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The energy and wave function of a harmonically confined two-electron system coupled to light is calculated by separating the wave functions of the relative and center of mass (CM) motions. The relative motion wave function has a known quasi-analytical solution. The light only couples to the CM variable and the coupled equation can be solved with diagonalization without approximations. The approach works for any coupling strength. Examples of wave functions of light-matter hybrid states are presented.

We present experimental evidence of incoherent Kondo scattering as the source of resistivity minima in bulk polycrystalline and nanocrystalline ${\mathrm{Pr}}_{2}{\mathrm{Ir}}_{2}{\mathrm{O}}_{7}$. The temperature dependence of thermopower shows a positive maximum at high temperature followed by a negative minimum at low temperature, with the sign inversion occurring at a much higher temperature than ${T}_{K}$. Moreover, we observe little correlation between ${T}_{K}$ and intersite coupling strength given by $|{\ensuremath{\theta}}_{\text{CW}}|$. We describe the temperature dependence of thermopower and resistivity within the framework of crystal field excitation in a Kondo lattice.

Floquet higher order topological insulators (FHOTIs) are a novel topological phase that can occur in periodically driven lattices. An appropriate experimental platform to realize FHOTIs has not yet been identified. We introduce a periodically-driven bipartite (two-band) system that hosts FHOTI phases, and predict that this lattice can be realized in experimentally-realistic optical waveguide arrays, similar to those previously used to study anomalous Floquet insulators. The model exhibits interesting phase transitions from first-order to second-order topological matter by tuning a coupling strength parameter, without breaking lattice symmetry. In the FHOTI phase, the lattice hosts corner modes at eigenphase $0$ or $\pi$, which are robust against disorder in the individual couplings., Comment: 5+ pages for the main text, plus 7 pages for the Supplementary Materials, to be submitted to Phys. Rev. B

Confinement of excitations induces quasilocalized dynamics in disorder-free isolated quantum many-body systems in one spatial dimension. This occurrence is signalled by severe suppression of quantum correlation spreading and of entanglement growth, long-time persistence of spatial inhomogeneities, and long-lived coherent oscillations of local observables. In this work, we present a unified understanding of these dramatic effects. The slow dynamical behavior is shown to be related to the Schwinger effect in quantum electrodynamics. We demonstrate that it is quantitatively captured for long time scales by effective Hamiltonians exhibiting Stark localization of excitations and weak growth of the entanglement entropy for arbitrary coupling strength. This analysis explains the phenomenology of real-time string dynamics investigated in a number of lattice gauge theories, as well as the anomalous dynamics observed in quantum Ising chains after quenches. Our findings establish confinement as a robust mechanism for hindering the approach to equilibrium in translationally-invariant quantum statistical systems with local interactions., 7+12 pages, 3+7 figures. v2: close to published version. Selected as "Editors' Suggestion" in Physical Review B

MnGe has been reported as a candidate for a three-dimensional (3D) skyrmion crystal in comparison to the two-dimensional (2D) skyrmion observed in most other B20 compounds like MnSi. In addition, the small-sized skyrmions in MnGe are desired properties for information storage. By performing the density functional theory (DFT) calculations and model simulations based on the DFT-informed tight-binding Hamiltonian, we explore the nature of the 3D skyrmion in MnGe. By invoking the dual nature of $d$ electrons on Mn atoms, we propose a strong-correlation-derived spin-fermion model with an antiferromagnetic coupling between the localized and itinerant moments. This model could explain the drastic difference in magnetic moments between MnGe and MnSi compounds. In addition, we find that the 3D or 2D nature of skyrmions is dependent on the coupling strength.

Spin pumping from the full Heusler alloy $\mathrm{C}{\mathrm{o}}_{2}\mathrm{FeAl}$ film into the transition-metal dichalcogenide $\mathrm{Mo}{\mathrm{S}}_{2}$ is reported. Plasma-assisted sulfurization of ion-beam sputtered Mo films of different nominal thicknesses is employed to first fabricate large area high-quality $\mathrm{Mo}{\mathrm{S}}_{2}$ sheets [thicknesses: 1, 2, 3, and 4 monolayers (MLs)] on $\mathrm{Si}{\mathrm{O}}_{2}/\mathrm{Si}$ substrates, followed by deposition of $\mathrm{C}{\mathrm{o}}_{2}\mathrm{FeAl}$ films with a fixed thickness of 8 nm. The spin pumping is investigated by measuring the changes in the damping constant in the Al(5 nm) capped $\mathrm{C}{\mathrm{o}}_{2}\mathrm{FeAl}/\mathrm{Mo}{\mathrm{S}}_{2}$ bilayers using ferromagnetic resonance spectroscopy. The study demonstrates that even 1 ML of $\mathrm{Mo}{\mathrm{S}}_{2}$ possesses high enough spin-orbit coupling strength to enhance damping from $5.5(\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ in $\mathrm{Al}(5\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{C}{\mathrm{o}}_{2}\mathrm{FeAl}(8\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$ to a nearly saturated value of $8.3(\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ in $\mathrm{Al}(5\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{C}{\mathrm{o}}_{2}\mathrm{FeAl}(8\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Mo}{\mathrm{S}}_{2}(1\phantom{\rule{0.16em}{0ex}}\mathrm{ML})$, which is suppressed by inserting a thin Al layer at the $\mathrm{C}{\mathrm{o}}_{2}\mathrm{FeAl}/\mathrm{Mo}{\mathrm{S}}_{2}$ interface. The observed enhancement in damping is in agreement with the results from first-principles calculations based on density functional theory. These results open up a paradigm for designing spintronic devices based on heterostructures comprising a full Heusler alloy and the inherently stable $\mathrm{Mo}{\mathrm{S}}_{2}$.

