The longitudinal spin Seebeck effect refers to the generation of a spin current when heat flows across a normal metal/magnetic insulator interface. Most explanations of the spin Seebeck effect use the interfacial temperature difference as the conversion mechanism between heat and spin fluxes. However, recent theoretical and experimental works claim that a magnon spin current is generated in the bulk of a magnetic insulator even in the absence of an interface. This is the so-called intrinsic spin Seebeck effect. Here, by utilizing a nonlocal spin Seebeck geometry, we provide additional evidence that the total magnon spin current in the ferrimagnetic insulator yttrium iron garnet (YIG) actually contains two distinct terms: one proportional to the gradient in the magnon chemical potential (pure magnon spin diffusion), and a second proportional to the gradient in magnon temperature ($\nabla T_m$). We observe two characteristic decay lengths for magnon spin currents in YIG with distinct temperature dependences: a temperature independent decay length of $\sim 10\mu \mathrm{m}$ consistent with earlier measurements of pure ($\nabla T_m=0$) magnon spin diffusion, and a longer decay length ranging from about $20\mu \mathrm{m}$ around 250 K and exceeding $80\mu \mathrm{m}$ at 10 K. The coupled spin-heat transport processes are modeled using a finite element method revealing that the longer-range magnon spin current is attributable to the intrinsic spin Seebeck effect ($\nabla T_m\ne 0$), whose length scale increases at lower temperatures in agreement with our experimental data.

We present an approach and a theoretical framework for generating high-order exceptional points of degeneracy (EPDs) in photonic structures based on periodic coupled resonator optical waveguides (CROWs). Such EPDs involve the coalescence of Floquet-Bloch eigenwaves in CROWs, without the presence of gain and loss, which contrasts with the parity-time symmetry required to develop exceptional points based on gain and loss balance. The EPDs arise here by introducing symmetry breaking in a conventional chain of coupled resonators through periodic coupling to an adjacent uniform optical waveguide, which leads to unique modal characteristics that cannot be realized in conventional CROWs. Such remarkable characteristics include high quality factors ($Q$ factors) and strong field enhancement, even without any mirrors at the two ends of a cavity. We show for the first time the capability of CROWs to exhibit EPDs of various orders, including the degenerate band edge (DBE) and the stationary inflection point. The proposed CROW of finite length shows an enhanced quality factor when operating near the DBE, and the $Q$ factor exhibits an unconventional scaling with the CROW's length. We develop the theory of EPDs in such unconventional CROW using coupled-wave equations, and we derive an analytical expression for the dispersion relation. The proposed unconventional CROW concepts have various potential applications including $Q$ switching, nonlinear devices, lasers, and extremely sensitive sensors.

We study the topography of the spin-polarized bound states of magnetic impurities embedded in a triangular lattice of a superconducting host. Such states have been observed experimentally in $2\mathrm{H}\text{−}{\mathrm{NbSe}}_2$ crystal [G. C. Ménard et al., Nat. Phys. 11, 1013 (2015)], and they revealed oscillating particle-hole asymmetry extending to tens of nanometers. Using the Bogoliubov–de Gennes approach, we explore the Yu-Shiba-Rusinov states in the presence of spin-orbit interaction. We also study the bound states of double impurities for several relative positions in a triangular lattice.

The electronic ground state of ${\mathrm{Ca}}_3{\mathrm{LiOsO}}_6$ was recently considered within an intermediate coupling regime that revealed $J=3/2$ spin-orbit entangled magnetic moments. Through inelastic neutron scattering and density functional theory we investigate the magnetic interactions and probe how the magnetism is influenced by the change in hierarchy of interactions as we move from ${\mathrm{Ca}}_3{\mathrm{LiOsO}}_6$ ($5d^3$) to ${\mathrm{Ca}}_3{\mathrm{LiRuO}}_6$ ($4d^3$). An alteration of the spin gap and ordered local moment is observed, however the magnetic structure, Néel temperature, and exchange interactions are unaltered. To explain this behavior it is necessary to include both spin-orbit coupling and hybridization, indicating the importance of an intermediate coupling approach when describing $5d$ oxides.

