We report a ^{11}B NMR measurement on the recently discovered superconductor
Li_2Pd_3B. The nuclear spin lattice relaxation rate 1/T_1 shows a well-defined
coherence peak just below T_c (H=1.46 T)=5.7 K, and the spin susceptibility
measured by the Knight shift also decreases below T_c. These results indicate
that the superconductivity is of conventional nature, with an isotropic gap.
Our results also suggest that the $p$-electrons of boron and the d-electrons of
palladium that hybridize with boron $p$-electrons are primarily responsible for
the superconductivity.Comment: 4 pages, 5 figure

D. E. Maclaughlin

Guo-qing Zheng

M. Nishiyama

Y. Inada

English

arXiv

We report a B-11 NMR measurement on the recently discovered superconductor Li2Pd3B. The nuclear spin lattice relaxation rate 1/T-1 shows a well-defined coherence peak just below T-c(H=1.46 T)=5.7 K, and the spin susceptibility measured by the Knight shift also decreases below T-c. These results indicate that the superconductivity is of conventional nature, with an isotropic gap. Our results also suggest that the p-electrons of boron and the d-electrons of palladium that hybridize with boron p-electrons are primarily responsible for the superconductivity.</p

Nishiyama, M

Inada, Y

Zheng, Guo-qing

Okayama University Scientific Achievement Repository

PhysicsPhysics fieldsOkayama University Year 2005Superconductivity of the ternary borideLi2Pd3B probed by B-11 NMRM Nishiyama Y InadaOkayama University Okayama UniversityGuo-qing ZhengOkayama UniversityThis paper is posted at eScholarship@OUDIR : Okayama University Digital InformationRepository.http://escholarship.lib.okayama-u.ac.jp/physics general/15Superconductivity of the ternary boride Li2Pd3B probed by 11B NMRM. Nishiyama,1 Y. Inada,2 and Guo-qing Zheng11Department of Physics, Okayama University, Okayama 700-8530, Japan2Department of Science Education, Faculty of Education, Okayama University, Okayama 700-8530, JapansReceived 16 March 2005; published 7 June 2005dWe report a 11B NMR measurement on the recently discovered superconductor Li2Pd3B. The nuclear spinlattice relaxation rate 1 /T1 shows a well-defined coherence peak just below TcsH=1.46 Td=5.7 K, and the spinsusceptibility measured by the Knight shift also decreases below Tc. These results indicate that the supercon-ductivity is of conventional nature, with an isotropic gap. Our results also suggest that the p-electrons of boronand the d-electrons of palladium that hybridize with boron p-electrons are primarily responsible for thesuperconductivity.DOI: 10.1103/PhysRevB.71.220505 PACS numberssd: 74.70.Dd, 76.60.CqSince the discovery of high temperature superconductivityin copper oxides,1 compounds containing transition metal el-ements have become targets for searching new strongly-correlated superconductors. In fact, superconductivity wasdiscovered in the cobalt oxide Na0.3CoO2·1.3H2O sRef. 2d,which was found to be of unconventional nature with strongelectron correlations in the normal state.3 Meanwhile, thediscovery of superconductivity at 40 K in MgB2 sRef. 4d hasgenerated recurred interest on the physical properties ofborides. Very recently, Togano et al. found that the ternarymetallic compounds containing boron and palladium,Li2Pd3B, is superconducting at Tc, 7 K sRef. 5d. This com-pound has a cubic structure with the space group of P4332,containing a distorted Pd6B octahedra, which is structurallysimilar to the superconductor MgCNi3 sTc=8 Kd sRef. 6dand in some sense also similar to the high-Tc copper oxideswhere the key structure is the oxygen-containing octahedra.Although the physical properties of this new compound areunexplored, it has been proposed that the correlations of Pdd-electrons may be dominant in the electronic properties andmay also be responsible for the superconductivity.7 Band cal-culation has shown that Pd d-electrons contribute signifi-cantly to the density of states at the Fermi level.8In this Rapid Communication, we report the first 11Bnuclear magnetic resonance sNMRd measurement in thesuperconducting and the normal states. We find that the su-perconductivity is of Bardeen-Cooper-Schrieffer sBCSd typewith an isotropic energy gap. In the normal state, thetemperature sTd dependencce of the nuclear