Browsing by Author "Hakonen, Pertti"
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Item Acoustic resonances in quartz tuning fork container cavity in 4He gas(2014-10-31) Kamppinen, Timo; Eltsov, Vladimir; Perustieteiden korkeakoulu; Hakonen, PerttiItem Aluminium microelectromechanical resonator as a probe of superfluid helium(2017-11-14) Kamppinen, Timo; Eltsov, Vladimir; Perustieteiden korkeakoulu; Hakonen, PerttiIn this work, the response of magnetomotively driven MEMS resonators have been measured at low temperatures in vacuum (16 mK – 4 K) and superfluid 4He (18 mK – 1 K). The goal post shaped devices (dimensions ∼ 10 μm) are made from suspended aluminium beams with rectangular cross section 0.15 μm × 1 μm and their operation frequencies range from 300 kHz to 500 kHz. We believe that the force resolution ∆F ∼ 1 fN of the devices is sufficient for studying dynamics of a single vortex pinned to the device. In vacuum, the dissipation follows a power law and the resonance frequency increases logarithmically with temperature. The behaviour is attributed to tunneling two level systems present in the structure. In superfluid 4He, the dissipation increases further as a consequence of momentum transfer in collisions with ballistic phonons and rotons. At large oscillation amplitudes, the devices show nonlinear Duffing-like behaviour with an amplitude dependent quadratic frequency shift. In superfluid 4He at T = 18 mK we observe a collapse of the oscillations at vc = 0.17 m/s independently of the driving force. We interpret this as a sudden increase of the drag force, when the critical velocity for the emission of vortex rings is exceeded.Item Chemical vapor deposition setup for graphene and single-walled carbon nanotube growth(2011) Häkkinen, Pasi; Hakonen, Pertti; Perustieteiden korkeakoulu; Dijken, Sebastiaan vanItem Computing Surface Wave Resonances on Superfluid He-4 by Finite Element Method(2016-08-10) Ranni, Antti; Tuoriniemi, Juha; Perustieteiden korkeakoulu; Hakonen, PerttiItem Contact doping, Klein tunneling, and asymmetry of shot noise in suspended graphene(2016-03-09) Laitinen, Antti; Paraoanu, G. S.; Oksanen, Mika; Craciun, Monica F.; Russo, Saverio; Sonin, Edouard; Hakonen, Pertti; Department of Applied Physics; Quantum Circuits and Correlations; Superconducting Qubits and Circuit QED; University of Exeter; Hebrew University of JerusalemThe inherent asymmetry of the electric transport in graphene is attributed to Klein tunneling across barriers defined by pn interfaces between positively and negatively charged regions. By combining conductance and shot noise experiments, we determine the main characteristics of the tunneling barrier (height and slope) in a high-quality suspended sample with Au/Cr/Au contacts. We observe an asymmetric resistance Rodd=100-70Ω across the Dirac point of the suspended graphene at carrier density |nG|=(0.3-4)×1011cm-2, while the Fano factor displays a nonmonotonic asymmetry in the range Fodd∼0.03-0.1. Our findings agree with analytical calculations based on the Dirac equation with a trapezoidal barrier. Comparison between the model and the data yields the barrier height for tunneling, an estimate of the thickness of the pn interface dItem Continuous-Time Monitoring of Landau-Zener Interference in a Cooper-Pair Box(American Physical Society (APS), 2006) Sillanpää, Mika A.; Lehtinen, Teijo; Paila, Antti; Makhlin, Yuriy; Hakonen, Pertti; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceLandau-Zener (LZ) tunneling can occur with a certain probability when crossing energy levels of a quantum two-level system are swept across the minimum energy separation. Here we present experimental evidence of quantum interference effects in solid-state LZ tunneling. We used a Cooper-pair box qubit where the LZ tunneling occurs at the charge degeneracy. By employing a weak nondemolition monitoring, we observe interference between consecutive LZ-tunneling events; we find that the average level occupancies depend on the dynamical phase. The system’s unusually strong linear response is explained by interband relaxation. Our interferometer can be used as a high-resolution Mach-Zehnder–type detector for phase and charge.Item Critical current fluctuations in graphene Josephson junctions(Nature Publishing Group, 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; Centre of Excellence in Quantum Technology, QTF; Quantum Circuits and Correlations; Kavli Institute of Nanoscience Delft; National Institute for Materials Science; Karlsruhe Institute of Technology; Department of Applied PhysicsWe 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.Item 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; National University of Defense Technology; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsDefects 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.Item Design and operation of a broadband Josephson parametric amplifier based on lumped elements(2018-10-18) Lilja, Ilari; Elo, Teemu; Perustieteiden korkeakoulu; Hakonen, PerttiItem Design and study of