SrMgxRu1-xO3 ceramics for 0 <~ x <~ 0.15
have been prepared by conventional solid-state reaction. The samples have
been studied by x-ray diffraction, dc resistivity, magnetic susceptibility,
and optical reflectance. Mg substitution produces little change in the lattice
parameters and does not destroy the ferromagnetic transition but renders the
material insulating.
Phys. Rev. B, 65, 224407, 2002.
The far-infrared c-axis reflectance of Sr2RuO4 has been measured above and
below the 2.5 K superconducting transition temperature of the sample. A plasma
edge develops near 70 cm(-1) as the temperature is lowered, which corresponds
to a Drude peak in the real optical conductivity associated with the onset of
the coherent motion of the carriers. A gaplike suppression of the
frequency-dependent scattering rate at low temperatures indicates that the
coherent transport is related to a loss of scattering below 60 cm(-1). A
strong resonance near 9 meV is responsible for the scattering that destroys
coherence.
Phys. Rev. Lett, 87, 227002:1-4, 2001.
The Raman scattering spectra of Ba3MoNbO8.5 and
Ba7Nb4MoO20, two cation-deficient
perovskite-like oxides, are presented for the first time and analyzed in an attempt to
confirm the distribution of octahedra and tetrahedra proposed in recent structural
investigations. The Raman spectra are discussed in relation to spectra of compounds of the
hexagonal perovskite-like oxides with the general formula A(n)B(n-delta)O(3n-x) (delta
greater than or equal to 1, x greater than or equal to 0).
While the spectra of all compounds are found to be dominated by strong tetrahedral bands
near 300 and 800 cm(-1), modes of octahedral symmetry are evident between 400 and 700
cm(-1) in those compounds which contain octahedral [BO6] groups according to structural
studies. The Raman spectrum of Ba3MoNbO8 contains modes in the
region attributed to vibrations of octahedral symmetry although it is stoichiometrically
identical to the palmierite structure which has only tetrahedrally coordinated B-sites.
This observation is in disagreement with structural work previously reported in the
literature but provides strong corroborating evidence for a new study which indicates a
variant stacking arrangement of hexagonal and cubic close-packed layers giving rise to an
equal number of octahedrally and tetrahedrally coordinated B-sites in this compound.
Chem. Mater., 12, 2287-91, 2000.
The polarized reflectance of nominally stoichiometric SmTiO3 has been measured
in the far-infrared at temperatures above and below the antiferromagnetic Neel temperature
T-N of 50 K. The appearance of a feature near 120 cm(-1) when the electric field vector
is polarized along the b axis correlates with the onset of magnetic ordering in the Ti and
Sm sublattices, suggesting it is likely due to a magnetic transition. Its large linewidth
and relatively high-energy scale point to a two-magnon process. Because the RTiO3
family of compounds (where R is a rare earth or Y) is susceptible to doping by vacancies
on the R site, the nominally stoichiometric sample examined has a small, but finite
background conductivity of the order 20 Omega(-1)cm(-1) at low frequencies. This
background conductivity, like that along the semiconducting c axis in underdoped cuprate
superconductors appears to be of an incoherent nature with a magnitude below Mott's
minimum metallic conductivity. At temperatures below T-N there is evidence that the continuum
interacts strongly with a nearby allowed phonon mode causing considerable asymmetry. The
correlation between the onset of magnetic ordering and the appearance of asymmetry in the allowed
phonon mode suggests that this continuum, to which the mode appears to be strongly coupled,
may be of magnetic origin.
Phys. Rev. B, 59, 6938-42, 1999.
The optical conductivity of underdoped
Pb2Sr2(Y/Ca)Cu3O8 has been obtained from
40 to 700 cm-1 via Kramers-Kronig analysis of the c-axis-polarized
reflectance measured at temperatures between 10 and 300 K. In the normal state at the
lowest and highest frequencies where it is least obscured by a large phonon
contribution, the background em>c-axis optical conductivity of underdoped
Pb2Sr2(Y/Ca)Cu3O8 is observed to decrease
systematically as the temperature is lowereed from 200 to near 150 K. Upon lowering
the temperature further, the optical conductivity remains constant until the
superconducting transition temperature of 65 K is reached, whereupon, at the lowest
frequencies only, a further depression is observed due to the removal of spectral
weight into the superconducting condensate. the spectral weight which is redistributed
into the delta-function condensate is estimated to derive from frequencies below 425
cm-1. The depressed conductivity in the normal state is attributed to the
formation of a pseudogap similar to that previously observed for underdoped
YBa2Cu3O7-y, YBa2Cu4O8,
and La2-xSrxCuO4. The temperature dependence suggests
that the pseudogap is fully formed at a temperature of the order 150 K. This
corresponds closely to the temperature range where a change in slope of the
ab-plane dc resistivity, and a decrease in the 63Cu NMR Knight shift
are observed.
