325 research outputs found
C60 adsorption on an aperiodically modulated Cu surface
Copper deposited on the ve-fold surface of icosahedral Al-Pd-Mn forms domains of a structure whose surface has a one-dimensional aperiodic modulation. It is shown that C60
deposited on this aperiodic film has highly reduced mobility as compared to C60 deposited on periodic Cu surfaces. This fnding is explained in terms of the recently proposed structural model of this system
Magnetic and structural transitions in LaNaFeAs single crystals
LaNaFeAs single crystals have been grown out of an
NaAs flux in an alumina crucible and characterized by measuring magnetic
susceptibility, electrical resistivity, specific heat, as well as single
crystal x-ray and neutron diffraction. LaNaFeAs single
crystals show a structural phase transition from a high temperature tetragonal
phase to a low-temperature orthorhombic phase at T\,=\,125\,K. This
structural transition is accompanied by an anomaly in the temperature
dependence of electrical resistivity, anisotropic magnetic susceptibility, and
specific heat. Concomitant with the structural phase transition, the Fe moments
order along the \emph{a} direction with an ordered moment of
0.7(1)\, at \emph{T}\,=\,5 K. The low temperature stripe
antiferromagnetic structure is the same as that in other
\emph{A}FeAs (\emph{A}\,=\,Ca, Sr, Ba) compounds.
LaNaFeAs provides a new material platform for the
study of iron-based superconductors where the electron-hole asymmetry could be
studied by simply varying La/Na ratio.Comment: 9 pages, 7 figures, to appear in Physical Review
High Pressure studies of the magnetic phase transition in MnSi: revisited
New measurements of AC magnetic susceptibility and DC resistivity of a high
quality single crystal MnSi were carried out at high pressure making use of
helium as a pressure medium. The form of the AC magnetic susceptibility curves
at the magnetic phase transition suddenly changes upon helium solidification.
This implies strong sensitivity of magnetic properties of MnSi to non
hydrostatic stresses and suggests that the early claims on the existence of a
tricritical point at the phase transition line are probably a result of
misinterpretation of the experimental data. At the same time resistivity
behavior at the phase transition does not show such a significant influence of
helium solidification. The sharp peak at the temperature derivative of
resistivity, signifying the first order nature of the phase transition in MnSi
successfully survived helium crystallization and continued the same way to the
highest pressure.Comment: 4 pages, 6 figure
Two-dimensional magnetic interactions in LaFeAsO
Inelastic neutron scattering measurements demonstrate that the magnetic interactions in antiferromagnetic LaFeAsO are two dimensional. Spin-wave velocities within the Fe layer and the magnitude of the spin gap are similar to the AFe2As2 based materials. However, the ratio of interlayer and intralayer exchange is found to be less than ∼10−4 in LaFeAsO, very similar to the cuprates, and ∼100 times smaller than that found in AFe2As2 compounds. The results suggest that the effective dimensionality of the magnetic system is highly variable in the parent compounds of the iron arsenides and weak three-dimensional interactions may limit the maximum attainable superconducting Tc
Periodic step arrays on the aperiodic i-Al-Pd-Mn quasicrystal surface at high temperature
We have observed the configuration and motion of surface steps on the aperiodic icosahedral (i-) Al-Pd-Mn quasicrystal using low-energy electron microscopy and scanning tunneling microscopy. As the quasicrystal is cooled from high temperature, bulk vacancies migrate to the surface causing the surface to be etched. Surprisingly, this etching occurs by two types of steps with different heights moving in different directions with different velocities. The steady-state surface morphology is a uniformly spaced rhomboidal step network. This network requires that the layer stacking near the surface deviates from the bulk quasicrystal stacking
High superconducting anisotropy and weak vortex pinning in Co doped LaFeAsO
Here, we present an electrical transport study in single crystals of
LaFeCoAsO ( K) under high magnetic fields. In
contrast to most of the previously reported Fe based superconductors, and
despite its relatively low , LaFeCoAsO shows a superconducting
anisotropy which is comparable to those seen for instance in the cuprates or
, where
is the effective mass anisotropy. Although, in the present case and as in all
Fe based superconductors, as . Under
the application of an external field, we also observe a remarkable broadening
of the superconducting transition particularly for fields applied along the
inter-planar direction. Both observations indicate that the low dimensionality
of LaFeCoAsO is likely to lead to a more complex vortex
phase-diagram when compared to the other Fe arsenides and consequently, to a
pronounced dissipation associated with the movement of vortices in a possible
vortex liquid phase. When compared to, for instance, F-doped compounds
pertaining to same family, we obtain rather small activation energies for the
motion of vortices. This suggests that the disorder introduced by doping
LaFeAsO with F is more effective in pinning the vortices than alloying it with
Co.Comment: 7 figures, 7 pages, Phys. Rev. B (in press
Irreversible Structural Transformation of five fold i-AlPdMn Quasicrystals after Ion Bombardment and Annealing
Five fold i-AlPdMn surface prepared under UHV by ion bombardment and annealing was so far considered to be bulk terminated. This result was substantially based on a quantitative LEED analyses [1]. Analysis of the specular rod in a X ray diffraction experiment at grazing incidence supported this result [2]. We present a new study of this surface by high resolution X ray diffraction at normal incidence. In this Bragg configuration the diffraction peak 18 – 29 for instance is at a photon energy of 2.873keV, the 72 – 116 reflection at 5.725keV. This results in an analyzed thickness of the sample surface of a few micrometers.
The surface was cleaned by ion bombardment. During annealing (T≅880K), we clearly observed the progressive disappearance of the initial Bragg peak characteristic of the as cast bulk sample. Conversely a new Bragg peak grows at an energy position shifted by 1eV compared to the position of the original Bragg peak. This is a clear signature for an irreversible structural transformation which takes place on at least the micron thickness. On the transformed surface, both, a LEED pattern and a RHEED pattern, characteristic for a five fold surface were easily obtained.
This high resolution experiment (the relative Bragg peak shift is 3ׁ10−4) was reproduced on samples from different initial compositions. This shows that five fold i-AlPdMn surface changes after preparation by ion bombardment and annealing at 900K on a micrometer thickness. This is not consistent with the conclusion that the surface is simply terminated by a cut of the original bulk. We conclude that a reorganization process of the quasicrystalline structure during annealing proceeds in the surface vicinity (probed depth is close to a few microns)
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