773 research outputs found
Mapping the B,T phase diagram of frustrated metamagnet CuFeO2
The magnetic phase diagram of CuFeO2 as a function of applied magnetic field
and temperature is thoroughly explored and expanded, both for magnetic fields
applied parallel and perpendicular to the material's c-axis. Pulsed field
magnetization measurements extend the typical magnetic staircase of CuFeO2 at
various temperatures, demonstrating the persistence of the recently discovered
high field metamagnetic transition up to Tn2 ~ 11 K in both field
configurations. An extension of the previously introduced phenomenological spin
model used to describe the high field magnetization process (Phys. Rev. B, 80,
012406 (2009)) is applied to each of the consecutive low-field commensurate
spin structures, yielding a semi-quantitative simulation and intuitive
description of the entire experimental magnetization process in both relevant
field directions with a single set of parameters.Comment: 14 pages, 11 figures, submitted to Phys. Rev.
Photo-induced magnetization enhancement in two-dimensional weakly anisotropic Heisenberg magnets
By comparing the photo-induced magnetization dynamics in simple layered
systems we show how light-induced modifications of the magnetic anisotropy
directly enhance the magnetization. It is observed that the spin precession in
(CH3NH3)2CuCl4, initiated by a light pulse, increases in amplitude at the
critical temperature TC. The phenomenon is related to the dependence of the
critical temperature on the axial magnetic anisotropy. The present results
underline the possibility and the importance of the optical modifications of
the anisotropy, opening new paths toward the control of the magnetization state
for ultrafast memories.Comment: 5 pages, 3 figures, supplementary info as SIr.pd
High-field recovery of the undistorted triangular lattice in the frustrated metamagnet CuFeO2
Pulsed field magnetization experiments extend the typical metamagnetic
staircase of CuFeO2 up to 58 T to reveal an additional first order phase
transition at high field for both the parallel and perpendicular field
configuration. Virtually complete isotropic behavior is retrieved only above
this transition, indicating the high-field recovery of the undistorted
triangular lattice. A consistent phenomenological rationalization for the field
dependence and metamagnetism crossover of the system is provided, demonstrating
the importance of both spin-phonon coupling and a small field-dependent
easy-axis anisotropy in accurately describing the magnetization process of
CuFeO2.Comment: 4 pages, 4 figure
Coherent amplitudon generation in K_0.3MoO_3 through ultrafast inter-band quasi particle decay
The charge density wave system K_0.3MoO_3 has been studied using variable
energy pump-probe spectroscopy, ellipsometry, and inelastic light scattering.
The observed transient reflectivity response exhibits quite a complex behavior,
containing contributions due to quasi particle excitations, coherent
amplitudons and phonons, and heating effects. The generation of coherent
amplitudons is discussed in terms of relaxation of photo-excited quasi
particles, and is found to be resonant with the interband plasmon frequency.
Two additional coherent excitations observed in the transients are assigned to
zone-folding modes of the charge density wave state
Magneto-optical readout of dark exciton distribution in cuprous oxide
An experimental study of the yellow exciton series in Cu2O in strong magnetic
fields up to 32 T shows the optical activation of direct and phonon-assisted
paraexciton luminescence due to mixing with the quadruple allowed orthoexciton
state. The observed phonon-assisted luminescence yields information on the
statistical distribution of occupied states. Additional time-resolved
experiments provide a unique opportunity to directly determine the time
evolution of the thermodynamical properties of the paraexciton gas. Because the
lifetime of paraexciton is hardly affected by the optical activation in a
strong magnetic field, this opens new possibilities for studies aiming at
Bose-Einstein condensation of excitons in bulk semiconductors.Comment: 14 pages, 6 figure
Electron-phonon and spin-phonon coupling in NaVO: charge fluctuations effect
We show that the asymmetric crystal environment of the V site in the ladder
compound NaVO leads to a strong coupling of vanadium 3d electrons
to phonons. This coupling causes fluctuations of the charge on the V ions, and
favors a transition to a charge-ordered state at low temperatures. In the low
temperature phase the charge fluctuations modulate the spin-spin superexchange
interaction, resulting in a strong spin-phonon coupling.Comment: Europhysics Letters, to be publishe
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
Compact cryogenic Kerr microscope for time-resolved studies of electron spin transport in microstructures
A compact cryogenic Kerr microscope for operation in the small volume of
high-field magnets is described. It is suited for measurements both in Voigt
and Faraday configuration. Coupled with a pulsed laser source, the microscope
is used to measure the time-resolved Kerr rotation response of semiconductor
microstructures with ~1 micron spatial resolution. The microscope was designed
to study spin transport, a critical issue in the field of spintronics. It is
thus possible to generate spin polarization at a given location on a
microstructure and probe it at a different location. The operation of the
microscope is demonstrated by time-resolved measurements of micrometer distance
diffusion of spin polarized electrons in a GaAs/AlGaAs heterojunction quantum
well at 4.2 K and 7 Tesla
Impact of tortuous flow on bacteria streamer development in microfluidic system during filtration
The way in which bacterial communities colonize flow in porous media is of importance but basic knowledge on the dynamic of these phenomena is still missing. The aim of this work is to develop microfluidic experiments in order to progress in the understanding of bacteria capture in filters and membranes. PDMS microfluidic devices mimicking filtration processes have been developed to allow a direct dynamic observation of bacteria across 10 or 20 micrometers width microchannels. When filtered in such devices, bacteria behave surprisingly: Escherichia coli, Pseudomonas aeruginosa or Staphylococcus aureus accumulate in the downstream zone of the filter and form large streamers which oscillate in the flow. In this study streamer formation is put in evidence for bacteria suspension in non nutritive conditions in less than one hour. This result is totally different from the one observed in same system with “inert” particles or dead bacteria which are captured in the bottleneck zone and are accumulated in the upstream zone. Observations within different flow geometries (straight channels, connected channels, staggered row pillars) show that the bacteria streamer development is influenced by the flow configuration and, particularly by the presence of tortuosity within the microchannels zone. These results are discussed at the light of 3D flow simulations. In confined systems and in laminar flow there is secondary flow (z-velocities) superimposed to the streamwise motion (in xy plane). The presence of the secondary flow in the microsystems has an effect on the bacterial adhesion. A scenario in three steps is established to describe the formation of the streamers and to explain the positive effect of tortuous flow on the development kinetics
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