Hole Spin Relaxation Time Research Articles

Hole Spin Coherence in InAs/InAlGaAs Self‐Assembled Quantum Dots Emitting at Telecom Wavelengths

Measurements of the longitudinal and transverse spin relaxation times of holes in an ensemble of vertically tunnel‐coupled self‐assembled InAs/InAlGaAs quantum dots (QDs), emitting in the telecom spectral range, are reported. The spin coherence is determined using the spin mode‐locking method in the inhomogeneous ensemble of QDs. Modeling the signal allows us to extract the hole spin coherence time to be in the range of μs. The longitudinal spin relaxation time μs is measured using the spin inertia method. The parameters investigated allow us to suggest that the main relaxation mechanism at low magnetic fields is related to the electron–hole spin exchange.

Coherent Carrier Spin Dynamics in FAPbBr3 Perovskite Crystals.

Coherent spin dynamics of electrons and holes are studied in hybrid organic-inorganic lead halide perovskite FAPbBr3 bulk single crystals using the time-resolved Kerr ellipticity technique at cryogenic temperatures. The Larmor spin precession of the carrier spins in a magnetic field is monitored to measure the Landé g-factors of electrons (+2.44) and holes (+0.41). These g-factors are highly isotropic. The measured spin dephasing times amount to a few nanoseconds, and the longitudinal hole spin relaxation time is 470 ns. The important role of the strong hyperfine interaction between carrier spins and nuclear spins is demonstrated via dynamic nuclear polarization. At low temperatures, electron and hole spin relaxation predominantly occurs via the hyperfine interaction, whose importance significantly decreases at temperatures above 12 K. We overview the spin dynamics in various lead halide perovskite crystals and polycrystalline films and conclude on their common features provided by charge carrier localization at cryogenic temperatures.

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.

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 of holes in In0.53Ga0.47As/InP quantum wells

Polarization-resolved photoluminescence was used to study spin relaxation of photoexcited holes in In0.53Ga0.47As/InP quantum wells in a quantizing magnetic field as a function of temperature. At a temperature below 10 K, the circular polarization of the photoluminescence due to the spin-split valence band Landau levels was found temperature-independent. In this temperature range fast hole spin relaxation as compared to their lifetime leads to the photoluminescence circular polarization determined by the ratio of these times. Increasing temperature resulted in efficient hole spin thermalization in the Zeeman split valence band Landau levels and as a consequence, in vanishing photoluminescence polarization. Fits of the experimental data by the theory allowed a determination of the hole spin relaxation times related to different Landau levels and the corresponding hole effective g-factor. Direct measurements of the hole spin relaxation times prove the obtained results.

Nanosecond Spin Coherence Time of Nonradiative Excitons in GaAs/AlGaAs Quantum Wells.

We report on the experimental evidence for a nanosecond timescale spin memory based on nonradiative excitons with large in-plane wave vector. The effect manifests itself in magnetic-field-induced oscillations of the energy of the optically active (radiative) excitons. The oscillations detected by a spectrally resolved pump-probe technique applied to a GaAs/AlGaAs quantum well structure in a transverse magnetic field persist over a timescale, which is orders of magnitude longer than the characteristic decoherence time in the system. The effect is attributed to the spin-dependent electron-electron exchange interaction of the optically active and inactive excitons. The spin relaxation time of the electrons belonging to nonradiative excitons appears to be much longer than the hole spin relaxation time.

Single-Shot Readout of Hole Spins in Ge.

The strong atomistic spin–orbit coupling of holes makes single-shot spin readout measurements difficult because it reduces the spin lifetimes. By integrating the charge sensor into a high bandwidth radio frequency reflectometry setup, we were able to demonstrate single-shot readout of a germanium quantum dot hole spin and measure the spin lifetime. Hole spin relaxation times of about 90 μs at 500 mT are reported, with a total readout visibility of about 70%. By analyzing separately the spin-to-charge conversion and charge readout fidelities, we have obtained insight into the processes limiting the visibilities of hole spins. The analyses suggest that high hole visibilities are feasible at realistic experimental conditions, underlying the potential of hole spins for the realization of viable qubit devices.

