Session B72: 2D Quantum Materials

Graphene-Based Moire Superconductors: Platform for New Discoveries and Applications

Twisted bilayer graphene can host an abundance of correlated insulating, topological, and superconducting

states of matter. While many recent results strongly point towards the unconventional nature of the

superconductivity in this material, the precise pairing mechanism and superconducting order remain elusive.

This talk will first discuss our current understanding of electronic properties in this and the related

broader family of graphene-based moire superconductors obtained by transport and local scanning tunneling 

microscopy measurements. In particular, I will present results on how the additional layers can improve 

the tunability of the superconducting phase and its overall robustness. In the second part, I will discuss how

we might use this novel highly tunable materials platform for quantum science applications.     

Gate-controlled quantum dots in monolayer WSe2

Quantum confinement in two-dimensional (2D) transition metal dichalcogenides (TMDs) offers the opportunity to create unique quantum states that can be practical for quantum technologies. The interplay between charge carrier spin and valley, as well as the possibility to address their quantum states electrically and optically, makes 2D TMDs an emerging platform for the development of quantum devices.

In this talk, we present the fabrication of a fully encapsulated monolayer tungsten diselenide (WSe₂) based device in which we realize gate-controlled hole quantum dots. We demonstrate how our device architecture allows us to identify and control the quantum dots formed in the local minima of electrostatic potential fluctuations in the WSe₂ using gates. Coulomb blockade peaks and diamonds are observed which allow us to extract information about the dot diameter and its charging energy. Furthermore, we demonstrate how the transport passing through the channel formed by two gates is sensitive to the occupation of a nearby quantum dot. Additionally, we show how this channel can be tuned to be in the charge detection or the Coulomb blockade regime. Finally, we present a new device architecture which exhibits quantized conductance plateaus over a channel length of 600 nm at a temperature of 4 K. Quantized conductance over such a long channel provides an opportunity to incorporate gate defined quantum dot circuits without the nuisance of inhomogeneity within the channel.   

Artificial Atom on a Chip Based on Coupling Between 2DEG and Piezo Resonator

A key requirement for designing a two-level quantum system for generating a qubit is ensuring anharmonic energy levels, such that a unique energy gap separates the target energy level pair for hosting a qubit. Atoms serve as a natural example of such a system but face challenges regarding scalability for quantum computing. Superconducting qubits provide a more scalable alternative but are sensitive to charge noise. Here, we propose a piezoelectric resonator coupled to a 2D electron gas (2DEG) as a scalable and resilient qubit-hosting platform. The quasi-acoustic field stored in the piezoelectric resonator manifests itself as a harmonic ladder of states. This quasi-acoustic wave contains an electric field, which interacts with the 2DEG electrons, giving rise to an anharmonic energy ladder of composite states. We determine the energy spectrum for this composite system in two steps: first, by introducing the second quantization and deriving the harmonic frequency for the quasi-acoustic field, and second, by solving for the piezo-2DEG coupling coefficient. We also show that, for an appropriate set of material dimensions and parameters, the energy-anharmonicity of the composite states is robust to the decoherence-induced spectral broadening.   

Experimental Measurements of Electron-Electron Interactions in Two-Dimensional Electron Systems

Electron–electron (e-e) interactions have a fundamental role in determining the quasiparticle lifetime in Fermi liquid theory, but do not affect electron mobility because the interactions conserve the total system momentum. Hence precision measurements were only recently achieved [e.g. 1,2]. Here we investigate e-e interactions using variable temperature (4-36 K) transverse magnetic focusing (TMF) measurements [1] on a high-mobility 2D electron system in a GaAs/AlGaAs heterostructure with conjoined high-resolution kinetic simulations. The ballistic nature of TMF brings out the importance of e-e interactions as dominant scattering mechanism in high-mobility materials, demonstrating that TMF can be used as a precision technique for probing e-e interactions. The measurements show deviations from the theory by Giuliani & Quinn, which is followed only up to a multiplicative constant. Deviations have also been noted in other recent experimental work. The possible origin of the systematic variance of the theory will be discussed.   

Photon statistics and coherence properties of hBN quantum emitters

Photonic quantum technologies require on-demand, indistinguishable photons (space, time, energy, and polarization) at high repetition rate. Recent discovery of quantum emission from atomically thin crystals of hexagonal boron nitride (hBN) has offered a promising candidate for single photon sources (SPSs). The realization of an efficient hBN SPS that delivers light pulses with one and only one photon requires full control over their photophysical properties and  integrability with conventional device substrates. 

