arXiv subjects

R. Valdés Aguilar

Publications and source records attributed to R. Valdés Aguilar.

23 records · Page 2Linked to original sources

arXiv2010

Observation of a coherence peak and pair-breaking effects in THz conductivity of BaFe$_{2-2x}$Co$_{2x}$As$_2$

We report a study of high quality pnictide superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ thin films using time-domain THz spectroscopy. Near T$_c$ we find evidence for a coherence peak and qualitative agreement with the weak-coupling Mattis-Bardeen form of the conductivity. At low temperature, we find that the real part of the THz conductivity is not fully suppressed and $σ_2$ is significantly smaller than the Matthis-Bardeen expectation. The temperature dependence of the penetration depth $λ$ follows a power law with an unusually high exponent of 3.1. We interpret these results as consistent with impurity scattering induced pair-breaking. Taken together our results are strong evidence for an extended s$\pm$ symmetry order parameter.

cond-mat.supr-con↗

arXiv2009

Origin of electromagnon excitations in \textit{R}MnO$_3$

The origin of electromagnon excitations in cycloidal \textit{R}MnO$_3$ is explained in terms of the Heisenberg coupling between spins despite the fact that the static polarization arises from the much weaker Dzyaloshinskii-Moriya (DM) exchange interaction. We present a model that incorporates structural characteristics of this family of manganites that is confirmed by far infrared transmission data as a function of temperature and magnetic field and inelastic neutron scattering results. A deep connection is found between the magnetoelectric dynamics of the spiral phase and the static magnetoelectric coupling in the collinear E-phase of this family of manganites.

cond-mat.str-el↗

arXiv2008

Spin phonon coupling in frustrated magnet CdCr$_2$O$_4$

The infrared phonon spectrum of the spinel CdCr2O4 is measured as a function temperature from 6 K to 300K. The triply degenerate Cr phonons soften in the paramagnetic phase as temperature is lowered below 100 K and then split into a singlet and doublet in the low T antiferromagnetic phase which is tetragonally distorted to relieve the geometric frustration in the pyrochlore lattice of Cr$^{3+}$ ions. The phonon splitting is inconsistent with the simple increase (decrease) in the force constants due to deceasing (increasing) bond lengths in the tetragonal phase. Rather they correspond to changes in the force constants due to the magnetic order in the antiferromagnetic state. The phonon splitting in this system is opposite of that observed earlier in ZnCr2O4 as predicted by theory. The magnitude of the splitting gives a measure of the spin phonon coupling strength which is smaller than in the case of ZnCr2O4.

cond-mat.mtrl-sci↗

arXiv2007

Colossal magnon-phonon coupling in multiferroic Eu$_{0.75}$Y$_{0.25}$MnO$_3$

We report the spectra of magnetically induced electric dipole absorption in Eu$_{0.75}$Y$_{0.25}$MnO$_3$ from temperature dependent far infrared spectroscopy (10-250 cm$^{-1}$). These spectra, which occur only in the $e||a$ polarization, consist of two relatively narrow electromagnon features that onset at $T_{FE}=30$ K and a broad absorption band that persists to temperatures well above $T_N=47$ K. The observed excitations account for the step up of the static dielectric constant in the ferroelectric phase. The electromagnon at 80 cm$^{-1}$ is observed to be strongly coupled to the nearby lowest optical phonon which transfers more than 1/2 of its spectral weight to the magnon. We attribute the origin of the broad background absorption to the two magnon emission decay process of the phonon.

cond-mat.mtrl-sci↗

arXiv2006

IR Phonon signatures of multiferroicity in TbMn$_{2}$O$_{5}$

The infrared (IR) active phonons in multiferroic TbMn$_2$O$_5$ are studied as a function of temperature. Most of the symmetry allowed IR modes polarized along the \textit{a} and \textit{b} crystal axes are observed and the behavior of several \textit{b} polarized phonons is correlated with the magnetic and ferroelectric transitions. A high frequency \textit{b} polarized phonon only Raman allowed in the paraelectric phase becomes IR active at the ferroelectric transition. The IR strength of this mode is proportional to the square of the ferroelectric order parameter and gives a sensitive measure of the symmetry lowering lattice distortions in the ferroelectric phase.

cond-mat.mtrl-sci↗