Physical Review B

Ab initio electronic structure calculations on potassium at high pressure reveal that the orthorhombic oP8 structure, found recently in sodium, has a lower enthalpy in potassium than do other candidate crystal structures recently reported to be stable at high pressure [Y. Ma, A. R. Oganov, and Y. ...

The issue of melting of pure iron and iron alloyed with lighter elements at high pressure is critical to the physics of the Earth. The iron melting curve in the relevant pressure range between 3 and 4 Mbar is reasonably well established from the theoretical point of view. However, so far no one atte...

First-principles calculations are used to investigate the effects of the epitaxial strain on the structure of the perovskite oxide CaTiO₃ . At 1.5% tensile strain, we find an epitaxial orientation transition between the ab-ePbnm phase favored for compressive strains and the c-ePbnm phase. While...

We investigate Josephson junctions with superconducting ferropnictides, both in the diffusive and ballistic limit. We focus on the proposed s_± -wave state and find that the relative phase shift intrinsic to the s_± -wave state may provide 0-π oscillations in the Josephson current. This feature...

We have studied the in- and out-of-plane magnetic penetration depths in the hole-doped iron-based superconductor Ba_1−xK_xFe₂As₂ (T_c≈30 K) . Single crystals grown from different fluxes and by different groups showed nearly identical results. The in-plane London penetration depth λ_ab is not...

The in-phase (rectangular) fluxon lattice is required for achieving coherent THz emission from stacked Josephson junctions. Unfortunately, it is usually unstable due to mutual repulsion of fluxons in neighbor junctions, which favors the out-of-phase (triangular) lattice. Here we experimentally study...

We present tunneling data from superconducting BaFe_1.86Co_0.14As₂ and its parent compound, BaFe₂As₂ . In the superconductor, clear coherencelike peaks are seen across the whole field of view, and their analysis reveals nanoscale variations in the superconducting gap value, Δ /span>. The average peak-t...

We present detailed heat capacity measurements for Ba(Fe_1−xCo_x)₂As₂ and Ba(Fe_1−xNi_x)₂As₂ single crystals in the vicinity of the superconducting transitions. The specific-heat jump at the superconducting transition temperature (T_c), ΔC_p/T_c, changes by a factor of ∼10 across these series. The Î...

The coexistence of charge-density waves and superconductivity in compressed 1T-TaS₂ is investigated through density-functional calculations. It has been speculated that this coexistence arises from a real-space phase separation of insulating domains of the commensurate-charge-density-wave phase an...

CoO₂ is the parent compound for the superconductor Na_xCoO₂⋅1.3H₂O and was widely believed to be a Mott insulator. We performed ⁵⁹Co nuclear magnetic resonance and nuclear quadrupole resonance studies on Li_xCoO₂ ( x=0.35 , 0.25, 0.12, and 0.0) to uncover the electronic state and spin correl...

We consider local magnetic moments coupled to conduction electrons with on-site attraction in order to discuss the interplay between pairing and magnetic order. We probe the ground-state properties of this model on a one-dimensional lattice through pair binding energies and several correlation funct...

The superconductor Sr₄V₂O₆Fe₂As₂ 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 Å and c=15.6732 Å . The observed large diamagnetization signal and zero resistance demons...

When localized spins on different d orbitals prefer different types of antiferromagnetic ordering, the Hund’s rule coupling creates frustration. Using spin-wave theory we study the case of two such orbitals on a square lattice coupled through Hund’s rule such that the first one couples antifer...

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 transit...

In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position suggests a significant relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high-temperature tetragonal phase of CaFe₂As₂ using inelastic x-ray scat...

We report upper critical-field measurements in the metal-free-all-organic superconductor β^″-(ET)₂SF₅CH₂CF₂SO₃ 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 uppe...

We consider the behavior of the Grüneisen parameter, the ratio between thermal expansion and specific heat, at pressure-tuned infinite-randomness quantum-critical points and in the associated quantum Griffiths phases. We find that the Grüneisen parameter diverges as ln(T₀/T) with vanishing tempe...

We have performed measurements of the scattering mean free path (ℓ_s) in photonic crystals with different and controlled amounts of disorder. In the most perfect crystals, 1 order of magnitude chromatic variation in ℓ_s for just 3% shift around the band gap (27 nm in wavelength) is obtained. I...

