Session B14: Focus Session: Spins in Semiconductors - Spin Relaxation in Semiconductors and Diamond

3D Electron Spin Relaxation Control by Electric Field in Quantum Wells

We have measured the electron spin relaxation time in (111)-oriented GaAs quantum wells by time-resolved photoluminescence. By embedding the QWs in a PIN or NIP structure we demonstrate the tuning of the conduction band spin splitting and hence the spin relaxation time with an applied external electric field applied along the growth z direction . The application of an external electric field of 50 kV/cm yields a two-order of magnitude increase of the spin relaxation time which can reach values larger than 30 ns; this is a consequence of the electric field tuning of the spin-orbit conduction band splitting which can almost vanish when the Rashba term compensates exactly the Dresselhaus one [1]. The spin quantum beats measurements under transverse magnetic field prove that the D'Yakonov-Perel (DP) spin relaxation time is not only increased for the Sz electron spin component but also for both Sx and Sy. These results contrast drastically with the (001) and (110) quantum wells.The role of the cubic Dresselhaus terms on the spin relaxation anisotropy will finally be discussed. The tuning or suppression of the DP electron spin relaxation demonstrated here for GaAs/AlGaAs quantum wells grown on (111) substrates is also possible in many other III-V and II-VI zinc-blende nanostructures since the principle relies only on symmetry considerations. \\[4pt] [1] A. Balocchi, Q. H. Duong, P. Renucci, B. L. Liu, C. Fontaine, T. Amand, D. Lagarde, and X. Marie, Phys. Rev. Lett 107, 136604(2011)   

Rotating Frame Spin dynamics of a Single Nitrogen Vacancy Center in Diamond Nanocrystal

We investigate the spin dynamics of a nitrogen-vacancy (NV) center contained in individual diamond nanocrystals with an average diameter of 70 $\pm $ 20 nm in the presence of continuous microwave excitation. Upon periodic reversal of the microwave phase, we observe a train of rotary (Solomon) echoes that effectively extends the system coherence lifetime to reach several tens of microseconds, depending on the microwave power and phase inversion rate [1]. Starting from a model where the NV center interacts with a bath of paramagnetic defects on the nanocrystal surface, we use average Hamiltonian theory to compute the signal envelope from its amplitude at the echo maxima. A comparison between the effective Rabi and Solomon propagators shows that the observed response can be understood as a form of higher-order decoupling from the spin bath. The observed rotary echoes can be thought of as the rotating frame analog of Hahn's spin echoes, implying that the present scheme may find use for nanodiamond-based magnetic sensing. [1] A. Laraoui, C. A. Meriles, Phys. Rev. B \textbf{84}, 161403(R) (2011).   

Persistence of single spin coherence above room temperature in diamond

The nitrogen vacancy (NV) center in diamond is unique among single spin systems because of its robust optical spin initialization, optical spin readout, and room temperature spin coherence. However, there is not yet an understanding of what processes limit the NV center's spin properties at higher temperatures. We address this question by performing pulsed electron spin resonance and spin-resolved optical lifetime measurements on single defects at elevated temperatures [1]. The measurements demonstrate that the NV center's spin coherence remains robust at high temperatures while its spin-dependent photoluminescence diminishes above 600 K due to nonradiative relaxation. These results provide an enhanced understanding of the NV center orbital structure and also suggest the possibility of using single spins in diamond for nanoscale thermometry with sensitivities on the order of 100 mK Hz$^{-1/2}$ over a broad temperature range. \\[4pt] [1] D. M. Toyli*, D. J. Christle*, A. Alkauskas, B. B. Buckley, C. G. Van De Walle, D. D. Awschalom, (\emph{submitted}).   

Optically trapped fluorescent nanodiamonds

The electronic spin state of the nitrogen-vacancy (NV) center in diamond has gained considerable interest because it can be optically initialized, coherently manipulated, and optically read out at room temperature. In addition, nanoparticle diamonds containing NV centers can be integrated with biological and microfluidic systems. We have constructed and characterized an optical tweezers apparatus to trap fluorescent nanodiamonds in a fluid and measure their fluorescence. Particles are held and moved in three dimensions using an infrared trapping laser. Fluorescent detection of these optically trapped nanodiamonds enables us to observe nanoparticle dynamics and to measure electron spin resonance of NV centers. We will discuss applications using the electron spin resonance of trapped NV centers in nanodiamonds for magnetic field imaging in fluidic environments.   

