  A light-beam-assisted temperature-control system operating within a
temperature range from 77 to 180 K is developed for investigating the
absorption of electromagnetic waves by high-temperature superconductors in the
vicinity of superconducting transition with an ESR spectrometer. The advantage
of this system is the feasibility of modulating the temperature of a sample
with a frequency of 80 Hz and amplitude of 0.01 -0.1 K. The rms temperature
instability over a 5-min time interval is within 0.06 K, the temperature
gradient in a sample is 0.01 K/mm for T=90 K, and the system relaxation time is
1 - 10 s. The microwave absorption peaks in the vicinity of the superconducting
transition of an Y-Ba-Cu-O, single crystal were first measured at (the
simultaneous application of magnetic and temperature modulations.
