[Paper Review] Observation of Majorana Fermions in a Nb-InSb Nanowire-Nb Hybrid Quantum Device

M. T. Deng, Chunlin Yu+4 more|arXiv (Cornell University)|Apr 18, 2012

Topological Materials and PhenomenaPhysics and Astronomy29 references636 citations

TL;DR

This study reports the experimental observation of Majorana fermions in a Nb-InSb nanowire-Nb hybrid quantum device, where proximity-induced superconductivity and a Zeeman field from an external magnetic field drive the system into a nontrivial topological superconductor phase. A robust zero-bias conductance plateau at 0.9–2.6 T, dependent on gate voltage and suppressed at higher fields, provides strong evidence for Majorana bound states at the nanowire ends.

ABSTRACT

We report on the observation of excitation of Majorana fermions in a Nb-InSb nanowire quantum dot-Nb hybrid system. The InSb nanowire quantum dot is formed between the two Nb contacts by weak Schottky barriers and is thus in the regime of strong couplings to the contacts. Due to the proximity effect, the InSb nanowire segments covered by superconductor Nb contacts turn to superconductors with a superconducting energy gap $Δ^*$. Under an applied magnetic field larger than a critical value for which the Zeeman energy in the InSb nanowire is $E_z\sim Δ^*$, the entire InSb nanowire is found to be in a nontrivial topological superconductor phase, supporting a pair of Majorana fermions, and Cooper pairs can transport between the superconductor Nb contacts via the Majorana fermion states. This transport process will be suppressed when the applied magnetic field becomes larger than a second critical value at which the transition to a trivial topological superconductor phase occurs in the system. This physical scenario has been observed in our experiment. We have found that the measured zero-bias conductance for our hybrid device shows a conductance plateau in a range of the applied magnetic field in quasi-particle Coulomb blockade regions.

Motivation & Objective

• To demonstrate a scalable, transport-based method for detecting Majorana fermions in solid-state systems using a hybrid superconductor-semiconductor nanowire device.
• To overcome challenges in magnetic field penetration and detection in fully superconducting-wrapped nanowires by using a Josephson junction configuration with two Nb contacts.
• To establish a platform where Majorana fermions can be probed via standard electrical transport measurements in a well-defined topological phase.
• To verify the emergence of Majorana bound states through a quantized zero-bias conductance plateau under controlled magnetic fields and gate voltages.

Proposed method

• Fabricated a Nb-InSb nanowire-Nb Josephson junction device using electron beam lithography, sputtering, and lift-off to define 80 nm-wide Nb contacts on an InSb segment of an InAs/InSb heterostructure nanowire.
• Employed surface chemical treatment with (NH₄)₂Sₓ to remove native oxide and ensure strong proximity coupling between Nb and InSb.
• Used a back-gated design to tune the electron density and control the phase difference between the two Nb superconducting contacts.
• Performed low-temperature transport measurements (down to 20 mK) in a ³He/⁴He dilution refrigerator to probe differential conductance and Josephson current.
• Applied perpendicular magnetic fields to induce Zeeman splitting and drive the system into a topological superconductor phase when E_z ≈ Δ*.
• Analyzed the evolution of the differential conductance and zero-bias peak to identify signatures of Majorana fermions, including conductance plateau and gap suppression.

Experimental results

Research questions

• RQ1 Can Majorana fermions be experimentally detected in a hybrid superconductor-semiconductor nanowire device using standard electrical transport?
• RQ2 Does a zero-bias conductance plateau emerge in a magnetic field range consistent with the topological superconductor phase transition?
• RQ3 How does the conductance plateau depend on gate voltage and magnetic field, and what does this imply about the phase coherence and topological nature of the system?
• RQ4 What is the role of the Josephson junction configuration in enabling detection of Majorana states despite the Meissner effect in fully superconducting contacts?
• RQ5 Can the observed conductance features be explained by a transition from a nontrivial to a trivial topological superconductor phase?

Key findings

• A robust zero-bias conductance plateau was observed in the differential conductance between 0.9 T and 2.6 T, indicating the presence of Majorana bound states.
• The plateau height varied with back-gate voltage, suggesting dependence on the phase difference between the two Nb superconducting contacts.
• At zero magnetic field, the device exhibited Josephson supercurrent and multiple Andreev reflection, confirming proximity-induced superconductivity in the InSb nanowire.
• The superconducting gap of Nb (Δ_Nb) and the InSb segment (Δ_InSb) were experimentally resolved from the multiple Andreev reflection features.
• As the magnetic field increased beyond 2.6 T, the zero-bias conductance dropped sharply, indicating a transition to a trivial topological superconductor phase.
• The evolution of the superconducting gap—nearly vanishing at the edges of the plateau and reopening at higher fields—matched theoretical predictions for topological phase transitions.