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Exciton condensation and fractional charge in a bilayer two-dimension electron gas adjacent to a superconductor film  [PDF]
Ningning Hao,Ping Zhang,Jian Li,Wei Zhang,Yupeng Wang
Physics , 2010, DOI: 10.1103/PhysRevB.82.165317
Abstract: We study the exciton condensate (EC) in a bilayer two-dimension-electron-gas (2DEG) adjacent to a type-II superconductor thin film with an array of pinned vortex lattices. By applying continuum low energy theory and carrying numerical simulations of lattice model within mean-field approximation, we find that if the order parameter of EC has a vortex profile, there are exact zero modes and associated \emph{rational} fractional charge for zero pseudospin potential ($\mu$) and average chemical potential ($h$): $\mu$=0 and $h$=0; while for $\mu\mathtt{\neq}0$ and $h$=0, intervalley mixing splits the zero energy levels, and the system exhibits \emph{irrational} fractional \emph{axial} charge.
Effective field theory for the bulk and edge states of quantum Hall states in unpolarized single layer and bilayer systems  [PDF]
Ana Lopez,Eduardo Fradkin
Physics , 2000, DOI: 10.1103/PhysRevB.63.085306
Abstract: We present an effective theory for the bulk Fractional Quantum Hall states in spin-polarized bilayer and spin-1/2 single layer two-dimensional electron gases (2DEG) in high magnetic fields consistent with the requirement of global gauge invariance on systems with periodic boundary conditions. We derive the theory for the edge states that follows naturally from this bulk theory. We find that the minimal effective theory contains two propagating edge modes that carry charge and energy, and two non-propagating topological modes responsible for the statistics of the excitations. We give a detailed description of the effective theory for the spin-singlet states, the symmetric bilayer states and for the $(m,m,m)$ states. We calculate explicitly, for a number of cases of interest, the operators that create the elementary excitations, their bound states, and the electron. We also discuss the scaling behavior of the tunneling conductances in different situations: internal tunneling, tunneling between identical edges and tunneling into a FQH state from a Fermi liquid.
Coulomb screening and collective excitations in a graphene bilayer  [PDF]
Xue-Feng Wang,Tapash Chakraborty
Physics , 2006, DOI: 10.1103/PhysRevB.75.041404
Abstract: We have investigated the Coulomb screening properties and collective excitations in a graphene bilayer. The static screening effect is anisotropic and is much stronger in the undoped graphene bilayer than in a monolayer graphene [1]. The dynamic screening shows the properties of a Dirac gas in the low frequency and that of a Fermi gas in the high frequency. The transition from the Dirac to the Fermi gas is also observed in the plasmon spectrum. Finally, we find that an electron gas in a doped graphene bilayer has quite similar properties as those of a Fermi gas in materials containing two energy valleys.
Topological phases and fractional excitations of the exciton condensate in a special class of bilayer systems  [PDF]
Ningning Hao,Ping Zhang,Yupeng Wang
Physics , 2010, DOI: 10.1103/PhysRevB.84.155447
Abstract: We study the exciton condensate in zero temperature limit in a special class of electron-hole bilayer systems adjacent to insulating ferromagnetic films. With the self-consistent mean-field approximation, we find that the Rashba spin-orbit interaction in the electron and hole layers can induce the p \pm ip or p pairing states depending on the different magnetization of the overlapped ferromagnetic films. Correspondingly, the topologically nontrivial or trivial phases emerge. Furthermore, in the topologically nontrivial phase, the quasiparticle excitations of the U(1) vortex are attached to fractional quantum numbers and obey Abelian statistics.
Coulomb screening and collective excitations in biased bilayer graphene  [PDF]
Xue-Feng Wang,Tapash Chakraborty
Physics , 2010, DOI: 10.1103/PhysRevB.81.081402
Abstract: We have investigated the Coulomb screening properties and plasmon spectrum in a bilayer graphene under a perpendicular electric bias. The bias voltage applied between the two graphene layers opens a gap in the single particle energy spectrum and modifies the many-body correlations and collective excitations. The energy gap can soften the plasmon modes and lead to a crossover of the plasmons from a Landau damped mode to being undamped. Plasmon modes of long lifetime may be observable in experiments and may have potentials for device applications.
