1,795 research outputs found
Multiplexable Kinetic Inductance Detectors
We are starting to investigate a novel multiplexable readout method that can be applied to a large class of superconducting pair-breaking detectors. This readout method is completely different from those currently used with STJ and TES detectors, and in principle could deliver large pixel counts, high sensitivity, and Fano-limited spectral resolution. The readout is based on the fact that the kinetic surface inductance L_s of a superconductor is a function of the density of quasiparticles n, even at temperatures far below T_c. An efficient way to measure changes in the kinetic inductance is to monitor the transmission phase of a resonant circuit. By working at microwave frequencies and using thin films, the kinetic inductance can be a significant part of the total inductance L, and the volume of the inductor can be made quite small, on the order of 1 µm^3. As is done with other superconducting detectors, trapping could be used to concentrate the quasiparticles into the small volume of the inductor. However, the most intriguing aspect of the concept is that passive frequency multiplexing could be used to read out ~10^3 detectors with a single HEMT amplifier
Toward one-band superconductivity in MgB2
The two-gap model for superconductivity in MgB2 predicts that interband
impurity scattering should be pair breaking, reducing the critical temperature.
This is perhaps the only prediction of the model that has not been confirmed
experimentally. It was previously shown theoretically that common
substitutional impurities lead to negligible interband scattering - if the
lattice is assumed not to distort. Here we report theoretical results showing
that certain impurities can indeed produce lattice distortions sufficiently
large to create measurable interband scattering. On this basis, we predict that
isoelectronic codoping with Al and Na will provide a decisive test of the
two-gap model.Comment: 4 pages, 2 figures, to appear in Phys. Rev.
Josephson Junctions with a synthetic antiferromagnetic interlayer
We report measurements of the critical current vs. Co thickness in
Nb/Cu/Co/Ru/Co/Cu/Nb Josephson junctions, where the inner Co/Ru/Co trilayer is
a "synthetic antiferromagnet" with the magnetizations of the two Co layers
coupled antiparallel to each other via the 0.6 nm-thick Ru layer. Due to the
antiparallel magnetization alignment, the net intrinsic magnetic flux in the
junction is nearly zero, and such junctions exhibit excellent Fraunhofer
patterns in the critical current vs. applied magnetic field, even with total Co
thicknesses as large as 23 nm. There are no apparent oscillations in the
critical current vs. Co thickness, consistent with theoretical expectations for
this situation. The critical current of the junctions decays over 4 orders of
magnitude as the total Co thickness increases from 3 to 23 nm. These junctions
may serve as useful templates for future explorations of spin-triplet
superconducting correlations, which are predicted to occur in supercon-
ducting/ferromagnetic hybrid systems in the presence of certain types of
magnetic inhomogeneity.Comment: 4 pages, 5 figure
Electronic structure and magnetism in the frustrated antiferromagnet LiCrO2
LiCrO2 is a 2D triangular antiferromagnet, isostructural with the common
battery material LiCoO2 and a well-known Jahn-Teller antiferromagnet NaNiO2. As
opposed to the latter, LiCrO2 exibits antiferromagnetic exchange in Cr planes,
which has been ascribed to direct Cr-Cr d-d overlap. Using LDA and LDA+U first
principles calculations I confirm this conjecture and show that (a) direct d-d
overlap is indeed enhanced compared to isostructural Ni and Cr compounds, (b)
p-d charge transfer gap is also enhanced, thus suppressing the ferromagnetic
superexchange, (c) the calculated magnetic Hamiltonian maps well onto the
nearest neighbors Heisenberg exchange model and (d) interplanar inteaction is
antiferromagnetic.Comment: 5 pages, 4 figure
The challenge of unravelling magnetic properties in LaFeAsO
First principles calculations of magnetic and, to a lesser extent, electronic
properties of the novel LaFeAsO-based superconductors show substantial apparent
controversy, as opposed to most weakly or strongly correlated materials. Not
only do different reports disagree about quantitative values, there is also a
schism in terms of interpreting the basic physics of the magnetic interactions
in this system. In this paper, we present a systematic analysis using four
different first principles methods and show that while there is an unusual
sensitivity to computational details, well-converged full-potential
all-electron results are fully consistent among themselves. What makes results
so sensitive and the system so different from simple local magnetic moments
interacting via basic superexchange mechanisms is the itinerant character of
the calculated magnetic ground state, where very soft magnetic moments and
long-range interactions are characterized by a particular structure in the
reciprocal (as opposed to real) space. Therefore, unravelling the magnetic
interactions in their full richness remains a challenging, but utterly
important task
Accounting for spin fluctuations beyond LSDA in the density functional theory
We present a method to correct the magnetic properties of itinerant systems
in local spin density approximation (LSDA) and we apply it to the
ferromagnetic-paramagnetic transition under pressure in a typical itinerant
system, NiAl. We obtain a scaling of the critical fluctuations as a
function of pressure equivalent to the one obtained within Moryia's theory.
Moreover we show that in this material the role of the bandstructure is crucial
in driving the transition. Finally we calculate the magnetic moment as a
function of pressure, and find that it gives a scaling of the Curie temperature
that is in good agreement with the experiment. The method can be easily
extended to the antiferromagnetic case and applied, for instance, to the
Fe-pnictides in order to correct the LSDA magnetic moment.Comment: 7 pages, 4 figure
Break in the VHE spectrum of PG 1553+113: new upper limit on its redshift?
PG 1553+113 is a known BL Lac object, newly detected in the GeV-TeV energy
range by H.E.S.S and MAGIC. The redshift of this source is unknown and a lower
limit of was recently estimated. The very high energy (VHE) spectrum
of PG 1553+113 is attenuated due to the absorption by the low energy photon
field of the extragalactic background light (EBL). Here we correct the combined
H.E.S.S and MAGIC spectrum of PG 1553+113 for this absorption assuming a
minimum density of the evolving EBL. We use an argument that the intrinsic
photon index cannot be harder than and derive an upper limit on
the redshift of . Moreover, we find that a redshift above
implies a possible break of the intrinsic spectrum at about 200 GeV. Assuming
that such a break is absent, we derive a much stronger upper limit of . Alternatively, this break might be attributed to an additional emission
component in the jet of PG 1553+113. This would be the first evidence for a
second component is detected in the VHE spectrum of a blazar.Comment: revised version submitted to Ap
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