Dominant Relaxation Mechanism Research Articles

Dynamic behavior of crosslinked amphiphilic block copolymer nanofibers dispersed in liquid poly(ethylene oxide) below and above their glass transition temperature

Crosslinked polystyrene nanofibers were synthesized via RAFT-mediated aqueous emulsion polymerization and showed good shape stability in both the presence of solvent and at high temperature (typically above the Tg of polystyrene). The viscoelastic properties of their suspension in a low molar mass poly(ethylene glycol) matrix (PEG400) was studied as a function of the temperature, i.e., below the Tg of polystyrene at 25 °C, and above at 130 °C. Below Tg, the critical concentration ϕ* between the dilute and the semi-dilute regimes determined from the crossover of the scaling law on the zero shear viscosity for the dilute regime and the semi-dilute regime showed that a Brownian motion of the nanofibers was the dominant mechanism of relaxation according to the Doi–Edwards theory. Above Tg, the nanofibers are flexible and their Brownian dynamics did not obey the Doi–Edwards theory anymore. From the concentration dependence of the zero shear viscosity, it appeared that their dynamics obey the power laws for polymer chains in solution. Moreover, the flow activation energy at T > Tg was drastically dependent on the nanofiber concentrations whereas it was observed to be constant at T < Tg as expected from the Doi–Edwards theory. Finally, the flow activation energy became closer to the flow activation energy of polystyrene at concentrations above ϕ = 15%. Such qualitative agreement with the dynamics of flexible chains has never been observed before and might be fortuitous; it would therefore require further theoretical investigations.

Spin transport in silicene and germanene

In this article, we have investigated spin polarized electronic transport in two dimensional sheet materials of silicon and germanium, such as, monolayer silicene and germanene, using semi-classical Monte-Carlo approach. Monte-Carlo simulations are used to model spin transport along with spin density matrix calculations in the devices. Dephasing of the spin vectors in silicene and germanene are due to primarily D'yakonov–Perel (DP) and Elliott–Yafet (EY) relaxation mechanisms. But, as silicene and germanene has high carrier mobility the latter one is not the dominant relaxation mechanism. In this work, we studied spin polarized transport along the length of the planar monolayer silicene, buckled monolayer silicene and germanene monolayer structure and the spin dephasing length is estimated to be in the range of 0.5 μm for silicene and 1.5 μm for germanene. We also investigated the ensemble averaged spin vector variation along the length of the planar and buckled monolayer silicene and germanene with varying temperature. Finally, we studied the effect of variation of vertical electric field (Ez) on spin dephasing length in silicene buckled monolayer structure. Silicene and germanene show significant potential for future nanoelectronics devices.

Electron spin resonance and exchange paths in the orthorhombic dimer system Sr2VO4

We report on magnetization and electron spin resonance (ESR) measurements of Sr$_{2}$VO$_4$ with orthorhombic symmetry. In this dimer system the $V^{4+}$ ions are in tetrahedral environment and are coupled by an antiferromagnetic intra-dimer exchange constant $J/k_B \approx$ 100 K to form a singlet ground state without any phase transitions between room temperature and 2 K. Based on an extended-H\"{u}ckel-Tight-Binding analysis we identify the strongest exchange interaction to occur between two inequivalent vanadium sites via two intermediate oxygen ions. The ESR absorption spectra can be well described by a single Lorentzian line with an effective g-factor $g$ = 1.89. The temperature dependence of the ESR intensity is well described by a dimer model in agreement with the magnetization data. The temperature dependence of the ESR linewidth can be modeled by a superposition of a linear increase with temperature with a slope $\alpha$ = 1.35 Oe/K and a thermally activated behavior with an activation energy $\Delta/k_B$ = 1418 K, both of which point to spin-phonon coupling as the dominant relaxation mechanism in this compound.

