Superconductivity-enhanced conductance fluctuations in few-layer graphene

Area of Science:

• Condensed Matter Physics
• Mesoscopic Physics
• Materials Science

Background:

• Few-layer graphene (FLG) exhibits unique electronic properties influenced by mesoscopic disorder.
• Superconducting contacts introduce quantum phenomena like Andreev reflection at the graphene-superconductor interface.
• Understanding conductance fluctuations is key to characterizing disorder and quantum transport in nanoscale devices.

Purpose of the Study:

• To investigate the mesoscopic disorder-induced root-mean-square (rms) conductance variance (delta G) in short few-layer graphene flakes.
• To analyze the dependence of delta G on temperature (T) and source-drain voltage (Vsd).
• To explore the role of superconducting contacts and Andreev reflection in conductance fluctuations.

Main Methods:

• Fabrication of short few-layer graphene flakes contacted by superconducting Ti/Al electrodes.
• Measurement of two-terminal conductance (G) and its fluctuations (delta G) via back-gate voltage sweeping.
• Systematic variation of temperature (T) and source-drain voltage (Vsd) to study their influence on delta G.
• Analysis of conductance variance in relation to the superconducting energy gap (Delta).

Main Results:

• Pronounced conductance fluctuations were observed, superimposed on a linear background conductance.
• delta G increases with decreasing temperature (T) and decreasing absolute source-drain voltage (|Vsd|).
• A significant cross-over in delta G was detected at a voltage corresponding to the superconducting energy gap (Delta).
• The conductance variance at the graphene-superconductor (G-S) interface (G(GS)) reached 0.58 e(2)/h at 230 mK.
• Conductance variance in the sub-gap region was 1.4-1.8 times larger than in the normal state.

Conclusions:

• The observed enhancement of conductance fluctuations in the superconducting state is attributed to phase-coherent charge transfer via Andreev reflection.
• Mesoscopic disorder significantly impacts quantum transport in few-layer graphene, especially when coupled to superconductors.
• Andreev reflection at the graphene-superconductor interface plays a crucial role in modulating conductance fluctuations in the sub-gap regime.