We present a theoretical study on the Landau levels (LLs) and magneto-optical absorption of a two-dimensional semi-Dirac electron system under a perpendicular magnetic field. Based on an effective k$·\mathrm{p}$ Hamiltonian, we find that the LLs are proportional to the two-thirds power law of the magnetic field and level index, which can be understood as a hybridization of the LL of Schrödinger and Dirac electrons with new features. With the help of the Kubo formula, we find the selection rule for interband (intraband) magneto-optical transition is anisotropic (isotropic). Specifically, the selection rules for interband magneto-optical transitions are $\mathrm{\Delta }n=0$, $\pm 2$ $(\pm 2,$ $\pm 4)$ for linearly polarized light along the linear (parabolic) dispersion direction, while the selection rules for the intraband transition are $\mathrm{\Delta }n=\pm 1,$ $\pm 3$ regardless of the polarization direction of the light. Further, the magneto-optical conductivity for interband (intraband) transition excited by linearly polarized light along the linear dispersion direction is two (one) orders of magnitude larger than that along the parabolic dispersion direction. This anisotropic magneto-optical absorption spectrum clearly reflects the structure of the LLs, and results in a strong linear dichroism. Importantly, a perfect linear dichroism with magnetic-field tunable wavelength can be achieved by using the interband transition between the two lowest LLs, i.e., from $E_{v0}$ to $E_{c0}$. Our results shed light on the magneto-optical property of the two-dimensional semi-Dirac electron systems and pave the way to design magnetically controlled optical devices.

We provide a systematic analysis of finite-temperature magnetic properties of random alloys ${\mathrm{Fe}}_x{\mathrm{Ni}}_{1-x}$ with the face-centered-cubic structure over a broad concentration range $x$. By means of the spin-polarized relativistic Korringa-Kohn-Rostoker method we calculate the electronic structure of disordered iron-nickel alloys and discuss how a composition change affects magnetic moments of Fe and Ni and the density of states. We investigate how the Curie temperature depends on Fe concentration using conventional approaches, such as mean-field approximation or Monte Carlo simulations, and dynamic spin-fluctuation theory. Being devised to account for spin fluctuations explicitly, the latter method shows the best fit to experimental results.

We study third-harmonic generation (THG) in an excitonic insulator (EI) described in a two-band correlated electron model. Employing the perturbative expansion with respect to the external electric field, we derive the THG susceptibility taking into account the collective dynamics of the excitonic order parameter. In the inversion-symmetric EI, the collective order parameter motion is activated at second order of the external field and its effects arise in THG. We find three peaks in the THG susceptibility at energies $\hslash \mathrm{\Omega }={\mathrm{\Delta }}_g/3$, ${\mathrm{\Delta }}_g/2$, and ${\mathrm{\Delta }}_g$, where ${\mathrm{\Delta }}_g$ is the band gap. While the THG response at ${\mathrm{\Delta }}_g/3$ is caused by bare three-photon excitation of the independent particle across the band gap, the latter two peaks involve the motion of the order parameter activated at second order. The resulting resonant peaks are prominent in particular in the BCS regime but they become less significant in the BEC regime. We demonstrate that the resonant peak originated by the collective excitation is observable in the temperature profile of the THG intensity. Our study suggests that the THG measurement should be promising for detecting the excitonic collective nature of materials.

The spinel-type ${\mathrm{LiV}}_2{\mathrm{O}}_4$ exhibits heavy-fermion metallic behaviors at low temperatures, but the effect of composition modulation on them is unknown. We realized the epitaxial growth of highly crystalline ${\mathrm{LiV}}_2{\mathrm{O}}_4$ films and conducted the electrochemical Li-ion intercalation (electron doping) to investigate systematic variation of transport properties in ${\mathrm{Li}}_{1+x}{\mathrm{V}}_2{\mathrm{O}}_4$. We found that adjustment of Li content during pulsed-laser deposition was important to obtain films exhibiting the heavy-fermion metallic behavior comparable to that of a bulk single crystal. At low doping regime $(x\le 0.5)$, resistivity increased with increasing electron filling. The slope of their linear $T^2$ dependence also increased in accordance of a Fermi-liquid model. At high doping regime $\left(x>0.5\right)$, a Mott transition was observed over a range of Li content where a new spinel phase and the original one coexisted. The results revealed that the electron doping induced enhancement of the electron correlation and the resultant Mott transition.