Qubit relaxation from evanescent-wave Johnson noise

In many quantum computer architectures, the qubits are in close proximity to metallic device elements. Metals have a high density of photon modes, and the fields spill out of the bulk metal because of the evanescent-wave component. Thus thermal and quantum electromagnetic Johnson-type noise from metallic device elements can decohere nearby qubits. In this Rapid Communication we use quantum electrodynamics to compute the strength of this evanescent-wave Johnson noise as a function of distance $z$ from a metallic half space. Previous treatments have shown unphysical divergences at $z=0$. We remedy this by using a proper nonlocal dielectric function. Decoherence rates of local qubits are proportional to the magnitude of electric or magnetic correlation functions evaluated at the qubit position. We present formulas for the decoherence rates. These formulas serve as an important constraint on future device architectures. Comparison with single-electron spin relaxation measurements shows that evanescent-wave Johnson noise may constitute the dominant relaxation mechanism in experiments performed at low magnetic field.

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.

In-plane interdot carrier transfer in InAs/GaAs quantum dots

Using time resolved photoluminescence (PL) quenching measurements, we investigated inplane carrier transfer in InAs/GaAs self-assembled quantum dots (QDs). THz pulses from a free-electron laser tuned to the intersublevel transition energy were used to excite carriers to higher levels causing quenching in the PL. These carriers could either fall back to the lower energy states and recombine or get transferred to adjacent QDs. The relaxation of the carriers was directly reflected in the recovery of the PL signal. Comparing measurements from two samples, we found that the redistribution of carriers into the neighbouring QDs is the dominant mechanism of carrier relaxation. The data were fitted using a rate equation model to estimate the PL recovery time which we attribute to the interdot carrier transfer time.

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.

Investigation of possible microcavity effect on lasing threshold of nonradiative-scattering-dominated semiconductor lasers

The effect of enhanced rate of spontaneous emission on gain and lasing threshold of semiconductor microcavity lasers has not been discussed clearly. Some reports have suggested that the lasing threshold in microcavities could possibly be lowered due to the so-called Purcell effect. Here, we argue that gain in weakly coupled semiconductor cavities is a local phenomenon, which occurs due to stimulated emission induced by an electromagnetic excitation and remains unaffected by the cavity boundary conditions. Hence, the Purcell effect in microcavities filled uniformly with a gain medium should not lead to a reduction in the laser’s threshold pump density, provided radiative scattering is not the dominant relaxation mechanism in the excited state. A systematic experimental investigation of laser threshold in parallel-plate semiconductor microcavity terahertz quantum-cascade lasers of different dimensions was found to be in accordance with our arguments.

Dielectric relaxation under pressure in granular dielectrics containing water: Compensation rule for the activation parameters

The dielectric relaxation of water-filled granular dielectrics under hydrostatic compression is investigated in the frequency domain. The activation volume and activation enthalpy governing a dominant relaxation mechanism of sandstone partially saturated with water are determined experimentally for the first time. The present results combined with recently published results for various wetted porous dielectrics indicate that, despite the complexity of this class of matter, such as heterogeneity of composition and micro-structural landscape and their disordered environment, a compensation rule between the absolute value of the activation volume and enthalpy is likely to be valid. The proportionality constant is determined by the isothermal bulk modulus and its pressure derivative, according to an earlier thermodynamic elastic model.

ChemInform Abstract: Spin Transport and Relaxation in Graphene

AbstractReview: 120 refs.

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.

6Li 1D EXSY NMR Spectroscopy: A New Tool for Studying Lithium Dynamics in Paramagnetic Materials Applied to Monoclinic Li2VPO4F

6Li selective inversion NMR studies are used to probe details of Li mobility in Li2VPO4F. Two crystallographically unique Li sites were resolved under magic-angle spinning (25–40 kHz) with paramagnetic shifts arising at 46 and −47 ppm (330 K). The rate of exchange between these sites was evaluated using selective inversion (or one-dimensional exchange (1D EXSY)) NMR. This methodology relies on relaxation-based experiments that provide a means for mobility time scales to be determined for materials in which Li+ ions exchange slowly relative to their T1 spin–lattice relaxation. This situation is particularly relevant to cathode materials for lithium ion batteries, where the unpaired electrons of the transition-metal centers provide a dominant mechanism for rapid relaxation. In Li2VPO4F, Li1–Li2 exchange pair jump rates extend from 24 (±1) to 55 (±4) Hz over a temperature range of 330–350 K. The activation energy for this ion exchange process was measured to be 0.44 (±0.06) eV.

