Domain pattern variations with temperature were studied by polarizing light microscopy for the morphotropic phase boundary composition $0.67\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_3-0.33\mathrm{PbTi}{\mathrm{O}}_3$ (PMN-0.33PT) single crystal. At room temperature, the monoclinic ${\mathrm{M}}_{\mathrm{A}}$ phase is the dominant phase in the unpoled crystal, which coexists with a small fraction of the tetragonal (T) phase. The orientation of spontaneous polarization was calculated to be $\sim 4.8^{\circ }$ away from the pseudocubic ${\langle 111\rangle }_C$ in the ${\left\{001\right\}}_C$ plane family. Under an electric field of 6 kV/cm along ${\left[011\right]}_C$, a single domain orthorhombic (O) phase was induced but partially switched back to ${\mathrm{M}}_{\mathrm{A}}$ a few hours after the removal of the $E$ field. It was found that the temperature induced phase transition sequence of the ${\left[011\right]}_C$ poled PMN-0.33PT single crystal is strongly hysteretic. On heating, the phase transition sequence is as follows: coexistence of O phase and ${\mathrm{M}}_{\mathrm{A}}\to \mathrm{rhombohedral}\to \mathrm{monoclinic}{\mathrm{M}}_{\mathrm{C}}\to \mathrm{cubic}\left(\mathrm{C}\right)$. On cooling, the phase transition sequence is given by $\mathrm{C}\to \mathrm{T}\to {\mathrm{M}}_{\mathrm{A}}$. The complete set of dielectric, piezoelectric, and elastic constants for the ${\left[011\right]}_C$ poled PMN-0.33PT single crystal was measured, which showed the strong feature of the single domain O phase with high shear $(d_{15}=2321\mathrm{pC}/\mathrm{N},d_{24}=1941\mathrm{pC}/\mathrm{N})$ and low longitudinal piezoelectric coefficients $(d_{33}=165\mathrm{pC}/\mathrm{N})$.

We study the parity effect and transport due to quasiparticles in circuits comprised of many superconducting islands. We develop a general approach and show that it is equivalent to previous methods for describing the parity effect in their more limited regimes of validity. As an example we study transport through linear arrays of Josephson junctions in the limit of negligible Josephson energy and observe the emergence of the parity effect with decreasing number of nonequilibrium quasiparticles. Due to the exponential increase in the number of relevant charge states with increasing length, in multijunction arrays the parity effect manifests in qualitatively different ways to the two-junction case. The role of charge disorder is also studied as this hides much of the parity physics that would otherwise be observed. Nonetheless, we see that the current through a multijunction array at low bias is limited by the formation of metastable even-parity states.

Recent experiments in the cuprates have seen evidence of a transient superconducting state upon optical excitation polarized along the $c$ axis [R. Mankowsky et al., Nature (London) 516, 71 (2014)]. Motivated by these experiments, we propose an extension of the single-layer $t-J-V$ model of cuprates to three dimensions in order to study the effects of interplane tunneling on the competition between superconductivity and bond density wave order. We find that an optical pump can suppress the charge order and simultaneously enhance superconductivity, due to the inherent competition between the two. We also provide an intuitive picture of the physical mechanism underlying this effect. Furthermore, based on a simple Floquet theory, we estimate the magnitude of the enhancement.

The influence of intrinsic Fe defects in FeO (either single cation vacancies or prototypical 4:1 vacancy clusters) on electronic and dielectric properties is studied within density-functional theory. The importance of local Coulomb interactions at Fe atoms is highlighted and shown to be responsible for the observed insulating Mott gap in FeO, which is reduced by the presence of defects. We investigate nonstoichiometric configurations of ${\mathrm{Fe}}_{1-x}\mathrm{O}$ with $x$ ranging from 3% to 9%, and we find the aliovalent Fe cations in both the regular and interstitial lattice sites of the considered configurations. Furthermore, we show that the trivalent Fe ions, induced by both isolated and clustered Fe vacancies, introduce the empty band states inside the insulating gap, which decreases monotonically with increasing cation vacancy concentration. The ${\mathrm{Fe}}_{1-x}\mathrm{O}$ systems with high defect content become metallic for small values of the Coulomb interaction $U$, yielding an increase in the dielectric functions and optical reflectivity at low energies, in agreement with the experimental data. Due to the crystal defects, the infrared-active transverse optic phonons split and distribute over a wide range of frequencies, clarifying the origin of the exceptionally large spectral linewidths of the dielectric loss functions observed for wüstite in recent experiments.