We study ultrafast relaxation of nonthermal electrons confined within the Γ valley of GaAs, based on the momentum-resolved transient electron distribution functions determined using time- and angle-resolved photoemission spectroscopy at 293 K. To elucidate the fundamental processes that lead to the electronic quasithermalization at the nonthermal regime of relaxation, the dynamics in both $p$- and $n$-type samples of GaAs are investigated. In $p$-type GaAs, photoinjected electrons form the electron ensembles quasiequilibrated only in the momentum space within 100 fs of excitation by the Coulomb interaction among photoinjected electrons. However, the ensembles maintain strong nonthermal populations in the energy space. They are quasithermalized only after certain time delays of a few hundred femtoseconds, which depend on the excess energy of photoinjected electrons. These features in $p$-type samples are not sensitively dependent on the excitation density in the range from $1.5\times {10}^{16}$ to $5\times {10}^{17}\mathrm{c}{\mathrm{m}}^{\text{–}3}$. On the other hand, in $n$-type samples, which include the cold electrons populated near the conduction-band minimum with the density of $\sim 1\times {10}^{18}\mathrm{c}{\mathrm{m}}^{\text{–}3}$, photoinjected electrons are quasithermalized within 170 fs of excitation. The main interaction for the fast quasithermalization is attributed to the inelastic scattering among the cold electrons and photoinjected electrons by a dynamics screened Coulomb interaction like the electronic thermalization in metals.

Symmetry-protected topological order in one dimension leads to protected degeneracies between symmetry blocks of the reduced density matrix. In the presence of periodic driving, topological Floquet phases can be identified in terms of a cycling of these symmetry blocks between different charge quantum numbers. We discuss an example of this phenomenon with an Ising ${\mathbb{Z}}_2$ symmetry, using both analytic methods and real quantum computers. By adiabatically moving along the phase diagram, we demonstrate that the cycling periodicity is broken in Floquet topological phase transitions. An equivalent signature of the topological Floquet phase is identified as a computational power allowing for the teleportation of quantum information.

The minerals $A_2{\mathrm{Cu}}_3\mathrm{O}{\left({\mathrm{SO}}_4\right)}_3(A_2={\mathrm{Na}}_2,\mathrm{NaK},{\mathrm{K}}_2)$ constitute quantum spin systems with copper hexamers as basic structural units. Strong intrahexamer spin couplings give rise to an effective triplet ground state. Weak interhexamer spin couplings are responsible for two-dimensional long-range magnetic order in the ($b,c$) plane below $3.0<T_c<4.7\mathrm{K}$. We investigated the magnetic excitations at $T=1.5$ K by inelastic neutron scattering (INS). The INS technique was based on the observation of wave vector dependent slices in reciprocal space in order to selectively probe the magnetic signals in different Brillouin zones with different weight. Due to the imbalance of the spin couplings, the data analysis relies on a model in which the interhexamer spin couplings are treated perturbatively on top of the exact $S=1$ ground state. The interhexamer spin couplings turn out to be ferromagnetic.

${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ (MBT) is a promising van der Waals layered antiferromagnetic (AFM) topological insulator that combines a topologically nontrivial inverted Bi-Te band gap with ferromagnetic (FM) layers of Mn ions. The inelastic neutron scattering on single crystals reported here describes rather complex magnetism in MBT. The magnetic anisotropy that controls the bulk and surface magnetic field response of MBT is found to have contributions from both single-ion and interlayer two-ion terms. A description of the quasi-two-dimensional intralayer FM spin waves requires long-range, competing FM and AFM interactions and anomalous damping. While this might suggest carrier-mediated magnetic coupling, $\mathit{ab}\mathit{initio}$ calculations in insulating MBT also find long-range interactions, and classical spin dynamics simulations suggest that magnetic vacancies are at least partially responsible for observations of anomalous damping near the zone boundary.

We investigate the thermodynamic properties of the ytterbium-based triangular lattice compound ${\mathrm{Ba}}_3\mathrm{Yb}{\left(\mathrm{B}{\mathrm{O}}_3\right)}_3$. The results demonstrate the absence of any long-range ordering down to 56 mK. Analysis of the magnetization, susceptibility, and specific heat measurements suggests that ${\mathrm{Ba}}_3\mathrm{Yb}{\left(\mathrm{B}{\mathrm{O}}_3\right)}_3$ may realize an $S=\frac{1}{2}$ quantum dipole lattice, in which the dominant interaction is the long-range dipole-dipole coupling on the geometrically frustrated triangular lattice, and exchange interactions are subdominant or negligible.

