Magnetic Field Induced Density of States in Superconducting MgB${}_2$: Measurement of Conduction Electron Spin-Susceptibility

The magnetic field dependence of the spin-susceptibility, ${\chi }_s$ was measured in the superconducting state of high purity MgB${}_2$ fine powders below 1.3 T. ${\chi }_s$ was determined from the intensity of the conduction electron spin resonance spectra at 3.8, 9.4, and 35 GHz. At the lowest magnetic fields (0.14 T), a gap opens in the density of states at the Fermi energy and, accordingly, ${\chi }_s(T)$ is small at low temperatures. Fields above 0.2 T (about 15 % of $H_{c2}^c$, the minimum upper critical field), destroy the gap. The field induced ${\chi }_s$ is much larger than expected from current superconductor models of MgB${}_2$.