35 research outputs found
Spin-State Transition and Metal-Insulator Transition in LaEuCoO}
We present a study of the structure, the electric resistivity, the magnetic
susceptibility, and the thermal expansion of LaEuCoO. LaCoO
shows a temperature-induced spin-state transition around 100 K and a
metal-insulator transition around 500 K. Partial substitution of La by
the smaller Eu causes chemical pressure and leads to a drastic increase
of the spin gap from about 190 K in LaCoO to about 2000 K in EuCoO, so
that the spin-state transition is shifted to much higher temperatures. A
combined analysis of thermal expansion and susceptibility gives evidence that
the spin-state transition has to be attributed to a population of an
intermediate-spin state with orbital order for and without orbital
order for larger . In contrast to the spin-state transition, the
metal-insulator transition is shifted only moderately to higher temperatures
with increasing Eu content, showing that the metal-insulator transition occurs
independently from the spin-state distribution of the Co ions. Around
the metal-insulator transition the magnetic susceptibility shows a similar
increase for all and approaches a doping-independent value around 1000 K
indicating that well above the metal-insulator transition the same spin state
is approached for all .Comment: 10 pages, 6 figure
Epitaxial and layer-by-layer growth of EuO thin films on yttria-stabilized cubic zirconia (001) using MBE distillation
We have succeeded in growing epitaxial and highly stoichiometric films of EuO
on yttria-stabilized cubic zirconia (YSZ) (001). The use of the Eu-distillation
process during the molecular beam epitaxy assisted growth enables the
consistent achievement of stoichiometry. We have also succeeded in growing the
films in a layer-by-layer fashion by fine tuning the Eu vs. oxygen deposition
rates. The initial stages of growth involve the limited supply of oxygen from
the YSZ substrate, but the EuO stoichiometry can still be well maintained. The
films grown were sufficiently smooth so that the capping with a thin layer of
aluminum was leak tight and enabled ex situ experiments free from trivalent Eu
species. The findings were used to obtain recipes for better epitaxial growth
of EuO on MgO (001).Comment: 10 pages, 15 figure
Epitaxy, stoichiometry, and magnetic properties of Gd-doped EuO films on YSZ (001)
We have succeeded in preparing high-quality Gd-doped single-crystalline EuO
films. Using Eu-distillation-assisted molecular beam epitaxy and a systematic
variation in the Gd and oxygen deposition rates, we have been able to observe
sustained layer-by-layer epitaxial growth on yttria-stabilized cubic zirconia
(001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We
used soft x-ray absorption spectroscopy at the Eu and Gd M4,5 edges to confirm
the absence of Eu3+ contaminants and to determine the actual Gd concentration.
The distillation process ensures the absence of oxygen vacancies in the films.
From magnetization measurements we found the Curie temperature to increase
smoothly as a function of doping from 70 K up to a maximum of 125 K. A
threshold behavior was not observed for concentrations as low as 0.2%.Comment: 8 pages, 9 figure
Charge transfer in dissociating iodomethane and fluoromethane molecules ionized by intense femtosecond X-ray pulses
Citation: Boll, R., Erk, B., Coffee, R., Trippel, S., Kierspel, T., Bomme, C., . . . Rudenko, A. (2016). Charge transfer in dissociating iodomethane and fluoromethane molecules ionized by intense femtosecond X-ray pulses. Structural Dynamics, 3(4). doi:10.1063/1.4944344Additional Authors: Marchenko, T.;Miron, C.;Patanen, M.;Osipov, T.;Schorb, S.;Simon, M.;Swiggers, M.;Techert, S.;Ueda, K.;Bostedt, C.;Rolles, D.;Rudenko, A.Ultrafast electron transfer in dissociating iodomethane and fluoromethane molecules was studied at the Linac Coherent Light Source free-electron laser using an ultraviolet-pump, X-ray-probe scheme. The results for both molecules are discussed with respect to the nature of their UV excitation and different chemical properties. Signatures of long-distance intramolecular charge transfer are observed for both species, and a quantitative analysis of its distance dependence in iodomethane is carried out for charge states up to I21+. The reconstructed critical distances for electron transfer are in good agreement with a classical over-the-barrier model and with an earlier experiment employing a near-infrared pump pulse. © 2016 Author(s)
Strong damping of phononic heat current by magnetic excitations in SrCu_2(BO_3)_2
Measurements of the thermal conductivity as a function of temperature and
magnetic field in the 2D dimer spin system SrCu(BO) are presented.
