172,903 research outputs found

    Giant Tunneling Magnetoresistance, Glassiness, and the Energy Landscape at Nanoscale Cluster Coexistence

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    We present microscopic results on the giant tunneling magnetoresistance that arises from the nanoscale coexistence of ferromagnetic metallic (FMM) and antiferromagnetic insulating (AFI) clusters in a disordered two dimensional electron system with competing double exchange and superexchange interactions. Our Monte Carlo study allows us to map out the different field regimes in magnetotransport and correlate it with the evolution of spatial structures. At coexistence, the isotropic O(3) model shows signs of slow relaxation, and has a high density of low energy metastable states, but no genuine glassiness. However, in the presence of weak magnetic anisotropy, and below a field dependent irreversibility temperature TirrT_{irr}, the response on field cooling (FC) differs distinctly from that on zero field cooling (ZFC). We map out the phase diagram of this `phase coexistence glass', highlight how its response differs from that of a standard spin glass, and compare our results with data on the manganites.Comment: Final published versio

    Can re-entrance be observed in force induced transitions?

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    A large conformational change in the reaction co-ordinate and the role of the solvent in the formation of base-pairing are combined to settle a long standing issue {\it i.e.} prediction of re-entrance in the force induced transition of DNA. A direct way to observe the re-entrance, i.e a strand goes to the closed state from the open state and again to the open state with temperature, appears difficult to be achieved in the laboratory. An experimental protocol (in direct way) in the constant force ensemble is being proposed for the first time that will enable the observation of the re-entrance behavior in the force-temperature plane. Our exact results for small oligonucleotide that forms a hairpin structure provide the evidence that re-entrance can be observed.Comment: 12 pages and 5 figures (RevTex4). Accepted in Europhys Lett. (2009

    Spinning flavour branes and fermion pairing instabilities

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    We consider probe Dp-branes, p=3,5,7, in global AdS_5 x S^5, rotating along an internal direction in the S^5. These are dual to strongly interacting N=4 SYM on S^3 with massless fundamental flavours, in the presence of an R-symmetry chemical potential for flavour fermions. For massless, "AdS-filling" Dp-brane embeddings at zero temperature, we find an infinite set of threshold values of the chemical potential at which instabilities are triggered. The onset of instability is always preceded by metastability of the zero density state. From the onset values of the chemical potential, we infer that unstable directions favour a homogeneous condensate of a bilinear made from fermion harmonics, or Cooper pairing. We confirm this picture both numerically and analytically. The linearized analysis showing the appearance of instabilities involves a charged scalar in global AdS space coupled to a (large) constant background gauge potential. The resulting frequency space correlator of the fermion bilinear at strong coupling displays poles in the upper half plane. In contrast, the correlator at zero coupling exhibits Pauli blocking due to occupation of states below the Fermi level, but no instabilities. The end-point of the strong coupling instability is not visible in our setup.Comment: 44 pages, 10 figures, uses late
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