Longitudinal Spin Relaxation Time T1 Research Articles

Coherent Spin Dynamics of Electrons in CdSe Colloidal Nanoplatelets

Coherent spin dynamics of electrons in CdSe colloidal nanoplatelets are investigated by time-resolved pump–probe Faraday rotation at room and cryogenic temperatures. We measure electron spin precession in a magnetic field and determine g-factors of 1.83 and 1.72 at low temperatures for nanoplatelets with a thickness of 3 and 4 monolayers, respectively. The dephasing time of spin precession T2* amounts to a few nanoseconds and has a weak dependence on temperature, while the longitudinal spin relaxation time T1 exceeds 10 ns even at room temperature. Observations of single and double electron spin–flips confirm that the nanoplatelets are negatively charged. The spin–flip Raman scattering technique reveals g-factor anisotropy by up to 10% in nanoplatelets with thicknesses of 3, 4, and 5 monolayers. In the ensemble with a random orientation of nanoplatelets, our theoretical analysis shows that the measured Larmor precession frequency corresponds to the in-plane electron g-factor. We conclude that the experimentally observed electron spin dephasing and its acceleration in the magnetic field are not provided by the electron g-factor anisotropy and can be related to the localization of the resident electrons and fluctuations of the localization potential.

Dipolar spin relaxation of divacancy qubits in silicon carbide

Divacancy spins implement qubits with outstanding characteristics and capabilities in an industrial semiconductor host. On the other hand, there are still numerous open questions about the physics of these important defects, for instance, spin relaxation has not been thoroughly studied yet. Here, we carry out a theoretical study on environmental spin-induced spin relaxation processes of divacancy qubits in the 4H polytype of silicon carbide (4H-SiC). We reveal all the relevant magnetic field values where the longitudinal spin relaxation time T1 drops resonantly due to the coupling to either nuclear spins or electron spins. We quantitatively analyze the dependence of the T1 time on the concentration of point defect spins and the applied magnetic field and provide an analytical expression. We demonstrate that dipolar spin relaxation plays a significant role both in as-grown and ion-implanted samples and it often limits the coherence time of divacancy qubits in 4H-SiC.

Measurement of a129Xe transverse relaxation rate without the influence of Rb polarization-induced magnetic gradient.

We demonstrate the measurement of the transverse spin relaxation rate of Xe without the influence of the Rb polarization-induced magnetic field gradient. The optical pumping beam and probe beam turn on and off repeatedly during the measurement to reduce the relaxation originating from the Rb polarization-induced magnetic field gradient. During the absence of the optical pumping beam, the nuclear spin of the noble gas atom does not experience the alkali-polarization-induced magnetic field gradient so that the transverse spin relaxation rate (1/T2) decreases. When the duty cycle reaches zero, the measured transverse relaxation time T2 approaches the longitudinal spin relaxation time T1. Our method helps in roughly estimating the longitudinal spin relaxation time with a single measurement of the free induction decay.

Exciton spin–flip processes in single quantum dots

We study the relaxation of the exciton spin (longitudinal spin–relaxation time T1) in single asymmetrical quantum dots (QD) at low temperatures. The main relaxation mechanism is due to the exciton spin–acoustic phonon coupling via the strain–dependent exchange interaction; piezoelectric coupling is less efficient. For zero magnetic field, relaxation within the radiative exciton–doublet for typical QDs is very slow compared to the exciton lifetime. Relaxation to nonradiative states becomes important for QDs with large singlet–triplet splitting of a few meVs. The calculated relaxation times strongly decrease in high magnetic fields. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Sol-gel coated glass cells for spin-exchange polarized He3

We have developed a high-purity sol-gel coating for the interior surface of glass cells used for polarizing He3 by spin-exchange optical pumping. The coating is designed to minimize spin relaxation due to wall collisions. A longitudinal spin-relaxation time T1 in a sol-gel coated Pyrex cell of 344±8 h was achieved, the longest T1 we have ever recorded for a gaseous sample. Repeated trials indicated that the coating was quite robust. Results using an uncoated Pyrex cell were also quite good, although inferior to the performance of the coated cell.

H and Mu diffusion in ice

Detailed studies of muonium in ice carried out at TRIUMF in the mid eighties directly inspired EPR investigation of H and D atoms in ice at the Argonne National Laboratory, using a pulsed EPR spectrometer in conjunction with a Van de Graaff electron accelerator. T2 measurements were made by the spin-echo method, Subsequent work resulted in a measurement of the longitudinal spin relaxation time T1 at a single temperature. In combination with T2 this permits an unambiguous determination of the strength of the magnetic interaction giving rise to relaxation, and hence permits conversion of all the existing T2 data to diffusional correlation times, τ. We have recently initiated a further study of muonium in ice, this time concentrating on T1 measurements. Preliminary work shows the feasibility of determining τ from the field dependence of T1 at a given temperature. The result is in good agreement with our earlier, less direct determinations.

Study of surface relaxation mechanism of liquid3He in porous glass by using SQUID NMR

Nuclear spin relaxation of liquid3He in porous glass has been studied. In addition to measurements of the longitudinal spin relaxation timeT 1 by a usual pulsed SQUID NMR, measurements of the transverse spin relaxation timeT 2 have been performed by using a newly developed SQUID NMR method to observe a spin echo signal. Temperature and frequency dependences ofT 1 andT 2 have been measured. A simple model is proposed which explains the main features of the experimental results.