arXiv subjects

J. P. Pekola

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

At least 91 records · Page 5Linked to original sources

arXiv2011

Magnetic-Field-Induced Stabilization of Non-Equilibrium Superconductivity

A small magnetic field is found to enhance relaxation processes in a superconductor thus stabilizing superconductivity in non-equilibrium conditions. In a normal-metal (N) - insulator - superconductor (S) tunnel junction, applying a field of the order of 100 μT leads to significantly improved cooling of the N island by quasiparticle (QP) tunneling. These findings are attributed to faster QP relaxation within the S electrodes as a result of enhanced QP drain through regions with locally suppressed energy gap due to magnetic vortices in the S leads at some distance from the junction.

cond-mat.supr-con↗

arXiv2011

Tunnel Spectroscopy of a Proximity Josephson Junction

We present tunnel spectroscopy experiments on the proximity effect in lateral superconductor-normal metal-superconductor (SNS) Josephson junctions. Our weak link is embedded into a superconducting (S) ring allowing phase biasing of the Josephson junction by an external magnetic field. We explore the temperature and phase dependence of both the induced mini-gap and the modification of the density of states in the normal (N) metal. Our results agree with a model based on the quasiclassical theory in the diffusive limit. The device presents an advanced version of the superconducting quantum interference proximity transistor (SQUIPT), now reaching flux sensitivities of 3 nA$/Φ_0$ where $Φ_0$ is the flux quantum.

cond-mat.supr-con↗

arXiv2011

Brownian refrigeration by hybrid tunnel junctions

Voltage fluctuations generated in a hot resistor can cause extraction of heat from a colder normal metal electrode of a hybrid tunnel junction between a normal metal and a superconductor. We extend the analysis presented in [Phys. Rev. Lett. 98, 210604 (2007)] of this heat rectifying system, bearing resemblance to a Maxwell's demon. Explicit analytic calculations show that the entropy of the total system is always increasing. We then consider a single electron transistor configuration with two hybrid junctions in series, and show how the cooling is influenced by charging effects. We analyze also the cooling effect from nonequilibrium fluctuations instead of thermal noise, focusing on the shot noise generated in another tunnel junction. We conclude by discussing limitations for an experimental observation of the effect.

cond-mat.mes-hall↗

arXiv2011

Laterally proximized aluminum tunnel junctions

This letter presents experiments on junctions fabricated by a new technique that enables the use of high quality aluminum oxide tunnel barriers with normal metal electrodes at low temperatures. Inverse proximity effect is applied to diminish the superconductivity of an aluminum dot through a clean lateral connection to a normal metal electrode. To demonstrate the effectiveness of this method, fully normal-state single electron transistors (SET) and normal metal-insulator-superconductor (NIS) junctions applying proximized Al junctions were fabricated. The transport characteristics of the junctions were similar to those obtained from standard theoretical models of regular SETs and NIS junctions.

cond-mat.mes-hall↗

arXiv2011

Real-time observation of discrete Andreev tunneling events

We provide a direct proof of two-electron Andreev transitions in a superconductor - normal metal tunnel junction by detecting them in a real-time electron counting experiment. Our results are consistent with ballistic Andreev transport with an order of magnitude higher rate than expected for a uniform barrier, suggesting that only part of the interface is effectively contributing to the transport. These findings are quantitatively supported by our direct current measurements in single-electron transistors with similar tunnel barriers.

cond-mat.mes-hall↗

arXiv2011

Decoherence of adiabatically steered quantum systems

We study the effect of Markovian environmental noise on the dynamics of a two-level quantum system which is steered adiabatically by an external driving field. We express the master equation taking consistently into account all the contributions to the lowest non-vanishing order in the coupling to the Markovian environment. We study the master equation numerically and analytically and we find that, in the adiabatic limit, a zero-temperature environment does not affect the ground state evolution. As a physical application, we discuss extensively how the environment affects Cooper pair pumping. The adiabatic ground state pumping appears to be robust against environmental noise. In fact, the relaxation due to the environment is required to avoid the accumulation of small errors from each pumping cycle. We show that neglecting the non-secular terms in the master equation leads to unphysical results, such as charge non-conservation. We discuss also a possible way to control the environmental noise in a realistic physical setup and its influence on the pumping process.

quant-ph↗

arXiv2011

Capacitively Enhanced Thermal Escape in Underdamped Josephson Junctions

We have studied experimentally the escape dynamics in underdamped capacitively shunted and unshunted Josephson junctions with submicroampere critical currents below 0.5 K temperatures. In the shunted junctions, thermal activation process was preserved up to the highest temperature where the escape in the unshunted junctions exhibits the phase diffusion. Our observations in the shunted junctions are in good agreement with the standard thermal activation escape, unlike the results in the unshunted junctions.

cond-mat.mes-hall↗

arXiv2010

Environmentally Activated Tunneling Events in a Hybrid Single-Electron Box

We have measured individual tunneling events and Coulomb step shapes in single-electron boxes with opaque superconductor-normal metal tunnel junctions. We observe anomalous broadening of the Coulomb step with decreasing temperature in a manner that is consistent with activation of first-order tunneling events by an external dissipative electromagnetic environment. We demonstrate that the rates for energetically unfavourable tunneling events saturate to finite values at low temperatures, and that the saturation level can be suppressed by more than an order of magnitude by a capacitive shunt near the device. The findings are important in assessing the performance limits of any single-electronic device. In particular, master equation based simulations show that the electromagnetic environment realized in the capacitively shunted devices allows for a metrologically accurate charge pump based on hybrid tunnnel junctions.

