Search Results (60 total)

We report on a theoretical and experimental study of the growth, electronic structure, and thermal stability of ultrathin films of nonmagnetic iron nitride. The films are grown by evaporation of Fe, in a flux of atomic N produced by a plasma source, onto a Cu(001) surface kept at 300 K and characterized by x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and x-ray diffraction. The compound formed is identified as γ^″-FeN(001) growing epitaxially on the substrate. Its electronic structure and thermal stability has been studied and compared with first-principles calculations. This nitride film transforms to magnetic γ^′-Fe₄N phase by annealing above 650 K, opening new perspectives for applications of these films in functional magnetic nanostructures.

Persistent photoconductivity and optical quenching phenomena in boron-doped homoepitaxial diamond thin films grown on type Ib (100) substrates were investigated. Boron doping induced positive persistent photoconductivity (PPPC) and photocurrent gain upon the illumination of deep ultraviolet (DUV) light. A deep defect with a thermal energy of around 1.37 eV was deduced from the results of thermally stimulated current measurements. An optical quenching experiment revealed a threshold photon energy of around 2 eV. The difference between the thermal and optical energies of the deep defect suggests a large lattice relaxation. An absolute transient negative photocurrent (TNPC) was observed for zero or weak electric fields with DUV light illumination after optically quenching the PPPC. The absolute TNPC was due to transient electron filling of the charged nitrogen in the substrate. The combination of the nitrogen in the substrate and the boron-related defects in the epilayer was apparently responsible for the PPPC and TNPC effects. Therefore, the overall photoresponse properties of a thin epilayer deposited on a type Ib diamond substrate can be tailored by adjusting the concentration of nitrogen in the substrate and of boron in the epilayer.

In the space of thermodynamic equilibrium states we introduce a Legendre invariant metric which contains all the information about the thermodynamics of black holes. The curvature of this thermodynamic metric becomes singular at those points where, according to the analysis of the heat capacities, phase transitions occur. This result is valid for the Kerr-Newman black hole and all its special cases and, therefore, provides a unified description of black hole phase transitions in terms of curvature singularities.

Highly homogeneous, ultrathin films of copper nitride (Cu₃N) have been grown on Fe(001) at room temperature using a Cu evaporator and a radio-frequency plasma source to obtain atomic nitrogen in a UHV environment. Cu₃N is a semiconductor with the valence band edge at −0.65±0.05  eV below the Fermi Level. The formation of copper nitride can be detected spectroscopically by the shape of the Cu LVV-Auger electron transition, which changes sensibly in shape and position compared to metallic Cu. Cu₃N grows epitaxially with the substrate forming flat disklike mosaic blocks, (001) oriented. Both x-ray core level photoelectron spectroscopy and ultraviolet photoelectron spectroscopy photoemission experiments have been used to study the electronic structure. A first-principles calculation has been performed and compared with the measured spectra.

We discuss the fixed-point Hamiltonian and the spectrum of excitations of a quasi-bidimensional electronic system supporting simultaneously antiferromagnetic ordering and superconductivity. The coexistence of these two order parameters in a single phase is possible because the magnetic order is linked to the formation of a metallic spin density wave, and its order parameter is not associated to a spectral gap but to an energy shift of the paramagnetic bands. This peculiarity entails several distinct features in the phase diagram and the spectral properties of the model, which may have been observed in CeRhIn₅. Apart from the coexistence, we find an abrupt suppression of the spin density wave when the superconducting and magnetic ordering temperatures are equal. The divergence of the cyclotron mass extracted from de Haas–van Alphen experiments is also analyzed in the same framework.

The electronic structure of ultrathin films of γ^′-Fe₄N(100) deposited on Cu(100) has been characterized by a combination of photoelectron spectroscopies, scanning tunneling microscopy, and diffraction techniques. The comparison of the data with first-principles simulations sheds light on magnetic moments, type of bonding, and charge transfer. N atoms residing in the bulk or at the surface are found to be distinguishable. The γ^′-Fe₄N(100) surface is laterally heterogeneous and contains both areas reconstructed with a p4gm(2×2) symmetry and bulklike terminated. The densities of states of the reconstructed and unreconstructed areas of the surface are obtained and compared with the experiment. Comparison with c(2×2)N∕Fe(100) provides spectroscopic evidence that a subsurface excess of N drives the p4gm(2×2) reconstruction.

Temperature dependent high resolution photoemission spectra of quasi-one-dimensional Li_0.9Mo₆O₁₇ evince a strong renormalization of its Luttinger-liquid density-of-states anomalous exponent. We trace this new effect to interacting charge neutral critical modes that emerge naturally from the two-band nature of the material. Li_0.9Mo₆O₁₇ is shown thereby to be a paradigm material that is capable of revealing new Luttinger physics.

