Mario Freamat

Dynamics of the Electro-Optic Response of Blue Bronze

We have studied the charge density wave (CDW) repolarization dynamics in blue bronze by applying square-wave voltages of different frequencies to the sample and measuring the changes in infrared transmittance, proportional to CDW strain. The frequency dependence of the electro-transmittance was fit to a modified harmonic oscillator response and the evolution of the parameters as functions of voltage, position, and temperature are discussed. Resonant frequencies decrease with distance from the current contacts, indicating that the resulting delays are intrinsic to the CDW with the strain effectively flowing from the contact. For a fixed position, the average relaxation time has a voltage dependence given by tau_0~V^-p, with 1<p<2. The temperature dependence of the fitting parameters shows that the dynamics are governed by both the force on the CDW and the CDW current: for a given force and position, both the relaxation and delay times are inversely proportional to the CDW current as temperature is varied. The long relaxation and delay times (~ 1 ms) suggest that the strain response involves the motion of macroscopic objects, presumably CDW phase dislocation lines.

Temperature and field dependence of the energy gap of MgB 2 / Pb planar junctions

We have constructed MgB 2 /Pb planar junctions for both temperature and field dependence studies. Our results show that the small gap is a true bulk property of MgB 2 superconductor, not due to surface effects. The temperature dependence of the energy gap manifests a nearly BCS-like behavior. Analysis of the effect of magnetic field on junctions suggests that the energy gap of MgB 2 depends nonlinearly on the magnetic field. Moreover, MgB 2 has an upper critical field of 15 T, in agreement with some reported H c 2 from transport measurements.

Order parameter oscillations in Fe/Ag/Bi2Sr2CaCu2O8+δ tunnel junctions

We have performed temperature-dependent tunneling conductance spectroscopy on Fe/Ag/Bi 2 Sr 2 CaCu 2 O 8 + δ (BSCCO) planar junctions. The multilayered Fe counterelectrode was designed to probe the proximity region of the abplane of BSCCO. The spectra manifested a coherent oscillatory behavior with magnitude and sign dependent on the energy, decaying with increasing distance from the junction barrier, in conjunction with the theoretical predictions involving d-wave superconductors coupled with ferromagnets. The conductance oscillates in antiphase at E=0 and E= ′Δ. The zero-bias conductance peak behavior is explained by a spin-dependent energy shift and the presence of a broken time-reversal pairing symmetry, which do not depend on the geometrical characteristics of the ferromagnetic film.

Andreev bound states in normal and ferromagnet/high-Tc superconducting tunnel junctions

Ag/BSCCO and Fe/Ag/BSCCO planar tunnel junctions were constructed in order to study experimentally the effect of an exchange potential on the spin polarized current transported through Andreev bound states appearing at the interface with a superconductor with broken time reversal pairing symmetry. The zero bias conductance peak (ZBCP) resulting from the Andreev bound states (ABS) is split into two symmetric peaks shifted at finite energies when the counterelectrode is normal. Four asymmetric peaks are observed when the ferromagnetic spin polarized charge reservoir is added, due to the combined effect of a spin filtering exchange energy in the barrier, which is a spin dependent phenomenon, and the spin independent effect of a broken time reversal symmetry (BTRS). The polarization in the iron layer leads to asymmetry. Due to the shift of ABS peaks to finite energies, the conductance at zero energy behaves as predicted by recent theoretical developments for pure d-wave junctions without Andreev reflections.

Temperature and field dependence of the energy gap of MgB2/Pb planar junctions

We have constructed MgB 2 /Pb planar junctions for both temperature and field dependence studies. Our results show that the small gap is a true bulk property of MgB 2 superconductor, not due to surface effects. The temperature dependence of the energy gap manifests a nearly BCS-like behavior. Analysis of the effect of magnetic field on junctions suggests that the energy gap of MgB 2 depends nonlinearly on the magnetic field. Moreover, MgB 2 has an upper critical field of 15 T, in agreement with some reported H c 2 from transport measurements.

Temperature and field dependence of the energy gap ofMgB2/Pbplanar junctions

We have constructed MgB 2 /Pb planar junctions for both temperature and field dependence studies. Our results show that the small gap is a true bulk property of MgB 2 superconductor, not due to surface effects. The temperature dependence of the energy gap manifests a nearly BCS-like behavior. Analysis of the effect of magnetic field on junctions suggests that the energy gap of MgB 2 depends nonlinearly on the magnetic field. Moreover, MgB 2 has an upper critical field of 15 T, in agreement with some reported H c 2 from transport measurements.