The effect of Rashba spin-orbit interaction and anisotropic elastic strain on the electronic, optical, and thermodynamic properties of an artificial graphenelike superlattice composed of InAs/GaAs quantum dots has been theoretically considered. The electronic energy dispersions have been obtained using Green's function formalism in combination with the Fourier transformation to the reciprocal space and an exact diagonalization technique. We have observed a splitting of Dirac points and the appearance of additional Dirac-like points due to the Rashba spin-orbit interaction. Furthermore, a breaking of the hexagonal symmetry of the dispersion surfaces caused by the strain anisotropy is observed as well. It is shown that both the spin-orbit interaction and strain anisotropy have a qualitative impact on the measurable characteristics of the considered structure and can be used as effective tools to control the performance of devices based on artificial graphene.

Motivated by recent photoemission and pump-probe experiments, we report determinant quantum Monte Carlo simulations of hybridization fluctuations in the half-filled periodic Anderson model. A tentative phase diagram is constructed based solely on hybridization fluctuation spectra and reveals a crossover regime between an unhybridized selective Mott state and a fully hybridized Kondo insulating state. This intermediate phase exhibits quantum hybridization fluctuations and consequentially the so-called “band bending” and a direct hybridization gap as observed in angle-resolved photoemission spectroscopy and optical conductivity. This connects the band bending with the hybridization fluctuations as proposed in a recent ultrafast optical pump-probe experiment. The Kondo insulating state is only established at lower temperatures with the development of sufficiently strong intersite hybridization correlations. Our work suggests a unified picture for interpreting recent photoemission, pump-probe, and optical observations and provides numerical evidences for the importance of hybridization fluctuations in heavy fermion physics.

We predict ${\mathrm{NiTe}}_2$ to be a type-II Dirac semimetal based on ab initio calculations and explore its bulk and spin-polarized surface states using spin- and angle-resolved photoemission spectroscopy (spin-ARPES). Our results show that, unlike ${\mathrm{PtTe}}_2, {\mathrm{PtSe}}_2$, and ${\mathrm{PdTe}}_2$, the Dirac node in ${\mathrm{NiTe}}_2$ is located in close vicinity to the Fermi energy. Additionally, ${\mathrm{NiTe}}_2$ also hosts a pair of band inversions below the Fermi level along the $\mathrm{\Gamma }-A$ high-symmetry direction, with one of them leading to a Dirac cone in the surface states. The bulk Dirac nodes and the ladder of band inversions in ${\mathrm{NiTe}}_2$ support unique topological surface states with chiral spin texture over a wide range of energies. Our work paves the way for the exploitation of the low-energy type-II Dirac fermions in ${\mathrm{NiTe}}_2$ in the fields of spintronics, infrared plasmonics, and ultrafast optoelectronics.

We consider a double quantum dot whose energy detuning is controlled by an ac electric field. We demonstrate an energy configuration for which the ac-induced current flowing through the double dot directly probes the spin-orbit anticrossing point for small ac field amplitudes. On the contrary, as the ac amplitude increases, a current antiresonance is formed and the direct information about the spin-orbit interaction is lost. This result indicates that a large ac amplitude is not necessarily advantageous for the spectroscopy of spin-orbit-coupled two-spin states. Moreover, we investigate the ac-induced current peaks versus the ac amplitude and show a current suppression when the ac field forms spin-blocked states. This effect gives rise to a characteristic pattern for the current which can be controlled at will by tuning the ac amplitude. Our results can be explored by performing electronic transport measurements in the spin-blockade regime.

We study the optical properties of ${\mathrm{Cs}}_3{\mathrm{Bi}}_2{\mathrm{I}}_9$ nanoplatelets using a combination of first-principles density functional theory, $\mathit{GW}$ plus Bethe-Salpeter equation calculations, and spectroscopic experiments. We show that the material exhibits flat bands and hence high effective masses. This manifests itself in the lowest-energy transition in the absorption spectrum arising from excitons with a high binding energy of 300 meV and a Bohr radius smaller than 6 nm. Due to the indirect band gap, electrons and holes are efficiently separated in reciprocal space and recombine slowly across the band gap, leading to very weak photoluminescence. Our results resolve inconsistencies in previous studies on ${\mathrm{Cs}}_3{\mathrm{Bi}}_2{\mathrm{I}}_9$ and lay the groundwork for further applications of this material, reliant on charge separation.

