Signatures of Andreev Blockade in a Double Quantum Dot Coupled to a Superconductor

At a Glance

Metadata	Details
Publication Date	2022-01-25
Journal	Physical Review Letters
Authors	Po Zhang, Hao Wu, Jun Chen, Sabbir A Khan, Peter Krogstrup
Institutions	University of Copenhagen, University of Pittsburgh
Citations	17
Analysis	Full AI Review Included

6CCVD Technical Documentation: Evidence of Andreev Blockade in a Double Quantum Dot Coupled to a Superconductor

This documentation analyzes the key technical requirements and outcomes of the published research on Andreev Blockade (arXiv:2102.03283v1) and connects them to 6CCVD’s advanced MPCVD diamond capabilities for potential replication and future development in quantum devices.

Executive Summary

The research successfully demonstrated Andreev Blockade (AB), a quantum phenomenon where Andreev reflections are suppressed by a localized spin-triplet state in a double quantum dot (DQD) coupled to a superconductor (S).

Core Mechanism: AB arises from the suppression of Cooper pair formation when the DQD holds a spin-triplet configuration, proving highly sensitive to spin parity mismatch at the DQD/S interface.
Device Architecture: An InAs semiconductor nanowire featuring an epitaxial Aluminum (Al) shell (the Superconductor, S) was used, with DQD states defined electrostatically by nanoscale gates.
Key Achievement: Verified the four predicted signatures (A-D) of AB, including quadruple charge degeneracy points and current asymmetry dependent on source-drain bias.
Environmental Sensitivity: Blockade signatures disappeared when superconductivity was suppressed, either by raising the temperature above the bulk critical temperature of Al (1.2 K) or applying a magnetic field (0.6 T).
Material Limitations & Future Path: Observed leakage currents indicate partial suppression of blockade, necessitating the use of larger induced-gap superconductors (like Sn or Pb, up to 1.2 mV gap) to achieve stronger blockade and increase the operating temperature range.
Application: The finding offers a novel method for studying spin-resolved transport in hybrid quantum devices, with direct relevance to developing Majorana zero modes and Andreev qubits.

Technical Specifications

The following hard data points were extracted, detailing device performance, material configurations, and environmental conditions required for the observation of Andreev Blockade.

Parameter	Value	Unit	Context
Base Measurement Temperature (T)	40	mK	Operational temperature in a dilution refrigerator.
Aluminum (Al) Bulk T_c	1.2	K	Critical temperature above which superconductivity/AB signatures disappear.
Induced Superconducting Gap (Δ/e)	0.2 (Consistent with 0.2-0.24)	mV	Observed sub-gap transport region in the QDs.
Inter-dot Charging Energy ($U_{NS}$)	10	µeV	Set point for the weakly coupled double dot regime.
Maximum Applied Magnetic Field (B)	0.6	T	Field required to suppress superconductivity/AB signatures.
Gate Metalization Bilayer	1.5 / 6	nm	Thicknesses of Ti / PdAu (E-beam evaporated).
Gate Dielectric Material / Thickness	HfO₂ / 10	nm	Atomic Layer Deposition (ALD) for gate isolation.
Source/Drain Lead Metalization	10 / 130	nm	Thicknesses of Ti / Au (E-beam evaporated).
Gate Pitch Resolution	60	nm	Required spatial resolution for electrostatic definition of dots.
QDN Charging Energy (E_c)	~4	mV	Charging energy of the Normal Dot (QDN).

Key Methodologies

The experiment utilized sophisticated epitaxial growth and high-resolution lithography to create the hybrid superconductor-semiconductor nanowire device.

InAs Nanowire Growth: InAs nanowires were grown using Molecular Beam Epitaxy (MBE) employing a Vapor-Liquid-Solid mechanism catalyzed by predefined Au droplets.
Superconducting Shell Epitaxy: An Aluminum (Al) shell (~15 nm) was grown in situ via MBE immediately after nanowire growth to ensure a high-quality, clean InAs/Al interface necessary for inducing a hard superconducting gap.
Gate Definition (EBL): Electrostatic gates (60 nm pitch) were patterned using 100 kV E-beam Lithography (EBL) with PMMA 950 A1 resist developed at -15 °C for enhanced resolution.
Gate Fabrication: A bilayer of 1.5 nm Titanium (Ti) / 6 nm Palladium-Gold (PdAu) was deposited via e-beam evaporation, followed by the deposition of 10 nm HfO₂ using Atomic Layer Deposition (ALD, 100 cycles) as the dielectric insulator.
Selective Etching: A critical step involved selectively etching the Al shell (Superconductor) over the QDN region using MF CD-26 developer/DI water (1:20 ratio) to create the normal metal interface without damaging the InAs core.
Lead Fabrication: Source and drain leads were defined by EBL, followed by Argon (Ar) cleaning and e-beam evaporation of 10 nm Ti / 130 nm Au contacts.
Measurement: Differential conductance spectra were taken in a dilution refrigerator at a base temperature of 40 mK, tuned by electrostatic gate voltages ($V_{N}$ and $V_{S}$).

