ELFT HÍRADÓ

[Andras Palyi <palyi AT mail.bme.hu>]

Dear Colleagues, 

Next Monday Balazs Gulacsi will give a talk at the BME Exotic Quantum Phases 
seminar.

Speaker: Balazs Gulacsi (BME)
Title: Collective modes for helical edge states interacting with quantum light
Time: June 24, Monday, 14:15
Location: seminar room of the theory department (Department of Theoretical 
Physics, Building F, stairway III., 1111 Budapest, Budafoki ut 8.)

Abstract: "We investigate the light-matter interaction between the edge state 
of a 2D topological insulator and quantum electromagnetic field. The 
interaction originates from the Zeeman term between the spin of the edge 
electrons and the magnetic field, and also through the Peierls substitution. 
The continuous U(1) symmetry of the system in the absence of the vector 
potential reduces into discrete time reversal symmetry in the presence of the 
vector potential. Due to light-matter interaction, a superradiant ground 
state emerges with spontaneously broken time reversal symmetry, accompanied 
by a net photocurrent along the edge, generated by the vector potential of 
the quantum light. The spectral function of the photon field reveals 
polariton continuum excitations above a threshold energy, corresponding to a 
Higgs mode and another low energy collective mode due to the phase 
fluctuations of the ground state. This collective mode is a zero energy 
Goldstone mode that arises from the broken continuous U(1) symmetry in the 
absence of the vector potential and acquires finite a gap in the presence of 
the vector potential. The optical conductivity of the edge electrons is 
calculated using the random phase approximation by taking the fluctuation of 
the order parameter into account. It contains the collective modes as a Drude 
peak with renormalized effective mass, which moves to finite frequencies as 
the symmetry of the system is lowered by the inclusion of the vector 
potential."

Reference: 
Balázs Gulácsi and Balázs Dóra
Collective modes for helical edge states interacting with quantum light
Phys. Rev. B 99, 245137 – Published 20 June 2019
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.99.245137

Best regards,
Andras Palyi