1,646 research outputs found
Structural, magnetic, electric, dielectric, and thermodynamic properties of multiferroic GeV4S8
The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the
Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below
about 14 K. In addition to this orbitally driven ferroelectricity, lacunar
spinels are an interesting material class, as the vanadium ions form V4
clusters representing stable molecular entities with a common electron
distribution and a well-defined level scheme of molecular states resulting in a
unique spin state per V4 molecule. Here we report detailed x-ray, magnetic
susceptibility, electrical resistivity, heat capacity, thermal expansion, and
dielectric results to characterize the structural, electric, dielectric,
magnetic, and thermodynamic properties of this interesting material, which also
exhibits strong electronic correlations. From the magnetic susceptibility, we
determine a negative Curie-Weiss temperature, indicative for antiferromagnetic
exchange and a paramagnetic moment close to a spin S = 1 of the V4 molecular
clusters. The low-temperature heat capacity provides experimental evidence for
gapped magnon excitations. From the entropy release, we conclude about strong
correlations between magnetic order and lattice distortions. In addition, the
observed anomalies at the phase transitions also indicate strong coupling
between structural and electronic degrees of freedom. Utilizing dielectric
spectroscopy, we find the onset of significant dispersion effects at the polar
Jahn-Teller transition. The dispersion becomes fully suppressed again with the
onset of spin order. In addition, the temperature dependencies of dielectric
constant and specific heat possibly indicate a sequential appearance of orbital
and polar order.Comment: 15 pages, 9 figure
Weak ferromagnetism and glassy state in kappa-(BEDT-TTF)2Hg(SCN)2Br
Since the first observation of weak ferromagnetism in the charge-transfer
salt kappa-(BEDT-TTF)2-Cu[N(CN)2]Cl [U. Welp et al., Phys. Rev. Lett. 69, 840
(1992)], no further evidence of ferromagnetism in this class of organic
materials has been reported. Here we present static and dynamic spin
susceptibility measurements on kappa-(BEDT-TTF)2Hg(SCN)2Br revealing weak
ferromagnetism below about TWF = 20 K. We suggest that frustrated spins in the
molecular dimers suppress long-range order, forming a spin-glass ground state
in the insulating phase
Systematic study of the decay rates of antiprotonic helium states
A systematic study of the decay rates of antiprotonic helium (\pbhef and
\pbhet) at CERN AD (Antiproton Decelerator) has been made by a laser
spectroscopic method. The decay rates of some of its short-lived states, namely
those for which the Auger rates are much larger than
their radiative decay rates ( s),
were determined from the time distributions of the antiproton annihilation
signals induced by laser beams, and the widths of the atomic resonance lines.
The magnitude of the decay rates, especially their relation with the transition
multipolarity, is discussed and compared with theoretical calculations.Comment: 6 pages, 5 figures, and 1 tabl
Perspectives of Nuclear Physics in Europe: NuPECC Long Range Plan 2010
The goal of this European Science Foundation Forward Look into the future of Nuclear Physics is to bring together
the entire Nuclear Physics community in Europe to formulate a coherent plan of the best way to develop the field in
the coming decade and beyond.<p></p>
The primary aim of Nuclear Physics is to understand the origin, evolution, structure and phases of strongly interacting matter, which constitutes nearly 100% of the visible matter in the universe. This is an immensely important and challenging task that requires the concerted effort of scientists working in both theory and experiment, funding agencies, politicians and the public.<p></p>
Nuclear Physics projects are often “big science”, which implies large investments and long lead times. They need careful forward planning and strong support from policy makers. This Forward Look provides an excellent tool to achieve this. It represents the outcome of detailed scrutiny by Europe’s leading experts and will help focus the views of the scientific community on the most promising directions in the field and create the basis for funding agencies to provide adequate support.<p></p>
The current NuPECC Long Range Plan 2010 “Perspectives of Nuclear Physics in Europe” resulted from consultation
with close to 6 000 scientists and engineers over a period of approximately one year. Its detailed recommendations
are presented on the following pages. For the interested public, a short summary brochure has been produced to
accompany the Forward Look.<p></p>
Sub-gap optical response across the structural phase transition in van der Waals layered \alpha-RuCl
We report magnetic, thermodynamic, thermal expansion, and on detailed optical
experiments on the layered compound -RuCl focusing on the THz and
sub-gap optical response across the structural phase transition from the
monoclinic high-temperature to the rhombohedral low-temperature structure,
where the stacking sequence of the molecular layers is changed. This type of
phase transition is characteristic for a variety of tri-halides crystallizing
in a layered honeycomb-type structure and so far is unique, as the
low-temperature phase exhibits the higher symmetry. One motivation is to
unravel the microscopic nature of spin-orbital excitations via a study of
temperature and symmetry-induced changes. We document a number of highly
unusual findings: A characteristic two-step hysteresis of the structural phase
transition, accompanied by a dramatic change of the reflectivity. An electronic
excitation, which appears in a narrow temperature range just across the
structural phase transition, and a complex dielectric loss spectrum in the THz
regime, which could indicate remnants of Kitaev physics. Despite significant
symmetry changes across the monoclinic to rhombohedral phase transition, phonon
eigenfrequencies and the majority of spin-orbital excitations are not strongly
influenced. Obviously, the symmetry of the single molecular layers determine
the eigenfrequencies of most of these excitations. Finally, from this combined
terahertz, far- and mid-infrared study we try to shed some light on the so far
unsolved low energy (< 1eV) electronic structure of the ruthenium
electrons in -RuCl.Comment: 22 pages, 9 figure
Hyperfine structure of antiprotonic helium revealed by a laser-microwave-laser resonance method
Using a newly developed laser-microwave-laser resonance method, we observed a
pair of microwave transitions between hyperfine levels of the
state of antiprotonic helium. This experiment confirms the quadruplet hyperfine
structure due to the interaction of the antiproton orbital angular momentum,
the electron spin and the antiproton spin as predicted by Bakalov and Korobov.
