Physical Review B

We report on polarized neutron-scattering and piezoresponse force microscopy studies of millimeter-sized single crystals of multiferroic BiFeO₃ . The crystals, grown below the Curie temperature, consist of a single ferroelectric domain. Two unique electric polarization directions, as well as the po...

Site-specific dynamics of oxygen atoms in a network silicate liquid (6.4%K₂O-93.6%SiO₂) has been studied in the glass transition region using high-temperature (up to 973 K) ¹⁷O nuclear-magnetic-resonance spectroscopy. Chemical exchange between bridging and nonbridging oxygen environments is obse...

We consider the finite-temperature dynamical susceptibility χ(ω,q,T) of gapped quantum spin chains such as the integer spin Heisenberg and transverse field Ising models. At zero temperature χ in these models is dominated by a delta-function line arising from the coherent propagation of single...

The electronic structure of the anomalous δ phase of Pu is analyzed by a general and exact reformulation of the exchange energy of the f shell. It is found that the dominating contribution to the exchange energy is a polarization of orbital spin-currents that preserves the time-reversal symmetr...

The three-dimensional classical dimer model with interactions shows an unexpected continuous phase transition between an ordered dimer crystal and a Coulomb liquid. A detailed analysis of the critical dimer and monomer correlation functions points to a subtle interplay between the fluctuations of th...

We provide evidence for the existence of a quantum critical point at the metallization of magnetite Fe₃O₄ at pressure p_c≈8 GPa . We show that the present ac magnetic-susceptibility data support earlier resistivity data. The Verwey temperature scales with pressure T_V∼(1−p/p_c)^ν with νâ...

Neutron diffraction measurements of a high-quality single crystal of CaFe₂As₂ are reported. A sharp transition was observed between the high-temperature tetragonal and low-temperature orthorhombic structures at T_S=172.5 K (on cooling) and 173.5 K (on warming). Concomitant with the structural t...

Samples of Nd_2−xCe_xCuO₄ , an electron-doped high-temperature superconducting cuprate, near optimal doping at x=0.155 were measured via angle-resolved photoemission. We report a renormalization feature in the self-energy (“kink”) in the band dispersion at ≈50–60 meV present in nodal ...

We have carried out measurements of the pressure dependence to 1.2 GPa of the oxygen isotope effect on T_c in the high- T_c superconductor YBa₂Cu₄O₈ using a clamp cell in a superconducting quantum interference device magnetometer. This compound lies close to, but just above, the 1/8th doping po...

Resistivity, specific-heat, and magnetic-susceptibility measurements performed on SrFe₂As₂ samples evidence a behavior very similar to that observed in LaFeAsO and BaFe₂As₂ , with the difference being that the formation of the spin-density wave and the lattice deformation occur in a pronounced fi...

We have performed ultrahigh energy- and momentum-resolved angle-resolved photoemission spectroscopy of impurity-doped Bi₂Sr₂CaCu₂O₈ . We found that the point node of the superconducting gap near the (π/2,π/2) point in a pristine sample gradually evolves into a small gapless Fermi arc upon Zn or...

We report on ¹⁹F nuclear magnetic resonance (NMR) investigation of the high-temperature superconductor LaFeAsO_0.89F_0.11 (T_c∼28 K) . We demonstrate that low-frequency spin fluctuations exhibit pseudogap behavior above T_c . We also deduce the London penetration depth λ from NMR line broad...

We explain the recently observed linear temperature dependence of the nodal Fermi velocity v_F(T) in nearly optimally doped cuprates. We argue that it originates from electron-electron interaction and it is a fundamental property of an arbitrary two-dimensional (2D) Fermi liquid. We consider a spin...

The optical conductivity σ(ω) is calculated at finite temperature T for CuO₂ chain clusters within a pd -Hubbard model. Data at T=300 K for Li₂CuO₂ are reanalyzed within this approach. The relative weights of Zhang-Rice singlet and triplet charge excitations near 2.5 and 4 eV, respect...

Superconductivity in Mo₃Sb₇ is analyzed using the combined electronic structure and phonon calculations, and the electron-phonon coupling constant λ_ph=0.54 is determined from first principles. This value explains the experimental value of the superconducting critical temperature T_c=2.2 K . T...

