Browsing by Author "Zhang, Kenneth Yi-Jie"

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Quantum criticality out of equilibrium in the pseudogap Kondo model
(2012-05-03)
Chung, Chung-Hou
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Zhang, Kenneth Yi-Jie
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電子物理學系
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Department of Electrophysics
Quantum phase transitions out of equilibrium are outstanding emergent subjects in condensed matter physics with great fundamental importance and challenges. We theoretically investigate here the nonequilibrium quantum phase transition in a generic nano-setup: the pseudogap Kondo model where a Kondo quantum dot couples to two-left (L) and right (R)-voltage-biased fermionic leads with power-law density of states (DOS) with respect to their Fermi levels mu(L/R), rho(c),(L(R))(omega) proportional to broken vertical bar omega - mu(L(R))broken vertical bar(r) with 0 < r < 1. In equilibrium (mu(L) - mu(R) = 0) and for 0 < r < 1/2, with increasing Kondo correlations this model exhibits a quantum phase transition from a unscreened local moment (LM) phase to the Kondo screened phase. At finite bias voltages and near criticality, we discover new nonequilibrium universal scaling behaviors in conductance, conduction electron T matrix, and local spin susceptibility via a controlled frequency-dependent renormalization group (RG) approach. The current-induced decoherence is key to understanding these distinct universal nonequilibrium quantum critical regimes. The relevance of our results to experiment is discussed.
Universal scaling of nonlinear conductance in the two-channel pseudogap Anderson model: Application for gate-tuned Kondo effect in magnetically doped graphene
(2013-08-26)
Lee, Tsung-Han
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Zhang, Kenneth Yi-Jie
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Chung, Chung-Hou
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Kirchner, Stefan
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電子物理學系
Based on the noncrossing approximation, we calculate both the linear and nonlinear conductance within the two-lead two-channel single-impurity Anderson model where the conduction electron density of states vanishes in a power-law fashion proportional to vertical bar omega - mu(F)vertical bar(r) with r = 1 near the Fermi energy, appropriate for a hexagonal system. For given gate voltage, we address the universal crossover from a two-channel Kondo phase, argued to occur in doped graphene, to an unscreened local moment phase. We extract universal scaling functions in conductance governing charge transfer through the two-channel pseudogap Kondo impurity and discuss our results in the context of a recent scanning tunneling spectroscopy experiment on Co-doped graphene.