75 research outputs found
Superconducting gap and pair breaking in CeRu2 studied by point contacts
The superconducting gap in a CeRu single crystal is investigated by
point contacts. BCS-like behavior of the gap in the temperature range
below TT, where T is the critical temperature, is
established, indicating the presence of a gapless superconductivity region
(between T and T). The pair-breaking effect of paramagnetic
impurities, supposedly Ce ions, is taken into consideration using the
Scalski-Betbeder-Matibet-Weiss approach based on Abrikosov-Gorkov theory. It
allows us to recalculate the superconducting order parameter (in the presence of paramagnetic impurities) and the gap (in
the pure case) for the single crystal and for the previously studied
polycrystalline CeRu. The value 2(0)2 meV, with
2(0)kT3.75, is found in both cases,
indicating that CeRu is a ``moderate'' strong-coupling superconductor.Comment: 4 pages incl. 3 figs., publ. in Fiz. Nizk. Temp.
(http://fnte.ilt.kharkov.ua/list.html
Point-contact study of ReFeAs(1-x)Fx (Re=La, Sm) superconducting films
Point-contact (PC) Andreev-reflection (AR) measurements of the
superconducting gap in iron-oxipnictide ReFeAsO_{1-x}F_x (Re=La, Sm) films have
been carried out. The value of the gap is distributed in the range 2\Delta
\simeq 5-10 meV (for Re=Sm) with a maximum in the distribution around 6 meV.
Temperature dependence of the gap \Delta(T) can be fitted well by BCS curve
giving reduced gap ratio 2\Delta /kT_c^*\simeq 3.5 (here T_c^* is the critical
temperature from the BCS fit). At the same time, an expected second larger gap
feature was difficult to resolve distinctly on the AR spectra making
determination reliability of the second gap detection questionable. Possible
reasons for this and the origin of other features like clear-cut asymmetry in
the AR spectra and current regime in PCs are discussed.Comment: 6 two-column pages, 6 figs., 26 Refs., to be published in
Superconductor Science and Technolog
Triple-gap superconductivity of MgB2 - (La,Sr)MnO3 composite. Which of the gaps is proximity induced?
Interplay of superconductivity and magnetism in a composite prepared of the
ferromagnetic half-metallic La_0.67Sr_0.33MnO (LSMO) nanoparticles and the
conventional s-wave superconductor MgB_2 has been studied. A few principal
effects have been found in bulk samples. With an onset of the MgB_2
superconductivity, a spectacular drop of the sample resistance has been
detected and superconductivity has been observed at temperature up to 20K.
Point-contact (PC) spectroscopy has been used to measure directly the
superconducting energy coupling. For small voltage, an excess current and
doubling of the PC's normal state conductance have been found. Conductance
peaks corresponding to three energy gaps are clearly observed. Two of these
gaps we identified as enhanced \Delta_{\pi} and \Delta_{\sigma} gaps
originating from the MgB_2; the third gap \Delta_{tr} is more than three times
larger than the largest MgB_2 gap. The experimental results provide unambiguous
evidences for a new type of proximity effect which follows the phase coherency
scenario of proximity induced superconductivity. Specifically, at low
temperature, the p-wave spin-triplet condensate with pairing energy \Delta_{tr}
is essentially sustained in LSMO but is incapable to display long-range
supercurrent response because of a phase-disordering state. The proximity
coupling to MgB_2 restores the long-range phase coherency of the triplet
superconducting state, which, in turn, enhances superconducting state of the
MgB_2.Comment: 10 pages, 6 figure
Point contact spectroscopy of hopping transport: effects of a magnetic field
The conductance of a point contact between two hopping insulators is expected
to be dominated by the individual localized states in its vicinity. Here we
study the additional effects due to an external magnetic field. Combined with
the measured conductance, the measured magnetoresistance provides detailed
information on these states (e.g. their localization length, the energy
difference and the hopping distance between them). We also calculate the
statistics of this magnetoresistance, which can be collected by changing the
gate voltage in a single device. Since the conductance is dominated by the
quantum interference of particular mesoscopic structures near the point
contact, it is predicted to exhibit Aharonov-Bohm oscillations, which yield
information on the geometry of these structures. These oscillations also depend
on local spin accumulation and correlations, which can be modified by the
external field. Finally, we also estimate the mesoscopic Hall voltage due to
these structures.Comment: 7 pages, 5 figur
Andreev reflection and order parameter symmetry in heavy-fermion superconductors: the case of CeCoIn
We review the current status of Andreev reflection spectroscopy on the heavy
fermions, mostly focusing on the case of CeCoIn, a heavy-fermion
superconductor with a critical temperature of 2.3 K. This is a well-established
technique to investigate superconducting order parameters via measurements of
the differential conductance from nanoscale metallic junctions. Andreev
reflection is clearly observed in CeCoIn as in other heavy-fermion
superconductors. The measured Andreev signal is highly reduced to the order of
maximum 13% compared to the theoretically predicted value (100%).