Nematic superconductors are characterized by an apparent crystal symmetry breaking that results in the anisotropy of the in-plane upper critical magnetic field $H_{c2}$. The symmetry breaking is usually attributed to the strain of the crystal lattice. The nature and the value of the strain are debatable. We perform systematic measurements of the $H_{c2}$ anisotropy in the high-quality ${\mathrm{Sr}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ single crystals in the temperature range 1.8 $\mathrm{K}<T<T_c\approx 2.7$ K using temperature stabilization with an accuracy of 0.0001 K. We observe that in all tested samples the anisotropy is weakly temperature dependent when $T<0.8T_c$ and smoothly decreases at higher temperatures without any sign of singularity when $T\to T_c$. Such a behavior is in a drastic contradiction with the prediction of the Ginzburg-Landau theory for the nematic superconductors. We discuss possible reasons for this discrepancy.

Electronic nematicity is a ubiquitous phenomenon in iron-based superconductors but its origin is still debated. Most models consider either spin or orbital degrees of freedom as the driving force but typically do not take electronic correlations into account. However, mass enhancements, coherent-incoherent crossovers, and the strong orbital differentiation can only be understood using correlations in a Hund's metal framework. Here, we study the influence of nematicity on the quasiparticle coherence in detwinned FeSe using angle-resolved photoemission spectroscopy (ARPES). We compare photoemission spectral weight from $d_{xz}$ and $d_{yz}$ orbitals in the coherent quasiparticle peak and in the incoherent Hubbard band and find an anisotropy between the two orbitals. We interpret our observation in terms of a more coherent $d_{xz}$ orbital compared to the $d_{yz}$ orbital inside the nematic phase. This result is in contrast to earlier predictions of an incoherent $d_{xz}$ orbital and highlights the importance of electronic correlations in the description of nematicity.

The goal of this paper is to investigate the characteristics of high-frequency whispering-gallery modes in embedded elastic spheres, that is, surrounded by an infinite elastic matrix. Due to several modeling difficulties, the high-frequency regime of embedded spheres has remained unexplored in elasticity. Our approach consists of formulating a specific finite-element method in spherical coordinates. The basic idea is to discretize only the radial coordinate while describing analytically the angular distribution of elastodynamic fields. Then, we also introduce a radial perfectly matched layer to cope with the unbounded nature of the external medium. Our approach yields a linear matrix eigensystem, simple and costless to solve. In order to identify general trends, both stiff and soft configurations are considered, corresponding to a sphere stiffer and softer than the external medium, respectively. Including material loss, our results highlight the behavior of leaky elastic whispering-gallery modes in the high-frequency regime. This work is motivated by the well-known behavior of whispering-gallery modes in optical resonators, reaching high $Q$ factors as the frequency increases. Identifying high-$Q$-factor whispering-gallery modes in elastic spheres could find promising applications for sensing the mechanical properties of external media.

We propose a generalization of the coherent anomaly method to extract the critical exponents of a phase transition occurring in the steady-state of an open quantum many-body system. The method, originally developed by Suzuki [J. Phys. Soc. Jpn. 55, 4205 (1986)] for equilibrium systems, is based on the scaling properties of the singularity in the response functions determined through cluster mean-field calculations. We apply this method to the dissipative transverse-field Ising model and the dissipative $XYZ$ model in two dimensions obtaining convergent results already with small clusters.

A key parameter for the low-temperature magnetic coupling of in dinuclear lanthanide single-molecule magnets (SMMs) is the barrier $U_{FA}$ resulting from the exchange and dipole interactions between the two $4f$ moments. Here we extend the pseudospin model previously used to describe the ground state of dinuclear endofullerenes to account for variations in the orientation of the single-ion anisotropy axes and apply it to the two SMMs ${\mathrm{Dy}}_2\mathrm{ScN}$@${\mathrm{C}}_{80}$ and ${\mathrm{Dy}}_2\mathrm{TiC}$@${\mathrm{C}}_{80}$. While x-ray magnetic circular dichroism (XMCD) indicates the same $J_z=15/2$ Dy ground state in both molecules, the Dy-Dy coupling strength and the stability of magnetization is distinct. We demonstrate that both the magnitude of the barrier $U_{FA}$ and the angle between the two $4f$ moments are determined directly from precise temperature-dependent magnetization data to an accuracy better than $1^{\circ }$. The experimentally found angles between the $4f$ moments are in excellent agreement with calculated angles between the quantization axes of the two Dy ions. Theory indicates a larger deviation of the orientation of the Dy magnetic moments from the Dy bond axes to the central ion in ${\mathrm{Dy}}_2\mathrm{TiC}$@${\mathrm{C}}_{80}$. This may explain the lower stability of the magnetization in ${\mathrm{Dy}}_2\mathrm{TiC}$@${\mathrm{C}}_{80}$, although it exhibits a $\sim 49\%$ stronger exchange coupling than in ${\mathrm{Dy}}_2\mathrm{ScN}$@${\mathrm{C}}_{80}$.

