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).]