The nonlocal transport of thermally generated magnons not only unveils the underlying mechanism of the spin Seebeck effect, but also allows for the extraction of the magnon relaxation length (${\lambda }_m$) in a magnetic material, the average distance over which thermal magnons can propagate. In this study, we experimentally explore in yttrium iron garnet (YIG)/platinum systems much further ranges compared with previous investigations. We observe that the nonlocal SSE signals at long distances ($d$) clearly deviate from a typical exponential decay. Instead, they can be dominated by the nonlocal generation of magnon accumulation as a result of the temperature gradient present away from the heater, and decay geometrically as $1/d^2$. We emphasize the importance of looking only into the exponential regime (i.e., the intermediate distance regime) to extract ${\lambda }_m$. With this principle, we study ${\lambda }_m$ as a function of temperature in two YIG films which are 2.7 and $50\mu \mathrm{m}$ in thickness, respectively. We find ${\lambda }_m$ to be around $15\mu \mathrm{m}$ at room temperature and it increases to $40\mu \mathrm{m}$ at $T=3.5$ K. Finite element modeling results agree with experimental studies qualitatively, showing also a geometrical decay beyond the exponential regime. Based on both experimental and modeling results, we put forward a general guideline for extracting ${\lambda }_m$ from the nonlocal spin Seebeck effect.

The hour-glass-like dispersion of spin excitations is a common feature of underdoped cuprates. It was qualitatively explained by the random-phase approximation based on various ordered states with some phenomenological parameters; however, its origin remains elusive. Here, we present a numerical study of spin dynamics in the $t\text{−}J$ model using the variational Monte Carlo method. This parameter-free method satisfies the no double-occupancy constraint of the model and thus provides a better evaluation on the spin dynamics with respect to various mean-field trial states. We conclude that the lower branch of the hour-glass dispersion is a collective mode and the upper branch more than the other candidates is likely the consequence of the stripe state.

We study how superconducting $T_c$ is affected as an electronic system in a tetragonal environment is tuned to a nematic quantum critical point (QCP). Including coupling of the electronic nematic variable to the relevant lattice strain restricts criticality only to certain high symmetry directions. This allows a weak-coupling treatment, even at the QCP. We develop a criterion distinguishing weak and strong $T_c$ enhancements upon approaching the QCP. We show that negligible $T_c$ enhancement occurs only if pairing is dominated by a non-nematic interaction away from the QCP, and simultaneously if the electron-strain coupling is sufficiently strong. We argue this is the case of the iron superconductors.

Recent experiments using the quantum dot coupled to the topological superconducting nanowire [Deng et al., Science 354, 1557 (2016)] revealed that the zero-energy bound state coalesces from the Andreev bound states. Such quasiparticle states, present in the quantum dot, can be controlled by magnetic and electrostatic means. We use a microscopic model of the quantum-dot–nanowire structure to reproduce the experimental results, applying the Bogoliubov–de Gennes technique. This is done by studying the gate voltage dependence of the various types of bound states and mutual influence between them. We show that the zero-energy bound states can emerge from the Andreev bound states in the topologically trivial phase and can be controlled using various means. In the nontrivial topological phase we show the possible resonance between these zero-energy levels with Majorana bound states. We discuss and explain this phenomenon as a result of dominant spin character of discussed bound states. Presented results can be applied in experimental studies by using the proposed nanodevice.

The generation of photons in a microcavity coupled to a laser-driven quantum dot interacting with longitudinal acoustic (LA) phonons is studied in the regime of simultaneously strong driving and strong dot-cavity coupling. The stationary cavity photon number is found to depend in a nontrivial way on the detuning between the laser and the exciton transition in the dot. In particular, the maximal efficiency of the cavity feeding is obtained for detunings corresponding to transition energies between cavity-dressed states with excitation numbers larger than one. Phonons significantly enhance the cavity feeding at large detunings. In the strong-driving, strong-coupling limit, the photon statistics is highly non-Poissonian. While without phonons a double-peaked structure in the photon distribution is predicted, phonons make the photon statistics thermal-like with very high effective temperatures $\sim {10}^5$ K, even for low phonon temperatures $\sim 4$ K. These results were obtained by numerical calculations where the driving, the dot-cavity coupling, and the dot-phonon interactions are taken into account without approximations. This is achieved by a reformulation of an exact iterative path-integral scheme which is applicable to a large class of quantum-dissipative systems and which in our case reduces the numerical demands by 15 orders of magnitude.

