共查询到20条相似文献,搜索用时 15 毫秒
1.
M. Idrish Miah 《Optics Communications》2011,284(5):1254-1256
Optically oriented electron spin lifetime in n-doped gallium arsenide was measured via depolarization of the photoluminescence (PL) in a transverse magnetic field (Hanle effect). In order to measure the PL polarization, a time-resolved pump-probe experiment, where a pump pulse generates spin-polarized electrons and a probe pulse monitors their polarization, was employed. The PL polarization in dependences of the pump-probe delay, external magnetic field as well as of the sample temperature was studied. The PL polarization was found to decay exponentially with the pump-probe delay, from which the spin lifetime of the electrons was measured. The measured value was found to depend on the strength of the magnetic field and sample temperature. 相似文献
2.
M. Idrish Miah 《Physics letters. A》2008,372(46):6981-6985
High field electron-spin transport in low temperature-grown gallium arsenide is studied. We generate electron spins in the samples by optical pumping. During transport, we observe the Dyakonov-Perel (DP) [M.I. Dyakonov, V.I. Perel, Zh. Eksp. Teor. Fiz. 60 (1971) 1954] spin relaxation of the drifting electrons. The results are discussed and are compared with those obtained in calculations of the DP spin relaxation frequency of the hot electrons. A good agreement is obtained. 相似文献
3.
S. Laurent O. Krebs S. Cortez M. Senes X. Marie T. Amand P. Voisin J. -M. Grard 《Physica E: Low-dimensional Systems and Nanostructures》2004,20(3-4):404
We report on optical orientation of electrons in n-doped InAs/GaAs quantum dots. Under non-resonant cw optical pumping, we measure a negative circular polarization of the luminescence of charged excitons (or trions) at low temperature (T=10 K). The dynamics of the recombination and of the circular polarization is studied by time-resolved spectroscopy. We discuss a simple theoretical model for the trion relaxation, that accounts for this remarkable polarization reversal. The interpretation relies on the bypass of Pauli blocking allowed by the anisotropic electron–hole exchange. Eventually, the spin relaxation time of doping electrons trapped in quantum dots is measured by a non-resonant pump–probe experiment. 相似文献
4.
D. Steiauf 《Journal of magnetism and magnetic materials》2010,322(6):L5
By making use of Kramer's degeneracy of the electronic states in a nonmagnetic material, Yafet has derived an expression for the spin relaxation time T1 due to scattering of electrons at phonons which involves the properties of electronic and phononic states and the matrix elements for the scattering. It is shown that an analogous expression for T1 can be derived for ferromagnets (where Kramer's degeneracy does not hold) when taking into account the conservation of the total number of electrons. This expression can be used as a starting point for the ab initio calculation of T1, and this quantity is required for an interpretation of the ultrafast demagnetization of ferromagnets after excitation with a femtosecond laser pulse. 相似文献
5.
Molecular reorientations and internal conformational transitions of an aligned chiral liquid crystal (LC) 10B1M7 are studied by means of deuterium spin-lattice relaxation in its smectic A (SmA) and smectic C* (SmC*) phase. The motional model which is applicable to uniaxial phases of many LCs is found to be adequate even when the phase is a tilted SmC* phase. The deuterium NMR spectrum in this phase cannot discern rotations of the molecular director about the pitch axis. The basic assumption is that the phase biaxiality is practically unobservable. However, the relaxation rates can be accounted for by the tilt angle between the molecular director and the layer normal in the SmC* phase. The tumbling motion appears to show a higher activation energy upon entering from the uniaxial SmA into the SmC* phase. 相似文献
6.
Bertram R Asbury T Fabiola F Quine JR Cross TA Chapman MS 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,163(2):300-309
The orientation data provided by solid-state NMR can provide a great deal of structural information about membrane proteins. The quality of the information provided is, however, somewhat degraded by sign degeneracies in measurements of the dipolar coupling tensor. This is reflected in the dipolar coupling penalty function used in atomic refinement, which is less capable of properly restraining atoms when dipolar sign degeneracies are present. In this report we generate simulated solid-state NMR data using a variety of procedures, including back-calculation from crystal structures of alpha-helical and beta-sheet membrane proteins. We demonstrate that a large fraction of the dipolar sign degeneracies are resolved if anisotropic dipolar coupling measurements are correlated with anisotropic chemical shift measurements, and that all sign degeneracies can be resolved if three data types are correlated. The advantages of correlating data are demonstrated with atomic refinement of two test membrane proteins. When refinement is performed using correlated dipolar couplings and chemical shifts, perturbed structures converge to conformations with a larger fraction of correct dipolar signs than when data are uncorrelated. In addition, the final structures are closer to the original unperturbed structures when correlated data are used in the refinement. Thus, refinement with correlated data leads to improved atomic structures. The software used to correlate dipolar coupling and chemical shift data and to set up energy functions and their derivatives for refinement, CNS-SS02, is available at our web site. 相似文献
7.
