The interaction of rare-earth magnetism and superconductivity has been a topic of interest for many years. In classical magnetic superconductors (Chevrel phases, ternary rhodium borides, etc) as well as in the high-Tc cuprates the superconducting state usually coexists with antiferromagnetic order on the rare-earth sublattice. In these compounds the magnetic ordering temperature TN is much below the superconducting transition temperature Tc. The discovery of superconducting borocarbides RT2B2C with R = Sc, Y, La, Th, Dy, Ho, Er, Tm or Lu and T = Ni, Ru, Pd or Pt (where not all of these combinations of R and T result in superconductivity) has reanimated the research on the coexistence of superconductivity and magnetic order. Most of these borocarbides crystallize in the tetragonal LuNi2B2C type structure which is an interstitial modification of the ThCr2Si2 type. Contrary to the behaviour of Cu in the cuprates Ni does not carry a magnetic moment in the borocarbides. Various types of antiferromagnetic structures on the rare-earth sublattice have been found to coexist with superconductivity in RNi2B2C for R = Tm, Er, Ho and Dy. Particularly of interest is the case of HoNi2B2C for which three different types of antiferromagnetic structures have been observed: (i) a commensurate one with Ho moments aligned ferromagnetically within layers perpendicular to the tetragonal c axis where consecutive layers are aligned in opposite directions, (ii) an incommensurate spiral along the c axis and (iii) an incommensurate a-axis-modulated structure with a modulation vector τ≈(0.55,0,0). This wave vector emerges in various RNi2B2C compounds with magnetic as well as nonmagnetic R elements and is connected with Fermi surface nesting. Both incommensurate magnetization structures have been shown to be related to the near-reentrant behaviour observed in HoNi2B2C whereas the commensurate structure coexists well with the superconducting state in this compound. The variation of TN and Tc with the de Gennes factor can roughly be drawn on straight lines from Lu to Gd and from Lu to Tb, respectively, with the exception of Yb. Consequently, Tc>TN holds for Tm, Er, Ho and Tc<TN for Dy. However, the study of various pseudoquaternary (R,R')Ni2B2C compounds has shown that this so-called de Gennes scaling is not universal for the borocarbides and it breaks down in some cases, which is attributed to effects of details of the electron structure, crystalline electric fields, the difference in the R and R' ionic radii or to the effect of nonmagnetic impurities in an antiferromagnetic superconductor. In an external magnetic field some of the RNi2B2C compounds show metamagnetic transitions combined with a large negative magnetoresistance. A small net magnetization found at low temperatures and zero magnetic field in TbNi2B2C has been interpreted as weak ferromagnetism of Dzyaloshinsky-Moriya type. A similar phenomenon observed for superconducting ErNi2B2C is still under discussion.

https://iopscience.iop.org/article/10.1088/0034-4885/64/8/202/pdf

Interaction of superconductivity and magnetism in borocarbide superconductors

We report a detailed comparison of experimental data and theoretical predictions for the dendritic flux instability, believed to be a generic behavior of type-II superconducting films. It is shown that a thermomagnetic model published very recently [Phys. Rev. B 73, 014512 (2006)] gives an excellent quantitative description of key features like the stability onset (first dendrite appearance) magnetic field, and how the onset field depends on both temperature and sample size. The measurements were made using magneto-optical imaging on a series of different strip-shaped samples of ${\mathrm{MgB}}_{2}$. Excellent agreement is also obtained by reanalyzing data previously published for Nb.

http://arxiv.org/pdf/cond-mat/0605425.pdf

Onset of dendritic flux avalanches in superconducting films.

Superconducting flux pumps are the kind of devices which can generate direct current into superconducting circuit using external magnetic field. The key point is how to induce a dc voltage across the superconducting load by ac fields. Giaever (1966 IEEE Spectr. 3 117) pointed out flux motion in superconductors will induce a dc voltage, and demonstrated a rectifier model which depended on breaking superconductivity. van de Klundert et al (1981 Cryogenics 21 195, 267) in their review(s) described various configurations for flux pumps all of which relied on inducing the normal state in at least part of the superconductor. In this letter, following their work, we reveal that a variation in the resistivity of type II superconductors is sufficient to induce a dc voltage in flux pumps and it is not necessary to break superconductivity. This variation in resistivity is due to the fact that flux flow is influenced by current density, field intensity, and field rate of change. We propose a general circuit analogy for travelling wave flux pumps, and provide a mathematical analysis to explain the dc voltage. Several existing superconducting flux pumps which rely on the use of a travelling magnetic wave can be explained using the analysis enclosed. This work can also throw light on the design and optimization of flux pumps.

