Time-of-flight inelastic neutron scattering measurements on ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ single crystals were performed to access the spin Hamiltonian in this canonical $J_{\mathrm{eff}}=1/2$ spin-orbital Mott insulator. The momentum of magnetic scattering at all inelastic energies measured is revealed to be $L$ independent, indicative of idealized two-dimensional in-plane correlations. We model the in-plane energy and momentum dependence of the excitations, that were measured up to $\sim 80$ meV, and define a spin gap of 0.6(1) meV. Collectively, the results indicate that despite the strongly spin-orbit entangled isospins, an isotropic two-dimensional $S=1/2$ Heisenberg model Hamiltonian accurately describes the magnetic interactions, confirming a robust analogy with unconventional superconducting cuprates.

We present a strong-coupling dynamical theory of the superconducting transition in a metal near a quantum-critical point toward $Q=0$ nematic order. We use a fermion-boson model, in which we treat the ratio of effective boson-fermion coupling and the Fermi energy as a small parameter $\lambda$. We solve, both analytically and numerically, the linearized Eliashberg equation. Our solution takes into account both strong fluctuations at small momentum transfers $\sim \lambda k_F$ and weaker fluctuations at large momentum transfers. The strong fluctuations determine $T_c$, which is of order ${\lambda }^2{E}_F$ for both $s$- and $d$-wave pairing. The weaker fluctuations determine the angular structure of the superconducting order parameter $F\left({\theta }_k\right)$ along the Fermi surface, separating between hot and lukewarm regions. In the hot regions $F\left({\theta }_k\right)$ is largest and approximately constant. Beyond the hot region, whose width is ${\theta }_h\sim {\lambda }^{1/3}$, $F\left({\theta }_k\right)$ drops by a factor ${\lambda }^{4/3}$. The $s$- and $d$-wave states are not degenerate but the relative difference $(T_c^s-T_c^d)/T_c^s\sim {\lambda }^2$ is small.

FeSe(001) with a high defect density was studied in the superconductiong phase by means of scanning tunneling spectroscopy at low temperature. Quasiparticle excitations surrounding structural defects are found inside the superconducting gap. This spectral intensity is used for revealing additional scattering channels for the quasiparticle excitations. The related wave vectors are located around the $M$ points of the Brillouin zone and follow $C_2$ symmetry. We attribute the symmetry and the observed lack of dispersion of the scattering channels to the interaction of Bogoliubov quasiparticles with nematic order.

We describe, within the framework of quantum electrodynamics, an interaction between a nonresonant hard x-ray pulse and an electronic system in the presence of a temporally periodic laser field driving electron dynamics in this system. We apply Floquet theory to describe the laser-driven electronic system, and then obtain the scattering probability of an arbitrary nonresonant x-ray pulse from such a system employing the density-matrix formalism. We show that the scattering probability can be connected to the time-dependent electron density of the driven electronic system only under certain conditions, in particular, if the bandwidth of the probe x-ray pulse is sufficiently narrow to spectroscopically resolve transitions to different final states. A special focus is laid on application of the theory to laser-driven crystals in a strongly nonperturbative regime. We show how the time-dependent electron density of a crystal can be reconstructed from energy-resolved scattering patterns. This is illustrated by a calculation of a diffraction signal from a driven MgO crystal.

While recent experiments on the spin Seebeck effect have revealed the decisive role of the magnon contribution to the heat current $Q$ in hybrid systems containing thin ferromagnetic layers, the available acoustic mismatch theory does not account for their magnetic properties. Here, we analyze theoretically the heat transfer through an insulating ferromagnet (F) sandwiched between two insulators (I). Depending on the relation between the F thickness $d$, and the mean free path of phonons generated by magnons $l_{ls}$, we reveal two qualitatively different regimes in the nonlinear heat transport through the F/I interfaces. Namely, in thick F layers the regime of conventional “Joule” heating with $Q\propto T_s^4$ is realized, in which the detailed structure of the F/I interfaces is inessential. Here $T_s$ is the magnon temperature. By contrast, in thin F layers with $d\ll l_{ls}$, most of the phonons emitted by magnons can leave F without being absorbed in its interior, giving rise to the magnon overheating regime with $Q\propto T_s^m$ and $m\gtrsim 7$. Conditions for the examination of both regimes and the determination of $T_s$ from experiments are discussed. The reported results are relevant for the theoretical analysis of the spin Seebeck effect and the development of magnon-based spin caloritronic devices.

