arXiv subjects

J. P. Pekola

Publications and source records attributed to J. P. Pekola.

At least 109 records · Page 6Linked to original sources

arXiv2009

Experimental investigation of hybrid single-electron turnstiles with high charging energy

We present an experimental study of hybrid turnstiles with high charging energies in comparison to the superconducting gap. The device is modeled with the sequential tunneling approximation. The backtunneling effect is shown to limit the amplitude of the gate drive and thereby the maximum pumped current of the turnstile. We compare results obtained with sine and square wave drive and show how a fast rise time can suppress errors due to leakage current. Quantized current plateaus up to 160 pA are demonstrated.

cond-mat.mes-hall↗

arXiv2009

Calorimetric readout of a superconducting proximity-effect thermometer

A proximity-effect thermometer measures the temperature dependent critical supercurrent in a long superconductor - normal metal - superconductor (SNS) Josephson junction. Typically, the transition from the superconducting to the normal state is detected by monitoring the appearance of a voltage across the junction. We describe a new approach to detect the transition based on the temperature increase in the resistive state due to Joule heating. Our method increases the sensitivity and is especially applicable for temperatures below about 300 mK.

cond-mat.supr-con↗

arXiv2008

Electronic cooling of a submicron-sized metallic beam

We demonstrate electronic cooling of a suspended AuPd island using superconductor-insulator-normal metal tunnel junctions. This was achieved by developing a simple fabrication method for reliably releasing narrow submicron sized metal beams. The process is based on reactive ion etching and uses a conducting substrate to avoid charge-up damage and is compatible with e.g. conventional e-beam lithography, shadow-angle metal deposition and oxide tunnel junctions. The devices function well and exhibit clear cooling; up to factor of two at sub-kelvin temperatures.

cond-mat.mes-hall↗

arXiv2008

Manipulation and Generation of Supercurrent in Out-of-Equilibrium Josephson Tunnel Nanojunctions

We demonstrate experimentally manipulation of supercurrent in Al-AlO_x-Ti Josephson tunnel junctions by injecting quasiparticles in a Ti island from two additional tunnel-coupled Al superconducting reservoirs. Both supercurrent enhancement and quenching with respect to equilibrium are achieved. We demonstrate cooling of the Ti line by quasiparticle injection from the normal state deep into the superconducting phase. A model based on heat transport and non-monotonic current-voltage characteristic of a Josephson junction satisfactorily accounts for our findings.

cond-mat.mes-hall↗

arXiv2008

Origin of Hysteresis in a Proximity Josephson Junction

We 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.

cond-mat.supr-con↗

arXiv2008

Recombination limited energy relaxation in a BCS superconductor

We study quasiparticle energy relaxation at sub-kelvin temperatures by injecting hot electrons into an aluminium island and measuring the energy flux from electrons into phonons both in the superconducting and in the normal state. The data show strong reduction of the flux at low temperatures in the superconducting state, in qualitative agreement with the presented quasiclassical theory for clean superconductors. Quantitatively, the energy flux exceeds that from the theory both in the superconducting and in the normal state, possibly suggesting an enhanced or additional relaxation process.

cond-mat.mes-hall↗

arXiv2008

Violation of the Wiedemann-Franz Law in a Single-Electron Transistor

We study the influence of Coulomb interaction on the thermoelectric transport coefficients for a metallic single-electron transistor. By performing a perturbation expansion up to second order in the tunnel-barrier conductance, we include sequential and cotunneling processes as well as quantum fluctuations that renormalize the charging energy and the tunnel conductance. We find that Coulomb interaction leads to a strong violation of the Wiedemann-Franz law: the Lorenz ratio becomes gate-voltage dependent for sequential tunneling, and is increased by a factor 9/5 in the cotunneling regime. Finally, we suggest a measurement scheme for an experimental realization.

cond-mat.mes-hall↗

arXiv2008

Influence of photon-assisted tunneling on heat flow in a normal metal - superconductor tunnel junction

We have investigated theoretically the influence of an AC drive on heat transport in a hybrid normal metal - superconductor tunnel junction in the photon-assisted tunneling regime. We find that the useful heat flux out from the normal metal is always reduced as compared to its magnitude under the static and quasi-static drive conditions. Our results are useful to predict the operative conditions of AC driven superconducting electron refrigerators.

cond-mat.mes-hall↗

arXiv2008

Ultrasensitive Proximity Josephson Sensor with Kinetic Inductance Read-Out

We propose a mesoscopic kinetic-inductance radiation detector based on a long superconductor--normal metal--superconductor Josephson junction. The operation of this proximity Josephson sensor (PJS) relies on large kinetic inductance variations under irradiation due to the exponential temperature dependence of the critical current. Coupled with a dc SQUID readout, the PJS is able to provide a signal to noise (S/N) ratio up to ~10^3 in the THz regime if operated as calorimeter, while electrical noise equivalent power (NEP) as low as ~7x10^{-20} W(Hz)^(-1/2) at 200 mK can be achieved in the bolometer operation. The high performance together with the ease of fabrication make this structure attractive as an ultrasensitive cryogenic detector of THz electromagnetic radiation.

cond-mat.mes-hall↗

arXiv2007

Electron-phonon coupling and longitudinal mechanical-mode cooling in a metallic nanowire

