The spin Seebeck effect provides an easy way to create and manipulate spin current. Schreier et al. experimentally observed a decrease of spin Seebeck voltage at high frequencies in the yttrium iron garnet/Pt system [Phys. Rev. B 93, 224430 (2016)]. The frequency dependence was attributed to the magnon spectrum. Here we provide a different explanation using the frequency dependence of the ac spin current. By combining a phenomenological two-temperature model with the ac- temperature method, our calculation shows that the frequency dependence of the spin current is determined by the thermal wavelength and the thermal boundary conditions of the phonons and the magnons.

Motivated by the recent experimental observation of anomalous magnetotransport properties near the Mott quantum critical point (QCP) of pyrochlore iridates, we study the generic topological band structure near QCP in the presence of magnetic field. We have found that the competition between different energy scales can generate various topological semimetal phases near QCP. Here the central role is played by the presence of a quadratic band crossing (QBC) with fourfold degeneracy in the paramagnetic band structure. Due to the large band degeneracy and strong spin-orbit coupling, the degenerate states at QBC can show an anisotropic Zeeman effect as well as the conventional isotropic Zeeman effect. Through the competition between three different magnetic energy scales including the exchange energy between Ir electrons and two Zeeman energies, various topological semimetals can be generated near QCP. Moreover, we have shown that these three magnetic energy scales can be controlled by modulating the magnetic multipole moment (MMM) of the cluster of spins in a unit cell, which can couple to the intrinsic MMM of the degenerate states at QBC. We propose the general topological band structure under magnetic field achievable near QCP, which would facilitate the experimental discovery of novel topological semimetal states in pyrochlore iridates.

We have studied long-range orders of electric quadrupole moments described by an effective pseudospin-$\frac{1}{2}$ Hamiltonian representing pyrochlore magnets with non-Kramers ions under [111] magnetic field, in relevance to ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. Order parameters and phase transitions of this frustrated system are investigated using classical Monte Carlo simulations. In zero field, the model undergoes a first-order phase transition from a paramagnetic state to an ordered state with an antiparallel arrangement of pseudospins. This pseudospin order is characterized by the wave vector $\mathit{k}=0$ and is selected by an energetic or an order-by-disorder mechanism from degenerate $\mathit{k}=(h,h,h)$ mean-field orders. Under [111] magnetic field this three-dimensional quadrupole order is transformed to a quasi-two-dimensional quadrupole order on each kagome lattice separated by field-induced ferromagnetic triangular lattices. We discuss the implication of the simulation results with respect to experimental data of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$.

Recent observation of intrinsic ferromagnetism in two-dimensional (2D) ${\mathrm{CrI}}_3$ is associated with the large magnetic anisotropy due to strong spin-orbit coupling of I. Magnetic anisotropy energy (MAE) defines the stability of magnetization in a specific direction with respect to the crystal lattice and is an important parameter for nanoscale applications. In this work we apply the density functional theory to study the strain dependence of MAE in 2D monolayer chromium trihalides $\mathrm{Cr}{X}_3$ (with $X$ = Cl, Br, and I). Detailed calculations of their energetics, atomic structures, and electronic structures under the influence of a biaxial strain $\varepsilon$ have been carried out. It is found that all three compounds exhibit ferromagnetic ordering at the ground state (with $\varepsilon =0$), and upon applying a compressive strain, phase transition to the antiferromagnetic state occurs. Unlike in ${\mathrm{CrCl}}_3$ and ${\mathrm{CrBr}}_3$, the electronic band gap in ${\mathrm{CrI}}_3$ increases when a tensile strain is applied. The MAE also exhibits a strain dependence in the chromium trihalides: it increases when a compressive strain is applied in ${\mathrm{CrI}}_3$, while an opposite trend is observed in the other two compounds. In particular, the MAE of ${\mathrm{CrI}}_3$ can be increased by 47% with a compressive strain of $\varepsilon$ = 5%.

