We employ polarization-resolved Raman spectroscopy to study the multiband stoichiometric $T_c=35$ K superconductor ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$. We do not detect Pomeranchuk-like electronic nematic fluctuations which were universally observed in the $XY\left(B_{2g}\right)$ symmetry Raman response for most families of the Fe-based superconductors. In the superconducting state we observe, consistent with a nodeless order parameter, a full spectral weight suppression at low energies and the emergence of a composite pair-breaking coherence feature at energies between 12 and 20 meV. We analyze the superconductivity-induced phonon self-energy effects and give an estimation for the electron-phonon coupling constant ${\lambda }^{\mathrm{\Gamma }}=0.0015$ which is insufficient to provide attraction for high-$T_c$ pairing.

We have performed high-resolution angle-resolved photoemission spectroscopy of the quasi-one-dimensional (1D) topological superconductor candidate ${\mathrm{Tl}}_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$ consisting of weakly coupled ${\mathrm{Mo}}_3{\mathrm{Se}}_3$ chains. We found a quasi-1D Fermi surface arising from a Dirac-like energy band, which is associated with the nonsymmorphic screw symmetry of the chains and predicted to trigger topological superconductivity. We observed a significant spectral-weight reduction over a wide energy range, together with a tiny Fermi-edge structure which exhibits a signature of a superconducting-gap opening below the superconducting-transition temperature. The observed quasi-1D Dirac-like band and its very small density of states point to an unconventional nature of superconductivity in ${\mathrm{Tl}}_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$.

The method of numerical analysis of experimental ferromagnetic resonance (FMR) spectra in thin films is developed and applied to (Ga,Mn)As thin films. Specifically, it starts with the finding of numerical solutions of Smit-Beljers (SB) equation and continues with their subsequent statistical analysis within the cross-validation (CV) approach taken from machine learning techniques. As a result of this treatment, we are able to reinterpret the available FMR experimental results in diluted ferromagnetic semiconductor (Ga,Mn)As thin films with the resulting determination of magnetocrystalline anisotropy constants. The outcome of CV analysis points out that it is necessary to take into account terms describing the bulk cubic anisotropy up to the fourth order to reproduce FMR experimental results for (Ga,Mn)As correctly. This finding contradicts the widespread conviction in the literature that only the first-order cubic anisotropy term is important in this material. We also provide numerical values of these higher order cubic anisotropy constants for (Ga,Mn)As thin films resulting from the SB-CV approach.

We obtained, within a path-integral formalism (mean-field variational Gaussian approximation), analytical expressions for thermodynamic quantities such as magnetization, heat capacity, and the magnetic Grüneisen parameter ${\mathrm{\Gamma }}_H$ of the system of triplons in spin gapped quantum magnets. Near the critical temperature ${\mathrm{\Gamma }}_H$ is discontinuous and changes its sign upon the Bose-Einstein condensation (BEC) of triplons. We predict that in the limit of low temperature $T$ and near the critical magnetic field $H_c,{\mathrm{\Gamma }}_H$ diverges as ${\mathrm{\Gamma }}_H\sim 1/T^2$, while it scales as ${\mathrm{\Gamma }}_H\sim 1/(H-H_c)$ as the magnetic field approaches $H_c$.

One of the most important discoveries in spintronics is the spin Seebeck effect (SSE), recently observed in both insulating and (semi)conducting magnets. However, the very existence of the effect in transverse configuration is still a subject of current debate. In this paper, motivated by the concept and the formulation of the mode-dependent magnon temperature introduced recently [Yan et al., Phys. Rev. B 95, 024417 (2017)], we develop a wave theory to explain the SSE by highlighting the interplay between wave localization and turbulence. We show that the emerging SSE with a sign change in the high/low-temperature regions is closely related to the extendedness of the spin wave that senses an average temperature of the system. On the one hand, ubiquitous disorders (or magnetic field gradients) can strongly suppress the transverse spin Seebeck effect (TSSE) due to the Anderson (or Wannier-Zeeman) spin-wave localization. On the other hand, the competing spin-wave turbulence from the magnetic anisotropy tends to delocalize the wave, and thus remarkably revives the TSSE before the magnon self-trapping. Our findings reveal the significant role of the magnetic anisotropy played in SSE and provide an appealing way to enhance the TSSE signal by choosing proper materials and sample shape.

