OPTICAL BANDGAP OF HIGH PURITY SINGLE CRYSTAL EuSe IN HIGH MAGNETIC FIELD UP TO 30T

Abstract

The optical bandgap of high purity EuSe single crystals were studied in high magnetic fields up to 30T in a wide temperature range from 2 K to 280 K. The optical bandgap increased monotonically with decreasing temperature down to a temperature around 20 K which is far above the Neel temperature TN=4.6 K, and started to derease at lower temperatures down to 2 K. We found the optical bandgap decreased from 1.969 to 1.862 eV with increasing magnetic field from 0 to 30 T at a fixed temperature T=12 K and from 1.950 to 1.870 eV at T=4.2 K. From these results, we obtained the spin exhange interaction constant of 5d band conduction electrons with 4f local spins, JCdf=61 meV and for donor electrons (activation energy=24 mY) JDdf=52 meV, using the value of JCdf-JCdf= 9 meV previously obtained by transport measurements. We also determined the spin exchange constants for trapped electrons distributed in the midgap. These values decreased with increasing activation energies which reflect the spatial extentions of the wave functions.