The oxide superconductors, particularly those recently discovered that are based on La2CuO4have a set of peculiarities that suggest a common, unique mechanism: they tend in every case to occur near a metal-insulator transition into an odd-electron insulator with peculiar magnetic properties. This insulating phase is proposed to be the long-sought “resonating-valence-bond” state or “quantum spin liquid” hypothesized in 1973. This insulating magnetic phase is favored by low spin, low dimensionality, and magnetic frustration. The preexisting magnetic singlet pairs of the insulating state become charged superconducting pairs when the insulator is doped sufficiently strongly. The mechanism for superconductivity is hence predominantly electronic and magnetic, although weak phonon interactions may favor the state. Many unusual properties are predicted, especially of the insulating state.

http://science.sciencemag.org/content/sci/235/4793/1196.full.pdf

The resonating valence bond state in La2CuO4 and superconductivity

The magnetoelectric effect--the induction of magnetization by means of an electric field and induction of polarization by means of a magnetic field--was first presumed to exist by Pierre Curie, and subsequently attracted a great deal of interest in the 1960s and 1970s (refs 2-4). More recently, related studies on magnetic ferroelectrics have signalled a revival of interest in this phenomenon. From a technological point of view, the mutual control of electric and magnetic properties is an attractive possibility, but the number of candidate materials is limited and the effects are typically too small to be useful in applications. Here we report the discovery of ferroelectricity in a perovskite manganite, TbMnO3, where the effect of spin frustration causes sinusoidal antiferromagnetic ordering. The modulated magnetic structure is accompanied by a magnetoelastically induced lattice modulation, and with the emergence of a spontaneous polarization. In the magnetic ferroelectric TbMnO3, we found gigantic magnetoelectric and magnetocapacitance effects, which can be attributed to switching of the electric polarization induced by magnetic fields. Frustrated spin systems therefore provide a new area to search for magnetoelectric media.

Magnetic control of ferroelectric polarization

Ferroelectric Metal–Organic Frameworks

Holography, coherent light amplification and optical phase conjugation with photorefractive materials

The review dwells on the problems of high-temperature superconductivity. Ideas encouraging the search for high-temperature superconductivity are elucidated. The way from cubical alloys, containing niobium, to laminated copper-containing oxides with the perovskite-type structure is shown. Properties of new laminated oxide superconductors are described.

Perovskite‐Type Oxides—the New Approach to High—Tc Superconductivity

The lattice dynamical properties have been studied by Raman spectroscopy in bismuth layer-structure oxides Bi4Ti3O12, and ABi4Ti4O15 (A=Ba, Ca, Pb, Sr). The rigid-layer modes were observed in all compounds in the range 57–62.5 cm-1. The lowest mode of Bi4Ti3O12 shows marked softening, and the square frequency of the lowest mode is linearly related to the square of the spontaneous polarization. As for CaBi4Ti4O15 and SrBi4Ti4O15 the underdamped soft modes were also clearly observed. In contrast, in BaBi4Ti4O15 only low-frequency overdamped modes have been observed. In PbBi4Ti4O15 the low-frequency spectra have a complicated structure and the softening of the underdamped modes was not observed clearly.

https://iopscience.iop.org/article/10.1143/JJAP.33.5559/pdf

Raman Scattering Study of Bismuth Layer-Structure Ferroelectrics

The magnetic susceptibility of ceramic samples in the metallic BaLaCuO system has been measured as a function of temperature. This system had earlier shown characteristic sharp drops in resistivity at low temperatures. It is found that, for small magnetic fields of less than 0.1 T, the samples become diamagnetic at somewhat lower temperatures than the resistivity drop. The highest-temperature diamagnetic shift occurs at (33 ± 2) K, and may be related to shielding currents at the onset of percolative superconductivity. The diamagnetic susceptibility can be suppressed with external fields of 1 to 5 T.

http://www.w2agz.com/Library/HTSC%20History/(1987)%20Bednorz,%20Takashige%20and%20Mueller,%20Europhysics%20Letters%203,%20379-385%20(1987).pdf

Susceptibility Measurements Support High-Tc Superconductivity in the Ba-La-Cu-O System

It has been found that Rb 2 ZnCl 4 shows phase transitions at about -81 and 29°C, respectively and is ferroelectric in the a direction below -81°C.

Ferroelectricity in Rb2ZnCl4

Tetramethylammonium tetrachlorozincate, {N(CH 3 ) 4 } 2 ZnCl 4 has been found to undergo successive phase transitions at about -112, -92, 3.3, 6.0 and 20°C and to show ferroelectricity along the a axis in the temperature range between 3.3 and 6.0°C.

Ferroelectricity in {N(CH3)4}2ZnCl4

It has been found that in NH 4 LiSO 4 another transition point 186.5°C exists in addition to the already known one 10°C and this substance is ferroelectric between these two transition points. Anomalies of dielectric constant at the transition points are very small as compared with those of proper ferroelectrics. The Curie-Weiss law holds within a narrow temperature range in the phase above the upper transition, where the Curie constant has a very small value of 5.6 K. Moreover the spontaneous polarization shows a curious increase with lowering temperature. Thus NH 4 LiSO 4 can be said to be one of improper ferroelectrics.

Masaaki Takashige

Papers

Raman Scattering Study of Bismuth Layer-Structure Ferroelectrics

Susceptibility Measurements Support High-Tc Superconductivity in the Ba-La-Cu-O System

Ferroelectricity in Rb2ZnCl4

Ferroelectricity in {N(CH3)4}2ZnCl4

Ferroelectricity in NH 4 IO 3