130 research outputs found
Low-temperature phase diagram of Fe1+yTe
We used low-temperature synchrotron x-ray diffraction to investigate the
structural phase transitions of Fe1+yTe in the vicinity of a tricitical point
in the phase diagram. Detailed analysis of the powder diffraction patterns and
temperature dependence of the peak-widths in Fe1+yTe showed that two-step
structural and magnetic phase transitions occur within the compositional range
0.11 0.13. The phase transitions are sluggish indicating a
strong competition between the orthorhombic and the monoclinic phases. We
combine high-resolution diffraction experiments with specific heat,
resistivity, and magnetization measurements and present a revised
temperature-composition phase diagram for Fe1+yTe.Comment: 10 pages, 14 figure
Pressure-induced phase transitions and high-pressure tetragonal phase of Fe1.08Te
We report the effects of hydrostatic pressure on the temperature-induced
phase transitions in Fe1.08Te in the pressure range 0-3 GPa using synchrotron
powder x-ray diffraction (XRD). The results reveal a plethora of phase
transitions. At ambient pressure, Fe1.08Te undergoes simultaneous first-order
structural symmetry-breaking and magnetic phase transitions, namely from the
paramagnetic tetragonal (P4/nmm) to the antiferromagnetic monoclinic (P2_1/m)
phase. We show that, at a pressure of 1.33 GPa, the low temperature structure
adopts an orthorhombic symmetry. More importantly, for pressures of 2.29 GPa
and higher, a symmetry-conserving tetragonal-tetragonal phase transition has
been identified from a change in the c/a ratio of the lattice parameters. The
succession of different pressure and temperature-induced structural and
magnetic phases indicates the presence of strong magneto-elastic coupling
effects in this material.Comment: 11 page
Pressure-induced ferromagnetism due to an anisotropic electronic topological transition in Fe1.08Te
A rapid and anisotropic modification of the Fermi-surface shape can be
associated with abrupt changes in crystalline lattice geometry or in the
magnetic state of a material. In this study we show that such an electronic
topological transition is at the basis of the formation of an unusual
pressure-induced tetragonal ferromagnetic phase in FeTe. Around 2 GPa,
the orthorhombic and incommensurate antiferromagnetic ground-state of
FeTe is transformed upon increasing pressure into a tetragonal
ferromagnetic state via a conventional first-order transition. On the other
hand, an isostructural transition takes place from the paramagnetic
high-temperature state into the ferromagnetic phase as a rare case of a `type
0' transformation with anisotropic properties. Electronic-structure
calculations in combination with electrical resistivity, magnetization, and
x-ray diffraction experiments show that the electronic system of FeTe
is instable with respect to profound topological transitions that can drive
fundamental changes of the lattice anisotropy and the associated magnetic
order.Comment: 7 pages, 4 figur
Nematic state of the FeSe superconductor
We study the crystal structure of the tetragonal iron selenide FeSe and its nematic phase transition to the low-temperature orthorhombic structure using synchrotron x-ray and neutron scattering analyzed in both real space and reciprocal space. We show that in the local structure the orthorhombic distortion associated with the electronically driven nematic order is more pronounced at short length scales. It also survives to temperatures above 90 K, where reciprocal-space analysis suggests tetragonal symmetry. Additionally, the real-space pair distribution function analysis of the synchrotron x-ray diffraction data reveals a tiny broadening of the peaks corresponding to the nearest Fe-Fe, nearest Fe-Se, and next-nearest Fe-Se bond distances as well as the tetrahedral torsion angles at a short length scale of 20 Å. This broadening appears below 20 K and is attributed to a pseudogap. However, we did not observe any further reduction in local symmetry below orthorhombic down to 3 K. Our results suggest that the superconducting gap anisotropy in FeSe is not associated with any symmetry-lowering short-range structural correlations
Nematic state of the FeSe superconductor
We study the crystal structure of the tetragonal iron selenide FeSe and its
nematic phase transition to the low-temperature orthorhombic structure using
synchrotron x-ray and neutron scattering analyzed in both real and reciprocal
space. We show that in the local structure the orthorhombic distortion
associated with the electronically driven nematic order is more pronounced at
short length scales. It also survives up to temperatures above 90 K where
reciprocal-space analysis suggests tetragonal symmetry. Additionally, the
real-space pair distribution function analysis of the synchrotron x-ray
diffraction data reveals a tiny broadening of the peaks corresponding to the
nearest Fe-Fe, nearest Fe-Se, and the next-nearest Fe-Se bond distances as well
as the tetrahedral torsion angles at a short length scale of 20 angstr\"om.
