44,839 research outputs found
H-NMR spin-echo measurements of the static and dynamic spin properties in -(BETS)FeCl
H-NMR spin-echo measurements of the spin-echo decay with a
decay rate 1/ and the frequency shift under applied
magnetic field = 9 T along the a-axis over a temperature
range 2.0180 K are reported for a single crystal of the organic conductor
-(BETS)FeCl. It provides the spin dynamic and static
properties in the paramagnetic metal (PM) and antiferromagnetic insulator (AFI)
states as well as across the PMAFI phase transition. A large slow beat
structure in the spin-echo decay is observed with a typical beat frequency of
7 kHz and it varies across the spectrum. Its origin is attributed to
the HH dipole interactions rather than to the much larger
dipolar field contribution from the Fe electrons (spin = 5/2). A
simple phenomenological model provides an excellent fit to the data. The
dominant H-NMR frequency shift comes from the dipolar field from the 3d
Fe ions, and the Fe Fe exchange interactions ()
( includes the dd exchange interactions through the electrons)
have a substantial effect to the local field at the proton sites expecially at
low temperatures. A good fit is obtained with = - 1.7 K. The data of
the spin-echo decay rate 1/ indicates that there is a significant change
in the slow fluctuations of the local magnetic field at the H-sites on
traversing the PM to AFI phase. This evidence supports earlier reports that the
PMAFI phase transition in -(BETS)FeCl is driven
magnetically and first order.Comment: 9 pages, 10 figures, resubmitted to Phys. Rev. B in response to
comments of Editor and reviewers on March 23, 200
Microsecond resolution of quasiparticle tunneling in the single-Cooper-pair-transistor
We present radio-frequency measurements on a single-Cooper-pair-transistor in
which individual quasiparticle poisoning events were observed with microsecond
temporal resolution. Thermal activation of the quasiparticle dynamics is
investigated, and consequently, we are able to determine energetics of the
poisoning and un-poisoning processes. In particular, we are able to assign an
effective quasiparticle temperature to parameterize the poisoning rate.Comment: 4 pages, 4 fig
Strain bursts in plastically deforming Molybdenum micro- and nanopillars
Plastic deformation of micron and sub-micron scale specimens is characterized
by intermittent sequences of large strain bursts (dislocation avalanches) which
are separated by regions of near-elastic loading. In the present investigation
we perform a statistical characterization of strain bursts observed in
stress-controlled compressive deformation of monocrystalline Molybdenum
micropillars. We characterize the bursts in terms of the associated elongation
increments and peak deformation rates, and demonstrate that these quantities
follow power-law distributions that do not depend on specimen orientation or
stress rate. We also investigate the statistics of stress increments in between
the bursts, which are found to be Weibull distributed and exhibit a
characteristic size effect. We discuss our findings in view of observations of
deformation bursts in other materials, such as face-centered cubic and
hexagonal metals.Comment: 14 pages, 8 figures, submitted to Phil Ma
Proton NMR measurements of the local magnetic field in the paramagnetic metal and antiferromagnetic insulator phases of -(BETS)FeCl
Measurements of the H-NMR spectrum of a small ( 4 g) single
crystal of the organic conductor -(BETS)FeCl are reported
with an applied magnetic field = 9 T parallel to the a-axis in
the -plane over a temperature range 2.0 180 K. They provide the
distribution of the static local magnetic field at the proton sites in the
paramagnetic metal (PM) and antiferromagnetic insulator (AFI) phases, along
with the changes that occur at the PMAFI phase transition. The spectra have
six main peaks that are significantly broadened and shifted at low . The
origin of these features is attributed to the large dipolar field from the 3d
Fe ion moments (spin = 5/2). Their amplitude and
dependence are modeled using a modified Brillouin function that includes a
mean field approximation for the total exchange interaction () between
one Fe ion and its two nearest neighbors. A good fit is obtained using
= 1.7 K. At temperatures below the PMAFI transition temperature
= 3.5 K, an extra peak appears on the high frequency side of the
spectrum and the details of the spectrum become smeared. Also, the rms
linewidth and the frequency shift of the spectral distribution are
discontinuous, consistent with the transition being first-order. These
measurements verify that the dominant local magnetic field contribution is from
the Fe ions and indicate that there is a significant change in the
static local magnetic field distribution at the proton sites on traversing the
PM to AFI phase transition.Comment: 11 pages, 7 figures. Revised version of cond-mat/0605044 resubmitted
to Phys. Rev. B in response to comments of Editor and reviewer
Quantum measurement of coherence in coupled quantum dots
We describe the conditional and unconditional dynamics of two coupled quantum
dots when one dot is subjected to a measurement of its occupation number using
a single electron transistor (SET). The measurement is made when the bare
tunneling rate though the SET is changed by the occupation number of one of the
dots. We show that there is a difference between the time scale for the
measurement-induced decoherence between the localized states of the dots and
the time scale on which the system becomes localized due to the measurement. A
comparison between theory and current experiments is made.Comment: 12 pages, 7 figure
Effects of random localizing events on matter waves: formalism and examples
A formalism is introduced to describe a number of physical processes that may
break down the coherence of a matter wave over a characteristic length scale l.
In a second-quantized description, an appropriate master equation for a set of
bosonic "modes" (such as atoms in a lattice, in a tight-binding approximation)
is derived. Two kinds of "localizing processes" are discussed in some detail
and shown to lead to master equations of this general form: spontaneous
emission (more precisely, light scattering), and modulation by external random
potentials. Some of the dynamical consequences of these processes are
considered: in particular, it is shown that they generically lead to a damping
of the motion of the matter-wave currents, and may also cause a "flattening" of
the density distribution of a trapped condensate at rest.Comment: v3; a few corrections, especially in Sections IV and
The Wilson Effective K\"ahler Potential For Supersymmetric Nonlinear Sigma Models
Renormalization group methods are used to determine the evolution of the low
energy Wilson effective action for supersymmetric nonlinear sigma models in
four dimensions. For the case of supersymmetric models, the
K\"ahler potential is determined exactly and is shown to exhibit a nontrivial
ultraviolet fixed point in addition to a trivial infrared fixed point. The
strong coupling behavior of the theory suggests the possible existence of
additional relevant operators or nonperturbative degrees of freedom.Comment: 9 pages, LaTeX, 1 eps figur
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