Point contact spectroscopy studies of the iron based superconductors

Point contact spectroscopy (PCS) is used to probe both the normal and superconducting phases of the iron based superconductors. It is shown that apart from probing superconductors by Andreev reflection, PCS is also a useful technique to study exotic electron matter in correlated materials. The point contact junctions are made by two separate techniques: needle-anvil PCS and soft PCS. $dI/dV$ measured in the superconducting phase is sensitive to the magnitude and symmetry of the superconducting order parameter. Andreev reflection spectra for the 122 and 111 families of the iron based superconductors is presented. The 122 crystals probed include electron doped \rm{Ba(Fe_1-xCo_x)₂As₂} ($\rm{x = 0.05, 0.055, 0.07, 0.08}$), hole doped \rm{Ba_0.8K_0.2Fe₂As₂}, and isoelectronic doped \rm{BaFe₂(As_1-xP_x)₂} ($\rm{x = 0.24, 0.43}$). The 111 crystals studied are electron doped \rm{NaFe_1-xCo_xAs} ($\rm{x = 0.02, 0.06}$). The Andreev spectra show clear features corresponding to multiple, nodeless superconducting gaps. The $dI/dV$ curves are fit to the independent two band BTK model assuming isotropic s-wave order parameters. The normal state spectra of certain iron based superconductors shows a conductance enhancement around zero bias above the structural transition temperature, T_S. Theoretical analysis showing that this enhancement is likely a consequence of orbital fluctuations is discussed. The nonsuperconducting compounds probed are \rm{AEFe₂As₂} ($\rm{AE=Ca,Sr,Ba}$) and \rm{Fe_1+yTe}. For $\rm{AE=Sr,Ba}$ orbital fluctuations are detected above T_S while for $\rm{AE=Ca}$ these fluctuations start below T_S. Co doping preserves the orbital fluctuations while K doping suppresses it. The fluctuations are only seen at those dopings and temperatures where for detwinned crystals, an in-plane resistive anisotropy is known to exist. The normal state spectra thus provides evidence that PCS is sensitive to orbital fluctuations in the iron based superconductors. A thorough analysis of \rm{Ba(Fe_1-xCo_x)₂As₂} is performed and a new region on the phase diagram is defined where PCS picks up orbital fluctuations. Diagnostics performed to ensure the quality of the PCS junctions are discussed in detail. Preliminary data for $dI/dV$ under applied magnetic field and compressive stress is also presented.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2014

Author dc:creator

• Arham, Hamood Zafir

Contributors dc:contributor

• Greene, Laura H.
• Mason, Nadya
• Dahmen, Karin A.
• DeMarco, Brian L.

• Superconductivity
• Point Contact Spectroscopy
• Iron Based Superconductors
• Andreev Reflection
• Orbital Fluctuations
• Nematicity
• Correlated Electron Matter

Statement dc:rights

• Copyright 2014 Hamood Zafir Arham

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/49451

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/49451