In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here, we report a notable exception in an insulator-ytterbium dodecaboride (YbB). The resistivity of YbB, which is of a much larger magnitude than the resistivity in metals, exhibits distinct quantum oscillations. These unconventional oscillations arise from the insulating bulk, even though the temperature dependence of the oscillation amplitude follows the conventional Fermi liquid theory of metals with a large effective mass. Quantum oscillations in the magnetic torque are also observed, albeit with a lighter effective mass.
We have studied metallic SrVO3 and CaVO3 by inverse photoemission and high-resolution photoemission. In going from Sr to Ca, considerable spectral weight is transferred from the coherent band to the upper and lower Hubbard bands. Meanwhile, the overall intensity rather than the width of the coherent band decreases, implying that the bandwidth remains finite as the system approaches the Mott transition. The result implies that the e6'ect of long-range Coulomb interaction as well as short-range interaction becomes increasingly important towards the transition.It is well known that electron correlation enhances the conduction electron mass m * in metals. The mass should diverge toward a metal-insulator transition if the transition is of second order and the carrier number remains finite up to the transition point. ' Recent studies of a filling-control system La& Sr"Ti03 suggest that such a mass divergence indeed occurs in the electronic specific heats and the magnetic susceptibilities. ' In bandwidthcontrol systems, where the ratio between the interaction strength U and the bandwidth 8' is varied for a fixed band filling, a mass divergence has also been predicted by Brinkman and Rice. Recent studies of the infinitedimension Hubbard model at half filling indicate that the coherent quasiparticle (QP) band, which crosses the Fermi level (EF), is narrowed with U/W while the spectral intensity at E~r emains unaltered.In previous work, ' we made photoemission studies of various compounds with a d ' configuration ranging from a Mott-Hubbard insulator to a normal metal (YTi03, LaTi03, SrVO3, VO2, and Re03) in order to investigate how the single-particle spectral function p(co) evolves with U/W across the transition. The result has shown that, with increasing U/W; spectral weight is transferred from the coherent part (QP excitations) around E~to the incoherent part (reminiscent of the lower Hubbard band) -1.5 eV below EF, but that the overall intensity rather than the bandwidth of the coherent part appears to decrease in contrast to what has been predicted by the Hubbard-model calculations. From analysis of the photoemission spectra using a phenomenological self-energy correction, it has been suggested that the mass enhancement associated with the spectral weight transfer is largely compensated for by the band widening which is represented by an increasing degree of the k dependence of the self-energy. However, the limited energy resolution in the previous work ' has precluded detailed and unambiguous information about the low-energy electronic structure of these compounds such as the presence or absence of the narrow QP band at Ez, as predicted theoretically.In order to clarify these points, we have performed detailed photoemission and inverse-photoemission spectroscopy [bremsstrahlung isochromat spectroscopy (BIS)] studies of two metallic compounds CaVO& and SrVO3.With much improved energy resolution, we have established that the intensity at EF indeed decreases with U/W, and that there is no detectable narrow peak in the...
Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator YbB12 in order to obtain its optical conductivity, σ(ω). Upon cooling below 70 K, a strong supression of σ(ω) is seen in the far-infrared region, indicating the opening of an energy gap of ∼ 25 meV. This gap development is coincident with a rapid decrease in the magnetic susceptibility, which shows that the gap opening has significant influence on magnetic properties. A narrow, asymmetric peak is observed at ∼ 40 meV in σ(ω), which is attributed to optical transitions between the Yb 4fderived states across the gap. In addition, a broad peak is observed at ∼ 0.25 eV. This peak is attributed to transitions between Yb 4f -derived states and p-d band, and is reminiscent of similar peaks previously observed for rare-earth hexaborides.
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