We study theoretically the ultrafast spin dynamics of II-VI diluted magnetic semiconductors in the presence of spin-orbit interaction. Our goal is to explore the interplay or competition between the exchange $\mathrm{sd}$ coupling and the spin-orbit interaction in both bulk and quantum-well systems. For bulk materials we concentrate on ${\mathrm{Zn}}_{1-x}{\mathrm{Mn}}_x\mathrm{Se}$ and take into account the Dresselhaus interaction, while for quantum wells we examine ${\mathrm{Hg}}_{1-x-y}{\mathrm{Mn}}_x{\mathrm{Cd}}_y\text{Te}$ systems with a strong Rashba coupling. Our calculations were performed with a recently developed formalism which incorporates electronic correlations beyond mean-field theory originating from the exchange $\mathrm{sd}$ coupling. For both bulk and quasi-two-dimensional systems we find that, by varying the system parameters within realistic ranges, either of the two interactions can be chosen to play a dominant role or they can compete on an equal footing with each other. The most notable effect of the spin-orbit interaction in both types of system is the appearance of strong oscillations where the exchange $\mathrm{sd}$ coupling by itself causes only an exponential decay of the mean electronic spin components. The mean-field approximation is also studied and an analytical interpretation is given as to why it shows a strong suppression of the spin-orbit-induced dephasing of the spin component parallel to the Mn magnetic field.

While the microscopic transfer properties of carriers are of primary importance for carrier transport of organic semiconductors, the mesoscopic features including the morphologies of grains and the structure of grain boundaries limit the overall carrier transport particularly in polycrystalline organic thin films. Thus the conventional evaluation methods of carrier mobility that rely on macroscopic properties such as $I\text{−}V$ curves of devices are not capable to determine carrier transfer probability at the molecular level. Here, we present a method for evaluating the relative strengths of transfer integrals using charge-modulation spectroscopy on thin-film transistors of dinaphtho[$2,3\text{−}b$:2′,3′-$f$]thieno[$3,2\text{−}b$]thiophene (DNTT) and its alkylated derivatives (${\mathrm{C}}_n$-DNTT, $n=8$, 10, and 12). The band edges of absorption spectra of holes at around 1.9 eV show bathochromic shifts with increasing length of alkyl chains introduced at both ends of a DNTT chromophore. Applying a two-dimensional model with Holstein-type Hamiltonians to electronic transitions of holes, we have been able to simulate the features of the absorption band edges observed. The simulations indicate that the bathochromic shifts are due to an increase in the transfer integrals of holes with increasing length of alkyl chains. Thus this analysis confirmed that the subtle changes in the mutual orientations between adjacent DNTT chromophores induced by alkyl chains enhance the microscopic hole transfer rate. Although this fastener effect has been suggested by hole mobility measurements by $I\text{−}V$ curves, the spectral analysis in this study gives clear evidence of this effect at the molecular level.

Optical second-harmonic generation (SHG) has been measured in a reflection from the nanometer-thick films (6 to 40 nm) of the topological insulator $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ using 1.51 eV (820 nm) Ti:Sapphire laser photons and revealed a strong dependence of the integral SHG intensity on the film thickness. The integral SHG intensity was determined by area integration of the SHG rotational anisotropy patterns measured for different input-output light polarization geometries. A ∼100-fold enhancement of the integral SHG intensity with decreasing film thickness has been suggested to result from the dc-electric-field-induced SHG (EFISHG) effects. Two sources of dynamically created dc electric field were proposed: (i) the capacitor-type dc electric field that gradually increases with decreasing film thickness from 40 to 6 nm due to a dynamical imbalance of photoexcited long-lived carriers between the opposite-surface Dirac surface states and (ii) a dc electric field associated with a nonlinearly excited Dirac plasmon, which is responsible for the resonant enhancement of the integral SHG intensity for the 10 nm thick film with a Lorentz-shaped resonance of ∼1.6 nm full width at half maximum. In addition to the general SHG enhancement trends with decreasing film thickness, a relative decrease of the out-of-plane contribution with respect to the in-plane contribution was observed. Using a theoretical treatment of the measured SHG rotational anisotropy patterns, this effect has been suggested to result from the joint contributions of the linear and quadratic dc electric field effects to the EFISHG response.

