10,782 research outputs found
Topological Transitions for Lattice Bosons in a Magnetic Field
We study the Hall response of the Bose-Hubbard model subjected to a magnetic
field. We show that the Hall conductivity is proportional to the particle
density plus an integer. The phase diagram is intersected by topological
transitions between different integer values. These transitions originate from
points in the phase diagram with effective charge conjugation symmetry, and are
attributed to degeneracies in the many body spectrum which serve as sources for
the Berry curvature. We find that extensive regions in the phase diagram
exhibit a negative Hall conductivity, implying that flux flow is reversed in
these regions - vortices there flow upstream. We discuss experimental
implications of our findings.Comment: 11 pages, 7 figure
Driving induced many-body localization
Subjecting a many-body localized system to a time-periodic drive generically
leads to delocalization and a transition to ergodic behavior if the drive is
sufficiently strong or of sufficiently low frequency. Here we show that a
specific drive can have an opposite effect, taking a static delocalized system
into the many-body localized phase. We demonstrate this effect using a
one-dimensional system of interacting hardcore bosons subject to an oscillating
linear potential. The system is weakly disordered, and is ergodic absent the
driving. The time-periodic linear potential leads to a suppression of the
effective static hopping amplitude, increasing the relative strengths of
disorder and interactions. Using numerical simulations, we find a transition
into the many-body localized phase above a critical driving frequency and in a
range of driving amplitudes. Our findings highlight the potential of driving
schemes exploiting the coherent suppression of tunneling for engineering
long-lived Floquet phases.Comment: 9 pages, 9 figure
Electroweak Absolute, Meta-, and Thermal Stability in Neutrino Mass Models
We analyze the stability of the electroweak vacuum in neutrino mass models
containing right handed neutrinos or fermionic isotriplets. In addition to
considering absolute stability, we place limits on the Yukawa couplings of new
fermions based on metastability and thermal stability in the early Universe.
Our results reveal that the upper limits on the neutrino Yukawa couplings can
change significantly when the top quark mass is allowed to vary within the
experimental range of uncertainty in its determination.Comment: 7 pages, 4 figures, match published versio
Disorder induced transitions in resonantly driven Floquet Topological Insulators
We investigate the effects of disorder in Floquet topological insulators
(FTIs) occurring in semiconductor quantum wells. Such FTIs are induced by
resonantly driving a transition between the valence and conduction band. We
show that when disorder is added, the topological nature of such FTIs persists
as long as there is a mobility gap at the resonant quasi-energy. For strong
enough disorder, this gap closes and all the states become localized as the
system undergoes a transition to a trivial insulator. Interestingly, the
effects of disorder are not necessarily adverse: we show that in the same
quantum well, disorder can also induce a transition from a trivial to a
topological system, thereby establishing a Floquet Topological Anderson
Insulator (FTAI). We identify the conditions on the driving field necessary for
observing such a transition.Comment: 18 pages, 13 figure
Testing quantum superpositions of the gravitational field with Bose-Einstein condensates
We consider the gravity field of a Bose-Einstein condensate in a quantum
superposition. The gravity field then is also in a quantum superposition which
is in principle observable. Hence we have ``quantum gravity'' far away from the
so-called Planck scale
Floquet metal to insulator phase transitions in semiconductor nanowires
We study steady-states of semiconductor nanowires subjected to strong
resonant time-periodic drives. The steady-states arise from the balance between
electron-phonon scattering, electron-hole recombination via photo-emission, and
Auger scattering processes. We show that tuning the strength of the driving
field drives a transition between an electron-hole metal (EHM) phase and a
Floquet insulator (FI) phase. We study the critical point controlling this
transition. The EHM-to-FI transition can be observed by monitoring the presence
of peaks in the density-density response function which are associated with the
Fermi momentum of the EHM phase, and are absent in the FI phase. Our results
may help guide future studies towards inducing novel non-equilibrium phases of
matter by periodic driving.Comment: 10 pages including appendice
Metastable π Junction between an s±-Wave and an s-Wave Superconductor
We examine a contact between a superconductor whose order parameter changes sign across the Brillioun zone, and an ordinary, uniform-sign superconductor. Within a Ginzburg-Landau-type model, we find that if the barrier between the two superconductors is not too high, the frustration of the Josephson coupling between different portions of the Fermi surface across the contact can lead to surprising consequences. These include time-reversal symmetry breaking at the interface and unusual energy-phase relations with multiple local minima. We propose this mechanism as a possible explanation for the half-integer flux quantum transitions in composite niobium-iron pnictide superconducting loops, which were discovered in recent experiments [C.-T. Chen et al., Nature Phys. 6, 260 (2010).]
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