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FP6

EUROMAGNET Informe resumido

Project ID: 506239
Financiado con arreglo a: FP6-INFRASTRUCTURES
País: Netherlands

Final Report Summary - EUROMAGNET (A coordinated approach to access, experimental development and scientific exploitation of european large infrastructures for high magnetic fields)

The objectives of the project were the following:
- to coordinate the transnational access to three main large infrastructures of high static and pulsed magnetic fields (RU-HFML, Nijmegen, the Netherlands; CNRS-LNCMP, Toulouse, France; and IFW-DD / HLD-FZD, Dresden, Germany) as well as a number of smaller pulsed-field installations (UBER-MG, Berlin, Germany; LVSM, Leuven, Belgium; and LCMIZ, Zaragoza, Spain);
- to develop new experimental possibilities at these infrastructures by two Joint research activities (JRAs) involving other groups;
- to foster exchange of information between the infrastructures and between various user groups by forming a network of scientific groups using high magnetic fields.

Transnational access was offered to static magnetic fields up to 33 T, and pulsed magnetic fields to the 60 - 100 T range.

Two JRAs were undertaken in order to create new experimental possibilities with the purpose of serving new user communities, as well as improving the quality of research in high magnetic fields in Europe. The development programs focused on high-field Nuclear magnetic resonance (NMR) and Infrared (IR) spectroscopy in pulsed fields.

JRA-NMR aimed at developing NMR spectroscopy at very high magnetic fields in resistive magnets up to 40 T and in pulsed magnets to 60 T. The project united all high-field NMR initiatives in Europe.

JRA-IR aimed at improving IR spectroscopy techniques with pulsed magnetic fields at frequencies which coincide with typical energies of cyclotron resonance and spin resonance at fields in the 20 - 100 T range. An important longer term goal was to make experiments possible which combine Free electron lasers (FEL) as a source of IR radiation with pulsed fields.

Science in high magnetic fields covers a broad spectrum since a magnetic field constitutes a very general, thermodynamic parameter applicable to systems in combination with many different experimental techniques. Therefore the community of researchers using high magnetic fields as an important tool for their research is vast and varied. The Science collaboration network (SCN) was set up to stimulate exchange of knowledge, information and techniques among this wide community and to establish a competitive European scientific user community.

The SCN undertook the following activities:
- an exchange program with bilateral research visits for collaboration and / or training of young scientists;
- a number of topical workshops, about one per year, centred around a specific research topic or around a new experimental technique;
- a school for young scientists (100 participants) to teach physics in high magnetic fields;
- a delocalised theory support group.

Access to continuous fields at RU-HFML, to pulsed fields at CNRS-LNCMP, IFW-DD, FZD-HLD, LVSM, LCMIZ or UBER-MG was given, provided that a research proposal for access was positively rated by a user selection panel based on the following selection criteria:
- scientific quality, relevance and originality of the proposal;
- necessity for the use of magnetic field strengths uniquely available at the infrastructures;
- track record of the user group, in particular proper reporting on possible earlier experiments;
- proper preparation by preliminary studies at lower fields at their home-institutions or elsewhere.

The most notable achievements were:
- the discovery of the quantum Hall effect in single- and bilayer graphite samples, and the observation that the quantum Hall effect persists to room temperature in graphene, by the Manchester group at RU-HFML;
- the establishment of the phase diagram of the frustrated antiferromagnetic triangular metal AgNiO2, by the Bristol group at RU-HFML;
- the observation of multiple Shubnikov-de Haas oscillations in underdoped YBaCuO superconductor, at CNRS-LNCMP;
- demonstration of critical fields over 70 T in carbon-doped thin films of MgB2; by the Genova group at CNRS-LNCMP;
- observation of quantum oscillations in overdoped Tl2Ba2CuO6+d, at CNRS-LNCMP.

The discovered, truly two-dimensional system of one and two-monolayers thick films of graphite presents an interesting and previously unachievable case of linear (relativistic) dispersion relation for quasi-particles, so their behaviour is described by the relativistic Dirac equation, rather than the Schrödinger equation. In particular, this leads to an unusual case of Quantum Hall effect (QHE) - half-integer QHE in case single-layer graphene, and strong degeneracy of Landau levels around the Dirac point.

