Thursday's seminar

[Cond.Matt. & Stat.Mech.Section]

SEMINAR on   Disorder and strong electron correlations





Thursday, 22 October      -  11:00 a.m.



Seminar Room,  Leonardo Building - first floor



Kamran BEHNIA     ( ESPCI, Paris )



"Nernst effect and dimensionality in the   quantum limit and beyond"



Abstract



Because of their very small carrier density, bismuth and graphite  
allow to attain the quantum limit in a bulk metal with available  
magnetic fields.  The fate of a three-dimensional electron gas pushed  
to this ultra-quantum regime remains an open question.

Recent measurements of the Nernst effect in graphite and in graphene  
combined to what was reported in bulk bismuth a few years ago, and in  
two-dimensional electron systems a decade earlier, permits to draw a  
striking correlation between the field profile of the Nernst response  
and dimensionality.  When a Landau level intersects the Fermi level,  
the Nernst signal peaks in three-dimensional systems and vanishes in  
two-dimensional systems.   We argue that this qualitative difference  
is a signature of a topological phase transition exclusive to three- 
dimensional systems.

Beyond the quantum limit, when all carriers are confined to the first  
Landau level, Nernst effect in bismuth detects field scales, which are  
not expected in the one-particle picture.  The field profile of these  
anomalies points to a bulk origin. According to our recent angular- 
dependent Nernst measurements, the band picture is quite successful in  
explaining the complex electronic spectrum of bismuth up to 9 T, but  
is inadequate as the quantum limit is crossed.  In particular, in the  
vicinity of 40 T, that is more than four times the quantum limit,  
large anomalies in all transport properties point to an enigmatic  
reorganization of bulk electrons. Unexpectedly, this electronic  
instability leads to an enhanced conductivity and a better  metallicity.