Using Wannier function formulation and total energy calculations by first-principles density functional theory (DFT), we derive the underlying spin model of a recently synthesized compound, ${\mathrm{Cs}}_2{\mathrm{CuAl}}_4{\mathrm{O}}_8$, having zeolitelike network structure. The computed magnetic interactions show that the interchain Cu-Cu interactions are negligibly small compared to intrachain Cu-Cu interactions, thus characterizing ${\mathrm{Cs}}_2{\mathrm{CuAl}}_4{\mathrm{O}}_8$ as a prototypical one-dimensional (1D) spin-$\frac{1}{2}$ system. Interestingly, the DFT-derived 1D spin model features a combination of alternating ferromagnetic-antiferromagnetic interactions, together with the presence of both nearest- and next-nearest-neighbor interactions, making it an unprecedented case. The solution of the derived spin model of the compound using the quantum Monte Carlo technique shows reasonably good agreement with the experimental susceptibility data, measured in the presence of the magnetic field. The presence of spin gap is suggested by quantum Monte Carlo simulations in the zero-field condition, which is cross-checked by a more rigorous exact diagonalization study. Motivated by the intricacy of the derived spin model, we further examine the ground-state properties of this model in the parameter space of exchange interactions, which shows the possibility of driving quantum phase transition between gapped and gapless spin excitation. Our study is expected to shed light on the fascinating world of 1D quantum spin systems.

The high-temperature superconductivity in copper oxides emerges under strong influence of spin correlations in doped Mott insulators. Recent discoveries of charge-order (CO) correlations in Y-based hole-doped cuprates as well as in electron-doped cuprates suggest that charge correlations should also play an important role on the electronic states of cuprates, although those correlations have been so far detected mainly by x-ray scattering measurements. Here we show signatures of CO correlations in transport properties of electron-doped cuprates as an anomalous enhancement of the metal-to-insulator crossover temperature ($T_{min}$), which appears in the limited doping range near the onset of superconductivity, while it decreases exactly when superconductivity sets in. We explain this nonmonotonous peak-like behavior of $T_{min}$ in terms of the evolution of the electronic states through development of CO correlations and appearance of the hole pockets in the folded Fermi surface, which impact on transport properties consecutively at different locations in the momentum space.

${\mathrm{SrMnSb}}_2$ is a candidate Dirac semimetal whose electrons near the $Y$ point have the linear dispersion and low mass of a Dirac cone. Here we demonstrate that ultrafast, 800-nm optical pulses can launch coherent phonon oscillations in ${\mathrm{Sr}}_{0.94}{\mathrm{Mn}}_{0.92}{\mathrm{Sb}}_2$, particularly an $A_g$ mode at 4.4 THz. Through first-principles calculations of the electronic and phononic structure of ${\mathrm{SrMnSb}}_2$, we show that high-amplitude oscillations of this mode would displace the atoms in a way that transiently opens and closes a gap at the node of the Dirac cone. The ability to control the nodal gap on a subpicosecond timescale could create opportunities for the design and manipulation of Dirac fermions.

Electric and thermoelectric transport properties are mutually intertangled in diffusive transport equations. In particular, in high mobility multiband systems an anomalous behavior may occur, which can be tracked down to the properties of the individual bands. Here, we present magnetoelectric and magnetothermoelectric transport properties of a ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ high-quality single crystal, for different magnetic field directions up to 30 T. We detect a giant Nernst effect and an anomalous field dependence of the Seebeck coefficient. The extraction of the Peltier tensor coefficients ${\alpha }_{xx}, {\alpha }_{xy}$, and ${\alpha }_{xz}$ allows us to disentangle the main transport mechanisms in play. The large ${\alpha }_{xy}$ and ${\alpha }_{xz}$ values and their field dependence provide evidence of the presence of a high mobility band, compatible with a Dirac dispersion band, crossing the Fermi level, and suggest a possible three-dimensional nature of the Dirac fermions.

We apply the dual fermion approach with a second-order approximation to the self-energy to the Mott transition in the two-dimensional Hubbard model. The approximation captures nonlocal dynamical short-range correlations as well as several features observed in studies using cluster dynamical mean-field theory. This includes a strong reduction of the critical interaction and inversion of the slope of the transition lines with respect to single-site dynamical mean-field theory. We show that these effects coincide with a much smaller momentum differentiation compared to cluster methods. We further discuss the role of the self-consistency condition and show that the approximation behaves as an asymptotic series at low temperature.

