25 research outputs found
A gentle introduction to the functional renormalization group: the Kondo effect in quantum dots
The functional renormalization group provides an efficient description of the
interplay and competition of correlations on different energy scales in
interacting Fermi systems. An exact hierarchy of flow equations yields the
gradual evolution from a microscopic model Hamiltonian to the effective action
as a function of a continuously decreasing energy cutoff. Practical
implementations rely on suitable truncations of the hierarchy, which capture
nonuniversal properties at higher energy scales in addition to the universal
low-energy asymptotics. As a specific example we study transport properties
through a single-level quantum dot coupled to Fermi liquid leads. In
particular, we focus on the temperature T=0 gate voltage dependence of the
linear conductance. A comparison with exact results shows that the functional
renormalization group approach captures the broad resonance plateau as well as
the emergence of the Kondo scale. It can be easily extended to more complex
setups of quantum dots.Comment: contribution to Les Houches proceedings 2006, Springer styl
Observation of a One-Dimensional Spin-Orbit Gap in a Quantum Wire
Understanding the flow of spins in magnetic layered structures has enabled an
increase in data storage density in hard drives over the past decade of more
than two orders of magnitude1. Following this remarkable success, the field of
'spintronics' or spin-based electronics is moving beyond effects based on local
spin polarisation and is turning its attention to spin-orbit interaction (SOI)
effects, which hold promise for the production, detection and manipulation of
spin currents, allowing coherent transmission of information within a device.
While SOI-induced spin transport effects have been observed in two- and
three-dimensional samples, these have been subtle and elusive, often detected
only indirectly in electrical transport or else with more sophisticated
techniques. Here we present the first observation of a predicted 'spin-orbit
gap' in a one-dimensional sample, where counter-propagating spins, constituting
a spin current, are accompanied by a clear signal in the easily-measured linear
conductance of the system.Comment: 10 pages, 5 figures, supplementary informatio
Quantum Point Contacts and Coherent Electron Focusing
I. Introduction
II. Electrons at the Fermi level
III. Conductance quantization of a quantum point contact
IV. Optical analogue of the conductance quantization
V. Classical electron focusing
VI. Electron focusing as a transmission problem
VII. Coherent electron focusing (Experiment, Skipping orbits and magnetic
edge states, Mode-interference and coherent electron focusing)
VIII. Other mode-interference phenomenaComment: #3 of a series of 4 legacy reviews on QPC'
Supercurrent reversal in quantum dots
When two superconductors become electrically connected by a weak link a
zero-resistance supercurrent can flow. This supercurrent is carried by Cooper
pairs of electrons with a combined charge of twice the elementary charge, e.
The 2e charge quantum is clearly visible in the height of Shapiro steps in
Josephson junctions under microwave irradiation and in the magnetic flux
periodicity of h/2e in superconducting quantum interference devices. Several
different materials have been used to weakly couple superconductors, such as
tunnel barriers, normal metals, or semiconductors. Here, we study supercurrents
through a quantum dot created in a semiconductor nanowire by local
electrostatic gating. Due to strong Coulomb interaction, electrons only tunnel
one-by-one through the discrete energy levels of the quantum dot. This
nevertheless can yield a supercurrent when subsequent tunnel events are
coherent. These quantum coherent tunnelling processes can result in either a
positive or a negative supercurrent, i.e. in a normal or a pi-junction,
respectively. We demonstrate that the supercurrent reverses sign by adding a
single electron spin to the quantum dot. When excited states of the quantum dot
are involved in transport, the supercurrent sign also depends on the character
of the orbital wavefunctions
Keldysh technique and non-linear sigma-model: basic principles and applications
The purpose of this review is to provide a comprehensive pedagogical
introduction into Keldysh technique for interacting out-of-equilibrium
fermionic and bosonic systems. The emphasis is placed on a functional integral
representation of underlying microscopic models. A large part of the review is
devoted to derivation and applications of the non-linear sigma-model for
disordered metals and superconductors. We discuss such topics as transport
properties, mesoscopic effects, counting statistics, interaction corrections,
kinetic equation, etc. The sections devoted to disordered superconductors
include Usadel equation, fluctuation corrections, time-dependent
Ginzburg-Landau theory, proximity and Josephson effects, etc. (This review is a
substantial extension of arXiv:cond-mat/0412296.)Comment: Review: 103 pages, 19 figure
Industrialised building system in Malaysia: Issues for research in a changing financial and property market
Industrialised Building System in its different guises appears to have gained popularity
in recent years. Through a thorough review of literature, this paper presents a historical
appraisal of the definitions offered for the different guises of IBS, the characteristics of
IBS, and the benefits it purports to offer. In the same vein, the factors that have fuelled
the popularity of IBS is discussed together with the impact of the changing financial and
property market crisis on the future of IBS. The implications of the current financial and
property market prices on IBS in Malaysia shall also be discussed. The paper concludes
by drawing inferences on the implications of the current financial and property market
crisis on research in IBS from a Malaysian construction industry perspective