Wei HanK.M. McCreary K. PiW.H. Wang Yan LiH. Wen J.R. ChenR.K. Kawakami 《Journal of magnetism and magnetic materials》2012,324(4):369-381
We review our recent work on spin injection, transport and relaxation in graphene. The spin injection and transport in single layer graphene (SLG) were investigated using nonlocal magnetoresistance (MR) measurements. Spin injection was performed using either transparent contacts (Co/SLG) or tunneling contacts (Co/MgO/SLG). With tunneling contacts, the nonlocal MR was increased by a factor of ∼1000 and the spin injection/detection efficiency was greatly enhanced from ∼1% (transparent contacts) to ∼30%. Spin relaxation was investigated on graphene spin valves using nonlocal Hanle measurements. For transparent contacts, the spin lifetime was in the range of 50-100 ps. The effects of surface chemical doping showed that for spin lifetimes in the order of 100 ps, charged impurity scattering (Au) was not the dominant mechanism for spin relaxation. While using tunneling contacts to suppress the contact-induced spin relaxation, we observed the spin lifetimes as long as 771 ps at room temperature, 1.2 ns at 4 K in SLG, and 6.2 ns at 20 K in bilayer graphene (BLG). Furthermore, contrasting spin relaxation behaviors were observed in SLG and BLG. We found that Elliot-Yafet spin relaxation dominated in SLG at low temperatures whereas Dyakonov-Perel spin relaxation dominated in BLG at low temperatures. Gate tunable spin transport was studied using the SLG property of gate tunable conductivity and incorporating different types of contacts (transparent and tunneling contacts). Consistent with theoretical predictions, the nonlocal MR was proportional to the SLG conductivity for transparent contacts and varied inversely with the SLG conductivity for tunneling contacts. Finally, bipolar spin transport in SLG was studied and an electron-hole asymmetry was observed for SLG spin valves with transparent contacts, in which nonlocal MR was roughly independent of DC bias current for electrons, but varied significantly with DC bias current for holes. These results are very important for the use of graphene for spin-based logic and information storage applications. 相似文献
8.
A.M. Tyryshkin S.A. Lyon T. Schenkel J. Bokor J. Chu W. Jantsch F. Schffler J.L. Truitt S.N. Coppersmith M.A. Eriksson 《Physica E: Low-dimensional Systems and Nanostructures》2006,35(2):257
We discuss pulsed electron spin resonance measurements of electrons in Si and determine the spin coherence from the decay of the spin echo signals. Tightly bound donor electrons in isotopically enriched 28Si are found to have exceptionally long spin coherence. Placing the donors near a surface or interface is found to decrease the spin coherence time, but it is still in the range of milliseconds. Unbound two-dimensional electrons have shorter coherence times of a few microseconds, though still long compared to the Zeeman frequency or the typical time to manipulate a spin with microwave pulses. Longer spin coherence is expected in two-dimensional systems patterned into quantum dots, but relatively small dots will be required. Data from dots with a lithographic size of 400 nm do not yet show longer spin coherence. 相似文献
9.
《Current Applied Physics》2014,14(3):516-520
In this article, we employ the semiclassical Monte Carlo approach to study the spin polarized electron transport in single layer graphene channel. The Monte Carlo method can treat non-equilibrium carrier transport and effects of external electric and magnetic fields on carrier transport can be incorporated in the formalism. Graphene is the ideal material for spintronics application due to very low Spin Orbit Interaction. Spin relaxation in graphene is caused by D'yakonov-Perel (DP) relaxation and Elliott-Yafet (EY) relaxation. We study effect of electron electron scattering, temperature, magnetic field and driving electric field on spin relaxation length in single layer graphene. We have considered injection polarization along z-direction which is perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. This theoretical investigation is particularly important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene. 相似文献
10.
