Dominant Relaxation Mechanism Research Articles

Effects of heavy p doping on the polarized emission spectra and low-temperature luminescence spectra of GaAs/GaAsP strained-layer structures

The optical orientation of electron spins in heavily doped, strained GaAs/GaAsP layers with a deformation-split valence band is studied experimentally. The observed polarized luminescence spectra and polarized photoemission (electron emission) spectra are shown to be described well by a model which allows for smearing of the edges of the bands by the fluctuation potential due to impurities, degeneracy of the carriers at low temperatures, and indirect electron-phonon optical transitions. The dominant mechanism of electron spin relaxation in strained layers is found to be the Bir-Aronov-Pikus mechanism. The parameters of the fluctuation potential and the parameters governing carrier spin relaxation are determined.

Atomic diffusion, step relaxation, and step fluctuations.

We show that the dynamics of the pair correlation function in a step train can pinpoint the dominant relaxation mechanism occurring at a crystal surface. Evaporation-condensation and step-edge diffusion do not produce dynamical correlations between neighboring steps, while terrace diffusion may lead to correlations which fall off like a power law with distance and which are peaked at a characteristic time. We derive these results within a "real space" Langevin formalism which is based on diffusion kernels which are different for each mass transport process. We validate this formalism by reproducing the step fluctuation autocorrelation function. We then derive results on the pair correlation between different steps. Results for solvable limiting cases are summarized in Tables I and II of the paper. As an intermediate step in the analysis we also find expressions for the relaxation time tau(pq) of a mode of wave number q along the steps and wave number p perpendicular to the steps, which we also discuss and compare with prior work.

High field NMR studies of conduction electron dynamics in metallic polypyrrole-PF6

We report measurements of the proton spin-lattice relaxation rate 1/T1 at the frequencies (ω/2π) 0.96 GHz (temperature T = 1.7 −55 K) and 0.38 GHz (1.7 −200 K) in metallic polypyrrole-PF6 doped to ~ 25% PF6. Within a factor of two, T1T is constant over the full temperature range for each frequency. The rate for the lower frequency is 2 – 8 times faster, depending on T. Although the microscopic details do not fit the model for 1D diffusion, the near constancy of T1T is a strong indication that the dominant proton relaxation mechanism is conduction electron motion. An advantage of the high field measurements is that below 15 K, saturation of any local electron moment magnetization eliminates nuclear spin diffusion to paramagnetic centers as a competing relaxation mechanism, leaving only relaxation by translational motion of the electons.

129Xespin relaxation in frozen xenon

We discuss the longitudinal spin relaxation of ${}^{129}\mathrm{Xe}$ nuclei in frozen xenon. Over a large range of temperatures and magnetic fields, the dominant spin-lattice relaxation mechanism is shown to be nuclear spin-flip Raman scattering of lattice phonons. Two closely related interactions couple the lattice phonons to the spins of ${}^{129}\mathrm{Xe}$ nuclei: (1) the nuclear spin-rotation interaction between nearest-neighbor atoms, and (2) the paramagnetic antishielding of the externally applied field at the site of ${}^{129}\mathrm{Xe}$ nuclei by the electrons of neighboring Xe atoms. We show that relaxation rates can be predicted by using measured chemical shifts of gaseous and condensed xenon. The predicted relaxation rates are in good agreement with measurements. We outline a simple way to estimate the spin-rotation coupling and paramagnetic antishielding in terms of the small perturbations of the outermost electron orbitals of one xenon atom due to a neighboring atom.

Heat pulse studies of the energy relaxation rate of hot electrons in n-type GaN epilayers

We have measured the energy relaxation rate of hot electrons in GaN over the temperature range 1–130 K using a non-equilibrium pulse technique. The samples were fabricated from an n + GaN epilayer grown by MBE on a c-axis sapphire substrate. In addition we were able to observe directly the emitted phonons. Above 10 K the relaxation rate showed a linear dependence characteristic of the equipartition regime. Below 10 K, the dependence was P∝ T 4 e, consistent with piezoelectric coupling in the so-called `dirty’ limit. In contrast to GaAs, coupling to acoustic phonons was the dominant energy relaxation mechanism up to the maximum temperature of the experiment.