Measurements of the persistent singlet state of N2O in blood and other solvents—Potential as a magnetic tracer

The development of hyperpolarized tracers has been limited by short nuclear polarization lifetimes. The dominant relaxation mechanism for many hyperpolarized agents in solution arises from intramolecular nuclear dipole-dipole coupling modulated by molecular motion. It has been previously demonstrated that nuclear spin relaxation due to this mechanism can be removed by storing the nuclear polarization in long-lived, singlet-like states. In the case of N(2)O, storing the polarization of the nitrogen nuclei has been shown to substantially increase the polarization lifetime. The feasibility of utilizing N(2)O as a tracer is investigated by measuring the singlet-state lifetime of the N(2)O when dissolved in a variety of solvents including whole blood. Comparison of the singlet lifetime to longitudinal relaxation and between protonated and deuterated solvents is consistent with the dominance of spin-rotation relaxation, except in the case of blood.

Decay of nuclear hyperpolarization in silicon microparticles

We investigate the low-field relaxation of nuclear hyperpolarization in undoped and highly doped silicon microparticles at room temperature following removal from high field. For nominally undoped particles, two relaxation time scales are identified for ambient fields above 0.2 mT. The slower, ${T}_{1,\mathrm{s}}$, is roughly independent of ambient field; the faster, ${T}_{1,\mathrm{f}}$, decreases with increasing ambient field. A model in which nuclear spin relaxation occurs at the particle surface via a two-electron mechanism is shown to be in good agreement with the experimental data, particularly the field independence of ${T}_{1,\mathrm{s}}$. For boron-doped particles, a single relaxation time scale is observed. This suggests that for doped particles, mobile carriers and bulk ionized acceptor sites, rather than paramagnetic surface states, are the dominant relaxation mechanisms. Relaxation times for the undoped particles are not affected by tumbling in a liquid solution.

Nuclear Magnetic Resonance Study of the Raman Spin-Phonon Processes in the Relaxation Mechanisms of Double Sulfate Li3Rb(SO4)2Single Crystals

The NMR spectra of <TEX>$Li_3Rb(SO_4)_2$</TEX> crystals and their relaxation processes were investigated by using <TEX>$^7Li$</TEX> and <TEX>$^{87}Rb$</TEX> NMR. The relaxation rates of the <TEX>$^7Li$</TEX> and <TEX>$^{87}Rb$</TEX> nuclei in the crystals were found to increase with increasing temperature, and can be described by the relation <TEX>$T_1^{-1}{\propto}AT^2$</TEX>. The dominant relaxation mechanism for these nuclei with electric quadrupole moments is provided by the coupling of these moments to the thermal fluctuations of the local electric field gradient via Raman spin-phonon processes.

Relaxation of hot electrons in a degenerate two-dimensional electron system: Transition to one-dimensional scattering

The energy relaxation channels of hot electrons far from thermal equilibrium in a degenerate two-dimensional electron system are investigated in transport experiments in a mesoscopic three-terminal device. We observe a transition from two dimensions at zero magnetic field to quasi--one-dimensional scattering of the hot electrons in a strong magnetic field. In the two-dimensional case electron-electron scattering is the dominant relaxation mechanism, while the emission of optical phonons becomes more and more important as the magnetic field is increased. The observation of up to 11 optical phonons emitted per hot electron allows us to determine the onset energy of LO phonons in GaAs at cryogenic temperatures with a high precision, $\eph=36.0\pm0.1\,$meV. Numerical calculations of electron-electron scattering and the emission of optical phonons underline our interpretation in terms of a transition to one-dimensional dynamics.

