Interfacial effects on the superconducting properties of $\mathrm{LaS}{\mathrm{i}}_{2}(112)$ films on Si(111)

Interfacial effects on the superconducting properties of $LaS i_{2} (112)$ films on Si(111)

Lixia Liu, Guangyao Miao, Bing Liu, Pengfei Nan, Yade Wang, Binghui Ge, Xuetao Zhu, Fang Yang, Weihua Wang, and Jiandong Guo

Phys. Rev. B 100, 165308 – Published 30 October 2019

Abstract

Superconducting films are fascinating due to their tunability associated with the interfacial effects. Here we report the superconductivity of single-crystalline $LaS i_{2} (112)$ films epitaxially grown on Si(111) substrates, which exhibit the superconducting transition temperature $(T_{c})$ of 3.2 K in films thicker than 21 monolayers (ML) in the strong-coupling regime. At the interface, the $LaS i_{2}$ films bear the tensile strain, leading to the formation of thickness-dependent stripes. The local superconducting gap varies in real space in accordance with the stripes. In some areas with the thickness of 10–21 ML the superconductivity is enhanced, while in films thinner than 10 ML the superconductivity is suppressed due to the suppression of electronic states at the Fermi level. Our work provides insights into the interfacial effects on the superconductivity of epitaxial compound films from both lattice and electronic aspects.

Received 8 April 2019
Revised 25 September 2019

DOI:https://doi.org/10.1103/PhysRevB.100.165308

Physics Subject Headings (PhySH)

Crystal structure Superconducting phase transition Surface & interfacial phenomena

Low-temperature superconductors Ultrathin films

Molecular beam epitaxy Scanning tunneling spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Lixia Liu^1,2, Guangyao Miao^1,2, Bing Liu^1,2, Pengfei Nan¹, Yade Wang^1,2, Binghui Ge³, Xuetao Zhu^1,2,4, Fang Yang¹, Weihua Wang^1,*, and Jiandong Guo^1,2,4,5,†

¹Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
²School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
³Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, China
⁴Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China
⁵Beijing Academy of Quantum Information Sciences, Beijing 100193, China

^*weihuawang@iphy.ac.cn
^†jdguo@iphy.ac.cn

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 100, Iss. 16 — 15 October 2019