Motivated by the observation of a giant Nernst effect in URu₂Si₂ , the thermoelectric response of the related system UPt₂Si₂ was investigated using thermal and electric transport properties such as the Nernst and Seebeck effects, thermal conductivity, Hall effect, and electrical resistivity. Unli...

We have experimentally realized a two-dimensional partial focusing within a planar waveguide using complementary indefinite metamaterials. When the electric fields emitted from the dipole are TE polarized, the focusing condition requires negative magnetic response in the propagation direction of the...

Optical metamaterials can exhibit negative index of refraction when both the effective permittivity and permeability are negative. The negative permeability is usually considered to be associated with a magnetic dipole resonance and the contribution from electric quadrupoles is neglected. Here, we s...

We have calculated static and dynamic response properties for several semiconducting and insulating solids using hybrid functionals, which admix a small fraction of nonlocal Fock exchange to an otherwise semilocal density functional. The calculated static and dynamic properties are clearly improved ...

Coherent x-ray scattering in a grazing-incidence geometry was used to image SiGe nanoislands grown by liquid phase epitaxy on Si(001). Due to their narrow size distribution, identical shape, and orientation, the total scattered intensity obtained in this geometry represents a coherent diffraction pa...

Electrical injection from the Heusler alloy Co₂FeSi into (Al,Ga)As is investigated for different growth temperatures T_G of the injector layer. Depending on T_G , the spin polarization of injected electrons in the semiconductor is determined by two competing mechanisms: actual spin injection at t...

Time-resolved microscopic optical second-harmonic generation (TRM-SHG) technique provides a different approach to study the dynamics of carriers in organic field-effect transistor (OFET). Different from many common methods, the TRM SHG directly probes the transient electric-field distribution in the...

We observe oscillatory magnetoresistance in double quantum wells under microwave irradiation. The results are explained in terms of the influence of subband coupling on the frequency dependent photoinduced part of the electron distribution function. As a consequence, the magnetoresistance demonstrat...

The detailed knowledge of a system’s ground state is the most important prerequisite to understand its physical properties. We have extended the cluster-expansion formalism to apply it to adsorbate systems with an in-principle arbitrary number of adsorbate sites. The formalism will be applied to a...

We study the spin Hall effect (SHE) in graphene using a realistic multiorbital tight-binding model that includes the atomic spin-orbit interaction. The SHE is found to be induced by the spin-dependent Aharonov-Bohm phase. In the metallic case, the calculated values for the spin Hall conductivity (SH...

The nature of local atomic and electronic structure at the nanoscale is of both fundamental and technological importance. For example, supported metal nanoclusters exhibit a number of unusual phenomena including large structural disorder and bond-length contraction with increasing temperature. We in...

We perform electrical transport measurements in graphene with several sample geometries. In particular, we design “invasive” probes crossing the whole graphene sheet as well as “external” probes connected through graphene side arms. The four-probe conductance measured between external probes...

Because graphene is an atomically two-dimensional gapless semiconductor with nearly identical conduction and valence bands, graphene-based bilayers are attractive candidates for high-temperature electron-hole pair condensation. We present estimates which suggest that the Kosterlitz-Thouless temperat...

Large-scale atomistic simulations considering a 5 nm in radius H-passivated Si nanosphere that impacts with relatively low energies onto a H-passivated Si substrate reveal a transition between two fundamental collision modes. At impacting speeds of less than ∼1000 m/s particle-reflection domin...

We compute the electron-phonon coupling (EPC) of selected phonon modes in graphene and graphite using various ab initio methods. The inclusion of nonlocal exchange-correlation effects within the GW approach strongly renormalizes the square EPC of the A₁^′ K mode by almost 80% with respect to ...

We report a scanning tunneling spectroscopy investigation of polycrystalline SmFeAsO_0.85 having a superconducting transition at 52 K. On large regions of the sample surface the tunneling spectra exhibited V -shaped gap structures with no coherence peaks, indicating degraded surface properties. In...

We apply a semiclassical method to the problem of field angle-dependent oscillations of the density of states and thermal conductivity for nodal superconductors and apply our results to the superconductor PrOs₄Sb₁₂ . The oscillatory contributions to the thermal conductivity for all possible point n...