We explore stochastic models for the study of ion transport in biological cells. Analysis of these models explains and explores an interesting feature of ion transport observed by biophysicists. Namely, the average time it takes ions to cross certain ion channels is the same in either direction, even if there is an electric potential difference across the channels. It is shown for simple single ion models that the distribution of a path (i.e., the history of location versus time) of an ion crossing the channel in one direction has the same distribution as the time-reversed path of an ion crossing the channel in the reverse direction. Therefore, not only is the mean duration of these paths equal, but other measures, such as the variance of passage time or the mean time a path spends within a specified section of the channel, are also the same for both directions of traversal. The feature is also explored for channels with interacting ions. If a system of interacting ions is in reversible equilibrium (net flux is zero), then the equivalence of the left-to-right trans paths with the time-reversed right-to-left trans paths still holds. However, if the system is in equilibrium, but not reversible equilibrium, then such equivalence need not hold.

The Küppers-Lortz instability occurs in rotating Rayleigh-Bénard convection and is a paradigmatic example of spatiotemporal chaos. Since the steady state of convection rolls is unstable to disturbance rolls oriented with an angle of about 60° with respect to the given rolls in the prograde direction [G. Küppers and D. Lortz, J. Fluid Mech. 35, 609 (1969)], a spatiotemporally chaotic pattern is realized with patches of rolls continuously replaced by other patches in which the roll axis is switched by about 60°. Surprisingly and contrary to this established scenario, Bajaj [Phys. Rev. Lett. 81 (1998)] observed experimentally square patterns in a cylindrical layer in the range of parameters where Küppers-Lortz instability was expected. In this paper we present square patterns which we have obtained in a numerical study by taking into account realistic boundary conditions. The Navier-Stokes and heat transport equations have been solved in the Oberbeck-Boussinesq approximation. The numerical method is pseudospectral and second order accurate in time. The rotation velocity of the square pattern increases linearly with the control parameter ε=Ra∕Ra_c−1, as in the experiment of Bajaj Furthermore, it was observed that this velocity decreases when the aspect ratio of the cylinder increases. These results indicate that the square pattern appears when the flow is laterally confined. The range of ε for which this pattern is stable tends to vanish for more extended layers.

A microscopic model for the diluted spin-mixed compounds (R_xY_1-x)₂BaNiO₅ (R=magnetic rare earth) is studied using quantum Monte Carlo simulations. The ordering temperature is shown to be a universal function of the impurity concentration x and the intrinsic Ni-chain correlation length. An effective model for the critical modes is derived. The possibility of a quantum critical point driven by the rare-earth concentration and the existence of a quantum Griffiths phase in the high dilution limit is investigated. Several possible experimental approaches to verify the results are put forward.

The zero-temperature phase diagram of carbon nanotube ropes is studied using a computational framework that incorporates the renormalization of intratube interactions and the effect of intertube Coulomb screening. This allows us to undertake a systematic analysis as a function of the number of metallic nanotubes in the rope and the effective strength of the phonon-mediated interaction. We find that there is in general a weak-coupling regime of the interactions, corresponding to Luttinger-liquid behavior in thin ropes and to superconducting behavior in sufficiently thick ropes. Furthermore, we show the existence of exotic phases in the strong-coupling regime, characterized by the appearance of charge instabilities that depend on the helicity and the doping level of the nanotubes in the rope. Our approach allows for the simultaneous analysis of the scaling of the Cooper-pair tunneling amplitude between metallic nanotubes, making it possible to discern the crossover from purely one-dimensional physics to the setting of three-dimensional Cooper-pair coherence. We provide then good estimates of the superconducting transition temperature and discuss the connection of our results with recent experiments.

A Reply to the Comment by F. Heidrich-Meisner et al.

“In situ” conductivity measurements on Al-Pd-Mn quasicrystals have been combined with synchrotron x-ray-diffraction experiments for studying its behavior during phase transformations. In order to confidently reduce the possible contribution to the conductivity of defects and grain boundaries, single-crystal samples have been annealed in situ under UHV conditions. This combined experiment allows us to affirm unambiguously that the structural state of the i-Al-Pd-Mn single crystal influences the electrical conductivity.

The superconducting properties of carbon nanotube ropes are studied using a new computational framework that incorporates the renormalization of intratube interactions and the effect of intertube Coulomb screening. This method allows one to study both the limits of thin and thick ropes ranging from purely one-dimensional physics to the setting of 3D Cooper-pair coherence, providing good estimates of the critical temperature as a function of the rope physical parameters. We discuss the connection of our results with recent experiments.

