Heavy ferrous metal

from Wikipedia, the free encyclopedia

Heavy-ferrous metals are metal alloys that have unusual properties due to their strongly correlated electron systems. They are mostly compounds with lanthanoids such as cerium or actinides such as uranium . Some show unconventional superconductivity ( heavy fermion superconductors ).

The term heavy fermion refers to the effective mass that the electrons (belonging to the fermions) acquire through their interaction with the periodic potential of the solid or with other electrons. As a result, they behave like free electrons, only with a modified mass ( quasi- electron ). It has nothing to do with the heavy fermions of elementary particle physics , i.e. the heavy quarks and leptons of the second and third generation ; Like all other ordinary matter, heavy fermion metals are built up from the light elementary particles.

Heavy ferrous metals show unusually high contributions of electrons to specific heat at low temperatures, which is up to a thousand times greater than expected from the Sommerfeld theory of metals . This corresponds to electrons with a high effective mass. There are also differences to normal metals in the magnetic susceptibility and the temperature dependence of the conductivity. There are various theories about the cause of the unusual behavior in which the partially filled 4f and 5f shells of the characteristic alloy components (lanthanides, actinides), which form strongly correlated systems with the conduction electrons, play a role. The Coulomb repulsion of the electrons in the partially filled shells and the mixture with the valence orbitals of the other alloy components work together.

Heavy fermion superconductors

Some heavy ferrous metals show an unconventional form of superconductivity , which was first discovered by Frank Steglich in 1979 on CeCu 2 Si 2 at a superconductor transition temperature of 0.7 Kelvin . Further examples are UPt 3 and UPd 2 Al 3 with a 2 Kelvin transition temperature. The highest value of the transition temperature so far was 2.3 Kelvin for CeCoIn 5 (Cer-Cobalt-Indium5). They are examples of unconventional superconductors that are not based on the exchange of lattice vibrations ( phonons ) like ordinary superconductors. There the electrons move much faster than the phonons, which enables the binding of Cooper pairs via phonons (the Coulomb repulsion does not prevent their formation due to the delayed interaction via the phonon exchange). With heavy fermions, however, the speed of the electrons is greatly reduced, so that the usual BCS mechanism of superconductivity does not take effect. There is evidence that spin density waves play a role instead. CeCu 2 Si 2 was also a surprising example of the non-existent coexistence of magnetism and superconductivity in ordinary superconductors, since the cerium ions have a local magnetic moment. The magnetic moments are even essential for superconductivity: if a few percent of the cerium is replaced by lanthanum (not magnetic, the corresponding compound LaCu 2 Si 2 is also not a superconductor), the substance loses its superconducting properties. Conversely, even a few percent of cerium in ordinary superconductors destroy the superconducting properties.

Quantum criticality

There can be quantum phase transitions and very pronounced quantum-critical behavior in the phase diagrams of heavy ferrous metals. The quantum phase transitions are often antiferromagnetic transitions , the Néel temperature of which is suppressed by an external parameter (e.g. pressure, doping, magnetic field) down to a temperature of 0 K. In the corresponding phase diagram, in the vicinity of such a quantum phase transition, there are often strong deviations from the classical predictions of the theory of Fermi liquids . Therefore, corresponding heavy fermion metals are referred to as “non-Fermi liquids” in these regimes.

literature

  • Frank Steglich: Heavy fermions superconductivity. From unconventional pair formation and quantum critical points , Physik Journal, Volume 3, 2004, No. 8/9, p. 61, online
  • M. Amusia, K. Popov, V. Shaginyan, V. Stephanovich: Theory of Heavy-Fermion Compounds - Theory of Strongly Correlated Fermi-Systems , Springer Series in Solid-State Sciences. 182, Springer 2015
  • Prasanta Misra: Heavy-Fermion Sytstems , Handbook of Metal Physics, Elsevier 2008
  • Yoshishika Onuki: Physics of Heavy Fermions , World Scientific 2018

Web links

Individual evidence

  1. ^ F. Steglich, J. Aarts, CD Bredl, W. Lieke, D. Meschede, W. Franz, H. Schäfer: Superconductivity in the Presence of Strong Pauli Paramagnetism: CeCu 2 Si 2 . In: Physical Review Letters . 43, No. 25, December 17, 1979, pp. 1892-1896. bibcode : 1979PhRvL..43.1892S . doi : 10.1103 / PhysRevLett.43.1892 .
  2. ↑ C. Petrovic, PG Pagliuso, MF Hundley, R. Movshovich, JL Sarrao, Thompson JD, Z. Fisk, P. Monthoux: Heavy-fermion superconductivity in CeCoIn 5 at 2.3 K . In: J. Phys .: Condens. Matter . 13, 2001, p. L337. arxiv : cond-mat / 0103168 . bibcode : 2001JPCM ... 13L.337P . doi : 10.1088 / 0953-8984 / 13/17/103 .
  3. ↑ a b Philipp Gegenwart, Qimiao Si, Frank Steglich: Quantum criticality in heavy-fermion metals . In: Nature Physics . 4, 2008, pp. 186-197. arxiv : 0712.2045 . doi : 10.1038 / nphys892 .