21 research outputs found
Structural and magnetic dynamics of a laser induced phase transition in FeRh
We use time-resolved x-ray diffraction and magnetic optical Kerr effect to
study the laser induced antiferromagnetic to ferromagnetic phase transition in
FeRh. The structural response is given by the nucleation of independent
ferromagnetic domains (\tau_1 ~ 30ps). This is significantly faster than the
magnetic response (\tau_2 ~ 60ps) given by the subsequent domain realignment.
X-ray diffraction shows that the two phases co-exist on short time-scales and
that the phase transition is limited by the speed of sound. A nucleation model
describing both the structural and magnetic dynamics is presented.Comment: 5 pages, 3 figures - changed to reflect version accepted for PR
Identification of coherent lattice modulations coupled to charge and orbital order in a manganite
We apply grazing-incidence femtosecond x-ray diffraction to investigate the
details of the atomic motion connected with a displacively excited coherent
optical phonon. We concentrate on the low frequency phonon associated with the
charge and orbital order in the mixed valence manganite
La0.25Pr0.375Ca0.375MnO3 for T < 210 K. We measure the response of three
superlattice reflections that feature different sensitivities to the motion of
the unit cell constituents. The results support the assignment to a
translational mode of the Mn4+ atoms together with the oxygen atoms connecting
adjacent Mn4+ sites.Comment: 13 pages, 3 figure
Restoring interlayer Josephson coupling in La1.885Ba0.115CuO4 by charge transfer melting of stripe order
We show that disruption of charge-density-wave (stripe) order by charge transfer excitation, enhances the superconducting phase rigidity in La1.885Ba0.115CuO4. Time-resolved resonant soft x-ray diffraction demonstrates that charge order melting is prompt following near-infrared photoexcitation whereas the crystal structure remains intact for moderate fluences. THz time-domain spectroscopy reveals that, for the first 2 ps following photoexcitation, a new Josephson plasma resonance edge, at higher frequency with respect to the equilibrium edge, is induced indicating enhanced superconducting interlayer coupling. The fluence dependence of the charge-order melting and the enhanced superconducting interlayer coupling are correlated with a saturation limit of âŒ0.5mJ/cm2. Using a combination of x-ray and optical spectroscopies we establish a hierarchy of timescales between enhanced superconductivity, melting of charge order, and rearrangement of the crystal structure
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
Femtosecond dynamics of the collinear-to-spiral antiferromagnetic phase transition in CuO
We report on the ultrafast dynamics of magnetic order in a single crystal of
CuO at a temperature of 207 K in response to strong optical excitation using
femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser
pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a
short delay ranging from 400 fs to 2 ps, we observe changes in the relative
intensity of the magnetic ordering diffraction peaks that indicate a shift from
a collinear commensurate phase to a spiral incommensurate phase. These results
indicate that the ultimate speed for this antiferromagnetic re-orientation
transition in CuO is limited by the long-wavelength magnetic excitation
connecting the two phases.Comment: Accepted by Physical Review Letters (Dec. 2, 2011
Probing the Interplay between Quantum Charge Fluctuations and Magnetic Ordering in LuFe2O4
Ferroelectric and ferromagnetic materials possess spontaneous electric and
magnetic order, respectively, which can be switched by the corresponding
applied electric and magnetic fields. Multiferroics combine these properties in
a single material, providing an avenue for controlling electric polarization
with a magnetic field and magnetism with an electric field. These materials
have been intensively studied in recent years, both for their fundamental
scientific interest as well as their potential applications in a broad range of
magnetoelectric devices [1, 2, 3, 4]. However, the microscopic origins of
magnetism and ferroelectricity are quite different, and the mechanisms
producing strong coupling between them are not always well understood. Hence,
gaining a deeper understanding of magnetoelectric coupling in these materials
is the key to their rational design. Here, we use ultrafast optical
spectroscopy to show that quantum charge fluctuations can govern the interplay
between electric polarization and magnetic ordering in the charge-ordered
multiferroic LuFe2O4