Phys. Rev. B, 56, 9129-33, 1997.
We report on NMR measurements in underdoped
Pb2Sr2(Y/Ca)Cu3O8+dcrystals. A pseudogap is observed in
the Knight shift and spin-lattice relaxation rate. In contrast to other underdoped compounds,
the pseudogap observed in the Knight shift is weak and occurs at a significantly lower temperature.
On the other hand, the effect the pseudogap has on spin-lattice relaxation is quite similar to that
in other compounds. The contrast between weak and strong pseudogaps is discussed.
Phys. Rev. B, 56, R8511-4, 1997.
Raman-scattering measurements have been carried out for crystals of the RTiO3
R=La,Ce,Pr,Nd,Sm,Gd) system whose members are Mott-Hubbard insulators. RTiO3
hasan orthorhombically distorted perovskite unit cell. The distortion increases systematically from
LaTiO3 to GdTiO3 and is accompanied by changes in electronic structure (decreasing
W/U ratio). As a consequence of the changing electronic properties, he Raman spectrum shows an
interesting evolution of both the phonon features and the electronic continuum. Most notable are (1)
a redistribution in the spectral shape of the electronic background, (2) a systematic change in line
shape, and a dramatic increase in the center frequency of one of the phonon modes from 187 cm-1
in LaTiO3 to 385 cm-1 in GdTiO3, and (3) the observation of resonance
effects in the most insulating members of the series. The appearance of a free-carrier component in the
electronic-scattering background, which seems to be related to systematic self-energy effects of the
phonon near 300 cm-1, is unexpected. It is likely the result of increased doping due to a
greater facility for rare-earth vacancies to form in large R3+ionic radius members
of the series. A systematic increase in the continuum scattering rate is also observed and indicates
that the free carriers are not scattering off rare-earth vacancies but rather that the scattering
mechanism originates from changes in electronic structure.
Phys. Rev. B, 55, 1442-8, 1997.
The one- and two-phonon Raman scattering spectrum of single crystal
Pb2Sr2RCu3O8, where R is a rare earth element, is examined under
resonance conditions. Strong features appear in the one-phonon spectrum
at the positions of longitudinal optical (LO) infrared modes as the laser
energy approaches that of the charge transfer (CT) gap. These modes,
which we interpret as Fröhlich-interaction-induced, are accompanied
by a second order spectrum composed of overtones and combinations of the
LO and ordinary Raman-allowed phonons. The forbidden LO scattering
weakens with increasing metallicity of the samples due to the vanishing
of the CT gap and the screening of the Fröhlich interaction. We
examine the dependence on incident energy of both the lineshape and
center position of the highest frequency two-phonon feature, which can be
unambiguously attributed to a pure overtone of an infrared LO vibration.
Due to the two-phonon nature of the scattering process the crystal
momentum, Q, is no longer restricted to small wave vectors and one can
expect to observe effects of phonon dispersion. We find, indeed, changes
in the lineshape as the gap is approached, accompanied by a decrease in
the center frequency to a value comparable to twice that of the
Q ~ 0 one-phonon LO mode.
Phys. C, 235-240, 1165-6, 1994.
Abstracts
The one-and two-phonon Raman-scattering spectrum of single-crystal
Pb2Sr2PrCu3O8 is examined under resonance conditions. Strong features
appear in the one-phonon spectrum at the positions of longitudinal
optical (LO) infrared modes as the laser energy approaches that of the
charge transfer gap. These Fröhlich-interaction-induced modes are
accompanied by a second-order spectrum composed of overtones and
combinations of the LO and ordinary Raman-allowed phonons. We examine
the dependence on incident photon energy of both the line shape and
center position of the highest frequency two-phonon feature, which can be
unambiguously attributed to a pure overtone of an infrared LO vibration.
Due to the two-phonon nature of the scattering process the crystal
momentum q is no longer restricted to small wave vectors and one
can expect to observe effects of phonon dispersion. We find, indeed,
changes in the line shape as the gap is approached, accompanied by a
decrease in the center frequency to a value close to twice that of the
q~0 one-LO-phonon (1-LO) mode. A detailed comparison to the free
electron theory for two-LO-phonon (2-LO) scattering by Zeyher
[Phys. Rev. B 9, 4439 (1974)] is carried out. Relationships
between the effective lattice mass characterizing the dispersion of this
LO branch and the electron and hole effective band masses are
established, and an estimate of the effective electron and hole band
masses is made based on the magnitude of the phonon dispersion in
YBa2Cu3O6 as measured by neutron scattering experiments.
Phys. Rev. B, 50, 13762-7, 1994.
