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Magnetic polaron effects in photoconductivity of EuSe
x
CONFERENCE ON PHOTOCONDUCTIVITY
the F centers. Drastic decreases of the photocurrent (up to the two orders of magnitude) are however observed below -100°C if F centers in KC1 : Li are optically converted into the FA (Li) type II centers. Parallel measurements of the F center luminescence show a linear relationship in the decrease of photocurrent and F center luminescence during this F -. F a conversion. These measurements clearly show that the current decrease is caused by a decrease of the electron source (F center) and that the formed FA (Li) center has a lower lying excited state which does not produce a photoelectron in this temperature range. An observed increase of the photoconductivity at higher temperatures is now under careful investigation to clarify if it can be contributed to the onset of thermal ionization of this low-lying excited F~ state. 1.
3-7.
LUTY F., P h y s i c s of Color Centers (Edited by FOWLER W,B.) Chapter 3. Academic Press, New York (1968). MAGNETIC POLARON E F F E C T S IN PHOTOCONDUCTIVITY OF EuSe T. Penney (IBM Watson Research Center, Yorktown Heights, N.Y.)
Giant magneto-conductance effects in magnetic semiconductors, such as Gd-doped EuSe, have been discussed in terms of a magnetic polaron model 1 or a magnetic impurity state. Z We have found that the photoconductivity of pure EuSe shows similar magnetic trapping effects without the presence of impurity ions. In the paramagnetic region, between 77°K and 25°K, the photoconductivity of EuSe follows exp ( - K / k T ) and increases in a magnetic field. The activation energy, E, decreases from about 0.03 eV in zero field to about 0.02 eV in 20 kOe, as found previously for the dark conductivity in 1% Gd doped EuSe. 1 In the antiferromagnetic region below 4.6°K, the photoconductivity is very small. It increases sharply at the transition to ferromagnetic order between 3 and 4 kOe, and rises further with increasing field strength. This behavior is analogous to tile conductivity in Gd-doped samples.
Vol. 7, No. 11
Since these results show that the presence of Gd impurities is apparently not necessary for the occurrence of giant magneto-conductance effects, a spin polaron model is a more general description of magnetic trapping. 1.
VON MOLNAR S. and METHFESSEL S., J. appl. Phys. 38, 959 (1967).
2.
KASUYA T. and YANASE A., Rev. rood. Phys. 40, 684 (1968).
3-8.
EXCITON-PHONON COUPLING E F F E C T S IN THE ABSORPTION SPECTRUM OF AgCt Kerry L. Shaklee (Bell Telephone Laboratories, Incorporated Holmdel, New Jersey) and John E. Rowe (Physics Department, Brown University, Providence, R.I.)
In indirect-bandgap materials exciton absorption threshoIds appear as peaks in the wavelength-derivative absorption (WDA) spectrum. We have observed a number of these peaks in AgC1 at 1.5~K using a double-beam WDA technique. The lowest energy peaks axe due to the formation of 1S-exciton states split by the valley-orbit interaction. Line-shape analysis indicates that some structure at higher energies is due to polaron singularities. Oscillatory structure spaced at approximately the LO phonon energy has also been observed in the WDA spectrum. This additional structure is, attributed to the mobile exciton-phonon bound state of the T o y o z a w a - H e r m a n s o n theory which has previously been observed only in ionic crystals with a direct bandgap such as ZnO. 3-9.
STRONG PHONON INTERACTIONS IN PHOTO-HALL MOBILITY G.C. Smith (Sandia Laboratories, Albuquerque, New Mexico)
Ionic crystals, such as KI, exhibit large anomalous Hall mobilities of both signs at temperatures near those corresponding to the .longitudinal optical lattice modes. Coupled fundamental excitations, part polaron and part phonon, are thermally excited in the vicinity of
CONFERENCE ON PHOTOCONDUCTIVITY
the F centers. Drastic decreases of the photocurrent (up to the two orders of magnitude) are however observed below -100°C if F centers in KC1 : Li are optically converted into the FA (Li) type II centers. Parallel measurements of the F center luminescence show a linear relationship in the decrease of photocurrent and F center luminescence during this F -. F a conversion. These measurements clearly show that the current decrease is caused by a decrease of the electron source (F center) and that the formed FA (Li) center has a lower lying excited state which does not produce a photoelectron in this temperature range. An observed increase of the photoconductivity at higher temperatures is now under careful investigation to clarify if it can be contributed to the onset of thermal ionization of this low-lying excited F~ state. 1.
3-7.
LUTY F., P h y s i c s of Color Centers (Edited by FOWLER W,B.) Chapter 3. Academic Press, New York (1968). MAGNETIC POLARON E F F E C T S IN PHOTOCONDUCTIVITY OF EuSe T. Penney (IBM Watson Research Center, Yorktown Heights, N.Y.)
Giant magneto-conductance effects in magnetic semiconductors, such as Gd-doped EuSe, have been discussed in terms of a magnetic polaron model 1 or a magnetic impurity state. Z We have found that the photoconductivity of pure EuSe shows similar magnetic trapping effects without the presence of impurity ions. In the paramagnetic region, between 77°K and 25°K, the photoconductivity of EuSe follows exp ( - K / k T ) and increases in a magnetic field. The activation energy, E, decreases from about 0.03 eV in zero field to about 0.02 eV in 20 kOe, as found previously for the dark conductivity in 1% Gd doped EuSe. 1 In the antiferromagnetic region below 4.6°K, the photoconductivity is very small. It increases sharply at the transition to ferromagnetic order between 3 and 4 kOe, and rises further with increasing field strength. This behavior is analogous to tile conductivity in Gd-doped samples.
Vol. 7, No. 11
Since these results show that the presence of Gd impurities is apparently not necessary for the occurrence of giant magneto-conductance effects, a spin polaron model is a more general description of magnetic trapping. 1.
VON MOLNAR S. and METHFESSEL S., J. appl. Phys. 38, 959 (1967).
2.
KASUYA T. and YANASE A., Rev. rood. Phys. 40, 684 (1968).
3-8.
EXCITON-PHONON COUPLING E F F E C T S IN THE ABSORPTION SPECTRUM OF AgCt Kerry L. Shaklee (Bell Telephone Laboratories, Incorporated Holmdel, New Jersey) and John E. Rowe (Physics Department, Brown University, Providence, R.I.)
In indirect-bandgap materials exciton absorption threshoIds appear as peaks in the wavelength-derivative absorption (WDA) spectrum. We have observed a number of these peaks in AgC1 at 1.5~K using a double-beam WDA technique. The lowest energy peaks axe due to the formation of 1S-exciton states split by the valley-orbit interaction. Line-shape analysis indicates that some structure at higher energies is due to polaron singularities. Oscillatory structure spaced at approximately the LO phonon energy has also been observed in the WDA spectrum. This additional structure is, attributed to the mobile exciton-phonon bound state of the T o y o z a w a - H e r m a n s o n theory which has previously been observed only in ionic crystals with a direct bandgap such as ZnO. 3-9.
STRONG PHONON INTERACTIONS IN PHOTO-HALL MOBILITY G.C. Smith (Sandia Laboratories, Albuquerque, New Mexico)
Ionic crystals, such as KI, exhibit large anomalous Hall mobilities of both signs at temperatures near those corresponding to the .longitudinal optical lattice modes. Coupled fundamental excitations, part polaron and part phonon, are thermally excited in the vicinity of