We study a statistical mechanics model of two species of bosons with mutual statistics $\theta =2\pi /n$ in (2+1) dimensions. This model realizes a fractionalized topological phase of bosons, which is a fractionalized version of the boson integer quantum Hall effect. The model can be studied with sign-free Monte Carlo simulations. We study the phase transitions between the fractionalized topological phase and a trivial insulator, and between different topological phases. We find that these transitions are continuous, and we measure their critical exponents.

We propose a continuous-variable analog of quantum controlled-not gates based on a system of exciton polaritons in semiconductor microcavities. This can be realized by engineering a parametric interaction between control and target polariton modes, which can be varied in time. As an explicit setup we use a system of dipolaritons, which allows for enhancement of parametric interaction by auxiliary classical fields and scalable multigate system realization. We find that high operational fidelity for the proposed gate is achievable for realistic system parameters.

Defect centers in diamond are exceptional solid-state quantum systems that can have exceedingly long electron and nuclear spin coherence times. So far, single-qubit gates for the nitrogen nuclear spin, a two-qubit gate with a nitrogen-vacancy (NV) center electron spin, and entanglement between nearby nitrogen nuclear spins have been demonstrated. Here, we develop a scheme to implement a universal two-qubit gate between two distant nitrogen nuclear spins. Virtual excitation of an NV center that is embedded in an optical cavity can scatter a laser photon into the cavity mode; we show that this process depends on the nuclear spin state of the nitrogen atom. If two NV centers are simultaneously coupled to a common cavity mode and individually excited, virtual cavity photon exchange can mediate an effective interaction between the nuclear spin qubits, conditioned on the spin state of both nuclei, which implements a universal controlled-$\mathit{Z}$ gate. We predict operation times below $10\mu \mathrm{s}$, which is four orders of magnitude faster than the decoherence time of nuclear spin qubits in diamond.

Inelastic scattering of electrons incident on a solid surface is determined by two properties: (i) electronic response of the target system and (ii) the detailed quantum-mechanical motion of the projectile electron inside and in the vicinity of the target. We emphasize the equal importance of the second ingredient, pointing out the fundamental limitations of the conventionally used theoretical description of the electron energy-loss spectroscopy (EELS) in terms of the “energy-loss functions.” Our approach encompasses the dipole and impact scattering as specific cases, with the emphasis on the quantum-mechanical treatment of the probe electron. Applied to the high-resolution EELS of Ag surface, our theory largely agrees with recent experiments, while some instructive exceptions are rationalized.

Numerical and closed-form analytic expressions for plasmon dispersion relations and rates of dissipation are first obtained at finite temperatures for free-standing gapped graphene. These closed-system results are generalized to an open system with Coulomb coupling of graphene electrons to an external electron reservoir. Plasmon modes, as well as plasmon dissipation channels, are found in this open system, including significant modifications arising from the combined effect of thermal excitation of electrons and an energy band gap in gapped graphene. Moreover, the characteristics of the plasmon mode and the additional plasmon dissipation may be fully controlled by adjusting the separation between the graphene layer from the surface of a thick conductor. Numerical results for the thermal shift of plasmon frequency in a doped gapped graphene layer, along with its sensitivity to the local environment, are demonstrated and analyzed. Such phenomenon associated with the frequency shift of plasmons may be applied to direct optical measurement of local electron temperature in transistors and nanoplasmonic structures.

We use scanning tunneling microscopy and spectroscopy to reveal a unique stripy charge order in a parent phase of iron-based superconductors in stoichiometric FeTe epitaxy films. The charge order has unusually the same—usually half—period as the spin order. We also found highly anisotropic electron band dispersions being large and little along the ferromagnetic (crystallographic $b$) and antiferromagnetic ($a$) directions, respectively. Our data suggest that the microscopic mechanism is likely of the Stoner type driven by interatomic Coulomb repulsion $V_{ij}$, and that $V_{ij}$ and charge fluctuations, so far much neglected, are important to the understanding of iron-based superconductors.

In this paper we discuss how far a single-particle picture can be applied for the description of photoelectron spectroscopy of large $\pi$-conjugated molecules on metal substrates measured near a $K$ edge. In this context we focus on particular spectroscopic features, which only exist at the immediate interface and show a constant kinetic energy when the photon energy is varied immediately above a resonance. We demonstrate that the kinetic energy of these features can be calculated on the basis of a single-particle picture for molecule-metal interface systems without charge transfer in the ground state. Similar features are also observed in the case of molecule-metal interfaces which show an occupation of the formerly lowest unoccupied molecular orbital in direct photoelectron spectroscopy as a result of charge transfer in the ground state. However, an analogous calculation of the kinetic energy of these signals fails here, indicating that a single-particle picture is not sufficient for these systems, for which many-body effects have to be adequately taken into account.

