Spin Memory Times Research Articles

Coupling Sub-nanoliter BDPA Organic Radical Spin Ensembles with YBCO Inverse Anapole Resonators

We report the development and test of planar microwave Inverse Anapole Resonators (IARs) made of superconducting Yttrium Barium Copper Oxide (YBCO) for electron spin resonance spectroscopy on small samples. We first characterize our resonators in zero field and then by carrying out transmission spectroscopy on a diluted $$\alpha ,\gamma $$ -bisdiphenylene- $$\beta $$ -phenylally (BDPA) organic radical spin ensemble in an applied magnetic field. These IARs allow us to carry out electron spin resonance spectroscopy both in continuous-wave and pulsed-wave mode, and to estimate the spin memory time of BDPA. The comparison with the results obtained for the same sample on typical linear coplanar resonators shows an improvement by $$\approx 2\text { - up to}\,3$$ – orders of magnitude in spin sensitivity, with effective sensing volumes below 1 nanoliter. The best sensitivity we achieved is $$S\approx \,10^{7}\,\text {spin}/\sqrt{\mathrm{Hz}}$$ in the pulsed-wave regime. These results compare well with similar experiments reported in the literature.

Effect of electric current on the optical orientation of interface electrons in AlGaAs/GaAs heterostructures

The effect of a lateral electric current on the photoluminescence H-band of an AlGaAs/GaAs heterostructure is investigated. The photoluminescence intensity and optical orientation of electrons contributing to the H-band are studied by means of continuous wave and time-resolved photoluminescence spectroscopy and time-resolved Kerr rotation. It is shown that the H-band is due to recombination of the heavy holes localized at the heterointerface with photoexcited electrons attracted to the heterointerface from the GaAs layer. Two lines with significantly different decay times constitute the H-band: a short-lived high-energy one and a long-lived low-energy one. The high-energy line originates from recombination of electrons freely moving along the structure plane, while the low-energy one is due to recombination of donor-bound electrons near the interface. Application of the lateral electric field of ~ 100-200 V/cm results in a quenching of both lines. This quenching is due to a decrease of electron concentration near the heterointerface as a result of a photocurrent-induced heating of electrons in the GaAs layer. On the contrary, electrons near the heterointerface are effectively cooled, so the donors near the interface are not completely empty up to ~ 100 V/cm, which is in stark contrast with the case of bulk materials. The optical spin polarization of the donor-bound electrons near the heterointerface weakly depends on the electric field. Their polarization kinetics is determined by the spin dephasing in the hyperfine fields of the lattice nuclei. The long spin memory time (> 40 ns) can be associated with suppression of the Bir-Aronov-Pikus mechanism of spin relaxation for electrons.

Enhancing sensitivity of Double Electron-Electron Resonance (DEER) by using Relaxation-Optimized Acquisition Length Distribution (RELOAD) scheme.

Over the past decades pulsed electron-electron double resonance (PELDOR), often called double electron-electron resonance (DEER), became one of the major spectroscopic tools for measurements of nanometer-scale distances and distance distributions in non-crystalline biological and chemical systems. The method is based on detecting the amplitude of the primary (3-pulse DEER) or refocused (4-pulse DEER) spin echo for the so-called "observer" spins when the other spins coupled to the former by a dipolar interaction are flipped by a "pump" pulse at another EPR frequency. While the timing of the pump pulse is varied in steps, the positions of the observer pulses are typically fixed. For such a detection scheme the total length of the observer pulse train and the electron spin memory time determine the amplitude of the detected echo signal. Usually, the distance range considerations in DEER experiments dictate the total length of the observer pulse train to exceed the phase memory time by a factor of few and this leads to a dramatic loss of the signal-to-noise ratio (SNR). While the acquisition of the DEER signal seems to be irrational under such conditions, it is currently the preferred way to conduct DEER because of an effective filtering out of all other unwanted interactions. Here we propose a novel albeit simple approach to improve DEER sensitivity and decrease data acquisition time by introducing the signal acquisition scheme based on RELaxation Optimized Acquisition (Length) Distribution (DEER-RELOAD). In DEER-RELOAD the dipolar phase evolution signal is acquired in multiple segments in which the observer pulses are fixed at the positions to optimize SNR just for that specific segment. The length of the segment is chosen to maximize the signal acquisition efficiency according the phase relaxation properties of the spin system. The total DEER trace is then obtained by "stitching" the multiple segments into a one continuous trace. The utility of the DEER-RELOAD acquisition scheme has been demonstrated on an example of the standard 4-pulse DEER sequence applied to two membrane protein complexes labeled with nitroxides. While theoretical gains from the DEER-RELOAD scheme increase with the number of stitched segments, in practice, even dividing the acquisition of the DEER trace into two segments may improve SNR by a factor of >3, as it has been demonstrated for one of these two membrane proteins.

