33 research outputs found
Infrared study of the phonon modes in PrMnO and CaMnO
The infrared (IR) reflectivity spectra of orthorhombic manganese perovskites
PrMnO and CaMnO are studied in the frequency range of optical phonon
modes at temperatures varying from 300 to 4 K. The IR phonon spectra of these
two materials are analyzed by a fitting procedure based on a Lorentz model, and
assigned to definite vibrational modes of structures by comparison with
the results of lattice dynamical calculations. The calculations have been
performed in the framework of a shell model using short range
Born-Mayer-Buckingham and long range Coulomb potentials, whose parameters have
been optimized in order that the calculated Raman and IR active phonon
frequencies, and lattice parameters match with their experimental values. We
find a close correspondence between the values of the IR phonon frequencies of
PrMnO and CaMnO, which shows that the substitution of the Pr
ions with Ca results in a reduction of the frequency of medium- and
high-energy IR phonons, and an increase of the frequency of those of
low-energy. Nevertheless, the experimentally obtained IR phonon amplitudes of
the two materials appear to be unrelated. A comparative study of the
vibrational patterns of these modes reveals that most of them correspond to
complex atomic vibrations significantly different from PrMnO to CaMnO
which cannot be assigned only to a given type of vibration (external, bending,
or stretching modes). In particular, these results confirm that the structure
of CaMnO is quite far from the ideal (cubic) perovskite structure.Comment: 10 pages, 6 figure
Observation of charge-density-wave excitations in manganites
In the optical conductivity of four different manganites with commensurate
charge order (CO), strong peaks appear in the meV range below the ordering
temperature T_{CO}. They are similar to those reported for one-dimensional
charge density waves (CDW) and are assigned to pinned phasons. The peaks and
their overtones allow one to obtain, for La{1-n/8}Ca{n/8}$MnO{3} with n = 5, 6,
the electron-phonon coupling, the effective mass of the CO system, and its
contribution to the dielectric constant. These results support a description of
the CO in La-Ca manganites in terms of moderately weak-coupling and of the CDW
theory.Comment: To be published on Phys. Rev. Let
Pressure dependence of the single particle excitation in the charge-density-wave CeTe system
We present new data on the pressure dependence at 300 K of the optical
reflectivity of CeTe, which undergoes a charge-density-wave (CDW) phase
transition well above room temperature. The collected data cover an
unprecedented broad spectral range from the infrared up to the ultraviolet,
which allows a robust determination of the gap as well as of the fraction of
the Fermi surface affected by the formation of the CDW condensate. Upon
compressing the lattice there is a progressive closing of the gap inducing a
transfer of spectral weight from the gap feature into the Drude component. At
frequencies above the CDW gap we also identify a power-law behavior, consistent
with findings along the Te series (i.e., chemical pressure) and
suggestive of a Tomonaga-Luttinger liquid scenario at high energy scales. This
newest set of data is placed in the context of our previous investigations of
this class of materials and allows us to revisit important concepts for the
physics of CDW state in layered-like two-dimensional systems
Electrodynamic properties of an artificial heterostructured superconducting cuprate
We perform infrared conductivity measurements on a series of CaCuO2/SrTiO3 heterostructures made by the insulating cuprate CaCuO2 (CCO) and the insulating perovkite SrTiO3 (STO). We estimate the carrier density of various heterostructures, with different level of hole doping from the integral of the optical conductivity and we measure the corresponding degree of correlation by estimating the ratio between the Drude weight and the integral of the infrared spectrum. The analysis demonstrates a large degree of correlation, which increases as the doping is reduced. The experimental results can be reproduced by Dynamical Mean-Field Theory calculations which strongly support the role of correlations in the CCO/STO heterostructures and their similarities with the most common cuprate superconductors. Our results suggest that cuprate superconductors can be looked at as natural superlattices, where the properties of the CuO2 conducting planes and charge reservoir blocks can be completely disentangled
Pressure and alloying effects on the metal to insulator transition in NiS{2-x}Se{x} studied by infrared spectroscopy
The metal to insulator transition in the charge transfer NiS{2-x}Se{x}
compound has been investigated through infrared reflectivity. Measurements
performed by applying pressure to pure NiS2 (lattice contraction) and by
Se-alloying (lattice expansion) reveal that in both cases an anomalous metallic
state is obtained. We find that optical results are not compatible with the
linear Se-alloying vs Pressure scaling relation previously established through
transport, thus pointing out the substantially different microscopic origin of
the two transitions.Comment: Accepted for publication in Phys. Rev.
Subterahertz electrodynamics of the graphenelike superconductor CaAlSi
We report an optical study of CaAlSi, a superconductor which displays both the crystal structure of MgB(2) and the electronic band structure of intercalated graphites. The reflectivity of a CaAlSi single crystal was measured down to subterahertz frequencies and to 3.3 K, with the use of coherent synchrotron radiation. A single superconducting gap in the hexagonal planes and two gaps along the c axis were found and measured consistently with the structure of the CaAlSi Fermi surface. The normal-state optical conductivity is also anisotropic: in the ab plane, the plasma frequency is larger by more than a factor of 2 than along the c axis. An analysis of the ab-plane spectral weight in comparison with the corresponding quantity in a cuprate such as La(2-x)Sr(x)CuO(4) shows that in CaAlSi the correlation effects are negligible
Identification of spin wave resonances and crystal field levels in simple chromites RCrO<sub>3</sub> (R = Pr, Sm, Er) at low temperatures in the THz spectral region
We report on THz absorption spectroscopy combined with high magnetic fields of polycrystalline RCrO3 (R = Pr, Sm, Er) aiming understanding spin wave resonances at their low temperature magnetic phases. Our measurements show that the temperature, and the implicit anisotropies at which the Cr3+ spin reorientation at TSR takes place, are determinant on the ferromagnetic-like (FM) and the antiferromagnetic-like (AFM) spin modes being optically active. It is found that they are dependent on Rare Earth 4f moment and ion size. We also studied temperature and field dependence of crystal field levels in the same spectroscopic region. Pr3+ non-Kramers emerges at 100 K and Zeeman splits. An observed absence of spin wave resonances in PrCrO3 is attributed to Pr3+ remaining paramagnetic. In SmCrO3 near cancelation of the spin and orbital moments is proposed as the possible reason for not detecting Sm3+ ground state transitions. Here, the FM and AFM resonant modes harden when the temperature decreases and split linearly under applied fields at 5 K and below. In ErCrO3 the Er3+ Kramers doublet becomes active at about the TSR onset. Each line further experiences Zeeman splitting under magnetic fields while an spin reversal induced by a ∼2.5 T field, back to the Γ4 (Fz) from the Γ1 phase at 2 K, produces a secondary splitting. The 5 K AFM and FM excitations in ErCrO3 have a concerted frequency-intensity temperature dependence and a shoulder pointing to the Er3+ smaller ion size also disrupting the two magnetic sublattice approximation. Both resonances reduce to one when the temperature is lowered to 2 K in the Γ1 representation. Our findings have important implications on the complex interplay in the magneto-electrodynamics associated with the Rare-Earth 4f – 3d transition metal spin coupling and the structural A site instabilities in perovskite multiferroics.Centro de Química Inorgánic