The Fermi and Rashba energies of surface states in the ${\text{Bi}}_x{\text{Pb}}_{1-x}/\text{Ag}\left(111\right)$ alloy can be tuned simultaneously by changing the composition parameter $x$. We report on unconventional Fermi surface spin textures observed by spin and angle-resolved photoemission spectroscopy that are correlated with a topological transition of the Fermi surface occurring at $x=0.5$. We show that the surface states remain fully spin polarized upon alloying and that the spin-polarization vectors are approximately tangential to the constant energy contours. We discuss the implications of the topological transition for the transport of spin.

We study the spectral function of a single hole doped into the $ab$ plane of the Mott insulator ${\text{LaVO}}_3$, with antiferromagnetic (AF) spin order of $S=1$ spins accompanied by alternating orbital (AO) order of active $\left\{d_{yz},d_{zx}\right\}$ orbitals. Starting from the respective $t\text{−}J$ model, with spin-orbital superexchange and effective three-site hopping terms, we derive the polaron Hamiltonian and show that a hole couples simultaneously to the collective excitations of the AF/AO phase, magnons, and orbitons. Next, we solve this polaron problem using the self-consistent Born approximation and find a stable quasiparticle solution—a spin-orbital polaron. We show that the spin-orbital polaron resembles the orbital polaron found in $e_g$ systems, as e.g., in ${\text{K}}_2{\text{CuF}}_4$ or (to some extent) in ${\text{LaMnO}}_3$, and that the hole may be seen as confined in a stringlike potential. However, the spins also play a crucial role in the formation of this polaron—we explain how the orbital degrees of freedom: (i) confine the spin dynamics acting on the hole as the classical Ising spins and (ii) generate the string potential which is of the joint spin-orbital character. Finally, we discuss the impact of the results presented here on the understanding of the phase diagrams of the lightly doped cubic vanadates.

At low energy, electrons in doped graphene sheets behave like massless Dirac fermions with a Fermi velocity, which does not depend on carrier density. Here we show that modulating a two-dimensional electron gas with a long-wavelength periodic potential with honeycomb symmetry can lead to the creation of isolated massless Dirac points with tunable Fermi velocity. We provide detailed theoretical estimates to realize such artificial graphenelike system and discuss an experimental realization in a modulation-doped GaAs quantum well. Ultrahigh-mobility electrons with linearly dispersing bands might open new venues for the studies of Dirac-fermion physics in semiconductors.

The in-plane dielectric and ferroelectric properties of coherent anisotropically strained ${\text{SrTiO}}_3$ thin films grown on orthorhombic (101) ${\text{DyScO}}_3$ substrates were examined as a function of the angle between the applied electric field and the principal directions of the substrate. The dielectric permittivity revealed two distinct maxima as a function of temperature along the ${\left[100\right]}_p$ and ${\left[010\right]}_p$ ${\text{SrTiO}}_3$ pseudocubic directions. These data, in conjunction with optical second-harmonic generation, show that the switchable ferroelectric polarization develops first predominantly along the in-plane axis with the larger tensile strain before developing a polarization component along the perpendicular direction with smaller strain as well, leading to domain twinning at the lower temperature. Finally, weak signatures in the dielectric and second-harmonic generation response were detected at the ${\text{SrTiO}}_3$ tilt transition close to 165 K. These studies indicate that anisotropic biaxial strain can lead to new ferroelectric domain reorientation transitions that are not observed in isotropically strained films.

In many transition-metal oxides the electrical resistance is observed to undergo dramatic changes induced by large biases. In magnetite, ${\text{Fe}}_3{\text{O}}_4$, below the Verwey temperature, an electric-field-driven transition to a state of lower resistance was recently found, with hysteretic current-voltage response. We report the results of pulsed electrical conduction measurements in epitaxial magnetite thin films. We show that while the high- to low-resistance transition is driven by electric field, the hysteresis observed in $I\text{−}V$ curves results from Joule heating in the low-resistance state. The shape of the hysteresis loop depends on pulse parameters and reduces to a hysteresis-free “jump” of the current provided thermal relaxation is rapid compared to the time between voltage pulses. A simple relaxation-time thermal model is proposed that captures the essentials of the hysteresis mechanism.

