Differential Bloch Oscillating Transistor Pair

We examine a Bloch Oscillating Transistor pair as a differential stage for cryogenic low-noise measurements. Using two oppositely biased, nearly symmetric Bloch Oscillating Transistors, we measured the sum and difference signals in the current gain and transconductance modes while changing the common mode signal, either voltage or current. From the common mode rejection ratio we find values $\sim 20$ dB even under non-optimal conditions. We also characterize the noise properties and obtain excellent noise performance for measurements having source impedances in the M$\Omega$ range.Comment: 8 pages, 3 figure

Wigner molecules in polygonal quantum dots: A density functional study

We investigate the properties of many-electron systems in two-dimensional polygonal (triangle, square, pentagon, hexagon) potential wells by using the density functional theory. The development of the ground state electronic structure as a function of the dot size is of particular interest. First we show that in the case of two electrons, the Wigner molecule formation agrees with the previous exact diagonalization studies. Then we present in detail how the spin symmetry breaks in polygonal geometries as the spin density functional theory is applied. In several cases with more than two electrons, we find a transition to the crystallized state, yielding coincidence with the number of density maxima and the electron number. We show that this transition density, which agrees reasonably well with previous estimations, is rather insensitive to both the shape of the dot and the electron number.Comment: 8 pages, 11 figure

Electonic transport properties of nitrate-doped carbon nanotube networks

The conductivity of carbon nanotube (CNT) networks can be improved markedly by doping with nitric acid. In the present work, CNTs and junctions of CNTs functionalized with NO$_3$ molecules are investigated to understand the microscopic mechanism of nitric acid doping. According to our density functional theory band structure calculations, there is charge transfer from the CNT to adsorbed molecules indicating p-type doping. The average doping efficiency of the NO$_3$ molecules is higher if the NO$_3$ molecules form complexes with water molecules. In addition to electron transport along individual CNTs, we have also studied electron transport between different types (metallic, semiconducting) of CNTs. Reflecting the differences in the electronic structures of semiconducting and metallic CNTs, we have found that besides turning semiconducting CNTs metallic, doping further increases electron transport most efficiently along semiconducting CNTs as well as through a junction between them.Comment: 13 pages, 12 figure

Local Semiconducting Transition in Armchair Carbon Nanotubes: The Effect of Periodic Bi-site Perturbation on Electronic and Transport Properties of Carbon Nanotubes

In carbon nanotubes, the most abundant defects, caused for example by irradiation or chemisorption treatments, are small perturbing clusters, i.e. bi-site defects, extending over both A and B sites. The relative positions of these perturbing clusters play a crucial role in determining the electronic properties of carbon nanotubes. Using bandstructure and electronic transport calculations, we find out that in the case of armchair metallic nanotubes a band gap opens up when the clusters fulfill a certain periodicity condition. This phenomenon might be used in future nanoelectronic devices in which certain regions of single metallic nanotubes could be turned to semiconducting ones. Although in this work we study specifically the effect of hydrogen adatom clusters, the phenomenon is general for different types of defects. Moreover, we study the influence of the length and randomness of the defected region on the electron transport through it.Comment: 5 Pages, 5 Figure

Lämmityskuvioiden optimointi lämpötomografiassa

Thermal tomography is a promising method for non-destructive testing of materials based on measuring the boundary temperature of an object that is exposed to a known heat flux. The estimation of the internal structure, i.e. the spatially varying heat capacity and thermal conductivity, is an inverse problem, to which the statistical inversion approach is applied. The question of optimal experiment design is how to conduct an experiment so that the maximum amount of information is gained. In this thesis, the goal is to optimize the time-dependent heating patterns in thermal tomography. The actual measurements were numerically simulated using finite element modeling. The resulting parameter reconstructions with the optimized heating patterns were compared to a reference pattern to see if, on average, the optimized patterns resulted in smaller reconstruction errors. The results indicate that on average the reconstruction error was mostly dependent on the rate of increase in the heating and the total amount of heat transferred. The optimization procedure also consistently resulted in patterns with maximum heating within the given constraints.Lämpötomografia on lupaava menetelmä materiaalien rikkomattomaan aineen- koetukseen. Siinä kappaleeseen kohdistetaan tunnettu lämpövuo, ja mitataan kappaleen lämpötilaa sen reunoilla. Kappaleen sisäisen rakenteen estimointi on erääntyyppinen inversio-ongelma, johon käytetään tilastollista lähestymistapaa. Optimaalisessa koesuunnittelussa tavoitteena on suunnittella suoritettava koe siten, että se tuottaa mahdollisimman paljon informaatiota. Tässä työssä tavoitteena on optimoida lämmityskuvioita. Työssä mittausta simuloitiin numeerisesti käyttäen elementtimenetelmää. Optimoi- duilla lämmityskuvioilla saatuja parametrirekonstruktioita verrattiin verrokkiku- vioilla saatuihin arvoihin. Kysymyksenä oli, johtavatko optimoidut lämmityskuviot pienempiin virheisiin. Tulokset indikoivat että keskimääräisen rekonstruktiovirheen pieneneminen oli enimmäkseen riippuvainen lämmityksen kasvunopeudesta sekä siirretystä koko- naislämmöstä. Optimointimenetelmä myös säännöllisesti tuotti lämmityskuvioita, joissa lämmitys oli suurin mahdollinen rajoitusehtojen puitteissa