This lecture presents a brief survey of spin physics in semiconductors together with the historic roots of the recent activity in investigating spin-related phenomena. 相似文献
11.
Ulmer TS Campbell ID Boyd J 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2004,166(2):114-201
Proton NMR longitudinal and transverse relaxation rates of unlabelled proteins are generally dominated by the many 1H-1H dipolar interactions so that spin diffusion, rather than molecular or internal motions, governs longitudinal relaxation. Here, relaxation measurements of backbone amide proton (1H(N)) magnetisations have been carried out employing the 99% 2H, 98% 15N labelled, small 2F2 protein domain in 10%/90% H(2)O/D(2)O solution. Under these conditions, the longitudinal relaxation rates exhibit time constants, T(1)*=1/R(1)* if described by a mono-exponential, within the range of 3.0 to 18.7s-a wide range which indicates that the phenomenon of spin diffusion has been greatly reduced. The majority of 1H(N) nuclei in this sample (pH 4.0 and 5 degrees C) exhibit chemical exchange with solvent that couples their longitudinal relaxation to that of the solvent. For the subset of 1H(N) nuclei not undergoing detectable solvent chemical exchange, the R(1)* rates correlate well with their individual 1H(N,O)/2H(N,O) structural environments. The correlation for corresponding transverse relaxation rates, R(2)* was found to be less good. Longitudinal relaxation measurements in 1%/99% H(2)O/D(2)O solution identify a further subset of 1H(N) nuclei which exhibit essentially indistinguishable R(1)* rates in both 1% and 10% H(2)O, implying that averaging of rates from spin diffusion processes and different 2F2 isotopomer populations are negligible for these 1H(N) sites. In addition to a high sensitivity to structural parameters, model calculations predict 1H(N) relaxation rates to exhibit pronounced sensitivity to internal dynamics. 相似文献
12.
Mainali L Feix JB Hyde JS Subczynski WK 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2011,212(2):418-425
There are no easily obtainable EPR spectral parameters for lipid spin labels that describe profiles of membrane fluidity. The order parameter, which is most often used as a measure of membrane fluidity, describes the amplitude of wobbling motion of alkyl chains relative to the membrane normal and does not contain explicitly time or velocity. Thus, this parameter can be considered as nondynamic. The spin-lattice relaxation rate () obtained from saturation-recovery EPR measurements of lipid spin labels in deoxygenated samples depends primarily on the rotational correlation time of the nitroxide moiety within the lipid bilayer. Thus, can be used as a convenient quantitative measure of membrane fluidity that reflects local membrane dynamics. profiles obtained for 1-palmitoyl-2-(n-doxylstearoyl)phosphatidylcholine (n-PC) spin labels in dimyristoylphosphatidylcholine (DMPC) membranes with and without 50 mol% cholesterol are presented in parallel with profiles of the rotational diffusion coefficient, R⊥, obtained from simulation of EPR spectra using Freed’s model. These profiles are compared with profiles of the order parameter obtained directly from EPR spectra and with profiles of the order parameter obtained from simulation of EPR spectra. It is shown that and R⊥ profiles reveal changes in membrane fluidity that depend on the motional properties of the lipid alkyl chain. We find that cholesterol has a rigidifying effect only to the depth occupied by the rigid steroid ring structure and a fluidizing effect at deeper locations. These effects cannot be differentiated by profiles of the order parameter. All profiles in this study were obtained at X-band (9.5 GHz). 相似文献
13.
M. D. Martín L. Via J. K. Son R. Ruf E. E. Mendez 《Physica E: Low-dimensional Systems and Nanostructures》2000,6(1-4)
We have used time-resolved photoluminescence spectroscopy to study the light emission dynamics in a semiconductor microcavity as a function of excitation density and exciton-cavity detuning. We paid special attention to polariton spin relaxation by using circularly polarized excitation. We have found a striking behavior of the photoluminescence degree of polarization, which reaches its maximum value at a finite time. As the excitation density is increased and the system enters the stimulated emission regime, this maximum is followed by a negative dip, whose depth strongly depends on exciton-cavity detuning. 相似文献
14.
