49 research outputs found
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-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
Polaronic Signatures in Mid-Infrared Spectra: Prediction for LaMnO3 and CaMnO3
Hole-doped LaMnO3 and electron-doped CaMnO3 form self-trapped electronic
states. The spectra of these states have been calculated using a two orbital
(Mn eg Jahn-Teller) model, from which the non-adiabatic optical conductivity
spectra are obtained. In both cases the optical spectrum contains weight in the
gap region, whose observation will indicate the self-trapped nature of the
carrier states. The predicted spectra are proportional to the concentration of
the doped carriers in the dilute regime, with coefficients calculated with no
further model parameters.Comment: 6 pages with 3 figures imbedde
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
LaSrNiO, polaron localization is found to suppress the
low-energy conductivity through the opening of a gap and to split the
- vibrational manifold of the oxygen octahedra. Similar effects
are detected in SrLaMnO and in LaNiO, 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
Dielectric properties and dynamical conductivity of LaTiO3: From dc to optical frequencies
We provide a complete and detailed characterization of the
temperature-dependent response to ac electrical fields of LaTiO3, a
Mott-Hubbard insulator close to the metal-insulator transition. We present
combined dc, broadband dielectric, mm-wave, and infrared spectra of ac
conductivity and dielectric constant, covering an overall frequency range of 17
decades. The dc and dielectric measurements reveal information on the
semiconducting charge-transport properties of LaTiO3, indicating the importance
of Anderson localization, and on the dielectric response due to ionic
polarization. In the infrared region, the temperature dependence of the phonon
modes gives strong hints for a structural phase transition at the magnetic
ordering temperature. In addition, a gap-like electronic excitation following
the phonon region is analyzed in detail. We compare the results to the
soft-edge behavior of the optical spectra characteristic for Mott-Hubbard
insulators. Overall a consistent picture of the charge-transport mechanisms in
LaTiO3 emerges.Comment: 11 pages, 8 figures, 1 tabl
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
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 metallic alloy system
CaSrVO were synthesized in order to investigate metallic
properties near the Mott transition. The substitution of a Ca ion for a
Sr ion reduces the band width due to a buckling of the V-O-V bond
angle from for SrVO to for CaVO. Thus,
the value of can be systematically controlled without changing the number
of electrons making CaSrVO: one of the most ideal systems for
studying band-width effects. The Sommerfeld-Wilson's ratio (), the
Kadowaki-Woods ratio (in the same region as heavy Fermion systems), and a large
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 to CaVO, 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
Signatures of polaronic excitations in quasi-one-dimensional LaTiO
The optical properties of quasi-one-dimensional metallic LaTiO are
studied for the polarization along the and 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
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.