Query: Two-dimensional epitaxial superconductor-semiconductor heterostructures: A platform for topological superconducting networks

Candidate papers:

abstract: Progress in the emergent field of topological superconductivity relies on synthesis of new material combinations, combining superconductivity, low density, and spin orbit coupling (SOC) For example, theory [1 4] indicates that the interface between a one dimensional (1D) semiconductor (Sm) with strong SOC and a superconductor (S) hosts Majorana modes with nontrivial topological properties [5 8] Recently, epitaxial growth of Al on InAs nanowires was shown to yield a high quality S Sm system with uniformly transparent interfaces [9] and a hard induced gap, indicted by strongly suppressed sub gap tunneling conductance [10] Here we report the realization of a two dimensional (2D) InAs/InGaAs heterostructure with epitaxial Al, yielding a planar S Sm system with structural and transport characteristics as good as the epitaxial wires. The realization of 2D epitaxial S Sm systems represent a significant advance over wires, allowing extended networks via top down processing. Among numerous potential applications, this new material system can serve as a platform for complex networks of topological superconductors with gate controlled Majorana zero modes [1 4] We demonstrate gateable Josephson junctions and a highly transparent 2D S Sm interface based on the product of excess current and normal state resistance.
author_names: Javad Shabani

Morten Kjaergaard

Henri J Suominen

Younghyun Kim

F Nichele

K Pakrouski

Tomas Stankevic

R M Lutchyn

Peter Krogstrup

Robert Feidenhans'l

Steffen Kraemer

C Nayak

Matthias Troyer

Charles M Marcus

Chris J Palmstrom
title: Two dimensional epitaxial superconductor semiconductor heterostructures: A platform for topological superconducting networks
year: 2016

author_names: Javad Shabani

Morten Kjaergaard

Henri J Suominen

Younghyun Kim

F Nichele

K Pakrouski

Tomas Stankevic

R M Lutchyn

Peter Krogstrup

Robert Feidenhans'l

Steffen Kraemer

C Nayak

Matthias Troyer

Charles M Marcus

Chris J Palmstrom
title: Publisher's Note: Two dimensional epitaxial superconductor semiconductor heterostructures: A platform for topological superconducting networks [Phys. Rev. B 93 155402 (2016)
year: 2016

abstract: One dimensional (1D) electronic transport and induced superconductivity in semiconductor nanostructures are crucial ingredients to realize topological superconductivity. Our approach for topological superconductivity employs a two dimensional electron gas (2DEG) formed by an InAs quantum well, cleanly interfaced with an epitaxial superconductor (epi Al) This epi Al/InAs quantum well heterostructure is advantageous for fabricating large scale nanostructures consisting of multiple Majorana zero modes. Here, we demonstrate transport studies of building blocks using a high quality epi Al/InAs 2DEG heterostructure, which could be put together to realize various proposed 1D nanowire based nanostructures and 2DEG based networks that could host multiple Majorana zero modes. The studies include (1) gate defined quasi 1D channels in the InAs 2DEG and (2) quantum point contacts for tunneling spectroscopy, as well as induced superconductivity in (3) a ballistic Al InAs 2DEG Al Josephson junction. From 1D transport, systematic evolution of conductance plateaus in half integer conductance quanta is observed with Lande g factor of 17, indicating the strong spin orbit coupling and high quality of the InAs 2DEG. The improved 2DEG quality leads to ballistic Josephson junctions with enhanced characteristic parameters such as Ic Rn and Iexc Rn, the product of superconducting critical current Ic (and excess current Iexc) and normal resistance Rn. Our results of electronic transport studies based on the 2D approach suggest that the epitaxial superconductor/2D semiconductor system with improved 2DEG quality is suitable for realizing large scale nanostructures for quantum computing applications.
author_names: Joon Sue Lee

Borzoyeh Shojaei

Mihir Pendharkar

Anthony P McFadden

Younghyun Kim

Henri J Suominen

Morten Kjaergaard

F Nichele

Hao Zhang

Charles M Marcus

Chris J Palmstrom
title: Transport Studies of Epi Al/InAs Two Dimensional Electron Gas Systems for Required Building Blocks in Topological Superconductor Networks.
venue: Nano letters
year: 2019

abstract: We model theoretically a two dimensional electron gas (2DEG) covered by a superconductor and demonstrate that topological superconducting channels are formed when stripes of the superconducting layer are removed. As a consequence, Majorana bound states (MBSs) are created at the ends of the stripes. We calculate the topological invariant and energy gap of a single stripe, using realistic values for an InAs 2DEG proximitized by an epitaxial Al layer. We show that the topological gap is enhanced when the structure is made asymmetric. This can be achieved either by imposing a phase difference (by driving a supercurrent or using a magnetic flux loop) over the strip or by replacing one superconductor by a metallic gate. Both strategies also enable control over the MBS splitting, thereby facilitating braiding and readout schemes based on controlled fusion of MBSs. Finally, we outline how a network of Majorana stripes can be designed.
author_names: Michael Hell

