Mott Insulator versus Bose glass in nanostructured Josephson junction chains

Speaker: 
Timothy Duty, EQuS - ARC Centre for Engineered Quantum Systems and School of Physics, University of New South Wales
Event Date and Time: 
Thu, 2017-02-16 14:00 - 15:00
Location: 
Hennings 318
Local Contact: 
Doug Bonn
Intended Audience: 
Graduate
The low-temperature states of bosonic condensates exhibit fundamental quantum
effects at the macroscopic scale such as supercurrents. The combined effects of
interaction and disorder in these can have drastic consequences, leading to the
Mott insulator and Bose-Anderson glass. The latter is thought to describe helium-4
in porous media, cold atoms in disordered optical potentials, disordered magnetic
insulators, and thin superconducting films. The prototypical Bose-Hubbard model
without disorder predicts a BKT quantum phase transition between superfluid and
Mott insulator. Experimental implementation using arrays of Josephson junctions
has been explored, however, the possibility of the insulating glass has not been
considered. The ubiquity of such a phase in one dimensional chains has important
implications for their proposed use as a fundamental current standard, which is
based on synchronisation of a ‘dual’ Josephson effect, envisioned to arise from
coherent tunnelling of flux quanta (quantum phase slips).
We have measured critical voltages for a large number of simple chains of
sub-micron Josephson junctions with significantly varying energy scales. We
observe universal scaling of critical voltage with single-junction Bloch bandwidth.
Our measurements reveal a localisation length exponent that steepens with
Luttinger parameter, arising from precursor quantum fluctuations of the Bose glasssuperfluid
transition. This contrasts with the fixed exponent found for classical
pinning of charge density waves, vortex lattices and disordered spin systems, and
is in excellent agreement with the quantum theory of one-dimensional disordered
bosonic insulators. We thereby demonstrate a unique signature of the charged
Bose glass in insulating Josephson-junction chains. We have also recently
extended our measurements to SQUID chains, finding somewhat unexpected and
tantalising behaviour.
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