We have studied the frequency and voltage dependences of voltage-induced torsional strains in orthorhombic TaS₃ [V. Y. Pokrovskii, S. G. Zybtsev, and I. G. Gorlova, Phys. Rev. Lett. 98, 206404 (2007)] by measuring the modulation of the resonant frequency of an rf cavity containing the sample. The ...

We demonstrate experimentally and theoretically dielectric metamaterials exhibiting a tunable range of negative effective permeability in the terahertz spectral region (0.2–0.36 THz). Our structures consist of an array of intrinsically nonmagnetic rods made of an incipient ferroelectric SrTiO₃ w...

We study electron transport in a double quantum dot in the Pauli spin blockade regime in the presence of strong spin-orbit coupling. The effect of spin-orbit coupling is incorporated into a modified interdot tunnel coupling. We elucidate the role of the external magnetic field, the nuclear fields in...

We use the density functional theory/local-density approximation (DFT/LDA)-1/2 method [L. G. Ferreira , Phys. Rev. B 78, 125116 (2008)], which attempts to fix the electron self-energy deficiency of DFT/LDA by half-ionizing the whole Bloch band of the crystal, to calculate the band offsets of two Si...

The Fermi and Rashba energies of surface states in the Bi_xPb_1−x/Ag(111) 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 t...

Many calculations require a simple classical model for the interactions between sp² -bonded carbon atoms, as in graphene or carbon nanotubes. Here we present a valence force model to describe these interactions. The calculated phonon spectrum of graphene and the nanotube breathing-mode energy agree...

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 creatio...

The trigonal zigzag nanodisk with size N has N localized spins. We investigate its thermodynamical properties with and without external leads. Leads are made of zigzag graphene nanoribbons or ordinary metallic wires. There exists a quasiphase transition between the quasiferromagnet and quasipara...

The low-energy theory of graphene exhibits spontaneous chiral symmetry breaking due to pairing of quasiparticles and holes, corresponding to a semimetal-insulator transition at strong Coulomb coupling. We report a lattice Monte Carlo study of the critical exponents of this transition as a function o...

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 wa...

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 hei...

Nanomechanical shuttles transferring electrons from one electrode to another, in groups or individually, offer a solution to the problem of controlled charge transport. We report the fabrication of shuttle junctions consisting of a gold nanoparticle embedded within the gap between two gold electrode...

We investigated the in-plane angular magnetoresistance (AMR) of T^′ -phase La_2−xCe_xCuO₄ thin films (x fabricated by a pulsed laser deposition technique. The in-plane AMR with H∥ab shows a twofold symmetry instead of the fourfold behavior found in other electron-doped cuprate...

Vortex lattice structures in Rashba noncentrosymmetric superconductors in magnetic fields parallel to the basal plane (H⊥c) are examined based on the BCS-like Hamiltonian and the resulting Ginzburg-Landau functional. Due to the momentum-dependent anisotropy of the Zeeman effect induced by the br...

We prove exponential decay of the off-diagonal correlation function in the two-dimensional homogeneous Bose gas when a²ρ is small and the temperature T satisfies Tgt;4πρ / ln|ln(a²ρ)| . Here, a is the scattering length of the repulsive interaction potential and ρ is the densit...

We investigate Josephson junctions with superconducting ferropnictides, both in the diffusive and ballistic limit. We focus on the proposed s_± -wave state and find that the relative phase shift intrinsic to the s_± -wave state may provide 0-π oscillations in the Josephson current. This feature...

C₆₀ and C₇₀ form with cubane (C₈H₈) molecules rotor-stator phases in which the fullerenes are rotating and the cubanes are static. Heating the rotor-stator phase up to 470 K a fulleride copolymer, poly (C₆₀C₈H₈) , is obtained. Here, we study using microwave conductivity measurements, electron...

We calculate the phonon spectrum for a graphene sheet resulting from the model proposed by Lenosky [Nature (London) 355, 333 (1992)] for the free energy of the lattice. This model takes into account not only the usual bond-bending and stretching terms, but it also captures the possible misalignment...

The adsorption of CO on the Cu(111) surface is investigated in the random phase approximation (RPA) as formulated within the adiabatic connection fluctuation-dissipation theorem. The RPA adsorption energy is obtained by adding a “local exchange-correlation correction” that is extrapolated from c...

