4,145 research outputs found
Magnetic ordering in GdNi2B2C revisited by resonant x-ray scattering: evidence for the double-q model
Recent theoretical efforts aimed at understanding the nature of
antiferromagnetic ordering in GdNi2B2C predicted double-q ordering. Here we
employ resonant elastic x-ray scattering to test this theory against the
formerly proposed, single-q ordering scenario. Our study reveals a satellite
reflection associated with a mixed-order component propagation wave vector,
viz., (q_a,2q_b,0) with q_b = q_a approx= 0.55 reciprocal lattice units, the
presence of which is incompatible with single-q ordering but is expected from
the double-q model. A (3q_a,0,0) wave vector (i.e., third-order) satellite is
also observed, again in line with the double-q model. The temperature
dependencies of these along with that of a first-order satellite are compared
with calculations based on the double-q model and reasonable qualitative
agreement is found. By examining the azimuthal dependence of first-order
satellite scattering, we show the magnetic order to be, as predicted,
elliptically polarized at base temperature and find the temperature dependence
of the "out of a-b plane" moment component to be in fairly good agreement with
calculation. Our results provide qualitative support for the double-q model and
thus in turn corroborate the explanation for the "magnetoelastic paradox"
offered by this model.Comment: 8 pages, 5 figures. Submitted to Phys. Rev.
Scanning Tunneling Microscopy in the superconductor LaSb2
We present very low temperature (0.15 K) scanning tunneling microscopy and
spectroscopy experiments in the layered superconductor LaSb. We obtain
topographic microscopy images with surfaces showing hexagonal and square atomic
size patterns, and observe in the tunneling conductance a superconducting gap.
We find well defined quasiparticle peaks located at a bias voltage comparable
to the weak coupling s-wave BCS expected gap value (0.17 meV). The amount of
states at the Fermi level is however large and the curves are significantly
broadened. We find T of 1.2 K by following the tunneling conductance with
temperature.Comment: 5 pages, 4 figure
Alternating magnetic anisotropy of Li(Li)N with = Mn, Fe, Co, and Ni
Substantial amounts of the transition metals Mn, Fe, Co, and Ni can be
substituted for Li in single crystalline Li(Li)N. Isothermal and
temperature-dependent magnetization measurements reveal local magnetic moments
with magnitudes significantly exceeding the spin-only value. The additional
contributions stem from unquenched orbital moments that lead to rare-earth-like
behavior of the magnetic properties. Accordingly, extremely large magnetic
anisotropies have been found. Most notably, the magnetic anisotropy alternates
as easy-plane easy-axis easy-plane
easy-axis when progressing from = Mn Fe Co
Ni. This behavior can be understood based on a perturbation
approach in an analytical, single-ion model. The calculated magnetic
anisotropies show a surprisingly good agreement with the experiment and capture
the basic features observed for the different transition metals.Comment: 5 pages, 3 figures, published as PRB Rapid Communication, Fig. 3
update
Local superconducting density of states of ErNi2B2C
We present local tunnelling microscopy and spectroscopy measurements at low
temperatures in single crystalline samples of the magnetic superconductor
ErNi2B2C. The electronic local density of states shows a striking departure
from s-wave BCS theory with a finite value at the Fermi level, which amounts to
half of the normal phase density of states.Comment: 9 pages, 3 figure
The biogeochemistry of microbial mats, stromatolites and the ancient biosphere
Stromatolites offer an unparalleled geologic record of early life, because they constitute the oldest and most abundant recognizable remains of microbial ecosystems. Microbial mats are living homologs of stromatolites; thus, the physiology of the microbiota as well as the processes which create those features of mats (e.g., biomarker organic compounds, elemental and stable isotopic compositions) which are preserved in the ancient record. Observations of the carbon isotopic composition (delta C-13) of stromatolites and microbial mats were made and are consistent with the hypothesis that atmospheric CO2 concentrations have declined by at least one to two orders of magnitude during the past 2.5 Ga. Whereas delta C-13 values of carbonate carbon average about 0 permil