Browsing by Author "Chen, Liyang"

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    Percolation and nanosecond fluctuators in V2O3 films within the metal–insulator transition
    (AIP Publishing LLC, 2020) Chen, Liyang; Zhou, Panpan; Kalcheim, Yoav; Schuller, Ivan K.; Natelson, Douglas
    Vanadium sesquioxide (V2O3) exhibits a metal–insulator transition (MIT) at 160 K between a low temperature, monoclinic, antiferromagnetic Mott insulator and a high temperature, rhombohedral, paramagnetic, metallic phase. In thin films, a percolative transition takes place over a finite temperature range of phase coexistence. We study the fluctuating dynamics of this percolative MIT by measuring voltage noise spectra at both low frequencies (up to 100 kHz) and radio frequencies (between 10 MHz and 1 GHz). Noise intensity quadratic in bias is observed in the MIT region, as expected for resistive fluctuations probed nonperturbatively by the current. The low frequency noise resembles flicker-type 1/fβ noise, often taking on the form of Lorentzian noise dominated by a small number of fluctuators as the volume fraction of the insulating phase dominates. Radio frequency noise intensity also quadratic in the bias current allows the identification of resistance fluctuations with lifetimes below 1 ns, approaching timescales seen in non-equilibrium pump–probe studies of the transition. We find quantitative consistency with a model for fluctuations in the percolative fraction. The thermodynamics of the MIT suggests that dominant fluctuations are ones that alter small volumes affecting the connectivity of domain boundaries. This noise serves as a sensitive and nonperturbative probe for the dynamics of switching phenomena in this system.
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    Spin Seebeck effect at low temperatures in the nominally paramagnetic insulating state of vanadium dioxide
    (AIP, 2022) Luo, Renjie; Zhao, Xuanhan; Chen, Liyang; Legvold, Tanner J.; Navarro, Henry; Schuller, Ivan K.; Natelson, Douglas
    The low temperature monoclinic, insulating phase of vanadium dioxide is ordinarily considered nonmagnetic, with dimerized vanadium atoms forming spin singlets, though paramagnetic response is seen at low temperatures. We find a nonlocal spin Seebeck signal in VO2 films that appears below 30 K and that increases with a decrease in temperature. The spin Seebeck response has a nonhysteretic dependence on the in-plane external magnetic field. This paramagnetic spin Seebeck response is discussed in terms of prior findings on paramagnetic spin Seebeck effects and expected magnetic excitations of the monoclinic ground state.
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    Tunneling noise and defects in exfoliated hexagonal boron nitride
    (AIP Publishing LLC, 2019) Zhao, Xuanhan; Zhou, Panpan; Chen, Liyang; Watanabe, Kenji; Taniguchi, Takashi; Natelson, Douglas
    Hexagonal boron nitride (hBN) has become a mainstay as an insulating barrier in stackable nanoelectronics because of its large bandgap and chemical stability. At mono- and bilayer thicknesses, hBN can function as a tunnel barrier for electronic spectroscopy measurements. Noise spectroscopy is of particular interest, as noise can be a sensitive probe for electronic correlations not detectable by first-moment current measurements. In addition to the expected Johnson-Nyquist thermal noise and nonequilibrium shot noise, low frequency (<100 kHz) noise measurements in Au/hBN/Au tunneling structures as a function of temperature and bias reveal the presence of thermally excited dynamic defects, as manifested through a flicker noise contribution at high bias that freezes out as temperature is decreased. In contrast, broad-band high frequency (∼250MHz – 580MHz) measurements on the same device show shot noise with no flicker noise contribution. The presence of the flicker noise through multiple fabrication approaches and processing treatments suggests that the fluctuators are in the hBN layer itself. Device-to-device variation and the approximate 1/f dependence of the flicker noise constrain the fluctuator density to on the order of a few per square micron.
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    Tunneling spectroscopy of c-axis epitaxial cuprate junctions
    (American Physical Society, 2020) Zhou, Panpan; Chen, Liyang; Sochnikov, Ilya; Wu, Tsz Chun; Foster, Matthew S.; Bollinger, Anthony T.; He, Xi; Božović, Ivan; Natelson, Douglas
    Atomically precise epitaxial structures are unique systems for tunneling spectroscopy that minimize extrinsic effects of disorder. We present a systematic tunneling spectroscopy study, over a broad doping, temperature, and bias range, in epitaxial c-axis La2−xSrxCuO4/La2CuO4/La2−xSrxCuO4 heterostructures. The behavior of these superconductor/insulator/superconductor (SIS) devices is unusual. Down to 20 mK there is complete suppression of c-axis Josephson critical current with a barrier of only 2 nm of La2CuO4, and the zero-bias conductance remains at 20–30% of the normal-state conductance, implying a substantial population of in-gap states. Tunneling spectra show greatly suppressed coherence peaks. As the temperature is raised, the superconducting gap fills in rather than closing at Tc. For all doping levels, the spectra show an inelastic tunneling feature at ∼80 meV, suppressed as T exceeds Tc. These nominally simple epitaxial cuprate junctions deviate markedly from expectations based on the standard Bardeen-Cooper-Schrieffer theory.