We analyze the effect of tensional strain in the electronic structure of graphene. In the absence of electron-electron interactions, within linear elasticity theory, and a tight-binding approach, we observe that strain can generate a bulk spectral gap. However, this gap is critical, requiring thresh...

We study the behavior of α cristobalite under anisotropic stresses using constant-pressure ab initio simulations and observe the formation of the anataselike, stishovite, and CaCl₂ -type structures depending on the degree of hydrostatic compression. These phase transformations proceed via a tetr...

Cluster expansions have proven to be a valuable tool in alloy theory and other problems in materials science but the generation of cluster expansions can be a computationally expensive and time-consuming process. We present a Bayesian framework for developing cluster expansions that explicitly incor...

We study the dynamics of open quantum many-body systems driven across a critical point by quenching a parameter of the Hamiltonian at a certain velocity. General scaling laws are derived for the density of excitations and of energy produced during the quench as a function of the quench velocity and ...

X-ray absorption and x-ray magnetic circular dichroism (XMCD) spectra have been recorded at the Pt L_2,3 edges of Pt₁₃ and hydrogenated Pt₁₃H_m clusters dispersed in NaY zeolite. A Pt foil was also measured as reference for comparison with bulk Pt. From the x-ray absorption near edge structure m...

We present a spin ladder with antiferromagnetic Ising ZZ interactions along the legs and interactions on the rungs which interpolate between the Ising ladder and the quantum compass ladder. We show that the entire energy spectrum of the ladder may be determined exactly for finite number of spins ...

Dissipative spin-tunneling transitions at biased resonances in molecular magnets such as Mn₁₂ Ac are controlled by the dipolar field that can bring the system on and off resonance. It is shown that this leads to spin relaxation in form of propagating fronts of tunneling, with the dipolar field adj...

Using micron-sized thermometers and Hall bars, we report time resolved studies of the local temperature and local magnetization for two types of magnetic avalanches (abrupt spin reversals) in the molecular magnet Mn₁₂ acetate, corresponding to avalanches of the main slow-relaxing crystalline form ...

We report a detailed examination of the magnetic structure of anhydrous cupric chloride CuCl₂ carried out by powder neutron diffraction, magnetic susceptibility, and specific heat measurements on polycrystalline and single-crystal samples as well as an evaluation of the spin-exchange interactions ...

We report on the relationship between resonant tunneling, resistive switching, and memory phenomena in tunnel junctions with epitaxial SrTiO₃ barriers. Opening and closing of tunneling channels in these barriers are correlated with resonant tunneling from a specific defect that can be eliminated b...

Using density-functional calculations we have examined the evolution of the electronic structure of SrRuO₃ films grown on SrTiO₃ substrates as a function of film thickness. At the ultrathin limit of two monolayers ( RuO₂ -terminated surface) the films are found to be at the brink of a spin-state...

Direct analytic continuation of the self-energy is used to determine the effect of antiferromagnetic ordering on the spectral function and optical conductivity of a Mott insulator. Comparison of several methods shows that the most robust estimation of the gap value is obtained by use of the real par...

The spontaneous decay of a V -type three-level atom placed in a negative-refractive-index waveguide is analyzed. We find that in thin waveguides, highly efficient surface-guided modes are supported, which do not occur in positive-index waveguides. In addition, at low absorption, the mode density an...

Employing oval-shaped quantum billiards connected by quantum wires as the building blocks of a linear quantum-dot array, we calculate the ballistic magnetoconductance in the linear-response regime. Optimizing the geometry of the billiards, we aim at a maximal finite over zero-field ratio of the magn...

We present and solve a model for the vortex configuration of a disordered quantum Hall bilayer in the limit of strong and smooth disorder. We argue that there is a characteristic disorder strength below which vortices will be rare and above which they proliferate. We predict that this can be observe...

We study a two-dimensional cylindrically symmetric electron droplet separated from a surrounding electron ring by a tunable barrier using the exact diagonalization method. The magnetic field is assumed strong so that the electrons become spin polarized and reside on the lowest Fock-Darwin band. We c...

We introduce a novel concept of "coherent" and #239;ncoherent" metmaterials, which show distinctly different resonant behavior upon disordering of their initially regular lattices. In the case of a coherent metamaterial a regular ensemble of meta-molecules exhibits a collective narrow-band response that becomes broader and eventually disappears with increasing disorder, while in an incoherent metamaterial the disorder has little effect on the structure's resonant properties.

