We study the polarization-dependent polariton-polariton interaction through its effect on a parametric scattering process in a microcavity (MC). The ratio of the anticircular interaction strength $V_2$ over its cocircular counterpart $V_1$ is involved in defining the regime in which many nonlinear processes arise in MCs, such as parametric conversion or condensation. We measure the ratio $V_2/V_1$ using a stimulated energy-degenerate parametric scattering process in a multiple MC. The sample is pumped at normal incidence, probed with a nonzero angle, and the phase-matched idler is observed at the opposite angle. The idler behavior, both in power and polarization, is compared to a Hamiltonian interaction model that takes into account the two polarization-dependent parametric scattering channels characterized by $V_1$ and $V_2$. The proposed method to measure the ratio $V_2/V_1$ is convenient and precise. The flexibility of the triple MC allows us to observe the process and measure this ratio over a large range of detunings, where we find it to be highly dependent on the detuning. These measurements complement the previous study of Vladimirova et al. [Phys. Rev. B 82, 075301 (2010)] with an original approach and for detunings that were unexplored up to now.

We study the scattering phase shift of Dirac fermions at graphene edge. We find that when a plane wave of a Dirac fermion is reflected at an edge of graphene, its reflection phase is shifted by the geometric phase resulting from the change of the pseudospin of the Dirac fermion in the reflection. The geometric phase is the Pancharatnam-Berry phase that equals the half of the solid angle on Bloch sphere determined by the propagation direction of the incident wave and also by the orientation angle of the graphene edge. The geometric phase is finite at the zigzag edge in general, while it always vanishes at the armchair edge because of intervalley mixing. To demonstrate its physical effects, we first connect the geometric phase with the energy band structure of graphene nanoribbon with the zigzag edge. The magnitude of the band gap of the nanoribbon, that opens in the presence of the staggered sublattice potential induced by edge magnetization, is related to the geometric phase. Second, we numerically study the effect of the geometric phase on the Veselago lens formed in a graphene nanoribbon. The interference pattern of the lens is distinguished between armchair and zigzag nanoribbons, which is useful for detecting the geometric phase.

We report on a detailed quasiparticle interference (QPI) scattering study of the Ni(111) surface by low-temperature scanning tunneling spectroscopy (LT-STS). While conventional constant-separation STS shows two broad features, which are interpreted as the ${\Lambda }_3$ bulk band and $sp$-like Shockley-type surface state ($sp$-SS), energy-dependent Fourier-transformed QPI maps reveal the band dispersion of the underlying surface electronic features. We find two electronlike branches in the $sp$-SS dispersion, which are interpreted as the exchange-split minority and majority spin part. The exchange splitting is determined to $\Delta E_{\mathrm{ex}}=100\pm 8$ meV. In addition, a holelike $d$-derived surface resonance is found. Band onsets and effective electron masses are determined by fitting the band dispersion with a parabola at small $k$ values. Hybridization effects with bulk electronic states are observed towards larger $k$ values. Prominent quantum confinement phenomena of the $sp$-SS are observed in STS data obtained within vacancy islands. The results can be interpreted within a one-dimensional quantum-well model.

X-ray absorption and magnetic circular dichroism spectra at the $L_3$ edge of Yb have been studied theoretically in the mixed-valent compounds ${\mathrm{YbAgCu}}_4$ and ${\mathrm{YbInCu}}_4$ using the fully relativistic Dirac linear muffin-tin orbital band structure method. The effect of the spin-orbit (SO) interaction and Coulomb repulsion $U$ on the electronic structure of ${\mathrm{YbAgCu}}_4$ and ${\mathrm{YbInCu}}_4$ is examined in the frame of the local spin-density approximation (LSDA)+SO+$U$ method. The core-hole effect in the final states as well as the effect of the electric quadrupole $E_2$ and magnetic dipole $M_1$ transitions have been investigated. The calculated results are compared with available experimental data.

Using an error-counting scheme, we evaluated the accuracy of single-electron (SE) transfer in Si tunable-barrier turnstiles in a dilution refrigerator. The error counting was performed by shuttling SEs between a lead and a charge-accumulating node and detecting in real time the number of electrons in the node with an SE-resolution charge sensor. The best experimentally obtained error rate is about 100 parts per million (ppm), where the SE capture in the SE island occurs in thermal equilibrium due to the heating effect on SEs caused by pulse voltages applied to modulate the potential barrier. When we reduce the heating effect by suppressing the pulse voltages, there is a change in the SE transfer mechanism to nonequilibrium SE capture. We theoretically discuss the crossover point of the change. Moreover, at the minimum pulse voltage, the theoretical lower bound of the error rate estimated by fitting is on the order of 0.01 ppm. This suggests that Si SE-transfer devices are promising candidates for use as quantum current standards in metrology.

