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Author:Puska, Antti
Title:Mesoscopic shot noise and cross-correlation measurements on multiterminal thin films
Raekohina ja ristikorrelaatiot moniporttisissa ohutkalvoissa
Publication type:Master's thesis
Publication year:2010
Pages:v + 62      Language:   eng
Department/School:Teknillisen fysiikan laitos
Main subject:Fysiikka   (Tfy-3)
Supervisor:Dijken, Sebastiaan van
Instructor:Hakonen, Pertti
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark Aalto  264   | Archive
Keywords:mesoscopic physics
shot noise
cross-correlation
diffusive metallic resistor
shot noise universality
Hanbury Brown - Twiss exchange correction
mesoskooppinen fysiikka
virtakohina
ristikorrelaatio
diffusiivinen metallivastus
virtakohinan universaalius
Hanbury Brown - Twiss -kojaustekijä
Abstract (eng): Shot noise is a phenomenon that cannot be seen in macroscopically large resistors but can be found in mesoscopic thin and short metal wires at low temperatures.
The requirement for shot noise to appear in metallic resistors is the resistor size to be below electron-phonon scattering length specific for the material at certain temperature.
Measurements of Fano factor along with cross-correlation measurements on a multiterminal thin film metallic sample can reveal the regime and thus the physical processes behind the noise behaviour as well as a picture of the effective operational area of the sample.

In this Master's thesis a thin film copper cross was fabricated by electron beam lithography.
The measurements were done in liquid helium bath at 4.2 K temperature where copper is estimated to have 2.4µm electron-phonon scattering length Le-ph and 1µm electron-electron inelastic collision length Le-e, about the size of the cross.
Theoretically the Fano factor is sqrt(3)/4 in the hot-electron regime between Le-ph and Le -e and 1/3 in diffusive regime below Le-e.
The measured Fano factors were of the order of 0.2, below the estimates.
Different connection schemes of the cross showed the Fano factor to remain the same in rotations of the multiterminal sample in accordance with shot noise universality claims but predicted increase in the Fano factor was not seen as some of the cross arms were left open instead of being grounded.

Cross-correlation measurements from two arms of the cross in three different DC biasing schemes gave a Hanbury Brown - Twiss exchange correction of -0.20.
Negative correction value indicates that the sample behaved like a diffusive box.
Positive correction factor would indicate a diffusive cross regime while in systems of classical statistics there exist no exchange effects and the correction becomes zero.
Abstract (fin): Makroskooppisissa vastuksissa on mahdotonta havaita virtakohinaa mutta mesoskooppisissa hyvin ohuissa ja lyhyissä metalli langoissa tämä on mahdollista matalissa lämpötiloissa.
Virtakohinan erottumisen edellytys on vastuksen pieni koko; koon tulee olla langan lämpötilassa alle materiaalin elektroni-fononi-sirontapituuden.
Fano-tekijän ja ristikorrelaatioiden mittaukset monihaaraisessa ohuessa metallikalvo näytteessä kertovat kohinan laadusta sekä. sen synnyttävistä fysikaalisista ilmiöistä ja voivat antaa kuvan näytteen varsinaisesta toiminnallisesta alueesta.

Tässä diplomityössä valmistettiin kuparista ohut ristin muotoinen vastus elektronisuihkulitografian keinoin.
Mittaukset tehtiin 4.2 K lämpötilassa nesteheliumissa, jolloin arvioitiin kuparin elektroni-fononi-sirontapituudeksi Le-ph = 2.4 µm ja epäelastiseksi elektroni-elektroni-sirontapituudeksi Le-e 1µm, mikä on myös ristin koko.
Teoreettinen Fano-tekijä on sqrt(3)/4 kun näytteen koko on Le-ph. ja Le-e välillä.
Diffusiivisille alle Le-e kokoisille näytteille Fano-tekijä on 1/3.
Mitatut Fano-tekijät olivat luokkaa 0.2, selvästi alle arvioiden.
Fano-tekijän havaittiin säilyvän samana kierrettäessä näytettä tietyssä kytkennässä mikä vahvistaa virtakohinan universaaliuden mutta jätettäessä ristin haaroja kytkemättä Fano-tekijän ei havaittu kasvavan kuten oli ennustettu.

Ristin kahdesta haarasta tehtyjen ristikorrelaatiomittauksen perusteella saatiin määritettyä näytteelle Hanbury Brown - Twiss -korjaustekijä -0.20.
Negatiivisen korjaustekijän perusteella näyte muistutti diffusiivista laatikkoa.
Korjaustekijä on positiivinen rakenteeltaan diffusiiviselle ristille ja klassisille vastuksille korjaustekijä on nolla.
ED:2013-11-06
INSSI record number: 47434
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