Electronic phase diagram of the layered cobalt oxide system, LixCoO2 (0.0 <= x <= 1.0)

Here we report the magnetic properties of the layered cobalt oxide system, LixCoO2, in the whole range of Li composition, 0 <= x <= 1. Based on dc-magnetic susceptibility data, combined with results of 59Co-NMR/NQR observations, the electronic phase diagram of LixCoO2 has been established. As in the related material NaxCoO2, a magnetic critical point is found to exist between x = 0.35 and 0.40, which separates a Pauli-paramagnetic and a Curie-Weiss metals. In the Pauli-paramagnetic regime (x <= 0.35), the antiferromagnetic spin correlations systematically increase with decreasing x. Nevertheless, CoO2, the x = 0 end member is a non-correlated metal in the whole temperature range studied. In the Curie-Weiss regime (x >= 0.40), on the other hand, various phase transitions are observed. For x = 0.40, a susceptibility hump is seen at 30 K, suggesting the onset of static AF order. A magnetic jump, which is likely to be triggered by charge ordering, is clearly observed at Tt = 175 K in samples with x = 0.50 (= 1/2) and 0.67 (= 2/3), while only a tiny kink appears at T = 210 K in the sample with an intermediate Li composition, x = 0.60. Thus, the phase diagram of the LixCoO2 system is complex, and the electronic properties are sensitively influenced by the Li content (x).Comment: 29 pages, 1 table, 9 figure

Electric-field dependent spin diffusion and spin injection into semiconductors

We derive a drift-diffusion equation for spin polarization in semiconductors by consistently taking into account electric-field effects and nondegenerate electron statistics. We identify a high-field diffusive regime which has no analogue in metals. In this regime there are two distinct spin diffusion lengths. Furthermore, spin injection from a ferromagnetic metal into a semiconductor is enhanced by several orders of magnitude and spins can be transported over distances much greater than the low-field spin diffusion length.Comment: 5 pages, 3 eps figure

Relativistic quantum model of confinement and the current quark masses

We consider a relativistic quantum model of confined massive spinning quarks and antiquarks which describes leading Regge trajectories of mesons. The quarks are described by the Dirac equations and the gluon contribution is approximated by the Nambu-Goto straight-line string. The string tension and the current quark masses are the main parameters of the model. Additional parameters are phenomenological constants which approximate nonstring short-range contributions. Comparison of the measured meson masses with the model predictions allows one to determine the current quark masses (in MeV) to be $m_s = 227 \pm 5,~ m_c = 1440 \pm 10,~ m_b = 4715 \pm 20$. The chiral $SU_3$ model[23] makes it possible to estimate from here the $u$- and $d$-quark masses to be $m_u = 6.2 \pm 0.2$~ Mev and $m_d = 11.1 \pm 0.4$ Mev.Comment: 15 pages, LATEX, 2 tables. (submitted to Phys.Rev.D

Electron spin relaxation by nuclei in semiconductor quantum dots

We have studied theoretically the electron spin relaxation in semiconductor quantum dots via interaction with nuclear spins. The relaxation is shown to be determined by three processes: (i) -- the precession of the electron spin in the hyperfine field of the frozen fluctuation of the nuclear spins; (ii) -- the precession of the nuclear spins in the hyperfine field of the electron; and (iii) -- the precession of the nuclear spin in the dipole field of its nuclear neighbors. In external magnetic fields the relaxation of electron spins directed along the magnetic field is suppressed. Electron spins directed transverse to the magnetic field relax completely in a time on the order of the precession period of its spin in the field of the frozen fluctuation of the nuclear spins. Comparison with experiment shows that the hyperfine interaction with nuclei may be the dominant mechanism of electron spin relaxation in quantum dots

Oscillating magnetoresistance in diluted magnetic semiconductor barrier structures

Ballistic spin polarized transport through diluted magnetic semiconductor (DMS) single and double barrier structures is investigated theoretically using a two-component model. The tunneling magnetoresistance (TMR) of the system exhibits oscillating behavior when the magnetic field are varied. An interesting beat pattern in the TMR and spin polarization is found for different NMS/DMS double barrier structures which arises from an interplay between the spin-up and spin-down electron channels which are splitted by the s-d exchange interaction.Comment: 4 pages, 6 figures, submitted to Phys. Rev.

Dynamic Kerr Effect and Spectral Weight Transfer in the Manganites

We perform pump-probe Kerr spectroscopy in the colossally magnetoresistive manganite Pr0.67Ca0.33MnO3. Kerr effects uncover surface magnetic dynamics undetected by established methods based on reflectivity and optical spectral weight transfer. Our findings indicate the connection between spin and charge dynamics in the manganites may be weaker than previously thought. Additionally, important differences between this system and conventional ferromagnetic metals manifest as long-lived, magneto-optical coupling transients, which may be generic to all manganites.Comment: 12 text pages, 4 figure