Combining density functional theory calculations and microscopic tight-binding models, we investigate theoretically the electronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity of the (110) surface of GaAs. For the case of the ${\left[{\mathrm{Mn}}^{2+}\right]}^0$ plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity $d$ electrons are in good agreement with approaches that treat the spin of the impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic ${\left[{\mathrm{Fe}}^{3+}\right]}^0$ and the ionized ${\left[{\mathrm{Fe}}^{2+}\right]}^{-}$ states are relevant to address scanning tunneling microscopy (STM) experiments, the inclusion of $d$ orbitals is essential. We find that the in-gap electronic structure of Fe impurities is significantly modified by surface effects. For the neutral acceptor state ${[{\mathrm{Fe}}^{2+},h]}^0$, the magnetic-anisotropy dependence on the impurity sublayer resembles the case of ${[{\mathrm{Mn}}^{2+},h]}^0$. In contrast, for ${\left[{\mathrm{Fe}}^{3+}\right]}^0$ electronic configuration the magnetic anisotropy behaves differently and it is considerably smaller. For this state we predict that it is possible to manipulate the Fe moment, e.g., by an external magnetic field, with detectable consequences in the local density of states probed by STM.

The lithographic fabrication of metal nanoparticles usually involves the thermal vacuum deposition of metals, which leads to polycrystallinity and surface roughness. In recent years, strong efforts have been made to clarify the role of such roughness in surface-enhanced Raman scattering (SERS). In this paper, we provide a systematic experimental and theoretical study of single lithographically fabricated nanoparticles to unravel the role of surface roughness and morphology on the optical far- and near-field properties. We find that the experimentally observed reduction of the SERS signal upon thermal annealing of particle arrays is caused by a complex interplay of changes in the dielectric response of gold, the resonance wavelength, and the reduced nanoscopic roughness.

We demonstrate strain tuning of magnetocrystalline anisotropy over a range of more than 1000 G in epitaxial $Y_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ films of excellent crystalline quality grown on lattice-mismatched Y${}_3$Al${}_5{\mathrm{O}}_{12}$ substrates. Ferromagnetic resonance (FMR) measurements reveal a linear dependence of both out-of-plane and in-plane uniaxial anisotropy on the strain-induced tetragonal distortion of $Y_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. Importantly, we find the spin mixing conductance $G_r$ determined from inverse spin Hall effect and FMR linewidth broadening remains large: $G_r$ = 3.33 × 10${}^{14}$ Ω${}^{-1}$ m${}^{-2}$ in Pt/$Y_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$/Y${}_3$Al${}_5{\mathrm{O}}_{12}$ heterostructures, quite comparable to the value found in Pt/$Y_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ grown on lattice-matched Gd${}_3$Ga${}_5{\mathrm{O}}_{12}$ substrates.

Large high-quality single crystals of hole-doped iron-based superconductor (Ba${}_{1-x}$K${}_x$)Fe${}_2$As${}_2$ were grown over a broad composition range $0.22\le x\le 1$ by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and an adjusted temperature window of the growth are necessary to obtain single crystals of heavily K-doped crystals ($0.65\le x\le 0.92$) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the $c$-axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity $\rho \left(T\right)$ found doping-independent, constant within error bars, resistivity at room temperature, $\rho \left(300\mathrm{K}\right)$, in sharp contrast with the significant doping dependence in electron and isovalent substituted BaFe${}_2$As${}_2$ based compositions. The shape of the temperature-dependent resistivity, $\rho \left(T\right)$, shows systematic doping-evolution, being close to $T^2$ in overdoped and revealing significant contribution of the $T$-linear component at optimum doping. The slope of the upper critical field, $d{H}_{c2}/dT$, scales linearly with $T_c$ for both $H\parallel c$, $H_{c2,c}$, and $H\parallel ab$, $H_{c2,ab}$. The anisotropy of the upper critical field $\gamma \equiv H_{c2,ab}/H_{c2,c}$ determined near zero-field $T_c$ increases from $\sim$2 to 4–5 with increasing K doping level from optimal $x\sim 0.4$ to strongly overdoped $x=1$.

