Functional renormalization group study of the Anderson--Holstein model

We present a comprehensive study of the spectral and transport properties in the Anderson--Holstein model both in and out of equilibrium using the functional renormalization group (FRG). We show how the previously established machinery of Matsubara and Keldysh FRG can be extended to include the local phonon mode. Based on the analysis of spectral properties in equilibrium we identify different regimes depending on the strength of the electron--phonon interaction and the frequency of the phonon mode. We supplement these considerations with analytical results from the Kondo model. We also calculate the non-linear differential conductance through the Anderson--Holstein quantum dot and find clear signatures of the presence of the phonon mode.Comment: 19 pages, 8 figure

Physics of Proximity Josephson Sensor

We study the proximity Josephson sensor (PJS) in both bolometric and calorimetric operation and optimize it for different temperature ranges between 25 mK and a few Kelvin. We investigate how the radiation power is absorbed in the sensor and find that the irradiated sensor is typically in a weak nonequilibrium state. We show in detail how the proximity of the superconductors affects the device response: for example via changes in electron-phonon coupling and out-of-equilibrium noise. In addition, we estimate the applicability of graphene as the absorber material.Comment: 13 pages, 11 figures, submitted to Journal of Applied Physics, v2: Addition of a new section discussing the radiation coupling to the device, several minor change

Nonequilibrium characteristics in all-superconducting tunnel structures

We study the nonequilibrium characteristics of superconducting tunnel structures in the case when one of the superconductors is a small island confined between large superconductors. The state of this island can be probed for example via the supercurrent flowing through it. We study both the far-from-equilibrium limit when the rate of injection for the electrons into the island exceeds the energy relaxation inside it, and the quasiequilibrium limit when the electrons equilibrate between themselves. We also address the crossover between these limits employing the collision integral derived for the superconducting case. The clearest signatures of the nonequilibrium limit are the anomalous heating effects seen as a supercurrent suppression at low voltages, and the hysteresis at voltages close to the gap edge $2\Delta/e$, resulting from the peculiar form of the nonequilibrium distribution function.Comment: 8 pages, 10 figure

Theory of temperature fluctuation statistics in superconductor-normal metal tunnel structures

We describe the statistics of temperature fluctuations in a SINIS structure, where a normal metal island (N) is coupled by tunnel junctions (I) to two superconducting leads (S). We specify conditions under which this structure exhibits manifestly non-Gaussian fluctuations of temperature. We consider both the Gaussian and non-Gaussian regimes of these fluctuations, and the current fluctuations that are caused by the fluctuating temperature. We also describe a measurement setup that could be used to observe the temperature fluctuations.Comment: 10 pages, 9 figures, final versio

Fully Overheated Single-Electron Transistor

We consider the fully overheated single-electron transistor, where the heat balance is determined entirely by electron transfers. We find three distinct transport regimes corresponding to cotunneling, single-electron tunneling, and a competition between the two. We find an anomalous sensitivity to temperature fluctuations at the crossover between the two latter regimes that manifests in an exceptionally large Fano factor of current noise.Comment: 6 pages, 3 figures, includes Appendi

Dali server update

The Dali server (http://ekhidna2.biocenter.helsinki.fi/dali) is a network service for comparing protein structures in 3D. In favourable cases, comparing 3D structures may reveal biologically interesting similarities that are not detectable by comparing sequences. The Dali server has been running in various places for over 20 years and is used routinely by crystallographers on newly solved structures. The latest update of the server provides enhanced analytics for the study of sequence and structure conservation. The server performs three types of structure comparisons: (i) Protein Data Bank (PDB) search compares one query structure against those in the PDB and returns a list of similar structures; (ii) pairwise comparison compares one query structure against a list of structures specified by the user; and (iii) all against all structure comparison returns a structural similarity matrix, a dendrogram and a multidimensional scaling projection of a set of structures specified by the user. Structural superimpositions are visualized using the Java-free WebGL viewer PV. The structural alignment view is enhanced by sequence similarity searches against Uniprot. The combined structure-sequence alignment information is compressed to a stack of aligned sequence logos. In the stack, each structure is structurally aligned to the query protein and represented by a sequence logo.Peer reviewe

The use of laptop computers in programming lectures

This research explores the effect of the use of laptop computers on students&rsquo; learning experiences during lectures. Our methodology involves embedding laptops with visualization software as a learning aid during lectures. We then employ a framework of seven principles of good practice in higher education to evaluate the impact of the use of laptop computers on the learning experience of computer programming students. Overall, we found that students were highly motivated and supportive of this innovative use of laptop computers with lectures.<br /

