We investigate a disorder-free quantum Ising chain subject to a time-periodic drive that rotates each spin by an angle $\pi (1-{\epsilon }_i)$. In case all spins experience the same deviation $\epsilon$ and the system is initialized in a fully polarized state, the dynamics is known to be time crystalline: the magnetization of the system exhibits period-doubled oscillations for timescales that grow exponentially with the length of the chain. In this work, we study the effect of a deviation $\epsilon$ that differs between spins. We find that reducing $\epsilon$ for a single spin drastically enhances the lifetime of spatiotemporal order, suggesting the name metronome spin. Employing perturbative arguments in an average Hamiltonian picture, we explain this observation for initial states with macroscopic bulk magnetization. Furthermore, in the case of random bit-string initial states, we report the enhancement of the lifetime of a topological edge mode, which can also be understood in the same picture. Finally, we discuss an altered geometry in which the metronome spin is not directly part of the chain, affecting the dynamics in different ways in the two scenarios considered. Our findings unveil the intricate dynamics that emerge in Floquet systems under the influence of a spatially varying drive, thereby uncovering new avenues for Floquet engineering.

We investigate collective spin excitations of graphene electrons with short-ranged interactions and subject to the external Zeeman magnetic field. We find that in addition to the familiar Silin spin wave, a collective spin-flip excitation that reduces to the uniform precession when the wave's momentum approaches zero, the magnetized graphene supports another collective mode visible in the transverse spin susceptibility: a collective spin-current mode. Unlike the Silin wave, this mode is not dictated by the spin-rotational symmetry but rather owns its existence to the pseudospin structure of the graphene lattice. We find the collective excitation to become sharply defined in a finite interval of the wave's momenta, the range of which is determined by the interaction and the magnetization.

The spin Hall effect (SHE) is one of the most important spin-to-charge conversion mechanisms in spintronics and has been intensively investigated in nonmagnetic materials. However, the SHE in ferromagnetic metals has been rarely studied and remains obscure. Here, we clarify that the SHE in ferromagnetic metals can be classified into three categories including conventional-, spin anomalous-, and magnetic spin Hall effect. By using first-principles calculations, we find the above three spin Hall mechanisms do coexist in common ferromagnetic metals. Particularly, for Pt-based ferromagnetic alloys, comparable conventional and magnetic spin Hall angles have been predicted. In addition, we investigate the temperature dependence and scaling behavior of different types of spin Hall conductivity by taking into account the lattice vibration and spin disorder scattering. Finally, we argue that ferromagnetic metals with remarkable unconventional SHE may be efficient spin current sources with out-of-plane spin polarization for spin-orbit torque-induced switching and spin terahertz emission. This work provides a comprehensive understanding of the SHE in ferromagnetic metals and may pave the way for promising applications in spin-to-charge conversion spintronics devices.

We describe the effect of the Fe layer thickness on the antiferromagnetic interlayer exchange coupling in ${[\mathrm{Fe}/\mathrm{MgO}]}_N$ superlattices. An increase in coupling strength with increasing Fe layer thickness is observed, which highlights the need for including the extension of both layers when discussing the interlayer exchange coupling in superlattices.

Superconductor-semiconductor hybrid devices, involving quantum dots interfaced with floating and/or grounded superconductors, have reached a level of complexity, which calls for the development of versatile and numerically efficient modeling tools. Here, we propose an extension of the surrogate model solver for subgap states [Phys. Rev. B 108, L220506 (2023)], which is able to handle floating superconducting islands with finite-charging energy. Upon eliminating all finite-size effects of the computationally demanding Richardson model approach, we achieve a more efficient way of calculating the subgap spectra and related observables without compromising their accuracy. We provide a number of benchmarks between the two approaches and showcase the versatility of the extended surrogate model solver by studying the stability of spin-triplet ground states in various tunable devices. The methods introduced here set the stage for reliable microscopic simulations of complex superconducting quantum circuits across all their relevant parameter regimes.

