{"id":{"repo_id":"uiuc","oai_identifier":"oai:www.ideals.illinois.edu:2142/15554"},"canonical_url":"https://search.dev.ndltd.org/etd/uiuc/oai:www.ideals.illinois.edu:2142/15554","repository":{"repo_id":"uiuc","name":"University of Illinois - Urbana-Champaign","base_url":"https://www.ideals.illinois.edu/oai-pmh"},"display":{"title":"Point-contact Andreev reflection studies on superconductors","abstract":"Point-contact Andreev reflection spectroscopy (PCARS) and planar tunneling spectroscopy are used to investigate the density of states and superconducting gap structure of several novel superconducting systems. Theoretical and experimental background is given with an emphasis on PCARS as applied to various unconventional superconductors, such as NbSe$_2$, the borocarbide family RNi$_2$B$_2$C (R=Lu, Y) and the recently-discovered, iron-based superconductors AFe$_2$As$_2$ (A=Ba, Sr) with both hole and electron doping. The superconducting gap distribution observed from point-contacts on NbSe$_2$, with energies between 0.9 and 1.5 meV at 2 K, is consistent with the multigap scenario claimed by other experiments. The superconducting gap follows the BCS temperature dependence. PCARS probes a small gap anisotropy in both LuNi$_2$B$_2$C and YNi$_2$B$_2$C for the three major crystallographic orientations, while point-nodes are reported to exist in \\textit{a} and \\textit{b} axes and predicted to originate from the antiferromagnetic fluctuations due to nesting structure on the 17th band Fermi surface. This can be explained by the other Fermi surface sheets, with $\\Delta\\geq$2.1 meV in [100] direction, masking the point-node feature and resulting in a small gap anisotropy, as observed in our PCARS. A large tunneling cone in PCARS measurements, which would smear the differences in three directions, may be another origin of the observed small gap anisotropy. For the Fe-122 family of the iron-based superconductors, Andreev reflection is present for PCARS on (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ and Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ crystals but measured conductances are distinct. A V-shape conductance valley (VCV) feature is observed for point-contacts on Sr(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ and (Sr$_{0.6}$Na$_{0.4}$)Fe$_2$As$_2$ crystals. This VCV feature is also observed for the non-superconducting parent compound BaFe$_2$As$_2$ and the superconducting (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ crystals. The coexistence of phase-separated magnetic and superconducting orders on the mesoscopic scale is considered to explain the range of behaviors in the superconducting samples.","abstract_html":"Point-contact Andreev reflection spectroscopy (PCARS) and planar tunneling spectroscopy are used to investigate the density of states and superconducting gap structure of several novel superconducting systems. Theoretical and experimental background is given with an emphasis on PCARS as applied to various unconventional superconductors, such as NbSe<span class=\"etd-inline-math\"><sub>2</sub></span>, the borocarbide family RNi<span class=\"etd-inline-math\"><sub>2</sub></span>B<span class=\"etd-inline-math\"><sub>2</sub></span>C (R=Lu, Y) and the recently-discovered, iron-based superconductors AFe<span class=\"etd-inline-math\"><sub>2</sub></span>As<span class=\"etd-inline-math\"><sub>2</sub></span> (A=Ba, Sr) with both hole and electron doping. The superconducting gap distribution observed from point-contacts on NbSe<span class=\"etd-inline-math\"><sub>2</sub></span>, with energies between 0.9 and 1.5 meV at 2 K, is consistent with the multigap scenario claimed by other experiments. The superconducting gap follows the BCS temperature dependence. PCARS probes a small gap anisotropy in both LuNi<span class=\"etd-inline-math\"><sub>2</sub></span>B<span class=\"etd-inline-math\"><sub>2</sub></span>C and YNi<span class=\"etd-inline-math\"><sub>2</sub></span>B<span class=\"etd-inline-math\"><sub>2</sub></span>C for the three major crystallographic orientations, while point-nodes are reported to exist in \\textit{a} and \\textit{b} axes and predicted to originate from the antiferromagnetic