Session M18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Diamond

Nitrogen-vacancy centers in diamond: a local probe to study magnetic oxides

We report on the development of a diamond-based scanning probe magnetometer (SPM) that operates over a wide range of temperature from 300 K to 4 K. The magnetic sensor is a nitrogen-vacancy (NV) center in diamond, which is read out via optically detected magnetic resonance. This sensor promises non-invasive imaging with single spin sensitivity and spatial resolution down to $\sim$ 10 nm. We have fabricated single-crystal diamond scanning probes with an embedded RF antenna for coherent manipulation of the NV electronic spin. The SPM is integrated into a variable temperature transport set-up in order to study interface magnetism in complex oxide heterostructures.   

Probing dynamics of a spin ensemble of P1 centers in diamond using a superconducting resonator

Solid-state spin ensembles are promising candidates for realizing a quantum memory for superconducting circuits. Understanding the dynamics of such ensembles is a necessary step towards achieving this goal. Here, we investigate the dynamics of an ensemble of nitrogen impurities (P1 centers) in diamond using magnetic-field controlled coupling to the first two modes of a superconducting (NbTiN) coplanar waveguide resonator. Three hyperfine-split spin sub-ensembles are clearly resolved in the 0.25-1.2 K temperature range, with a collective coupling strength extrapolating to 23 MHz at full polarization. The coupling to multiple modes allows us to distinguish the contributions of dipolar broadening and magnetic field inhomogeneity to the spin linewidth. We find the spin polarization recovery rate to be temperature independent below 1 K and conclude that spin out-diffusion across the resonator mode volume provides the mechanism for spin relaxation of the ensemble. Furthermore, by pumping spins in one sub-ensemble and probing the spins in the other sub-ensembles, we observe fast steady-state cross-relaxation (compared to spin repolarization) across the hyperfine transitions. These observations have important implications for using the three sub-ensembles as independent quantum memories.   

High-Resolution Correlation Spectroscopy of $^{13}$C Spins Near a Nitrogen-Vacancy Center in Diamond

We use a pulse protocol to monitor the time evolution of the 13C ensemble in the vicinity of a NV center. We observe time correlations in the nuclear spin dynamics that extend over several milliseconds exceeding the color center coherence lifetime by more than an order of magnitude. Upon Fourier transform, we separate 13C spins exhibiting differing coupling constants with a frequency resolution inversely proportional to the NV spin-lattice relaxation time. Further, we use the nuclear spin of the host nitrogen as a quantum register during the correlation interval and demonstrate that hyperfine-shifted resonances in this spectrum can be separated from the bare carbon peak upon proper initialization of the NV. Intriguingly, we find that the amplitude of the correlation signal exhibits a sharp dependence on the applied magnetic field, virtually disappearing below a critical field common to all centers. The value of this transition field can be `tuned' by properly adjusting the timing within our correlation scheme. We discuss these observations in the context of the `quantum-to-classical' transition proposed recently to explain the combined dynamics of the NV spin and the 13C bath at variable magnetic field.   

Measurement and control of single spins in diamond above 600 K

The nitrogen vacancy (NV) center in diamond stands out among spin qubit systems in large part because its spin can be controlled under ambient conditions whereas most other solid state qubits operate only at cryogenic temperatures. However, despite the intense interest in the NV center's room temperature properties for nanoscale sensing and quantum information applications, the ultimate thermal limits to its measurement and control have been largely unknown. We demonstrate that the NV center's spin can be optically addressed and coherently controlled at temperatures exceeding 600 K and show that its addressability is eventually limited by thermal quenching of the optical spin readout [1]. These measurements, in combination with computational studies, provide important information about the electronic states that facilitate the optical spin measurement and, moreover, suggest that the coherence of the NV center's spin states could be utilized for thermometry. We infer that single spins in diamond offer temperature sensitivities better than 100 mK/$\mathrm{\sqrt{Hz}}$ up to 600 K using conventional sensing techniques and show that advanced measurement schemes provide a pathway to reach 10 mK/$\mathrm{\sqrt{Hz}}$ sensitivities. Together with diamond's ideal thermal and mechanical properties, these results suggest that NV center thermometers could be applied in cellular thermometry and scanning thermal microscopy. \\[4pt] [1] D. M. Toyli*, D. J. Christle*, A. Alkauskas, B. B. Buckley, C. G. Van de Walle, and D. D. Awschalom, \emph{Phys. Rev. X} \textbf{2}, 031001 (2012).   