6CCVD Solutions & Capabilities

6CCVD provides the enabling material platforms, processing expertise, and engineering support necessary to replicate this complex hybrid system and advance next-generation quantum research, particularly concerning spin-based transport and Andreev phenomena.

The current research highlights the need for a stable substrate/dielectric, pristine interfaces, and robust thermal management—all areas where 6CCVD’s MPCVD diamond excels.

Applicable Materials

To achieve the “larger hard gaps” proposed for stronger blockade (e.g., interfacing with Sn or Pb superconductors) and increased operational temperature, exceptional thermal management and dielectric stability are critical.

Optical Grade Single Crystal Diamond (SCD): Offers unparalleled thermal conductivity (up to 2000 W/mK) to efficiently sink localized heat and stabilize superconducting interfaces, essential for maximizing the induced hard gap (Δ). Available in thicknesses from 0.1 µm to 500 µm.
High-Purity SCD Wafers: Provide an ultra-stable, low-defect platform, superior to typical semiconductor substrates, crucial for maintaining the precise electrostatic potentials needed to define stable quantum dots over large arrays.

Customization Potential

The experiment relied on specific gate and lead metalizations (Ti/PdAu, Ti/Au). 6CCVD can replicate and optimize these integrations directly onto diamond substrates.

Precision Polishing and Surface Prep: 6CCVD guarantees ultra-low surface roughness (Ra < 1 nm for SCD), providing an ideal, flat surface for high-quality epitaxial growth of nanowires (InAs) or deposition of superconducting thin films (Al, Sn, Pb).
Custom Metalization: 6CCVD offers in-house deposition of all critical materials used in this study (Ti, Au, Pd, Pt, W, Cu) allowing for rapid prototyping of custom gate and lead configurations required for complex N-QDN-QDS-S device geometry.
Custom Dimensions and Etching: We provide custom SCD and PCD plates up to 125 mm and precise laser cutting and etching services necessary to integrate diamond into unique device architectures, such as complex multi-terminal quantum transport setups.

Engineering Support

The successful extension of this work requires sophisticated material integration, particularly solving the challenges associated with creating harder superconducting gaps and managing interface defects mentioned in the paper (e.g., residual Al grains, wet etch damage).

Hybrid Material Consultation: 6CCVD’s in-house PhD engineering team specializes in material selection and integration for hybrid superconductor-semiconductor (S-Sm) systems and quantum dot fabrication, helping researchers transition from small-scale nanowire tests to robust, scalable diamond-based quantum circuits.
Defect Mitigation: We assist clients in designing material stacks and processing recipes that leverage diamond’s chemical inertness and high purity to mitigate mesoscopic factors and material defects that complicate the observation of clear Andreev blockade.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

We investigate an electron transport blockade regime in which a spin triplet localized in the path of current is forbidden from entering a spin-singlet superconductor. To stabilize the triplet, a double quantum dot is created electrostatically near a superconducting Al lead in an InAs nanowire. The quantum dot closest to the normal lead exhibits Coulomb diamonds, and the dot closest to the superconducting lead exhibits Andreev bound states and an induced gap. The experimental observations compare favorably to a theoretical model of Andreev blockade, named so because the triplet double dot configuration suppresses Andreev reflections. Observed leakage currents can be accounted for by finite temperature. We observe the predicted quadruple level degeneracy points of high current and a periodic conductance pattern controlled by the occupation of the normal dot. Even-odd transport asymmetry is lifted with increased temperature and magnetic field. This blockade phenomenon can be used to study spin structure of superconductors. It may also find utility in quantum computing devices that use Andreev or Majorana states.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevlett.128.046801