472 research outputs found
Evolution of two-gap behavior of the superconductor FeSe_1-x
The superfluid density, \rho_s, of the iron chalcogenide superconductor,
FeSe_1-x, was studied as a function of pressure by means of muon-spin rotation.
The zero-temperature value of \rho_s increases with increasing transition
temperature T_c (increasing pressure) following the tendency observed for
various Fe-based and cuprate superconductors. The analysis of \rho_s(T) within
the two-gap scheme reveals that the effect on both, T_c and \rho_s(0), is
entirely determined by the band(s) where the large superconducting gap
develops, while the band(s) with the small gap become practically unaffected.Comment: 5 pages, 3 figure
Magnetic order and spin dynamics across a ferromagnetic quantum critical point: SR investigations of YbNi(PAs)
In the quasi-1D heavy-fermion system YbNi(PAs) the
presence of a ferromagnetic (FM) quantum critical point (QCP) at with unconventional quantum critical exponents in the thermodynamic
properties has been recently reported. Here, we present muon-spin relaxation
(SR) experiments on polycrystals of this series to study the magnetic
order and the low energy 4-electronic spin dynamics across the FM QCP. The
zero field SR measurements on pure YbNi(P proved static long
range magnetic order and suggested a strongly reduced ordered Yb moment of
about 0.04. With increasing As substitution the ordered moment is
reduced by half at and to less than 0.005 at . The
dynamic behavior in the SR response show that magnetism remains
homogeneous upon As substitution, without evidence for disorder effect. In the
paramagnetic state across the FM QCP the dynamic muon-spin relaxation rate
follows 1/ with . The critical fluctuations are very slow and are even becoming slower
when approaching the QCP.Comment: 6 pages, 4 figure
Formation of hydrogen impurity states in silicon and insulators at low implantation energies
The formation of hydrogen-like muonium (Mu) has been studied as a function of
implantation energy in intrinsic Si, thin films of condensed van der Waals
gases (N2, Ne, Ar, Xe), fused and crystalline quartz and sapphire. By varying
the initial energy of positive muons (mu+) between 1 and 30 keV the number of
electron-hole pairs generated in the ionization track of the mu+ can be tuned
between a few and several thousand. The results show the strong suppression of
the formation of those Mu states that depend on the availability of excess
electrons. This indicates, that the role of H-impurity states in determining
electric properties of semiconductors and insulators depends on the way how
atomic H is introduced into the material.Comment: 4 pages, 4 enscapulated postscript figures, uses revtex4 twocolumn
style to be published in Physical Review Letter
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
High temperature ferromagnetism of Li-doped vanadium oxide nanotubes
The nature of a puzzling high temperature ferromagnetism of doped
mixed-valent vanadium oxide nanotubes reported earlier by Krusin-Elbaum et al.,
Nature 431 (2004) 672, has been addressed by static magnetization, muon spin
relaxation, nuclear magnetic and electron spin resonance spectroscopy
techniques. A precise control of the charge doping was achieved by
electrochemical Li intercalation. We find that it provides excess electrons,
thereby increasing the number of interacting magnetic vanadium sites, and, at a
certain doping level, yields a ferromagnetic-like response persisting up to
room temperature. Thus we confirm the surprising previous results on the
samples prepared by a completely different intercalation method. Moreover our
spectroscopic data provide first ample evidence for the bulk nature of the
effect. In particular, they enable a conclusion that the Li nucleates
superparamagnetic nanosize spin clusters around the intercalation site which
are responsible for the unusual high temperature ferromagnetism of vanadium
oxide nanotubes.Comment: with some amendments published in Europhysics Letters (EPL) 88 (2009)
57002; http://epljournal.edpsciences.or
Microscopic Evidence of Spin State Order and Spin State Phase Separation in Layered Cobaltites RBaCo2O5.5 with R=Y, Tb, Dy, and Ho
We report muon spin relaxation measurements on the magnetic structures of
RBaCo_2O_5.5 with R=Y, Tb, Dy, and Ho. Three different phases, one
ferrimagnetic and two antiferromagnetic, are identified below 300 K. They
consist of different ordered spin state arrangements of high-, intermediate-,
and low-spin Co^3+ of CoO_6 octahedra. Phase separation into well separated
regions with different spin state order is observed in the antiferromagnetic
phases. The unusual strongly anisotropic magnetoresistance and its onset at the
FM-AFM phase boundary is explained.Comment: 4 pages, accepted for publication in Phys. Rev. Let
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