How long spin relaxation by phonon excitation?4 answersSpin relaxation by phonon excitation can vary significantly depending on the system studied. In the case of the negatively charged nitrogen vacancy (NV-) center in diamond, exceptionally long longitudinal relaxation times of up to 8 hours have been observed due to the low phononic density of states at the NV- transition frequency, enabling spin polarization to persist over macroscopic timescales. On the other hand, in undoped quasi-spherical CdS quantum dots, spin relaxation at zero magnetic fields has been investigated, revealing a room-temperature spin-relaxation time of several nanoseconds. This shorter timescale is attributed to a novel mechanism involving intralevel exciton transitions with phonon emission and absorption, coupled with electron spin flips driven by electron-hole exchange interactions.
What are the potential benefits of using EIT in brain disease?5 answersElectrical impedance tomography (EIT) has potential benefits in brain disease. EIT can be used for real-time supervision and early recognition of cerebral brain infarction, hemorrhage, and other diseases, making it extremely practical for brain imaging and diagnosing stroke. EIT offers advantages over other neuroimaging technologies and has been applied in epilepsy imaging, stroke monitoring, and detecting cerebral edema. EIT can reflect pathological changes in the cerebral region, offering an alternative for timely diagnosis and treatment of cerebral disease. EIT algorithms and hardware can be developed and improved to increase accuracy and decrease reconstruction time, making it a promising technique for functional brain imaging applications.
When is the Relaxation time approximation for the calculation of thermal conductivity valid?4 answersThe relaxation time approximation for the calculation of thermal conductivity is valid in various situations. It has been shown to be applicable even under large nonequilibrium conditions, where phonon modes are subject to large excitations. The approximation is also valid when defects are homogeneously distributed and linear response to the external electric field is assumed. Additionally, the relaxation time approximation can be used to model band structures and extract scattering rates with high accuracy. However, it is important to note that the validity of the relaxation time approximation depends on the specific material and system being studied. For example, in an interacting mixture, the relaxation time approximation is valid, but the transition time scale may affect certain transport coefficients. Therefore, the applicability of the relaxation time approximation should be carefully considered in each specific case.
What is the momentum relaxation time in graphene?5 answersThe momentum relaxation time in graphene refers to the time it takes for the momentum of photoexcited graphene to relax in the THz frequency range. The momentum relaxation time of graphene exhibits a threshold behavior with respect to the optical pumping intensity. The relaxation time of the conduction electrons in graphene is of the order of 5-30 ns, depending on the concentration of paramagnetic centers. In the mid-infrared region, the relaxation kinetics of carriers in graphene films on CaF2 show two decay times, with a fast component of ~0.2 ps and a slow component of ~1.5 ps. The fast component is attributed to ultrafast intraband and interband decay channels, while the slow component is primarily due to optical phonon-acoustic phonon scattering. The relaxation dynamics of nonequilibrium carriers in graphene have been studied using high-resolution pump-probe experiments, revealing a bi-exponential decay with a fast component in the range of tens of femtoseconds and a slower component in the range of picoseconds. The relaxation dynamics in doped graphene have also been investigated, with a focus on carrier multiplication induced by Coulomb-induced intraband scattering. The spin relaxation time in graphene has been measured to be in the range of 100-200 ps, weakly dependent on charge density and temperature.
What is the relationship between the relaxation times of atomic vapors and the coherence time of EIT?5 answersThe relaxation times of atomic vapors, specifically the ground state relaxation time, are closely related to the coherence time of electromagnetically induced transparency (EIT). In EIT, the diffusion of atomic coherence in and out of the laser beam plays a crucial role in determining the EIT resonance lineshape and the stored light lifetime. The recovery time of EIT, which decreases with increasing optical power, is determined by the ground relaxation time of the atoms. The optically-detected spin-echo (ODSE) method can be used to measure the ground-state relaxation times of atomic vapors, including the coherence relaxation time relevant for the clock transition in rubidium (Rb) atomic clocks. The relaxation times of noble-gas atoms are important indicators of vapor cell performance, which is crucial for high-precision micro-miniature gyroscopes.
The importance of measuring relaxation times in atomic vapors and connection with EIT?4 answersRelaxation times in atomic vapors are important for accurately characterizing the performance of vapor cells used in gyroscopes. These relaxation times, which indicate the time it takes for the atoms to return to their ground state after being excited, can be measured using techniques such as free induction decay (FID) and the π pulse method. Additionally, the study of relaxation dynamics in atomic systems has provided insights into phenomena such as electromagnetically induced transparency (EIT). EIT is a process where the absorption of light by a medium is reduced due to the interference between different atomic energy levels. The time response of EIT to changes in optical phase can be used to investigate ground state relaxation and control EIT. Diffusion of atomic coherence in and out of the laser beam also plays a role in determining the EIT resonance lineshape and the lifetime of stored light.