Recent years have witnessed a steady progress towards blending two-dimensional quantum materials into technology, with future applications often rooted in the electronic structure. Since crossings and inversions of electronic bands with different orbital characters determine intrinsic quantum transport properties, knowledge of the orbital character is essential. Here, we benchmark angle-resolved photoelectron emission spectroscopy (ARPES) as a tool to experimentally derive orbital characters. For this purpose we study the valence electronic structure of two technologically relevant quantum materials, graphene and ${\mathrm{WSe}}_2$, and focus on circular dichroism that is believed to provide sensitivity to the orbital angular momentum. We analyze the contributions related to angular atomic photoionization profiles, interatomic interference, and multiple scattering. Regimes in which initial-state properties could be disentangled from the ARPES maps are critically discussed and the potential of using circular dichroic ARPES as a tool to investigate the spin polarization of initial bands is explored. For the purpose of generalization, results from two additional materials, ${\mathrm{GdMn}}_6{\mathrm{Sn}}_6$ and ${\mathrm{PtTe}}_2$, are presented in addition. This research demonstrates rich complexity of the underlying physics of circular dichroic ARPES, providing insights that will shape the interpretation of both past and future circular-dichroic ARPES studies.

A significant increase in resistance in response to an external magnetic field, often called large magnetoresistance (MR), is a key area of focus in both fundamental solid-state research and modern technological applications. Understanding the interplay between MR, resistivity upturn, and carrier compensation remains complex yet essential in materials with intricate electronic structures. Here, we report the transport properties of a transition metal stannide ${\mathrm{ZrSn}}_2$, which exhibits a resistivity upturn and large magnetoresistance (1750% at 2 K and 14 T). We demonstrate that when the ratio of hole to electron concentration is close to unity, the materials display a resistivity upturn under an applied magnetic field. The carrier concentrations estimated from the field-dependent Hall resistivity measurements are nearly compensated, which satisfies the condition necessary for both resistivity upturn and large MR. We also propose a method based on Monte Carlo simulations to calculate the complex Fermi surface volume from first-principles calculation which further reveals carrier compensation in ${\mathrm{ZrSn}}_2$.

We characterize the dynamic universality classes of a relaxational dynamics under equilibrium conditions at the continuous transitions of three-dimensional (3D) spin systems with ${\mathbb{Z}}_2$ gauge symmetry. In particular, we consider the pure lattice ${\mathbb{Z}}_2$ gauge model and the lattice ${\mathbb{Z}}_2$ gauge XY model, which present various types of transitions: topological transitions without a local order parameter and transitions characterized by both gauge-invariant and non-gauge-invariant XY order parameters. We consider a standard relaxational (locally reversible) Metropolis dynamics and determine the dynamic critical exponent $z$ that characterizes the critical slowing down of the dynamics as the continuous transition is approached. At the topological ${\mathbb{Z}}_2$ gauge transitions we find $z=2.55\left(6\right)$. Therefore, the dynamics is significantly slower than in Ising systems—$z\approx 2.02$ for the 3D Ising universality class—although 3D ${\mathbb{Z}}_2$ gauge systems and Ising systems have the same static critical behavior because of duality. As for the nontopological transitions in the 3D ${\mathbb{Z}}_2$ gauge XY model, we find that their critical dynamics belong to the same dynamic universality class as the relaxational dynamics in ungauged XY systems independently of the gauge-invariant or non-gauge-invariant nature of the order parameter at the transition.

In this work, we apply the $\text{DFT}+U$ approach for a detailed ab initio study of the refined structure of the low-temperature phase of magnetite [M. S. Senn et al., Nature (London) 481, 173 (2012)]. We compare the electronic properties of this structure and several alternatives with respect to the presence of trimeron ordering and the bandgap properties. The connection of the trimeron arrangement with site-selective doping of magnetite is discussed. Calculations of the polaron hopping energy allow us to make one step forward toward understanding the complex interplay of polaronic and bandgap contributions to electronic properties of the magnetite.

Qubit performance is often reported in terms of a variety of single-value metrics, each providing a facet of the underlying noise mechanism limiting performance. However, the value of these metrics may drift over long timescales, and reporting a single number for qubit performance fails to account for the low-frequency noise processes that give rise to this drift. In this work, we demonstrate how we can use the distribution of these values to validate or invalidate candidate noise models. We focus on the case of randomized benchmarking (RB), where typically a single error rate is reported but this error rate can drift over time when multiple passes of RB are performed. We show that using a statistical test as simple as the Kolmogorov-Smirnov statistic on the distribution of RB error rates can be used to rule out noise models, assuming the experiment is performed over a long enough time interval to capture relevant low frequency noise. With confidence in a noise model, we show how care must be exercised when performing error attribution using the distribution of drifting RB error rate.