The giant planar Hall effect arising from a chiral anomaly, and that is related to the Berry curvature, has been predicted but never observed in nonmagnetic type-II Dirac/Weyl semimetals. Here, we report an observation of the anisotropic planar Hall effect in type-II Weyl semimetal ${\mathrm{WTe}}_2$. Interestingly, we observe a chiral-anomaly-induced sinusoidal angular-dependent planar Hall effect when the electric field is parallel to the tilting direction of Weyl cone, i.e., the $b$ axis of ${\mathrm{WTe}}_2$. The planar Hall effect amplitude is linearly dependent on the magnetic fields and decreases gradually as the temperature increases across the topological phase transition temperature. Our observations clearly reveal the footprints on transport from the chiral anomaly feature in type-II Weyl semimetals.

The ground state of two-electron quantum dots in single-valley materials like GaAs is always a spin singlet regardless of what the potential and interactions are. This statement cannot be generalized to the multivalley materials like $n$-doped Si. Here we calculate numerically the spectrum of a two-electron Si quantum dot and show that the dot with the lateral size of several nm can have the spin triplet ground state which is impossible in the single-valley materials. Predicted singlet-triplet level crossing in two-electron Si quantum dots can potentially establish the platform for quantum simulation of magnetic many body systems based on quantum dots. We suggest several examples of such systems that open a way to controlled quantum simulations within the condensed matter setting.

The anisotropic exchange splitting of the $\mathrm{\Gamma }$ exciton ${\delta }_1$ is measured in (In,Al)As/AlAs quantum dots with a type-I band alignment by means of two photoluminescence techniques: The macroscopic technique exploits the competition between the anisotropic exchange interaction and the Zeeman splitting, whereas with the microscopic technique the energy splitting of the exciton fine-structure in a single quantum dot is measured directly. We find that in the spectral region of the $\mathrm{\Gamma }X$ mixing the anisotropic exchange splitting decreases strongly.

The field of nanophotonics has long sought to identify mechanisms to realize dynamical control of optical modes. In most approaches, the magnitude of tuning is dependent upon the degree to which the optical permittivity is malleable upon some material change, such as carrier concentration. Here, through a multiwavelength Raman spectroscopic examination of 4H-SiC nanopillars, momentum is identified as an alternative means to enhance spectral tunability of nanophotonic modes, owing to the spatial dispersion implicit in the infrared (IR) optical permittivity of polar semiconductors. Experimentally, this is deduced through the observation of a “forbidden” Raman mode at $\approx 780{\mathrm{cm}}^{-1}$ and the emergence of the surface-optical phonon polariton at $\approx 950 {\mathrm{cm}}^{-1}$, which evolved with intensities dependent upon the nanopillar diameter and the wavelength of the incident light. The evolution of these modes is accompanied by a redshift and spectral narrowing of the longitudinal-optical plasmon coupled (LOPC) mode exhibiting a similar wavelength and diameter dependence. Mie resonances, identified using ultraviolet-visible spectroscopy and excited by the visible laser excitation of the Raman experiment, acted to vary the momentum sampled during the Raman experiment leading to these spectral dependencies. This was deduced by fitting the Raman response accounting for both the presence of the surface phonon and the overdamped LOPC mode under the Lindhard-Mermin approximation. This fitting not only explains the Raman response, but also clearly exhibits the spatially disperse permittivity of the SiC, which is shown to have a momentum-dependent sensitivity to carrier concentration. Such sensitivity, in turn, highlights the potential of spatial dispersion as a means to accentuate the performance of active IR nanophotonic approaches employing phonon polaritons.

