The DRIFT Directional Dark Matter Detector and First Studies of the Head-Tail Effect

Measurement of the direction of the elastic nuclear recoil track and ionization charge distribution along it, gives unique possibility for unambiguous detection of the dark matter WIMP particle. Within current radiation detection technologies only Time Projection Chambers filled with low pressure gas are capable of such measurement. Due to the character of the electronic and nuclear stopping powers of low energy nuclear recoils in the gas, an asymmetric ionization charge distribution along their tracks may be expected. Preliminary study of this effect, called Head-Tail, has been carried out here using the SRIM simulation program for Carbon and Sulfur in 40 Torr carbon disulfide, as relevant to the DRIFT detector. Investigations were focused on ion tracks projected onto the axis of the initial direction of motion in the energy range between 10 and 400 keV. Results indicate the likely existence of an asymmetry influenced by two competing effects: the nature of the stopping power and range straggling. The former tends to result in the Tail being greater than the Head and the latter the reverse. It has been found that for projected tracks the mean position of the ionization charge flows from 'head' to 'tail' with the magnitude depending on the ion type and its energy.Comment: To appear in the proceedings of Dark 2007 Sixth International Heidelberg conference on "Dark Matter in Astro & Particle Physics", Sydney, Australia 24th-28th September 200

Origin of bulk uniaxial anisotropy in zinc-blende dilute magnetic semiconductors

It is demonstrated that the nearest neighbor Mn pair on the GaAs (001) surface has a lower energy for the [-110] direction comparing to the [110] case. According to the group theory and the Luttinger's method of invariants, this specific Mn distribution results in bulk uniaxial in-plane and out-of-plane anisotropies. The sign and magnitude of the corresponding anisotropy energies determined by a perturbation method and ab initio computations are consistent with experimental results.Comment: 5 pages, 1 figur

Spin-torque efficiency enhanced by Rashba spin splitting in three dimensions

We examine a spin torque induced by the Rashba spin-orbit coupling in three dimensions within the Boltzmann transport theory. We analytically calculate the spin torque and show how its behavior is related with the spin topology in the Fermi surfaces by studying the Fermi-energy dependence of the spin torque. Moreover we discuss the spin-torque efficiency which is the spin torque divided by the applied electric current in association with the current-induced magnetization reversal. It is found that high spin-torque efficiency is achieved when the Fermi energy lies on only the lower band and there exists an optimal value for the Rashba parameter, where the spin-torque efficiency becomes maximum.Comment: 7 pages, 5 figure

Raising Bi-O bands above the Fermi energy level of hole-doped Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} and other cuprate superconductors

The Fermi surface (FS) of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) predicted by band theory displays Bi-related pockets around the $(\pi,0)$ point, which have never been observed experimentally. We show that when the effects of hole doping either by substituting Pb for Bi or by adding excess O in Bi2212 are included, the Bi-O bands are lifted above the Fermi energy ($E_F$) and the resulting first-principles FS is in remarkable accord with measurements. With decreasing hole-doping the Bi-O bands drop below $E_F$ and the system self-dopes below a critical hole concentration. Computations on other Bi- as well as Tl- and Hg-based compounds indicate that lifting of the cation-derived band with hole doping is a general property of the electronic structures of the cuprates.Comment: 4 pages, 4 figures; PRL (2006, in press

Spin-dependent tunneling in modulated structures of (Ga,Mn)As

A model of coherent tunneling, which combines multi-orbital tight-binding approximation with Landauer-B\"uttiker formalism, is developed and applied to all-semiconductor heterostructures containing (Ga,Mn)As ferromagnetic layers. A comparison of theoretical predictions and experimental results on spin-dependent Zener tunneling, tunneling magnetoresistance (TMR), and anisotropic magnetoresistance (TAMR) is presented. The dependence of spin current on carrier density, magnetization orientation, strain, voltage bias, and spacer thickness is examined theoretically in order to optimize device design and performance.Comment: 9 pages, 13 figures, submitted to PR

Kinematics in Kapteyn's Selected Area 76: Orbital Motions Within the Highly Substructured Anticenter Stream

We have measured the mean three-dimensional kinematics of stars in Kapteyn's Selected Area (SA) 76 (l=209.3, b=26.4 degrees) that were selected to be Anticenter Stream (ACS) members on the basis of their radial velocities, proper motions, and location in the color-magnitude diagram. From a total of 31 stars ascertained to be ACS members primarily from its main sequence turnoff, a mean ACS radial velocity (derived from spectra obtained with the Hydra multi-object spectrograph on the WIYN 3.5m telescope) of V_helio = 97.0 +/- 2.8 km/s was determined, with an intrinsic velocity dispersion sigma_0 = 12.8 \pm 2.1 km/s. The mean absolute proper motions of these 31 ACS members are mu_alpha cos (delta) = -1.20 +/- 0.34 mas/yr and mu_delta = -0.78 \pm 0.36 mas/yr. At a distance to the ACS of 10 \pm 3 kpc, these measured kinematical quantities produce an orbit that deviates by ~30 degrees from the well-defined swath of stellar overdensity constituting the Anticenter Stream in the western portion of the Sloan Digital Sky Survey footprint. We explore possible explanations for this, and suggest that our data in SA 76 are measuring the motion of a kinematically cold sub-stream among the ACS debris that was likely a fragment of the same infalling structure that created the larger ACS system. The ACS is clearly separated spatially from the majority of claimed Monoceros ring detections in this region of the sky; however, with the data in hand, we are unable to either confirm or rule out an association between the ACS and the poorly-understood Monoceros structure.Comment: Accepted to ApJ. 48 pages, 20 figures, preprint forma