during both the early and mid-Proterozoic, the delta C-13 values of stromatolitic organic matter increase from an average of -35 between 2.0 and 2.6 Ga ago to an average of about -28 about 1.0 Ga ago. Modern microbial mats in hypersaline environments have delta C-13 values typically in the range of -5 to -9, relative to an inorganic bicarbonate source at 0 permil. Both microbial mats and pur cultures of cyanobacteria grown in waters in near equilibrium with current atmospheric CO2 levels exhibit minimal discrimination against C-13. In contrast, hot spring cyanobacterial mats or cyanobacterial cultures grown under higher CO2 levels exhibit substantially greater discrimination. If care is taken to compare modern mats with stromatolites from comparable environments, it might be possible to estimate ancient levels of atmospheric CO2. In modern microbial mats, a tight coupling exists between photosynthetic organic carbon production and subsequent carbon oxidation, mostly by sulfate reduction. The rate of one process fuels a high rate of the other, with much of the sulfate reduction occurring within the same depth interval as oxygenic photosynthesis. Other aspects of this study are presented
Strong Enhancement of the Critical Current at the Antiferromagnetic Transition in ErNi2B2C Single Crystals
We report on transport and magnetization measurements of the critical current
density Jc in ErNi2B2C single crystals that show strongly enhanced vortex
pinning at the Neel temperature TN and low applied fields. The height of the
observed Jc peak decreases with increasing magnetic field in clear contrast
with that of the peak effect found at the upper critical field. We also
performed the first angular transport measurements of Jc ever conducted on this
compound. They reveal the correlated nature of this pinning enhancement, which
we attribute to the formation of antiphase boundaries at TN.Comment: 3 figure
The equilibrium attained for the system hydrogen, bromine vapor and hydrogen bromide under the influence of the corona discharge
Hedgehog Spin-vortex Crystal Antiferromagnetic Quantum Criticality in CaK(Fe1-xNix)4As4 Revealed by NMR
Two ordering states, antiferromagnetism and nematicity, have been observed in
most iron-based superconductors (SCs). In contrast to those SCs, the newly
discovered SC CaK(FeNi)As exhibits an antiferromagnetic
(AFM) state, called hedgehog spin-vortex crystal structure, without nematic
order, providing the opportunity for the investigation into the relationship
between spin fluctuations and SC without any effects of nematic fluctuations.
Our As nuclear magnetic resonance studies on
CaK(FeNi)As (0 0.049) revealed that
CaKFeAs is located close to a hidden hedgehog SVC AFM quantum-critical
point (QCP). The magnetic QCP without nematicity in
CaK(FeNi)As highlights the close connection of spin
fluctuations and superconductivity in iron-based SCs. The advantage of
stoichiometric composition also makes CaKFeAs an ideal platform for
further detailed investigation of the relationship between magnetic QCP and
superconductivity in iron-based SCs without disorder effects.Comment: 6 pages, 5 figures, accepted for publication in Phys. Rev. Let
Magnetic and superconducting phase diagrams in ErNi2B2C
We present measurements of the superconducting upper critical field Hc2(T)
and the magnetic phase diagram of the superconductor ErNi2B2C made with a
scanning tunneling microscope (STM). The magnetic field was applied in the
basal plane of the tetragonal crystal structure. We have found large gapless
regions in the superconducting phase diagram of ErNi2B2C, extending between
different magnetic transitions. A close correlation between magnetic
transitions and Hc2(T) is found, showing that superconductivity is strongly
linked to magnetism.Comment: 5 pages, 4 figure
Strong interaction between electrons and collective excitations in multiband superconductor MgB2
We use a tunable laser ARPES to study the electronic properties of the
prototypical multiband BCS superconductor MgB2. Our data reveal a strong
renormalization of the dispersion (kink) at ~65 meV, which is caused by
coupling of electrons to the E2g phonon mode. In contrast to cuprates, the 65
meV kink in MgB2 does not change significantly across Tc. More interestingly,
we observe strong coupling to a second, lower energy collective mode at binding
energy of 10 meV. This excitation vanishes above Tc and is likely a signature
of the elusive Leggett mode.Comment: 10 fig
- …