fluctuations due to nesting structure on the 17th band Fermi surface. This can be explained by the other Fermi surface sheets, with $\\Delta\\geq$2.1 meV in [100] direction, masking the point-node feature and resulting in a small gap anisotropy, as observed in our PCARS. A large tunneling cone in PCARS measurements, which would smear the differences in three directions, may be another origin of the observed small gap anisotropy. For the Fe-122 family of the iron-based superconductors, Andreev reflection is present for PCARS on (Ba<span class=\"etd-inline-math\"><sub>0.6</sub></span>K<span class=\"etd-inline-math\"><sub>0.4</sub></span>)Fe<span class=\"etd-inline-math\"><sub>2</sub></span>As<span class=\"etd-inline-math\"><sub>2</sub></span> and Ba(Fe<span class=\"etd-inline-math\"><sub>0.9</sub></span>Co<span class=\"etd-inline-math\"><sub>0.1</sub></span>)<span class=\"etd-inline-math\"><sub>2</sub></span>As<span class=\"etd-inline-math\"><sub>2</sub></span> crystals but measured conductances are distinct. A V-shape conductance valley (VCV) feature is observed for point-contacts on Sr(Fe<span class=\"etd-inline-math\"><sub>0.9</sub></span>Co<span class=\"etd-inline-math\"><sub>0.1</sub></span>)<span class=\"etd-inline-math\"><sub>2</sub></span>As<span class=\"etd-inline-math\"><sub>2</sub></span> and (Sr<span class=\"etd-inline-math\"><sub>0.6</sub></span>Na<span class=\"etd-inline-math\"><sub>0.4</sub></span>)Fe<span class=\"etd-inline-math\"><sub>2</sub></span>As<span class=\"etd-inline-math\"><sub>2</sub></span> crystals. This VCV feature is also observed for the non-superconducting parent compound BaFe<span class=\"etd-inline-math\"><sub>2</sub></span>As<span class=\"etd-inline-math\"><sub>2</sub></span> and the superconducting (Ba<span class=\"etd-inline-math\"><sub>0.6</sub></span>K<span class=\"etd-inline-math\"><sub>0.4</sub></span>)Fe<span class=\"etd-inline-math\"><sub>2</sub></span>As<span class=\"etd-inline-math\"><sub>2</sub></span> crystals. The coexistence of phase-separated magnetic and superconducting orders on the mesoscopic scale is considered to explain the range of behaviors in the superconducting samples.","abstract_has_math":true,"creators":["Lu, Xin"],"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.","Eckstein, James N.","Peng, Jen-Chieh","Leggett, Anthony J."],"advisors":[],"committee_chairs":[],"committee_members":[],"year":2010,"date_issued":"2010-05-14T20:50:00Z","date_published":"2010-05-14T20:50:00Z","updated_at":"2026-07-22T22:25:08Z","subjects":["Point-contact Andreev reflection spectroscopy (PCARS)","superconductors","superconducting gap","order parameter symmetry","Sharvin ballistic limit","planar tunneling","density of states (DOS)","Blonder-Thinkham-Klapwijk Model","NbSe2","borocarbide","iron-based superconductor"],"languages":["en"],"rights":["Copyright 2010 Xin Lu"],"rights_urls":[],"identifier_entries":[]},"links":{"outbound_url":"http://hdl.handle.net/2142/15554","outbound_label":"Handle","outbound_source":"dc:identifier"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor","label":"Contributor","values":["Greene, Laura H.","Eckstein, James N.","Peng, Jen-Chieh","Leggett, Anthony J."]},{"key":"dc:creator","label":"Author","values":["Lu, Xin"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date","label":"Dc Date","values":["2010-05-14T20:50:00Z","2012-05-15T10:00:31Z","2010-5"]},{"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":["Point-contact Andreev reflection spectroscopy (PCARS)","superconductors","superconducting gap","order parameter symmetry","Sharvin ballistic limit","planar tunneling","density of states (DOS)","Blonder-Thinkham-Klapwijk Model","NbSe2","borocarbide","iron-based superconductor"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["en"]},{"key":"dc:rights","label":"Dc Rights","values":["Copyright 2010 Xin Lu"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier","label":"Identifier","values":["http://hdl.handle.net/2142/15554"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description","label":"Description","values":["Point-contact Andreev reflection spectroscopy (PCARS) and planar tunneling spectroscopy are used to investigate the density