16 research outputs found
Phonons and Hybrid Modes in the High and Low Temperature Far Infrared Dynamics of Hexagonal TmMnO3
We report on TmMnO3 far infrared emissivity and reflectivity spectra from
1910 K to 4 K. At the highest temperature the number of infrared bands is lower
than that predicted for centrosymmetric P63/mmc (D6h4) (Z=2) space group due
high temperature anharmonicity and possible defect induced bitetrahedra
misalignments. On cooling, at ~1600 K TmMnO3 goes from non-polar to an
antiferroelectric-ferroelectric polar phase reaching the ferroelectric onset at
the ~700 K. The 300 K reflectivity is fitted using 19 oscillators and this
number of phonons is maintained down to 4 K. A weak phonon anomaly in the band
profile at 217 cm-1 (4 K) suggests subtle Rare Earth magnetoelectric couplings
at ~TN and below. A low energy collective excitation is identified as a THz
instability associated with room temperature eg electrons in a d-orbital
fluctuating environment. It condenses into two modes that emerge pinned to the
E-type antiferromagmetic order hardening simultaneously down to 4 K. They obey
power laws with TN as the critical temperature and match known zone center
magnons. The one peaking at 26 cm-1, with critical exponent \b{eta}=0.42 as for
antiferromagnetic order in a hexagonal lattice, is dependent on the Rare Earth.
The band at ~50 cm-1, with \b{eta}=0.25, splits at ~TN into two peaks. The
weaker band of the two is assimilated to the upper branch of gap opening in the
transverse acoustical (TA) phonon branch crossing the magnetic dispersion found
in YMnO3. (Petit et al, 2007 PRL 99, 266604). The stronger second at ~36 cm-1
corresponds to the lower branch of the TA gap. We assign both excitations as
zone center magnetoelectric hybrid quasiparticles concluding that in NdMnO3
perovskite the equivalent picture corresponds to an instability which may be
driven by an external field to transform NdMnO3 into a multiferroic compound by
perturbation enhancing the TA phonon-magnetic correlation.Comment: 39 pages, 9 Figure
High Temperature Far Infrared Dynamics of Orthorhombic NdMnO3: Emissivity and Reflectivity
We report on near normal far- and mid-infrared emission and reflectivity of
NdMnO3 perovskite from room temperature to sample decomposition above 1800 K.
At 300 K the number infrared active phonons is in close agreement with the 25
calculated for the orthorhombic D2h16-Pbnm (Z=4) space group. Their number
gradually decreases as we approach the temperature of orbital disorder at ~1023
K where the orthorhombic O' lower temperature cooperative phase coexists with
the cubic orthorhombic O. At above ~1200 K, the three infrared active phonons
coincide with the expected for cubic Pm-3m (Z=1) in the high temperature
insulating regime. Heating samples in dry air triggers double exchange
conductivity by Mn3+ and Mn4+ ions and a small polaron mid-infrared band. Fits
to the optical conductivity single out the octahedral antisymmetric and
symmetric vibrational modes as main phonons in the electron-phonon interactions
at 875 K. For 1745 K, it is enough to consider the symmetric stretching
internal mode. An overdamped defect induced Drude component is clearly outlined
at the highest temperatures. We conclude that Rare Earth manganites eg
electrons are prone to spin, charge, orbital, and lattice couplings in an
intrinsic orbital distorted perovskite lattice favoring embryonic low energy
collective excitations.Comment: 32 pages with 5 figure
Reflectividad infrarroja de SrRuO₃
En este trabajo presentamos los espectros de reflectividad infrarroja a temperatura ambiente y a 80K piara la perovskita distorsionada SrRuO₃. Con las relaciones de Krammers-Kronig calculamos las funciones ópticas y reconstruimos el espectro de reflectividad con un modelo basado en la generalización de la relación de Lyddane- Sachs-Teller y el modelo de Drude. Con estos datos calculamos el número de portadores, la movilidad efectiva y la conductividad óptica, comparando nuestros resultados con los conocidos para diferentes compuestos.Centro de QuÃmica Inorgánic
Electron Dynamics in Films Made of Transition Metal Nanograins Embedded in SiO2:Infrared Reflectivity and Nanoplasma Infrared Resonance
We report on near normal infrared reflectivity spectra of ~550 nm thick films
made of cosputtered transition metal nanograins and SiO2 in a wide range of
metal fractions. Co0.85(SiO2)0.15,with conductivity well above the percolation
threshold has a frequency and temperature behavior according to what it is find
in conducting metal oxides. The electron scattering rate displays an unique
relaxation time characteristic of single type of carriers experiencing strong
electron-phonon interactions. Using small polaron fits we identify those
phonons as glass vibrational modes. Ni0.61(SiO2)0.39, with a metal fraction
closer to the percolation threshold, undergoes a metal-non metal transition at
~77 K. Here, as it is suggested by the scattering rate nearly quadratic
dependence, we broadly identify two relaxation times (two carrier
contributions) associated to a Drude mode and a mid-infrared overdamped band,
respectively. Disorder induced, the mid-infrared contribution drives the phase
transition by thermal electron localization. Co0.51(SiO2)0.49 has the
reflectivity of an insulator with a distinctive band at ~1450cm\^{-1}
originating in electron promotion, localization, and defect induced polaron
formation. Angle dependent oblique reflectivity of globally insulating
Co0.38(SiO2)0.62, Fe0.34(SiO2)0.66, and Ni0.28(SiO2)0.72, reveals a remarkable
resonance at that band threshold. We understand this as due to the excitation
by normal to the film electric fields of defect localized electrons in the
metallic nanoparticle
Collective Phase-like Mode and the Role of Lattice Distortions at TN~TC in RMn2O5 (R= Pr, Sm, Gd, Tb, Bi)
We report on electronic collective excitations in RMn2O5 (R= Pr, Sm, Gd, Tb)
showing condensation starting at and below TN\simTC\sim40-50 K. Its origin is
understood as partial delocalized eg electron orbitals in the Jahn-Teller
distortion of the pyramids dimmer with strong hybridized Mn3+-O bonds. Our
local probes, Raman, infrared, and X-ray absorption, back the conclusion by
which there is no structural phase transition at TN\simTC. Ferroelectricity is
magnetically assisted by electron localization triggering lattice
polarizability by unscreening. We have also found phonon hardening as the rare
earth is sequentially replaced. This is understood as consequence of lanthanide
contraction. It is suggested that partially f-electron screened Rare Earth
nuclei might be introducing a perturbation to eg electrons prone to delocalize
as the superexchange interaction takes place.Comment: Journal of Physics Cond. Matter April 12, 2012. In pres
Stability of the Ni sites across the pressure-induced metallization in YNiO3
The local environment of nickel atoms in Y NiO3 across the pressure- induced
insulator to metal (IM) transition was studied using X-ray absorption
spectroscopy (XAS) supported by ab initio calculations. The monotonic
contraction of the NiO6 units under applied pressure observed up to 13 GPa,
stops in a limited pressure domain around 14 GPa, before resuming above 16 GPa.
