1,264 research outputs found
Electronic nematic susceptibility of iron-based superconductors
We review our recent experimental results on the electronic nematic phase in
electron- and hole-doped BaFeAs and FeSe. The nematic susceptibility is
extracted from shear-modulus data (obtained using a three-point-bending method
in a capacitance dilatometer) using Landau theory and is compared to the
nematic susceptibility obtained from elastoresistivity and Raman data. FeSe is
particularly interesting in this context, because of a large nematic, i.e., a
structurally distorted but paramagnetic, region in its phase diagram. Scaling
of the nematic susceptibility with the spin lattice relaxation rate from NMR,
as predicted by the spin-nematic theory, is found in both electron- and
hole-doped BaFeAs, but not in FeSe. The intricate relationship of the
nematic susceptibility to spin and orbital degrees of freedom is discussed.Comment: Invited review article for a special issue on Fe-based
superconductors in Comptes Rendus Physiqu
Nematicity, magnetism and superconductivity in FeSe
Iron-based superconductors are well known for their complex interplay between
structure, magnetism and superconductivity. FeSe offers a particularly
fascinating example. This material has been intensely discussed because of its
extended nematic phase, whose relationship with magnetism is not obvious.
Superconductivity in FeSe is highly tunable, with the superconducting
transition temperature, , ranging from 8 K in bulk single
crystals at ambient pressure to almost 40 K under pressure or in intercalated
systems, and to even higher temperatures in thin films. In this topical review,
we present an overview of nematicity, magnetism and superconductivity, and
discuss the interplay of these phases in FeSe. We focus on bulk FeSe and the
effects of physical pressure and chemical substitutions as tuning parameters.
The experimental results are discussed in the context of the well-studied
iron-pnictide superconductors and interpretations from theoretical approaches
are presented.Comment: Topical Review submitted to Journal of Physics: Condensed Matte
Bounds on the basic physical parameters for anisotropic compact general relativistic objects
We derive upper and lower limits for the basic physical parameters
(mass-radius ratio, anisotropy, redshift and total energy) for arbitrary
anisotropic general relativistic matter distributions in the presence of a
cosmological constant. The values of these quantities are strongly dependent on
the value of the anisotropy parameter (the difference between the tangential
and radial pressure) at the surface of the star. In the presence of the
cosmological constant, a minimum mass configuration with given anisotropy does
exist. Anisotropic compact stellar type objects can be much more compact than
the isotropic ones, and their radii may be close to their corresponding
Schwarzschild radii. Upper bounds for the anisotropy parameter are also
obtained from the analysis of the curvature invariants. General restrictions
for the redshift and the total energy (including the gravitational
contribution) for anisotropic stars are obtained in terms of the anisotropy
parameter. Values of the surface redshift parameter greater than two could be
the main observational signature for anisotropic stellar type objects.Comment: 18 pages, no figures, accepted for publication in CQ
On the relation between mass of pion, fundamental physical constants and cosmological parameters
In this article we reconsider the old mysterious relation, advocated by Dirac
and Weinberg, between the mass of the pion, the fundamental physical constants,
and the Hubble parameter. By introducing the cosmological density parameters,
we show how the corresponding equation may be written in a form that is
invariant with respect to the expansion of the Universe and without invoking a
varying gravitational "constant", as was originaly proposed by Dirac. It is
suggest that, through this relation, Nature gives a hint that virtual pions
dominante the "content" of the quantum vacuum
NMR evidence for static local nematicity and its cooperative interplay with low-energy magnetic fluctuations in FeSe under pressure
We present Se-NMR measurements on single-crystalline FeSe under
pressures up to 2 GPa. Based on the observation of the splitting and broadening
of the NMR spectrum due to structural twin domains, we discovered that static,
local nematic ordering exists well above the bulk nematic ordering temperature,
. The static, local nematic order and the low-energy stripe-type
antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation
rate measurements, are both insensitive to pressure application. These NMR
results provide clear evidence for the microscopic cooperation between
magnetism and local nematicity in FeSe.Comment: 5 pages, 5 figures, accepted for publication in Phys. Rev. B rapid
communicatio
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