of states and superconducting gap structure of several novel superconducting systems. Theoretical and experimental background is given with an emphasis on PCARS as applied to various unconventional superconductors, such as NbSe$_2$, the borocarbide family RNi$_2$B$_2$C (R=Lu, Y) and the recently-discovered, iron-based superconductors AFe$_2$As$_2$ (A=Ba, Sr) with both hole and electron doping. The superconducting gap distribution observed from point-contacts on NbSe$_2$, with energies between 0.9 and 1.5 meV at 2 K, is consistent with the multigap scenario claimed by other experiments. The superconducting gap follows the BCS temperature dependence. PCARS probes a small gap anisotropy in both LuNi$_2$B$_2$C and YNi$_2$B$_2$C for the three major crystallographic orientations, while point-nodes are reported to exist in \\textit{a} and \\textit{b} axes and predicted to originate from the antiferromagnetic fluctuations due to nesting structure on the 17th band Fermi surface. This can be explained by the other Fermi surface sheets, with $\\Delta\\geq$2.1 meV in [100] direction, masking the point-node feature and resulting in a small gap anisotropy, as observed in our PCARS. A large tunneling cone in PCARS measurements, which would smear the differences in three directions, may be another origin of the observed small gap anisotropy. For the Fe-122 family of the iron-based superconductors, Andreev reflection is present for PCARS on (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ and Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ crystals but measured conductances are distinct. A V-shape conductance valley (VCV) feature is observed for point-contacts on Sr(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ and (Sr$_{0.6}$Na$_{0.4}$)Fe$_2$As$_2$ crystals. This VCV feature is also observed for the non-superconducting parent compound BaFe$_2$As$_2$ and the superconducting (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ crystals. The coexistence of phase-separated magnetic and superconducting orders on the mesoscopic scale is considered to explain the range of behaviors in the superconducting samples.","Item withdrawn by Rebecca Bryant (rabryant@illinois.edu) on 2009-12-23T15:25:11Z Item was in collections: University of Illinois Theses & Dissertations (ID: 1) No. of bitstreams: 2 Lu_Xin.pdf: 6095023 bytes, checksum: 52cdb170ece73de2db6c73beb9a59c94 (MD5) Lu_Xin_dissertation.zipx: 41365355 bytes, checksum: 54b95d7cb2fb131b7e4b46007a0cffaa (MD5)","Made available in DSpace on 2010-05-14T20:50:00Z (GMT). No. of bitstreams: 3 Lu_Xin_dissertation.zipx: 41365355 bytes, checksum: 54b95d7cb2fb131b7e4b46007a0cffaa (MD5) Lu_Xin.pdf: 6095023 bytes, checksum: 52cdb170ece73de2db6c73beb9a59c94 (MD5) license.txt: 4053 bytes, checksum: 44c13066b3232d77c58ba453615063ee (MD5)","Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by William Ingram (wingram2@illinois.edu) on 2010-05-14T20:52:45Z Item is restricted until 2012-05-14T20:52:43Z","Item reinstated by Sarah Shreeves (sshreeve@illinois.edu) on 2012-05-15T10:00:31Z Item was in collections: University of Illinois Dissertations and Theses (ID: 204) Dissertations and Theses - Physics (ID: 445) No. of bitstreams: 4 Lu_Xin.pdf.txt: 213063 bytes, checksum: 72ea54a41130b52d22d02b4863578e7f (MD5) Lu_Xin_dissertation.zipx: 41365355 bytes, checksum: 54b95d7cb2fb131b7e4b46007a0cffaa (MD5) Lu_Xin.pdf: 6095023 bytes, checksum: 52cdb170ece73de2db6c73beb9a59c94 (MD5) license.txt: 4053 bytes, checksum: 44c13066b3232d77c58ba453615063ee (MD5)","Item released from any restrictions by Sarah Shreeves (sshreeve@illinois.edu) on 2012-05-15T10:00:31Z"]},{"key":"dc:title","label":"Title","values":["Point-contact Andreev reflection studies on superconductors"]}]}],"canonical_facts":{"dc:contributor":["Greene, Laura H.","Eckstein, James N.","Peng, Jen-Chieh","Leggett, Anthony J."],"dc:creator":["Lu, Xin"],"dc:date":["2010-05-14T20:50:00Z","2012-05-15T10:00:31Z","2010-5"],"dc:description":["Point-contact Andreev reflection spectroscopy (PCARS) and planar tunneling spectroscopy are used to investigate the density of states and superconducting gap structure of several novel superconducting systems. Theoretical