8,870 research outputs found
Competition between spin density wave order and superconductivity in the underdoped cuprates
We describe the interplay between d-wave superconductivity and spin density
wave (SDW) order in a theory of the hole-doped cuprates at hole densities below
optimal doping. The theory assumes local SDW order, and associated electron and
hole pocket Fermi surfaces of charge carriers in the normal state. We describe
quantum and thermal fluctuations in the orientation of the local SDW order,
which lead to d-wave superconductivity: we compute the superconducting critical
temperature and magnetic field in a `minimal' universal theory. We also
describe the back-action of the superconductivity on the SDW order, showing
that SDW order is more stable in the metal. Our results capture key aspects of
the phase diagram of Demler et al. (cond-mat/0103192) obtained in a
phenomenological quantum theory of competing orders. Finally, we propose a
finite temperature crossover phase diagram for the cuprates. In the metallic
state, these are controlled by a `hidden' quantum critical point near optimal
doping involving the onset of SDW order in a metal. However, the onset of
superconductivity results in a decrease in stability of the SDW order, and
consequently the actual SDW quantum critical point appears at a significantly
lower doping.
All our analysis is placed in the context of recent experimental results.Comment: 27 pages, 11 figures; (v2) added clarifications and refs, and
corrected numerical errors (thanks to A. Chubukov
Non-perturbative models of intermittency in edge turbulence
A theory of the probability distribution function (PDF) tails of the blob
density in plasma edge turbulence is provided. A simplified model of the fast
convective radial transport is used. The theoretically predicted PDF tails
corroborate earlier measurements of edge transport, further confirming the
strongly non-Gaussian feature of edge transport. It is found that increasing
the cross sectional spatial scale length ( and ) of the blob results
in larger transport whereas increasing the toroidal scale length ()
decreases the PDF. The results imply that the PDF decreases for larger blob
speed .Comment: 19 pages, 2 figure
Interference of nematic quantum critical quasiparticles: a route to the octet model
Repeated observations of inhomogeneity in cuperate superconductors[1-5] make
one immediately question the existance of coherent quasiparticles(qp's) and the
applicability of a momentum space picture. Yet, obversations of interference
effects[6-9] suggest that the qp's maintain a remarkable coherence under
special circumstances. In particular, quasi-particle interference (QPI) imaging
using scanning tunneling spectroscopy revealed a highly unusual form of
coherence: accumulation of coherence only at special points in momentum space
with a particular energy dispersion[5-7]. Here we show that nematic quantum
critical fluctuations[10], combined with the known extreme velocity
anisotropy[11] provide a natural mechanism for the accumulation of coherence at
those special points. Our results raise the intriguing question of whether the
nematic fluctuations provide the unique mechanism for such a phenomenon.Comment: 4 pages, 3 figure
The Spitzer c2d Survey Of Nearby Dense Cores. VII. Chemistry And Dynamics In L43
We present results from the Spitzer Space Telescope and molecular line observations of nine species toward the dark cloud L43. The Spitzer images and molecular line maps suggest that it has a starless core and a Class I protostar evolving in the same environment. CO depletion is seen in both sources, and DCO(+) lines are stronger toward the starless core. With a goal of testing the chemical characteristics from pre- to protostellar stages, we adopt an evolutionary chemical model to calculate the molecular abundances and compare with our observations. Among the different model parameters we tested, the best-fit model suggests a longer total timescale at the pre-protostellar stage, but with faster evolution at the later steps with higher densities.NSF AST-0307250, AST0607793NASA NNX07AJ72GNational Research Foundation of Korea (NRF) government (MEST) 2009-0062865KOSEF R012007- 000-20336-0Astronom
Resonance enhanced turbulent transport
The effect of oscillatory shear flows on turbulent transport of passive scalar fields is studied by numerical computations based on the results provided by E. Kim [Physics of Plasmas 13, 022308 (2006)] . Turbulent diffusion is found to depend crucially on the competition between suppression due to shearing and enhancement due to resonances, depending on the characteristic time and length scales of shear flow and turbulence. Enhancements in transport occur for turbulence with finite memory time either due to Doppler or parametric resonances. Scalings of turbulence amplitude and transport are provided in different parameter spaces. The results suggest that oscillatory shear flows are not only less efficient in regulating turbulence, but also can enhance the value of turbulent diffusion, accelerating turbulent transport
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