·
Dmitri Averin:
Error-correction in Josephson-junction qubits
D.V. Averin and R. Fazio
Simple majority code correcting $k$ dephasing errors by
encoding
a qubit of information into $2k+1$ physical qubits is
studied
quantitatively. We derive an equation for quasicontinuous
evolution
of the density matrix under the error-correction procedure.
The
equation describes an effective reduced dephasing rate and
allows
to optimize the size of the code for a given rate of environmental
dephasing and accuracy of the error-correcting
transformations.
We present specific design of the Josephson-junction circuit
implementing this error-correction scheme.
·
Wolfgang Belzig:
Noise in
Superconductor-Normal-Metal-Heterostructures
Current
noise provides useful information on correlations
important
for transport properties of mesoscopic systems. Of
particular
interest, both experimentally and theoretically, are
heterostructures
of normal metals and superconductors. We use an
extended
Keldysh-Green's function approach to calculate the current
correlations
fully accounting for the superconducting proximity
effect.
The shot noise in diffusive wires shows a reentrant behaviour,
similar to
the reentrance effect of the conductance. At intermediate
energies
the effective charge (the ratio of the differential noise and
differential
conductance) is suppressed below the value of the
Cooperpair
charge 2e due to higher correlations In superconducting
heterostructures
with more than two normal leads, we address the
question
of the sign of correlations between currents in different
terminals.
We show that positive cross correlations are a generic
feature in
these structures, which should be easily observable
experimentally.
·
Lev Boulaevskii:
Tunneling
measurements of a single spin
L.N.
Bulaevskii and G. Ortiz
Los Alamos
National Laboratory.
Measurement
of tunneling current via a molecule or an atom with localized
spin
provides information on the orientation of that single spin. Such
measurement
constitutes an example of indirect continuous
quantum
measurement and is of particular importance for reading and
writing of
a single qubit. How does the tunneling current reflects the
information
stored in that single qubit? This is the main concern in this
talk.
Dependence of the tunneling current on spin orientation and the
backward
effect on the spin dynamics will be discussed.
·
Alexandre Buzdin:
New superconducting phases in field-induced organic Superconductor
A. Buzdin, M. Houzet, CPMOH,
University of Bordeaux I, France
L. Bulaevskii, M. Maley,^{ }Los Alamos
National Laboratory, USA
Very
recently the magnetic-field-induced superconductivity has been observed in the
quasi two-dimensional (2d) organic conductor
lBETS)_{2}FeCl_{4}.
Superconductivity is induced in this compound at B » 17 T and
the maximum critical temperature T_{c }» 4K is reached at B » 33 T and T_{c} (B) drops to zero at B » 45 T .
Such unusual behaviour was interpreted as the manifestation of the Jaccarino-Peter
effect, where the exchange field of aligned Fe^{3+} ions is compensated
by the external field acting on the electron spins. We derive the parallel
upper critical field B_{c2}(T) in this compound accounting for the
formation of the nonuniform Larkin-Ovchinnikov-Fulde-Ferrell state. In the
tilted field we predict the formation of new superconducting states described
by higher Landau level functions. Corresponding vortex states are quite
peculiar and reveal the zeros of superconducting order parameter with high winding
numbers. The predicted quasi-oscillatory angular and temperature dependence of
B_{c2},
as well as
a cascade of first order transitions must permit the unambiguous identification
of mysterious Larkin-Ovchinnikov-Fulde-Ferrell state.
·
Hervé Courtois
Coherent low-energy charge
transport in a diffusive S-N-S junction
We have
studied the current voltage characteristics of diffusive
mesoscopic
Nb-Cu-Nb Josephson junctions with highly-transparent Nb-Cu
interfaces.
We consider the low-voltage and high-temperature regime
eV<epsilon_c<kT
where epsilon_c is the Thouless energy. The observed
excess
current as well as the observed sub-harmonic Shapiro steps
under
microwave irradiation suggest the occurrence of low-energy
coherent
Multiple Andreev Reflection (MAR).
·
Pino Falci:
Decoherence due to 1/f noise
in superconducting nanocircuits
·
Rosario Fazio:
Interplay of magnetism and
superconductivity in small metallic grains
I would
like to give the talk on the second week. Due
to teaching
duties, most probably I cannot be at the
workshop
from the beginning. I will know this details
before the
end of the week and I will let you know in order
to change
the hotel reservation.