/pdf/origin-of-dc-voltage-in-type-ii-superconducting-flux-pumps-2sxoldxpcr.pdf

Origin of DC voltage in type II superconducting flux pumps: field, field rate of change, and current density dependence of resistivity

https://iopscience.iop.org/article/10.1088/1361-6668/ab6fe9/pdf

A temperature-dependent multilayer model for direct current carrying HTS coated-conductors under perpendicular AC magnetic fields

We present results of an in-field directional point-contact spectroscopy study of the quaternary borocarbide superconductor $\mathrm{Y}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$, which is characterized by a highly anisotropic superconducting gap function. For $I\ensuremath{\Vert}a$, the superconducting energy gap $(\ensuremath{\Delta})$, decreases linearly with magnetic field and vanishes at around $3.25\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, which is well below the upper critical field $({\mathbf{H}}_{c2}\ensuremath{\sim}6\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ measured at the same temperature $(2\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. For $I\phantom{\rule{0.1em}{0ex}}\ensuremath{\Vert}c$, on the other hand, $\ensuremath{\Delta}$ decreases weakly with magnetic field but the broadening parameter $(\ensuremath{\Gamma})$ increases rapidly with magnetic field with the absence of any resolvable feature above $3.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. From an analysis of the field variation of energy gaps and the zero-bias density of states we show that the unconventional gap function observed in this material could originate from multiband superconductivity.

/pdf/magnetic-field-dependence-of-superconducting-energy-gaps-in-6tzuxmg91h.pdf

Magnetic-field dependence of superconducting energy gaps in Y Ni 2 B 2 C : Evidence of multiband superconductivity

A polycrystalline sample of the MgB_2 superconductor was investigated by measurements of the electrical resistivity, the thermopower and the thermal conductivity in the temperature range between 1.8K and 300K in zero magnetic field. The electrical resistivity shows a superconducting transition at T_c=38.7K and, similarly to borocarbides, a T^2.4 behaviour up to 200K. The electron diffusion thermopower and its bandstructure-derived value indicate the dominant hole character of the charge carriers. The total thermopower can be explained by the diffusion term renormalized by a significant electron-phonon interaction and a phonon drag term. In the thermal conductivity, for decreasing temperature, a significant decrease below T_c is observed resulting in a T^3 behaviour below 7K. The reduced Lorenz number exhibits values smaller than 1 and a characteristic minimum which resembles the behaviour of non-magnetic borocarbides.

https://arxiv.org/pdf/cond-mat/0105429.pdf

Heat and Charge Transport Properties of MgB2

The critical current density and the resistive transition curves of Ag-sheathed, textured (Bi/sub 1-x/Pb/sub x/)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ tapes have been studied in magnetic fields up to 12 T. The results can be described by the modified flux creep model proposed by E. Zeldov et al. (1990). It is shown that this model yields a simple explanation for the exponential field dependence of the critical current density in high-temperature superconductors. >

Critical current densities and activation energy of BiPbSrCaCuO tapes

The irreversibility line of silver-sheathed Bi-2223 tapes and of melt-processed Bi-2223 samples doped with Te has been investigated by AC susceptibility, DC magnetization and transport measurements. The temperature dependence of the irreversibility field B ∗ follows a relation B ∗ (T) ∝ (T c /T−1) 1 n with n = 0.6 which can be interpreted in the framework of a flux-creep model with an expression for the activation energy U ∝ (1− T / T c )/ B n .

Irreversibility line of polycrystalline Bi-2223 samples

Preparation and critical current density of melt-processed YBaCuO thick films and AgPd-sheathed tapes☆

Both thermal conductivity λ and specific heat cp of amorphous polystyrene were measured in the temperature range 1.7 K K at three different pressure values up to p=0.4 GPa . The thermal conductivity data show that the influence of pressure is most pronounced at the plateau of λ at 2 K K . Fit calculations were carried out to receive values for P γ 2 and W. As pressure is applied the specific heat is clearly reduced below T=20 K .

Influence of high pressure on thermal properties of amorphous polystyrene

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