64 research outputs found
Ultrafast Laser-Induced Melting of Long-Range Magnetic Order in Multiferroic TbMnO3
We performed ultrafast time-resolved near-infrared pump, resonant soft X-ray
diffraction probe measurements to investigate the coupling between the
photoexcited electronic system and the spin cycloid magnetic order in
multiferroic TbMnO3 at low temperatures. We observe melting of the long range
antiferromagnetic order at low excitation fluences with a decay time constant
of 22.3 +- 1.1 ps, which is much slower than the ~1 ps melting times previously
observed in other systems. To explain the data we propose a simple model of the
melting process where the pump laser pulse directly excites the electronic
system, which then leads to an increase in the effective temperature of the
spin system via a slower relaxation mechanism. Despite this apparent increase
in the effective spin temperature, we do not observe changes in the wavevector
q of the antiferromagnetic spin order that would typically correlate with an
increase in temperature under equilibrium conditions. We suggest that this
behavior results from the extremely low magnon group velocity that hinders a
change in the spin-spiral wavevector on these time scales.Comment: 9 pages, 4 figure
Optically induced lattice deformations, electronic structure changes, and enhanced superconductivity in YBa2Cu3O6.48
Resonant optical excitation of apical oxygen vibrational modes in the normal
state of underdoped YBa2Cu3O6+x induces a transient state with optical
properties similar to those of the equilibrium superconducting state. Amongst
these, a divergent imaginary conductivity and a plasma edge are transiently
observed in the photo-stimulated state. Femtosecond hard x-ray diffraction
experiments have been used in the past to identify the transient crystal
structure in this non-equilibrium state. Here, we start from these
crystallographic features and theoretically predict the corresponding
electronic rearrangements that accompany these structural deformations. Using
density functional theory, we predict enhanced hole-doping of the CuO2 planes.
The empty chain Cu dy2-z2 orbital is calculated to strongly reduce in energy,
which would increase c-axis transport and potentially enhance the interlayer
Josephson coupling as observed in the THz-frequency response. From these
calculations, we predict changes in the soft x-ray absorption spectra at the Cu
L-edge. Femtosecond x-ray pulses from a free electron laser are used to probe
these changes in absorption at two photon energies along this spectrum, and
provide data consistent with these predictions.Comment: 20 pages with 6 figure
Probing the interplay between lattice dynamics and short-range magnetic correlations in CuGeO3 with femtosecond RIXS
Investigations of magnetically ordered phases on the femtosecond timescale
have provided significant insights into the influence of charge and lattice
degrees of freedom on the magnetic sub-system. However, short-range magnetic
correlations occurring in the absence of long-range order, for example in
spin-frustrated systems, are inaccessible to many ultrafast techniques. Here,
we show how time-resolved resonant inelastic X-ray scattering (trRIXS) is
capable of probing such short-ranged magnetic dynamics in a charge-transfer
insulator through the detection of a Zhang-Rice singlet exciton. Utilizing
trRIXS measurements at the O K-edge, and in combination with model
calculations, we probe the short-range spin-correlations in the frustrated spin
chain material CuGeO3 following photo-excitation, revealing a strong coupling
between the local lattice and spin sub-systems
Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and \u3cem\u3eAb Initio\u3c/em\u3e Simulations
We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5σ and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process
Nematicity dynamics in the charge-density-wave phase of a cuprate superconductor
Understanding the interplay between charge, nematic, and structural ordering
tendencies in cuprate superconductors is critical to unraveling their complex
phase diagram. Using pump-probe time-resolved resonant x-ray scattering on the
(0 0 1) Bragg peak at the Cu L3 and oxygen K resonances, we investigate
non-equilibrium dynamics of Qa = Qb = 0 nematic order and its association with
both charge density wave (CDW) order and lattice dynamics in
La1.65Eu0.2Sr0.15CuO4. In contrast to the slow lattice dynamics probed at the
apical oxygen K resonance, fast nematicity dynamics are observed at the Cu L3
and planar oxygen K resonances. The temperature dependence of the nematicity
dynamics is correlated with the onset of CDW order. These findings
unambiguously indicate that the CDW phase, typically evidenced by translational
symmetry breaking, includes a significant electronic nematic component.Comment: 16 pages, 4 figure
X ray emission spectroscopy of bulk liquid water in no man s land
The structure of bulk liquid water was recently probed by x ray scattering below the temperature limit of homogeneous nucleation TH of amp; 8764;232 K [J. A. Sellberg et al., Nature 510, 381 384 2014 ]. Here, we utilize a similar approach to study the structure of bulk liquid water below TH using oxygen K edge x ray emission spectroscopy XES . Based on previous XES experiments [T. Tokushima et al., Chem. Phys. Lett. 460, 387 400 2008 ] at higher temperatures, we expected the ratio of the 1b1 amp; 8242; and 1b1 amp; 8242; amp; 8242; peaks associated with the lone pair orbital in water to change strongly upon deep supercooling as the coordination of the hydrogen H bonds becomes tetrahedral. In contrast, we observed only minor changes in the lone pair spectral region, challenging an interpretation in terms of two interconverting species. A number of alternative hypotheses to explain the results are put forward and discussed. Although the spectra can be explained by various contributions from these hypotheses, we here emphasize the interpretation that the line shape of each component changes dramatically when approaching lower temperatures, where, in particular, the peak assigned to the proposed disordered component would become more symmetrical as vibrational interference becomes more importan
Light-enhanced Charge Density Wave Coherence in a High-Temperature Superconductor
In high-T cuprates, superconductivity and charge density waves (CDW)
are competitive, yet coexisting orders. To understand their microscopic
interdependence a probe capable of discerning their interaction on its natural
length and time scales is necessary. Here we use ultrafast resonant soft x-ray
scattering to track the transient evolution of CDW correlations in
YBaCuO following the quench of superconductivity by an
infrared laser pulse. We observe a picosecond non-thermal response of the CDW
order, characterized by a large enhancement of spatial coherence, nearly
doubling the CDW correlation length, while only marginally affecting its
amplitude. This ultrafast snapshot of the interaction between order parameters
demonstrates that their competition manifests inhomogeneously through
disruption of spatial coherence, and indicates the role of superconductivity in
stabilizing topological defects within CDW domains.Comment: 29 pages, 9 figures, Main text and Supplementary Material
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