1,831 research outputs found
Theoretical study of impurity-induced magnetism in FeSe
Experimental evidence suggests that FeSe is close to a magnetic instability,
and recent scanning tunneling microscopy (STM) measurements on FeSe multilayer
films have revealed stripe order locally pinned near defect sites. Motivated by
these findings, we perform a theoretical study of locally induced magnetic
order near nonmagnetic impurities in a model relevant for FeSe. We find that
relatively weak repulsive impurities indeed are capable of generating
short-range magnetism, and explain the driving mechanism for the local order by
resonant eg-orbital states. In addition, we investigate the importance of
orbital-selective self-energy effects relevant for Hund's metals, and show how
the structure of the induced magnetization cloud gets modified by orbital
selectivity. Finally, we make concrete connection to STM measurements of
iron-based superconductors by symmetry arguments of the induced magnetic order,
and the basic properties of the Fe Wannier functions relevant for tunneling
spectroscopy.Comment: 10 pages, 4 figure
Temperature-dependent Raman scattering of DyScO3 and GdScO3 single crystals
We report a temperature-dependent Raman scattering investigation of DyScO3
and GdScO3 single crystals from room temperature up to 1200 {\deg}C. With
increasing temperature, all modes decrease monotonously in wavenumber without
anomaly, which attests the absence of a structural phase transition. The high
temperature spectral signature and extrapolation of band positions to higher
temperatures suggest a decreasing orthorhombic distortion towards the ideal
cubic structure. Our study indicates that this orthorhombic-to-cubic phase
transition is close to or higher than the melting point of both rare-earth
scandates (\approx 2100 {\deg}C), which might exclude the possibility of the
experimental observation of such a phase transition before melting. The
temperature-dependent shift of Raman phonons is also discussed in the context
of thermal expansion
Rules and mechanisms governing octahedral tilts in perovskites under pressure
The rotation of octahedra (octahedral tilting) is common in ABO3 perovskites
and relevant to many physical phenomena, ranging from electronic and magnetic
properties, metal-insulator transitions to improper ferroelectricity.
Hydrostatic pressure is an efficient way to tune and control octahedral
tiltings. However, the pressure behavior of such tiltings can dramatically
differ from one material to another, with the origins of such differences
remaining controversial. In this work, we discover several new mechanisms and
formulate a set of simple rules that allow to understand how pressure affects
oxygen octahedral tiltings, via the use and analysis of first-principles
results for a variety of compounds. Besides the known A-O interactions, we
reveal that the interactions between specific B-ions and oxygen ions contribute
to the tilting instability. We explain the previously reported trend that the
derivative of the oxygen octahedral tilting with respect to pressure (dR/dP)
usually decreases with both the tolerance factor and the ionization state of
the A-ion, by illustrating the key role of A-O interactions and their change
under pressure. Furthermore, three new mechanisms/rules are discovered. We
further predict that the polarization associated with the so-called hybrid
improper ferroelectricity could be manipulated by hydrostatic pressure, by
indirectly controlling the amplitude of octahedral rotations.Comment: Submitted to Phys. Re
Robustness of Quasiparticle Interference Test for Sign-changing Gaps in Multiband Superconductors
Recently, a test for a sign-changing gap function in a candidate multiband
unconventional superconductor involving quasiparticle interference data was
proposed. The test was based on the antisymmetric, Fourier transformed
conductance maps integrated over a range of momenta corresponding to
interband processes, which was argued to display a particular resonant form,
provided the gaps changed sign between the Fermi surface sheets connected by
. The calculation was performed for a single impurity, however, raising
the question of how robust this measure is as a test of sign-changing pairing
in a realistic system with many impurities. Here we reproduce the results of
the previous work within a model with two distinct Fermi surface sheets, and
show explicitly that the previous result, while exact for a single nonmagnetic
scatterer and also in the limit of a dense set of random impurities, can be
difficult to implement for a few dilute impurities. In this case, however,
appropriate isolation of a single impurity is sufficient to recover the
expected result, allowing a robust statement about the gap signs to be made.Comment: 9 pages, 12 figure
An algorithmic approach to the existence of ideal objects in commutative algebra
The existence of ideal objects, such as maximal ideals in nonzero rings,
plays a crucial role in commutative algebra. These are typically justified
using Zorn's lemma, and thus pose a challenge from a computational point of
view. Giving a constructive meaning to ideal objects is a problem which dates
back to Hilbert's program, and today is still a central theme in the area of
dynamical algebra, which focuses on the elimination of ideal objects via
syntactic methods. In this paper, we take an alternative approach based on
Kreisel's no counterexample interpretation and sequential algorithms. We first
give a computational interpretation to an abstract maximality principle in the
countable setting via an intuitive, state based algorithm. We then carry out a
concrete case study, in which we give an algorithmic account of the result that
in any commutative ring, the intersection of all prime ideals is contained in
its nilradical
Existential witness extraction in classical realizability and via a negative translation
We show how to extract existential witnesses from classical proofs using
Krivine's classical realizability---where classical proofs are interpreted as
lambda-terms with the call/cc control operator. We first recall the basic
framework of classical realizability (in classical second-order arithmetic) and
show how to extend it with primitive numerals for faster computations. Then we
show how to perform witness extraction in this framework, by discussing several
techniques depending on the shape of the existential formula. In particular, we
show that in the Sigma01-case, Krivine's witness extraction method reduces to
Friedman's through a well-suited negative translation to intuitionistic
second-order arithmetic. Finally we discuss the advantages of using call/cc
rather than a negative translation, especially from the point of view of an
implementation.Comment: 52 pages. Accepted in Logical Methods for Computer Science (LMCS),
201
Mast cell leukemia with prolonged survival on PKC412/midostaurin
Mast cell leukemia (MCL) is a rare and aggressive form of systemic mastocytosis. There are approximately 50 reported cases since 1950s. MCL is refractory to cytoreduction chemotherapy and the average survival is only six months. We report a MCL case in a 71 year-old woman with high tumor load at the initial presentation in 2005, who did not respond to either interleukin-2 or dasatinib therapy. After enrolled in a clinical trial of PKC412 (or Midostaurin) with a daily dose of 100 mg, the patient responded well to PKC412 and became transfusion independent in three months. Since then, her disease had been stably controlled. This is the first report of a high-tumor-load MCL case which achieved prolonged survival (101 months) by PKC 412. The 101-month overall survival is the longest among reported MCL cases in the English literature
On a class of invariant coframe operators with application to gravity
Let a differential 4D-manifold with a smooth coframe field be given. Consider
the operators on it that are linear in the second order derivatives or
quadratic in the first order derivatives of the coframe, both with coefficients
that depend on the coframe variables. The paper exhibits the class of operators
that are invariant under a general change of coordinates, and, also, invariant
under the global SO(1,3)-transformation of the coframe. A general class of
field equations is constructed. We display two subclasses in it. The subclass
of field equations that are derivable from action principles by free variations
and the subclass of field equations for which spherical-symmetric solutions,
Minkowskian at infinity exist. Then, for the spherical-symmetric solutions, the
resulting metric is computed. Invoking the Geodesic Postulate, we find all the
equations that are experimentally (by the 3 classical tests) indistinguishable
from Einstein field equations. This family includes, of course, also Einstein
equations. Moreover, it is shown, explicitly, how to exhibit it. The basic tool
employed in the paper is an invariant formulation reminiscent of Cartan's
structural equations. The article sheds light on the possibilities and
limitations of the coframe gravity. It may also serve as a general procedure to
derive covariant field equations
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