201 research outputs found
Precursor phenomena at the magnetic ordering of the cubic helimagnet FeGe
We report on detailed magnetic measurements on the cubic helimagnet FeGe in
external magnetic fields and temperatures near the onset of long-range magnetic
order at K. Precursor phenomena display a complex succession of
temperature-driven crossovers and phase transitions in the vicinity of .
The A-phase region, present below and fields kOe, is split in
several pockets. Relying on a modified phenomenological theory for chiral
magnets, the main part of the A-phase could indicate the existence of a
Skyrmion lattice, the adjacent A pocket, however, appears to be related to
helicoids propagating in directions perpendicular to the applied field.Comment: 5 pages, 4 figure
4f-derived Fermi Surfaces of CeRu2(Si[1-x]Ge[x])2 near the Quantum Critical Point: Resonant Soft X-ray ARPES Study
Angle-resolved photoelectron spectroscopy in the Ce 3d-4f excitation region
was measured for the paramagnetic state of CeRu2Si2, CeRu2(Si0.82Ge0.18)2, and
LaRu2Si2 to investigate the changes of the 4 electron Fermi surfaces around
the quantum critical point. While the difference of the Fermi surfaces between
CeRu2Si2 and LaRu2Si2 was experimentally confirmed, a strong 4f-electron
character was observed in the band structures and the Fermi surfaces of
CeRu2Si2 and CeRu2(Si0.82Ge0.18)2, consequently indicating a delocalized nature
of the 4 electrons in both compounds. The absence of Fermi surface
reconstruction across the critical composition suggests that SDW quantum
criticality is more appropriate than local quantum criticality in
CeRu2(Si[1-x]Ge[x])2.Comment: 4 pages, 5 figure
Further Evidence for the Decay K+ to pi+ neutrino-antineutrino
Additional evidence for the rare kaon decay K+ to pi+ neutrino-antineutrino
has been found in a new data set with comparable sensitivity to the previously
reported result. One new event was observed in the pion momentum region
examined, 211<P<229 MeV/c, bringing the total for the combined data set to two.
Including all data taken, the backgrounds were estimated to contribute 0.15 pm
0.05 events. The branching ratio is B=1.57^{+1.75}_{-0.82} 10^{-10}.Comment: 10 pages, 2 figure
Search for the decay in the momentum region
We have searched for the decay in the kinematic
region with pion momentum below the peak. One event was
observed, consistent with the background estimate of . This
implies an upper limit on
(90% C.L.), consistent with the recently measured branching ratio of
, obtained using the standard model
spectrum and the kinematic region above the peak. The
same data were used to search for , where is a weakly
interacting neutral particle or system of particles with .Comment: 4 pages, 2 figure
Search for the decay K+ to pi+ gamma gamma in the pi+ momentum region P>213 MeV/c
We have searched for the K+ to pi+ gamma gamma decay in the kinematic region
with pi+ momentum close to the end point. No events were observed, and the 90%
confidence-level upper limit on the partial branching ratio was obtained, B(K+
to pi+ gamma gamma, P>213 MeV/c) < 8.3 x 10-9 under the assumption of chiral
perturbation theory including next-to-leading order ``unitarity'' corrections.
The same data were used to determine an upper limit on the K+ to pi+ gamma
branching ratio of 2.3 x 10-9 at the 90% confidence level.Comment: 15 pages, 3 figures; no change in the results, accepted for
publication in Physics Letters
Magnetoelectric effects in single crystals of the cubic ferrimagnetic helimagnet Cu2OSeO3
We present magnetodielectric measurements in single crystals of the cubic
spin-1/2 compound CuOSeO. A magnetic field-induced electric
polarization () and a finite magnetocapacitance (MC) is observed at
the onset of the magnetically ordered state ( K). Both and
MC are explored in considerable detail as a function of temperature (T),
applied field , and relative field orientations with respect to the
crystallographic axes. The magnetodielectric data show a number of anomalies
which signal magnetic phase transitions, and allow to map out the phase diagram
of the system in the -T plane. Below the 3up-1down collinear ferrimagnetic
phase, we find two additional magnetic phases. We demonstrate that these are
related to the field-driven evolution of a long-period helical phase, which is
stabilized by the chiral Dzyalozinskii-Moriya term D \vec{M}
\cdot(\bs{\nabla}\times\vec{M}) that is present in this non-centrosymmetric
compound. We also present a phenomenological Landau-Ginzburg theory for the
ME effect, which is in excellent agreement with experimental data, and
shows three novel features: (i) the polarization has a uniform as
well as a long-wavelength spatial component that is given by the pitch of the
magnetic helices, (ii) the uniform component of points along the
vector , and (iii) its strength is proportional to
, where is the longitudinal
and is the transverse (and spiraling) component of the magnetic
ordering. Hence, the field dependence of P provides a clear signature of the
evolution of a conical helix under a magnetic field. A similar phenomenological
theory is discussed for the MC
Measurement of Direct Photon Emission in K^+ -> pi^+ pi^0 gamma Decay
We have performed a measurement of the K^+ -> pi^+ pi^0 gamma decay and have
observed 2 X 10^4 events. The best fit to the decay spectrum gives a branching
ratio for direct photon emission of (4.7\pm0.8\pm0.3) X 10^{-6} in the pi^+
kinetic energy region of 55 to 90 MeV and requires no component due to
interference with inner bremsstrahlung.Comment: 9 pages, 3 figures. To be submitted to PR
- …