We model a change of the spin configuration in a two-dimensional square array, or a lateral superlattice, of quantum rings in an external perpendicular homogeneous magnetic field. The electron system is placed in a circular cylindrical far-infrared photon cavity with a single circularly symmetric photon mode. Our numerical results reveal that the spin ordering of the two-dimensional electron gas in each quantum ring can be influenced or controlled by the electron-photon coupling strength and the energy of the photons. The Coulomb interaction between the electrons is described by a spin-density functional approach, but the para- and diamagnetic electron-photon interactions are modeled via a configuration interaction formalism in a truncated many-body Fock-space, which is updated in each iteration step of the density functional approach. In the absence of external electromagnetic pulses this reordering of the spin configuration is replicated in the orbital magnetization of the rings. The change in the spin configuration can be suppressed by a strong electron-photon interaction. In addition, fluctuations in the spin configuration are found in dynamical calculations, where the system is excited by a time-dependent coupling scheme to a cylindrical cavity mode for emphasizing the diamagnetic electron-photon interaction not leading to simple electrical dipole oscillations. The diamagnetic interaction is enhanced by the rotational electric field of the particular cavity mode.

This work investigates the impact of particle agglomeration on thermal emission from polaritonic dielectric particles supporting localized surface phonons (LSPhs) in the infrared. Specifically, the spectral, hemispherical emissivity from 50- and 500-nm-radius $\mathrm{Si}{\mathrm{O}}_2$ particle chains is predicted via the discrete system Green's function method based on fluctuational electrodynamics. It is shown that contacting and near-contacting particles support collective LSPhs (cLSPhs) that cause significant spectral broadening and resonance damping of the emissivity. cLSPhs, arising from the coupling of LSPhs between closely spaced particles, can travel across particle chains. The dipolar contribution to the cLSPh dispersion relations reveals that the resonance damping and spectral broadening are mediated by long-range cLSPh modes in longitudinal polarization. These modes contribute to thermal emission when the particle edge-to-edge spacing to radius ratio, $d/R$, is smaller than approximately 1 and 4 for particle radii of 50 and 500 nm, respectively. Beyond these thresholds, LSPh coupling between particles is negligible, and thermal emission from multiple particles becomes additive. These findings have implications for manufacturing dielectric-based metasurfaces and metamaterials used for controlling thermal emission in energy conversion and thermal management systems.

Semiconductor InSb nanowires present a highly intriguing platform with immense potential for applications in spintronics and topological quantum devices. The narrow band gap exhibited by InSb allows for precise tuning of these nanowires, facilitating smooth transitions between the electron-transport region and the hole-transport region. In this paper, we demonstrate quantum transport measurements obtained from a high-quality InSb nanowire quantum dot device. By utilizing a back gate, this device can be adjusted from an electron-populated quantum dot regime to a hole-populated one. Within both regimes, we have observed dozens of consecutive quantum levels without any charge rearrangement or impurity-induced interruptions. Our investigations in the electron-transport regime have explored phenomena such as the Coulomb blockade effect, Zeeman effect, and Kondo effect. Meanwhile, in the hole-transport regime, we have identified conductance peaks induced by lead states. Particularly, we have created a tomographic analysis method of these lead states by tracking the behavior of these conductance peaks across consecutive Coulomb diamond structures.

Twisted multilayer graphene provides an unprecedented platform for studying strongly correlated and topological physics, such as twisted monobilayer graphene (TMBG) featured with electric-field tunable superconductivity. Adding an extra layer to TMBG, twisted tetralayer graphene emerges with more fascinating correlated phases related to crystal symmetry and band structure. In this study, by using scanning tunneling microscope, we thoroughly investigated twisted Bernal-stacked tetralayer graphene with a small twist angle in three distinct configurations: a twist between monotrilayer (A/BAB), bibilayer (AB/AB), and trimonolayer (ABA/B). All three types host almost the same twist angle induced by as-grown grain boundaries. We detected two distinct scanning tunneling spectroscopy spectra on the A/BAB- and ABA/B-type tetralayer graphene, indicative of mirror-symmetry breaking. Notably, both flat and linear bands coexist in the tetralayer system with a small twist angle, which supports emerging correlated phases. Moreover, our result revealed layer-dependent distribution of local density of states assigned to the flat and linear bands in twisted tetralayer graphene. Intriguingly, the flat-band localization becomes weakened as the twisted interface recedes from the top to bottom layers. This work not only sheds light on structural and electronic properties of twisted tetralayer graphene in distinct configurations, but also opens up further opportunities in understanding correlated physics in twisted multilayer graphene system.