spin-latticerelaxation rate 1 /T1 and the Knight shift due to spin suscep-tibility, Ks satisfies the so-called Korringa relation,T1TKs2=const, with no signature of electron correlations. Ourresults suggest that the superconductivity in Li2Pd3B isphonon-mediated.The samples were prepared by the arc-melting methodwith starting materials of Li s99.9% purityd, Pd s99.95%d andB s99.5%d. The two-step arc melting process5 was used. Thestarting melt composition of Li:Pd:B=2.1:3 :1 was adoptedin the light of high vapor pressure of Li. The x-ray diffrac-tion chart shows that the sample is single phase. For NMRmeasurements, the sample was crushed into powder. Figure 1shows the ac susceptibility measured using the NMR coil.The Tc is about 7.1 K at zero magnetic field. A standardphase-coherent pulsed NMR spectrometer was used to col-lect data. The NMR spectra were obtained by fast Fouriertransform sFFTd of the spin echo taken at a constant mag-netic field of 1.4629 T. The nuclear spin-lattice relaxationrate, 1 /T1, was measured by using a single saturation pulseand fitting the recovery of the nuclear magnetization after thesaturation pulse.Figure 2 shows the spectrum at T=10 K. The full width atthe half maximum sFWHMd of the NMR line is less than 5kHz. The very sharp transition ensures the high quality of thesample. The temperature dependence of the FWHM is shownin Fig. 3. It can be seen that the spectrum broadens belowTc sH=1.46 Td=5.7 K, at which temperature both 1/T1 andthe Knight shift shows anomaly ssee belowd.Figure 4 shows the temperature dependence of 1 /T1. Inthe normal state above Tc, 1 /T1 varies in proportion to T, asseen in conventional metals. Just below TcsH=1.46 Td=5.7 K, however, 1 /T1 increases as T is reduced, showing acoherence peak, then decreases exponentially upon furtherlowing T. This is the characteristic of s-wave, isotropic su-perconductivity. The 1/T1s in the superconducting state isexpressed asFIG. 1. The ac susceptibility measured using the NMR coil forLi2Pd3B at zero magnetic field.PHYSICAL REVIEW B 71, 220505sRd s2005dRAPID COMMUNICATIONS1098-0121/2005/71s22d/220505s3d/$23.00 ©2005 The American Physical Society220505-1T1sT = TcdT1s=2kBTcE sNssEd2 + MssEd2dfsEds1 − fsEdddE ,s1dwhere NssEd=N0E / sE2−D2d1/2 is the superconducting den-sity of states sDOSd with D being the BCS gap, MssEd=N0D / sE2−D2d1/2 is the anomalous DOS due to the coher-ence factor,9 N0 is the DOS in the normal state and fsEd isthe Fermi function. Both MssEd and NssEd diverge at E=D,so theoretically 1/T1 diverges just below Tc. In real materi-als, however, broadening of the energy level and some an-isotropy in the energy gap will remove the singularity at E=D so that 1 /T1 only shows a peak just below Tc. FollowingHebel,10 we convolute MssEd and NssEd with a broadeningfunction BsEd which is approximated with a rectangularfunction centered at E with a height of 1 /2d. The solid curvebelow Tc shown in Fig. 4 is a calculation with 2Ds0d=2.2kBTc and r;Ds0d /d=2. It fits the experimental data rea-sonably well. The parameter 2Ds0d is substantially smallerthan the BCS value of 3.5kBTcsHd. This could be due to theeffect of the magnetic field, which usually reduces the gapsize.9Figure 5 shows the Knight shift, K, as a function of tem-perature. The shift is small and T-independent within theexperimental error in the normal state, but changes abruptlyat Tc where 1/T1 shows the coherence peak. Note that thechange is positive, namely, K increases below Tc. The shiftmay be expressed as,K = Korb + Ks, s2dKs = Ahfxs, s3dwhere Korb is the contribution due to orbital sVan Vleckdsusceptibility which is T-independent, Ahf is the hyperfinecoupling constant and xs is the spin susceptibility. Ks may bedecomposed intoFIG. 