Search results for "CONDUCTIVITY"

Vortex-liquid entanglement inBi2Sr2CaCu2O8+δfilms in the presence of quenched disorder

1998

We have investigated the thermally activated behavior of the in-plane electrical resistivity of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ films for magnetic fields $Bl~{10}^{4}\mathrm{G}$ applied parallel to the $c$ axis. The activation energy in the vortex-liquid state changes suddenly at a crossover field ${B}_{\mathrm{cr}}.$ The anisotropy reduction generated by oxygen annealing leads to the increase of the crossover field. For $Bl{B}_{\mathrm{cr}},$ the activation energy $U$ is weakly magnetic-field dependent. For $Bg{B}_{\mathrm{cr}},$ $U(B,T)\ensuremath{\sim}(1\ensuremath{-}{T/T}_{c0}{)/B}^{1/2},$ which corresponds to an entan…

Optical conductivity inYBa2Cu3O7−δthin films

1999

The real and imaginary parts of the optical conductivity ${\ensuremath{\sigma}}_{1}+i{\ensuremath{\sigma}}_{2}$ of partly untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ thin films were measured by submillimeter spectroscopy for frequencies from 100 GHz to 1 THz. The frequency dependence of the conductivity below ${T}_{C}$ can be described by a narrow Drude-like peak with strongly temperature-dependent relaxation rates. ${\ensuremath{\sigma}}_{1}(\ensuremath{\nu})$ does not extrapolate to the universal d-wave value at 0 K. Impurity scattering significantly different to the unitary limit has to be assumed to achieve agreement with recent theor…

2018

We apply homogenization theory to calculate the effective electric conductivity and Hall coefficient tensor of passive three-dimensionally periodic metamaterials subject to a weak external static homogeneous magnetic field. We not only allow for variations of the conductivity and the Hall coefficient of the constituent material(s) within the metamaterial unit cells, but also for spatial variations of the magnetic permeability. We present four results. First, our findings are consistent with previous numerical calculations for finite-size structures as well as with recent experiments. This provides a sound theoretical justification for describing such metamaterials in terms of effective mate…

2008

ZnO doped with a few per cent (<10%) of magnetic ions such as Co exhibit room temperature (RT) ferromagnetism, transforming it into a very promising candidate for future spin electronic applications. We present x-ray magnetic circular dichroism (XMCD) spectroscopy, which has been used in total electron yield, total fluorescence yield, and reflection mode to investigate the origin of ferromagnetism in such diluted magnetic semiconductor materials in a surface, bulk and interface sensitive way, respectively. We investigated three different types of samples: ZnO doped with 5% Co, artificially layered films, and layered films with additional co-doping of 10% Li. These films are prepared by puls…

2015

The dynamics of emergent magnetic quasiparticles, such as vortices, domain walls, and bubbles are studied by scanning transmission x-ray microscopy (STXM), combining magnetic (XMCD) contrast with about 25 nm lateral resolution as well as 70 ps time resolution. Essential progress in the understanding of magnetic vortex dynamics is achieved by vortex core reversal observed by sub-GHz excitation of the vortex gyromode, either by ac magnetic fields or spin transfer torque. The basic switching scheme for this vortex core reversal is the generation of a vortex-antivortex pair. Much faster vortex core reversal is obtained by exciting azimuthal spin wave modes with (multi-GHz) rotating magnetic fie…

Frequency-dependent conductivity of UPd2Al3 films

1998

The transmission of UPd2Al3 films was studied (4 K < T < 300 K) in the frequency range from 4 to 32 cm−1 by using a coherent source interferometer which allows for measuring both, amplitude and phase. In addition we report on radio frequency and optical measurements. Below 20 K the conductivity and dielectric constant show strong deviations from the behavior of a normal metal which cannot simply be explained by a single renormalized Drude model with an enhanced mass and reduced scattering rate. Instead, we find evidence for the opening of a pseudogap with a gap energy of 6 cm−1 and an extremely narrow ω = 0 mode which is responsible for the large DC conductivity.

Jahn-Teller effect in the phonon properties of defective SrTiO3from first principles

2012

the Jahn–Teller effect occurs, thus reducing the cubic symmetry of a perfect crystal and leading to the appearance of both Raman- and infrared-active vibrational modes. The calculated phonon densities of states and group-theoretical analysis of defect-induced phonon frequencies were used for the interpretation of the relevant experimental data, once defect-induced local modes are identified. The temperature dependence of the Vo formation energy based on the calculated Gibbs free energy was also compared with experiments, and the phonon contribution therein estimated.

On time-resolved approach for phonon assisted interband transitions

2015

Photoexcited dynamics of electrons and holes in two-band dielectric, with special emphasis on back reaction of phonons are developed by combining the quantum electrodynamics and Baker-Campbell-Hausdorff (BCH) canonical transformation. These methods create an explicit time-domain representation of photoinduced processes and contribute in unifying phonon-assisted description of distribution functions of electron and hole quasiparticles for the description of observable effects of photoinduced processes in dielectrics.

Electronic structure studies ofBaFe2As2by angle-resolved photoemission spectroscopy

2009

We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of ${\text{BaFe}}_{2}{\text{As}}_{2}$, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the $\ensuremath{\Gamma}$ point and an electron pocket at each of the $X$ points. The topology of the pockets has been conclu…

Refrigeration of an Array of Cylindrical Nanosystems by Flowing Superfluid Helium

2016

We consider the refrigeration of an array of heat-dissipating cylindrical nanosystems as a simplified model of computer refrigeration. We explore the use of He II as cooling fluid, taking into account forced convection and heat conduction. The main conceptual and practical difficulties arise in the calculation of the effective thermal conductivity. Since He II does not follow Fourier’s law, the effective geometry-dependent conductivity must be extracted from a more general equation for heat transfer. Furthermore, we impose the restrictions that the maximum temperature along the array should be less than (Formula presented.) transition temperature and that quantum turbulence is avoided, in o…