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ABSTRACT: Using the quantum transport of chiral Dirac fermions in graphene, we investigate the normal-state conductance and thermoelectric effect of a nano device under the ballistic superconductor-graphene-superconductor (SGS) model. Because of the Josephson effect and Andreev reflections, there exists an oscillatory behavior of the normal-state conductance flowing through the successive discrete energy levels on a finite-sized graphene contacted to the superconducting leads. The normal-state conductance displays a rich structure of subharmonic gaps controlled by means of a gate voltage on the discrete energy levels near the Fermi energy. Since the Fermi energy is an essential factor in determining the nature of conduction such as n or p type, we study the thermoelectric effect over the graphene-based nano device. It is shown that the thermoelectric effect can provide information on the location of the Fermi energy with respect to the energy levels of the finite-sized graphene.
Journal of Nanoscience and Nanotechnology 10/2008; 8(9):4692-7. · 1.56 Impact Factor
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ABSTRACT: The peculiar behaviors in two-dimensional graphene are fundamentally governed by the relativis-tic Dirac fermions in 2 + 1 dimensions. We exploit the peculiar transport phenomena of massless Dirac fermions to explore the application of the graphene to nanoelectrionics. Electronic transport through graphene is affected by superconducting electrodes and environments. If the interface be-tween graphene and a superconducting lead is transparently clean, a dissipationless bipolar current can flow through a discrete density of states near the Fermi level in the nanoelectronics due to the Josephson effects and Andreev's reflection, even in the presence of low dissipation. The successive discrete energy levels result in an oscillatory behavior of the critical current and the normal-state conductance through a finite-sized graphene attached to the superconducting leads. Hence, we propose the possibility of realizing a dissipationless nano transistor made of a superconductor-graphene-superconductor junction.
Journal- Korean Physical Society 07/2007; 50(73). · 0.45 Impact Factor
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ABSTRACT: To understand the unusual quantum Hall effects in graphene, we exploit the supersymmetric quantum mechanics on the basis of the index theorem and a higher degree of symmetry. The index theorem relates the zero-energy states to the topology of the compact lattice. We claim that the zero-energy state emerges naturally, provided the Zeeman splitting becomes as large as the Landau level separation at the Fermi points in graphene. This results in the fact that supersymmetry is a good symmetry. In the case of nonzero energy, the up-spin and the down-spin states provide the exact higher symmetry of spin, forming a supersymmetric multiplet. We describe briefly a unified picture for the peculiar and unconventional quantum Hall effects in graphene on the basis of the supersymmetric quantum mechanics.
Journal- Korean Physical Society 07/2007; 50(73). · 0.45 Impact Factor
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ABSTRACT: The quantum entanglement of the ground state and the thermal entanglement of the mixed thermal state of a three-qubit system with the Heisenberg interaction are investigated in detail in the presence of an inhomogeneous magnetic field. The concurrence of the model is compared with the magnetization, and the threshold temperature of the entanglement is discussed. The effects of the magnetic field and the coupling coefficient in the thermal entanglement are also examined by considering the concurrence of the three-qubit and two-qubit systems. We found that the thermal entanglement of the system exists at temperatures below the threshold temperature even if the ground state is unentangled.
Journal of Physics A Mathematical and Theoretical 06/2007; 40(26):7283. · 1.56 Impact Factor
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ABSTRACT: Quantum Hall effects in graphene occur in the unconventional form of 4e 2 h (n + 1/2), which is shifted by a half integer on the Landau levels. The peculiar behaviors are fundamentally governed by massless Dirac fermions and the nontrivial topology of the band structure on the graphene in 2 + 1 dimensions. Charge carriers in bilayer graphene reveal chiral fermions with a parabolic energy spectrum and a zero-level anomaly accompanied by metallic conductivity. We also discuss briefly the possibility of studying a variety of unusual transport properties in the graphene system from the topological point of view and the techniques developed in the theories of 2 + 1 dimensional gravity.
Journal- Korean Physical Society 01/2007; 49. · 0.45 Impact Factor
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ABSTRACT: The role of edge states in the electronic specific heat and magnetic ordering of graphene strip is investigated by diagonalizing a lattice Hamiltonian for graphene strips. The magnetization of the edge sites is evaluated by employing the on-site Coulomb repulsion U within a mean-field approximation. We find that edge states give extra strong enhancement in the specific heat of zigzag strips at very low temperatures in addition to the Cel≃γTα behavior of two-dimensional (2D) massless fermions in graphene with and α≈2.27 at low temperatures. The onset value Uc for ferrimagnetic ordering in zigzag strips depends notably on the next nearest neighbour (NNN) hopping strength t2 but not much on the Rashba spin–orbit coupling VR for a given t2.
Physica E Low-dimensional Systems and Nanostructures 40(5):1715-1717. · 1.53 Impact Factor
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ABSTRACT: We investigate low-energy electronic states and heat capacity of graphene strips. The density of states of graphene is obtained by diagonalizing a lattice Hamiltonian for graphene strips with zigzag and armchair edges, respectively. The modes localized at strip edges in zigzag geometry reveal a peculiar density of states near the zero-energy line. We find that the overall behavior of the heat capacity Cel(T) in graphene is similar to that of a two-level system, and the low-temperature heat capacity shows unconventional behavior of Cel∝Tα with α≈2.2 varying slightly for different choices of various band structure parameters and the width of the strip. Unusual strong enhancement is observed in the heat capacity at low temperatures for zigzag strips, in addition to the Tα behavior of the two-dimensional graphene. This enhancement stems from the states localized along the edges in the strips with zigzag terminations.
Phys. Rev. B. 76(11).
