Browsing by Author "Peltonen, Joonas"
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- Active Quasiparticle Suppression in a Non-Equilibrium Superconductor
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-07-08) Marin Suarez, Marco; Peltonen, Joonas; Pekola, J.P.Quasiparticle (qp) poisoning is a major issue that impairs the operation of various superconducting devices. Even though these devices are often operated at temperatures well below the critical point where the number density of excitations is expected to be exponentially suppressed, their bare operation and stray microwave radiation excite the non-equilibrium qp’s. Here we use voltage-biased superconducting junctions to demonstrate and quantify qp extraction in the turnstile operation of a superconductor–insulator–normal metal–insulator–superconductor single-electron transistor. In this operation regime, excitations are injected into the superconducting leads at a rate proportional to the driving frequency. We reach a reduction of density by an order of magnitude even for the highest injection rate of 2.4 × 108 qp’s per second when extraction is turned on. - Cascade Electronic Refrigerator Using Superconducting Tunnel Junctions
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-11-23) Nguyen, Hung; Peltonen, Joonas; Meschke, M.; Pekola, J. P.Microrefrigerators that operate in the subkelvin regime are key devices in quantum technology. A well-studied candidate, an electronic cooler using normal-metal-insulator-superconductor (N-I-S) tunnel junctions, offers substantial performance and power. However, its superconducting electrodes are severely overheated due to exponential suppression of their thermal conductance towards low temperatures, and the cooler performs unsatisfactorily - especially in powerful devices needed for practical applications. We employ a second N-I-S cooling stage to thermalize the hot superconductor at the backside of the main N-I-S cooler. Not only providing a lower bath temperature, the second-stage cooler actively evacuates quasiparticles out of the hot superconductor, especially in the low-temperature limit. We demonstrate the apparent advantage of our approach. This cascade design can also be employed to manage excess heat in other cryoelectronic devices. - Distribution of current fluctuations in a bistable conductor
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-12-27) Singh, S.; Peltonen, Joonas; Khaymovich, I. M.; Koski, J. V.; Flindt, C.; Pekola, J. P.We measure the full distribution of current fluctuations in a single-electron transistor with a controllable bistability. The conductance switches randomly between two levels due to the tunneling of single electrons in a separate single-electron box. The electrical fluctuations are detected over a wide range of time scales and excellent agreement with theoretical predictions is found. For long integration times, the distribution of the time-averaged current obeys the large-deviation principle. We formulate and verify a fluctuation relation for the bistable region of the current distribution. - Fluctuations, relaxation and proximity effect in superconducting circuits
Perustieteiden korkeakoulu | Doctoral dissertation (article-based)(2011) Peltonen, JoonasMesoscopic physics investigates structures smaller than the everyday macroscopic scale but larger than the scale of individual atoms, with properties that can often only be explained in terms of the laws of quantum mechanics. A typical mesoscopic electrical component is a tunnel junction, formed by a thin insulating oxide layer separating two metallic electrodes. In this thesis, various mesoscopic circuits containing sub-micron tunnel junctions between normal (N) and superconducting (S) metals are studied experimentally at sub-kelvin temperatures. An emphasis is placed on the influence of electrical fluctuations on the systems, as well as the strong connection between electrical and thermal transport in them. We first demonstrate that a Josephson tunnel junction between two S electrodes functions as an on-chip detector of current fluctuations in a wide band of frequencies, potentially useful for studying charge transport in various mesoscopic systems. The lifetime of the zero-voltage state in a current-biased junction is very sensitive to the fluctuations in the bias current. We are able to observe the non-Gaussian nature of the shot noise generated by electrons tunneling across another nearby tunnel junction coupled to the detector junction. Several of the experiments in this thesis probe how the transport close to a transparent interface between a superconducting and a normal conducting electrode is modified by the phenomenon of superconducting proximity effect. We present direct measurements of electron overheating in a normal metal weak link between two superconductors, explaining the routinely