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Organic Superconductors
Published in David A. Cardwell, David C. Larbalestier, I. Braginski Aleksander, Handbook of Superconductivity, 2023
Many organic SCs have been obtained through the suppression of several types of phase transitions other than the Peierls, SDW, and AF transitions. The dimensionalities of the electronic band structure, spin lattice, and elasticity of the crystal lattice are important factors that induce or suppress the phase transitions commonly observed in organic conductors. The Peierls and SDW transitions are known as metal–insulator transitions (TMI: metal−insulator transition temperature). The order–disorder (OD) transition of the anion molecule, which is classified within the Peierls transition, and charge ordering (CO; charge localization, charge disproportionation) are sometimes manifested as a metal−insulator transition. These transitions are observed in TMTSF and TMTTF systems. The Mott transition is an insulator−metal transition, and some ET Mott insulators exhibit a superconducting state next to the Mott insulating state. Regarding insulator−insulator transitions, the AF and spin-Peierls transitions derived from a Mott insulator are common in organic conductors. Some ET Mott insulators exhibit the phase sequence such as Mott insulator → AF insulator → SC, Mott insulator → spin-Peierls insulator → AF insulator → SC, or Mott insulator → QSL insulator → SC depending on external conditions.
Electronic Properties of Perovskite Oxides
Published in Gibin George, Sivasankara Rao Ede, Zhiping Luo, Fundamentals of Perovskite Oxides, 2020
Gibin George, Sivasankara Rao Ede, Zhiping Luo
The smaller the ionic radius of the A-site, the smaller the electronic bandwidth, W. The double-exchange interaction in Ln1−xAxMnO3 is the resultant of itinerancy of the doped holes, which favors a reduction in W, so as the charge-ordered state. At the same time, the ferromagnetic state is destabilized. When the charge-ordering occurs, the resistivity increases abruptly by several orders of magnitude, depending on the crystallographic and/or magnetic structures. In accordance with the electronic change, the ferromagnetic to antiferromagnetic transition occurs simultaneously. The ferromagnetic transition temperature Tc changes steeply as a function of the tolerance factor, and the competition between the ferromagnetic double-exchange and antiferromagnetic charge order interactions is sensitive to W (Kuwahara et al. 1997).
Elastic Control of Magnetic Order at Oxide Interfaces
Published in Tamalika Banerjee, Oxide Spintronics, 2019
Nickelates receive strong interest because they show a metal–insulator transition driven by strong electron correlations, which is sensitive to dimensionality and elastic strain [31, 43, 44]. LaNiO3 is metallic and paramagnetic down to low temperatures, whereas rare earth metals at the A site result in an insulating antiferromagnetic ground state and a metal–insulator transition at a temperature TMI. The effect of the rare earth metal on B site magnetic order is essentially an elastic one due to its lower ionic radius. The Néel temperature TN and TMI can differ by more than 100 K with TN ≤ TMI [31, 32, 44]. Metallic phases of ANiO3 seem to show no magnetic long-range order. LaNiO3 is rhombohedral with the rotation pattern a−a−a− and pseudocubic lattice parameter of 3.84 Å. Ni3+ ions have a nominal electronic 3d7 configuration with one electron in the eg level and fully occupied t2g levels (S = ½). Ni can take an electron configuration near 3d8 if a ligand hole at an adjacent oxygen ion is formed. In the insulating state, Ni ions show charge disproportionation associated with charge ordering, i.e., two different valence states of Ni ions exist and are regularly ordered [44, 45].
Strongly correlated oxides for energy harvesting
Published in Science and Technology of Advanced Materials, 2018
Jobu Matsuno, Jun Fujioka, Tetsuji Okuda, Kazunori Ueno, Takashi Mizokawa, Takuro Katsufuji
Let us first discuss the orbital and charge degrees of freedom itself before discussing their fluctuation. As discussed above, there is degeneracy of states for the d orbitals in transition metals, for example, three-fold degeneracy in the states and the two-fold degeneracy in the states. If there is an electron in such degenerate states, the electron can choose which states to occupy because energetically they are equivalent, and this is called orbital degree of freedom. The interaction between the orbital degrees of freedom at the neighboring sites arises from the second order perturbation energy given by the transfer integral of electrons t, the on-site Coulomb repulsion U, and the Hund coupling (Kugel-Khomskii interaction [18]).With this interaction, the orbital degree of freedom orders at low temperatures (orbital ordering). Furthermore, if the average valence of the transition metal is not integer, each transition metal can take one of the two (or more) different valences, and this is called charge degree of freedom. This can also order at low temperatures (charge ordering), where the d electrons of the transition metals are periodically aligned [19,20]. In some compounds, the orbital ordering and the charge ordering occur simultaneously [21].
Suppression of charge ordering and thermal hysteresis of electronic transport and magnetisation in La0.5Ca0.5Mn1−xNixO3
Published in Philosophical Magazine, 2018
Ankam Bhaskar, M.-S. Huang, Chia-Jyi Liu
The charge ordering plays a crucial role in manganites. However, either magnetic field or partial replacement of transition metal for Mn could melt or suppress the charge ordering state [8]. The charge ordering is a long-range order phenomenon where electrons are arranged periodically in a crystal lattice, especially the electrostatic interaction between Mn3+ and Mn4+ ions. Chen et al. [9] reported that an incommensurate–commensurate charge-ordering temperature occurs at ~149 and ~200 K for the La0.5Ca0.5MnO3 upon cooling and warming, respectively. Cox et al. [10,11] reported that the La0.5Ca0.5MnO3 exhibits a charge density waves (CDW) state. The charge density wave is characterised by a complex order parameter [12],