Electronic Relaxation Rate Research Articles

The Dependence of the Plasmon Field Induced Nonradiative Electronic Relaxation Mechanisms on the Gold Shell Thickness in Vertically Aligned CdTe−Au Core−Shell Nanorods

The dependence of the plasmon field enhancement of the nonradiative relaxation rate of the band gap electrons in vertically aligned CdTe-Au core-shell nanorods on the plasmonic gold nanoshell thickness is examined. Increasing the thickness of the gold nanoshell from 15 to 26 nm is found to change the decay curve from being nonexponential and anisotropic to one that is fully exponential and isotropic (i.e., independent of the nanorod orientation with respect to the exciting light polarization direction). Analysis of the kinetics of the possible electronic relaxation enhancement mechanisms is carried out, and DDA simulated properties of the induced plasmonic field of the thin and thick gold nanoshells are determined. On the basis of the conclusions of these treatments and the experimental results, it is concluded that by increasing the nanoshell thickness the relaxation processes evolve from multiple enhancement mechanisms, dominated by highly anisotropic Auger processes, to mechanism(s) involving first-order excited electron ejection process(es). The former is shown to give rise to nonexponential anisotropic decays in the dipolar plasmon field of the thin nanoshell, while the latter exhibits an exponential isotropic decay in the unpolarized plasmonic field of the thick nanoshell.

Soft modes in electronic stripe phases and their consequences for thermodynamics and transport

The Goldstone mode due to stripe or unidirectional charge-density-wave order in electron systems is found to have the same functional form as the one in classical smectic liquid crystals. It is very similar to the Goldstone mode that results from helical magnetic order. This allows for an effective theory that provides a quasiparticle description of either stripe phases or helimagnets in the low-energy regime. The most remarkable observable consequence is an electronic relaxation rate in d=2 that is 1/\tau ~ T\ln T in clean systems and 1/\tau ~ \sqrt{T} in weakly disordered ones. The corresponding results in d=3 are 1/\tau ~ T^{3/2} and 1/\tau ~ T, respectively.

SPIN DYNAMICS IN III-V/II-VI: Mn HETEROVALENT QUANTUM WELLS

We report on the spectroscopic magnetooptical studies of spin dynamics in diluted magnetic semiconductor (DMS) GaAs / AlGaAs / ZnSe / ZnCdMnSe heterovalent double quantum wells (QW). The transients of circularly polarized photoluminescence in an external magnetic field are detected in the structures with different widths of the GaAs QW. The analysis of the data, performed within the rate-equation model, has allowed separate estimations of the spin relaxation rate of localized electrons and holes. The spin flip of the electrons confined in the DMS ZnCdMnSe QW is faster than 20 ps, whereas the spin flip of the heavy hole localized in the GaAs QW is as long as ~9 ns. The long spin flip of the holes is presumably governed by their strong 3-dimensional localization.

Probing theYb3+spin relaxation inY0.98Yb0.02Ba2Cu3Oxby electron paramagnetic resonance

The relaxation of ${\text{Yb}}^{3+}$ in ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{x}$ $(6<x<7)$ was studied using electron paramagnetic resonance. It was found that both electronic and phononic processes contribute to the ${\text{Yb}}^{3+}$ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy $(\ensuremath{\sim}500\text{ }\text{K})$ optical phonons or an Orbach-type process via the excited vibronic levels of the ${\text{Cu}}^{2+}$ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping $x=6.98$, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below ${T}_{c}$. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of ${\text{Yb}}^{3+}$ relaxation in ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6.98}$. It was shown that the electronic part of the ${\text{Yb}}^{3+}$ relaxation in the superconducting state follows the same temperature dependence as $^{63}\text{C}\text{u}$ and $^{17}\text{O}$ nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.

