Scientific context
The nanoscale confinement phenomena have recently shifted into the focus of modern condensed matter physics, and a very
intense research on confined condensates has already started across the world. This brings us to
the main objective of the proposed programme
: to investigate the effect of the nanoscale confinement of condensate and flux on superconductivity in order to reveal its
nanoscale evolution and to determine the fundamental relations between quantized confined states and the physical properties
of these systems, enabling “quantum design” of their properties.
Along the line of the main objective, the proposed
research will be focused on the following topics:
- Evolution of superconductivity at nanoscale, superfluidity in restricted geometries (Task 1).
The correlation between the nanograin size and the superconducting gap and the critical temperature Tc will be
investigated theoretically and experimentally. We will systematically reduce the characteristic size of superconducting
grains and clusters in order to reveal the crossover between the bulk superconducting regime and fluctuation-dominated
superconductivity regime. For comparison, superfluidity in nanopores will also be studied as a function of
the size of the nanopores.
- Superconductivity in hybrid superconducting – normal (SN) and superconducting – magnet (SM)
nanosystems with tuneable boundary condition (Task 2).
Confined condensate will be studied in superconducting
nano-islands surrounded by normal metallic or magnetic material. The role of proximity effects and the Andreev reflection
in modifying the transparency of the sample boundaries will be revealed. The variation of the superfluid density near the
boundary will be mapped by using the local scanning tunnelling spectroscopy (STS) techniques. Different vortex configurations,
including those with symmetry induced antivortices, and their dynamics will be investigated in individual nanostructures of
different geometries. Here we expect to find strong effect of the specific boundary conditions on confined flux
and condensate.
- Confined flux in nanostructured superconductors and hybrid SN and SM nanosystems (Task 3).
Three different types of
nanostructured superconductors will be investigated: individual nanoplaquettes of different topology, their clusters and huge arrays.
By using local probe techniques, such as STM and scanning Hall-probe microscope, the distributions of the order parameter density
and local magnetic fields will be mapped simultaneously and then compared with the calculations of these parameters based
on the solution of the GL equations with the realistic boundary conditions imposed though nanostructuring. Hybrid SN and SM
arrays will be also studied. Magnetic dots will be used to generate local vortex-antivortex dipole loops, which will be strongly
interacting with the flux lines in superconductors, creating a tunable magnetic periodic confinement. Different novel flux phases,
including stable vortex-antivortex patterns, will be studied. Here we can anticipate a very interesting interplay between
flux generated by an applied field and magnetic dipoles, which can substantially enhance flux pinning. Magnetic domains will
be used to achieve vortex manipulation.
Using the recent progress in nanoengineered pinning arrays in superconductors,
similar structures can be made to confine flux in rotating superfluids. Keeping in mind that the coherence length for the 3He
superfluids is much longer than for 4He, it seems to be much easier to fabricate periodic pinning arrays for 3He.
Instead of antidots used in superconductors, an adequate choice here is the periodic array of nanopillars. Here we expect to find
out novel flux phases, which are otherwise not stable in a reference superfluid without a periodic pinning array.
- Josephson effects and tunneling in weakly coupled condensates (Task 4).
We shall investigate a variety of Josephson phenomena and phase shifting effects in coupled superconducting condensates,
where nanoscale coupling can be provided through an insulating, metallic or magnetic layer. Hybrid structures are essential
here in order to tune the coupling strength. These phenomena will be compared with Josephson effects in coupled superfluids,
mostly based on 3He.
- Fundamentals of fluxonics, superconducting devices (Task 5).
We will study the devices that control the motion of flux quanta in superconductors and could address a central problem in
many superconducting devices; namely, the removal of trapped magnetic flux that produces noise. The controllable vortex
motion will be used in nanostructured superconductors for making pumps, diodes and lenses of quantized magnetic flux.
Vortex ratchets effects will be studied and then used to achieve vortex manipulation.
