Browsing by Author "Kumar, Manohar"

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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.
  • Coupling between electrons and optical phonons in suspended bilayer graphene
    (2015) Laitinen, Antti; Kumar, Manohar; Oksanen, Mika; Placais, Bernard; Virtanen, Pauli; Hakonen, Pertti J.
     School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Using electrical transport experiments and shot noise thermometry, we investigate electron-phonon heat transfer rate in a suspended bilayer graphene. Contrary to monolayer graphene with heat flow via three-body supercollision scattering, we find that regular electron–optical-phonon scattering in bilayer graphene provides the dominant scattering process at electron energies ≳0.15 eV. We determine the strength of these intrinsic heat flow processes of bilayer graphene and find good agreement with theoretical estimates when both zone edge and zone center optical phonons are taken into account.
  • Critical current fluctuations in graphene Josephson junctions
    (2021-10-06) Haque, Mohammad T.; Will, Marco; Tomi, Matti; Pandey, Preeti; Kumar, Manohar; Schmidt, Felix; Watanabe, Kenji; Taniguchi, Takashi; Danneau, Romain; Steele, Gary; Hakonen, Pertti
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have studied 1/f noise in critical current Ic in h-BN encapsulated monolayer graphene contacted by NbTiN electrodes. The sample is close to diffusive limit and the switching supercurrent with hysteresis at Dirac point amounts to ≃ 5 nA. The low frequency noise in the superconducting state is measured by tracking the variation in magnitude and phase of a reflection carrier signal vrf at 600–650 MHz. We find 1/f critical current fluctuations on the order of δIc/ Ic≃ 10 - 3 per unit band at 1 Hz. The noise power spectrum of critical current fluctuations SIc measured near the Dirac point at large, sub-critical rf-carrier amplitudes obeys the law SIc/Ic2=a/fβ where a≃ 4 × 10 - 6 and β≃ 1 at f> 0.1 Hz. Our results point towards significant fluctuations in Ic originating from variation of the proximity induced gap in the graphene junction.
  • 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.
  • Designing a Platform for Microwaving Quantum Hall States
    (2023-04-05) Frei Nadarajah, Kristiana
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Emerging quantum hybrid systems for non-Abelian-state manipulation
    (2023) Muralidharan, Bhaskaran; Kumar, Manohar; Li, Chuan
     A2 Katsausartikkeli tieteellisessä aikakauslehdessä
    The non-Abelian state has garnered considerable interest in the field of fundamental physics and future applications in quantum computing. In this review, we introduce the basic ideas of constructing the non-Abelian states in various systems from 1D to 3D and discuss the possible approaches to detect these states, including the Majorana bound states in a hybrid device and the v = 5/2 state in a fractional quantum Hall system.
  • Gyrotropic Zener tunneling and nonlinear IV curves in the zero-energy Landau level of graphene in a strong magnetic field
    (2018-12-01) Laitinen, Antti; Kumar, Manohar; Hakonen, Pertti; Sonin, Edouard
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have investigated tunneling current through a suspended graphene Corbino disk in high magnetic fields at the Dirac point, i.e. at filling factor ν = 0. At the onset of the dielectric breakdown the current through the disk grows exponentially before ohmic behaviour, but in a manner distinct from thermal activation. We find that Zener tunneling between Landau sublevels dominates, facilitated by tilting of the source-drain bias potential. According to our analytic modelling, the Zener tunneling is strongly affected by the gyrotropic force (Lorentz force) due to the high magnetic field.
  • Mitigating vortex losses in superconducting aluminum coplanar waveguide resonators
    (2024-11-15) Frei, Kristiana
     School of Science | Master's thesis
    Superconducting coplanar waveguide resonators play a crucial role in the advancement of quantum technology as building blocks for quantum computers. Critical for the development of the next generation of quantum systems, these resonators need to be versatile, scalable, and able to sustain high performance even in demanding environments. External magnetic fields are especially detrimental to their operation due to the nucleation of vortices inside the super- conducting film. Unwanted fields can be minimized through adequate external shielding. However when a microwave cavity is used to probe a system that operates in an external field, the inherent magnetic field resilience of the cavity needs to be improved. By taking advantage of material properties and designing resonators with cross sections below the width required for the generation of vortices, we see a greatly enhanced performance of aluminum coplanar waveguide resonators in a perpendicular magnetic field. In addition to this, we developed a backstitching sweeping technique that shuffles the vortices penetrating the film into more favorable positions, strongly suppressing the losses at a given field. Combining the optimized resonator design with this sweeping technique, we measure internal quality factors exceeding 10^4 in magnetic fields above 7.2 mT. This is above 70% of the critical magnetic field for bulk aluminum (Bc = 10 mT), among the highest ratios to Bc reported for superconducting waveguides. In future work, we are planning to use these resonators to probe emergent quantum anomalous Hall transport phases that arise at low fields in topological materials.
