We numerically calculate the local density of states (LDOS) of a one-dimensional Mott insulator with open boundaries, which is modelled microscopically by a (extended) Hubbard chain at half filling. In the Fourier transform of the LDOS we identify several dispersing features corresponding to propagating charge and spin degrees of freedom, thus providing a visualization of the spin-charge separation in the system. We also consider the effect of an additional boundary potential, which, if sufficiently strong, leads to the formation of a boundary bound state which is clearly visible in the LDOS as a nondispersing feature inside the Mott gap.

Multiband touchings together with the emergence of fermions exhibiting linear dispersions have recently been predicted and realized in various materials. We first investigate the Adler-Bell-Jackiw chiral anomaly in these multiband touching semimetals when they are described by the pseudospin operator in high-dimensional representation. By evaluating the Chern number, we show that the anomalous Hall effect is enhanced depending on the magnitude of the pseudospin. It is also confirmed by the analysis of the Landau levels when magnetic field is applied. Namely, charge pumping occurs from one multiband touching point to another through multichannel Landau levels in the presence of parallel electric and magnetic fields. We also show a pair annihilation of two multiband touching points by photoirradiation. Furthermore, we propose generalizations of Dirac semimetals, multiple Weyl semimetals, and loop-nodal semimetals to those composed of fermions carrying pseudospins in high-dimensional representation. Finally we investigate the three-band touching protected by the $C_3$ symmetry. We show that the three-band touching point is broken into two Weyl points by photoirradiation.

A general theory of photon-mediated energy and momentum transfer in $N$-body planar systems out of thermal equilibrium is introduced. It is based on the combination of the scattering theory and the fluctuational-electrodynamics approach in many-body systems. By making a Landauer-like formulation of the heat transfer problem, explicit formulas for the energy transmission coefficients between two distinct slabs as well as the self-coupling coefficients are derived and expressed in terms of the reflection and transmission coefficients of the single bodies. We also show how to calculate local equilibrium temperatures in such systems. An analogous formulation is introduced to quantify momentum transfer coefficients describing Casimir-Lifshitz forces out of thermal equilibrium. Forces at thermal equilibrium are readily obtained as a particular case. As an illustration of this general theoretical framework, we show on three-body systems how the presence of a fourth slab can impact equilibrium temperatures in heat-transfer problems and equilibrium positions resulting from the forces acting on the system.

We extend the existing theoretical model for determining the characteristic features of magnetic deflagration in nanomagnet crystals. For the first time, all energy levels are accounted for calculation of the the Zeeman energy, the deflagration velocity, and other parameters. It reduces the final temperature and significantly changes the propagation velocity of the spin-flipping front. We also consider the effect of a strong transverse magnetic field, and show that the latter significantly modifies the spin-state structure, leading to an uncertainty concerning the activation energy of the spin flipping. Our front velocity prediction for a crystal of ${\mathrm{Mn}}_{12}$ acetate in a longitudinal magnetic field is in much better agreement with experimental data than the previous reduced-model results.

The dielectric properties of diamond in excited state modulated by an ultrashort and intense single-cycle laser field are investigated based on the real-time time-dependent density functional theory. The electron dynamics of diamond is analyzed within ultrashort time resolution. The change of dielectric properties are demonstrated by studying the effect of easily tunable laser parameters including intensity, frequency, and duration time. The extracted dielectric function shows anisotropy even in an centrosymmetric diamond when going beyond the linear response regime. We also demonstrate control of the dielectric functions by the delay time between pump and probe pulse. It is concluded that a tailored single-cycle laser field can be used to effectively manipulate the dielectric properties of wide-band gap materials.

Autoresonance is a self-sustained resonance mechanism due to a driving force whose frequency monotonically varies with time. We theoretically show that the autoresonance mechanism allows an efficient switching of perpendicular magnetization by spin-orbit spin-transfer torques. We find that a threshold current for the autoresonant switching can be much smaller than that of conventional spin-orbit torque switching driven by a DC current. Moreover, the suggested scheme allows fully deterministic switching without the help of any external field.

