471 research outputs found
Pressure Tuning of an Ionic Insulator into a Heavy Electron Metal: An Infrared Study of YbS
Optical conductivity [] of YbS has been measured under
pressure up to 20 GPa. Below 8 GPa, is low since YbS is an
insulator with an energy gap between fully occupied 4 state and unoccupied
conduction () band. Above 8 GPa, however, increases
dramatically, developing a Drude component due to heavy carriers and
characteristic infrared peaks. It is shown that increasing pressure has caused
an energy overlap and hybridization between the band and 4 state, thus
driving the initially ionic and insulating YbS into a correlated metal with
heavy carriers
Possible Kondo resonance in PrFe4P12 studied by bulk-sensitive photoemission
Pr 4f electronic states in Pr-based filled skutterudites PrT4X12(T=Fe and Ru;
X=P and Sb) have been studied by high-resolution bulk-sensitive Pr 3d-4f
resonance photoemission. A very strong spectral intensity is observed just
below the Fermi level in the heavy-fermion system PrFe4P12. The increase of its
intensity at lower temperatures is observed. We speculate that this is the
Kondo resonance of Pr, the origin of which is attributed to the strong
hybridization between the Pr 4f and the conduction electrons.Comment: 4 pages(camera ready format), 4 figures, ReVTeX
Infrared study of valence transition compound YbInCu4 using cleaved surfaces
Optical reflectivity R(w) of YbInCu4 single crystals has been measured across
its first-order valence transition at T_v ~ 42 K, using both polished and
cleaved surfaces. R(w) measured on cleaved surfaces Rc(w) was found much lower
than that on polished surface Rp(w) over the entire infrared region. Upon
cooling through T_v, Rc(w) showed a rapid change over a temperature range of
less than 2 K, and showed only minor changes with further cooling. In contrast,
Rp(w) showed much more gradual and continuous changes across T_v, similarly to
previously reported data on polished surfaces. The present result on cleaved
surfaces demonstrates that the microscopic electronic structures of YbInCu4
observed with infrared spectroscopy indeed undergo a sudden change upon the
valence transition. The gradual temperature-evolution of Rp(w) is most likely
due to the compositional and/or Yb-In site disorders caused by polishing.Comment: 4 pages, 4 figures, Fig.1(a) correcte
Optical conductivity of the Kondo insulator YbB_12: Gap formation and low-energy excitations
Optical reflectivity experiments have been conducted on single crystals of
the Kondo insulator YbB_12 in order to obtain its optical conductivity,
\sigma(\omega). Upon cooling below 70 K, a strong supression of \sigma(\omega)
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 \sigma(\omega), which is attributed to optical transitions between the
Yb 4f-derived 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.Comment: 4 pages, 4 figure
Theory of Metal-Insulator Transition in PrRu4P12 and PrFe4P12
All symmetry allowed couplings between the 4f^2-electron ground state doublet
of trivalent praseodymium in PrRu4P12 and PrFe4P12 and displacements of the
phosphorus, iron or ruthenium ions are considered. Two types of displacements
can change the crystal lattice from body-centred cubic to simple orthorhombic
or to simple cubic. The first type lowers the point group symmetry from
tetrahedral to orthorhombic, while the second type leaves it unchanged, with
corresponding space group reductions Im3 --> Pmmm and Im3 --> Pm3 respectively.
In former case, the lower point-group symmetry splits the degeneracy of the
4f^2 doublet into states with opposite quadrupole moment, which then leads to
anti-quadrupolar ordering, as in PrFe4P12. Either kind of displacement may
conspire with nesting of the Fermi surface to cause the metal-insulator or
partial metal-insulator transition observed in PrFe4P12 and PrRu4P12. We
investigate this scenario using band-structure calculations, and it is found
that displacements of the phosphorus ions in PrRu4P12 (with space group
reduction Im3 --> Pm3) open a gap everywhere on the Fermi surface.Comment: 6 page
Metal-insulator transition in PrRuP and SmRuP investigated by optical spectroscopy
Electronic structures of the filled-skutterudite compounds PrRuP
and SmRuP, which undergo a metal-insulator transition (MIT) at
= 60 K and 16 K, respectively, have been studied by means of
optical spectroscopy. Their optical conductivity spectra develop an energy gap
of 10 meV below . The observed characteristics of the energy
gap are qualitatively different from those of the Kondo semiconductors. In
addition, optical phonon peaks in the spectra show anomalies upon the MIT,
including broadening and shifts at and an appearance of new peaks
below . These results are discussed in terms of density waves or
orbital ordering previously predicted for these compounds.Comment: 4pages, 4figures, submitted to Physical Review
Bulk-sensitive Photoemission of Mn5Si3
We have carried out a bulk-sensitive high-resolution photoemission experiment
on Mn5Si3. The measurements are performed for both core level and valence band
states. The Mn core level spectra are deconvoluted into two components
corresponding to different crystallographic sites. The asymmetry of each
component is of noticeable magnitude. In contrast, the Si 2p spectrum shows a
simple Lorentzian shape with low asymmetry. The peaks of the valence band
spectrum correspond well to the peak positions predicted by the former band
calculation.Comment: To be published in: Solid State Communication
Electron self-trapping in intermediate-valent SmB6
SmB6 exhibits intermediate valence in the ground state and unusual behaviour
at low temperatures. The resistivity and the Hall effect cannot be explained
either by conventional sf-hybridization or by hopping transport in an impurity
band. At least three different energy scales determine three temperature
regimes of electron transport in this system. We consider the ground state
properties, the soft valence fluctuations and the spectrum of band carriers in
n-doped SmB6. The behaviour of excess conduction electrons in the presence of
soft valence fluctuations and the origin of the three energy scales in the
spectrum of elementary excitations is discussed. The carriers which determine
the low-temperature transport in this system are self-trapped electron-polaron
complexes rather than simply electrons in an impurity band. The mechanism of
electron trapping is the interaction with soft valence fluctuations.Comment: 12 pages, 3 figure
Possible Kondo resonance in PrFe4P12 studied by bulk-sensitive photoemission
Pr 4f electronic states in Pr-based filled skutterudites PrT4X12 (T = Fe and Ru; X = P and Sb) have been studied by high-resolution bulk-sensitive Pr 3d-->4f resonance photoemission. A very strong spectral intensity is observed just below the Fermi level in the heavy-fermion system PrFe4P12. The increase of its intensity at lower temperatures is observed. We speculate that this is the Kondo resonance of Pr, the origin of which is attributed to the strong hybridization between the Pr 4f and the conduction electrons
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