37 research outputs found
Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5
THz-frequency optical pulses can resonantly drive selected vibrational modes
in solids and deform their crystal structure. In complex oxides, this method
has been used to melt electronic orders, drive insulator to metal transitions
or induce superconductivity. Strikingly, coherent interlayer transport strongly
reminiscent of superconductivity can be transiently induced up to room
temperature in YBa2Cu3O6+x. By combining femtosecond X-ray diffraction and ab
initio density functional theory calculations, we determine here the crystal
structure of this exotic non-equilibrium state. We find that nonlinear lattice
excitation in normal-state YBa2Cu3O6+x at 100 K causes a staggered
dilation/contraction of the Cu-O2 intra/inter- bilayer distances, accompanied
by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional
theory calculations indicate that these motions cause dramatic changes in the
electronic structure. Amongst these, the enhancement in the dx2-y2 character of
the in-plane electronic structure is likely to favor superconductivity.Comment: 28 pages, including Supplemen
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
Watching the birth of a charge density wave order: diffraction study on nanometer-and picosecond-scales
Femtosecond time-resolved X-ray diffraction is used to study a photo-induced
phase transition between two charge density wave (CDW) states in 1T-TaS,
namely the nearly commensurate (NC) and the incommensurate (I) CDW states.
Structural modulations associated with the NC-CDW order are found to disappear
within 400 fs. The photo-induced I-CDW phase then develops through a
nucleation/growth process which ends 100 ps after laser excitation. We
demonstrate that the newly formed I-CDW phase is fragmented into several
nanometric domains that are growing through a coarsening process. The
coarsening dynamics is found to follow the universal Lifshitz-Allen-Cahn growth
law, which describes the ordering kinetics in systems exhibiting a
non-conservative order parameter.Comment: 6 pages, 5 figure
Observation of a temperature dependent asymmetry in the domain structure of a Pd-doped FeRh epilayer
Using X-ray photoelectron emission microscopy we have observed the
coexistence of ferromagnetic and antiferromagnetic phases in a (3 at.%)Pd-doped
FeRh epilayer. By quantitatively analyzing the resultant images we observe that
as the epilayer transforms there is a change in magnetic domain symmetry from
predominantly twofold at lower temperatures through to an equally weighted
combination of both four and twofold symmetries at higher temperature. It is
postulated that the lowered symmetry Ising-like nematic phase resides at the
near-surface of the epilayer. This behavior is different to that of undoped
FeRh suggesting that the variation in symmetry is driven by the competing
structural and electronic interactions in the nanoscale FeRh film coupled with
the effect of the chemical doping disorder.Comment: 10 pages, 8 figures, 1 tabl
Ferromagnetic GaAs/GaMnAs Core−Shell Nanowires Grown by Molecular Beam Epitaxy
GaAs/GaMnAs core−shell nanowires were grown by molecular beam epitaxy. The core GaAs nanowires were synthesized under typical nanowire growth conditions using gold as catalyst. For the GaMnAs shell the temperature was drastically reduced to achieve low-temperature growth conditions known to be crucial for high-quality GaMnAs. The GaMnAs shell grows epitaxially on the side facets of the core GaAs nanowires. A ferromagnetic transition temperature of 20 K is obtained. Magnetic anisotropy studies indicate a magnetic easy axis parallel to the nanowire axis