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This article has described the discovery of superconductivity in Y-Ni-B-C system and shown that it is an important landmark in the field of superconductivity. It can be seen from the above discussions and from other contributions ...
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This article has described the discovery of superconductivity in Y-Ni-B-C system and shown that it is an important landmark in the field of superconductivity. It can be seen from the above discussions and from other contributions in this workshop that, research in QBCs over the past seven years since the discovery, has continually produced many surprising results leading to the new area of research -superconductivity and magnetism in Quaternary Borocarbides. There are still problems to be solved, and there is potential for new materials, possibly multinaries too. QBCs have provided a new impetus for enhancing our understanding of superconductivity, magnetism and their interplay.
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To determine low lying crystal field (CF) excitations for the class of RNi_2~(11)B_2C compounds with R = Pr, Tb, Dy, Ho, Er, Tm, Yb we performed inelastic neutron scattering experiments on the magnetically diluted R_xR'_(1-x)Ni_2~...
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To determine low lying crystal field (CF) excitations for the class of RNi_2~(11)B_2C compounds with R = Pr, Tb, Dy, Ho, Er, Tm, Yb we performed inelastic neutron scattering experiments on the magnetically diluted R_xR'_(1-x)Ni_2~(11)B_2C alloys with R'= Y and Lu. Excitations of the low lying CF-levels (< 10 meV) have been observed and are discussed in comparison with available sets of CF-parameters.
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Our investigations of CeNi_4B, while pursuing new valence fluctuation materials, motivated us to study YNi_4B as a reference material which led us to the discovery of weak signals of superconductivity in YNi_4B and bulk supercondu...
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Our investigations of CeNi_4B, while pursuing new valence fluctuation materials, motivated us to study YNi_4B as a reference material which led us to the discovery of weak signals of superconductivity in YNi_4B and bulk superconductivity in multi- phase samples of the quaternary borocarbide system Y-Ni-B-C. As future directions in magnetic superconducting borocarbides, I emphasize the necessity of synthesizing new quaternaries and, possibly, multinaries. I, then, suggest several new experiments on interplay of superconductivity and magnetism in borocarbides. If carried out successfully, results of these experiments should be of crucial importance with respect to phenomenology of superconductivity.
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The recently discovered, superconducting compounds RNi_2B_2C, where R is rare earths (Lu, Tm, Er, Ho and Dy) and Y, still remain a hot topic of intensive research. Their crystallographic structure resembles that of high-T_c materi...
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The recently discovered, superconducting compounds RNi_2B_2C, where R is rare earths (Lu, Tm, Er, Ho and Dy) and Y, still remain a hot topic of intensive research. Their crystallographic structure resembles that of high-T_c materials, albeit being 3D in electronic properties. The mechanism of Cooper pairing in these compounds is not known in detail although they are commonly believed to be mediated by the ordinary electron-phonon interaction, like A15. The boron isotope effect clearly points that the phonons are involved in superconductivity . Whether these compounds are s- or d-wave superconductors is still under debate. The important subclass in these materials is presented by the magnetic compounds (R =Tm, Er, Ho, and Dy), with the antiferromagnetic (AFM) order and weak ferromagnetism (Er) coexisting with superconductivity. The measurements of the quasiparticle density of states (DOS) by tunneling spectroscopy encounters experimental difficulties, especially these refer to the magnetic superconductors. To our knowledge, no information is available about the electron-quasiparticle-interaction (EQI) spectral function measured by tunneling spectroscopy in the superconducting state, which unambiguously allows one to determine the mechanism of Cooper pairing.
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Superconductivity (SC) and magnetism are two different ordered states into which substances can condense at low temperatures and in general these states are inimical to one another. Competition between SC and magnetism has been on...