Role of Spin-Coupled Polaron Pairs in the Recombination of Charges in Electroluminescent Conjugated Polymers

We study the delayed fluorescence following geminate recombination of photogenerated polaron pairs (PPs) in films of a model conjugated polymer F8BT. Doping with tiny gold nanoparticles also enables us to observe phosphorescence from triplet states that arise via intersystem crossing from the directly photoexcited singlet state. Small magnetic fields ∼5 mT have large effects on the delayed fluorescence by causing demixing of the hyperfine coupled singlet and triplet PP configurations, implying PP spin equilibration times much shorter than 1 μs. Nevertheless, the magnetic field effect on the fluorescence persists to tens of microseconds, and we argue that this apparent inconsistency implies that photogenerated polarons reside primarily as uncoupled spin half particles with long spin memory times. We consider the implications for whether recombination of charges in organic light-emitting diodes is likely to be governed by quantum spin statistics.

Optoelectronic spin memories of electrons in semiconductors

We optically generate electron spins in semiconductors and apply an external magnetic field perpendicularly to them. Time-resolved photoluminescence measurements, pumped with a circularly polarized light, are performed to study the spin polarization and spin memory times in the semiconducting host. The measured spin polarization is found to be an exponential decay with the time delay of the probe. It is also found that the spin memory times, extracted from the polarization decays, enhance with the strength of the external magnetic field. However, at higher fields, the memory times get saturated to sub-{\mu}s because of the coupling for interacting electrons with the local nuclear field.

Optically Controlled Nanomagnets

AbstractWe report on single dot photoluminescence imaging and spectroscopy at B=0T on magnetically doped CdMnTe self-assembled quantum dots with average Mn concentration of several percent. Quasi-resonant excitation with circularly polarized laser leads to formation of magnetic polarons with magnetization induced by the laser light. In this case all quantum dots are polarized in the same direction. In contrast, when the dots are populated using above the barrier excitation, with randomly polarized excitons, the resultant magnetization is random and varies from dot to dot. These experiments demonstrate a way to control the magnetization of magnetically doped quantum dots by means of light excitation. In addition, they point towards extremely long spin memory times in these structures, reaching hundreds of microseconds, making CdMnTe quantum dots promising candidates for local magnetic field sources on the nanoscale.

Gated spin relaxation in (1 1 0)-oriented quantum wells

Growth, photoluminescence characterisation and time-resolved optical measurements of electron spin dynamics in (1 1 0)-oriented GaAs/AlGaAs quantum wells are described. Conditions are given for MBE growth of good-quality quantum wells, judged by the width of low-temperature excitonic photoluminescence. At 170 K the electron spin relaxation rate in (1 1 0)-oriented wells shows a 100-fold reduction compared to equivalent (1 0 0)-oriented wells and also a 10-fold increase with applied electric field from 20 to 80 kV cm −1 . There is evidence for similar dramatic effects at 300 K . Spin relaxation is field independent below 20 kV cm −1 reflecting quantum well asymmetry. The results indicate the achievability of voltage-gateable quantum well spin memory time longer than 10 ns at room temperature simultaneously with high electron mobility.

High temperature gate control of quantum well spin memory.

Time-resolved optical measurements in (110)-oriented GaAs/AlGaAs quantum wells show a tenfold increase of the spin-relaxation rate as a function of applied electric field from 20 to 80 kV cm(-1) at 170 K and indicate a similar variation at 300 K, in agreement with calculations based on the Rashba effect. Spin relaxation is almost field independent below 20 kV cm(-1) reflecting quantum well interface asymmetry. The results indicate the achievability of a voltage-gateable spin-memory time longer than 3 ns simultaneously with a high electron mobility.

Long electron spin memory times in gallium arsenide

Extremely long electron spin memory times in GaAs are reported. It was established by the optical orientation method that the spin relaxation time of electrons localized at shallow donors in n-type gallium arsenide (N d −N A ≈1014 cm−3) is 290±30 ns at a temperature of 4.2 K. The exchange interaction of quasi-free electrons and electrons at donors suppresses the main spin-loss channel for electrons localized at donors—spin relaxation due to the hyperfine interaction with lattice nuclei.