Using Cu-$L_3$ edge resonant inelastic x-ray scattering (RIXS) we measured the dispersion and damping of spin excitations (magnons and paramagnons) in the high-$T_{\mathrm{c}}$ superconductor ${\left(\text{Bi,Pb}\right)}_2{\left(\text{Sr,La}\right)}_2{\mathrm{CuO}}_{6+\delta }$ (Bi2201), for a large doping range across the phase diagram $\left(0.03\lesssim p\lesssim 0.21\right)$. Selected measurements with full polarization analysis unambiguously demonstrate the spin-flip character of these excitations, even in the overdoped sample. We find that the undamped frequencies increase slightly with doping for all accessible momenta, while the damping grows rapidly, faster in the $(0,0)\to (0.5,0.5)$ nodal direction than in the $(0,0)\to (0.5,0)$ antinodal direction. We compare the experimental results to numerically exact determinant quantum Monte Carlo (DQMC) calculations that provide the spin dynamical structure factor $S(\mathbf{Q},\omega )$ of the three-band Hubbard model. The theory reproduces well the momentum and doping dependence of the dispersions and spectral weights of magnetic excitations. These results provide compelling evidence that paramagnons, although increasingly damped, persist across the superconducting dome of the cuprate phase diagram; this implies that long-range antiferromagnetic correlations are quickly washed away, while short-range magnetic interactions are little affected by doping.

We introduce a frustration-free, one-dimensional model of spinless fermions with hopping, $p$-wave superconducting pairing and alternating chemical potentials. The model possesses two exactly degenerate ground states even for finite system sizes. We present analytical results for the strong Majorana zero modes, the phase diagram, and the topological order. Furthermore, we generalize our results to include interactions.

We study quantum dots defined electrostatically in silicene. We determine the spin-valley structure of confined single- and two-electron systems, and quantify the effects of the intervalley scattering by the electron-electron interaction potential and the crystal edge. The double quantum dots are discussed in the context of the spatial symmetry of the extended orbitals. We determine the charge, spin, and valley transitions induced by alternate electric fields. We show that the valley transition times can be changed within several orders of magnitude by tuning the confinement potential and the coupling of the confined states to the edge of the flake. The spin transition rates can be enhanced by orders of magnitude by the coupling of the bonding and antibonding orbitals mediated by the Rashba spin-orbit interaction.

In 1964, W. E. Lamb introduced a mode-coupling constant $C$ to characterize the stability of a dual-mode laser. Considering quantum-dot semiconductor lasers, we calculate analytically $C$ in the framework of a rate-equation model, which includes both the homogeneous broadening of the quantum-dot emission and the dot-to-dot carrier exchange due to wetting-layer-assisted lateral coupling. Although first established using fully symmetric laser parameters for both modes, this result is then extended numerically to nonsymmetric parameters and shows that $C$ remains unchanged when the gain/losses are adjusted so that the two laser modes are brought to oscillate simultaneously. Finally, $C$ is shown to depend on two parameters only encompassing the pumping, the gain material mainly through the homogeneous broadening and the dot-to-dot carrier exchange, and the cavity design. Above laser threshold, the analytic result predicts a stable dual-mode behavior whatever the conditions but with a margin that decreases drastically close to lasing threshold or at small beating frequencies.

The collective excitation spectrum of two-dimensional (2D) antimonene is calculated beyond the low-energy continuum approximation. The dynamical polarizability is computed using a six-orbital tight-binding model that properly accounts for the band structure of antimonene in a broad energy range. Electron-electron interaction is considered within the random phase approximation. The obtained spectrum is rich, containing the standard intraband 2D plasmon and a set of single interband modes. We find that spin-orbit interaction plays a fundamental role in the reconstruction of the excitation spectrum, with the emergence of novel interband branches in the continuum that interact with the plasmon.

