35 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
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
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Resonant inelastic x-ray incarnation of Young’s double-slit experiment
Young’s 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 Young’s 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 “slits” 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 Melilite‐Type Crystals Ca2MgSi2O7 and Ca2Ga2SiO7
χ(3)‐nonlinear optical interactions in two melilite‐type stimulated Raman scattering (SRS)‐active non‐centrosymmetric crystals, Ca2MgSi2O7 and Ca2Ga2SiO7, formerly known as Nd3+‐laser media, are presented. Under picosecond pumping at 1.064 and 0.532 µm cascaded and cross‐cascaded effects occur in these tetragonal silicates. Besides the SRS‐promoting phonon modes with energy of ωSRS1 ≈ 908 cm−1 and ωSRS2 ≈ 668 cm−1 for Ca2MgSi2O7, and ωSRS1 ≈ 720 cm−1 and ωSRS2 ≈ 550 cm−1 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 χ(3)‐nonlinear properties expand the functionality of the studied silicates and foreshadow their use in self‐frequency Raman laser converters (self‐SRS lasers)
Whispering gallery resonator from lithium tetraborate for nonlinear optics
For second-order nonlinear-optical processes in the ultraviolet, appropriate materials with a sufficiently large band-gap typically exhibit smaller nonlinear coefficients than materials with comparably smaller band-gap. Whispering gallery resonators, with their outstanding quality factors, provide field enhancement and can compensate for these small coefficients. We report on the successful fabrication of a whispering gallery resonator made of lithium tetraborate, a suitable material for ultraviolet applications with a small nonlinear coefficient of d31 = 0.073 pm/V. Quality factors of the order of 108 are observed from the ultraviolet to the near-infrared spectrum. The inferred absorption coefficients of lithium tetraborate are below 0.2 m-1 in the visible and near-infrared. Continuous-wave second harmonic generation from 490 nm light to 245 nm is observed with conversion efficiencies up to 2.2 %
Infrared-active phonon modes in monoclinic multiferroic MnWO
We report on polarized infrared reflectivity measurements of multiferroic,
monoclinic MnWO4 between 10K and 295K. The full dielectric tensor and the
frequency dependence of the orientation of the principal axes have been
determined in the frequency range of the phonons. All infrared-active phonon
modes (7 A_u modes and 8 B_u modes) are unambiguously identified. In particular
the strongest B_u modes have been overlooked in previous studies, in which the
monoclinic symmetry was neglected in the analysis. Using a generalized
Drude-Lorentz model, we determine the temperature dependence of the phonon
parameters, including the orientation of the B_u modes within the ac plane. The
phonon parameters and their temperature dependence have been discussed
controversially in previous studies, which do not include a full polarization
analysis. Our data does not confirm any of the anomalies reported above 20K.
However, in the paramagnetic phase we find a drastic reduction of the spectral
weights of the weakest A_u mode and of the weakest B_u mode with increasing
temperature. Below 20K, the parameters of the A_u phonon modes for E||b show
only subtle changes, which demonstrate a finite but weak coupling between
lattice dynamics and magnetism in MnWO4. A quantitative comparison of our
infrared data with the quasi-static dielectric constant epsilon_b indicates
that the spectral weight of the electromagnon is tiny for E||b, in particular
much smaller than the weak changes of the spectral weight of the phonons.Comment: published version, minor changes, 1 figure revised, 16 pages, 15
figure
Humidification Technique Using New Modified MiniModule Membrane Contactors for Air Cooling
An experimental study is conducted to cool the ambient air using a new humidification technique. A wind tunnel is built with a test section formed by four modified MiniModule membrane contactors. An ambient air passes over the membrane contactors (cross flow) while water pumps through the contactors. Air temperature and relative humidity are measured upstream and downstream of the membrane contactors array which was used to humidify and cool the outdoor air. Five average air velocities (3.03, 3.33, 3.95, 4.52, and 5.04 m/s) and four water flow rates (0.0, 0.013, 0.019, and 0.025 kg/s) are used. Air velocity is measured at different locations along the centerline of the cross section. Using the modified MiniModule membrane contactors array dropped the air temperature by a maximum and minimum of 10.77°C and 3.44°C, respectively, depending on the outdoor air. The corresponding maximum increase of the relative humidity is 4.65% which depends on the ambient condition. It is noticed that the evaporation process does not follow the isenthalpic lines therefore; heat transfers from the air as latent and sensible heats
Thermal and electromechanical properties of melilite-type piezoelectric single crystals
Melilite-type crystals, including Ca2Ga2SiO7, Ca2MgSi2O7, and CaNdGa3O7, were successfully grown by the Czochralski technique. Thermal properties were investigated and full matrices of electromechanical constants of these melilite crystals were evaluated by impedance method, with d14 being on the order of 5.3-9.3 pC/N. The relationship between microstructure and electromechanical properties of the three kinds of crystals was established, in order to explore the piezoelectric origin and further optimize the piezoelectric properties. The distortion of eight-fold antiprisms and the distortion/rotation of tetrahedrons were thought to contribute to the piezoelectric d14 and d36, respectively. In addition, the layered structure leads to strong anisotropic behavior, accounts for the high resistivity along Z direction in Ca2MgSi2O7, while the disordered structure accounts for the degraded resistivity in CaNdGa3O7. The properties at elevated temperature were investigated, where the elastic constants show high thermal stability over the range of 25-500 C, with minimal variation of 6%