Physics of High Temperature Superconductivity

SCy

What's the latest in the search to understand the secrets
of high temperature superconductivity which can't be
explained by the BCS theory of cooper pairs bonded
by phonons? What's the leading theory or best candidate
that has ample scientific and experimental support?

SCy

Igor Khavkine

On 2005-06-21, SCy <magnetofeynman@yahoo.com> wrote:
> What's the latest in the search to understand the secrets
> of high temperature superconductivity which can't be
> explained by the BCS theory of cooper pairs bonded
> by phonons? What's the leading theory or best candidate
> that has ample scientific and experimental support?

The latest is: still searching.

One of the main problems is the sheer complexity of the phenomenon and
its rich phenomenology. The unit cell of a YBCO crystal, one of the
first discovered high-Tc superconductors, contains on the order of 100
atoms. Elementary excitations on top of the superconducting state are
well described by BCS-like quasiparticles with a d-wave gap (4-fold
rotational symmetry, nodes along the diagonals of the Brillouin zone
along wich the gap vanishes). There are important structures in 1D (Cu-O
chains), in 2D (Cu-O planes, considered most important), and 3D
(coupling between Cu-O layers). The presence of superconductivity
depends on a certain amount of disorder. The phase diagram of high-Tc
materials contains unusual regions such as an antiferromagnetic
insulator, the mysterious "pseudogap" state, a non-metallic normal
phase, as well as a doped conductor phase.

As superconductors, high-Tc materials don't differ much from Type-II
superconductors described by BCS theory. So many believe that the
superconducting state is due to Cooper-like pairing, except that it is
mediated not by phonons, but by as yet an unknown agent.

There are many proposals for a microscopic theory of high-Tc
superconductivity. But none of them are yet successful. One reason is
that the important properties of these materials stem from strong
correlations between their electrons. In other words, the electrons
cannot be treated as non- or weakly interacting, like in normal metals.
This imposes difficulties theoretically since simple models with weak
coupling are not expected to be appropriate, and strongly coupled
interacting systems are always difficult to work with. Instead, there
are many phenomenological theories describing different phases in the
phase diagram, with varying degrees of success. But no theory has come
close to, say, accurately predicting the transition temperature.

A recent review article focusing on the state of the theory is
cond-mat/0309094 by Yanase et al.

Hope this helps.

Igor