59 research outputs found
Critical velocity and event horizon in pair-correlated systems with "relativistic" fermionic quasiparticles
The condition for the appearance of event horizon is considered in such
pair-correlated systems (superfluids and superconductors) where the fermionic
quasiparticles obey the "relativistic" equations. In these systems, the Landau
critical velocity of superflow corresponds to the speed of light. In
conventional systems, such as s-wave superconductors, the superflow remains
stable even above the Landau treshold. We showed that in the "relativistic"
systems however the quantum vacuum becomes unstable and the superflow collapses
after the "speed of light" is reached, so that horizon cannot appear. Thus an
equilibrium dissipationless superfluid flow state and the horizon are
incompatible due to quantum effects. This negative result is consistent with
the quantum Hawking radiation from the horizon, which would lead to the
dissipation of the flow.Comment: RevTex file, 4 pages, 1 Figur
Expanding the Repertoire of Natural Product-Inspired Ring Pairs for Molecular Recognition of DNA
A furan amino acid, inspired by the recently discovered proximicin natural products, was incorporated into the scaffold of a DNA-binding hairpin polyamide. While unpaired oligomers of 2,4-disubstituted furan amino acids show poor DNA-binding activity, furan (Fn) carboxamides paired with N-methylpyrrole (Py) and N-methylimidazole (Im) rings demonstrate excellent stabilization of duplex DNA as well as discrimination of noncognate sequences, consistent with function as a Py mimic according to the Py/Im polyamide pairing rules
Effects of Magnetic Order on the Upper Critical Field of UPt
I present a Ginzburg-Landau theory for hexagonal oscillations of the upper
critical field of UPt near . The model is based on a
representation for the superconducting order parameter,
, coupled to an in-plane AFM order parameter,
. Hexagonal anisotropy of arises from the weak in-plane
anisotropy energy of the AFM state and the coupling of the superconducting
order parameter to the staggered field. The model explains the important
features of the observed hexagonal anisotropy [N. Keller, {\it et al.}, Phys.
Rev. Lett. {\bf 73}, 2364 (1994).] including: (i) the small magnitude, (ii)
persistence of the oscillations for , and (iii) the change in
sign of the oscillations for and (the temperature at the
tetracritical point). I also show that there is a low-field crossover
(observable only very near ) below which the oscillations should vanish.Comment: 9 pages in a RevTex (3.0) file plus 2 postscript figures (uuencoded).
Submitted to Physical Review B (December 20, 1994)
Microwave Assisted Synthesis of Py-Im Polyamides
Microwave synthesis was utilized to rapidly build Py-Im polyamides in high yields and purity using Boc-protection chemistry on Kaiser oxime resin. A representative polyamide targeting the 5′-WGWWCW-3′ (W = A or T) subset of the consensus Androgen and Glucocorticoid Response Elements was synthesized in 56% yield after 20 linear steps and HPLC purification. It was confirmed by Mosher amide derivatization of the polyamide that a chiral α-amino acid does not racemize after several additional coupling steps
Quantization of Superflow Circulation and Magnetic Flux with a Tunable Offset
Quantization of superflow-circulation and of magnetic-flux are considered for
systems, such as superfluid He-A and unconventional superconductors, having
nonscalar order parameters. The circulation is shown to be the anholonomy in
the parallel transport of the order parameter. For multiply-connected samples
free of distributed vorticity, circulation and flux are predicted to be
quantized, but generically to nonintegral values that are tunably offset from
integers. This amounts to a version of Aharonov-Bohm physics. Experimental
settings for testing these issues are discussed.Comment: 5 two-column pages, ReVTeX, figure available upon request (to
[email protected]
BCS and generalized BCS superconductivity in relativistic quantum field theory. I. formulation
We investigate the BCS and generalized BCS theories in the relativistic
quantum field theory. We select the gauge freedom as U(1), and introduce a
BCS-type effective attractive interaction. After introducing the Gor'kov
formalism and performing the group theoretical consideration of the mean
fields, we solve the relativistic Gor'kov equation and obtain the Green's
functions in analytical forms. We obtain various types of gap equations.Comment: 31 page
The Nonlinear Meissner Effect in Unconventional Superconductors
We examine the long-wavelength current response in anisotropic
superconductors and show how the field-dependence of the Meissner penetration
length can be used to detect the structure of the order parameter. Nodes in the
excitation gap lead to a nonlinear current-velocity constitutive equation at
low temperatures which is distinct for each symmetry class of the order
parameter. The effective Meissner penetration length is linear in and
exhibits a characteristic anisotropy for fields in the -plane that is
determined by the positions of the nodes in momentum space. The nonlinear
current-velocity relation also leads to an intrinsic magnetic torque for
in-plane fields that are not parallel to a nodal or antinodal direction. The
torque scales as for and has a characteristic angular
dependence. We analyze the effects of thermal excitations, impurity scattering
and geometry on the current response of a superconductor, and
discuss our results in light of recent measurements of the low-temperature
penetration length and in-plane magnetization of single-crystals of
and .Comment: 30 pages, RevTeX file with 16 postscript figures. Submitted to Phys.
