nanoelectronics - Institut d'Études Scientifiques de Cargèse (IESC)

Posters
Nonclassical photon-pair production in a voltage-biased Josephson junction
Juha Leppäkangas 1 , Göran Johansson 1 , Michael Marthaler 2 , and Mikael Fogelström 1
1
Microtechnology and Nanoscience, MC2, Chalmers University of Technology, SE-412 96
Göteborg, Sweden
2
Institut für Theoretische Festkörperphysik and DFG-Center for Functional Nanostructures
(CFN), Karlsruhe Institute of Technology, D-76128 Karlsruhe, Germany
We investigate electromagnetic radiation emitted by a small voltage-biased Josephson
junction connected to a superconducting transmission line. For weak tunneling couplings and
frequencies below the well known emission peak at the Josephson frequency (2eV/h), extra
radiation is triggered by quantum noise in the electromagnetic environment. For typical ohmic
transmission lines, the corresponding photon flux spectrum becomes symmetric around half
the Josephson frequency, indicating that the photons are predominately created in pairs. By
establishing an input-output formalism for the microwave field in the transmission line, we
give further evidence for this non-classical photon pair production, demonstrating that it
violates the classical Cauchy-Schwarz inequality for two-mode flux cross correlations. In
connection to recent experiments [1], we also consider a stepped transmission line, where
resonances increase the signal-to-noise ratio.
[1] M. Hofheinz, F. Portier, Q. Baudouin, P. Joyez, D. Vion, P. Bertet, P. Roche, and D.
Esteve Bright Side of the Coulomb Blockade, Phys. Rev. Lett. 106 217005 (2011)
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Gap Generation in Topological Insulator Surface States by non-Ferromagnetic
Magnets
László Oroszlány 1 , Alberto Cortijo 2
1
Department of Physics of Complex Systems, Eotvos University,
H-1117 Budapest, Pázmány Péter sétány 1/A, Hungary
2
Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain.
Within a tight binding approach, it is shown that, contrary to the naive expectation, single
particle spectral gaps can be opened on the surface states of three dimensional topological
insulators by using commensurate out- and in-plane antiferromagnetic or ferromagnetic
insulating thin films [1].
[1] L. Oroszlány, A. Cortijo arXiv:1208.4615
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Optical conductivity of bilayer graphene in the far-infrared (FIR) spectral range
Gábor Széchenyi, József Cserti
Department of Physics of Complex Systems, Eötvös University, Budapest, Hungary
Using Kubo formula we study theoretically the frequency dependent optical conductivity in
bilayer graphene with symmetry breaking mechanisms. We demonstrate that in the FIR
spectral range (1.2-80 meV) the spectrum is strongly sensitive to the rate of the symmetry
breaking, the trigonal warping and the polarization of the light. We show analytically a
connection between the minimal conductivity and the zero frequency limit of the optical
spectrum.