Martin Leijnse

Karsten Flensberg
title: Two Dimensional Platform for Networks of Majorana Bound States.
venue: Physical review letters
year: 2017

abstract: Theory suggests that the interface between a one dimensional semiconductor (Sm) with strong spin orbit coupling and a superconductor (S) hosts Majorana modes with nontrivial topological properties. A key challenge in fabrication of such hybrid devices is forming highly transparent contacts between the active electrons in the semiconductor and the superconducting metal. Recently, it has been shown that a near perfect interface and a highly transparent contact can be achieved using epitaxial growth of aluminum on InAs nanowires. In this work, we present the first two dimensional epitaxial superconductor semiconductor material system that can serve as a platform for topological superconductivity. We show that our material system, Al InAs, satisfies all the requirements necessary to reach into the topological superconducting regime by individual characterization of the semiconductor two dimensional electron system, superconductivity of Al and performance of S Sm S junctions. This exciting development might lead to a number of useful applications ranging from spintronics to quantum computing.
author_names: Javad Shabani
title: Epitaxial superconductor semiconductor two dimensional systems: platforms for quantum circuits (Conference Presentation)
venue: NanoScience Engineering
year: 2016

abstract: To form a coherent quantum transport in hybrid superconductor semiconductor (S Sm) junctions, the formation of a homogeneous and barrier free interface between two different materials is necessary. The S Sm junction with high interface transparency will then facilitate the observation of the induced hard superconducting gap, which is the key requirement to access the topological phases (TPs) and observation of exotic quasiparticles such as Majorana zero modes (MZM) in hybrid systems. A material platform that can support observation of TPs and allows the realization of complex and branched geometries is therefore highly demanding in quantum processing and computing science and technology. Here, we introduce a two dimensional material system and study the proximity induced superconductivity in semiconducting two dimensional electron gas (2DEG) that is the basis of a hybrid quantum integrated circuit (QIC) The 2DEG is a 30 nm thick In0.75Ga0.25As quantum well that is buried between two In0.75Al0.25As barriers in a heterostructure. Niobium (Nb) films are used as the superconducting electrodes to form Nb In0.75Ga0.25As Nb Josephson junctions (JJs) that are symmetric, planar and ballistic. Two different approaches were used to form the JJs and QICs. The long junctions were fabricated photolithographically, but e beam lithography was used for short junctions' fabrication. The coherent quantum transport measurements as a function of temperature in the presence/absence of magnetic field B are discussed. In both device fabrication approaches, the proximity induced superconducting properties were observed in the In0.75Ga0.25As 2DEG. It was found that e beam lithographically patterned JJs of shorter lengths result in observation of induced superconducting gap at much higher temperature ranges. The results that are reproducible and clean suggesting that the hybrid 2D JJs and QICs based on In0.75Ga0.25As quantum wells could be a promising material platform to realize the real complex and scalable electronic and photonic quantum circuitry and devices.
author_names: Kaveh Delfanazari

P Ma

Reuben Kahan Puddy

Teng Yi

Moda Cao

Yilmaz Gul

Carly Richardson

Ian Farrer

David W Ritchie

Hannah J Joyce

Michael Joseph Kelly

Charles G Smith
title: Scalable Quantum Integrated Circuits on Superconducting Two Dimensional Electron Gas Platform.
venue: Journal of visualized experiments JoVE
year: 2019

abstract: The quality of interfaces and surfaces is crucial for the performance of nanoscale devices. A pertinent example is the close tie between current progress in gate tunable and topological superconductivity using semiconductor/superconductor electronic devices and the hard proximity induced superconducting gap obtained from epitaxial indium arsenide/aluminium heterostructures. Fabrication of devices requires selective etch processes; these only exist for InAs/Al hybrids, which precludes the use of other, potentially better material combinations in functional devices. We present a crystal growth platform based on three dimensional structuring of growth substrates for synthesising semiconductor nanowires with in situ patterned superconductor shells, which enables independent choice of material by eliminating etching. We realise and characterise all the most frequently used architectures in superconducting hybrid devices, finding increased yield and electrostatic stability compared to etched devices, along with evidence of ballistic superconductivity. In addition to aluminium, we present hybrid devices based on tantalum, niobium and vanadium.
author_names: Damon J Carrad