We report the results from a Cu nuclear magnetic resonance (NMR) and magnetization study of the electron-doped high-temperature superconducting cuprate (HTSC) Sr_0.9La_0.1Cu_1−xNi_xO₂ . This compound shows a large suppression of superconductivity by Ni [ΔT_c/Δx=18 K/%(Ni)] that is comparable to ...

We study the heat transfer from femtosecond laser-heated hot electrons in a metal to adsorbates in the presence of vibrational-mode coupling. The theory is successfully applied to the experimental result of atomic oxygen hopping on a vicinal Pt(111) surface. The effective friction coupling between h...

We study a family of non-Abelian topological models in a lattice that arise by modifying the Kitaev model through the introduction of single-qudit terms. The effect of these terms amounts to a reduction in the discrete gauge symmetry with respect to the original systems, which corresponds to a gener...

Magnetoabsorption spectra of bilayer graphene ribbons with Bernal stacking are studied by the Peierls-coupling tight-binding method. When the magnetic confinement prevails over the quantum confinement, low-energy spectra chiefly exhibit many Landau peaks, which are strongly modified by the inter-rib...

The ionization of xenon Rydberg atoms excited to the lowest-lying state in the n=17 and 20 Stark manifolds at Au(111) surfaces is investigated as a function of the angle of incidence. Analysis of the data points to the presence of localized stray fields at the surface associated with surface inhom...

The detailed knowledge of a system’s ground state is the most important prerequisite to understand its physical properties. We have extended the cluster-expansion formalism to apply it to adsorbate systems with an in-principle arbitrary number of adsorbate sites. The formalism will be applied to a...

Effects of a Kekule distortion on exciton instability in single-layer graphene are discussed. In the framework of quantum electrodynamics the mass of the electron generated dynamically is worked out using a Schwinger-Dyson equation. For homogeneous lattice distortion it is shown that the generated m...

The energy of formation of self-interstitial atoms has been experimentally determined using an energy analysis of low-temperature field evaporation of tungsten in a field-ion microscope. An experimental approach, based on the strong dependence of the threshold field for evaporation on the total ener...

We present a theory of the finite temperature thermoelectric response functions of graphene in the hydrodynamic regime where electron-electron collisions dominate the scattering. In moderate magnetic fields, the Dirac particles undergo a collective cyclotron motion with a temperature-dependent relat...

Work functions of NiPt alloys with different compositions are investigated using first-principles methods based on density-functional theory. Results of our calculations reveal that surface alloy composition has a significant effect on the work function of the NiPt alloy. However, for a given surfac...

The stress-induced zircon-scheelite phase transition is theoretically studied by means of ab initio and shell-model calculations. It is shown that this phase transition may originate from shear elastic strains which bring together the structures of both phases and draw them unstable against the SiO_...

Based on density functional theory calculations, we study cleavage under loading mode I, in which the atomic layers are allowed to relax after a crack with a given opening was initiated. By introducing the new materials parameter l_r , the critical opening for relaxed cleavage, we derive a simple an...

The frequency dependence of the dynamical-mode-locking phenomena in the presence of noise is studied in the one-dimensional ac driven dissipative Frenkel-Kontorova model. It was found that, besides producing the melting of Shapiro steps and decrease in the critical depinning force, noise may transfe...

Structural and magnetic properties of the RCr₂Si₂C compounds ( R=Y , La–Sm, Gd–Er) with the carbon-filled CeMg₂Si₂ -type structure (space group P4/mmm ) have been studied by means of powder and single-crystal x-ray diffractions, magnetic measurements, and neutron powder diffraction. The comp...

In the first part of this paper, we study the spin- S Kitaev model using spin-wave theory. We discover a remarkable geometry of the minimum-energy surface in the N -spin space. The classical ground states, called Cartesian or CN-ground states, whose number grows exponentially with the number of sp...

The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in La_1−xCa_xMnO₃ for x=0.3 and 0.2 was investigated by dc conductivity, ellipsometry, and vacuum ultraviolet reflectance for energies between 0 and 22 eV. A stabili...

A cone-shaped cloak whose cross section gradually increases along the axial direction ( z direction) is proposed in this paper. We present full wave analysis of this cloak in response to electromagnetic waves, showing that a perfect conical cloak can support the propagation of any kind of fields. I...

Motivated by both experimental and theoretical activities, we discuss the fate of Kondo screening and possible quantum phase transitions in antiferromagnetically ordered phases of Kondo lattices. While transitions with topological changes of the Fermi surface may occur, we demonstrate that an entire...

