20 research outputs found
Point-contact spectroscopy of the borocarbide superconductor YNi2B2C
Point-contact (PC) spectroscopy measurements on YNi2B2C single crystals in
the normal and superconducting (SC) state (T_c=15.4K) for the main
crystallographic directions are reported. The PC study reveals the
electron-phonon interaction (EPI) function with a dominant maximum around 12meV
and a further weak structure (kink or shallow broad maximum) at higher energy
at about 50meV. Other phonon maxima at 20, 24 and 32meV specified in the phonon
DOS of YNi2B2C by neutron measurements [PRB, V.55, 9058 (1997)] are not
resolved in the PC spectra pointing out to the main role of the low energy
phonon modes in EPI. Directional study of the SC gap results in
\Delta_[100]=1.5meV for the a- direction and \Delta_[001]=2.4meV along the
c-axis which may point to anisotropic and/or multiband behavior. Noteworthy,
the critical temperature T_c in all cases corresponds to that of bulk samples.
The value 2\Delta_[001]/kT_c=3.6 is close to the BCS one of 3.52, and the
temperature dependence \Delta(T) is BCS-like, while for the a-direction
\Delta(T) deviates from mean-field BCS behavior above T_c/2. The directional
variation in \Delta can be attributed to the multiband nature of the SC state
in YNi2B2C predicted 10 years ago (PRL, V.80, 1730 (1998)).Comment: 3 figs, 2 pages, presented on M2S-HTSC Conference, July 9-14, 2006,
Dresde
The superconducting gaps in FeSe studied by soft point-contact Andreev reflection spectroscopy
FeSe single crystals have been studied by soft point-contact
Andreev-reflection spectroscopy. Superconducting gap features in the
differential resistance dV/dI(V) of point contacts such as a characteristic
Andreev-reflection double-minimum structure have been measured versus
temperature and magnetic field. Analyzing dV/dI within the extended two-gap
Blonder-Tinkham-Klapwijk model allows to extract both the temperature and
magnetic field dependence of the superconducting gaps. The temperature
dependence of both gaps is close to the standard BCS behavior. Remarkably, the
magnitude of the double-minimum structure gradually vanishes in magnetic field,
while the minima position only slightly shifts with field indicating a weak
decrease of the superconducting gaps. Analyzing the dV/dI(V) spectra for 25
point contacts results in the averaged gap values = 1.8+/-0.4meV and
=1.0+/-0.2 meV and reduced values 2/kTc=4.2+/-0.9 and
2/kTc=2.3+/-0.5 for the large (L) and small (S) gap, respectively.
Additionally, the small gap contribution was found to be within tens of percent
decreasing with both temperature and magnetic field. No signatures in the dV/dI
spectra were observed testifying a gapless superconductivity or presence of
even smaller gaps.Comment: 8 pages, 4 figs., 3 tables. Shortened version without fig.4 and Table
3 is accepted for publication in Phys. Rev.
Point-contact spectroscopy of the nickel borocarbide superconductor YNi2B2C in the normal and superconducting state
Point-contact (PC) spectroscopy measurements of YNi2B2C single crystals in
the normal and superconducting (SC) state (T_c=15.4K) for the main
crystallographic directions are reported. The PC study reveals the
electron-phonon interaction (EPI) spectral function with dominant phonon
maximum around 12 meV and further weak structures (hump or kink) at higher
energy at about 50 meV. No "soft" modes below 12 meV are resolved in the normal
state. The PC EPI spectra are qualitatively similar for the different
directions. Contrary, directional study of the SC gap results in
\Delta_[100]=1.5 meV for the a direction and \Delta_[001]=2.3 meV along the c
axis; however the critical temperature T_c in PC in all cases is near to that
in the bulk sample. The value 2\Delta_[001]/kT_c=3.6 is close to the BCS value
of 3.52, and the temperature dependence \Delta_[001](T) is BCS-like, while the
for small gap \Delta_[100](T) is below BCS behavior at T>T_c/2 similarly as in
the two-gap superconductor MgB2. It is supposed that the directional variation
\Delta can be attributed to a multiband nature of the SC state in YNi2B2C.Comment: 9 pages, 10 figures, to be published in a special issue of J. Low
Temp. Phys. in honour of Prof. H. von Loehneyse
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
Distribution of the superconducting gap in an YNi2B2C film studied by point contact spectroscopy
The differential resistances of point contacts between a
normal metal and a c axis oriented YNi2B2C film ( = 15.2K) in the
superconducting (SC) state have been investigated. contains clear
"gap" features connected with processes of Andreev reflection at the boundary
between normal metal and superconductor that allow the determination of the SC
gap and its temperature and magnetic field dependence. A distribution
of from 1.5 meV to 2.4 meV is
revealed; however the critical temperature in all cases corresponded to
that of the film. The value 23.66 is close to the BCS
value of 3.52, and the temperature dependence is BCS-like,
irrespective of the actual value. It is supposed that the distribution
of can be attributed to a gap anisotropy or to a multiband nature of
the SC state in YNi2B2C, rather than to the presence of nodes in the gap.Comment: 6 two-column pages, 7 figs; V2: as published, Fig.4 is modifie