Magnetic Correlations in a d-Electron Heavy Fermion: LiV₂O₄

S.-H. Lee, NCNR/CHRNS and University of Maryland
C. Broholm and Y. Qiu, Johns Hopkins University
Y. Ueda, University of Tokyo, Japan
J. Rush, NCNR/CHRNS

Heavy fermion behaviors are usually found in Ce and U based rare earth compounds that have two different types of electrons near Fermi level E_F: localized f-electrons and conduction (s,p) electrons. Competition between the Kondo screening of the localized electron by the conduction electrons and the RKKY interactions among neighboring localized spins leads to anomalous properties at low temperatures. It is therefore very surprising that LiV₂O₄ which has only d-electrons near E_F becomes a heavy fermion below T_c~16 K. The system behaves as a Fermi liquid at low temperatures exhibiting linear in temperature specific heat with a Sommerfeld constant (1K) = 350 mJ/mol K². The electronic specific heat coefficient is as large as that of UPt₃ and this is very unusual for a d-electron system. In LiV₂O₄ magnetic V^3.5+ ions form a network of corner-sharing tetrahedra, which is a frustrating lattice for antiferromagnetic nearest neighbor interactions. The topology of the lattice prohibits antiferromagnetically interacting spins from ordering and induces macroscopic degeneracy in the ground state. Such a macroscopic ground state degeneracy leads to a large enhancement of spin fluctuations at low temperature and heavy-mass quasiparticles. For instance, V₂O₃ with the corundum structure has ~ 37 mJ/mol K² which is significantly larger than those for ordinary transition metal oxides. The specific heat coefficient for LiV₂O₄ is however larger by an order of magnitude than that for V₂O₃. There are two important questions to ask. (1) By what mechanism does LiV₂O₄ become a heavy fermion? (2) Why is the effective mass of quasiparticles in LiV₂O₄ as large as in actinide heavy fermion systems?

We investigated these issues by performing inelastic neutron scattering measurements on a powder sample of LiV₂O₄.[1] We utilized the multiplexing detection system with a position sensitive detector and a large flat analyzer to rapidly map out a broad range of energy and wave vector phase space. Figure 2 shows contour images of the resulting neutron scattering intensities at four different temperatures spanning T_c. At 1.4 K << T_c, there is strong magnetic inelastic scattering centered at Q_c ~ 0.55 Å^-1 which indicates antiferromagnetic small spin clusters most likely correlated V^3.5+ tetrahedra. Upon warming, however, the scattering at Q_c weakens and intensity starts to increase at lower Q. Finally at T = 80 K >> T_c the low Q scattering becomes dominant and the scattering intensity decreases monotonically with Q in the range of wave vectors probed. We have confirmed by inelastic polarized neutron scattering technique that the low Q scattering is magnetic in origin. The strong low Q scattering indicates that ferromagnetic interactions may be developing at high temperatures between magnetic V^3.5+ ions. It appears that as the system enters the heavy fermion phase the dominant interactions change from ferromagnetic to antiferromagnetic. A possible mechnism leading to competing many body states starts with consideration of the crystal field states for vanadium in LiV₂O₄. It has been argued that in the trigonal ligand environment one electron of the 1.5 d-electrons / V^3.5+ ion is localized in a nondegenerate A_1g orbital and the remaining 0.5 electron is itinerant in a doubly degenerate e_g orbital.[2] At high temperatures, the itinerant electrons may hop from site to site giving rise to ferromagnetic double exchange interactions. At low temperatures, however, partial screening develops and the ferromagnetic interaction is less efficient. Screening, however, is not complete because there are more localized electrons than itinerant electrons. The uncompensated spins of V^3.5+ ions interact antiferromagnetically with each other leading to a tendency towards antiferromagnetic long range order. However, the topology of the magnetic lattice, corner-sharing tetrahedra, prevents ordering and induces magnetic fluctuations that enhance the specific heat at low temperatures. The relaxation rate, , of antiferromagnetic correlations varies linearly with temperature: = ₀ + C k_BT with the residual relaxation rate ₀ = 17(1) K. This temperature dependence is different from the usual square-root behavior in other heavy fermion systems but is common in geometrically frustrated magnets.

Figure 2. Color images of neutron scattering intensities from a powder of LiV₂O₄ at four different temperatures.

In summary, our neutron scattering studies have shown that the nature of the heavy fermion phase of the d-electron system LiV₂O₄ is different from that of the other Ce and U based heavy fermion systems. Ferromagnetic double exchange, geometrical frustration and incomplete screening are all important ingredients in the physics of LiV₂O₄.

[1] S.-H. Lee, C. Broholm, Y. Qiu, Y. Ueda, and J. Rush, In Press Phys. Rev. Lett. (2001).
[2] V.I. Anisimov, et al., Phys. Rev. Lett. 85, 364 (1999).

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