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Condensed Matter Theory Seminar

Time: 16h00-17h00
Place: Buys Ballot Building (BBG) room 7.12, Princetonplein 5, Utrecht
Organizer: Lars Fritz

Wednesday, 28 February 2018
Time & Place: 16h00-17h30 in BBG 7.12
Speaker: Fabian Hassler (RWTH Aachen University)
Title: Quantum phases of the Majorana toric code in the presence of finite Cooper-pair tunneling

Abstract: The toric code based on Majorana fermions on mesoscopic superconducting islands is a promising candidate for quantum information processing. In the limit of vanishing Cooper-pair tunneling, it has been argued that the phase transition separating the topologically ordered phase of the toric code from the trivial one is in the universality class of (2+1)D-XY. On the other hand, in the limit of infinitely large Cooper-pair tunneling, the phase transition is in the universality class of (2+1)D-Ising. In this work, we treat the case of finite Cooper-pair tunneling and address the question of how the continuous XY symmetry breaking phase transition turns into a discrete ℤ2 symmetry breaking one when the Cooper-pair tunneling rate is increased. We show that this happens through a couple of tricritical points and first order phase transitions. Using a Jordan-Wigner transformation, we map the problem to that of spins coupled to quantum rotors and subsequently, propose a Landau field theory for this model that matches the known results in the respective limits. We calculate the effective field theories and provide the relevant critical exponents for the different phase transitions.

Archives - Condensed Matter Theory Seminar

Schedule 2018:

Date      Host  
28.02.18 Schuricht
14.03.18 Duine
28.03.18 Ortix
11.04.18 Fritz
25.04.18 Stoof
09.05.18 Morais Smith
23.05.18 Schuricht
06.06.18 Duine
20.06.18 Ortix