We argue from both technical and physical points of view that the main result shown in the Comment above as well as the authors' interpretations of the result are not sufficient to draw the conclusion that the scaling law at the mobility edge takes the form T\infty1/L2 . On the other hand, we believe that the result shows some evidence of T\inftylnL/L2 behavior found in Ref. 2. More calculations with even larger Ls are necessary to give a more definitive answer to this question.

Sample-to-sample fluctuations of the time-dependent conductance of a system with static disorder have been studied by means of diagrammatic theory and microwave pulsed transmission measurements. The fluctuations of time-dependent conductance are not universal, i.e., depend on sample parameters, in contrast to the universal conductance fluctuations in the steady-state regime. The variance of normalized conductance, determined by the infinite-range intensity correlation C3(t), is found to increase as a third power of delay time from an exciting pulse, t. C3(t) grows larger than the long-range intensity correlation C2(t) after a time tq ~ {g }nbsp;tD (tD being the diffusion time, g being the average dimensionless conductance).

We study in detail out-of-plane losses of photonic crystal membranes located at optical distance above a substrate by using approximate and rigorous methods. We reveal a resonance mechanism in the air gap, separating membrane and substrate, being responsible for a nonmonotonic loss dependence on wavelength and gap width. We show that by taking advantage of this effect and by carefully adjusting the gap width the losses can even be less than those for an isolated membrane.

Self-diffusion in ion beam sputtered nanocrystalline Fe is studied between 310 and 510 C degrees, using neutron reflectometry on [natFe (7 nm)/57Fe(3 nm)]15 isotope multilayers. Neutron reflectometry has the advantage over other methods of diffusivity determination, that diffusion lengths in the order of 1 nm and below can be determined. This enables diffusion experiments in a nanostructure which is not significantly modified by grain growth during annealing. The determined diffusivities are time depended and decrease by more than two orders of magnitude during isothermal annealing. In early stages, diffusion is controlled by frozen-in non-equilibrium point defects (interstitials or vacancies) present after deposition. The decrease of the diffusivities can be attributed to the annihilation of these point defects. For very long annealing times the diffusivities above 400 C degrees are in good agreement with volume diffusivities measured in single crystals given in literature. However, at a temperature of 400 C degrees and below the diffusivities are still higher than extrapolated literature data also after more than 8 days of annealing, indicating that defect annihilation is still going on.

First-principles density-functional theory calculations reveal significantly different behavior between group IIIA and IIIB delafossites CuMO2. The group IIIA delafossites have indirect bandgaps with large differences between the direct and indirect bandgaps. However, this difference is small for the group- IIIB delafossites: only 0.22 eV for CuScO2 and it diminishes further for CuYO2 and CuLaO2. Also, whereas group IIIA prefers rhombohedral stacking, group IIIB stabilizes in hexagonal structures. We further find that CuScO2 has the highest calculated fundamental bandgap among all the delafossite oxides. In addition, CuLaO2 is found to have a direct bandgap. These differences are explained by the different atomic configurations between the group IIIA and IIIB elements. Our understanding of these delafossites provides general guidance for proper selection of delafossites for suitable applications in optoelectronic devices.

We study the tunneling current between edge states of quantum Hall liquids across a single long contact region, and predict a resonance at a bias voltage set by the scale of the edge velocity. For typical devices and edge velocities associated with charged modes, this resonance occurs outside the physically accessible bias domain. However, for edge states that are expected to support neutral modes, such as the n = 2/3, and n = 5/2 Pfaffian and anti-Pfaffian states, the neutral velocity can be orders of magnitude smaller than the charged mode and if so the resonance would be accessible. Therefore, such long tunneling contacts can resolve the presence of neutral edge modes in certain quantum Hall liquids.

Current auto- and cross-correlations are studied in a system of two capacitively coupled quantum dots. We are interested in a role of Coulomb interaction in dynamical correlations, which occur outside the Coulomb blockade region (for high bias). After decomposition of the current correlation functions into contributions between individual tunneling events, we can show which of them are relevant and lead to sub-/supper-Poissonian shot noise and negative/positive cross-correlations. The results are differentiated for a weak and strong inter-dot coupling. Interesting results are for the strong coupling case when electron transfer in one of the channel is strongly correlated with charge drag in the second channel. We show that cross-correlations are non-monotonic functions of bias voltage and they are in general negative (except some cases with asymmetric tunnel resistances). This is effect of local potential fluctuations correlated by Coulomb interaction, which mimics the Pauli exclusion principle.

