T. Penney

Band tails and the insulator-metal transition in the persistent photoconductor Cd1-xMnxTe:In

Abstract Persistent photoconductivity has been demonstrated in the dilute magnetic semiconductor Cd 1-x Mn x Te:In and has been used to elucidate the shape of the band tail. It is further shown that the insulator-metal transition may be approached, in one sample, by controlling the persistent carrier density with illumination.

Diluted magnetic III-V semiconductor structures

Abstract Diluted magnetic III–V semiconductors are a class of new semiconductor alloys containing a large amount of magnetic ions in the host III–V semiconductors. This was made possible by growing the epitaxial films at relatively low temperatures in molecular beam epitaxy, as specifically exemplified by the (In, Mn)As alloy system. This paper reviews electronic and magnetic properties, including a striking cooperative effect of carrier induced magnetism, in both bulk films and heterostructures.

Hall effect in the heavy fermion systems CeCu6 and UBe13

Abstract The Hall effect in both UBe13 and CeCu6 becomes very large at low temperature showing, with the resistivity and specific heat, the transition into the heavy fermion state. The Hall constant of CeCu6 changes sign, on cooling into the coherently scattering regime.

Soft bulk modulus at the configurational phase transition in Sm1−xYxS

Solid solutions of Sm1−xYxS undergo a configurational phase transition with increasing x, similar to the transition in SmS under pressure. We have measured the sound velocities and bulk modulus of Sm1−xYxS and find that the bulk modulus is soft in the gold, mixed valent phase. We consider a simple model for SmS under pressure to illustrate the relevant physical processes involved.

Hole concentration, Hall number, and disorder in compensation doped NdBa2Cu3Oy

Abstract The compensation doped systems Nd (1+x)−c Ca c Ba 2−x Cu 3 O y and Nd 1+x Ba 2−x Cu 3−z Zn z O y are studied to elucidate both the dependence of T c on the Cu O plane hole concentration and the effect of disorder on the planes. Nd substitution for Ba reduces T c and the plane holes. Ca substitution compensates for Nd, raising T c and the plane holes. Zn has a small effect on the hole concentration. More importantly, Zn causes disorder on the Cu O planes, suppressing T c . The choice of these Nd systems is very important, because, contrary to the YBa 2 Cu 3−x Zn x O y based systems, the Hall number can be unambiguously interpreted and is in good agreement with the plane hole concentration derived from chemical analysis and the method of Tokura et al. for dividing the total holes into mobile plane holes and local chain holes.

OBSERVATION OF A COHERENT QUASIPARTICLE BAND IN THE PERIODIC HEAVY FERMION SYSTEM CECU6

Abstract The question addressed here is whether there is structure in the narrow renormalized quasiparticle bands due to coherence in a periodic heavy fermion lattice. In CeCu6 the specific heat indicates that the Fermi temperature, TF, is about 3 K. We find that for a crystal of CeCu6, well below TF, the Hall constant shows changes in behavior at about 400 mK and again at 25 mK. This result is evidence that there is structure in the band on a scale smaller than TF due to the periodicity of the Kondo lattice system.

Photoconductivity Mechanisms in the Magnetic Semiconductor EuSe

In the europium chalcogenides, the 4f spin order is strongly coupled to electronic conduction. The results of static and transient photoconductivity measurements in EuSe show that there is thermally activated conduction in the paramagnetic region which increases with increasing magnetic field. These effects are similar to previous dark conductivity measurements in Gd‐doped EuSe. The transient photoconductivity measurements show that it is the mobility and not the lifetime which is dependent on temperature and magnetic order. The results are explained using a magnetic impurity state model, MIS, in which spin clusters form around crystal defects. The observed distribution of long photodecay times is associated with relaxation between the MIS on a neutral complex defect and charged defect. The thermally activated conduction may be due to either hopping or quasithermal activation to the conduction band.

The Mott Mobility Edge and the Magnetic Polaron

This paper reviews the metal-insulator transition in a magnetic semiconductor with random potential fluctuations. The transition is explained in terms of Mott’s mobility edge in disordered systems and the magnetic polaron.

Surface and bulk valence of Y0.9Yb0.1Al2 and La0.9Yb0.1Al2

Photoemission and susceptibility measurements have been made on alloys of 10% YbAl2 with 90% YAl2 or LaAl2. Similar to YbAl2, Yb in the YAl2 host is intermediate valent in the bulk consistent with a single ion picture. Yb in the LaAl2 host has a valence of 2.0–2.1 in the bulk. In both cases the surface is completely divalent and the surface 4f emission is shifted to higher binding energies relative to the bulk 4f emission.

Specific heat measurements on (YSe)0.8(TmSe)0.2 and (LaSe)0.9(TmSe)0.1 alloys

Abstract Specific heats of the alloys (YSe) 0.8 (TmSe) 0.2 and (LaSe) 0.9 (TmSe) 0.1 have been measured. A low temperature upturn in Y-based alloy is attributed to the appearance of magnetic order. The magnetic contribution increases above 16 K and is attributed to crystalline field effect. A bump, similar to that of TmTe is observed.