Session J65: Semiconductors, Superconductors, and Molecular Qubits

Magnetoelectric behavior in metal-organic complexes including spin crossovers compounds

We explore metal-organic complexes that show unconventional routes to magnetoelectric behavior. Traditionally, magnetoelectric behavior has been most commonly studied in inorganic oxides, where ferro or antiferromagnetism couples to dielectric or ferroelectric properties. Such coupling allows an electric field to control and switch e.g. a quantum magnetic state or vice versa. This field contains a wealth of intriguing fundamental science, as well as applications being targeted in ultra-low power sensing, qubits, memories, tunable antennas, and other devices. In this talk I will review several of our recent efforts to extend this field of magnetoelectrics to metal-organic complexes, which contain transition metal ions and organic ligands. These compounds tend to have soft lattices that mediate magnetoelectric coupling, and a broad array of intriguing forms of magnetism going beyond (anti) ferromagnetism. I will begin with spin crossovers where the spin state of a transition metal ion changes with applied magnetic fields, thereby modifying the electric polarization. I will show results on Mn-based spin crossovers where magnetic fields toggle the spin state and thereby induce structural phase transitions or Jahn Teller effects. These structural changes result in very strong magnetoelectric coupling as well as complex phase diagrams. I will also discuss metal-organic frameworks that are effectively intrinsic heterostructures where magnetic and electric properties can be combined with flexible architectures. I will review the state of this nascent field and potential new directions.   

On-chip Integrable Highly Spectrally Uniform Ordered Semiconductor Quantum Dot Single Photon Source Arrays for Scalable Quantum Optical Networks

Scalable optical networks demand ordered and spectrally uniform single photon source (SPS) array integrable on-chip with photon manipulating units. Recently we demonstrated such SPSs based on a new class of spatially ordered and spectrally uniform InGaAs/GaAs mesa top single quantum dots (MTSQDs) [1,2] that are readily integrable, on-chip, with dielectric light manipulating units (LMUs) [1,3] based on 2D photonic crystal platform or collective Mie-like resonance of dielectric building block based metastructures. These MTSQDs have single photon emission purity > 99% (g⁽²⁾(0)<0.02) at 9.4K [2]. The spectral uniformity of the nominally In_0.5Ga_0.5As MTSQD array is ~8nm limited by alloy fluctuation, but importantly reveal pairs of MTSQDs emitting within 300μeV [1,3]. In this talk we report studies on binary InAs/GaAs MTSQD in 5X8 arrays demonstrating striking spectral uniformity of 1.8nm (<2meV) over 1000um² area. The results highlight the potential of the spatially-ordered MTSQDs-LMU integrated system for realizing quantum optical circuits [3].
[1] J.Zhang et.al,J.Appl.Phys.120,243103(2016)
[2] J.Zhang et.al,Appl.Phys.Lett.114,071102(2019)
[3] S.Chattaraj et.al,IEEE J.Quant.Electron(2019), accepted for publication
  

Fused LiNbO3-(Al)GaAs hybrids for quantum dots optomechanics

Surface acoustic waves (SAW) are a useful tool to control the emission of quantum dots (QDs). In particular, SAWs enable the injection of charge carriers into the dot or the modulation of their energy levels [1,2,3]. Here, we explore the possibility to enhance the interaction between the SAW and the QDs by transferring them on a strong piezoelectric LiNbO₃ substrate by epitaxial lift-off [4]. By employing multiharmonic transducers, we generate SAWs on LiNbO₃ over a wide range of radiofrequencies. We monitor their coupling to and propagation across the semiconductor membrane, both in the electrical and optical domain. We demonstrate the enhanced optomechanical tuning of the embedded QDs with increasing frequencies. This effect is verified by finite element modelling of our device geometry and attributed to an increased localization of the acoustic field within the semiconductor membrane [5]. The results of our study clearly show the large potential of our hybrid platform to integrate complex phononic and optomechanical circuitry with integrated QDs to study sound-matter coupling.
[1] Appl. Phys. Lett. 93, 081115 (2008), [2] Nano Lett. 10, 3399-3407 (2010), [3] Phys. Rev. B 88, 085307 (2013), [4] Appl. Phys. Lett. 106, 013107 (2015), [5] J. Phys. D: Appl. Phys. 50, 43LT01 (2017)   

Optomechanical single photon frequency division multiplexing

Here we report on the implementation of a hybrid nonlinear optomechanical scheme to generate frequency multiplexed of single photons from a single quantum dot. To this end, the excitonic optical transition of a single semiconductor quantum dot (QD) is coherently driven by a laser with a frequency of approximately 330THz and simultaneously dynamically strained by a surface acoustic wave (SAW) in the low GHz regime. The coherent optomechanical interaction caused by the QD mixes optical and acoustic frequencies, resulting in the generation of a frequency comb. Thus the collected resonance fluorescence signal, consisting of single photons, is emitted into these frequency bins, precisely split by the frequency ω_SAW of the SAW.
Furthermore the nonlinear interaction between two mutually coherent SAWs with frequencies ω_SAW and 2ω_SAW allow for the intensity modulation of certain teeth of the comb. The probability of a single photon being emitted in a given frequency bin can be programed, by the relative phase between the two SAWs. Introducing a frequency detuning of 50μHz between the two SAWs an intensity modulation of the teeth of the frequency comb with a period of approximately 5½ hours can directly be observed, verifying the extraordinary stability of our scheme.   

