[Paper Review] Signatures of Chiral Superconductivity in Rhombohedral Graphene

Tonghang Han, Zhengguang Lu+21 more|arXiv (Cornell University)|Aug 27, 2024

Graphene research and applicationsMaterials Science3 citations

TL;DR

This study reports robust unconventional superconductivity in rhombohedral tetra- and pentalayer graphene without moiré superlattices, demonstrating signatures of chiral superconductivity through spontaneous time-reversal symmetry breaking, magnetic hysteresis in resistivity under out-of-plane fields, immunity to in-plane magnetic fields, and a critical field up to 1.4 T. The findings establish a pure carbon platform for topological superconductivity and Majorana fermion research.

ABSTRACT

Chiral superconductors are unconventional superconducting states that break time reversal symmetry spontaneously and typically feature Cooper pairing at non-zero angular momentum. Such states may host Majorana fermions and provide an important platform for topological physics research and fault-tolerant quantum computing. Despite intensive search and prolonged studies of several candidate systems, chiral superconductivity has remained elusive so far. Here we report the discovery of robust unconventional superconductivity in rhombohedral tetra- and penta-layer graphene in the absence of moiré superlattice effects. We observed two superconducting states in the gate-induced flat conduction bands with Tc up to 300 mK and charge density ne as low as 2.4*1011 cm-2 in three tetralayer and two pentalayer devices. Spontaneous time-reversal-symmetry-breaking (TRSB) due to electron's orbital motion is found, and several observations indicate the chiral nature of these superconducting states, including: 1. In the superconducting state, Rxx shows magnetic hysteresis in varying out-of-plane magnetic field B, which is absent from all other superconductors; 2. the superconducting states are immune to in-plane magnetic field and are developed within a spin- and valley-polarized quarter-metal phase; 3. the normal states show anomalous Hall signals at zero magnetic field and magnetic hysteresis. We also observed a critical B of up to 1.4 Tesla, higher than any graphene superconductivity reported so far and indicates a strong-coupling superconductivity close to the BCS-BEC crossover. Our observations establish a pure carbon material for the study of topological superconductivity, and pave the way to explore Majorana modes and topological quantum computing.

Motivation & Objective

• To identify and characterize unconventional superconductivity in rhombohedral multilayer graphene without moiré superlattices.
• To determine whether the observed superconducting states break time-reversal symmetry spontaneously.
• To investigate the chiral nature of Cooper pairing in flat-band graphene systems.
• To explore the potential for hosting Majorana fermions in a carbon-based platform.
• To establish a strong-coupling superconducting state near the BCS-BEC crossover in a van der Waals heterostructure.

Proposed method

• Gate-tuning of rhombohedral tetra- and pentalayer graphene devices to induce flat conduction bands with low carrier density (ne = 2.4×10¹¹ cm⁻²).
• Measurement of electrical transport properties, including longitudinal resistance Rxx, under varying out-of-plane and in-plane magnetic fields.
• Detection of anomalous Hall signals in the normal state and magnetic hysteresis in Rxx under out-of-plane B-field sweeps.
• Analysis of superconducting transition temperature Tc up to 300 mK and critical field Bc up to 1.4 T.
• Use of spin- and valley-polarized quarter-metal phase as a precursor to chiral superconductivity.
• Comparison of transport responses across multiple devices to confirm reproducibility of time-reversal-symmetry-breaking signatures.

Experimental results

Research questions

• RQ1 Does superconductivity in rhombohedral multilayer graphene exhibit spontaneous time-reversal symmetry breaking?
• RQ2 Are the observed superconducting states chiral, as indicated by magnetic hysteresis in Rxx under out-of-plane magnetic fields?
• RQ3 Can chiral superconductivity emerge in a pure carbon system without moiré superlattices?
• RQ4 What is the strength of the superconducting critical field, and does it indicate strong coupling near the BCS-BEC crossover?
• RQ5 Is the superconducting state robust against in-plane magnetic fields, consistent with chiral pairing symmetry?

Key findings

• Two superconducting states were observed in rhombohedral tetra- and pentalayer graphene with Tc up to 300 mK and carrier density as low as 2.4×10¹¹ cm⁻².
• Magnetic hysteresis in Rxx under out-of-plane magnetic field was observed exclusively in the superconducting state, indicating spontaneous time-reversal symmetry breaking.
• The superconducting states are robust against in-plane magnetic fields, consistent with chiral pairing symmetry.
• Anomalous Hall signals and magnetic hysteresis in the normal state suggest a pre-existing chiral state, possibly a quarter-metal phase.
• The critical magnetic field reaches up to 1.4 T, the highest reported in graphene superconductors, indicating strong-coupling behavior near the BCS-BEC crossover.
• The absence of moiré superlattices confirms that chiral superconductivity arises from intrinsic electronic correlations in rhombohedral stacking.