240 research outputs found
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 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 m at 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 is converging towards any
value from an infinite discrete set of energies: . If
, 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
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