Electronic-vibrational coupling in single-molecule junctions may result in current-induced bond rupture and is thus an important mechanism for the stability of molecular junctions. We use the hierarchical quantum master equation (HQME) method in combination with the quasi-classical Ehrenfest approach for the nuclear degrees of freedom to simulate current-induced bond rupture in single-molecule junctions. Employing generic models for molecular junctions with dissociative nuclear potentials, we analyze the underlying mechanisms. In particular, we investigate the dependence of the dissociation probability on the applied bias voltage and the molecule-lead coupling strength. The results show that an applied bias voltage can not only lead to dissociation of the molecular junction, but under certain conditions can also increase the stability of the molecule.

We numerically examine the manipulation of superconducting vortices interacting with a moving trap representing a magnetic force tip translating across a superconducting sample containing a periodic array of pinning sites. As a function of the tip velocity and coupling strength, we find five distinct dynamic phases, including a decoupled regime where the vortices are dragged a short distance within a pinning site, an intermediate coupling regime where vortices in neighboring pinning sites exchange places, an intermediate trapping regime where individual vortices are dragged longer distances and exchange modes of vortices occur in the surrounding pins, an intermittent multiple trapping regime where the trap switches between capturing one or two vortices, and a strong coupling regime in which the trap permanently captures and drags two vortices. In some regimes we observe the counterintuitive behavior that slow moving traps couple less strongly to vortices than faster moving traps; however, the fastest moving traps are generally decoupled. The different phases can be characterized by the distances the vortices are displaced and the force fluctuations exerted on the trap. We find different types of stick-slip motion depending on whether vortices are moving into and out of pinning sites, undergoing exchange, or performing correlated motion induced by vortices outside of the trap. Our results are general to the manipulation of other types of particle-based systems interacting with periodic trap arrays, such as colloidal particles or certain types of frictional systems.

According to standard theory the magnetoresistance magnitude in ferromagnetic manganites crucially depends on the electron-phonon coupling strength. We showed that in La0.7Sr0.3MnO3 the phonon renormalization is strong, despite its relatively small magnetoresistance. Here, we report results of a similar inelastic neutron-scattering investigation of a closely related compound, La0.8Sr0.2MnO3, where the magnetoresistance is enhanced. We find similar phonon renormalization and dynamic CE-type polaron correlations as in La0.7Sr0.3MnO3. However, quantitative comparison of the results for the two samples shows that only polaron lifetime is well correlated with the strength of the colossal magnetoresistance.

Using exact diagonalization, we study the spin-orbit coupling and interaction-induced mixing between ${t}_{2g}$ and ${e}_{g}\phantom{\rule{4pt}{0ex}}d$-orbital states in a cubic crystalline environment, as commonly occurs in transition metal oxides. We make a direct comparison with the widely used ${t}_{2g}$-only or ${e}_{g}$-only models, depending on electronic filling. We consider all electron fillings of the $d$ shell and compute the total magnetic moment, the spin, the occupancy of each orbital, and the effective spin-orbit coupling strength (renormalized through interaction effects) in terms of the bare interaction parameters, spin-orbit coupling, and crystal-field splitting, focusing on the parameter ranges relevant to $4d$ and $5d$ transition metal oxides. In various limits, we provide perturbative results consistent with our numerical calculations. We find that the ${t}_{2g}\ensuremath{-}{e}_{g}$ mixing can be large, with up to 20% occupation of orbitals that are nominally ``empty,'' which has experimental implications for the interpretation of the branching ratio in experiments, and can impact the effective local moment Hamiltonian used to study magnetic phases and magnetic excitations in transition metal oxides. Our results can aid the theoretical interpretation of experiments on these materials, which often fall in a regime of intermediate coupling with respect to electron-electron interactions.

We report the discovery of superconductivity in pressurized CeRhGe3, until now the only remaining non-superconducting member of the isostructural family of non-centrosymmetric heavy-fermion compounds CeTX3 (T = Co, Rh, Ir and X = Si, Ge). Superconductivity appears in CeRhGe3 at a pressure of 19.6 GPa and the transition temperature Tc reaches a maximum value of 1.3 K at 21.5 GPa. This finding provides an opportunity to establish systematic correlations between superconductivity and materials properties within this family. Though ambient-pressure unit-cell volumes and critical pressures for superconductivity vary substantially across the series, all family members reach a maximum Tcmax at a common critical cell volume Vcrit, and Tcmax at Vcrit increases with increasing spin-orbit coupling strength of the d-electrons. These correlations show that substantial Kondo hybridization and spin-orbit coupling favor superconductivity in this family, the latter reflecting the role of broken centro-symmetry.

We theoretically demonstrate that the in-plane magnetization induced quantum anomalous Hall effect (QAHE) can be realized in atomic crystal layers of group-V elements with buckled honeycomb lattice. We first construct a general tight-binding Hamiltonian with $sp^3$ orbitals via Slater-Koster two-center approximation, and then numerically show that for weak and strong spin-orbit couplings the systems harbor QAHEs with Chern numbers of $\mathcal{C}=\pm1$ and $\pm2$ , respectively. For the $\mathcal{C}=\pm1$ phases, we find the critical phase-transition magnetization from a trivial insulator to QAHE can become extremely small by tuning the spin-orbit coupling strength. Although the resulting band gap is small, it can be remarkably enhanced by orders via tilting the magnetization slightly away from the in-plane orientation. For the $\mathcal{C}=\pm2$ phases, we find that the band gap is large enough for the room-temperature observation. Although the critical magnetization is relatively large, it can be effectively decreased by applying a strain. All these suggest that it is experimentally feasible to realize high-temperature QAHE from in-plane magnetization in atomic crystal layers of group-V elements., 6 pages, 3 figures

