Visualizing the Odd-Parity Superconducting Order Parameter and Its Quasiparticle Surface Band in UTe2

A distinctive identifier of nodal intrinsic topological superconductivity (ITS) would the appearance of an Andreev bound state on crystal surfaces parallel to the nodal axis, in the form of a topological quasiparticle surface band (QSB) appearing only for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>T</mml:mi> <mml:mo><</mml:mo> <mml:msub><mml:mi>T</mml:mi> <mml:mi>C</mml:mi></mml:msub> </mml:mrow> </mml:math> . Moreover, the theory shows that specific QSB characteristics observable in tunneling to an <i>s</i>-wave superconductor can distinguish between chiral and non-chiral ITS order parameter <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>Δ</mml:mi> <mml:mrow><mml:mi>k</mml:mi></mml:mrow> </mml:msub> </mml:math> . To search for such phenomena in UTe₂, <i>s</i>-wave superconductive scan-tip scanning tunneling microscopy (STM) imaging was employed. It reveals an intense zero-energy Andreev conductance maximum at the UTe₂ (0-11) crystal termination. The development of the zero-energy Andreev conductance peak into two finite-energy particle-hole symmetric conductance maxima as the tunnel barrier is reduced and then signifies that UTe₂ superconductivity is non-chiral. Quasiparticle interference imaging (QPI) for an ITS material should be dominated by the QSB for energies within the superconductive energy gap <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mfenced><mml:mi>E</mml:mi></mml:mfenced> <mml:mo>≤</mml:mo> <mml:mi>Δ</mml:mi></mml:mrow> </mml:math> , so that bulk <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>Δ</mml:mi> <mml:mrow><mml:mi>k</mml:mi></mml:mrow> </mml:msub> </mml:math> characteristics of the ITS can only be detected excursively. Again using a superconducting scan-tip, the in-gap quasiparticle interference patterns of the QSB of UTe₂ were visualized. Specifically, a band of Bogoliubov quasiparticles appears as a characteristic sextet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub><mml:mrow><mml:mi>q</mml:mi></mml:mrow> <mml:mi>i</mml:mi></mml:msub> <mml:mo>:</mml:mo> <mml:mi>i</mml:mi> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mo>-</mml:mo> <mml:mn>6</mml:mn> <mml:mrow></mml:mrow></mml:mrow> </mml:math> of interference wavevectors, showing that QSB dispersions <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>k</mml:mi></mml:mrow> </mml:math> (<i>E</i>) occur only for energies <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mfenced><mml:mi>E</mml:mi></mml:mfenced> <mml:mo>≤</mml:mo> <mml:msub><mml:mi>Δ</mml:mi> <mml:mo>max</mml:mo></mml:msub> </mml:mrow> </mml:math> and only within the range of Fermi momenta projected onto the (0-11) crystal surface. In combination, these phenomena are consistent with a bulk <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>Δ</mml:mi> <mml:mrow><mml:mi>k</mml:mi></mml:mrow> </mml:msub> </mml:math> exhibiting spin-triplet, time-reversal conserving, odd-parity, <i>a</i>-axis nodal, <i>B</i> _<i>3u</i> symmetry in UTe₂.