We investigate electron-phonon coupling in a narrow suspended metallic wire, in which the phonon modes are restricted to one dimension but the electrons behave three-dimensionally. Explicit theoretical results related to the known bulk properties are derived. We find out that longitudinal vibration modes can be cooled by electronic tunnel refrigeration far below the bath temperature provided the mechanical quality factors of the modes are sufficiently high. The obtained results apply to feasible experimental configurations.

cond-mat.mes-hall↗

arXiv2007

Parity effect in Al and Nb single electron transistors in a tunable environment

Two different types of Cooper pair transistors, with Al and Nb islands, have been investigated in a tunable electromagnetic environment. The device with an Al island demonstrates gate charge modulation with 2e-periodicity in a wide range of environmental impedances at bath temperatures below 340 mK. Contrary to the results of the Al sample, we were not able to detect 2e-periodicity under any conditions on similar samples with Nb island. We attribute this to the material properties of Nb.

cond-mat.supr-con↗

arXiv2007

Information entropic superconducting microcooler

We consider a design for a cyclic microrefrigerator using a superconducting flux qubit. Adiabatic modulation of the flux combined with thermalization can be used to transfer energy from a lower temperature normal metal thin film resistor to another one at higher temperature. The frequency selectivity of photonic heat conduction is achieved by including the hot resistor as part of a high frequency LC resonator and the cold one as part of a low-frequency oscillator while keeping both circuits in the underdamped regime. We discuss the performance of the device in an experimentally realistic setting. This device illustrates the complementarity of information and thermodynamic entropy as the erasure of the quantum bit directly relates to the cooling of the resistor.

cond-mat.supr-con↗

arXiv2007

Normal metal - superconductor tunnel junction as a Brownian refrigerator

Thermal noise generated by a hot resistor (resistance $R$) can, under proper conditions, catalyze heat removal from a cold normal metal (N) in contact with a superconductor (S) via a tunnel barrier. Such a NIS junction acts as Maxwell's demon, rectifying the heat flow. Upon reversal of the temperature gradient between the resistor and the junction the heat fluxes are reversed: this presents a regime which is not accessible in an ordinary voltage-biased NIS structure. We obtain analytical results for the cooling performance in an idealized high impedance environment, and perform numerical calculations for general $R$. We conclude by assessing the experimental feasibility of the proposed effect.

cond-mat.mes-hall↗

arXiv2007

Qubits as devices to detect the third moment of current fluctuations

Under appropriate conditions controllable two-level systems can be used to detect the third moment of current fluctuations. We derive a Master Equation for a quantum system coupled to a bath valid to the third order in the coupling between the system and the environment. In this approximation the reduced dynamics of the quantum system depends on the frequency dependent third moment. Specializing to the case of a controllable two-level system (a qubit) and in the limit in which the splitting between the levels is much smaller than the characteristic frequency of the third moment, it is possible to show that the decay of the qubit has additional oscillations whose amplitude is directly proportional to the value of the third moment. We discuss an experimental setup where this effect can be seen.

cond-mat.mes-hall↗

arXiv2006

Wide-band detection of the third moment of shot noise by a hysteretic Josephson junction

We 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.

cond-mat.mes-hall↗

arXiv2006

Detecting Current Noise with a Josephson Junction in the Macroscopic Quantum Tunneling Regime

We discuss the use of a hysteretic Josephson junction to detect current fluctuations with frequencies below the plasma frequency of the junction. These adiabatic fluctuations are probed by switching measurements observing the noise-affected average rate of macroscopic quantum tunneling of the detector junction out of its zero-voltage state. In a proposed experimental scheme, frequencies of the noise are limited by an on-chip filtering circuit. The third cumulant of current fluctuations at the detector is related to an asymmetry of the switching rates.

cond-mat.mes-hall↗

arXiv2006

High resolution superconducting single flux quantum comparator for sub kelvin temperatures

A design of subkelvin single flux quantum (SFQ) circuits with reduced power dissipation and additional cooling of shunt resistors for superconducting qubit control circuits has been developed and characterized. We demonstrate operation of SFQ comparators with current resolution of 40 nA at 2 GHz sampling rate. Due to improved cooling the electron temperature in shunt resistors of a SFQ comparator is below 50 mK when the bath temperature is about 30 mK.

cond-mat.supr-con↗

arXiv2006

Frequency-dependent current correlation functions from scattering theory

We present a general formalism based on scattering theory to calculate quantum correlation functions involving several time-dependent current operators. A key ingredient is the causality of the scattering matrix, which allows one to deal with arbitrary correlation functions. Our formalism might be useful in view of recent developments in full counting statistics of charge transfer, where detecting schemes have been proposed for measurement of frequency dependent spectra of higher moments. Some of these schemes are different from the well-known fictitious spin-detector and therefore generally involve calculation of non-Keldysh-contour-ordered correlation functions. As an illustration of our method we consider various third order correlation functions of current, including the usual third cumulant of current statistics. We investigate the frequency dependence of these correlation functions explicitly in the case of energy-independent scattering. The results can easily be generalized to the calculation of arbitrary n-th order correlation functions, or to include the effect of interactions.

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