We investigate the chiral electronic modes at the interface between two regions of a threefold topological semimetal, which is illuminated by left and right handed elliptically polarized waves. The radiation effects on the band structure of semimetal are analyzed by using Floquet theory. Two distinct solutions of the interface modes are found with the chirality depending on the phase of the irradiation. We also consider the anomalous Hall response, which is attributed to the interband contribution between the dispersionless flat band and conic bands.

Light absorption in graphene nanoribbons is studied analytically and numerically. Due to size quantization, the electronic states in the nanoribbon form a set of one-dimensional subbands modifying remarkably the absorption coefficient. The electronic subbands are calculated using the Berry-Mondragon boundary conditions at the nanoribbon edges, which are applicable for nanoribbons of the armchair type. Numerical results on pseudospin (sublattice) density generation by light absorption are presented and discussed.

The electronic surface state of a three-dimensional strong topological insulator (TI) is characterized by an isotropic Dirac cone at the $\mathrm{\Gamma }$ point and an ideally perpendicular spin-momentum locking. Away from this point, the lattice anisotropy becomes effective where deviations from the ideal picture are observed. In particular, weak violations of the spin-momentum orthogonality is observed in many experiments. In parallel to these, the existence of the electron-phonon interaction has been verified by a number of experimental groups. In this article we devise an interacting theory of the surface spin. We observe that the interactions, while weakly affecting the energy bands, can yield changes in the spin texture in the presence of a Fermi surface anisotropy. In particular, using the coupling of a specific optical phonon experimentally discovered in the ${\mathrm{Bi}}_{2-y}{\mathrm{Sb}}_y{\mathrm{Se}}_{3-x}{\mathrm{Te}}_x$ family to the surface state, we obtain a sixfold nonorthogonal modulations in the in-plane spin-momentum pair. The out-of-plane spin experiments are also discussed in this context. Our results indicate that there is an additionally new mechanism contributing to the deviations in the spin texture beyond the conventional independent particle picture in topological materials where interactions are observed to be important. Recent experimental reports on the spin-momentum nonorthogonality in other symmetries, and even in nearly isotropic Fermi surfaces, indicate that the theory introduced here may be relevant to a larger set of interacting Dirac materials and deserves further exploration.

We study the optical and magneto-optical properties of low-concentration arrays of ultrathin nickel nanowires by means of Brillouin spectroscopy of thermal magnons. Brillouin spectroscopy in such quasitransparent magneto-optical structures is dominated by the Bragg phase synchronism mechanism. At variance with ultrathin cobalt nanowire arrays, the Stokes/anti-Stokes scattering pattern is practically symmetrical. This feature is attributed to their peculiar optical properties.

Spin-transfer torque is one of the important physical quantities to understand for successful application of topological insulators to spintronics. In this paper we present analytical expressions of the spin-transfer torques on a surface of a magnetic topological insulator by including the higher order contributions of momentum, $k^2$ term, and the hexagonal warping. We obtain six different types of the spin-transfer torque including both the fieldlike and the dampinglike torques; the four of them appear only when the higher order momentum contributions are included. In addition, we discuss the dynamics of magnetic skyrmions driven by the spin-transfer torques on the surface of the topological insulator. Unlike the skyrmion dynamics in conventional metals, we find that the dynamics significantly depends on the internal structure of magnetic textures.

We report on the experimental observation of nonlocal spin-dependent thermoelectric effects in superconductor-ferromagnet multiterminal structures. Our samples consist of a thin superconducting aluminum wire with several ferromagnetic tunnel junctions attached to it. When a thermal excitation is applied to one of the junctions in the presence of a Zeeman splitting of the density of states of the superconductor, a thermoelectric current is observed in remote junctions at distances exceeding $10\mu \mathrm{m}$. The results can be explained by recent theories of coupled spin and heat transport in high-field superconductors.