In this work, we show that a giant spin current can be injected into a nodal topological superconductor, using a normal paramagnetic lead, through a large number of zero-energy Majorana fermions at the superconductor edge. The giant spin current is caused by the selective equal-spin Andreev reflections (SESARs) induced by Majorana fermions. In each SESAR event, a pair of electrons with certain spin polarization are injected into the nodal topological superconductor, even though the pairing in the bulk of the nodal superconductor is spin-singlet $s$ wave. We further explain the origin of the spin current by showing that the pairing correlation at the edge of a nodal topological superconductor is predominantly equal-spin triplet at zero energy. The experimental consequences of SESARs in nodal topological superconductors are discussed.

We investigate, using the density-matrix renormalization group, the evolution of the Nagaoka state with $t^{\prime }$ hopping that frustrates the hole kinetic energy in the $U=\infty$ Hubbard model on the square and anisotropic triangular lattices. We find that the Nagaoka ferromagnet survives up to a rather small $t_c^{\prime }/t\sim 0.2.$ At this critical value, there is a transition to an antiferromagnetic phase that depends on the lattice: a $\mathbf{Q}=(Q,0)$ spiral order, which continuously evolves with $t^{\prime }$, for the triangular lattice and the usual $\mathbf{Q}=(\pi ,\pi )$ Néel order for the square lattice. Remarkably, the local magnetization takes its classical value for all considered $t^{\prime }$ ($t^{\prime }/t\le 1$). Our results show that the recently found classical kinetic antiferromagnetism, a perfect counterpart of Nagaoka ferromagnetism, is a generic phenomenon in these kinetically frustrated electronic systems.

The physics of charge separation in organic semiconductors is a topic of ongoing research of relevance to material and device engineering. Herein, we present experimental observations of the field and temperature dependence of charge separation from singlet excitons in PTB7 and ${\mathrm{PC}}_{71}\mathrm{BM}$, and from charge-transfer states created across interfaces in $\mathrm{PTB}7/{\mathrm{PC}}_{71}\mathrm{BM}$ bulk heterojunction solar cells. We obtain this experimental data by time-resolving the near infrared emission of the states from 10 K to room temperature and electric fields from 0 to $2.5{\mathrm{MV}\mathrm{cm}}^{-1}$. Examining how the luminescence is quenched by field and temperature gives direct insight into the underlying physics. We observe that singlet excitons can be split by high fields, and that disorder broadens the high threshold fields needed to split the excitons. Charge-transfer (CT) states, on the other hand, can be separated by both field and temperature. Also, the data imply a strong reduction of the activation barrier for charge splitting from the CT state relative to the exciton state. The observations provided herein of the field-dependent separation of CT states as a function of temperature offer a rich data set against which theoretical models of charge separation can be rigorously tested; it should be useful for developing the more advanced theoretical models of charge separation.

We describe spin and charge dynamics of the system of two electrons confined within a double quantum dot defined in a quantum wire. The spin dynamics is driven by the electron motion in the presence of the spin-orbit interaction and the randomly varying local Overhauser field due to the nuclear spins. The Schroedinger equation is solved with the time-dependent configuration interaction method that allows for an exact description of the system dynamics. The procedures of the spin separation, exchange and read-out by the spin to charge conversion all induced by the detuning variation are simulated. The rates of the potential variation that are necessary for the spin separation and spin to charge conversion in the context of the Landau-Zener transitions are determined. The average over random configurations of the hyperfine field produces spin exchange results which qualitatively agree with the experimental data.