We study the finite-temperature transition in a model $XY$ antiferromagnet on a pyrochlore lattice, which describes the pyrochlore material ${\mathrm{Er}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. The ordered magnetic structure selected by thermal fluctuations is sixfold degenerate. Nevertheless, our classical Monte Carlo simulations show that the critical behavior corresponds to the three-dimensional $XY$ universality class. We determine an additional critical exponent ${\nu }_6=0.75>\nu$ characteristic of a dangerously irrelevant scaling variable. Persistent thermal fluctuations in the ordered phase are revealed in Monte Carlo simulations by the peculiar coexistence of Bragg peaks and diffuse magnetic scattering, the feature also observed in neutron diffraction experiments.

Recent work shows that highly excited many-body localized eigenstates can exhibit broken symmetries and topological order, including in dimensions where such order would be forbidden in equilibrium. In this paper we extend this analysis to discrete symmetry-protected order via the explicit examples of the Haldane phase of one-dimensional spin chains and the topological Ising paramagnet in two dimensions. We comment on the challenge of extending these results to cases where the protecting symmetry is continuous.

Recently an extended series of equally spaced vibrational modes was observed in uranium nitride (UN) by performing neutron spectroscopy measurements using the ARCS and SEQUOIA time-of-flight chopper spectrometers [A. A. Aczel et al., Nat. Commun. 3, 1124 (2012)]. These modes are well described by three-dimensional isotropic quantum harmonic oscillator (QHO) behavior of the nitrogen atoms, but there are additional contributions to the scattering that complicate the measured response. In an effort to better characterize the observed neutron scattering spectrum of UN, we have performed Monte Carlo ray tracing simulations of the ARCS and SEQUOIA experiments with various sample kernels, accounting for nitrogen QHO scattering, contributions that arise from the acoustic portion of the partial phonon density of states, and multiple scattering. These simulations demonstrate that the U and N motions can be treated independently, and show that multiple scattering contributes an approximate $Q$-independent background to the spectrum at the oscillator mode positions. Temperature-dependent studies of the lowest few oscillator modes have also been made with SEQUOIA, and our simulations indicate that the $T$ dependence of the scattering from these modes is strongly influenced by the uranium lattice.

The time dependence of switching multiferroic domains in ${\mathrm{MnWO}}_4$ has been studied by time-resolved polarized neutron diffraction. Inverting an external electric field inverts the chiral magnetic component within rise times ranging between a few and some tens of milliseconds in perfect agreement with macroscopic techniques. There is no evidence for any faster process in the inversion of the chiral magnetic structure. The time dependence is well described by a temperature-dependent rise time suggesting a well-defined process of domain reversion. As expected, the rise times decrease when heating towards the upper boundary of the ferroelectric phase. However, switching also becomes faster upon cooling towards the lower boundary, which is associated with a first-order phase transition.

We investigate the fate of the orthorhombic stripe-type magnetic state [ordering vectors $(\pi ,0)$/$(0,\pi )$], observed in most iron-pnictide superconductors, in the presence of localized magnetic moments that tend to form a Néel state [ordering vector $(\pi ,\pi )$]. We show that before long-range Néel order sets in, the coupling between the conduction electrons and the fluctuations of the local moments favors an unusual magnetic state consisting of a coherent superposition of the $(\pi ,0)$ and $(0,\pi )$ orders that preserves tetragonal symmetry. The magnetization of this state is nonuniform and induces a simultaneous checkerboard charge order. We discuss signatures of this magnetic configuration on the electronic spectrum and its impact on the superconducting state, showing that its phase space for coexistence with the $s^{+-}$ state is smaller than the stripe-type state. Our results shed light on recent experimental observations on $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-\mathrm{x}}{\mathrm{Mn}}_{\mathrm{x}}\right)}_2{\mathrm{As}}_2$ compounds, where the Néel-type local Mn moments interact with the Fe conduction electrons.

Density functional theory was used to study the nonmagnetic (NM) and ferromagnetic (FM) phases of face-centered cubic cerium. Functionals of four levels of approximation for the exchange-correlation energy were used: LDA, PBE, LDA/PBE + $U$, and YS-PBEh. The latter two contain an adjustable parameter, the onsite Coulomb repulsion parameter $U$ for LDA/PBE + $U$, and the fraction ${\alpha }_{\text{x}}$ of Hartree-Fock exchange for YS-PBEh, which were varied in order to study their influence on the results. By supposing that, as a first approximation, the NM and FM solutions can be identified to the observed $\alpha$ and $\gamma$ phases, respectively, it is concluded that while a small value of $U$ or ${\alpha }_{\text{x}}$ leads to the correct trend for the stability ordering of the two phases, larger values are necessary for a more appropriate (but still not satisfying) description of the electronic structure.