Tag Archive: estado sólido

Prezados Professores e Estudantes do Departamento de Física-UFC,
temos o prazer de convidá-los para o Seminário dessa semana do nosso Programa de Pós-Graduação:


Dr. John R. deBruyn, University of Western Ontario

Local e Data: Sala de Seminários, sexta-feira, 25/11/11, 16 horas.

Contamos com a presença de todos.

Coordenação dos Seminários do Departamento de Física



Seminarista: Dr. LUCIAN COVACI, Universiteit Antwerpen, Belgium

Local e Data: Sala de Seminários, Dia 04/11/11, sexta-feira às 16 horas


Lucian Covaci and Francois Peeters
Universiteit Antwerpen, Belgium

Phase fluctuations in high-Tc superconductors are believed to play an important role in the underdoped regime. Various experimental measurements (Nernst effect, STM, resistivity, etc.) suggest that a superconducting state which has a finite order parameter but acquires a disordered phase is responsible for the peculiar properties of the pseudogap regime. Recently, STM measurements [1] on structures made of superconducting LSCO put in contact with a metallic layer (either overdoped LSCO or Au) revealed that the STM gap located at the Fermi level survives above the superconducting critical temperature when the LSCO layer is in the pseudogap phase. The location of the gap remains pinned to the Fermi level even if the nature of the metallic layer is changed (overdoped LSCO or Au). In a previous study [2] we showed that a gap induced by spin density wave order will not follow the Fermi level when the band structure of the metallic layer is modified. One needs an order with Q=0 in order to explain the experimental findings, such an order is the phase fluctuating superconductor. We consider a model in which the mean-field transition temperature is higher than the phase ordering temperature. Regions with size on the order of the coherence length are next considered as spins in a 2D-XY model which will have a lower critical temperature. Using a Monte-Carlo procedure for the 2D-XY model we extract for each temperature a set of phase configurations with which we compute the average LDOS at the surface of the metallic layer. We show that at the phase ordering temperature there is little change in the LDOS gap, similar to experimental findings. Above the phase ordering temperature vortex-antivortex pairs will be unbound and rapid changes of the phase will induce zero energy bound states in the metallic region which will then, on average, fill the LDOS gap. Due to the requirement of large system size needed in order to describe the proximity effect in this system we use the Chebyshev-BdG method [3] previously developed in our group. The Chebyshev-BdG methods is expanded to computations on Graphics Processing Units (GPU) to give impressive speed-up times (on the order of x1000 on a server with three GTX 580 GPUs).

[1] O. Yuli, I. Asulin, Y. Kalcheim, G. Koren, and O. Millo, Phys.
Rev. Lett. 103, 197003 (2009).
[2] G-Q. Zha, L. Covaci, S-P. Zhou and F.M. Peeters, Phys. Rev. B 82,
140502(R) (2010).
[3] L. Covaci, F.M. Peeters and M. Berciu, Phys. Rev. Lett. 105, 167006 (2010).

Proximity effect in graphene under inhomogeneous strain

L. Covaci and F. M. Peeters

ABSTRACT: The interplay between quantum Hall states and Cooper pairs is usually hindered by the suppression of the superconducting state due to the strong magnetic fields needed to observe the quantum Hall effect. From this point of view graphene is special since it allows the creation of strong pseudo-magnetic fields due to strain. We show that in a Josephson junction made of strained graphene, Cooper pairs will diffuse into the strained region. The pair correlation function will be sub-lattice polarized due to the polarization of the local density of states in the zero pseudo-Landau level. We uncover two regimes;

1) one in which the cyclotron radius is larger than the junction length in which case the supercurrent will be enhanced, and

2) the long junction regime where the supercurrent is strongly suppressed because the junction becomes an insulator.

In the latter case quantized Hall states form and Andreev scattering at the normal/superconducting interface will induce edge states. Our numerical calculation has become possible due to an extension of the Chebyshev Bogoliubov-de Gennes method to computations on video cards (GPUs).

Prezados professores e estudantes do Departamento de Física – UFC,

Gostaríamos de convidar a todos para o Seminário da semana do
Departamento de Física da UFC.


Prof. João Milton Pereira Junior
Departamento de Física – Universidade Federal do Ceará

Local: Sala de Seminários, Departamento de Física
Data: Sexta-feira, 02/09/2011 às 16 horas.