Electron Spin Relaxation Time Research Articles

Spin-Lattice Relaxation and Spin-Phonon Coupling of ns1 Metal Ions at the Surface.

To use transition metal ions for spin-based applications, it is essential to understand fundamental contributions to electron spin relaxation in different ligand environments. For example, to serve as building blocks for a device, transition metal ion-based molecular qubits must be organized on surfaces and preserve long electron spin relaxation times, up to room temperature. Here we propose monovalent group 12 ions (Zn+ and Cd+) as potential electronic metal qubits with an ns1 ground state. The relaxation properties of Zn+ and Cd+, stabilized at the interface of porous aluminosilicates, are investigated and benchmarked against vanadium (3d1) and copper (3d9) ions. The spin-phonon coupling has been evaluated through DFT modeling and found to be negligible for the ns1 states, explaining the long coherence time, up to 2 μs, at room temperature. These so far unexplored metal qubits may represent viable candidates for room temperature quantum operations and sensing.

Surface optimization of nanodiamonds using non-thermal plasma

Sensitive local monitoring of intracellular processes by quantum sensing utilizing nitrogen-vacancy (NV) centers in nanodiamonds (NDs), would greatly advance cell biology and medicine. However, NDs still fall behind in sensitivity compared to bulk diamond because of their much shorter NV relaxation times. As suggested in theoretical studies, prolongation of NV relaxation times should be achievable by surface optimization creating mixed H/O surface termination consisting of hydrogen atoms, hydroxyl groups, and C–O–C ether bridges. Here we target such chemistry by employing a non-thermal plasma (NTP) in a point-to-plain discharge configuration in aqueous solution. We have devised a set of experiments with different types of nanodiamond samples (of HPHT or detonation origin and with an H- or O-terminated surface) and four working gases (air, O2, He, and H2). Using FTIR, we have found that NTP modification induces a relative increase of O–H and C–H bonds with respect to CO bonds. We have observed the biggest changes in FTIR spectra and the greatest decrease in zeta potential with oxidized NDs (both detonation and HPHT) and hydrogen as the working gas. NV electron spin relaxation times of thus modified HPHT NDs indicate an improvement of the T1 time by ∼17%–∼29% and the T2 time prolongation of 40%.

Measurement of the Lifetime and of the Spin-Relaxation Time of Electrons in Semiconductors by the Optical-Orientation Method

It was shown recently [1, 2] that in semiconducting crystals it is possible to attain an appreciable orientation of the spins of the non-equilibrium carriers as a result of interband transitions caused by absorption of polarized light. Observation of optical orientation makes it possible to extend to semiconductors the research methods widely used in atomic spectroscopy [3], and particularly to investigate relaxation processes in a crystal under stationary conditions.

Electron spin dynamics in multi-energy level systems under the excitation of circular polarization modulated continuous-wave laser

Time-resolved photoluminescence spectroscopy under the excitation of circular polarization modulated continuous-wave laser can quantitatively extract the electron spin polarization degrees and electron spin relaxation time, providing an important tool to detect electron spin dynamics. We theoretically analyze the electron spin dynamics in four-, six- and eight-energy level systems under the excitation of circular polarization modulated continuous-wave laser by solving the rate equations of electron population. The relationship between the time-resolved photoluminescence spectra and the spin information of the spin system is provided. The electron spin polarization degrees of the ground and excited states, the establishment time of the steady-state spin polarization and the electron spin relaxation time can be directly derived from the time-resolved photoluminescence spectra. And the theoretical model is verified experimentally by the measurement of electron spin dynamics in Ce3+: YAG crystal.

Optical and electronic spin properties of fluorescent micro- and nanodiamonds upon prolonged ultrahigh-temperature annealing.