The measured frequencies of GHz and
GHz agree with recent theoretical
calculations on a level of .Comment: 4 pages, 4 figures, 1 tabl
Anisotropic magnetism, superconductivity, and the phase diagram of Rb(1-x)Fe(2-y)Se2
We report the crystal growth and structural, magnetic, conductivity, and
specific heat investigations of Rb(1-x)Fe(2-y)Se2 single crystals with varying
stoichiometry prepared by self-flux and Bridgman methods. The system exhibits a
strongly anisotropic antiferromagnetic behavior below 400 K. For 1.53 <2-y< 1.6
superconductivity is found, whereas for Fe concentrations 2-y 1.6
insulating and semiconducting behavior is observed, respectively. The sharpest
transition to the superconducting state and the highest transition temperature
T_c of 32.4 K is found for compositions close to Rb2Fe4Se5. A critical current
density j_c of 1.6*10^4 A/cm^2 (at 2 K) is obtained in the superconducting
samples with the highest diamagnetic response. In these samples additional
intense x-ray reflexes appear, which are incompatible with the tetragonal
symmetry. Upper critical fields H_{c2} of ~250 kOe for the in-plane and 630 kOe
for the inter-plane configurations are estimated from resistivity in magnetic
fields parallel and perpendicular to the c-axis, respectively. In the
non-superconducting samples with the Fe concentration below 1.45 both specific
heat and susceptibility revealed an anomaly at 220 K which is not related to
antiferromagnetic or structural transformations. Comparison with the magnetic
behavior of non-superconducting samples provides evidence for the coexistence
of superconductivity and static antiferromagnetic order.Comment: 12 pages, 16 figures, 2 table
On the multiferroic skyrmion-host GaV4S8
The lacunar spinel GaV4S8 exhibits orbital ordering at 44 K and shows a
complex magnetic phase diagram below 12.7 K, which includes ferromagnetic and
cycloidal spin order. At low but finite external magnetic fields, N\'eel-type
skyrmions are formed in this material. Skyrmions are whirl-like spin vortices
that have received great theoretical interest because of their non-trivial spin
topology and that are also considered as basic entities for new data-storage
technologies. Interestingly, we found that the orbitally ordered phase shows
sizable ferroelectric polarization and that excess spin-driven polarizations
appear in all magnetic phases, including the skyrmion-lattice phase. Hence,
GaV4S8 shows simultaneous magnetic and polar order and belongs to the class of
multiferroics, materials that attracted enormous attention in recent years.
Here, we summarize the existing experimental information on the magnetic,
electronic, and dielectric properties of GaV4S8. By performing detailed
magnetic susceptibility, resistivity, specific heat, and dielectric
experiments, we complement the low-temperature phase diagram. Specifically, we
show that the low-temperature and low-field ground state of GaV4S8 seems to
have a more complex spin configuration than purely collinear ferromagnetic spin
order. In addition, at the structural Jahn-Teller transition the magnetic
exchange interaction changes from antiferromagnetic to ferromagnetic. We also
provide experimental evidence that the vanadium V4 clusters in GaV4S8 can be
regarded as molecular units with spin 1/2. However, at high temperatures
deviations in the susceptibility show up, indicating that either the magnetic
moments of the vanadium atoms fluctuate independently or excited states of the
V4 molecule become relevant.Comment: 17 pages, 10 figures, submitted to Phil. Ma
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