631 research outputs found
Why of (CaFeAs)PtAs is twice as high as (CaFePtAs)PtAs
Recently discovered (CaFePtAs)PtAs and
(CaFeAs)PtAs superconductors are very similar materials
having the same elemental composition and structurally similar superconducting
FeAs slabs. Yet the maximal critical temperature achieved by changing Pt
concentration is approximately twice higher in the latter. Using angle-resolved
photoemission spectroscopy(ARPES) we compare the electronic structure of their
optimally doped compounds and find drastic differences. Our results highlight
the sensitivity of critical temperature to the details of fermiology and point
to the decisive role of band-edge singularities in the mechanism of high-
superconductivity
ARPES on HTSC: simplicity vs. complexity
A notable role in understanding of microscopic electronic properties of high
temperature superconductors (HTSC) belongs to angle resolved photoemission
spectroscopy (ARPES). This technique supplies a direct window into reciprocal
space of solids: the momentum-energy space where quasiparticles (the electrons
dressed in clouds of interactions) dwell. Any interaction in the electronic
system, e.g. superconducting pairing, leads to modification of the
quasi-particle spectrum--to redistribution of the spectral weight over the
momentum-energy space probed by ARPES. A continued development of the technique
had an effect that the picture seen through the ARPES window became clearer and
sharper until the complexity of the electronic band structure of the cuprates
had been resolved. Now, in an optimal for superconductivity doping range, the
cuprates much resemble a normal metal with well predicted electronic structure,
though with rather strong electron-electron interaction. This principal
disentanglement of the complex physics from complex structure reduced the
mystery of HTSC to a tangible problem of interaction responsible for
quasi-particle formation. Here we present a short overview of resent ARPES
results, which, we believe, denote a way to resolve the HTSC puzzle.Comment: A review written for a special issue of FN
TaIrTe4 a ternary Type-II Weyl semi-metal
In metallic condensed matter systems two different types of Weyl fermions can
in principle emerge, with either a vanishing (type-I) or with a finite
(type-II) density of states at the Weyl node energy. So far only WTe2 and MoTe2
were predicted to be type-II Weyl semi-metals. Here we identify TaIrTe4 as a
third member of this family of topological semi-metals. TaIrTe4 has the
attractive feature that it hosts only four well-separated Weyl points, the
minimum imposed by symmetry. Moreover, the resulting topological surface states
- Fermi arcs connecting Weyl nodes of opposite chirality - extend to about 1/3
of the surface Brillouin zone. This large momentum-space separation is very
favorable for detecting the Fermi arcs spectroscopically and in transport
experiments
Anomalously enhanced photoemission from the Dirac point and symmetry of the self-energy variations for the surface states in Bi2Se3
Accurate analysis of the photoemission intensity from the surface states of
Bi2Se3 reveals two unusual features: spectral line asymmetry and anomalously
enhanced photoemission from the Dirac point. The former indicates a certain
symmetry of a scattering process, which results in strongly k\omega-dependent
contribution to the imaginary part of the self-energy that changes sign while
crossing both the dispersion curves and the energy of the Dirac point. The
latter is hard to describe by one particle spectral function while a final
state interference seems to be plausible explanation
A description of a system of programs for mathematically processing on unified series (YeS) computers photographic images of the Earth taken from spacecraft
A description of a batch of programs for the YeS-1040 computer combined into an automated system for processing photo (and video) images of the Earth's surface, taken from spacecraft, is presented. Individual programs with the detailed discussion of the algorithmic and programmatic facilities needed by the user are presented. The basic principles for assembling the system, and the control programs are included. The exchange format within whose framework the cataloging of any programs recommended for the system of processing will be activated in the future is displayed
High-temperature superconductivity from fine-tuning of Fermi-surface singularities in iron oxypnictides
In the family of the iron-based superconductors, the FeAsO-type compounds
(with being a rare-earth metal) exhibit the highest bulk superconducting
transition temperatures () up to and thus hold
the key to the elusive pairing mechanism. Recently, it has been demonstrated
that the intrinsic electronic structure of SmFeCoAsO
() is highly nontrivial and consists of multiple
band-edge singularities in close proximity to the Fermi level. However, it
remains unclear whether these singularities are generic to the FeAsO-type
materials and if so, whether their exact topology is responsible for the
aforementioned record . In this work, we use angle-resolved
photoemission spectroscopy (ARPES) to investigate the inherent electronic
structure of the NdFeAsOF compound with a twice higher
. We find a similarly singular Fermi surface and
further demonstrate that the dramatic enhancement of superconductivity in this
compound correlates closely with the fine-tuning of one of the band-edge
singularities to within a fraction of the superconducting energy gap
below the Fermi level. Our results provide compelling evidence that the
band-structure singularities near the Fermi level in the iron-based
superconductors must be explicitly accounted for in any attempt to understand
the mechanism of superconducting pairing in these materials.Comment: Open access article available online at
http://www.nature.com/articles/srep1827
Interaction-induced singular Fermi surface in a high-temperature oxypnictide superconductor
In the family of iron-based superconductors, LaFeAsO-type materials possess
the simplest electronic structure due to their pronounced two-dimensionality.
And yet they host superconductivity with the highest transition temperature
Tc=55K. Early theoretical predictions of their electronic structure revealed
multiple large circular portions of the Fermi surface with a very good
geometrical overlap (nesting), believed to enhance the pairing interaction and
thus superconductivity. The prevalence of such large circular features in the
Fermi surface has since been associated with many other iron-based compounds
and has grown to be generally accepted in the field. In this work we show that
a prototypical compound of the 1111-type, SmFe0.92Co0.08AsO, is at odds with
this description and possesses a distinctly different Fermi surface, which
consists of two singular constructs formed by the edges of several bands,
pulled to the Fermi level from the depths of the theoretically predicted band
structure by strong electronic interactions. Such singularities dramatically
affect the low-energy electronic properties of the material, including
superconductivity. We further argue that occurrence of these singularities
correlates with the maximum superconducting transition temperature attainable
in each material class over the entire family of iron-based superconductors.Comment: Open access article available online at
http://www.nature.com/srep/2015/150521/srep10392/full/srep10392.htm
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