In this paper, we study the properties of two-dimensional lattices in the presence of non-Hermitian disorder. In the context of coupled mode theory, we consider random gain-loss distributions on every waveguide channel (on site disorder). Our work provides a systematic study of the interplay between disorder and non-Hermiticity. In particular, we study the eigenspectrum in the complex frequency plane and we examine the localization properties of the eigenstates, either by the participation ratio or the level spacing, defined in the complex plane. A modified level distribution function vs disorder seems to fit our computational results.

We investigate the Loschmidt amplitude and dynamical quantum phase transitions in multiband one-dimensional topological insulators. For this purpose we introduce a new solvable multiband model based on the Su-Schrieffer-Heeger model, generalized to unit cells containing many atoms but with the same symmetry properties. Such models have a richer structure of dynamical quantum phase transitions than the simple two-band topological insulator models typically considered previously, with both quasiperiodic and aperiodic dynamical quantum phase transitions present. Moreover, the aperiodic transitions can still occur for quenches within a single topological phase. We also investigate the boundary contributions from the presence of the topologically protected edge states of this model. Plateaus in the boundary return rate are related to the topology of the time-evolving Hamiltonian and hence to a dynamical bulk-boundary correspondence. We go on to consider the dynamics of the entanglement entropy generated after a quench and its potential relation to the critical times of the dynamical quantum phase transitions. Finally, we investigate the fidelity susceptibility as an indicator of the topological phase transitions and find a simple scaling law as a function of the number of bands of our multiband model which is found to be the same for both bulk and boundary fidelity susceptibilities.

The simultaneous occurrence of direct and inverse magnetostriction in transversely isotropic hexagonal crystal is theoretically investigated. Particular emphasis is here given to the need of identifying the fourth-order magnetostriction tensor, as it represents the most primitive object from which all related physical quantities of interest in micromagnetism are deduced. For hexagonal crystals, the magnetostriction tensor is expressed in terms of six independent magnetostrictive coefficients whose values are, so far, unknown. Indeed, the existing literature provides just four independent constraints that are extracted from the expression of the differential scalar strain in a given direction. In this work, the two extra conditions required to solve this identification problem are obtained by deducing the explicit functional dependence of the main features characterizing the motion of a magnetic domain wall along the major axis of a thin magnetostrictive nanostrip placed on the top of a thick piezoelectric actuator. The results of our analysis reveal that such two conditions may be associated to the effective anisotropy coefficient and the domain-wall width. To validate our proposal, a comparison with some recent experimental results is also successfully addressed.

While many-body effects in flat-band systems are receiving renewed hot interest in condensed-matter physics for superconducting and topological properties as well as for magnetism, studies have primarily been restricted to multiband systems (with coexisting flat and dispersive bands). Here we focus on one-band systems where a band is “partially flat,” comprising flat and dispersive portions in $k$ space to reveal whether intriguing correlation effects can already arise on the simplest possible one-band level. For that, the two-dimensional repulsive Hubbard model is studied for two models having different flat areas in an intermediate-coupling regime with the dynamical mean-field theory combined with the fluctuation exchange approximation. We have a crossover from ferromagnetic to antiferromagnetic spin fluctuations as the band filling is varied and this triggers, for the model with a wider flat portion, a triplet-pair superconductivity favored over an unusually wide filling region, which is taken over by a sharply growing singlet pairing. For the model with a narrower flat portion, $T_C$ against filling exhibits an unusual double-peaked $T_C$ dome, associated with different numbers of nodes in the gap function having remarkably extended pairs in real space. We identify these as a manifestation of the physics outside the conventional nesting physics where only the pair scattering across the Fermi surface in designated (hot) spots is relevant. Another correlation effect arising from the flattened band is found in a non-Fermi-liquid behavior as detected in the momentum distribution function, frequency dependence of the self-energy, and spectral function. These indicate that unusual correlation physics can indeed occur in flat-band systems.

The understanding of the superconducting phase under pressure in the famous iron-based spin ladder ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ commands a perfect understanding of the structural properties. We present an accurate single crystal x-ray analysis which demonstrates that the structure admitted until now at room temperature is totally ruled out. We show that the system has a polar structure already at 300 K and is thus ferroelectric. Its structure is modified below the Néel transition and the polarization is intensified. These features unambiguously prove the multiferroic character of ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ as proposed theoretically. This system evidences a competition between multiferroicity and superconductivity.