Analysis of the conductance spectra using the extended BTK model provides a
qualitative measure for the superconducting order parameter symmetry, which is
determined to be -wave in CeCoIn. A phenomenological model is
proposed employing a Fano interference effect between two conductance channels
in order to explain both the conductance asymmetry and the reduced Andreev
signal. This model appears plausible not only because it provides good fits to
the data but also because it is highly likely that the electrical conduction
occurs via two channels, one into the heavy electron liquid and the other into
the conduction electron continuum. Further experimental and theoretical
investigations will shed new light on the mechanism of how the coherent
heavy-electron liquid emerges out of the Kondo lattice, a prototypical strongly
correlated electron system. Unresolved issues and future directions are also
discussed.Comment: Topical Review published in JPCM (see below), 28 pages, 9 figure
Superconducting transition temperatures and coherence length in non s-wave pairing materials correlated with spin-fluctuation mediated interaction
Following earlier work on electron or hole liquids flowing through assemblies
with magnetic fluctuations, we have recently exposed a marked correlation of
the superconducting temperature Tc, for non s-wave pairing materials, with
coherence length xi and effective mass m*. The very recent study of Abanov et
al. [Europhys. Lett. 54, 488 (2001)] and the prior investigation of Monthoux
and Lonzarich [Phys. Rev. B 59, 14598 (1999)] have each focussed on the concept
of a spin-fluctuation temperature T_sf, which again is intimately related to
Tc. For the d-wave pairing via antiferromagnetic spin fluctuations in the
cuprates, these studies are brought into close contact with our own work, and
the result is that k_B T_sf ~ hbar^2 / m* xi^2. This demonstrates that xi is
also determined by such antiferromagnetic spin-fluctuation mediated pair
interaction. The coherence length in units of the lattice spacing is then
essentially given in the cuprates as the square root of the ratio of two
characteristic energies, namely: the kinetic energy of localization of a charge
carrier of mass m* in a specified magnetic correlation length to the hopping
energy. The quasi-2D ruthenate Sr_2RuO_4, with Tc ~ 1.3 K, has p-wave
spin-triplet pairing and so is also briefly discussed here.Comment: Accepted for publication in Phys. Rev.
Small-Angle Neutron Scattering and Magnetization Study of HoNi2B2C
The superconducting and magnetic properties of HoNi2B2C single crystals are
investigated through transport, magnetometry and small-angle neutron scattering
measurements. In the magnetic phases that enter below the superconducting
critical temperature, the small-angle neutron scattering data uncover networks
of magnetic surfaces. These likely originate from uncompensated moments e.g. at
domain walls pinned to crystallographic grain boundaries. The field and
temperature dependent behaviour appears consistent with the metamagnetic
transitions reported in earlier works.Comment: 11 pages , 4 figures, submitted to Low Temperature Physic
Peculiarities of electron transport and resistive switching in point contacts on TiSe2, TiSeS and CuxTiSe2
TiSe2 has received much attention among the transition metals chalcogenides
because of its thrilling physical properties concerning atypical resistivity
behavior, emerging of charge density wave (CDW) state, induced
superconductivity etc. Here, we report discovery of new feature of TiSe2,
namely, observation of resistive switching in voltage biased point contacts
(PCs) based on TiSe2 and its derivatives doped by S and Cu (TiSeS, CuxTiSe2).
The switching is taking place between a low resistive mainly metallic-type
state and a high resistive semiconducting-type state by applying bias voltage
(usually below 0.5V), while reverse switching takes place by applying voltage
of opposite polarity (usually below 0.5V). The difference in resistance between
these two states can reach up to two orders of magnitude at the room
temperature. The origin of the effect can be attributed to the variation of
stoichiometry in PC core due to drift/displacement of Se/Ti vacancies under
high electric field. Additionally, we demonstrated, that heating takes place in
PC core, which can facilitate the electric field induced effect. At the same
time, we did not found any evidence for CDW spectral features in our PC spectra
for TiSe2. The observed resistive switching allows to propose TiSe2 and their
derivatives as the promising materials, e.g., for non-volatile resistive random
access memory (ReRAM) engineering.Comment: 11 pages, 6 figure
The Superconducting Gap Behavior in the Antiferromagnetic Nickel-Borocarbide Compounds RNi2B2C (R=Dy, Ho, Er, Tm) Studied by Point-Contacts Spectroscopy
An general survey of the superconducting (SC) gap study in the title
compounds by point-contact (PC) spectroscopy is presented. The SC gap was
determined from dV/dI of PCs employing the well-known theory of conductivity
for normal metal-superconductor PCs accounting Andreev reflection. The theory
was modified by including pair-breaking effects considering the presence of
magnetic rare-earth ions. A possible multiband structure of these compounds was
also taken into account. The PC study of the gap in the Er-compound
(TN=6K<Tc=11K) gives =evidence for the presence of two SC gaps. Additionally, a
distinct decrease of both gaps is revealed for R = Er in the antiferromagnetic
(AF) state. For R = Tm (TN=1.5K<Tc=10.5K) a decrease of the SC gap is observed
below 4-5K, while for R = Dy (TN=10.5K>Tc=6.5K) the SC gap has a BCS-like
dependence in the AF state. The SC gap for R = Ho (TN=5.2K<Tc=8.5K) exhibits
below T*=5.6K a single-band BCS-like dependence vanishing above T*, where a
specific magnetic order occurs. The difference in the SC gap behavior in the
title compounds is attributed to different AF ordering.Comment: 4 pages, 4 figs., submitted to LT25; V2. missed author is adde
Phonon structure in I-V characteristic of MgB point-contacts
The search of the phonon structure at the above-gap energies was carried out
for spectra of MgB point contacts with a normal metal.
The two-band model is assumed not only for the gap structure in
-characteristics, but also for phonons in
point-contact spectra, with up to the maximum lattice vibration energy. Since
the current is carried mostly by charges of 3D-band, whereas the strong
electron-phonon interaction occurs in 2D-band, we observe the phonon
peculiarities due to ''proximity'' effect in {\it k}-space, which depends on
the variation of interband coupling through the elastic scattering.Comment: 6 pages, 4 figures, revtex4, reported in International Conference
"Modern Problems in Superconductivity", 9-13 September, Yalta, Ukrain
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