The electronic structure of oxygen vacancy and proton defects close to grain boundaries in MgO are calculated using first principles methods. These defects, in various charge states, favorably segregate to grain boundaries and can trap electrons. The interplay between electron and defect segregation provides a mechanism for charge to build up at boundaries, for example, under irradiation or applied electrical voltage. The theoretical calculations presented provide insight into the complex electronic properties of metal-oxide grain boundaries that can be difficult to obtain by experiment alone but which are important for many applications.

A systematic far-infrared optical study has been performed on multiferroic oxides ${\text{Eu}}_{1-x}{\text{Y}}_x{\text{MnO}}_3$ ($x=0.1$, 0.2, 0.3, 0.4, and 0.45) in which versatile characteristics of magnetic orders including the $ab$- and $bc$-plane spiral, $A$-type antiferromagnetic, and collinear sinusoidal states show up, depending on Y content $\left(x\right)$ and temperature. We have observed evolution of various absorption bands below $100{\text{cm}}^{-1}$, which critically depends on the nature of magnetic order and hence can be assigned to magnetic excitations. The spiral-spin structures yield the most intense absorption composed of two pronounced peak structures around 20 and $65{\text{cm}}^{-1}$, while the spectra for the collinear sinusoidal spin structure exhibit a plateaulike shape below $80{\text{cm}}^{-1}$. In contrast, the $A$-type antiferromagnetic phase shows no electrically active spin excitation. A quantitative analysis shows their strong mutual coupling in the spiral-spin phases as manifested by the transfer of the spectral weight between the electrically active spin excitations and the lowest optical-phonon mode.

The spins of a pair of spin-orbit split surface states at a metal surface are usually antiparallelly aligned, in accord with the Rashba model for a two-dimensional electron gas. By first-principles calculations and two-photon photoemission experiments we provide evidence that in the surface alloy Bi/Cu(111) the spins of an unoccupied pair of surface states are parallelly aligned. This unconventional spin polarization, which is not consistent with that imposed by the Rashba model, is explained by hybridization of surface states with different orbital character and is attributed to the spin-orbit interaction. Since hybridization is a fundamental effect our findings are relevant for spin electronics in general.

Recent experiments showed that the shear modulus of solid ${}^4\text{H}\text{e}$ stiffens in the same temperature range (below 200 mK) where mass decoupling and supersolidity have been inferred from torsional oscillator measurements. The two phenomena are clearly related and crystal defects, particularly dislocations, appear to be involved in both. We have studied the effects of annealing and the effects of applying large stresses on the elastic properties of solid ${}^4\text{H}\text{e}$, using both acoustic resonances and direct low-frequency and low-amplitude measurements of the shear modulus. Both annealing and stressing affect the shear modulus, as expected if dislocations are responsible. However, it is the high-temperature modulus which is affected; the low-temperature behavior is unchanged and appears to reflect the intrinsic modulus of solid helium. We interpret this behavior in terms of dislocations which are pinned by isotopic ${}^3\text{H}\text{e}$ impurities at low temperatures and so have no effect on the shear modulus. At higher temperatures they become mobile and weaken the solid. Stressing the crystal at low temperatures appears to introduce new defects or additional pinning sites for the dislocation network but these effects can be reversed by heating the crystal above 500 mK. This is in contrast to dislocations produced during crystal growth, which are only annealed at temperatures close to melting.

We report on the resonant coupling between localized surface plasmon resonances (LSPRs) in nanostructured Ag films and an adsorbed monolayer of Rhodamine $6G$ dye. Hybridization of the plasmons and molecular excitons creates new coupled polaritonic modes, which have been tuned by varying the LSPR wavelength. The resulting polariton dispersion curve shows an anticrossing behavior which is very well fit by a simple coupled-oscillator Hamiltonian, giving a giant Rabi-splitting energy of $\sim 400\text{meV}$. The strength of this coupling is shown to be proportional to the square root of the molecular density. The Raman spectra of R6G on these films show an enhancement of many orders of magnitude due to surface enhanced scattering mechanisms; we find a maximum signal when a polariton mode lies in the middle of the Stokes shifted emission band.