cond-mat.mes-hall↗

arXiv2010

Photon assisted tunneling as an origin of the Dynes density of states

We show that the effect of a high-temperature environment in current transport through a normal metal-insulator-superconductor tunnel junction can be described by an effective density of states (DOS) in the superconductor. In the limit of a resistive low-ohmic environment, this DOS reduces into the well-known Dynes form. Our theoretical result is supported by experiments in engineered environments. We apply our findings to improve the performance of a single-electron turnstile, a potential candidate for a metrological current source.

cond-mat.supr-con↗

arXiv2010

Decoherence in adiabatic quantum evolution - application to Cooper pair pumping

One of the challenges of adiabatic control theory is the proper inclusion of the effects of dissipation. Here, we study the adiabatic dynamics of an open two-level quantum system deriving a generalized master equation to consistently account for the combined action of the driving and dissipation. We demonstrate that in the zero temperature limit the ground state dynamics is not affected by environment. As an example, we apply our theory to Cooper pair pumping which demonstrates the robustness of ground state adiabatic evolution.

cond-mat.mes-hall↗

arXiv2010

Ex-situ Tunnel Junction Process Technique Characterized by Coulomb Blockade Thermometry

We investigate a wafer scale tunnel junction fabrication method, where a plasma etched via through a dielectric layer covering bottom Al electrode defines the tunnel junction area. The ex-situ tunnel barrier is formed by oxidation of the bottom electrode in the junction area. Room temperature resistance mapping over a 150 mm wafer give local deviation values of the tunnel junction resistance that fall below 7.5 % with an average of 1.3 %. The deviation is further investigated by sub-1 K measurements of a device, which has one tunnel junction connected to four arrays consisting of N junctions (N = 41, junction diameter 700 nm). The differential conductance is measured in single-junction and array Coulomb blockade thermometer operation modes. By fitting the experimental data to the theoretical models we found an upper limit for the local tunnel junction resistance deviation of ~5 % for the array of 2N+1 junctions. This value is of the same order as the minimum detectable deviation defined by the accuracy of our experimental setup.

cond-mat.mtrl-sci↗

arXiv2010

Detection of mechanical resonance of a single-electron transistor by direct current

We have suspended an Al based single-electron transistor whose island can resonate freely between the source and drain leads forming the clamps. In addition to the regular side gate, a bottom gate with a larger capacitance to the SET island is placed underneath to increase the SET coupling to mechanical motion. The device can be considered as a doubly clamped Al beam that can transduce mechanical vibrations into variations of the SET current. Our simulations based on the orthodox model, with the SET parameters estimated from the experiment, reproduce the observed transport characteristics in detail.

cond-mat.mes-hall↗

arXiv2010

Non-Abelian geometric phases in ground state Josephson devices

We present a superconducting circuit in which non-Abelian geometric transformations can be realized using an adiabatic parameter cycle. In contrast to previous proposals, we employ quantum evolution in the ground state. We propose an experiment in which the transition from non-Abelian to Abelian cycles can be observed by measuring the pumped charge as a function of the period of the cycle. Alternatively, the non-Abelian phase can be detected using a single-electron transistor working as a charge sensor.

cond-mat.supr-con↗

arXiv2010

Thermal conductance of a proximity superconductor

We study heat transport in hybrid normal metal - superconductor - normal metal (NSN) structures. We find the thermal conductance of a short superconducting wire to be strongly enhanced beyond the BCS value due to inverse proximity effect. The measurements agree with a model based on the quasiclassical theory of superconductivity in the diffusive limit. We determine a crossover temperature below which quasiparticle heat conduction dominates over the electron-phonon relaxation.

cond-mat.supr-con↗

arXiv2009

SQUIPT - Superconducting Quantum Interference Proximity Transistor

We present the realization and characterization of a novel-concept interferometer, the superconducting quantum interference proximity transistor (SQUIPT). Its operation relies on the modulation with the magnetic field of the density of states of a proximized metallic wire embedded in a superconducting ring. Flux sensitivities down to $\sim 10^{-5} Φ_0$Hz$^{-1/2}$ can be achieved even for a non-optimized design, with an intrinsic dissipation ($\sim 100$ fW) which is several orders of magnitude smaller than in conventional superconducting interferometers. Our results are in agreement with the theoretical prediction of the SQUIPT behavior, and suggest that optimization of the device parameters would lead to a large enhancement of sensitivity for the detection of tiny magnetic fields. The features of this setup and their potential relevance for applications are further discussed.

cond-mat.mes-hall↗

arXiv2009

Parallel pumping of electrons

We present simultaneous operation of ten single-electron turnstiles leading to one order of magnitude increase in current level up to 100 pA. Our analysis of device uniformity and background charge stability implies that the parallelization can be made without compromising the strict requirements of accuracy and current level set by quantum metrology. In addition, we discuss how offset charge instability limits the integration scale of single-electron turnstiles.

cond-mat.mes-hall↗

arXiv2009

Electronic Radio-Frequency Refrigerator

We demonstrate experimentally that a hybrid single-electron transistor with superconducting leads and a normal-metal island can be refrigerated by an alternating voltage applied to the gate electrode. The simultaneous measurement of the dc current induced by the rf gate through the device at a small bias voltage serves as an in-situ thermometer.

cond-mat.mes-hall↗

arXiv2009

Pumping properties of the hybrid single-electron transistor in dissipative environment

Pumping characteristics were studied of the hybrid normal-metal/superconductor single-electron transistor embedded in a high-ohmic environment. Two 3 micrometer-long microstrip resistors of CrOx with a sum resistance R=80kOhm were placed adjacent to this hybrid device. Substantial improvement of pumping and reduction of the subgap leakage were observed in the low-MHz range. At higher frequencies 0.1-1GHz, a slowdown of tunneling due to the enhanced damping and electron heating negatively affected the pumping, as compared to the reference bare devices.

cond-mat.supr-con↗