graphene twisted bilayer nanostructures(2023-10-10) Piedra Marin, Luis; Krupke, Ralph; Danneau, Romain; Kemian tekniikan korkeakoulu; Hakonen, PerttiTwisted bilayer graphene (TBG) has emerged as a captivating platform in the field of nanotechnology due to its unique electronic properties which arise from the interplay of lattice misalignment and moiré superlattices. TBG nanostructures are highly sensitive to the twist angle between the top and bottom graphene layers. At the magic angle of 1.1°, TBG exhibits a flat electronic band structure which can lead to the emergence of correlated electronic states and superconductivity. The remarkable tunability of TBG electronic structure via twist angle manipulation and gate voltage has paved the way for designing novel nanoscale electronic devices. In this thesis, the aim relies on the nanofabrication of TBG nanodevices starting by employing a stacking process for the formation of the heterostructure h-BN/TBG/h-BN. Followed by the e-beam lithography technique to transfer the desired pattern onto the surface of the substrate to create the electrodes, top gate, and back gate which are the essential components of the nanodevice. Different metal deposition techniques were employed including MBE, ALD, and sputtering. Finally, four nanodevices were fabricated and only two showed no structural damage. These two nanodevices could be used as platforms for investigating different quantum phenomena including ballistic transport, induced superconductivity, and correlated insulating states. However, due to lack of time and technical problems transport measurements could not be performed. Lastly, CVD monolayer graphene FETs were fabricated using an unconventional approach where the Al2O3 dielectric top gate was created by oxidation of the aluminum gate electrode. Additionally, transport measurements were performed where hysteresis was found due to charge traps at the interface graphene/SiO2 from the back gate.Item Development of a fully superconducting Bloch oscillating transistor(2009) Korhonen, Laura; Hakonen, Pertti; Informaatio- ja luonnontieteiden tiedekunta; Teknillinen korkeakoulu; Helsinki University of Technology; Nieminen, RistoMesoscopic Josephson junctions are the basis of the Bloch oscillating transistor (BOT). It is a mesoscopic three-terminal amplifier that converts a current of single electrons into a notably larger supercurrent of Cooper pairs. It consists of three tunnel junctions, one is normal metal - insulator - superconductor (NIS) junction in the base terminal, and the other two are Josephson junctions in a SQUID-like structure in the emitter terminal. The collector terminal is formed by a thin-film chromium resistor. The implementation of BOT is carried out by electron beam lithography. We have changed the base NIS junction to a superconductor - insulator - superconductor (SIS) junction to simplify the evaporation process. One application of BOT is modern quantum metrology where amplification of small currents is required. This application also sets a demand for the input noise of BOT, 1 fA/pHz at 1 Hz. The IV characteristics of BOT can be modelled by means of P(E)-theory, and we simulated the two samples presented in this work. In the experimental IV characteristics, we discovered a new feature, a two-step peak, whereas P(E)-theory only gives one peak. We believe the second peak is related to supercurrent passing through the emitter and base Josephson junctions. In the experiments, the highest current gain was found to be 33 reached in the limit of hysteretic behaviour, which could create instability problems in applications. At the operation point, the noise of BOT was 1/f2-type. The lowest input referred current noise was 3 fA/pHz at 95 Hz, which predicts more than ten times higher noise than the maximum allowed 1 fA/pHz at 1 Hz. The noise temperature was 0.26 K at 95 Hz. The SIS base BOT did not meet the requirements of metrology, which makes the NIS base BOT still the better candidate.Item Electrical Low-Frequency 1/fγ Noise Due to Surface Diffusion of Scatterers on an Ultra-low-Noise Graphene Platform(AMERICAN CHEMICAL SOCIETY, 2021-09-22) Kamada, Masahiro; Laitinen, Antti; Zeng, Weijun; Will, Marco; Sarkar, Jayanta; Tappura, Kirsi; Seppä, Heikki; Hakonen, Pertti; Department of Applied Physics; Quantum Circuits and Correlations; Centre of Excellence in Quantum Technology, QTF; VTT Technical Research Centre of FinlandLow-frequency 1/f γ noise is ubiquitous, even in high-end electronic devices. Recently, it was found that adsorbed O2 molecules provide the dominant contribution to flux noise in superconducting quantum interference devices. To clarify the basic principles of such adsorbate noise, we have investigated low-frequency noise, while the mobility of surface adsorbates is varied by temperature. We measured low-frequency current noise in suspended monolayer graphene Corbino samples under the influence of adsorbed Ne atoms. Owing to the extremely small intrinsic noise of suspended graphene, we could resolve a combination of 