Crystal growth, structural studies and superconducting properties of beta-pyrochlore KOs2O6

TL;DR

This study reports the growth, structural analysis, and superconducting properties of KOs2O6 crystals, revealing non-centrosymmetric structure and high upper critical fields indicative of potential unconventional superconductivity.

Contribution

The paper provides detailed structural characterization and superconducting measurements of KOs2O6, highlighting its non-centrosymmetric structure and high critical fields, which are novel insights in beta-pyrochlore superconductors.

Findings

Crystals show metallic conductivity and superconductivity at 9.65 K.

Upper critical field estimated at 230 kOe, close to Pauli limit.

Structural analysis reveals non-centrosymmetric F-43m symmetry.

Abstract

Single crystals of KOs2O6 have been grown in a sealed quartz ampoule. Detailed single crystal X-ray diffraction studies at room temperature show Bragg peaks that violate Fd-3m symmetry. With a comparative structure refinement the structure is identified as non-centrosymmetric (F-43m). Compared to the ideal beta-pyrochlore lattice (Fd-3m), both Os tetrahedral and O octahedral network exhibit breathing mode like volume changes accompanied by strong anisotropic character of the K channels. The crystals show metallic conductivity and a sharp transition to the superconducting state at Tc = 9.65 K. Superconducting properties have been investigated by magnetization measurements performed in a temperature range from 2 to 12 K and in magnetic fields from 0 to 60 kOe. The temperature dependence of the upper critical field Hc2(T) has been determined and the initial slope (dHc2/dT)Tc = -33.3 kOe/K has been obtained near Tc. The upper critical field at zero temperature was estimated to be Hc2(0) \cong 230 kOe, which is a value close to the Pauli paramagnetic limiting field Hp(0)\cong 250 kOe. Then, the Ginzburg-Landau (GL) coherence length xi GL(0) \approx 3.8 nm was calculated, and the Maki parameter alpha \approx \sqrt 2 was obtained, suggesting the possibility that KOs2O6 might behave unconventionally at low temperatures and high magnetic fields.