Frequency offset dependence of adiabatic rotating frame relaxation rate constants: relevance to MRS investigations of metabolite dynamics in vivo

In this work, we investigated the frequency-offset dependence of the rotating frame longitudinal (R(1ρ)) and transverse (R(2ρ)) relaxation rate constants when using hyperbolic-secant adiabatic full passage pulses or continuous-wave spin-lock irradiation. Phantom and in vivo measurements were performed to validate theoretical predictions of the dominant relaxation mechanisms existing during adiabatic full passage pulses when using different settings of the frequency offset relative to the carrier. In addition, adiabatic R(1ρ) and R(2ρ) values of total creatine and N-acetylaspartate were measured in vivo from the human brain at 4 T. When the continuous-wave pulse power was limited to safe specific absorption rates for humans, simulations revealed a strong dependence of R(1ρ) and R(2ρ) values on the frequency offset for both dipolar interactions and anisochronous exchange mechanisms. By contrast, theoretical and experimental results showed adiabatic R(1ρ) and R(2ρ) values to be practically invariant within the large subregion of the bandwidth of the hyperbolic-secant pulse where complete inversion was achieved. However, adiabatic R(1ρ) and R(2ρ) values of the methyl protons of total creatine (at 3.03 ppm) were almost doubled when compared with those of the methyl protons of N-acetylaspartate (at 2.01 ppm) in spite of the fact that these resonances were in the flat region of the inversion band of the adiabatic full passage pulses. We conclude that differences in adiabatic R(1ρ) and R(2ρ) values of human brain metabolites are not a result of their chemical shifts, but instead reflect differences in dynamics.

Spin relaxation of a short-lived radical in zero magnetic field

A short-lived radical containing only one I = 1/2 nucleus, the muoniated 1,2-dicarboxyvinyl radical dianion, was produced in an aqueous solution by the reaction of muonium with the dicarboxyacetylene dianion. The identity of the radical was confirmed by measuring the muon hyperfine coupling constant (hfcc) by transverse field muon spin rotation spectroscopy and comparing this value with the hfcc obtained from DFT calculations. The muon spin relaxation rate of this radical was measured as a function of temperature in zero magnetic field by the zero field muon spin relaxation technique. The results have been interpreted using the theoretical model of Fedin et al. (J. Chem. Phys., 2003, 118, 192). The muon spin polarization decreases exponentially with time after muon implantation and the temperature dependence of the spin relaxation rate indicates that the dominant relaxation mechanism is the modulation of the anisotropic hyperfine interaction due to molecular rotation. The effective radius of the radical in solution was determined to be 1.12 ± 0.04 nm from the dependence of the muon spin relaxation rate on the temperature and viscosity of the solution, and is approximately 3.6 times larger than the value obtained from DFT calculations.

Stress relaxation behavior of Cu/Sn/Cu micro-connect after electrical current

The effect of electromigration on stress relaxation behavior of pure tin solder joints was investigated. It was found that the stress relaxation rate was accelerated significantly after the sample was subjected to current stressing. The accelerating effect increased with the current stressing time. Measurements of the activation energy and stress exponent suggested that the dominant mechanism of the stress relaxation of pure tin solder joint went from dislocation climb to grain boundary diffusion after electromigration. As a result of grain boundary diffusion and sliding, grain boundary grooves were observed on the surface of the tin solder joints after electromigration. The groove was associated with the divergence of vacancy concentration at the grain boundaries. The vacancy concentration at the grain boundaries, which increased with the current stressing time, promoted the atomic diffusion along the grain boundaries, resulting in a higher stress relaxation rate.

Hydration water dynamics in biopolymers from NMR relaxation in the rotating frame

Assuming dipole–dipole interaction as the dominant relaxation mechanism of protons of water molecules adsorbed onto macromolecule (biopolymer) surfaces we have been able to model the dependences of relaxation rates on temperature and frequency. For adsorbed water molecules the correlation times are of the order of 10 −5 s, for which the dispersion region of spin–lattice relaxation rates in the rotating frame R 1 ρ = 1/ T 1 ρ appears over a range of easily accessible B 1 values. Measurements of T 1 ρ at constant temperature and different B 1 values then give the “dispersion profiles” for biopolymers. Fitting a theoretical relaxation model to these profiles allows for the estimation of correlation times. This way of obtaining the correlation time is easier and faster than approaches involving measurements of the temperature dependence of R 1 = 1/ T 1. The T 1 ρ dispersion approach, as a tool for molecular dynamics study, has been demonstrated for several hydrated biopolymer systems including crystalline cellulose, starch of different origins (potato, corn, oat, wheat), paper (modern, old) and lyophilized proteins (albumin, lysozyme).