412 research outputs found
Raman signatures of charge ordering in K0.3WO3
We present polarization- and temperature-dependent Raman spectroscopic study of hexagonal tungsten bronze, K0.3WO3. The observed asymmetry in phonon line shapes indicate the presence of strong lattice anharmonicity arising due to the nonstoichiometry of the material. We observed a broad multipeak Raman feature at low frequency due to the local modes of K atoms known as local structural excitations. The observed vibrational features indicate a second-order phase transition around T=200 K accompanied by a frequency softening of low-frequency phonon modes. The observed phonon anomalies hint a physical picture involving a continuous symmetry change toward a charge-ordered state below 200 K. These observations indicate that K0.3WO3 may exhibit a weak charge-density-wave ground state at low temperatures.
Evidence for differentiation in the iron-helicoidal-chain in GdFe(BO)
We report on a single-crystal X-ray structure study of
at room temperature and at T=90 K. At room temperature
crystallizes in a trigonal space group R32 (No. 155), the same as found for
other members of iron-borate family . At 90 K the
structure of has transformed to the space group
(No. 152). The low-temperature structure determination gives new
insight into the weakly first-order structural phase transition at 156 K and
into the related Raman phonon anomalies. The discovery of two inequivalent iron
chains in the low temperature structure provide new point of view on the
low-temperature magnetic properties.Comment: Subm. to Acta Cryst.
A Raman study of the Charge-Density-Wave State in AMoO (A = K,Rb)
We report a comparative Raman spectroscopic study of the
quasi-one-dimensional charge-density-wave systems \ab (A = K, Rb). The
temperature and polarization dependent experiments reveal charge-coupled
vibrational Raman features. The strongly temperature-dependent collective
amplitudon mode in both materials differ by about 3 cm, thus revealing the role
of alkali atom. We discus the observed vibrational features in terms of
charge-density-wave ground state accompanied by change in the crystal symmetry.
A frequency-kink in some modes seen in \bb between T = 80 K and 100 K supports
the first-order lock-in transition, unlike \rb. The unusually sharp Raman
lines(limited by the instrumental response) at very low temperatures and their
temperature evolution suggests that the decay of the low energy phonons is
strongly influenced by the presence of the temperature dependent charge density
wave gap.Comment: 13 pages, 7 figure
Phonon and crystal field excitations in geometrically frustrated rare earth titanates
The phonon and crystal field excitations in several rare earth titanate
pyrochlores are investigated. Magnetic measurements on single crystals of
Gd2Ti2O7, Tb2Ti2O7, Dy2Ti2O7 and Ho2Ti2O7 are used for characterization, while
Raman spectroscopy and terahertz time domain spectroscopy are employed to probe
the excitations of the materials. The lattice excitations are found to be
analogous across the compounds over the whole temperature range investigated
(295-4 K). The resulting full phononic characterization of the R2Ti2O7
pyrochlore structure is then used to identify crystal field excitations
observed in the materials. Several crystal field excitations have been observed
in Tb2Ti2O7 in Raman spectroscopy for the first time, among which all of the
previously reported excitations. The presence of additional crystal field
excitations, however, suggests the presence of two inequivalent Tb3+ sites in
the low temperature structure. Furthermore, the crystal field level at
approximately 13 cm-1 is found to be both Raman and dipole active, indicating
broken inversion symmetry in the system and thus undermining its current
symmetry interpretation. In addition, evidence is found for a significant
crystal field-phonon coupling in Tb2Ti2O7. These findings call for a careful
reassessment of the low temperature structure of Tb2Ti2O7, which may serve to
improve its theoretical understanding.Comment: 13 pages, 7 figure
Femtosecond Covariance Spectroscopy
The success of non-linear optics relies largely on pulse-to-pulse
consistency. In contrast, covariance based techniques used in photoionization
electron spectroscopy and mass spectrometry have shown that wealth of
information can be extracted from noise that is lost when averaging multiple
measurements. Here, we apply covariance based detection to nonlinear optical
spectroscopy, and show that noise in a femtosecond laser is not necessarily a
liability to be mitigated, but can act as a unique and powerful asset. As a
proof of principle we apply this approach to the process of stimulated Raman
scattering in alpha-quartz. Our results demonstrate how nonlinear processes in
the sample can encode correlations between the spectral components of
ultrashort pulses with uncorrelated stochastic fluctuations. This in turn
provides richer information compared to the standard non-linear optics
techniques that are based on averages over many repetitions with well-behaved
laser pulses. These proof-of-principle results suggest that covariance based
