24 research outputs found
Comment on "Experimental determination of superconducting parameters for the intermetallic perovskite superconductor MgCNi"
In a recent paper (Phys. Rev. {\bf B 67}, 094502 (2003)) Mao et al.
investigated the bias-dependent conductance of mechanical junctions between
superconducting MgCNi and a sharp W tip. They interpreted their results in
terms of 'single-particle tunneling'. We show it is more likely that current
transport through those junctions is determined by thermal effects due to the
huge normal-state resistivity of MgCNi. Therefore no conclusion can be
drawn about the possible unconventional pairing or strong-coupling
superconductivity in MgCNi.Comment: 2 pages, 1 Fig. Comment on Z. Q. Mao et al. (Phys. Rev. {\bf B 67},
094502 (2003)
Point-contact investigations of challenging superconductors: two-band MgB2, antiferromagnetic HoNi2B2C, heavy fermion UPd2Al3, paramagnetic MgCNi3
An overview on recent efforts in point-contact (PC) spectroscopy of title
superconductors is given. Distinct phonon features and
crystalline-electric-field effects are observed in PC spectra of HoNi2B2C.
Results of study of superconducting (SC) gap and excess current versus
temperature and magnetic field reflecting specific multi-band electronic
structure in MgB2 are presented. The nature of the extremely nonlinear I(V)
curves in the antiferromagnetic (AF) and SC state are elucidated for UPd2Al3
break-junctions and MgCNi3 point contacts.Comment: 2 pages, 4 figs., presented on SCES`04 (26-30 July, Karlsruhe,
Germany), accepted for publicatio
Advances in point-contact spectroscopy: two-band superconductor MgB₂ (Review Article)
Analysis of the point-contact spectroscopy (PCS) data on the new dramatic high-Tc superconductor
magnesium diboride MgB₂ reveals quite different behavior of two disconnected σ and π electronic
bands, deriving from their anisotropy, different dimensionality, and electron—phonon interaction.
PCS allows direct registration of both the superconducting gaps and electron—phonon
interaction spectral function of the two-dimensional σ and three-dimensional π band, establishing
correlation between the gap value and intensity of the high-Tc driving force — the E₂g boron vibration
mode. PCS data on some nonsuperconducting transition-metal diborides are surveyed for comparison
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
Excess current in point contacts on two-band superconductor MgB in magnetic field
Series of characteristic and bias-dependent differential resistance
for point-contacts on the single crystals of two-band superconductor
MgB were measured in magnetic field up to 9 T. We have obtained magnetic
field dependences of the excess current on the curves and analyzed them
using Koshelev and Golubov (Phys. Rev. Lett. {\bf 90}, 177002 (2003))
theoretical results for the mixed state of a dirty two-band superconductor.
Introducing a simple model for the excess current in the point contact in the
mixed state our data can be qualitatively described utilizing theoretical
magnetic filed behavior of the and -band superconducting order
parameters and the corresponding averaged electronic density of states in
MgB.Comment: 4 pages, 3 figs, RevTex4; V2: plain text improved, BTK fit is added
to insets of Fig.
Current-field diagram of magnetic states of a surface spin valve in a point contact with a single ferromagnetic film
We present a study of the influence of an external magnetic field H and an electric current I on the spin-valve
(SV) effect in point contact between a ferromagnetic thin film (F) and a sharp tip of a nonmagnetic metal (N). To
explain our observations, we propose a model of a local surface SV which is formed in such a N/F contact. In
this model, a ferromagnetic cluster at the N/F interface plays the role of the free layer in this SV. This cluster exhibits
a larger coercive field than the bulk of the ferromagnetic film, presumably due to its nanoscale nature. Finally,
we construct a magnetic state diagram of the surface SV as a function of I and H
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
Analysis of nonlinear conductivity of point contacts on the base of FeSe in the normal and superconducting state
Nonlinear conductivity of point contacts (PCs) on the base of FeSe single crystals has been investigated.
Measured dV/dI dependencies demonstrate the prevailing contribution to the PC conductivity caused by the
degraded surface. Superconducting (SC) feature in dV/dI like a sharp zero-bias minimum develops for relatively
low ohmic PCs, where the deep areas of FeSe are involved. Analysis of dV/dI has shown that the origin
of the zero-bias minimum is connected with the Maxwell part of the PC resistance, what masks energy dependent
spectral peculiarities. Even so, we have found the specific features in dV/dI — the sharp side maxima,
which may have connection to the SC gap, since their position follows the BCS temperature dependence.
Exploring the dV/dI spectra of the rare occurrence with Andreev-like structure, the two gaps with Δ = 2.5 and
3.5 meV were identified
Observation of anisotropic effect of antiferromagnetic ordering on the superconducting gap in ErNi₂B₂C
The point-contact spectra of the Andreev reflection dV / dI curves of the superconducting rare-earth nickel borocarbide ErNi₂B₂C (Tc≈11K) have been analyzed in the «one-gap» and «two-gap» approximations using the generalized Blonder–Tinkham–Klapwijk model and the Beloborod'ko model allowing for the pair-breaking effect of magnetic impurities. Experimental and calculated curves have been compared not only in shape, but in magnitude as well, which provide more reliable data for determining the temperature dependence of the energy gap (or superconducting order parameter) Δ(T). The anisotropic effect of antiferromagnetic ordering at TN≈6K on the superconducting gap/order parameter has been determined: as the temperature is lowered, Δ decreases by∼25% in the c-direction and only by∼4% in the ab-plane. It is found that the pair-breaking parameter increases in the vicinity of the magnetic transitions, the increase being more pronounced in the c-direction. The efficiency of the models was tested for providing Δ(T) data for ErNi₂B₂C from Andreev reflection spectra
Josephson effect and Andreev reflection in Ba₁₋xNaxFe₂As₂ (x = 0.25 and 0.35) point contacts
I(V) characteristics and their first derivatives of ScS and ScN-type (S is superconductor, c is constriction, N is
normal metal) point contacts (PCs) based on Ba₁₋xNaxFe₂As₂ (x = 0.25 and 0.35) were studied. ScS-type PCs with
S = Nb,Ta, and Pb show Josephson-like resistively shunted I(V) curves with microwave induced Shapiro steps
which satisfy relation 2eV = ω. The IcRN product (Ic is critical current, RN is normal state PC resistance) in these
PCs is found to be up to 1.2 mV. All this data with the observed dependence of the Ic on the microwave power of
ScS PCs with Pb counterelectrode indicates the presence of the singlet s-wave type pairing in Ba₁₋xNaxFe₂As₂.
From the dV/dI(V) curves of ScN-type PCs demonstrating Andreev-reflection like features, the superconducting gap
∆ ratio 2∆/kBTc = 3.6 ± 1 for the compound with x = 0.35 was evaluated. Analysis of these dV/dI(V) at high biases
V, that is well above ∆, testifies transition to the thermal regime in PCs with a voltage increase