Spin Relaxation Mechanism Research Articles

Comparison of spin lifetimes in n-Ge characterized between three-terminal and four-terminal nonlocal Hanle measurements

We compared the temperature dependence of spin lifetime in n-Ge characterized from three-terminal (3T) and four-terminal (4T) Hanle measurements using single-crystalline Fe/MgO/n-Ge tunnel junctions. The bias conditions of the two schemes were chosen to be about the same in order to compare the spin lifetimes (τ3T and τ4T). The temperature dependences of τ3T and τ4T behave in a very similar way at the low temperature region (T ≤ 10 K), and both τ3T and τ4T decrease as the temperature increases, which is consistent with the dominating Elliot–Yafet spin relaxation mechanism in bulk Ge. However, when the temperature is higher than 10 K, τ4T is longer than τ3T, which may be explained by the fact that 3T Hanle measurements are more easily affected by additional scattering effects caused by the accompanied charge current and electric field in the 3T geometry.

Valley polarization and intervalley scattering in monolayer MoS2

We probe the degree of circular polarization of the emitted photoluminescence from a single layer of MoS2 as a function of the circularly polarized photo-excitation energy. A Single layer of MoS2 has strong emission at around 1.9 eV associated with a direct transition at the K-point of the Brillouin zone. The circular polarization of the photoluminescence is very high for excitation near the bandgap and has a power-law decrease as the excitation energy increases. We identify phonon-assisted intervalley scattering as the primary spin relaxation mechanism and present a model that explains the wide variation in values for the polarization reported in the literature.

Albert Warner Overhauser

Albert Warner Overhauser

Monte Carlo simulation study of spin transport in trilayer graphene: A comparison between ABA and ABC stacking

In this work, we have used semi-classical Monte Carlo simulations to model spin transport in trilayer graphene (TLG) with ABA as well as ABC stacking. We have taken into consideration both the D'yakonov–Perel (DP) and Elliot–Yafet (EY) mechanisms of spin relaxation for modeling purposes. The two different stacking orders, ABA and ABC, have different band-structures, and we have studied the effect of the change in band structure on spin transport. Further, we have compared these results with bilayer graphene and single layer graphene and tried to explain the differences in the spin relaxation lengths in terms of band structure. We observe that TLG with ABC stacking exhibits a significantly higher spin relaxation length than TLG with ABA stacking.

Suppression of electron spin relaxation in a wide GaAs/AlGaAs quantum well with a lateral confining potential

Coherent spin dynamics of electrons laterally confined in a wide GaAs quantum well using a special mosaic electrode deposited onto the sample plane has been investigated. Electron spin lifetime measurements have been carried out in structures with different sizes of orifices in the mosaic electrode using the time-resolved Kerr rotation technique. It has been found that the electron spin lifetime in the lateral traps increases significantly with decreasing orifice size in the mosaic electrode. The observed electron spin lifetime dependence can be interpreted in terms of D'yakonov–Perel' spin relaxation mechanism suppression.

Growth direction dependence of the electron spin dynamics in {111} GaAs quantum wells

The electron spin dynamics is studied by time-resolved Kerr rotation in GaAs/AlGaAs quantum wells embedded in NIP structures grown on (111)A or (111)B-oriented substrates. In both cases the spin lifetimes are significantly increased by applying an external electric field, but this field has to point along the growth direction for structures grown on (111)A and opposite to it for the ones grown on (111)B. This extended electron spin lifetime is the result of the suppression of the D’yakonov-Perel spin relaxation mechanism [Sov. Phys. Solid State 13, 3023 (1972)] due to the cancellation effect of the internal Dresselhaus term [Phys. Rev. 100, 580 (1955)] with the external electric field induced Rashba one [J. Phys. C 17, 6039 (1984)], both governing the conduction band spin-orbit splitting. These results demonstrate the key role played by the growth direction in the design of spintronic devices.