·
Dima Feldman:
D.E.
Feldman and V.M. Vinokur:
Destruction of bulk ordering
by surface randomness
We
demonstrate that arbitrarily weak quenched disorder on a surface of a
system of
continuous symmetry destroys long range order in the bulk, and,
instead,
quasi-long range order emerges. Correlation functions are
calculated
exactly for the weakly disordered two- and three-dimensional XY
model with
surface randomness via the functional renormalization group. Even
at strong
quenched disorder the three-dimensional XY model possesses
topological
order. Experimental realizations include
superconductors
with columnar defects.
·
Yuri Galperin:
Is weak temperature
dependence of dephasing possible?
In
collaboration with J. Bergli, V. Afonin, V. Gurevich and V. Kozub
A
first-principle theory of the electron dephasing by disorder-induced
two-state
fluctuators is developed. Two channels of the dephasing can
be
discriminated. They are, respectively, due to (i) direct
transitions
between the defect levels caused by inelastic
electron-defect
scattering, and to a "breathing" scattering potential
produced
by the fluctuators which breaks the symmetry with respect to
the time
reversal. Both mechanisms predict weak temperature dependence
of
dephasing rate at low temperatures. The quantitative
estimates
based on the experimental data concerning the fluctuators
density
show that the disorder-induced dephasing can dominate at low
enough
temperatures. It is capable to explain the experimental
low-temperature
behavior of magnetoresistance of two-dimensional
electron
gas.
·
Yuval Gefen:
My work
on spin+ interferometry+ quantum dots
·
Milena Grifoni:
Quantum ratchets
Periodic
systems lacking inversion symmetry (ratchets), have recently
attracted
much interest, due to their potential application ranging from
electronic
to biological systems.
While much
is known about classical ratchets, there is poor knowledge
about
ratchet systems being ruled by quantum equilibrium fluctuations.
We propose
a new theory capable to describe
vibrational
relaxation and tunneling in nonadiabatically driven quantum
ratchets.
Thus, our new method is not restricted to the semiclassical
requirement
of thermal equilibrium, and it accounts for interband
tunneling
transitions being necessary to properly describe the ratchet
dynamics.
We apply
our theory to describe current rectification
in
recently realized superconducting quantum ratchet devices.
The
ratchet potential is experienced
by
vortices moving in a quasi-one dimensional array of Josephson
junctions.
Due to their
small mass
and to the low temperatures of the experiments, the vortices
behave
quantum mechanically.
·
Frank Hekking:
Entanglement and
quantum measurements in Josephson junction circuits
F.W.J. Hekking,
F. Faure, LPMMC-CNRS and Université Joseph Fourier, P.O. Box 166, 38042
Grenoble, France
O. Buisson, F.
Balestro, CRTBT-CNRS P.O. Box 166, 38042, Grenoble, France
J. Pekola, Department
of Physics, University of Jyväskylä, P.O. Box 35, 40351 Jyväskylä, Finland
We theoretically study the dynamics of a
quantum superconducting circuit which consists of a Josephson charge qubit,
coupled capacitively to a current biased Josephson junction. Under certain
conditions, the eigenstates of the qubit and the junction become entangled. We
obtain the time evolution of these states in the limit of weak coupling. Rabi
oscillations occur, as a result of the spontaneous emission and re-absorption
of a single oscillation quantum in the junction. We present a way to determine
the quantum state of the junction, and hence to observe the Rabi oscillations.
The detection method is based on macroscopic quantum tunnelling (MQT). We are
currently implementing this method experimentally, performing one-shot readout
quantum measurements, using MQT escape with ultra-short pulses on a
current-biased DC-SQUID.
·
Hans Hilgenkamp:
Low Temperature
Division, Department of Applied Physics and MESA+ Research Institute,
University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Josephson
devices with intrinsic π-phase shifts
The
predominant d-wave order parameter symmetry of the high T_{c}
cuprates provides the opportunity to configure Josephson devices in which part
of the junctions is biased with an additional π-phase shift. Schulz et
al. realized for example all-high T_{c}
dc π-SQUIDs, with characteristics complementary to that of standard SQUIDs
[1]. Recently, more complex geometries have been realized, using thin film
Josephson junctions connecting high T_{c}
and low T_{c} superconductors [2].
