[Paper Review] Magnetic-field-free nonreciprocal transport in graphene multi-terminal Josephson junctions

Fan Zhang, Asmaul Smitha Rashid+7 more|arXiv (Cornell University)|Jan 12, 2023

Quantum and electron transport phenomena47 references4 citations

TL;DR

This paper demonstrates magnetic-field-free, reconfigurable nonreciprocal transport in multi-terminal graphene Josephson junctions by breaking spatial mirror symmetry in asymmetric graphene channels. The nonreciprocity arises from unequal critical currents between ports (I_c,jk ≠ I_c,kj) due to asymmetric Josephson junctions, achieving up to 30% theoretical efficiency in a circuit-network model, with experimental validation showing ~24% efficiency in three-terminal devices.

ABSTRACT

Nonreciprocal superconducting devices have attracted growing interest in recent years as they potentially enable directional charge transport for applications in superconducting quantum circuits. Specifically, the superconducting diode effect has been explored in two-terminal devices that exhibit superconducting transport in one current direction while showing dissipative transport in the opposite direction. Here, we exploit multi-terminal Josephson junctions (MTJJs) to engineer magnetic-field-free nonreciprocity in multi-port networks. We show that when treated as a two-port electrical network, a three-terminal Josephson junction (JJ) with an asymmetric graphene region exhibits reconfigurable two-port nonreciprocity. We observe nonreciprocal (reciprocal) transport between superconducting terminals with broken (preserved) spatial mirror symmetry. We explain our observations by considering a circuit-network of JJs with different critical currents.

Motivation & Objective

• To achieve nonreciprocal superconducting transport without external magnetic fields or current bias in multi-terminal Josephson junctions.
• To demonstrate that spatial mirror symmetry breaking in asymmetric graphene channels induces two-port nonreciprocity.
• To develop a circuit-network model of Josephson junctions with differing critical currents to explain and predict nonreciprocal behavior.
• To quantify the efficiency of nonreciprocal transport and show its dependence on junction asymmetry.
• To validate the theoretical model experimentally in three- and four-terminal graphene-based Josephson junctions.

Proposed method

• Fabricated hBN/graphene/hBN van der Waals heterostructures with edge contacts to form multi-terminal Josephson junctions.
• Engineered asymmetric graphene channel geometries to break spatial mirror symmetry between specific terminals.
• Used a resistively shunted junction (RSJ) model with zero capacitance to simulate voltage-current characteristics and critical currents.
• Defined nonreciprocity via unequal critical currents: I_c,jk ≠ I_c,kj, where I_c,kj is the maximum current at terminal k when current is injected at terminal j.
• Modelled the system as a network of Josephson junctions with distinct critical currents (I_c1, I_c2, I_c3), using the current-phase relation I = I_c sin(ϕ) + G dϕ/dt.
• Varied the asymmetry parameter x = I_c3 / I_c1 to study its effect on nonreciprocal efficiency η, with numerical simulations showing a plateau at ~30%.

Experimental results

Research questions

• RQ1 Can nonreciprocal transport be achieved in multi-terminal Josephson junctions without applying an external magnetic field or current bias?
• RQ2 How does spatial mirror symmetry breaking in the graphene channel influence two-port nonreciprocity?
• RQ3 What is the theoretical maximum efficiency of nonreciprocal transport in such systems, and how does it depend on junction asymmetry?
• RQ4 Can a circuit-network model of Josephson junctions with different critical currents quantitatively explain the observed nonreciprocity?
• RQ5 Is the single-port definition of nonreciprocity (I_c^+ ≠ I_c^-) valid for multi-port superconducting networks?

Key findings

• Nonreciprocal transport was experimentally observed in three- and four-terminal graphene Josephson junctions without any external magnetic field or bias current.
• Nonreciprocity emerged only between terminals where spatial mirror symmetry was broken, while reciprocal transport persisted between symmetric terminals.
• The nonreciprocal efficiency reached approximately 24% in the three-terminal device, closely matching the theoretical maximum of 30% predicted by the RSJ model.
• The efficiency increased with increasing asymmetry (x = I_c3 / I_c1) and saturated at ~30% for high x, indicating a fundamental upper bound.
• A four-terminal device with higher channel symmetry showed only ~7% efficiency, confirming the critical role of mirror symmetry breaking.
• The study demonstrated that the single-port definition of nonreciprocity fails in multi-port networks, as reciprocal junctions can appear nonreciprocal under such definitions.