Spin relaxation mechanism in silver nanowires covered with MgO protection layer

Spin-flip mechanism in Ag nanowires with MgO surface protection layers has been investigated by nonlocal spin injection using permalloy/Ag lateral spin valves. The spin flip events mediated by surface scattering are effectively suppressed by the MgO capping layer. The spin relaxation process was found to be well described in the framework of Elliott-Yafet mechanism (R. J. Elliott, Phys. Rev. 96, 266 (1954); Y. Yafet, in Solid State Physics, edited by F. Seitz and D. Turnbull (Academic, New York, 1963), pp. 1–98) and then the probabilities of spin-filp scattering for phonon or impurity mediated momentum scattering is precisely determined in the nanowires. The temperature dependent spin-lattice relaxation follows the Bloch-Grüneisen theory (V. F. Bloch, Z. Phys. 59, 208 (1930); V. E. Grüneisen, Ann. Phys. 5, 530 (1933)) and falls on to a universal curve of Ag as in the conduction-electron-spin resonance data for bulk.

Testing the Elliott-Yafet spin-relaxation mechanism in KC8: A model system of biased graphene

Temperature dependent electron spin resonance (ESR) measurements are reported on stage 1 potassium doped graphite, a model system of biased graphene. The ESR linewidth is nearly isotropic and although the g-factor has a sizeable anisotropy, its majority is shown to arise due to macroscopic magnetization. Albeit the homogeneous ESR linewidth shows an unusual, non-linear temperature dependence, it appears to be proportional to the resistivity which is a quadratic function of the temperature. These observations suggests the validity of the Elliott-Yafet relaxation mechanism in KC8 and allows to place KC8 on the empirical Beuneu-Monod plot among ordinary elemental metals.

Electron tunneling in lithium-ammonia solutions probed by frequency-dependent electron spin relaxation studies.

Electron transfer or quantum tunneling dynamics for excess or solvated electrons in dilute lithium-ammonia solutions have been studied by pulse electron paramagnetic resonance (EPR) spectroscopy at both X- (9.7 GHz) and W-band (94 GHz) frequencies. The electron spin-lattice (T(1)) and spin-spin (T(2)) relaxation data indicate an extremely fast transfer or quantum tunneling rate of the solvated electron in these solutions which serves to modulate the hyperfine (Fermi-contact) interaction with nitrogen nuclei in the solvation shells of ammonia molecules surrounding the localized, solvated electron. The donor and acceptor states of the solvated electron in these solutions are the initial and final electron solvation sites found before, and after, the transfer or tunneling process. To interpret and model our electron spin relaxation data from the two observation EPR frequencies requires a consideration of a multiexponential correlation function. The electron transfer or tunneling process that we monitor through the correlation time of the nitrogen Fermi-contact interaction has a time scale of (1-10) × 10(-12) s over a temperature range 230-290 K in our most dilute solution of lithium in ammonia. Two types of electron-solvent interaction mechanisms are proposed to account for our experimental findings. The dominant electron spin relaxation mechanism results from an electron tunneling process characterized by a variable donor-acceptor distance or range (consistent with such a rapidly fluctuating liquid structure) in which the solvent shell that ultimately accepts the transferring electron is formed from random, thermal fluctuations of the liquid structure in, and around, a natural hole or Bjerrum-like defect vacancy in the liquid. Following transfer and capture of the tunneling electron, further solvent-cage relaxation with a time scale of ∼10(-13) s results in a minor contribution to the electron spin relaxation times. This investigation illustrates the great potential of multifrequency EPR measurements to interrogate the microscopic nature and dynamics of ultrafast electron transfer or quantum-tunneling processes in liquids. Our results also impact on the universal issue of the role of a host solvent (or host matrix, e.g. a semiconductor) in mediating long-range electron transfer processes and we discuss the implications of our results with a range of other materials and systems exhibiting the phenomenon of electron transfer.

Temperature dependent spin precession measurements in trilayer graphene utilizing co/graphene contacts

The temperature dependence of the spin lifetime and spin diffusion coefficient of exfoliated multilayer graphene is measured using nonlocal spin detection and spin precession measurements. Low impedance cobalt contacts are utilized for spin injection and readout. A decrease in spin lifetime with increasing temperature is observed as well as an increase in the spin diffusion coefficient with increasing temperature. This observation provides some insight into the relevant spin relaxation mechanisms that are occurring in this trilayer graphene sample.