Symmetric band structures and asymmetric ultrafast electron and hole relaxations in silicon and germanium quantum dots: time-domain ab initio simulation

State-of-the-art time domain density functional theory and non-adiabatic (NA) molecular dynamic simulations are used to study phonon-induced relaxation of photoexcited electrons and holes in Ge and Si quantum dots (QDs). The relaxation competes with productive processes and causes energy and voltage losses in QD solar cells. The ab initio calculations show that quantum confinement makes the electron and hole density of states (DOS) more symmetric in Si and Ge QDs compared to bulk. Surprisingly, in spite of the symmetric DOS, the electron and hole relaxations are quite asymmetric: the electrons decay faster than the holes. The asymmetry arises due to stronger NA coupling in the conduction band (CB) than in the valence band (VB). The stronger NA coupling of the electrons compared to the holes is rationalized by the larger contribution of the high-frequency Ge-H and Si-H surface passivating bonds to the CB relative to the VB. Linear relationships between the electron and hole relaxation rates and the CB and VB DOS are found in agreement with Fermi's golden rule. The faster relaxation of the electrons compared to the holes in the Ge and Si QDs is unexpected and is in contrast with the corresponding dynamics in the majority of binary QDs, such as CdSe. It suggests that Auger processes will transfer energy from holes to electrons rather than in the opposite direction as in CdSe, and that a larger fraction of the photoexcitation energy will be transferred to phonons coupled with electrons rather than holes. The difference in the phonon-induced electron and hole decay rates is larger in Ge than Si, indicating that the Auger processes should be particularly important in Ge QDs. The simulations provide direct evidence that the high-frequency ligand modes on the QD surface play a pivotal role in the electron-phonon relaxation dynamics of semiconductor QDs.

Internalization of annexin A5‐functionalized iron oxide particles by apoptotic Jurkat cells

Apoptosis plays an important role in the etiology of various diseases. Several studies have reported on the use of annexin A5-functionalized iron oxide particles for the detection of apoptosis with MRI, both in vitro and in vivo. The protein annexin A5 binds with high affinity to the phospholipid phosphatidylserine, which is exposed in the outer leaflet of the apoptotic cell membrane. When co-exposed to apoptotic stimuli, this protein was shown to internalize into endocytic vesicles. Therefore in the present study we investigated the possible internalization of commercially available annexin A5-functionalized iron oxide particles (r1 = 34.0 +/- 2.1 and r2 = 205.0 +/- 10.4 mm(-1) s(-1) at 20 MHz), and the effects of their spatial distribution on relaxation rates R2*, R2 and R1. Two different incubation procedures were performed, where (1) Jurkat cells were either incubated with the contrast agent after induction of apoptosis or (2) Jurkat cells were simultaneously incubated with the apoptotic stimulus and the contrast agent. Transmission electron microscopy images and relaxation rates showed that the first incubation strategy mainly resulted in binding of the annexin A5-iron oxide particles to the cell membrane, whereas the second procedure allowed extensive membrane-association as well as a small amount of internalization. Owing to the small extent of internalization, only minor differences were observed between the DeltaR2*/DeltaR2 and DeltaR2/DeltaR1 ratios of cell pellets with membrane-associated or internalized annexin A5 particles. Only the increase in R1 (DeltaR1) appeared to be diminished by the internalization. Internalization of annexin A5-iron oxide particles is also expected to occur in vivo, where the apoptotic stimulus and the contrast agent are simultaneously present. Where the extent of internalization in vivo is similar to that observed in the present study, both T2- and T2*-weighted MR sequences are considered suitable for the detection of these particles in vivo.

Probing the Yb3+ spin relaxation in Y0.98Yb0.02Ba2Cu3Ox by electron paramagnetic resonance

The relaxation of Yb3+ in YBa2Cu3Ox (6<x<7) was studied using electron paramagnetic resonance. It was found that both electronic and phononic processes contribute to the Yb3+ relaxation. The phononic part of the relaxation has an exponential temperature dependence, which can be explained by a Raman process via the coupling to high-energy (~500 K) optical phonons or an Orbach-type process via the excited vibronic levels of the Cu2+ ions (localized Slonczewski-modes). In a sample with a maximum oxygen doping x=6.98, the electronic part of the relaxation follows a Korringa law in the normal state and strongly decreases below Tc. Comparison of the samples with and without Zn doping proved that the superconducting gap opening is responsible for the sharp decrease of Yb3+ relaxation in YBa2Cu3O6.98. It was shown that the electronic part of the Yb3+ relaxation in the superconducting state follows the same temperature dependence as 63Cu and 17O nuclear relaxations despite the huge difference between the corresponding electronic and nuclear relaxation rates.

