Magnetotransport properties of a polarization-doped three-dimensional electron slab

We present evidence of strong Shubnikov-de-Haas magnetoresistance oscillations in a polarization-doped degenerate three-dimensional electron slab in an Al$_{x}$Ga$_{1-x}$N semiconductor system. The degenerate free carriers are generated by a novel technique by grading a polar alloy semiconductor with spatially changing polarization. Analysis of the magnetotransport data enables us to extract an effective mass of $m^{\star}=0.19 m_{0}$ and a quantum scattering time of $\tau_{q}= 0.3 ps$. Analysis of scattering processes helps us extract an alloy scattering parameter for the Al$_{x}$Ga$_{1-x}$N material system to be $V_{0}=1.8eV$

AAV2-mediated gene transfer of GDNF to the striatum of MPTP monkeys enhances the survival and outgrowth of co-implanted fetal dopamine neurons

Neural transplantation offers the potential of treating Parkinson’s disease by grafting fetal dopamine neurons to depleted regions of the brain. However, clinical studies of neural grafting in Parkinson’s disease have produced only modest improvements. One of the main reasons for this is the low survival rate of transplanted neurons. The inadequate supply of critical neurotrophic factors in the adult brain is likely to be a major cause of early cell death and restricted outgrowth of fetal grafts placed into the mature striatum. Glial derived neurotrophic factor (GDNF) is a potent neurotrophic factor that is crucial to the survival, outgrowth and maintenance of dopamine neurons, and so is a candidate for protecting grafted fetal dopamine neurons in the adult brain. We found that implantation of adeno-associated virus type 2 encoding GDNF (AAV2-GDNF) in the normal monkey caudate nucleus induced over-expression of GDNF that persisted for at least 6 months after injection. In a 6-month within-animal controlled study, AAV2-GDNF enhanced the survival of fetal dopamine neurons by 4-fold, and increased the outgrowth of grafted fetal dopamine neurons by almost 3-fold in the caudate nucleus of MPTP-treated monkeys, compared with control grafts in the other caudate nucleus. Thus, the addition of GDNF gene therapy to neural transplantation may be a useful strategy to improve treatment for Parkinson’s disease

ULTRASONIC PHONON VELOCITIES IN Cdl-xMnxTe BETWEEN 1.5 AND 96 K : ANOMALIES NEAR THE MAGNETIC TRANSITION

Transit times of 30 MHz ultrasonic waves have been measured from 1.5 K to 96 K for Cdl-xMnxTe samples with 0.05 ≤ x ≤ 0.65. Below 60 K the velocities for higher concentrations exhibit anomalous features which depend on x and are stronger for shear modes than for longitudinal modes. The shear mode anomalies are flattening of the temperature dependence for x = 0.20 and 0.35 at low temperatures and a wide minimum centered around 22 K for samples with x ≥ 0.55. The location of each minimum indicates that it is connected with a transition to the spin glass or antiferromagnetic state. The depth of the minimum is much greater than the elastic constant depression observed in transition metal alloy spin glasses (TMASG). This may be due to electrons (or holes) in large orbits around shallow-level impurities affecting the Mn magnetic moments

ULTRASONIC PHONONS IN Hg0.8Mn0.2Te : DEPENDENCES OF ELASTIC MODULI ON PRESSURE AND TEMPERATURE

Ultrasonic transit times have been measured for Hg0.8Mn0.2Te, a semimagnetic semiconductor, as a function of pressure up to 4 kbar at 296 K and of temperature from 1.5 to 296 K. They are used to deduce the second order elastic stiffness constants SOEC and also force constants and third order elastic constants at 296 K. The shear modulus Cs = (C11 - C12)/2 is decreased by pressure implying a structural transformation at 10 kbar. No elastic effects attributable to the magnetic moments of the Mn ions are observed even at low temperatures

ULTRASONIC ATTENUATION AND VELOCITY IN (AgI)x (AgPO3)1-x GLASSES

We have measured the attenuation, α, and velocity, v, of ultrasonic waves in (AgI)x(AgPO3)1-x glasses, with x = 0, 0.3, and 0.4, in some range of temperature, T, between 80 K and 360 K. For x = 0.3 and 0.4 there is a very broad, double or lop-sided attenuation peak and velocity inflection. The peak is larger and at lower T for x = 0.4, is at a higher T the higher the frequency, and is higher for shear than for longitudinal waves, indicating that relaxation of Ag ions and/or entities involving Ag ions are responsible. The role of structure in our results is discussed also

ULTRASONIC ATTENUATION AND VELOCITY IN Cd0.55 Mn0.45Te+

We deduce the electromechanical coupling factor, piezoelectric constant (e14), and electrical conductivity of Cd0.55Mn0.45Te from the attenuation maximum and velocity change of piezoelectrically-active [110][001] ultrasonic shear waves which occur as a function of temperature, T. Electromechanical coupling and el4 are much larger in Cd0.55Mn0.45Te than in CdTe due to less bond charge transfer in the former because, we believe, of hybridization of Mn 3d orbitals into the tetrahedral bonds. The dc resistivity was measured and found to be an exponential function of 1/T in accord with the electrical conductivity deduced from our ultrasonic data. The conductivity activation energy we identify with the ionization energy of centers which provide mobile charge carriers (holes)