For distribution

[CM ICTP - Trieste]

									06/09

JOINT ICTP/SISSA CONDENSED MATTER SEMINARS
Academic Year 2006/07





Seminar Room - Main Building   (first floor)



Wednesday, 13 December -     4:00 p.m.




A. TSVELIK   ( Brookhaven National Laboratory, Upton )





" Phenomenological theory of underdoped state of the cuprates "


Abstract

We model the Fermi surface of the cuprates by 1D nested parts near the 
antinodal regions and unnested parts near the zone diagonals.  
Electrons in the nested regions form 1D spin liquids and develop 
spectral gaps below some T*, but superconducting order is prevented by 
quantum fluctuations.  It is shown that Josephson coupling between 
order parameters formed at  antinodes locks their relative phase at a 
crossover scale T** < T*.  Below  T** the system's response becomes 2D 
and it displays Nernst effect.  The total phase gets locked at T_c 
where the system develops true long range order.

====





CONDENSED MATTER AND STATISTICAL PHYSICS SECTION
	
INFORMAL SEMINAR on

Disorder and strong electron correlations






Thursday, 14 December    -    11:00 a.m.




Lecture Room 'C',  Main Bldg.- terrace level




C. MUDRY  ( Paul Scherrer Institut, Villigen )



" Electron fractionalization in  two-dimensional graphene-like 
structures "


Abstract

Electron fractionalization is intimately related to topology.  In 
one-dimensional systems, fractionally charged states exist at domain 
walls between degenerate vaccua.  In two-dimensional systems, 
fractionalization exists in quantum Hall fluids, where time-reversal 
symmetry is broken by a large external magnetic field.  Recently, there 
has been a tremendous effort in the search for examples of 
fractionalization in two-dimensional systems with time-reversal 
symmetry.  In this paper, we show that fractionally charged topological 
excitations exist on graphene-like structures, where quasiparticles are 
described by two flavors of Dirac fermions and time-reversal symmetry 
is respected.  The topological zero-modes are mathematically similar to 
fractional vortices in p-wave superconductors.  They correspond to a 
twist in the phase in the mass of the Dirac fermions, akin to cosmic 
strings in particle physics.