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Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 05/05 Podcast
Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts

Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts

Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 05/05 vor 12 Jahren

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A Quantum point contact (QPC) is a one dimensional constriction,
separating two extended electron systems allowing transport between
them only though a short and narrow channel. The linear conductance
of QPCs is quantized in units of the conductance quantum
G_Q=2e^2/h, where e is the electron charge and h is Planck's
constant. Thus the conductance shows a staircase when plotted as a
function of gate-voltage which defines the width of the channel. In
addition measured curves show a shoulder-like step around 0.7G_Q.
In this regime QPCs show anomalous behaviour in quantities like
electrical or thermal conductance, noise, and thermopower, as a
function of external parameters such as temperature, magnetic
field, or applied voltage. These phenomena, collectively known as
the 0.7-anomaly in QPCs are subject of controversial discussion.
This thesis offers a detailed description of QPCs in the parameter
regime of the 0.7-anomaly. A model is presented which reproduces
the phenomenology of the 0.7-anomaly. We give an intuitive picture
and a detailed description of the microscopic mechanism leading to
the anomalous behavior. Further, we offer detailed predictions for
the behavior of the 0.7-anomaly in the presence of spin-orbit
interactions. Our best theoretical results were achieved using an
approximation scheme within the functional renormalization group
(fRG) which we developed to treat inhomogeneous interacting fermi
systems. This scheme, called the coupled ladder approximation
(CLA), allows the flow of the two-particle vertex to be
incorporated even if the number of interacting sites N, is large,
by reducing the number of independent variables which represent the
two-particle vertex from O(N^4) to O (N^2).