46 research outputs found
Critical slowing down near the multiferroic phase transition in MnWO
By using broadband dielectric spectroscopy in the radiofrequency and
microwave range we studied the magnetoelectric dynamics in the multiferroic
chiral antiferromagnet MnWO. Above the multiferroic phase transition at
K we observe a critical slowing down of the corresponding
magnetoelectric fluctuations resembling the soft-mode behavior in canonical
ferroelectrics. This electric field driven excitation carries much less
spectral weight than ordinary phonon modes. Also the critical slowing down of
this mode scales with an exponent larger than one which is expected for
magnetic second order phase transition scenarios. Therefore the investigated
dynamics have to be interpreted as the softening of an electrically active
magnetic excitation, an electromagnon.Comment: 5 pages, 4 figures, appendi
The biaxial nonlinear crystal BiB3O6 as a polarization entangled photon source using non-collinear type-II parametric down-conversion
We describe the full characterization of the biaxial nonlinear crystal BiB3O6
(BiBO) as a polarization entangled photon source using non-collinear type-II
parametric down-conversion. We consider the relevant parameters for crystal
design, such as cutting angles, polarization of the photons, effective
nonlinearity, spatial and temporal walk-offs, crystal thickness and the effect
of the pump laser bandwidth. Experimental results showing entanglement
generation with high rates and a comparison to the well investigated
beta-BaB2O4 (BBO) crystal are presented as well. Changing the down-conversion
crystal of a polarization entangled photon source from BBO to BiBO enhances the
generation rate as if the pump power was increased by more than three times.
Such an improvement is currently required for the generation of multiphoton
entangled states.Comment: 15 pages, 13 figures, published versio
The spin- XXZ chain system CsCoCl in a transverse magnetic field
Comparing high-resolution specific heat and thermal expansion measurements to
exact finite-size diagonalization, we demonstrate that CsCoCl for a
magnetic field along the crystallographic b axis realizes the
spin- XXZ chain in a transverse field. Exploiting both thermal as
well as virtual excitations of higher crystal field states, we find that the
spin chain is in the XY-limit with an anisotropy
substantially smaller than previously believed. A spin-flop Ising quantum phase
transition occurs at a critical field of T
before around 3.5 T the description in terms of an effective spin-
chain becomes inapplicable.Comment: 5 pages, 3 figure
Second harmonic generation on incommensurate structures: The case of multiferroic MnWO4
A comprehensive analysis of optical second harmonic generation (SHG) on an
incommensurate (IC) magnetically ordered state is presented using multiferroic
MnWO4 as model compound. Two fundamentally different SHG contributions coupling
to the primary IC magnetic order or to secondary commensurate projections of
the IC state, respectively, are distinguished. Whereas the latter can be
described within the formalism of the 122 commensurate magnetic point groups
the former involves a breakdown of the conventional macroscopic symmetry
analysis because of its sensitivity to the lower symmetry of the local
environment in a crystal lattice. Our analysis thus foreshadows the fusion of
the hitherto disjunct fields of nonlinear optics and IC order in
condensed-matter systems
Spin-orbit coupling in a half-filled shell: the case of KReCl
The half-filled shell of the configuration usually, in LS
coupling, hosts a S = 3/2 ground state with quenched orbital moment. This state
is not Jahn-Teller active. Sufficiently large spin-orbit coupling has
been predicted to change this picture by mixing in orbital moment, giving rise
to a sizable Jahn-Teller distortion. In KReCl we study the
electronic excitations using resonant inelastic x-ray scattering (RIXS) and
optical spectroscopy. We observe on-site intra- excitations below 2 eV
and corresponding overtones with two intra- excitations on adjacent
sites, the Mott gap at 2.7 eV, -to- excitations above 3 eV, and
charge-transfer excitations at still higher energy. The intra-
excitation energies are a sensitive measure of and Hund's coupling
. The sizable value of 0.29 eV places KReCl into
the intermediate coupling regime, but is not
sufficiently large to drive a pronounced Jahn-Teller effect. We discuss the
ground state wavefunction in a Kanamori picture and find that the S = 3/2
multiplet still carries about 97 % of the weight. However, the finite admixture
of orbital moment allows for subtle effects. We discuss small
temperature-induced changes of the optical data and find evidence for a
lowering of the ground state by about 3 meV below the structural phase
transitions.Comment: 16 pages, 14 figure
Recommended from our members
Resonant inelastic x-ray incarnation of Youngâs double-slit experiment
Youngâs archetypal double-slit experiment forms the basis for modern diffraction techniques: The elastic scattering of waves yields an interference pattern that captures the real-space structure. Here, we report on an inelastic incarnation of Youngâs experiment and demonstrate that resonant inelastic x-ray scattering (RIXS) measures interference patterns, which reveal the symmetry and character of electronic excited states in the same way as elastic scattering does for the ground state. A prototypical example is provided by the quasi-molecular electronic structure of insulating Ba 3 CeIr 2 O 9 with structural Ir dimers and strong spin-orbit coupling. The double âslitsâ in this resonant experiment are the highly localized core levels of the two Ir atoms within a dimer. The clear double-slit-type sinusoidal interference patterns that we observe allow us to characterize the electronic excitations, demonstrating the power of RIXS interferometry to unravel the electronic structure of solids containing, e.g., dimers, trimers, ladders, or other superstructures
Stimulated Raman Scattering in MeliliteâType Crystals Ca2MgSi2O7 and Ca2Ga2SiO7
Ï(3)ânonlinear optical interactions in two meliliteâtype stimulated Raman scattering (SRS)âactive nonâcentrosymmetric crystals, Ca2MgSi2O7 and Ca2Ga2SiO7, formerly known as Nd3+âlaser media, are presented. Under picosecond pumping at 1.064 and 0.532 ”m cascaded and crossâcascaded effects occur in these tetragonal silicates. Besides the SRSâpromoting phonon modes with energy of ÏSRS1 â 908 cmâ1 and ÏSRS2 â 668 cmâ1 for Ca2MgSi2O7, and ÏSRS1 â 720 cmâ1 and ÏSRS2 â 550 cmâ1 for Ca2Ga2SiO7, respectively, combined phonon modes are observed. For Ca2MgSi2O7 new data in a broad wavelength range of refractive indices and their dispersion are given as well. The observed Ï(3)ânonlinear properties expand the functionality of the studied silicates and foreshadow their use in selfâfrequency Raman laser converters (selfâSRS lasers)