We report a morphological transition in the magnetic domain pattern exhibited by perpendicular anisotropy ferromagnetic ${\left[\mathrm{Co}/\mathrm{Pt}\right]}_{50}$ multilayer films at room temperature and remanence. We found that the remanent magnetic domain morphology and the associated domain density, defined as the number of domains of a given magnetization direction per area, strongly depend on the magnetic history. When the magnitude of the previously applied external field approaches a specific value, typically 75–95% of the saturation field, the magnetic pattern, which generally forms a maze of interconnected stripe domains, decays into a shorter stripe pattern, and the domain density increases. We mapped out this morphological transition as a function of the previously applied field magnitude as well as the Co thickness. We found that a Co thickness close to 30 Å yields the highest domain density with the formation of a pure bubble domain state. Three-dimensional micromagnetic simulations confirm the formation of a pure bubble state in that parameter region and allow an estimation of the perpendicular anisotropy (here $2\times {10}^5\mathrm{J}/{\mathrm{m}}^3$ for an input magnetization of 1080 kA/m), as well as the interpretation of distinct features of the samples’ hysteresis loop based on the corresponding domain pattern.

We have studied the magnetic order of the rare-earth ferroborate ${\mathrm{CeFe}}_3{\left({\mathrm{BO}}_3\right)}_4$ through the thermodynamic and the neutron diffraction measurements. The heat capacity and the magnetic susceptibility revealed antiferromagnetic magnetic ordering at 29 K. In the neutron powder diffraction data, we observed the magnetic Bragg peaks indexed by the commensurate (CM) propagation vector ${\mathit{k}}_{\mathrm{CM}}=(0,0,\frac{3}{2})$ and the incommensurate (ICM) vector ${\mathit{k}}_{\mathrm{ICM}}=(0,0,\frac{3}{2}+\delta )$. The incommensurability $\delta$ increases with decreasing the temperature, and is evaluated to be 0.04556(16) at 3.7 K. Magnetic structure analysis reveals that the magnetic moments aligning in the $ab$ plane form the collinear antiferromagnetic structure having ${\mathit{k}}_{\mathrm{CM}}$ and helical structure having ${\mathit{k}}_{\mathrm{ICM}}$. Detailed measurements of the magnetic susceptibility exhibit an additional anomaly at 27 K. Furthermore, the temperature dependence of the neutron diffraction profile on the single-crystal sample shows that the ICM and CM ordering occurs at 29 and 26 K, respectively. These results suggest a phase separation state between the collinear and helical structures. The multiferroicity of ${\mathrm{CeFe}}_3{\left({\mathrm{BO}}_3\right)}_4$ is discussed on the basis of the determined magnetic structure.

We derive the effective Ginzburg-Landau (GL) theory for finite-momentum [Fulde-Ferrell-Larkin-Ovchinnikov/pair-density wave (FFLO/PDW)] superconductivity without spin population imbalance from a model with local attraction and repulsive pair hopping. We find that the GL free energy must include up to sixth-order derivatives of the order parameter, providing a unified description of the interdependency of zero and finite-momentum superconductivity. For weak pair hopping the phase diagram contains a line of Lifshitz points where vanishing superfluid stiffness induces a continuous change to a long wavelength FF state. For larger pair hopping there is a bicritical region where the pair momentum changes discontinuously. Here the FF type state is near degenerate with the LO or PDW type state. At the intersection of these two regimes there is a “super-Lifshitz” point with extra soft fluctuations. The instability to finite-momentum superconductivity occurs for arbitrarily weak pair hopping for sufficiently large attraction suggesting that even a small repulsive pair hopping may be significant in a microscopic model of strongly correlated superconductivity. Several generic features of the model may have bearing on the cuprate superconductors, including the suppression of superfluid stiffness in proximity to a Lifshitz point as well as the existence of subleading FFLO order (or vice versa) in the bicritical regime.

