The ab-plane infrared and visible (3 meV - 3 eV) response of Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+δ</sub> (Bi-2212) thin films (prepared by r.f. sputtering on SrTiO<sub>3</sub>) has been measured between 300 K and 10 K for different doping levels. In the superconducting state, dramatic differences appear between the underdoped and overdoped regimes regarding the electrodynamics of the formation of the superfluid condensate. In the over-doped regime, the superfluid grows up by removing states from energies below 60 meV. This energy is of the order of a few times the superconducting gap. In this respect, overdoped Bi-2212 exhibits a conventional behavior. In the underdoped regime, states extending up to 2 eV contribute to the superfluid. This anomalously large energy scale may be assigned to a change of electronic kinetic energy at the superconducting transition, and is compatible with an electronic pairing mechanism.
A facility for performing time-resolved infrared spectroscopy has been developed at the NSLS, primarily at beamline U12IR. The pulsed IR light from the synchrotron is used to perform pump-probe spectroscopy. We present here a description of the facility and results for the relaxation of photoexcitations in both a semiconductor and superconductor.
Coherent synchrotron radiation from the NSLS VUV ring has been detected and partially characterized. The observations have been performed at the new far infrared beamline U12IR. The coherent radiation is peaked near a wavelength of 7 mm and occurs in short duration bursts. The bursts occur only when the electron beam current (I) exceeds a threshold value (I<SUB>th</SUB>), which itself varies with ring operating conditions. Beyond threshold, the average intensity of the emission is found to increase as (I-I<SUB>th</SUB>)<SUP>2</SUP>. The coherent emission implies micro-bunching of the electron beam due to a longitudinal instability.
Electron synchrotron storage rings, such as the VUV ring at the National Synchrotron Light Source, product short pulses of IR radiation suitable for investigating time-dependent phenomena in a variety of interesting experimental systems. In contrast to other pulsed sources of IR, the synchrotron produces a continuum spectral output over the entire IR (and beyond), though at power levels typically below those obtained from laser systems. The infrared synchrotron radiation source is therefore well-suited as a probe using standard FTIR spectroscopic techniques. Here we describe the pump-probe spectroscopy facility being established at the NSLS and demonstrate the technique by measuring the photocarrier decay in a semiconductor.
The infrared reflection spectra of a YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-(delta</SUB> ) single crystal with a large surface containing the c-axis have been recorded from 4 k to room temperature. Thermal treatment of the sample allowed us to obtain spectra for several T<SUB>c</SUB>s, ranging from 50 to 91.3 K on the same sample. The obtained results along the c-axis show an extra band in the superconducting phase which can be attributed to the formation of Cooper pairs. By decoupling this extra absorption from phonons and unpaired charge carries, we have determined the thermal evolution of the London penetration depth and the concentration of pairs along the c-axis as a function of T<SUB>c</SUB>. The observed effects are compared to the BCS theory.