Presentation on theme: "Normal metal - superconductor tunnel junctions as kT and e pumps"— Presentation transcript:
1Normal metal - superconductor tunnel junctions as kT and e pumps Jukka PekolaLow Temperature Laboratory, Helsinki University of TechnologyCoulomb blockade and electronic refrigerationRadiofrequency single-electron refrigeratorHeat transistorHybrid single-electron turnstile for electronsCollaborators:M. Meschke, O.-P. Saira, A. Savin, M. Möttönen, J. Vartiainen, A. Timofeev, M. Helle, N. Kopnin (LTL), A. Kemppinen (Mikes)F. Giazotto (SNS Pisa), D. Averin (SUNY Stony Brook), F. Hekking (CNRS Grenoble)
7Quantitative performance of SER Frequency dependence of cooling powerCharge and heat flux under typical operation conditionsInfluence of photon assisted tunnelling: N. Kopnin et al., Phys. Rev. B 77, (2008)
8Heat transistor – Combining Coulomb blockade and electronic refrigeration VDSSNSMAXIMUM COOLINGPOWERCgVg = (n+1/2)eVDSSNSMINIMUM COOLINGPOWERCgVg = ne
9Influence of charging energy The first demonstration of gate controlled refrigerationO.-P.Saira et al., PRL 99, (2007)NS contacts
11Brownian refrigerator COOLING POWER OF N (fW)J.P. and F. Hekking, PRL 98, (2007); see poster by Andrey Timofeev today
12Electron pumps Towards frequency-to- current conversion Semiconductor, travelling wave:J.Shilton et al., J. Phys. Condens. Matter 8, L531 (1996)M. Blumenthal, S. Giblin et al., Nature Physics 3, 343 (2007)Fast, but needs still improvementR-pumps:S. Lotkhov et al.Fully superconducting pumps:Fast, hard (but not impossible!) to make accurateNormal single-electron pump: I =efM. W. Keller et al., APL 69, 1804 (1996).High accuracy but still slow: I < 10 pA
14Hybrid single-electron turnstile (SINIS or NISIN) J.P. Pekola, J.J. Vartiainen, M. Möttönen, O.-P. Saira, M. Meschke, and D.V. Averin, Nature Physics 4, 120 (2008)
15Stability diagrams Hybrid SET (SINIS or NISIN) Normal SET Important qualitative difference: stability regions overlap in a hybrid SET unlike in a normal SET
16Operation cycle Basic operation cycle Exactly one electron is transferred through the turnstile in each cycle:I = ef.
17Expected behaviour based on ”classical” tunnelling BLACK – HYBRID SETRED – NORMAL SETParameters chosen to correspond to the experiment to be presented.DC gate positions are 0, 0.1e, 0.2e, 0.3e and 0.4e (hybrid)
18Dependences from the measurement f = 12.5 MHzf = 20 MHz
19Bias and frequency dependence of the turnstile current Parameters of the turnstile:RT = 350 kWEC = 2 K
20Low leakage NIS junctions IMPROVED JUNCTIONS:A. Kemppinen et al., arXiv:g = 10-5g = 10-6THE FIRST EXPERIMENTS, g > 10-4
21Error rates (1)Probability (per cycle) of tunnelling in wrong direction is approximatelyProbability (per cycle) of tunnelling an extra electron in forward direction is approximatelyOptimum operation point is therefore at eV = D, where the error rate isAt typical temperatures (< 100 mK), with aluminium, this error is << 10-8
22Error rates (2) Missed tunnelling events due to high frequency: D = EC assumed above.Frequency cut-off can be compensated by parallelisation: compared to N-pump, N parallel turnstiles yield N2 higher current (with the same level of complexity)
24Error rates (3) Possible overheating of the island: The island can cool also!
25Error rates: quantum tunnelling Higher order tunnelling processes:In NISIN elastic virtual processes are harmfulIn SINIS these do not contributeInfluence of various inelastic processes?
