Chiral multifold fermions are quasiparticles described by higher spin generalizations of the Weyl equation and are realized as low energy excitations near symmetry protected band crossings in certain chiral crystals. In this work we calculate the linear optical conductivity of all chiral multifold fermions. We show that it is enhanced with respect to that of Weyl fermions with the same Fermi velocity and features characteristic activation frequencies for each multifold fermion class, providing an experimental fingerprint to detect them. We calculate the conductivity for realistic chiral multifold semimetals by using lattice tight-binding Hamiltonians that match the effective models of multifold fermions at low energies, for space groups 199 and 198. The latter includes RhSi, for which we give quantitative predictions, and also CoSi and AlPt. Our predictions can be tested in absorption or penetration depth measurements and are necessary to extract the recently proposed quantized photocurrents from experiments.

Nonlinear phenomena are inherent in most systems in nature. Second- or higher-order harmonic generations, three-wave mixing, and four-wave mixing are typical phenomena in nonlinear optics. To obtain a nonzero signal for second-harmonic generation in the long-wavelength limit ($q\to 0$), the breaking of inversion symmetry is required. In topological materials, a hexagonal warping term that breaks the rotation symmetry of the Fermi surface is observed by angular-resolved photoemission spectroscopy. If a gap opens (e.g., by doping with magnetic impurities), the inversion symmetry will be broken. Here we use a nonlinear response theory based on a generalized Kubo formula to explain the frequency up-conversion in topological materials.

Recent experiments have suggested that the electron-phonon coupling may play an important role in the $\gamma \to \alpha$ volume collapse transition in cerium. A minimal model for the description of such a transition is the periodic Anderson model. In order to better understand the effect of the electron-phonon interaction on the volume collapse transition, we study the periodic Anderson model with coupling between Holstein phonons and electrons in the conduction band. We find that the electron-phonon coupling enhances the volume collapse, which is consistent with experiments in cerium. While we start with the Kondo volume collapse scenario in mind, our results capture some interesting features of the Mott scenario, such as a gap in the conduction electron spectra which grows with the effective electron-phonon coupling.

We formulate a method of deriving an effective low-energy Hamiltonian for nonperiodic systems such as interfaces for strongly correlated electron systems by extending a conventional multiscale ab initio scheme for correlated electrons (MACE). We apply the formalism to copper-oxide high $T_{\mathrm{c}}$ superconductors in an example of the interface between overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ and Mott insulating ${\mathrm{La}}_2{\mathrm{CuO}}_4$ recently realized experimentally and derive the two-band effective Hamiltonian ($E_g$ Hamiltonian) from the Cu $3d_{x^2-y^2}$-like and $3d_{3r^2-z^2}$-like orbitals near the Fermi level. We show that the parameters of the $E_g$ Hamiltonian derived for the ${\mathrm{La}}_2{\mathrm{CuO}}_4/{\mathrm{La}}_{1.55}{\mathrm{Sr}}_{0.45}{\mathrm{CuO}}_4$ superlattice differ considerably from those for the bulk ${\mathrm{La}}_2{\mathrm{CuO}}_4$, particularly significant in the partially screened Coulomb parameters and the level offset between the $d_{x^2-y^2}$ and $d_{z^2}$ orbitals, $\mathrm{\Delta }E$. In addition, we investigate the effect of the lattice relaxation on the $E_g$ Hamiltonian by carefully comparing the parameters derived before and after the structure optimization. We find that the ${\mathrm{CuO}}_6$ octahedra distort after the relaxation as a consequence of the Madelung potential difference between the ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and ${\mathrm{La}}_{1.55}{\mathrm{Sr}}_{0.45}{\mathrm{CuO}}_4$ sides, by which the layer dependence of the hopping and Coulomb parameters becomes more gradual than the unrelaxed case. Furthermore, the structure relaxation dramatically changes the $\mathrm{\Delta }E$ value and the occupation number at the interface. This study not only evidences the importance of the ionic relaxation around interfaces but also provides a set of layer-dependent parameters of the $E_g$ Hamiltonian, which is expected to provide further insight into the interfacial superconductivity when solved with low-energy solvers.

