9,855 research outputs found
Dynamical effects of self-generated magnetic fields in cosmic ray modified shocks
Recent observations of greatly amplified magnetic fields () around supernova shocks are consistent with the predictions of the
non-linear theory of particle acceleration (NLT), if the field is generated
upstream of the shock by cosmic ray induced streaming instability. The high
acceleration efficiencies and large shock modifications predicted by NLT need
however to be mitigated to confront observations, and this is usually assumed
to be accomplished by some form of turbulent heating. We show here that
magnetic fields with the strength inferred from observations have an important
dynamical role on the shock, and imply a shock modification substantially
reduced with respect to the naive unmagnetized case. The effect appears as soon
as the pressure in the turbulent magnetic field becomes comparable with the
pressure of the thermal gas. The relative importance of this unavoidable effect
and of the poorly known turbulent heating is assessed. More specifically we
conclude that even in the cases in which turbulent heating may be of some
importance, the dynamical reaction of the field cannot be neglected, as instead
is usually done in most current calculations.Comment: 4 pages, 1 figure, accepted for publication in ApJ Letter
Weak magnetism phenomena in heavy-fermion superconductors: selected SR studies
The behavior of the so-called weak moment antiferromagnetic states, observed
in the heavy-fermion superconductors UPt and URuSi, is discussed in
view of recent SR results obtained as function of control parameters like
chemical substitution and external pressure. In UPt, the Pd substitution
for Pt reveals the dynamical character of the weak moment order. On the other
hand, SR measurements performed on samples in which Th substitutes U
suggest that crystallographic disorder on the magnetic sites deeply affects the
fluctuation timescale. In URuSi, a phase separation between the
so-called hidden order state, present at ambient pressure, and an
antiferromagnetic state, occurring under pressure, is observed. In view of the
pressure-temperature phase diagram obtained by SR, it is deduced that the
respective order parameters have different symmetries.Comment: To appear in: J. Phys.: Cond. Matte
The new versatile general purpose surface-muon instrument (GPS) based on silicon photomultipliers for SR measurements on a continuous-wave beam
We report on the design and commissioning of a new spectrometer for muon-spin
relaxation/rotation studies installed at the Swiss Muon Source (SS) of the
Paul Scherrer Institute (PSI, Switzerland). This new instrument is essentially
a new design and replaces the old general-purpose surface-muon instrument (GPS)
which has been for long the workhorse of the SR user facility at PSI. By
making use of muon and positron detectors made of plastic scintillators read
out by silicon photomultipliers (SiPMs), a time resolution of the complete
instrument of about 160 ps (standard deviation) could be achieved. In addition,
the absence of light guides, which are needed in traditionally built SR
instrument to deliver the scintillation light to photomultiplier tubes located
outside magnetic fields applied, allowed us to design a compact instrument with
a detector set covering an increased solid angle compared to the old GPS.Comment: 11 pages, 11 figure
Probing the phase diagram of CeRu_2Ge_2 by thermopower at high pressure
The temperature dependence of the thermoelectric power, S(T), and the
electrical resistivity of the magnetically ordered CeRu_2Ge_2 (T_N=8.55 K and
T_C=7.40 K) were measured for pressures p < 16 GPa in the temperature range 1.2
K < T < 300 K. Long-range magnetic order is suppressed at a p_c of
approximately 6.4 GPa. Pressure drives S(T) through a sequence of temperature
dependences, ranging from a behaviour characteristic for magnetically ordered
heavy fermion compounds to a typical behaviour of intermediate-valent systems.
At intermediate pressures a large positive maximum develops above 10 K in S(T).
Its origin is attributed to the Kondo effect and its position is assumed to
reflect the Kondo temperature T_K. The pressure dependence of T_K is discussed
in a revised and extended (T,p) phase diagram of CeRu_2Ge_2.Comment: 7 pages, 6 figure
Evidence for spin liquid ground state in SrDyO frustrated magnet probed by muSR
Muon spin relaxation (SR) measurements were carried out on
SrDyO, a frustrated magnet featuring short range magnetic correlations
at low temperatures. Zero-field muon spin depolarization measurements
demonstrate that fast magnetic fluctuations are present from K down to
20 mK. The coexistence of short range magnetic correlations and fluctuations at
mK indicates that SrDyO features a spin liquid ground state.
Large longitudinal fields affect weakly the muon spin depolarization, also
suggesting the presence of fast fluctuations. For a longitudinal field of
T, a non-relaxing asymmetry contribution appears below K,
indicating considerable slowing down of the magnetic fluctuations as
field-induced magnetically-ordered phases are approached.Comment: 6 pages, 4 figures, to be published as a proceeding of HFM2016 in
Journal of Physics: Conference Series (JPCS
Precursor Plerionic Activity and High Energy Gamma-Ray Emission in the Supranova Model of Gamma-Ray Bursts
The supranova model of gamma-ray bursts (GRBs), in which the GRB event is
preceded by a supernova (SN) explosion by a few months to years, has recently
gained support from Fe line detections in X-ray afterglows. A crucial
ingredient of this model yet to be studied is the fast-rotating pulsar that
should be active during the time interval between the SN and the GRB, driving a
powerful wind and a luminous plerionic nebula. We discuss some observational
consequences of this precursor plerion, which should provide important tests
for the supranova model: 1) the fragmentation of the outlying SN ejecta
material by the plerion and its implications for Fe line emission; and 2) the
effect of inverse Compton cooling and emission in the GRB external shock due to
the plerion radiation field. The plerion-induced inverse Compton emission can
dominate in the GeV-TeV energy range during the afterglow, being detectable by
GLAST from redshifts and distinguishable from self-Compton
emission by its spectrum and light curve. The prospects for direct detection
and identification of the precursor plerion emission are also briefly
considered.Comment: ApJ vol.583, in pres
Magnetic quantum critical point and superconductivity in UPt3 doped with Pd
Transverse-field muon spin relaxation measurements have been carried out on
the heavy-fermion superconductor UPt3 doped with small amounts of Pd. We find
that the critical Pd concentration for the emergence of the large-moment
antiferromagnetic phase is ~0.6 at.%Pd. At the same Pd content,
superconductivity is completely suppressed. The existence of a magnetic quantum
critical point in the phase diagram, which coincides with the critical point
for superconductivity, provides evidence for ferromagnetic spin-fluctuation
mediated odd-parity superconductivity, which competes with antiferromagnetic
order.Comment: 4 pages (includes 3 figures); postscript fil
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