Magnetic multilayers with perpendicular anisotropy and an interfacial Dzyaloshinskii-Moriya interaction (DMI) contain chiral domain walls and skyrmions that are promising for applications. Here, we measure the temperature dependence of the DMI in Pt/CoFeB/Ir and Pt/CoB/Ir multilayers by means of static domain imaging. First, the temperature dependences of saturation magnetization $\left(M_{\mathrm{S}}\right)$, exchange stiffness $\left(A\right)$, and intrinsic perpendicular anisotropy $\left(K_{\mathrm{u}}\right)$ are determined. Then the demagnetized domain pattern in each multilayer is imaged by wide-field Kerr microscopy in the temperature range 9–290 K, and the characteristic domain period at each temperature is determined. We calculate the DMI constant $D$ from an analytical expression for the domain wall energy density that treats the multilayer as a uniform medium. Scaling laws for $K_{\mathrm{u}}$ and $D$ with the magnetization are established from the experiments. While the scaling of $K_{\mathrm{u}}$ is consistent with Callen-Callen theory, we find that the scaling of $D$ is like that of $A$ predicted theoretically $(\sim 1.8)$.

We present a computational study of ${\mathrm{PbCoO}}_3$ at ambient and elevated pressure. We employ the static and dynamic treatment of local correlation in the form of density functional theory $+$ $U$ $(\mathrm{DFT}+U)$ and $+$ dynamical mean-field theory $(\mathrm{DFT}+\mathrm{DMFT})$. Our results capture the experimentally observed crystal structures and identify the unsaturated Pb $6s–\mathrm{O}$ $2p$ bonds as the driving force beyond the complex physics of ${\mathrm{PbCoO}}_3$. We provide a geometrical analysis of the structural distortions and we discuss their implications, in particular the internal doping, which triggers a transition between phases with and without local moments and a site-selective Mott transition in the low-pressure phase.

In this paper we address, both experimentally and theoretically, the very grazing scattering of He atoms off KCl(001) with incidence along the $\langle 100\rangle$ channel. Our theoretical model combines a semiquantum description of the scattering dynamics and a high-precision interaction potential. By means of a thorough analysis of the quantum phase for in-plane scattering and rainbow trajectories, we are able to connect the presence of the physisorption well with the significant enhancements of the corrugation and rainbow angle, relative to the hard corrugated wall predictions. We trace this connection to dynamical effects on the incident and scattered beams due to their traversing of the physisorption well. Finally, we show that the inclusion of van der Waals interactions in the potential improves the theoretical accord with experiments for both the corrugation and the rainbow angle.

Ternary telluride ${\mathrm{Ta}}_4{\mathrm{Pd}}_3{\mathrm{Te}}_{16}$ is a layered superconductor with quasi-one-dimensional characteristics. It has attracted great research interests due to the possible nodal superconductivity and its interplay with adjacent charge-density wave as well as proposed nontrivial band topology. Here, we report low-energy electronic band structure of ${\mathrm{Ta}}_4{\mathrm{Pd}}_3{\mathrm{Te}}_{16}$ by means of high-resolution laser-based angle-resolved photoemission spectroscopy. We acquired the Fermi surface and detailed band dispersions, directly reflecting its multiband nature. Furthermore, dispersion kinks at ∼8 meV below the Fermi level are unambiguously observed, indicative of strong electron-boson coupling. We suggest these kinks are probably due to electron-phonon coupling with effective coupling constants quantified to be ∼0.72 and ∼0.48 for the ${\gamma }_1$ and ${\gamma }_2$ bands, respectively. Our results provide valuable information for future studies on the mechanism of superconductivity as well as its intimacy to charge-density wave in ${\mathrm{Ta}}_4{\mathrm{Pd}}_3{\mathrm{Te}}_{16}$.

We describe the effects of the crossing of the exciton levels in moiré quantum dots in an external magnetic field. We demonstrate that the unpolarized light can create significant dynamic valley polarization of interlayer excitons in the conditions where the Zeeman energy is much smaller than the thermal energy. The angular momentum is gained by the excitons from the nuclear spin fluctuations. In the $n$-type moiré quantum dots for the typical experimental parameters, the unpolarized light can lead to the complete valley polarization of the resident electrons.