Scanning tunneling microscopy of superconducting topological surface states in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$

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Scanning tunneling microscopy of superconducting topological surface states in ${Bi}_{2} {Se}_{3}$

Ian M. Dayton, Nicholas Sedlmayr, Victor Ramirez, Thomas C. Chasapis, Reza Loloee, Mercouri G. Kanatzidis, Alex Levchenko, and Stuart H. Tessmer

Phys. Rev. B 93, 220506(R) – Published 20 June 2016

Abstract

In this Rapid Communication we present scanning tunneling microscopy of a large ${Bi}_{2} {Se}_{3}$ crystal with superconducting PbBi islands deposited on the surface. Local density of states measurements are consistent with induced superconductivity in the topological surface state with a coherence length of order 540 nm. At energies above the gap the density of states exhibits oscillations due to scattering caused by a nonuniform order parameter. Strikingly, the spectra taken on islands also display similar oscillations along with traces of the Dirac cone, suggesting an inverse topological proximity effect.

Received 25 April 2016

DOI:https://doi.org/10.1103/PhysRevB.93.220506

Physics Subject Headings (PhySH)

Island Proximity effect

Superconductors Topological insulators

Scanning tunneling microscopy Scanning tunneling spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Ian M. Dayton^1,*, Nicholas Sedlmayr¹, Victor Ramirez¹, Thomas C. Chasapis², Reza Loloee¹, Mercouri G. Kanatzidis², Alex Levchenko³, and Stuart H. Tessmer^1,†

¹Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA
²Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA
³Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA

^*ian.m.dayton@gmail.com
^†tessmer@pa.msu.edu

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Vol. 93, Iss. 22 — 1 June 2016

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Figure 1
(a) A schematic of the standard proximity effect picture showing the induced superconducting energy gap [34] $Δ$ at an interface as a function of position $x$ , where $F$ is the superconducting condensate amplitude. The pairing interaction constant $g$ is generally taken to have the form of a clear step function, but at small scales, the step actually has a finite slope due to electronic screening [35]. (b) A schematic illustration for the geometry of the STM probe scanning over the surface of TI ${Bi}_{2} {Se}_{3}$ in proximity to a PbBi island. In this experiment, we are interested in measuring the superconducting coherence length in the plane parallel to the TI surface, not in the normal direction to the surface as done in many other experiments.
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Figure 2
(a) and (b) Atomic force microscopy (AFM) topographs of a thermally deposited PbBi dot. We can clearly see a very grainy appearance to the overall dot, but upon closer inspection it is clear that the dot comprises many small superconducting droplets. (c) STM topograph of a PbBi droplet with its respective height profile trace shown in (d). The radius of curvature at the base of the droplet is an artifact due to the radius of the STM tip. The actual interface between the droplet and the TI is very sharp, which is ideal for our measurement.
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Figure 3
The right panel represents $d I / d V$ curves measured at 4.2 K taken at various distances from a PbBi island. The LDOS displays clear signature of the induced superconducting gap, with small arrows marking the decoherence peaks. Another notable feature of the presented data is visible oscillations. The inset shows the Fourier transform of the 40 nm data as the solid curve; here the arrow marks the most prominent resonance, corresponding to a periodicity of 9 mV. The left panel represents $d I / d V$ curves measured on different islands at nominally the same conditions. Above the gap one can see not only oscillations but also traces of the Dirac cone at higher energies. All data are normalized so that ${d I / d V |}_{- 20 mV} = 1$ . We attribute the asymmetry which can be seen in the left panel to mV-scale offsets between the Fermi energy and the minimum of the Dirac cone, caused by the states leaking onto the superconducting island.
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Figure 4
(a), (b) The local density of states in the diffusive limit as a function of energy (a) at different distances from the boundary of $0.5 R, 1 R, 1.5 R$ , and as a function of position (b) at energies $0.1 E_{Th}, 0.8 E_{Th}, 1.5 E_{Th}$ , with lines representing the linearized analytical result, and symbols the numerical result of full nonlinear Eq. (1). The inset show a comparison between the gap found experimentally (blue diamonds) and the analytical result (solid line). (c), (d) The local density of states as a function of energy for the topological insulator surface states. A phenomenological damping has been included, and the bulk density of states are included as a comparison.
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Physical Review B

covering condensed matter and materials physics

Scanning tunneling microscopy of superconducting topological surface states in ${Bi}_{2} {Se}_{3}$

Ian M. Dayton, Nicholas Sedlmayr, Victor Ramirez, Thomas C. Chasapis, Reza Loloee, Mercouri G. Kanatzidis, Alex Levchenko, and Stuart H. Tessmer

Phys. Rev. B 93, 220506(R) – Published 20 June 2016

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Physical Review B

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Scanning tunneling microscopy of superconducting topological surface states in Bi2Se3

Ian M. Dayton, Nicholas Sedlmayr, Victor Ramirez, Thomas C. Chasapis, Reza Loloee, Mercouri G. Kanatzidis, Alex Levchenko, and Stuart H. Tessmer

Phys. Rev. B 93, 220506(R) – Published 20 June 2016

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Scanning tunneling microscopy of superconducting topological surface states in ${Bi}_{2} {Se}_{3}$