High-temperature annealing is a promising but still mainly unexplored method for enhancing spin properties of negatively charged nitrogen-vacancy (NV) centers in diamond particles. After high-energy irradiation, the formation of NV centers in diamond particles is typically accomplished via annealing at temperatures in the range of 800-900 °C for 1-2 h to promote vacancy diffusion. Here, we investigate the effects of conventional annealing (900 °C for 2 h) against annealing at a much higher temperature of 1600 °C for the same annealing duration for particles ranging in size from 100 nm to 15 μm using electron paramagnetic resonance and optical characterization. At this high temperature, the vacancy-assisted diffusion of nitrogen can occur. Previously, the annealing of diamond particles at this temperature was performed over short time scales because of concerns of particle graphitization. Our results demonstrate that particles that survive this prolonged 1600 °C annealing show increased NV T1 and T2 electron spin relaxation times in 1 and 15 μm particles, due to the removal of fast relaxing spins. Additionally, this high-temperature annealing also boosts magnetically induced fluorescence contrast of NV centers for particle sizes ranging from 100 nm to 15 μm. At the same time, the content of NV centers is decreased fewfold and reaches a level of <0.5 ppm. The results provide guidance for future studies and the optimization of high-temperature annealing of fluorescent diamond particles for applications relying on the spin properties of NV centers in the host crystals.

Origin of metallic-like behavior in disordered carbon nano-onions

Carbon nano-onions are a class of nanomaterials that can exhibit long electron spin relaxation times at room temperature and thus hold promise as potential building blocks for spintronics and quantum information processing devices. Despite first being synthesized 30 years ago, there exists a gap in understanding electronic and magnetic properties of these nanostructures. Here we investigate the origin of the metallic-like behavior that has been observed experimentally in disordered nano-onions. Employing a density functional tight-binding approach and starting from ordered multi-shell fullerenes, we develop realistic models of highly disordered nano-onions comprised of nanometer-scale graphitic flakes. We find that multiple parameters such as flake size, edge structure and substituent groups can give rise to in-gap metallic states, effectively closing the HOMO-LUMO gap.

Statistical Amplification of the Effects of Weak Magnetic Fields in Cellular Translation.

We assume that the enzymatic processes of recognition of amino acids and their addition to the synthesized molecule in cellular translation include the formation of intermediate pairs of radicals with spin-correlated electrons. The mathematical model presented describes the changes in the probability of incorrectly synthesized molecules in response to a change in the external weak magnetic field. A relatively high chance of errors has been shown to arise from the statistical enhancement of the low probability of local incorporation errors. This statistical mechanism does not require a long thermal relaxation time of electron spins of about 1 μs-a conjecture often used to match theoretical models of magnetoreception with experiments. The statistical mechanism allows for experimental verification by testing the usual Radical Pair Mechanism properties. In addition, this mechanism localizes the site where magnetic effects originate, the ribosome, which makes it possible to verify it by biochemical methods. This mechanism predicts a random nature of the nonspecific effects caused by weak and hypomagnetic fields and agrees with the diversity of biological responses to a weak magnetic field.

Electronic properties of graphene/CdX (X=S, Se, and Te) semiconductor heterostructure and a proposal of all-optical injection and detection of electron spins in graphene

Graphene was predicted to have a long spin relaxation time in the range of microseconds to milliseconds due to its weak spin-orbit coupling and hyperfine interaction. However, very short spin relaxation times were measured experimentally on the order of nanoseconds. Magnetic proximity effect from spin valves were considered strongly to influence the electronic and spin properties in graphene. Here, a graphene/nonmagnetic semiconductor CdX heterostructure is proposed to eliminate the magnetic effect and to fulfill optical injection of spin polarization. Based on the first-principles calculation within the density functional theory, we perform a systematic study on the structural, electronic properties and band structures of the heterostructures, and find that graphene on CdX (X = S, Se and Te) slabs preserves its linear Dirac band structure within the bandgap of CdX, and a built-in electric field occurs near the interface. The built-in electric field drives spin polarized electrons into graphene preferably. An optical detection scheme of spin relaxation time of graphene is proposed based on Faraday and Hanle effects. • A graphene/nonmagnetic semiconductor heterostructure constructed to eliminate strong magnetic effects on spin relaxation time of electrons in graphene. • First-principles calculations reveal Graphene on CdX (X = S, Se and Te) slabs preserves its linear Dirac band structure which locates within the bandgap of CdX. • A built-in electric field exists within interface, which favor spin polarized electrons transported into graphene. • A new scheme of all-optical injection and detection of electron spin is proposed to measure true long spin relaxation time of graphene.