2. The Fourier transformed 11B NMR spectrum measuredat T=10 K at a magnetic field of H=1.4629 T.FIG. 3. The temperature dependence of the full width at the halfmaximum sFWHMd of the 11B NMR spectrum.FIG. 4. sColor onlined The temperature dependence of the spinlattice relaxation rate, 1 /T1, measured at a field of H=1.4629 T.The straight line above Tc represents the T1T=const relation. Thesolid curve below Tc is a calculation assuming the BCS gap func-tion with the parameter shown in the figure. For details, see the text.FIG. 5. sColor onlined The temperature dependence of theKnight shift measured at a constant field of H=1.4629 T. Thestraight line above Tc is a guide to the eyes. The curve below Tc isthe calculated result assuming a BCS gap with the same parameteras in the calculation of T1.NISHIYAMA, INADA, AND ZHENG PHYSICAL REVIEW B 71, 220505sRd s2005dRAPID COMMUNICATIONS220505-2Ks = Kcp + K0, s4dwhere Kcp is the shift due to core polarization interactionbetween the boron p electrons and the nuclear spins, which isnegative, and K0 is due to other interactions including theusual Fermi contact interaction. The result that Ks increasesbelow Tc indicates that Kcp is dominant over K0. Therefore,the DOS that decreases due to the onset of superconductivityare due to boron p electrons, and/or d electrons of palladiumwhich hybridize with boron p electrons. If we assume thatthe spin susceptibility vanishes completely, due to singletpairing, at T=1.6 K which is well below Tc, then Korb isabout 0.085±0.003% and Ks=−0.012±0.003%. The curvebelow Tc depicts the calculated Knight shift in the supercon-ducting stateKssc ~ xssc ~ −E NssEds] fsEd/]EddE s5dusing the same gap parameter as in Fig. 4.Finally, when the electrons responsible for the spin latticerelaxation and the Knight shift are s-electrons, T1TKs2=" / s4pkBdsge /gNd2=2.55310−6 sec·K sKorringa constantd,where ge and gN are the gyromagnetic ratio of electron and11B nuclear spins, respectively. The observed value of T1TKs2in the present case is 3.21310−7 sec·K, which is muchsmaller than the Korringa constant. This result also supportsthat the dominant electrons that participate in the relaxationand the Knight shift are p electrons, in which case the excessrelaxation such as the orbital relaxation is important so thatthe observed T1TKs2 deviates from " / s4pkBdsge /gNd2.In summary, through the measurements of 11B NMR, wefind that the spin-lattice relaxation rate 1 /T1 in the newlydiscovered Pd-containing boride Li2Pd3B exhibits a well-defined coherence peak just below Tc and decreases expo-nentially with further decreasing temperature. The spin sus-ceptibility as measured by the Knight shift Ks also decreasesbelow Tc, suggesting a singlet spin pairing. In the normalstate, the Korringa relation, namely, T1TKs2=const is obeyed,indicating the lack of electron correlations. Our results indi-cate that this compound is a superconductor with an isotropicgap and that the superconductivity occurs in the absence ofelectron correlations.We thank T. Kato and K. Matano for assistance in some ofthe measurements. This work was supported in part by aGrant-in-Aid for Scientific Research from MEXT.1 J. G. Bednorz and K. A. Muller, Z. Phys. B: Condens. Matter 64,189 s1986d.2 K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R. A.Dilanian, and T. Sasaki, Nature sLondond 422, 53 s2003d.3 T. Fujimoto, G.-q. Zheng, Y. Kitaoka, R. L. Meng, J. Cmaidalka,and C. W. Chu, Phys. Rev. Lett. 92, 047004 s2004d.4 J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, and J.Akimitsu, Nature sLondond 410, 63 s2001d.5 K. Togano, P. Badica, Y. Nakamori, S. Orimo, H. Takeya, and H.Hirata, Phys. Rev. Lett. 93, 247004 s2004d.6 T. He, Q. Huang, A. P. Ramirez, Y. Wang, K. A. Regan, N.Rogado, M. A. Hayward, M. K. Haas, J. S. Slusky, K. Inumaru,H. W. Zandbergen, N. P. Ong, and R. J. Cava, Nature sLondond411, 54 s2001d.7 M. Sardar and D. Sa, cond-mat/0404168 sunpublishedd.8 Sharat Chandra, S. Mathi Jaya, and M. C. Valsakumar,cond-mat/0502525 sunpublishedd.9 D. E. Maclaughlin, in Solid State Physics, edited by H.Ehrenreich et al. sAcademic New York, 1976d, Vol. 31, p. 1.10 L. C. Hebel, Phys. Rev. 116, 79 s1959d.SUPERCONDUCTIVITY OF THE TERNARY BORIDE… PHYSICAL REVIEW B 71, 220505sRd s2005dRAPID COMMUNICATIONS220505-3

Superconductivity of the ternary boride Li2Pd3B probed by B-11 NMR

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Superconductivity of the ternary boride
                    
                      
                        
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Superconductivity of the Ternary Boride Li_2Pd_3B Probed by ^{11}B NMR

Superconductivity of the Ternary Boride Li_2Pd_3B Probed by ^{11}B NMR

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