In this talk, we present recent results on the measurement of the position and  dipole orientation of hBN quantum emitters with accuracy down to ~ 7 nm [1]. Room temperature photon statistics, quantified by Mandel-Q parameter, of these quantum emitters will be discussed. We report transformation of sub-Poissonian statistics of intensity fluctuations observed over the shorter (tens of nanosecond) times to super-Poissonian over the longer (tens of microseconds) times. Finally, we will present our on-going work on characterizing coherence properties of hBN quantum emitters, using folded scanning Michelson interferometer, on the way towards attaining near-transformed limited emitters for applications in remote sensing and imaging.         

Influence of substrates on properties of defect-based quantum emitters in hexagonal boron nitride

Over the last two decades, the search for room-temperature qubit candidates has revived interest in the study of deep-defect centers in semiconductors. Amongst 3D crystals, deep defects in diamond and silicon carbide have been shown to be promising spin-qubits. However, there is an increasing interest in exploring quantum emitters in layered 2D semiconductors, such as hexagonal boron nitride (hBN) and transition metal dichalcogenides. Unlike 3D semiconductors, a 2D layered material offers greater potential for the deterministic placement of a deep defect in the 2D-matrix, providing a scalable platform for quantum applications. In addition, properties of the layered materials and hence, their defects, can be tuned by: (a) applying strain [Phys. Rev. Research 2, 022050(R) (2020)], and (b) by controlling the composition via the number of the layers and/or choice of the substrate. Although, observed in experiments [ACS Nano 11, 3328 (2017)], the substrate effects have remained relatively unexplored in theoretical works. Using silicon dioxide as a prototype substrate in our density functional theory-based calculations, we show how substrates can affect the ground- and excited-state properties of deep defects in hBN.   

Evaluating dielectric loss in hexagonal boron nitride for high-performace quantum devices

Two-dimensional van der Waals (vdWs) materials offer a promising platform for quantum information devices owing to their crystallinity and ultra-clean interfaces. Hexagonal boron nitride (hBN) is a widely studied van der Waals dielectric material. We compared the loss tangents in hBN against other dielectric materials by fabricating parallel plate capacitors with various combinations of superconductors. We measured an ultra-low dielectric loss of 2.4e-5 with a dielectric constant of 4.4 ± 1.1 for hBN at microwave frequencies. Further, we used this parallel plate capacitor as a shunt capacitor in superconducting qubits and calculated an energy relaxation time of > 1 μs. The control experiments suggest that the measured dielectric loss is limited by fabrication process and not by the intrinsic material properties of hBN. This demonstrates that hexagonal boron nitride is an excellent material for use as a dielectric in fabricating quantum devices.   

Dependence of the electronic structure of the EuS/InAs interface on the bonding configuration

Recently, the EuS/InAs interface has attracted attention for the possibility of inducing magnetic exchange correlations in a strong spin-orbit semiconductor, which could be useful for topological quantum devices. We use density functional theory with a machine-learned Hubbard U correction [Yu et al., npj Comput. Mater. 6, 180 (2020)] to elucidate the effect of the bonding configuration at the interface on the electronic structure. For all interface configurations considered here, we find that the EuS valence band maximum (VBM) lies below the InAs VBM. In addition, dispersed states emerge at the top of the InAs VBM at the interface, which do not exist in either material separately. These states are contributed mainly by the InAs layer adjacent to the interface. They are localized at the interface and may be attributed to charge transfer from the EuS to the InAs. The interface configuration affects the position of the EuS VBM with respect to the InAs VBM, as well as the dispersion of the interface states. For all interface configurations studied here, the induced magnetic moment in the InAs is small. Our results suggest that this interface, in its coherent form studied here, may not be promising for inducing equilibrium magnetic properties in InAs.   

Engineering Localized Carbon-based Quantum Emitting Defects in Pristine Hexagonal Boron Nitride Crystals

Emerging quantum information technologies require robust and tunable single-photon sources in precise locations. Solid-state single photon emitters (SPEs) hosted by mid-bandgap defects in 2D material hexagonal boron nitride (hBN) are bright and stable at room temperature and demonstrate a wide range of strain tunability. While recent studies have narrowed the range of possible defect candidates by demonstrating the role of carbon in hBN SPEs, the methods to engineer carbon-based defects in hBN either produce randomly located emitters or require bottom-up crystal growth on structured substrates. In this work, we achieve patterned arrays of SPEs via focused ion beam (FIB) milling followed by chemical vapor deposition (CVD) of carbon, and find that both techniques are necessary for significant and repeatable creation of SPEs. Furthermore, we utilize the SPE arrays and the rich parameter space of FIB and CVD techniques to elucidate the material and process conditions most desirable for hBN defect engineering. Our results provide important insights into the hBN SPE formation process and simplify the fabrication techniques to pattern carbon-based hBN SPEs for devices such as photonic circuits and quantum transducers.   