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Structure and morphology of $LaS i_{2}$ . (a) Lattice schematics of $LaS i_{2}$ in three-dimensional, top, and side views. The blue and yellow spheres represent La and Si atoms, respectively. (b) STM topographic image (−2.4 V/50 pA) of the $LaS i_{2} (112)$ film with the thickness of 108 ML. The upper inset shows the LEED patterns of the $LaS i_{2} (112)$ film, while the lower inset is the height profile of the step marked by the blue arrow. (c) High-resolution STM image (0.5 V/500 pA) of the $LaS i_{2} (112)$ surface, with the in-plane rectangle unit cell labeled. The inset shows the fast Fourier transform of the image. (d) XRD spectra of the 270-ML-thick $LaS i_{2} (112)$ film on Si(111) and the bare Si(111) substrate. The peak at ∼26° is the accompanying diffraction of the $Cu K_{β}$ line. The results of STEM-EDS of the $LaS i_{2}$ film with the thickness of 165 ML on Si(111): (e) high-angle annular dark-field image of the $LaS i_{2}$ film on Si(111); (f), (g) EDS mappings of Si and La, respectively.
Reuse & Permissions
Figure 2
Crystalline structure of $LaS i_{2} (112)$ epitaxy on Si(111). (a) Lattice schematics of $LaS i_{2} (112)$ in top and side views. The crystalline orientation indexes are labeled relative to the $LaS i_{2}$ lattice. (b) Schematic of the in-plane epitaxial relationship of $LaS i_{2} (112)$ on Si(111). Three equivalent domains (with the unit cells labeled by the black rectangles) may be formed associated with the threefold symmetry of Si(111). The honeycomb lattice is the Si(111) bilayer with the unit cell labeled by the dashed green rhombus. The green dashed rectangle illustrates the growth template supplied by Si(111) lattice ( $2 a_{0}$ and $\sqrt{3} a_{0}$ along $[1 \bar{1} 0]$ and $[11 \bar{2}]$ directions, respectively, $a_{0} = 3.86 Å$ ) for a “fully strained” epitaxial film. The black rectangle labels the unit cell of $LaS i_{2} (112)$ . The orientation indexes of Si are also labeled in green for reference. (c) STEM image of the $LaS i_{2} (112) / Si (111)$ along $[1 \bar{1} 0]$ of $LaS i_{2}$ . The lattice schematics of $LaS i_{2}$ and Si are superimposed. (d) Schematic drawing of the lattice relaxation of $LaS i_{2} (112) / Si (111)$ interface along $[1 \bar{1} 0]$ of $LaS i_{2}$ . The red and black dots illustrate the lattice alignment.
Reuse & Permissions
Figure 3
Stripes on the $LaS i_{2} (112)$ films with different thickness. (a)–(e) The STM images (−1 V/50 pA) of the $LaS i_{2}$ surfaces with different thickness. The inset of (a) shows the zoomed-in image. The orientation of the stripes is about $10^{\circ}$ off the $a$ direction of $LaS i_{2}$ lattice. With the film thickness increasing from 4 to 70 ML, the interspacing of the stripes $d$ increases from 5.2 to $> 70 nm$ . (f) The inverse of the stripe interspacing $(1 / d)$ as a function of $t$ , fitted by an exponential function ( $1 / d = 0.33 e^{- t / 11.77}$ ).
Reuse & Permissions
Figure 4
Superconductivity of $LaS i_{2} (112)$ films measured by electrical transport and STS. (a) $ρ_{a b} - T$ of the 90-ML $LaS i_{2} (112)$ film. The inset shows the zoomed-in curve around $T_{c}$ . (b), (c) The temperature-dependent $d I / d V$ spectra taken in zero magnetic field and the magnetic-field-dependent $d I / d V$ spectra taken at 2.3 K on the 126-ML $LaS i_{2} (112)$ film, respectively. The spectra are shifted vertically for clear presentation. The black dashed lines indicate the position of the coherent peaks. (d) BCS fitting (red solid curve) of measured superconducting gap (black dots) at 2.3 K of the 126-ML film.
Reuse & Permissions
Figure 5
Interfacial effects on the superconductivity of $LaS i_{2} (112)$ films on Si(111). (a) Thickness-dependent $1 / d$ (the blue line is guide to the eye), superconducting gap (the maximum values of $Δ_{\max}$ are taken with the spatial inhomogeneity neglected; see the main text), and $T_{c}$ (taken as the gap-opening temperature). (b) Experimental $d I / d V$ spectra of the films with different thickness measured at 2.3 K. (c) STM image of the stripes on the 7-ML $LaS i_{2}$ surface. (d) The $d I / d V$ spectra at 2.3 K taken at the positions marked by the arrows in the corresponding colors in (c). All the spectra are normalized following the method described in Ref. [37]. The superconducting gaps are labeled by the dashed lines and arrows, respectively. Note that all the spectra in (b) and (d) are shifted vertically for clear presentation.
Reuse & Permissions
Figure 6
(a) $d I / d V$ spectra taken at 4.3 K on the $LaS i_{2} (112)$ films with different thickness. The spectra are shifted vertically for clear presentation. The olive curve at the bottom is the calculated DOS of the 1-ML $LaS i_{2} (112)$ on Si(111) for comparison. The reduced DOS of unoccupied states might be related to the interfacial charge redistribution, since such DOS reduction is absent in multilayered $LaS i_{2}$ (not shown). The vertical dashed line indicates the DOS suppression at $E_{F}$ in the thin films and the calculated model. (b) Calculated charge redistribution at the $LaS i_{2} / Si (111)$ interface. Cyan (yellow) color represents charge depletion (accumulation) region relative to the pristine $LaS i_{2}$ and Si, respectively. Blue and gray circles are La and Si atoms, respectively.
Reuse & Permissions

Physical Review B

covering condensed matter and materials physics

Interfacial effects on the superconducting properties of $LaS i_{2} (112)$ films on Si(111)

Lixia Liu, Guangyao Miao, Bing Liu, Pengfei Nan, Yade Wang, Binghui Ge, Xuetao Zhu, Fang Yang, Weihua Wang, and Jiandong Guo

Phys. Rev. B 100, 165308 – Published 30 October 2019

Abstract

Physics Subject Headings (PhySH)

Authors & Affiliations

Article Text (Subscription Required)

References (Subscription Required)

Issue

Access Options

Physical Review B

covering condensed matter and materials physics

Interfacial effects on the superconducting properties of LaSi2(112) films on Si(111)

Lixia Liu, Guangyao Miao, Bing Liu, Pengfei Nan, Yade Wang, Binghui Ge, Xuetao Zhu, Fang Yang, Weihua Wang, and Jiandong Guo

Phys. Rev. B 100, 165308 – Published 30 October 2019

Abstract

Physics Subject Headings (PhySH)

Authors & Affiliations

Article Text (Subscription Required)

References (Subscription Required)

Issue

Access Options

Authorization Required

Other Options

Download & Share

Images

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Log In

Search

Article Lookup

Paste a citation or DOI

Enter a citation

Interfacial effects on the superconducting properties of $LaS i_{2} (112)$ films on Si(111)