Experiments and simulations in solid-state high harmonic generation often make use of the distinction between interband and intraband currents. These two contributions to the total current have been associated with qualitatively different processes as well as physically measurable signatures, for example, in the spectral phase of harmonic emission. However, it was recently argued [P. Földi, Phys. Rev. B 96, 035112 (2017)] that these quantities can depend on the gauge employed in calculations. Since physical quantities are expected to have gauge-independent values, this raises the question of whether the decomposition of the total current into interband and intraband contributions is physically meaningful or merely a feature of a particular mathematical representation of nature. In this article, we explore this apparent ambiguity. We show that a closely related issue arises when calculating instantaneous band populations. In both the case of inter/intraband currents and the case of instantaneous band populations, we propose definitions that are gauge invariant and thus allow these quantities to be calculated consistently in any gauge.

We present a comprehensive theory of the spin- and angle-resolved photoemission spectroscopy (SARPES) of materials with glide-mirror symmetry, focusing on the role of glide symmetry in the spin selection rule. In the glide-symmetric SARPES configuration, where the surface of a material, the incoming light, and the outgoing photoelectrons are invariant under a glide reflection, the spin polarization of photoelectrons is determined by the glide eigenvalue of the initial state, which makes SARPES a powerful tool for studying topological phases protected by glide symmetry. We also show that, due to the nonsymmorphic character of glide symmetry, the spin polarization of a photoelectron whose momentum is in the second surface Brillouin zone is the opposite of the spin polarization of a photoelectron which is ejected from the same initial Bloch state but whose momentum is in the first zone. This momentum dependence of the spin selection rule clearly distinguishes glide symmetry from mirror symmetry and is particularly important if the Bloch wave vector of the initial state is close to the first surface Brillouin zone boundary. As a proof of principle, we simulate the SARPES from the surface states of KHgSb (010) and investigate how the spin selection rule imposed by the glide symmetry manifests itself in a real material.

The structural relaxation of multilayer graphene is essential in describing the interesting electronic properties induced by intentional misalignment of successive layers, including the recently reported superconductivity in twisted bilayer graphene. This is difficult to accomplish without an accurate interatomic potential. Here, we present a new, registry-dependent Kolmogorov-Crespi-type interatomic potential to model interlayer interactions in multilayer graphene structures. It consists of two parts, representing attractive interaction due to dispersion and repulsive interaction due to anisotropic overlap of electronic orbitals. An important new feature is a dihedral-angle-dependent term that is added to the repulsive part to describe correctly several distinct stacking states that the original Kolmogorov-Crespi potential cannot distinguish. We refer to the new model as the dihedral-angle-corrected registry-dependent interlayer potential (DRIP). Computations for several test problems show that DRIP correctly reproduces the binding, sliding, and twisting energies and forces obtained from ab initio total-energy calculations based on density-functional theory. We use the new potential to study the structural properties of a twisted graphene bilayer and the exfoliation of graphene from graphite. Our potential is available through the OpenKIM interatomic potential repository at https://openkim.org.

Open fermion systems with energy-independent bilinear coupling to a fermionic environment have been shown to obey a general duality relation [J. Schulenborg et al., Phys. Rev. B 93, 081411 (2016)] which allows for a drastic simplification of time-evolution calculations. In the weak-coupling limit, such a system can be associated with a unique dual physical system in which all energies are inverted, in particular the internal interaction. This paper generalizes this fermionic duality in two ways: we allow for weak coupling with arbitrary energy dependence and describe both occupations and coherences coupled by a quantum master equation for the density operator. We also show that whenever generalized detailed balance holds (Kolmogorov criterion), the stationary probabilities for the dual system can be expressed explicitly in terms of the stationary recurrence times of the original system, even at large bias. We illustrate the generalized duality by a detailed analysis of the rate equation for a quantum dot with strong onsite Coulomb repulsion, going beyond the commonly assumed wide-band limit. We present predictions for (i) the decay rates for transient charge and heat currents after a gate-voltage quench and (ii) the thermoelectric linear-response coefficients in the stationary limit. We show that even for pronouncedly energy-dependent coupling, all nontrivial parameter dependence in these problems is entirely captured by just two well-understood stationary variables, the average charge of the system and of the dual system. Remarkably, it is the latter that often dictates the most striking features of the measurable quantities (e.g., positions of resonances), underscoring the importance of the dual system for understanding the actual one.