EPR and Related Magnetic Resonance Imaging Techniques in Cancer Research.

Imaging tumor microenvironments such as hypoxia, oxygenation, redox status, and/or glycolytic metabolism in tissues/cells is useful for diagnostic and prognostic purposes. New imaging modalities are under development for imaging various aspects of tumor microenvironments. Electron Paramagnetic Resonance Imaging (EPRI) though similar to NMR/MRI is unique in its ability to provide quantitative images of pO2 in vivo. The short electron spin relaxation times have been posing formidable challenge to the technology development for clinical application. With the availability of the narrow line width trityl compounds, pulsed EPR imaging techniques were developed for pO2 imaging. EPRI visualizes the exogenously administered spin probes/contrast agents and hence lacks the complementary morphological information. Dynamic nuclear polarization (DNP), a phenomenon that transfers the high electron spin polarization to the surrounding nuclear spins (1H and 13C) opened new capabilities in molecular imaging. DNP of 13C nuclei is utilized in metabolic imaging of 13C-labeled compounds by imaging specific enzyme kinetics. In this article, imaging strategies mapping physiologic and metabolic aspects in vivo are reviewed within the framework of their application in cancer research, highlighting the potential and challenges of each of them.

Fast electron and slow hole spin relaxation in CsPbI3 nanocrystals

Spin-dependent properties of lead halide perovskites (LHPs) have recently gained significant attention paving their way toward spin-optoelectronic applications. However, separate measurements of the electron and hole spin relaxation rates are so far missing in LHPs. The knowledge of the electron and hole spin relaxation timescales is necessary to understand the spin-dependent properties of LHPs. Here, we report on the spin polarization dynamics in CsPbI3 nanocrystals (NCs). We employ polarization dependent ultrafast differential transmission spectroscopy (DTS) at room temperature to study the spin polarization dynamics in this system. In the case of pure CsPbI3 NCs, it is not possible to measure separately electron and hole spin relaxation rates from the polarization dependent DTS. Here, we introduce the soluble fullerene derivative PC60BM as an electron acceptor along with CsPbI3 to create an imbalance between the photoexcited electrons and holes in the NCs and, thus, affecting their spin-dependent carrier distribution. CsPbI3:PC60BM blend sample shows a distinct difference in the spin dependent kinetics of the DTS spectra as compared to the NCs-only sample. With the help of a kinetic model for the spin-dependent charge carrier distributions, we separately determine the electron and hole spin relaxation times in CsPbI3 NCs. We find that the room temperature hole spin lifetime (τh ∼ 5 ps) is ∼13 times longer than the electron spin lifetime (τe ∼ 0.4 ps). We ascribe the fast electron spin relaxation to the presence of strong spin–orbit coupling in the conduction band, which is ineffective for holes in the s-type valence band.

A frequency-swept, longitudinal detection EPR system for measuring short electron spin relaxation times at ultra-low fields

A frequency-swept longitudinal detection (LOD) EPR system is described for ultra-low field spectroscopy and relaxometry. With the capability of performing simultaneous transmit and receive with −80 dB isolation, this LOD-EPR can capture signals with decay constants in the nanosecond range and in theory even sub-nanosecond range, at fields close to the earth’s magnetic field. The theoretical principles underlying this LOD-EPR are based on a fictitious field that accounts for the Z-axis magnetization polarized by a radiofrequency field alone. The electron spin relaxation time is obtained directly from a previously derived equation that describes the relationship between the relaxation time and the spectral peak position. Herein, the first frequency-swept LOD-EPR system is described in detail, along with experimental measurements of the short relaxation time (∼30 ns) of the free radical, 2,2-diphenyl-1-picrylhydrazyl, at zero to low field.

Observation of gamma radiation-induced defects in radiation-intolerant and radiation-tolerant Si/SiO2 by electron spins resonance and quantitative interpretation

The gamma radiation-induced defects in radiation intolerant and radiation tolerant Si/SiO2 were observed by electron spin resonance (ESR). The Pb and È defects appeared in radiation-intolerant Si/SiO2 after radiation, but only Pb defects appeared in radiation-tolerant Si/SiO2 before and after radiation. The experimental results showed that these defects were correlated with the way of oxidation, the dosage of 60Co radiation and the electrical bias field during radiation. The higher the radiation dosage, the higher the defect concentration. A quantitative relationship was found between defect concentration and radiation dosage. Besides, the △B (peak to peak) of Pb and È indicated that Pb is the defect with a slow electron spin relaxation time, while È is the defect with a fast electron spin relaxation time. Finally, the experimental results are explained qualitatively.