The orthorhombic uranium dichalcogenide ${\mathrm{UTe}}_2$ displays superconductivity below 1.7 K, with the anomalous feature of retaining 50% of normal state (ungapped) carriers, according to heat capacity data from two groups. Incoherent transport that crosses over from above 50 K toward a low-temperature, Kondo lattice Fermi liquid regime indicates strong magnetic fluctuations and the need to include correlation effects in theoretical modeling. We report density functional theory plus Hubbard $U$ ($\mathrm{DFT}+U$) results for ${\mathrm{UTe}}_2$ to provide a platform for modeling its unusual behavior, focusing on ferromagnetic (FM, time-reversal breaking) long-range correlations along the $\widehat{a}$ axis as established by magnetization measurements and confirmed by our calculations. States near the Fermi level are dominated by the $j=\frac{5}{2}$ configuration, with the $j_z=\pm \frac{1}{2}$ sectors being effectively degenerate and half-filled. Unlike the small-gap insulating nonmagnetic electronic spectrum, the FM Fermi surfaces are large (strongly metallic) and display low-dimensional features, reminiscent of the FM superconductor ${\mathrm{UGe}}_2$.

The occurrence of superconductivity in topological materials is considered as a promising route for realizing topological superconductors, a platform able to host the long-sought Majorana fermions in condensed matter. In this work, by using electrical transport and heat-capacity measurements, as well as first-principles band-structure calculations, we investigate the physical properties of TaOsSi, a superconductor with $T_c\approx 5.8$ K. The behavior of both its upper critical field and low-temperature heat-capacity suggest the existence of two superconducting gaps. More strikingly, first-principles calculations reveal gapless topological surface states in the present material. The evolution of the electrical resistivity with pressure (up to 50 GPa) was also investigated, and a “V-shaped” diagram of $T_c$ vs $P$ was found. Overall, our data suggest that TaOsSi is a new system where multiband superconductivity and topological surface states coexist and, hence, it may serve as a possible candidate in the search for topological superconductivity.

The adiabatic charge pumping of a nonequilibrium state of spinless fermions in a one-dimensional lattice is investigated, with an emphasis placed on its usefulness in revealing many-body interaction effects on interband coherence. For a noninteracting system, the pumped charge per adiabatic cycle depends not only on the topology of the occupied bands but also on the interband coherence in the initial state. This insight leads to an interesting opportunity for quantitatively observing how quantum coherence is affected by many-body interaction that is switched on for a varying duration prior to adiabatic pumping. In particular, interband coherence effects can be clearly observed by adjusting the switch-on rates with different adiabatic pumping protocols and by scanning the duration of many-body interaction prior to adiabatic pumping. The time dependence of single-particle interband coherence in the presence of many-body interaction can then be examined in detail. As an interesting side result, for relatively weak interaction strength, it is found that the difference in the pumped charges between different pumping protocols vanishes if a coherence measure defined by the single-particle density matrix in the sublattice representation reaches its local minima. Our results hence provide an interesting means to quantitatively probe the dynamics of quantum coherence in the presence of many-body interaction (e.g., in a thermalization process).

We investigate the potential for optical quantum technologies of ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{O}}_3$ in the form of monodisperse spherical nanoparticles. We measured optical inhomogeneous lines of 27 GHz and optical homogeneous linewidths of 108 and 315 kHz in particles with 400- and 150-nm average diameters, respectively, for the ${}^{1}D_2\left(0\right)\leftrightarrow {}^{3}H_4\left(0\right)$ transition at 1.4 K. Furthermore, ground-state and ${}^{1}D_2$ excited-state hyperfine structures in ${\mathrm{Y}}_2{\mathrm{O}}_3$ are here determined by spectral hole burning and modeled by complete Hamiltonian calculations. Ground-state spin transitions have energies of 5.99 and 10.42 MHz, for which we demonstrate spin inhomogeneous linewidths of 42 and 45 kHz, respectively. Spin $T_2$ up to $880\mu \mathrm{s}$ was obtained for the $\pm 3/2\leftrightarrow \pm 5/2$ transition at 10.42 MHz, a value which exceeds that of bulk ${\mathrm{Pr}}^{3+}$-doped crystals reported so far. These promising results confirm nanoscale ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{O}}_3$ is a very appealing candidate to integrate quantum devices. In particular, we discuss the possibility of using this material for realizing spin-photon interfaces emitting indistinguishable single photons.