Carrier mobilities of rubrene films are among the highest values reported for any organic semiconductor. Here, we probe with first-principles calculations the sensitivity of rubrene crystals on impurities. We find that isolated oxygen impurities create distinct peaks in the electronic density of sta...

Lanthanum telluride (La_3−xTe₄) has been synthesized via mechanical alloying and characterized for thermoelectric performance. This work confirms prior reports of lanthanum telluride as a good high-temperature thermoelectric material, with zT∼1.1 obtained at 1275 K. The thermoelectric perform...

Effects of correlated disorder on wave localization have attracted considerable interest. Motivated by the importance of studies of quantum transport in rough nanowires, here we examine how colored surface roughness impacts the conductance of two-dimensional quantum waveguides, using direct-scatteri...

Coherent x-ray scattering in a grazing-incidence geometry was used to image SiGe nanoislands grown by liquid phase epitaxy on Si(001). Due to their narrow size distribution, identical shape, and orientation, the total scattered intensity obtained in this geometry represents a coherent diffraction pa...

We show via first principles calculations that the electronic structure of ZrPd2, which does not form a hydride, can be modified by partial substitution of Fe for Pd, leading to a material that forms a hydride. We also show that PdZr2, which forms a very stable hydride, can also be modified by Fe addition to lower the enthalpy of hydride formation. These results are explained in terms of electronic structure, specifically electronegativity, charge transfer and s-d bonding, and clearly have implications in the search of new materials for hydrogen storage.

This article considers idealized solar cells whose absorbers are intermediate bands (IB) media with finite bandwidths that permit both interband and intraband photo-induced electronic transitions at states within the IB. To comprehend the effect of the IB width, three classes of IB absorbers are constructed, where each class is distinguished from the others by its spectral selectivity. It is shown that (i) the maximum-power efficiency tends gradually towards zero with increasing bandwidth when photo-induced interband transitions and intraband transitions are equally likely; (ii) with respect to the former, a relative efficiency enhancement may occur when photo-induced intraband transitions dominate interband transitions; and (iii) though thermodynamically consistent, efficiencies may be physically inconsistent without including photo-induced intraband transitions. Resulting from the solar surface temperature of 6000 K, the authors conclude that the largest efficiencies result when the IB width is roughly equal to or less than 800 meV.

We present density functional theory calculations for the geometry and conductance of 4,4-bipyridine (BPD) nano junctions with Au and Pt electrodes. The fact that transport takes place via bipyridine's lowest unoccupied molecular orbital (LUMO) suggests that the Au-BPD junction should have larger conductance than the Pt-BPD junction due to the smaller work function of Au as compared to Pt. On the other hand coupling to the local d-band is stronger in the case of Pt and this broadens the LUMO resonance. We find that these effects largely outbalance each other leading to conductances of 0.01G0 and 0.02G0 for the Au and Pt contacts, respectively (G0=2e2/h is the conductance quantum). The effect of coupling to the electrodes is investigated by means of the group orbital which makes precise the concept of the local band. The construction allows us to explain and rationalize the first-principles results within a simple single-level model.

We study the ground state properties of the geometrically frustrated Hubbard model on the anisotropic checkerboard lattice with nearest-neighbor hopping t and next nearest-neighbor hopping t. By using the path-integral renormalization group method, we study the phase diagram in the parameter space of the Hubbard interaction U and the frustration-control parameter t/t. Close examinations of the effective hopping, the double occupancy, the momentum distribution and the spin/charge correlation functions allow us to determine the phase diagram at zero temperature, where the plaquette-singlet insulator emerges besides the antiferromagnetic insulator and the paramagnetic metal. Spin-liquid insulating states without any kind of symmetry breaking cannot be found in our frustrated model.

We analyze spin-charge separation in molecular wires using a combination of real-time density functional simulations and model Hamiltonian calculations. By considering the ab initio electron dynamics of positively charged (C50H52+) and negatively charged (C50H52-) polyacetylene chains under a chemical potential bias, we are able to extract information about the mobility of electrons, holes and spins in these molecules. Our results indicate that charges move more rapidly than spins in these molecules. We further supplement our ab initio data with empirical calculations employing the Pariser-Parr-Pople (PPP) model Hamiltonian. Our modeling indicates that the degree of spin-charge separation responds very strongly to the non-local exchange interaction, while showing little sensitivity to Coulombic forces. In particular, in order to reproduce the B3LYP results within the PPP model, it is necessary to reduce the strength of the exchange interaction by ca. 50% in the latter. We therefore conclude that many of the features present in the B3LYP spin current response are a direct result of self-interaction error in the functional.

