ELFT HÍRADÓ

[tcsaba AT eik.bme.hu]

                       M E G H Í V Ó - I N V I T A T I O N

     Seminar series of the Department of Theoretical Physics at the
           Budapest University of Technology and Economics


                             Gergely Zaránd
                 (Department of Theoretical Physics,
            Budapest University of Technology and Economics)

Universal Fermi liquid crossover and quantum criticality in a  
mesoscopic device


The microscopic origins of quantum phase transitions (QPTs) in complex  
materials and the fate of the Fermi liquid state at quantum  
criticality are often debated. Quantum dots and mesoscopic circuits  
provide an experimental framework for realizing known quantum impurity  
Hamiltonians that can feature tunable second-order QPTs. Here we  
report on the theoretical and experimental investigation  of the  
quantum phase transitions occurring in a mesoscopic system, a quantum  
dot coupled to a metallic grain and to lead electrodes. We establish  
theoretically the complex phase diagram of this device through  
detailed numerical renormalization group calculations and resolve a  
former controversy: We show that, counter- intuitively, stable lines  
of non-Fermi liquid spin and charge two-channel Kondo states emerge  
and coexist with SU(4) physics in this simple device. We demonstrate  
experimentally in unprecedented detail, with support from numerical  
computations, a universal crossover from a quantum critical non-Fermi  
liquid behavior to distinct Fermi liquid ground states in a regime,  
where our device realizes a spin-1/2 impurity exchange-coupled equally  
to two independent electronic reservoirs. Arbitrarily small detuning  
of the exchange couplings results in conven- tional screening of the  
spin by the more strongly coupled channel for energies below a Fermi  
liquid scale T*. We extract a quadratic dependence of T* on gate  
voltage close to criticality and validate an asymptotically exact  
conformal field theory description of the universal crossover between  
strongly correlated non-Fermi liquid and Fermi liquid states.



Idöpont: 2015. november 13., 10:15
Helyszín: BME Fizikai Intézet, Elméleti Fizika Tanszék,
Budafoki út 8. F-épület, III lépcsöház, szemináriumi szoba


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