Recent scanning tunneling microscopy experiments have found Kekulé-distorted (KD) ordering in graphene subjected to strong magnetic fields, a departure from the antiferromagnetic (AF) state identified in earlier transport experiments on double-encapsulated devices with larger dielectric screening constant $\epsilon$. This variation suggests that the magnetic anisotropic energy is sensitive to the dielectric screening constant. To calculate the magnetic anisotropic energy without resorting to perturbation theory, we adopted a two-step approach. First, we derived the bare valley-sublattice-dependent interaction coupling constants from microscopic calculations and accounted for the leading logarithmic divergences arising from quantum fluctuations by solving renormalization group flow equations in the absence of a magnetic field, from the carbon lattice scale up to the much larger magnetic length. Subsequently, we used these renormalized coupling constants to perform nonperturbative, self-consistent Hartree-Fock calculations. Our results demonstrate that the ground state at neutrality ($\nu =0$) transitions from an AF state to a spin-singlet KD state when dielectric screening and magnetic fields become small, consistent with experimental observations. For filling fraction $\nu =\pm 1$, we predict a transition from spin-polarized charge-density wave states to a spin-polarized KD state when dielectric screening and magnetic fields become small. Our self-consistent Hartree-Fock calculations, which encompass a large number of Landau levels, reveal that the magnetic anisotropic energy receives substantial contributions from the Dirac sea when $\epsilon$ is small. Our paper provides insights into how the Dirac sea, which contributes one electron per graphene unit cell, affects the small magnetic anisotropic energy in graphene.

It is well known that macroscopically normalizable zero-energy wave functions of spin-$\frac{1}{2}$ particles in a two-dimensional inhomogeneous magnetic field are spin-polarized and exactly calculable with degeneracy equaling the number of flux quanta linking the whole system. Extending this argument to massless Dirac fermions subjected to magnetic fields that have zero net flux but are doubly periodic in real space, we show that there exist only two Bloch-normalizable zero-energy eigenstates, one for each spin flavor. This result is immediately relevant to graphene multilayer systems subjected to doubly periodic strain fields, which at low energies enter the Hamiltonian as periodic pseudogauge vector potentials. Furthermore, we explore various related settings including nonlinearly dispersing band structure models and systems with singly periodic magnetic fields.

In-plane magnetic fields offer a relatively unexplored opportunity to alter the band structure of stacks of two-dimensional (2D) materials so that they exhibit the desired physical properties. Here we show that an in-plane magnetic field combined with a transverse electric field can induce an insulator-metal (IM) transition in bilayer graphene. Our study of the magnetic response reveals that the orbital magnetic susceptibility changes from diamagnetic to paramagnetic around the transition point. We discuss several strategies to observe the IM transition, switch the diamagnetism, and more generally control the band structure of stacked 2D materials at experimentally accessible magnetic fields.

Spin-orbit interaction (SOI) is a crucial ingredient for many potential applications of quantum devices, such as the use of semiconductor nanostructures for quantum computing. It is known that nonlinear conductivities are sensitive to the strength and type of SOI, however, many calculations of nonlinear transport coefficients are based on the semiclassical Boltzmann theory and make simplifying assumptions about scattering effects due to disorder. In this paper, we develop and employ a microscopic theory based on the Keldysh formalism that goes beyond simple semiclassical approximations. This approach, for instance, naturally takes into account the effects of interband transitions, Berry curvature, and allows for a more precise treatment of impurity scattering. As a test of this formalism, we consider the nonlinear transport properties in an effective two-band model of one-dimensional nanowires (1DNWs) and two-dimensional hole gases (2DHGs) in the presence of a magnetic field causing Zeeman splitting of the spin states. We find that the small energy scales in nanostructures can result in interband contributions that are relevant, especially in the dirty limit, and therefore could modify qualitative features found using a purely semiclassical approach. Nonetheless, we find that different types of SOI (linear or cubic) still result in remarkably pronounced different in-plane field angle dependences, which survive even when interband effects are relevant. Our results provide a detailed understanding of when interband effects become important for nonlinear transport and can serve as the basis for a microscopic description to predict other nonlinear transport effects in materials and devices.

We study the effects of $d$-wave altermagnetism on the thermal transport properties in Josephson junctions based on altermagnets. Owing to the anisotropic spin splitting in the altermagnet, the phase coherence of the thermal transport can be effectively modulated by the crystal orientation of the altermagnet. For the junction with pure $d_{xy}$-wave altermagnetism, despite the spin splitting of altermagnet, the spin-up and -down bands contribute equally to the thermal transport, and the total thermal conductance is insensitive to the altermagnetic exchange field strength. On the contrary, in the presence of $d_{x^2-y^2}$-wave altermagnetism, the spin-up and -down bands manifest themselves distinctly in the thermal transport, and the total thermal conductance possesses pronounced oscillating profiles by varying the altermagnetic exchange field strength. These findings suggest that the altermagnet-based Josephson junction can serve a promising platform for realizing controllable phase-coherent thermal transport.