The tetragonal to orthorhombic ferroelastic phase transition between rutile-type and CaCl2-type SiO2 at high pressures is studied using first-principles calculations and the Landau free energy expansion. The phase transition is systematically investigated in terms of characteristic phonon modes with B1g and Ag symmetries, shear moduli, transverse acoustic mode, rotation angle of the SiO6 octahedra, spontaneous symmetry-breaking strain and volume strain, and enthalpy. The results show that these physical behaviors at the transition are well described using the Landau free energy expansion parameterized by the first-principles calculations.

We present evidence of in-plane coupling in relaxor PbMg\text1/3Nb\text2/3O\text3 /ferroelectric PbTiO\text3 superlattices [PMN\text(1-x)L /PT\textxL]\text10. For constant superlattice wavelength = d\textPMN + d\textPT , we find that by varying the constituents thicknesses d\textPMN and d\textPT a significant effect is observed on the internal a\text1/a\text2 domain structure of the PT layers as well as on the appearance of domains in PMN. Since the strain effects on the structural patterns can be modulated by adjusting x, this is one way to control the polarization axis on a nanoscale level.

A method for calculating free-energy differences based on a free-energy perturbation (FEP) formalism in an alloy system described by two different Hamiltonians is reported. The intended application is the calculation of solid-liquid phase equilibria in alloys with the accuracy of first-principles electronic density-functional theory (DFT). For this purpose free energies are derived with a classical interatomic potential and FEP calculations are used to compute corrections to these reference values. For practical applications of this approach, due to the relatively high computational cost of DFT calculations, it is critical that the FEP calculations converge rapidly in terms of the number of samples used to estimate relevant ensemble averages. This issue is investigated in the current study employing two classical interatomic potential models for Ni-Cu. These models yield differences in predicted phase boundary temperatures of approximately 100 K, comparable to those that might be expected between a DFT Hamiltonian and a well-fit classical potential. We show that for pure elements the FEP calculations converge rapidly with the number of samples, yielding free energy differences converged to within a fraction of a meV/atom in a few dozen energy calculations. For a concentrated equiatomic alloy similar precision requires roughly a hundred samples. The results suggest that the proposed methodology could provide a computationally tractable framework for calculating solid-liquid phase equilibria in concentrated alloys with DFT accuracy.

By combining the methods of isotopic substitution in neutron diffraction and high-energy x-ray diffraction we have determined partial structure factors of vitreous SiO2. No first sharp diffraction peak is observed in the concentration-concentration partial structure factor, ruling out the void-cluster based model as a possible explanation for the origin of intermediate range order. However, the data are consistent with a cage model in which the intermediate range order arises from the periodicity of boundaries between small cages in the network, and the second diffraction peak is associated with chemical ordering of SiO4 tetrahedra within continuous regions of the network between cages.

The aim of the present work is to complete a preliminary study concerning electronic band structure calculations using the ab-initio TBLMTO method and X-ray Photoelectron Spectroscopy (XPS) investigations on NaxCu1-xIn5S8 compounds with 0x1, which are expected to be formed at the Cu(In,Ga)Se2/In2S3 interface. The band structure calculations demonstrate that for the compounds containing both Na and Cu, as the Cu-content increases the band gap tends to decreases and XPS measurements show that this variation is mainly due to valence-band-maximum shift along the solid solution. The band gap strongly depends on the nature of the monovalent cation and band structure calculations demonstrate that the d electrons of copper are responsible for the shift of the valence band. In addition, it is worth noting that the Cu-containing compounds have indirect gaps.

The domain wall induced reversal dynamics in compressively strained Ga1-xMnxAs was studied employing the magneto-optical Kerr effect and Kerr microscopy. Due to the influence of an uniaxial part in the in-plane magnetic anisotropy 90d domain walls with considerably different dynamic behavior are observed. While the 90 +d reversal is identified to be propagation dominated with a small number of domains, the case of 90-d reversal involves a larger number of nucleation centers. The domain wall nucleation/propagation energies e for both transitions are estimated using model calculations from which we conclude that single domain devices can be achievable using the 90+d mode.

