Current
Activity
- Colossal
magnetoresistance oxides:-
The
very actively explored family of doped manganites, namely Re1-xAx
MnO3 where Re is a trivalent rare earth and A
are alkaline earths, pose a new and difficult challenge in many
body physics because local (Jahn-Teller) electron lattice
coupling, electron electron correlation, Hund's rule ferromagnetic
spin exchange, are all present and strong. A new
microscopic two fluid approach has been developed which has the
potential to illuminate the entire field. The idea is the
necessary coexistence of two different kinds of orbital states at
each site. With one, a large Jahn-Teller distortion and consequent
energy lowering is associated and with the other, not. The former
leads, in a lattice, to nearest localized states in the relevant
antiadiabatic regime, and the latter to a correlated broad band.
The simultaneous but necessary coexistence of these two
qualitatively different kinds of electron fluids, possible for
strong Jahn-Teller coupling and on including quantum lattice
effects, is shown to lie behind the very rich and hitherto
puzzling phenomena in manganites. For example, the insulator to
ferromagnetic metal transition as a function of doping x
and temperature T, colossal negative magnetoresistance,
transport properties of the ferromagnetic and paramagnetic phases,
the ubiquitous `two phase' coexistence, large quantum effects
(isotopic mass) on the ferromagnetic Tc, and properties of
electron doped manganites have all been qualitatively and
quantitatively explained including the strong correlation effects
nonperturbatively. Extensions to orbital correlations/order,
electronic inhomogeneities, polaron dynamics etc. are in progress.
2.
High Temperature Superconductivity:-
(a)
Effective low energy theory for cuprates: Role of
internal and overall phase fluctuations and implications for the
pseudogap phase, theory of the underdoped regime.
(b)
Electronic properties of the superconducting phase:
Quasiparticles and
quasiparticle transport in the mixed state, vortex dynamics,
inelastic neutron peak, phase fluctuation effects, effect of
impurities (esp. Zn), vortex induced idxy order,
and bound electronic states in the vortex core.
3. Biophysics:-
Origin
and significance of long tailed distributions in gene expression.
Chromatin, gene transcription, coarse
grained models for DNA.
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