All-electron density functional theory calculations were performed aiming to determine the equation of state and the dependence with pressure of the $c/a$ ratio and the anisotropic compressibility for hexagonal-close-packed rhenium. Calculations were carried out up to maximum compression of 0.46, and the resulting total energy versus volume is well described by a Vinet equation of state with $B_0$ = 367(5) GPa and $B_0^{\prime }$ = 4.64(3), valid up to 1.5 TPa. The agreement with two recent experimental studies [S. Anzellini et al., Equation of state of rhenium and application for ultra high pressure calibration, J. Appl. Phys. 115, 043511 (2014); T. Sakai et al., High pressure generation using double-stage diamond anvil technique: Problems and equations of state of rhenium, High Pressure Res. 38, 107 (2018)] supports their conclusion that the pressure in previous experiments with a double-stage diamond-anvil cell [L. Dubrovinsky et al., Implementation of micro-ball nanodiamond anvils for high-pressure studies above 6 Mbar, Nat. Commun. 3, 1163 (2012)] was significantly overestimated.

The magnetic quantum states of holmium single atom magnets on MgO(100) have proven extremely robust when exposed to high magnetic fields and temperatures up to 35 K. Here we address the stability of Ho at small magnetic fields, where the hyperfine interaction creates several avoided level crossings. Using spin-polarized scanning tunneling microscopy, we demonstrate quantum state control via Landau-Zener tunneling and stable magnetization at zero field. Our observations indicate a total spin ground state of $J_z=\pm 8$. Combined quantum and classical control render Ho a promising qubit candidate.

We study the correlated quantum magnet ${\mathrm{YbCl}}_3$ with neutron scattering, magnetic susceptibility, and heat capacity measurements. The crystal field Hamiltonian is determined through simultaneous refinements of the inelastic neutron scattering and magnetization data. The ground-state doublet is well isolated from the other crystal field levels and results in an effective spin-1/2 system with local easy plane anisotropy at low temperature. Cold neutron spectroscopy shows low-energy excitations peaked at 0.5 meV that are consistent with nearest-neighbor antiferromagnetic correlations.

We found that a high-mobility semimetal $1T^{\prime }\text{−}\mathrm{MoT}{\mathrm{e}}_2$ shows a significant pressure-dependent change in the cryogenic thermopower in the vicinity of the critical pressure, where the polar structural transition disappears. With the application of a high pressure of 0.75 GPa, while the resistivity becomes as low as 10 µΩ cm, thermopower reached the maximum value of $60\mu \mathrm{V}{\mathrm{K}}^{-1}$ at 25 K, leading to a giant thermoelectric power factor of $300\mu \mathrm{W}{\mathrm{K}}^{-2}\mathrm{c}{\mathrm{m}}^{-1}$. Based on semiquantitative analyses, the origin of this behavior is discussed in terms of inelastic electron-phonon scattering enhanced by the softening of zone-center phonon modes associated with the polar structural instability.

We study a frustrated two-dimensional lattice spin model with Kitaev-type interaction. The lattice is obtained from the honeycomb lattice by replacing half of its sites with triangles. Using the SO(3) Majorana representation of spin, the model is exactly mapped into a $Z_2$ lattice gauge theory of complex fermions with standard Gauss law constraint. We show that the ground state of the model is a chiral spin liquid, and it has gapless excitations. The Chern number of the ground state is $\pm 1$, implying the existence of a chiral edge mode. The vortex excitation of the model is bound with a Majorana zero mode, such mode behaves as non-Abelian anyons.

The electronic and optical conductivities for anisotropic tilted Dirac semimetals are calculated using the Kubo formula. It is shown that, as in graphene, the minimal conductivity is sensitive to the order in which different temperature, frequency, and scattering limits are taken. Both intraband and interband conductivities are found to be direction dependent. In the high-frequency and low-temperature limit the geometric mean $\sqrt{{\sigma }_{xx}{\sigma }_{yy}}$ reaches the same constant value as in graphene. This results from the fact that, in the zero-temperature limit, interband transitions are not affected by the tilt in the dispersion, a result that is physically interpreted as a global tilt in the allowed transitions. Such a result is verified by an independent and direct calculation of the absorption coefficient using Fermi's golden rule. However, as the temperature is raised, an interesting minimum is observed in the interband conductivity, interpreted here as a consequence of the interplay between the tilt in the allowed transitions and the chemical potential increasing with the temperature. This effect is a signature of the global tilt in the allowed optical transitions and can be measured by a suitable optical experiment.

