137 research outputs found
Complex and strongly anisotropic magnetism in the pure spin system EuRh2Si2
In divalent Eu systems, the 4f local moment has a pure spin state J=S=7/2.
Although the absence of orbital moment precludes crystal electric field
effects, we report a sizeable magnetic anisotropy in single crystals of
EuRh2Si2. We observed a surprisingly complex magnetic behavior with three
succesive phase transitions. The Eu2+ moments order in a likely
amplitude-modulated structure below 24.5K, undergoing a further transition to a
structure that is possibly of the equal moment type, and a first order
transition at lower temperatures, presumably into a spin spiral structure. The
sharp metamagnetic transition observed at low fields applied perpendicular to
the hard axis is consistent with a change from a spiral to a fan structure.
These magnetic structures are presumably formed by ferromagnetic planes
perpendicular to the c axis, stacked antiferromagnetically along c but not of
type I, at least just below the ordering temperature. Since EuRh2Si2 is
isoelectronic to EuFe2As2, our results are also relevant for the complex
Eu-magnetism observed there, especially for the transition from an
antiferromagnetic to a ferromagnetic state observed in EuFe2P2 upon
substituting As by P.Comment: submitted to Journal of Physics: Condensed Matte
Measurement of climate-relevant trace gases via infrared spectroscopy
The goal of this thesis was to develop and build a fully automated FTIR system for total column measurements of atmospheric trace gases. As part of TCCON, it is planned to install the system in the tropics where such measurements are very sparse.
The designated site is Ascension Island, a British oversea territory in the South Atlantic. This unique location should provide excellent observation conditions for the FTIR instrument. Due to its small size and very scarce vegetation, the influence from local sources and sinks on the CO2 and CH4 measurements should be minimal.
This work describes the history of the system in three parts: Development, Calibration, and Testing.
Chapter 2 introduces the design of the system: chosen components and self- made parts like a custom solar tracker protection dome are explained and the automation concept of the system is illustrated. Besides that, first results of total column measurements in Jena are presented.
In 2009, after the Jena FTIR system was completed, it took part in the IMECC aircraft calibration campaign. The integration of the FTIR measurements into the existing ground-based in-situ network requires a calibration. Earlier campaigns (Wunch et al., 2010) pointed out that it is highly likely that there is a species-specific uniform calibration factor for all TCCON FTIR systems. The goal of the IMECC aircraft campaign was to verify these findings and to calibrate six of the European TCCON stations. The results of the CH4 calibration are presented in Chapter 3.
After successfully taking part in the calibration campaign, the FTIR system was shipped to Australia for a test campaign. The aim was to prove the systems functionality and to compare the measurements of the system to the ones performed by a similar instrument operated by the University of Wollongong (UoW), Australia. The results of this campaign are presented in Chapter 4
Magnetic anisotropy of YbNi4P2
We report on transport and magnetic measurements between 1.8 and 400 K on
single crystalline YbNi4P2, which was recently reported to be a heavy fermion
system with a low lying ferromagnetic transition at T_C=0.17 K, based on data
from polycrystals. The tetragonal crystal structure of YbNi4P2 presents
quasi-one-dimensional Yb chains along the c direction. Here we show that at
high temperatures, the magnetic anisotropy of YbNi4P2 is dominated by the
crystal electrical field effect with an Ising-type behaviour, which gets more
pronounced towards lower temperatures. The electrical resistivity also reflects
the strong anisotropy of the crystal structure and favours transport along c,
the direction of the Yb chains.Comment: SCES 2011 proceedings, in pres
Charge, lattice and magnetism across the valence crossover in EuIrSi single crystals
We present a detailed study of the temperature evolution of the crystal
structure, specific heat, magnetic susceptibility and resistivity of single
crystals of the paradigmatic valence-fluctuating compound EuIrSi. A
comparison to stable-valent isostructural compounds EuCoSi (with
Eu), and EuRhSi, (with Eu) reveals an anomalously large
thermal expansion indicative of the lattice softening associated to valence
fluctuations. A marked broad peak at temperatures around 65-75 K is observed in
specific heat, susceptibility and the derivative of resistivity, as thermal
energy becomes large enough to excite Eu into a divalent state, which localizes
one f electron and increases scattering of conduction electrons. In addition,
the intermediate valence at low temperatures manifests in a moderately
renormalized electron mass, with enhanced values of the Sommerfeld coefficient
in the specific heat and a Fermi-liquid-like dependence of resistivity at low
temperatures. The high residual magnetic susceptibility is mainly ascribed to a
Van Vleck contribution. Although the intermediate/fluctuating valence duality
is to some extent represented in the interconfiguration fluctuation model
commonly used to analyze data on valence-fluctuating systems, we show that this
model cannot describe the different physical properties of EuIrSi with