We observe via polarized far-infrared-reflectance measurements of
single-crystal samples of Pb2Sr2RCu3O8,
where R
is a rare-earth element, significant doping and temperature-induced
changes in two c-axis phonons assigned to motion within the
structural block containing the double CuO2 planes. In the normal state
both phonons shift to lower frequency as the level of doping increases,
with a monotonic transfer of oscillator strength from the lower [bending
motion along c of the planar oxygen, O(3)] to the higher
[vibrations of the apical oxygen, O(1)]. This doping-induced transfer of
oscillator strength to the apical oxygen, accompanied by a sudden
increase in asymmetry in the normal-state line shape of metallic samples
which exhibit superconductivity at low temperatures, suggests that this
process may play a role in the mechanism leading to the superconducting
pairing. The linewidth of these two phonons shows a maximum as a
function of doping near which, upon entry into the superconducting state,
there is further anomalous broadening. A third phonon, attributed to
vibrations of oxygen within the O(2)-Cu(1)-O(2) "sticks" and thus
spatially removed from the CuO2 planes, shows no unusual behavior. These
observations are consistent with a strong interaction between the phonons
and the carriers of the structural block containing the CuO2 planes.
Phys. Rev. B, 49, 15984-92, 1994.
We show that the strong absorption structure observed in the
a-b-plane optical conductivity of the high-Tc
superconductors is correlated with c-axis longitudinal
optical (LOc) phonons. This suggests a resolution to the long-standing
controversy over the origin of these features. The interaction with LOc
phonons is forbidden when the incident wave vector q is normal to
the c axis which leads to the surprising result,
confirmed with experiments, that the optical properties along a
or b are different when measured on the
a-b-plane face and a face containing the c
axis.
Phys. Rev. Lett., 69, 2705-8, 1992. Reply to
Comment: Phys. Rev. Lett. 71, 2677, 1993.
Details are given of a technique for measuring the reflectance at
near-normal incidence of small, irregular, submillimeter-sized samples
from the far IR (40 cm-1) to the visible (40000 cm-1) between 10 and 300
K by using a modified Michelson interferometer or grating spectrometer.
The sample and a reference mirror are mounted on nonreflecting cones.
At the focus the size of the beam is larger than either the sample or the
reference, so that the entire area of the sample is utilized. The
positions are interchanged by a 90 degree rotation by using preset
mechanical stops. The scattering caused by geometrical effects is
corrected for by the in situ evaporation of gold or aluminum
onto the sample. The effect of diffraction is estimated from Mie theory
by assuming the sample and reference to be spheres. For frequencies
above ~40 cm-1 and sample diameters of ~1 mm with a detector field of
view of 30 degrees, the calculations show that the ratio of the
backscattered intensities gives a good approximation of the specular
reflectance.
Appl. Opt., 32, 2976-83, 1993.
The ab-plane dc resistivity and reflectivity of single
crystals of the Ca-free Pb2Sr2LCu3O8
series, where
L=Y, Dy, Eu, Sm, Nd, and Pr, has been investigated. A trend
is observed from insulating to metallic and superconducting behavior with
decreasing size of the rare-earth constituent. A transfer of spectral
weight from the Cu-O charge-transfer band to the mid- and far-infrared
regions accompanies the progression to metallic character. As the
electronic background develops, phonons visible in the far-infrared
reflectance of the insulating members (Nd and Pr) become increasingly more
screened, while concurrently, there is an unexpected appearance of
phonons in the 400-600-cm-1 region of the spectrum. These phonons
develop an antiresonantlike line shape at higher doping levels that we
attribute to coupling to the mid-infrared continuum. Comparison to Raman
and neutron work suggests the presence of a universal band near 400
cm-1.
Phys. Rev. B, 45, 7406-13, 1992.
Magnetization measurements of single-crystal Pb2Sr2(Y,Ca)Cu3O8 from 5 to
250 K in a field of 1.5 T show that, with the field oriented perpendicular
to the c axis of the crystal, there is a sharply defined
onset of diamagnetism at Tc while with the field parallel to
the c axis, the transition is rounded and the diamagnetism
present above Tc as a result of superconducting fluctuations.
The three-dimensional critical region just above Tc is
found in the latter case to be ~2 K in width. Via analysis of the
two-dimensional regime above this critical region, a value of 11
angstroms for the zero-temperature abplane coherence length
is obtained. Using this value for the zero-temperature
abplane coherence the
temperature-dependent magnitization in the vicinity of
Tc for various field strengths is analyzed to extract
the temperature dependence of the in-plane upper critical field
near Tc. In addition, the temperature dependence of
both the in-plane upper critical field and the out-of-plane critical field is
determined directly from the linear onset of diamagnetism
and the critical-field slope for the in-plane upper critical field is in
good agreement with that derived from the fluctuation analysis.
Phys. Rev. B, 45, 10057-61, 1992.