Quasiparticle excitations beyond the Dirac-Weyl-Majorana classification can appear in lattice systems due to a lesser symmetry constraint compared with Poincaré symmetry in high-energy physics. In particular, fermions with an integer spin can appear in a variety of lattices. Here, we show that two-dimensional spin-1 fermions may coexist with Dirac fermions in the triangular kagome lattice (TKL). We derive a four-band effective model that hosts both types of fermions. The effective model can be used to study the interplay between spin-1 and spin-1/2 fermions. As an example, using this model we show that spin-nonconserving Klein tunneling can occur in the TKL, which has a transmission coefficient $T=1$ for normal incidence. Our findings pave a way to the study of the interaction and interplay between different types of fermions in lattice systems.

The second-order conductivity of a material, ${\sigma }^{\left(2\right)}$, relating current to the square of electric field, is nonzero only when inversion symmetry is broken, unlike the conventional linear conductivity. Second-order nonlinear optical responses are thus powerful tools in basic research as probes of symmetry breaking; they are also central to optical technology as the basis for generating photocurrents and frequency doubling. The recent surge of interest in Weyl semimetals with acentric crystal structures has led to the discovery of a host of ${\sigma }^{\left(2\right)}$-related phenomena in this class of materials, such as polarization-selective conversion of light to dc current (photogalvanic effects) and the observation of giant second-harmonic generation (SHG) efficiency in TaAs at photon energy 1.5 eV. Here, we present measurements of the SHG spectrum of TaAs, revealing that the response at 1.5 eV corresponds to the high-energy tail of a resonance at 0.7 eV, at which point the second harmonic conductivity is approximately 200 times larger than seen in the standard candle nonlinear crystal, GaAs. This remarkably large SHG response provokes the question of ultimate limits on ${\sigma }^{\left(2\right)}$, which we address by a new theorem relating frequency-integrated nonlinear response functions to the third cumulant (or “skewness”) of the polarization distribution function in the ground state.

Understanding complex correlated oxides and their phase transitions has long been a challenge. The difficulty largely arises from the intriguing interplay between multiple degrees of freedoms. While degeneracy can play an important role in determining material characteristics, there is no well-defined way to quantify and to unveil its role in real materials having complicated band structures. Here we suggest a way to quantify the “effective degeneracy” relevant to metal-insulator transition by introducing entropylike terms. This new quantity well describes the electronic behaviors of transition-metal oxides as a function of external and internal parameters. With $3d$ titanates, $4d$ ruthenates, and $5d$ iridates as our examples, we show that this new effective quantity provides useful insights to understand these systems and their phase transitions. For ${\mathrm{LaTiO}}_3/{\mathrm{LaAlO}}_3$ superlattice, we suggest a novel “degeneracy control” metal-insulator transition.

Due to the absence of local order parameters, it is a challenging task to characterize the quantum topological phase transitions between topologically ordered phases in two dimensions. In this paper, we construct a topologically ordered tensor network wave function with one parameter $\lambda$, describing both the toric code state $\left(\lambda =1\right)$ and double semion state $\left(\lambda =-1\right)$. Via calculating the correlation length defined from the one-dimensional quantum transfer operator of the wave function norm, we can map out the complete phase diagram in terms of the parameter $\lambda$, and three different quantum critical points (QCPs) at $\lambda =0,\pm 1.73$ are identified. The first one separates the toric code phase and double semion phase, while latter two describe the topological phase transitions from the toric code phase or double semion phase to the symmetry-breaking phase, respectively. When mapping the quantum tensor network wave function to the exactly solved statistical model, the norm of the wave function is identified as the partition function of the classical eight-vertex model, and both QCPs at $\lambda =\pm 1.73$ correspond to the eight-vertex model at the critical point $\lambda =\sqrt{3}$, while the QCP at $\lambda =0$ corresponds to the critical six-vertex model. Actually such a quantum-classical mapping cannot yield the complete low-energy excitations at these three QCPs. We further demonstrate that the full eigenvalue spectra of the transfer operators without/with the flux insertions can give rise to the complete quantum criticalities, which are described by the (2+0)-dimensional free boson conformal field theories (CFTs) compactified on a circle with the radius $R=\sqrt{6}$ at $\lambda =\pm \sqrt{3}$ and $R=\sqrt{8/3}$ at $\lambda =0$. From the complete transfer operator spectra, the finite-size spectra of the CFTs for the critical eight-vertex model are obtained, and the topological sectors of anyonic excitations are yielded as well. Furthermore, for the QCP at $\lambda =0$, no anyon condensation occurs, but the emerged symmetries of the matrix product operators significantly enrich the topological sectors of the CFT spectra. Finally, we provide our understanding of the (2+0)-dimensional conformal quantum criticalities and their possible connection with the generic (2+1)-dimensional CFTs for quantum topological phase transitions.