observed hysteretic current-voltage characteristic. Electronic temperature is probed locally by contacting the N island to an additional S electrode via an oxide barrier (I), thereby forming an NIS tunnel junction. Connecting the S electrodes of such a proximity SNS weak link into a closed loop, we further demonstrate use of the structure as a sensitive magnetometer with low dissipation. We probe also the electronic thermal conduction of short S wires between two N terminals. Due to the inverse proximity effect, the thermal conductance is found to be strongly enhanced beyond the value for a bulk superconductor. We consider theoretically the prospects for rectifying thermal fluctuations by an NIS junction in a suitable electromagnetic environment, thereby realizing a Brownian refrigerator. Finally, we report the observation of increased cooling power in a voltage-biased NIS junction in small applied magnetic fields. This is attributed to enhanced relaxation of the hot electrons injected into the S lead of the junction. - Josephson junctions as detectors of non-Gaussian noise
Helsinki University of Technology | Master's thesis(2008) Peltonen, JoonasMesoskooppisissa johteissa varauksenkuljettajien diskreetin luonteen takia syntyvä sähkövirran raekohina on tällä hetkellä voimakkaan kokeellisen ja teoreettisen tutkimuksen kohteena. Pelkän kohinan varianssin lisäksi on alettu kiinnittää huomiota myös virran korkeamman asteen korrelaatiofunktioihin, jotka viittaavat kohinan ei-gaussiseen jakaumaan. Tässä diplomityössä käsitellään suprajohtavien Josephson-liitosten hyödyntämistä raekohinan ilmaisimina. Kohinasta ja sen lähteestä saadaan tietoa epäsuorasti mittaamalla lähteeseen vahvasti kytketyn ilmaisinliitoksen vastetta. Josephson-liitos muodostuu yhdistettäessä kaksi suprajohtavaa metallista elektrodia toisiinsa heikon linkin kuten ohuen eristävän oksidikerroksen välityksellä. Liitoksen metastabiilin supravirtatilan hajoamisnopeus ja siten sen elinikä riippuvat eksponentiaalisesti rakenteen läpi kulkevasta keskimääräisestä virrasta, jolloin se soveltuu herkäksi virran kynnysilmaisimeksi. Jos biasvirrassa ei esiinny raekohinaa, hajoamisnopeuden määrää lämpötilasta riippuen klassinen terminen aktivaatio potentiaalivallin yli tai makroskooppinen kvanttitunnelointi. Raekohina vaikuttaa vahvasti tähän hajoamisnopeuteen, ja rakenne toimii myös kohinan mittarina. Työssä tarkastellaan aiemmin esitettyjä menetelmiä liitoksen supravirtatilan hajoamisnopeuden laskemiseksi sekä klassisella että kvanttimekaanisella rajalla, kun huomioon otetaan raekohinan toinen ja kolmas kumulantti. Kokeellisia tarkasteluja varten valmistettiin elektronisuihkulitografiaa ja monikulmahöyrystystä käyttäen näytteitä, joissa ilmaisinliitos on kytkettävissä virtakohinan lähteenä toimivaan normaalimetallin ja suprajohteen väliseen tunnelointiliitokseen. Mittaukset suoritettiin 3He-4He -diluutiojäähdyttimessä noin sadan millikelvinin lämpötilassa. Työn olennaisin kokeellinen tulos on detektorin supravirtatilan hajoamisnopeuksien eron luotettava mittaaminen asetelmassa, jossa kohinalähteen esijännite käännetään, mutta ilmaisimen läpi kulkeva keskimääräinen virta pidetään tarkasti vakiona. Lisäksi tutkittiin kohinalähteen konduktanssin vaikutusta detektorin vasteeseen. Mittaustulokset noudattavat mallia, jossa detektoriliitoksen dynamiikkaa kuvataan efektiivisellä raekohinan varianssista riippuvalla lämpötilalla, ja jossa kohinan kolmas kumulantti aiheuttaa havaitun hajoamisnopeuksien eron. Kohinalähdettä kuvaavien Fano-tekijöiden tai muiden suureiden kvantitatiivinen määritys ei ole vielä mahdollista ilmaisinliitoksen vaikeasta kalibroitavuudesta johtuen. Tähän tullaan kiinnittämään huomiota tulevissa kokeissa, joissa on suunniteltu käytettäväksi säädettäviä kohinalähteitä kuten kvanttipistekontakteja, ja tutkittavaksi tarkemmin pienillä kohinavirroilla ja matalissa lämpötiloissa olennaisia kvanttimekaanisia ilmiöitä. - Low Temperature Primary Coulomb Blockade Thermometry
Perustieteiden korkeakoulu | Master's thesis(2019-12-17) Muhojoki, JesseIn the light of the revision of the SI-system in May 2019, we aim to improve low temperature primary thermometry by increasing the temperature range of a Coulomb blockade thermometer (CBT) down to low millikelvin range. In this work we fabricate and measure a novel two junction CBT, where the influence of the environment is suppressed with a capacitive shunt. This allows improved thermalization between the substrate phonons and the CBT electrons by fabricating one large cooling fin, insted of smaller ones between multiple junctions that are traditionally required to suppress the coupling to the environment. Furthermore, the island can be cooled by the direct heat flow through the junctions, which becomes relevant at low temperatures in a two junction CBT. The temperature readings from the samples of the first