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Valence Bond Theory02:42

Valence Bond Theory

Coordination compounds and complexes exhibit different colors, geometries, and magnetic behavior, depending on the metal atom/ion and ligands from which they are composed. In an attempt to explain the bonding and structure of coordination complexes, Linus Pauling proposed the valence bond theory, or VBT, using the concepts of hybridization and the overlapping of the atomic orbitals. According to VBT, the central metal atom or ion (Lewis acid) hybridizes to provide empty orbitals of suitable...
Ionic Crystal Structures02:42

Ionic Crystal Structures

Ionic crystals consist of two or more different kinds of ions that usually have different sizes. The packing of these ions into a crystal structure is more complex than the packing of metal atoms that are the same size.
Most monatomic ions behave as charged spheres, and their attraction for ions of opposite charge is the same in every direction. Consequently, stable structures for ionic compounds result (1) when ions of one charge are surrounded by as many ions as possible of the opposite...
Crystal Field Theory - Tetrahedral and Square Planar Complexes02:46

Crystal Field Theory - Tetrahedral and Square Planar Complexes

Tetrahedral Complexes
Crystal field theory (CFT) is applicable to molecules in geometries other than octahedral. In octahedral complexes, the lobes of the dx2−y2 and dz2 orbitals point directly at the ligands. For tetrahedral complexes, the d orbitals remain in place, but with only four ligands located between the axes. None of the orbitals points directly at the tetrahedral ligands. However, the dx2−y2 and dz2 orbitals (along the Cartesian axes) overlap with the ligands less than the dxy,...
Imperfections in Crystal Structure: Stoichiometric Point Defects01:26

Imperfections in Crystal Structure: Stoichiometric Point Defects

Schottky defects arise when some lattice points in a crystal, such as those in NaCl, remain unoccupied, creating lattice vacancies without disturbing the overall electrical neutrality of the crystal. This defect is common in ionic crystals where the positive and negative ions are similar in size, as seen in sodium chloride and cesium chloride. The presence of Schottky defects enables the crystal to conduct electricity to a small extent through an ionic mechanism. Electric fields cause nearby...
Periodic Classification of the Elements04:00

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their electron configurations are added to the table, a periodic recurrence of similar electron configurations in the outer shells of these elements is observed. Because they are in the outer shells of an atom, valence electrons play the most important role in chemical reactions. The outer electrons have the highest energy of the electrons in an atom...
Colors and Magnetism03:02

Colors and Magnetism

Color in Coordination Complexes
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human eye.

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Visualizing Uniaxial-strain Manipulation of Antiferromagnetic Domains in Fe1+YTe Using a Spin-polarized Scanning Tunneling Microscope
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La estructura del nodo de la línea octeta del parámetro de orden superconductor en KFe2As22.

K Okazaki1, Y Ota, Y Kotani

  • 1Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan. okazaki@issp.u-tokyo.ac.jp

Science (New York, N.Y.)
|September 18, 2012
PubMed
Resumen

KFe(2)As(2) exhibe superconductividad a pesar de carecer de las superficies de Fermi de electrones. Su naturaleza nodal de onda s revela una estructura única de múltiples brechas con características nodiales selectivas de FS, lo que sugiere una simetría universal A (g) en el hierro-pnictides.

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Área de la Ciencia:

  • Física de la materia condensada Física de la materia condensada
  • Ciencia de los materiales Ciencia de los materiales.
  • La superconductividad es la superconductividad.

Sus antecedentes:

  • Los superconductores de pnictida de hierro generalmente dependen de las interacciones entre bandas entre las superficies de Fermi (FS) de agujero y electrón.
  • KFe(2)As(2) presenta una anomalía, posee FSs de agujero múltiple pero no FS de electrón, sin embargo, todavía muestra superconductividad.

Objetivo del estudio:

  • Para investigar la superconductividad no convencional en KFe(2)As(2).
  • Para dilucidar la estructura de la superficie de Fermi (FS) y la simetría de la brecha superconductora en KFe(2)As(2).

Principales métodos:

  • Espectroscopia de fotoemisión con resolución de ángulo láser de ultraalta resolución (ARPES).

Principales resultados:

  • KFe(2)As(2) es identificado como un superconductor nodal de onda s.
  • Se observó una estructura de múltiples brechas altamente inusual y selectiva para FS: una brecha sin nodos en la FS interna, una brecha de "nodo de línea de octeto" en la FS media y una brecha casi nula en la FS externa.
  • La estructura de la brecha observada sugiere la frustración entre las interacciones de emparejamiento competitivas en los FS de agujero, lo que lleva a una inversión de signo ocho veces mayor.

Conclusiones:

  • Los hallazgos desafían los modelos convencionales de superconductividad del pnictide de hierro.
  • Se propone la simetría superconductora universal A(1g) para los pnictidos de hierro, independientemente de las diversas funciones de hueco.
  • Este estudio proporciona información crítica sobre los complejos mecanismos de emparejamiento en los superconductores basados en hierro.