We report on strong transport anisotropy in a two-dimensional hole gas in a Ge/SiGe quantum well, which emerges only when both perpendicular and in-plane magnetic fields are present. The ratio of resistances, measured along and perpendicular to the in-plane field, can exceed $3\times {10}^4$. The anisotropy occurs in a wide range of filling factors where it is determined primarily by the tilt angle. The lack of significant anisotropy without an in-plane field, easy tunability, and persistence to higher temperatures and filling factors set this anisotropy apart from nematic phases in GaAs/AlGaAs.

An interband hybridization gap with spin-dependent size was detected in a low-dimensional electron system influenced by strong spin-orbit coupling. Energy gaps between hybridizing states are distinctly influenced by strong spin-orbit coupling. If the size of these gaps depends on the spin direction of the states, this may lead to fully spin-polarized valleylike structures. As a model system to study the hybridization mechanism behind such valleys, we investigate the unoccupied electronic band structure of Pb/Cu(111) by spin- and angle-resolved inverse photoemission in combination with first-principles calculations. Here, we find a hybridization gap with a splitting of about 200 meV for the one and even larger than 500 meV for the other spin direction. We develop an effective two-band tight-binding model and demonstrate that the interplay of adlayer and substrate states is crucial to induce a sizable spin-orbit coupling and hybridization strength to the interacting states.

This paper reports on a study of the resistivity and Hall effect of the ferromagnetic pyrochlore $\mathrm{L}{\mathrm{u}}_2{\mathrm{V}}_2{\mathrm{O}}_7$. The temperature dependence of the resistivity of single crystalline $\mathrm{L}{\mathrm{u}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ exhibits overall activation behavior with a metalliclike exception at intermediate temperatures near the Curie temperature $\left(T_C\right)$. This temperature dependence bears a surprising resemblance to that of doped semiconductors. The ferromagnetic oxide shows a negative magnetoresistance (MR) which scales quadratically with the reduced magnetization at temperatures above $T_C$; however, the scaling factor is significantly smaller than the value expected for a ferromagnetic system in the pure spin scattering regime, which suggests that other scattering processes may be at work. Concomitant with the negative MR, a distinct switch in the Hall resistivity slope is observed at temperatures near $T_C$. Our analysis suggests that the nonlinear Hall effect is associated with a change in the effective carrier density at a constant critical magnetization induced by an external magnetic field. We argue that within a picture that incorporates high temperature activation transport with a magnetization-driven charge percolation transition, the observed complex electronic transport in the ferromagnetic pyrochlore can be quantitatively described.

It is proposed that the ground-state manifold of the neutral nitrogen-vacancy center in diamond could be used as a quantum two-level system in a solid-state-based implementation of a broadband noise-free quantum optical memory. The proposal is based on the same-spin $\Lambda$-type three-level system created between the two $E$ orbital ground states and the $A_1$ orbital excited state of the center, and the cross-linear polarization selection rules obtained with the application of a transverse electric field or uniaxial stress. Possible decay and decoherence mechanisms of this system are discussed, and it is shown that high-efficiency, noise-free storage of photons as short as a few tens of picoseconds for at least a few nanoseconds could be possible at low temperature.

Classical nonlocality in conducting nanostructures has been shown to dramatically alter the linear optical response by placing a fundamental limit on the maximum field enhancement that can be achieved. This limit directly extends to all nonlinear processes, which depend on field amplitudes. A numerical study of third-harmonic generation in metal film-coupled nanowires reveals that for subnanometer vacuum gaps, the nonlocality may boost the effective nonlinearity by 5 orders of magnitude as the field penetrates deeper inside the metal than that predicted assuming a purely local electronic response. We also study the impact of a nonlinear dielectric placed in the gap region. In this case the effect of nonlocality could be masked by the third-harmonic signal generated by the spacer. By etching the dielectric underneath the nanowire, however, it is possible to muffle such contributions. Calculations are performed for both silver and gold nanowire.