Field-Induced Negative Differential Spin Lifetime in Silicon

We show that the electric-field-induced thermal asymmetry between the electron and lattice systems in pure silicon substantially impacts the identity of the dominant spin relaxation mechanism. Comparison of empirical results from long-distance spin transport devices with detailed MonteCarlo simulations confirms a strong spin depolarization beyond what is expected from the standard Elliott-Yafet theory even at low temperatures. The enhanced spin-flip mechanism is attributed to phonon emission processes during which electrons are scattered between conduction band valleys that reside on different crystal axes. This leads to anomalous behavior, where (beyond a critical field) reduction of the transit time between spin-injector and spin-detector is accompanied by a counterintuitive reduction in spin polarization and an apparent negative spin lifetime.

Electron spin relaxation in graphene with random Rashba field: comparison of the D'yakonov–Perel' and Elliott–Yafet-like mechanisms

To understand the main spin relaxation mechanism in graphene, we investigate the spin relaxation with random Rashba field (RRF) induced by both adatoms and a substrate using the kinetic spin Bloch equation approach. The charged adatoms, on the one hand, enhance the Rashba spin–orbit coupling locally and, on the other hand, serve as Coulomb potential scatterers. Both effects contribute to spin relaxation limited by the D'yakonov–Perel' (DP) mechanism. In addition, the RRF also causes spin relaxation by spin-flip scattering, manifesting itself as an Elliott–Yafet (EY)-like mechanism. Both mechanisms are sensitive to the correlation length of the RRF, which may be affected by environmental parameters such as electron density and temperature. Fitting and comparing the experiments of the Groningen group (Józsa et al 2009 Phys. Rev. B 80 241403(R)) and the Riverside group (Pi et al 2010 Phys. Rev. Lett.104 187201; Han and Kawakami 2011 Phys. Rev. Lett.107 047207), which show either DP (with the spin relaxation rate being inversely proportional to the momentum scattering rate) or EY-like (with the spin relaxation rate being proportional to the momentum scattering rate) properties, we suggest that the DP mechanism dominates the spin relaxation in graphene. The latest experimental finding of a nonmonotonic dependence of spin relaxation time on diffusion coefficient by Jo et al 2011 (Phys. Rev. B 84 075453) is also well reproduced by our model.

NaDyF4Nanoparticles as T2Contrast Agents for Ultrahigh Field Magnetic Resonance Imaging

A major limitation of the commonly used clinical MRI contrast agents (CAs) suitable at lower magnetic field strengths (<3.0 T) is their inefficiency at higher fields (>7 T), where next-generation MRI scanners are going. We present dysprosium nanoparticles (β-NaDyF4 NPs) as T2 CAs suitable at ultrahigh fields (9.4 T). These NPs effectively enhance T2 contrast at 9.4 T, which is 10-fold higher than the clinically used T2 CA (Resovist). Evaluation of the relaxivities at 3 and 9.4 T show that the T2 contrast enhances with an increase in NP size and field strength. Specifically, the transverse relaxivity (r2) values at 9.4 T were ∼64 times higher per NP (20.3 nm) and ∼6 times higher per Dy(3+) ion compared to that at 3 T, which is attributed to the Curie spin relaxation mechanism. These results and confirming phantom MR images demonstrate their effectiveness as T2 CAs in ultrahigh field MRIs.

Electron spins in an array of tunnel-coupled quantum dots

Mechanisms of electron spin relaxation in semiconductor arrays with tunnel-coupled quantum dots are reviewed. The contribution for anisotropic exchange interaction is shown for asymmetrical quantum dots having no inversion axis relative to their plane. The configuration of vertically coupled double Ge/Si quantum dots is found where anisotropic exchange coupling does not contribute to spin decoherence. It could be a basic configuration of spin-based quantum computation schemes.