Two magnets in relative motion interact through their dipolar fields, making individual magnetic moments dynamically adapt to the changes in the energy landscape and bringing about collective magnetization dynamics. Some of the energy of the system is irrevocably lost through various coupling mechanisms between the spin degrees of freedom and those of the underlying lattice, resulting in magnetic friction. In this work, we use micromagnetic simulations to study magnetic friction in a system of two thin ferromagnetic films containing quenched disorder mimicking a polycrystalline structure. We observe bursts of magnetic activity resulting from repeated domain wall pinning due to the disorder and subsequent depinning triggered by the dipolar interaction between the moving films. These domain wall jumps result in strong energy dissipation peaks. We study how the properties of the polycrystalline structure such as grain size and strength of the disorder, along with the driving velocity and the width of the films, affect the magnetization dynamics, average energy dissipation, and the statistical properties of the energy dissipation bursts.

Electron-phonon interaction in copper oxide superconductors is still enigmatic. Strong coupling for certain optic phonons is now well established experimentally, but a theoretical understanding is challenging. Scattering of electrons near the Fermi surface by the longitudinal acoustic (LA) phonons is expected from basic theory because these phonons modulate electron density. We used inelastic neutron scattering on a large single-crystal sample of optimally doped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ to show that low-energy LA phonons could couple to electronic density fluctuations only at small phonon wave vectors, which naturally limits any interaction to forward scattering. Such scattering should not be pair-breaking in the case of the $d$-wave gap. We also found that the previously reported low-energy phonon spectral weight half-way to the zone boundary is consistent with conventional lattice dynamics and does not reflect an incipient charge density wave.

By constructing an effective model based on recently calculated ab initio bare interaction parameters, we study the phase diagram of alkali-doped fullerides as a function of temperature and internal pressure. We use a slave-rotor mean-field approach at the weak and intermediate coupling limits and a variational mean-field approach at the strong coupling limit, and find a good agreement with experimental phase diagram. We explain the unified description of the phase diagram including the proximity of $s$-wave superconducting state and the Mott-insulating state, and the existence of Jahn-Teller distorted metallic state using orbital selective physics. We argue that the double electronic occupation of two degenerate orbitals triggers both $s$-wave superconductivity and Jahn-Teller distortion. While the orbital ordering of two electrons causes the distortion, the remaining single electron in the third orbital causes the metal-insulator transition.

Inspired by a recently constructed commuting-projector Hamiltonian for a two-dimensional (2D) time-reversal-invariant topological superconductor [Z. Wang et al., Phys. Rev. B 98, 094502 (2018)], we introduce a commuting-projector model that describes an interacting yet exactly solvable 2D topological insulator. We explicitly show that both the gapped and gapless boundaries of our model are consistent with those of band-theoretic, weakly interacting topological insulators. Interestingly, on certain lattices our time-reversal-symmetric models also enjoy $\mathcal{CP}$ symmetry, leading to intuitive interpretations of the bulk invariant for a $\mathcal{CP}$-symmetric topological insulator upon putting the system on a Klein bottle. We also briefly discuss how these many-body invariants may be able to characterize models with only time-reversal symmetry.

Radiation induced magnetoresistance oscillations are well known and readily observable in the high-quality two-dimensional (2D) electron system. Here, we report unexpected photoexcited transport, under a low-carrier density, low-mobility condition, in the high-quality GaAs/AlGaAs devices that exhibit the above-mentioned oscillations. This study reveals enhanced nonresonant magnetoresistance as well as microwave resonance that appear unrelated to the above-mentioned radiation-induced magnetoresistance oscillations. The positions of the resonance along the $B$ axis depends on the microwave power, the microwave frequency, and the direction of sweep of the magnetic field, as the photoexcited diagonal resistance is prominently enhanced over the dark value. Most remarkably, the observed microwave-induced resonance becomes unobservable below a characteristic cutoff frequency. These features suggest a bulk magnetoplasmon origin for the observed resonances. A picture including bottlenecks in transport within 2D electron systems and microwave modified tunneling at bottlenecks is suggested to help understand some aspects of experiment.