We study the Anderson single-level quantum dot attached to two BCS superconducting leads with the same gap size. We reveal that a system with asymmetric tunnel coupling to the leads ($\Gamma_{L}\neq\Gamma_{R}$) can be related to the symmetric system with the same net coupling strength $\Gamma=\Gamma_{L}+\Gamma_{R}$. Surprisingly, it is the symmetric case which is the most general, meaning that all physical quantities in case of asymmetric coupling are fully determined by the symmetric ones. We give ready-to-use conversion formulas for the $0-\pi$ phase transition boundary, on-dot quantities, and the Josephson current, and illustrate them on the NRG results of Oguri, Tanaka and Bauer [Phys. Rev. B 87, 075432 (2013)] for the three-terminal setup. We apply our theory to the recent $0-\pi$ transition measurement of Delagrange et al. [Phys. Rev. B 93, 196437 (2016)] and determine the asymmetry of the experimental setup from the measured transition width. Finally, we establish that the widely assumed Kondo "universality" of physical quantities depending only on the ratio of the Kondo temperature and the superconducting gap $T_{K}/\Delta$ cannot hold for asymmetric junctions., Comment: 10 pages, 5 figures v3: revised and extended version; sec. III and Appendix B were largely modified, abstract and conclusions have been updated, minor text and graphical improvements

A model describing the ferromagnetic resonance of multilayer structures is used to characterize the interface anisotropy constant and interlayer exchange coupling strength associated to individual ...

The interlayer couplings in commensurate and incommensurate bilayer structures of transition metal dichalcogenides are investigated with perturbative treatment. The interlayer coupling in \pm K valleys can be decomposed into a series of hopping terms with distinct phase factors. In H-type and R-type commensurate bilayers, the interference between the three main terms leads to a sensitive dependence of the interlayer coupling strength on the translation. The magnitudes of the main hopping terms are found to some dozen of meV by fitting to the ab initio results. The interlayer couplings in \Gamma valley of valence band and Q valley of conduction band are also studied. The obtained strong coupling strengths of several hundred meV can play important roles in mediating the ultrafast interlayer charge transfer in heterobilayers of transition metal dichalcogenides., Comment: 12 pages, 8 figures

Most theoretical studies of nanoscale transport in molecular junctions rely on the combination of the Landauer formalism with Kohn-Sham density functional theory (DFT) using standard local and semilocal functionals to approximate exchange and correlation effects. In many cases, the resulting conductance is overestimated with respect to experiments. Recent works have demonstrated that this discrepancy may be reduced when including many-body corrections on top of DFT. Here we study benzene-dithiol (BDT) gold junctions and analyze the effect of many-body perturbation theory (MBPT) on the calculation of the conductance with respect to different bonding geometries. We find that the many-body corrections to the conductance strongly depend on the metal-molecule coupling strength. In the BDT junction with the lowest coupling, many-body corrections reduce the overestimation on the conductance to a factor two, improving the agreement with experiments. In contrast, in the strongest coupling cases, many-body corrections on the conductance are found to be sensibly smaller and standard DFT reveals a valid approach., Comment: 9 pages, 4 figures

We find that if solid xenon is formed from liquid xenon, denoted ``ice,'' there is a 10% increase in $^{129}\mathrm{Xe}$ longitudinal relaxation ${T}_{1}$ time (taken at 77 K and 2 T) over a trickle-freeze formation, denoted ``snow.'' Forming xenon ice also gives an unprecedented reproducibility of $^{129}\mathrm{Xe}$ ${T}_{1}$ measurements across a range of 77--150 K. This temperature dependence roughly follows the theory of spin rotation mediated by Raman scattering of harmonic phonons, though it results in a smaller-than-predicted spin-rotation coupling strength ${c}_{K0}/h$. Enriched ice $^{129}\mathrm{Xe}$ ${T}_{1}$ experiments show no isotopic dependence of bulk relaxation mechanisms at 77 K and at kilogauss fields.

In the present work, the problem of an all-coupling analytic description for the optical conductivity of the Froehlich polaron is treated, with the goal being to bridge the gap in validity range that exists between two complementary methods: on the one hand the memory function formalism and on the other hand the strong-coupling expansion based on the Franck-Condon picture for the polaron response. At intermediate coupling, both methods were found to fail as they do not reproduce Diagrammatic Quantum Monte Carlo results. To resolve this, we modify the memory function formalism with respect to the Feynman-Hellwarth-Iddings-Platzman (FHIP) approach, in order to take into account a non-quadratic interaction in a model system for the polaron. The strong-coupling expansion is extended beyond the adiabatic approximation, by including into the treatment non-adiabatic transitions between excited polaron states. The polaron optical conductivity that we obtain by combining the two extended methods agree well, both qualitatively and quantitatively, with the Diagrammatic Quantum Monte Carlo results in the whole available range of the electron-phonon coupling strength., Comment: 13 pages, 4 figures