26Error rates (4) Threshold: eV = 2D INELASTIC COTUNNELLING OF QUASIPARTICLES IN A SYMMETRIC SINIS STRUCTURE IS EFFICIENTLY SUPPRESSEDeVDS N SThreshold: eV = 2D
27Two-electron process and Cooper pair – electron cotunnelling D. Averin and J. Pekola, arXiv:METROLOGICAL REQUIREMENTS SATISFIED IN THEORY
28SummaryRefrigeration by hybrid tunnel junctions is already a well-established technique as such - Interplay of energy filtering and Coulomb blockade leads to new phenomena and devicesPresented a cyclic electron refrigerator, a heat transistor and a Brownian refrigeratorHybrid SINIS turnstile looks promisingSimple design and operationErrors can be suppressed efficientlySeems straightforward to run many turnstiles in parallelPossibility for error counting and correction
29Gate modulation of the SET-transistor Normal SETHybrid SET(this is one of the measured turnstiles)
30Raw experimental data Parameters of the turnstile: RT = 350 kW EC = 2 K
32Energy relaxation of electrons in metal In thermal equilibrium:Electron-electron collisionse’-we+ww- drive f to feq(e,T) (T= Tph generally)ee’e+wElectron-phonon collisionsweffective at high temperaturesdrive f to feq(e,Tph)eAt low T electron-phonon relaxation becomes extremely weak
33Entropy production in the Brownian refrigerator SSpecial case:ALWAYS ≥ 0
34Possible implications of the presented effect Until now good thermal isolation at low T has been taken for granted (vanishing electron-phonon rate, superconductivity,...)Consequences of e-photon coupling:Increased heat load and noise of micro-bolometers and calorimetersA way to tune thermal coupling (heat switches, optimization of bolometers)Another channel to remove heat from dissipative elements, like shunt resistors of SQUIDs at low TActs as a mediator of increased decoherence?
35Amplitude of temperature variation in response to magnetic flux enSymbols: experimentLines: theoretical model with the same parameters as in the previous plot
36NIS-junctionSuperconducting gap yields non-linear temperature-dependent IV characteristics
37Cooling power Cooling power of a double-NIS device: Optimum cooling power is obtained at V 2D/e:Optimum cooling power per junction at low temperatures
38Experimental status Refrigeration of lattice (membrane) M. Nahum et al (NIS)M. Leivo, J. Pekola and D. Averin, 1996 (SINIS)A. Manninen et al (SIS’IS), see also Chi and Clarke 1979 and Blamire et al. 1991L. Kuzmin et al., cooler + bolometersA. Luukanen et al (membrane refrigeration by SINIS)A. Savin et al (S – Schottky – Semic – Schottky – S)A. Clark et al (x-ray detector refrigerated by SINIS)Refrigeration of lattice (membrane)Refrigeration of a bulk objectA. Clark et al., Appl. Phys. Lett. 86, (2005).A. Luukanen et al., J. Low Temp. Phys. 120, 281 (2000).For a review, see F. Giazotto et al., Rev. Mod. Phys. 78, 217 (2006).
39Single-mode heat conduction by photons ElectronsystemElectricalenvironmentLatticeM. Meschke, W. Guichard and J. Pekola, Nature 444, 187 (2006).
40Quantized conductance Electrical conductance in a ballistic contact:Quantum of thermal conductance:GQ and sQ related by Wiedemann-Franz lawExpression of GQ is expected to hold for carriers obeying arbitrary statistics, in particular for electrons, phonons, photons (Pendry 1983, Greiner et al. 1997, Rego and Kirczenow 1999, Blencowe and Vitelli 1999).
41Example of quantized thermal conductance: phonons in a nanobridge K. Schwab et al., Nature 404, 974 (2000).
42Heat transported between two resistors Radiative contribution to net heat flow between electrons of 1 and 2:Impedance matching:Linear response for small temperature differenceDT = Te1 – Te2:D. Schmidt, A. Cleland and R. Schoelkopf, Phys. Rev. Lett. 93, (2004).
43Our experimental set-up M. Meschke et al., Nature 444, 187 (2006).Tunable impedance matching using DC-SQUIDsFIsland size: 6 mm x 0.75 mm x 15 nmMaterial: PdAu
44Measured variation of island temperature Vary bath temperatureLine: P1 = 1 fW, P2 =0enThermal model:
45Heat flows from hot to cold by photon radiation The situation is nearly the same if we replace one resistor by an ordinary tunnel junctionThis happens between two resistors
46Harmonic vs stochastic drive in refrigeration Sinusoidal bias –Refrigerates N if frequency and amplitude are not too highStochastic drive –Refrigerates N if spectrum is ”suitable”Brownian refrigerator?