SP Seminar 6 April @ ICTP, Luigi Stasi Room, Leonardo: Alexander NERSESYAN

[Condensed Matter Section]

JOINT ICTP/SISSA STATISTICAL PHYSICS SEMINAR

Wednesday 6 April - 16:30 hrs.

Luigi Stasi Seminar Room, 1st floor ICTP Leonardo Building


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Alexander NERSESYAN
Andronikashvili Institute of Physics, Condensed Matter Physics, Tbilisi, 
Georgia


Title: Classical Impurities and Boundary Majorana Zero Modes in Quantum 
Chains


_Abstract:_

We study the response of classical impurities in quantum Ising 
chains.The Z2 degeneracy they
entail renders the existence of twodecoupled Majorana modes at zero 
energy an exact property
of a finitesystem at arbitrary values of its bulk parameters. We trace 
theevolution of these modes
across the transition from the disordered phaseto the ordered one and 
analyze the concomitant
qualitative changes of localmagnetic properties of an isolated impurity. 
In the disordered phase,
the two ground states differ only close to the impurity. In this phase 
the local transverse spin
susceptibility follows a Curie law.The criticalresponse of a boundary 
impurity is logarithmically
divergent and maps tothe two-channel Kondo problem, while it saturates 
for critical bulkimpurities,
as well as in the ordered phase. The results for the Isingchain 
translate to the related problem
of a resonant level coupled to a1d p-wave superconductor or a Peierls 
chain, whereby the magnetic
order ismapped to topological order.We find that the topological phase 
alwaysexhibits a continuous
impurity response to local fields as a result of thelevel repulsion of 
local levels from the boundary
Majorana zero mode. Incontrast, the disordered phase generically 
features a discontinuous magnetization
or charging response. This difference constitutes a general 
thermodynamic fingerprint of topological order
in phases with abulk gap.