The anomalous features in diffraction patterns first observed by Wood over a century ago have been the subject of many investigations, both experimental and theoretical. The sharp, narrow structures—and the large resonances with which they are sometimes associated—arise in numerous studies in optics and photonics. In this paper we present an analytical method to study diffracted fields of optically thin gratings that highlights the nonanalyticities associated with the anomalies. Using this approach we can immediately derive diffracted fields for any polarization in a compact notation. While our equations are approximate, they fully respect energy conservation in the electromagnetic field, and describe the large exchanges of energy between incident and diffracted fields that can arise even for thin gratings.

We present determinant quantum Monte Carlo simulations of the hole-doped single-band Hubbard-Holstein model on a square lattice, to investigate how quasiparticles emerge when doping a Mott insulator (MI) or a Peierls insulator (PI). The MI regime at large Hubbard interaction $U$ and small relative e-ph coupling strength $\lambda$ is quickly suppressed upon doping, by drawing spectral weight from the upper Hubbard band and shifting the lower Hubbard band towards the Fermi level, leading to a metallic state with emergent quasiparticles at the Fermi level. On the other hand, the PI regime at large $\lambda$ and small $U$ persists out to relatively high doping levels. We study the evolution of the $d$-wave superconducting susceptibility with doping, and find that it increases with lowering temperature in a regime of intermediate values of $U$ and $\lambda$.

The extension of the linear flavor-wave theory to fully antisymmetric irreducible representations (irreps) of $\mathrm{SU}\left(N\right)$ is presented in order to investigate the color order of $\mathrm{SU}\left(N\right)$ antiferromagnetic Heisenberg models in several two-dimensional geometries. The square, triangular, and honeycomb lattices are considered with $m$ fermionic particles per site. We present two different methods: the first method is the generalization of the multiboson spin-wave approach to $\mathrm{SU}\left(N\right)$ which consists of associating a Schwinger boson to each state on a site. The second method adopts the Read and Sachdev bosons which are an extension of the Schwinger bosons that introduces one boson for each color and each line of the Young tableau. The two methods yield the same dispersing modes, a good indication that they properly capture the semiclassical fluctuations, but the first one leads to spurious flat modes of finite frequency not present in the second one. Both methods lead to the same physical conclusions otherwise: long-range Néel-type order is likely for the square lattice for SU(4) with two particles per site, but quantum fluctuations probably destroy order for more than two particles per site, with $N=2m$. By contrast, quantum fluctuations always lead to corrections larger than the classical order parameter for the tripartite triangular lattice (with $N=3m$) or the bipartite honeycomb lattice (with $N=2m$) for more than one particle per site, $m>1$, making the presence of color very unlikely except maybe for $m=2$ on the honeycomb lattice, for which the correction is only marginally larger than the classical order parameter.

Plasmons in two-dimensional (2D) materials have emerged as a new source of physical phenomena and optoelectronic applications due in part to the relatively small number of charge carriers on which they are supported. Unlike conventional plasmonic materials, they possess a large Fermi wavelength, which can be comparable with the plasmon wavelength, thus leading to unusually strong nonlocal effects. Here, we study the optical response of a selection of 2D crystal layers (graphene, ${\mathrm{MoS}}_2$, and black phosphorus) with inclusion of nonlocal and thermal effects. We extensively analyze their plasmon dispersion relations and focus on the Purcell factor for the decay of an optical emitter in close proximity to the material as a way to probe nonlocal and thermal effects, with emphasis placed on the interplay between temperature and doping. The results are based on tight-binding modeling of the electronic structure combined with the random-phase approximation response function in which the temperature enters through the Fermi-Dirac electronic occupation distribution. Our study provides a route map for the exploration and exploitation of the ultrafast optical response of 2D materials.

Transmission through a quantum point contact (QPC) in the quantum Hall regime usually exhibits multiple resonances as a function of gate voltage and high nonlinearity in bias. Such behavior is unpredictable and changes sample by sample. Here, we report the observation of a sharp transition of the transmission through an open QPC at finite bias, which was observed consistently for all the tested QPCs. It is found that the bias dependence of the transition can be fitted to the Fermi-Dirac distribution function through universal scaling. The fitted temperature matches quite nicely to the electron temperature measured via shot-noise thermometry. While the origin of the transition is unclear, we propose a phenomenological model based on our experimental results that may help to understand such a sharp transition. Similar transitions are observed in the fractional quantum Hall regime, and it is found that the temperature of the system can be measured by rescaling the quasiparticle energy with the effective charge ($e^{*}=e/3$). We believe that the observed phenomena can be exploited as a tool for measuring the electron temperature of the system and for studying the quasiparticle charges of the fractional quantum Hall states.