${\mathrm{Re}}_{6}\mathrm{Hf}$, which crystallizes in $\ensuremath{\alpha}$-Mn structure(space group $I\overline{4}3m)$ without a spatial inversion center, is a superconductor with a superconducting transition temperature ${T}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}6.2\phantom{\rule{4pt}{0ex}}\mathrm{K}$. The measurements of magnetic susceptibility $(\ensuremath{\chi})$, resistivity $(\ensuremath{\rho})$, and specific heat capacity $(C)$ were carried out. Bulk superconductivity is revealed by the jump at ${T}_{c}$ of the specific heat with $\mathrm{\ensuremath{\Delta}}C/{\ensuremath{\gamma}}_{n}{T}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}\phantom{\rule{4pt}{0ex}}1.63$, suggesting moderate electron-electron coupling strength in this system. The upper critical field ${\ensuremath{\mu}}_{0}{H}_{c2}^{WHH}(0)$ was estimated to be of 89 kOe, and ${\ensuremath{\mu}}_{0}{H}_{c2}^{GL}(0)=107$ kOe, which is close to the Pauli limiting field. The Ginzburg Landau parameter ${\ensuremath{\kappa}}_{GL}=50.2$, indicates that ${\mathrm{Re}}_{6}\mathrm{Hf}$ is a type-II superconductor. The temperature dependence of the electronic specific heat ${C}_{el}(T)$ in the superconducting state can be explained by BCS theory. Furthermore, the magnetic-field dependence of $\ensuremath{\gamma}(H)$ is found to be linear with respect to $H$. These results imply a dominant $s$-wave superconductivity in ${\mathrm{Re}}_{6}\mathrm{Hf}$.

We analyze theoretically optical generation of a spin-polarized charge current (photogalvanic effect) and spin polarization in graphene with Rashba spin-orbit coupling. An external magnetic field is applied in the graphene plane, which plays a crucial role in the mechanism of current generation. We predict a highly efficient resonant-like photogalvanic effect in a narrow frequency range which is determined by the magnetic field. A relatively less efficient photogalvanic effect appears in a broader frequency range, determined by the electron concentration and spin-orbit coupling strength.

Positive-muon (${\ensuremath{\mu}}^{+}$) Knight shifts have been measured in the paramagnetic states of ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Nd}}_{x}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$ alloys, where $x=0$, 0.25, 0.45, 0.50, 0.55, 0.75, and 1.00. In Pr-substituted ${\mathrm{NdOs}}_{4}{\mathrm{Sb}}_{12}$ ($x\ensuremath{\le}0.75$), but not in ${\mathrm{NdOs}}_{4}{\mathrm{Sb}}_{12}$, Clogston-Jaccarino plots of ${\ensuremath{\mu}}^{+}$ Knight shift $K$ versus magnetic susceptibility $\ensuremath{\chi}$ exhibit an anomalous saturation of $K(\ensuremath{\chi})$ at $\ensuremath{\sim}\ensuremath{-}0.5$% for large susceptibilities (low temperatures), indicating a reduction of the coupling strength between ${\ensuremath{\mu}}^{+}$ spins and $4f$ paramagnetism for temperatures $\ensuremath{\lesssim}15$ K. We speculate that itinerant ${\mathrm{Pr}}^{3+}$ quadrupolar excitations, invoked to mediate the superconducting Cooper-pair interaction, might modify the ${\ensuremath{\mu}}^{+}$-$4f$ ion indirect spin-spin interaction.

Despite considerable research efforts, a clear understanding of superconductivity in Ba${}_{0.5}$K${}_{0.5}$BiO${}_{3}$ has been elusive. Recent studies showed that although electron-correlation effects in this compound can significantly increase the electron-phonon coupling, they do not reproduce the measured Eliashberg spectral function (${\ensuremath{\alpha}}^{2}F$). We show that the oxygen octahedra tilts in Ba${}_{0.5}$K${}_{0.5}$BiO${}_{3}$ increase ${\ensuremath{\alpha}}^{2}F$ in the range of frequencies near 30 meV, even on the level of the generalized gradient approximation. This increase in ${\ensuremath{\alpha}}^{2}F$ changes its shape to provide better agreement with experiment and results in a 50--60% increase of the average electron-phonon coupling strength $\ensuremath{\lambda}$. We use the Wannier interpolation technique to determine the electron-phonon coupling with high precision.

Numerical simulations have been used to investigate the dynamics of a pair of resistively linked Josephson junctions with ac bias. For suitable choices of parameters, the chaotic states of the two junctions become intermittently synchronized. Intervals of synchronization are interleaved between bursts of desynchronized activity. The distributions of these laminar times and their dependence on the coupling strength are determined. The role of phase winding in the definition of synchronization intervals is considered.

The interband polarizations induced by two phase-locked pulses in a semiconductor show strong interference effects depending on the time tau_1 separating the pulses. The four-wave mixing signal diffracted from a third pulse delayed by tau is coherently controlled by tuning tau_1. The four-wave mixing response is evaluated exactly for a two-level system coupled to a single LO phonon. In the weak coupling regime it shows oscillations with the phonon frequency which turn into sharp peaks at multiples of the phonon period for a larger coupling strength. Destructive interferences between the two phase-locked pulses produce a splitting of the phonon peaks into a doublet. For fixed tau but varying tau_1 the signal shows rapid oscillations at the interband-transition frequency, whose amplitude exhibits bursts at multiples of the phonon period., 4 pages, 4 figures, RevTex, content changed

We study the optical conductivity in ${A}_{3}{\mathrm{C}}_{60}$ ($A=\mathrm{K},$Rb). The effects of the electron-phonon interaction are included to lowest order in the coupling strength \ensuremath{\lambda}. It is shown that this leads to a narrowing of the Drude peak by a factor $1+\ensuremath{\lambda}$ and a transfer of weight to a midinfrared peak at somewhat larger energies than the phonon energy. Although this goes in the right direction, it is not sufficient to describe experiment.