Spin-lattice relaxation of Co(II) in hexaaquocobalt(II) picrylsulphonate tetrahydrate: An estimate from EPR line width of the dopant, Mn(II)

Single crystal EPR of Mn(II)-doped [Co(H 2O) 6] (picsul) 2·4H 2O is studied in the temperature range of 123–300 K. The angular variation of the zero-field transitions, observed from a single magnetically distinct site, is successfully simulated with an asymmetric zero-field tensor with D=436×10 −4 cm −1 and E=79×10 −4 cm −1. Sharp EPR spectra observed for Mn(II), a paramagnetic dopant in the paramagnetic host lattice, are attributed to host-spin-lattice relaxation narrowing. Based on the theory of exchange narrowing of dipolar coupling, the T 1 of Co(II) has been estimated to be 0.54×10 −12 s at 300 K. From the temperature dependence of T 1, it is postulated that two-phonon Raman process is the dominant mechanism for spin-lattice relaxation above 250 K, and below 250 K a cross-relaxation process is evident.

Temperature dependence of carrier relaxation in strain-induced quantum dots

We report experimental observation and theoretical interpretation of temperature-dependent, time-resolved luminescence from strain-induced quantum dots. The experimental results are well described by a master equation model for the electrons. The intraband relaxation in the conduction band and the radiative recombination rate are governed by the hole populations resulting in prominent temperature dependence of the relaxation process. Even when only a few electrons and holes are confined in a single quantum dot the Auger-like process provides a rapid intraband relaxation channel for electrons that can replace the phonon scattering as the dominant relaxation mechanism.

Electron paramagnetic resonance study of atomic phosphorus encapsulated in [60]fullerene

Well resolved EPR spectra of P@C60 in solution have been recorded, proving that the encased phosphorus atoms are in their quartet spin ground state. The isotropic hyperfine interaction is increased by a factor of 2.5 compared with the values measured for free atoms. An analysis of spin relaxation data reveals that fluctuating zero field splitting (ZFS) interaction induced by collision-induced deformations of the carbon shell constitutes the dominant relaxation mechanism. The variance of the time fluctuating ZFS interaction is about a factor of 10 larger than that observed for N@C60 under identical conditions. Values for the correlation time of the deformation of the fullerene cage range from 5ps to 32 ps in the temperature interval 190-300 K in toluene.

Electron paramagnetic resonance study of atomic phosphorus encapsulated in [60]fullerene

Well resolved EPR spectra of P@C60 in solution have been recorded, proving that the encased phosphorus atoms are in their quartet spin ground state. The isotropic hyperfine interaction is increased by a factor of 2.5 compared with the values measured for free atoms. An analysis of spin relaxation data reveals that fluctuating zero field splitting (ZFS) interaction induced by collision-induced deformations of the carbon shell constitutes the dominant relaxation mechanism. The variance of the time fluctuating ZFS interaction is about a factor of 10 larger than that observed for N@C60 under identical conditions. Values for the correlation time of the deformation of the fullerene cage range from 5ps to 32 ps in the temperature interval 190-300 K in toluene.

A theoretical approach to the thermal transient mechanical loss in Mg matrix composites

In metal matrix composites, temperature changes build up thermal stresses at the ceramic–metal interfaces, which result from the thermal expansion coefficient mismatch between the two phases. Depending on the relaxation mechanisms, such high stresses can degrade the mechanical properties of the composite. During mechanical spectroscopy measurements, thermal stress relaxation gives rise to an additional transient mechanical loss response. In order to interpret the dominant mechanism of stress relaxation, different models have been proposed, which relate the transient loss with interface debonding, growth of a plastic zone around the fibres, or creation and motion of dislocations. In the case of the magnesium-based composites, a nonlinear relation is observed between the transient loss and the measurement parameters, which cannot be explained by the existing models. In order to interpret the mechanism of stress relaxation in these composites, a new theoretical approach has been developed. The model takes into account a relaxation mechanism due to the motion of the existing dislocations. It allows one to distinguish between the case where the dislocation motion is controlled by a viscous force such as due to the dragging of solute atoms, and the case where the motion is controlled by a solid friction mechanism such as the breakaway from pinning points. In the case of the magnesium-based composites, the model allows one to conclude that the dominant mechanism of thermal stress relaxation is the motion of existing dislocations, which is controlled by a solid friction mechanism.

Dysprosium-DOTA-PAMAM dendrimers as macromolecular T2 contrast agents. Preparation and relaxometry.

