Band-width control in a perovskite-type 3d^1 correlated metal Ca_1-xSr_xVO_3. II. Optical spectroscopy investigation

Optical conductivity spectra of single crystals of Ca_1-xSr_xVO_3 have been studied to elucidate how the electronic behavior depends on the strength of the electron correlation without changing the nominal number of electrons per vanadium atom. The effective mass deduced by the analysis of the Drude-like contribution do not show critical enhancement, even though the system is close to the Mott transition. Besides the Drude-like contribution, two anomalous features were observed in the optical conductivity spectra of the intraband transition within the 3d band. These features can be assigned to transitions involving the incoherent and coherent bands near the Fermi level. The large spectral weight redistribution in this system, however, does not involve a large mass enhancement.Comment: 12 pages in a Phys. Rev. B camera-ready format with 16 EPS figures embedded. LaTeX 2.09 source file using "camera.sty" and "prbplug.sty" provided by N. Shirakawa. For OzTeX (Macintosh), use "ozfig.sty" instead of "psfig.sty". "ozfig.sty" can be also obtained by e-mail request to N. Shirakawa: . Submitted to Phys. Rev. B. See "Part I (by Inoue et al.)" at cond-mat/980107

Infrared response of ordered polarons in layered perovskites

We report on the infrared absorption spectra of three oxides where charged superlattices have been recently observed in diffraction experiments. In La$_{1.67}$Sr$_{0.33}$NiO$_4$, polaron localization is found to suppress the low-energy conductivity through the opening of a gap and to split the $E_{2u}$-$A_{2u}$ vibrational manifold of the oxygen octahedra. Similar effects are detected in Sr$_{1.5}$La$_{0.5}$MnO$_4$ and in La$_2$NiO$_{4+y}$, with peculiar differences related to the type of charge ordering.Comment: File latex, 11 p. + 3 Figures, to appear on Phys. Rev. B (Rapid Commun.), 1 Oct. 1996. The figures will be faxed upon request. E-mail:[email protected] Fax: +39-6-446315

Signatures of polaronic excitations in quasi-one-dimensional LaTiO3.41_{3.41}

The optical properties of quasi-one-dimensional metallic LaTiO$_{3.41}$ are studied for the polarization along the $a$ and $b$ axes. With decreasing temperature modes appear along both directions suggestive for a phase transition. The broadness of these modes along the conducting axis might be due to the coupling of the phonons to low-energy electronic excitations across an energy gap. We observe a pronounced midinfrared band with a temperature dependence consistent with (interacting) polaron models. The polaronic picture is corroborated by the presence of strong electron-phonon coupling and the temperature dependence of the dc conductivity.Comment: 5 pages, 5 figure

Localization of vibrational modes in high-entropy oxides

The recently-discovered high-entropy oxides offer a paradoxical combination of crystalline arrangement and high disorder. They differ qualitatively from established paradigms for disordered solids such as glasses and alloys. In these latter systems, it is well known that disorder induces localized vibrational excitations. In this article, we explore the possibility of disorder-induced localization in (MgCoCuNiZn)O, the prototypical high-entropy oxide with rock-salt structure. To describe phononic excitations, we model the interatomic potentials for the cation-oxygen interactions by fitting to the physical properties of the parent binary oxides. We validate our model against the experimentally determined crystal structure, bond lengths, and optical conductivity. The resulting phonon spectrum shows wave-like propagating modes at low energies and localized modes at high energies. Localization is reflected in signatures such as participation ratio and correlation amplitude. Finally, we explore the possibility of increased mass disorder in the oxygen sublattice. Admixing sulphur or tellurium atoms with oxygen enhances localization. It even leads to localized modes in the middle of the spectrum. Our results suggest that high-entropy oxides are a promising platform to study Anderson localization of phonons.Comment: 13 pages, 17 figures. To be submitted to Physical Review

Band-width control in a perovskite-type 3d^1 correlated metal Ca_{1-x}Sr_xVO_3. I. Evolution of the electronic properties and effective mass

Single crystals of the perovskite-type $3d^{1}$ metallic alloy system Ca$_{1-x}$Sr$_x$VO$_3$ were synthesized in order to investigate metallic properties near the Mott transition. The substitution of a Ca$^{2+}$ ion for a Sr$^{2+}$ ion reduces the band width $W$ due to a buckling of the V-O-V bond angle from $\sim180^\circ$ for SrVO$_3$ to $\sim160^\circ$ for CaVO$_3$. Thus, the value of $W$ can be systematically controlled without changing the number of electrons making Ca$_{1-x}$Sr$_x$VO$_3$: one of the most ideal systems for studying band-width effects. The Sommerfeld-Wilson's ratio ($\simeq2$), the Kadowaki-Woods ratio (in the same region as heavy Fermion systems), and a large $T^{2}$ term in the electric resistivity, even at 300 K, substantiate a large electron correlation in this system, though the effective mass, obtained by thermodynamic and magnetic measurements, shows only a systematic but moderate increase in going from SrVO$_3$ to CaVO$_3$, in contrast to the critical enhancement expected from the Brinkmann-Rice picture. It is proposed that the metallic properties observed in this system near the Mott transition can be explained by considering the effect of a non-local electron correlation.Comment: 14 pages in a Phys. Rev. B camera-ready format with 10 EPS figures embedded. LaTeX 2.09 source file using "camera.sty" and "prbplug.sty" provided by N. Shirakawa. For OzTeX (Macintosh), use "ozfig.sty" instead of "psfig.sty". "ozfig.sty" can be also obtained by e-mail request to N. Shirakawa: . Submitted to Phys. Rev.

