Resistive phase transition of the superconducting Si(111)-( )-In surface
Nanoscale Research Letters
Resistive phase transition of the √ superconducting Si(111)-( 7 ×
Takashi Uchihashi 0
Puneet Mishra 0
Tomonobu Nakayama 0
0 International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS) , 1-1, Namiki, Tsukuba, Ibaraki 305-0044 , Japan
Recently, superconductivity was found on semiconductor surface reconstructions induced by metal adatoms, promising a new field of research where superconductors can be studied from the atomic level. Here we measure the electron transport properties of the Si(111)-(√7 × √3)-In surface near the resistive phase transition and analyze the data in terms of theories of two-dimensional (2D) superconductors. In the normal state, the sheet resistances (2D resistivities) R of the samples decrease significantly between 20 and 5 K, suggesting the importance of the electron-electron scattering in electron transport phenomena. The decrease in R is progressively accelerated just above the transition temperature (Tc) due to the direct (Aslamazov-Larkin term) and the indirect (Maki-Thompson term) superconducting fluctuation effects. A minute but finite resistance tail is found below Tc down to the lowest temperature of 1.8 K, which may be ascribed to a dissipation due to free vortex flow. The present study lays the ground for a future research aiming to find new superconductors in this class of materials.
Surface reconstruction; Silicon; Indium; Superconductivity; Electron transport; Fluctuation effects; Vortex flow
-
Background
Semiconductor surface reconstructions induced by metal
adatoms constitute a class of two-dimensional (2D)
materials with an immense variety [1,2]. They are considered
one form of atomic layer materials which can possess
novel electronic properties and device applications [3,4].
Recently, superconductivity was measured by scanning
tunneling microscopy (STM) for atomically thin Pb films
[5,6] and three kinds of Si(11√1) surface reconstructions:
√ √ √
SIC-Pb, ( 7 × 3)-Pb, and ( 7 × 3)-In [7]. This
discovery was followed by a demonstration√of ma√croscopic
superconducting currents on Si(111)-( 7 × 3)-In by
direct electron transport measurements [8]. These
findings are important because they enable us to create
superconductors from the atomic level using state-of-the-art
nanotechnology. In addition, the space inversion
symmetry breaking due to the presence of surface naturally leads
to the Rashba spin splitting [9,10] and may consequently
help realize exotic superconductors [11].
In reference[8], we√have unambiguously clarified the
√ √
p√resence of Si(111)-( 7 × 3)-In (referred to as ( 7 ×
3)-In here) superconductivity. However, systematic
analysis on electron transport properties above and below
the transition temperature (Tc) is still lacking. For
example, 2D superconductors are known to exhibit the
precursor of phase transition due to the thermal fluctuation
effects just above Tc [12-14]. Superconductivity is
established below Tc, but vortices can be thermally excited in
a 2D system. Their possible motions can cause the phase
fluctuation and limit the ideal superconducting property
of perfect zero resistance [15]. These fundamental
properties should be revealed before one proceeds to search for
new superconductors in this class of 2D materials. √
√In this paper, the resistive phase transition of the ( 7 ×
3)-In surface is studied in detail for a series of samples.
In the normal state, the sheet resistances (2D resistivities)
R of the samples decrease significantly between 20 and
5 K, which amounts to 5% to 15% of the residual resis
tivity Rn,res. Their characteristic temperature dependence
suggests the importance of electron-electron scattering
in electron transport phenomena, which are generally
marginal for conventional metal thin films. Tc is
determined to be 2.64 to 2.99 K and is found to poorly
correlate with Rn,res. The decrease in R is progressively
accelerated just above Tc due to the superconducting
fluctuation effects. Quantitative analysis indicates the
parallel contributions of fluctuating Cooper pairs due to the
direct (Aslamazov-Larkin term) and the indirect
(MakiThompson term) effects. A minute but finite resistance
tail is found below Tc down to the lowest temperature of
1.8 K, which may be ascribed to a dissipation due to free vortex flow.
Methods
The experimental method basically follows the procedure
described in reference [8] but includes some
modifications. The whole procedure from the sample preparation
through the transport measurement was performed in a
home-built ultrahigh vacuum (UHV) apparatus wi√thout
breaking vacuum (see Figure 1a) [16,17]. First, the ( 7 ×
√
3)-In surface was prepared by thermal evaporation of
In onto a clean Si(111) substrate, followed by annealing
at around 300°C for approximately 10 s in UHV [18-20],
and was subsequently confirmed by low-energy electron
diffraction and STM. The sample was then patterned by
Ar+ sputtering through a shadow (...truncated)