Tailoring Nanoscale Crystal Polytype Selection for Quantum Confinement Engineering

Electrically-controlled and integrable single photon sources are essential components of quantum information systems. Although semiconductor quantum dots (QDs), including InGaN/GaN QDs-in-nanowires, are promising for room temperature applications, their scalability is limited by alloy composition fluctuations. Here, we aim to create alloy-fluctuation-free QD arrays, namely poly-type QDs-in-NWs, using self-catalyzed NW growth at vapor-liquid-solid (VLS) triple-junctions. We demonstrate, for the first time, epitaxy of GaN NW ensembles and films with a significant ZB content on Si(001). In addition to presenting our novel two-step molecular-beam epitaxy (TS-MBE) approach to control SixNy layer formation and subsequent GaN phase selection, we reveal new insight into the relative roles of surface and step-edge diffusion on film to nanowire transitions. The NWs exhibit remarkable photoluminescence (PL) characteristics consisting of distinct emission from donor-acceptor pairs (DAP) and excitonic transitions related to both ZB and WZ GaN. This first demonstration of epitaxial growth and PL emission from ZB GaN NWs on Si(001) provides a crucial step toward the realization of GaN QDs-in-NWs for single photon emitters.   

Multiferroic triangular molecular qubits

The frustrated triangular single molecule magnets with half integer spins from an important class of molecular magnets that have potential application as qubits in quantum computing. The lack of inversion symmetry allows these molecular qubits to be manipulated by an external electric field. Among several candidate triangular molecules, Fe₃O(NC₅H₅)₃(O₂CC₆H₅)₆ molecular cation is particularly exciting since to date this is the only molecule in which the spin-electric coupling effect is observed experimentally. In this work, using standard density-functional methods, we demonstrate that the spin-electric behavior of this molecule could be even more interesting as there are energetically competitive reference states associated with both high and low local spins (S=5/2 vs. S=1/2) on the Fe³⁺ ions. Each of these spin structures allow spin-electric coupling in their respective chiral ground states. The presence of nearly degenerate reference states, composed of high-spin and low-spin centers, is of potential interest for quantum sensing applications. In this talk we will also discuss the possibility of inducing spin-crossover between the low and high spin configurations by an external electric field.   

Supercurrent in All-Van-der-Waals Josephson Tunnel Junction

Superconducting quantum circuits have demonstrated tremendous progress over the past decade. However, current state-of-art of superconducting qubits still suffer strongly from dielectric two-level-systems (TLS), a major cause of qubit decoherence. Further advancement of qubit lifetime ultimately requires material and fabrication improvements that reduce TLS loss.
In this work, we construct superconducting quantum devices with van der Waals (vdW) materials and their heterostructures, which feature crystalline layers with atomically precise interface. We fabricate high-quality, all-vdW parallel-plate capacitors and Josephson junctions – two key components in superconducting qubits. We then characterize the dielectric loss of the vdW capacitors and measure the Josephson response of the vdW tunnel junctions. We expect the advent of vdW qubits will enable both high coherence and small form factors.   

Shadow lithography for in-situ growth of generic semiconductor/superconductor devices

Current progress in Majorana and Andreev qubits and devices using semiconductor/superconductor nanowires is built on the hard proximity-induced superconducting gap obtained from epitaxial indium arsenide/aluminium interfaces. Devices are conventionally obtained by selectively etching superconductor segments from the semiconductor. However, this is currently only possible for InAs/Al hybrids, excluding the use of potentially more desirable materials (e.g. Nb, InSb) in functional devices. Here, we present a crystal growth platform based on three-dimensional structuring of growth substrates for synthesising semiconductor nanowires with in-situ patterned superconductor shells. The shadow lithography platform enables independent choice of material since etching is no longer required, and is highly flexible with regard to device geometry. We grow and characterise tunnel probe devices, Josephson junctions and Majorana islands using aluminium, niobium, tantalum and vanadium. The devices exhibit increased yield and electrostatic stability compared to etched devices, which we attribute to the obviation of damaging etch processes. The shadow lithography platform promises high yield, stable, reproducible Majorana devices using the best possible materials.   

