We study low-temperature electron transport in $p$-wave superconductor-insulator-normal metal junctions. In diffusive metals, the $p$-wave component of the order parameter is strongly suppressed at distances greater than the mean free path $l$. At the superconductor-normal metal boundary, due to spin-orbit interaction, there is a triplet to singlet conversion of the superconducting order parameter. The singlet component survives at distances much larger than $l$ from the boundary. It is this component that controls the low-temperature resistance of the junctions. As a result, the resistance of the system strongly depends on the angle between the insulating boundary and the $\mathbf{d}$ vector characterizing the spin structure of the triplet superconducting order parameter. We also analyze the spatial dependence of the electric potential in the presence of the current and show that the electric field is suppressed in the insulating boundary as well as in the normal metal at distances of order of the coherence length away from the boundary. This is very different from the case of the normal metal-insulator-normal metal junctions, where the voltage drop takes place predominantly at the insulator.

We identify $dd$ excitations in the quasi-one-dimensional compound Ca${}_2$Y${}_2$Cu${}_5$O${}_{10}$ using resonant inelastic x-ray scattering. By tuning across the Cu $L_3$ edge, we observe abrupt shifts in the $dd$-peak positions as a function of incident photon energy. This observation demonstrates orbital-specific coupling of the high-energy excited states of the system to the low-energy degrees of freedom. A Franck-Condon treatment of electron-lattice coupling, consistent with other measurements in this compound, reproduces these shifts, explains the Gaussian line shapes, and highlights charge-orbital-lattice renormalization in the high-energy $d$ manifold.

Here we present a systematic study of the dynamics of a single quantum emitter near a flat metal-dielectric interface. We identify the key elements that determine the onset of reversibility in these systems by using a formalism suited for absorbing media and through an exact integration of the dynamics. Moreover, when the quantum emitter separation from the surface is small, we are able to describe the dynamics within a pseudomode description that yields analytical understanding and allows more powerful calculations.

At the spin transition point of $\nu =2/3$ quantum Hall states, nuclear spins in a two-dimensional electron gas are polarized by an electric current. Using GaAs/AlGaAs double-quantum-well samples, we first observed the spatial diffusion of nuclear spin polarization between the two layers when the nuclear spin polarization is current induced in one layer. By numerical simulation, we estimated the diffusion constant of the nuclear spin polarization to be $15\pm 7{\mathrm{nm}}^2/\mathrm{s}$.

The spin of a single electron in a suspended carbon nanotube can be read out by using its coupling to the nanomechanical motion of the nanotube. To show this, we consider a single electron confined within a quantum dot formed by the suspended carbon nanotube. The spin-orbit interaction induces a coupling between the spin and one of the bending modes of the suspended part of the nanotube. We calculate the response of the system to pulsed external driving of the mechanical motion using a Jaynes-Cummings model. To account for resonator damping, we solve a quantum master equation, with parameters comparable to those used in recent experiments, and show how information about the spin state of the system can be acquired by measuring the mechanical motion of the nanotube. The latter can be detected by observing the current through a nearby charge sensor.

We report the results of a zero-field muon-spin relaxation (ZF-$\mu$SR) study of superconducting Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ ($0.5\le x\le 0.9$) in search of weak spontaneous internal magnetic fields associated with proposed time-reversal-symmetry-breaking mixed pairing states. The measurements were performed on polycrystalline samples, which do not exhibit the mesoscopic phase separation previously observed in single crystals of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$. No evidence of spontaneous internal magnetic fields is found in any of the samples at temperatures down to $T\sim 0.02$ K.

We report on the doping-induced antiferromagnetic state and Fermi-liquid state that are connected by a superconducting region in a series of CeIrIn${}_{5-x}$Hg${}_x$, CeIrIn${}_{5-x}$Sn${}_x$, and CeIr${}_{1-x}$Pt${}_x$In${}_5$ single crystals. Measurements of the specific heat $C\left(T\right)$ and electrical resistivity $\rho \left(T\right)$ demonstrate that hole doping via Hg/In substitution gives rise to an antiferromagnetic ground state, but substitutions of In by Sn or Ir by Pt (electron doping) favor a paramagnetic Fermi-liquid state. A conelike non-Fermi-liquid region is observed near CeIrIn${}_5$, showing a diverging effective mass on the slightly Hg-doped side. The obtained temperature-doping phase diagram suggests that CeIrIn${}_5$ is in proximity to an antiferromagnetic quantum critical point, and heavy fermion superconductivity in this compound is mediated by magnetic quantum fluctuations rather than by valence fluctuations.

