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Coexistence of Superconductivity and Magnetism

#2774


Theoretical and Experimental Study of the Coexistence and Interplay of Superconductivity and Magnetism

Tech Area / Field

  • PHY-SSP/Solid State Physics/Physics

Status
3 Approved without Funding

Registration date
20.05.2003

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov

Supporting institutes

  • FIAN Lebedev, Russia, Moscow

Collaborators

  • Kyoto University / Department of Physics, Japan, Kyoto\nIowa State University of Science and Technology / Ames Laboratory, USA, IA, Ames\nUniversity of Saskatchewan / Department of Physics and Engineering Physics, Canada, SK, Saskatoon\nEcole Normale Supérieure / Laboratoire de Physique Statistique, France, Paris\nMax Plank Institute of Solid State Physics, Germany, Stuttgart

Project summary

The goal of this Project is to obtain different compounds at high pressures and to investigate coexistence and mutual interplay of superconductivity and magnetism in them. The study of superconductivity, magnetism, and their mutual influence is very important for understanding physics of the two phenomena, for explaining numerous experimental results, as well as for possible engineering applications. Superconductivity and magnetism are to a great extent antagonistic phenomena and their coexistence in homogeneous materials requires that special, difficultly realizable conditions be fulfilled. To take an example, for these conditions to be met, layered superlattices consisting of alternating, rather thin layers of magnetic and superconducting materials are specially prepared.

The antagonism between the ferromagnetic and superconducting long-range orders in a homogeneous system can partly be weakened through a mutual accommodation of the magnetic and superconducting subsystems. This accommodation can be reached due to the appearance of a nonuniform modulation of the magnetic order parameter and/or a state with a nonuniform superconducting order parameter.

Modulated magnetic structure is a form of coexistence of superconductivity and magnetism. Such ‘combined phases’ with modulated magnetic structures are experimentally found mainly in compounds like RRh4B4 or RMo6S8 (R is a rare-earth element). As for the mutual accommodation of superconductivity and magnetism, rare-earth boron-nickel carbides RNi2B2C should be also mentioned. Because of the alternation of ferromagnetic planes R–C with superconducting layers Ni2B2, these compounds are natural microscopic analogs of above-described superlattices.

In addition to the above-listed borides, borocarbides, and chalcogenides, there are two more classes of compounds which, in principle, may exhibit the coexistence of magnetism and superconductivity. These are intermetallic compounds RRu2 with Laves-phase structures, as well as stannides of rare earths and transition d metals crystallizing in cubic Remeika-phase structures. Up to now, however, systems with coexisting superconductivity and magnetism are limited in number, because they are difficult to obtain.

In particular, the experimenters who participate in this Project have demonstrated that in CeRu2 compound synthesized at a high pressure of ~8 GPa, the magnetic and charge states of Ru ion change strongly in comparison to those in CeRu2 produced at normal pressure. In connection with this observation, it is supposed to synthesize under high pressure a series of ternary compounds CeRu2–xGax and to investigate the mutual influence of superconductivity and magnetism in these systems for various crystal structures and charge states of Ru and Ga ions.

Thus, one of the goals of this project is the further development of methods for high-pressure synthesis of various compounds, which would allow one to extend the class of compounds exhibiting the coexistence of superconductivity with magnetism. It is also supposed to study this phenomenon in detail, both theoretically and experimentally, including joint research with foreign collaborators.

One of the goals of theoretical study proposed is to establish the reasons for change in magnetic and charge states of various ions in compounds synthesized under high pressure. It is also supposed to calculate electronic band structure and related properties for the purpose of revealing the effects of strong exchange-correlation interaction, as well as the role of immediate surroundings of an ion in the change of its state. Furthermore, it is supposed to establish the validity range of the local approximation for the exchange-correlation energy and to establish, whether it is possible to go beyond this approximation.

