25MgNMR study of theMgB2superconductor

Abstract

25 Mg NMR spectra and nuclear spin-lattice relaxation time, T1, have been measured in polycrystalline MgB2 with a superconducting transition temperature Tc = 39 K in zero magnetic field. From the first order and second order quadrupole perturbed NMR spectrum a quadrupole coupling frequency νQ = 222.0(1.5) kHz is obtained. T1T = 1090(50) sK and Knight shift Kc = 242(4) ppm are temperature independent in the normal conducting phase. The 25 Mg Korringa ratio equals to 0.95 which is very close to the ideal value of unity for s-electrons. The comparison of the experimental νQ, T1T , and Kc with the corresponding values obtained by LDA calculations shows an excellent agreement for all three quantities.PACS numbers: 74.70. Ad, 74.25.Jb, 74.25.Nf, 76.60.Cq, The recent discovery of superconductivity in MgB 2 with remarkably high T c = 39 K [1] has attracted much attention. In particular the observation of a sizeable boron [2] isotope effect strongly suggests that this simple layered intermetallic compound belongs to the conventional family of phonon mediated BCS superconductors. The relevant electron-phonon coupling constant is proportional to the density of states (DOS) at the Fermi level. So it is important to have experimental data of this quantity in MgB 2 .Nuclear magnetic resonance (NMR) in metals probes the DOS at the Fermi level. The measured quantities, the spin lattice relaxation rate, 1/T 1 , and the Knight shift, K, are related to the electron spin susceptibility (χ(q, ω)) of electrons close to the Fermi level, specifically 1/(T 1 T ) ∝ lim ω→0 q Imχ(q, ω)/ω, and K ∝ Reχ(0, 0). In case the atomic site symmetry in the crystal structure is less than cubic and the atom has a nuclear quadrupole moment as it applies for Mg and B in MgB 2 , quadrupolar disturbed NMR delivers in addition valuable information on the electric field gradient (EFG) at the specific nuclear site. 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. 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. These large discrepancies most likely come from the choice of * Electronic address: mali@physik.unizh.ch different materials as references for the Knight shift, as well as from difficulties to measure the small 11 B Knight shift of a broad and in addition quadrupolarly shifted 11 B central line powder spectrum.As far as we know, there are no experimental data concerning the NMR quantities at...

Evidence for two energy gaps and Fermi liquid behavior in the SrPt2As2superconductor

“…This linear relation yields T 1 T = 0.23 s K for 195 Pt. Interestingly, this type of behavior had also been observed in MgB 2 by 25 Mg and 11 B NMR measurements [21,22]. The validity of the Korringa relation in this compound, together with its T 2 resistivity at low temperatures, is indicative of a Fermi liquid ground state.…”

Recent Advances in Solid-State 25Mg NMR Spectroscopy

“…Inset: The same spectrum is recorded with a higher spectral width, allowing the detection of the inner ST singularities. Reprinted with permission from Mali et al, 140 Copyright 2002 by the American Physical Society. observed powder pattern with respect to ideal second-order quadrupolar powder patterns, a small KSA was estimated as K ab ÀK c %10ppm.…”

“…The quadrupole coupling constant was calculated from the separation of the inner ST singularities, giving C q ¼1.49MHz (and q ¼0), in good agreement with the results by Mali et al 140 The isotropic Knight shift was determined by fitting the quadrupole perturbed CT lineshape, giving K iso ¼260(10)ppm. KSA was also estimated from this powder pattern, but the obtained value K ax ¼50 (10)ppm was of relatively low accuracy and did not agree with the results by Mali et al 140 or with theoretical predictions (see below).…”

“…139 Soon after the discovery of this phenomenon in March 2001, two independent 25 Mg NMR reports appeared (as well as many other 11 B NMR studies), with complementary and consistent results. The first one was by Mali et al, 140 who performed variabletemperature static experiments on a powdered MgB 2 sample. The room temperature 25 Mg NMR spectrum of the CT is shown in Figure 15; the inset shows the spectrum recorded in a wider spectral range, from which the inner ST singularities can clearly be observed.…”

“…For temperatures below 19K, the shape of the CT spectrum changed, which was interpreted as a signature of the onset of superconductivity for crystallites oriented perpendicular to the magnetic field. 140 Spin-lattice relaxation was also studied, evidencing a relaxation mechanism dominated by purely magnetic interactions. From the determined T 1 parameter, the Korringa product T 1 T was calculated, which remained nearly constant in the temperature range studied (30-294K).…”