Determining the Anisotropic Exchange Coupling of CrO_2 via First-Principles Density Functional Theory Calculations

We report a study of the anisotropic exchange interactions in bulk CrO_2 calculated from first principles within density functional theory. We determine the exchange coupling energies, using both the experimental lattice parameters and those obtained within DFT, within a modified Heisenberg model Hamiltonian in two ways. We employ a supercell method in which certain spins within a cell are rotated and the energy dependence is calculated and a spin-spiral method that modifies the periodic boundary conditions of the problem to allow for an overall rotation of the spins between unit cells. Using the results from each of these methods, we calculate the spin-wave stiffness constant D from the exchange energies using the magnon dispersion relation. We employ a Monte Carlo method to determine the DFT-predicted Curie temperature from these calculated energies and compare with accepted values. Finally, we offer an evaluation of the accuracy of the DFT-based methods and suggest implications of the competing ferro- and antiferromagnetic interactions.Comment: 10 pages, 13 figure

Crystal growth and ambient and high pressure study of the reentrant superconductor Tm_2Fe_3Si_5

We report single crystal growth of the reentrant superconductor Tm_2Fe_3Si_5, and measurements of the anisotropic static magnetic susceptibility \chi(T) and isothermal magnetization M(H), ac susceptibility \chi_ac(T), electrical resistivity \rho(T) and heat capacity C(T) at ambient pressure and \chi_ac(T) at high pressure. The magnetic susceptibility along the c-axis \chi_c(T) shows a small maximum around 250 K and does not follow the Curie-Weiss behavior while the magnetic susceptibility along the a-axis \chi_a(T) follows a Curie-Weiss behavior between 130 K and 300 K with a Weiss temperature \theta and an effective magnetic moment \mu_eff which depend on the temperature range of the fit. The easy axis of magnetization is perpendicular to the c-axis and \chi_a/\chi_c = 3.2 at 1.8 K. The ambient pressure \chi_ac(T) and C(T) measurements confirm bulk antiferromagnetic ordering at T_N = 1.1 K. The sharp drop in \chi_ac below T_N is suggestive of the existence of a spin-gap. We observe superconductivity only under applied pressures P\geq 2 kbar. The temperature-pressure phase diagram showing the non-monotonic dependence of the superconducting transition temperature T_c on pressure P is presented.Comment: 7 pages, 8 figure

Probing quasiparticle excitations in a hybrid single electron transistor

We investigate the behavior of quasiparticles in a hybrid electron turnstile with the aim of improving its performance as a metrological current source. The device is used to directly probe the density of quasiparticles and monitor their relaxation into normal metal traps. We compare different trap geometries and reach quasiparticle densities below 3um^-3 for pumping frequencies of 20 MHz. Our data show that quasiparticles are excited both by the device operation itself and by the electromagnetic environment of the sample. Our observations can be modelled on a quantitative level with a sequential tunneling model and a simple diffusion equation

A Fresh Look at Entropy and the Second Law of Thermodynamics

This paper is a non-technical, informal presentation of our theory of the second law of thermodynamics as a law that is independent of statistical mechanics and that is derivable solely from certain simple assumptions about adiabatic processes for macroscopic systems. It is not necessary to assume a-priori concepts such as "heat", "hot and cold", "temperature". These are derivable from entropy, whose existence we derive from the basic assumptions. See cond-mat/9708200 and math-ph/9805005.Comment: LaTex file. To appear in the April 2000 issue of PHYSICS TODA

Propagation of optical excitations by dipolar interactions in metal nanoparticle chains

Dispersion relations for dipolar modes propagating along a chain of metal nanoparticles are calculated by solving the full Maxwell equations, including radiation damping. The nanoparticles are treated as point dipoles, which means the results are valid only for a/d <= 1/3, where a is the particle radius and d the spacing. The discrete modes for a finite chain are first calculated, then these are mapped onto the dispersion relations appropriate for the infinite chain. Computed results are given for a chain of 50-nm diameter Ag spheres spaced by 75 nm. We find large deviations from previous quasistatic results: Transverse modes interact strongly with the light line. Longitudinal modes develop a bandwidth more than twice as large, resulting in a group velocity that is more than doubled. All modes for which k_mode <= w/c show strongly enhanced decay due to radiation damping.Comment: 26 pages, 7 figures, 2 tables. to appear in Phys. Rev.