The authors have investigated dysprosium [Dy]-DOTA-PAMAM, generation 5 (G = 5) dendrimers as a possible new class of macromolecular T2 contrast agents. The use of DOTA provides a metal complex with greater stability than can be achieved using DTPA as ligand, an important factor in the design of blood pool agents with long half-lives. Generation 5 ammonia-core PAMAM dendrimers were linked to the bifunctional ligand p-SCN-Bz-DOTA. After determination of the number of conjugated DOTA molecules by 1H nuclear magnetic resonance, Dy3+ was titrated at a 90% molar ratio. For comparison, single ionic chelates of Dy-DTPA and Dy-DOTA also were prepared. Using a variable field relaxometer, T1 and T2 relaxation times were measured at 13 different field strengths from 0.05 to 1.5 T and temperatures of 3, 10, 20 and 37 degrees C. The synthesis resulted in a preparation with 76 DOTA and 68 Dy3+ ions per dendrimer molecule. The T1 relaxivity values for Dy-DTPA, Dy-DOTA, and the Dy-DOTA-based dendrimer all were independent of field strength, with values between 0.12 and 0.20 mM-1 sec-1. At lower fields (0.05-0.1 T), 1/T2 was identical to 1/T1. At higher fields, however, 1/T2 increased quadratically with field strength, with a strong dependence on temperature. The field-dependent component of 1/T2 was up to three times higher for the Dy-DOTA-based dendrimer compared with the single chelate molecules, with coefficients of 0.37 to 0.03 sec-1/Tesla2 for T = 3 to 37 degrees C. The results are interpreted with the "inner sphere" theory of susceptibility effects (Curie spin relaxation). The large temperature dependence suggests that the dominant mechanism of relaxation is the contact interaction effect, with the proton residence time as the primary time constant. This largely unexplored relaxation mechanism has the potential to create a new class of T2-selective contrast agents.

Nuclear magnetic resonance and electron spin resonance of amorphous hydrogenated carbon

The temperature dependences of the and magnetization recovery and spin-lattice relaxation times as well as the NMR spectra have been studied between 300 K and 4 K. The observed short and nearly temperature independent proton and spin-lattice relaxation times demonstrate that the dominant spin-lattice relaxation mechanism is spin diffusion to paramagnetic impurities. The fact that the magnetization recovery curves clearly deviate from the single-exponential form expected for the case of spin diffusion and randomly distributed paramagnetic centres demonstrates that the paramagnetic centres aggregate in clusters. Superparamagnetic freezing effects expected for such an inhomogeneous distribution are indeed seen below 50 K in the temperature dependence of the electron spin-resonance (ESR) intensity, the NMR spectra and the SQUID magnetization measurements which show a distinct magnetic hysteresis loop.

Clear Evidence of Reptation in Polyethylene from Neutron Spin-Echo Spectroscopy

The dynamic structure factor $S(q,t)$ of polyethylene (PEB-2) was measured by neutron spin echo in the Fourier time range of $t\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.3--175$ nsec and for momentum transfers $q$ between 0.05 and $0.145{\AA{}}^{\ensuremath{-}1}$ to test the validity of competing phenomenological theories of relaxation in polymer melts. Previous spin-echo experiments limited to $t<25$ nsec were equally well described by a variety of models. This ambiguity has now been lifted, and the experiment clearly favors the reptation model, showing that the dominant relaxation mechanism in entangled linear polymers is via reptation.

13C and 1H spin-lattice relaxation in amorphous hydrogenated carbon

The temperature dependences of the 13C and 1H magnetization recovery and spin-lattice relaxation times have been measured in amorphous hydrogenated carbon ( a-C : H). The observed short and nearly temperature independent proton and 13C spin-lattice relaxation times demonstrate that the dominant spin-lattice relaxation mechanism is spin diffusion to paramagnetic impurities. The fact that the magnetization recovery curves clearly deviate from the single exponential form demonstrates that the paramagnetic centers aggregate in clusters so that the unpaired electron distribution in a-C : H is strongly inhomogeneous.

Hot carrier relaxation in InAs/GaAs quantum dots

Excited states and energy relaxation processes are studied for self-organized InAs/GaAs quantum dots (QDs). Photoluminescence excitation (PLE) spectra show sample-dependent either multi-LO-phonon resonances or excited state transitions, revealing the dominant carrier relaxation mechanism or the size-dependent excited state splitting of the QDs, respectively. Time-resolved photoluminescence results indicate sample-dependent non-radiative recombination, suggesting a model for the observed PLE behavior, analogous to hot carrier relaxation in higher-dimensional systems. Carrier relaxation in the self-organized InAs/GaAs QDs proceeds by multi-LO-phonon scattering on a 40 ps time scale, which is short compared to radiative (>500 ps) and non-radiative (>100 ps) recombination times, accounting for the absence of a phonon bottleneck effect in PL spectra. However, carrier relaxation might effect the stimulated emission region.

