A hundred years after discovery of superconductivity, one fundamental prediction of the theory, the coherent quantum phase slip (CQPS), has not been observed. CQPS is a phenomenon exactly dual1 to the Josephson effect: whilst the latter is a coherent transfer of charges between superconducting contacts 2,3 , the former is a

We show that the effect of a high-temperature environment in current transport through a normal metalinsulator-superconductor tunnel junction can be described by an effective density of states in the superconductor. In the limit of a resistive low-Ohmic environment, this density of states 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. DOI: 10.1103/PhysRevLett.105.026803 PACS numbers: 73.40.Gk, 06.20.Jr, 72.70.+m, 73.20.At Introduction.-The density of states (DOS) of the carriers governs the transport rates in a mesoscopic conductor [1], e.g., in a tunnel junction. Understanding the current transport in a junction in detail is of fundamental interest, but it plays a central role also in practical applications, for instance, in the performance of superconducting qubits [2], of electronic coolers and thermometers [3], and of a singleelectron turnstile to be discussed in this Letter [4]. When one or both of the contacts of a junction are superconducting, the one-electron rates at small energy bias should vanish at low temperatures because of the gap in the Bardeen-Cooper-Schrieffer (BCS) DOS [5]. Yet, a small linear in voltage leakage current persists in the experiments [3,6-10] that can often be attributed to the Dynes DOS, a BCS-like expression with lifetime broadening [11,12]. A junction between two leads admits carriers to pass at a rate that depends on the DOS of the conductors, the occupation of the energy levels, and the number of conduction channels in the junction [13]. In general, basic one-electron tunneling coexists with many-electron tunneling, for instance, cotunneling in multijunction systems [14], or Andreev reflection in superconductors [15,16]. However, when the junction is made sufficiently opaque, a common situation in practice, only one-electron tunneling governed by the Fermi golden rule should persist. We demonstrate experimentally that the subgap current in a high-quality opaque tunnel junction between a normal metal and a superconductor can be ascribed to photon-assisted tunneling. We show theoretically that this leads exactly to the Dynes DOS with an inverse lifetime of e 2 k B T env R=@ 2 , where T env and R are the temperature and effective resistance of the environment.We employ a tunnel junction with a normal metalinsulator-superconductor (NIS) structure; see Fig. 1(a). The essentially constant DOS in the normal metal renders

We report on single molecule electron transport measurements of two oligophenylenevinylene (OPV3) derivatives placed in a nanogap between gold (Au) or lead (Pb) electrodes in a field effect transistor device. Both derivatives contain thiol end groups that allow chemical binding to the electrodes. One derivative has additional methylene groups separating the thiols from the delocalized pi-electron system. The insertion of methylene groups changes the open state conductance by 3-4 orders of magnitude and changes the transport mechanism from a coherent regime with finite zero-bias conductance to sequential tunneling and Coulomb blockade behavior.

We present the fabrication and measurement of a radio frequency single electron transistor (rf-SET), that displays a very high charge sensitivity of 1.9 µe/ √ Hz at 4.2 K. At 40 mK, the charge sensitivity is 0.9 and 1.0 µe/ √ Hz in the superconducting and normal state respectively. The sensitivity was measured as a function of radio frequency amplitude at three different temperatures; 40 mK, 1.8 K and 4.2 K.

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.Realization of a standard for electrical current based on discreteness of the electron charge e is one of the major goals of modern metrology. The theoretical basis for obtaining current I = nef when n electrons are sequentially transferred at frequency f has been well known for more than two decades [1,2,3]. With multijunction devices it has been possible to demonstrate pumping with relative accuracy of 10 −8 up to picoampere level [4]. Although the accuracy is more than an order of magnitude better than the present definition of ampere, the output current level is, however, too small for applications apart from the capacitance standard [5]. In order to create large enough current in this fashion with the desired accuracy, parallelization of multiple pumps is inevitable. In this letter, we demonstrate parallel electron pumps with quantized current plateaus. The parallelization is done up to ten devices leading to a current level exceeding 100 pA. This is already enough for the closure of the so-called quantum metrological triangle [6,7] which would then justify the current standard based on single electron transport.The idea in quantum metrological triangle is to probe the consistency of the current from an electron pump against two other quantum phenomena, resistance from Quantum Hall effect and voltage from the AC Josephson effect. This verification would yield a consistency check for two fundamental physical constants, the charge of electron e and Planck's constant , and enable one to define the SI-units of electrical quantities directly from quantum mechanics. To obtain higher current levels, various approaches have been studied [8,9,10,11,12,13,14], such as surface acoustic waves, superconducting devices and semiconductor quantum dots but still, the present accuracy of these devices is limited. Two separate semiconductor quantum dot devices have recently been operated in parallel [15]. The hybrid turnstile [16,17] used in this work holds the promise of achieving extremely low pumping errors [18], similar to the multijunction circuits. In addition, due to the simplicity, the turnstiles can be scaled up to higher integration levels for parallel operation as they require only one tuning signal per device.The scheme of parallel turnstiles is shown in a scanning electron micrograph in Fig. 1 (a). The samples used in this letter were fabricated with standard electron beam lithography on top of a spin-on-glass insulator layer as explained in supporting information. In each of the repeated cells there is one individual device. It is a single-electron transi...

Superconducting quantum devices offer numerous applications, from electrical metrology and magnetic sensing to energy-efficient high-end computing and advanced quantum information processing. The key elements of quantum circuits are (single and double) Josephson junctions controllable either by electric current or magnetic field. The voltage control, commonly used in semiconductor-based devices via the electrostatic field effect, would be far more versatile and practical. Hence, the field effect recently reported in superconducting devices may revolutionise the whole field of superconductor electronics provided it is confirmed. Here we show that the suppression of the critical current attributed to the field effect, can be explained by quasiparticle excitations in the constriction of superconducting devices. Our results demonstrate that a miniscule leakage current between the gate and the constriction of devices perfectly follows the Fowler-Nordheim model of electron field emission from a metal electrode and injects quasiparticles with energies sufficient to weaken or even suppress superconductivity.

We present transport measurements of single-molecule junctions bridged by a molecule with three benzene rings connected by two double bonds and with thiol end-groups that allow chemical binding to gold electrodes. The I-V curves show switching behavior between two distinct states. By statistical analysis of the switching events, we show that a 300 meV mode mediates the transition between the two states. We propose that breaking and reformation of a S-H bond in the contact zone between molecule and electrode explains the observed bistability.

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.