Highlights from activities in 2005

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Highlights from activities in 2005
the group of Yuri Galperin and Tom H. Johansen
Main research topics

• Mesoscopic flux dynamics in superconductors
• Physics of granular and other hopping systems
• Nano-magnetomechanics
• Devices for quantum computation
• Physics of semiconductor nanodevices

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Mesoscopic flux dynamics in superconductors
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Fingering instability
The theory of fingering instability governing
magnetic field penetration in superconducting
films is developed.
Thin films are found to be much more unstable
than bulk superconductors, and have a stronger
tendency for formation of fingering (dendritic)
pattern.
4
We discovered that NbN below 5.5 K has a magnetic
instability in external fields, which results in
abrupt formation of dendritic flux patterns.
Quantitative analysis shows that the flux
structures have a fractal geometry. The fractal
dimension varies from D1.04 at low temperatures,
to D1.77 at 4.8 K.
5
We address here a claim made in the literature
that the carbon contents in MgB2 films is a main
cause of the dendritic flux instability.
We had MgB2 films made ultra-clean and free of
carbon, and found that such films behave as
ordinary MgB2 films as far as the instability is
concerned.
By coating MgB2 films with a layer of gold, the
instability could be suppressed, thus supporting
a thermal instability scenario, which we have
advocated previously.
6
Magneto-optical imaging is used to visualize
vortex avalanches in MgB2 films at 4 K. The size
distribution function has a clear peak whose
position moves towards larger sizes as the
applied field increases. This field dependence
as well as variation of flux density profile
during an avalanche is well described by a
proposed model assuming a thermal origin of the
avalanches. The model is based on the adiabatic
approach and takes into account nonlocal
electrodynamics in thin superconductors. The
threshold field for thermal avalanches is
predicted to be much smaller than that for thick
superconductors, in agreement with the experiment.
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Physics of granular and other hopping systems
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The Rashba and Dresselhaus spin-orbit
interactions are both shown to yield the low
temperature spin-Hall effect for strongly
localized electrons coupled to phonons. A
frequency-dependent electric field E generates a
spin-polarization current, normal to E, due to
interference of hopping paths.
The spin-Hall effects is extensively discussed in
connection with materials and devices for
spintronics.
9
High-frequency (HF) conductivity in systems with
a dense array of self-organized Ge0.7Si0.3
quantum dots in silicon with different boron
concentrations is determined by acoustic methods
in magnetic fields of up to 18 T in the
temperature interval from 1 to 20 K.
The EU STREP proposal for using such systems for
single-photon detectors has successfully passed
the first round and now is under consideration.
10
Cond-mat/0411379, accepted to Phys. Rev. Lett.
Mechanism
We have shown that the crossed Andreev reflection
is responsible for anomalous interface
magnetoresistance between a hopping semiconductor
and superconducting leads (Al, In).
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Nano-magnetomechanics
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Guided transport of paramagnetic particles over
the surface of complex patterns of magnetic walls
was studied. The complexity of the maze-type of
patterns was controlled by external fields, and a
range of novel phenomena were studied
MagnetoMechanical configuration
Mobile magnetic domain structure in ferrite
garnet films can serve as magnetomechanical
manipulator of nanoparticles, here suspended in
water
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The nano-squeezer, recently discovered by us, was
used to study a buckling-like transition in a
linear array of small paramagnetic particles. The
dipolar interactions between the particles could
be adjusted, and the effect on the critical
parameters controlling the transition was
determined. Model calculations explained the
observations
MagnetoMechanical Squeezer
A pair of Bloch walls, aligned perpendicular to a
stress-line in a ferrite garnet film, can be set
in anti-phase oscillation by an ac-magnetic
field. Trapped magnetic particles experience a
tunable compressive force
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Devices for quantum computation
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A theoretical interpretation of the recent
experiments on the T1-relaxation rate
in Josephson charge qubits is proposed. The
experimentally observed reproducible
non-monotonic dependence of T1 on the splitting
EJ of the qubit levels suggests further
specification of the previously proposed models
of the background charge noise. Within this model
one can naturally explain both the average linear
T1(EJ) dependence and the irregular fluctuations.
Crossed Andreev tunneling
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The problem of Rabi oscillations in a qubit
coupled to a fluctuator and in contact with a
heath bath is considered. A scheme is developed
for taking into account both phase and energy
relaxation in a phenomenological way, while
taking full account of the quantum dynamics of
the four-level system subject to a driving AC
field. The effect of the fluctuator state on the
read-out signal is discussed. This effect is
shown to modify the observed signal
significantly. This may be relevant to recent
experiments by Simmonds et al. (Phys. Rev. Lett.
93 (2004) 077003)
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Physics of semiconductor nanodevices
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We have observed an unusual dc current
spontaneously generated in the conducting channel
of a short-gated transistor. The magnitude and
direction of this current critically depend upon
the voltage applied to the gate. We propose that
it is initiated by the injection of hot electrons
from the gate that relax via phonon emission. The
phonons then excite secondary electrons from
asymmetrically distributed impurities in the
channel, which leads to the observed current.
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Special recognition
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Our magneto-optical image of Abrikosov vortices
selected as one of the 33 most beautiful pictures
in the history of science