The electronic structure and the magnetism of the ferromagnetic semiconductor (Ga,Fe)Sb, whose Curie temperature $T_{\mathrm{C}}$ can exceed room temperature, were investigated by means of x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and resonance photoemission spectroscopy (RPES). The line-shape analyses of the XAS and XMCD spectra suggest that the ferromagnetism is of intrinsic origin. The orbital magnetic moments deduced using XMCD sum rules were found to be large, indicating that there is a considerable $3d^6$ contribution to the ground state of Fe. From RPES, we observed a strong dispersive Auger peak and nondispersive resonantly enhanced peaks in the valence-band spectra. The latter is a fingerprint of the correlated nature of Fe $3d$ electrons, whereas the former indicates their itinerant nature. It was also found that the Fe $3d$ states have a finite contribution to the density of states at the Fermi energy. These states, presumably consisting of majority-spin $p\text{−}d$ hybridized states or minority-spin $e$ states, would be responsible for the ferromagnetic order in this material.

Building on the discovery that a Weyl superconductor in a magnetic field supports chiral Landau-level motion along the vortex lines, we investigate its transport properties out of equilibrium. We show that the vortex lattice carries an electric current $I=\frac{1}{2}(Q_{\mathrm{eff}}^2/h)(\mathrm{\Phi }/{\mathrm{\Phi }}_0)V$ between two normal-metal contacts at voltage difference $V$, with $\mathrm{\Phi }$ the magnetic flux through the system, ${\mathrm{\Phi }}_0$ the superconducting flux quantum, and $Q_{\mathrm{eff}}<e$ the renormalized charge of the Weyl fermions in the superconducting Landau level. Because the charge renormalization is energy dependent, a nonzero thermoelectric coefficient appears even in the absence of energy-dependent scattering processes.

Luttinger liquid theory of one-dimensional quantum systems ignores exponentially weak backscattering of particles. This endows Luttinger liquids with superfluid properties. The corresponding two-fluid hydrodynamic description available at present applies only to Galilean-invariant systems, whereas most experimental realizations of one-dimensional quantum liquids lack Galilean invariance. Here we develop the two-fluid theory of such quantum liquids. In the low-frequency limit the theory reduces to single-fluid hydrodynamics. However, the absence of Galilean invariance brings about three new transport coefficients. We obtain expressions for these coefficients in terms of the backscattering rate.

The anomalous Hall effect (AHE) is studied on the surface of a 3D magnetic topological insulator. By applying a modified semiclassical framework, all three contributions to the AHE, the Berry curvature effect, the side-jump effect and the skew scattering effects are systematically treated, and analytical expressions for the conductivities are obtained in terms of the Fermi level, the spatial orientation of the surface magnetization and the concentration of magnetic and nonmagnetic impurities. We demonstrate that the AHE can change sign by altering the orientation of the surface magnetization, the concentration of the impurities and also the position of the Fermi level, in agreement with recent experimental observations. We show how each contribution to the AHE, or even the whole AHE, can be turned off by properly adjusting the given parameters. For example, one can turn off the anomalous hall conductivity in a system with in-plane magnetization by pushing the system into the fully metallic regime.

As an analogy to the Weyl point in $k$-space, we search for energy levels which close at a single point as a function of a three-dimensional parameter space. Such points are topologically protected in the sense that any perturbation which acts on the two-level subsystem can be corrected by tuning the control parameters. We find that parameter-controlled Weyl points are ubiquitous in semiconductor-superconductor quantum dots and that they are deeply related to Majorana zero modes. In this paper, we present several semiconductor-superconductor quantum-dot devices which host parameter-controlled Weyl points. Furthermore, we show how these points can be observed experimentally via conductance measurements.