Many quantum technologies, including quantum computers, quantum heat engines, and quantum sensors, rely on operating conditions in the subkelvin regime. It is therefore desirable to develop practical tools and methods for the precise control of the temperature in nanoscale quantum systems. Here, we present a proposal for fast, accurate, and local temperature control using qubits, which regulate the flow of heat between a quantum system and its thermal environment. The qubits are kept in a thermal state with a temperature that is controlled in an interplay between work done on the qubits by changing their energy splittings and the flow of heat between the qubits and the environment. Using only a few qubits, it is possible to control the thermal environment of another quantum system, which can be heated or cooled by the qubits. As an example, we show how a quantum system at subkelvin temperatures can be significantly and accurately cooled on a nanosecond timescale. Our proposal can potentially be realized with superconducting flux qubits, charge qubits, or spin qubits, which can now be fabricated and manipulated with exquisite control.

The transport of Bloch electrons under strong fields is traditionally understood through two mechanisms: intraband Bloch oscillations and interband Zener tunneling. Here, we propose an oscillation mechanism induced by the interband quantum metric, which would significantly affect the electron dynamics under strong fields. By considering the multiband dynamics to the second order of the density matrix, we reveal that quantum metric induced oscillations (QMOs) persist regardless of band dispersion, even in exactly dispersionless flat bands. The resultant drift current can reach a magnitude comparable to the Bloch oscillation induced drift current in systems where interband tunneling is negligible. Notably, the QMO induced drift current increases linearly with electric field strength under the constraints of time-reversal or spatial-inversion symmetry, emerging as the primary delocalized current. We further show that both one-dimensional and two-dimensional superlattices are potential platforms for investigating QMO.

Two-dimensional (2D) ferroelectric materials hold great promise for ultrahigh-density data storage and ultrafast memory applications in flexible or wearable electronic devices. Because of depolarizing field effects and high-energy barriers for switching, only a handful of 2D ferroelectrics have been reported experimentally, such as ${\mathrm{In}}_2{\mathrm{Se}}_3$ and ${\mathrm{CuInP}}_2{\mathrm{S}}_6$. Through high-throughput calculations we identified a catalog of 25 synthesizable sliding ferroelectrics with large out-of-plane polarization and low sliding energy barriers. The high-throughput process started with 6351 monolayer materials in 2dmatpedia, narrowed down to 79 monolayer semiconductors with honeycomb structures, leading to the construction of 474 homobilayers sliding ferroelectrics, and ultimately identifying 25 high-performance 2D sliding ferroelectrics. Using big-data analysis and machine learning, we also revealed strong correlations between polarization and key physical properties, especially a hidden factor of effective van der Waals radius, which has been overlooked in previous fewer-sample studies. Based on these mechanistic insights, we proposed machine learning descriptors for predicting sliding ferroelectric properties.

Valley degree of freedom, as the energy extreme point of $K$ and $K^{\prime }$ momenta that are distinctly separated in reciprocal space, gives rise to negligible valley scattering, and thus the robust valley edge states occur at the interface between two domains with opposite valley Chern numbers. The valley Chern numbers are the integration of nonzero Berry curvature around $K$ and $K^{\prime }$. In this work we report an acoustic scheme of achieving the valley Hall effect without localized Berry curvature at $K$ and $K^{\prime }$ in a bilayer graphene phononic crystal. Since the valley Chern number is zero, nontrivial valley Euler numbers are employed to characterize the topology of the phononic crystals. We finally demonstrate that the pseudospin and edge state arise from the topological phase transition of the nonzero valley Euler numbers. Our work provides a macroscopic platform to explore the intriguing properties of the valley Hall effect.

In this work, we theoretically study the spin transport of different chiral magnons injected from a ferrimagnet to an antiferromagnet with interfacial spin canting and present a chirality-dependent magnon transport. Specifically, we find that the penetration length of the right-handed magnons can be either longer or shorter than that of the left-handed ones, depending on whether the interfacial coupling is antiferromagnetic or ferromagnetic. This phenomenon is attributed to the mode match between the chiral magnons and the right-handed dynamics of the net magnetization in the canted antiferromagnet. The explicit values of the magnon penetration lengths are found to depend on the penetration length of the canting angle instead of its magnitude and hence can be efficiently modulated by the exchange parameters in the antiferromagnet, while the strength of the interfacial exchange coupling plays a minor role. This result suggests antiferromagnets with a larger penetration length of interfacial canting, corresponding to a larger intrasublattice ferromagnetic coupling, is favored for the chirality-sensitive magnon transport, which could be useful in chirality-based magnonics.