Hikaru Takeda

An orbital-selective spin liquid in a frustrated heavy fermion spinel LiV2O4

The pronounced enhancement of the effective mass is the primary phenomenon associated with strongly correlated electrons. In the presence of local moments, the large effective mass is thought to arise from Kondo coupling, the interaction between itinerant and localized electrons. However, in d electron systems, the origin is not clear because of the competing Hunds rule coupling. Here we experimentally address the microscopic origin for the heaviest d fermion in a vanadium spinel LiV(2)O(4) having geometrical frustration. Utilizing orbital-selective (51)V NMR, we elucidate the orbital-dependent local moment that exhibits no long-range magnetic order despite persistent antiferromagnetic correlations. A frustrated spin liquid, Hund-coupled to itinerant electrons, has a crucial role in forming heavy fermions with large residual entropy. Our method is important for the microscopic observation of the orbital-selective localization in a wide range of materials including iron pnictides, cobaltates, manganites and ruthnates.

Magnetic interactions and orbital state in double chain systems investigated by NMR measurements

We have made NMR measurements to clarify the relationship between the orbital state and the magnetic properties of the double chain compounds NaV2O4 and K2Cr8O16. In both the oxides, the t2g orbital is split into a non-degenerated dxy and a degenerated dyz,zx orbital due to the lattice symmetry. Based on the anisotropy of the hyperfine coupling tensor determined by the present NMR measurements, we conclude the predominant dxy orbital and the partially occupied dyz,zx orbital, naturally leading to the double exchange mechanism of the ferromagnetic interaction. In addition to the ferromagnetic interaction, there is an antiferromagnetic interaction via the direct dxy-dxy path along the double chain direction. Thus, the ferromagnetic transition takes place in K2Cr8O16, whereas the helical spin order appears in NaV2O4 due to the competition between the ferromagnetic and antiferromagnetic interactions.

Magnetic frustration effect in the multi-band vanadate NaV2O4

We have performed 23Na and 51V NMR measurements to study the magnetic frustration effect on the local magnetic properties of NaV2O4 with the double chain structure. In the paramagnetic state, we find the presence of the ferromagnetic interaction, which competes with the antiferromagnetic interaction, coming from the multi-band effect. The spin structure in the magnetically ordered state, an incommensurate helical structure which appears due to the competition between the magnetic interactions, is proposed. Thus the magnetic frustration closely related to the multi-band effect is concluded to play an essential role for the magnetic properties of NaV2O4.

Electronic States of Half-Metallic Chromium Oxides Probed by 53Cr NMR

We have performed 53Cr NMR measurements to investigate local electronic states of K2Cr8O16 and CrO2 which are ferromagnetic half-metals with high valent chromium ions. In the ferromagnetic metallic phases of both oxides, we observed at least two 53Cr NMR spectra which are inconsistent with crystallographically inequivalent one chromium site on a tetragonal lattice, the hollandite structure (symmetry I4/m) of K2Cr8O16 and the rutile structure (P42mnm) of CrO2. The origin of this anomalous electronic state may be associated with an electronic phase separation in double exchange systems with the mixed valence.

Anisotropic hyperfine coupling in vanadium oxides

We have investigated orbital occupancies in metallic states of the vanadium oxides by 51V NMR measurements on the single crystals of V2O3, K2V8016, and LiV204, having ground states of an antiferromagnetic Mott insulator, a spin singlet insulator, and a heavy-mass Fermi liquid, respectively. The Knight shift K vs the magnetic susceptibility plots for the principal axes give the axially anisotropic and isotropic parts of the hyperfine coupling constant, Aax and Aiso In V2O3, Aax diminishes owing to the equivalent contribution of three-hold t2g orbitals to the spin susceptibility. The orbital mixing is also significant in K2V8O16 having a small Aax/Aiso ~ 0.1, which is expected to have 0.5 – 0.6 for 3dxy1. In LiV204, Aax/Aiso has a large negative value, as expected in the a1g orbital. The result suggests the presence of the localized spin hidden in the itinerancy, giving the high-entropy state of LiV2O4 at low temperatures.

Local magnetic properties of Na1−xCaxV2O4 with Double Chains Invesigated by 51V NMR

We have performed 51V NMR measurements to investigate local magnetic properties of Na1−xCaxV2O4 with the CaFe2O4 type structure composed of double chains. The 51V Knight shift due to the spin susceptibility in an itinerant anfierromagnet NaV2O4 is found to obey the Curie-Weiss law with the positive Weiss temperature, indicating the ferromagnetic correlation in the paramagnetic state. The NMR spectrum at 4.2 K may be reproduced by an incommensurate spin structure in the antiferromagnetic state of NaV2O4. Also the NMR results at 4.2 K on Na1−xCaxV2O4 are consistent with the phase diagram proposed from the μ+SR measurements, namely, antiferromagnetic, spin-glass-like and mixture phases.