a single set of parameters.Comment: 12 pages, 4 figures, 1 tabl
Exploring the spin-1/2 frustrated square lattice model with high-field magnetization measurements
We report on high-field magnetization measurements for a number of layered
vanadium phosphates that were recently recognized as spin-1/2 frustrated square
lattice compounds with ferromagnetic nearest-neighbor couplings (J_1) and
antiferromagnetic next-nearest-neighbor couplings (J_2). The saturation fields
of the materials lie in the range from 4 to 24 T and show excellent agreement
with the previous estimates of the exchange couplings deduced from low-field
thermodynamic measurements. The consistency of the high-field data with the
regular frustrated square lattice model provides experimental evidence for a
weak impact of spatial anisotropy on the nearest-neighbor couplings in layered
vanadium phosphates. The variation of the J_2/J_1 ratio within the compound
family facilitates the experimental access to the evolution of the
magnetization curve upon the change of the frustration magnitude. Our results
support the recent theoretical prediction by Thalmeier et al. [Phys. Rev. B,
77, 104441 (2008)] and give evidence for the enhanced bending of the
magnetization curves due to the increasing frustration of the underlying spin
system.Comment: Brief Report: 4 pages, 3 figures, 1 tabl
Magnetization study of the energy scales in YbRhSi under chemical pressure
We present a systematic study of the magnetization in YbRhSi
under slightly negative (6?% Ir substitution) and positive (7% Co substitution)
chemical pressure. We show how the critical field , associated with the
high-field Lifshitz transitions, is shifted to lower (higher) values with Co
(Ir) substitution. The critical field , which identifies the
boundary line of the antiferromagnetic (AFM) phase
increases with positive pressure and it approaches zero with 6% Ir
substitution. On the other side, the crossover field , associated with
the energy scale where a reconstruction of the Fermi surface has
been observed, is not much influenced by the chemical substitution.}{Following
the analysis proposed in
Refs.\,\cite{Paschen2004,Gegenwart2007,Friedemann2009,Tokiwa2009a} we have
fitted the quantity with a crossover function to
indentify . The line follows an almost linear -dependence
at sufficiently high fields outside the AFM phase, but it deviates from
linearity at and in
Yb(RhCo)Si it changes slope clearly inside the
AFM phase. Moreover, the FWHM of the fit function depends linearly on
temperature outside the phase, but remains constant inside, suggesting either
that such an analysis is valid only for or that the
Fermi surface changes continuously at inside the AFM phase.}}Comment: 6 pages, 4 figure
Unusual weak magnetic exchange in two different structure types: YbPtSn and YbPtIn
We present the structural, magnetic, thermodynamic, and transport properties
of the two new compounds YbPtSn and YbPtIn. X-ray powder diffraction
shows that they crystallize in different structure types, the hexagonal
ZrPtAl and the cubic Heusler type, respectively. Despite quite different
lattice types, both compounds present very similar magnetic properties: a
stable trivalent Yb, no evidence for a sizeable Kondo interaction, and
very weak exchange interactions with a strength below 1K as deduced from
specific heat . Broad anomalies in suggest short range magnetic
ordering at about 250mK and 180mK for YbPtSn and YbPtIn, respectively.
The weak exchange and the low ordering temperature result in a large
magnetocaloric effect as deduced from the magnetic field dependence of ,
making these compounds interesting candidates for magnetic cooling. In addition
we found in YbPtIn evidences for a charge density wave transition at about
290K. The occurrence of such transitions within several RETX compound
series (RE = rare earth, T = noble metal, X = In, Sn) is analyzed.Comment: 16 pages, 7 figure
Quantum criticality in Yb(Rh0.97Co0.03)2Si2 probed by low-temperature resistivity
Quantum criticality in Yb(Rh0.97Co0.03)2Si2 is investigated by means of
resistivity and magnetoresistance. The partial substitution of Co leads to a
stabilization of the magnetism as expected according to the application of
chemical pressure for Yb systems. However, the signature of the Kondo-breakdown
remains at the same position in the temperature-magnetic field phase diagram
compared to stoichiometric YbRh2Si2. As a consequence, the Kondo-breakdown is
situated within the antiferromagnetic phase. These results fit well within the
global phase diagram under chemical pressure [1].Comment: 4 pages, 4 figures, submitted to ICM/SCES200
Signatures of phase transitions in the microwave response of YbRh2Si2
We used a spectroscopic microwave technique utilizing superconducting
stripline resonators at frequencies between 3 GHz and 15 GHz to examine the
charge dynamics of YbRh2Si2 at temperatures and magnetic fields close to the
quantum critical point. The different electronic phases of this heavy-fermion
compound, in particular the antiferromagnetic, Fermi-liquid, and
non-Fermi-liquid regimes, were probed with temperature-dependent microwave
measurements between 40 mK and 600 mK at a set of different magnetic fields up
to 140 mT. Signatures of phase transitions were observed, which give
information about the dynamic response of this peculiar material that exhibits
field-tuned quantum criticality and pronounced deviations from Fermi-liquid
theory.Comment: 5 pages, 3 figure
- …