Recent theoretical studies [G. Chen et al., Phys. Rev. B 82, 174440 (2010); H. Ishizuka et al., Phys. Rev. B 90, 184422 (2014)] for the magnetic Mott insulator ${\mathrm{Ba}}_2\mathrm{Na}\mathrm{Os}{\mathrm{O}}_6$ have proposed a low-temperature order parameter that breaks lattice rotational symmetry without breaking time reversal symmetry, leading to a nematic phase just above the magnetic ordering temperature. We present high-resolution calorimetric and magnetization data of the same ${\mathrm{Ba}}_2\mathrm{Na}\mathrm{Os}{\mathrm{O}}_6$ single crystal and show evidence for a weakly field-dependent phase transition occurring at a temperature of $T_s\approx 9.5\mathrm{K}$, above the magnetic ordering temperature of $T_c\approx 7.5\mathrm{K}$. This transition appears as a broadened step in the low-field temperature dependence of the specific heat. The evolution of the phase boundary with applied magnetic field suggests that this phase coincides with the phase of broken local point symmetry seen in NMR experiments at high fields [L. Lu et al., Nat. Commun. 8, 14407 (2017)]. Furthermore, the magnetic field dependence of the specific heat provides clear indications for magnetic correlations persisting at temperatures between $T_c$ and $T_s$ where long-range magnetic order is absent, giving support for the existence of the proposed nematic phase.

The only alkali metal known to be superconducting at ambient pressure is Li at 0.4 mK. Under 30 GPa pressure $T_{\text{c}}$ for Li rises to 14 K. In addition, nearly 50 years ago the heavy alkali metal Cs was reported to become superconducting near 1.3 K at 12 GPa. In the present experiment the superconductivity of Cs under pressure is confirmed. In addition, strong evidence is presented in electrical resistivity measurements that neighboring Rb also becomes superconducting near 2 K at 55 GPa as it enters the oC16 phase, as for Cs, where $T_{\text{c}}$ decreases under the application of pressure. It would seem likely that under the right temperature/pressure conditions all alkali metals, including metallic hydrogen, will join the ranks of the superconducting elements. With the addition of Rb, 55 of the 92 naturally occurring elements are superconducting at ambient or high pressure.

We develop a methodology for extracting the Kohn-Sham angular momentum of excitons in realistic systems from time-dependent density functional theory. For small systems the exciton populations can be calculated analytically, which allows us to test the methodology for a three-arm ${\mathrm{H}}_2$ molecular ring and a pair of such rings. For larger systems the developed methodology opens a venue to determine the angular momentum of excitons by first principles calculations. A chain of twenty three-arm ${\mathrm{H}}_2$ molecular rings and a triphenylphosphine molecule are investigated as illustrative examples. It is demonstrated that the angular momentum is conserved during the absorption of twisted light.

By employing the time-dependent exact diagonalization method, we investigate the photoexcited states of the excitonic insulator in the extended Falicov-Kimball model (EFKM). We here show that the pulse irradiation can induce the interband electron-electron pair correlation in the photoexcited states, while the excitonic electron-hole pair correlation in the initial ground state is strongly suppressed. We also show that the photoexcited states contain the eigenstates of the EFKM with a finite number of interband electron-electron pairs, which are responsible for the enhancement of the electron-electron pair correlation. The mechanism found here is due to the presence of the internal SU(2) pairing structure in the EFKM and thus it is essentially the same as that for the photoinduced $\eta$ pairing in the repulsive Hubbard model reported recently [T. Kaneko et al., Phys. Rev. Lett. 122, 077002 (2019)]. This also explains why the nonlinear optical response is effective to induce the electron-electron pairs in the photoexcited states of the EFKM. Furthermore, we show that, unlike the $\eta$ pairing in the Hubbard model, the internal SU(2) structure is preserved even for a nonbipartite lattice when the EFKM has the direct-type band structure, in which the pulse irradiation can induce the electron-electron pair correlation with momentum $\mathit{q}=\mathit{0}$ in the photoexcited states. We also discuss briefly the effect of a perturbation that breaks the internal SU(2) structure.