Real and momentum space spectrally resolved images of microcavity polariton emission in the regime of condensation are investigated under non resonant excitation using a laser source with reduced intensity fluctuations on the timescale of the exciton lifetime. We observe that the polariton emission consists of many macroscopically occupied modes. Lower energy modes are strongly localized by the spatial polaritonic potential disorder on a scale of few microns. Higher energy modes have finite k-vectors and are delocalized over 10-15 mm. All the modes exhibit long range spatial coherence comparable to their size. We provide a theoretical model describing the behavior of the system with the results of the simulations in good agreement with the experimental observations. We show that the multimode emission of the polariton condensate is a result of its nonequilibrium character, the interaction with the local polaritonic potential and the reduced intensity fluctuations of the excitation laser.

We demonstrate the coupling of a single InAs/InP quantum dash, emitting around 1.55 \mu m, to a slow-light mode in a 2D photonic crystal on Bragg reflector. These surface addressable 2.5D photonic crystal band-edge modes present the advantages of a vertical emission and the mode area and localization may be controlled, leading to a less critical spatial alignment with the emitter. An increase of the spontaneous emission rate by a factor 1.5-2, is measured at low temperature and is compared to the Purcell factor predicted by 3D time-domain electromagnetic simulations.

We analyze the full counting statistics (FCS) of a single-site quantum dot coupled to a local Holstein phonon for arbitrary transmission and weak electron-phonon coupling. We identify explicitly the contributions due to quasielastic and inelastic transport processes in the cumulant generating function and discuss their influence on the transport properties of the dot. We find that in the low-energy sector, i.e.nbsp;for bias voltage and phonon frequency much smaller than the dot-electrode contact transparency, the inelastic term causes a sign change in the shot noise correction at certain universal values of the transmission. Furthermore, we show that when the correction to the current due to inelastic processes vanishes, all the odd order cumulants vanish as well.

We report on theoretical study of the magnetic properties of a magnetic metal d layer embedded into a nonmagnetic nondegenerated semiconductor, taking into account the diffusion smearing, that is unavoidable in the case of delta doping. The system of interest is modelled by the d layer core, enriched in metal atoms, and a nearly depleted smeared periphery. Confinement states in the form of two-dimensional spin-polarized subbands within the semiconductor band gap arise from potential and exchange scattering of carriers at the core. The mechanism of indirect exchange between impurity spins placed within the peripheral region of the d layer, via partially occupied confinement states, is analyzed. It is shown that, in the case of a ferromagnetic core, the impurity spins align parallel or antiparallel to the core magnetization, due to the polarization of carriers in the confinement states. Allowing for the confinement mechanism of interaction between the impurity spins as well as for the exchange mechanism through deep levels of the semiconductor host, the magnetic configuration of the impurity spins in the peripheral region of the d layer is obtained in the framework of a phenomenological approach.

Using a recently developed broadband microwave measurement technique, we have studied the hysteretic appearance and disappearance with in-plane magnetic field of the uniform ferromagnetic resonance mode of a patterned permalloy disc array. The observed features are consistent with our micromagnetic simulations (performed on an infinite array of such discs), which predict that on decreasing the magnetic field from a positively magnetized state at positive fields the array will: i) pass continuously into a double vortex state; ii) followed by a discontinuous transition to a single vortex state; and finally iii) discontinuously into a negatively magnetized state at some negative field. The hysteretic counterpart occurs on reversing the field sweep and returning to positive fields. The FMR data are consistent with the hysteretic d.c. magnetization measurements performed earlier on samples patterned in an identical manner.

Spin-state transitions, observed in many transition metal compounds containing Co3+ and Fe2+, may occur with the change of temperature, pressure, but also with doping, in which case the competition of single-site effects and kinetic energy of doped carriers can favor a change in the spin state. We consider this situation in a simple model, formally resembling that used for manganites in Ref.nbsp;. Based on such a model, we predict the possibility of a jump-like change in the number of Co3+ ions undergoing spin-state transition caused by hole doping. A tendency to the electronic phase separation within a wide doping range is demonstrated. Phase diagrams with the regions of phase separation are constructed at different values of the characteristic parameters of the model.