Quantum criticality provides an important route to revealing universal nonequilibrium behavior. A canonical example of a critical point is the Bose-Hubbard model, which we study under the application of an electric field. A Boltzmann transport formalism and $\epsilon$ expansion are used to obtain the nonequilibrium conductivity and current noise. This approach allows us to explicitly identify how a universal nonequilibrium steady state is maintained, by identifying the rate-limiting step in balancing Joule heating and dissipation to a heat bath. It also reveals that the nonequilibrium distribution function is very far from a thermal distribution.

We study quasiparticle dynamics in graphene exposed to a linearly polarized electromagnetic wave of very large intensity. We demonstrate that low-energy transport in such system can be described by an effective time-independent Hamiltonian, characterized by multiple Dirac points in the first Brillouin zone. Around each Dirac point the spectrum is anisotropic: the velocity along the polarization of the radiation significantly exceeds the velocity in the perpendicular direction. Moreover, in some of the points the transverse velocity oscillates as a function of the radiation intensity. We find that the conductance of a graphene p-n junction in the regime of strong irradiation depends on the polarization as $G\left(\theta \right)\propto {|sin\theta |}^{3/2}$, where $\theta$ is the angle between the polarization and the p-n interface, and oscillates as a function of the radiation intensity.

We show that there exist two-dimensional (2D) time-reversal invariant fractionalized insulators with the property that both their boundary with the vacuum and their boundary with a topological insulator can be fully gapped without breaking time-reversal or charge conservation symmetry. This result leads us to an apparent paradox: we consider a geometry in which a disklike region made up of a topological insulator is surrounded by an annular strip of a fractionalized insulator, which is, in turn, surrounded by the vacuum. If we gap both boundaries of the strip, we naively obtain an example of a gapped interface between a topological insulator and the vacuum that does not break any symmetries—an impossibility. The resolution of this paradox is that this system spontaneously breaks time-reversal symmetry in an unusual way, which we call weak symmetry breaking. In particular, we find that the only order parameters that are sensitive to the symmetry breaking are nonlocal operators that describe quasiparticle tunneling processes between the two edges of the strip; expectation values of local order parameters vanish exponentially in the limit of a wide strip. Also, we find that the symmetry breaking in our system comes with a ground-state degeneracy, but this ground-state degeneracy is topologically protected, rather than symmetry protected. We show that this kind of symmetry breaking can also occur at the edge of 2D fractional topological insulators.

The layered FeTe${}_2$O${}_5$Cl compound was studied by specific-heat, muon-spin relaxation, nuclear magnetic resonance, and dielectric as well as neutron and synchrotron x-ray diffraction measurements, and the results were compared to isostructural FeTe${}_2$O${}_5$Br. We find that the low-temperature ordered state, similarly as in FeTe${}_2$O${}_5$Br, is multiferroic: the elliptical amplitude-modulated magnetic cycloid and the electric polarization simultaneously develop below 11 K. However, compared to FeTe${}_2$O${}_5$Br, the magnetic elliptical envelope rotates by 75(4)${}^{\circ }$ and the orientation of the electric polarization is much more sensitive to the applied electric field. We propose that the observed differences between the two isostructural compounds arise from geometric frustration, which enhances the effects of otherwise subtle Fe${}^{3+}$ ($S=\frac{5}{2}$) magnetic anisotropies. Finally, x-ray diffraction results imply that, on the microscopic scale, the magnetoelectric coupling is driven by shifts of the O${}_1$ atoms, as a response to the polarization of the Te${}^{4+}$ lone-pair electrons involved in the Fe-O-Te-O-Fe exchange bridges.

