Upper Critical Field Peculiarities of SuperconductingYNi2B2Cand

Abstract

We present new upper critical field Hc2(T ) data in a broad temperature region 0.3K ≤ T ≤ Tc for LuNi2B2C and YNi2B2C single crystals with well characterized low impurity scattering rates.The absolute values for all T , in particular Hc2(0), and the sizeable positive curvature (PC) of Hc2(T ) at high and intermediate T are explained quantitatively within an effective two-band model. The failure of the isotropic single band approach is discussed in detail. Supported by de Haas van Alphen data, the superconductivity reveals direct insight into details of the electronic structure. The observed maximal PC near Tc gives strong evidence for clean limit type II superconductors. 74.60.Ec, 74.70.Ad, 74.72Ny The discovery [1,2] of superconductivity in transition metal borocarbides has generated large interest due to their relatively high transition temperatures T c ∼ 15 to 23 K and due to the relation between the mechanisms of superconductivity in these compounds, in cuprates, and in ordinary transition metals. Another highlight is the coexistence of magnetism and superconductivity in some of these compounds containing rare earth elements [3][4][5]. A study of the non-magnetic compounds such as LNi 2 B 2 C, with L=Lu,Y,Th,Sc [6], is a prerequisite for the understanding of their magnetic counterparts. Experimental data for LuNi 2 B 2 C [7] demonstrate beside a maximal positive curvature (PC) of H c2 (T ) near T c , observed also for YNi 2 B 2 C [8,4,9], a weak T -dependent anisotropy within the tetragonal basal plane and a Tindependent out-of-plane anisotropy of the upper critical field H c2 . Both anisotropies have been described [7] in terms of nonlocal corrections to the Ginzburg-Landau (GL) equations. In this picture the PC of H c c2 ( H to the tetragonal c-axis) is caused, almost purely, by the basal plane anisotropy. However, it should be noted that the reported anisotropy of H c2 for YNi 2 B 2 C is significantly smaller than for LuNi 2 B 2 C [7,9,10] whereas its PC is comparable or even larger. Further explanations of the unusual PC of H c2 (T ), such as quasi-2D fluctuations [11], are excluded by the underestimation of H c2 (T ) at low-T [9] and the observed weak anisotropy. The quantum critical point scenario [12] as well as the bipolaronic one [13] can be disregarded because the slope of H c2 (T ) decreases for T →0 (see Fig. 1). Local density approximation (LDA) band structure calculations [14,15] predict a nearly isotropic electronic structure with rather complicated bands near the Fermi level E F . However, in analyzing the superconductivity in terms of an isotropic single-band (ISB) Eliashberg model, the multi-band character and the anisotropic Fermi surface have been widely ignored so far.Here we present and analyze theoretically new data of H c2 (T ) in a broad interval 0.3K≤ T ≤ T c for high purity LuNi 2 B 2 C and YNi 2 B 2 C single crystals. We show that typical features of both compounds, such as H c2 (0) ∼ 8 to 10 T and the unusual PC of H c2 (T ) for T > ∼ 0.5T c , cannot in any way be explained ...

Evidence of multiband behavior in the superconducting alloy Zr0.96V0.04B2Renosto1, Consoline2, Santos3 et al. 2013Phys. Rev. B284251View full textAdd to dashboardCite

“…9,22 Hence, the twoband Eliashberg theory is generally taken as the appropriate description rather than the isotropic single band model. 23 The TB Eliashberg model was applied for determining the behavior of H c2 in YNi 2 B 2 C and LuNi 2 B 2 C, and MgB 2 . 23,25 The temperature dependence of the upper critical field present an upward curvature, provided by electrons at distinctly bands.…”

“…23 The TB Eliashberg model was applied for determining the behavior of H c2 in YNi 2 B 2 C and LuNi 2 B 2 C, and MgB 2 . 23,25 The temperature dependence of the upper critical field present an upward curvature, provided by electrons at distinctly bands. This can be explained by (i) significantly different Fermi velocities (V F ) at the two bands and (ii) strong coupling in the small-V F band and sizable coupling between the small-V F and large-V F bands, where V F is the Fermi velocity.…”

Pressure dependence of critical temperature in MgB2 and two-bands Eliashberg theory

“…In the following I will refer to the paper of A.F.Goncharov [5] because in there are present both measurement of the variation of critical temperature and of phonon mode by means of Raman measurement, with the pressure and so I mainly refer to these experimental data. In fact only in this work there are all input parameters necessary to my model.Let us start from the generalization of the Eliashberg theory [7,8] for systems with two bands [9], that has already been used with success to study the MgB 2 and related systems [10,11,12,13,14,15]. To obtain the gaps and the critical temperature within the s-wave, two-band Eliashberg model one has to solve four coupled integral equations for the gaps ∆ i (iω n ) and the renormalization functions Z i (iω n ):…”

“…The solution of Eqs. 1,2 requires as input: i) the four (but only three independent [9]) electron-phonon spectral functions α 2 ij (ω)F (ω); ii) the four (but only three independent [9]) elements of the Coulomb pseudopotential matrix µ * (ω c ).…”

The Properties of the Superconducting State in YNi2B2C: The One-Band Eliashberg Approach

“…In the literature related to the topic there are some reports that the simplest version of the Eliashberg equations might not be able to cope with the description of all relevant physical quantities. In particular, the dependence of the upper critical field (H C2 ) on the temperature [11,12] or the results obtained with a use of the directional point-contact spectroscopy [13,14] suggest the necessity of the two-band model being used. On the other hand, some researchers basing on the thermal and spectroscopic experiments are moving toward the direction of the one-band models with a non-trivial wave symmetry (s + g or even d-wave symmetry) [15][16][17][18].…”