We calculate an exact expression for the probability propagator for a noisy electric field driven tight-binding lattice. The noise considered is a two level jump process or a telegraph process (TP) which jumps randomly between two values $\pm \mu$. In the absence of a static field, and in the limit of zero jump rate of the noisy field, we find that the dynamics yields Bloch oscillations with frequency $\mu$, while with an additional static field $\epsilon$ we find oscillatory motion with a superposition of frequencies $(\epsilon \pm \mu )$. On the other hand, when the jump rate is rapid, and in the absence of a static field, the stochastic field averages to zero if the two states of the TP are equally probable a priori. In that case we see a delocalization effect. The intimate relationship between the rapid relaxation case and the zero field case seems to be a generic effect found in a wide variety of systems. It is interesting to note that even for zero static field and rapid relaxation, Bloch oscillations ensue if there is a bias $\delta p$ in the probabilities of the two levels. Remarkably, the Wannier-Stark localization caused by an additional static field is destroyed if the latter is tuned to be exactly equal and opposite to the average stochastic field $\mu \delta p$. This is an example of incoherent destruction of Wannier-Stark localization.

Using density functional theory (DFT) calculations and the Greens's function formalism, we report the existence of magnetic edge states with a noncollinear spin texture present on different edges of the $1T^{\prime }$ phase of the three monolayer transition metal dichalcogenides (TMDs): ${\mathrm{M}oS}_2, {\mathrm{M}oTe}_2$, and ${\mathrm{W}Te}_2$. The magnetic states are gapless and accompanied by a spontaneous breaking of the time-reversal symmetry. This may have an impact on the prospects of utilizing ${\mathrm{W}Te}_2$ as a quantum spin Hall insulator. It has previously been suggested that the topologically protected edge states of the $1T^{\prime }$ TMDs could be switched off by applying a perpendicular electric field [X. Qian, J. Liu, L. Fu, and J. Li, Science 346, 1344 (2014)]. We confirm with fully self-consistent DFT calculations that the topological edge states can be switched off. The investigated magnetic edge states are seen to be robust and remain gapless when applying a field.

We propose a protocol to prepare a state in the left and center quantum dots of a triple dot array and transfer it directly to the center and right dots. Initially the state is prepared combining the exchange interaction and magnetic field gradients. Once in the desired state, ac gate voltages in the outer dots are switched on, allowing to select a given photoassisted long-range path and to transfer the prepared state directly from one edge to the other with high fidelity. We investigate the effect of charge noise on the protocol and propose a configuration in which the transfer can be performed with high fidelity. Our proposal can be experimentally implemented and is a promising avenue for transferring quantum states between two spatially separated two-level systems.

We study plasmon propagation on a metallic two-dimensional surface partially coated with a thin film of anomalous-Hall material. The resulting three regions, separated by two sharp interfaces, are characterized by different Hall conductivities but identical normal conductivities. A single bound mode is found, which can localize to either interface and has an asymmetric potential profile across the region. For propagating modes, we calculate the reflection and transmission coefficients through the magnetic region. We find Airy transmission patterns with sharp maxima and minima as a function of the plasmon incidence angle. The system therefore behaves as a high-quality filter.

Intentional doping and unintentional impurities in intrinsic semiconductors generate carriers that enable device operations. Fundamental to the electronic activity of dopants and impurities is the introduction of defect states inside the forbidden energy gap of semiconductors having shallow and/or deep characteristics, which fundamentally define the ability to engineer its physical properties and associated device performance. Here we demonstrate that unintentional electron doping by oxygen (${\mathrm{O}}_{\mathrm{N}}$) impurities and intentional magnesium hole doping ($\mathrm{M}{\mathrm{g}}_{\mathrm{Sc}}$) in scandium nitride (ScN) do not introduce any defect states inside its fundamental bandgap and that the rigid-band electronic structure remains unchanged. Employing a combination of spectroscopic techniques as well as first-principles density functional theory analysis, we show that the ${\mathrm{O}}_{\mathrm{N}}$ and $\mathrm{M}{\mathrm{g}}_{\mathrm{Sc}}$ defect states in ScN are located inside the bands, which leaves behind the virgin ScN bandgap as well as the valence and conduction band edges that are important for electronic transport. The rigid-band electronic structure of ScN with respect to the electron and hole doping results in high electron and hole concentrations due to the free movement of Fermi level and results in tunable electronic and thermoelectric properties necessary for device applications.