Martin Bjergfelt

Thomas Kanne

Martin Aagesen

Filip Krizek

Elisabetta M Fiordaliso

Erik V Johnson

Jesper Nygard

Thomas Sand Jespersen
title: Shadow lithography for in situ growth of generic semiconductor/superconductor devices
year: 2019

abstract: We develop a unified numerical approach for modeling semiconductor superconductor heterostructures. All the key physical ingredients of these systems orbital effect of magnetic field, superconducting proximity effect, and electrostatic environment are taken into account on equal footing in a realistic device geometry. As a model system, we consider indium arsenide (InAs) nanowires with an epitaxial aluminum (Al) shell, which is one of the most promising platforms for Majorana zero modes. We demonstrate qualitative and quantitative agreement of the obtained results with the existing experimental data. Finally, we characterize the topological superconducting phase emerging in a finite magnetic field and calculate the corresponding topological phase diagram.
author_names: G W Winkler

A E Antipov

Bernard van Heck

Alexey A Soluyanov

Leonid I Glazman

Michael Wimmer

R M Lutchyn
title: Unified numerical approach to topological semiconductor superconductor heterostructures
venue: Physical Review B
year: 2019

abstract: Superconductors with topological surface or edge states have been intensively explored for the prospect of realizing Majorana bound states, which obey non Abelian statistics and are crucial for topological quantum computation. The traditional routes for making topological insulator/superconductor and semiconductor/superconductor heterostructures suffer fabrication difficulties and can only work at low temperature. Here, we use angle resolved photoemission spectroscopy to directly observe the evolution of a topological transition of band structure nearby the Fermi level in two dimensional high {T}_{c} superconductor \mathrm{FeT}\mathrm{e}}_{1\ensuremath{ }x}\mathrm{S}\mathrm{e}}_{x}\mathrm{SrTi}\mathrm{O}}_{3}(001) monolayers, fully consistent with our theoretical calculations. Furthermore, evidence of edge states is revealed by scanning tunneling spectroscopy with assistance of theoretical calculations. Our study provides a simple and tunable platform for realizing and manipulating Majorana states at high temperature.
author_names: X -L Peng

Yaqing Li

X -X Wu

H -B Deng

X Shi

Wenhui Fan

M Li

Y -B Huang

Tian Qian

Pierre Richard

J -P Hu

Shuheng H Pan

H Q Mao

Yj Sun

Hong Ding
title: Observation of topological transition in high Tc superconducting monolayer FeTe1 xSex films on SrTiO3(001)
venue: Physical Review B
year: 2019

abstract: Achievements in the growth of ultra pure III V semiconductor materials using state of the art molecular beam epitaxy (MBE) machine has led to the discovery of new physics and technological innovations. High mobility two dimensional electron gas (2DEG) embedded in GaAs/AlxGa1 xAs heterostructures provides an unparalleled platform for many body physics including fractional quantum Hall effect. On the other hand, single electron devices fabricated on modulation doped GaAs/AlxGa1 xAs heterostructures have been extensively used for fabrication of quantum devices such as spin qubit with application in quantum computing. Furthermore, epitaxial hybrid superconductor semiconductor heterostructures with ultra clean superconductor semiconductor interface have been grown using MBE technique to explore rare physical quantum state of the matter namely Majorana zero modes with non abelian exchange statistics.Chapter 1 in the manuscript starts with description of GaAs MBE system at Purdue University and continues with the modifications have been made to MBE hardware and growth conditions for growing heterostrcutures with 2DEG mobility exceeding 35 x 106 cm 2/V s. Utilizing an ultra high pure Ga source material and its further purification by thermal evaporation in the vacuum are determined to have major impact on growth of high mobility GaAs/AlxGa1 xAs heterostructures.Chapter 2 reports a systematic study on the effect of silicon doping density on low frequency charge noise and conductance drift in laterally gated nanostructures fabricated on modulation doped GaAs/AlxGa1 xAs heterostructures grown by Molecular Beam Epitaxy (MBE) The primary result of this study is that both charge noise and conductance drift are strongly impacted by the silicon doping used to create the two dimensional electron gas. These findings shed light on the physical origin of the defect states responsible for charge noise and conductance drift. This is especially significant for spin qubit devices, which require minimization of conductance drift and charge noise for stable operation and good coherence. Chapter 3 demonstrates measurements of the induced superconducting gap in 2D hybrid Al/Al0.15In0.85As/InAs heterostructures which is a promising platform for scaling topological qubits based on Majorana zero modes. The 2DEG lies in an InAs quantum well and is separated from the epitaxial Al layer by a barrier of Al0.15In0.85As with thickness d. Due to hybridization between the wave functions of 2DEG and superconductor, the strength of induced gap in the 2DEG largely depends on the barrier thickness. This chapter presents a systematic study of the strength of the induced gap in hybrid Al/Al0.15In0.85As/InAs superconductor/semiconductor heterostructures as a function of barrier thickness.
author_names: Saeed Fallahi
title: Growth and characterization of two dimensional III V semiconductor platforms for mesoscopic physics and quantum devices
year: 2019