Engineered Chirality of One-Dimensional Nanowires

Quantum transport in 1D geometries is fascinating in its own right, but it can also be regarded as a building-block for creating and exploring a variety of quantum systems. We have developed a flexible platform for creating 1D nanostructures at the LaAlO₃/SrTiO₃ interface using a conductive atomic force microscope lithography technique. Straight nanowire segments behave as electron waveguides with subband occupation that can be tuned with a gate and an external magnetic field. We can periodically perturb this waveguide, with 10 nm periodicity, to yield a chiral nanowire which exhibits striking oscillatory transmission as a function of both magnetic field and chemical potential. We discuss these results in terms of an engineered axial in-plane spin-orbit interaction within the spiral electron waveguide. These findings represent an important advance in the ability to design new families of quantum materials with emergent properties, and mark a milestone in the development of a solid-state 1D quantum simulation platform.   

InSb Nanostructures for Advanced Quantum Devices

Indium-antimonide (InSb) nanowires (NWs) constitute a suitable platform for hosting Majorana zero modes, a key requirement for fault-tolerant topological quantum computing, due to their high electron mobility and strong spin-orbit coupling.
Building devices based on InSb nanostructures, while maintaining their pristine quality has proven to be rather challenging. Firstly, brought forth by the difficulties to obtain high quality nanostructures and secondly due to the fabrication steps required to transform a single nanostructure into a working device.
Here, we show the growth of pure zinc blende InSb NWs and nanosheets. The high chemical purity of these nanostructures is reflected in the higher electron mobility values as compared to so far reported values for InSb nanostructures. Further, we use the high degree of control over growing both 1-dimensional (1D) and 2-dimensional (2D) nanostructures on the same substrate, to design in-situ quantum device structures. In particular, by relying on both InSb NWs and nanosheets to shadow deposit superconductors and normal metals we design a variety of devices ranging from Josephson Junctions to hybrid superconductor/semiconductor devices, while circumventing the harsh processing steps which degrade the device quality.
  

InSbAs two-dimensional electron gases as a platform for topological superconductivity

Majorana zero modes, the fundamental building blocks of topologically protected qubits, can be realized in semiconductor-superconductor hybrid systems. Two-dimensional electron gases (2DEGs) with strong spin-orbit coupling offer a scalable platform for Majorana devices. Here, we investigate a new 2D hybrid system consisting of ternary InSbAs 2DEGs with epitaxially grown aluminum. Studying Josephson junctions in these systems allows us to estimate a large induced gap (200 μeV) and high interface transparencies (≈0.9). Furthermore, through weak anti-localization and quantum Hall measurements, we show that InSbAs has exceptionally strong spin-orbit interaction and a g-factor comparable to pure InSb. This unique combination of properties make InSbAs/Al 2DEGs a promising platform to study topological superconductivity.   

Stable quantum dots in an InSb two-dimensional electron gas

Indium antimonide (InSb) two-dimensional electron gases (2DEGs) have a unique combination of material properties: high electron mobility, strong spin-orbit interaction, large Landé g-factor, and small effective mass. This makes them an attractive platform to explore a variety of mesoscopic phenomena ranging from spintronics to topological superconductivity. However, there exist limited studies of quantum confined systems in these 2DEGs, often attributed to charge instabilities and gate drifts. We overcome this by removing the δ-doping layer from the heterostructure, and induce carriers electrostatically. This allows us to perform the first detailed study of stable gate-defined quantum dots in InSb 2DEGs. We demonstrate two distinct strategies for carrier confinement and study the charge stability of the dots. The small effective mass results in a relatively large single particle spacing, allowing for the observation of an even-odd variation in the addition energy. By tracking the Coulomb oscillations in a parallel magnetic field we determine the ground state spin configuration and show that the large g-factor (∼30) results in a singlet-triplet transition at magnetic fields as low as 0.3 T.   

Electron-phonon coupling in nitride superconductors from first principles: The effect of epitaxial strain and nitrogen concentration on superconducting properties

The recent all-epitaxial integration of NbN superconductors with the III-Nitride family of semiconductors by molecular beam epitaxy has created an opportunity for scalable, integrated semiconductor/superconductor devices with access to industrial fabrication processes [Nature 555, 183–189 (2018)]. NbN has a large superconducting critical temperature (T_c) that makes it a useful superconducting component at liquid helium temperature, but its structural, metallic, and superconducting properties are extremely sensitive to growth conditions and nitrogen concentration. Group IV-B Nitrides (Ti, Zr, Hf) are chemically more stable than NbN, but their T_c’s are markedly smaller. Can the recent demonstration of epitaxial integration of transition metal nitrides with III-Nitrides be used as a strategy for tailoring their superconducting properties?

Using first-principles approaches we explore the effect of epitaxial strain and nitrogen concentration on Group IV-B Nitride superconducting properties. We find that both epitaxial strain and nitrogen concentration can drastically alter the electron-phonon coupling and potentially increase T_c to temperature ranges sensible for superconducting devices.