Recently, we have found an additional spin-orbit (SO) interaction in quantum wells with two subbands [Phys. Rev. Lett. 99, 076603 (2007)]. This new SO term is non-zero even in symmetric geometries, as it arises from the intersubband coupling between confined states of distinct parities, and its strength is comparable to that of the ordinary Rashba. Starting from the 8 ×8 Kane model, here we present a detailed derivation of this new SO Hamiltonian and the corresponding SO coupling. In addition, within the self-consistent Hartree approximation, we calculate the strength of this new SO coupling for realistic symmetric modulation-doped wells with two subbands. We consider gated structures with either a constant areal electron density or a constant chemical potential. In the parameter range studied, both models give similar results. By considering the effects of an external applied bias, which breaks the structural inversion symmetry of the wells, we also calculate the strength of the resulting induced Rashba couplings within each subband. Interestingly, we find that for double wells the Rashba couplings for the first and second subbands interchange signs abruptly across the zero bias, while the intersubband SO coupling exhibits a resonant behavior near this symmetric configuration. For completeness we also determine the strength of the Dresselhaus couplings and find them essentially constant as function of the applied bias.

Adsorption of benzene on the SiC(001)-(3×2) surface is investigated employing density-functional theory within the generalized gradient approximation (DFT-GGA). Due to the relatively large Si dimer distance on the clean surface, benzene appears to adsorb only in a standard butterfly configuration on single dimers. Such a well-defined structure might turn out to be of particular interest for fundamental experimental investigations, as well as for surface functionalization by organic molecules. The reaction pathway of benzene from an initial state in vacuum to the final adsorption state in the standard butterfly configuration on the surface is explored and the reaction process is analyzed employing maximally localized Wannier functions. The electronic structure of the optimized adsorption configuration is discussed in terms of the surface band structure, charge and state densities, as well as scanning tunneling microscopy images.

Excellent mechanical properties of amorphous/glassy metals along with the capability to fabricate sub -50 nm patterns promise their applications as a futuristic material for production of various nano/micro-electromechanical systems. In addition to their applications, these materials impose lot of challenges in understanding their fundamental properties because of random structure and involvement of three or more different kinds of atoms. The amorphous FeZr system which is reported to exhibit superconductivity, ferromagnetism, spin glass and antiferromagnetism is an ideal system to understand. The controversial magnetic properties of this system are still a subject of debate. In the present paper we revisited FeZr system and studied their detailed magnetic properties in the temperature range of 5 to 330K. FexZr100-x (x = 63, 68, 76, 86, 92 and 93 atomic co-sputtering technique. Curie temperature (Tc) is shown to increase almost linearly with an increase in Fe concentration, but it decreases rapidly for the Fe rich films. The films with x up to   86 at.strange shape of the hysteresis loop and are shown to exhibit perpendicular magnetic anisotropy with a stripe domain structure. Hysteresis loop shapes are correlated with the magnetic structure of the films, and provide an easy understanding of magnetic properties. It has been shown that the Fe rich films are composed of antiferromagnetic Fe clusters which are embedded in the ferromagnetic amorphous FeZr.

We have studied the paramagnetic-to-antiferromagnetic phase transition in spherical b-MnS nanoparticles of well defined diameters in the range of 3 to 11 nm. The MnS nanoparticles were obtained by intra-pore synthesis inside mesoporous silica matrices. Electron spin resonance and magnetization measurements reveal that no antiferromagnetic order is established in MnS spheres of 3 nm down to 2 K and that the antiferromagnetic order is gradually recovered on increasing the particle diameter to 11 nm. Photoluminescence excitation spectroscopy proves that in all MnS nanostructures the nearest-neighbor coupling between the Mn-ions remains the same as in bulk suggesting that the suppression of the phase transition arises due to geometric restrictions alone.

In the presence of spatial variation of the magnetization direction, electric current noise causes a fluctuating spin-transfer torque that increases the fluctuations of the ferromagnetic order parameter. By the fluctuation-dissipation theorem, the fluctuations at thermal equilibrium are related to the viscous magnetization damping, which in non-uniform ferromagnets acquires a nonlocal tensor structure. At the hand of spin spirals, we demonstrate that the current-induced noise and damping increase with the gradient of the magnetization texture and becomes significant for narrow domain walls. .