One of the important aspects of this work is to investigate superconducting nanostructured materials for which the
confinement of the condensate inside the samples can be controlled by imposing the proper boundary conditions for the order
parameter at the nanofabricated boundaries. Remarkably, the order parameter, the analogue of the wave function for
normal quantum mechanical systems, obeys the Ginzburg-Landau equations, which play a role similar to that
of the Schrödinger equation. This gives a theoretical background for proving the feasibility of the fundamentals
of the quantum design and nanoengineering of the superconducting critical parameters. The concept of quantum
design is now the backbone for developing new elements and systems for microelectronics and information
technology (quantum computing, SQUIDS with improved sensitivity, sensors, etc).
Summarizing the proposed tasks in NES on nanostructured superconductors, we can say that the core of the proposal
will be focused on the development of the fundamental principles of the “quantum design” of two important superconducting
critical parameters - critical currents and critical fields - through the optimization of the flux and condensate confinement.
The nanoscale evolution of superconductivity will be investigated. In individual nanostructures topology- and geometry-dependent
critical fields, as well as to the symmetry induced antivortices will be investigated. In nanostructured superconductors
a rich variety of novel flux phases and patterns will be studied in order to master vortex behaviour and develop fundamentals
of fluxonics. Superconducting elements for quantum computing will be designed and investigated.
Facilities and expertise which will be accessible within the ESF NES Programme
Schematic illustration of the NES - Integrated Research Facilities
In order to carry out successfully the planned joint research, the integration of the research facilities
of the NES- teams will be achieved at five different levels:
- Integration of modern sample preparation and nanostructuring techniques (ground floor).
Molecular Beam Epitaxy –MBE, Sputtering, Thermal evaporation, Laser ablation, Clean Room, Reflection High Energy
Electron Diffraction –RHEED, Auger spectroscopy, Infrared Spectra, X-ray Photoelectron Spectroscopy –XPS, Energy Dispersive
X-Ray Spectroscopy –EDS, Rutherford Backscattering Spectrometry –RBS, E-beam lithography, Ion beam etching,
Scanning Tunneling Microscopy –STM writing, Bottom-up methods of self assembly, X-ray diffraction, Ion Implanter,
Irradiation.
- Integration of local probing techniques enabling vortex visualization and condensate wave function mapping with a nanoscale resolution (first floor).
Local techniques are a key factor for achieving the scientific objectives, since these technologies provide an important
microscopic information:
(Low Temperature) STM, (Low Temperature) Scanning Tunneling Spectroscopy –STS, Force Microscopy – FM, Low Temperature Laser
Microscopy –LT laserM, Low Temperature Electron Microscopy –LTEM, Scanning Electron Micropscopy –SEM, Micro-Raman,
Scanning Hall Probe or (array) Hall micro-magnetometry, Magnetic decoration, Scanning Superconducting Quantum Interference
Device –Scanning SQUID, Magneto-Optical Imaging –MOI, Low energy muon spin rotation (LE-µSR), Transmission Electron Microscopy-TEM.
- The next level of the shared research facilities is bulk integrated response. (second floor).
The techniques needed for the experimental studies on nanostructured superconductors are:
SQUID, Vibrating Sample Magnetometry –VSM, Torque Magnetometry, AC-susceptibility, Noise measurements, MOKE,
Thermal conductivity, Electrical transport measurements (including high frequency responses), Ultra Low Temperature Systems,
Ultrasonic resonance, Specific heat, Neutron scattering, Synchrotron radiation, Far-infrared magnetooptics –FIR-MO,
Nuclear Magnetic Resonance –NMR.
- A test platform for the development of new applications. (third floor).
Josephson junctions technology, Ultra sensitive SQUID magnetometers, Superconducting SC – qubits, flux –logics -lenses -diodes –transistors.
- The theoretical methods and techniques will be integrated in order to interact continuously with the experimental NES-teams (fourth floor).
The most important approaches describing the physics of individual nanostructured superconductors are:
Bardeen-Cooper and Schrieffer –BSC, (Time Dependent) Ginzburg-Landau -(TD)GL, Bogolubov-de-Gennes, Richardson’s
approach to the solution of the BCS Hamiltonian, Molecular dynamics simulations, Group theory and Topology,
Monte Carlo simulations, bosonization, renormalization group calculations, Keldysh-formalisms, Sine-Gordon-Equation.
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