nonlinear spectroscopy will improve the applicability of fs non-linear
spectroscopy in wavelength ranges where stable, transform limited pulses are
not available such as, for example, x-ray free electron lasers which naturally
have spectrally noisy pulses ideally suited for this approach
Magnetodielectric and magnetoelastic coupling in TbFe3(BO3)4
We have studied the magnetodielectric and magnetoelastic coupling in
TbFe3(BO3)4 single crystals by means of capacitance, magnetostriction and Raman
spectroscopy measurements. The data reveal strong magnetic field effects on the
dielectric constant and on the macroscopic sample length which are associated
to long range magnetic ordering and a field-driven metamagnetic transition. We
discuss the coupling of the dielectric, structural, and magnetic order
parameters and attribute the origin of the magnetodielectric coupling to phonon
mode shifts according to the Lyddane-Sachs-Teller (LST) relation.Comment: Accepted for publication in Physical Review
Raman scattering from phonons and magnons in RFe3)BO3)4
Inelastic light scattering spectra of several members of the RFe3(BO3)4
family reveal a cascade of phase transitions as a function of temperature,
starting with a structural, weakly first order, phase transition followed by
two magnetic phase transitions. Those consist of the ordering of the Fe-spin
sublattice revealed by all the compound, and a subsequent spin-reorientational
transition for GdFe3(BO3)4. The Raman data evidence a strong coupling between
the lattice and magnetic degrees of freedom in these borates. The Fe-sublattice
ordering leads to a strong suppression of the low energy magnetic scattering,
and a multiple peaked two-magnon scattering continuum is observed. Evidence for
short-range correlations is found in the `paramagnetic' phase by the
observation of a broad magnetic continuum in the Raman data, which persists up
to surprisingly high temperatures.Comment: 17 pages, 13 figure
Ultrafast changes in lattice symmetry probed by coherent phonons
The electronic and structural properties of a material are strongly
determined by its symmetry. Changing the symmetry via a photoinduced phase
transition offers new ways to manipulate material properties on ultrafast
timescales. However, in order to identify when and how fast these phase
transitions occur, methods that can probe the symmetry change in the time
domain are required. We show that a time-dependent change in the coherent
phonon spectrum can probe a change in symmetry of the lattice potential, thus
providing an all-optical probe of structural transitions. We examine the
photoinduced structural phase transition in VO2 and show that, above the phase
transition threshold, photoexcitation completely changes the lattice potential
on an ultrafast timescale. The loss of the equilibrium-phase phonon modes
occurs promptly, indicating a non-thermal pathway for the photoinduced phase
transition, where a strong perturbation to the lattice potential changes its
symmetry before ionic rearrangement has occurred.Comment: 14 pages 4 figure
Manipulation of Charge Delocalization in a Bulk Heterojunction Material Using a Mid-Infrared Push Pulse
In organic bulk heterojunction materials, charge delocalization has been
proposed to play a vital role in the generation of free carriers by reducing
the Coulomb attraction via an interfacial charge transfer exciton (CTX).
Pump-push-probe (PPP) experiments produced evidence that the excess energy
given by a push pulse enhances delocalization, thereby increasing photocurrent.
However, previous studies have employed near-IR push pulses in the range
0.4-0.6 eV which is larger than the binding energy of a typical CTX. This
raises the doubt that the push pulse may directly promote dissociation without
involving delocalized states. Here, we perform PPP experiments with mid-IR push
pulses at energies that are well below the binding energy of a CTX state
(0.12-0.25 eV). We identify three types of CTX: delocalized, localized, and
trapped. The excitation resides over multiple polymer chains in delocalized
CTXs, while is restricted to a single chain (albeit maintaining a degree of
intrachain delocalization) in localized CTXs. Trapped CTXs are instead
completely localized. The pump pulse generates a hot delocalized CTX, which
relaxes to a localized CTX, and eventually to trapped states. We find that
photo-exciting localized CTXs with push pulses resonant to the mid-IR charge
transfer absorption can promote delocalization and contribute to the formation
of long-lived charge separated states. On the other hand, we found that trapped
CTX are non-responsive to the push pulses. We hypothesize that delocalized
states identified in prior studies are only accessible in systems where there
is significant interchain electronic coupling or regioregularity that supports
either interchain or intrachain polaron delocalization. This emphasizes the
importance of engineering the micromorphology and energetics of the
donor-acceptor interface to exploit a full potential of a material for
photovoltaic applications
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