The extremely stringent requirement on material quality has hindered the investigation and potential applications of exotic chiral magnetic effect in Dirac semimetals. Here, we propose that gray tin is a perfect candidate for observing the chiral anomaly effect and Shubnikov-de-Haas (SdH) oscillation at relatively low magnetic field. Based on effective $k·p$ analysis and first-principles calculations, we discover that gray tin becomes a Dirac semimetal under tensile uniaxial strain, in contrast to a topological insulator under compressive uniaxial strain as known before. In this newly found Dirac semimetal state, two Dirac points which are tunable by tensile [001] strains lie in the $k_z$ axis and Fermi arcs appear in the (010) surface. Due to the low carrier concentration and high mobility of gray tin, a large chiral anomaly induced negative magnetoresistance and a strong SdH oscillation are anticipated in this half of the strain spectrum. Comparing to other Dirac semimetals, the proposed Dirac semimetal state in the nontoxic elemental gray tin can be more easily manipulated and accurately controlled. We envision that gray tin provides a perfect platform for strain engineering of chiral magnetic effects by sweeping through the strain spectrum from positive to negative and vice versa.

We explore the far-field scattering properties of anisotropic two-dimensional materials in ribbon array configuration. Our study reveals the plasmon-enhanced linear birefringence in these ultrathin metasurfaces, where linearly polarized incident light can be scattered into its orthogonal polarization or be converted into circular polarized light. We found wide modulation in both amplitude and phase of the scattered light via tuning the operating frequency or material's anisotropy and develop models to explain the observed scattering behavior.

We describe a variational theory of multiband two-dimensional electron gases that captures the interplay between electrostatic confining potentials, orbital-dependent interlayer electronic hopping, and electron-electron interactions and apply it to the $d$-band two-dimensional electron gases that form near perovskite oxide surfaces and heterojunctions. These multiband two-dimensional electron gases are prone to the formation of Coulomb-interaction-driven orbitally-ordered nematic ground states. We find that as the electron density is lowered and interaction effects strengthen, spontaneous orbital order occurs first, followed by spin order. We compare our results with known properties of single-component two-dimensional electron gas systems and comment on closely related physics in semiconductor quantum wells and van der Waals heterostructures.

We present results of resistance fluctuation spectroscopy on single crystals of the predicted Kondo topological insulator material ${\mathrm{SmB}}_6$. Our measurements show that at low temperatures, transport in this system takes place only through surface states. The measured noise in this temperature range arises due to universal conductance fluctuations whose statistics was found to be consistent with theoretical predictions for that of two-dimensional systems in the symplectic symmetry class. At higher temperatures, we find signatures of glassy dynamics and establish that the measured noise is caused by mobility fluctuations in the bulk. We find that, unlike the topological insulators of the dichalcogenide family, the noises in surface and bulk conduction channels in ${\mathrm{SmB}}_6$ are completely uncorrelated. Our measurements establish that at sufficiently low temperatures, the bulk has no discernible contribution to electrical transport in ${\mathrm{SmB}}_6$, making it an ideal platform for probing the physics of topological surface states.

$\mathrm{B}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_2\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_4\mathrm{O}$ is a self-doped superconductor exhibiting a $T_c\sim 21.5\mathrm{K}$ and containing, distinctively with respect to other Fe-based superconductors, not only $\left[\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2\right]$ layers but also conducting $\left[\mathrm{T}{\mathrm{i}}_2\mathrm{O}\right]$ sheets. This compound exhibits a transition at $T^{*}\sim 125\mathrm{K}$, which has tentatively been assigned in the literature to a possible density-wave order. However, the nature of this density wave (whether it is charge- or spin-induced) is still under debate. Magnetism in $\mathrm{B}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_2\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_4\mathrm{O}$ has never been experimentally confirmed, which raises the question whether this superconductor might be nonmagnetic or exhibiting a very weak magnetism. Here, we report evidence from inelastic neutron scattering (INS) measurements and ab initio calculations of phonon spectra pointing towards a possible absence of magnetism in $\mathrm{B}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_2\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_4\mathrm{O}$. The INS measurements did not reveal any noticeable magnetic effects in $\mathrm{B}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_2\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_4\mathrm{O}$, within the accessible ($Q, E$) space. The effect of magnetism on describing phonon spectra was further investigated by performing ab initio calculations. In this context, nonmagnetic calculations reproduced well the measured phonon spectra. Therefore, our results indicate a nonmagnetic character of the superconductor $\mathrm{B}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_2\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_4\mathrm{O}$.

