1,769 research outputs found
Experimental Constraints on the Pairing State of the Cuprate Superconductors: an Emerging Consensus
We present a critical discussion of recent experimental probes of the pairing
state of the high temperature superconductors, focusing primarily, but not
exclusively, on \Yba, where the best data currently exist. Penetration depth
measurements near \Tc\ give no indication of an extra transition, indicating
that the pairing state is a one-dimensional representation of the crystal
symmetry. Penetration depth measurements at low temperatures provide strong
evidence for a change in sign of the gap function over the Fermi surface.
Quantum mechanical phase interference experiments generally confirm this and in
addition show that the nodal positions are consistent with a \dx2y2\ pairing
state. This pairing state is consistent with photoemission measurements of the
gap function, Raman scattering, the effect on \Tc\ of impurities, and many
other data (reviewed by two of us previously) which indicate the presence of
low lying excitations in the superconducting state. We also discuss evidence
that apparently does not fit in with a \dx2y2\ pairing state, and we describe
possible alternative scenarios.Comment: 81 pages, macro package (modified version of uiucmac.tex) included in
submission, figures NOT available (but not essential
Universal Sound Absorption in Amorphous Solids: A Theory of Elastically Coupled Generic Blocks
Glasses are known to exhibit quantitative universalities at low temperatures,
the most striking of which is the ultrasonic attenuation coefficient 1/Q. In
this work we develop a theory of coupled generic blocks with a certain
randomness property to show that universality emerges essentially due to the
interactions between elastic blocks, regardless of their microscopic nature.Comment: (Revised) 16 pages, 2 figures. To appear in Journal of
Non-Crystalline Solid
Universal Properties of the Ultra-Cold Fermi Gas
We present some general considerations on the properties of a two-component
ultra-cold Fermi gas along the BEC-BCS crossover. It is shown that the
interaction energy and the ground state energy can be written in terms of a
single dimensionless function , where and
. The function incorporates all the many-body physics
and naturally occurs in other physical quantities as well. In particular, we
show that the RF-spectroscopy shift \bar{\d\o}(\xi,\tau) and the molecular
fraction in the closed channel can be expressed in terms of
and thus have identical temperature dependence. The conclusions
should have testable consequences in future experiments
BEC-BCS Crossover with Feshbach Resonance for a Three-Hyperfine-Species Model
We consider the behavior of an ultracold Fermi gas across a narrow Feshbach
resonance, where the occupation of the closed channel may not be negligible.
While the corrections to the single-channel formulae associated with the
nonzero chemical potential and with particle conservation have been considered
in the existing literature, there is a further effect, namely the
"inter-channel Pauli exclusion principle" associated with the fact that a
single hyperfine species may be common to the two channels. We focus on this
effect and show that, as intuitively expected, the resulting corrections are of
order , where is the Fermi energy of the gas in the absence of
interactions and is the Zeeman energy difference between the two
channels. We also consider the related corrections to the fermionic excitation
spectrum, and briefly discuss the collective modes of the system
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