batch deviated considerably from the reference, too much to be explainable with coupling to the environment alone. However the two measured sensors agreed with each other well, despite having order of magnitude difference in tunneling resistance and being measured in different cryostats. To find possible errors in the device, the CBT was measured without the magnetic field keeping the leads in the normal state. These measurements revealed nothing out of the ordinary for a SINIS device. As the reason for the deviation of the temperature remained unclear, a new patch of CBTs were fabricated with a slightly altered geometry, where the distance from bonding pad and the ground-plane capacitor to the junction was shorter. This should not influence the results, as the propagation time in the lead is shorter that the tunneling time in all measured samples and at all temperatures investigated. However, the new sensors did not show any systematic error in the and only deviated from the reference by less than 5 % in the temperature range of 25 - 75 mK. The accuracy was especially good at sub 50 mK temperatures, which is promising for future experiments closer to the target temperatures. This also points to the long distance between the ground plane overlap and junctions being the reason for the increased temperature reading, but further tests are needed to confirm this. - Low-temperature characterization of Nb-Cu-Nb weak links with Ar ion-cleaned interfaces
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2016-01-25) Najafi Jabdaraghi, Robab; Peltonen, Joonas; Saira, Olli-Pentti; Pekola, JukkaWe characterize niobium-based lateral Superconductor (S)-Normal metal (N)-Superconductor (SNS) weak links through low-temperature switching current measurements and tunnel spectroscopy. We fabricate the SNS devices in two separate lithography and deposition steps, combined with strong argon ion cleaning before the normal metal deposition in the last step. Our SNS weak link consists of high-quality sputtered Nb electrodes that have contacted with evaporated Cu. The two-step fabrication flow enables more flexibility in the choice of materials and pattern design. A comparison of the temperature-dependent equilibrium critical supercurrent with theoretical predictions indicates that the quality of the Nb-Cu interface is similar to that of evaporated Al-Cu weak links. We further demonstrate a hybrid magnetic flux sensor based on an Nb-Cu-Nb SNS junction, where the phase-dependent normal metal density of states is probed with an Al tunnel junction. - Noise of a superconducting magnetic flux sensor based on a proximity Josephson junction
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-08-14) Najafi Jabdaraghi, Robab; Golubev, Dmitry; Pekola, J.P.; Peltonen, JoonasWe demonstrate simultaneous measurements of DC transport properties and flux noise of a hybrid superconducting magnetometer based on the proximity effect (superconducting quantum interference proximity transistor, SQUIPT). The noise is probed by a cryogenic amplifier operating in the frequency range of a few MHz. In our non-optimized device, we achieve minimum flux noise ~4 μΦ0/Hz1/2, set by the shot noise of the probe tunnel junction. The flux noise performance can be improved by further optimization of the SQUIPT parameters, primarily minimization of the proximity junction length and cross section. Furthermore, the experiment demonstrates that the setup can be used to investigate shot noise in other nonlinear devices with high impedance. This technique opens the opportunity to measure sensitive magnetometers including SQUIPT devices with very low dissipation. - Origin of hysteresis in proximity Josephson junction
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2008-08-08) Courtois, H; Meschke, Matthias; Peltonen, Joonas; Pekola, JukkaWe investigate hysteresis in the transport properties of superconductor–normal-metal–superconductor (S-N-S) junctions at low temperatures by measuring directly the electron temperature in the normal metal. Our results demonstrate unambiguously that the hysteresis results from an increase of the normal-metal electron temperature once the junction switches to the resistive state. In our geometry, the electron temperature increase is governed by the thermal resistance of the superconducting electrodes of the junction. - Quasiparticle density in hybrid single electron turnstiles
Perustieteiden korkeakoulu | Master's thesis(2017-02-14) Moisio, AnttiSI-järjestelmän perusyksikköhin kuuluva ampeeri tullaan lähitulevaisuudessa määrittelemään uudestaan kiinnittämällä alkeisvarauksen arvo. Uuden määritelmän perusteella ampeeri voidaan toteuttaa siirtämällä yksittäisiä elektroneja kiinteällä taajuudella, mikä tarjoaa suoran taajuus-virta-muunnoksen. Eräs lupaava kandidaatti uudeksi virtastandardiksi on SINIS-hybridikiertompumppu, joka koostuu pienestä normaalimetallisaarekkeesta, joka on yhdistetty kahteen suprajohtavaan johtimeen siten, että metallien välissä on eristekerros. Eräs suurimmista ongelmista pumpuille on pumppaksen aikana syntyvät kvasipartikkelit, jotka voivat tunneloitua eristekerroksen läpi kasvattaen laitteen läpi kulkevaa sähkövirtaa. Jotta kvasipartikkelien lukumäärää tunneliliitosten läheisy - Thermal conductance by the inverse proximity effects in a superconductor
