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 EF: 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 EF becomes a heavy fermion below Tc~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 V3.5+ 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. 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 Tc. At 1.4 K << Tc, there is strong magnetic inelastic scattering centered at Qc ~ 0.55 Å-1 which indicates antiferromagnetic small spin clusters most likely correlated V3.5+ tetrahedra. Upon warming, however, the scattering at Qc weakens and intensity starts to increase at lower Q. Finally at T = 80 K >> Tc 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 V3.5+ 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 / V3.5+ ion is localized in a nondegenerate A1g orbital and the remaining 0.5 electron is itinerant in a doubly degenerate eg orbital. 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 V3.5+ 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 kBT 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.
 S.-H. Lee, C. Broholm, Y. Qiu, Y. Ueda, and J. Rush, In Press Phys. Rev. Lett. (2001).
 V.I. Anisimov, et al., Phys. Rev. Lett. 85, 364 (1999).
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