For distribution - thanks

[CM ICTP - Trieste]

SEMINAR on  Disorder and strong electron correlations



Thursday, 11 October   -    11:00 a.m.


Seminar Room,  Main Bldg.- first floor



G. VOLOVIK   ( Helsinki University of Technology )


"On Larkin-Imry-Ma state of superfluid 3He"

Abstract

	Superfluid 3He-A shares the properties of the spin nematic 
characterized by unit vector d of spin anisotropy and the chiral 
orbital ferromagnet characterized by vector l of the orbital angular 
momentum of Cooper pairs.  This doubly anisotropic superfluid, when it 
is confined in aerogel, represents the most interesting example of a 
system  with continuous symmetry in the presence of random anisotropy 
disorder, which is provided by aerogel strands.
	Recent NMR experiments allow us to discuss the state of 3He-A in 
aerogel in terms of the Larkin-Imry-Ma (LIM) state.  The LIM state is 
characterized by the short-range orientational order of the l-vector, 
while the long-range orientational order is destroyed by the collective 
action of the randomly oriented aerogel strings.  Since 3He-A is a 
chiral superfluid, the disordered LIM state can be represented as  a 
system of randomly distributed skyrmions -- vortices with continuous 
cores.  The characteristic distance between the vortex-skyrmions and 
the size of their cores are determined by the LIM length which is of 
order of few microns.  It is much bigger than the microscopic scales -- 
the distance between the impurities-strands.
	A regular anisotropy may destroy the LIM effect, and this does take 
place when one applies a uni-axial deformation to the aerogel sample.  
Even less than 1% deformation leads to the restoration of the uniform 
orientation of the  l-vector in the sample.  As a result the value of 
the transverse NMR frequency shift is enhanced by an order of magnitude 
compared to that in the LIM state.  When the deformation leads to 
orientation of  l along the magnetic field, it becomes possible to 
study novel phenomena which only occur under this arrangement and thus 
are not possible in bulk 3He-A.  One of these phenomena is the 
phase-coherent spin precession recently observed in 3He-A in the 
deformed aerogel.  This presents another realization of the 
Bose-Einstein condensation (BEC) of magnons, which is different from 
the magnon BEC found twenty years ago in 3He-B and known as the 
homogeneously precessing domain.  This geometry will allow us to 
stabilize and observe Alice strings -- half-quantum vortices -- in 
rotating 3He-A.