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2010-08-27) Peltonen, Joonas; Virtanen, Pauli; Meschke, Matthias; Koski, J.V.; Heikkilä, Tero; Pekola, JukkaWe study heat transport in hybrid lateral normal-metal–superconductor–normal-metal structures. We find the thermal conductance of a short superconducting wire to be strongly enhanced beyond the BCS value due to the inverse proximity effect, resulting from contributions of elastic cotunneling and crossed Andreev reflection of quasiparticles. Our measurements agree with a model based on the quasiclassical theory of inhomogeneous superconductivity in the diffusive limit. - Thermal Conductance of a Single-Electron Transistor
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-08-15) Dutta, B.; Peltonen, Joonas; Antonenko, D. S.; Meschke, M.; Skvortsov, M. A.; Kubala, Björn; König, J.; Winkelmann, Clemens B.; Courtois, H; Pekola, J. P.We report on combined measurements of heat and charge transport through a single-electron transistor. The device acts as a heat switch actuated by the voltage applied on the gate. The Wiedemann-Franz law for the ratio of heat and charge conductances is found to be systematically violated away from the charge degeneracy points. The observed deviation agrees well with the theoretical expectation. With a large temperature drop between the source and drain, the heat current away from degeneracy deviates from the standard quadratic dependence in the two temperatures. - Thermal conductance of Nb thin films at sub-kelvin temperatures
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2017-02-03) Feshchenko, A. V.; Saira, O. P.; Peltonen, Joonas; Pekola, J. P.We determine the thermal conductance of thin niobium (Nb) wires on a silica substrate in the temperature range of 0.1-0.6 K using electron thermometry based on normal metal-insulator-superconductor tunnel junctions. We find that at 0.6 K, the thermal conductance of Nb is two orders of magnitude lower than that of Al in the superconducting state, and two orders of magnitude below the Wiedemann-Franz conductance calculated with the normal state resistance of the wire. The measured thermal conductance exceeds the prediction of the Bardeen-Cooper-Schrieffer theory, and demonstrates a power law dependence on temperature as T4.5, instead of an exponential one. At the same time, we monitor the temperature profile of the substrate along the Nb wire to observe possible overheating of the phonon bath. We show that Nb can be successfully used for thermal insulation in a nanoscale circuit while simultaneously providing an electrical connection. - Wideband detection of the third moment of shot noise by a hysteretic Josephson junction
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2007-05-16) Timofeev, A.V.; Meschke, Matthias; Peltonen, Joonas; Heikkilä, Tero; Pekola, JukkaWe use a hysteretic Josephson junction as an on-chip detector of the third moment of shot noise of a tunnel junction. The detectable bandwidth is determined by the plasma frequency of the detector, which is about 50 GHz in the present experiment. The third moment of shot noise results in a measurable change of the switching rate when reversing polarity of the current through the noise source. We analyze the observed asymmetry assuming adiabatic response of the detector. - Zero-average Bias Bidirectional Single-electron Current Generation in a Hybrid Turnstile
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023-01) Marin Suarez, Marco; Pashkin, Yuri A.; Peltonen, Joonas; Pekola, J.P.Hybrid turnstiles have proven to generate accurate single-electron currents. The usual operation consists of applying a periodic modulation to a capacitively coupled gate electrode and requires a nonzero DC source-drain bias voltage. Under this operation, a current of the same magnitude and opposite direction can be generated by flipping the polarity of the bias. Here, we demonstrate that accurate single-electron currents can be generated under zero average bias voltage. We achieve this by applying an extra periodic modulation with twice the frequency of the gate signal and zero DC level to the source electrode. This creates a time interval, which is otherwise zero, between the crossings of tunnelling thresholds that enable single-electron tunnelling. Furthermore, we show that within this operation the current direction can be reversed by only shifting the phase of the source signal.