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High Temperature Superconducting Ceramics
Published in Lionel M. Levinson, Electronic Ceramics, 2020
Since the new high-Tc superconductors are clearly unusual, one is fully justified in asking whether any of the standard assumptions/findings of BCS are applicable. This has certainly been ample motivation for theoretical speculation on new mechanisms in the high-Tc materials, In fact, even in previously known low-Tc materials there has been room to speculate. For instance, the so-called heavy fermion materials like UPt3 (Stewart, 1984) have sufficiently unusual properties that some think that the Cooper pairs may be in a higher angular momentum state (p or d) and that their attraction may be caused by electronic rather than phonon interactions (Varma, 1984). This latter speculation, the mediation of the pair attraction by electronic interaction, often is called "excitonic" superconductivity and will be mentioned again later. It is also known that He3 enters a BCS-type p-wave-paired state at a few milli-Kelvin (Anderson, 1984). Although He3 is a Fermi liquid, it is not charged, so the state is superfluid rather than superconducting. Since it has no lattice (is a liquid), its Cooper pairs of He3 atoms are produced by "excitonic" interaction, i.e., by the van der Waals attraction between atoms (modified by the nuclear spin interaction).
Unconventional Superconductivity in Heavy Fermion and Ruthenate Materials
Published in David A. Cardwell, David C. Larbalestier, I. Braginski Aleksander, Handbook of Superconductivity, 2023
Heavy fermion systems comprise a class of crystalline intermetallic compounds containing a mixture of elements that have partially filled f-orbitals (typically Ce, U, or Yb), together with non-magnetic elements. The role of the non-magnetic elements is firstly to separate the f-ions thereby suppressing their tendency to order magnetically, and secondly to supply conduction electrons. The essential physics of heavy fermions comes from the coupling of their conduction electrons to partially filled f-orbitals.
Neutron Techniques: Flux-Line Lattice
Published in David A. Cardwell, David C. Larbalestier, Aleksander I. Braginski, Handbook of Superconductivity, 2022
In this entry, we have showcased the neutron techniques presently being used to study the flux-line lattice (FLL) in Type II superconductors. Due to the crystallisation of the FLL with a mesoscopic length scale, the workhorse technique of small-angle neutron scattering (SANS) continues to be the choice probe for studying FLLs and their properties in many superconducting classes, ranging from conventional superconductors such as elemental Nb (Laver et al., 2006), (Mühlbauer et al., 2009), to more exotic systems such as borocarbides (Yaron et al., 1996), (Eskildsen et al., 1998a), High-Tc cuprates (Cubitt et al., 1993), (Chang et al., 2012), pnictides (Eskildsen et al., 2011) and heavy-fermion materials (Bianchi et al., 2008), (Gannon et al., 2015). As an experimental probe of FLLs, SANS has a special status; in contrast to surface-sensitive techniques, SANS is a non-perturbative bulk probe of the FLL that allows a direct imaging of the FLL coordination and alignment. In favourable situations, information concerning the superconducting gap function of the material can be obtained. In addition, SANS studies can also be conducted for a wide range of applied magnetic fields ranging from typical values for Hc1 up to 17 T. This field range overlaps with, and even expands beyond those accessible by other probes of the FLL, enabling SANS to provide unique and complementary information concerning the microscopic FLL structure and perfection both in the bulk, and under extreme conditions.
Structural differences between single crystal and polycrystalline UBe13
Published in Philosophical Magazine, 2018
H. M. Volz, S. C. Vogel, A. I. Smith, J. L. Smith, Z. Fisk, B. Winkler, M. R. Dirmyer, E. Judge
Then in 1983, the superconductivity of UBe13 was discovered [4] with very widely spaced U atoms. However, the specific heat was anomalously large, possessing more similarities to magnets. In time, work with actinide and cerium compounds led to a class of heavy fermion superconductors, which show very high electrical resistivities from intense electron scattering that is still not fully understood and in most cases a tendency to become magnetic with small perturbations. With small amounts of substitutional thorium, i.e. as U1-xThxBe13, this compound still exhibits superconductivity, albeit altered with another transition in the specific heat [5–8].