The effect of interface roughness on the induced polarization of V in polycrystalline V/Fe/V trilayers was investigated with x-ray magnetic circular dichroism (XMCD), x-ray resonant magnetic scattering (XRMS) and polarized neutron reflectometry (PNR). Trilayer samples were sputter-deposited onto Si substrates at room temperature to minimize inter-diffusion. The films were polycrystalline and exhibited an average 0.5 nm root mean square (rms) interfacial roughness at the Fe/V interfaces. The induced polarization found in V was constrained to the Fe/V interface extending approximately up to 2-3 monolayers into the V, and exhibited antiferromagnetic alignment to the Fe layer. A magnetic moment for V ranging between -0.46 to -0.86 \mu B/V atom is consistent with the neutron and resonant x-ray data. Notably, this value for structurally rough interfaces is significantly larger than that reported for samples with atomically flat Fe/V interfaces.

We have performed x-ray linear and circular magnetic dichroism experiments at the Mn L2,3-edge of the La0.7Sr0.3MnO3 ultra thin films. Our measurements show that the antiferromagnetic (AF) insulating phase is stabilized by the interfacial rearrangement of the Mn 3d orbitals, despite the relevant magnetostriction anisotropic effect on the double-exchange ferromagnetic (FM) metallic phase. As a consequence, the Mn atomic magnetic moment orientation and how it reacts to strain differ in the FM and AF phases. In some cases a FM insulating (FMI) phase adds to the AF and FM. Its peculiar magnetic properties include in-plane magnetic anisotropy and partial release of the orbital moment quenching. Nevertheless the FMI phase appears little coupled to the other ones. PACS numbers: 75.47.Lx, 78.70.Dm

In spin-driven ferroelectrics, often referred to as magnetoelectric (ME) multiferroics, a domain wall separates two magnetic as well as ferroelectric domains. Therefore, it is expected that the control of such an ME domain wall could result in a large ME effect. Here, we show that the direction of a slanted magnetic field controls whether the electric polarization (\bm P) rotates by +90 or -90 degrees upon the \bm P-flop transition in a prototypical spiral magnetic ferroelectric TbMnO3. This behavior is discussed in terms of the domain-wall energy of a spiral magnet in a slanted magnetic field.

Theory of the electron spin relaxation in graphene on the SiO2 substrate is developed. Charged impurities and polar optical surface phonons in the substrate induce an effective random Bychkov-Rashba-like spin-orbit coupling field which leads to spin relaxation by the D'yakonov-Perel' mechanism. Analytical estimates and Monte Carlo simulations show that the corresponding spin relaxation times are between micro- to milliseconds, being only weakly temperature dependent. It is also argued that the presence of adatoms on graphene can lead to spin lifetimes shorter than nanoseconds.

An interplay between the increase in the number of carriers and the decrease in the scattering time is expected to result in a saturation of the in-plane resistivity, rab, in graphite above room temperature. Contrary to this expectation, we observe a pronounced increase in rab in the interval between 300 and 900 K. We provide a theory of this effect based on intervalley scattering of charge carriers by high-frequency, graphene-like optical phonons.

We investigate electronic transport through single-wall carbon-nanotube quantum dots weakly coupled to metallic leads in the Coulomb-blockade regime. While sequential tunneling is suppressed, the transport, due to correlated tunneling of two electrons - cotunneling, is possible. We report on a pronounced current peak for the singlet-triplet transition mediated by inelastic cotunneling processes that allows for their separation from the elastic components. Using the second order perturbation theory in the tunnel-coupling strength we are able to fit it into the experimental data, explain the details of the line shape, and extract values of parameters that describe pseudo-spin asymmetry.

Recent experiments on quantum turbulence in superfluid helium made use of small tracer particles to track the motion of quantized vortices, determine velocity statistics and visualize vortex reconnections. A problem with this visualization technique is that it may change the turbulent flow which it visualizes. To address this problem, we derive and solve the equations of motion of a quantized vortex ring which contains a given number of particles trapped on the vortex core, hence derive a quantitative criterion to determine in which measure small particles trapped in quantized vortices affect vortex motion. Finally we apply the criterion to a recent experiment.