Nuclear Polarization of Nanodiamond

Nanoparticles with long nuclear spin relaxation times are candidates for use in the context of targeted therapeutic delivery [1] and magnetic resonance imaging [1,2]. We report progress towards the development of contrast agents [3] based on 13C in nanodiamond. Nuclear relaxation and electron spin resonance data is presented for particles produced using detonation and the high-pressure high temperature technique. We describe the development of a milli-Kelvin nuclear polarization setup that makes use of a dilution refrigerator and X-band microwave resonator with fast sample exchange. [1] Huang H., Pierstorff E., Osawa E., Ho D., ``Active Nanodiamond Hydrogels for Chemotherapeutic Delivery'', Nano Lett, 7, 3305-3314 (2007). [2] Aptekar J.W., Cassidy M. C., Johnson A. C., Barton R. A., Lee M. Y., Ogier A. C., Vo C., Anahtar M. N., Ren Y., Bhatia S. N., Ramanathan C., Cory D. G., Hill A. L., Mair R. W., Rosen M. S., Walsworth R. L., Marcus C. M., ``Silicon nanoparticles as hyperpolarized magnetic resonance imaging agents'', ACS Nano, 3, 4003-4008 (2009). [3] Manus L. M., Mastarone D. J., Waters E. A., Zhang X., Schultz-Sikma E. A., MacRenaris K. W., Ho D., Meade T. J., ``Gd(III)-nanodiamond conjugates for MRI contrast enhancement'', Nano Lett, 10, 484-489 (2010).   

Spin Lifetime Measurements of GaAsBi Films

Substituting a small amount of As with Bi, the largest non-radioactive group V element, leads to a large reduction in the GaAs band gap and expected large spin-orbit effects \footnote{B. Fluegel et al., \textbf{Giant Spin-Orbit Bowing in GaAs$_{1-x}$Bi$_{x}$}, Phys. Rev. Lett. \textbf{97}, 067205 (2006).}. Both properties are advantageous with potential applications ranging from infrared detectors to spin valves. Compressively strained GaAsBi films with varying bismuth compositions were grown on GaAs using molecular-beam epitaxy. Spin lifetimes were measured using the Hanle effect, a magneto-optical technique where an out-of-plane spin polarization is generated by circularly polarized light and then made to precess about an in-plane magnetic field. A Lorentzian lineshape can be fit to the field-dependent photoluminescence polarization to extract $gT_s$, where $g$ is the Lande g-factor and $T_s$ is a function of the carrier recombination time and spin dephasing time and provides a lower bound for both. Temperature and power dependent measurements were conducted and our extracted values for $gT_s$ vary from 100ps to 1ns.   

Carrier and Spin Dynamics in Narrow Gap Multi Quantum Well Structures

We studied carrier/spin dynamics in several $InSb/Al_{x}In_{1-x}Sb$ multiple-quantum well structures using several time resolved differential transmission schemes in the mid-infrared. Our results demonstrate the unique and complex dynamics in InSb heterostructures that can be important for electronic and optoelectronic devices. We present experimental observations and compare them with theoretical calculations. Calculations are based on the 8-band Pidgeon-Brown model generalized to include confinement potential as well as pseudomorphic strain. Optical properties are calculated within the golden rule approximation and compared with one and two color, time-resolved pump-probe differential transmission.   

Electric field-driven coherent spin reorientation and spin rephasing of optically generated electron spin packets in InGaAs

Full electric-field control of spin orientations is one of the key tasks in semiconductor spintronics. We demonstrate that electric field pulses can be utilized for phase-coherent 2$\pi$ spin rotation of optically generated electron spin packets in InGaAs epilayers using time-resolved Faraday rotation. Through spin-orbit interaction, the electric-field pulses act as local magnetic field pulses (LMFP). By the temporal control of the LMFP, we can turn on and off electron spin precession and thereby rotate the spin direction into arbitrary orientations in a 2-dimensional plane [1]. Moreover, using two subsequent pulses of opposite polarity allows us to perform spin echo measurements by reversing the spin precession direction. Although our spin transport experiment is in the diffusive regime, we unexpectedly observe that electric field-induced spin dephasing is reversible to a large extend.\\ $[1]$ S. Kuhlen \textit{et al}. arXiv 1107.4307   

Using spin fluctuations to reveal long hole spin lifetimes and hole-nuclear coupling in (In,Ga)As quantum dots