We present the first results of numerical simulations of a 2+1 dimensional fermion field theory based on a recent proposal for a model of graphene, consisting of Nf four-component Dirac fermions moving in the plane and interacting via an instantaneous Coulomb interaction. In the strong-coupling limit we identify a critical number of flavors Nfc=4.8(2) separating an insulating from a conducting phase. This transition corresponds to the location of a quantum critical point, and we use a fit to the equation of state for the chiral order parameter to estimate the critical exponents. Next we simulate Nf=2 corresponding to real graphene, and approximately locate a transition from strong to weak coupling behaviour. Strong correlations are evident in the weak-coupling regime.

Grazing incidence ion bombardment results in the formation of nanoripples that induce an anisotropic optical reflection The evolution of the reflectance anisotropy has been monitored in-situ with reflectance anisotropy spectroscopy. The Rayleigh-Rice theory (RRT) has been used to analyze the optical spectra quantitatively and provides the evolution of the average ripple period and root mean squared surface roughness. After an incipient phase, both the increase of the periodicity and the roughness vary roughly with the square root of the sputter time. Additional high-resolution low-energy electron diffraction (HR-LEED) measurements have been performed to characterize details of the average structure created by ion bombardment.

We present the results of calculations of surface relaxations, energetics, and bonding properties for CaTiO3 (001) and (011) surfaces using a hybrid B3PW description of exchange and correlation. We consider both CaO and TiO2 terminations of the non-polar (001) surface, and Ca, TiO and O terminations of the polar (011) surface. On the (001) surfaces, we find that all upper-layer atoms relax inwards on the CaO-terminated surface, while outward relaxations of all atoms in the second layer are found for both terminations. For the TiO2-terminated (001) surface, the largest relaxations are on the second-layer atoms. The surface rumpling is much larger for the CaO-terminated than for the TiO2-terminated (001) surface, but their surface energies are quite similar at 0.94 eV/cell and 1.13 eV/cell respectively. In contrast, different terminations of the (011) CaTiO3 surface lead to very different surface energies of 1.86 eV/cell, 1.91 eV/cell, and 3.13 eV/cell for the O-terminated, Ca-terminated, and TiO-terminated (011) surface respectively. Our results for surface energies contrast sharply with those of Zhang et al. [Phys. Rev. B 76, 115426 (2007)], where the authors found a rather different pattern of surface energies. We predict a considerable increase of the Ti-O chemical bond covalency near the (011) surface as compared both to the bulk and to the (001) surface.

A monolayer of C60 molecules on a flat surface provides an unusual substrate for the adsorption of simple gases. Both the lattice constant and the corrugation are larger than is typical of most traditional surfaces. These differences give rise to new phenomena, such as unusual commensurate phases. This paper discusses the ordering transition of various gases, corresponding to the filling of the honeycomb array of three-fold coordinated, hollow sites located between C60 molecules. That transition is investigated with Monte Carlo simulations and analytical (lattice-gas) models. The value of the resulting transition temperature (Tc) depends on the form assumed for the long-range interaction and the role of many-body effects, i.e. screening, due to the gas molecules' close proximity to the C60 layer. The effect of three-body interactions is found to be large in all cases considered.

A quantum hydrodynamic (fluid) model, derived from the Wigner-Poisson equations, is used to investigate the ultrafast electron dynamics in thin metal films. The hydrodynamic equations, which include exchange and correlation effects, can be combined into a single nonlinear Schrödinger-type equation. The fluid model is first benchmarked against a density-functional calculation for the ground state, with good agreement between the two approaches. The ultrafast nonlinear electron dynamics is then investigated and compared to recent semiclassical results obtained with a Vlasov-Poisson approach.

We have measured small-amplitude rf penetration depth l(H,T) in pure and C, Li and (Li+C) doped single crystals of MgB2. The effect of doping on the critical temperature Tc and on the upper critical field Hc2 was found to be in good agreement with previous results. We report the presence of clear signatures of irreversibility in l(H,T), associated with the peak effect. Carbon doping enhances the observed feature and shifts its position on the H-T phase diagram to higher temperatures. In contrast, Li substitution suppresses the peak effect moving it to lower temperatures. Analysis of both zero field (ZFC) and field cooled (FC) measurements suggests that the hysteresis associated with the peak effect is due to macroscopic supercurrents induced due to vortex pinning upon the magnetic field ramp.