Rapid scan ESR: A versatile tool for the spin relaxation studies at (sub)THz frequencies

The development of pulse electron spin resonance (ESR) spectroscopy at microwave frequencies above 100 GHz is rather challenging and expensive due to the low output power of modern high-frequency solid state electronics. However, there is a number of scientific problems that require spin relaxation measurements at THz frequencies. The rapid scan ESR is an alternative technique that does not require high microwave power and still provides information on the spin relaxation times. The method takes advantage of fast sweeps of the excitation microwave frequency over the ESR line. When the frequency sweep reaches a sufficiently high rate, distinct oscillations (also called wiggles) appear in the ESR spectrum. These oscillations bear information about T2 electron spin relaxation time, which can be extracted via fitting the rapid scan spectrum using the modified Bloch equations. In this Perspective Letter, we introduce the recent advances in this technique and discuss the future steps necessary to make the THz rapid scan ESR a convenient and easy to use tool.

Spin-voltage-driven efficient terahertz spin currents from the magnetic Weyl semimetals Co2MnGa and Co2MnAl

Magnetic Weyl semimetals are an emerging material class that combines magnetic order and a topologically non-trivial band structure. Here, we study ultrafast optically driven spin injection from thin films of the magnetic Weyl semimetals Co2MnGa and Co2MnAl into an adjacent Pt layer by means of terahertz emission spectroscopy. We find that (i) Co2MnGa and Co2MnAl are efficient terahertz spin-current generators reaching efficiencies of typical 3d-transition-metal ferromagnets such as Fe. (ii) The relaxation of the spin current provides an estimate of the electron-spin relaxation time of Co2MnGa (170 fs) and Co2MnAl (100 fs), which is comparable to Fe (90 fs). Both observations are consistent with a simple analytical model and highlight the large potential of magnetic Weyl semimetals as spin-current sources in terahertz spintronic devices. Finally, our results provide a strategy to identify magnetic materials that offer maximum spin-current amplitudes for a given deposited optical energy density.

Non-equilibrium spin noise spectroscopy of a single quantum dot operating at fiber telecommunication wavelengths

We report on the spin and occupation noise of a single, positively charged (InGa)As quantum dot emitting photons in the telecommunication C-band. The spin noise spectroscopy measurements are carried out at a temperature of 4.2 K in dependence on intensity and detuning in the regime beyond thermal equilibrium. The spin noise spectra yield in combination with an elaborate theoretical model the hole-spin relaxation time of the positively charged quantum dot and the Auger recombination and the electron-spin relaxation time of the trion state. The extracted Auger recombination time of this quantum dot emitting at 1.55μm is comparable to the typical Auger recombination times on the order of a few μs measured in traditionally grown InAs/GaAs quantum dots emitting at around 900 nm.

Spin relaxation induced by interfacial effects in GaN/Al0.25Ga0.75N heterostructures

Spin relaxation induced by the interfacial effects in GaN/Al0.25Ga0.75N heterostructures was carefully investigated using a photon-energy-dependent time-resolved Kerr rotation spectrum. The existence of the interfacial localized states with potential fluctuations at the GaN/AlGaN heterointerface leads to photoluminescence peaks showing blue and S-shaped shifts owing to the excitation power and temperature, respectively. Photoexcited electrons in the localized states show a spin relaxation time longer than 1 ns because of the suppression of the D'yakonov–Perel’ (DP) scattering, while the spin relaxation time of free electrons was approximately only 10 ps because of the giant Rashba spin-orbit coupling induced by the interfacial polarization field under the framework of the DP scattering mechanism. Furthermore, it is found that the high electron mobility at the heterointerface results in a long spin diffusion length of 300 nm at high temperatures, which is promising for the development of GaN-based spintronic devices.