Factors of air ion balance in a coniferous forest according to measurements in Hyytiälä, Finland

A new mathematical model describing air ion balance was developed and tested. It has improved approximations and includes dry deposition of ions onto the forest canopy. The model leads to an explicit algebraic solution of the balance equations. This allows simple calculation of both the ionization rate and the average charge of aerosol particles from measurements of air ions and aerosol particles, with some parameters of the forest. Charged aerosol particles are distinguished from cluster ions by their size, which exceeds 1.6 nm diameter. The relative uncertainty of the ionization rate is about the same or less than the relative uncertainties of the measurements. The model was tested with specific air ion measurements carried out simultaneously at two heights at the Hyytiälä forest station, Finland. Earlier studies have shown a difference in the predictions of the ionization rate in the Hyytiälä forest when calculated in two different ways: based on the measurements of the environmental radioactivity and based on the air ion and aerosol measurements. The new model explains the difference as a consequence of neglecting dry deposition of ions in the earlier models. The ionization rate during the 16 h campaign was 5.6&plusmn;0.8 cm^&minus;3 s^&minus;1 at the height of 2 m and 3.9&plusmn;0.2 cm^&minus;3 s^&minus;1 at the height of 14 m, between the tops of the trees. The difference points out the necessity to consider the height variation when the ionization rate is used as a parameter in studies of ion-induced nucleation. Additional results are some estimates of the parameters of air ion balance. The recombination sink of cluster ions on the ions of opposite polarity made up 9&ndash;13%, the sink on aerosol particles 65&ndash;69%, and the sink on forest canopy 18&ndash;26% of the total sink of cluster ions. The average lifetime of cluster ions was about 130 s for positive and about 110 s for negative ions. At the height of 2 m, about 70% of the space charge of air was carried by aerosol particles, and at the height of 14 m, about 84%

Characterization of air ions in boreal forest air during BIOFOR III campaign

International audienceThe behavior of the concentration of positive small (or cluster) air ions and naturally charged nanometer aerosol particles (aerosol ions) has been studied on the basis of measurements carried out in a boreal forest at the Hyytiälä SMEAR station, Finland, during the BIOFOR III campaign in spring 1999. Statistical characteristics of the concentrations of cluster ions, two classes of aerosol ions of the sizes of 2.5?8 nm and 8?ca 20 nm and the quantities that determine the balance of small ions in the atmosphere have been given for the nucleation event days and non-event days. The dependence of small ion concentration on the ion loss (sink) due to aerosol particles was investigated applying a model of bipolar diffusion charging of particles by small ions. The small ion concentration and the ion sink were closely correlated (correlation coefficient 87%) when the fog events and the hours of high relative humidity (above 97%), as well as nocturnal calms and weak wind (wind speed-1 had been excluded. In the case of nucleation burst events, variations in the concentration of small positive ions were in accordance with the changes caused by the ion sink due to aerosols; no clear indication of positive ion depletion by ion-induced nucleation was found. The estimated average ionization rate of air at the Hyytiälä station in early spring, when the ground was partly covered with snow, was about 4.8 ion pairs cm-3 s-1. The study of the charging state of nanometer aerosol particles (2.5?8 nm) revealed a strong correlation (correlation coefficient 88%) between the concentrations of particles and their charged fraction (positive air ions) during nucleation bursts. The estimated charged fraction of particles, which varied from 3% to 6% considering various nucleation event days, confirms that these particles are almost quasi-steady state charged. Also the particles and air ions in the size range of 8?ca 20 nm showed a good qualitative consistency; the correlation coefficient was 92%

Giant current fluctuations in an overheated single electron transistor

Interplay of cotunneling and single-electron tunneling in a thermally isolated single-electron transistor (SET) leads to peculiar overheating effects. In particular, there is an interesting crossover interval where the competition between cotunneling and single-electron tunneling changes to the dominance of the latter. In this interval, the current exhibits anomalous sensitivity to the effective electron temperature of the transistor island and its fluctuations. We present a detailed study of the current and temperature fluctuations at this interesting point. The methods implemented allow for a complete characterization of the distribution of the fluctuating quantities, well beyond the Gaussian approximation. We reveal and explore the parameter range where, for sufficiently small transistor islands, the current fluctuations become gigantic. In this regime, the optimal value of the current, its expectation value, and its standard deviation differ from each other by parametrically large factors. This situation is unique for transport in nanostructures and for electron transport in general. The origin of this spectacular effect is the exponential sensitivity of the current to the fluctuating effective temperature.Comment: 10 pages, 11 figure