These
developments are leading to new building blocks for superconducting
sensor / electronic devices, and facilitate further detailed studies
on the physics of high T_{c}
superconductivity, such as on the admixture of other symmetry components to the
d-wave order parameter. The current status on these activities will be
discussed.
Various
intruiging prospects arise for studies on vortex-physics using these d-wave
devices, such as the creation of Josephson contacts with designed alternations
in the sign of the critical current density, or the generation of fractional
flux-quanta at selected positions.
[1]
R.R. Schulz, B. Chesca, B. Goetz, C.W. Schneider, A. Schmehl, H. Bielefeldt, H.
Hilgenkamp, J. Mannhart and C.C. Tsuei, Design and realization of an all
d-wave dc π-superconducting quantum interference device, Appl. Phys.
Lett. 76, 912 (2000).
[2] H.J.H.
Smilde, Ariando, D.H.A. Blank, G.J. Gerritsma, H. Hilgenkamp and H. Rogalla, d-Wave-induced
Josephson current counterflow in YBa_{2}Cu_{3}O_{7} / Nb
Zigzag junctions, Phys. Rev. Lett. 88, 057004 (2002).
·
Allard Hoekstra:
Kamerlingh Onnes Laboratory,
Leiden University
At the other side of the
switchable mirror: scaling in the insulating phase of YH_{x}
How does a material change from a shiny, conducting metal
into a transparent insulator in a continuous fashion? Thin-film hydride
switchable mirrors provide a new opportunity to address this question, which is
one of the most intriguing and long-standing ones in condensed matter physics.
Such a metal-insulator transition is an example of a continuous T = 0 K quantum
phase transition. Instead of hydrogen loading at room temperature, we therefore
used persistent photoconductivity at a lowest temperature of T = 0.35 K to fine
tune the charge carrier density of an YH_{x} thin film [1], the
prototypal switchable mirror. Thus we drove a single sample from the insulating
to the metallic state while performing resistivity and Hall measurements. As
has been shown [2], dynamical scaling analysis applies to the transport data in
the metallic phase: the large values found for the critical exponents signify
the important role of electron-electron interactions. However, to analyse
recent measurements deep in the insulating phase, corrections to scaling are
required. The outcome of these corrections confirm the critical exponents in
the metallic phase, but indicate a smaller extent of the scaling regime in the
insulating phase. Open theoretical questions, invoked by these results, will be
put forward.
^{1}D. Voss,
Science 292 (2001) 1987
^{2}A.F.Th.
Hoekstra et al., Phys.Rev.Lett. 86 (2001) 5349
·
Lev
Ioffe
Possible realization of an ideal quantum comptuer in Josephson
junction array
·
Teun Klapwijk
Delft University of
technology
"Metal"-to-insulator
transition in a strongly interacting 2-dimensional electron gas
A summary
will be provided of experimental results on the metal-
insulator
transition in a low density 2 dimensional electron gas in Si
MOSFETs.
The results accumulated in the past 2 years and their
critical
evaluation appears to lead to a gradual acceptance that
competing
ground states may play a key role. After a general
overview
of the peculiar observation in these systems, the
arguments
which point to critical behavior will be outlined. A
connection
will be drawn to emerging theoretical treatments.
·
Nikolai Kopnin:
Spectral flow dissipation in
superconducting point contacts
We find
that multiple Andreev reflections (MAR) mediating the
transport
in superconducting point contacts are strongly affected
by a small
amount of impurities in the area of the contact.
The
associated momentum exchange competes with the ballistic MAR
and
results in a new linear regime in the I-V dependence
with a
conductance much larger than that in the Ohmic
regime.
Crossing vortex lattices in
layered superconductors
A.E.Koshelev
Materials
Science Division, Argonne National Laboratory,
Argonne,
Illinois 60439
In very
anisotropic layered superconductors a magnetic field
tilted
with respect to the c-axis creates a unique vortex state
consisting
of two qualitatively different interpenetrating
sublattices.
