Electronic Raman Scattering of OverdopedTl2Ba2CuO6+δin High Magnetic Fields

The excitations across the superconducting gap ( $2\Delta$-peak in $B_{1g}$-symmetry Raman spectra) in an overdoped ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_{6+\delta }$ single crystal have been studied as a function of temperature and field. The $2\Delta$-peak intensity, which we interpret as the density of the superconducting condensate around the gap antinodes, is a linear function of temperature, but a nonlinear function of the field. The latter is attributed to renormalization of quasiparticle spectra in the vicinity of vortex lines in the mixed state. The $2\Delta$-peak is present above the irreversibility line and yields a conventional temperature dependence of the effective upper critical field.