48 research outputs found
Link between the diversity, heterogeneity and kinetic properties of amorphous ice structures
Based on neutron wide-angle diffraction and small-angle neutron scattering
experiments, we show that there is a correlation between the preparational
conditions of amorphous ice structures, their microscopic structural
properties, the extent of heterogeneities on a mesoscopic spatial scale and the
transformation kinetics. There are only two modifications that can be
identified as homogeneous disordered structures, namely the very high-density
vHDA and the low-density amorphous LDA ice. Structures showing an intermediate
static structure factor with respect to vHDA and LDA are heterogeneous phases.
This holds independently from their preparation procedure, i.e. either obtained
by pressure amorphisation of ice I_h or by heating of vHDA. The degree of
heterogeneity can be progressively suppressed when higher pressures and
temperatures are applied for the sample preparation. In accordance with the
suppressed heterogeneity the maximum of the static structure factor displays a
pronounced narrowing of the first strong peak, shifting towards higher
Q-numbers. Moreover, the less heterogeneous the obtained structures are the
slower is the transformation kinetics from the high--density modifications into
LDA. The well known high-density amorphous structure HDA does not constitute
any particular state of the amorphous water network. It is formed due to the
preparational procedure working in liquid nitrogen as thermal bath, i.e. at
about 77 K
Understanding the magnetism in noncentrosymmetric CeIrGe3 Muon spin relaxation and neutron scattering studies
The magnetic properties of a pressure induced noncentrosymmetric
heavy-fermion superconductor CeIrGe have been investigated by muon spin
relaxation (SR), powder neutron diffraction (ND) and inelastic neutron
scattering (INS) techniques at ambient pressure. For completeness we have also
measured the ac magnetic susceptibility , dc magnetic
susceptibility , dc isothermal magnetization and heat capacity
down to 2 K. CeIrGe is known to exhibit pressure induced
superconductivity ( K) at a pressure of 20 GPa and
antiferromagnetic ordering at 8.7 K, 4.7 K and 0.7 K at ambient pressure. Our
and data show an additional anomaly near 6.2 K
which is also captured in data. From ,
and measurements we infer three antiferromagnetic
transitions above 2 K at K, K and K. Our SR study also confirms the presence of three transitions
through the observation of one frequency for ,
two frequencies for and three frequencies for
in the oscillatory asymmetry. The ND data reveal an
incommensurate nature of the magnetic ordering at K with the propagation
vector k = (0,0,0.688(3)), and a commensurate magnetic structure at K
with the propagation vector locked to the value k = (0,0,2/3) and magnetic
moments oriented along the axis. The commensurate structure couples a
macroscopic ferromagnetic component, resulting in a strong dependence of the
lock-in transition temperature on external magnetic field. The INS data show
two well defined crystal electric field (CEF) excitations arising from the
CEF-split Kramers doublet ground state of Ce.Comment: 13 pages, 16 figures, 1 tabl
Magnetic structures and excitations in CePd2(Al, Ga)2 series: Development of the "vibron" states
CePd2Al2-xGax compounds crystallizing in the tetragonal CaBe2Ge2-type structure (space group P4/nmm) and undergoing a structural phase transition to an orthorhombic structure (Cmme) at low temperatures were studied by means of neutron scattering. The amplitude-modulated magnetic structure of CePd2Al2 is described by an incommensurate propagation vector k - =(dx, 12+dy, 0) with dx=0.06 and dy=0.04. The magnetic moments order antiferromagnetically within the ab planes stacked along the c axis and are arranged along the direction close to the orthorhombic a axis with a maximum value of 1.5(1) µB/Ce3+. CePd2Ga2 reveals a magnetic structure composed of two components: the first is described by the propagation vector k1 - =(12, 12, 0), and the second one propagates with k2 - =(0, 12, 0). The magnetic moments of both components are aligned along the same direction - the orthorhombic 100] direction - and their total amplitude varies depending on the mutual phase of magnetic moment components on each Ce site. The propagation vectors k1 - and k2 - describe also the magnetic structure of substituted CePd2Al2-xGax compounds, except the one with x=0.1.CePd2Al1.9Ga0.1 with magnetic structure described by k - and k1 - stays on the border between pure CePd2Al2 and the rest of the series. Determined magnetic structures are compared with other Ce 112 compounds. Inelastic neutron scattering experiments