We study the transverse dynamical spin susceptibility of the two-dimensional U(1) spinon Fermi-surface spin liquid in a small applied Zeeman field. We show that both short-range interactions, present in a generic Fermi liquid, as well as gauge fluctuations, characteristic of the U(1) spin liquid, qualitatively change the result based on the frequently assumed noninteracting spinon approximation. The short-range part of the interaction lead to a collective “spinon spin wave” mode, which splits off from the two-spinon continuum at a small momentum and disperses downward. Gauge fluctuations renormalize the susceptibility, providing nonzero power-law weight in the region outside the spinon continuum and giving the spin wave a finite lifetime, which scales as momentum squared. We also study the effect of Dzyaloshinskii-Moriya anisotropy on the zero momentum susceptibility, which is measured in electron spin resonance (ESR), and obtain a resonance linewidth linear in temperature and varying as $B^{2/3}$ with magnetic field $B$ at low temperatures. Our results form the basis for a theory of inelastic neutron scattering, ESR, and resonant inelastic x-ray scattering studies of this quantum spin-liquid state.

We have deposited polycrystalline Re-doped ${\left(\mathrm{F}{\mathrm{e}}_{65}\mathrm{C}{\mathrm{o}}_{35}\right)}_{\left(100\text{−}x\right)}\mathrm{R}{\mathrm{e}}_x$ ($0\le x\le 12.6$ at. %) thin films grown under identical conditions and sandwiched between thin layers of Ru in order to study the phenomenon of spin pumping as a function of Re concentration. In-plane and out-of-plane ferromagnetic resonance spectroscopy results show an enhancement of the Gilbert damping with an increase in Re doping. We find 98% enhancement in the real part of effective spin mixing conductance $\left[\mathrm{Re}\left(g_{\mathrm{eff}}^{\uparrow \downarrow }\right)\right]$ with Re doping. Conversely, the $\mathrm{Re}\left(g_{\mathrm{eff}}^{\uparrow \downarrow }\right)$ does not change with Re doping in $\mathrm{F}{\mathrm{e}}_{65}\mathrm{C}{\mathrm{o}}_{35}$ thin films which are seeded and capped with Cu layers. The enhancement in $\mathrm{Re}\left(g_{\mathrm{eff}}^{\uparrow \downarrow }\right)$ of Re-doped $\mathrm{F}{\mathrm{e}}_{65}\mathrm{C}{\mathrm{o}}_{35}$ thin films sandwiched between thin layers of Ru is linked to the Re doping-induced change of the interface electronic structure in the nonmagnetic Ru layer. The saturation magnetization decreases 35% with increasing Re doping up to 12.6 at. %. This study opens a direction of tuning the spin mixing conductance in magnetic heterostructures by doping of the ferromagnetic layer, which is essential for the realization of energy-efficient operation of spintronic devices.

We investigate the impact of electron-lattice coupling on the stability of various magnetic orders in rare-earth nickelates. We use the Hartree-Fock approximation, at zero temperature, to study an effective two-band model with correlations characterized by a Hubbard $U$ and a Hund's $J$. This is coupled to breathing-mode distortions of the octahedral oxygen cages, described semiclassically with a Holstein term. We analyze the effect of the various parameters on the resulting phase diagram, in particular, on the degree of charge disproportionation and on the magnetic order. We confirm that the coupling to the lattice cooperates with Hund's coupling and thus encourages charge disproportionation. We also find that it favors the four-site periodic magnetic order of type $\Uparrow 0\Downarrow 0$. Other convergent magnetic phases, such as the collinear $\uparrow \uparrow \downarrow \downarrow$ and noncollinear $\uparrow \to \downarrow \leftarrow$ states, do not couple to the lattice because of their lack of charge disproportionation. Novel phases, e.g., with charge disproportionation but no magnetic order, are found to be stabilized in specific conditions.

We study the self-energy of phonons, magnetoelastically coupled to the two-dimensional Kitaev spin model on the honeycomb lattice. Fractionalization of magnetic moments into mobile Majorana matter and a static ${\mathbb{Z}}_2$ gauge field lead to a continuum of relaxation processes comprising two channels. Thermal flux excitations, which act as an emergent disorder, strongly affect the phonon renormalization. Above the flux proliferation temperature, the dispersion of a narrow quasiparticle-hole channel is suppressed in favor of broad and only weakly momentum-dependent features, covering large spectral ranges. Our analysis is based on complementary calculations in the low-temperature homogeneous gauge and a mean-field treatment of thermal gauge fluctuations, valid at intermediate and high temperatures.