Topological crystalline insulators (TCIs) are insulating electronic phases of matter with nontrivial topology originating from crystalline symmetries. Recent theoretical advances have proposed new TCI states protected by rotational symmetries and provided powerful guidelines to search for TCIs in real materials. Building upon recent theoretical works, we demonstrate a feasible method to identify new TCI states based on first-principles calculations. We systematically unveil the topological properties of the TCI states in ${\mathrm{Ca}}_2\mathrm{As}$. On both top and side surfaces, we observe topological surface states protected independently by rotational and mirror symmetries. We show that a particular lattice distortion can single out the newly proposed topological protection by the rotational symmetry. As a result, the Dirac points of the topological surface states are moved to generic locations in momentum space away from any high-symmetry lines. Such topological surface states have not been seen before. Moreover, the other family members, including ${\mathrm{Ca}}_2\mathrm{Sb}$, ${\mathrm{Ca}}_2\mathrm{Bi}$, and ${\mathrm{Sr}}_2\mathrm{Sb}$, feature different topological surface states due to their distinct topological invariants. We thus further propose topological phase transitions in the pseudobinary systems such as ${\left({\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x\right)}_2\mathrm{As}$ and ${\mathrm{Ca}}_2{\mathrm{As}}_x{\mathrm{Sb}}_{1-x}$. Our work reveals rich and exotic TCI physics across the ${\mathrm{Ca}}_2\mathrm{As}$ family of materials and demonstrates a complete roadmap for uncovering TCIs topological nature based on first-principles calculations. Such a method can be broadly applied in searching for new TCIs.

Symmetry and dimensionality are essential factors defining lattice dynamics and conductivity (κ). Here, we critically examine these via ab initio Boltzmann transport applied to single chain and bulk electride $\mathrm{B}{\mathrm{a}}_3\mathrm{N}$ and $\mathrm{B}{\mathrm{a}}_3\mathrm{N}X$ ($X=\mathrm{Sb}$,Bi). Chiral phonons in one-dimensional chains obey new symmetry-based scattering rules that limit thermal resistance. Weak chain coupling breaks these in the bulk, giving lower κ and large κ anisotropy. Curiously, intercalation of large X atoms binds chains more strongly, reducing the volume, yet gives lower κ and transforms the electronic behavior. Insights developed here can be more generally applied to other materials and provide avenues for predictive materials design.

Ultrafast characterization and control of many-body interactions and elementary excitations are critical to understanding and manipulating emergent phenomena in strongly correlated systems. In particular, spin interaction plays an important role in unconventional superconductivity, but efficient tools for probing spin dynamics, especially out of equilibrium, are still lacking. To address this question, we develop a theory for nonresonant time-resolved Raman scattering, which can be a generic and powerful tool for nonequilibrium studies. We also use exact diagonalization to simulate the pump-probe dynamics of correlated electrons in the square-lattice single-band Hubbard model. Different ultrafast processes are shown to exist in the time-resolved Raman spectra and dominate under different pump conditions. For high-frequency and off-resonance pumps, we show that the Floquet theory works well in capturing the softening of bimagnon excitation. By comparing the Stokes and anti-Stokes spectra, we also show that effective heating dominates at small pump fluences, while a coherent many-body effect starts to take over at larger pump amplitudes and frequencies on resonance to the Mott gap. Time-resolved Raman scattering thereby provides the platform to explore different ultrafast processes and design material properties out of equilibrium.

Combining the process-chain method with a symbolized evaluation, we work out in detail a high-order symbolic strong-coupling expansion (HSSCE) for determining the quantum phase boundaries between the Mott insulator and the superfluid phase of the Bose-Hubbard model for different fillings in hypercubic lattices of different dimensions. With a subsequent Padé approximation we achieve for the quantum phase boundaries a high accuracy, which is comparable to high-precision quantum Monte-Carlo simulations, and show that all the Mott lobes can be rescaled to a single one. As a further cross-check, we find that the HSSCE results coincide with a hopping expansion of the quantum phase boundaries, which follow from the effective potential Landau theory.