Fidelity recovery in chaotic systems and the Debye-Waller factor

Using supersymmetry calculations and random matrix simulations, we studied the decay of the average of the fidelity amplitude f_epsilon(tau)=<psi(0)| exp(2 pi i H_epsilon tau) exp(-2 pi i H_0 tau) |psi(0)>, where H_epsilon differs from H_0 by a slight perturbation characterized by the parameter epsilon. For strong perturbations a recovery of f_epsilon(tau) at the Heisenberg time tau=1 is found. It is most pronounced for the Gaussian symplectic ensemble, and least for the Gaussian orthogonal one. Using Dyson's Brownian motion model for an eigenvalue crystal, the recovery is interpreted in terms of a spectral analogue of the Debye-Waller factor known from solid state physics, describing the decrease of X-ray and neutron diffraction peaks with temperature due to lattice vibrations.Comment: revised version (major changes), 4 pages, 4 figure

Diluted Graphene Antiferromagnet

We study RKKY interactions between local magnetic moments for both doped and undoped graphene. We find in both cases that the interactions are primarily ferromagnetic for moments on the same sublattice, and antiferromagnetic for moments on opposite sublattices. This suggests that at sufficiently low temperatures dilute magnetic moments embedded in graphene can order into a state analogous to that of a dilute antiferromagnet. We find that in the undoped case one expects no net magnetic moment, and demonstrate numerically that this effect generalizes to ribbons where the magnetic response is strongest at the edge, suggesting the possibility of an unusual spin-transfer device. For doped graphene we find that moments at definite lattice sites interact over longer distances than those placed in interstitial sites of the lattice ($1/R^2$ vs. $1/R^3$) because the former support a Kohn anomaly that is suppressed in the latter due to the absence of backscattering.Comment: 5 pages, two figures include

First principles study of intrinsic point defects in hexagonal barium titanate

Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both mono- and di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of vacancies in the system are presented for numerous temperatures. Comparisons are drawn with the cubic polymorph as well as with previous potential-based simulations and experimental results

Three-terminal thermoelectric transport through a molecule placed on an Aharonov-Bohm ring

The thermoelectric transport through a ring threaded by an Aharonov-Bohm flux, with a molecular bridge on one of its arms, is analyzed. The transport electrons also interact with the vibrational excitations of that molecule. This nano-system is connected to three terminals: two are electronic reservoirs, which supply the transport electrons, and the third is the phonon bath which thermalizes the molecular vibrations. Expressions for the transport coefficients, relating all charge and heat currents to the temperature and chemical potential differences between the terminals, are derived to second order in the electron-vibration coupling. At linear response, all these coefficients obey the full Onsager-Casimir relations. When the phonon bath is held at a temperature different from those of the electronic reservoirs, a heat current exchanged between the molecular vibrations and the transport electrons can be converted into electric and/or heat electronic currents. The related transport coefficients, which exist only due to the electron-vibration coupling, change sign under the interchange between the electronic terminals and the sign change of the magnetic flux. It is also demonstrated that the Aharonov-Bohm flux can enhance this type of conversion.Comment: Added clearer kists of the new result

The Electron-Phonon Interaction of Low-Dimensional and Multi-Dimensional Materials from He Atom Scattering

Atom scattering is becoming recognized as a sensitive probe of the electron-phonon interaction parameter $\lambda$ at metal and metal-overlayer surfaces. Here, the theory is developed linking $\lambda$ to the thermal attenuation of atom scattering spectra (in particular, the Debye-Waller factor), to conducting materials of different dimensions, from quasi-one dimensional systems such as W(110):H(1$\times$1) and Bi(114), to quasi-two dimensional layered chalcogenides and high-dimensional surfaces such as quasicrystalline 2ML-Ba(0001)/Cu(001) and d-AlNiCo(00001). Values of $\lambda$ obtained using He atoms compare favorably with known values for the bulk materials. The corresponding analysis indicates in addition the number of layers contributing to the electron-phonon interaction that is measured in an atom surface collision.Comment: 23 pages, 5 figures, 1 tabl