A set of multilayer films of 20-\AA{} Pd(1.2-at. % Fe)/L \AA{} Pd with different values of the thickness, L, of the Pd spacer layer were prepared on Si(111) substrates. X-ray diffraction showed that the films have a predominantly Pd(111) texture. The dependence of the magnetization and the magnetic susceptibility on temperature was measured. Considering the multilayer (ml) film with L=200 \AA{} as a two-dimensional one (${\mathrm{\ensuremath{\chi}}}_{2\mathrm{D}}$), the interlayer coupling strength ${\mathrm{J}}_{\mathrm{\ensuremath{\perp}}}$ of the rest of the ml films (${\mathrm{\ensuremath{\chi}}}_{3\mathrm{D}}$) could be obtained from a mean-field equation relating ${\mathrm{\ensuremath{\chi}}}_{3\mathrm{d}}$, ${\mathrm{\ensuremath{\chi}}}_{2\mathrm{d}}$, and ${\mathrm{J}}_{\mathrm{\ensuremath{\perp}}}$. This ${\mathrm{J}}_{\mathrm{\ensuremath{\perp}}}$ decreases monotonically with increasing L, and it increases with increasing temperature for all the ml's. The order of ${\mathrm{J}}_{\mathrm{\ensuremath{\perp}}}$ is a few ${10}^{\mathrm{\ensuremath{-}}4}$ eV/moment pair, which is in a reasonable range from many other ml systems.

The magnetic interlayer coupling between two magnetic layers separated by an insulator was investigated on ${\mathrm{Co}}_{0.2}$${\mathrm{Fe}}_{2.8}$${\mathrm{O}}_{4}$/${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$/MgO/${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ samples grown by means of molecular beam epitaxy on (001) ${\mathrm{MgAl}}_{2}$${\mathrm{O}}_{4}$ substrates. The samples were designed to observe interlayer coupling of either sign. Hysteresis loop measurements show that the ${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$ layers are ferromagnetically coupled in the thickness range 0--45 nm MgO. Below a MgO spacer thickness of 1.3 nm, the coupling strength increases drastically with decreasing MgO thickness and is ascribed to the existence of ferromagnetic bridges through the MgO spacer. The small ferromagnetic coupling above 1.3 nm seems to arise from a magnetostatic coupling due to correlated interface irregularities.

The structure and electronic phenomena at the 180-degree domain wall in the rhombohedral phase of BaTiO3 are described using Landau-Ginzburg-Devonshire theory. Dependent on the wall orientation, two types of domain wall behaviors are identified. The low-energy "achiral" phase occurs in the vicinity of the {110} orientation and has odd polarization profile invariant with respect to inversion about the wall center. The "chiral" phase occurs around {211} wall orientations and corresponds to mixed parity domain walls. The temperature-induced transformation between the phases is abrupt and is accompanied with 20%-30% change of the domain wall thickness. This process gives rise to the significant changes of the electronic structure of the wall. Depending on the temperature and flexoelectric coupling strength, relative conductivity of the wall becomes at least one order of magnitude higher than in the single-domain region. The possible strategies for exploring these transitions based on direct measurements of domain wall width and conductive atomic force microscopy are discussed. DOI: 10.1103/PhysRevB.87.054111

Powder-in-the Ag tube processed ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ and (Bi,Pb${)}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ tapes were irradiated with 5.8-GeV Pb ions along the tape normal for the ion fluences of 0.68, 1.0, and 2.0\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The angular (\ensuremath{\theta}) dependence of critical current densities ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta},B,T) for these tapes was measured at temperature T=27 K and 55--80 K. Cusplike sharp peaks in ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta},B,T) were observed for T\ensuremath{\geqslant}55 K when the applied magnetic field B is oriented parallel to the direction of the irradiation (\ensuremath{\theta}=90\ifmmode^\circ\else\textdegree\fi{}), and ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) did not scale with the perpendicular component of B, (${\mathit{B}}_{\mathrm{\ensuremath{\perp}}}$=B sin\ensuremath{\theta}). Based on this evidence, it was concluded that the magnetic vortices behave as linelike (three-dimensional) rather than the pancakelike [two-dimensional (2D)] objects in these irradiated superconductors. However, at 27 K, neither the peaks in ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) nor the scaling of ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) with ${\mathit{B}}_{\mathrm{\ensuremath{\perp}}}$ was observed. We attribute this to the increased strength of the isotropic pinning by the small irradiation-induced defect debris relative to that of the columns and due to the increased interlayer coupling strength at reduced temperature, but not as evidence for the pancakelike 2D vortices at low temperatures. Also, for T\ensuremath{\geqslant}55 K, the enhancement in the pinning strength qualitatively followed the theory by Nelson and Vinokur [Phys. Rev. Lett. 68, 2398 (1992); Phys. Rev. B 48, 13 060 (1993)]. \textcopyright{} 1996 The American Physical Society.

By intercalating large ${\mathrm{HgI}}_{2}$ molecules between the ${\mathrm{Bi}}_{2}$${\mathrm{O}}_{2}$ layers in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{\mathit{y}}$, the distance between the ${\mathrm{CuO}}_{2}$ blocks was significantly extended up to 7.2 \AA{}. The superconducting properties of this system were measured in various magnetic fields applied perpendicular to the ${\mathrm{CuO}}_{2}$ planes. This system showed large positional fluctuation of the vortex even far below the transition temperature due to the reduction of dimensionality. The change of the thermodynamic parameters is well explained by the model of Josephson-coupled superconductivity. However, the interlayer coupling strength did not change the transition temperature drastically, which does not support the suggestion of Wheatley, Hsu, and Anderson of coherent hopping of the valence bond pair between the ${\mathrm{CuO}}_{2}$ layers. \textcopyright{} 1996 The American Physical Society.

