211 research outputs found
Giant Magnetoresistance Effect in the Metal-Insulator Transition of Pyrochlore Oxide Nd2Ir2O7
We investigated the magnetoresistance (MR) effect of the pyrochlore oxide
Nd2Ir2O7, which shows a metal-insulator transition at T_MI =33 K. A small
positive MR effect was observed in the metallic state above T_MI, while a large
negative MR effect was observed in the insulating state below T_MI . MR effects
exceeding 3000% were found at 1 K at a field of 9 T. As a result, we confirmed
the crossover from the insulating state to a state with a small or partial band
gap in a field up to 56 T. Furthermore, from the MR effect in Eu2Ir2O7 (T_MI =
120 K) and GdIrO (T_MI = 127 K), we revealed that the large
negative MR effect of the pyrochlore iridate Ln2Ir2O7 depends on the magnetism
of the lanthanide Ln^{3+} ion. The d-f interaction plays a significant role in
the large negative MR effect in the insulating state.Comment: 10 pages, 4 figure
Possible Excitonic Phase of Graphite in the Quantum Limit State
The in-plane resistivity, Hall resistivity and magnetization of graphite were
investigated in pulsed magnetic fields applied along the \textit{c}-axis. The
Hall resistivity approaches zero at around 53 T where the in-plane and
out-of-plane resistivities steeply decrease. The differential magnetization
also shows an anomaly at around this field with a similar amplitude compared to
that of de Haas-van Alphen oscillations at lower fields. This transition field
appears insensitive to disorder, but reduces with doping holes. These results
suggest the realization of the quantum limit states above 53 T. As a plausible
explanation for the observed gapped out-of-plane conduction above 53 T, the
emergence of the excitonic BCS-like state in graphite is proposed.Comment: 15 pages, 6 figures, to be published in J. Phys. Soc. Jp
Magnetization Plateaus in the Spin-1/2 Kagome Antiferromagnets: Volborthite and Vesignieite
The magnetization of two spin-1/2 kagome antiferromagnets, volborthite and
vesignieite, has been measured in pulsed magnetic fields up to 68 T. A
magnetization plateau is observed for each compound near the highest magnetic
field. Magnetizations at saturation are approximately equal to 0.40Ms for both
compounds, where Ms is the fully saturated magnetization, irrespective of a
difference in the distortion of the kagome lattice between the two compounds.
It should be noted that these values of magnetizations are significantly larger
than Ms/3 predicted theoretically for the one-third magnetization plateau in
the spin-1/2 kagome antiferromagnet. The excess magnetization over Ms/3 is
nearly equal to the sum of the magnetizations gained at the second and third
magnetization steps in volborthite, suggesting that there is a common origin
for the excess magnetization and the magnetization steps.Comment: 4 pages, 4 figures. Phys. Rev. B, accepte
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