In this narrow pressure range, crystallographic modifications basically occur
in the medium/long range, not in the NiO6 octahedron, whereas the evolution of
the near-edge XAS features can be associated to metallization. Ab initio
calculations show that these features are related to medium range order,
provided that the Ni-O-Ni angle enables a proper overlap of the Ni eg and O 2p
orbitals. Metallization is then not directly related to modifications in the
average local geometry of the NiO6 units but more likely to an inter-octahedra
rearrangement. These outcomes provides evidences of the bandwidth driven nature
of the IM transition.Comment: 6 pages with figure
Raman and infrared spectroscopy of Sr2B′UO6 (B′ = Ni; Co) double perovskites
Temperature dependent normal modes and lattice thermal expansion of Sr 2B′UO6 (B′ = Ni, Co) double perovskites were investigated by Raman/infrared spectroscopies and synchrotron X-ray diffraction, respectively. Monoclinic crystal structures with space group P21/n were confirmed for both compounds, with no clear structural phase transition between 10 and 400 K. As predicted for this structure, the first-order Raman and infrared spectra show a plethora of active modes. In addition, the Raman spectra reveal an enhancement of the integrated area of an oxygen stretching mode, which is also observed in higher-order Raman modes, and an anomalous softening of ∼1 cm-1 upon cooling below T* ∼ 300 K. In contrast, the infrared spectra show conventional temperature dependence. The band profile phonon anomalies are possibly related to an unspecified electronic property of Sr2B′UO6 (B′ = Ni, Co).Centro de QuÃmica Inorgánic
Raman and infrared spectroscopy of Sr2B′UO6 (B′ = Ni; Co) double perovskites
Temperature dependent normal modes and lattice thermal expansion of Sr 2B′UO6 (B′ = Ni, Co) double perovskites were investigated by Raman/infrared spectroscopies and synchrotron X-ray diffraction, respectively. Monoclinic crystal structures with space group P21/n were confirmed for both compounds, with no clear structural phase transition between 10 and 400 K. As predicted for this structure, the first-order Raman and infrared spectra show a plethora of active modes. In addition, the Raman spectra reveal an enhancement of the integrated area of an oxygen stretching mode, which is also observed in higher-order Raman modes, and an anomalous softening of ∼1 cm-1 upon cooling below T* ∼ 300 K. In contrast, the infrared spectra show conventional temperature dependence. The band profile phonon anomalies are possibly related to an unspecified electronic property of Sr2B′UO6 (B′ = Ni, Co).Centro de QuÃmica Inorgánic
High Temperature Emissivity, Reflectivity, and X-ray absorption of BiFeO3
We report on the lattice evolution of BiFeO3 as function of temperature using
far infrared emissivity, reflectivity, and X-ray absorption local structure. A
power law fit to the lowest frequency soft phonon in the magnetic ordered phase
yields an exponent {\beta}=0.25 as for a tricritical point. At about 200 K
below TN~640 K it ceases softening as consequence of BiFeO3 metastability. We
identified this temperature as corresponding to a crossover transition to an
order-disorder regime. Above ~700 K strong band overlapping, merging, and
smearing of modes are consequence of thermal fluctuations and chemical
disorder. Vibrational modes show band splits in the ferroelectric phase as
emerging from triple degenerated species as from a paraelectric cubic phase
above TC~1090 K. Temperature dependent X-ray absorption near edge structure
(XANES) at the Fe K-edge shows that lower temperature Fe3+ turns into Fe2+.
While this matches the FeO w\"ustite XANES profile, the Bi LIII-edge downshift
suggests a high temperature very complex bond configuration at the distorted A
perovskite site. Overall, our local structural measurements reveal high
temperature defect-induced irreversible lattice changes, below, and above the
ferroelectric transition, in an environment lacking of long-range coherence. We
did not find an insulator to metal transition prior to melting.Comment: Accepted for publicatio
TTF-DDQ: Two "Green'' Synthetic Routes, Crystal Structure and Band Gap from FT-IR Spectroscopy
International audience[No abstract