  • Strong magnetoresistance in a graphene Corbino disk at low magnetic fields
    (2021-09-29) Kamada, Masahiro; Gall, Vanessa; Sarkar, Jayanta; Kumar, Manohar; Laitinen, Antti; Gornyi, Igor; Hakonen, Pertti
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have measured magnetoresistance of suspended graphene in the Corbino geometry at magnetic fields up to B = 0.15 T, i.e., in a regime uninfluenced by Shubnikov-de Haas oscillations. The low-temperature relative magnetoresistance [R(B) - R(0)]/R(0) is strong, approaching 100% at the highest magnetic field studied, with a quite weak temperature dependence below 30 K. A decrease in the relative magnetoresistance by a factor of two is found when charge carrier density is increased to vertical bar n vertical bar similar or equal to 3 x 10(10) cm(-2). Furthermore, we find a shift in the position of the charge neutrality point with increasing magnetic field, which suggests that magnetic field changes the screening of Coulomb impurities around the Dirac point. The gate dependence of the magnetoresistance allows us to characterize the role of scattering on long-range (Coulomb impurities, ripples) and short-range disorder (adatoms, atomic defects), as well as to separate the bulk resistance from the contact one. Based on the analysis of the magnetoresistance, we propose a more reliable method to extract the bulk mobility, which does not require prior knowledge of the contact resistance. It is thus demonstrated that studying magnetoresistance in the Corbino geometry is an extremely valuable tool to characterize high-mobility graphene samples, in particular, in the vicinity of the Dirac point.
  • Twin techniques narrow search for elusive Majorana particles
    (2022-12-15) Kumar, Manohar; Li, Chuan
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Nanowire eliminates false-positive signals of quasiparticles. [Figure not available: see fulltext.].
  • Ultra low 1/f noise in suspended bilayer graphene
    (2015) Kumar, Manohar; Laitinen, Antti; Cox, Daniel; Hakonen, Pertti J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We have studied 1/f noise power SI in suspended bilayer graphene devices. Around the Dirac point, we observe ultra low noise amplitude on the order of f*SI/I2b=10−9 . The low frequency noise level is barely sensitive to intrinsic carrier density, but temperature and external doping are found to influence the noise power. In our current-annealed samples, the 1/f noise is dominated by resistance fluctuations at the contacts. Temperature dependence of the 1/f noise suggests the presence of trap states in the contact regions, with a nearly exponential distribution function displaying a characteristic energy of 0.12 eV. At 80 K, the noise displays an air pressure sensitivity that corresponds to ∼0.3 ppm gas detection sensitivity; this indicates the potential of suspended graphene as a platform for gas sensing applications.
  • Unconventional fractional quantum Hall states and Wigner crystallization in suspended Corbino graphene
    (2018-12-01) Kumar, Manohar; Laitinen, Antti; Hakonen, Pertti
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Competition between liquid and solid states in two-dimensional electron systems is an intriguing problem in condensed matter physics. We have investigated competing Wigner crystal and fractional quantum Hall (FQH) liquid phases in atomically thin suspended graphene devices in Corbino geometry. Lowerature magnetoconductance and transconductance measurements along with IV characteristics all indicate strong charge density dependent modulation of electron transport. Our results show unconventional FQH phases which do not fit the standard Jain's series for conventional FQH states, instead they appear to originate from residual interactions of composite fermions in partially filled Landau levels. Also at very low charge density with filling factors ν ≲, 1/5ν 1 -5, electrons crystallize into an ordered Wigner solid which eventually transforms into an incompressible Hall liquid at filling factors around ν ≤ 1/7. Building on the unique Corbino sample structure, our experiments pave the way for enhanced understanding of the ordered phases of interacting electrons.
  • Weak antilocalization of composite fermions in graphene
    (2018-02-08) Laitinen, Antti; Kumar, Manohar; Hakonen, Pertti J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We demonstrate experimentally that composite fermions in monolayer graphene display weak antilocalization. Our experiments deal with fractional quantum Hall (FQH) states in high-mobility, suspended graphene Corbino disks in the vicinity of ν=1/2. We find a strong temperature dependence of conductivity σ away from half filling, which is consistent with the expected electron-electron interaction-induced gaps in the FQH state. At half filling, however, the temperature dependence of conductivity σ(T) becomes quite weak, as anticipated for a Fermi sea of composite fermions, and we find a logarithmic dependence of σ on T. The sign of this quantum correction coincides with the weak antilocalization of graphene composite fermions, indigenous to chiral Dirac particles.