We study the magnetic susceptibility in various three-dimensional gapless systems, including Dirac and Weyl semimetals, and a line-node semimetal. The susceptibility is decomposed into the orbital term, the spin term and also the spin-orbit cross term, which is caused by the spin-orbit interaction. We show that the orbital susceptibility logarithmically diverges at the band touching energy in the point-node case, while it exhibits a stronger $\delta$-function singularity in the line node case. The spin-orbit cross term is shown to be paramagnetic in the electron side while diamagnetic in the hole side, in contrast with other two terms which are both even functions in Fermi energy. The spin-orbit cross term in the nodal semimetal is found to be directly related to the chiral surface current induced by the topological surface modes.

A microscopic study of the individual annealed (In,Ga)As/GaAs quantum dots is done by means of high-resolution transmission electron microscopy. The Cauchy-Green strain-tensor component distribution and the chemical composition of the (In,Ga)As alloy are extracted from the microscopy images. The image processing allows for the reconstruction of the strain-induced electric-field gradients at the individual atomic columns extracting thereby the magnitude and asymmetry parameter of the nuclear quadrupole interaction. Nuclear magnetic resonance absorption spectra are analyzed for parallel and transverse mutual orientations of the electric-field gradient and a static magnetic field.

Systems with purely off-diagonal disorder may have peculiar features such as the localization-delocalization transition and long-range correlations in their wave functions. To motivate possible experimental studies of the physics of off-diagonal disorder (e.g., in systems with random nearest-neighbor hopping), we study in detail disordered discrete-time quantum walk in a finite chain. Starting from a transfer matrix approach, we show, both theoretically and computationally, that the dynamics of the quantum walk with disorder manifests all the main features of systems with off-diagonal disorder. We also propose how to prepare a remarkable delocalized zero mode from a localized and easy-to-prepare initial zero mode using an adiabatic protocol that changes the disorder strength slowly. Numerical experiments are also performed with encouraging results.

I propose a method for ultrafast switching of ferroelectric polarization using midinfrared pulses. This involves selectively exciting the highest frequency $A_1$ phonon mode of a ferroelectric material with an intense midinfrared pulse. Large amplitude oscillations of this mode provides a unidirectional force to the lattice such that it displaces along the lowest frequency $A_1$ phonon mode coordinate because of a nonlinear coupling of the type $g{Q}_{\mathrm{P}}{Q}_{\mathrm{IR}}^2$ between the two modes. First-principles calculations show that this coupling is large in perovskite transition-metal oxide ferroelectrics, and the sign of the coupling is such that the lattice displaces in the switching direction. Furthermore, I find that the lowest frequency $A_1$ mode has a large $Q_{\mathrm{P}}^3$ order anharmonicity, which causes a discontinuous switch of electric polarization as the pump amplitude is continuously increased.

We report measurements of the polar Kerr effect, proportional to the out-of-plane component of the magnetization, in thin films of the magnetically doped topological insulator ${\left({\text{Cr}}_{0.12}{\text{Bi}}_{0.26}{\text{Sb}}_{0.62}\right)}_2{\text{Te}}_3$. Measurements of the complex Kerr angle ${\mathrm{\Theta }}_K$ were performed as a function of photon energy in the range $0.8\text{eV}<\hslash \omega <3.0\text{eV}$. We observed a peak in the real part of ${\mathrm{\Theta }}_K\left(\omega \right)$ and zero crossing in the imaginary part that we attribute to a resonant interaction with a spin-orbit avoided crossing located $\approx 1.6$ eV above the Fermi energy. The resonant enhancement allows measurement of the temperature and magnetic field dependence of ${\mathrm{\Theta }}_K$ in the ultrathin film limit, $d\ge 2$ quintuple layers (QL). We find a sharp transition to zero remanent magnetization at 6 K for $d<8$ QL, consistent with theories of the dependence of impurity spin interactions on film thickness and their location relative to topological insulator surfaces.

Spin excitations were studied by absorption spectroscopy in ${\mathrm{CaBaCo}}_4{\mathrm{O}}_7$ which is a type-I multiferroic compound with the largest magnetic-order induced ferroelectric polarization ($\mathrm{\Delta }P=17$ mC/${\mathrm{m}}^2$) reported, so far. We observed two optical magnon branches: a solely electric dipole allowed one and a mixed magnetoelectric resonance. The entangled magnetization and polarization dynamics of the magnetoelectric resonance gives rise to unidirectional light absorption, i.e., that magnon mode absorbs the electromagnetic radiation for one propagation direction but not for the opposite direction. Our systematic study of the magnetic field and temperature dependence of magnon modes provides information about the energies and symmetries of spin excitations, which is required to develop a microscopic spin model of ${\mathrm{CaBaCo}}_4{\mathrm{O}}_7$.

