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Semiconductor quantum dots provide a highly controllable environment to study strongly correlated phenomena and quantum phase transitions. In a parallel double-quantum-dot device in which dot 1 is in the Kondo regime and dot 2 behaves as a non-interacting resonant level, varying the dot-1 energy ε1 drives the system through a pair of first-order quantum phase transitions separating Kondo-screened and local-moment phases [1]. In this work, we use the numerical renormalization-group approach to explore the effect of a nonzero Coulomb interaction U2 in dot 2. For fixed U2, there is a range of ε1 and dot-2 energy ε2 (centered around the particle-hole-symmetric point ε1=-U1/2,&epsilon2=-U2/2) within which the ground state retains an unquenched spin-1/2 degree of freedom. This free-moment phase, which is separated from a surrounding Kondo-screened phase by quantum phase transitions of the Kosterlitz-Thouless type, grows in area on the &epsilon1-ε2 plane with increasing U2. Within the free-moment phase, analysis of thermodynamic properties and static spin-spin correlations between the two dots and between each dot and the leads reveals a crossover from localization of the residual spin on dot 1 (when dot 2 is in its mixed valence regime) to an underscreened spin-1 Kondo effect (when both dots are in their Kondo regimes). Signatures of these behaviors can be experimentally identified through the conductance of the system at finite temperatures [2].
[1] L. G. G. V. Dias da Silva, N. P. Sandler, K. Ingersent, and S. E. Ulloa, Phys. Rev. Lett. 97, 096603 (2006). [2] A. Wong, W. B. Lane, L. G. G. V. Dias da Silva, K. Ingersent, N. P. Sandler and S.E. Ulloa (in preparation). |
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