Electron Refrigeration in the Tunneling Approach

The qualities of electron refrigeration by means of tunnel junctions between superconducting and normal--metal electrodes are studied theoretically. A suitable approximation of the basic expression for the heat current across those tunnel junctions allows the investigation of several features of the device such as its optimal bias voltage, its maximal heat current, its optimal working point, and the maximally gained temperature reduction. Fortunately, the obtained results can be compared with those of a recent experiment.Comment: 4 pages, 4 Postscript figures, uses eps

Effect of a thin AlO_x layer on transition-edge sensor properties

We have studied the physics of transition-edge sensor (TES) devices with an insulating AlOx layer on top of the device to allow implementation of more complex detector geometries. By comparing devices with and without the insulating film, we have observed significant additional noise apparently caused by the insulator layer. In addition, AlOx was found to be a relatively good thermal conductor. This adds an unforeseen internal thermal feature to the system.Comment: 6 pages, 5 figures, Low Temperature Detectors 14 conferenc

Efficient Peltier refrigeration by a pair of normal metal/ insulator/superconductor junctions

We suggest and demonstrate in experiment that two normal metal /insulator/ superconductor (NIS) tunnel junctions combined in series to form a symmetric SINIS structure can operate as an efficient Peltier refrigerator. Specifically, it is shown that the SINIS structure with normal-state junction resistances 1.0 and 1.1 k$\Omega$ is capable of reaching a temperature of about 100 mK starting from 300 mK. We estimate the corresponding cooling power to be 1.5 pW per total junction area of 0.8 $\mu$m$^2$ at $T= 300$ mK.Comment: 7 pages, revtex, 3 figures by fax/conventional mail upon reques

Nonequilibrium Superconductor-Normal Metal Tunnel Contact and the Phonon Deficit Effect

We consider tunnel microrefrigerators at low temperature. There is a number of experimental studies performed on microrefrigeration in tunneling superconductor--normal metal (SN) structures. Related to these experiments, only the electron subsystem has been considered theoretically. Independently, the phonon deficit effect has been studied a while ago in superconductor-superconductor tunnel junctions. It can be regarded as a possible prototype scheme for superconducting microrefrigerators. We try to provide the missing link between experiments on the SN tunnel junction refrigerators and the theory which includes microscopically phonons in combination with the mechanism of the phonon deficit effect.Comment: 2 pages, 1 figur

Microrefrigeration by a pair of normal metal/insulator/superconductor junctions

We suggest and demonstrate experimentally that two normal metal/insulator/superconductor (NIS) tunnel junctions combined in series to form a symmetric SINIS structure can operate as an efficient Peltier refrigerator. Specifically, it is shown that the SINIS structure with normal-state junction resistences of 1.0 and 1.1 kOmega is capable of reaching a temperature of about 100 mK starting from 300 mK. We estimate the corresponding cooling power to be 1.5 pW per total junction area of 0.8 micrometers(exp 2) at T = 300 mK. This cooling power density implies that scaling of junction area up to about 1 mm(exp 2) should bring the cooling power into the microW range

Quantization of the elastic modes in an isotropic plate

We quantize the elastic modes in a plate. For this, we find a complete, orthogonal set of eigenfunctions of the elastic equations and we normalize them. These are the phonon modes in the plate and their specific forms and dispersion relations are manifested in low temperature experiments in ultra-thin membranes.Comment: 14 pages, 2 figure

Uusliberalismi suomalaisissa yliopistoissa:retorinen diskurssianalyysi kantaaottavasta lehtikirjoittelusta

