Browsing by Author "Zhao, Xuanhan"

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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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    Spin Shot Noise measurement
    (2022-02-23) Zhao, Xuanhan; Natelson, Douglas
    Charge shot noise, the intrinsic fluctuations in the electrical current resulting from the statistics of the arrival of discrete electronic charges, was predicted more than a century ago2 and has been an invaluable tool for examining the mechanism of charge transport and the nature of current-carrying excitations.3-8 A noise in a nonequilibrium angular momentum current analogous to shot noise seen in charge flow is expected, since the carrier of angular momentum, magnon, also has a discrete nature. In this dissertation, I mainly studied the nonequilibrium noise due to fluctuations in angular momentum current. We first explored the possibility of identifying the effect of spin accumulation on charge shot noise in Pt/h-BN/Au tunnel junctions, where a flicker noise is caused by the thermally excited dynamic defects in the h-BN layer itself. The defect density is on the order of a few per square micron. We also detected a spin Seebeck voltage noise in structures designed to measure the longitudinal spin Seebeck effect in yttrium iron garnet (YIG) on gadolinium gallium garnet (GGG). Measurements have shown indications at low temperatures of a contribution to the spin Seebeck voltage noise in the inverse spin Hall detector that is present when the magnetization of the underlying YIG/GGG is oriented transverse to the Pt wire and absent when the magnetization is oriented along the Pt wire.
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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.