The nature and extent of the spin-entanglement in the triplet-triplet biexciton with total spin zero in correlated-electron $\pi$-conjugated systems continues to be an enigma. Differences in the ultrafast transient absorption spectra of free triplets versus the triplet-triplet can give a measure of the entanglement. This, however, requires theoretical understandings of transient absorptions from the optical spin-singlet, the lowest spin-triplet exciton, as well as from the triplet-triplet state, whose spectra are often overlapping and hence difficult to distinguish. We present a many-electron theory of the electronic structure of the triplet-triplet, and of complete wavelength-dependent excited state absorptions (ESAs) from all three states in a heteroacene dimer of interest in the field of intramolecular singlet fission. The theory allows direct comparisons of ESAs with existing experiments as well as experimental predictions, and gives physical understandings of transient absorptions within a pictorial exciton basis that can be carried over to other experimental systems.

We report a single-crystal neutron diffraction study of the magnetic structure of the multiferroic compound ${\mathrm{YbMnO}}_3$, a member of the hexagonal manganite family, in zero field and under a magnetic field applied along the $c$ axis. We propose a scenario for the zero-field magnetic ordering and for the field-induced magnetic reorientation of the Mn atom and of the two Yb atoms on distinct crystallographic sites, compatible with the macroscopic measurements, as well as with previous powder neutron diffraction experiments and results from other techniques (optical second-harmonic generation and Mössbauer spectroscopy). Our study should contribute to settling some debated issues regarding the magnetic properties of this material as part of a broader investigation of the entire hexagonal $R{\mathrm{MnO}}_3(R$ = Dy, Ho, Er, Tm, Yb, Lu, Y) family.

Neutron inelastic scattering has been used to measure the magnetic excitations in powdered ${\mathrm{NiPS}}_3$, a quasi-two-dimensional antiferromagnet with spin $S=1$ on a honeycomb lattice. The spectra show clear, dispersive magnons with a $\sim 7$ meV gap at the Brillouin zone center. The data were fitted using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb planes. Magnetic exchange interactions up to the third intraplanar nearest neighbor were required. The fits show robustly that ${\mathrm{NiPS}}_3$ has an easy-axis anisotropy with $\mathrm{\Delta }=0.3$ meV and that the third nearest neighbor has a strong antiferromagnetic exchange of $J_3=-6.90$ meV. The data can be fitted reasonably well with either $J_1<0$ or $J_1>0$, however, the best quantitative agreement with high-resolution data indicates that the nearest-neighbor interaction is ferromagnetic with $J_1=1.9$ meV and that the second nearest-neighbor exchange is small and antiferromagnetic with $J_2=-0.1$ meV. The dispersion has a minimum in the Brillouin zone corner that is slightly larger than that at the Brillouin zone center, indicating that the magnetic structure of ${\mathrm{NiPS}}_3$ is close to being unstable.

A single-channel quantum wire with two well-separated Zeeman subbands and in the presence of weak spin-orbit coupling is considered. An impurity level which is split off the upper subband is degenerate with the continuum of the lower subband. We show that, when the Fermi level lies in the vicinity of the impurity level, the transport is completely blocked. This is the manifestation of the effect of resonant reflection and can be viewed as resonant tunneling between left-moving and right-moving electrons via the impurity level. We incorporate electron-electron interactions and study their effect on the shape of the resonant-reflection profile. This profile becomes a two-peak structure, where one peak is caused by resonant reflection itself, while the origin of the other peak is reflection from the Friedel oscillations of the electron density surrounding the impurity.