Unconventional superconductivity in CuxBi2Se3 from magnetic susceptibility and electrical transport

Medientyp: E-Artikel
Titel: Unconventional superconductivity in CuxBi2Se3 from magnetic susceptibility and electrical transport
Beteiligte: Fang, Yifei; You, Wen-Long; Li, Mingtao
Erschienen: IOP Publishing, 2020
Erschienen in: New Journal of Physics
Sprache: Nicht zu entscheiden
DOI: 10.1088/1367-2630/ab7fca
ISSN: 1367-2630
Schlagwörter: General Physics and Astronomy
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Beschreibung: <jats:title>Abstract</jats:title> <jats:p>Although the Cu doped Bi<jats:sub>2</jats:sub>Se<jats:sub>3</jats:sub> topological insulator was discovered and intensively studied for almost a decade, its electrical and magnetic properties in normal state, and the mechanism of ‘high-<jats:italic>T</jats:italic> <jats:sub>c</jats:sub>’ superconductivity regarding the relatively low-carrier density are still not addressed yet. In this work, we report a systematic investigation of magnetic susceptibility, critical fields, and electrical transport on the nominal Cu<jats:sub>0.20</jats:sub>Bi<jats:sub>2</jats:sub>Se<jats:sub>3</jats:sub> single crystals with <jats:inline-formula> <jats:tex-math><?CDATA ${T}_{\mathrm{c}}^{\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{e}\mathrm{t}}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">o</mml:mi> <mml:mi mathvariant="normal">n</mml:mi> <mml:mi mathvariant="normal">s</mml:mi> <mml:mi mathvariant="normal">e</mml:mi> <mml:mi mathvariant="normal">t</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="njpab7fcaieqn1.gif" xlink:type="simple" /> </jats:inline-formula> = 4.18 K, the highest so far. The composition analysis yields the Cu stoichiometry of <jats:italic>x</jats:italic> = 0.09(1). The magnetic susceptibility shows considerable anisotropy and an obvious kink at around 96 K was observed in the magnetic susceptibility for <jats:italic>H</jats:italic>∥<jats:italic>c</jats:italic>, which indicates a charge density anomaly. The electrical transport measurements indicate the two-dimensional (2D) Fermi liquid behavior at low temperatures with a high Kadowaki–Woods ratio, <jats:italic>A</jats:italic>/<jats:italic>γ</jats:italic> <jats:sup>2</jats:sup> = 30.3<jats:italic>a</jats:italic> <jats:sub>0</jats:sub>. The lower critical field at 0 K limit was extracted to be 6.0 Oe for <jats:italic>H</jats:italic>∥<jats:italic>ab</jats:italic>. In the clean limit, the ratio of energy gap to <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> was determined to be Δ<jats:sub>0</jats:sub>/<jats:italic>k</jats:italic> <jats:sub>B</jats:sub> <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> = 2.029 ± 0.124 exceeding the standard BCS value 1.764, suggesting Cu<jats:sub>0.09</jats:sub>Bi<jats:sub>2</jats:sub>Se<jats:sub>3</jats:sub> is a strong-coupling superconductor. The in-plane penetration depth at 0 K was calculated to be 1541.57 nm, resulting in an unprecedented high ratio of <jats:italic>T</jats:italic> <jats:sub>c</jats:sub>/<jats:italic>λ</jats:italic> <jats:sup>−2</jats:sup>(0) ≅ 9.86. Moreover, the ratio of <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> to Fermi temperature is estimated to be <jats:inline-formula> <jats:tex-math><?CDATA ${T}_{\mathrm{c}}/{T}_{\mathrm{F}}^{2\mathrm{D}}$?></jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">F</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> <mml:mi mathvariant="normal">D</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="njpab7fcaieqn2.gif" xlink:type="simple" /> </jats:inline-formula> = 0.034. Both ratios fall into the region of unconventional superconductivity according to Uemura’s regime, supporting the unconventional superconducting mechanism in Cu<jats:sub> <jats:italic>x</jats:italic> </jats:sub>Bi<jats:sub>2</jats:sub>Se<jats:sub>3</jats:sub>. Finally, the enhanced <jats:italic>T</jats:italic> <jats:sub>c</jats:sub> value higher than 4 K is proposed to arise from the increased density of states at Fermi energy and strong electron–phonon interaction induced by the charge density instability.</jats:p>
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