The extent of the vortex liquid state in underdoped single crystals of the oxypnictide superconductors NdFeAs(O,F) and (Ba,K)Fe2As2 is investigated using specific heat (Cp) and Hall probe magnetization experiments. In both materials, the vortex liquid lies entirely in the regime where the three-dimensional Lowest Landau Level (3D-LLL) approximation is valid, and both systems present a very small shift of the specific heat anomaly with increasing field. The irreversibility line, defined as the onset of diamagnetic response, is very rapidly shifted towards lower temperatures in NdFeAs(O,F), but remains close to the Cp anomaly in (Ba,K)Fe2As2. These measurements strongly suggest that a vortex liquid phase occupies a large portion of the mixed state phase diagram of NdFeAs(O,F), but not in (Ba,K)Fe2As2. This difference can be attributed to different Ginzburg numbers Gi, the latter being about 100 times larger in NdFeAs(O,F) than in (Ba,K)Fe2As2. The angular dependence of the upper critical field, derived from 3D-LLL scaling of the irreversibility lines, presents deviations from the standard 3D effective mass model in both materials, with an anisotropy being about 3 times smaller in (Ba,K)Fe2As2 (g ~ 2.5) than in Nd(F,O)FeAs (g ~ 7.5).

We apply the time-dependent Ginzburg-Landau approach to describe the nucleation of vortex-antivortex pairs in a hybrid system, formed by a thin superconducting film with a single in-plane magnetic dipole on top of it, and to simulate the vortex dynamics in the presence of a direct transport current. At current densities, sufficient to depin vortices and antivortices from a magnetic dipole, it periodically generates vortex-antivortex pairs, producing vortex and antivortex streams which eventually coalesce into phase-slip lines with further increasing current. We demonstrate the possibility to efficiently control the generation rate and trajectories of emitted vortices by the applied current density and an additional weak homogeneous magnetic field.

We use high resolution angle-resolved photoemission to study the electronic structure of the nbsp;pnictides. We observe two electron bands and two hole bands near the X-point, (p,p) of the Brillouin zone, in the paramagnetic state for electron-doped , undoped , and hole-doped . Among these bands, only the electron bands cross the Fermi level, forming two electron pockets around X, while the hole bands approach but never reach the Fermi level. We show that the band structure of the nbsp;family matches reasonably well with the prediction of LDA calculations after a momentum-dependent shift and renormalization. Our finding resolves a number of inconsistencies regarding the electronic structure of pnictides.

Direct analytic continuation of the self-energy is used to determine the effect of antiferromagnetic ordering on the spectral function and optical conductivity of a Mott insulator. Comparison of several methods shows that the most robust estimation of the gap value is obtained by use of the real par...

Cluster expansions have proven to be a valuable tool in alloy theory and other problems in materials science but the generation of cluster expansions can be a computationally expensive and time-consuming process. We present a Bayesian framework for developing cluster expansions that explicitly incor...

We study electron transport in a double quantum dot in the Pauli spin blockade regime in the presence of strong spin-orbit coupling. The effect of spin-orbit coupling is incorporated into a modified interdot tunnel coupling. We elucidate the role of the external magnetic field, the nuclear fields in...

Ab initio electronic structure calculations on potassium at high pressure reveal that the orthorhombic oP8 structure, found recently in sodium, has a lower enthalpy in potassium than do other candidate crystal structures recently reported to be stable at high pressure [Y. Ma, A. R. Oganov, and Y. ...

The Fermi and Rashba energies of surface states in the Bi_xPb_1−x/Ag(111) 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 t...

We have studied the frequency and voltage dependences of voltage-induced torsional strains in orthorhombic TaS₃ [V. Y. Pokrovskii, S. G. Zybtsev, and I. G. Gorlova, Phys. Rev. Lett. 98, 206404 (2007)] by measuring the modulation of the resonant frequency of an rf cavity containing the sample. The ...

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 creatio...

We study the spectral function of a single hole doped into the ab plane of the Mott insulator LaVO₃ , with antiferromagnetic (AF) spin order of S=1 spins accompanied by alternating orbital (AO) order of active {d_yz,d_zx} orbitals. Starting from the respective t-J model, with spin-orbital sup...

In many transition-metal oxides the electrical resistance is observed to undergo dramatic changes induced by large biases. In magnetite, Fe₃O₄ , below the Verwey temperature, an electric-field-driven transition to a state of lower resistance was recently found, with hysteretic current-voltage respo...

The in-plane dielectric and ferroelectric properties of coherent anisotropically strained SrTiO₃ thin films grown on orthorhombic (101) DyScO₃ substrates were examined as a function of the angle between the applied electric field and the principal directions of the substrate. The dielectric perm...