This broadening appears below 20 K and is attributed to a pseudogap. However,
we did not observe any further reduction in local symmetry below orthorhombic
down to 3 K. Our results suggest that the superconducting gap anisotropy in
FeSe is not associated with any symmetry-lowering short-range structural
correlations.Comment: 9 pages, 6 figure
Leaving the ISCO: the inner edge of a black-hole accretion disk at various luminosities
The "radiation inner edge" of an accretion disk is defined as the inner
boundary of the region from which most of the luminosity emerges. Similarly,
the "reflection edge" is the smallest radius capable of producing a significant
X-ray reflection of the fluorescent iron line. For black hole accretion disks
with very sub-Eddington luminosities these and all other "inner edges" locate
at ISCO. Thus, in this case, one may rightly consider ISCO as the unique inner
edge of the black hole accretion disk. However, even for moderate luminosities,
there is no such unique inner edge as differently defined edges locate at
different places. Several of them are significantly closer to the black hole
than ISCO. The differences grow with the increasing luminosity. For nearly
Eddington luminosities, they are so huge that the notion of the inner edge
losses all practical significance.Comment: 12 pages, 15 figures, submitted to A&
How large should whales be?
The evolution and distribution of species body sizes for terrestrial mammals
is well-explained by a macroevolutionary tradeoff between short-term selective
advantages and long-term extinction risks from increased species body size,
unfolding above the 2g minimum size induced by thermoregulation in air. Here,
we consider whether this same tradeoff, formalized as a constrained
convection-reaction-diffusion system, can also explain the sizes of fully
aquatic mammals, which have not previously been considered. By replacing the
terrestrial minimum with a pelagic one, at roughly 7000g, the terrestrial
mammal tradeoff model accurately predicts, with no tunable parameters, the
observed body masses of all extant cetacean species, including the 175,000,000g
Blue Whale. This strong agreement between theory and data suggests that a
universal macroevolutionary tradeoff governs body size evolution for all
mammals, regardless of their habitat. The dramatic sizes of cetaceans can thus
be attributed mainly to the increased convective heat loss is water, which
shifts the species size distribution upward and pushes its right tail into
ranges inaccessible to terrestrial mammals. Under this macroevolutionary
tradeoff, the largest expected species occurs where the rate at which
smaller-bodied species move up into large-bodied niches approximately equals
the rate at which extinction removes them.Comment: 7 pages, 3 figures, 2 data table
<i>Spitzer</i> microlens measurement of a massive remnant in a well-separated binary
We report the detection and mass measurement of a binary lens OGLE-2015-BLG-1285La,b, with the more massive component having M1 > 1.35 M⊙ (80% probability). A main-sequence star in this mass range is ruled out by limits on blue light, meaning that a primary in this mass range must be a neutron star (NS) or black hole (BH). The system has a projected separation r⊥ = 6.1 ± 0.4 AU and lies in the Galactic bulge. These measurements are based on the "microlens parallax" effect, i.e., comparing the microlensing light curve as seen from Spitzer, which lay at 1.25 AU projected from Earth, to the light curves from four ground-based surveys, three in the optical and one in the near-infrared. Future adaptive optics imaging of the companion by 30 m class telescopes will yield a much more accurate measurement of the primary mass. This discovery both opens the path and defines the challenges to detecting and characterizing BHs and NSs in wide binaries, with either dark or luminous companions. In particular, we discuss lessons that can be applied to future Spitzer and Kepler K2 microlensing parallax observations
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