``Spin noise spectroscopy'' is a recently-developed technique for passively measuring the spin dynamics of electrons and holes via their intrinsic random spin fluctuations. In accord with the fluctuation-dissipation theorem, the frequency spectra of this spin noise alone reveals spin dephasing times and Land\'{e} $g$-factors. Using these methods we measure hole spins confined in self-assembled (In,Ga)As/GaAs quantum dots (QDs). Owing to their \emph{p}-type wavefunctions, holes experience much less hyperfine interaction with lattice nuclei as compared with confined electrons, leading in principle to long spin decoherence times which are favorable for potential qubit applications. We observe very long hole spin correlation times ($\sim$400 ns) in zero magnetic field, ultimately limited by dephasing from hole-nuclear hyperfine interactions. Suppressing this dephasing with small longitudinal fields ($<$ 100 G) directly reveals the hyperfine coupling strength, and unveils intrinsic hole spin relaxation times up to $\sim$5 $\mu$s. Importantly, the lineshape of the noise evolves from a Lorentzian to a power-law as the hole-nuclear dephasing is suppressed.   

Spin lifetimes in InGaAs quantum wells

We have carried out Hanle spin relaxation time measurements in InGaAs quantum well structures as well as in Fe-based spin-LEDs that incorporate InGaAs QWs. In the InGaAs QW structures the spin lifetime T$_{S}$ at T = 5 K is equal to 2 ns while in spin LEDs T$_{S}$ is only 0.17 ns. For the undoped QWs, T$_{S}$ increases from 2ns at 5K to 4.5ns at 15K and then decreases monotonically. On the other hand, T$_{S}$ in the LEDs increases monotonically with temperature. The origin for the difference in the spin lifetimes between undoped QWs and that within a LED structure can be understood by considering the corresponding differences in the band structure between the symmetric quantum well and the spin-LED. In the latter, the Rashba spin-orbit interaction due to the built-in asymmetry brings about a significant enhancement in spin relaxation. In addition, bias conditions in the spin-LED play a crucial role in determining the temperature trend of the spin relaxation. The bias voltage tunes the electron density and accordingly the spin dephasing could be either suppressed or enhanced by electron-electron scattering.   

T1 spin lifetimes in n-doped quantum wells and dots

We have used a pump probe technique to measure $T_1$ spin lifetimes in $n$-type GaAs quantum wells and InAs self-assembled quantum dots. The circularly polarized pump laser pulse aligns the spins; the linearly polarized probe laser pulse probes the spin states of the selected well (or dots) via the Kerr (or Faraday) effect at some later time. Results for the quantum well sample include a spin-filling effect that depends on the direction from which the probe laser wavelength approaches that of the well, and spin lifetimes ranging from 50 to 2000 ns (depending on temperature and field conditions). The InAs quantum dots, doped such that each dot has approximately one extra electron, display $T_1$ lifetimes longer than 5 ms at 1 T and 1.5 K.   

Frequency-domain optical probing of coherent spins in semiconductors

We have demonstrated a technique for measuring GHz-scale coherent spin dynamics in semiconductors using narrow-linewidth modulated lasers. This scheme, based on the Faraday effect, is carried out by adding a sinusoidal modulation at frequency $\Omega$ to continuous-wave pump and probe lasers. The result is to effectively shift the Fourier component of the Faraday rotation signal at $\Omega$ to zero frequency, where it can be measured by a low-bandwidth detector. By producing the Fourier transform of the time-domain spin dynamics, this method yields the same information as pulsed pump-probe measurements without the need for complex pulsed lasers, and with minimal spectral bandwidth, allowing for high-resolution spectroscopic measurements. The ability to perform Faraday rotation measurements with narrow-linewidth lasers is essential for observing spins in individual quantum dots, or for avoiding unintentional carrier excitation. We have employed this technique to observe coherent spin dynamics in CdSe nanocrystals using standard diode lasers. By fitting these results to the expected model, we can extract electron g-factors, and spin coherence and dephasing times in agreement with time-domain measurements.   

Quadratic magnetic field dependence of magnetoelectric photocurrent

We experimentally study the spin and electric photocurrents excited by a linearly polarized light via direct interband transitions in an InGaAs/InAlAs quantum well. In the absence of a magnetic field, the linearly polarized light induces a pure spin current due to the spin-orbit coupling, which may be transformed into a measurable electric current by applying an in-plane magnetic field. The induced electric photocurrent is linear with the in-plane magnetic field. Here, we report a quadratic magnetic field dependence of the photocurrent in the presence of an additional perpendicular component of the magnetic field. We attribute the observation to the Hall effect of magnetoelectric photocurrent.