First-principles predictions of out-of-plane group IV and V dimers as high-symmetry high-spin defects in hexagonal boron nitride

Hexagonal boron nitride (h-BN) has been recently found to host a variety of quantum point defects, which are promising candidates as single-photon sources for solid-state quantum nanophotonics applications. Most recently, optically addressable spin qubits in h-BN have been the focus of intensive research due to their unique potential in quantum computing, communication, and sensing. However, the number of high-symmetry high-spin defects that are desirable for developing spin qubits in h-BN is highly limited. Here, we combine density functional theory (DFT) and quantum embedding theories (QET) to show that out-of-plane X_NY_i dimer defects (X, Y=C, N, P, Si) form a new class of stable C_3v spin-triplet defects in h-BN. We find that the dimer defects have a robust ³A₂ ground state and ³E excited state, both of which are isolated from the h-BN bulk states. We show that ¹E and ¹A shelving states exist and they are positioned between the ³E and ³A₂ states for all the dimer defects considered in this study. To support future experimental identification of the X_NY_i dimer defects, we provide an extensive characterization of the defects in terms of their spin and optical properties. We predict that the zero-phonon line of the spin-triplet X_NY_i defects lies in the visible range (800 nm – 500 nm) range. We compute the zero-field splitting of the dimers’ spin to range from 1.79 GHz (Si_NP_i⁰) to 29.5 GHz (C_NN_i⁰). Our results broaden the scope of high-spin defect candidates that would be useful for the development of spin-based solid-state quantum technologies in two-dimensional hexagonal boron nitride.   

Theoretical study of spin decoherence in transition metal dichalcogenides

Transition metal dichalcogenides (TMDC) have recently emerged as potential candidates to host coherent qubits to realize quantum information technologies in two-dimensional materials platforms [1]. Prospective qubit candidates include quantum dots, valley qubits, and spin defects [2,3,4]. Notably, a recent experimental study reported creation of a localized electron spin by using a carbon radical ion in WS₂, which could be further developed to a spin qubit system [5]. In this study, we theoretically investigate the decoherence time of spin defects in MX₂ TMDC materials (M=Mo, W and X=S, Se, Te). We compute the Hahn-echo decoherence time (T₂) of an electron spin associated with carbon radical ions in TMDC by combining a cluster correlation expansion method (CCE) and density functional theory [6]. We found that the T₂ time of TMDC materials ranges from 1.6 ms to 36 ms, and the longest T₂ time was found for WS₂. We also discuss the microscopic mechanism of the decoherence in TMDC materials by analyzing their spin Hamiltonian terms. Our results show that TMDCs are promising materials to host robust spin qubits with long coherence time, which would be crucial for their potential applications in quantum sensing and quantum information processing.

[1] Liu, X., Hersam and M.C., Nat. Rev. Mater. 4, 669–684 (2019).

[2] Ye, M., Seo, H. and Galli, G., npj Comp. Mater. 5, 44 (2019).

[3] Onizhuk, M. and Galli, G., Appl. Phys. Lett. 118, 154003 (2021).

[4] Kormányos, A. et al., Phys. Rev. X 4, 011034 (2014).

[5] Cochrane, K. A. et al., arXiv:2008.12196 (2020).

[6] Seo, H. et al., Nat. comm. 7, 12935 (2016).   

Extending the coherence of spin qubits in hexagonal boron nitride by materials engineering: a cluster expansion theory

Negatively charged boron vacancy (V_B^-) in hexagonal boron nitride (h-BN) has recently emerged as a new promising spin qubit candidate in 2-dimensional materials hosts for solid-state quantum applications [1]. Their spin coherence time (T₂), however, was measured to be very short limited to a few microseconds [2], which is mainly due to their interaction to the inherent dense nuclear spin bath of the h-BN host [3]. In this study, we theoretically propose ways to enhance the quantum coherence of the V_B^- spin in h-BN by using isotopic and strain engineering. We combine density functional theory and cluster correlation expansion to compute the decoherence of V_B^- spins induced by the dense nuclear spin bath of h-BN. We show that inhomogeneous strain can create spatially varying nuclear quadrupole interaction in h-BN, which can significantly suppress the nuclear spin flip-flop dynamics in the bath. In addition, we find that the coherence time of the V_B^- spin can be effectively engineered by adjusting the ratio of ¹⁰B and ¹¹B isotopes in h-BN. We show that the combination of the two methods could increase the T₂ time by 4.5 times larger than the T₂ time in a pristine h-BN bulk. Our results provide not only a fundamental understanding of the decoherence of V_B^- spins in h-BN, but pave the way to engineer their T₂ time, which is crucial for their practical applications.