Skip to main content
eScholarship
Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2018)

Due to the low dimensionality of molecular conductors, they are categorized into a class of materials with strong electron correlation, namely Coulomb repulsion. The pairing mechanism of unconventional superconductors (SCs), which commonly arises in these compounds, is a central and unresolved problem in condensed matter physics. Whereas numerous unconventional SCs exist in the vicinity of magnetic ground states, we investigate whether magnetic ordering or fluctuation is a necessary ingredient of unconventional superconductivity, as well as whether other mechanisms such as the charge degree of freedom can play a role. Moreover, the low dimensionality of molecular conductors also allows us to study a long-sought-after inhomogeneous high field superconducting phase, the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. $\beta"$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$ ($\beta"$-SC) that belongs to the family of molecular conductors, is an ideal candidate for studies of both topics. Nuclear magnetic resonance (NMR) is one of the few possible microscopic experimental probes for studying molecular conductors. In this dissertation, interesting physics in $\beta"$-SC, in the contexts such as charge-fluctuation mediated superconductivity and the fluctuating nature of the FFLO state, is revealed by NMR.

Specifically, our results on $\beta"$-SC are consistent with an unconventional superconducting state with singlet pairing and nodal superconducting gap. Moreover, absent or barely distinguishable evidence of magnetic fluctuations are observed above superconducting transition temperature $T_c$, unlike in most other unconventional SCs, including high $T_c$ cuprate SCs. In the normal state, $\beta"$-SC is a charge-ordered metal revealed by NMR spin lattice relaxation rate $(T_1T)^{-1}$. The charge disproportionation forms a stripe pattern along the $a$ axis, whereas the disproportionation increases monotonically upon cooling to $T_c$. Above $T_c$, $\beta"$-SC exhibits only moderate antiferromagetic correlation from analysis of Korringa ratio $\varkappa\sim2$. Motion of the ethylene end groups is studied by $^2$H NMR above 100 K, and is found to induce an enhanced $(T_1T)^{-1}$ on $^{13}$C through modifying the electron correlation. The response of $(T_1T)^{-1}$ to pressure shows that charge disproportionation is suppressed at high pressures as well as $T_c$ according to previous reports, indicating the possible interplay between charge disproportionation and superconductivity.

In the FFLO state, when the orbital pair breaking effect is suppressed under in-plane magnetic field, we observe an unusual enhancement of $(T_1T)^{-1}$ above the normal state $(T_1T)^{-1}$, at the verge of the FFLO--uniform SC phase boundary. Moreover, we observe an enhancement that is 2.5 times the normal state $(T_1T)^{-1}$ under 1 degree misaligned field at $B=9.25$ T. We conclude that both in-plane and out-of-plane $(T_1T)^{-1}$ enhancements in the FFLO state originate from hyperfine coupling to the electrons. We propose a picture of a fluctuating FFLO state where the enhancement of $(T_1T)^{-1}$ is caused by a fluctuating magnetic field induced by dynamics of short-range modulations of the superconducting order parameter.

    Cover page: NMR Study on a Quarter-Filled Organic Superconductor -- Charge Ordering, Superconductivity, and the FFLO state in a Layered Superconductor
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2013)
    III-V photovoltaics (PVs) have demonstrated the highest power conversion efficiencies for both single- and multi-junction cells. However, expensive epitaxial growth substrates, low precursor utilization rates, long growth times, and large equipment investments restrict applications to concentrated and space photovoltaics (PVs). Here, we demonstrate the first vapor-liquid-solid (VLS) growth of high-quality III-V thin-films on metal foils as a promising platform for large-area terrestrial PVs overcoming the above obstacles. We demonstrate 1-3 μm thick InP thin-films on Mo foils with ultra-large grain size up to 100 μm, which is ~100 times larger than those obtained by conventional growth processes. The films exhibit electron mobilities as high as 500 cm²/V-s and minority carrier lifetimes as long as 2.5 ns. Furthermore, under 1-sun equivalent illumination, photoluminescence efficiency measurements indicate that an open circuit voltage of up to 930 mV can be achieved, only 40 mV lower than measured on a single crystal reference wafer.
      Cover page: A direct thin-film path towards low-cost large-area III-V photovoltaics.
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2015)
      Perovskite photovoltaics offer a compelling combination of extremely low-cost, ease of processing and high device performance. The optoelectronic properties of the prototypical CH3NH3PbI3 can be further adjusted by introducing other extrinsic ions. Specifically, chlorine incorporation has been shown to affect the morphological development of perovksite films, which results in improved optoelectronic characteristics for high efficiency. However, it requires a deep understanding to the role of extrinsic halide, especially in the absence of unpredictable morphological influence during film growth. Here we report an effective strategy to investigate the role of the extrinsic ion in the context of optoelectronic properties, in which the morphological factors that closely correlate to device performance are mostly decoupled. The chlorine incorporation is found to mainly improve the carrier transport across the heterojunction interfaces, rather than within the perovskite crystals. Further optimization according this protocol leads to solar cells achieving power conversion efficiency of 17.91%.
        Cover page: The optoelectronic role of chlorine in CH3NH3PbI3(Cl)-based perovskite solar cells
        Top