We report Aharonov-Bohm (AB) type quantum interference in simply connected devices created at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface using conductive atomic force microscope (c-AFM) lithography. The oscillations are multiperiodic functions of magnetic field strength, and they exhibit a substantial magnetic hysteresis with frequencies that depend on the magnetic sweep direction. The oscillation amplitude for the lowest two frequencies approaches $e^2/h$, consistent with the theoretical maximum for the AB effect, and harmonics up to third order are observable. Broadband quasiperiodic behavior is reported in a fraction of simply connected electron waveguide devices that exhibit magnetic asymmetries. Curiously, nanoscale ring devices that are multiply connected lack signatures of AB quantum interference. The interference phenomena are associated with an inhomogeneous magnetic landscape within the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ nanostructures.

The spin-orbital-entangled Kramers doublet, known as the $J_{\mathbf{eff}}=1/2$ pseudospin driven by large spin-orbit coupling (SOC), appears in layered iridates and $\alpha \text{−}{\mathrm{RuCl}}_3$, manifesting a relativistic Mott insulating phase. Such entanglement, however, seems barely attainable in $3d$ transition metal oxides, where the SOC is small and the orbital angular momentum is easily quenched. Based on the density-functional-theory calculations, we report the ${\mathrm{CuAl}}_2{\mathrm{O}}_4$ spinel as the possible example of a $J_{\mathbf{eff}}=1/2$ Mott insulator in $3d$ transition metal compounds. With the help of strong electron correlations, the $J_{\mathbf{eff}}=1/2$ state can survive the competition with an orbital-momentum-quenched $S=1/2$ state in the $d^9$ configuration of ${\mathrm{CuO}}_4$ tetrahedron. From the dynamical mean-field theory calculations, the electron-addition spectra probing unoccupied states are well described by the $j_{\mathbf{eff}}=1/2$ hole state, whereas electron-removal spectra have a rich multiplet structure. The fully relativistic entity found in ${\mathrm{CuAl}}_2{\mathrm{O}}_4$ provides insight into the untapped regime where the spin-orbital-entangled Kramers pair coexists with strong electron correlation.

We have carried out Raman spectroscopy experiments to investigate two-magnon excitations in epitaxial thin films of the quasi-two-dimensional antiferromagnetic Mott insulator $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ under in-plane misfit strain. With in-plane biaxial compression, the energy of the two-magnon peak increases and the peak remains observable over a wider temperature range above the Néel temperature, indicating a strain-induced enhancement of the superexchange interactions between $J_{\mathrm{eff}}=1/2$ pseudospins. From density functional theory calculations, we have found an increase of the nearest-neighbor hopping parameter and exchange interaction with increasing biaxial compressive strain, in agreement with the experimental observations. Our experimental and theoretical results provide perspectives for systematic, theory-guided strain control of the primary exchange interactions in $5d$ transition-metal oxides.

We investigate the erbium flip-flop dynamics as a limiting factor of the electron-spin lifetime and more generally as an indirect source of decoherence in rare-earth-doped insulators. Despite the random isotropic arrangement of dopants in the host crystal, the dipolar interaction strongly depends on the magnetic field orientation following the strong anisotropy of the $g$ factor. In ${\mathrm{Er}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$, we observe in a 10-ppm doped sample a 3 orders-of-magnitude variation of the erbium flip-flop rate, as expected from the dipolar coupling term between identical spins with an anisotropic $g$ tensor. We can estimate the correct order of magnitude for a 50-ppm concentration, but we can only reproduce qualitatively the orientational variation of the flip-flop rate.