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Recent NMR Results in NCKU

Recent NMR Results in NCKU. C. S. Lue ( 呂欽山 ) Department of Physics, National Cheng Kung University ( 國立成功大學物理系 ). Outline:. I: Fundamental NMR principles: (i) NMR frequency shifts (ii) Quadrupole interactions (ii) Spin-lattice relaxation rates II: Studied systems:

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Recent NMR Results in NCKU

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  1. Recent NMR Results in NCKU C. S. Lue (呂欽山) Department of Physics, National Cheng Kung University (國立成功大學物理系)

  2. Outline: I: Fundamental NMR principles: (i) NMR frequency shifts (ii) Quadrupole interactions (ii) Spin-lattice relaxation rates II: Studied systems: (i) 27Al NMR study of electronic structure of Al3M (ii) 51V NMR study of spin gap nature of BaCu2V2O8 (iii) 51V NMR study of pseudogap characteristics of Fe2VAl

  3. Bulk Properties:collective response of the system to an external perturbation • Electronic property:  =E/J • Magnetic property: = M/H • Thermal property: C = U/T • Optical property:

  4. Merits of NMR:local probe of electronic and magnetic features • Site selected • Impurity phase isolated • Sensitiveto the excitation near the Fermi-level

  5. Central transition 71Ga NMR line shape in NbGa3 D022 crystal structure

  6. Simple resonance theory: Zeeman energy: E = oh = nHo Nuclear spin I: 2I +1 energy states For I = 3/2, H = 0 H = Ho  (MHz) n no

  7. NMR in Solids: (i) Magnetic hyperfine interactions:couplings between nuclear magnetic moment n and electronic magnetic momente Fermi-contactdipolarorbital s-like e-non-s-character e- Ks Kan Korb

  8. Site-II Site-I o NMR signal NMR frequency shifts: Site-I Site-II n0 n (MHz)

  9. NMR Shift & Magnetic susceptibility (a) Simple metals: (s-electrons) (b) d-electron based materials: Note: Bulkdiamagnetic term L does not enter because of the small hyperfine field.

  10. -q -q +q +q +q +q -q -q (ii) Electric hyperfine interactions:couplings between nuclear quadrupolemoment eQ and electric field gradient (For I > 1/2 in the non-cubic environments with axial symmetry ) -q -q -q +q +q +q -q -q Ea > Eb

  11. Satellite Lines:I = 3/2 EFG = 0 EFG  0 no no-nQ/2 no no+nQ/2

  12. 27Al (I = 5/2) NMR powder pattern in Cr2AlC

  13. M(t) t Spin-lattice relaxation time (T1):

  14. Recovery curve of 11B in ZrB2

  15. T1 & Electronic origins

  16. r ~ 2A and m~ 10-3mB Hloc ~ m/r3 ~ 1 gauss Ho = 1 T = 104 gauss Hloc /Ho = Dn/no ~ 10-4 If no ~ 10 - 100 MHz, intrinsic line width Dn ~ 1 - 10 kHz Magnetic dipolar broadening of rigid lattices:(Simplest case: cubic) Motional narrowing: motional effects narrow the line width in normal liquids.

  17. Varian 300 Solid-State NMR Home-built NMR probe-head (Top-loaded) 7.05 T superconducting magnet

  18. D023-type Al3Zr & Al3Hf Potential aerospace applications: High melting point Low mass density Large elastic modulus Shortage: Poor ductility Interesting issues: Electronic properties Structural stability

  19. 27Al NMR central transitions of Al3Zr & Al3Hf Central transition line shapes: Anisotropic Knight shift & Quadrupole effects High-frequency peak: Al-III Low-frequency part: Al-I & Al-II

  20. Satellite lines for the three Al sites in Al3Zr and Al3Hf

  21. Partial 27Al NMR results of Al3Zr & Al3Hf Fermi-level s-DOS (states/eV atom) for each Al crystallographic site Smaller Fermi-level DOS in Al3Zr → Al3Zr is more stable than Al3Hf with respect to the D023 structure, consistent with the fact that Al3Hf becomes more favorable with D022 as T > 650 C.

  22. Oxidation states: Magnetic Cu2+ (S = ½) Nonmagnetic V5+ Spin chains: CuO4 square plaquette + edge-sharing V(I)O4 tetrahedra Alternating couplingratio J2/J1 = 0.2 Spin gap D = 230 K

  23. Bulk magnetic susceptibility of BaCu2V2O8 He et al. PRB 69 (2004) Ghoshray et al. PRB 71 (2005)

  24. Models for the S=1/2 one-dimensional spin chain compounds • Alternating-chain model • Dimer-chain model J1 J1 J1 J2 aJ J J aJ J From the analyses of the bulk susceptibility and heat capacity, He et al. concluded that the alternating chain model is more suitable for the understanding of the gap characteristics of BaCu2V2O8.

  25. 51V NMR investigation of BaCu2V2O8

  26. T-dependent NMR shifts of BaCu2V2O8

  27. T-dependent NMR T1 of BaCu2V2O8

  28. Alternating-chain model: DK(I) = 360 K DK(II) = 370 K DR(II) = 440 K For V-II, DR/DK ~ 1.2 Dimer-chain model: DK(I) = 460 K DK(II) = 470 K DR(II) = 450 K For V-II, DR/DK ~ 1 NMR parameters of BaCu2V2O8 Summary: Both models seem to be suitable for the understanding of the spin gap nature in BaCu2V2O8.

  29. L21 Heusler-type Fe2VAl • Transport: semi-conducting behavior • Magnetism: paramagnetic behavior (Pauli or Van-Vleck?) • Low-T specific heat: possible 3d heavy fermion • (g= 14 mJ/mol K2 mass enhancement m*/m ~ 20 -70) • LiV2O4: 3d heavy fermion? • FeSi: 3d Kondo insulator

  30. Theoretical calculations on Fe2VAl • G. Y. Guo, G. A. Botton, and Y. Nishino, J. Phys.: Condens. Matter 10, L119 (1998). • D. J. Singh and I. I. Mazin, Phys. Rev. B 57, 14352 (1998). • R. Weht and W. E. Pickett, Phys. Rev. B 58, 6855 (1998). • M. Weinert and R. E. Watson, Phys. Rev. B 58, 9732 (1998). • A. Bansil, S. Kaprzyk, P. E. Mijnarends, and J. Tobola, Phys. Rev. B 60, 13396 (1999).

  31. 1. Narrow NMR line width: nonmagnetic 2. NMR shifts: For 51V, Ko= 0.61% is not likely due to the Pauli paramagnetism. → Van-Vleck mechanism dominated Band splitting: Eg ~ 0.22 eV

  32. T-dependent NMR T1 of Fe2VAl Eg ~ 0.27 eV Low-T data: V partial Fermi-level DOS D(EF) = 0.023 states/eV atom Total Fermi-level DOS D(EF) = 0.055 states/eV atom →Semi-metallic characteristics

  33. 1. Sample-dependent heat capacity 2. Solid line: C(T) = gT +bT3+dT5 Small g= 1.5 mJ/mol K2 3. Magnetic cluster inducedlow-T upturn in g

  34. Field-dependent specific heat in Fe2VAl • Multi-level Schottky anomaly: • Conclusions: the reported g • enhancement is not intrinsic • → Fe2VAl is a false d-electron • heavy fermion.

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