Time-reversal symmetry (TRS) may be broken in superconductors with three or more condensates interacting repulsively, yielding two degenerate states specified by chirality of gap functions. We consider a loop of such a superconductor with two halves occupied by the two states with opposite chiralities. Fractional flux plateaus are found in a magnetization curve associated with free-energy minima, where the two domain walls between the two halves of loop accommodate different intercomponent phase kinks leading to finite winding numbers around the loop only in a part of all condensates. Fractional flux plateaus form pairs related by the flux quantum ${\mathrm{\Phi }}_0=hc/2e$, although they individually take arbitrary values depending on material parameters and temperature. This phenomenon is a clear evidence of TRS broken superconductivity, and in a general point of view it provides a novel chance to explore relative phase difference, phase kink and soliton in ubiquitous multicomponent superconductivity such as that in iron pnicitides.

Zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) provides a new regime for the measurement of nuclear spin-spin interactions free from the effects of large magnetic fields, such as truncation of terms that do not commute with the Zeeman Hamiltonian. One such interaction, the magnetic dipole-dipole coupling, is a valuable source of spatial information in NMR, though many terms are unobservable in high-field NMR, and the coupling averages to zero under isotropic molecular tumbling. Under partial alignment, this information is retained in the form of so-called residual dipolar couplings. We report zero- to ultralow-field NMR measurements of residual dipolar couplings in acetonitrile-2-${}^{13}\mathrm{C}$ aligned in stretched polyvinyl acetate gels. This permits the investigation of dipolar couplings as a perturbation on the indirect spin-spin $J$ coupling in the absence of an applied magnetic field. As a consequence of working at zero magnetic field, we observe terms of the dipole-dipole coupling Hamiltonian that are invisible in conventional high-field NMR. This technique expands the capabilities of zero- to ultralow-field NMR and has potential applications in precision measurement of subtle physical interactions, chemical analysis, and characterization of local mesoscale structure in materials.

We present a set of three-body interaction models based on the Slater-Kirkwood (SK) potential that are suitable for the study of the energy, structural, and elastic properties of solid ${}^4\mathrm{He}$ at high pressure. Our effective three-body potentials are obtained from the fit to total energies and atomic forces computed with the van der Waals density functional theory method due to Grimme, and represent an improvement with respect to previously reported three-body interaction models. In particular, we show that some of the introduced SK three-body potentials reproduce closely the experimental equation of state and bulk modulus of solid helium up to a pressure of $\sim 60$ GPa, when used in combination with standard pairwise interaction models in diffusion Monte Carlo simulations. Importantly, we find that recent predictions reporting a surprisingly small variation of the kinetic energy and Lindeman ratio on quantum crystals under increasing pressure are likely to be artifacts deriving from the use of incomplete interaction models. Also, we show that the experimental variation of the shear modulus, $C_{44}$, at pressures $0\le P\le 25$ GPa can be quantitatively described by our set of SK three-body potentials. At higher compression, however, the agreement between our $C_{44}$ calculations and experiments deteriorates and thus we argue that higher order many-body terms in the expansion of the atomic interactions probably are necessary in order to better describe elasticity in very dense solid ${}^4\mathrm{He}$.

We theoretically demonstrate the Ruderman-Kittel-Kasuya-Yosida interaction between magnetic impurities that is mediated by the Weyl fermions embedded inside a three-dimensional Weyl semimetal (WSM). The WSM is characterized by a pair of Weyl points separated in the momentum space. Using the Green's function method and a two-band model, we show that four terms contribute to the magnetic impurity interaction in the WSM phase: the Heisenberg, Dzyaloshinsky-Moriya, spin-frustrated, and Ising terms. Except for the last term which is vanishingly small in the plane perpendicular to the line connecting two Weyl points, all the other interaction terms are finite. Furthermore, the magnetic spins of the Dzyaloshinsky-Moriya and spin-frustrated terms lie in the plane perpendicular to the line connecting two Weyl points, but in this plane, the magnetic spins of the Ising term have no components. For each contribution, an analytical expression is obtained, falling off with a spatial dependence as $R^{-5}$ at Weyl points and showing beating behavior that depends on the direction between two magnetic impurities.