Exchange forces on the movable dot (“shuttle”) in a magnetic shuttle device depend on the parity of the number of shuttling electrons. The performance of such a device can therefore be tuned by changing the strength $U$ of Coulomb correlations to block or unblock parity fluctuations. We show that by increasing $U$ the spintromechanics of the device crosses over, at $U=U_c\left(T\right)$, from a mechanically stable regime to a regime of spin-induced shuttle instabilities (neglecting electric forces). This is due to enhanced spin-dependent mechanical forces as parity fluctuations are reduced by a Coulomb blockade of tunneling and demonstrates that single-electron manipulation of single-spin controlled nanomechanics is possible.

We consider a freestanding metallic nanofilm with a predominant intersubband pairing which emerges as a result of the confinement in the growth direction. We show that the Fermi wave vector mismatch between the subbands, detrimental to the intersubband pairing, can be compensated by the nonzero center-of-mass momentum of the Cooper pairs. This leads to the spontaneous appearance of the intersubband Fulde-Ferrell (IFF) state, even in the absence of an external magnetic field. Our study of the intrasubband pairing channel on the stability of the IFF phase shows that the former strongly competes with the intersubband pairing, which prohibits the coexistence of the two superconducting phases. Interestingly, upon application of the magnetic field we find a transition to an exotic mixed spin-singlet subband-triplet and spin-triplet subband-singlet paired state. Finally, we discuss the possibility of existence of the IFF pairing in novel superconducting materials.

We demonstrate a transition from weak, to strong, to ultrastrong coupling of Frenkel molecular excitons and Bloch surface wave photons at room temperature using a one-sided, all-dielectric optical structure. The all-dielectric structure comprises an organic semiconductor thin film on the surface of a distributed Bragg reflector. We investigate the evolution of multiple vibronic polariton branches and their dominant absorption peaks as a function of coupling and in-plane momentum, which is absent in previous ultrastrong coupling systems. Measurements are interpreted using both the transfer matrix method and a coupled-oscillator model without the rotating wave approximation. The dependence of Rabi splitting on the number of excitons and electrical field amplitude is also modeled showing a transition to ultrastrong coupling at film thicknesses $\ge 50$ nm. This low-loss polaritonic structure enables us to study phenomena such as organic exciton-polariton dynamics, ultralong range polariton propagation, and high efficiency energy transport in the ultrastrong coupling regime at room temperature.

The magneto-optical response of monolayer transition metal dichalcogenides, including excitonic effects, is studied using a nanoribbon geometry. We compute the diagonal optical conductivity and the Hall conductivity. Comparing the excitonic optical Hall conductivity to results obtained in the independent-particle approximation, we find an increase in the amplitude corresponding to one order of magnitude when excitonic effects are included. The Hall conductivities are used to calculate Faraday rotation spectra for ${\mathrm{MoS}}_2$ and ${\mathrm{WSe}}_2$. Finally, we have also calculated the diamagnetic shift of the exciton states of ${\mathrm{WSe}}_2$ in different dielectric environments. Comparing the calculated diamagnetic shift to recent experimental measurements, we find a very good agreement between the two.

We study a periodically driven central site coupled to a disordered environment. In comparison to the static model, transport features are either enhanced or reduced, depending on the frequency of the drive. We demonstrate this by analyzing the statistics of quasienergies and the logarithmic growth of bipartite entanglement entropy, which show similar features: For frequencies larger than disorder strength, localization is enhanced due to a reduced effective coupling to the central site. Remarkably, localization can even be increased up to almost perfect freezing at particular frequencies, at which the central site decouples due to the emergence of “dark Floquet states.” This high-frequency domain of our model is bounded from below by a critical frequency ${\omega }_c$, at which transport increases abruptly. We demonstrate that ${\omega }_c$ is determined by one-photon resonances, which connect states across the mobility edge and generically delocalize the system. This sensitive frequency dependence allows us to fine-tune transport properties of the driven central-site model with unprecedented precision.

