Metal-insulator transition in SrMgxRu1-xO3 system

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.

Far infrared normal state resonance in Sr2RuO4

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.

Raman scattering study of cation-deficient Ban(MoNb)n-dO3n-x and related perovskite-like oxides

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.

Far-infrared optical properties of antiferromagnetic SmTiO3

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.

Far-infrared investigation of the pseudogap in underdoped Pb2Sr2(Y/Ca)Cu3O8

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.

Weak pseudogap in crystals of Pb2Sr2(Y/Ca)Cu3O8+d

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 in the RTiO3 family of Mott-Hubbard insulators

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.

Forbidden Fröhlich-interaction-induced Z-LO Raman scattering in Pb2Sr2PrCu3O8 (R = rare earth element).

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.


Observation of the effects of phonon dispersion on the Fröhlich-interaction induced second-order Raman scattering in Pb2Sr2PrCu3O8 .

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.

Anomalous temperature- and doping-induced changes in the c-axis apical-oxygen phonon mode of Pb2Sr2RCu3O8 .

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.

Evidence for a-b-Plane coupling to Longitudinal c-Axis phonons in High-Tc Superconductors.

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.

Technique for measuring the reflectance of irregular, submillimeter-sized samples.

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.

Optical investigation of the metal-insulator transition in the Ca-free Pb2Sr2LCu3O8 Series (L=Y, Dy, Eu, Sm, Nd, Pr).

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+d: Determination of the coherence length and upper critical field.

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.