In most of the above examples of coexisting superconductivity and magnetism, the two phenomena were associated with two different electronic subsystems coupled only weakly to one another. The discovery of superconductivity in ‘heavy-fermion’ systems, high-temperature superconducting (HTSC) cuprates, and Sr2RuO4 compound introduced a new and very interesting aspect to the problem of coexistence of magnetism and superconductivity, namely, the question of nature of superconductivity in these systems. First of all, note that superconducting properties of such systems are essentially different from the properties of previously known superconductors. The difference is mainly in strong anisotropy of superconducting order parameter and in high critical temperatures Tc of superconducting transition.

Such a distinctive behavior of the superconducting order parameter in these systems has led a number of researchers to the conclusion that in these new superconductors, the mechanism of superconductivity is also completely new. Moreover, many authors are inclined to believe that this mechanism is magnetic by its origin. At present, the HTSC cuprates are the best-studied, and it is in this case that the strongest disagreement exists on both the superconductivity mechanism responsible for high Tc of HTSC cuprates and the properties of their normal state.

The following key problems are supposed to solve in this Project:

- Improving our technique for high-pressure synthesis of various compounds, which will make it possible to considerably extend the class of compounds exhibiting the coexistence of superconductivity and magnetism.

- Calculating the electronic band structure and related properties for both new compounds to be synthesized and a number of compounds currently available, with the aim of revealing the effects of strong exchange interaction.

- Establishing reasons for the change in magnetic and charge states of various ions in compounds synthesized under high pressure.

- Explaining the origin of superconducting order parameter in ‘unconventional’ high-temperature superconductors.

The research group from P.N. Lebedev Physical Institute of Russian Academy of Sciences (LPI) has accumulated an extensive experience in theoretical studies of high-temperature superconductivity. This work started in 1964, long before high-temperature superconductivity was experimentally discovered in 1986. After 1986, this theoretical study was being continued with a specific example of HTSC cuprates. Electronic band structure and the optical spectra of compounds La1.8Sr0.2CuO4 and YBa2Cu3O7 were calculated in the framework of standard density functional theory. The results of those calculations were confirmed by Japanese and Swiss experimentalists. The LPI team together with the researchers from Ecole Normale Superieure (France) and Cambridge University (Great Britain) have shown in their joint work that relaxation processes in the normal state of optimally doped systems are well described by a theoretical model involving strong electron–phonon interaction (EPI).

In the course of this Project, it is supposed to proceed with calculating electronic and optical spectra of HTSC compounds using appropriate computer programs available to the Project participants. Some of these programs have been developed by the LPI team in the course of previous ISTC project (# 207-98). We also plan to perform ab initio calculations of electron–phonon interaction, with the aim of clarifying the actual role of the EPI in HTSC systems. Furthermore, a number of models involving the combined effect of EPI and Coulomb exchange-correlation interaction on superconductivity will be studied with the aim of establishing the most probable mechanism of superconductivity in cuprates.

Expected Results:


- Publication of two review articles: on the current state of intermediate valence theory and on high- and intermediate-temperature superconductivity.
- Suggestions on further improvement and development of our technique for high-pressure synthesis of various compounds.
- High-pressure synthesis of a series of ternary compounds like CeRu2-xGax, as well as Laves-phase intermetallic compounds RRu2, and the study of relationship between magnetism and superconductivity in obtained compounds.
- Experimental and theoretical investigations of the physical properties of compounds like Ba1-xKxBiO3 and MgB2. Establishment of the nature of superconductivity in these compounds.
- Establishment of the role of surroundings of various ions in the origin of intermediate valence.
- Elucidation of the relationship of strong EPI with Coulomb correlations in the theory of high-temperature superconductivity.

It seems extremely useful to carry out all the investigations proposed in close cooperation of the two institutions, LPI and Russian Federal Nuclear Center–VNIIEF. Working in the field of basic research together with the LPI team that has extensive international relations will allow the VNIIEF scientists and engineers who previously developed nuclear weapons to redirect their effort to peaceful activities and to join the international scientific community.


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