Experimental search for the origin of low-energy modes in topological materials

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Experimental search for the origin of low-energy modes in topological materials

G. P. Mazur, K. Dybko, A. Szczerbakow, J. Z. Domagala, A. Kazakov, M. Zgirski, E. Lusakowska, S. Kret, J. Korczak, T. Story, M. Sawicki, and T. Dietl

Phys. Rev. B 100, 041408(R) – Published 26 July 2019

Abstract

Point-contact spectroscopy of several nonsuperconducting topological materials reveals a low-temperature phase transition that is characterized by a Bardeen-Cooper-Schrieffer type of criticality. We find such a behavior of differential conductance for topological surfaces of nonmagnetic and magnetic ${Pb}_{1 - y - x} {Sn}_{y} {Mn}_{x} Te$ . We examine a possible contribution from superconducting nanoparticles, and show to what extent our data are consistent with Brzezicki's et al. theory (arXiv:1812.02168), assigning the observations to a collective state adjacent to atomic steps at topological surfaces.

Received 14 December 2018
Revised 22 March 2019

DOI:https://doi.org/10.1103/PhysRevB.100.041408

Physics Subject Headings (PhySH)

Andreev reflection Edge states Electrical properties Magnetic phase transitions Surface states Topological materials

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

G. P. Mazur^1,*, K. Dybko^1,2,†, A. Szczerbakow², J. Z. Domagala², A. Kazakov¹, M. Zgirski², E. Lusakowska², S. Kret², J. Korczak^1,2, T. Story², M. Sawicki², and T. Dietl ^1,3,‡

¹International Research Centre MagTop, Institute of Physics, Polish Academy of Sciences, Aleja Lotnikow 32/46, PL-02668 Warsaw, Poland
²Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw, Poland
³WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

^*grzegorz.mazur@MagTop.ifpan.edu.pl
^†dybko@ifpan.edu.pl
^‡dietl@MagTop.ifpan.edu.pl

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Vol. 100, Iss. 4 — 15 July 2019

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Images

Figure 1
(a) Differential conductance $d I / d V$ at 50 mK normalized to its value at the normal state for the as-grown (001) ${Pb}_{1 - y} {Sn}_{y} Te$ with $y = 0$ , 0.20, 0.80, and 1. The spectrum is featureless for $y = 0$ (PbTe) and $y = 0.20$ but shows zero-energy mode characteristics for Sn content $(y = 0.80$ and 1, i.e., SnTe) corresponding to a topological crystalline insulator phase. Evolution of the spectrum with temperature (b) and the magnetic field (c) for $y = 0.80$ and two locations of the point contact on the sample surface, respectively. A magnetic field is applied perpendicularly to the (001) plane. (d) Resistance of this sample measured by a four-contact method with a current density as low as $2.5 \times 10^{- 3} A / {cm}^{2}$ . No global superconductivity is detected.
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Figure 2
Temperature dependence of differential conductance spectra for the etched (011) ${Pb}_{0.30} {Sn}_{0.67} {Mn}_{0.03} Te$ (a) in a spectroscopic regime and (011) ${Pb}_{0.16} {Sn}_{0.74} {Mn}_{0.10} Te$ (b) in a thermal regime, respectively. (c) Evolution of the spectrum with the magnetic field at 1.8 K for the cleaved (001) ${Pb}_{0.16} {Sn}_{0.74} {Mn}_{0.10} Te$ in a spectroscopic regime. A magnetic field is applied perpendicularly to the sample plane. (d) Resistance of ${Pb}_{0.16} {Sn}_{0.74} {Mn}_{0.10} Te$ measured by a four-contact method with current density $2.5 \times 10^{- 5} A / {cm}^{2}$ (noisy trace), and the solid line represents a numerical average over 40 temperature scans. Critical scattering at the Curie temperature $T_{Curie} = 14 K$ is observed but no global superconductivity is detected.
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Figure 3
(a) Temperature dependence of the point-contact resistance at the limit of zero bias pointing to a phase transition. (b), (c) Conductance gap $Δ$ evaluated from the differential conductance spectra for samples presented in Figs. 1 and 2 for ${Pb}_{1 - y - x} {Sn}_{y} {Mn}_{x} Te$ corresponding to the topological crystalline insulator phase vs magnetic field perpendicular to the surface plane and temperature, respectively. Solid lines in (b) are fits to $Δ (T, H) = Δ (T, H = 0) {(1 - H / H_{c})}^{1 / 2}$ . Solid lines in (c) are fits of the BCS formula for $Δ (T)$ to the experimental points treating $T_{c}$ and $C$ as adjustable parameters $(C = 4.35$ , 4.53, and 3.49 from top to bottom, respectively; $C = 1.76$ in the BCS theory).
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Figure 4
AFM images of the studied single-crystal surfaces. (a) Naturally grown (001) facet of ${Pb}_{0.20} {Sn}_{0.80} Te$ showing surface steps. (c) Cleaved (001) surface of ${Pb}_{0.16} {Sn}_{0.74} {Mn}_{0.10} Te$ showing multilayer steps and a higher roughness. (b) and (d) depict step height profiles. The obtained values of about 0.3 nm in (b) correspond to a single atomic step (315 pm).
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