The nature of polarons is studied in a four-site model containing one and two spinless fermions in the presence of fermion-phonon (f-ph) interactions and intersite hopping. In the four-site one-fermion model a small-to-large polaron transition associated with a sudden increase in polaronic hopping and a decrease in the polaronic binding energy is observed as the f-ph strength decreases below a particular value. The role of the modified Lang-Firsov transformation and two-phonon coherent states in lowering the energy of the system is discussed. For the four-site two-fermion system the nature and stability of bipolaronic states with the variation of f-ph coupling strength are studied. It is observed that bipolaronic states are stable only in the presence of intersite f-ph interactions.

It is shown that a correlation between the positions of the c-axis longitudinal optic (${\mathrm{LO}}_{\mathit{c}}$) phonons and ``notch''-like structures in the ab-plane conductivity of high-${\mathit{T}}_{\mathit{c}}$ superconductors results from phonon-mediated interaction between electrons in different layers. It is found that the relative size of the notches depends on ${\ensuremath{\lambda}}_{\mathrm{ph}}$(${\mathrm{\ensuremath{\Omega}}}_{\mathrm{ph}}$/${\ensuremath{\gamma}}_{\mathrm{ph}}$), where ${\ensuremath{\lambda}}_{\mathrm{ph}}$, ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{ph}}$, and ${\ensuremath{\gamma}}_{\mathrm{ph}}$ are the effective coupling strength, the frequency, and the width of the optical phonon which is responsible for the notch. Even for ${\ensuremath{\lambda}}_{\mathrm{ph}}$\ensuremath{\approxeq}0.01 the effect can be large if the phonon is very sharp. \textcopyright{} 1996 The American Physical Society.

Motivated by a recent optical experiment on ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}\mathrm{Bi}{\mathrm{O}}_{3}$, we calculate the far-infrared conductivity in the normal and superconducting states of an electron-phonon driven superconductor, and note the signatures of the electron-phonon interaction. We find that two methods in particular are effective ways to determine the electron-phonon coupling strength, both of which are applicable in the normal state. One relies on a sum rule, and requires independent knowledge of the plasma frequency, the other relies on the Drude-like width, and was used by Puchkov et al. in their infrared measurements. We note quantitative corrections to this latter method. However, we support their conclusion that the electron-phonon interaction in ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}\mathrm{Bi}{\mathrm{O}}_{3}$ is too weak to account for superconductivity. A third method relies on observing shifts of structure in the superconducting state. However, we find that for systems with intermediate-coupling strength, such structure can be very difficult to detect, especially if the system is moderately dirty.

We consider a simple model polariton problem and show that the phonons in this system will never exhibit sub-Poissonian statistics. We furthermore observe that the probability distribution of the phonons will be classical at all temperatures, although the polariton complex as a whole can have a nonclassical behavior below a threshold temperature whose value will depend on the photon-phonon coupling strength.

We use the coupled-cluster method in high orders of approximation to make a comprehensive study of the ground-state (GS) phase diagram of the spin-1/2 J1-J2-J3 model on a two-dimensional honeycomb lattice with antiferromagnetic (AFM) interactions up to third-nearest neighbors. Results are presented for the GS energy and the average local onsite magnetization. With the nearest-neighbor coupling strength J1≡1, we find four magnetically ordered phases in the parameter window J2,J3∈[0,1], namely, the Neel, striped, and Neel-II collinear AFM phases, plus a spiral phase. The Neel-II phase appears as a stable GS phase in the classical version of the model only for values J3

The recent paper by V. G. Kogan and J. Schmalian [Phys. Rev. B 83, 054515 (2011)] argues that the widely used two-component Ginzburg-Landau (GL) models are not correct, and further concludes that in the regime which is described by a GL theory there could be no disparity in the coherence lengths of two superconducting components. This would in particular imply that [in contrast to $U(1)\ifmmode\times\else\texttimes\fi{}U(1)$ superconductors] there could be no ``type-1.5'' superconducting regime in $U(1)$ multiband systems for any finite interband coupling strength. We point out that these claims are incorrect and based on an erroneous scheme of reduction of a two-component GL theory.

The spin-spin correlation function of the two-dimensional (2D) $XY$ model decays as a power law at all temperatures below the Berezinskii-Kosterlitz-Thouless transition point with a temperature-dependent exponent $\ensuremath{\eta}=\ensuremath{\eta}(T/J)$ ($J$ is the ferromagnetic coupling strength). It is known from computer experiments that in the 2D $XY$ model with site or bond dilution this exponent depends on the concentration $p$ of removed sites or bonds as well. Knowing the slope $\ensuremath{\partial}\ensuremath{\eta}/\ensuremath{\partial}p$ at point $p=0$, one can predict the value of the exponent for small dilution concentrations: ${\ensuremath{\eta}(p)\ensuremath{\simeq}\ensuremath{\eta}(0)+p(\ensuremath{\partial}\ensuremath{\eta}/\ensuremath{\partial}p)|}_{p=0}$. As shown in this paper, the spin-wave Hamiltonian allows one to obtain exact results for this slope: ${(\ensuremath{\partial}\ensuremath{\eta}/\ensuremath{\partial}p)|}_{p=0}=T/(2J)+O({(T/J)}^{2})$ and $T/(\ensuremath{\pi}J)+O({(T/J)}^{2})$ for site and for bond dilution, respectively.