Approach to Dark Spins Initialization in Nanodiamond

Diamond nanoparticles host a number of paramagnetic point defects and impurities--many of them adjacent to the surface--whose response to external stimuli could help probe the complex dynamics of the particle and its local, nanoscale environment. Here we use a Hartman-Hahn protocol to demonstrate spin polarization transfer from a single, optically-polarized nitrogen-vacancy (NV) center to the ensemble of paramagnetic defects hosted by an individual diamond nanocrystal (30 nm in diameter). Owing to the strong NV-bath coupling, the transfer takes place on a short, microsecond time scale. Upon fast repetition of the pulse sequence we observe strong polarization transfer blockade, which we interpret as an indication of spin bath cooling. Numerical simulations indicate that the spin bath polarization is non-uniform throughout the nanoparticle averaging approximately 2\% over the crystal volume, but reaching up to 20\% in the immediate vicinity of the NV. These observations may prove relevant to the planning of future bath-assisted magnetometry tests.   

Engineering shallow spins in diamond with nitrogen delta-doping

The excellent spin properties of diamond nitrogen-vacancy (NV) centers motivate applications from sensing to quantum information processing. Still, external electron and nuclear spin sensing are limited by weak magnetic dipole interactions, requiring NVs be within a few nm of the surface and retain long spin coherence times ($T_2$). We report a nitrogen delta-doping technique to create artificial NVs meeting these requirements. Isotopically pure $^{15}$N$_2$ gas is introduced to form a thin N-doped layer (1--2 nm thick) during chemical vapor deposition of a diamond film. Post growth electron irradiation creates vacancies and subsequent annealing forms NVs while mitigating crystal damage. We identified doped NVs through the hyperfine signature of the rare $^{15}$N isotope in electron spin resonance measurements. We confirm the doped NV depth dispersion is less than 4 nm by doping NVs in the $^{12}$C layer of an isotopically engineered $^{13}$C/$^{12}$C/$^{13}$C structure and probing the coupling between the doped NVs and the $^{13}$C nuclear spins. Furthermore, despite their surface proximity, doped NVs embedded in $^{12}$C films 5 (52) nm below the surface show $T_2$ greater than 100 (600) $\mu$s [1].\\[4pt] [1] K. Ohno \emph{et al.}, Appl. Phys. Lett. \textbf{101}, 082413 (2012).   

Optically trapped nanodiamonds with nitrogen-vacancy center spins for scanning magnetometry and thermometry

Nanodiamonds with nitrogen-vacancy (NV) centers are a versatile sensing platform that combines the optically addressable atom-like properties of embedded NV centers, which are sensitive to electromagnetic fields and temperature, with the physical size and mobility necessary for nanometer-scale spatial resolution. We constructed an optical tweezers apparatus that accomplishes position control of nanodiamonds in solution within a microfluidic circuit and enables simultaneous optical measurement and microwave manipulation of the NV centers' ground-state spins [1]. We observe nanodiamond fluorescence and trapping stability over many hours, and infer high d.c. magnetic field and temperature sensitivities from measured spin resonance spectra. Scanning the position of the trapped nanodiamonds enables us to map the magnetic field of current-carrying wires and magnetic nanostructures, and perform thermometry in liquid. This work provides an approach to three-dimensional spin-based scanning probe magnetometry and thermometry in fluids for applications in the biological and physical sciences. \\[4pt] [1] V.R. Horowitz, B.J. Alem\'{a}n, D.J. Christle, A.N. Cleland, and D.D. Awschalom, \emph{Proc. Natl. Acad. Sci. USA}, \textbf{109}, 13493 (2012).   

Exchange Interaction of Transition Metal Dopants in Diamond

Advances in single-ion implantation and spectroscopy have permitted direct observation of the exchange interaction between two dopant spins in a semiconductor[1], which is accurately described by tight-binding models of the semiconducting host[1,2]. These advances suggest controllable fabrication and utilization of few-dopant structures to explore fundamental properties and for applications[3]. Transition metal substitutional dopants in tetrahedrally-bonded semiconductors are good candidates for controllable spin manipulation and spin-spin interaction because they offer both highly-localized and much more extended spin-polarized states. For example, both the Ni and Cr dopant have spin-1 ground states in diamond, but with differing spatial extent[4]. We calculate the exchange interaction between pairs of Ni and Cr dopants in diamond using the technique of Ref. 2, but with an spds* tight-binding model. We find strong exchange interactions between pairs of Ni, and pairs of Cr, which are influenced by the differing symmetry of the dopants' ground state. [1] D. Kitchen et al., Nature 442, 436 (2006). [2] J.-M. Tang \& M.E. Flatt\'e, Phys. Rev. Lett. 92, 047201 (2004). [3] P. Koenraad \& M.E. Flatt\'e, Nat. Mat. 10, 91 (2011). [4] T. Chanier, et. al., Phys. Rev. B 86, 085203 (2012).   