As a simple metal, aluminum's valence band is usually described as a free-electron gas with three electrons per atom. The discrepancies between the experimental electronic Grüneisen parameter and heat capacity and their free-electron-gas counterparts are usually attributed to electron-phonon coupling. We recently calculated thermal electronic contributions to aluminum's internal energies with our average-atom code paradisio and obtained results that contradict this point of view. Our code also pointed out the overlap of the $sp$ valence band by the $3d$ one, resulting in an $sp$ to $d$ electron transfer. Applying Sommerfeld's temperature expansion method to the electron-electron Coulomb part of the internal energy, we relate the electronic Grüneisen parameter, $T=0$ K isotherm, and thermal contributions to the internal energy to a parameter $\alpha$ describing the fraction of $d$ electrons resulting from $p$ to $d$ transfer. Finally, we find a unique value of this parameter that provides a consistent explanation for the experimental Grüneisen parameter, for $T=0$ K energy deduced from experimental shock Hugoniot data, as well as for our average-atom thermal contributions to internal energy.

A theoretical formula is derived for the threshold current to switch a perpendicular magnetization in a ferromagnet by the spin Hall effect. The numerical simulation of the Landau-Lifshitz-Gilbert equation indicates that magnetization switching is achieved when the steady-state solution of the magnetization in the presence of the current is outside an energetically unstable region. Based on the numerical result, an analytical theory deriving the threshold current is developed by focusing on the first-order perturbation to the unstable state. The analytical formula clarifies that the magnitude of the magnetic field applied to the current direction should be larger than 15% of the perpendicular magnetic anisotropy field, and the current is less than the derived threshold value.

The magnetization, magnetoresistance, and Hall effect of the kagome structured material ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ is reported for high-quality single crystals. Previous investigations of ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ polycrystals detected ferromagnetism at $T_c\approx 657\mathrm{K}$ with moments along the easy $c$ axis and a moment rotation towards the $a-b$ plane upon cooling that culminates in moments freezing to form a spin glass at $T_F\approx 80\mathrm{K}$. The results presented here for single crystals show the lower value of $T_F=70\pm 2.5\mathrm{K}$, most likely due to the increased sample quality. Above $T_F$ we identify a topological contribution to the Hall resistivity coinciding with an anomalous magnetoresistance. This supports recent proposals that the magnetic structure contains magnetic skyrmions in this temperature range.

The magnetic structure of the “nonmetallic metal” FeCrAs, a compound with the characters of both metals and insulators, was examined as a function of temperature using single-crystal neutron diffraction. The magnetic propagation vector was found to be $k=(1/3,1/3,0)$, and the magnetic reflections disappeared above $T_N=116\left(1\right)\mathrm{K}$. In the ground state, the Cr sublattice shows an in-plane spiral antiferromagnetic order. The moment sizes of the Cr ions were found to be small due to strong magnetic frustration in the distorted kagome lattice or the itinerant nature of the Cr magnetism and vary between 0.8 and $1.4{\mu }_B$ on different sites as expected for a spin-density-wave- (SDW-) type order. The upper limit of the moment on the Fe sublattice is estimated to be less than $0.1{\mu }_B$. With increasing temperature up to 95 K, the Cr moments cant out of the $ab$ plane gradually with the in-plane components being suppressed and the out-of-plane components increasing in contrast. This spin reorientation of Cr moments can explain the dip in the $\mathit{c}$-direction magnetic susceptibility and the kink in the magnetic order parameter at $T_O\sim 100\mathrm{K}$, a second magnetic transition which was ignored before. We have also discussed the similarity between FeCrAs and the model itinerant magnet Cr, which exhibits spin-flip transitions and SDW-type antiferromagnetism.

We report on synthesis, crystal structure, magnetic, thermodynamic, and transport properties of the compound ${\mathrm{YbPd}}_2\mathrm{In}$, crystallizing as a Heusler structure type. A trivalent state of the rare earth was determined by fitting the magnetic susceptibility with a Curie-Weiss law. This compound is characterized by showing very weak magnetic interactions and a negligible Kondo effect. A specific-heat jump was observed at $T\approx 250\mathrm{mK}$, followed at higher temperature by a power-law decrease of $C_P\left(T\right)/T$. The resulting large electronic entropy increase at very low temperature is rapidly shifted to higher temperature by the application of magnetic field. This magnetocaloric effect places ${\mathrm{YbPd}}_2\mathrm{In}$ as a very good candidate for adiabatic demagnetization cooling processes.