232 research outputs found
Vortex Imaging in the pi-Band of Magnesium Diboride
We report scanning tunneling spectroscopy imaging of the vortex lattice in
single crystalline MgB2. By tunneling parallel to the c-axis, a single
superconducting gap (Delta = 2.2 meV) associated with the pi-band is observed.
The vortices in the pi-band have a large core size compared to estimates based
on Hc2, and show an absence of localized states in the core. Furthermore,
superconductivity between the vortices is rapidly suppressed by an applied
field. These results suggest that superconductivity in the pi-band is, at least
partially, induced by the intrinsically superconducting sigma-band.Comment: 4 pages, 3 figure
Determination of the Coherence Length and the Cooper-Pair Size in Unconventional Superconductors by Tunnelling Spectroscopy
The main purpose of the paper is to discuss a possibility of the
determination of the values of the coherence length and the Cooper-pair size in
unconventional superconductors by using tunnelling spectroscopy. In the mixed
state of type-II superconductors, an applied magnetic field penetrates the
superconductor in the form of vortices which form a regular lattice. In
unconventional superconductors, the inner structure of a vortex core has a
complex structure which is determined by the order parameter of the
superconducting state and by the pairing wavefunction of the Cooper pairs. In
clean superconductors, the spatial variations of the order parameter and the
pairing wavefunction occur over the distances of the order of the coherence
length and the Cooper-pair size, respectively. Therefore, by performing
tunnelling spectroscopy along a line passing through a vortex core, one is
able, in principle, to estimate the values of the coherent length and the
Cooper-pair size.Comment: 13 pages, including 17 figure
Independent determination of the two gaps by directional point-contact spectroscopy in MgB_2 single crystals
Directional point-contact spectroscopy measurements were performed for the
first time in state-of-the-art MgB_2 single crystals. The selective suppression
of the superconductivity in the "pi" band by means of a suitable magnetic field
allowed separating the partial contribution of each band to the total
point-contact conductance. By fitting the partial conductance curves
sigma_sigma(V) and sigma_pi(V), we got an independent determination of the two
gaps, Delta_sigma and Delta_pi, with a strong reduction of the experimental
uncertainty. Their temperature dependence was found to agree well with the
predictions of the two-band models for MgB_2.Comment: 6 pages, 4 eps figures. References added, abstract rewritten, text
slightly changed. Proceedings of the BOROMAG Conference, June 17-19, Genoa,
Ital
Anomalous Flux Flow Resistivity in Two Gap Superconductor MgB_2
The flux flow resistivity associated with purely viscous motion of vortices
in high-quality MgB_2 was measured by microwave surface impedance. Flux flow
resistivity exhibits unusual field dependence with strong enhancement at low
field, which is markedly different to conventional s-wave superconductors. A
crossover field which separates two distinct flux flow regimes having different
flux flow resistivity slopes was clearly observed in H//ab-plane. The unusual
H-dependence indicates that two very differently sized superconducting gaps in
MgB_2 manifest in the vortex dynamics and almost equally contribute to energy
dissipation. The carrier scattering rate in two different bands is also
discussed with the present results, compared to heat capacity and thermal
conductivity results.Comment: 4 pages, 3figure
Nonmagnetic impurity effects in MgB
We study nonmagnetic impurity effects in MgB using the quasiclassical
equations of superconductivity for a weak-coupling two-band model. Parameters
in the model are fixed so as to reproduce experiments on MgB as closely
as possible. The quasiparticle density of states and the specific heat are
calculated for various values of the interband impurity scattering. The density
of states changes gradually from a two-gap structure into the conventional
single-gap structure as the interband scattering increases. It is found that
the excitation threshold is not a monotonic function of the interband
scattering. Calculated results for the specific heat are in good agreements
with experiments on samples after irradiation
Can one extract the electron-phonon-interaction from tunneling data in case of the multigap superconductor MgB?
