136,829 research outputs found
Fate of the Bose insulator in the limit of strong localization and low Cooper-pair density in ultrathin films
A Bose insulator composed of a low density of strongly localized Cooper pairs develops at the two-dimensional superconductor to insulator transition (SIT) in a number of thin film systems. Investigations of ultrathin amorphous PbBi films far from the SIT described here provide evidence that the Bose insulator gives way to a second insulating phase with decreasing film thickness. At a critical film thickness dc the magnetoresistance changes sign from positive, as expected for boson transport, to negative, as expected for fermion transport, signs of local Cooper-pair phase coherence effects on transport vanish, and the transport activation energy exhibits a kink. Below dc pairing fluctuation effects remain visible in the high-temperature transport while the activation energy continues to rise. These features show that Cooper pairing persists and suggest that the localized unpaired electron states involved in transport are interspersed among regions of strongly localized Cooper pairs in this strongly localized, low Cooper-pair density phase
Collapse of the Cooper pair phase coherence length at a superconductor to insulator transition
We present investigations of the superconductor to insulator transition (SIT)
of uniform a-Bi films using a technique sensitive to Cooper pair phase
coherence. The films are perforated with a nanohoneycomb array of holes to form
a multiply connected geometry and subjected to a perpendicular magnetic field.
Film magnetoresistances on the superconducting side of the SIT oscillate with a
period dictated by the superconducting flux quantum and the areal hole density.
The oscillations disappear close to the SIT critical point to leave a
monotonically rising magnetoresistance that persists in the insulating phase.
These observations indicate that the Cooper pair phase coherence length, which
is infinite in the superconducting phase, collapses to a value less than the
interhole spacing at this SIT. This behavior is inconsistent with the gradual
reduction of the phase coherence length expected for a bosonic, phase
fluctuation driven SIT. This result starkly contrasts with previous
observations of oscillations persisting in the insulating phase of other films
implying that there must be at least two distinct classes of disorder tuned
SITs
Hyperfine quenching of the metastable states in divalent atoms
Hyperfine quenching rates of the lowest-energy metastable and
states of Mg, Ca, Sr, and Yb atoms are computed. The calculations are carried
out using ab initio relativistic many-body methods. The computed lifetimes may
be useful for designing novel ultra-precise optical clocks and trapping
experiments with the fermionic isotopes. The resulting natural widths of
the clock transition are 0.44 mHz for Mg, 2.2 mHz for
Ca, 7.6 mHz for Sr, 43.5 mHz for Yb, and 38.5 mHz for
Yb. Compared to the bosonic isotopes, the lifetime of the states
in fermionic isotopes is noticeably shortened by the hyperfine quenching but
still remains long enough for trapping experiments.Comment: 10 pages, 1 figure, submitted to Phys. Rev.
Phenomenological analysis of the double pion production in nucleon-nucleon collisions up to 2.2 GeV
With an effective Lagrangian approach, we analyze several NN \to NN\pi\pi
channels by including various resonances with mass up to 1.72 GeV. For the
channels with the pion pair of isospin zero, we confirm the dominance of
N*(1440)\to N\sigma in the near threshold region. At higher energies and for
channels with the final pion pair of isospin one, we find large contributions
from N*(1440)\to \Delta\pi, double-Delta, \Delta(1600) \to N*(1440)\pi,
\Delta(1600) \to \Delta\pi and \Delta(1620) \to \Delta\pi. There are also
sizeable contributions from \Delta \to \Delta\pi, \Delta \to N\pi, N \to
\Delta\pi and nucleon pole at energies close to the threshold. We well
reproduce the total cross sections up to beam energies of 2.2 GeV except for
the pp\to pp\pi^0\pi^0 channel at energies around 1.1 GeV and our results agree
with the existing data of differential cross sections of pp \to pp\pi^+\pi^-,
pp \to nn\pi^+\pi^+ and pp \to pp\pi^0\pi^0 which are measured at CELSIUS and
COSY.Comment: 36 pages, 18 figure
Role of Disorder in Mn:GaAs, Cr:GaAs, and Cr:GaN
We present calculations of magnetic exchange interactions and critical
temperature T_c in Mn:GaAs, Cr:GaAs and Cr:GaN. The local spin density
approximation is combined with a linear-response technique to map the magnetic
energy onto a Heisenberg hamiltonion, but no significant further approximations
are made. Special quasi-random structures in large unit cells are used to
accurately model the disorder. T_c is computed using both a spin-dynamics
approach and the cluster variation method developed for the classical
Heisenberg model.
We show the following: (i) configurational disorder results in large
dispersions in the pairwise exchange interactions; (ii) the disorder strongly
reduces T_c; (iii) clustering in the magnetic atoms, whose tendency is
predicted from total-energy considerations, further reduces T_c. Additionally
the exchange interactions J(R) are found to decay exponentially with distance
R^3 on average; and the mean-field approximation is found to be a very poor
predictor of T_c, particularly when J(R) decays rapidly. Finally the effect of
spin-orbit coupling on T_c is considered. With all these factors taken into
account, T_c is reasonably predicted by the local spin-density approximation in
MnGaAs without the need to invoke compensation by donor impurities.Comment: 10 pages, 3 figure
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