A model describing the effect of magnetic dipolar interactions on the susceptibility of magnetic nanopillars is presented. It is an extension of a recently reported model for three-dimensional randomlike dispersions of nearly spherical nanoparticles in equilibrium [Sánchez et al., Phys. Rev. B 95, 134421 (2017)], to well-ordered arrays of nanoparticles out of equilibrium. To test it, a high-quality benchmark consisting of a two-dimensional hexagonal arrangement of quasi-identical parallel nickel nanopillars embedded in a porous alumina template was fabricated. This model is based on an effective demagnetizing tensor, which only depends on a few morphological parameters of the sample, as the nearest-neighbor distance between pillars and the volume fraction of pillars in the specimen. It allows us to obtain the nanopillar intrinsic susceptibility tensor from the magnetic response of the nanopillar ensemble. The values of the in-plane and normal-to-plane susceptibility of the sample are successfully predicted by the model. Furthermore, the model reproduces the susceptibility in the applied field direction, measured for different applied field angles. In this way, it provides a simple and accurate treatment to account for the complex magnetic effects produced by dipolar interactions.

We present phase diagrams, free-energy landscapes, and order-parameter distributions for a model spin-crossover material with a two-step transition between the high-spin and low-spin states (a square-lattice Ising model with antiferromagnetic-like nearest-neighbor and ferromagnetic-like long-range interactions) [P. A. Rikvold et al., Phys. Rev. B 93, 064109 (2016)]. The results are obtained by a recently introduced, macroscopically constrained Wang-Landau Monte Carlo simulation method [Phys. Rev. E 95, 053302 (2017)]. The method's computational efficiency enables calculation of thermodynamic quantities for a wide range of temperatures, applied fields, and long-range interaction strengths. For long-range interactions of intermediate strength, tricritical points in the phase diagrams are replaced by pairs of critical end points and mean-field critical points that give rise to horn-shaped regions of metastability. The corresponding free-energy landscapes offer insights into the nature of asymmetric, multiple hysteresis loops that have been experimentally observed in spin-crossover materials characterized by competing short-range interactions and long-range elastic interactions.

Thermal quantum critical systems, with partition functions expressed as conformal tensor networks, are revealed to exhibit universal entropy corrections on nonorientable manifolds. Through high-precision tensor network simulations of several quantum chains, we identify the universal entropy $S_{\mathcal{K}}=lnk$ on the Klein bottle, where $k$ relates to quantum dimensions of the primary fields in conformal field theory (CFT). Different from the celebrated Affleck-Ludwig boundary entropy $lng$ $(g$ reflects noninteger ground-state degeneracy), $S_{\mathcal{K}}$ has $no$ boundary dependence or surface energy terms accompanying it, and can be very conveniently extracted from thermal data. On the Möbius-strip manifold, we uncover an entropy $S_{\mathcal{M}}=\frac{1}{2}(lng+lnk)$ in CFT, where $\frac{1}{2}lng$ is associated with the only open edge of the Möbius strip and $\frac{1}{2}lnk$ is associated with the nonorientable topology. As a useful application, we employ the universal entropy to accurately pinpoint the quantum phase transitions, even for those without local order parameters.

Preparing an integrable system in a mixed state described by a thermal density matrix, we subject it to a sudden quench and explore the subsequent unitary dynamics. To address the question of whether the nonanalyticities, namely, the dynamical quantum phase transitions (DQPTs), persist when the initial state is mixed, we consider two versions of the generalized Loschmidt overlap amplitude (GLOA). Our study shows that the GLOA constructed using the Uhlmann approach does not show any signature of DQPTs at any nonzero initial temperature. On the other hand, a GLOA defined in the interferometric phase approach through the purifications of the time-evolved density matrix, indeed shows that nonanalyiticies in the corresponding “dynamical free-energy density” persist, thereby establishing the existence of mixed state dynamical quantum phase transitions (MSDQPTs). Our work provides a framework that perfectly reproduces both the nonanalyticities and also the emergent topological structure in the pure state limit. These claims are corroborated by analyzing the nonequilibrium dynamics of a transverse Ising chain initially prepared in a thermal state and subjected to a sudden quench of the transverse field.