In the present work we calculate the tunneling density of states (DOS) of
MgB% for different tunneling directions by directly solving the two-band
Eliashberg equations (EE) in the real-axis formulation. This procedure reveals
the fine structures of the DOS due to the optical phonons. Then we show that
the numeric inversion of the standard \emph{single-band} EE (the only available
method), when applied to the \emph{two-band} DOS of MgB, may lead to
wrong estimates of the strength of certain phonon branches (e.g. the )
in the extracted electron-phonon spectral function . The
fine structures produced by the two-band interaction at energies between 20 and
100 meV turn out to be clearly observable only for tunneling along the
planes, when the extracted contains the combination
\textbf{+}, together with a minor \textbf{+} component. Only in this case
it is possible to extract information on the -band contribution to the
spectral functions. For any other tunneling direction, the -band
contribution (which does not determine the superconducting properties of
MgB) is dominant and almost coincides with the whole
for tunneling along the c axis. Our results are compared with recent
experimental tunneling and point-contact data.Comment: 5 pages, 3 figures. Submitted to Phys. Rev. B (Brief Reports
Low-lying Quasiparticle Excitations around a Vortex Core in Quantum Limit
Focusing on a quantum-limit behavior, we study a single vortex in a clean
s-wave type-II superconductor by self-consistently solving the Bogoliubov-de
Gennes equation. The discrete energy levels of the vortex bound states in the
quantum limit is discussed. The vortex core radius shrinks monotonically up to
an atomic-scale length on lowering the temperature T, and the shrinkage stops
to saturate at a lower T. The pair potential, supercurrent, and local density
of states around the vortex exhibit Friedel-like oscillations. The local
density of states has particle-hole asymmetry induced by the vortex. These are
potentially observed directly by STM.Comment: 4 pages, 6 figure
A study of the superconducting gap in RNiBC (R = Y, Lu) single crystals by inelastic light scattering
Superconductivity-induced changes in the electronic Raman scattering response
were observed for the RNiBC (R = Y, Lu) system in different scattering
geometries. In the superconducting state, 2-like peaks were observed in
A, B, and B spectra from single crystals. The peaks in
A and B symmetries are significantly sharper and stronger than
the peak in B symmetry. The temperature dependence of the frequencies of
the 2-like peaks shows typical BCS-type behavior, but the apparent
values of the gap are strongly anisotropic for both systems. In
addition, for both YNiBC and LuNiBC systems, there exists
reproducible scattering strength below the gap which is roughly
linear to the frequency in B and B symmetries. This discovery of
scattering below the gap in non-magnetic borocarbide superconductors, which are
thought to be conventional BCS-type superconductors, is a challenge for current
understanding of superconductivity in this system.Comment: Added text, changed a figure, and added references. Will appear in
Phys. Rev.
Predicting progression of mild cognitive impairment to dementia using neuropsychological data: a supervised learning approach using time windows
Background: Predicting progression from a stage of Mild Cognitive Impairment to dementia is a major pursuit in current research. It is broadly accepted that cognition declines with a continuum between MCI and dementia. As such, cohorts of MCI patients are usually heterogeneous, containing patients at different stages of the neurodegenerative process. This hampers the prognostic task. Nevertheless, when learning prognostic models, most studies use the entire cohort of MCI patients regardless of their disease stages. In this paper, we propose a Time Windows approach to predict conversion to dementia, learning with patients stratified using time windows, thus fine-tuning the prognosis regarding the time to conversion. Methods: In the proposed Time Windows approach, we grouped patients based on the clinical information of whether they converted (converter MCI) or remained MCI (stable MCI) within a specific time window. We tested time windows of 2, 3, 4 and 5 years. We developed a prognostic model for each time window using clinical and neuropsychological data and compared this approach with the commonly used in the literature, where all patients are used to learn the models, named as First Last approach. This enables to move from the traditional question "Will a MCI patient convert to dementia somewhere in the future" to the question "Will a MCI patient convert to dementia in a specific time window". Results: The proposed Time Windows approach outperformed the First Last approach. The results showed that we can predict conversion to dementia as early as 5 years before the event with an AUC of 0.88 in the cross-validation set and 0.76 in an independent validation set. Conclusions: Prognostic models using time windows have higher performance when predicting progression from MCI to dementia, when compared to the prognostic approach commonly used in the literature. Furthermore, the proposed Time Windows approach is more relevant from a clinical point of view, predicting conversion within a temporal interval rather than sometime in the future and allowing clinicians to timely adjust treatments and clinical appointments.FCT under the Neuroclinomics2 project [PTDC/EEI-SII/1937/2014, SFRH/BD/95846/2013]; INESC-ID plurianual [UID/CEC/50021/2013]; LASIGE Research Unit [UID/CEC/00408/2013
Magneto-electrodynamics at high frequencies in the antiferromagnetic and superconducting states of DyNi_2B_2C
We report the observation of novel behaviour in the radio frequency (rf) and
microwave response of DyNi_2B_2C over a wide range of temperature (T) and
magnetic field (H) in the antiferromagnetic (AFM) and superconducting (SC)
states. At microwave frequencies of 10 GHz, the T dependence of the surface
impedance Z_s=R_s+iX_s was measured which yields the T dependence of the
complex conductivity \sigma_1-i\sigma_2 in the SC and AFM states. At radio
frequencies (4 MHz), the H and T dependence of the penetration depth
\lambda(T,H) were measured. The establishment of antiferromagnetic order at
T_N=10.3 K results in a marked decrease in the scattering of charge carriers,
leading to sharp decreases in R_s and X_s. However, R_s and X_s differ from
each other in the AFM state. We show that the results are consistent with
conductivity relaxation whence the scattering rate becomes comparable to the
microwave frequency. The rf measurements yield a rich dependence of the
scattering on the magnetic field near and below T_N. Anomalous decrease of
scattering at moderate applied fields is observed at temperatures near and
above T_N, and arises due to a crossover from a negative magnetoresistance
state, possibly associated with a loss of spin disorder scattering at low
fields, to a positive magnetoresistance state associated with the metallic
nature. The normal state magnetoresistance is positive at all temperatures for
\mu_0H>2T and at all fields for T>15K. Several characteristic field and
temperature scales associated with metamagnetic transitions (H_M1(T), H_M2(T))
and onset of spin disorder H_D(T), in addition to T_c, T_N and H_c2(T) are
observed in the rf measurements.Comment: 9 pages, Latex, Uses REVTeX, This and related publications also
available at http://sagar.physics.neu.edu/ Submitted to Phys. Rev.
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