and experimental background is given with an emphasis on PCARS as applied to various unconventional superconductors, such as NbSe$_2$, the borocarbide family RNi$_2$B$_2$C (R=Lu, Y) and the recently-discovered, iron-based superconductors AFe$_2$As$_2$ (A=Ba, Sr) with both hole and electron doping. The superconducting gap distribution observed from point-contacts on NbSe$_2$, with energies between 0.9 and 1.5 meV at 2 K, is consistent with the multigap scenario claimed by other experiments. The superconducting gap follows the BCS temperature dependence. PCARS probes a small gap anisotropy in both LuNi$_2$B$_2$C and YNi$_2$B$_2$C for the three major crystallographic orientations, while point-nodes are reported to exist in \\textit{a} and \\textit{b} axes and predicted to originate from the antiferromagnetic fluctuations due to nesting structure on the 17th band Fermi surface. This can be explained by the other Fermi surface sheets, with $\\Delta\\geq$2.1 meV in [100] direction, masking the point-node feature and resulting in a small gap anisotropy, as observed in our PCARS. A large tunneling cone in PCARS measurements, which would smear the differences in three directions, may be another origin of the observed small gap anisotropy. For the Fe-122 family of the iron-based superconductors, Andreev reflection is present for PCARS on (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ and Ba(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ crystals but measured conductances are distinct. A V-shape conductance valley (VCV) feature is observed for point-contacts on Sr(Fe$_{0.9}$Co$_{0.1}$)$_2$As$_2$ and (Sr$_{0.6}$Na$_{0.4}$)Fe$_2$As$_2$ crystals. This VCV feature is also observed for the non-superconducting parent compound BaFe$_2$As$_2$ and the superconducting (Ba$_{0.6}$K$_{0.4}$)Fe$_2$As$_2$ crystals. The coexistence of phase-separated magnetic and superconducting orders on the mesoscopic scale is considered to explain the range of behaviors in the superconducting samples.","Item withdrawn by Rebecca Bryant (rabryant@illinois.edu) on 2009-12-23T15:25:11Z Item was in collections: University of Illinois Theses & Dissertations (ID: 1) No. of bitstreams: 2 Lu_Xin.pdf: 6095023 bytes, checksum: 52cdb170ece73de2db6c73beb9a59c94 (MD5) Lu_Xin_dissertation.zipx: 41365355 bytes, checksum: 54b95d7cb2fb131b7e4b46007a0cffaa (MD5)","Made available in DSpace on 2010-05-14T20:50:00Z (GMT). No. of bitstreams: 3 Lu_Xin_dissertation.zipx: 41365355 bytes, checksum: 54b95d7cb2fb131b7e4b46007a0cffaa (MD5) Lu_Xin.pdf: 6095023 bytes, checksum: 52cdb170ece73de2db6c73beb9a59c94 (MD5) license.txt: 4053 bytes, checksum: 44c13066b3232d77c58ba453615063ee (MD5)","Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by William Ingram (wingram2@illinois.edu) on 2010-05-14T20:52:45Z Item is restricted until 2012-05-14T20:52:43Z","Item reinstated by Sarah Shreeves (sshreeve@illinois.edu) on 2012-05-15T10:00:31Z Item was in collections: University of Illinois Dissertations and Theses (ID: 204) Dissertations and Theses - Physics (ID: 445) No. of bitstreams: 4 Lu_Xin.pdf.txt: 213063 bytes, checksum: 72ea54a41130b52d22d02b4863578e7f (MD5) Lu_Xin_dissertation.zipx: 41365355 bytes, checksum: 54b95d7cb2fb131b7e4b46007a0cffaa (MD5) Lu_Xin.pdf: 6095023 bytes, checksum: 52cdb170ece73de2db6c73beb9a59c94 (MD5) license.txt: 4053 bytes, checksum: 44c13066b3232d77c58ba453615063ee (MD5)","Item released from any restrictions by Sarah Shreeves (sshreeve@illinois.edu) on 2012-05-15T10:00:31Z"],"dc:identifier":["http://hdl.handle.net/2142/15554"],"dc:language":["en"],"dc:rights":["Copyright 2010 Xin Lu"],"dc:subject":["Point-contact Andreev reflection spectroscopy (PCARS)","superconductors","superconducting gap","order parameter symmetry","Sharvin ballistic limit","planar tunneling","density of states (DOS)","Blonder-Thinkham-Klapwijk Model","NbSe2","borocarbide","iron-based superconductor"],"dc:title":["Point-contact Andreev reflection studies on superconductors"],"thesis:degree_discipline":["Physics"],"thesis:degree_level":["Dissertation"],"thesis:degree_name":["Ph.D."],"thesis:institution_name":["University of Illinois at Urbana-Champaign"]},"updated_at":"2026-07-22T22:25:08Z"}