The magnetic, electric and thermal properties of the (Ln 1−y Y y ) 0.7 Ca 0.3 CoO 3 perovskites (Ln = Pr, Nd) were investigated down to very low temperatures. The main attention was given to a peculiar metal-insulator transition, which is observed in the praseodymium based samples with y = 0.075 and 0.15 at T M −I = 64 and 132 K, respectively. The study suggests that the transition, reported originally in Pr 0.5 Ca 0.5 CoO 3 , is not due to a mere change of cobalt ions from the intermediate-to the low-spin states, but is associated also with a significant electron transfer between Pr 3+ and Co 3+ /Co 4+ sites, so that the praseodymium ions occur below T M −I in a mixed Pr 3+ /Pr 4+ valence. The presence of Pr 4+ ions in the insulating phase of the yttrium doped samples (Pr 1−y Y y ) 0.7 Ca 0.3 CoO 3 is evidenced by Schottky peak originating in Zeeman splitting of the ground state Kramers doublet. The peak is absent in pure Pr 0.7 Ca 0.3 CoO 3 in which metallic phase, based solely on non-Kramers Pr 3+ ions, is retained down to the lowest temperature.

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We have studied a Pr-site substitution effect using various RE ions (RE ¼ Nd, Sm, Gd, and Y) on a simultaneous metal-insulator (MI) and spin-state (SS) transition in (Pr 1Ày RE y ) 1Àx Ca x CoO 3 using measurements of electrical resistivity, magnetization, and thermal dilatation. The MI-SS transition took place at the appropriate combination of x and y for samples of RE ¼ Sm, Gd, and Y. The MI-SS transition temperatures T MI{SS can be scaled universally by the average ionic radius hr A i of the A-site in the perovskite ACoO 3 , which is independent of x, y, and the RE ion species. The atomic randomness of the A-site, which is defined as the mean square deviation ' 2 , larger than the critical value ' 2 cr is also necessary for the occurrence of the MI-SS transition and T MI{SS increases with increasing ' 2 . In contrast, no MI-SS transition was observed in the RE ¼ Nd samples (x ¼ 0:2 { 0:4), which can be inferred from the small ' 2 value because of the small difference in ionic radius between Pr 3þ and Nd 3þ . The atomic randomness of the A-site might be an important parameter that dominates the MI-SS transition through the difference in electronic energy E between the spin states of Co 3þ ions.

Reflection-mode scattering-type scanning near-field optical microscope using a laser trapped gold colloidal particle as a scattering probe

We propose that heterodyne detection is useful for the scattering-type scanning near-field optical microscope (SNOM) to effectively remove the scattering from various sources on the sample which causes deterioration of the image resolution. First, we show in a simple model that this sample scattering cannot be completely removed by ac-mode detection of the SNOM signal for samples with high reflectivity, while only the probe-scattering signal can be extracted by heterodyne detection. Next, we show experimental results which compare the ac-mode and heterodyne images, and demonstrate the effectiveness of heterodyne detection.