Abstract
The temperature dependence of the electron paramagnetic resonance and surface resistance of MgB 2 and MgB 2 doped with lithium is studied. A single narrow symmetric line having an isotropic g value of 2.0036 is observed which is attributed to mobile spin 1/2 holes in the hexagonal boron planes. Both the g value and line width are independent of temperature in the normal state but there is a marked decrease in the intensity of the spectrum as the temperature is lowered to T c. The surface resistance is observed to decrease linearly with temperature in the normal state of the undoped material. In the doped material there is some deviation of the surface resistance from the linear temperature dependence starting at 150 K suggesting the possibility of the existence of a spin gap in the less than optimally doped material. Large changes in surface resistance as a function of dc magnetic field are observed in the superconducting state resulting from dissipation due to vortex movement.
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