Browsing by Author "Rice, W.D."

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    Magnetotransport in type-enriched single-wall carbon nanotube networks
    (American Physical Society, 2018) Wang, X.; Gao, W.; Li, X.; Zhang, Q.; Nanot, S.; Hároz, E.H.; Kono, J.; Rice, W.D.
    Single-wall carbon nanotubes (SWCNTs) exhibit a wide range of physical phenomena depending on their chirality. Nanotube networks typically contain a broad mixture of chiralities, which prevents an in-depth understanding of SWCNT ensemble properties. In particular, electronic-type mixing (the simultaneous presence of semiconductor and metallic nanotubes) in SWCNT networks remains the single largest hurdle to developing a comprehensive view of ensemble nanotube electrical transport, a critical step toward their use in optoelectronics. Here, we systematically study temperature-dependent magnetoconductivity (MC) in networks of highly enriched semiconductor and metal SWCNT films. In the semiconductor-enriched network, we observe two-dimensional variable-range hopping conduction from 5 to 290 K. Low-temperature MC measurements reveal a large, negative MC from which we determine the wave-function localization length and Fermi energy density of states. In contrast, the metal-enriched film exhibits positive MC that increases with decreasing temperature, a behavior attributed to two-dimensional weak localization. Using this model, we determine the details of the carrier phase coherence and fit the temperature-dependent conductivity. These extensive measurements on type-enriched SWCNT networks provide insights that pave the way for the use of SWCNTs in solid-state devices.
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    Observation of Forbidden Exciton Transitions Mediated by Coulomb Interactions in Photoexcited Semiconductor Quantum Wells
    (American Physical Society, 2013) Rice, W.D.; Kono, J.; Zybell, S.; Winnerl, S.; Bhattacharyya, J.; Schneider, H.; Helm, M.; Ewers, B.; Chernikov, A.; Koch, M.; Chatterjee, S.; Khitrova, G.; Gibbs, H.M.; Schneebeli, L.; Breddermann, B.; Kira, M.; Koch, S.W.
    We use terahertz pulses to induce resonant transitions between the eigenstates of optically generated exciton populations in a high-quality semiconductor quantum well sample. Monitoring the excitonic photoluminescence, we observe transient quenching of the 1s exciton emission, which we attribute to the terahertz-induced 1s-to-2p excitation. Simultaneously, a pronounced enhancement of the 2s exciton emission is observed, despite the 1s-to-2s transition being dipole forbidden. A microscopic many-body theory explains the experimental observations as a Coulomb-scattering mixing of the 2s and 2p states, yielding an effective terahertz transition between the 1s and 2s populations.
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    Spin relaxation times of single-wall carbon nanotubes
    (American Physical Society, 2013) Rice, W.D.; Weber, R.T.; Nikolaev, P.; Arepalli, S.; Berka, V.; Tsai, A.-L.; Kono, J.
    We have measured temperature (T )- and power-dependent electron spin resonance in bulk single-wall carbon nanotubes to determine both the spin-lattice and the spin-spin relaxation times, T1 and T2. We observe that T −1 1 increases linearly with T from 4 K to 100 K, whereas T −1 2 decreases by over a factor of two when T is increased from 3 K to 300 K. We interpret the T −1 1 ∝ T trend as spin-lattice relaxation via interaction with conduction electrons (Korringa law) and the decreasing T dependence of T −1 2 as motional narrowing. By analyzing the latter, we find the spin hopping frequency to be 285 GHz. Last, we show that the Dysonian line shape asymmetry follows a three-dimensional variable-range hopping behavior from 3 K to 20 K; from this scaling relation, we extract a localization length of the hopping spins to be ∼100 nm.