Tiivistelmä. Tässä pro gradu -tutkielmassa tutkitaan julkisuudessa esitettyjä näkemyksiä siitä, kuinka uusliberalismi on vaikuttanut suomalaisten yliopistojen aatteisiin ja toimintaan. Tutkimuksessa analysoidaan retorisella diskurssianalyysilla aihetta käsitteleviä kantaaottavia lehtikirjoituksia, jotka on julkaistu vuosien 2000–2018 aikana. Tutkimuskysymykset ovat seuraavat: millaisia diskursseja tuotetaan uusliberalismin vaikutuksista suomalaisten yliopistojen aatteisiin ja toimintaan sekä miten näihin liittyviä näkemyksiä argumentoidaan. Tutkimusaihetta voidaan pitää ajankohtaisena, sillä uusliberalistiset aatteet ovat vaikuttaneet jo vuosikymmenten ajan sekä Suomessa että maailmalla. Maailmanlaajuisesti ne ovat näkyneet 1980-luvulta lähtien organisaatioihin ja yksilöihin kohdistettuina tehokkuusvaatimuksina, kun taas Suomessa uusliberalistisia aatteita on voitu havaita erityisesti 1990-luvulla, kun esimerkiksi tutkijoilta on vaadittu tulosvastuullisuutta, kansainvälistymistä ja tehokkuutta. Tämän suuntaiset vaatimukset ovat vain kasvaneet 2000-luvulle tultaessa, kun yliopistoilta on edellytetty entistä nopeampaa ja joustavampaa reagointia ympäristönsä muutoksiin sekä tarpeellisuutensa jatkuvaa todistamista. Tämän voidaan taas ajatella herättävän ihmisten keskuudessa monenlaisia, yliopistojen uusliberalistiseen kehitykseen liittyviä näkemyksiä, joista saatua informaatiota voidaan pitää arvokkaana palautteena yliopistolaitoksen kehittämisen näkökulmasta. Tutkimuksen mukaan on löydettävissä neljä diskurssia, joissa esiintyy yliopistojen uusliberalistisia aatteita ja toimintamalleja puolustavia ja vastustavia argumentteja; ajan henki -, humanistinen-, kilpailukyky- sekä perinteinen yliopisto -diskurssi. Näistä ajan henki - ja humanistinen diskurssi ovat kaikista vahvimpia. Ajan henki -diskurssissa kuvataan yliopistojen uusliberalistista kehitystä ajankohtaisena ja jopa väistämättömänä asiana, mikä herättää diskurssissa monenlaisia mielipiteitä puolesta ja vastaan. Humanistisessa diskurssissa taas argumentoidaan yliopistojen uusliberalististen aatteiden ja toimintamallien puolesta ja niitä vastaan humanistisista, kuten inhimillisyyden, moraalin ja tasa-arvon näkökulmista. Kilpailukyky-diskurssissa kannatetaan kilpailun, tehokkuuden ja tuottavuuden näkymistä yliopistoissa, kun taas perinteinen yliopisto -diskurssissa argumentoidaan yliopistojen historiaan, traditioihin ja sivistysyliopistoon pohjautuvien aatteiden ja toimintamallien puolesta. Kaikissa diskursseissa käytetään monipuolisesti niiden argumentaatiota vahvistavia retorisia keinoja. Lisäksi voidaan yleisesti havaita, että yliopistojen uusliberalistisia aatteita ja toimintamalleja puolustetaan yhteensä yhdeksässä ja vastustetaan yhdessätoista lehtikirjoituksessa 20:stä

Efficiency in nanostructured thermionic and thermoelectric devices

Advances in solid-state device design now allow the spectrum of transmitted electrons in thermionic and thermoelectric devices to be engineered in ways that were not previously possible. Here we show that the shape of the electron energy spectrum in these devices has a significant impact on their performance. We distinguish between traditional thermionic devices where electron momentum is filtered in the direction of transport only and a second type, in which the electron filtering occurs according to total electron momentum. Such 'total momentum filtered' kr thermionic devices could potentially be implemented in, for example, quantum dot superlattices. It is shown that whilst total momentum filtered thermionic devices may achieve efficiency equal to the Carnot value, traditional thermionic devices are limited to efficiency below this. Our second main result is that the electronic efficiency of a device is not only improved by reducing the width of the transmission filter as has previously been shown, but also strongly depends on whether the transmission probability rises sharply from zero to full transmission. The benefit of increasing efficiency through a sharply rising transmission probability is that it can be achieved without sacrificing device power, in contrast to the use of a narrow transmission filter which can greatly reduce power. We show that devices which have a sharply-rising transmission probability significantly outperform those which do not and it is shown such transmission probabilities may be achieved with practical single and multibarrier devices. Finally, we comment on the implications of the effect the shape of the electron energy spectrum on the efficiency of thermoelectric devices.Comment: 11 pages, 15 figure

Dimensionality effects in restricted bosonic and fermionic systems

The phenomenon of Bose-like condensation, the continuous change of the dimensionality of the particle distribution as a consequence of freezing out of one or more degrees of freedom in the low particle density limit, is investigated theoretically in the case of closed systems of massive bosons and fermions, described by general single-particle hamiltonians. This phenomenon is similar for both types of particles and, for some energy spectra, exhibits features specific to multiple-step Bose-Einstein condensation, for instance the appearance of maxima in the specific heat. In the case of fermions, as the particle density increases, another phenomenon is also observed. For certain types of single particle hamiltonians, the specific heat is approaching asymptotically a divergent behavior at zero temperature, as the Fermi energy $\epsilon_{\rm F}$ is converging towards any value from an infinite discrete set of energies: ${\epsilon_i}_{i\ge 1}$. If $\epsilon_{\rm F}=\epsilon_i$, for any i, the specific heat is divergent at T=0 just in infinite systems, whereas for any finite system the specific heat approaches zero at low enough temperatures. The results are particularized for particles trapped inside parallelepipedic boxes and harmonic potentials. PACS numbers: 05.30.Ch, 64.90.+b, 05.30.Fk, 05.30.JpComment: 7 pages, 3 figures (included

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.Comment: 10 pages, 5 figures. To appear in the proceedings of the Conference on Micro- and Nanocryogenics (LT25 satellite) organized in Espoo, Finland (2008