4,601 research outputs found
Superfluid and Fermi liquid phases of Bose-Fermi mixtures in optical lattices
We describe interacting mixtures of ultracold bosonic and fermionic atoms in
harmonically confined optical lattices. For a suitable choice of parameters we
study the emergence of superfluid and Fermi liquid (non-insulating) regions out
of Bose-Mott and Fermi-band insulators, due to finite Boson and Fermion
hopping. We obtain the shell structure for the system and show that angular
momentum can be transferred to the non-insulating regions from
Laguerre-Gaussian beams, which combined with Bragg spectroscopy can reveal all
superfluid and Fermi liquid shells.Comment: 4 pages, 2 figure
Nanoscale Structure and Elasticity of Pillared DNA Nanotubes
We present an atomistic model of pillared DNA nanotubes (DNTs) and their
elastic properties which will facilitate further studies of these nanotubes in
several important nanotechnological and biological applications. In particular,
we introduce a computational design to create an atomistic model of a 6-helix
DNT (6HB) along with its two variants, 6HB flanked symmetrically by two double
helical DNA pillars (6HB+2) and 6HB flanked symmetrically by three double
helical DNA pillars (6HB+3). Analysis of 200 ns all-atom simulation
trajectories in the presence of explicit water and ions shows that these
structures are stable and well behaved in all three geometries. Hydrogen
bonding is well maintained for all variants of 6HB DNTs. We calculate the
persistence length of these nanotubes from their equilibrium bend angle
distributions. The values of persistence length are ~10 {\mu}m, which is 2
orders of magnitude larger than that of dsDNA. We also find a gradual increase
of persistence length with an increasing number of pillars, in quantitative
agreement with previous experimental findings. To have a quantitative
understanding of the stretch modulus of these tubes we carried out
nonequilibrium Steered Molecular Dynamics (SMD). The linear part of the force
extension plot gives stretch modulus in the range of 6500 pN for 6HB without
pillars which increases to 11,000 pN for tubes with three pillars. The values
of the stretch modulus calculated from contour length distributions obtained
from equilibrium MD simulations are similar to those obtained from
nonequilibrium SMD simulations. The addition of pillars makes these DNTs very
rigid.Comment: Published in ACS Nan
Magnonic spin-transfer torque MRAM with low power, high speed, and error-free switching
A new class of spin-transfer torque magnetic random access memory (STT-MRAM)
is discussed, in which writing is achieved using thermally initiated magnonic
current pulses as an alternative to conventional electric current pulses. The
magnonic pulses are used to destabilize the magnetic free layer from its
initial direction, and are followed immediately by a bipolar electric current
exerting conventional spin-transfer torque on the free layer. The combination
of thermal and electric currents greatly reduces switching errors, and
simultaneously reduces the electric switching current density by more than an
order of magnitude as compared to conventional STT-MRAM. The energy efficiency
of several possible electro-thermal circuit designs have been analyzed
numerically. As compared to STT-MRAM with perpendicular magnetic anisotropy,
magnonic STT-MRAM reduces the overall switching energy by almost 80%.
Furthermore, the lower electric current density allows the use of thicker
tunnel barriers, which should result in higher tunneling magneto-resistance and
improved tunnel barrier reliability. The combination of lower power, improved
reliability, higher integration density, and larger read margin make magnonic
STT-MRAM a promising choice for future non-volatile storage.Comment: 9 Pages, 11 Figure
Superfluid and Mott Insulating shells of bosons in harmonically confined optical lattices
Weakly interacting atomic or molecular bosons in quantum degenerate regime
and trapped in harmonically confined optical lattices, exhibit a wedding cake
structure consisting of insulating (Mott) shells. It is shown that superfluid
regions emerge between Mott shells as a result of fluctuations due to finite
hopping. It is found that the order parameter equation in the superfluid
regions is not of the Gross-Pitaeviskii type except near the insulator to
superfluid boundaries. The excitation spectra in the Mott and superfluid
regions are obtained, and it is shown that the superfluid shells posses low
energy sound modes with spatially dependent sound velocity described by a local
index of refraction directly related to the local superfluid density. Lastly,
the Berezinskii-Kosterlitz-Thouless transition and vortex-antivortex pairs are
discussed in thin (wide) superfluid shells (rings) limited by three (two)
dimensional Mott regions.Comment: 11 pages, 9 figures
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