The superconductor ${\text{Sr}}_4{\text{V}}_2{\text{O}}_6{\text{Fe}}_2{\text{As}}_2$ with transition temperature at 37.2 K was successfully fabricated. It has a layered structure with the space group of $p4/nmm$ and with the lattice constants $a=3.9296\text{Å}$ and $c=15.6732\text{Å}$. The observed large diamagnetization signal and zero resistance demonstrated the bulk superconductivity. The broadening of resistive transition was measured under different magnetic fields revealing a rather high upper critical field. The results also suggest a large vortex liquid region that reflects high anisotropy of the system. The Hall-effect measurements revealed dominantly electronlike charge carriers in this material. The superconductivity in the present system may be induced by oxygen deficiency or the multiple valence states of vanadium.

We present magnetotransport measurements performed on underdoped ${\text{Pr}}_{2-x}{\text{Ce}}_x{\text{CuO}}_4$ at THz frequencies as a function of temperature and doping. A rapidly decreasing Hall mass is observed as the doping is reduced consistent with the formation of small electron Fermi pockets. However, both dc and infrared magnetotransport data strongly deviate from the predictions of transport theory within the relaxation-time approximation based on angular-resolved photoemission data. Furthermore, the Hall mass remains negative with no signature of a change in Fermi-surface topology at or above the Néel temperature and the characteristic temperature at which the optical gap fully closes. In the paramagnetic state, this behavior of the Hall mass may be qualitatively understood as arising from current vertex corrections to the Hall conductivity due to magnetic fluctuations as observed in overdoped ${\text{Pr}}_{2-x}{\text{Ce}}_x{\text{CuO}}_4$ [G. S. Jenkins et al., arXiv:0901.1701 (unpublished)].

We measure tunneling through a single quantum level in a carbon nanotube quantum dot connected to resistive metal leads. For the electrons tunneling to/from the nanotube, the leads serve as a dissipative environment, which suppresses the tunneling rate. In the regime of sequential tunneling, the height of the single-electron conductance peaks increases as the temperature is lowered, although it scales more weakly than the conventional $\propto T^{-1}$. In the resonant tunneling regime (temperature smaller than the level width), the peak width approaches saturation, while the peak height starts to decrease. Overall, the peak height shows a nonmonotonic temperature dependence. We associate this unusual behavior with the transition from the sequential to the resonant tunneling through a single quantum level in a dissipative environment.

We present a study of the angular dependence of the resistivity tensor up to 35 T in elemental bismuth complemented by torque magnetometry measurements in a similar configuration. For at least two particular field orientations a few degrees off the trigonal axis, the Hall resistivity was found to become field independent within experimental resolution in a finite field window corresponding to a field which is roughly three times the frequency of quantum oscillations. The Hall plateaus rapidly vanish as the field is tilted off these magic angles. We identify two distinct particularities of these specific orientations, which may play a role in the emergence of the Hall plateaus.

One year after their initial discovery, two schools of thought have crystallized regarding the electronic structure and magnetic properties of ferropnictide systems. One postulates that these are itinerant weakly correlated metallic systems that become magnetic by virtue of spin-Peierls-type transition due to near nesting between the hole and the electron Fermi-surface pockets. The other argues that these materials are strongly or at least moderately correlated and the electrons are considerably localized and close to a Mott-Hubbard transition, with the local magnetic moments interacting via short-range superexchange. In this Rapid Communication we argue that neither picture is fully correct. The systems are moderately correlated but with correlations driven by Hund’s rule coupling rather than by the on-site Hubbard repulsion. The iron moments are largely local, driven by Hund’s intra-atomic exchange. Superexchange is not operative, and the interactions between the Fe moments are considerably long range and driven mostly by one-electron energies of all occupied states.

