{"id":{"repo_id":"uiuc","oai_identifier":"oai:www.ideals.illinois.edu:2142/49451"},"canonical_url":"https://search.dev.ndltd.org/etd/uiuc/oai:www.ideals.illinois.edu:2142/49451","repository":{"repo_id":"uiuc","name":"University of Illinois - Urbana-Champaign","base_url":"https://www.ideals.illinois.edu/oai-pmh"},"display":{"title":"Point contact spectroscopy studies of the iron based superconductors","abstract":"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 $\\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ ($\\rm{x = 0.05, 0.055, 0.07, 0.08}$), hole doped $\\rm{Ba_{0.8}K_{0.2}Fe_2As_2}$, and isoelectronic doped $\\rm{BaFe_2(As_{1-x}P_{x})_2}$ ($\\rm{x = 0.24, 0.43}$). The 111 crystals studied are electron doped $\\rm{NaFe_{1-x}Co_xAs}$ ($\\rm{x = 0.02, 0.06}$). 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 $\\rm{AEFe_2As_2}$ ($\\rm{AE=Ca,Sr,Ba}$) and $\\rm{Fe_{1+y}Te}$. 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 $\\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ 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.","abstract_html":"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 <span class=\"etd-inline-math\">\\rm{Ba(Fe<sub>1-x</sub>Co<sub>x</sub>)<sub>2</sub>As<sub>2</sub>}</span> ($\\rm{x = 0.05, 0.055, 0.07, 0.08}$), hole doped <span class=\"etd-inline-math\">\\rm{Ba<sub>0.8</sub>K<sub>0.2</sub>Fe<sub>2</sub>As<sub>2</sub>}</span>, and isoelectronic doped <span class=\"etd-inline-math\">\\rm{BaFe<sub>2</sub>(As<sub>1-x</sub>P<sub>x</sub>)<sub>2</sub>}</span> ($\\rm{x = 0.24, 0.43}$). The 111 crystals studied are electron doped <span class=\"etd-inline-math\">\\rm{NaFe<sub>1-x</sub>Co<sub>x</sub>As}</span> ($\\rm{x = 0.02, 0.06}$). 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, <span class=\"etd-inline-math\">T<sub>S</sub></span>. Theoretical analysis showing that this enhancement is likely a consequence of orbital fluctuations is discussed. The nonsuperconducting compounds probed are <span class=\"etd-inline-math\">\\rm{AEFe<sub>2</sub>As<sub>2</sub>}</span> ($\\rm{AE=Ca,Sr,Ba}$) and <span class=\"etd-inline-math\">\\rm{Fe<sub>1+y</sub>Te}</span>. For $\\rm{AE=Sr,Ba}$ orbital fluctuations are detected above <span class=\"etd-inline-math\">T<sub>S</sub></span> while for $\\rm{AE=Ca}$ these fluctuations start below <span class=\"etd-inline-math\">T<sub>S</sub></span>. 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 <span class=\"etd-inline-math\">\\rm{Ba(Fe<sub>1-x</sub>Co<sub>x</sub>)<sub>2</sub>As<sub>2</sub>}</span> 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.","abstract_has_math":true,"creators":["Arham, Hamood Zafir"],"institution":"University of Illinois at Urbana-Champaign","degree_name":"Ph.D.","degree_level":"Dissertation","degree_discipline":"Physics","degree_department":null,"school":null,"contributors":["Greene, Laura H.","Mason, Nadya","Dahmen, Karin A.","DeMarco, Brian L."],"advisors":[],"committee_chairs":[],"committee_members":[],"year":2014,"date_issued":"2014-05-30T16:44:43Z","date_published":"2014-05-30T16:44:43Z","updated_at":"2026-07-22T22:25:38Z","subjects":["Superconductivity","Point Contact Spectroscopy","Iron Based Superconductors","Andreev Reflection","Orbital Fluctuations","Nematicity","Correlated Electron Matter"],"languages":["en"],"rights":["Copyright 2014 Hamood Zafir Arham"],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"http://hdl.handle.net/2142/49451","outbound_label":"Handle","outbound_source":"dc:identifier"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Greene, Laura H.","Mason, Nadya","Dahmen, Karin A.","DeMarco, Brian L."]},{"key":"dc:creator","label":"Author","values":["Arham, Hamood Zafir"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date","label":"Dc Date","values":["2014-05-30T16:44:43Z","2014-05"]},{"key":"dc:type","label":"Dc Type","values":["text"]},{"key":"thesis:degree_discipline","label":"Discipline","values":["Physics"]},{"key":"thesis:degree_level","label":"Degree Level","values":["Dissertation"]},{"key":"thesis:degree_name","label":"Degree Name","values":["Ph.D."]