Spin Relaxation near the Metal-Insulator Transition: Dominance of the Dresselhaus Spin-Orbit Coupling

We identify the Dresselhaus spin-orbit coupling as the source of the dominant spin-relaxation mechanism in the impurity band of a wide class of n-doped zinc blende semiconductors. The Dresselhaus hopping terms are derived and incorporated into a tight-binding model of impurity sites, and they are shown to unexpectedly dominate the spin relaxation, leading to spin-relaxation times in good agreement with experimental values. This conclusion is drawn from two complementary approaches: an analytical diffusive-evolution calculation and a numerical finite-size scaling study of the spin-relaxation time.

ChemInform Abstract: Spin Transport and Relaxation in Graphene

AbstractReview: 120 refs.

Spin-phonon coupling in single Mn-doped CdTe quantum dot

The spin dynamics of a single Mn atom in a laser driven CdTe quantum dot is addressed theoretically. Recent experimental results\cite{Le-Gall_PRL_2009,Goryca_PRL_2009,Le-Gall_PRB_2010}show that it is possible to induce Mn spin polarization by means of circularly polarized optical pumping. Pumping is made possible by the faster Mn spin relaxation in the presence of the exciton. Here we discuss different Mn spin relaxation mechanisms. First, Mn-phonon coupling, which is enhanced in the presence of the exciton. Second, phonon-induced hole spin relaxation combined with carrier-Mn spin flip coupling and photon emission results in Mn spin relaxation. We model the Mn spin dynamics under the influence of a pumping laser that injects excitons into the dot, taking into account exciton-Mn exchange and phonon induced spin relaxation of both Mn and holes. Our simulations account for the optically induced Mn spin pumping.

Magnetoresistance effects and phase coherent transport phenomena in a magnetic nonplanar two-dimensional hole system

Phase coherent transport phenomena are studied in a magnetic, two-dimensional hole system confined in a strained InAs quantum-well structure. At low magnetic fields we observe a well pronounced weak antilocalization signature, demonstrating that magnetic ions in proximity to the itinerant holes preserve phase coherent transport, although magnetic effects like the anomalous and planar Hall effects are present at low temperatures. From fits to the conductivity correction, spin and phase relaxation times are extracted in a large temperature range from 200 mK to 16 K. From the dependency of the spin relaxation time on temperature and in-plane magnetic fields, we conclude that the Dyakonov-Perel mechanism is the dominant spin relaxation mechanism. The metamorphic growth of the heterostructure causes a nonplanar quantum well that enables the formation of magnetic barriers in the presence of a parallel applied magnetic field. Transport through these magnetic barriers causes a positive magnetoresistance superimposed on the weak antilocalization signal.

Electrical spin injection and transport in germanium

We report the first experimental demonstration of electrical spin injection, transport, and detection in bulk germanium (Ge). The nonlocal magnetoresistance (MR) in n-type Ge is observable up to 225 K. Our results indicate that the spin relaxation rate in the n-type Ge is closely related to the momentum scattering rate, which is consistent with the predicted Elliot-Yafet spin relaxation mechanism for Ge. The bias dependence of the nonlocal MR and the spin lifetime in n-type Ge is also investigated.

Lysine Nζ-Decarboxylation Switch and Activation of the β-Lactam Sensor Domain of BlaR1 Protein of Methicillin-resistant Staphylococcus aureus

The integral membrane protein BlaR1 of methicillin-resistant Staphylococcus aureus senses the presence of β-lactam antibiotics in the milieu and transduces the information to the cytoplasm, where the biochemical events that unleash induction of antibiotic resistance mechanisms take place. We report herein by two-dimensional and three-dimensional NMR experiments of the sensor domain of BlaR1 in solution and by determination of an x-ray structure for the apo protein that Lys-392 of the antibiotic-binding site is posttranslationally modified by N(ζ)-carboxylation. Additional crystallographic and NMR data reveal that on acylation of Ser-389 by antibiotics, Lys-392 experiences N(ζ)-decarboxylation. This unique process, termed the lysine N(ζ)-decarboxylation switch, arrests the sensor domain in the activated ("on") state, necessary for signal transduction and all the subsequent biochemical processes. We present structural information on how this receptor activation process takes place, imparting longevity to the antibiotic-receptor complex that is needed for the induction of the antibiotic-resistant phenotype in methicillin-resistant S. aureus.