Session L39: Defect Spins in Semiconductors

Understanding quantum materials using strain-sensitive x-ray diffraction imaging

Understanding the local crystalline strain around point defects, such as the nitrogen vacancy center in diamond and divacancy complexes in SiC, is a critical step toward improving spin coherence and optical properties. These lattice perturbations affect both the charge and the optical transition frequency stability of the defect, limiting their use as nanoscale sensors and quantum bits for quantum communication applications. While local strain can be mitigated using applied electric fields and external static strain, direct observation of inhomogeneous strain fields around these defects at the nanometer length scale remains challenging. Here we present work on the development of synchrotron-based, strain-sensitive x-ray imaging techniques which we use to map the local lattice perturbations within diamond and SiC crystals. These tools can help understand the interaction of defects with dynamically driven strain fields as well as probe the defect creation process to help improve the basic properties of quantum materials. We show two separate techniques: strain-sensitive Bragg coherent diffraction imaging (BCDI) that can measure the three-dimensional lattice strain of individual diamond and SiC nanoparticles as a function of annealing temperature [1,2], and a stroboscopic scanning x-ray diffraction microscopy (s-SXDM) imaging approach that can spatially map acoustic waves in SiC [3] and probe the local strain around crystalline defects. Combining these techniques with growth and implantation protocols could provide a direct means to understand the local crystalline environment surrounding point defect as well as a pathway towards improving their spin properties.

[1] S.O. Hruszkewycz, et al., APL Materials 5, 026105 (2017).
[2] S.O. Hruszkewycz, et al., Phys. Rev. Materials 2 (8), 086001 (2018).
[3] S. J. Whiteley, et al., arXiv:1808.04920

In collaboration with S. J. Whiteley, S. O. Hruszkewycz, M. V. Holt, & D. D. Awschalom.   

Optically-Addressable Spins in Hexagonal Boron Nitride: Creation, Identification, and Characterization

Optically-addressable spins associated with localized defects in wide-bandgap semiconductors are the basis for rapidly expanding quantum technologies in nanoscale sensing and quantum information processing. Whereas most research has focused on three-dimensional host materials such as diamond, the van der Waals material hexagonal boron nitride (hBN) has emerged as a robust host for bright, stable, room-temperature quantum emitters (QEs). However, many questions persist regarding the chemical and electronic structure of the defects responsible for emission as well as the potential role of spin-related effects. Significantly complicating the identification are the heterogeneity of optical characteristics observed for these QEs.

In this talk, I will describe the optical and magnetic properties of QEs in hBN characterized using confocal fluorescence microscopy on suspended hBN films. Qualitative similarities in excitation and emission polarization, spectral shape, and emission statistics are evident among QEs in hBN, even for large variations in emission energy [1]. Significantly, a small percentage of observed QEs exhibit strongly anisotropic photoluminescence modulation in response to an applied magnetic field in ambient conditions [2]. The magnetic-field-induced modulation is consistent with an electronic model featuring a spin-dependent inter-system crossing between triplet and singlet manifolds, suggesting that these defects host optically addressable spin states. This discovery enables the realization of spin-based quantum technologies with van der Waals heterostructures.

[1] Exarhos et al., ACS Nano 11, 3328 (2017).
[2] Exarhos et al., arXiv:1804.09061 [quant-ph] (2018). Nature Communications (accepted).

In collaboration with D. A. Hopper, R. N. Patel, R. R. Grote, A. Breitweiser, A. Alkauskas, M. W. Doherty, and L. C. Bassett.   

Diamond Nanomechanical Resonators with Narrow Linewidth NVs

Nitrogen vacancy centers in diamond are promising qubit candidates which have had recent work focusing on interfacing NVs with acoustic fields. One approach, which is monolithic and doesn’t require external driving of phonons, is coupling NVs to high quality factor mechanical resonators in diamond. This approach necessitates low mass, thin membranes which are generally fabricated by thinning down a thicker diamond sample with a plasma etch. However, plasma damage to the diamond results in deep charge defects and poor surface termination which affects the spin and optical properties of the NV center. Here we report on the fabrication of thin mechanical resonators imbedded in a phononic crystal and the measurement of improved optical properties of NVs in those membranes.   

Mechanical rotation via optical pumping of paramagnetic impurities

The microscopic understanding of magneto-mechanical processes — involving the inter-conversion between magnetization and angular momentum in a solid — has long been hampered by the complex dynamics governing the spin polarized electrons of a ferromagnetic material, collectively coupled via quantum exchange. Here we use a rotationally invariant Hamiltonian to study the spin dynamics of a pair of paramagnetic centers in an insulator — namely, the so-called P1 and NV centers in diamond — close enough to each other to interact via the dipolar coupling. We examine the interplay between magnetic and mechanical degrees of freedom, and theoretically show that in the presence of continuous optical illumination, cross-relaxation between the NV and P1 spins leads to a rigid rotation of the diamond crystal along with the generation of spin-polarized phonons. The effect should be observable using state-of-the-art torsional oscillators.   