Rev.
Infrared Conductivity in Layered -wave Superconductors
We calculate the infrared conductivity of a stack of coupled, two-dimensional
superconducting planes within the Fermi liquid theory of superconductivity. We
include the effects of random scattering processes and show that the presence
of even a small concentration of resonant impurities, in a -wave
superconductor, has an important effect on both the in-plane and -axis
transport properties, which could serve as signatures for -wave pairing.Comment: 18 pages in a RevTex (3.0) file plus 5 postscript figures
(uuencoded). Replaced with minor changes as it will appear in the Physical
Review B {\bf 52} issue 1 Oct. 199
Criticality and Superfluidity in liquid He-4 under Nonequilibrium Conditions
We review a striking array of recent experiments, and their theoretical
interpretations, on the superfluid transition in He in the presence of a
heat flux, . We define and evaluate a new set of critical point exponents.
The statics and dynamics of the superfluid-normal interface are discussed, with
special attention to the role of gravity. If is in the same direction as
gravity, a self-organized state can arise, in which the entire sample has a
uniform reduced temperature, on either the normal or superfluid side of the
transition. Finally, we review recent theory and experiment regarding the heat
capacity at constant . The excitement that surrounds this field arises from
the fact that advanced thermometry and the future availability of a
microgravity experimental platform aboard the International Space Station will
soon open to experimental exploration decades of reduced temperature that were
previously inaccessible.Comment: 16 pages, 9 figures, plus harvard.sty style file for references
Accepted for publication in Colloquia section of Reviews of Modern Physic
Minimal Cooling of Neutron Stars: A New Paradigm
A new classification of neutron star cooling scenarios, involving either
``minimal'' cooling or ``enhanced'' cooling is proposed. The minimal cooling
scenario replaces and extends the so-called standard cooling scenario to
include neutrino emission from the Cooper pair breaking and formation process.
This emission dominates that due to the modified Urca process for temperatures
close to the critical temperature for superfluid pairing. Minimal cooling is
distinguished from enhanced cooling by the absence of neutrino emission from
any direct Urca process, due either to nucleons or to exotica. Within the
minimal cooling scenario, theoretical cooling models can be considered to be a
four parameter family involving the equation of state of dense matter,
superfluid properties of dense matter, the composition of the neutron star
envelope, and the mass of the neutron star. Consequences of minimal cooling are
explored through extensive variations of these parameters. Results are compared
with the inferred properties of thermally-emitting neutron stars in order to
ascertain if enhanced cooling occurs in any of them. All stars for which
thermal emissions have been clearly detected are at least marginally consistent
with the lack of enhanced cooling. The two pulsars PSR 0833-45 (Vela) and PSR
1706-44 would require enhanced cooling in case their ages and/or temperatures
are on the lower side of their estimated values whereas the four stars PSR
0656+14, PSR 1055-52, Geminga, and RX J0720.4-3125 may require some source of
internal heating in case their age and/or luminosity are on the upper side of
their estimated values. The new upper limits on the thermal luminosity of PSR
J0205+6449 and RX J0007.0+7302 are indicative of the occurrence of some
enhanced neutrino emission beyond the minimal scenario.Comment: Version to appear in ApJ Supplements. Minor modifications in text and
discussion of updated data with new figure
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