+46 (0)46 222 0326
Your most visited
Theses, dissertations and research publications (including journal articles, conference abstracts and books) from Lund University are collected in this database. Where possible, the option to download a full text document is available. It is also possible to search for Lund University student theses in the student theses database.
|Title||Transport phenomena in quantum wells and wires in presence of disorder and interactions|
Mathematical Physics (Faculty of Science)
|Full-text||Available as PDF|
|Defence place||Lecture hall F, Sölvegatan 14A|
|Opponent||Professor Stefano Sanvito|
|Publisher||Department of Physics, Lund University|
|Popular science abstract||
All of our present information technology culture with computers, internet, smart-phones, Bluetooth links, 3D-Tv, iPad tablets, programmable washing/cooking machines, car engines, navigation computers, etc. (the list goes on and on) is based on small electrical circuits. The smaller these circuits can be made, the faster and the better microelectronics can perform.
There is much more round the corner: nano-chip technology could soon dim the
boundary between living and non-living entities, and perhaps even between us and
what is just outside our body: the external world. Some of our capabilities could
be improved or fully regained from deﬁcit situations (think of people recovering
neural abilities, improving their eyesight, using cyber-prostethics, having real-time monitoring of non-perfect vital functions, etc.)
It is fair to say that some of these developments could impel us to deal with
novel bio-ethical conﬂicts (voices of concern exist already), but science has forced us before to face dilemmas of this sort. Past experience over the last few millennia shows that each time humanity has made a great discovery (e.g. the ﬁre, the wheel, the printing press, the steam engine, the electricity, penicillin, the transistor, internet) the subsequent technological evolution has always proceeded in one
Regaining a more down-to-earth perspective, present-day electrical circuits
have reached such small dimensions that the laws of physics which govern the
microscopic world, called quantum mechanics, are becoming center-stage. Even
within the status-quo of technological development (we refer to it as "nanoelec-
tronics"), it is becoming increasingly important to have a basic understanding
of how small systems with a ﬁnite number of atoms and electrons behave when
subjected to perturbing agents, for example by electric current passing through
The knowledge we have of such systems relies, ﬁrst and foremost, on elaborate
and careful experiments. However, experimental data can be diﬃcult to interpret,
because even such small systems are in-fact many-particle systems. The analysis
can be (and usually is) further complicated by the fact that samples are "disor-
dered", i.e. we have incomplete knowledge and control of the kind of atoms and
their positions in the sample.
In principle, theoretical research can contribute signiﬁcantly to this endeavor,
by answering a number of important questions. In practice, often a major obstacle
is the lack of accurate theoretical information on how interactions among particles
and disorder aﬀect the results.
This thesis is about research work in this direction, namely theoretical investi-
gations of the electric current in diﬀerent nano-structures. We have analyzed quan-
tum wells (layered slices of semiconductors), and quantum wires (one-dimensional
conducting aggregates of atoms). Both are man-made artiﬁcial structures where,
as their name suggests, quantum eﬀect play an important role in the current trans-mission. These systems have great potential for technological breakthroughs. We
have employed rather diﬀerent theoretical techniques, aimed to look directly into
the behavior of the current in the steady-state (where the current does not change
in time), or to follow how the current changes in time to reach such steady state.
We have used the Boltzmann’s equation, a method with a long and eminent ser-
vice record in physics, but also a rather new approach (called "density functional
theory"), which uses the total electron density as a basic but only variable and
therefore requires signiﬁcantly less computing power than traditional methods.
In the end, the actual common denominator to the diﬀerent parts of our thesis
work is the presence of disorder in our systems. Disorder is ubiquitous in nature:
in fact, in many instances, the notion of order corresponds more to our need for
simple conceptualizations of reality, than to reality itself (that is, in most cases,
in nature, order exists only in an approximate way). Nanoscale systems are no
exception and, in fact, the eﬀects of disorder are expected to be strong in these
From the outside, and especially to the eye of the professional physicist, these
considerations can seem a rather tenuous link to thread together somewhat diﬀer-
ent subjects, systems and methodologies in the same thesis. For us, who worked
on these topics for several years, this thesis is a conﬁrmation that, as life itself,
scientiﬁc research is often made of pieces whose mutual connection is not imme-
diately apparent, and that, in the end, there is beauty in all diﬀerent parts of
Present-day electronics employ circuits of smaller and smaller dimensions, and today the length scales are so small that the laws of physics which rule micro-cosmos, quantum mechanics, become directly important. This thesis reports on theoretical work on electron
transport in different nanostructures. We have studied semiconductor quantum wells, layered materials where each layer can be only a few atomic layers thick, and transport in thin atomic wires. The layered materials have been studied semi-classically by means the so-called
Bolzmann equation and Monte-Carlo techniques. The works on layered
materials focused on effects of resonant scattering mechanisms on the
electron transport and the feasibility to use semiconductor super-
lattices for generating terahertz (THz)radiation. The quantum wires
were modeled by 1D Hubbard chains connected to semi-infinite leads and
were treated fully quantum-mechanically via the time-dependent density-
functional theory (TDDFT). Our TDDFT treatment appears to be able to
capture complex features due to competition between correlation and
disorder. The merits of the coherent-potential approximation are also
analyzed for contacted chains.
In total, four papers are included in the thesis.
In paper I, Monte Carlo simulations of transport in various two-
dimensional semiconductor hetero-structures, in particular in cases
where accurately calculated scattering probabilities are needed.
In paper II, we present result for electron transport in į-doped
Si/SiGe quantum wells at different temperatures and field strengths.
In paper III, we develop a Monte-Carlo technique to handle electron
transport between quantum-well layers when an electric field is applied
along the growth direction. We use this method to study scattering-
assisted transport under strong fields in the Wannier-Stark regime.
In paper IV, finally, the static and dynamical behavior of 1D Hubbard
chains are investigated. The focus is on how the interplay of
interactions and disorder affects the localization of fermions in
Hubbard chains contacted to semi-infinite leads.
Physics and Astronomy
|Keywords||disorder, time-dependent density-functional theory, electron correlation, Lowdimensional semiconducting systems, transport phenomena, Fysicumarkivet F:2012:Vettchinkina|
|Part of||Interacting fermions in 1D disordered lattices: Exploring localization and transport properties with lattice density-functional theories|
+46 (0)46 222 0326
Lund University's "ReSearch for the Future" magazine (Pdf, 10 Mb) presents a range of research from across the University.