429 research outputs found
Experimental evidence of enhancement without the influence of spin fluctuations: NMR study on LaFeAsO_{1-x}H_x under a pressure of 3.0 GPa
The electron-doped high-transition-temperature (T_c) iron-based pnictide
superconductor LaFeAsO_{1-x}H_x has a unique phase diagram: superconducting
(SC) double domes are sandwiched by antiferromagnetic phases at ambient
pressure and they turn to a single dome with a maximum T_c that exceeds 45K at
a pressure of 3.0 GPa. We studied whether spin fluctuations are involved in
increasing T_c under a pressure of 3.0 GPa by using ^{75}As nuclear magnetic
resonance (NMR) technique. The ^{75}As-NMR results for the powder samples show
that T_c increases up to 48 K without the influence of spin fluctuations. The
fact indicates that spin fluctuations are not involved in raising T_c, which
implies that other factors, such as orbital degrees of freedom, may be
important for achieving a high T_c of almost 50 K.Comment: Correponding Author: Naoki Fujiwar
Detection of antiferromagnetic ordering in heavily doped LaFeAsO1-xHx pnictide superconductors using nuclear-magnetic-resonance techniques
We studied double superconducting (SC) domes in LaFeAsO1-xHx by using 75As-
and 1H-nuclear magnetic resonance techniques, and unexpectedly discovered that
a new antiferromagnetic (AF) phase follows the double SC domes on further H
doping, forming a symmetric alignment of AF and SC phases in the electronic
phase diagram. We demonstrated that the new AF ordering originates from the
nesting between electron pockets, unlike the nesting between electron and hole
pockets as seen in the majority of undoped pnictides. The new AF ordering is
derived from the features common to high-Tc pnictides: however, it has not been
reported so far for other high-Tc pnictides because of their poor electron
doping capability.Comment: 5 figures, in press in PR
Quantum critical behavior in heavily doped LaFeAsOH pnictide superconductors analyzed using nuclear magnetic resonance
We studied the quantum critical behavior of the second antiferromagnetic (AF)
phase in the heavily electron-doped high- pnictide, LaFeAsOH
by using As and H nuclear-magnetic-resonance (NMR) technique. In
the second AF phase, we observed a spatially modulated spin-density-wave-like
state up to =0.6 from the NMR spectral lineshape and detected a low-energy
excitation gap from the nuclear relaxation time of As. The
excitation gap closes at the AF quantum critical point (QCP) at . The superconducting (SC) phase in a lower-doping regime contacts the
second AF phase only at the AF QCP, and both phases are segregated from each
other. The absence of AF critical fluctuations and the enhancement of the
in-plane electric anisotropy are key factors for the development of
superconductivity.Comment: accepted in Phys. Rev.
Crossover between magnetism and superconductivity in low H-doped LaFeAsO
By a systematic study of the hydrogen-doped LaFeAsO system by means of dc
resistivity, dc magnetometry, and muon-spin spectroscopy we addressed the
question of universality of the phase diagram of rare-earth-1111 pnictides. In
many respects, the behaviour of LaFeAsO_(1-x)H_(x) resembles that of its widely
studied F-doped counterpart, with H^- realizing a similar (or better)
electron-doping in the LaO planes. In a x = 0.01 sample we found a long-range
SDW order with T_n = 119 K, while at x = 0.05 the SDW establishes only at 38 K
and, below T_c = 10 K, it coexists at a nanoscopic scale with bulk
superconductivity. Unlike the abrupt M-SC transition found in the parent
La-1111 compound, the presence a crossover region makes the H-doped system
qualitatively similar to other Sm-, Ce-, or Nd-1111 families.Comment: to appear in Journal of Physics: Condensed Matte
Accurately accounting for effects on times-of-flight caused by finite field-transition times during the ejection of ions from a storage trap: A study for TOF and MRTOF mass spectrometry
In applied forms of time-of-flight mass spectrometry utilizing ion storage
devices prior to an analysis device, a non instantaneous electric ejection
pulse applied in the region of ion storage is used to accelerate ions into the
time-of-flight analyzer. The calculated mass value of the ions from the
time-of-flight is dependent on the duration of the field transition up to full
strength. For novel applications dedicated to precision measurements, such as
multi-reflection time-of-flight mass spectrometry of short-lived isotopes, the
goal is to continuously decrease the measurement uncertainty while providing a
mass accuracy on the same order. Even though dynamic-field models for
time-of-flight mass spectrometry have been considered in the past for
technological advances, it is important to study the accuracy of the measured
mass in this context. Using a simplified linear model for the field transition,
we provide a basic investigation of the scenario, and discuss the deviation
from the classical "mass-over-charge" dependency of the ions' time-of-flight,
which becomes violated. The emerging mass discrepancy depends on the distance
between the mass of the ion used for calibration and that of the ion of
interest and, in extreme cases, can increase to about one percent for systems
with short times-of-flight. However, for typical conditions in single-reference
multi-reflection time-of-flight mass spectrometry, mass deviations caused by
this effect typically remain below the 1 ppm level. If a mass calibration using
two or more ion species is possible during the measurement, the effect becomes
negligible for appropriate choices of reference masses.Comment: 14 pages, 9 figure
Nonmigrating semidiurnal tide over the Arctic determined from TIMED Doppler Interferometer wind observations
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94847/1/jgrd15899.pd
The new MRTOF mass spectrograph following the ZeroDegree spectrometer at RIKEN's RIBF facility
A newly assembled multi-reflection time-of-flight mass spectrograph
(MRTOF-MS) at RIKEN's RIBF facility became operational for the first time in
spring 2020; further modifications and performance tests using stable ions were
completed in early 2021. By using a pulsed-drift-tube technique to modify the
ions' kinetic energy in a wide range, we directly characterize the dispersion
function of the system for use in a new procedure for optimizing the voltages
applied to the electrostatic mirrors. Thus far, a mass resolving power of is reached within a total time-of-flight of only
, making the spectrometer capable of studying short-lived
nuclei possessing low-lying isomers. Detailed information about the setup and
measurement procedure is reported, and an alternative in-MRTOF ion selection
scheme to remove molecular contaminants in the absence of a dedicated
deflection device is introduced. The setup underwent an initial on-line
commissioning at the BigRIPS facility at the end of 2020, where more than 70
nuclear masses have been measured. A summary of the commissioning experiments
and results from a test of mass accuracy will be presented.Comment: 13 pages, 11 figure
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