Superconductivity under High Pressure in LaFeAsO

Abstract

Electrical resistivity measurements under high pressures up to 29 GPa were performed for oxypnictide compound LaFeAsO. We found a pressure-induced superconductivity in LaFeAsO. The maximum value of T c is 21 K at $12 GPa. The pressure dependence of the T c is similar to those of LaFeAsO 1Àx F x series reported previously.KEYWORDS: superconductivity, high pressure, electrical resistivity, oxypnictide, LaFeAsO DOI: 10.1143/JPSJ.77.113712Recent discovery of Fe-based oxypnictide superconductor LaFeAsO 1Àx F x with superconducting transition temperature T c of $26 K has attracted great interests in the field of condensed matter physics.1) The crystal structure of LaFeAsO 1Àx F x is a tetragonal ZrCuSiAs-type structure (P4=nmm), and is composed of a stack of insulating LaO and conducting FeAs layers. It is thought that conductive carriers are confined two dimensionally in the FeAs layers and the carrier concentration can be increased by the substitution of O 2À by F À . The parent compound LaFeAsO is metallic and undergoes a structural phase transition at $160 K and a magnetic phase transition at $140 K.2,3) The crystal structure changes from the tetragonal to orthorhombic, then the compound exhibits a spin density wave type antiferromagnetic (AFM) ordering. The structural distortion and the magnetic ordering are suppressed by the F substitution, resulting in the emergence of the superconductivity.Right after the discovery of the superconductivity in LaFeAsO 1Àx F x , it was reported that the T c of LaFeAsO 0:89 F 0:11 shows large pressure effect and increases to a maximum of 43 K at 4 GPa, suggesting that the lattice contraction is effective in the enhancement of the T c of oxypnictide compounds. 4) Actually, RFeAsO (R = rare earth) having smaller R atom than La atom shows the T c over 50 K. 5,6) It is known that external pressure can control carrier concentration in CuO 2 plane of high-T c cupper oxides. 7) In order to understand the effect of external pressure on T c in LaFeAsO 1Àx F x and clarify similarities and differences between the external pressure effect and the F substitution effect, we have performed electrical resistivity measurements under high pressures up to 29 GPa for the parent compound LaFeAsO. In this paper, we report a pressure-induced superconductivity in LaFeAsO with a maximum T c of 21 K.A polycrystalline sample was prepared by a solid state reaction method.1,2) The sample was confirmed to be mostly composed of a single phase of the ZrCuSiAs-type tetragonal structure with lattice constants of a ¼ 0:403268ð1Þ nm and c ¼ 0:874111ð4Þ nm at room temperature.2) Electrical resistivity measurements under high pressures up to 1.5 and 12 GPa were performed by a standard dc four-probe technique using a piston-cylinder type pressure cell and a cubic anvil cell, respectively. 8) A liquid pressure-transmitting medium (Fluorinert FC70 : FC77 ¼ 1 : 1) was used to maintain hydrostatic conditions. A diamond anvil cell (DAC) made of CuBe alloy was used for electrical resistivity measurements under high pressure...

Electronic structure of Fe-based superconductors

“…[92,93,94] Furthermore, at least in LaFeAsO and other oxy-pnictides, the SDW is preceded by a structural distortion that lowers the symmetry to that of the SDW state, but at a higher temperature than the long range magnetic orderings, [76] and importantly the largest signatures in transport and other properties are at the structural transition. The structural transition is explainable within DFT as a consequence of the magnetic ordering (with large moments), [29,51] and it has been argued that its occurrence reflects a fluctuating magnetic state closely related to the SDW but without long range static order.…”

Electrical resistivity of single crystals of LaFeAsO under applied pressure

“…16 Characterizing the evolution of SDW order under applied pressure provides important complementary information on the interplay of superconductivity and magnetism in ZrCuSiAs-type materials. The pressure-temperature phase diagram of the undoped parent compound LaFeAsO was previously examined by Okada et al 17 using polycrystalline samples. The resulting phase diagram appeared to indicate a smooth suppression of the SDW and a region of overlap between the SDW and superconductivity.…”

Effects of pressure on the electronic and structural properties of LaOFeAs

“…[19], pure LaOFeAs crystal becomes superconductor under pressure of ~ 12 GPa, with a superconducting temperature of ~ 20 K. This implies the disappearance of AFM order at temperatures lower than 20 K when applied pressure > 12 GPa. When a competition between the magnetic order and superconductivity takes place, DFT calculations overestimate the territory of magnetic order.…”