The coexistence of Rashba and Dresselhaus spin-orbit interactions (SOIs) in semiconductor quantum wells leads to an anisotropic effective field coupled to carriers’ spins. We demonstrate a gate-controlled anisotropy in Aharonov-Casher (AC) spin interferometry experiments with InGaAs mesoscopic rings by using an in-plane magnetic field as a probe. Supported by a perturbation-theory approach, we find that the Rashba SOI strength controls the AC resistance anisotropy via spin dynamic and geometric phases and establish ways to manipulate them by employing electric and magnetic tunings. Moreover, assisted by two-dimensional numerical simulations, we identify a remarkable anisotropy inversion in our experiments attributed to a sign change in the renormalized linear Dresselhaus SOI controlled by electrical means, which would open the door to new possibilities for spin manipulation.

In the resonant tunneling regime, sequential processes dominate single-electron transport through quantum dots or molecules that are weakly coupled to macroscopic electrodes. In the Coulomb blockade regime, however, cotunneling processes dominate. Cotunneling is an inherently quantum phenomenon and thus gives rise to interesting observations, such as an increase in the current shot noise. Since cotunneling processes are inherently fast compared to the sequential processes, it is of interest to examine the short time behavior of systems where cotunneling plays a role, and whether these systems display nonrenewal statistics. We consider three questions in this paper. Given that an electron has tunneled from the source to the drain via a cotunneling or sequential process, what is the waiting time until another electron cotunnels from the source to the drain? What are the statistical properties of these waiting time intervals? How does cotunneling affect the statistical properties of a system with strong inelastic electron-electron interactions? In answering these questions, we extend the existing formalism for waiting time distributions in single-electron transport to include cotunneling processes via an $n$-resolved Markovian master equation. We demonstrate that for a single resonant level, the analytic waiting time distribution including cotunneling processes yields information on individual tunneling amplitudes. For both a SRL and an Anderson impurity deep in the Coulomb blockade, there is a nonzero probability for two electrons to cotunnel to the drain with zero waiting time in between. Furthermore, we show that at high voltages, cotunneling processes slightly modify the nonrenewal behavior of an Anderson impurity with a strong inelastic electron-electron interaction.

Bound states in the continuum (BICs) in photonic crystals represent the unique solutions of wave equations possessing an infinite quality factor. We design a type of bilayer photonic crystal and study the influence of symmetry and coupling between TE and TM polarizations on BICs. The BIC modes possess $C_{3v}$ symmetry in the $x\text{−}y$ plane while the mirror-flip symmetry in the $z$ direction is broken, and they provide selective coupling into different layers by varying frequency. The enhanced TE-TM coupling due to broken mirror-flip symmetry in the $z$ direction gives rise to high-$Q$-factor BIC states with unique spatial characteristics. We show the emergence of such BIC states even in the presence of coupling between the TE- and TM-like modes, which is different from the existing single polarization BIC models. We propose to study BICs in multilayer systems, and the results may be helpful in designing photonic settings to observe and manipulate BICs with various symmetries and polarizations for practical applications.

Much recent experimental effort has focused on the realization of exotic quantum states and dynamics predicted to occur in periodically driven systems. But how robust are the sought-after features, such as Floquet topological surface states, against unavoidable imperfections in the periodic driving? In this paper, we address this question in a broader context and study the dynamics of quantum systems subject to noise with periodically recurring statistics. We show that the stroboscopic time evolution of such systems is described by a noise-averaged Floquet superoperator. The eigenvectors and -values of this superoperator generalize the familiar concepts of Floquet states and quasienergies and allow us to describe decoherence due to noise efficiently. Applying the general formalism to the example of a noisy Floquet topological chain, we rederive and corroborate our recent findings on the noise-induced decay of topologically protected end states. These results follow directly from an expansion of the end state in eigenvectors of the Floquet superoperator.