We calculate the upper critical field, Hc2(T), due to the orbital pair breaking in disordered superconductors without inversion symmetry. Differences from the usual centrosymmetric case are highlighted. The linearized gap equations in magnetic field, with the singlet and triplet pairing channels mixed by impurity scattering, are solved exactly for a cubic crystal.

By exciting a BCS superconductor with ultra-short pulses in the frequency range of the superconducting gap a nonadiabatic regime can be reached. In this regime the modulus of the order parameter oscillates in time. Although its average value can be identified with the gap in the absorption spectrum, the oscillation itself remains invisible in pump-probe spectra. In this paper we demonstrate that by employing a coherent control-type scheme of excitation by two phase-locked pump pulses this oscillation can be unveiled in the probe spectrum. We find that the reaction of the superconductor to a second pump pulse depends strongly on its instantaneous state at the time of the impact of the second pulse. Based on numerical calculations performed on the mean-field level it is shown that by varying the delay time between the two pump pulses the transient oscillation of the order parameter can be transformed into an oscillation of its long-time value, which shows up in the absorption spectrum of a subsequent weak probe pulse as an oscillation of the gap when plotted as a function of the delay time.

The electronic response of doped manganites at the transition from the paramagnetic insulating to the ferromagnetic metallic state in La_1−xCa_xMnO₃ for x=0.3 and 0.2 was investigated by dc conductivity, ellipsometry, and vacuum ultraviolet reflectance for energies between 0 and 22 eV. A stabili...

Electrical injection from the Heusler alloy Co₂FeSi into (Al,Ga)As is investigated for different growth temperatures T_G of the injector layer. Depending on T_G , the spin polarization of injected electrons in the semiconductor is determined by two competing mechanisms: actual spin injection at t...

The ground-state properties of a spin S=1/2 tetrameric Heisenberg antiferromagnetic chain with alternating couplings AF₁-AF₂-AF₁-F (AF and F denote antiferromagnetic and ferromagnetic couplings, respectively) are studied by means of the density-matrix renormalization-group method. Two plateaus o...

Temperature (5–250 K) and magnetic-field (0–70 kOe) variations of the low-energy (1–10 meV) electrodynamics of spin excitations have been investigated for a complete set of light-polarization configurations for a ferroelectric magnet DyMnO₃ by using terahertz time-domain spectroscopy. We ide...

The structural and the electronic properties of a hypothetical three-dimensional (3D) all- sp² carbon phase, called K₄ , are investigated using first-principles calculations. The cohesive energy per atom for this structure is found to be 1.3 eV lower than for graphite and diamond, but also more th...

We report density functional calculations of the electronic structure and Fermi surface of the BaFe₂As₂ and LiFeAs phases including doping via the virtual-crystal approximation. The results show that contrary to a rigid-band picture, the density of states at the Fermi energy is only weakly doping ...

We propose to generate terahertz surface plasmon polaritons (SPPs) on a metal wire by launching electron bunches onto a tapered end of the wire. To show the potential of this method, we solve Maxwell’s equations for the appropriate boundary conditions. The metal wire tip is modeled by a perfectly ...

Single crystals of BaRh₂As₂ have been synthesized from a Pb flux. We present the room-temperature crystal structure, single-crystal x-ray diffraction measurements as a function of temperature T , anisotropic magnetic susceptibility χ versus T , electrical resistivity in the ab plane ρ ve...

We report systematic low-temperature measurements of the dc magnetization, ac susceptibility, and heat capacity of dysprosium pyrogermanate (Dy₂Ge₂O₇) single crystal and powder samples. Our results confirm that Dy₂Ge₂O₇ is an anisotropic antiferromagnet. The isothermal field dependent magnetizat...

Neutron diffraction measurements of a high-quality single crystal of CaFe₂As₂ are reported. A sharp transition was observed between the high-temperature tetragonal and low-temperature orthorhombic structures at T_S=172.5 K (on cooling) and 173.5 K (on warming). Concomitant with the structural t...