1/f γ and Lorentzian noise induced by the presence of Ne. We find that the 1/f γ noise is caused by surface diffusion of Ne atoms and by temporary formation of few-Ne-atom clusters. Our results support the idea that clustering dynamics of defects is relevant for understanding of 1/f noise in metallic systems.Item Electrical Transport Experiments on Suspended Graphene(2014) Laitinen, Antti; Perustieteiden korkeakoulu; Perustieteiden korkeakoulu; Hakonen, PerttiGraphene, a hexagonal grid of carbon atoms arranged into an atomically thin 2D-lattice, is one of the strongest known materials with extraordinary electrical properties. These properties were investigated by probing a micron-sized graphene flake suspended between current leads using standard conductance measurements while the charge carrier density was tuned with a back gate. The gate dependence of conductance of the high-quality monolayer graphene sample was in line with the theory of ballistic graphene reaching mobilities up to 105 cm2/(Vs). Measurements as a function of temperature showed evidence of scattering from flexural phonons. At low temperatures, electronic multimode Fabry-Pérot interference was observed.Item Electrical transport measurements in disordered graphite(2023-11-02) Perälä, Ilkka; Sangani, Varma; Perustieteiden korkeakoulu; Hakonen, PerttiItem Electron transport and shot noise in graphene Josephson junctions(2016-10-25) Tomi, Matti; Hakonen, Pertti; Perustieteiden korkeakoulu; Hakonen, PerttiItem Electron–phonon coupling in copper intercalated Bi 2 Se 3(Nature Publishing Group, 2022-07-15) Wiesner, Maciej; Koski, Kristie; Laitinen, Antti; Manninen, Juuso; Zyuzin, Alexander A.; Hakonen, Pertti; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Quantum Circuits and Correlations; University of California, Davis; Adam Mickiewicz University PoznanWe report charge and heat transport studies in copper-intercalated topological insulator Bi2Se3 hybrid devices. Measured conductivity shows impact of quantum corrections, electron–electron and electron–phonon interactions. Our shot noise measurements reveal that heat flux displays a crossover between T2 and T4 with the increase of temperature. The results might be explained by a model of inelastic electron scattering on disorder, increasing the role of transverse acoustic phonons in the electron–phonon coupling process.Item Enhancement of the optomechanical coupling and Kerr nonlinearity using the Josephson capacitance of a Cooper-pair box(American Physical Society, 2022-04-01) Manninen, Juuso; Haque, Mohammad Tasnimul; Vitali, David; Hakonen, Pertti; Department of Applied Physics; Quantum Circuits and Correlations; Centre of Excellence in Quantum Technology, QTF; University of CamerinoWe propose a scheme for enhancing the optomechanical coupling between microwave and mechanical resonators by up to seven orders of magnitude to the ultrastrong coupling limit in a circuit optomechanical setting. The tripartite system considered here consists of a Josephson junction Cooper-pair box that mediates the coupling between the microwave cavity and the mechanical resonator. The optomechanical coupling can be modified by tuning the gate charge and the magnetic flux bias of the Cooper-pair box which in turn affect the Josephson capacitance of the Cooper-pair box. We additionally show that with a suitable choice of tuning parameters, the optomechanical coupling vanishes and the system purely exhibits a cross-Kerr type of nonlinearity between the cavity and the mechanical resonator. This allows the system to be used for phonon counting.Item Fabrication of Suspended Graphene Josephson Junctions(2015-11-30) Paananen, Topi; Oksanen, Mika; Perustieteiden korkeakoulu; Hakonen, PerttiItem Generation of multimode entanglement with a Josephson Parametric Oscillator(2021-12-14) Lilja, Ilari; Hakonen, Pertti; Perustieteiden korkeakoulu; Hakonen, PerttiQuantum information theory studies how quantum mechanics can be used to solve information processing tasks. The explotation of quantum mechanical properties can lead to quantum information protocols being more efficient than their classical counterparts. Continuous-Variable states offer an attractive platform for implementing various quantum information continuous variable protocols. In this thesis we show how Josephson Parametric Oscillators (JPO) can be used to generate multipartite entangled CV states for the use as a quantum resource in quantum information protocols. By simultaneously driving the JPO with two different pump tones, a tripartite entangled state is generated. The entanglement properties of the tripartite state are entirely contained in its 6-by-6 covariance matrix. Various entanglement measures are applied to the covariance matrix to demonstrate that the generated state exhibits genuine tripartite entanglement.Item Grafeeninäytteiden valmistus ja grafeenin raekohinamittaukset(2010) Salmilehto, Juha; Hakonen, Pertti; Informaatio- ja luonnontieteiden tiedekunta; Alava, Mikko
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