Resonant magnetic quantum tunneling through thermally activated states

A theory of the magnetic relaxation of a large spin (S=10) having a large uniaxial magnetocrystalline anisotropy is outlined. The theory explains magnetic relaxation observed in Mn12 acetate. The joint action of local fields and a fourth order distortion of the magnetocrystalline anisotropy is necessary to account for the tunneling that is observed, between the m=−4 and m=4 unperturbed states. Even in resonance, tunneling takes place mainly incoherently in Mn12 acetate. It proceeds through the lowest energy state doublet which is not blocked by longitudinal local fields. The relaxation rate Γ has been calculated using a master equation. The model gives magnetization hysteresis loops and ac magnetic susceptibility curves which are in quantitative agreement with experimental results. For temperatures below 0.5 K approximately, nonresonant tunneling from the ground state becomes the dominant relaxation mechanism.

13C NMR Investigation of Molecular Chain Diffusion in Semicrystalline Polyethylene. 1. Effect of Cross-Linking

Solid-state molecular chain diffusion in linear high-density polyethylene (HDPE) is established as the dominant mechanism for the crystalline 13C longitudinal relaxation at 60 °C, confirming previous work of Schmidt-Rohr and Spiess. A progressive saturation NMR experiment was undertaken on several samples of HDPE, where different degrees of cross-linking were introduced by electron irradiation. A decrease in the rate of recovery of the crystalline signal was observed with an increase in cross-link density, as measured by gel fraction. As irradiation forms cross-links on the fold surfaces of the lamellae, this behavior cannot be explained through a conventional dipolar spin−lattice mechanism or 13C spin diffusion. Transport of magnetization via chain diffusion to the amorphous region, where it experiences an efficient relaxation process, is consistent with the relaxation data and the Overhauser enhancement. As the gel fraction increased from zero to 87%, the effective diffusion coefficients almost halved from the nonirradiated sample (0.033 nm2·s-1) to 0.018 nm2·s-1.

Numerical investigation of the dynamics of a thin-film type-II superconductor with and without disorder

The equilibrium dynamics of a thin film type II superconductor with spherical geometry are investigated numerically in a simulation based on the lowest Landau level approximation to the time-dependent Ginzburg-Landau equation. Both the static and time-dependent density-density correlation functions of the superconducting order parameter have been investigated. As the temperature is lowered the correlation length, the length-scale over which the vortices have short-range crystalline order, increases but the introduction of quenched random disorder reduces this correlation length. We see no signs of a phase transition in either the pure or the disordered case. For the disordered system there is no evidence for the existence of a Bragg glass phase with quasi long-range correlations. The dynamics in both the pure and disordered systems is activated, and the barrier of the relaxation mechanism grows linearly with the correlation length. The self-diffusion time scale of the vortices was also measured and has the same temperature dependence as that of the longest time scales found in the time dependent density-density correlation function. The dominant relaxation mechanism observed is a change in orientation of a correlated region of size of the correlation length. A scaling argument is given to explain the value of the barrier exponent.

Spin-lattice relaxations of Be9 and Al27 in single crystalline alexandrite

Nuclear spin-lattice relaxations of Be9 and Al27 in an alexandrite (BeAl2−xCrxO4, x=0.02) single crystal have been investigated in the temperature range 140 K–400 K. The paramagnetic impurity effect on the relaxation is negligible for both nuclei. The relaxation of the Be9 nuclear spins is found single exponential in time when the whole resonance lines are uniformly excited. When only the central resonance line is excited for Al27, the relaxation is well fitted with a linear combination of three exponential functions given by Simmons et al. [Phys. Rev. 127, 1168 (1962)]. Since the spin-lattice relaxation rate is proportional to the square of temperature for both nuclei, the dominant relaxation mechanism in this temperature range can be inferred as the Raman process.

Fourier transform EPR study of N@C 60 in solution

Using pulsed EPR techniques, the electron spin relaxation properties of nitrogen atoms encapsulated in C 60 have been studied. Because of the high symmetry of the cage, most of the conventional relaxation mechanisms otherwise affecting electron spins in solution are absent, resulting in an exceptional homogeneous ERP linewidth of only 2.5 kHz. Apparently, solvent collision-induced deformations of the carbon shell which modulate the zero-field-splitting sensed by the quartet spin state of N@C 60 are the dominant spin relaxation mechanism.