Crystal and magnetic structures of the intermetallic ${\mathrm{HoNi}}_2{\mathrm{B}}_2\mathrm{C}$ have been investigated using different single-crystal diffraction techniques at both room and low temperatures (superconducting phase). Combined refinement of the neutron and x-ray diffraction data performed at room temperature shows that the crystal structure of ${\mathrm{HoNi}}_2{\mathrm{B}}_2\mathrm{C}$ is well described in the tetragonal space group $I4/mmm$, in agreement with the literature. The magnetic ordering at low temperatures is accompanied by a structural phase transition to the orthorhombic space group $Fmmm$. However, only noncharacteristic orbits are occupied in this space group, and the symmetry reduction is entirely due to a lifting of the symmetry-induced restrictions on the anisotropic displacement parameters. Based on magnetic symmetry analysis, two types of commensurate antiferromagnetic structures with different magnetic space groups (MSGs) are found below $T_{\mathrm{N}}\approx 5.2$ K: MSG $P_{I}nnm$ (Ho spins are along the [100] direction of the parent structure) and MSG $C_{A}mca$ (Ho spins are along [110] of the parent structure). Both models agree well with conventional unpolarized neutron diffraction data at low temperature (2.4 K) with ordered magnetic moments of 9.16(9) ${\mu }_{\mathrm{B}}$/${\mathrm{Ho}}^{3+}$ atom. Spherical neutron polarimetry, on the other hand, allowed us to unambiguously solve the magnetic structure (MSG $C_{A}mca$) as well as to determine the population of two types of ${90}^{\circ }$ antiferromagnetic domains (0.45/0.55).

We study dissipative spin-spin coupling in a dispersive regime mediated by virtual photons in a microwave cavity. Dissipative coupling between spin of each magnetic material and the cavity photons is established by means of two phase-shifted driving forces acting on each spin. We show that mode-level attraction between two spin modes can be reached when one of the spins is dissipatively coupled to the cavity. By tuning the phase parameter at each ferromagnetic insulator, we can shift the order of “dark” and “bright” collective modes with the phase difference equal to 0 or $\pi$. Moreover, by selectively applying the phase-shifted field, it is possible to construct dark and bright collective modes with the phase difference equal to $\pm \pi /2.$

Majorana mode based topological qubits are potentially subject to diabatic errors that in principle can limit the utility of topological quantum computation. Using a combination of analytical and numerical methods we study the diabatic errors in Majorana-based topological Y junction that are coupled to a Bosonic bath in the Markovian approximation. We find that in the absence of a bath, the error can be made exponentially small with increasing braiding time, only when the time variation in the Hamiltonian is completely smooth. The presence of a dominantly dissipative Markovian bath is found to eliminate this exponential scaling of error to a power-law scaling as $T^{-1}$ with $T$ being the braiding time. However, the inclusion of relaxation improves this scaling slightly to go as $T^{-2}$. Thus, coupling of topological systems to Bosonic baths can lead to power law in braiding time diabatic errors that might limit the speed of topologically protected operations using Majorana modes.

We present low-energy muon-spin rotation measurements on $\mathrm{Cu}/\mathrm{Nb}/{\mathrm{AlO}}_x/\mathrm{Co}$ thin films that probe the newly described electromagnetic (EM) proximity effect. By varying the thickness of the insulating ${\mathrm{AlO}}_x$ layer we control the degree of coupling between the superconductor and ferromagnet and thus the EM proximity effect. For barrier thicknesses up to 4 nm we find both a small contact-dependent reduction in the standard Meissner effect and a larger diamagnetic contribution originating at the $\mathrm{Nb}/{\mathrm{AlO}}_x/\mathrm{Co}$ interface which decays away over a lengthscale far exceeding the superconducting coherence length. This second component we attribute to the EM proximity effect. Our analysis provides compelling experimental evidence for previously neglected electromagnetic effects within proximity coupled systems.