Detection and Manipulation of Single NV Centers in Diamond

We use a scanning confocal microscope to investigate the fluorescence emission from nitrogen vacancy (NV) centers in diamond, a promising building block for quantum computing due to its long coherence time at room temperature. We demonstrate detection and coherent manipulation of a single NV center spin in synthetic diamond. Rabi oscillation data shows a modulation in the amplitude that is accounted for by simulating NV center spin dynamics in the presence of a proximal $^{14}$N nuclear spin. The hyperfine interaction opens up the possibility of coupling the electronic spin of an NV center to nearby nuclear spins, forming multi-qubit systems for quantum computation. For applications where a long coherence time is necessary, decoherence caused by the hyperfine interaction can be suppressed using a spin-echo pulse sequence, resulting in electron spin coherence times of over 1 $\mu$s at room temperature in type Ib diamond of high impurity content.   

Improving the Collection Efficiency of Bulk Diamond NV Center Fluorescence with Solid Immersion Lenses

The spin-dependent fluorescence of nitrogen vacancy (NV) centers in diamond makes them promising systems for a variety of applications ranging from magnetic field sensing to quantum information processing. The fidelity of optical detection of NV center spin states is therefore dependent on the collection efficiency of the NV fluorescence. While the crystal structure of diamond is useful in allowing for stable, room temperature measurements, its high index of refraction leads to a shallow critical angle of total internal reflection ($\sim24^\circ$) significantly limiting the optical collection efficiency. Here we develop a method for fabricating a solid immersion lens (SIL) on the surface of bulk diamonds. The hemispherical SILs, milled with high-energy gallium ions, are positioned such that the NV center of interest is at the origin of the sphere, thereby utilizing the full numerical aperture of the objective lens in the confocal microscope. Our lenses have already improved the collection efficiency by a factor of 2-3. With simple first order corrections to the milling process, higher collection efficiencies should be attainable. Further improvements in the lenses will allow single-shot readout of the spin states of NV centers.   

Towards Nuclear Polarization of Nanodiamond

Nanoparticles with long nuclear spin relaxation times [1] are candidates for use in targeted therapeutic delivery [2] and magnetic resonance imaging [3]. We report progress towards the development of contrast agents based on 13C in nanodiamond. Nuclear relaxation and electron spin resonance data is presented. We describe the development of a DNP setup at X band frequencies based on an ENDOR cavity, together with a novel brute force setup that combines milli-Kelvin temperatures of a dilution refrigerator, high magnetic fields and fast sample exchange. [1] J. Aptekar, {\it et al.}, ACS Nano, 3, 4003-4008 (2009). [2] H. Huang , E. Pierstorff, E. Osawa, and D. Ho, Nano Lett, 7, 3305-3314 (2007). [3] L Manus T. J. Meade, Nano Lett, 10, 484-489 (2010).   

Effective spin-orbit Hamiltonians and spin lifetimes for diamond and strontium titanate

The long spin coherence times of spin centers in diamond and the large Rashba coefficients and spin injection efficiencies in strontium titanate-based two-dimensional systems makes these wide band-gap semiconductors strong candidates for spintronics applications. To calculate the spin properties of these inversion-symmetric materials we have constructed a low-energy Hamiltonian, making use of a tight-binding model with atomic spin-orbit interactions. Furthermore we have derived and calculated the tensor that controls the form of the effective spin-orbit interaction in the non-spherical conduction bands of these materials. Finally we have computed the spin relaxation rates via the Elliott-Yafet mechanism through impurity scattering for diamond and uniaxially strained strontium titanate as a function of temperature and carrier density. Long spin lifetimes suggest the potential for novel spintronic applications of these wide bandgap semiconductors. This work was supported by an ARO MURI and an AFOSR MURI.   

Dynamic nuclear polarization of single nitrogen isoelectronic centers in GaAs

Due to their very long coherence time, nuclear spins of atomic systems represent good candidates for spin-based qubits in semiconductors. In this work, the dynamic nuclear polarization of isoelectronic centers formed from two nitrogen impurities in GaAs is investigated as a function of the external magnetic field and the polarization ellipticity of the exciting light. The nuclear spins of a single center are probed by the Overhauser shift of the neutral exciton and negatively charged exciton bound states. A nuclear magnetic field of 25 mT is measured at low external magnetic field and it decreases with this external field, indicating an efficiency loss in the exciton-nucleus spin-flip process. A peculiar Overhauser shift, scaling as the square of the ellipticity, is found for the exciton. A strong hysteretic behavior is also observed for both the neutral and charged excitons. These effects are believed to originate from the complex dynamic of the hyperfine interaction between the different excitonic spin states and nuclei. Our results show that dynamic nuclear polarization, much studied in quantum dots, is scalable to a single atomic-sized system. These results represent a first step towards the optical control of single nuclear spins in semiconductors.