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN are investigated with photon-energy-dependent time-resolved Kerr rotation spectroscopy at low temperatures. It is observed that the spin orientation generated with circularly polarized light can be manipulated with photon energy upon the excitation energy being resonated with various fine energy levels in the band structure. Three reversals of the spin orientation and a remarkable reduce of spin relaxation time are obtained, which is caused by the photon-energy dependent transitions from the heavy and light hole bands to the exciton levels and the conduction band edge. A long spin relaxation time of 1.7 ns is attributed to the spin polarization of donor-bound electrons as the formation of donor-bound excitons, while the spin relaxation time of the conduction band electrons is shorter and shows a nonmonotonic dependence on the optical excitation power, revealing the dominant role of the D'yakonov-Perel' spin relaxation mechanism in wurtzite GaN.

Impacts of Crystal Quality on Carrier Recombination and Spin Dynamics in (110)-Oriented GaAs/AlGaAs Multiple Quantum Wells at Room Temperature

We have systematically investigated the structural properties, carrier lifetimes, namely, photoluminescence (PL) lifetimes (τPL), and electron spin relaxation times (τs) in (110) GaAs/AlGaAs multiple quantum wells (MQWs) by using time-resolved PL measurements. The MQWs were grown by molecular beam epitaxy within a wide range of the growth temperature Tg (430–600 °C) and a high V/III flux ratio using As2. At 530 °C < Tg < 580 °C, we found that the quality of the heterointerfaces is significantly improved, resulting in τPL~40 ns at RT, one order of magnitude longer than those reported so far. Long τs (~6 ns) is also observed at RT.

Signature of linear-in- k Dresselhaus splitting in the spin relaxation of X -valley electrons in indirect band gap AlGaAs

GaAs/AlGaAs quantum well (QW) system is utilized to investigate the electron spin relaxation in the satellite X-valley of indirect band gap Al0.63Ga0.37As epitaxial layers through polarization resolved photo-luminescence excitation spectroscopy. Solving the rate equations, steady state electronic distribution in various valleys of AlxGa1-xAs is estimated against continues photo carrier generation and an expression for the degree of circular polarization (DCP) of photoluminescence coming from the adjacent quantum well (QW) is derived. Amalgamating the experimental results with analytical expressions, the X-valley electron spin relaxation time ({\tau}_S^X) is determined to be 2.7 +/- 0.1 ps at 10 K. To crosscheck its validity, theoretical calculations are performed based on Density Functional Theory within the framework of the projector augmented wave method, which support the experimental result quite well. Further, temperature dependence of {\tau}_S^X is studied over 10-80 K range, which is explained by considering the intra-valley scattering of carriers in the X-valley of indirect band gap AlGaAs barrier layer. It is learnt that the strain induced modification of band structure lifts the degeneracy in X-valley, which dominates the electron spin relaxation beyond 50 K. Furthermore, the DCP spectra of hot electrons in indirect band gap AlGaAs layers is found to be significantly different compared to that of direct bandgap AlGaAs. It is understood as a consequence of linear k dependent Dresselhaus spin splitting and faster energy relaxation procedure in the X-valley. Findings of this work could provide a new horizon for the realization of next generation spin-photonic devices which are less sensitive to Joule heating.

Spin dynamics in GaN/Al0.1Ga0.9N quantum well with complex band edge structure

Spectrally distinguished spin relaxation dynamics in a single GaN/Al0.1Ga0.9N quantum well was investigated by a time-resolved Kerr rotation spectrum at room temperature. Three spin relaxation processes were well distinguished by a photon energy upon the excitation energy being resonated with the bandgap of various layers. It is observed that the electron spin relaxation time of 7 ps in a GaN quantum well is much shorter than that of 140 ps in an Al0.1Ga0.9N barrier layer due to the considerable polarization electric field at a GaN/Al0.1Ga0.9N heterointerface. For electrons in bulk GaN and Al0.1Ga0.9N, the dominant role of electron–photon scattering and alloy disorder scattering in the anisotropic D'yakonov–Perel’ (DP) relaxation was revealed by the photoexcited electron density and magnetic field dependence of the spin relaxation time.