arXiv subjects

Z. Gedik

Publications and source records attributed to Z. Gedik.

27 records · Page 2Linked to original sources

arXiv2008

Spin Bath Decoherence Mediated by Phonons

We introduce an exactly solvable model to study decoherence of a central spin interacting with a spin bath where the coupling is mediated by phonons which we assume to be in a coherent state or thermal distribution. For the coherent state case, we find that the decoherence factor decays in a Gaussian fashion and it becomes independent of the phonon frequencies at short times. If the phonon energies are much larger than spin-phonon coupling or bath spins are fully polarized, decoherence time becomes independent of the initial phonon state. For the thermal state case, phonons play more important role in decoherence with increasing temperature. We also discuss possible effects of the temperature on spin bath contribution to decoherence.

quant-ph↗

arXiv2002

Metal nanoring and tube formation on carbon nanotubes

The structural and electronic properties of aluminum covered single wall carbon nanotubes (SWNT) are studied from first-principles for a large number of coverage. Aluminum-aluminum interaction that is stronger than aluminum-tube interaction, prevents uniform metal coverage, and hence gives rise to the clustering. However, a stable aluminum ring and aluminum nanotube with well defined patterns can also form around the semiconducting SWNT and lead to metallization. The persistent current in the Al nanoring is discussed to show that a high magnetic field can be induced at the center of SWNT.

cond-mat.mtrl-sci↗

arXiv2001

Temperature and magnetic field dependence of superconductivity in nanoscopic metallic grains

We study pairing correlations in ultrasmall superconductor in the nanoscopic limit by means of a toy model where electrons are confined in a single, multiply degenerate energy level. We solve the model exactly to investigate the temperature and magnetic field dependence of number parity effect (dependence of ground state energy on evenness or oddness of the number of electrons). We find a different parity effect parameter to critical temperature ratio (=~ 4 rather than 3.5) which turns out to be consistent with exact solution of the BCS gap equation for our model. This suggest the equivalence between the parity effect parameter and the superconducting gap. We also find that magnetic field is suppressed as temperature increases.

cond-mat.supr-con↗

arXiv2000

Parity Effect in Mesoscopic and Nanoscopic Superconducting Particles

Superconductivity in small metallic specimens is studied with regard to the size dependence of the "parity gap" (Delta_P), a parameter distinguishing between the energy of even and odd number of electrons in the granule. Delta_P is shown to be an increasing function of level spacing delta. The energy gap of superconductor Delta, on the other hand, decreases with increasing delta and vanishes at delta=delta_c which is of the order of Delta. However, nonzero value of Delta_P persists above delta_c in a "gapless" superconducting-insulating state. Level degeneracy in small specimens having perfect geometry changes the size dependence of the parity gap, the Josephson effect, and flux quantization. Parity gap is evaluated using an interpolation procedure between the continuum limit (delta< >Delta), for which an exact solution to the pairing problem is provided with the numeric diagonalization of system Hamiltonian in a small metallic cluster.

cond-mat.str-el↗

arXiv1999

Superconductivity in Mesoscopic Metal Particles: The Role of Degeneracy

Recently, it has been possible to construct single-electron transistors to study electronic properties, including superconductivity, in metallic grains of nanometer size. Among several theoretical results are suppression of superconductivity with decreasing grain size and parity effect, i.e. dependence on the parity of the number of electrons on the grain. We study how these results are affected by degeneracy of energy levels. In addition to the time-reversal symmetry, for certain energy spectra and more generally for lattice symmetries, energy levels are strongly degenerate near the Fermi energy. For a parabolic dispersion, degeneracy d is of the order of k_FL while the typical distance between the levels is of the order of epsilon_F/(k_FL)^2 where k_F and epsilon_F are the Fermi wave vector and energy, respectively, and L is the particle size. First, using an exact solution method for BCS Hamiltonian with finite number of energy levels, for the well studied non-degenerate case we find a new feature. In that case, parity effect exhibits a minimum instead of a monotonic behavior. For d-fold degenerate states we find that the ratio of two successive parity effect parameters Delta_p is nearly 1+1/d. Our numerical solutions for the exact ground state energy of negative U Hubbard model on a cubic cluster also give very similar results. Hence we conclude that parity effect is a general property of small Fermi systems with attractive interaction, and it is closely related to degeneracy of energy levels.

cond-mat.supr-con↗

arXiv1998

Disorder and Localization in the Lowest Landau Level

We study the localization property of a two-dimensional noninteracting electron gas in the presence of randomly distributed short-range scatterers. We evaluate the participation number of the eigenstates obtained by exact diagonalization technique. At low impurity concentrations we obtain self-averaged values showing that all states, except those exactly at the Landau level, are localized with finite localization length. We conclude that there is no universal localization exponent and at least at low impurity concentrations localization length does not diverge.

cond-mat.mes-hall↗

arXiv1998

Suppression of superconductivity in high-$T_c$ cuprates due to nonmagnetic impurities: Implications for the order parameter symmetry

We studied the effects of nonmagnetic impurities on high-temperature superconductors by solving the Bogoliubov-de Gennes equations on a two-dimensional lattice via exact diagonalization technique in a fully self-consistent way. We found that s-wave order parameter is almost unaffected by impurities at low concentrations while $d_{x^2-y^2}$-wave order parameter exhibits a strong linear decrease with impurity concentration. We evaluated the critical impurity concentration $n_i^c$ at which superconductivity ceases to be 0.1 which is in good agreement with experimental values. We also investigated how the orthorhombic nature of the crystal structure affects the suppression of superconductivity and found that anisotropy induces an additional s-wave component. Our results support $d_{x^2-y^2}$-wave symmetry for tetragonal and $s+d_{x^2-y^2}$-wave symmetry for orthorhombic structure.

cond-mat.supr-con↗

arXiv1998

Bose-Einstein condensation in a one-dimensional interacting system due to power-law trapping potentials

We examine the possibility of Bose-Einstein condensation in one-dimensional interacting Bose gas subjected to confining potentials of the form $V_{\rm ext}(x)=V_0(|x|/a)^γ$, in which $γ< 2$, by solving the Gross-Pitaevskii equation within the semi-classical two-fluid model. The condensate fraction, chemical potential, ground state energy, and specific heat of the system are calculated for various values of interaction strengths. Our results show that a significant fraction of the particles is in the lowest energy state for finite number of particles at low temperature indicating a phase transition for weakly interacting systems.

cond-mat.stat-mech↗

arXiv1997

Energy spectrum for two-dimensional potentials in very high magnetic fields

A method, analogous to supersymmetry transformation in quantum mechanics, is developed for a particle in the lowest Landau level moving in an arbitrary potential. The method is applied to two-dimensional potentials formed by Dirac delta scattering centers. In the periodic case, the problem is solved exactly for rational values of the magnetic flux (in units of flux quantum) per unit cell. The spectrum is found to be self-similar, resembling the Hofstadter butterfly.

cond-mat.mes-hall↗