The knowledge of the value and anisotropy of the gyromagnetic factor in semiconducting nanowires (NWs) is crucial for their potential applications in several fields, such as spintronics and topological quantum computation. Here, we present a complete experimental and theoretical investigation of the Zeeman splitting of the fundamental exciton transition in an important material system: wurtzite (WZ) InP NWs. The excitonic $g$ factors are derived by the Zeeman splitting of the spin levels observed by photoluminescence measurements under magnetic fields $\vec{B}$ up to 29 T. In addition to being about three times greater than in zincblende InP, the $g$ factor in the WZ phase is strongly anisotropic (50%) upon variation of the direction of $\vec{B}$ from parallel to perpendicular to the NW axis. Moreover, it exhibits a marked sublinear dependence on $\vec{B}$ whenever $\vec{B}$ points along the NW axis, a feature common to other non-nitride III-V WZ NWs but never properly understood. All these features are well accounted for by a realistic $k·p$ modeling of the Landau levels in WZ InP with the envelope function approximation including excitonic effects. The nonlinearity is spin dependent and due to the coupling between the heavy-hole- and light-hole-like Landau levels. This is indeed a general signature of the bulk WZ structure not requiring quantum confinement nor NW geometry, and is demonstrated to hold also for GaAs, InAs and GaN WZ crystals as reported by Faria Junior et al. [Phys. Rev. B, present issue]. Our study solves the outstanding puzzle of the nonlinear Zeeman splitting found in several III-V WZ NWs.

Density functional theory is generalized to incorporate electron-phonon coupling. A Kohn-Sham equation yielding the electronic density $n_U\left(\mathbf{r}\right)$, a conditional probability density depending parametrically on the phonon normal mode amplitudes $U=\left\{U_{\mathbf{q}\lambda }\right\}$, is coupled to the nuclear Schrödinger equation of the exact factorization method. The phonon modes are defined from the harmonic expansion of the nuclear Schrödinger equation. A nonzero Berry curvature on nuclear configuration space affects the phonon modes, showing that the potential energy surface alone is generally not sufficient to define the phonons. An orbital-dependent functional approximation for the nonadiabatic exchange-correlation energy reproduces the leading-order nonadiabatic electron-phonon-induced band structure renormalization in the Fröhlich model.

The existence of many-body mobility edges in closed quantum systems has been the focus of intense debate after the emergence of the description of the many-body localization phenomenon. Here we propose that this issue can be settled in experiments by investigating the time evolution of local degrees of freedom, tailored for specific energies and initial states. An interacting model of spinless fermions with exponentially long-ranged tunneling amplitudes, whose noninteracting version is known to display single-particle mobility edges, is used as the starting point upon which nearest-neighbor interactions are included. We verify the manifestation of many-body mobility edges by using numerous probes, suggesting that one cannot explain their appearance as merely being a result of finite-size effects. Lastly, our scheme of following the quantum time evolution at different energies is generic and can be promptly used in a variety of models.

The $g$-factor anisotropy of the heavy quasiparticles in the hidden order state of ${\mathrm{URu}}_2{\mathrm{Si}}_2$ has been determined from the superconducting upper critical field and microscopically from Shubnikov–de Haas (SdH) oscillations. We present a detailed analysis of the $g$ factor for the $\alpha , \beta$, and $\gamma$ Fermi-surface pockets. Our results suggest a strong $g$-factor anisotropy between the $c$ axis and the basal plane for all observed Fermi-surface pockets. The observed anisotropy of the $g$ factor from the quantum oscillations is in good agreement with the anisotropy of the superconducting upper critical field at low temperatures, which is strongly limited by the paramagnetic pair breaking along the easy magnetization axis $c$. However, the anisotropy of the initial slope of the upper critical field near $T_c$ cannot be explained by the anisotropy of the effective masses and Fermi velocities derived from quantum oscillations.