},{"key":"thesis:institution_name","label":"Thesis Institution Name","values":["University of Illinois at Urbana-Champaign"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Superconductivity","Point Contact Spectroscopy","Iron Based Superconductors","Andreev Reflection","Orbital Fluctuations","Nematicity","Correlated Electron Matter"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["en"]},{"key":"dc:rights","label":"Dc Rights","values":["Copyright 2014 Hamood Zafir Arham"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["http://hdl.handle.net/2142/49451"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["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 $\\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ ($\\rm{x = 0.05, 0.055, 0.07, 0.08}$), hole doped $\\rm{Ba_{0.8}K_{0.2}Fe_2As_2}$, and isoelectronic doped $\\rm{BaFe_2(As_{1-x}P_{x})_2}$ ($\\rm{x = 0.24, 0.43}$). The 111 crystals studied are electron doped $\\rm{NaFe_{1-x}Co_xAs}$ ($\\rm{x = 0.02, 0.06}$). 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 $\\rm{AEFe_2As_2}$ ($\\rm{AE=Ca,Sr,Ba}$) and $\\rm{Fe_{1+y}Te}$. 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 $\\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ 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.","Item withdrawn by Laura Spradlin (lspradl2@illinois.edu) on 2013-12-17T14:39:51Z Item was in collections: University of Illinois Theses & Dissertations (ID: 1) No. of bitstreams: 68 Figure6p9.png: 158474 bytes, checksum: 5185b834ba9ae36055bce18ffd113259 (MD5) Figure6p8.png: 73547 bytes, checksum: 7968f6ceb560ee981f1f209d66903076 (MD5) Figure6p7.png: 67657 bytes, checksum: 78296c59081e80cbd0c4f28a1e31e097 (MD5) Figure6p6.png: 62123 bytes, checksum: f68513d7cfcab2cda6984bcfe2a9ad46 (MD5) Figure6p5.png: 117063 bytes, checksum: da403f0e5430a0c0330f14f65c7df2f9 (MD5) Figure6p4.png: 112160 bytes, checksum: 8c230f257c4e7d3075e8846196fa3035 (MD5) Figure6p3.png: 75301 bytes, checksum: 371596388c44312117d765370f73910c (MD5) Figure6p2.png: 128197 bytes, checksum: b1640611d615658d4d651e23c1166b24 (MD5) Figure6p1.png: 66614 bytes, checksum: 52eb9f7f26a70197c52badcb4db35495 (MD5) Figure5p8.png: 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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 $\\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ ($\\rm{x = 0.05, 0.055, 0.07, 0.08}$), hole doped $\\rm{Ba_{0.8}K_{0.2}Fe_2As_2}$, and isoelectronic doped $\\rm{BaFe_2(As_{1-x}P_{x})_2}$ ($\\rm{x = 0.24, 0.43}$). The 111 crystals studied are electron doped $\\rm{NaFe_{1-x}Co_xAs}$ ($\\rm{x = 0.02, 0.06}$). 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 $\\rm{AEFe_2As_2}$ ($\\rm{AE=Ca,Sr,Ba}$) and $\\rm{Fe_{1+y}Te}$. 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 $\\rm{Ba(Fe_{1-x}Co_x)_2As_2}$ 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.","Item withdrawn by Laura Spradlin (lspradl2@illinois.edu) on 2013-12-17T14:39:51Z Item was in collections: University of Illinois Theses & Dissertations (ID: 1) No. of bitstreams: 68 Figure6p9.png: 158474 bytes, checksum: 5185b834ba9ae36055bce18ffd113259 (MD5) Figure6p8.png: 73547 bytes, checksum: 7968f6ceb560ee981f1f209d66903076 (MD5) Figure6p7.png: 67657 bytes, checksum: 78296c59081e80cbd0c4f28a1e31e097 (MD5) Figure6p6.png: 62123 bytes, checksum: f68513d7cfcab2cda6984bcfe2a9ad46 (MD5) Figure6p5.png: 117063 bytes, checksum: da403f0e5430a0c0330f14f65c7df2f9 (MD5) Figure6p4.png: 112160 bytes, checksum: 8c230f257c4e7d3075e8846196fa3035 (MD5) Figure6p3.png: 75301 bytes, checksum: 371596388c44312117d765370f73910c (MD5) Figure6p2.png: 128197 bytes, checksum: b1640611d615658d4d651e23c1166b24 (MD5) Figure6p1.png: 66614 bytes, checksum: 52eb9f7f26a70197c52badcb4db35495 (MD5) Figure5p8.png: 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