Impurity scattering in unconventional density waves

We have investigated the effect of nonmagnetic impurities on the quasi-one-dimensional unconventional density wave (UDW) ground state. The thermodynamics were found to be close to those of a d-wave superconductor in the Born limit. Four different optical conductivity curves were found depending on the direction of the applied electric field and on the wavevector dependence of the gap.Comment: 14 pages, 9 figure

Threshold electric field in unconventional density waves

As it is well known most of charge density wave (CDW) and spin density wave (SDW) exhibit the nonlinear transport with well defined threshold electric field E_T. Here we study theoretically the threshold electric field of unconventional density waves. We find that the threshold field increases monotonically with temperature without divergent behaviour at T_c, unlike the one in conventional CDW. The present result in the 3D weak pinning limit appears to describe rather well the threshold electric field observed recently in the low-temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4.Comment: 4 pages, 2 figure

Quasiparticle spectrum of the hybrid s+g-wave superconductors YNi_2B_2C and LuNi_2B_2C

Recent experiments on single crystals of YNi$_2$B$_2$C have revealed the presence of point nodes in the superconducting energy gap Delta(k} at k = (1,0,0), (0,1,0), (-1,0,0), and (0,-1,0). In this paper we investigate the effects of impurity scattering on the quasiparticle spectrum in the vortex state of s+g-wave superconductors, which is found to be strongly modified in the presence of disorder. In particular, a gap in the quasiparticle energy spectrum is found to open even for infinitesimal impurity scattering, giving rise to exponentially activated thermodynamic response functions, such as the specific heat, the spin susceptibility, the superfluid density, and the nuclear spin lattice relaxation. Predictions derived from this study can be verified by measurements of the angular dependent magnetospecific heat and the magnetothermal conductivity.Comment: 8 pages, RevTex, 4 figure

The magnetic field dependence of the threshold electric field in unconventional charge density waves

Many experiments suggest that the unidentified low temperature phase (LTP) of alpha-(BEDT-TTF)_2KHg(SCN)_4 is most likely unconventional charge density wave (UCDW). To further this identification we present our theoretical study of the threshold electric field of UCDW in a magnetic field. The magnetic field-temperature phase diagram is very similar to those in a d-wave superconductor. We find a rather strong field dependence of the threshold electric field, which should be readily accessible experimentally.Comment: 7 pages, 6 figure

Imperfect nesting and transport properties in unconventional density waves

We consider the effect of imperfect nesting in quasi-one dimensional unconventional density waves. The phase diagram is very close to those in a conventional DW. The linear and non-linear aspects of the electric conductivity are discussed. At T=0 the frequency dependent electric conductivity develops a small dip at low frequencies. The threshold electric field depends strongly on the imperfect nesting parameter, allowing us to describe the measured threshold electric field in the low temperature phase of the quasi-two dimensional organic conductor, alpha-(BEDT-TTF)_2KHg(SCN)_4 very well.Comment: 9 pages, 9 figure

Impurity effects in unconventional density waves in the unitary limit

We investigate the effect of strong, nonmagnetic impurities on quasi-one-dimensional conventional and unconventional density waves (DW and UDW). The conventional case remains unaffected similarly to s-wave superconductors in the presence of weak, nonmagnetic impurities. The thermodynamic properties of UDW were found to be identical to those of a d-wave superconductor in the unitary limit. The real and imaginary part of the optical conductivity is determined for electric fields applied in the perpendicular directions. A new structure can be present corresponding to excitations from the bound state at the Fermi energy to the gap maximum in addition to the usual peak at 2\Delta. In the dc limit, universal electric conductivity is found.Comment: 9 pages, 5 figure

Existence of the Abrikosov vortex state in two-dimensional type-II superconductors without pinning

Theory alternative to the vortex lattice melting theories is advertised. The vortex lattice melting theories are science fiction cond-mat/9811051 because the Abrikosov state is not the vortex lattice with crystalline long-range order. Since the fluctuation correction to the Abrikosov solution is infinite in the thermodynamic limit (K.Maki and H.Takayama, 1972) any fluctuation theory of the mixed state should consider a superconductor with finite sizes. Such nonperturbative theory for the easiest case of two-dimensional superconductor in the lowest Landau level approximation is presented in this work. The thermodynamic averages of the spatial average order parameter and of the Abrikosov parameter $\beta_{a}$ are calculated. It is shown that the position H_{c4} of the transition into the Abrikosov state (i.e. in the mixed state with long-range phase coherence) depends strongly on sizes of two-dimensional superconductor. Fluctuations eliminate the Abrikosov vortex state in a wide region of the mixed state of thin films with real sizes and without pinning disorders, i.e. H_{c4} << H_{c2}. The latter has experimental corroboration in Phys.Rev.Lett. 75, 2586 (1995).Comment: 4 pages, 0 figure

Quantum dots in high magnetic fields: Rotating-Wigner-molecule versus composite-fermion approach

Exact diagonalization results are reported for the lowest rotational band of N=6 electrons in strong magnetic fields in the range of high angular momenta 70 <= L <= 140 (covering the corresponding range of fractional filling factors 1/5 >= nu >= 1/9). A detailed comparison of energetic, spectral, and transport properties (specifically, magic angular momenta, radial electron densities, occupation number distributions, overlaps and total energies, and exponents of current-voltage power law) shows that the recently discovered rotating-electron-molecule wave functions [Phys. Rev. B 66, 115315 (2002)] provide a superior description compared to the composite-fermion/Jastrow-Laughlin ones.Comment: Extensive clarifications were added (see new footnotes) regarding the difference between the rotating Wigner molecule and the bulk Wigner crystal; also regarding the influence of an external confining potential. 12 pages. Revtex4 with 6 EPS figures and 5 tables . For related papers, see http://www.prism.gatech.edu/~ph274c

Superconducting zero temperature phase transition in two dimensions and in the magnetic field

We derive the Ginzburg-Landau-Wilson theory for the superconducting phase transition in two dimensions and in the magnetic field. Without disorder the theory describes a fluctuation induced first-order quantum phase transition into the Abrikosov lattice. We propose a phenomenological criterion for determining the transition field and discuss the qualitative effects of disorder. Comparison with recent experiments on MoGe films is discussed.Comment: 7 pages, 2 figure

Topological Defects, Orientational Order, and Depinning of the Electron Solid in a Random Potential

We report on the results of molecular dynamics simulation (MD) studies of the classical two-dimensional electron crystal in the presence disorder. Our study is motivated by recent experiments on this system in modulation doped semiconductor systems in very strong magnetic fields, where the magnetic length is much smaller than the average interelectron spacing $a_0$, as well as by recent studies of electrons on the surface of helium. We investigate the low temperature state of this system using a simulated annealing method. We find that the low temperature state of the system always has isolated dislocations, even at the weakest disorder levels investigated. We also find evidence for a transition from a hexatic glass to an isotropic glass as the disorder is increased. The former is characterized by quasi-long range orientational order, and the absence of disclination defects in the low temperature state, and the latter by short range orientational order and the presence of these defects. The threshold electric field is also studied as a function of the disorder strength, and is shown to have a characteristic signature of the transition. Finally, the qualitative behavior of the electron flow in the depinned state is shown to change continuously from an elastic flow to a channel-like, plastic flow as the disorder strength is increased.Comment: 31 pages, RevTex 3.0, 15 figures upon request, accepted for publication in Phys. Rev. B., HAF94MD