Hexagonal boron nitride (h-BN) is an attractive van der Waals material for studying fluorescent defects due to its large band gap. In this work, we demonstrate enhanced pink color due to neutron irradiation and perform electron paramagnetic resonance (EPR) measurements. The point defects are tentatively assigned to doubly occupied nitrogen vacancies with ($S=1$) and a zero-field splitting ($D=1.2\mathrm{GHz}$). These defects are associated with a broad visible optical absorption band and a near-infrared photoluminescence band centered at ∼490 and 820 nm, respectively. The EPR signal intensities are strongly affected by thermal treatments in the temperature range between 600 °C and 800 °C, where also the irradiation-induced pink color is lost. Our results are important for understanding of point defects in h-BN and their deployment for quantum and integrated photonic applications.

We argue that macroscopic electrodynamics is unsuited to describe the process of radiative thermalization between a levitated nanoparticle in high vacuum and the thermal electromagnetic field. Based on physical arguments, we propose a model to describe such systems beyond the quasiequilibrium approximation. We use path integral techniques to analytically solve the model and exactly calculate the time evolution of the quantum degrees of freedom of the system. Free parameters of the microscopic quantum model are determined by matching analytical results to well-known macroscopic response functions. The time evolution of the internal energy of a levitated nanoparticle in a thermal electromagnetic field, as described by our model, qualitatively differs from macroscopic electrodynamics, a prediction that can be experimentally tested.

For the generation of spin-polarized photocurrents in topological insulators, a coupling between photon angular momentum and electron spin is often assumed. Such a coupling seems to be supported by dichroism reported in $E\left(k_y\right)$-intensity maps in photoemission. We show in three dimensional two-photon photoemission and one-step photoemission calculations that the circular dichroism is in fact threefold in $E(k_x,k_y)$ maps although it may appear antisymmetric in $E\left(k_y\right)$. The threefold symmetry is inconsistent with the previously assumed coupling between photon momentum and electron's chiral spin via the orbital momentum. Instead it reflects the surface point group. The only antisymmetric patterns appear in the energy range in which surface and bulk states hybridize. In general, a threefold-symmetric dichroic signal does not support unidirectional photocurrents. Nevertheless, the residual asymmetry of up to 3.5% in our photoemission spectra is compatible with previously observed helicity-dependent photocurrents.

We present a combination of quantum Monte Carlo methods and a finite-size extrapolation framework with which we calculate the thermodynamic limit of the exact correlation energy of the polarized electron gas at high densities to meV accuracy, $-40.44\left(5\right)$ and $-31.70\left(4\right)$ mHa at $r_{\mathrm{s}}=0.5$ and 1, respectively. The fixed-node error is characterized and found to exceed 1 mHa, and we show that the magnitude of the correlation energy of the polarized electron gas is underestimated by up to 6 meV by the Perdew-Wang parametrization, for which we suggest improvements.

One promising avenue to study one-dimensional (1D) topological phases is to realize them in synthetic materials such as cold atomic gases. Intriguingly, it is possible to realize Majorana boundary modes in a 1D number-conserving system consisting of two fermionic chains coupled only by pair-hopping processes [C. V. Kraus et al., Phys. Rev. Lett. 111, 173004 (2013)]. It is commonly believed that significant interchain single-particle tunneling necessarily destroys these Majorana modes, as it spoils the ${\mathbb{Z}}_2$ fermion-parity symmetry that protects them. In this Rapid Communication, we present a mechanism to overcome this obstacle, by piercing a (synthetic) magnetic $\pi$ flux through each plaquette of the Fermi ladder. Using bosonization, we show that in this case there exists an exact leg-interchange symmetry that is robust to interchain hopping, and acts as fermion parity at long wavelengths. We utilize density matrix renormalization group and exact diagonalization to verify that the resulting model exhibits Majorana boundary modes up to large single-particle tunnelings, comparable to the intrachain hopping strength. Our work highlights the unusual impacts of different topologically trivial band structures on these interaction-driven topological phases, and identifies a distinct route to stabilizing Majorana boundary modes in 1D fermionic ladders.