We report upper critical-field measurements in the metal-free-all-organic superconductor ${\beta }^{″}\text{−}{\left(\text{ET}\right)}_2{\text{SF}}_5{\text{CH}}_2{\text{CF}}_2{\text{SO}}_3$ obtained from measuring the in-plane penetration depth using the tunnel diode oscillator technique. For magnetic field applied parallel to the conducting planes the low-temperature upper critical fields are found to exceed the Pauli limiting field calculated by using a semiempirical method. Furthermore, we found a signature that could be the phase transition between the superconducting vortex state and the Fulde-Ferrell-Larkin-Ovchinnikov state in the form of a kink just below the upper critical field and only at temperatures below 1.23 K.

We study a quantum point contact in a quantum spin Hall insulator. It has recently been shown that the Luttinger liquid theory of such a structure maps to the theory of a weak link in a Luttinger liquid with spin with Luttinger liquid parameters $g_{\rho }=1/g_{\sigma }=g<1$. We show that for weak interactions, $1/2<g<1$, the pinch-off of the point contact as a function of gate voltage is controlled by a novel quantum critical point, which is a realization of a nontrivial intermediate fixed point found previously in the Luttinger liquid model with spin. We predict that the dependence of the conductance on gate voltage near the pinch-off transition for different temperatures collapses onto a universal curve described by a crossover scaling function associated with that fixed point. We compute the conductance and critical exponents of the critical point as well as the universal scaling function in solvable limits, which include $g=1-ϵ$, $g=1/2+ϵ$, and $g=1/\sqrt{3}$. These results, along with a general scaling analysis, provide an overall picture of the critical behavior as a function of $g$. In addition, we analyze the structure of the four-terminal conductance of the point contact in the weak tunneling and weak backscattering limits. We find that different components of the conductance can have different temperature dependences. In particular, we identify a skew conductance $G_{XY}$, which we predict vanishes as $T^{\gamma }$ with $\gamma \ge 2$. This behavior is a direct consequence of the unique edge state structure of the quantum spin Hall insulator. Finally, we show that for strong interactions, $g<1/2$, the presence of spin nonconserving spin-orbit interactions leads to a novel time-reversal-symmetry breaking insulating phase. In this phase, the transport is carried by spinless chargons and chargeless spinons. These lead to nontrivial correlations in the low frequency shot noise. Implications for experiments on HgCdTe quantum well structures will be discussed.

We discuss the existence of stable topological quantum memory at finite temperature. At stake here is the fundamental question of whether it is, in principle, possible to store quantum information for macroscopic times without the intervention from the external world, that is, without error correction. We study the toric code in two dimensions with an additional bosonic field that couples to the defects, in the presence of a generic environment at finite temperature: the toric-boson model. Although the coupling constants for the bare model are not finite in the thermodynamic limit, the model has a finite spectrum. We show that in the topological phase, there is a finite temperature below which open strings are confined and therefore the lifetime of the memory can be made arbitrarily (polynomially) long in system size. The interaction with the bosonic field yields a long-range attractive force between the end points of open strings but leaves closed strings and topological order intact.

We present a theory for charge transport in organic crystals. A mobility expression is derived based on the evaluation of the Kubo formula and the Holstein Hamiltonian. It covers the whole temperature range from low $T$, where it reproduces an expression from the Boltzmann equation for band transport, via elevated $T$, where it generalizes Holstein’s small-polaron theory to finite bandwidths, up to high $T$, for which a temperature dependence equal to Marcus’ electron transfer theory is obtained. The general expression treats coherent band transport and thermally induced hopping on equal footing. By avoiding the approximation of narrow polaron bands the theory allows for the description of large and small polarons.

A source of nanometer wavelength, transverse-polarized, sound waves based on stimulated emission of phonons in a semiconductor superlattice inside an acoustic cavity and pumped by a low-average power nanosecond pulsed laser is described. The experimental evidence for sound laser (saser) action in our device is a superlinear increase in emitted acoustic intensity and an increase in emission directionality above a threshold in the peak optical pumping power at $\sim 500\text{W}{\text{cm}}^{-2}$. We also show that saser action is theoretically possible in the device due to strong electron-phonon coupling, the achievement of population inversion under optical pumping and effective phonon confinement in an acoustic cavity.