44 research outputs found
Magnetic transitions in CaMn7O12 : a Raman observation of spin-phonon couplings
The quadruple Calcium manganite (CaMn7O12) is a multiferroic material that
exhibits a giant magnetically-induced ferroelectric polarization which makes it
very interesting for magnetoelectric applications. Here, we report the Raman
spectroscopy study on this compound of both the phonon modes and the low energy
excitations from 4 K to room temperature. A detailed study of the Raman active
phonon excitations shows that three phonon modes evidence a spin-phonon
coupling at TN2 = 50 K. In particular, we show that the mode at 432 cm-1
associated to Mn(B)O6 (B position of the perovskite) rotations around the [111]
cubic diagonal is impacted by the magnetic transition at 50 K and its coupling
to the new modulation of the Mn spin in the (a,b) plane. At low energies, two
large low energy excitations are observed at 25 and 47 cm-1. The first one
disappears at 50 K and the second one at 90 K. We have associated these
excitations to electro-magneto-active modes
Electromagnon and phonon excitations in multiferroic TbMnO3
We have performed Raman measurements on TbMnO3 single crystal under magnetic
field along the three crystallographic directions. The flip of the spin spiral
plane creates an electromagnon excitation. In addition to the electromagnons
induced by the Heisenberg coupling, we have detected the electromagnon created
by the Dzyaloshinskii-Moriya interaction along the c axis. We have identified
all the vibrational modes of TbMnO3. Their temperature dependences show that
only one phonon observed along the polarization axis is sensitive to the
ferroelectric transition. This mode is tied to the Tb3+ ion displacements that
contribute to the ferroelectric polarization
Doping dependence of the lattice dynamics in Ba(FeCo)As studied by Raman spectroscopy
We report Raman scattering spectra of iron-pnictide superconductor
Ba(FeCo)As single crystals with varying cobalt content.
Upon cooling through the tetragonal-to-orthorhombic transition, we observe a
large splitting of the E in-plane phonon modes involving Fe and As
displacements. The splitting of the in-plane phonons at the transition is
strongly reduced upon doping and disappears for qualitatively
following the trend displayed by the Fe magnetic moment. The origin of the
splitting is discussed in terms of magnetic frustration inherent to
iron-pnictide systems and we argue that such enhanced splitting may be linked
to strong spin-phonon coupling.Comment: 6 pages, 6 figure
Magneto-electric excitations in multiferroic TbMnO3 by Raman scattering
Low energy excitations in the multiferroic material TbMnO3 have been
investigated by Raman spectroscopy. Our observations reveal the existence of
two peaks at 30 cm-1 and 60 cm-1. They are observed in the cycloidal phase
below the Curie temperature but not in the sinusoidal phase, suggesting their
magnetoelectric origin. While the peak energies coincide with the frequencies
of electromagnons measured previously by transmission spectroscopy, they show
surprisingly different selection rules, with the 30 cm-1 excitation enhanced by
the electric field of light along the spontaneous polarization. The origins of
the modes are discussed under Raman and infrared selection rules
considerations
Three energy scales in the superconducting state of hole-doped cuprates detected by electronic Raman scattering
We explored by electronic Raman scattering the superconducting state of
Bi-2212 single crystal by performing a fine tuned doping study. We found three
distinct energy scales in A1g, B1g and B2g symmetries which show three distinct
doping dependencies. Above p=0.22 the three energies merge, below p=0.12, the
A1g scale is no more detectable while the B1g and B2g scales become constant in
energy. In between, the A1g and B1g scales increase monotonically with
under-doping while the B2g one exhibits a maximum at p=0.16. The three
superconducting energy scales appear to be an universal feature of hole-doped
cuprates. We propose that the non trivial doping dependence of the three scales
originates from Fermi surface topology changes and reveals competing orders
inside the superconducting dome.Comment: 6 pages, 5 figure