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The purpose of
mesoscopic physics is the observation of the condensed
matter properties of middle-sized objects or systems; that
is, objects that are larger than a single atom (~10-10
m) but smaller than a pinhead (~10-3 m). This
range is not arbitrary, but is set by the characteristic
length scales associated with the physical effects that want
to be observed. Typical condensed matter measurements aim at
revealing the electronic, optical, thermal, and
mechanical properties of objects and systems. In our group
the main interest is on electronic properties of metals, and
metallic and semiconducting structures. In such systems the
typical length scales are the mean free path (~40 nm in Au),
the phase coherence length (~15 um in Au), the thermal
diffusion length (~1 mm in Au at 80mK and ~20um at room
temperature), and the spin relaxation length (~500 nm in
Cu). Therefore our mesoscopic objects are semiconducting or
metallic structures with dimensions between 20 nm and
hundreds of microns. Furthermore, depending on the relative
size of the structures and the above mentioned length
scales, it is possible to isolate certain physical effects,
or even reduce the dimensionality of the system to 2D (as in
the case of two dimensional electron gases in semiconducting
heterostructures or thin metallic wires), 1D (point contacts
or thin and narrow metallic wires), and 0D (quantum dots).
At this mesoscopic length
scales, both quantum and new quasi-classical effects can
appear, but in many cases they are smeared out by the
thermal motion of the lattice, which thermalizes the charge
carriers. The necessity of cooling down the structures
becomes evident. For some systems, such as spin valves,
lowering the temperature only increases the size of the
measured effect (in this case, magnetoresistance). However
for other systems, like a point contact in a 2D electron gas
(with a Fermi energy of 10 meV), cooling down from room
temperature (40 meV) to liquid Helium temperature (0.6 meV)
brings out the full quantum behavior of the point contact
which can be observed as steps in the conductance. Other
effects which require low temperatures are
superconductivity, quantized hall effect, Bohm-Aharonov
oscillations, conductance fluctuations, and weak
localization, among others. For some experiments, such as
single spin detection and electron entanglement it is
necessary to make the coherence time of a quantum state as
long as possible. In this case it is necessary to use
dilution refrigerators to cool down the system to
temperatures of a few milikelvin.
Current efforts in our
group are oriented towards:
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The measurement of
electron transport in a time scale shorter than the
phase coherence time of individual electrons.
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Entanglement and
detection of electron spins in quantum dots.
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Investigation of
charge and spin transport in spin valve structures.
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