This set of crossing lattices contains a sublattice
of
Josephson vortices (JVs) generated by the component of the
field
parallel to the layers, coexisting with a sublattice of
stacks of
pancake vortices generated by the component of the field
perpendicular
to the layers. I will discuss properties of this
state. I
will consider interaction between the stack of pancake
vortices
and isolated JV and discuss observable consequences of
this
interaction. I will consider structure of isolated JV in
dense
pancake lattice, its energy, pinning of JV by pancake
lattice,
and viscous friction of moving JV due to pancake
displacements.
The phase field of the JV is composed of two types
of phase
deformations: the regular phase and vortex phase, and the
phase
deformations with smaller stiffness dominate. The
contribution
from the vortex phase smoothly takes over with
increase
of the magnetic field. We find that the structure of
cores
experiences smooth but qualitative evolution with increase
of the
ratio alpha=lambda/gamma <I>s</I>, where lambda is the
London
penetration depth, gamma is anisotropy, and <I>s</I> is
interlayer
spacing. At small alpha pancakes have only small
deformations
with respect to position of the ideal crystal while
at alpha
> 0.5 pancake stacks in the central row form
soliton-like
structure smoothly transferring between neighboring
lattice
position. I will also demonstrate that pancake vortices
strongly
pin JVs and strongly increase their viscous friction.
·
Leo Kouwenhoven
Technical University Delft,
The Netherlands
Electrons in Quantum Dots
Quantum
dots are nano-scale field-effect transistor devices, which only contain a small
number of electrons. This number can be changed with a gate voltage such that
one can create a box containing exactly one, two, three, etc. electrons. This
artificial, human-fabricated system has many similarities with atoms: the
electron energy spectrum is discrete with a shell structure, which is filled
with electrons according to Hund’s rules. We exploit the ability to tune
in-situ the quantum dot properties for a controlled study of electron-electron
interactions for a specific number of electrons. These interactions lead to
phenomena such as two-electron singlet and triplet states and the Kondo effect.
We will further discuss various quantum dot systems (semiconductor,
nanocrystals and carbon nanotubes) including some of the fabrication
procedures. Although these studies are presently pure scientific we will
speculate on electronic applications.
·
Serge Lemay
Two-Dimensional Imaging of
Electronic Wavefunctions in Carbon Nanotubes
We have obtained
two-dimensional maps of the electronic wavefunctions in
single-walled
carbon nanotubes (SWNTs). The measurements were performed by
shortening
individual metallic SWNTs so as to form discrete
"particle-in-a-box"
states, and imaging the spatial structure of the
corresponding
wavefunctions using scanning tunnelling spectroscopy. The
measured
spatial patterns, which differ markedly from the underlying atomic
lattice,
can be directly understood by considering the band structure of a
single
graphite sheet. For instance, Fourier analysis yields the expected
periodicities
for Bloch waves near the Fermi level. The wavefunction images
also
reveal delicate interference effects such as "beating" between
electron
waves with
slightly different wavevectors. We exploit the latter to directly
measure
the linear electronic dispersion relation of a metallic SWNT, and
deduce a
value for the Fermi velocity of (8.2 +/- 0.7) * 10^7 cm/s.
·
Andrei Lopatin
The theory
of quantum tunneling between paramagnetic and superconducting
states of
a nanometer-scale superconducting grain placed in a magnetic field
A.V.
Lopatin and V.M. Vinokur
We
consider the process of quantum tunnelling between the
superconducting
and paramagnetic states of
a nanometer-scale
superconducting grain placed in a magnetic
field. The
grain is supposed to be coupled via tunnelling junction
to a
normal metallic contact that plays a role of the spin
reservoir.
Using the instanton method we find the probability of
the quantum
tunnelling process and express it in terms of the
applied
magnetic field, order parameter of the superconducting
grain and
conductance of the tunnelling junction between the grain
and
metallic contact.