We conduct photoluminescence microscopy that is sensitive to both electron and nuclear spin polarization to investigate the changes that occur in the magnetic ordering in the vicinity of the first integer quantum Hall state in a GaAs two-dimensional electron system (2DES). We observe a discontinuity in the electron spin polarization and nuclear spin longitudinal relaxation time which heralds a spontaneous transition in the magnetic ordering. We image in real space the spin phase domains that coexist at this transition, and observe hysteresis in their formation as a function of the 2DES's chemical potential. Based on measurements in a tilted magnetic field orientation, we found that the transition is protected by an energy gap containing the Zeeman energy. We explain that these observations are consistent with a phase of skyrmions forming at the transition.

The phase diagram of correlated systems like cuprate or pnictide high-temperature superconductors is likely defined by a topological change of the Fermi surface under continuous variation of an external parameter, the so-called Lifshitz transition. However, a number of low-temperature instabilities and the interplay of multiple energy scales complicate the study of this phenomenon. Here we identify the optical signatures of a pressure-induced Lifshitz transition in a clean elemental system, black phosphorus. By applying external pressures above 1.5 GPa, we observe a change in the pressure-dependent Drude plasma frequency due to the appearance of massless Dirac fermions. At higher pressures, optical signatures of two structural phase transitions are also identified. Our findings suggest that a key fingerprint of the Lifshitz transition, in the absence of structural transitions, is a Drude plasma frequency discontinuity due to a change in the Fermi surface topology.

The persistent spin helix (PSH) state in III–V semiconductor quantum wells (QWs) is a promising candidate for spin-based applications because the PSH state realizes controllable spin orientation with long spin lifetime. Although the cubic Dresselhaus spin-orbit (SO) interaction is known for breaking the PSH state in both (001)-oriented and (110)-oriented QWs, it is not well understood how the distinct symmetry of cubic Dresselhaus terms ${\beta }_3$ between (001) and (110) QWs affects the robustness of the PSH state. Here we investigate robustness of the PSH state between (001) and (110) QWs under various strengths of cubic Dresselhaus SO interaction based on numerical Monte Carlo approach and magnetoconductance simulation, respectively, representing optical spin excitation/detection and weak localization/weak antilocalization in magnetotransport. For electron spins initialized along $z\parallel \left[001\right]$ in a (001) QW and $x\parallel \left[001\right]\mathrm{and}y\parallel \left[1\overline{1}0\right]$ in a (110) QW, where the spin distribution is developed with the helical spin mode, a (001) QW shows a more robust PSH state against the increase of ${\beta }_3$ than does a (110) QW. This phenomenon is contrary to numerically computed magnetoconductance, where weak localization is maintained on the variation of ${\beta }_3$ in a (110) QW, whereas weak antilocalization appears in a (001) QW. By deriving the spin lifetime of the PSH state in a (110) QW from a diffusion equation using a random-walk approach, we demonstrate that such a difference arises directly from the magnitude and orientation of third angular harmonics in cubic Dresselhaus fields for (001) and (110) QWs.

Highly excited states of excitons in cuprous oxide have recently been observed at a record quantum number of up to $n=25$. Here, we evaluate the long-range interactions between pairs of Rydberg excitons in ${\mathrm{Cu}}_2\mathrm{O}$, which are due to direct Coulomb forces rather than short-range collisions typically considered for ground-state excitons. A full numerical analysis is supplemented by the van der Waals asymptotics at large exciton separations, including the angular dependence of the potential surfaces.

With recent experiments investigating the optical properties of progressively smaller plasmonic particles, quantum effects become increasingly more relevant, requiring a microscopic description. Using the density matrix formalism we analyze the photoexcited few-electron dynamics of a small plasmonic nanosphere. Following the standard derivation of the bulk plasmon we particularly aim at elucidating the role of the Coulomb interaction. Calculating the dielectric susceptibility spectrum in the linear optical response we find discrete resonances resulting from a collective response mediated by the Coulomb interaction between the electrons. In the nonlinear regime, the occupations of the system exhibit oscillations between the interacting eigenstates. Our approach provides an ideal platform to study and explain nonlinear and quantum plasmonics, revealing that the photoexcited dynamics of plasmonic nanospheres has similarities with and combines characteristics of both the well-known two-level Rabi dynamics and the collective many-electron behavior typical of plasmons.