We study the thermal transport properties of several quantum-spin chains and ladders. We find indications for a diverging thermal conductivity at finite temperatures for the models examined. The temperature at which the nondiverging prefactor κ^(th)(T) peaks is, in general, substantially lower than the temperature at which the corresponding specific heat c_V(T) is maximal. We show that this result of the microscopic approach leads to a substantial reduction for estimates of the magnetic mean-free path λ extracted by analyzing recent experiments, as compared to similar analyses by phenomenological theories.

We discuss zero-frequency transport properties of various spin-1/2 chains. We show that a careful analysis of quantum Monte Carlo data on the imaginary axis allows to distinguish between intrinsic ballistic and diffusive transport. We determine the Drude weight, current-relaxation lifetime, and the mean free path for integrable and nonintegrable quantum spin chains. We discuss, in addition, some phenomenological relations between various transport-coefficients and thermal response functions.

We propose a microscopic model that describes the magnetic behavior of the mixed-spin quantum systems R₂BaNiO₅ (R=magnetic rare earth). An evaluation of the properties of this model by quantum Monte Carlo simulations shows remarkable good agreement with the experimental data, and provides insight into the physics of mixed-spin quantum magnets.

A technique to determine accurately transport properties of integrable and nonintegrable quantum-spin chains at finite temperatures by quantum Monte Carlo is presented. The reduction of the Drude weight by interactions in the integrable gapless regime is evaluated. Evidence for the absence of Drude weight in the gapless regime of a nonintegrable system with longer-ranged interactions is presented. We estimate the effect of the nonintegrability on the transport properties and compare with recent experiments on one-dimensional quantum-spin chains.

With standard and resonant x-ray magnetic diffraction we investigate the magnetic properties of Pt atoms in a Co/Pt(111) system. It is shown, that, depending on the thickness of the Co films, the magnetic anisotropy as monitored by the Pt magnetism changes from perpendicular to parallel, mimicking the behavior of the Co overlayer. Adsorption of carbon monoxide on very thin films has been found to change the easy magnetization axis. Moreover, the critical film thickness for the flipping of the easy direction of magnetization depends markedly on the growth temperature even in the vicinities of 300 K, where bulk alloying does not occur. A brief annealing to ∼375 K of a parallel Co film suffices to reverse its easy direction to perpendicular. By analyzing the magnetic crystal truncation rods of the Pt surface, it is concluded that the change of anisotropy is due to site exchange between interfacial Co and Pt atoms. The site exchange which affects up to 4% of the interface atoms has been related to previous scanning tunneling microscopy findings on the same system. Comparison of magneto-optical and x-ray magnetic diffraction measurements on the same film show that the responses to the externally applied field are not identical. A possible explanation is suggested.

We derive the one-loop renormalization equations for the shift in the Fermi wave vectors for one-dimensional interacting models with four Fermi points (two left and two right movers) and two Fermi velocities v₁ and v₂. We find the shift to be proportional to (v₁-v₂)U², where U is the Hubbard U. Our results apply to the Hubbard ladder and to the t₁-t₂ Hubbard model. The Fermi sea with fewer particles tends to empty. The stability of a saddle point due to shifts of the Fermi energy and the shift of the Fermi wave vector at the Mott-Hubbard transition are discussed.

We determine the electronic structure of the one-dimensional spin- 1 / 2 Heisenberg compound γ-LiV₂O₅, which has two inequivalent vanadium ions, V(1) and V(2), via density-functional calculations. We find a relative V(1)-V(2) charge ordering of roughly 70:30. We discuss the influence of the charge ordering on the electronic structure and the magnetic behavior. We give estimates of the basic hopping matrix elements and compare with the most studied α^′-NaV₂O₅.

Recent experimental evidence suggests the existence of three distinct V-valence states (V⁴⁺, V^4.5+, and V⁵⁺) in the low-temperature phase of NaV₂O₅ in apparent discrepancy with the observed spin gap. We investigate a spin cluster model, consisting of weakly coupled, frustrated four-spin clusters aligned along the crystallographic b axis that was recently proposed to reconcile these experimental observations. We have studied the phase diagram and the magnon dispersion relation of this model using DMRG, exact diagonalization, and a cluster-operator theory. We find a spin gap for all parameter values and two distinct phases, a cluster phase and a Haldane phase. We evaluate the size of the gap and the magnon dispersion and find no parameter regime which would reproduce the experimental results. We conclude that this model is inappropriate for the low-temperature regime of NaV₂O₅.

The Au(111)22×sqrt[3] reconstructed surface was prepared and characterized in UHV conditions with x-ray diffraction. Using a recently developed UHV/high pressure chamber, that allows one to increase the pressure from the UHV regime up to several atmospheres, the interaction of CO near atmospheric pressure with the Au surface was investigated. It was found that the interaction of CO with the substrate is thermally activated, and that it causes a substantial destabilization of the Au reconstruction.