Spin relaxation taking place during radiofrequency (RF) irradiation can be assessed by measuring the longitudinal and transverse rotating frame relaxation rate constants (R1ρ and R2ρ). These relaxation parameters can be altered by utilizing different settings of the RF irradiation, thus providing a useful tool to generate contrast in MRI. In this work, we investigate the dependencies of R1ρ and R2ρ due to dipolar interactions and anisochronous exchange (i.e., exchange between spins with different chemical shift δω≠0) on the properties of conventional spin-lock and adiabatic pulses, with particular emphasis on the latter ones which were not fully described previously. The results of simulations based on relaxation theory provide a foundation for formulating practical considerations for in vivo applications of rotating frame relaxation methods. Rotating frame relaxation measurements obtained from phantoms and from the human brain at 4 T are presented to confirm the theoretical predictions. 相似文献
15.
16.
J. O. Vigfusson 《Journal of statistical physics》1982,27(2):339-353
The time relaxation behavior of the solutions of certain classes of discrete master equations is studied in the limit of an infinite number of states. Depending on the range of the transition matrix, a relaxation behavior is found reaching from at
–1/2 law for short range, over enhanced relaxation to an exponential relaxation for the extreme long-range case. The behavior in the limit of a continuous family of states is also discussed. 相似文献
17.
The dynamical properties of a 2D Heisenberg model with dipolar interactions and perpendicular anisotropy are studied using Monte Carlo simulations in two different ordered regions of the equilibrium phase diagram. We find a temperature defining a dynamical transition below which the relaxation suddenly slows down and the system apart from the typical Arrhenius relaxation to a Vogel-Fulcher-Tamann law. This anomalous behavior is observed in the scaling of the magnetic relaxation and may eventually lead to a freezing of the system. Through the analysis of the domain structures we explain this behavior in terms of the domains dynamics. Moreover, we calculate the energy barriers distribution obtained from the data of the magnetic viscosity. Its shape supports our comprehension of both, the Vogel-Fulcher-Tamann dynamical slowing down and the freezing mechanism. 相似文献
18.
Carrier recombination and electron spin relaxation dynamics in asymmetric n-doped (110) GaAs/AlGaAs quantum wells are investigated with time-resolved pump-probe spectroscopy. The experiment results reveal that the measured carrier recombination time depends strongly on the polarization of pump pulse. With the same pump photon flux densities, the recombination time of spin-polarized carriers is always longer than that of the spin-balanced carriers except at low pump photon flux densities, this anomaly originates from the polarization-sensitive nonlinear absorption effect. Differing from the traditional views, in the low carrier density regime, the D'yakonov–Perel' (DP) mechanism can be more important than the Bir–Aronov–Pikus (BAP) mechanism, since the DP mechanism takes effect, the spin relaxation time in (110) GaAs QWs is shortened obviously via asymmetric doping. 相似文献
19.
Proton spectra of solids are usually broadened by strong proton homonuclear dipolar interactions. However, substantial line narrowing may be achieved by Magic Angle Spinning (MAS) in systems of low proton density or in systems in which rapid molecular motions occur. In such conditions, T1(H) measurements are often used to characterise the dynamics of each resolved proton site. We show that T1(H) values measured for solid organic compounds with high proton abundance, such as adamantane and glycine, may be strongly dependent on the spinning rate employed, so that care is required when values are compared. The effects of molecular motion and proton density on T1(H) and its dependence on spinning rate were investigated. We found that an increase in molecular motion leads to an increase of T1(H) at higher spinning rates. The opposite is found for systems with low proton densities which show relatively lower T1(H), at higher spinning rates. A possible interpretation is suggested in terms of the reduced spin diffusion efficiency at higher spinning rates. 相似文献
20.
Ranajeet Ghose James H. Prestegard 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1997,128(2):138-143
The effects of cross-correlation between Curie spin–nuclear dipole and nuclear dipole–nuclear dipole interactions on the linewidths and resonance frequencies of the individual lines of anAXmultiplet in paramagnetic systems have been calculated. The implication of the relaxation-induced frequency shift of the lines (dynamic frequency shift) for the accurate measurement of residual dipolar couplings in field-oriented systems has been discussed. Our simulations indicate that these effects may play a role in the precise measurement of residual dipolar couplings in systems which belong to the small and intermediate tumbling regime, i.e., correlation times less than 5 ns. 相似文献