Anisotropic irreversibility lines due to thermal fluctuation for quasi-two-dimensional high-${\mathit{T}}_{\mathit{c}}$ superconductors (HTS's) were observed in c-axis-aligned powders of the ${\mathrm{HgBa}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ compound with ${\mathit{T}}_{\mathit{c}}$=131\ifmmode\pm\else\textpm\fi{}0.5 K. The anisotropic ratio ${\mathit{H}}_{\mathrm{irr}}$(\ensuremath{\perp}c)/${\mathit{H}}_{\mathrm{irr}}$(\ensuremath{\parallel}c) below 2 was observed in all temperature regions. The simple three-dimensional-like power law ${\mathit{H}}_{\mathrm{irr}}$=a(1-T/${\mathit{T}}_{\mathit{c}}$${)}^{\mathit{n}}$ was observed only in the low-field region (\ensuremath{\le}2000 G) with n=1.40\ifmmode\pm\else\textpm\fi{}0.1 for H\ensuremath{\perp}c and n=1.63\ifmmode\pm\else\textpm\fi{}0.1 for H\ensuremath{\parallel}c. In the higher field region up to 5 T, the temperature dependence of ${\mathit{H}}_{\mathrm{irr}}$(T) lines changes into a two-dimensional-like exponential function ${\mathit{H}}_{\mathrm{irr}}$=a exp(-T/${\mathit{T}}_{0}$) due to the breakdown of the interlayer coupling of the conduction channel, which consists of three Cu-O planes, with ${\mathit{T}}_{0}$=16.0\ifmmode\pm\else\textpm\fi{}1.3 K for H\ensuremath{\perp}c and ${\mathit{T}}_{0}$=16.6\ifmmode\pm\else\textpm\fi{}1.1 K for H\ensuremath{\parallel}c. The exponent n is independent of the pinning strength or anisotropy related to interlayer coupling between ${\mathrm{CuO}}_{2}$ planes. The characteristic crossover field giving deviation from the power function depends strongly on the anisotropic nature of materials; e.g., \ensuremath{\ge}5 T for Y HTS's, \ensuremath{\sim}0.2 T for Hg HTS's, \ensuremath{\sim}0.02 T for Bi HTS's, and \ensuremath{\sim}0.01 T for Tl HTS's; this means that flux dynamics depend strongly on coupling strength between ${\mathrm{CuO}}_{2}$ blocks in the HTS.

In this work the short-time dynamics of quenched Frenkel excitons in binary, coiled chains is investigated by numerical calculations. Quenching is modeled by means of a non-Hermitian damping Hamiltonian accounting for interstitional traps. The dependence of donor and acceptor response functions to \ensuremath{\delta}-pulse excitation is examined as a function of diagonal disorder, off diagonal disorder induced by random non-nearest-neighbor contacts (degree of coiling), coupling strength, and two types of initial conditions (localized and nonlocalized). The results clearly demonstrate that the relaxation behavior shows significant oscillations even after averaging over a representative ensemble of chain configurations and over all possible initial localized conditions over a wide range of model parameters.

The specific heat coefficient ${C}_{p}/T$ was investigated for the optimally doped Ba${}_{0.6}$K${}_{0.4}$Fe${}_{2}$As${}_{2}$ and Sr${}_{0.55}$K${}_{0.45}$Fe${}_{2}$As${}_{2}$. Previously, both the single- and the two-gap pairing have been suggested for Ba${}_{0.6}$K${}_{0.4}$Fe${}_{2}$As${}_{2}$ single crystals. Our analysis reveals that the controversy is mainly caused by the differences in the adopted phonon background. This is especially true in the (Ba,K)Fe${}_{2}$As${}_{2}$ system, in which the phonon contribution below 20 K significantly deviates from the simplified Debye model and, in addition, strongly depends on the doping. The different pairing features reported previously can be reproduced from the same ${C}_{p}/T$ data set if the respective phonon baselines are adopted. The soft-phonon shifts, therefore, were examined on the (Ba,K)Fe${}_{2}$As${}_{2}$ and (Sr,K)Fe${}_{2}$As${}_{2}$ systems, as well as the (Ba${}_{0.6}$K${}_{0.4}$)(Fe,Co)${}_{2}$As${}_{2}$ system. The data show that although the K-Ba replacement may change the low-temperature slope $\ensuremath{\beta}=\frac{d\left({C}_{p}/T\right)}{d({T}^{2})}$ by 50%, the effects of both K-Sr and Co-Fe replacements are much weaker. The carrier part, ${C}_{e}/T$, is consequently extracted for both Sr${}_{0.55}$K${}_{0.45}$Fe${}_{2}$As${}_{2}$ and Ba${}_{0.6}$K${}_{0.4}$Fe${}_{2}$As${}_{2}$. The two-gap features appear in both cases, but the coupling strength is much stronger for the Ba-based superconductors.

The Raman spectra of pristine ${\mathrm{C}}_{70}$ films and ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$ doped to the maximum-conductivity phase (nominally ${\mathrm{K}}_{4}$${\mathrm{C}}_{70}$) are studied. All the Raman modes for ${\mathrm{C}}_{70}$ are only moderately broadened upon K doping, distinctively different from the superconducting ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ where several ${\mathit{H}}_{\mathit{g}}$ modes are broadened to the extent that they have almost disappeared from the spectrum. Assuming that the electron-phonon interaction is responsible for the broadening, the electron-phonon coupling strength is found to be comparable to the Coulomb repulsion. These results are important for understanding the absence of superconductivity in ${\mathrm{K}}_{\mathit{x}}$${\mathrm{C}}_{70}$.

We study the effect of dephasing on the Landau-Zener transition. Our approach is based on the interpretation of the transition as an interference process. We derive a phase-uncertainty factor for a general coupling to an environment, and demonstrate the enhancement of the asymptotic transition probability due to this phase uncertainty. We discuss the dependence of this enhancement on the properties of a specific environment at hand, the form of the coupling term, and the coupling strength. Given a certain type of coupling to the environment, we find the characteristic dephasing time (${\mathrm{\ensuremath{\tau}}}_{\mathrm{\ensuremath{\varphi}}}$) corresponding to the crossover from weak to strong dephasing. We argue that this crossover ${\mathrm{\ensuremath{\tau}}}_{\mathrm{\ensuremath{\varphi}}}$ can be regarded as a probe of the relevant time scale of phase oscillations, which are suppressed by the specific dephasing mechanism.