·
Piero Martinoli
Institut de Physique, Université
de Neuchatel, Neuchatel, Switzerland
Dimensional
crossover and hidden incommensurability in Josephson junction arrays of
periodically repeated Sierpinski gaskets
A study of
overdamped Josephson junction arrays with the geometry of
periodically
repeated Sierpinski gaskets is reported. These model
superconductors
share essential geometrical features with truly random
(percolative)
systems. When exposed to a perpendicular magnetic field
<i>B</i>,
their euclidian or fractal behavior depends on the relation between
the
intervortex distance (imposed by <i>B</>) and the size of a
constituent
gasket,
and was explored with high-resolution measurements of the
sample
magnetoinductance <i>L(B)</i>. In terms of the frustration
parameter
<i>f</i> expressing (in units of the superconducting flux
quantum)
the magnetic flux threading an elementary triangular cell
of a
gasket, the crossover between the two regimes occurs at
<i>f<sub>cN</sub>=1/(2
x 4<sup>N</sup>)</i>, where <i>N</i> is the
gasket order. In the
fractal
regime (<i>f>f<sub>cN</sub></i>) a sequence of
equally spaced
structures
corresponding to the set of states with unit cells not
larger
than a single gasket is observed at multiples of
<I>f<sub>cN</sub></I>,
as
predicted by theory. The fine structure of <i>L(f)</i> radically
changes in
the euclidian regime (<i>f<f<sub>cN</sub></i>),
where it is determined by
the
commensurability of the vortex lattice with the effective potential
created by
the array. Anomalies observed in both the periodicity and the
symmetry
of <i>L(f)</i> are attributed to the effect of a hidden
incommensurability,
which
arises from the deformation of the magnetic field distribution
caused by
the asymmetric diamagnetic response of the superconducting
islands
forming the arrays.
·
Hans Mooij
Quantum dynamics of
Josephson junctions
·
Yuli Nazarov:
Superconductivity-driven
absolute spin valve effect
Daniel
Huertas-Hernando, Yuli V. Nazarov, W. Belzig
Delft
University of Technology, the Netherlands
We investigate
spin dependent transport in hybrid
superconductor(S)--normal-metal(N)--ferromagnet(F)
structures under conditions of
proximity
effect. We demonstrate the feasibility of the absolute spin-valve effect
for a
certain interval of voltages in a system consisting of two coupled tri-layer
structures.
Our results are also valid for non-collinear magnetic configurations
of the
ferromagnets.
·
Peter Silvestrov:
Semiclassical
quantisation of Andreev billiards.
P.G.Silvestrov
Instituut-Lorentz,
Universiteit Leiden
We propose
a scheme leading to complete semiclassical solution
of
Bogoliubov-De Gennes equation in a ballistic chaotic cavity
connected
by a small contact to superconductor. Existing
solutions relate
the period of trajectory to the
energy of
corresponding Andreev level and then smooth density
of states
is found from the density of classical trajectories.
Our
approach goes further and allows to find the discrete set
of periods
corresponding to discrete eigenstates extended
over the
whole volume of the cavity.
The theory
is applied to the problems of calculating
a hard gap
in the semiclassical spectrum and crossover
between
semiclassical and random matrix description
of Andreev
billiards.
·
Herre van der Zant
Delft University of
Technology
Microscopic
charge-density-wave dynamics
We have
developed different ways of fabricating mesoscopic CDW devices including
patterning of CDW crystals by means of a focused-ion beam and reactive ion
etching. We find that CDW dynamics in small devices differ dramatically from
that of bulk crystals showing surprising new features. A few examples will be
discussed. (i) In (sub)micron NbSe_{3} wires, the conversion process of
normal electrons into the CDW condensate has been studied [1]. The data
indicate that single phase-slip events dominate the conversion process on
mesoscopic length scales. The addition and removal of CDW wave fronts may then
become correlated in time. (ii) Small gold wires are used to probe CDW dynamics
in TaS_{3} on a local scale smaller than the phase-coherent length.
When the probe spacing is of the order of a micron, the I(V) characteristics
may show a region with a negative resistance above the threshold field for
sliding [2]. (iii) In nano-CDW wires (NbSe3) we observe a metal-to-insulator
transition. The typical length scale is large compared to the (amplitude-)
coherence length and is of the order of 100 nm. On the insulating side of the
transition, the resistance follows a power-law dependence on temperature as
does the current as a function of voltage above a certain field. These
power-law dependencies may be a result of a recovery of the Luttinger-Liquid
properties of the metallic chains. No microscopic CDW theories exist as of yet
to explain these results.
[1] O.C. Mantel, F. Chalin, C.
Dekker, H.S.J. van der Zant, Yu. I. Latyshev, B. Pannetier and P. Monceau
Phys. Rev. Lett. 84 (2000) 538-541.
