Specific heat and disorder in the mixed state of non-magnetic borocarbides

Lipp, D.; Schneider, M.; Gladun, A.; Drechsler, S.‐L.; Freudenberger, J.; Fuchs, G.; Nenkov, K.; Müller, K.‐H.; Cichorek, T.; Gegenwart, P.

doi:10.1209/epl/i2002-00392-7

Cited by 26 publications

(15 citation statements)

References 37 publications

(28 reference statements)

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“…On the other hand, a recent study of magnetic field orientation dependence of the specific heat has indicated line nodes, where the lines are parallel to the k z axis and pass through the k x and k y axes in the Brillouin zone . We note that these point-node and line-node Nohara et al 1999Nohara et al , 2000; see also Lipp et al 2002). This work focussed on the magnetic field dependence of the specific heat g at T << T c , where the peaks for both of these materials (Mizuno et al 1999(Mizuno et al , 2000.…”

Section: Borocarbidesmentioning

confidence: 99%

“…This seems to support the conjecture.] There is some data in poor agreement with this model (Miller et al 2000, 2001, Schmiedeshoff et al 2001, Lipp et al 2002, but for these the smaller power-law exponents might possibly be due to having multiple conduction bands with different gap magnitudes (Nakai et al 2002, Boaknin et al 2002. (See also Amin et al 2000, Dahm et al 2002b, Kusunose et al 2002.)…”

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confidence: 90%

“…One study found evidence for a significant fraction of the sample remaining normal (suggested to be due to vacancies and/or local interchanges of boron and carbon ions), with the normal signal component dominating 1/T 1 below 6K (Kohara et al 1995). These variations of the 1/T 1 data likewise support a strongly anisotropic s-wave gap form for the intrinsic behavior, apparently with nodes.Strong gap anisotropy is also demonstrated by Raman scattering (Yang et al 2000), thermal conductivity (Boaknin et al 2001), and magnetic field dependence of specific heat (Nohara et al 1997, 2000, Izawa et al 2001, Lipp et al 2002 (Mizuno et al 1999(Mizuno et al , 2000. We attribute this similarity to a high degree of disorder in both samples.…”

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confidence: 98%

“…Strong gap anisotropy is also demonstrated by Raman scattering (Yang et al 2000), thermal conductivity (Boaknin et al 2001), and magnetic field dependence of specific heat (Nohara et al 1997, 2000, Izawa et al 2001, Lipp et al 2002 (Mizuno et al 1999(Mizuno et al , 2000. We attribute this similarity to a high degree of disorder in both samples.…”

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confidence: 98%

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Strongly anisotropic s-wave gaps in exotic superconductors

Brandow

2003

Philosophical Magazine

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The exotic superconductors, defined as those which follow the phenomenological trend of Uemura (T c approximately µ l L -2 ), presently constitute the most broad and general class of superconductors which can reasonably be considered "similar to the high-T c cuprates". It is therefore of much interest to determine the forms of their pairing gap functions. We examine evidence for the gap forms in non-cuprate exotics, and demonstrate some general features. The cubic materials often have highly anisotropic gaps. The planar materials tend to have even stronger gap anisotropy, to the extent that they often have gap nodes, but nevertheless they usually have an s-wave-like gap symmetry. There is good evidence for the latter even in the controversial cases of planar organics and nickel borocarbides. Exceptions to these generalizations are also pointed out and discussed. §1. INTRODUCTIONThere are now many unusual superconductors which are recognized to share some of the novel features of the high-T c cuprate superconductors. The possibility of further similarity to the cuprates, in particular the possibility of pairing by exchange of a virtual spin fluctuation or paramagnon, has motivated suggestions of d-wave gap forms in some of these other superconductors, in cases where there is evidence for gap nodes. This argument has been applied especially to the planar organics and sometimes also to the 2 nickel borocarbides, in addition to the well known cases of heavy-fermion materials. This line of reasoning calls for a careful examination of the evidence for the gap forms in these and other unusual superconductors.In this paper we focus on the exotic superconductors of Uemura and co-workers. 1 These exotics include a number of the unusual superconductors of earlier (pre-cuprate) interest, as well as the cuprates themselves and others discovered after the cuprates, and they include most of the materials previously suggested as being similar to the cuprates. 2 As discussed below, they now constitute the most broad and general class of superconductors that can reasonably be considered similar to the cuprates. (The variety of crystal structures and material chemistries in this class of materials is remarkably extensive.2 ) It is therefore very reasonable to focus on these materials. We now examine many of the non-cuprate exotic materials and material families, those for which there is reasonable gap-form evidence, in order to establish two general conclusions about their superconducting phenomenology: (1) In the cubic and nearly-cubic materials, which are conventionally expected to have nearly isotropic gaps, there is often evidence of very strong gap anisotropy. Sometimes there even appear to be gap nodes, although the gap symmetry remains s-wave-like. (2) In the planar (quasi-two-dimensional) materials there are often gap nodes, but nevertheless they typically still have an s-wave-like gap symmetry.[Here s-wave-like means having the full point-group symmetry of the crystal structure, a condition which does allow the p...

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Section: Borocarbidesmentioning

confidence: 99%

mentioning

confidence: 90%

mentioning

confidence: 98%

mentioning

confidence: 98%

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Strongly anisotropic s-wave gaps in exotic superconductors

Brandow

2003

Philosophical Magazine

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“…For example, variation of electronic specific heat C el as a function of temperature in the superconducting state in pure YNi 2 B 2 C follows a power law, C el ∼ (T/T c ) 3 (anisotropic energy gap) whereas it exhibits an exponential T-dependence in the material YNi 1.8 Pt 0.2 B 2 C (isotropic energy gap) [5,6]. T c is not affected significantly with the introduction of Pt (T c ∼ 12.2 K in YNi 1.8 Pt 0.2 B 2 C; T c = 15.5 K in YNi 2 B 2 C); however, variation of the upper critical field H c2 as a function of x clearly shows that the material passes from the clean limit to the dirty limit with the increase of x [7].…”

Section: Introductionmentioning

confidence: 95%