The dominant spin relaxation mechanism in compound organic semiconductors

Despite the recent interest in "organic spintronics", the dominant spin relaxation mechanism of electrons or holes in an organic compound semiconductor has not been conclusively identified. There have been sporadic suggestions that it might be hyperfine interaction caused by background nuclear spins, but no confirmatory evidence to support this has ever been presented. Here, we report the electric-field dependence of the spin diffusion length in an organic spin-valve structure consisting of an Alq3 spacer layer, and argue that this data, as well as available data on the temperature dependence of this length, contradict the notion that hyperfine interactions relax spin. Instead, they suggest that the Elliott-Yafet mechanism, arising from spin-orbit interaction, is more likely the dominant spin relaxing mechanism.Comment: Accepted for publication in Physical Review

Displaced Higgs production in type III seesaw

We point out that the type III seesaw mechanism introducing fermion triplets predicts peculiar Higgs boson signatures of displaced vertices with two b jets and one or two charged particles which can be cleanly identified. In a supersymmetric theory, the scalar partner of the fermion triplet contains a neutral dark matter candidate which is almost degenerate with its charged components. A Higgs boson can be produced together with such a dark matter triplet in the cascade decay chain of a strongly produced squark or gluino. When the next lightest supersymmetric particle (NLSP) is bino/wino-like, there appears a Higgs boson associated with two charged tracks of a charged lepton and a heavy charged scalar at a displacement larger than about 1 mm. The corresponding production cross-section is about 0.5 fb for the squark/gluino mass of 1 TeV. In the case of the stau NLSP, it decays mainly to a Higgs boson and a heavy charged scalar whose decay length is larger than 0.1 mm for the stau NLSP mixing with the left-handed stau smaller than 0.3. As this process can have a large cascade production $\sim 2$ pb for the squark/gluino mass $\sim 1$ TeV, one may be able to probe it at the early stage of the LHC experiment.Comment: 10 pages, 5 figure

Spectroscopic Identification of the Infrared Counterpart to GX5-1

Using CGS4 on UKIRT, we have obtained a 1.95-2.45 micron infrared spectrum of the primary candidate counterpart to the bright Z LMXB GX5-1. IR photometry by Naylor, Charles, & Longmore (1992) and the astrometry of Jonker et al. (2000) had previously identified this star as the most likely counterpart to GX5-1. The spectrum presented here clearly shows Brackett gamma and He lines in emission, for the first time confirming the identity of the counterpart. Similar to our previous spectroscopy of the Z source LMXBs Sco X-1 and Sco X-2 (Bandyopadhyay et al. 1999), the K-band spectrum of GX5-1 shows emission lines only. We briefly discuss the implications of this spectrum for the nature of the Z sources.Comment: accepted for publication as a Letter in MNRA

Are spin junction transistors suitable for signal processing?

A number of spintronic junction transistors, that exploit the spin degree of freedom of an electron in addition to the charge degree of freedom, have been proposed to provide simultaneous non-volatile storage and signal processing functionality. Here, we show that some of these transistors unfortunately may not have sufficient voltage and current gains for signal processing. This is primarily because of a large output ac conductance and poor isolation between input and output. The latter also hinders unidirectional propagation of logic signal from the input of a logic gate to the output. Other versions of these transistors appear to have better gain and isolation, but not better than those of a conventional transistor. Therefore, these devices may not improve state-of-the-art signal processing capability, although they may provide additional functionality by offering non-volatile storage. They may also have niche applications in non-linear circuits