We theoretically study the equilibrium and dynamic properties of nanoscale magnetic tunnel junctions (MTJs) and magnetic wires, in which an electric field controls the magnetic anisotropy through spin-orbit coupling. By performing micromagnetic simulations, we construct a rich phase diagram and find that, in particular, the equilibrium magnetic textures can be tuned between Néel and Bloch domain walls in an elliptical MTJ. Furthermore, we develop a phenomenological model of a quasi-one-dimensional domain wall confined by a parabolic potential and show that, near the Néel-to-Bloch-wall transition, a pulsed electric field induces precessional domain-wall motion which can be used to reverse the chirality of a Néel wall and even depin it. This domain-wall motion controlled by electric fields, in lieu of applied current, may provide a model for ultralow-power domain-wall memory and logic devices.

We have performed angle-resolved photoemission spectroscopy (ARPES) on the (111) surface of the topological crystalline insulator SnTe. Distinct from a pair of Dirac-cone surface states across the $\overline{X}$ point of the surface Brillouin zone on the (001) surface, we revealed two types of Dirac-cone surface states each centered at the $\overline{\Gamma }$ and $\overline{M}$ points, which originate from the bulk-band inversion at the $L$ points. We also found that the energy location of the Dirac point and the Dirac velocity are different from each other. This ARPES experiment demonstrates the surface states on different crystal faces of a topological material, and it elucidates how mirror-symmetry-protected Dirac cones of a topological crystalline insulator show up on surfaces with different symmetries.

We examine the low-energy behavior of a double quantum dot in a regime where spin and pseudospin excitations are degenerate. The individual quantum dots are described by Anderson impurity models with an on-site interaction $U$ which are capacitively coupled by an interdot interaction $U_{12}<U$. The low-energy response functions are expressed in terms of renormalized parameters, which can be deduced from an analysis of the fixed point in a numerical renormalization group calculation. At the point where the spin and pseudospin degrees of freedom become degenerate, the free quasiparticle excitations have a phase shift of $\pi /4$ and a 4-fold degeneracy. We find, however, when the quasiparticle interactions are included, that the low-energy effective model has SU(4) symmetry only in the special case $U_{12}=U$ unless both $U$ and $U_{12}$ are greater than $D$, the half bandwidth of the conduction electron bath. We show that the gate voltage dependence of the temperature-dependent differential conductance observed in recent experiments can be described by a quasiparticle density of states with temperature-dependent renormalized parameters.

We study the dynamics of an electron spin in a graphene quantum dot, which is interacting with a bath of less than ten nuclear spins via the anisotropic hyperfine interaction. Due to substantial progress in the fabrication of graphene quantum dots, the consideration of such a small number of nuclear spins is experimentally relevant. This choice allows us to use exact diagonalization to calculate the long-time average of the electron spin as well as its decoherence time. We investigate the dependence of spin observables on the initial states of nuclear spins and on the position of nuclear spins in the quantum dot. Moreover, we analyze the effects of the anisotropy of the hyperfine interaction for different orientations of the spin quantization axis with respect to the graphene plane. Interestingly, we then predict remarkable long decoherence times of more than 10 ms in the limit of few nuclear spins.

We introduce a methodology for constructing anharmonic vibrational Hamiltonians that are parametrized from first-principles electronic-structure calculations and can be used to study high-temperature properties of crystalline materials. Our method provides an accurate description of the Born-Oppenheimer potential energy surface of a crystal that can be systematically refined and is invariant to space-group symmetries of the ideal reference crystal and finite rigid-body rotations and translations. These features make it ideally suited for Monte Carlo or molecular dynamics simulations to predict finite-temperature thermodynamic properties, structural phase transitions, and thermal conductivity. We use this method to construct an anharmonic Hamiltonian for ZrH${}_2$, which exhibits a high-temperature cubic phase that undergoes a symmetry-breaking second-order transition to one of three equivalent tetragonal phases upon cooling. Although density functional theory predicts a zero-Kelvin dynamical instability of cubic ZrH${}_2$, we find via Monte Carlo simulation that the cubic phase can be anharmonically stabilized at high temperature and predict a cubic-to-tetragonal transition temperature that is in good agreement with extrapolation from experiments. We also calculate finite-temperature free energies for the cubic and tetragonal phases, finding that they are consistent with the phenomenological Landau theory of second-order phase transitions.