A Scaling Behavior of Spectral Weight Changes in Perovskite Manganites La_{0.7-y}Pr_{y}Ca_{0.3}MnO_3

Optical conductivity spectra of La_{0.7-y}Pr_{y}Ca_{0.3}MnO_3 were systematically investigated. For metallic samples, the spectral weight below 0.5 eV, whose magnitude can be represented by the effective carrier number N_{eff}(0.5 eV), increases as temperature becomes lower. Regardless of the Pr doping, all the measured values of N_{eff}(0.5 eV)/T_C fall into one scaling curve. This scaling behavior could be explained by the theoretical model by Roeder et al. [Phys. Rev. Lett. 76, 1356 (1996)], which includes spin double exchange and Jahn-Teller lattice coupling to holes. With the Pr doping, far-infrared conductivities were found to be suppressed, probably due to the Anderson localization.Comment: Latex 2e, 8 pages including 4 postscript figures, submitted at Apr 2

Mid-Infrared Conductivity from Mid-Gap States Associated with Charge Stripes

The optical conductivity of La(2-x)Sr(x)NiO(4) has been interpreted in various ways, but so far the proposed interpretations have neglected the fact that the holes doped into the NiO(2) planes order in diagonal stripes, as established by neutron and X-ray scattering. Here we present a study of optical conductivity in La(2)NiO(4+d) with d=2/15, a material in which the charge stripes order three-dimensionally. We show that the conductivity can be decomposed into two components, a mid-infrared peak that we attribute to transitions from the filled valence band into empty mid-gap states associated with the stripes, and a Drude peak that appears at higher temperatures as carriers are thermally excited into the mid-gap states. The shift of the mid-IR peak to lower energy with increasing temperature is explained in terms of the Franck-Condon effect. The relevance of these results to understanding the optical conductivity in the cuprates is discussed.Comment: final version of paper (minor changes from previous version

Neutron diffraction study of stripe order in La(2)NiO(4+d) with d=2/15

We report a detailed neutron scattering study of the ordering of spins and holes in oxygen-doped La(2)NiO(4.133). The single-crystal sample exhibits the same oxygen-interstitial order but better defined charge-stripe order than that studied previously in crystals with d = 0.125. In particular, charge order is observed up to a temperature at least twice that of the magnetic transition, T_m = 110.5 K. On cooling through T_m, the wave vector \epsilon, equal to half the charge-stripe density within an NiO(2) layer, jumps discontinuously from 1/3 to 0.2944. It continues to decrease with further cooling, showing several lock-in transitions on the way down to low temperature. To explain the observed lock-ins, a model is proposed in which each charge stripe is centered on either a row of Ni or a row of O ions. The model is shown to be consistent with the l-dependence of the magnetic peak intensities and with the relative intensities of the higher-order magnetic satellites. Analysis of the latter also provides evidence that the magnetic domain walls (charge stripes) are relatively narrow. In combination with a recent study of magnetic-field-induced effects, we find that the charge stripes are all O-centered at T>T_m, with a shift towards Ni centering at T<T_m. Inferences concerning the competing interactions responsible for the the temperature dependence of \epsilon and the localization of charge within the stripes are discussed.Comment: ReVTeX, 17 2-col pages, 10 eps figs. embedded with psfig, submitted to Phys. Rev.

Spin Dynamics in an Ordered Stripe Phase

Inelastic neutron scattering has been used to measure the low-energy spin excitations in the ordered charge-stripe phase of La(2)NiO(4+d) with d=0.133. Spin-wave-like excitations disperse away from the incommensurate magnetic superlattice points with a velocity ~60% of that in the d=0 compound. Incommensurate inelastic peaks remain well-resolved up to at least twice the magnetic ordering temperature. Paramagnetic scattering from a d=0.105 sample, which has a N\'eel-ordered ground state, shows anomalies suggestive of incipient stripe correlations. Similarities between these results and measurements on superconducting cuprates are discussed.Comment: 5 pp, 2 col., REVTeX, 4 epsf figures embedded with psfig; Abstract and introduction have been revise

Orbital Physics in the Perovskite Ti Oxides

In the perovskite Ti oxide RTiO3 (R=rare-earth ions), the Ti t2g orbitals and spins in the 3d^1 state couple each other through the strong electron correlations, resulting in a rich variety of orbital-spin phases. The origin and nature of orbital-spin states of these Mott insulators have been intensively studied. In this article, we review the studies on orbital physics in the perovskite titanates. We focus on the following three topics: (1) the origin and nature of the ferromagnetism as well as the orbital ordering in the compounds with relatively small R ions such as GdTiO3 and YTiO3, (2) the origin of the G-type antiferromagnetism and the orbital state in LaTiO3, and (3) the orbital-spin structures in other AFM(G) compounds with relatively large R ions (R=Ce, Pr, Nd and Sm). On the basis of these discussions, we discuss the whole phase diagram together with mechanisms of the magnetic phase transition. We also show that the Ti t2g degeneracy is inherently lifted in the titanates, which allows the single-band descriptions of the ground-state and low-energy electronic structures as a good starting point. Our analyses indicate that these compounds offer touchstone materials described by the single-band Hubbard model on the cubic lattice. From this insight, we also reanalyze the hole-doped titanates. Experimentally revealed filling-dependent and bandwidth-dependent properties and the critical behavior of the metal-insulator transitions are discussed in the light of theories based on the single-band Hubbard models.Comment: Review article, 26 pages, to appear in New Journal of Physic