50 research outputs found
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
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 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
Electronic shell effects and the stability of alkali nanowires
Experimental conductance histograms for Na nanowires are analyzed in detail
and compared to recent theoretical results on the stability of cylindrical and
elliptical nanowires, using the free-electron model. We find a one-to-one
correspondence between the peaks in the histograms and the most stable nanowire
geometries, indicating that several of the commonly observed nanowires have
elliptical cross sections
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
Electron-phonon interaction in transition metal diborides TB_2 (T=Zr, Nb, Ta) studied by point-contact spectroscopy
The electron-phonon interaction (EPI) in transition metal diborides TB_2
(T=Zr, Nb, Ta) is investigated by point-contact (PC) spectroscopy. The PC EPI
functions were recovered and the EPI parameters lambda<0.1 were estimated for
all three compounds. Common and distinctive features between the EPI functions
for those diborides are discussed also in connection with the superconductivity
in MgB_2.Comment: V2: minor changes, Ref.[21] added, publ. in PR
Recommended from our members
Stimulated emission and absorption of photons in magnetic point contacts
Point contacts between high anisotropy ferromagnetic SmCo5 and normal metal Cu are used to achieve a strong spin-population inversion in the contact core. Subjected to microwave irradiation in resonance with the Zeeman splitting in Cu, the inverted spin population relaxes through stimulated spin-flip photon emission, detected as peaks in the point-contact resistance. Resonant spin-flip photon absorption is detected as resistance minima, corresponding to sourcing the photon field energy into the electrical circuit. These results demonstrate fundamental mechanisms that are potentially useful in designing metallic spin-based lasers
Investigation of the superconducting energy gap in the compound LuNi₂B₂C by the method of point contact spectroscopy: two-gap approximation
It is shown that the two-gap approximation is applicable for describing the dV/dI(V) spectra
of LuNi₂B₂C–Ag point contacts in a wide interval of temperatures. The values and the temperature
dependences of the large and the small gaps in the ab plane and in the c direction were estimated
using the generalized BTK model [A. Plecenik, M. Grajacar, S. Benacka P. Seidel, A. Pfuch,
Phys. Rev. B49, 10016 (1994)]] and the equations of [S.I. Beloborodko, Fiz. Nizk. Temp. 29, 868
(2003) [Low Temp. Phys. 29, 650 (2003)]. In the BCS extrapolation the critical temperature of
the small gap is 10 K in the ab plane and 14.5 K in the c direction. The absolute values of the gaps
are ∆₀ab = 2.16 meV and ∆₀с = 1.94 meV. For the large gaps the critical temperature Tc coincides
with the bulk, Tc = 16.8 K, and their absolute values are very close, being about 3 meV in both
orientations. In the c direction the contributions to the conductivity from the small and the large
gaps remain practically identical up to 10–11 K. In the ab plane the contribution from the small
gap is much smaller and decreases rapidly as a temperature rises
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