disclosed three nondispersive magnetic excitations in the paramagnetic state of CePd2Al2, while only two crystal field (CF) excitations are expected from the splitting of ground state J=52 of the Ce3+ ion in a tetragonal/orthorhombic point symmetry. Three magnetic excitations at 1.4, 7.8, and 15.9 meV are observed in the tetragonal phase of CePd2Al2. A structural phase transition to an orthorhombic structure shifts the first excitation up to 3.7 meV, while the other two excitations remain at almost the same energy. The presence of an additional magnetic peak is discussed and described within the Thalmeier-Fulde CF-phonon coupling (i.e., magnetoelastic coupling) model generalized to the tetragonal point symmetry. The second parent compound CePd2Ga2 does not display any sign of additional magnetic excitation. The expected two CF excitations were observed. The development of magnetic excitations in the CePd2Al2-xGax series is discussed and crystal field parameters determined
Hopping motion of lattice gases through nonsymmetric potentials under strong bias conditions
The hopping motion of lattice gases through potentials without
mirror-reflection symmetry is investigated under various bias conditions. The
model of 2 particles on a ring with 4 sites is solved explicitly; the resulting
current in a sawtooth potential is discussed. The current of lattice gases in
extended systems consisting of periodic repetitions of segments with sawtooth
potentials is studied for different concentrations and values of the bias.
Rectification effects are observed, similar to the single-particle case. A
mean-field approximation for the current in the case of strong bias acting
against the highest barriers in the system is made and compared with numerical
simulations. The particle-vacancy symmetry of the model is discussed.Comment: 8 pages (incl. 6 eps figures); RevTeX 3.
Magnetic frustration in a metallic fcc lattice
Magnetic frustration in metals is scarce and hard to pinpoint, but exciting
due to the possibility of the emergence of fascinating novel phases. The cubic
intermetallic compound HoInCu with all holmium atoms on an fcc lattice,
exhibits partial magnetic frustration, yielding a ground state where half of
the Ho moments remain without long-range order, as evidenced by our neutron
scattering experiments. The substitution of In with Cd results in HoCdCu in
a full breakdown of magnetic frustration. Consequently we found a fully ordered
magnetic structure in our neutron diffraction experiments. These findings are
in agreement with the local energy scales and crystal electric field
excitations, which we determined from specific heat and inelastic neutron
scattering data. The electronic density of states for the itinerant bands acts
as tuning parameter for the ratio between nearest-neighbor and
next-nearest-neighbor interactions and thus for magnetic frustration
Search for and Using Genetic Programming Event Selection
We apply a genetic programming technique to search for the double Cabibbo
suppressed decays and .
We normalize these decays to their Cabibbo favored partners and find
\Lambda_c^+ \to p K^+ \pi^-\Lambda_c^+ \to p K^-
\pi^+ and D_s^+ \to K^+ K^+
\pi^-D_s^+ \to K^+ K^- \pi^+ where
the first errors are statistical and the second are systematic. Expressed as
90% confidence levels (CL), we find and respectively.
This is the first successful use of genetic programming in a high energy
physics data analysis.Comment: 10 page
Application of Genetic Programming to High Energy Physics Event Selection
We review genetic programming principles, their application to FOCUS data
samples, and use the method to study the doubly Cabibbo suppressed decay D+ ->
K+ pi+ pi- relative to its Cabibbo favored counterpart, D+ -> K- pi+ pi+. We
find that this technique is able to improve upon more traditional analysis
methods. To our knowledge, this is the first application of the genetic
programming technique to High Energy Physics data.Comment: 39 page
Complex atomic dynamics in a deep-eutectic binary metallic melt
Quasielastic neutron scattering (QENS) was used to explore relaxation processes in a deep-eutectic
Ce80Ni20 melt over a temperature range of 750 K, i.e., from 775 K to 1525 K. At low temperatures
(T = 890 K), the self-correlation function shows a fast b-relaxation process. The a-relaxation process
displays stretching of the self-correlation and obeys a time-temperature superposition principle that
extends over the entire measured temperature range. Even though the decay of the self-correlation
functions of the Ce80Ni20 melt exhibit that of glass-forming melts, the temperature dependence of
self-diffusivity displays an Arrhenius behavior, as observed for many simple, non-glass forming liquids