In zero magnetic field the thermal conductivity along and perpendicular to the
magnetic planes shows a pronounced double-peak structure as a function of
temperature. The low-temperature maximum is drastically suppressed with
increasing magnetic field. Our quantitative analysis reveals that the heat
current is due to phonons and that the double-peak structure arises from
pronounced resonant scattering of phonons by magnetic excitations.Comment: a bit more than 4 pages, 2 figures included; minor changes to improve
the clarity of the presentatio
Imaging Molecular Structure through Femtosecond Photoelectron Diffraction on Aligned and Oriented Gas-Phase Molecules
This paper gives an account of our progress towards performing femtosecond
time-resolved photoelectron diffraction on gas-phase molecules in a pump-probe
setup combining optical lasers and an X-ray Free-Electron Laser. We present
results of two experiments aimed at measuring photoelectron angular
distributions of laser-aligned 1-ethynyl-4-fluorobenzene (C8H5F) and
dissociating, laseraligned 1,4-dibromobenzene (C6H4Br2) molecules and discuss
them in the larger context of photoelectron diffraction on gas-phase molecules.
We also show how the strong nanosecond laser pulse used for adiabatically
laser-aligning the molecules influences the measured electron and ion spectra
and angular distributions, and discuss how this may affect the outcome of
future time-resolved photoelectron diffraction experiments.Comment: 24 pages, 10 figures, Faraday Discussions 17
Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser
Citation: Savelyev, E., Boll, R., Bomme, C., Schirmel, N., Redlin, H., Erk, B., . . . Rolles, D. (2017). Jitter-correction for IR/UV-XUV pump-probe experiments at the FLASH free-electron laser. New Journal of Physics, 19, 13. doi:10.1088/1367-2630/aa652dIn pump-probe experiments employing a free-electron laser (FEL) in combination with a synchronized optical femtosecond laser, the arrival-time jitter between the FEL pulse and the optical laser pulse often severely limits the temporal resolution that can be achieved. Here, we present a pump-probe experiment on the UV-induced dissociation of 2,6-difluoroiodobenzene (C6H3F2I) molecules performed at the FLASH FEL that takes advantage of recent upgrades of the FLASH timing and synchronization system to obtain high-quality data that are not limited by the FEL arrival-time jitter. Wediscuss in detail the necessary data analysis steps and describe the origin of the timedependent effects in the yields and kinetic energies of the fragment ions that we observe in the experiment
Single-molecule techniques in biophysics : a review of the progress in methods and applications
Single-molecule biophysics has transformed our understanding of the
fundamental molecular processes involved in living biological systems, but also
of the fascinating physics of life. Far more exotic than a collection of
exemplars of soft matter behaviour, active biological matter lives far from
thermal equilibrium, and typically covers multiple length scales from the
nanometre level of single molecules up several orders of magnitude to longer
length scales in emergent structures of cells, tissues and organisms.
Biological molecules are often characterized by an underlying instability, in
that multiple metastable free energy states exist which are separated by energy
levels of typically just a few multiples of the thermal energy scale of kBT,
where kB is the Boltzmann constant and T the absolute temperature, implying
complex, dynamic inter-conversion kinetics across this bumpy free energy
landscape in the relatively hot, wet environment of real, living biological
matter. The key utility of single-molecule biophysics lies in its ability to
probe the underlying heterogeneity of free energy states across a population of
molecules, which in general is too challenging for conventional ensemble level
approaches which measure mean average properties. Parallel developments in both
experimental and theoretical techniques have been key to the latest insights
and are enabling the development of highly-multiplexed, correlative techniques
to tackle previously intractable biological problems. Experimentally,
technological developments in the sensitivity and speed of biomolecular
detectors, the stability and efficiency of light sources, probes and
microfluidics, have enabled and driven the study of heterogeneous behaviours
both in vitro and in vivo that were previously undetectable by ensemble
methods..