Vortex Chain States in a Ferromagnet/Superconductor Bilayer  [PDF]
Serkan Erdin
Physics , 2006, DOI: 10.1103/PhysRevB.73.224506
Abstract: The discrete vortex lattices in a ferromagnet/superconductor bilayer are studied when the ferromagnet has periodic stripe domains with an out-of-plane magnetization. The vortices are assumed to be situated periodically on chains in the stripe domains. Only up to two vortex chains per domain configurations are considered. When the domain period is fixed, the threshold magnetization is calculated at which the transition from the Meissner state to the mixed state occurs. When the domain period is not fixed, the equilibrium domain size and vortex positions are calculated, depending on the domain's magnetization and the domain wall energy. In equilibrium, the vortices in the neighbor domains are half-way shifted, while they are next to each other in the same domain.
Vortices and Dissipation in a Bilayer Thin Film Superconductor  [PDF]
Wei Zhang,H. A. Fertig
Physics , 2005, DOI: 10.1103/PhysRevB.71.224514
Abstract: Vortex dynamics in a bilayer thin film superconductor are studied through a Josephson-coupled double layer XY model. A renormalization group analysis shows that there are three possible states associated with the relative phase of the layers: a free vortex phase, a logarithmically confined vortex-antivortex pair phase, and a linearly confined phase. The phases may be distinguished by measuring the resistance to counterflow current. For a geometry in which current is injected and removed from the two layers at the same edge by an ideal (dissipationless) lead, we argue that the three phases yield distinct behaviors: metallic conductivity in the free vortex phase, a power law I-V in the logarithmically confined phase, and true dissipationless superconductivity in the linearly confined phase. Numerical simulations of a resistively shunted Josephson junction model reveal size dependences for the resistance of this system that support these expectations.
Field induced superconducting phase in Superconductor-Normal metal and Superconductor-Superconductor bilayer  [PDF]
Xavier Montiel,Alexandre I. Buzdin
Physics , 2011, DOI: 10.1103/PhysRevB.84.054518
Abstract: We study the proximity effect in a superconductor (S)-normal metal (N) bilayer under in-plane magnetic field. A compensation between the Zeeman effect and the energy splitting between bonding and anti-bonding levels leads to a magnetic field induced superconducting phase in the (H,T) phase diagram of the S-N bilayer well above the standard paramagnetic limit. We demonstrate that the presence of the non-magnetic impurities shrink the region of field induced superconductivity existence in S-N and S-S bilayers.
Proximity effects in a superconductor-normal metal bilayer system  [PDF]
Naushad Ahmad Kamar,N. S. Vidhyadhiraja
Physics , 2014,
Abstract: In this letter, we have studied the effects of proximity of a superconductor to a normal metal. The system, represented by a bilayer attractive Hubbard model, is investigated using layer-dynamical mean field theory and iterated perturbation theory for superconductivity as an impurity solver. The bilayer system comprises a superconducting and a normal metallic layer, connected by an inter-planar hopping ($t_\perp$). It is found that superconductivity is induced in the normal layer for small $t_\perp$. With increasing inter-layer hopping, the bilayer system undergoes two transitions: a first order transition to a normal metallic phase, and subsequently a continuous crossover to a band insulator.
Low-lying excitations in superconducting bilayer systems  [PDF]
Marius Grigorescu
Physics , 1999, DOI: 10.1139/cjp-78-2-119
Abstract: The ground and first excited state of two superconducting layers in interaction are studied considering two different coupling terms, one represented by the standard Josephson interaction, and one new, which is a superexchange pairing force between bilayer pairs. It is shown that a moderate-to strong Josephson interaction produces a low-lying collective state, pictured as an out-of-phase oscillation of the BCS gauge angles of the two layers. This antisymmetric angular oscillation might explain the 41 meV resonance observed in the neutron scattering experiments. The bilayer pairs are formed by electrons from different layers with an antiparallel orientation of the spins, being related to the antiferromagnetic arrangement. The pair operators within the layers together with the bilayer pairs generate by commutation an so(5) algebra. It is shown that the transition between the superconducting and antiferromagnetic phases can be explained assuming the dependence on concentration of the bilayer pairing strength, with maximum at half-filling.
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