Electron spin relaxation andK39pulsed ENDOR studies onCr5+-dopedK3NbO8at 9.7 and 240 GHz

${\text{Cr}}^{5+}$-doped ${\text{K}}_{3}{\text{NbO}}_{8}$, considered to be useful as an electron spin qubit, has been investigated by pulsed $X$ band $(\ensuremath{\sim}9.7\text{ }\text{GHz})$ and 240 GHz electron paramagnetic resonance and electron nuclear double resonance (ENDOR). Comparison of the low temperature electronic spin-lattice relaxation rate $1/{T}_{1}$ at 9.7 and 240 GHz shows that it is 250 times faster at 240 GHz than at $X$ band. On the other hand, spin-spin relaxation rate $1/{T}_{2}$ appears largely frequency independent and is very likely related to the superhyperfine (SHF) coupling of the ${\text{Cr}}^{5+}$ electron with the surrounding potassium and niobium nuclei. This coupling was investigated by hyperfine sublevel correlation spectroscopy at 9.7 GHz and pulsed Mims ENDOR at 240 GHz. The use of high frequency and field enabled us to unambiguously measure the hyperfine and quadrupole couplings of the $^{39}\text{K}$ in spite of its small magnetic moment. We find that the largest $^{39}\text{K}$ SHF coupling is positive, with 0.522 and 0.20 MHz as its isotropic and dipolar parts, respectively. $^{93}\text{N}\text{b}$ ENDOR was dominantly due to its quadrupolar interaction with a coupling of about 0.8 MHz and a SHF coupling of about 0.08 MHz. The significance of these data to spin qubit studies is pointed out.

Suppression of Electron Spin Relaxation in Mn-Doped GaAs

We report a surprisingly long spin relaxation time of electrons in Mn-doped p-GaAs. The spin relaxation time scales with the optical pumping and increases from 12 ns in the dark to 160 ns upon saturation. This behavior is associated with the difference in spin relaxation rates of electrons precessing in the fluctuating fields of ionized or neutral Mn acceptors, respectively. For the latter, the antiferromagnetic exchange interaction between a Mn ion and a bound hole results in a partial compensation of these fluctuating fields, leading to the enhanced spin memory.

Plasmon Field Effects on the Nonradiative Relaxation of Hot Electrons in an Electronically Quantized System: CdTe−Au Core−Shell Nanowires

The intense electromagnetic fields of plasmonic nanoparticles, resulting from the excitation of their localized surface plasmon oscillations, are known to enhance radiative processes. Their effect on the nonradiative electronic processes, however, is not as well-documented. Here, we report on the enhancement of the nonradiative electronic relaxation rates in CdTe nanowires upon the addition of a thin gold nanoshell, especially at excitation energies overlapping with those of the surface plasmon oscillations. Some possible mechanisms by which localized surface plasmon fields can enhance nonradiative relaxation processes of any quantized electronic excitations are proposed.

Microwave-Absorption-Induced Heating of Surface State Electrons on Liquid 3He

A resonance-induced change in the resistivity of the surface state electrons (SSE) exposed to the microwave (MW) radiation is observed. The MW frequency corresponds to the transition energy between two lowest Rydberg energy levels. All measurements are done with electrons over liquid 3He in a temperature range 0.45-0.65 K, in which the electron relaxation time and the MW absorption linewidth are determined by collisions with helium vapor atoms. The input MW power is varied by two orders of magnitude, and the resistivity is always found to increase. This effect is attributed to the heating of electrons with the resonance MW radiation. The temperature and the momentum relaxation rate of the hot electrons are calculated as a function of the MW power in the cell, and the Rabi frequency is determined from the comparison of the theoretical result with the experiment. In addition, the broadening of the absorption signal caused by the heating is studied experimentally, and the results are found to be in good agreement with our calculations.

