beneficial for $\mu \mathrm{SR}$ experiment analysis and holds potential for broader application in studying the quantum effects of heavy, positively charged particles in various materials.

The ellipticity of a laser pulse offers a control knob to tune the efficiency and polarization of emitted higher-order harmonics. In this paper, we systematically investigate the ellipticity dependence of high-harmonic generation in various bilayer graphene systems, such as AA-stacked, AB-stacked, and twisted bilayer graphene. We demonstrate how the underlying symmetries and electronic band structures of different bilayer systems distinctly contribute to the characteristics of ellipticity dependence, such as elliptical enhancement and elliptical dichroism. Additionally, we showcase a unique applicability of high-harmonic spectroscopy as a tool for identifying stacking faults in bilayer graphene systems. Our findings offer valuable insights into the nonlinear optical interactions of twisted bilayer graphene with different twist angles.