Browsing by Author "Abhilash, T. S."

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  • Breakdown of Zero-Energy Quantum Hall State in Graphene in the Light of Current Fluctuations and Shot Noise
    (2018) Laitinen, Antti; Kumar, Manohar; Elo, Teemu; Liu, Ying; Abhilash, T. S.; Hakonen, Pertti J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have investigated the cross-over from Zener tunneling of single charge carriers to avalanche type of bunched electron transport in a suspended graphene Corbino disk in the zeroth Landau level. At low bias, we find a tunneling current that follows the gyrotropic Zener tunneling behavior. At larger bias, we find an avalanche type of transport that sets in at a smaller current the larger the magnetic field is. The low-frequency noise indicates strong bunching of the electrons in the avalanches. On the basis of the measured low-frequency switching noise power, we deduce the characteristic switching rates of the avalanche sequence. The simultaneous microwave shot noise measurement also reveals intrinsic correlations within the avalanche pulses and indicate a decrease in correlations with increasing bias.
  • Broadband lumped-element Josephson parametric amplifier with single-step lithography
    (2019-04-15) Elo, T.; Abhilash, T. S.; Perelshtein, M. R.; Lilja, I.; Korostylev, E. V.; Hakonen, P. J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We present a lumped-element Josephson parametric amplifier (JPA) fabricated using a straightforward e-beam lithography process. Our strongly coupled flux-pumped JPA achieves a gain of 20 dB with a bandwidth of 95 MHz around 5 GHz, while the center frequency is tunable by more than 1 GHz, with the additional possibility for rapid tuning by varying the pump frequency alone. Analytical calculations based on the input-output theory reproduce our measurement results closely.
  • Defects in h-BN tunnel barrier for local electrostatic probing of two dimensional materials
    (2018-09-01) Liu, Ying; Tan, Zhenbing; Kumar, Manohar; Abhilash, T. S.; Liu, Guan Jun; Hakonen, Pertti
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Defects in the hexagonal boron nitride (h-BN) layer can facilitate the tunneling current through thick h-BN tunneling barriers. We have investigated such current-mediating defects as local probes for materials in two dimensional heterostructure stacks. Besides IV characteristics and negative differential conductance, we have characterized the electrical properties of h-BN defects in vertical graphene-h-BN-Cr/Au tunnel junctions in terms of low frequency current noise. Our results indicate a charge sensitivity of 1.5×10-5 e/Hz at 10 Hz, which is equal to good metallic single electron transistors. The noise spectra at low frequency are governed by a few two-level fluctuators. For variations in the electrochemical potential, we achieve a sensitivity of 0.8 μeV/Hz.
  • Dry transfer method for suspended graphene on lift-off-resist: simple ballistic devices with Fabry-Pérot interference
    (2019-04-04) Liu, Ying; Abhilash, T. S.; Laitinen, Antti; Tan, Zhenbing; Liu, Guan-jun; Hakonen, Perth
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We demonstrate a fabrication scheme for clean suspended structures using chemical-vapor-deposition-grown graphene and a dry transfer method on lift-off-resist-coated substrates to facilitate suspended graphene nanoelectronic devices for technological applications. It encompasses the demands for scalable fabrication as well as for ultra-fast response due to weak coupling to environment. The fabricated devices exhibited initially a weak field-effect response with substantial positive (p) doping which transformed into weak negative (n) doping upon current annealing at the temperature of 4 K. With increased annealing current, n-doping gradually decreased while the Dirac peak position approached zero in gate voltage. An ultra-low residual charge density of 9 × 108 cm-2 and a mobility of 1.9 × 105 cm2 V-1 s-1 were observed. Our samples display clear Fabry-Pérot (FP) conductance oscillation which indicates ballistic electron transport. The spacings of the FP oscillations are found to depend on the charge density in a manner that agrees with theoretical modeling based on Klein tunneling of Dirac particles. The ultra-low residual charge, the FP oscillations with density dependent period, and the high mobility prove the excellent quality of our suspended graphene devices. Owing to its simplicity, scalability and robustness, this fabrication scheme enhances possibilities for production of suspended, high-quality, two-dimensional-material structures for novel electronic applications.
  • Topologically-imposed vacancies and mobile solid 3He on carbon nanotube
    (2022-10-05) Todoshchenko, I.; Kamada, M.; Kaikkonen, J. P.; Liao, Y.; Savin, A.; Will, M.; Sergeicheva, E.; Abhilash, T. S.; Kauppinen, E.; Hakonen, P. J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Low dimensional fermionic quantum systems are exceptionally interesting because they reveal distinctive physical phenomena, including among others, topologically protected excitations, edge states, frustration, and fractionalization. Our aim was to confine 3He on a suspended carbon nanotube to form 2-dimensional Fermi-system. Here we report our measurements of the mechanical resonance of the nanotube with adsorbed sub-monolayer down to 10 mK. At intermediate coverages we have observed the famous 1/3 commensurate solid. However, at larger monolayer densities we have observed a quantum phase transition from 1/3 solid to an unknown, soft, and mobile solid phase. We interpret this mobile solid phase as a bosonic commensurate crystal consisting of helium dimers with topologically-induced zero-point vacancies which are delocalized at low temperatures. We thus demonstrate that 3He on a nanotube merges both fermionic and bosonic phenomena, with a quantum phase transition between fermionic solid 1/3 phase and the observed bosonic dimer solid.