Understanding the heavy fermion behavior inCeInPt4

The cubic compound $\mathrm{Ce}\mathrm{In}{\mathrm{Pt}}_{4}$ exhibits heavy fermion behavior with a high extrapolated value of the electronic specific heat, $\ensuremath{\gamma}=2.5\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕\text{mole}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{2}$ as $T\ensuremath{\rightarrow}0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The specific heat does not exhibit any sign of long range magnetic ordering down to $100\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. In order to understand the origin of the high value of $\ensuremath{\gamma}$ and the nature of the $4f$ electrons of this compound, we have carried out neutron diffraction, low-field magnetic susceptibility, muon spin relaxation, and inelastic neutron scattering measurements on $\mathrm{Ce}\mathrm{In}{\mathrm{Pt}}_{4}$. Our susceptibility results show that for temperatures between 20 and $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a power law behavior is exhibited, $\ensuremath{\chi}(T)\ensuremath{\sim}{T}^{\ensuremath{-}\ensuremath{\beta}}$ with $\ensuremath{\beta}=0.5$. Below $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the susceptibility is almost temperature independent, again without any sign of magnetic ordering down to the lowest available temperature $(300\phantom{\rule{0.3em}{0ex}}\mathrm{mK})$. The muon spin relaxation measurements reveal that below $1\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the electronic relaxation rate strongly increases without any loss of muon initial asymmetry, indicating the presence of low energy spin fluctuations as an explanation for the high value of $\ensuremath{\gamma}$ in $\mathrm{Ce}\mathrm{In}{\mathrm{Pt}}_{4}$. Heat capacity data reveal a $\mathrm{log}(C∕T)\ensuremath{\propto}\mathrm{log}(T)$ behavior, indicating that $\mathrm{Ce}\mathrm{In}{\mathrm{Pt}}_{4}$ may exhibit non-Fermi-liquid behavior close to a $T\ensuremath{\rightarrow}0\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ quantum critical point. Our inelastic neutron scattering results reveal a broad crystal field excitation centered at $25\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, indicating the presence of strong hybridization between the $4f$ and the conduction electrons, which is consistent with the observed high value of the paramagnetic Curie temperature $({\ensuremath{\theta}}_{p}=\ensuremath{-}255\phantom{\rule{0.3em}{0ex}}\mathrm{K})$.

Nonlinear resistivity of a two-dimensional electron gas in a magnetic field

We develop a theory of nonlinear response to an electric field of a two-dimensional electron gas (2DEG) placed in a classically strong magnetic field. The latter leads to a nonlinear current-voltage characteristic at a relatively weak electric field. The origin of the nonlinearity is twofold: the formation of a nonequilibrium electron distribution function and the geometrical resonance in the inter-Landau-level transitions rates. We find the dependence of the current-voltage characteristics on the electron relaxation rates in the 2DEG.

Carrier capture in InGaAsSb∕InAs∕InGaSb type-II laser heterostructures

Experimental studies of the electron and hole concentration dynamics in the barrier of GaSb-based type-II quantum-well (QW) heterostructures were performed. Capture of electrons and holes was studied separately in specially designed and grown laser heterostructures with QWs only for electrons or only for holes. The difference between electron and hole relaxation rates is explained by corresponding QW carrier confinement energies.

Influence of electron–electron scattering on electron relaxation rates in three and four-level quantum cascade lasers in magnetic fields

The aim of this work is to calculate carrier relaxation rates from the upper laser level due to electron–electron interactions in three and four-level quantum cascade lasers (QCLs) in a strong magnetic field. The comparison between calculated results and previously obtained values for acoustical and optical-phonon scattering processes indicates that carrier–carrier scattering might have noticeable influence on laser output properties, depending on the structural design. Numerical results are presented for two λ∼9μm GaAs-based QCLs in magnetic fields between 20T and 60T and the band nonparabolicity is taken into account.

Relaxation theory of the electronic spin of a complexed paramagnetic metal ion in solution beyond the Redfield limit

The relaxation of the electronic spin S of a paramagnetic metal ion with fully quenched orbital angular momentum in its ground state is investigated in an external magnetic field through a systematic study of the time correlation functions governing the evolution of the statistical operator (density matrix). Let omega0 be the Larmor angular frequency of S. When the relaxation is induced by a time-fluctuating perturbing Hamiltonian hH1(t) of time correlation tauc, it is demonstrated that after a transient period the standard Redfield approximation is relevant to calculate the evolution of the populations of the spin states if parallelH1 parallel2tauc2/(1+omega0(2)tauc2)<<1 and that this transient period becomes shorter than tauc at sufficiently high field for a zero-field splitting perturbing Hamiltonian. This property, proven analytically and confirmed by numerical simulation, explains the surprising success of several simple expressions of the longitudinal electronic relaxation rate 1/T1e derived from the Redfield approximation well beyond its expected validity range parallelH1 paralleltauc<<1. It has favorable practical consequences on the interpretation of the paramagnetic relaxation enhancement of nuclei used for structural and dynamic studies.