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Superconductivity (SC) and magnetism are two different ordered states into which substances can condense at low temperatures and in general these states are inimical to one another. Competition between SC and magnetism has been one of the main topics of study in the field of SC research. In a limited class of intermetallic systems, SC occurs even though magnetic ions with a local moment occupy all of one specific crystallographic site, which is well isolated and decoupled from the conduction path. The RM_2B_2C is one of these systems, in which the three dimensional SC with relatively high T_c (6-16.6 K), coexists with long-range anti-ferromagnetic (AFM) ordering, with T_N in the range of T_N =1.5 -19 K. This short paper summarizes several Moessbauer effect (ME) measurements performed on RNi_2B_2C materials on both: (i) the 4f R ions, and (ii) on ~(57)Fe doped in the Ni sites, which provides a local probe for the study of electronic effects in the Ni-B planes. No attempt is made to survey the literature exhaustively, and to describe the effect of Fe substitution on other physically interesting properties, such as critical fields in the SC state or anisotropy phenomena in the AFM state etc. The goal is to acquaint the non-specialist reader with the main concepts, experimental results and theoretical interpretations in this important field. So far, it appears that the RNi_2B_2C compounds crystallize in the filled version of the body-centered tetragonal ThCr_2Si_2 -type crystal structure, space group I4/mmm, stabilized by the incorporation of additional carbon atom in each R layer. This structure consists of alternating layers of Ni_2B_2 and RC, and SC and AFM are confined to these layers respectively. SC mainly occurs for the heavy members of R~(3+) ions (R= Dy, Ho, Er ,Tm, Lu and Y) and their T_c values are relatively high (15.5 and 16.6 K for R=Y and Lu). The light members of the rare-earth elements do not super-conduct and some of them (R= Gd-Tb) are only AFM ordered. Surprisingly, SC does not appear in YbNi_2B_2C down to 0.35 K even though Yb seems to be close to trivalent. This may be related to moderately heavy fermion behavior. Most of the experimental and theoretical results tend to support a conventional intermetallic type II SC, driven by a standard electron phonon interaction, which is described by the BCS theory. The low boron mass is conducive to the occurrence of high phonon frequencies and consequently to high T_c values. Electronic band calculations performed on LuNi_2B_2C show a large density of states (DOS) at the Fermi level with a dominant contribution coming from the 3d Ni states. It is thus suggested that despite the layered structure, the RNi_2B_2C compounds are electronically 3D and in that sense are more similar to conventional intermetallic superconductors than to the cuprate HTSC. The T_N values occur at higher temperatures (with T_N = as high as 19.6 K for R= Gd) and these large values suggest magnetic interaction being mediated through conduction electrons via an RKKY mechanism, thus indicating a much more intimate interplay of SC and AFM than ever encountered before. The relatively high T_N and T_c values for RNi_2B_2C indicate, that the interplay between SC and AFM, should be on a comparable energetic footing. For all compounds, the ratio T_c/T_N >1 (7.0 for R= Tm), except for DyNi_2B_2C (T_N>T_c). The most interesting system appears to be HoNi_2B_2C which becomes SC at ～7.5 K, while developing long range AFM order at the same temperature. This compound reenters the normal state at ～5 K, only to quickly become SC again with farther reduction of temperature. This type of behavior is consistent with the coupling between conduction electrons and local magnetic moments giving rise to the long-range magnetic order via the RKKY interaction mentioned above The paper is organized as follows: We start (for the non-specialist) with basic concepts of hyperfine parameters deduced from the ME, next we describe t
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Here we review first briefly on the superconductivity in high-T_c cuprates and Sr_2RuO_4. Then we present theoretical expressions of the superfluid density and thermal conductivity tensor in the vortex state in a planar magnetic f...
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Here we review first briefly on the superconductivity in high-T_c cuprates and Sr_2RuO_4. Then we present theoretical expressions of the superfluid density and thermal conductivity tensor in the vortex state in a planar magnetic field for a variety of unconventional superconductors. These expression should be useful in identifying the unconventional superconductors. Finally we propose a possible model for borocarbides.
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An introduction in presented to the structure and chemistry of the nickel, palladium, and platinum borocarbide and boronitride superconductors and related phases.
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There are several clear experimental examples of the interaction between superconductivity and local moment magnetism in the R1221 series. Whereas these are now becoming well delineated experimentally they still need to be address...
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There are several clear experimental examples of the interaction between superconductivity and local moment magnetism in the R1221 series. Whereas these are now becoming well delineated experimentally they still need to be addressed in a detailed, theoretical manner. Some of the features that will have to be accounted for by any successful theory are (ⅰ) the effects of the CEF anisotropy on the suppression of T_c and H_(c2)(T) in the paramagnetic state and (ⅱ) the effects of the nature of the magnetic order on the superconducting ground state, specifically the effects associated with the nearly ubiquitous 0.55a~* state. In addition there is the need to account for the effects of the magnetically ordered state on the pinning of the vortex lattice, but this is probably a somewhat more complex problem.
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Crystallographic structure, magnetic ordering temperatures, structure of R magnetic moments and superconducting transition temperatures of RNiBC (R = Er, Ho, Dy, Tb, Gd) compounds are compared with the data of the corresponding RN...
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Crystallographic structure, magnetic ordering temperatures, structure of R magnetic moments and superconducting transition temperatures of RNiBC (R = Er, Ho, Dy, Tb, Gd) compounds are compared with the data of the corresponding RNi_2B_2C compounds. Information on local structure at the Ni site and on the order of the R magnetic moments are obtained from Moessbauer spectroscopy on ~(57)Fe doped (1 at %) samples. The local symmetries of RNi_2B_2C and RNiBC compounds, measured through the ΔE_Q, were found to be linearly correlated with the c'/a ratio (c' is the distance of the R-C layers between which the Ni_2-B_2 layers are sandwiched). A scaling of T_c with the local symmetry (c'/a) was found to be the same for all the RNi_2B_2C and RNiBC (except for Y) compounds.
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As an overview, we demonstrate that bandstructure calculations using the local density approximation are an appropriate tool to get detailed insight into the electronic structure and properties derived from of rare earth transitio...
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As an overview, we demonstrate that bandstructure calculations using the local density approximation are an appropriate tool to get detailed insight into the electronic structure and properties derived from of rare earth transition metal borocarbides. Structural parameters for ScNi_2B_2C derived from total energy calculation allow to discriminate conflicting experimental structural reports. A strong coupling of the charge degrees of freedom to the Raman active A_(1_g) boron-phonon is shown for YPd_2B_2C. The calculated Fermi surfaces show large anisotropies and extended nesting regions for various superconducting compounds, whereas for the non-superconducting LaNi_2B_2C no nesting was found. The influence of disorder on the electronic structure is investigated using the coherent potential approximation.
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