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Determining the crystal growth routes of the unconventional superconductors Ba1-xKxFe2As2

Grant number: 17/18754-2
Support type:Scholarships in Brazil - Scientific Initiation
Effective date (Start): February 01, 2018
Effective date (End): January 31, 2019
Field of knowledge:Physical Sciences and Mathematics - Physics
Principal Investigator:Cris Adriano
Grantee:Ana Flávia Pimenta Bonatto
Home Institution: Instituto de Física Gleb Wataghin (IFGW). Universidade Estadual de Campinas (UNICAMP). Campinas , SP, Brazil

Abstract

In this project we propose the development of new systematic routes of growth of monocrystalline samples of new unconventional superconductors of Ba1-xKxFe2As2 grown by the Indium metallic flux technique. So then one can correlate the changes in sample preparation with the modification in final parameters of the sample. The main objectives are: 1) to obtain samples with high crystalline quality; 2) to obtain samples with a typical size of ~ 3 mm2 of surface; 3) determining the optimum thermal treatment of the K-doped BaFe2As2 samples; 4) correlate the amount of K placed in the sample preparation with the amount of K that is actually incorporated into the crystal, i.e. to determine x. It is important to mention that, to date, our group has not yet been able to systematically synthesize K-doped BaFe2As2 samples by the In metallic flux technique. We believe that these samples, when synthesized by In flux, will have the potential to present the critical superconducting temperature higher than that presented by samples grown by other techniques (Sn flux and/or self-flux). The methodology to grow Ba1-xKxFe2As2 samples by the metal flux method consists of: Stochiometric weighting of the Ba-K-Fe-As-In materials, after which they are placed inside an alumina crucible with elements having higher melting point temperatures below and the flux of In at the top. The crucible is then placed in a quartz tube with quartz wool down to prevent the tube from breaking during heating due to the different coefficients of thermal expansion of the quartz and crucible. Another portion of quartz wool is placed on top of the crucible to serve as a filter at the time of removal of flux excess. The quartz tube is closed in a vacuum (or inert atmosphere) and goes into the furnace for thermal treatment. After the crystal has growth, but still at temperatures above the melting point of the In-flux, the tube is quickly withdrawn from the furnace, inverted and placed in a centrifuge. In the centrifugation process, the excess flux is left, which is still liquid through the quartz wool, leaving the crystal in the crucible on the other side. The samples were analyzed by electrical resistivity (measured using four metallic tips), and magnetic susceptibility for basic characterization of the existence of magnetic and/or superconducting phases and verification of value of the transition temperatures. These measurements are important to identify whether the dopant K is actually being incorporated into the crystalline structure of the BaFe2As2 sample. This is verified by the change in the magnetic transition temperature that must begin to decrease and at the onset of a superconducting phase. That is why these measures should be part of this project, because the first step is to obtain the crystals (main focus of this project), but the continuation of the work requires knowing if and how much potassium is entering the crystalline structure of the compound. Electrical resistivity measurements will be performed using the four electrical contacts technique in the temperature range of 2 - 300 K in a Quantum Design's Commercial Physical Equipment Measurement System (PPMS). Magnetic susceptibility measurements will be performed on Quantum Design's Commercial Magnetic Properties Measurement System (MPMS) in the temperature range from 2-300 K in low magnetic fields (of the order of 200 Oe) in field cooling and zero field cooling cicles to identify the superconducting transition phase temperature. (AU)