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SUMMARY:High temperature superfluidity in double bilayer graphene and in e
 lectron-hole bilayers:  comparison with diffusion Quantum Monte Carlo resu
 lts - Dr Andrea Perali - University of Camerino\, Italy
DTSTART:20130708T131500Z
DTEND:20130708T141500Z
UID:TALK46173@talks.cam.ac.uk
CONTACT:Teri Bartlett
DESCRIPTION:Exciton bound states in solids between electrons and holes are
  predicted to form a superfluid at high temperatures. We show that by empl
 oying atomically thin crystals such as a pair of adjacent bilayer graphene
  sheets\, equilibrium superfluidity of electron- hole pairs should be achi
 evable for the first time. The transition temperatures are well above liqu
 id helium temperatures.  \nWe find that Coulomb screening is progressively
  suppressed in the strongly coupled regime of pairing approaching low dens
 ities\, because of the large Fermi surface smearing that occurs when the s
 uperfluid gap is of order of the noninteracting Fermi energy and the pairs
  become more localized. This is a self-consistent effect\, which is includ
 ed in our calculation. Because the sample parameters needed for the device
  have already been attained in similar graphene devices\, our work suggest
 s a new route towards realizing high-temperature superfluidity in existing
  quality graphene samples [1].  \nOur proposed system consists of a pair o
 f parallel bilayer graphene sheets. The lower bilayer sheet is an electron
  bilayer and the upper bilayer sheet is a hole bilayer. The two bilayer sh
 eets are separated by a hexagonal boron nitride insulating barrier to prev
 ent tunneling between the sheets. There are separate electrical contacts t
 o the two layers and by tuning the biases on top and bottom metal gates a 
 wide range of carrier densities can be achieved. We discuss bilayer graphe
 ne here since it has been well characterized\, but a number of other such 
 crystals are possible. \nFinally\, we will report a quantitative compariso
 n between our theoretical results for the condensate fraction based on the
  zero temperature BCS-RPA approach and the recent difussion Quantum Monte 
 Carlo simulations for the same quantity [2]. We found a remarkable agreeme
 nt for the onset of the electron-hole superfluidity and a satisfactory des
 cription of the BCS-BEC crossover induced in this system by approaching th
 e low density regime [3].\n\n[1] A. Perali\, D. Neilson\, and A. R. Hamilt
 on\, Phys. Rev. Lett. 110\, 146803 (2013). [2] R. Maezono\, P. L. Rios\, T
 . Ogawa\, R. J. Needs\, Phys. Rev. Lett. 110\, 216407 (2013). [3] D. Neils
 on\, A. Perali\, in preparation.\n\nWeb site: http://www.supercondmat.org/
 perali\n
LOCATION:Mott Seminar Room\, Cavendish Laboratory\, Department of Physics
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