Microwave measurement of energy gap and penetration depth of MgB2 thin film

Abstract Using a dielectric resonator technique the magnetic field penetration depth λ and the energy gap Δ of two MgB 2 thin films were determined by measuring changes of resonance frequency with temperature. The exponential temperature dependence of λ is revealed at low temperatures ( T T c /4, T c is the critical temperature of the thin film). It indicates that a finite energy gap exists for thermal excitation of quasiparticles. From our measurements values of Δ / kT c =1.25 and λ 0 =290 nm for sample A and Δ / kT c =1.20 and λ 0 =160 nm for sample B give the best fit to the experimental results. The corresponding values for the energy gap Δ are 3.4 meV for sample A and 3.8 meV for sample B. In the high temperature region ( T > T c /2) the temperature dependence of λ can be well fitted by λ 0 /[1−( T / T c ) 2 ] 1/2 . Our results strongly support the absence of nodes in the gap and a possible multigap scenario.