The $\nu =5/2$ fractional quantum Hall effect is a system of intense experimental and theoretical interest as its ground state may host non-Abelian excitations, but the exact nature of the ground state is still undetermined. We present the results of an exact diagonalization study of an electron system in the disk configuration, including the effects of Landau level (LL) mixing and the finite thickness of the quantum well confining the electrons. The degeneracy between the two leading candidates for the ground state, the Pfaffian and anti-Pfaffian, is broken by interactions with a neutralizing background, in addition to the inclusion of two- and three-body interactions via LL mixing. As a result of the neutralizing background in the disk configuration, there is a phase transition from the anti-Pfaffian to the Pfaffian as LL mixing is turned on, in stark contrast to what is observed in spherical geometry. This behavior is in agreement with existing experiments, showing the appearance of the Pfaffian state at strong LL mixing before the system enters a compressible phase. The inclusion of LL mixing leads to an increased charge $e/4$ quasihole size. LL mixing interactions are also shown to overcome the effects of edge reconstruction. Due to finite thickness effects, these properties are enhanced dramatically. We also find that only the Pfaffian and anti-Pfaffian states continue to possess energy gaps at finite width, while gaps for compressible stripe states close, which is in agreement with available experimental data.

The centrosymmetric two-dimensional material bilayer graphene (BLG) does not show dipole-allowed second-harmonic generation (SHG) in its pristine form. However, the symmetry can be broken by applying an electric field perpendicular to the layer. Here, we present a theoretical study of SHG from biased BLG. We show that the sheet second-harmonic susceptibility reaches very large values of several hundred ${\mathrm{nm}}^2$/V in the midinfrared region. The SHG is tunable depending on the strength of the electric field. Furthermore, a strong, tunable double resonance appears in the spectrum. We believe that this study could spark interest in the nonlinear optical properties of biased BLG.

Spin-transfer torque (STT) effects on the stationary forced response of nanoscale ferromagnets subject to thermal fluctuations and driven by an ac magnetic field of arbitrary strength and direction are investigated via a generic nanopillar model of a spin-torque device comprising two ferromagnetic strata representing the free and fixed layers and a nonmagnetic conducting spacer all sandwiched between two Ohmic contacts. The STT effects are treated via Brown's magnetic Langevin equation generalized to include the Slonczewski STT term thereby extending the statistical moment method [Y. P. Kalmykov et al., Phys. Rev. B 88, 144406 (2013)] to the forced response of the most general version of the nanopillar model. The dynamic susceptibility, nonlinear frequency-dependent dc magnetization, dynamic hysteresis loops, etc. are then evaluated highlighting STT effects on both the low-frequency thermal relaxation processes and the high-frequency ferromagnetic resonance, etc., demonstrating a pronounced dependence of these on the spin polarization current and facilitating interpretation of STT experiments.

We present a field-theory analysis of a model of two $\mathrm{SU}\left(2n\right)$-invariant magnetic chains coupled by a generic interaction preserving time-reversal and inversion symmetry. Contrary to the SU(2)-invariant case the zero-temperature phase diagram of such two-leg spin ladder does not contain topological phases. Only generalized valence bond solid phases are stabilized when $n>1$ with different wave vectors and ground-state degeneracies. In particular, we find a phase which is made of a cluster of $2n$ spins put in an $\mathrm{SU}\left(2n\right)$ singlet state. For $n=3$, this cluster phase is relevant to ${}^{173}\mathrm{Yb}$ ultracold atoms, with an emergent SU(6) symmetry, loaded in a double-well optical lattice.

