31 research outputs found
Optical conductivity and penetration depth in MgB2
The complex conductivity of a MgB2 film has been investigated in the
frequency range 4 cm^{-1}< nu < 30 cm^{-1} and for temperatures 2.7 K < T <300
K. The overall temperature dependence of both components of the complex
conductivity is reminiscent of BCS-type behavior, although a detailed analysis
reveals a number of discrepancies. No characteristic feature of the isotropic
BCS gap temperature evolution is observed in the conductivity spectra in the
superconducting state. A peak in the temperature dependence of the real part of
the conductivity is detected for frequencies below 9 cm^{-1}. The
superconducting penetration depth follows a T^2 behavior at low temperatures.Comment: 4 pages, 4 figure
Anisotropic conductivity of Nd_{1.85}Ce_{0.15}CuO_{4-\delta} films at submillimeter wavelengths
The anisotropic conductivity of thin NdCeCuO
films was measured in the frequency range 8 cm 40 cm and
for temperatures 4 K K. A tilted sample geometry allowed to extract
both, in-plane and c-axis properties. The in-plane quasiparticle scattering
rate remains unchanged as the sample becomes superconducting. The temperature
dependence of the in-plane conductivity is reasonably well described using the
Born limit for a d-wave superconductor. Below T_{{\rm C}%} the c-axis
dielectric constant changes sign at the screened c-axis plasma
frequency. The temperature dependence of the c-axis conductivity closely
follows the linear in T behavior within the plane.Comment: 4 pages, 4 figure
Experimental evidence of two-band behavior of MgB2
The break-junction tunneling has been systematically investigated in MgB2.
Two types of the break-junction contacts have been exploited on the same
samples, which demonstrated tunnel contact like (SIS) and point contact like
(SnS) behavior. Both of them have shown the existence of the two distinct
energy gaps. We have observed also the peculiarities on the I(V)-
characteristics related to Leggett's collective mode assisted tunneling. -->
Corresponding author address: [email protected]: 14 pages, 6 figures, 1 table; corrected typos and fig
Effect of boron doping in the microwave surface resistance of neutron irradiated melt-textured Y_1.6Ba_2.3Cu_3.3O_7-x samples
We report on the microwave surface resistance of melt-textured
Y_1.6Ba_2.3Cu_3.3O_7-x samples, doped with different amount of B_2O_3 and,
subsequently, irradiated by thermal neutrons at the fluence of 1.476 \times
10^17 cm^-2. The microwave surface resistance has been measured as a function
of temperature and DC magnetic field. The experimental results are
quantitatively discussed in the framework of the Coffey and Clem theory,
properly adapted to take into account the d-wave nature of cuprate
superconductors. By fitting the experimental data at zero DC field, we have
highlighted the effects of the induced defects in the general properties of the
samples, including the intergranular region. The analysis of the results
obtained at high DC fields allowed us to investigate the fluxon dynamics and
deduce the depinning frequency; in particular, we have shown that the addition
of B_2O_3 up to 0.1 wt% increases the effectiveness of the defects to hinder
the fluxon motion induced by the microwave current.Comment: 9 pages, 8 embedded figures, accepted for publication in Physica
Superconducting energy gap in MgB2 film observed by infrared reflectance
Far-infrared reflectance of a MgB2 film has been measured by
Fourier-transform spectroscopy for frequencies 10 cm^{-1}<\nu <4000 cm^{-1}
above and below the superconducting transition. The data provide clear
experimental evidence for the onset of a superconducting gap at 24 cm^{-1} at
T=5 K. On increasing temperature the gap energy increases, contrary to what is
expected in isotropic BCS superconductors. The small zero-temperature gap value
and its unconventional increases on increasing temperature can only be
explained by a highly anisotropic or multiple gap function.Comment: 5 pages, figures include