SnTe hosts ferroelectricity that competes with its weak nontrivial band topology: in the high-symmetry rocksalt structure—in which its intrinsic electric dipole is quenched—this material develops metallic surface bands, but in its rhombic ground-state configuration—which hosts a nonzero spontaneous electric dipole—the crystalline symmetry is lowered, and the presence of surface electronic bands is not guaranteed. Here, the type of ferroelectric coupling and the atomistic and electronic structure of SnTe films ranging from 2 to 40 atomic layers (ALs) are examined on freestanding samples, to which atomic layers were gradually added. Four-AL SnTe films are antiferroelectrically coupled, while thicker freestanding SnTe films are ferroelectrically coupled. The electronic band gap reduces its magnitude in going from 2 to 40 ALs, but it does not close due to the rhombic nature of the structure. These results bridge the structure of SnTe films from the monolayer to the bulk.

We show that a quasiperiodic magnetic chain comprising magnetic atomic sites sequenced in a Fibonacci pattern can act as a prospective candidate for spin filters for particles with arbitrary spin states. This can be achieved by tuning a suitable correlation between the amplitude of the substrate magnetic field and the on-site potential of the magnetic sites, which can be controlled by an external gate voltage. Such correlation leads to a spin filtering effect in the system, allowing one of the spin components to completely pass through the system while blocking the others over the allowed range of energies. The underlying mechanism behind this phenomena holds true for particles with any arbitrary spin states $S=1,3/2,2,\cdots$, in addition to the canonical case of spin-half particles. Our results open up the interesting possibility of designing a spin demultiplexer using a simple quasiperiodic magnetic chain system. Experimental realization of this theoretical study might be possible by using ultracold quantum gases and can be useful in engineering new spintronic devices.

Bulk ${\mathrm{Fe}}_{1-x}{\mathrm{Ga}}_x$ alloys present a strong sensitivity of the magnetostrictive properties with Ga content with maximum magnetostriction near $x=0.19$. Here, we present magnetoelastic coefficients measured by the cantilever method on ${\mathrm{Fe}}_{1-x}{\mathrm{Ga}}_x$ thin films grown by molecular beam epitaxy on GaAs(001). We find that Ga-dependent magnetoelastic coefficients in nanometer thin films are comparable in magnitude to the respective bulk values. Moreover, we compare thin films with a tetragonal structure due to a slightly preferential alignment of Ga pairs along the growth direction with and a cubic structure. It turns out that magnetoelastic coefficients are unaffected by a preferential alignment of Ga pairs along the growth direction.

We have studied relaxation processes of the photocurrent and the light-induced giant injection magnetoresistance (${\mathrm{IMR}}^{\left(ph\right)}$) in ${\mathrm{SiO}}_2\left(\mathrm{Co}\right)/\mathrm{GaAs}$ heterostructures, where the ${\mathrm{SiO}}_2\left(\mathrm{Co}\right)$ structure is the granular ${\mathrm{SiO}}_2$ film with Co nanoparticles, and have found that the photocurrent is accompanied by relaxation oscillations. Relaxation oscillations are caused by transitions between the photocurrent and electrons on the highest level in the interface quantum well. The light-induced magnetoresistance ${\mathrm{IMR}}^{\left(ph\right)}$ reaches the maximum value in the avalanche onset region and has the local minimum at the higher voltage. It is found that the local minimum can been explained by delocalization of the highest level in the interface quantum well and by decrease of the probability of the backscattering process of injected electrons on deeper levels. ${\mathrm{SiO}}_2\left(\mathrm{Co}\right)/\mathrm{GaAs}$ heterostructures are proposed to use as efficient fast-response magnetic sensors operating without hysteresis at room temperature.