We report a simultaneous measurement of a longitudinal spin Seebeck effect (LSSE) and thermal conductivity in a Pt/${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG)/Pt system in a temperature range from 10 to 300 K. By directly monitoring the temperature difference in the system, we excluded thermal artifacts in the LSSE measurements. It is found that both the LSSE signal and the thermal conductivity of YIG exhibit sharp peaks at the same temperature, different from previous reports. The maximum LSSE coefficient is found to be $S_{\mathrm{LSSE}}>10\mu \mathrm{V}/\mathrm{K}$, one-order-of magnitude greater than the previously reported values. The concomitant enhancement of the LSSE and thermal conductivity of YIG suggests the strong correlation between magnon and phonon transport in the LSSE.

Spin-driven nematicity, or the breaking of the point-group symmetry of the lattice without long-range magnetic order, is clearly quite important in iron-based superconductors. From a symmetry point of view, nematic order can be described as a coherent locking of spin fluctuations in two interpenetrating Néel sublattices with ensuing nearest-neighbor bond order and an absence of static magnetism. Here, we argue that the low-temperature state of the recently discovered superconductor ${\mathrm{BaTi}}_2{\mathrm{Sb}}_2\mathrm{O}$ is a strong candidate for a more exotic form of spin-driven nematic order, in which fluctuations occurring in four Néel sublattices promote both nearest- and next-nearest-neighbor bond order. We develop a low-energy field theory of this state and show that it can have, as a function of temperature, up to two separate bond-order phase transitions, namely, one that breaks rotation symmetry and one that breaks reflection and translation symmetries of the lattice. The resulting state has an orthorhombic lattice distortion, an intra-unit-cell charge density wave, and no long-range magnetic order, all consistent with reported measurements of the low-temperature phase of ${\mathrm{BaTi}}_2{\mathrm{Sb}}_2\mathrm{O}$. We then use density functional theory calculations to extract exchange parameters to confirm that the model is applicable to ${\mathrm{BaTi}}_2{\mathrm{Sb}}_2\mathrm{O}$.

We use high-resolution angle-resolved photoemission spectroscopy to study the resonant, collective excitation mode in the superconducting state of Bi2212. By collecting very high-quality data we found noteworthy features in the self-energy in the antinodal region, where the interaction of electrons with the mode is the strongest. This interaction leads to a pronounced peak in the scattering rate and we demonstrate that this feature is directly responsible for the well-known peak-dip-hump structure in cuprates. By studying how the weight of this peak changes with temperature we unequivocally demonstrate that interaction of electrons with the resonant mode in cuprates vanishes at ${\mathrm{T}}_c$ and is very much localized in the momentum space close to the antinode. These findings present a consistent picture of line shape and self-energy signatures of the electron-boson coupling in cuprates and resolve long-standing controversy surrounding this issue. The momentum dependence of the strength of electron-mode interaction enables development of quantitative theory of this phenomenon in cuprates.

We report the observation of a magnetic-field-assisted transient superconducting state in the two-dimensional electron gas existing at the interface of ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterostructures. This metastable state depends critically on the density of charge carriers in the system. It appears concomitantly with a Lifshitz transition as a consequence of the interplay between ferromagnetism and superconductivity and the finite relaxation time of the in-plane magnetization. Our results clearly demonstrate the inherently metastable nature of the superconducting state competing with a magnetic order in these systems. The coexistence of superconductivity and ferromagnetism in the conducting electronic layer formed at the interface of insulating oxides has thrown up several intriguing and as yet unanswered questions. An open question in this field is the energetics of the interplay between these two competing orders and the present observation goes a long way in understanding the underlying mechanism.