79 research outputs found
2D superconductivity with strong spin-orbit interaction
We consider superconductivity confined at a two-dimensional interface with a
strong surface spin-orbit (Rashba) interaction. Some peculiar properties of
this system are investigated. In particular, we show that an in-plane Zeeman
field can induce a supercurrent flow.Comment: latex, 1 figure in ep
Enhancing Tc in field-doped Fullerenes by applying uniaxial stress
Capitalizing on the two-dimensional nature of superconductivity in
field-effect doped C60, we show that it should be possible to increase the
transition temperature Tc by applying uniaxial stress perpendicular to the gate
electrode. This method not only holds the promise of substantially enhancing Tc
(by about 30 K per GPa), but also provides a sensitive check of the current
understanding of superconductivity in the doped Fullerenes.Comment: 3 pages RevTe
Crystal Structures and Electronic Properties of Haloform-Intercalated C60
Using density functional methods we calculated structural and electronic
properties of bulk chloroform and bromoform intercalated C60, C60 2CHX3
(X=Cl,Br). Both compounds are narrow band insulator materials with a gap
between valence and conduction bands larger than 1 eV. The calculated widths of
the valence and conduction bands are 0.4-0.6 eV and 0.3-0.4 eV, respectively.
The orbitals of the haloform molecules overlap with the orbitals of the
fullerene molecules and the p-type orbitals of halogen atoms significantly
contribute to the valence and conduction bands of C60 2CHX3. Charging with
electrons and holes turns the systems to metals. Contrary to expectation, 10 to
20 % of the charge is on the haloform molecules and is thus not completely
localized on the fullerene molecules. Calculations on different crystal
structures of C60 2CHCl3 and C60 2CHBr3 revealed that the density of states at
the Fermi energy are sensitive to the orientation of the haloform and C60
molecules. At a charging of three holes, which corresponds to the
superconducting phase of pure C60 and C60 2CHX3, the calculated density of
states (DOS) at the Fermi energy increases in the sequence DOS(C60) < DOS(C60
2CHCl3) < DOS(C60 2CHBr3).Comment: 11 pages, 7 figures, 4 table
Two-Dimensional Wigner Crystal in Anisotropic Semiconductor
We investigate the effect of mass anisotropy on the Wigner crystallization
transition in a two-dimensional (2D) electron gas. The static and dynamical
properties of a 2D Wigner crystal have been calculated for arbitrary 2D Bravais
lattices in the presence of anisotropic mass, as may be obtainable in Si
MOSFETs with (110) surface. By studying the stability of all possible lattices,
we find significant change in the crystal structure and melting density of the
electron lattice with the lowest ground state energy.Comment: 4 pages, revtex, 4 figure
Sensitivity of the Mott Transition to Non-cubic Splitting of the Orbital Degeneracy: Application to NH3 K3C60
Within dynamical mean-field theory, we study the metal-insulator transition
of a twofold orbitally degenerate Hubbard model as a function of a splitting
\Delta of the degeneracy. The phase diagram in the U-\Delta plane exhibits
two-band and one-band metals, as well as the Mott insulator. The correlated
two-band metal is easily driven to the insulator state by a strikingly weak
splitting \Delta << W of the order of the Kondo-peak width zW, where z << 1 is
the metal quasiparticle weight. The possible relevance of this result to the
insulator-metal transition in the orthorhombic expanded fulleride NH3 K3C60 is
discussed.Comment: revtex, 15 pages including 6 ps figures. Submitted to Phys. Rev.
Small Fermi energy and phonon anharmonicity in MgB_2 and related compounds
The remarkable anharmonicity of the E_{2g} phonon in MgB_2 has been suggested
in literature to play a primary role in its superconducting pairing. We
investigate, by means of LDA calculations, the microscopic origin of such an
anharmonicity in MgB_2, AlB_2, and in hole-doped graphite. We find that the
anharmonic character of the E_{2g} phonon is essentially driven by the small
Fermi energy of the sigma holes. We present a simple analytic model which
allows us to understand in microscopic terms the role of the small Fermi energy
and of the electronic structure. The relation between anharmonicity and
nonadiabaticity is pointed out and discussed in relation to various materials.Comment: 5 pages, 2 figures replaced with final version, accepted on Physical
Review
Mesoscopic fluctuations of the Density of States and Conductivity in the middle of the band of Disordered Lattices
The mesoscopic fluctuations of the Density of electronic States (DoS) and of
the conductivity of two- and three- dimensional lattices with randomly
distributed substitutional impurities are studied. Correlations of the levels
lying above (or below) the Fermi surface, in addition to the correlations of
the levels lying on opposite sides of the Fermi surface, take place at half
filling due to nesting. The Bragg reflections mediate to increase static
fluctuations of the conductivity in the middle of the band which change the
distribution function of the conductivity at half- filling.Comment: 5 pages, 3 figure
Anderson localization of polaron states
Using the vanishing of the typical polaron tunneling rate as an indicator of
the breakdown of itinerancy, we study the localization of polaron states in a
generic model for a disordered polaronic material. We find that extremely small
disorder causes an Anderson localization of small polaron states. However, the
ratio between the critical disorder strength needed to localize all states in
the polaron band and the renormalized bandwidth is not necessarily smaller than
for a bare electron.Comment: 4 pages, 3 figure
Criticality in coupled quantum spin-chains with competing ladder-like and two-dimensional couplings
Motivated by the geometry of spins in the material CaCuO, we study a
two-layer, spin-half Heisenberg model, with nearest-neighbor exchange couplings
J and \alpha*J along the two axes in the plane and a coupling J_\perp
perpendicular to the planes. We study these class of models using the
Stochastic Series Expansion (SSE) Quantum Monte Carlo simulations at finite
temperatures and series expansion methods at T=0. The critical value of the
interlayer coupling, J_\perp^c, separating the N{\'e}el ordered and disordered
ground states, is found to follow very closely a square root dependence on
. Both T=0 and finite-temperature properties of the model are
presented.Comment: 9 pages, 11 figs., 1 tabl
Two-electron elastic tunneling in low-dimensional conductors
This article was published in the journal, Physical Review B [© American Physical Society]. It is also available at: http://link.aps.org/abstract/PRB/v65/e155209.We solve the Lippmann-Schwinger equation describing one-dimensional elastic scattering of preformed pairs (e.g., bipolarons) off a short-range scattering center, and find the two-particle transmission through a thin potential barrier. While the pair transmission is smaller than the single-electron transmission in the strong-coupling limit, it is remarkably larger in the weak-coupling limit. We also calculate current-voltage characteristics of a molecule-barrier-molecule junction. They show unusual temperature and voltage behaviors which are experimentally verifiable at low temperatures in bulk and nanoscale molecular conductors
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