Using the exact-diagonalization (ED) and mean-field (MF) approaches, we investigate the ground-state phase diagram of the interacting Haldane model on the honeycomb lattice, incorporating spin-dependent sublattice potentials ${\mathrm{\Delta }}_{\sigma ,\alpha }$. Here $\alpha =\text{A},\text{B}$ and $\sigma =\uparrow ,\downarrow$ denote the sublattice and spin components, respectively. Setting ${\mathrm{\Delta }}_{\sigma ,\text{A}}=+\mathrm{\Delta }$ $(-\mathrm{\Delta })$ and ${\mathrm{\Delta }}_{\sigma ,\text{B}}=-\mathrm{\Delta }$ $(+\mathrm{\Delta })$ for $\sigma =\uparrow$ $(\downarrow )$ results in the system favoring a spin ordered state. Conversely, introducing the nearest-neighbor Coulomb interaction can induce charge ordering in the system. Due to the competition between these factors, we observe that in both ED and MF approaches, an exotic state with Chern number $C=1$ survives amidst two locally ordered phases and a topologically ordered phase with $C=2$. In the ED method, various properties, such as the fidelity metric, the excitation gaps and the structure factors, are employed to identify critical points. In the MF method, using a sufficiently large lattice size, we define the local order parameters and band gaps to characterize the phase transitions. The interacting Haldane model and the spin-dependent lattice potential may be experimentally realized in an ultracold atom gas, providing a potential means to detect this intriguing state.

Projected entangled pair states (PEPS) offer memory-efficient representations of some quantum many-body states that obey an entanglement area law and are the basis for classical simulations of ground states in two-dimensional (2d) condensed matter systems. However, rigorous results show that exactly computing observables from a 2d PEPS state is generically a computationally hard problem. Yet approximation schemes for computing properties of 2d PEPS are regularly used, and empirically seen to succeed, for a large subclass of (“not too entangled”) condensed matter ground states. Adopting the philosophy of random matrix theory, in this work, we analyze the complexity of approximately contracting a 2d random PEPS by exploiting an analytic mapping to an effective replicated statistical mechanics model that permits a controlled analysis at a large bond dimension. Through this statistical-mechanics lens, we argue that (i) although approximately sampling wave-function amplitudes of random PEPS faces a computational-complexity phase transition above a critical bond dimension, and (ii) one can generically efficiently estimate the norm and correlation functions for any finite bond dimension. These results are supported numerically for various bond-dimension regimes. It is an important open question whether the above results for random PEPS apply more generally also to PEPS representing physically relevant ground states.

In strongly correlated systems such as heavy-fermion materials, the coherent superposition of localized and mobile spin states leads to the formation of Kondo resonant states, which on a dense, periodic array of Kondo ions develop lattice coherence. Characteristically, these quantum-coherent superposition states respond to a terahertz (THz) excitation by a delayed THz pulse on the scale of the material's Kondo energy scale and hence independent of the pump-light intensity. However, a delayed response is also typical for superradiance in an ensemble of excited atoms. In this case, quantum coherence is established by the coupling to an external, electromagnetic mode and hence dependent on the pump-light intensity. In the present paper, we investigate the physical origin of the delayed pulse, i.e., inherent, correlation-induced versus light-induced coherence, in the prototypical heavy-fermion compound ${\mathrm{CeCu}}_{5.9}{\mathrm{Au}}_{0.1}$. We study the delay, duration, and amplitude of the THz pulse at various temperatures in dependence on the electric-field strength of the incident THz excitation. We observe a robust delayed response at approximately 6 ps with an amplitude proportional to the amplitude of the incident THz wave. This is consistent with theoretical expectation for the Kondo-like coherence and thus provides compelling evidence for the dominance of condensed-matter versus optical coherence in the heavy-fermion compound.

We employ matrix product state simulations to study energy transport within the nonintegrable regime of the one-dimensional ${\mathbb{Z}}_3$ chiral clock model. To induce a nonequilibrium steady state throughout the system, we consider open system dynamics with boundary driving featuring jump operators with adjustable temperature and footprint in the system. Given a steady state, we diagnose the effective local temperature by minimizing the trace distance between the true local state and the local state of a uniform thermal ensemble. Via a scaling analysis, we extract the transport coefficients of the model at relatively high temperatures above both its gapless and gapped low-temperature phases. In the medium-to-high temperature regime we consider, diffusive transport is observed regardless of the low-temperature physics. We calculate the temperature dependence of the energy diffusion constant as a function of model parameters, including in the regime where the model is quantum critical at the low temperature. Notably, even within the gapless regime, an analysis based on power series expansion implies that intermediate-temperature transport can be accessed within a relatively confined setup. Although we are not yet able to reach temperatures where quantum critical scaling would be observed, our approach is able to access the transport properties of the model over a broad range of temperatures and parameters. We conclude by discussing the limitations of our method and potential extensions that could expand its scope, for example, to even lower temperatures.

