130 research outputs found
Persistence of phase boundaries between a topological and trivial Z2 insulator
When time reversal symmetry is present there is a sharp distinction between
topological and trivial band insulators which ensures that, as parameters are
varied, these phases are separated by a phase transition at which the bulk gap
closes. Surprisingly we find that even in the absence of time reversal
symmetry, gapless regions originating from the phase boundaries persist.
Moreover the critical line generically opens up to enclose Chern insulating
phases that are thin but of finite extent in the phase diagram. We explain the
topological origin of this effect in terms of quantized charge pumping, showing
in particular that it is robust to the effect of disorder and interactions
Solvable model for a dynamical quantum phase transition from fast to slow scrambling
We propose an extension of the Sachdev-Ye-Kitaev (SYK) model that exhibits a
quantum phase transition from the previously identified non-Fermi liquid fixed
point to a Fermi liquid like state, while still allowing an exact solution in a
suitable large limit. The extended model involves coupling the interacting
-site SYK model to a new set of peripheral sites with only quadratic
hopping terms between them. The conformal fixed point of the SYK model remains
a stable low energy phase below a critical ratio of peripheral sites
that depends on the fermion filling . The scrambling dynamics throughout the
non-Fermi liquid phase is characterized by a universal Lyapunov exponent
in the low temperature limit, however the temperature
scale marking the crossover to the conformal regime vanishes continuously at
the critical point . The residual entropy at , non zero in the
NFL, also vanishes continuously at the critical point. For the
quadratic sites effectively screen the SYK dynamics, leading to a quadratic
fixed point in the low temperature and frequency limit. The interactions have a
perturbative effect in this regime leading to scrambling with Lyapunov exponent
.Comment: 20 pages, 12 figures, added the calculation for Lyapunov exponent
away from the particle-hole symmetric situatio
Strong disorder renormalization group primer and the superfluid-insulator transition
This brief review introduces the method and application of real-space
renormalization group to strongly disordered quantum systems. The focus is on
recent applications of the strong disorder renormalization group to the physics
of disordered-boson systems and the superfluid-insulator transition in one
dimension. The fact that there is also a well understood weak disorder theory
for this problem allows to illustrate what aspects of the physics change at
strong disorder. In particular the strong disorder RG analysis suggests that
the transitions at weak disorder and strong disorder belong to distinct
universality classes, but this question remains under debate and is not fully
resolved to date. Further applications of the strong disorder renormalization
group to higher-dimensional Bose systems and to bosons coupled to dissipation
are also briefly reviewed
The superfluid insulator transition of ultra-cold bosons in disordered 1d traps
We derive an effective quantum Josephson array model for a weakly interacting
one-dimensional condensate that is fragmented into weakly coupled puddles by a
disorder potential. The distribution of coupling constants, obtained from first
principles, indicate that weakly interacting bosons in a disorder potential
undergo a superfluid insulator transition controlled by a strong randomness
fixed point [Phys. Rev. Lett. 93, 150402 (2004)]. We compute renormalization
group flows for concrete realizations of the disorder potential to facilitate
finite size scaling of experimental results and allow comparison to the
behavior dictated by the strong randomness fixed point. The phase diagram of
the system is obtained with corrections to mean-field results.Comment: 10 pages, 6 figures, expanded version including a calculation of a
global phase diagra
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