A spectacular consequence of the peculiar bandstructure in graphene is that the QHE (actually a typical low-temperature phenomenon) surprisingly becomes observable at room-temperature.

The dHvA effect was measured in small single crystals of AgNiO2 by using a piezocantilever torque method down to temperatures as low as 0.5 K and fields up to 33 T. The observed quantum oscillations determined by the conduction electrons gave evidence of a rich behaviour in field with several new phases stabilised at high field.

High-temperature superconductivity in copper oxides occurs when the materials are chemically tuned to have a carrier concentration intermediate between their metallic state at high doping and their insulating state at zero doping.

A negative Hall resistance was observed in the magnetic-field-induced normal state of YBa2Cu3Oy and YBa2Cu4O8, which reveals that these pockets are electron-like rather than hole-like.

In order to determine whether there are other closed FS, which is of fundamental importance to clarify the FS of High-temperature superconductors (HTSC) in the pseudogap phase, a team from the University of Bristol, together with scientists from the Université de Sherbrooke, the LNCMP and International Superconductivity Center in Tokyo, have performed high-precision measurements of the de Haas-van Alphen effect in underdoped YBa2Cu3Oy.

Magnesium diboride (MgB2) is a conventional superconductor with an astonishingly high critical temperature of 40 K. The crucial characteristic in enabling conventional phonon- mediated superconductivity up to such high temperatures is the multiband transport in MgB2; hence, the importance of exploring transport in the two different types of bands, namely the p bands formed by the pz orbitals of boron atoms, which are three-dimensional and weakly coupled to phonons, and the s bands formed by sp2-hybridised orbitals stretched along boron-boron bonds, which are two-dimensional and strongly coupled with the optical E2g phonon mode.

To resolve the puzzling problem of the upper critical field in MgB2 thin films, measurements were done on a set of samples in which neutron irradiation had induced a controlled amount of disorder.

The nature of the metallic phase in the HTSCs, as well as its evolution with carrier concentration, has been a long-standing mystery. A central question is how coherent electronic states, or quasiparticles, emerge from the antiferromagnetic insulator with doping.

For the observation of NMR in resistive magnets improvements must be made of the spatial and temporal homogeneity. The team at Nijmegen developed a D2O mapping probe with stepper motor control and synchronization with a Varian infinity plus NMR console, and improved the field homogeneity using a ferroshim.

At CNRS-GHMFL the emphasis was on probes for low temperatures and with sample rotation and multi-frequency NMR.

The Haase group in Dresden reported the first spectra of 63Cu NMR at fields up to 33T (370 MHz Larmor frequency) by using a pulsed field magnet. By calibrating the pulsed field sources and developing specially adapted NMR probes, 2H NMR signals were reported at fields up to 58 T (380 MHz).

In the JRA-IR project a compact spectrometer for infrared spectroscopy in pulsed magnetic fields was built and demonstrated.

The use of Quantum cascade lasers (QCL) as Mid-infrared (MIR) excitation source of cyclotron resonance were demonstrated for pulsed magnetic field experiments in which the QCL is placed close to the magnet bore centre.

In a collaboration between Leuven and Toulouse also a fibre-based ODR system was designed and developed, and ODR at longer wavelengths was further explored in Oxford.

The light of a FEL in Dresden was coupled to pulsed field magnets and successfully used for magneto-spectroscopic experiments in very high pulsed magnetic fields. The installations for guiding the FEL into the magnet lab were completed and transport of the FEL beam over the 50 m to the magnet lab was demonstrated over a large wavelength range (from 5 to 200 µm) with acceptable losses.

In the bilateral exchange program a basic contribution of EUR 2 000 Euro per month was offered to permit the secondment of scientists between institutions, for typically several weeks to a few months.

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Jan Cornelis MAAN, (Director)
Tel.: +31-243653422
Fax: +31-243652440
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