[Paper Review] Possible superconductivity in multi-layer-graphene by application of a gate voltage

A. Ballestar, P. Esquinazi+5 more|arXiv (Cornell University)|Feb 15, 2012

Graphene research and applications23 references3 citations

TL;DR

This study demonstrates gate voltage-induced superconductivity in multi-layer graphene (MLG) at temperatures up to ~17 K, driven by electric-field-controlled carrier density modulation. Using transport and surface potential microscopy, the authors identify inhomogeneous carrier doping that triggers a 3D-like superconducting transition, suppressed by both parallel and perpendicular magnetic fields, suggesting filamentary superconducting paths connecting intrinsic 2D superconducting regions via gate-induced 3D pathways.

ABSTRACT

The carrier density in tens of nanometers thick graphite samples (multi-layer-graphene, MLG) has been modified by applying a gate voltage ($V_g$) perpendicular to the graphene planes. Surface potential microscopy shows inhomogeneities in the carrier density ($n$) in the sample near surface region and under different values of $V_g$ at room temperature. Transport measurements on different MLG samples reveal that under a large enough applied electric field these regions undergo a superconducting-like transition at $T \lesssim 17$ K. A magnetic field applied parallel or normal to the graphene layers suppresses the transition without changing appreciably the transition temperature.

Motivation & Objective

• To explore electric-field-induced superconductivity in undoped, intrinsic multi-layer graphene (MLG) without chemical intercalation.
• To determine whether gate voltage can induce sufficient carrier density to trigger superconducting transitions in MLG at low temperatures.
• To investigate the magnetic field response and anisotropy of the superconducting transition to infer the dimensionality and nature of the superconducting state.
• To clarify whether the observed transition arises from intrinsic 2D superconducting regions or from 3D connecting paths formed by electrostatic doping.

Proposed method

• Application of a perpendicular gate voltage (Vg) to tens of nanometers thick MLG samples to modulate carrier density across the graphene planes.
• Use of surface potential microscopy to map spatial inhomogeneities in carrier density (n) at room temperature under varying Vg.
• Measurement of temperature-dependent resistance under various gate voltages to detect superconducting-like transitions.
• Application of magnetic fields both parallel and perpendicular to the graphene planes to probe the nature and dimensionality of the superconducting state.
• Comparison of resistance behavior under different field orientations to identify orbital or pair-breaking effects.
• Analysis of resistance minima and field evolution to infer the presence of filamentary 3D superconducting paths connecting intrinsic 2D superconducting regions.

Experimental results

Research questions

• RQ1 Can gate voltage alone induce superconductivity in undoped multi-layer graphene without chemical doping or intercalation?
• RQ2 What is the origin of the superconducting-like transition observed at ~17 K under negative gate voltage?
• RQ3 Why does the transition show weak anisotropy in magnetic field response, and what does this imply about the dimensionality of the superconducting state?
• RQ4 How do inhomogeneities in carrier density affect the formation and stability of superconducting paths in MLG?
• RQ5 What role do 3D connecting paths play in the observed superconducting behavior, and how do they differ from intrinsic 2D superconducting regions?

Key findings

• A superconducting-like transition was observed at T ≤ 17 K in multi-layer graphene samples under a gate voltage of -100 V, indicating a critical temperature near 17 K.
• Surface potential microscopy revealed spatial inhomogeneities in carrier density (n) at room temperature, suggesting non-uniform electrostatic doping.
• Magnetic fields applied both parallel and perpendicular to the graphene planes suppressed the transition without significantly shifting the critical temperature, indicating weak field anisotropy.
• A reentrant behavior in resistance was observed under normal magnetic fields, suggesting orbital effects or enhanced Andreev reflection probability at high fields.
• The suppression of the transition by a small perpendicular field (0.1 T) and the weak anisotropy point to the influence of 3D filamentary paths rather than purely 2D superconducting regions.
• The results support the hypothesis that gate voltage triggers 3D superconducting paths connecting intrinsic 2D superconducting regions, with the 3D transition temperature closer to that of intercalated graphite compounds than to 2D superconducting states.