Femtosecond electron and spin dynamics probed by nonlinear optics

A theoretical calculation is performed for the ultrafast spin dynamics in nickel using an exact diagonalization method. The present theory mainly focuses on a situation where the intrinsic charge and spin dynamics is probed by the nonlinear (magneto-)optical responses on the femtosecond time scale, i.e. optical second harmonic generation (SHG) and the nonlinear magneto-optical Kerr effect (NOLIMOKE). It is found that the ultrafast charge and spin dynamics are observable on the time scale of 10 fs. The charge dynamics proceeds ahead of the spin dynamics, which indicates the existence of a spin memory time. The fast decay results from the loss of coherence in the initial excited state. Both the material specific and experimental parameters affect the dynamics. We find that the increase of exchange interaction mainly accelerates the spin dynamics rather than the charge dynamics. A reduction of the hopping integrals, such as present at interfaces, slows down the spin dynamics significantly. Besides, it is found that a spectrally broad excitation yields the intrinsic speed limit of the charge (SHG) and spin dynamics (NOLIMOKE) while a narrower width prolongs the dynamics. This magnetic interface dynamics then should become accessible to state of art time resolved nonlinear-optical experiments.

Electron-spin echoes at 604 GHz using far infrared lasers.

We have observed electron-spin echoes at 604 GHz in a magnetic field of 21.5 T. This frequency and magnetic field are more than 4 times larger than frequencies and fields at which electron-spin echoes have been reported before. We have used far infrared lasers for the excitation pulses and have employed a heterodyne detection system with a Schottky diode as mixer element and a far infrared laser as local oscillator. For a relatively concentrated nitroxide free-radical sample, we show that the spin-memory time is longer at higher frequencies due to the increased spin polarization at high magnetic fields.

Selective study of paramagnetic centres in coals by e.s.r. and e.s.e. spectrometry

Electron spin resonance (e.s.r.) and electron spin echo (e.s.e.) studies of two natural and heat-treated bituminous coals are presented. In all samples, two main paramagnetic centres have been detected and studied selectively by obseving e.s.e. decays at different parts of the e.s.r. signal. These centres interact differently with neighbouring protons and the interaction is reduced in both centres by heat-treatment. The e.s.r. measurements reflect the heterogeneity of the region around the radical and are sensitive to the presence of proton hyperfine and dipolar broadening interactions. E.s.e. decays are sensitive to the presence of signals with longer spin memory time T M . The T M values obtained for all samples can be rationalized by assuming an exchange frequency in the range 1 × 10 8 − 1 × 10 9s −1.

Fluorescence detection of electron spin echoes in the triplet state of nonphosphorescent molecules and a photosynthetic bacterium

We have observed electron spin echo signals in zero magnetic field in the triplet state of the porphin free base and the photo-induced triplet state of the photosynthetic bacterium Rhodopseudomonas spheroides (wild type) via changes in the intensity of the fluorescence. With the help of the echo signals we have been able to determine the spin memory times of the triplet spins.

An exact dynamical solution to a linear chain doublet exciton with exponential memory

An exact analytic solution to the non-Markoffian spin one-half exciton linear chain system is found by diagonalising and Laplace transforming the coupled space and spin equations. As a particular example, numerical calculations of the time evolution of the two-site exciton system are presented for several realistic values of space and spin memory times. The unimportance of the cross-channel non-Markoffian behaviour for the cases where the space and spin memory times differ by at least a factor of ten is illustrated by comparison with the traditional Pauli master equation solutions. The memory can lead to oscillating populations which, on time scales shorter than or comparable with the relevant memory time, look like coherent transfer. Coherent motion can be observed, for certain values of the parameters, in the slower channel long after the more normal channel has become Markoffian. Interference regimes are possible, but require similar values of space and spin transfer integrals; this is improbable for organics.

Suppression of the Kondo Many-Body Scattering Effect

An experimental study of the electrical resistivity in dilute Cu:Mn doped with Pt impurities is presented. The results indicate a suppression of the Kondo divergence in the conduction-electron scattering amplitude as a result of the spin-orbit interaction at the Pt impurities. The results are interpreted in terms of a reduction in the spin memory time.