The longitudinal spin Seebeck effect (SSE) in magnetic insulator/nonmagnetic metal heterostructures has been theoretically studied, primarily with the assumption of an isotropic interfacial exchange coupling. Here, we present a general theory of the SSE in the case of an antisymmetric Dzyaloshinskii-Moriya interaction (DMI) at the interface in addition to the usual Heisenberg form. We numerically evaluate the dependence of the spin current on the temperature and bulk DMI using a pyrochlore iridate as a model insulator with an all-in-all-out ground-state configuration. We also compare the results of different crystalline surfaces arising from different crystalline orientations and conclude that the relative angles between the interfacial moments and Dzyaloshinskii-Moriya vectors play a significant role in the spin transfer. Our work extends the theory of the SSE by including the anisotropic nature of the interfacial Dzyaloshinskii-Moriya exchange interaction in magnetic insulator/nonmagnetic metal heterostructures and can suggest possible materials to optimize the interfacial spin transfer in spintronic devices.

We investigate the topological nodal structure of three-dimensional (3D) ${\mathrm{ZrTe}}_5$ driven by Zeeman splitting as a function of the direction of external magnetic (B) field by using a Wannier-function-based tight-binding (WFTB) model obtained from first-principles calculations. It is known that small external stimuli can drive 3D ${\mathrm{ZrTe}}_5$ into different topological phases including Dirac semimetal. In order to emphasize the effect of Zeeman splitting, we consider 3D ${\mathrm{ZrTe}}_5$ in a strong topological insulator phase with a small band gap. With Zeeman splitting greater than the band gap, the WFTB model suggests that a type-I nodal ring protected by (glide) mirror symmetry is formed when the B field aligns with the crystal $a$ or $b$ axes, and that a pair of type-I Weyl nodes are formed otherwise, when conduction and valence bands touch. We show that a pair of separate Weyl nodes can disappear through formation of a nodal ring, rather than requiring two Weyl nodes with opposite chirality to come together. Interestingly, a type-II nodal ring appears from crossings of the top two valence bands when the B field is applied along the $c$ axis. This nodal ring gaps out to form type-II Weyl nodes when the B field rotates in the $bc$ plane. Comparing the WFTB and linearized $k·p$ model, we find inadequacy of the latter at some $\mathbf{B}$ field directions. Further, using the WFTB model, we numerically compute the intrinsic anomalous Hall conductivity ${\sigma }_{ac}$ induced by Berry curvature as a function of chemical potential and B field direction. We find that ${\sigma }_{ac}$ increases abruptly when the B field is tilted from the $a$ axis within the $ab$ plane. Our WFTB model also shows significant anomalous Hall conductivity induced by avoided level crossings even in the absence of Weyl nodes.

The Mott metal-insulator transition in the Hubbard model is studied by constructing a dynamical slave-boson mean-field theory in the limit of large lattice coordination number $z$ that incorporates the binding between doubly occupied (doublon) and empty (holon) sites. On the Mott insulating side where all doublons and holons bond in real space into excitonic pairs leading to the charge gap, the theory simplifies considerably to leading order in $1/\sqrt{z}$ and becomes exact on the infinite-$z$ Bethe lattice. An asymptotic solution is obtained for a continuous Mott transition associated with the closing of the charge gap at a critical value of the Hubbard $U_c$ and the corresponding doublon density $n_d^c$, hopping ${\chi }_d^c$ and doublon-holon pairing ${\mathrm{\Delta }}_d^c$ amplitudes. We find $U_c=U_{\mathrm{BR}}[1-2n_d^c-\sqrt{z}({\chi }_d^c+{\mathrm{\Delta }}_d^c)]\simeq 0.8U_{\mathrm{BR}}$, where $U_{\mathrm{BR}}$ is the critical value for the Brinkman-Rice transition in the Gutzwiller approximation captured in the static mean-field solution of the slave-boson formulation of Kotliar and Ruckenstein. Thus, the Mott transition can be viewed as the quantum correction to the Brinkman-Rice transition due to doublon-holon binding. Quantitative comparisons are made to the results of the dynamical mean-field theory, showing good agreement. In the absence of magnetic order, the Mott insulator is a $U\left(1\right)$ quantum spin liquid with nonzero intersite spinon hopping that survives the large-$z$ limit and lifts the $2^N$-fold degeneracy of the local moments. We show that the spinons are coupled to the doublons/holons by a dissipative compact $U\left(1\right)$ gauge field in the deconfined phase, realizing the spin-charge separated gapless spin liquid Mott insulator.

