Detection of a Superconducting Phase in a Two-Atom Layer of Hexagonal Ga Film Grown on Semiconducting GaN(0001)

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Detection of a Superconducting Phase in a Two-Atom Layer of Hexagonal Ga Film Grown on Semiconducting GaN(0001)

Hui-Min Zhang, Yi Sun, Wei Li, Jun-Ping Peng, Can-Li Song, Ying Xing, Qinghua Zhang, Jiaqi Guan, Zhi Li, Yanfei Zhao, Shuaihua Ji, Lili Wang, Ke He, Xi Chen, Lin Gu, Langsheng Ling, Mingliang Tian, Lian Li, X. C. Xie, Jianping Liu, Hui Yang, Qi-Kun Xue, Jian Wang, and Xucun Ma

Phys. Rev. Lett. 114, 107003 – Published 10 March 2015

Abstract

The recent observation of the superconducting state at atomic scale has motivated the pursuit of exotic condensed phases in two-dimensional (2D) systems. Here we report on a superconducting phase in two-monolayer crystalline Ga films epitaxially grown on wide-band-gap semiconductor GaN(0001). This phase exhibits a hexagonal structure and only 0.552 nm in thickness, nevertheless, brings about a superconducting transition temperature $T_{c}$ as high as 5.4 K, confirmed by in situ scanning tunneling spectroscopy and ex situ electrical magnetotransport and magnetization measurements. The anisotropy of critical magnetic field and Berezinski-Kosterlitz-Thouless-like transition are observed, typical for the 2D superconductivity. Our results demonstrate a novel platform for exploring atomic-scale 2D superconductors, with great potential for understanding the interface superconductivity.

Received 4 October 2014

DOI:https://doi.org/10.1103/PhysRevLett.114.107003

Authors & Affiliations

Hui-Min Zhang¹, Yi Sun², Wei Li³, Jun-Ping Peng¹, Can-Li Song³, Ying Xing², Qinghua Zhang¹, Jiaqi Guan¹, Zhi Li¹, Yanfei Zhao², Shuaihua Ji^3,4, Lili Wang^3,4, Ke He^3,4, Xi Chen^3,4, Lin Gu^1,4, Langsheng Ling⁵, Mingliang Tian⁵, Lian Li⁶, X. C. Xie^2,4, Jianping Liu⁷, Hui Yang⁷, Qi-Kun Xue^3,4, Jian Wang^2,4,*, and Xucun Ma^1,3,4,†

¹Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
²International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China
³State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
⁴Collaborative Innovation Center of Quantum Matter, 100084 Beijing, China
⁵High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, Anhui, China
⁶Department of Physics, University of Wisconsin, Milwaukee, Wisconsin 53211, USA
⁷Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Jiangsu 215123, China

^*jianwangphysics@pku.edu.cn
^†xucunma@mail.tsinghua.edu.cn

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Vol. 114, Iss. 10 — 13 March 2015

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Images

Figure 1
(a) Topographic image (3.0 V, 0.05 nA, $1 \times 1 μ m^{2}$ ) of 2 ML Ga films, with a step height of 2.5 Å. (b) Atomic-resolution STM image (0.22 V, 0.05 nA, $8 \times 8 {nm}^{2}$ ) of a Ga film, and the inset is its corresponding FFT pattern. The bright spots correspond to Ga atoms at the top layer. (c) Cross-sectional high-angle annular dark-field image of $Ag / Ga / GaN (0001)$ heterostructure viewed from the $[11 \bar{2} 0]$ crystallographic direction, showing two Ga atomic layers just above the GaN substrate. (d) Schematic top (top panel) and section (bottom panel) views of 2 ML $Ga / GaN$ heterostructure. The average separations between various layers are $Z_{1} = 2.76$ , $Z_{2} = 2.76$ , and $Z_{3} = 2.67 Å$ .
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Figure 2
(a) A series of differential tunneling conductance spectra (set point: 10 mV, 0.1 nA) at various temperatures, normalized to the normal conductance spectrum at 10 K. (b) Temperature-dependent superconducting gap magnitude $Δ$ (dark squares) and their best fit to BCS gap function (red curve) for 2 ML Ga films. (c) Three-dimensional plots of tunneling conductance measured at various magnetic fields at 2.7 K. Spectra measured at 0, 1.2, and 5.0 T are labeled by black dashes.
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Figure 3
(a) $R_{sheet} − T$ curve at zero magnetic field, showing $T_{c}^{onset} = 5.4$ and $T_{c}^{zero} = 3.8 K$ , respectively. The inset schematically shows the diagram for all transport measurements, where indium has been used for all electrical contacts. (b) $R_{sheet} − T$ curves for various $B_{⊥}$ up to 10 T. (c),(d) Magnetoresistance (c) $R_{sheet} − B_{⊥}$ and (d) $R_{sheet} − B_{∥}$ at various temperatures ranging from 2.0 to 10 K. (e) Temperature dependence of magnetization measured under a 10 mT magnetic field normal to the sample surface, showing the Meissner effect. (f) Low-field $M (B_{⊥})$ at various temperatures from 1.9 to 2.5 K. Note that the magnetization signal below $\sim 4 mT$ is too small to be resolved in our measurement. Inset shows the temperature dependence of $B_{c 1} (T)$ . The excitation current of $5 μ A$ is used for all $R_{sheet} − T$ and $R_{sheet} − B$ measurements throughout this Letter.
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Figure 4
(a) $V (I)$ characteristics at various temperatures plotted on a double-logarithmic scale at $B = 0 T$ . Two dashed blue lines indicate the $V \sim I$ and $V \sim I^{3}$ curves, respectively. (b) Plot of the exponent $α$ as a function of temperature $T$ , extracted from the power-law fits in (a). $T_{BKT} = 4.4 K$ is defined as the temperature with $α = 3$ . (c) $[d \ln (R_{sheet}) / d T]^{- 2 / 3}$ plotted as a function of temperature. The solid line depicts the expected BKT-like transition behavior with $T_{BKT} = 4.6 K$ .
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Figure 5
Normalized $d I / d V$ spectra before and after Ag deposition. Inset shows 2 ML Ga (I) partially covered by 1 ML Ag (II) (1.8 V, 0.05 nA, $25 \times 10 {nm}^{2}$ ). Three spectra are acquired at pristine Ga films (solid black curve), regions I (black dashes) and II (blue dashes), respectively.
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