Conductance fluctuations in metallic nanogaps made by electromigration

We report on low temperature conductance measurements of gold nanogaps fabricated by controlled electromigration. Fluctuations of the conductance due to quantum interferences and depending both on bias voltage and magnetic field are observed. By analyzing the voltage and magnetoconductance correlation functions we determine the type of electron trajectories generating the observed quantum interferences and the effective characteristic time of phase coherence in our device.Comment: 5 pages, 4 figures, to appear in J. Appl. Phy

The Cooper Pair Pump as a Quantized Current Source

A new charge quantization in a phase-polarized Cooper Pair Pump (CPP) is proposed, based on the topological properties of its Hamiltonian ground state over a three-dimensional parameter space $\mathbb{P}$. The charge is quantized using a set of path in $\mathbb{P}$ covering the surface of a torus, and is a multiple of the integer Chern index $c_1$ of this surface. This quantization is asymptotic but the pumped charge converges rapidly to the quantized value with the increase in the path frequency. The topological nature of the current makes this CPP implementation an excellent candidate for a metrological current standard.Comment: 4 PRL page

Smearing of Coulomb Blockade by Resonant Tunneling

We study the Coulomb blockade in a grain coupled to a lead via a resonant impurity level. We show that the strong energy dependence of the transmission coefficient through the impurity level can have a dramatic effect on the quantization of the grain charge. In particular, if the resonance is sufficiently narrow, the Coulomb staircase shows very sharp steps even if the transmission through the impurity at the Fermi energy is perfect. This is in contrast to the naive expectation that perfect transmission should completely smear charging effects.Comment: 4 pages, 3 figure

Towards single-electron metrology

We review the status of the understanding of single-electron transport (SET) devices with respect to their applicability in metrology. Their envisioned role as the basis of a high-precision electrical standard is outlined and is discussed in the context of other standards. The operation principles of single electron transistors, turnstiles and pumps are explained and the fundamental limits of these devices are discussed in detail. We describe the various physical mechanisms that influence the device uncertainty and review the analytical and numerical methods needed to calculate the intrinsic uncertainty and to optimise the fabrication and operation parameters. Recent experimental results are evaluated and compared with theoretical predictions. Although there are discrepancies between theory and experiments, the intrinsic uncertainty is already small enough to start preparing for the first SET-based metrological applications.Comment: 39 pages, 14 figures. Review paper to be published in International Journal of Modern Physics

Exploring the feasibility and acceptability of the contents, design, and functionalities of an online intervention promoting mental health, wellbeing, and study skills in Higher Education students

Background: Substantial numbers of students in Higher Education (HE) are reporting mental health difficulties, such as mild to moderate symptoms of depression and anxiety. Coupled with academic skills challenges, these difficulties can lead to decreased academic performance, low levels of study satisfaction, and eventually drop out. Student support services are facing budget cuts and can only attend to limited numbers of students, usually the ones who present with more severe mental health problems. Moreover, face-to-face contact may not appeal to those students who feel embarrassed by their problems or are afraid of being stigmatised. To address this important problem, an online psychological wellbeing and study skills support system called MePlusMe, has been developed to provide personalised support to its users. In the present study we investigated the feasibility and acceptability of the contents, design, and functionalities of the system. Methods: An offline version of the system was introduced to 13 postgraduate and undergraduate students (mean age = 31.3 years, SD = 10.25 years; 4 males) in a UK HE Institution, who presented with mild or moderate mental health difficulties. The participants evaluated the design of the system, its functionalities, and contents at Baseline and at Weeks 2, 4, and 8. Results: Participants found the system easy to use, professional, and efficient and its contents non-judgemental and informative. Participants stated that engaging with and practicing the techniques targeted at mental health difficulties led to improvements in positive thinking and self-confidence, while the study skills techniques were practical. Suggestions for further improvement included the development of an app and an option for direct engagement with professionals. Conclusions: The findings confirmed the acceptability of the contents, design and functionalities of the system, while providing useful information to inform its further development. Next steps include a feasibility study, which will test and quantify the effects on everyday functioning, mood, mental wellbeing, and academic self-efficacy after using the system, and subsequently a randomized controlled trial, which will evaluate its effectiveness

Effect of Level Statistics on Superconductivity in Ultrasmall Metallic Grains

We examine the destruction of superconducting pairing in metallic grains as their size is decreased for both even and odd numbers of electrons. This occurs when the average level spacing d is of the same order as the BCS order parameter. The energy levels of these grains are randomly distributed according to random matrix theory, and we must work statistically. We find that the average value of the critical level spacing is larger than for the model of equally spaced levels for both parities, and derive numerically the probabilities $P_{o,e}(d)$ that a grain of mean level spacing d shows pairing.Comment: 12 pages, 2 PostScript files, RevTex format, submitted to PR

Part One: The Man Who Loved Cities

Part One: The Man Who Loved Cities -- Whyte on Whyte: A Walk in the City / Eugenie L. Birch -- Holly Whyte’s Journalism of Place / Charles E. Little -- The Energizer / Ann Louise Strong -- Sowing the Seeds / Thomas Balsley -- The Wit and Wisdom of Holly Whyte / Gathered by Albert LaFarge

Rf-induced transport of Cooper pairs in superconducting single electron transistors in a dissipative environment

We investigate low-temperature and low-voltage-bias charge transport in a superconducting Al single electron transistor in a dissipating environment, realized as on-chip high-ohmic Cr microstrips. In our samples with relatively large charging energy values Ec > EJ, where EJ is the energy of the Josephson coupling, two transport mechanisms were found to be dominating, both based on discrete tunneling of individual Cooper pairs: Depending on the gate voltage Vg, either sequential tunneling of pairs via the transistor island (in the open state of the transistor around the points Qg = CgVg = e mod(2e), where Cg is the gate capacitance) or their cotunneling through the transistor (for Qg away of these points) was found to prevail in the net current. As the open states of our transistors had been found to be unstable with respect to quasiparticle poisoning, high-frequency gate cycling (at f ~ 1 MHz) was applied to study the sequential tunneling mechanism. A simple model based on the master equation was found to be in a good agreement with the experimental data.Comment: 8 pages, 6 figure

New model for system of mesoscopic Josephson contacts

Quantum fluctuations of the phases of the order parameter in 2D arrays of mesoscopic Josephson junctions and their effect on the destruction of superconductivity in the system are investigated by means of a quantum-cosine model that is free of the incorrect application of the phase operator. The proposed model employs trigonometric phase operators and makes it possible to study arrays of small superconducting granules, pores filled with superfluid helium, or Josephson junctions in which the average number of particles $n_0$ (effective bosons, He atoms, and so on) is small, and the standard approach employing the phase operator and the particle number operator as conjugate ones is inapplicable. There is a large difference in the phase diagrams between arrays of macroscopic and mesoscopic objects for $n_0 < 5$ and $U<J$ ($U$ is the characteristic interaction energy of the particle per granule and $J$ is the Josephson coupling constant). Reentrant superconductivity phenomena are discussed.Comment: 4 pages, 3 Postscript figure