Electron spin relaxation in semiconducting carbon nanotubes: the role of hyperfine interaction

A theory of electron spin relaxation in semiconducting carbon nanotubes is developed based on the hyperfine interaction with disordered nuclei spins I=1/2 of $^{13}$C isotopes. It is shown that strong radial confinement of electrons enhances the electron-nuclear overlap and subsequently electron spin relaxation (via the hyperfine interaction) in the carbon nanotubes. The analysis also reveals an unusual temperature dependence of longitudinal (spin-flip) and transversal (dephasing) relaxation times: the relaxation becomes weaker with the increasing temperature as a consequence of the particularities in the electron density of states inherent in one-dimensional structures. Numerical estimations indicate relatively high efficiency of this relaxation mechanism compared to the similar processes in bulk diamond. However, the anticipated spin relaxation time of the order of 1 s in CNTs is still much longer than those found in conventional semiconductor structures.Comment: 11 pages, 2 figure

Collective character of spin excitations in a system of Mn2+^{2+} spins coupled to a two-dimensional electron gas

We have studied the low energy spin excitations in n-type CdMnTe based dilute magnetic semiconductor quantum wells. For magnetic fields for which the energies for the excitation of free carriers and Mn spins are almost identical an anomalously large Knight shift is observed. Our findings suggests the existence of a magnetic field induced ferromagnetic order in these structures, which is in agreement with recent theoretical predictions [J. K{\"o}nig and A. H. MacDonald, submitted Phys. Rev. Lett. (2002)]Comment: 4 figure

Strong reduction of V4+ amount in vanadium oxide/hexadecylamine nanotubes by doping with Co2+ and Ni2+ ions: EPR and magnetic studies

In this work we present a complete characterization and magnetic study of vanadium oxide/hexadecylamine nanotubes (VOx/Hexa NT's) doped with Co2+ and Ni2+ ions. The morphology of the NT's has been characterized by Transmission Electron Microscopy (TEM) while the metallic elements have been quantified by Instrumental Neutron Activation Analysis (INAA) technique. The static and dynamic magnetic properties were studied collecting data of magnetization as a function of magnetic field and temperature and by Electron Paramagnetic Resonance (EPR). We observed that the incorporation of metallic ions (Co2+, S=3/2 and Ni2+, S=1) decrease notably the amount of V4+ ions in the system, from 14-16% (non-doped case) to 2-4%, with respect to the total vanadium atoms into the tubular nanostructure, improving considerably their potential technological applications as Li-ion batteries cathodes.Comment: 24 pages 10 figue

Strongly exchange-coupled triplet pairs in an organic semiconductor

From biological complexes to devices based on organic semiconductors, spin interactions play a key role in the function of molecular systems. For instance, triplet-pair reactions impact operation of organic light-emitting diodes as well as photovoltaic devices. Conventional models for triplet pairs assume they interact only weakly. Here, using electron spin resonance, we observe long-lived, strongly-interacting triplet pairs in an organic semiconductor, generated via singlet fission. Using coherent spin-manipulation of these two-triplet states, we identify exchange-coupled (spin-2) quintet complexes co-existing with weakly coupled (spin-1) triplets. We measure strongly coupled pairs with a lifetime approaching 3 µs and a spin coherence time approaching 1 µs, at 10 K. Our results pave the way for the utilization of high-spin systems in organic semiconductors.Gates-Cambridge Trust, Winton Programme for the Physics of Sustainability, Freie Universität Berlin within the Excellence Initiative of the German Research Foundation, Engineering and Physical Sciences Research Council (Grant ID: EP/G060738/1)This is the author accepted manuscript. The final version is available from Nature Publishing Group at http://dx.doi.org/10.1038/nphys3908