40 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
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
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
Suppression of ferromagnetism in CeSi_1.81 under temperature and pressure
We have studied the pressure dependence of the magnetization of single
crystalline CeSi_1.81. At ambient pressure ferromagnetism develops below T_C =
9.5 Below ~ 5 K an additional shoulder in low-field hysteresis loops and a
metamagnetic crossover around 4 T suggest the appearance of an additional
magnetic modulation to the ferromagnetic state. The suppression of the magnetic
order in CeSi_1.81 as function of temperature at ambient pressure and as
function of pressure at low temperature are in remarkable qualitative
agreement. The continuous suppression of the ordered moment at p ~ 13.1 kbar
suggests the existence of a ferromagnetic quantum critical point in this
material.Comment: 9 pages, 9 figures, to be published in Physical Review
Born Effective Charges and Infrared Response of LiBC
Calculations of the zone center optical mode frequencies (including LO-TO
splitting), Born effective charges Z for each atom,
dielectric constants and , and the dielectric
response in the infrared, using density functional linear response theory, are
reported. Calculated Raman modes are in excellent agreement with experimental
values (170 cm and 1170 cm), while it will require better
experimental data to clarify the infrared active mode frequencies. The Born
effective charges Z (i) have surprisingly different values
for B and C, and (ii) show considerable anisotropy. Relationships between the
effective charges and LO-TO splitting are discussed, and the predicted
reflectivity in the range 0 -- 1400 cm is presented. These results hold
possible implications for Li removal in LiBC, and C substition for B in
MgB.Comment: 6 pages, 3 figure
Electronic and Structural Properties of a 4d-Perovskite: Cubic Phase of SrZrO
First-principles density functional calculations are performed within the
local density approximation to study the electronic properties of SrZrO, an
insulating 4d-perovskite, in its high-temperature cubic phase, above 1400 K, as
well as the generic 3d-perovskite SrTiO, which is also a d^0-insulator and
cubic above 105 K, for comparison reasons. The energy bands, density of states
and charge density distributions are obtained and a detailed comparison between
their band structures is presented. The results are discussed also in terms of
the existing data in the literature for both oxides.Comment: 5 pages, 2 figure
Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C in the normal and superconducting state
Point-contact (PC) spectroscopy measurements on antiferromagnetic (AF)
(T_N=5.2K) HoNi2B2C single crystals in the normal and two different
superconducting (SC) states (T_c=8.5K and T_c^*=5.6K<T_c, with 2\Delta/kT_c^*=3.9. The strong coupling
Eliashberg analysis of the low-temperature SC phase with T_c^*=5.6K =T_N,
coexisting with the commensurate AF structure, suggests a sizable value of the
EPI constant \lambda_s=0.93. We also provide strong support for the recently
proposed by us ''Fermi surface (FS) separation'' scenario for the coexistence
of magnetism and superconductivity in magnetic borocarbides, namely, that the
superconductivity in the commensurate AF phase survives at a special (nearly
isotropic) FS sheet without an admixture of Ho 5d states. Above T_c^* the SC
features in the PC characteristics are strongly suppressed pointing to a
specific weakened SC state between T_c* and T_c.Comment: 11 pages, 8 figs, to be published in PRB, Vol.75, Iss.2