Electronic states of boron in superconducting MgB2 studied by11B NMR

Abstract

Nuclear magnetic resonance (NMR) spectra and nuclear spin-lattice relaxation rate T,' of "B have been measured in superconducting polycrystalline Mg13 2 with T°"s = 39.5 K. It is shown that (T,T) -' and the Knight shift K, are independent of temperature and nearly isotropic above 1. Both of these quantities are decreased gradually in going to the superconducting state. According to NMR data the density of states near the Fermi level is flat at the scale of about 500 K. Some conclusions on the orbital content of the density of states at the Fermi level were drawn and compared with the results of the band structure calculations.

Anisotropy of conductingpstates and11Bnuclear spin-lattice relaxation inMg1−xBelashchenko1, Antropov2, Rashkeev3 2001Phys. Rev. B191140View full textAdd to dashboardCite

“…We will show that in MgB 2 the orbital relaxation rate is about three times larger than the dipolar and the Fermi-contact rates with all other contribution being much smaller, whereas the Fermi-contact mechanism is dominating in AlB 2 . Our calculated total rates are in fair agreement with those measured [4][5][6] , thus indicating that the calculated N in MgB 2 is basically correct, whereas in AlB 2 , due to an unusually high anisotropy of p states, we predict a much smaller relaxation rate fully determined by Fermi-contact mechanism.…”

“…Experimental numbers have been reported in Ref. [4][5][6] where values of T T 1 = 180, 155 and 165 K·sec for MgB 2 on 11 B (µ = 2.689µ N ) were obtained. These authors interpreted the relaxation rates in terms of dipolar and orbital contributions due to the corresponding Korringa ratio and the already known 2,3 theoretical total N .…”

Evidence for non-s-wave symmetry of theπgap inMgB2from intermodulation distortion measurements

“…A similar small Hebel-Schlichter peak was observed by another group 64 while other groups do not observe it at all. [65][66][67][68] The issue of the Hebel-Schlichter peak in MgB 2 is to our knowledge still open. 62 As for the low-field low-temperature variation in ͓T 1S ͑T͔͒ −1 , the data in Ref.…”

25MgNMR study of theMgB2superconductor

“…While there is reasonable agreement among 11 B ν Q 's and relaxation rates, there is a considerable controversy concerning the 11 B Knight shifts. While some authors [4] report in the normal conducting state a small temperature independent isotropic shift of 175 ppm, others [5] report a smaller and even temperature dependent shift of approximately 60 ppm, which they attribute to the Fermi-contact interaction. Finally there is also a report [6] of a negative 11 B shift of mere 5 ppm attributed to the core polarization.…”

“…Unlike 1/T 1 and K the EFG is determined by the distribution of all charges. Up to date only measurements of 1/T 1 , K, and of the quadrupole coupling to the EFG, expressed by quadrupole coupling frequency ν Q , of 11 B were performed [3,4,5,6]. While there is reasonable agreement among 11 B ν Q 's and relaxation rates, there is a considerable controversy concerning the 11 B Knight shifts.…”