The influence of magnetic layer thickness on the interlayer coupling through a tunnel barrier is investigated in Co/MgO/Co structures with perpendicular anisotropy. Despite the rather large MgO thickness, a clear antiferromagnetic coupling is observed. It oscillates with increasing magnetic layer thickness, in agreement with theories on indirect coupling through insulating spacers. Although the average interlayer coupling strength decreases with increasing annealing temperature, constant period and amplitude of these oscillations are observed for all annealing temperatures.

We explore the effect of a (non-)magnetic impurity on the thermal transport of the spin-$\frac{1}{2}$ Heisenberg chain model. This unique system allows to probe Kondo-type phenomena in a prototype strongly correlated system. Using numerical diagonalization techniques we study the scaling of the frequency dependent thermal conductivity with system size and host-impurity coupling strength as well as the dependence on temperature. We focus in particular on the analysis of ``cutting healing'' of weak links or a magnetic impurity by the host chain via Kondo-like screening as the temperature is lowered.

The $^{5}$${\mathit{D}}_{3}$(4${\mathit{f}}^{6}$)${\mathrm{\ensuremath{-}}}^{7}$${\mathit{F}}_{0}$(4${\mathit{f}}^{6}$) transition of ${\mathrm{Sm}}^{2+}$ in SrFCl is found to be anomalously strong and it exhibits an asymmetric line shape, which is interpreted in terms of resonant interaction between the $^{5}$${\mathit{D}}_{3}$ excited state and the vibronic levels of a low-lying 4${\mathit{f}}^{5}$5d electronic state. The electronic coupling strength is estimated to be of the order of 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The symmetry species of interacting levels are identified, and the possibility of an E\ifmmode\times\else\texttimes\fi{}(${\mathit{b}}_{1}$+${\mathit{b}}_{2}$) type of Jahn-Teller effect in the 4${\mathit{f}}^{5}$5d state is discussed.

The linewidths and frequencies of three Raman-active phonons have been measured in twin-free single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. Below ${\mathit{T}}_{\mathit{c}}$, the phonon at 340 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ sharpens dramatically, the phonon at 440 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ sharpens slightly, and the phonon at 500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ broadens. The temperature dependence of the phonon linewidths places the superconducting energy gap (2\ensuremath{\Delta}) between 440 and 500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, or 2\ensuremath{\Delta}=6.8${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$--7.7${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$. Based upon the observed change in linewidth, the electron-phonon coupling strength of the 340-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ phonon is estimated to be between 0.6 and 0.8.

The $\ensuremath{\beta}$ function of the multichannel Kondo model is calculated exactly in the limit of large spin N and channel number $M=\ensuremath{\gamma}N$, with constant $\ensuremath{\gamma}$. There are no corrections in any finite order of $1/N$. One zero is found at a finite coupling strength, showing directly the non-Fermi-liquid behavior of the model. This renormalization group flow allows introduction of a variational principle for the entropy, to obtain the low-temperature thermodynamics. Thus, in particular, the low-temperature thermodynamics of the noncrossing approximation to the Kondo model becomes accessible.

Low-temperature scanning tunneling spectroscopy and first-principles calculations are used to characterize electron transport through vibronic states of C60 molecules. This is achieved by positioning a C60 molecule on top of a molecular self-assembled template on Au(111). In these conditions, conductance spectra are shown to reveal the dynamic Jahn-Teller effect of the C60 molecule. This vibronic transport study helps in solving a long-standing debate [Phys. Rev. Lett. 74, 1875 (1995); Phys. Rev. Lett. 91, 196402 (2003)] on density-functional calculations of the C60 electron-phonon coupling strength., T.F. acknowledges support from the Danish FNU -Grant No. 272-07-0114. A.A. and T.F. thank partial financial support from UPV/EHU Grant No. IT-366-07 and MEC -Grant No. FIS2007-66711-C02-01. N.L. acknowledges support from MEC -Grant No. FIS2006-12117-C04-01. Financial support by the DFG through Contracts No. SPP 1243 and No. Sfb 658 is gratefully acknowledged.

We report structural, transport, and thermal properties of carrier-tuned Ba_8Ga_Sn_ single crystals with the type-1 clathrate structure (β phase), demonstrating that Ba_8Ga_Sn_ is a unique thermoelectric clathrate material wherein both the structure type and the carrier type are tunable. The results are compared with the properties of the better known type-8 structure (α phase) and of A_8Ga_Ge_ (A=Sr,Eu). Differential thermal analysis and powder x-ray diffraction show that both phases are stable up to their virtually identical melting point of 520±3 °C. Refinements of single-crystal x-ray diffraction data indicate that the Ba(2) guest ion in the tetrakaidecahedron occupies the off-center 24k sites which are 0.43–0.44 Å away from the centered 6d site. The temperature-linear coefficient of the specific heat is 29 mJ/mol K^2 for both n- and p-type carriers in the β phase, four times larger than that for the α phase, suggesting contributions from tunneling of the Ba(2) guest ions between off-center minima. Analysis of specific heat with a soft-potential model (SPM) gives a characteristic energy of 20 K for the Ba(2) vibration, significantly lower than 50 K for the α phase and in fact the lowest among type-1 clathrates. The lattice thermal conductivities κL for the β phase with both charge carrier types are very similar and show a glasslike temperature dependence. This behavior in κL(T) is also described using SPM analysis, where it is found that the coupling strength between guest modes and acoustic phonons for the β phase is significantly larger than that for Sr_8Ga_Ge_., 論文