The highest antiferromagnetic (AFM) temperature in Ce based compounds has been reported for CeScGe with $T_N=47\mathrm{K}$, but its local or itinerant nature has not been deeply investigated yet. In order to shed more light into this unusually high ordering temperature we have investigated structural, magnetic, transport, and thermal properties of ${\mathrm{CeTi}}_{1-x}{\mathrm{Sc}}_x\mathrm{Ge}$ alloys within the range of stability of the CeScSi-type structure: $0.25\le x\le 1$. Along this concentration range, this strongly anisotropic system presents a complex magnetic phase diagram with a continuous modification of its magnetic behavior, from ferromagnetism for $0.25\le x\le 0.50$ (with $7\text{K}\le T_C\le 16\mathrm{K}$) to AFM for $0.60\le x\le 1$ (with $19\text{K}\le T_N\le 47\mathrm{K}$). The onset of the AFM phase is associated to a metamagnetic transition with a critical field increasing from $H_{\mathrm{cr}}=0$ at $x\approx 0.55$ to $\approx 6\mathrm{T}$ at $x=1$, coincident with an increasing contribution of the first excited crystal electric field doublet. At a critical point $x_{\mathrm{cr}}\approx 0.65$ a second transition appears at $T_L\le T_N$. In contrast to observations in itinerant systems like ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$ or ${\mathrm{CeRh}}_3{\mathrm{B}}_2$, no evidences for significant hybridization of the $4f$ electrons at large Sc contents were found. Therefore, the exceptionally large $T_N$ of CeScGe can be attributed to an increasing Ruderman-Kittel-Kasuya-Yosida interaction between Ce double layers as Sc content grows.

The electronic structures of the ferromagnetic superconductors ${\mathrm{UGe}}_2$ and $\mathrm{UCoGe}$ in the paramagnetic phase were studied by angle-resolved photoelectron spectroscopy using soft x rays $\left(h\nu =400\text{–}500\mathrm{eV}\right)$. The quasiparticle bands with large contributions from $\mathrm{U}5f$ states were observed in the vicinity of $E_{\mathrm{F}}$, suggesting that the $\mathrm{U}5f$ electrons of these compounds have an itinerant character. Their overall band structures were explained by the band-structure calculations treating all the $\mathrm{U}5f$ electrons as being itinerant. Meanwhile, the states in the vicinity of $E_{\mathrm{F}}$ show considerable deviations from the results of band-structure calculations, suggesting that the shapes of Fermi surface of these compounds are qualitatively different from the calculations, possibly caused by electron correlation effect in the complicated band structures of the low-symmetry crystals. Strong hybridization between $\mathrm{U}5f$ and $\mathrm{Co}3d$ states in $\mathrm{UCoGe}$ were found by the $\mathrm{Co}2p-3d$ resonant photoemission experiment, suggesting that $\mathrm{Co}3d$ states have finite contributions to the magnetic, transport, and superconducting properties.

We study dephasing of a superconducting qubit due to quasiparticle tunneling through a Josephson junction. While qubit decay due to tunneling processes is well understood within a golden rule approximation, pure dephasing due to BCS quasiparticles gives rise to a divergent golden rule rate. We calculate qubit dephasing due to quasiparticle tunneling beyond lowest-order approximation in coupling between qubit and quasiparticles. Summing up a certain class of diagrams, we show that qubit dephasing due to purely longitudinal coupling to quasiparticles leads to dephasing $\sim exp[-x(t\left)\right]$ where $x\left(t\right)\propto t^{3/2}$ for short-time scales and $x\left(t\right)\propto tln\left(t\right)$ for long-time scales.

Hexagonal $\alpha$-Ru trichloride single crystals exhibit a strong magnetic anisotropy and we show that upon applying fields up to 14 T in the honeycomb plane the successive magnetic order at $T_1=14\mathrm{K}$ and $T_2=8\mathrm{K}$ could be completely suppressed, whereas in the perpendicular direction the magnetic order is robust. Furthermore, the field dependence of $\chi \left(\mathrm{T}\right)$ implies coexisting ferro- and antiferromagnetic exchange between in-plane components of ${\text{Ru}}^{3+}$ spins, whereas for out-of-plane components a strong antiferromagnetic exchange becomes evident. ${}^{101}\mathrm{Ru}$ zero-field nuclear magnetic resonance in the ordered state evidence a complex (probably noncoplanar chiral) long-range magnetic structure. The large orbital moment on ${\mathrm{Ru}}^{3+}$ is found in density-functional calculations.