Browsing by Author "Zhou, Panpan"

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    c-axis pressure-induced antiferromagnetic order in optimally P-doped BaFe2(As0.70P0.30)2 superconductor
    (Springer Nature, 2018) Hu, Ding; Wang, Weiyi; Zhang, Wenliang; Wei, Yuan; Gong, Dongliang; Tam, David W.; Zhou, Panpan; Li, Yu; Tan, Guotai; Song, Yu; Georgii, Robert; Pedersen, Björn; Cao, Huibo; Tian, Wei; Roessli, Bertrand; Yin, Zhiping; Dai, Pengcheng
    Superconductivity in BaFe2(As1−xPx)2 iron pnictides emerges when its in-plane two-dimensional (2D) orthorhombic lattice distortion associated with nematic phase at Ts and three-dimensional (3D) collinear antiferromagnetic order at TN (Ts = TN) are gradually suppressed with increasing x, reaching optimal superconductivity around x = 0.30 with Tc ≈ 30 K. Here we show that a moderate uniaxial pressure along the c-axis in BaFe2(As0.70P0.30)2 spontaneously induces a 3D collinear antiferromagnetic order with TN = Ts > 30 K, while only slightly suppresses Tc. Although a ~ 400 MPa pressure compresses the c-axis lattice while expanding the in-plane lattice and increasing the nearest-neighbor Fe–Fe distance, it barely changes the average iron-pnictogen height in BaFe2(As0.70P0.30)2. Therefore, the pressure-induced antiferromagnetic order must arise from a strong in-plane magnetoelastic coupling, suggesting that the 2D nematic phase is a competing state with superconductivity.
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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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    Shot noise detection in hBN-based tunnel junctions
    (AIP Publishing, 2017) Zhou, Panpan; Hardy, Will J.; Watanabe, Kenji; Taniguchi, Takashi; Natelson, Douglas; Applied Physics Program; Smalley-Curl Institute
    High quality Au/hBN/Au tunnel devices are fabricated using transferred atomically thin hexagonal boron nitride as the tunneling barrier. All tunnel junctions show tunneling resistance on the order of several kΩ/μm2. Ohmic I-V curves at small bias with no signs of resonances indicate the sparsity of defects. Tunneling current shot noise is measured in these devices, and the excess shot noise shows consistency with theoretical expectations. These results show that atomically thin hBN is an excellent tunnel barrier, especially for the study of shot noise properties, and this can enable the study of the tunneling density of states and shot noise spectroscopy in more complex systems.
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    Shot noise measurements in strongly correlated materials
    (2019-12-05) Zhou, Panpan; Natelson, Douglas
    In conventional metals where the interaction between electrons is weak, the low energy excitations can be well explained by Landau Fermi liquid theory. However, in many metallic materials with bulk d or f-electrons, such as transition metal oxides, conventional theories fail to effectively describe the electronic or spin properties due to the presence of strong electron-electron interactions. A better understanding of electronic behavior in strongly correlated systems has been a great challenge in modern physics. In this dissertation, I mainly focused on the studying of quasiparticle's effective charge in strongly correlated material by probing the shot noise--a current fuctuation that originates from the discrete nature of charge carriers. We firstly explored methods for fabricating tunnel junctions and found that hexagonal boron nitride (hBN) is a very good candidate for a tunneling barrier. The tunneling device made by Au/hBN/Au has well-behaved shot noise properties that match with single-particle tunneling predictions quantitatively. Shot noise is also studied in highquality LSCO/LCO/LSCO tunnel structures grown by the molecular beam epitaxy (MBE) technique at various doping levels from underdoped to nearly optimum doped. In those devices, the shot noise is found to be larger than single-electron tunneling prediction deep into the pseudogap region of temperature and bias, indicating pairing might exist in the pseudogap phase.
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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.