[2] H.S.J. van der
Zant, E. Slot, S.V. Zaitsev-Zotov, and S.N. Artemenko, Phys. Rev. Lett. 87 (2001) 126401
·
Jan van Ruitenbeek
Kamerlingh Onnes Laboratorium, Universiteit Leiden, The
Netherlands
Magic wires and the physics of conductance at the atomic
scale.
When a metallic contact is gradually broken, in the final
stages it consists
of only a few atoms. Such atomic scale contacts can now be
routinely
produced and their mechanical and electrical properties can
be studied using
a scanning tunneling microscope or a mechanically
controllable
break-junction technique. The number of conductance modes
through a single
atom contact is determined by the number of valence orbitals
of the metal
atom. This can be demonstrated by analyzing the subgap
structure for
superconducting atomic contacts, from shot noise, and from
conductance
fluctuations as a function of bias voltage. We show that
there is a
fundamental distinction between monovalent metals (gold)
where the
conductance channels tend to open one by one, and
multivalent metals
(aluminum, niobium) where this is not the case.
Guided by this knowledge, in experiments on gold we have
discovered that
during the contact breaking process the atoms in the contact
form stable
chains of single atoms being up to 7 atoms long. Such chains
constitute the
ultimate one-dimensional metallic nanowires. The mechanism behind
the
formation of chains is the same as that giving rise to the
surface
reconstructions on clean gold surfaces. This has led to the
discovery of
similar chain formation for Pt and Ir.
For alkali metals, which are the best approximation to
free-electron metals,
one observes an interaction between the electronic quantum
states and the
configurations that the wires assume. It is shown that the
most stable
"magic" wire diameters have a structure that is
similar to that of the
magic-number metal clusters.
·
Bart van Wees
Groningen University
Spin accumulation, spin transport and spin precession
in mesoscopic hybrid devices.
The
electron spin offers a new degree of freedom for designing and studying
electronic circuits. This is the research field of "spintronics" In
this talk I will give an overview of our recent results on spin injection, spin
transport, and spin precession.
By
designing and fabricating specially designed hybrid ferromagnet/non-magnetic
metal systems we were able to make a device where the output signal was
determined exclusively by the spin degree of freedom[1]. By using an applied
perpendicular magnetic field we were able to make the injected spins precess
through 180 degrees, and thus reverse the sign of the output. I will discuss the
relevant physics of spin accumulation and spin transport, and discuss
experiments in progress, including spin injection in superconductors, and
mesosopic spin dependent interference
experiments.
[1] F.J. Jedema et al., Nature 18 April 2002, to
be published
·
Anatoly Volkov :
The
effect of a nonhomogeneous magnetisation on equilibrium and transport
properties of ferromagnet/superconductor mesoscopic structures.
A.F.Volkov, F. S. Bergeret and K. B. Efetov
Ruhr-Universitat Bochum, D-44780 Bochum, Germany
The results of theoretical studies of F/S mesoscopic structures with a
nonhomogeneous magnetisaion M in the ferromagnet F are presented. The critical
Josephson current in a Josephson S/F/S junction with a rotating magnetisation
in F (a spiral structure of M(x)) is calculated [1]. In this case not only a
singlet component, but also a specific triplet component is induced in F which
is not affected by impurities. The critical current depends strongly on a characteristic
wave vector Q of the spiral structure. In particular, the inhomogeneity of the
magnetisation leads to a transition of the Josephson junction from the pi-state
into the ordinary 0-state. We also calculate the conductance of F/F' and F/S
mesoscopic structures in the presence of a domain wall in F [2]. If the domain
wall is located in the vicinity of the F/S interface, the triplet component is
induced in F and penetrates the ferromagnet over a long distance and leads to
an increase in the conductance.
·
Andrei Zaikin:
Superconducting Quantum Dot
in the Kondo Regime
Y.
Avishai, A. Golub and A.D. Zaikin
We have
developed a theoretical framework which allows
to analyse
an interplay between MAR and Coulomb effects
in a
superconducting quantum dot. The latter is modelled as
an
Anderson impurity situated between two superconductors.
We have
investigated electron transport through such dots
in the
Kondo regime. The current, shot noise power and Fano factor
are
displayed versus the applied voltage in the subgap region and
found to
be strongly dependent on the ratio between the
Kondo
temperature and the superconducting gap. In particular,
the I-V
curve exposes an excess current in
the limit
of high Kondo temperature.