Soft x-ray resonant magnetic scattering is an element-sensitive technique that enables the characterization of the magnetic properties of a wide variety of systems. Here we apply this technique to study lithographically produced artificial spin ice, a particular class of magnetically frustrated systems comprising arrays of nanomagnets. Using a CCD detector we can access a large fraction of the reciprocal space at once, allowing us to easily distinguish the signatures of the magnetic ground-state ordering. Comparing the dichroic signal at the position of the Bragg peaks with model calculations based on the kinematical theory of x-ray diffraction, we are able to determine the number of reversed moments as a function of applied magnetic field for each of the two sublattices. This study demonstrates the benefit of having direct access to a significant fraction of the reciprocal space, and opens the way towards more sophisticated x-ray based experiments on artificial spin ice such as scattering of coherent x-ray beams to explore the dynamics of thermally activated systems.

We derive from a microscopic model the effective theory of nematic order in a system with a spontaneous quantum anomalous Hall effect in two dimensions. Starting with a model of two-component fermions (a spinor field) with a quadratic band crossing and short-range four-fermion marginally relevant interactions we use $1/N$ expansion and bosonization methods to derive the effective field theory for the hydrodynamic modes associated with the conserved currents and with the local fluctuations of the nematic order parameter. We focus on the vicinity of the quantum phase transition from the isotropic Mott Chern insulating phase to a phase in which time-reversal symmetry breaking coexists with nematic order, the nematic Chern insulator. The topological sector of the effective field theory is a background field (BF)/Chern-Simons gauge theory. We show that the nematic order parameter field couples with the Maxwell-type terms of the gauge fields as the space components of a locally fluctuating metric tensor. The nematic field has $z=2$ dynamic scaling exponent. The low-energy dynamics of the nematic order parameter is found to be governed by a Berry phase term. By means of a detailed analysis of the coupling of the spinor field of the fermions to the changes of their local frames originating from long-wavelength lattice deformations, we calculate the Hall viscosity of this system and show that in this system it is not the same as the Berry phase term in the effective action of the nematic field, but both are related to the concept of torque Hall viscosity, which we introduce here.

We investigate persistent currents in metallic rings interrupted by a Coulomb-blockaded topological superconducting segment. We show that the presence of Majorana bound states in the superconductor is reflected in the emergence of an $h/e$ harmonic in the persistent current. The Majorana bound states further render the current finite at zero flux, with a sign that is determined by the fermion parity of the superconductor. The resulting peculiar symmetry of the persistent current is compatible with a free energy that is even in time-reversal symmetry-breaking fields. These unique features of the persistent currents are robust against disorder and provide unambiguous signatures of the presence of Majorana fermions.

Nonlinear optics of semiconductors is an important field of fundamental and applied research, but surprisingly the role of excitons in the coherent processes leading to harmonics generation has remained essentially unexplored. Here we report results of a comprehensive experimental and theoretical study of the three-photon process of optical second-harmonic generation (SHG) involving the exciton resonances of the noncentrosymmetric hexagonal wide-band-gap semiconductor ZnO in the photon energy range of $3.2$–$3.5$ eV. Resonant crystallographic SHG is observed for the $1s(A,B)$, $2s(A,B)$, $2p(A,B)$, and $1s\left(C\right)$ excitons. We show that SHG signals at these exciton resonances are induced by the application of a magnetic field when the incident and the SHG light wave vectors are along the crystal $z$ axis where the crystallographic SHG response vanishes. A microscopic theory of SHG through excitons is developed, which shows that the nonlinear interaction of coherent light with excitons has to be considered beyond the electric-dipole approximation. Depending on the particular symmetry of the exciton states SHG can originate from the electric- and magnetic-field-induced perturbations of the excitons due to the Stark effect, the spin as well as orbital Zeeman effects, or the magneto-Stark effect. The importance of each mechanism is analyzed and discussed by confronting experimental data and theoretical results for the dependences of the SHG signals on photon energy, magnetic field, electric field, crystal temperature, and light polarization. Good agreement is obtained between experiment and theory proving the validity of our approach to the complex problem of nonlinear interaction of light with ZnO excitons. This general approach can be applied also to other semiconductors.