We study the topological properties of the Haldane and modified Haldane models in $\alpha \text{−}{T}_3$ lattice. The band structures and phase diagrams of the system are investigated. Individually, each model undergoes a distinct phase transition: (i) the Haldane-only model experiences a topological phase transition from the Chern insulator $(\mathcal{C}=1)$ phase to the higher Chern insulator $(\mathcal{C}=2)$ phase; while (ii) the modified-Haldane-only model experiences a phase transition from the topological metal phase to the higher Chern insulator phase with identical Chern number $\mathcal{C}=2$, indicating that $\mathcal{C}$ is insufficient to characterize this system because of the indirect band gap. By plotting the Chern number and $\mathcal{C}$ phase diagram, we show that in the presence of both Haldane and modified Haldane models in the $\alpha \text{−}{T}_3$ lattice, the interplay between the two models manifests three distinct topological phases, namely the $\mathcal{C}=1$ Chern insulator (CI) phase, $\mathcal{C}=2$ higher Chern insulator (HCI) phase, and $\mathcal{C}=2$ topological metal (TM) phase. These results are further supported by the $\alpha \text{−}{T}_3$ Hall conductance, zigzag, and armchair edge states calculations. This work elucidates the rich phase evolution of the Haldane and modified Haldane models as $\alpha$ varies continuously from 0 to 1 in an $\alpha \text{−}{T}_3$ model, thus suggesting $\alpha \text{−}{T}_3$ lattice as a versatile condensed matter platform for studying topological phase transitions.

The concept of proximity coupling is a promising approach to specifically modify the properties of epitaxial graphene layers. In order to introduce spin-orbit coupling into graphene, we have deposited Bi with an average thickness of up to 3.6 bilayers (BL) on epitaxial monolayer graphene (MLG) on SiC(0001) and perform magnetotransport measurements in magnetic fields up to 4 T. Upon adsorption, epitaxial Bi(110) islands are formed, which change the initial $n$-type doping of MLG locally. The formation of inhomogeneous carrier concentration profiles on MLG results in a positive and linear magnetoresistivity effect with increasing Bi coverage. Along with this, the slope of the Hall resistivity decreases, suggesting a rise of the carrier concentration. However, by extracting the carrier concentration from the Shubnikov–de Haas oscillations, we confirm that the carrier concentration in the uncovered regions remains constant and that the change of the Hall slope is solely an effect of the inhomogeneity. Also, the conductivity of MLG has not changed drastically and even at coverages as high as 2.4 BL the mobility in the noncovered region is reduced by only about 15% of its original value. Moreover, the signatures of weak localization in the magnetoresistivity vanish with increasing Bi coverage, while no signs of weak antilocalization were found at all. Apparently, the proximitized Bi islands induce a well-defined lateral doping profile so that the electrons are not penetrating into the areas of the Bi islands, thus mimicking antidots, but are reflected at their edges. This scattering process seems to be phase breaking, thus suppressing the weak localization effect. Our results show clearly that both the coupling but also the homogeneity at the interface is crucial for proximity coupling.

In the context of $\theta$ electrodynamics we find transverse electromagnetic wave solutions forbidden in Maxwell electrodynamics. Our results attest to different signatures of the topological magnetoelectric effect in topological insulators, resulting from a polarization rotation of an external electromagnetic field. Unlike Faraday and Kerr rotations, the effect does not rely on a longitudinal magnetic field, the reflected field, or birefringence. The rotation occurs due to transversal discontinuities of the topological magnetoelectric parameter in cylindrical geometries. The dispersion relation is linear, and birefringence is absent. Exact transversality allows electromagnetic waves to propagate in an optical fiber without successive total internal reflections, diminishing losses, and regardless of acute bends of the fiber. These results may open other possibilities in optics and photonics by utilizing topological insulators to manipulate light.

Ab initio calculations of electron-phonon interactions in materials without adjustable parameters have provided microscopic insights into their charge-transport properties. Other transport properties such as the diffusion coefficient provide additional microscopic information and are readily accessible experimentally, but few ab initio calculations of these properties have been performed. Here, we report first-principles calculations of the hot electron diffusion coefficient in Si and its dependence on electric field over temperatures from 77–300 K. While qualitative agreement in trends such as anisotropy at high electric fields is obtained, the quantitative agreement that is routinely achieved for low-field mobility is lacking. We examine whether the discrepancy can be attributed to an inaccurate description of $f$-type intervalley scattering by computing the microwave-frequency noise spectrum and piezoresistivity. These calculations indicate that any error in the strength of $f$-type scattering is insufficient to explain the diffusion coefficient discrepancies. Our findings suggest that the measured diffusion coefficient is influenced by factors such as space-charge effects, which are not included in ab initio calculations, impacting the interpretation of experimental measurements in terms of microscopic charge-transport processes.