Shot noise spectroscopy of electronic spin flips in quantum dots

Spin decoherence and spin flips crucially affect the tunneling transport of spin-polarized electrons through a quantum dot connected to magnetic leads. Here, the authors show that the low-frequency shot noise in such structures diverges as the spin relaxation rate for electrons on the dot goes to zero, reaching giant super-Poissonian values for realistic spin-flip rates. It is also shown that combined measurements of the average current and the shot noise as a function of bias voltage and external magnetic field offer a spectroscopic tool for studying electronic spin relaxation rates in this system.

Mixed Frequency/Time Domain Optical Analogues of Heteronuclear Multidimensional NMR

Ultrafast spectroscopy is dominated by time domain methods such as pump-probe and, more recently, 2D-IR spectroscopies. In this paper, we demonstrate that a mixed frequency/time domain ultrafast four wave mixing (FWM) approach not only provides similar capabilities, but it also provides optical analogues of multiple- and zero-quantum heteronuclear nuclear magnetic resonance (NMR). The method requires phase coherence between the excitation pulses only over the dephasing time of the coherences. It uses twelve coherence pathways that include four with populations, four with zero-quantum coherences, and four with double-quantum coherences. Each pathway provides different capabilities. The population pathways correspond to those of two-dimensional (2D) time domain spectroscopies, while the double- and zero-quantum coherence pathways access the coherent dynamics of coupled quantum states. The three spectral and two temporal dimensions enable the isolation and characterization of the spectral correlations between different vibrational and/or electronic states, coherence and population relaxation rates, and coupling strengths. Quantum-level interference between the direct and free-induction decay components gives a spectral resolution that exceeds that of the excitation pulses. Appropriate parameter choices allow isolation of individual coherence pathways. The mixed frequency/time domain approach allows one to access any set of quantum states with coherent multidimensional spectroscopy.

Simulation of the photodynamics of azobenzene on its first excited state: Comparison of full multiple spawning and surface hopping treatments

We have studied the cis-->trans and trans-->cis photoisomerization of azobenzene after n-->pi* excitation using the full multiple spawning (FMS) method for nonadiabatic wave-packet dynamics with potential-energy surfaces and couplings determined "on the fly" from a reparametrized multiconfigurational semiempirical method. We compare the FMS results with a previous direct dynamics treatment using the same potential-energy surfaces and couplings, but with the nonadiabatic dynamics modeled using a semiclassical surface hopping (SH) method. We concentrate on the dynamical effects that determine the photoisomerization quantum yields, namely, the rate of radiationless electronic relaxation and the character of motion along the reaction coordinate. The quantal and semiclassical results are in good general agreement, confirming our previous analysis of the photodynamics. The SH method slightly overestimates the rate of excited state decay, leading in this case to lower quantum yields.

Practical Route to Relative Diffusion Coefficients and Electronic Relaxation Rates of Paramagnetic Metal Complexes in Solution by Model-Independent Outer-Sphere NMRD. Potentiality for MRI Contrast Agents

The relaxation of electronic spins S of paramagnetic species is studied by the field-dependence of the longitudinal, transverse, and longitudinal in the rotating frame relaxation rates R1, R2, and R1rho of nuclear spins I carried by dissolved probe solutes. The method rests on the model-independent low-frequency dispersions of the outer-sphere (OS) paramagnetic relaxation enhancement (PRE) of these rates due to the three-dimensional relative diffusion of the complex with respect to the probe solute. We propose simple analytical formulas to calculate these enhancements in terms of the relative diffusion coefficient D, the longitudinal electronic relaxation time T1e, and the time integral of the time correlation function of the I-S dipolar magnetic interaction. In the domain of vanishing magnetic field, these parameters can be derived from the low-frequency dispersion of R1 thanks to sensitivity improvements of fast field-cycling nuclear relaxometers. At medium field, we present various approaches to obtain these parameters by combining the rates R1, R2, and R1rho. The method is illustrated by a careful study of the proton PREs of deuterated water HOD, methanol CH3OD, and tert-butyl alcohol (CH3)3COD in heavy water in the presence of a recently reported nonacoordinate Gd(III) complex. The exceptionally slow electronic relaxation of the Gd(III) spin in this complex is confirmed and used to test the accuracy of the method through the self-consistency of the low- and medium-field results. The study of molecular diffusion at a few nanometer scale and of the electronic spin relaxation of other complexed metal ions is discussed.