Observing the charge state of substitutional nitrogen via nitrogen vacancy magnetometry

Defects in diamond have been widely explored as a potential system for quantum information processing. The negatively charged Nitrogen Vacancy (NV^-) center is of particular interest because its spin can be operated at ambient conditions and addressed optically with the help of low frequency microwave. It has been suggested that the spin state of the donor electron of the substitutional nitrogen defect (N_s) can be utilized as a quantum register for a spintronics bus [1]. Here, we use the NV center as a local probe to examine the ionization of small N_s ensembles in type 1b diamond. To this end, we implement double electron-electron resonance (DEER), [2] which we articulate with multi-color laser excitation to prepare and subsequently probe the local charge state in the N_s ensemble. We will also discuss the use of polarization transfer techniques from the NVs as a route to probe the spin resilience of photo-generated carriers throughout cycles of ionization from and re-trapping by N_s.
1. M. W. Doherty et al. Physical Review X, 6(4), 041035 (2016)
2. A. Laraoui et al. Nano Letters, 12(7), 3477 (2012)   

Polarization transfer between electric and nuclear spins at the level anti-crossing of the NV center in diamond

Point defect qubits in semiconductors offer new, highly polarizable and controllable electron spin resources to the be utilized in nuclear spin polarization experiment. The NV center in diamond holds a great promise for implementing various dynamic nuclear polarization (DNP) mechanisms to achieve high degree of hyperpolarization at elevated temperatures. For example, the level anti-crossing of the electron spin states at moderate magnetic field allows all-optical DNP processes [1] with a mechanism of microwave-free polarization inversion [2]. In my talk, I summarize our most recent theoretical results on all-optical DNP at the level anti-crossing of NV center in diamond of different ¹³C nuclear spin concentrations. We demonstrate that the increasing nuclear spin concentration has a negative impact on the efficiency of nuclear polarization transfer that explains the lack of hyperpolarization in ¹³C enriched samples. Additionally, I assess the requirements and discuss possible layouts for hyperpolarizing external nuclear spins for sensitivity enhanced NMR and MRI measurements.

References:
[1] Ivady et.al., Phys. Rev. B 92, 115206 (2015)
[2] Ivady et.al., Phys. Rev. Lett. 117, 220503 (2017)   

First-principle study of spin-strain coupling in defect spin qubits

Spin defects in semiconductors are promising platforms for quantum information processing and are useful components of hybrid quantum devices. Transition energies between different defect states are sensitive to external perturbations and hence lattice strains can be utilized for mechanical control of qubits. In this work, we use density functional theory to predict the coupling strength between spin and mechanical degrees of freedom for prototypical defect spin qubits including the nitrogen-vacancy centers in diamond and divacancies in silicon carbide, and we compare our results with recent experiments [1].

[1] Whiteley, S. J., Wolfowicz, G., Anderson, C. P., Bourassa, A., Ma, H., Ye, M., Koolstra, G., Satzinger, K. J., Holt, M. V., Heremans, F. J., Cleland, A. N., Schuster, D. I., Galli, G., Awschalom D. D. (2018) arXiv:1804.10996.
  

Exploring solid-state defects with a microwave-modulated spectroscopy technique

We have developed a microwave-based spectroscopy technique to determine charge state of nitrogen-vacancy ensembles in diamond. The technique isolates, in situ, the spectral shape of the fluorescence contribution from neutral and negatively-charged defects, producing sample-specific results which take into account the effects of experimental conditions (eg. illumination intensity and wavelength) and material properties (such as local strain and electric fields). Here, we use this technique to explore how ensemble charge state is affected by experimental and material parameters, and to study the physics of NV ionization from the negative charge state.
We also apply a variation of our technique to silicon vacancies (SiV) in 4H silicon carbide. The technique may be used to isolate the spectra of V1 and V2-type defects at room temperature -- these are two silicon vacancies that occur at in-equivalent lattice sites but have closely-spaced spectral signatures. We explore the modulation of the SiV signals in both bulk SiC and in nanobeam optical resonators that include the vacancies.   

Coherent electrical driving of quantum spins via localized magnons

Spin-spin interactions in quantum-classical spin hybrids are promising enablers of quantum spintronics. For example, dipolar interactions between classical magnets and quantum dot (QD) spins have enabled electrically driven high-fidelity quantum processors [Nat. Nano 13, 102 (2018)]. Motivated by higher operating temperatures, dipolar interactions in quantum impurity (QI) spin [like Nitrogen vacancy (NV) center]-classical magnet hybrids have been used for quantum sensing of ground state configurations-solitons [Nature 549, 252 (2017); Nat. Commun. 9, 2712 (2018)], as well as, excitations-magnons [Science 357, 195 (2017)]. In addition, magnons have mediated long-distance coherent coupling between NVs and microwave antennas [npj Quantum Info. 3, 28 (2017)]. In contrast to QD spin-magnet hybrids, coherent electrical driving of QI spins is, however, missing. Here, we propose to utilize spin-orbit induced electrical driving of magnons [Nano Lett. 2017, 17, 1, 572], to mitigate this challenge. Specifically, we study coherent and incoherent coupling of QI spins in proximity to electrically controlled localized magnons in nanomagnets-showing coherent electrical driving of QI spins.   

Probing Coherent Spin Dynamics of Isolated Exchange Coupled Defects in III-V Semiconductors

Individual magnetic impurities or small collections of magnetic impurities in III-V semiconductors can be identified via scanning tunneling microscopy (STM) [1,2]. Their exchange interaction can be measured [3], and they can have remarkably long spin coherence times [4]. Through low-field magnetoresistance calculations we find that exchange coupled defects generate a resonance feature in the current at critical values of the applied magnetic field, termed "exchange resistance". Using a single site approximation [5,6], the signatures of hyperfine interaction and the influence of g-factor fluctuation on exchange resistance are examined. By including a non-trivial spin manifold as the ground state this work becomes applicable to defects like divacancies in silicon carbide and Mn+hole complexes in gallium arsenide.

[1] J. M. Tang and M. E. Flatté, Phys. Rev. Lett. 92, 047201 (2004).
[2] A. M. Yakunin et al., Phys. Rev. Lett. 92, 216806 (2004).
[3] D. Kitchen et al., Nature 442, 436 (2006).
[4] R. C. Myers et al., Nature Materials 7, 203 (2008).
[5] H. Inoue et al., Phys. Rev. X 5, 041023 (2015)
[6] N. J. Harmon and M. E. Flatté, Phys. Rev. B 98, 035412 (2018)