Understanding the nature of low thermal conductivity and phonon transport properties is critical for the design of potential thermoelectric materials. Based on density-functional theory combined with the phonon Peierls-Boltzmann transport equation, we reveal that Ag-based intercalated layered materials $\mathrm{AgBi}{X}_2$ (X = S, Se, and Te) have inherently low lattice thermal conductivity, which is mainly attributed to the anticrossing behavior of low-frequency optical phonons and longitudinal acoustic phonons induced by the rattling mode of the cations. It is found that the optical phonons in $\mathrm{AgBi}{\mathrm{S}}_2$ contribute dominantly (up to 65%) to the total thermal conductivity, originating from the weak bonding nature of intercalated Ag atoms that leads to strong anharmonicity and softening of transverse acoustic phonons. Electronic relaxation times under acoustic deformation potential scattering, polar optical phonon scattering, and ionized impurity scattering are considered to obtain reasonable electron transport properties. The ZT value of $p$-type $\mathrm{AgBi}{\mathrm{S}}_2$ reaches 1.77 for optimal doping at room temperature, which can further be enhanced to ∼2.6 through strain engineering. The present work demonstrates that chemically controlled weak bonding in Ag-based intercalated layered structure produces intrinsically low thermal conductivity and provides important theoretical insight for thermal insulator and thermoelectric applications.

In this study, we address selection rules with respect to the polarization of the optical excitation of two color centers in $4H\text{−}\mathrm{SiC}$ and $6H\text{−}\mathrm{SiC}$ with potential for applications in quantum technology, the divacancy and the nitrogen-vacancy pair. We show that the photoluminescence of the axial configurations of higher symmetry $\left(C_{3v}\right)$ than the basal ones $\left(C_{1h}\right)$ can be canceled using any excitation (resonant or nonresonant) with polarization parallel to the crystal axis $\left(E_L\left|\right|c\right)$. The polarization selection rules are determined using group-theoretical analysis and simple physical arguments showing that phonon-assisted absorption with $E_L\left|\right|c$ is prohibited despite being formally allowed by group theory. A comparison with the selection rules for the silicon vacancy, another defect with $C_{3v}$ symmetry, is also carried out. Using the selection rules, we demonstrate selective excitation of only one basal divacancy configuration in $4H\text{−}\mathrm{SiC}$, the PL3 line, and discuss the higher contrast and increased Debye-Waller factor in the selectively excited spectrum.

In this work we prescribe a theoretical framework aiming at predicting the position of monovacancy defects at the edges of zigzag graphene nanoribbons (ZGNRs) using Floquet-Bloch formalism, which can be experimentally observed through time- and angle-resolved photoemission spectroscopy (tr-ARPES). Our methodology involves an in-depth investigation of the Floquet quasienergy band spectrum influenced by light with varying polarization across a range of frequencies. Particularly under the influence of circularly polarized light with a frequency comparable to the bandwidth of the system, our findings suggest a promising approach for locating monovacancy defects at either edge, a challenge that proves intricate to predict from the ARPES spectrum of ZGNRs with monovacancy defects. This has been achieved by analyzing the orientation of the Floquet edge state and the appearance of new Dirac points in the vicinity of the Fermi level. The real-world applications of these captivating characteristics underscore the importance and pertinence of our theoretical framework, paving the way for additional exploration and practical use. Our approach, employing the Floquet formalism, is not limited to monovacancy-type defects; rather, it can be expanded to encompass various types of vacancy defects.

Electronic structure calculations based on many-body perturbation theory [e.g., GW or the random-phase approximation (RPA)] require function evaluations in the complex time and frequency domain, for example, inhomogeneous Fourier transforms or analytic continuation from the imaginary axis to the real axis. For inhomogeneous Fourier transforms, the time-frequency component of the GreenX library provides time-frequency grids that can be utilized in low-scaling RPA and GW implementations. In addition, the adoption of the compact frequency grids provided by our library also reduces the computational overhead in RPA implementations with conventional scaling. In this paper, we present low-scaling GW and conventional RPA benchmark calculations using the GreenX grids with different codes (FHI-aims, CP2K, and ABINIT) for molecules, two-dimensional materials and solids. Very small integration errors are observed when using 30 time-frequency points for our test cases, namely $<{10}^{-8}$ eV/electron for the RPA correlation energies, and $\le 10$ meV for the $GW$ quasiparticle energies.