We have studied the fundamental properties of weak-link ${\mathrm{Sr}}_2{\mathrm{RuO}}_4/{\mathrm{Sr}}_2{\mathrm{RuO}}_4$ Josephson junctions fabricated by making a narrow constriction on superconducting ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ films through laser micropatterning. The junctions show a typical overdamped behavior with much higher critical current density compared with those previously reported for bulk ${\mathrm{Sr}}_2{\mathrm{RuO}}_4/s$-wave-superconductor junctions. Observed magnetic-field and temperature dependencies of the Josephson critical current suggest that the chiral $p$ wave is unlikely for the superconducting symmetry, encouraging further theoretical calculations of the ${\mathrm{Sr}}_2{\mathrm{RuO}}_4/{\mathrm{Sr}}_2{\mathrm{RuO}}_4$-type junctions.

Using an imaginary-time matrix-product state (MPS) based quantum impurity solver we perform a realistic dynamical mean-field theory (DMFT) calculation combined with density functional theory (DFT) for Sr2RuO4. We take the full Hubbard-Kanamori interactions and spin-orbit coupling (SOC) into account. The MPS impurity solver works at essentially zero temperature in the presence of SOC, a regime of parameters currently inaccessible to continuous-time quantum Monte Carlo methods, due to a severe sign problem. We show that earlier results obtained at high temperature, namely, that the diagonal self-energies are nearly unaffected by SOC and that interactions lead to an effective enhancement of the SOC, hold even at low temperature. We observe that realism makes the numerical solution of the impurity model with MPS much more demanding in comparison to earlier works on Bethe lattice models, requiring several algorithmic improvements.

Kondo correlations are responsible for the emergence of a zero-bias peak in the low temperature differential conductance of Coulomb blockaded quantum dots. In the presence of a global $\mathrm{SU}\left(2\right)\otimes \mathrm{SU}$(2) symmetry, which can be realized in carbon nanotubes, they also inhibit inelastic transitions which preserve the Kramers pseudospins associated to the symmetry. We report on magnetotransport experiments on a Kondo correlated carbon nanotube where resonant features at the bias corresponding to the pseudospin-preserving transitions are observed. We attribute this effect to a simultaneous enhancement of pseudospin-nonpreserving transitions occurring at that bias. This process is boosted by asymmetric tunneling couplings of the two Kramers doublets to the leads and by asymmetries in the potential drops at the leads. Hence, the present work discloses a fundamental microscopic mechanisms ruling transport in Kondo systems far from equilibrium.

Topological crystalline insulators (TCIs) are usually described with topological protection imposed by the crystalline symmetry. In general, however, the existence of TCI states does not necessitate the periodicity of crystals as long as an essential lattice symmetry can be identified. Here we demonstrate the compatibility of TCIs with aperiodic systems, as exemplified by an octagonal quasicrystal. The aperiodic TCIs we proposed are attributed to a band inversion mechanism, which inverts states with the same parity but opposite eigenvalues of a specific symmetry (such as mirror reflection). The nontrivial topology is characterized by a nonzero integer “mirror Bott index.” Moreover, we demonstrate that the topological edge states and quantized conductance of the aperiodic TCI, which are robust against disorder, can be effectively manipulated by external electric fields. Our findings not only provide a better understanding of electronic topology in relation to symmetry but also extend the experimental realization of topological states to much broader material categories beyond crystals.

We propose a variational approximation to the ground state energy of a one-dimensional gas of interacting bosons on the continuum based on the Bethe ansatz ground state wave function of the Lieb-Liniger model. We apply our variational approximation to a gas of dipolar bosons in the single mode approximation and obtain its ground state energy per unit length. This allows for the calculation of the Tomonaga-Luttinger exponent as a function of density and the determination of the structure factor at small momenta. Moreover, in the case of attractive dipolar interaction, an instability is predicted at a critical density, which could be accessed in lanthanide atoms.