Spin relaxation dynamics of holes in intrinsic GaAs quantum wells studied by transient circular dichromatic absorption spectroscopy at room temperature

Spin relaxation dynamics of holes in intrinsic GaAs quantum wells is studied using time-resolved circular dichromatic absorption spectroscopy at room temperature. It is found that ultrafast dynamics is dominated by the cooperative contributions of band filling and many-body effects. The relative contribution of the two effects is opposite in strength for electrons and holes. As a result, transient circular dichromatic differential transmission (TCD-DT) with co- and cross-circularly polarized pump and probe presents different strength at several picosecond delay time. Ultrafast spin relaxation dynamics of excited holes is sensitively reflected in TCD-DT with cross-circularly polarized pump and probe. A model, including coherent artifact, thermalization of nonthermal carriers and the cooperative contribution of band filling and many-body effects, is developed, and used to fit TCD-DT with cross-circularly polarized pump and probe. Spin relaxation time of holes is achieved as a function of excited hole density for the first time at room temperature, and increases with hole density, which disagrees with a theoretical prediction based on EY spin relaxation mechanism, implying that EY mechanism may be not dominant hole spin relaxation mechanism at room temperature, but DP mechanism is dominant possibly.

Spin properties of localized hole and resident electron in ZnO epilayers

AbstractThe spin dynamic of the hole localized inside the donor bound exciton (D0X) and donor electrons is studied in ZnO epilayer by time‐resolved optical orientation of the D0X photoluminescence and the absorption bleaching of the spin‐sensitive D0X transition, respectively. A D0X life time of 140 ps and hole spin relaxation time of 80 ps are extracted. The comparable times allows us for an effective pumping of the donor electron spin via circularly polarized excitation.Long electron spin relaxation times in the 1 μs range are uncovered if the hyperfine interaction is suppressed by a proper longitudinal magnetic field B‖. A field strength of ≈2 mT is large enough to achieve that proving the extremely small Overhauser field BN acting on localized electrons. This is a direct result of the small abundance of magnetic nuclei interacting inside the small donor volume both specific for ZnO. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin splitting in 2D monochalcogenide semiconductors.

We report ab initio calculations of the spin splitting of the uppermost valence band (UVB) and the lowermost conduction band (LCB) in bulk and atomically thin GaS, GaSe, GaTe, and InSe. These layered monochalcogenides appear in four major polytypes depending on the stacking order, except for the monoclinic GaTe. Bulk and few-layer ε-and γ -type, and odd-number β-type GaS, GaSe, and InSe crystals are noncentrosymmetric. The spin splittings of the UVB and the LCB near the Γ-point in the Brillouin zone are finite, but still smaller than those in a zinc-blende semiconductor such as GaAs. On the other hand, the spin splitting is zero in centrosymmetric bulk and even-number few-layer β-type GaS, GaSe, and InSe, owing to the constraint of spatial inversion symmetry. By contrast, GaTe exhibits zero spin splitting because it is centrosymmetric down to a single layer. In these monochalcogenide semiconductors, the separation of the non-degenerate conduction and valence bands from adjacent bands results in the suppression of Elliot-Yafet spin relaxation mechanism. Therefore, the electron- and hole-spin relaxation times in these systems with zero or minimal spin splittings are expected to exceed those in GaAs when the D’yakonov-Perel’ spin relaxation mechanism is also suppressed.

Tuning of the hole spin relaxation time in single self-assembled In1−xGaxAs/GaAs quantum dots by electric field

We investigate the electric field tuning of the phonon-assisted hole spin relaxation in single self-assembled In1−xGaxAs/GaAs quantum dots (QDs), using an atomistic empirical pseudopotential method. We find that the electric field along the growth direction can tune the hole spin relaxation time for more than one order of magnitude. The electric field can prolong or shorten the hole spin lifetime and the tuning shows an asymmetry in terms of the field direction. The asymmetry is more pronounced for the taller dot. The results show that the electric field is an effective way to tune the hole spin-relaxation in self-assembled QDs.

Spin-lasers: From threshold reduction to large-signal analysis

Lasers in which spin-polarized carriers are injected provide paths to different practical room temperature spintronic devices, not limited to magnetoresistive effects. Unlike the conventional understanding of spintronic devices, an optimal performance of such spin-lasers can arise for finite, not infinite, spin relaxation time. By considering spin-relaxation times of both electrons and holes, we elucidate advantages of spin-lasers over their conventional (spin-unpolarized) counterparts. In addition to the steady-state threshold reduction, spin-lasers can improve transient operation leading to shorter turn-on delay times, reduced ringing of emitted light, and an enhanced bandwidth.

Optical spin noise of a single hole spin localized in an (InGa)As quantum dot.

We advance spin noise spectroscopy to the ultimate limit of single spin detection. This technique enables the measurement of the spin dynamic of a single heavy hole localized in a flat (InGa)As quantum dot. Magnetic field and light intensity dependent studies reveal even at low magnetic fields a strong magnetic field dependence of the longitudinal heavy hole spin relaxation time with an extremely long T1 of ≥180 μs at 31mT and 5K. The wavelength dependence of the spin noise power discloses for finite light intensities an inhomogeneous single quantum dot spin noise spectrum which is explained by charge fluctuations in the direct neighborhood of the quantum dot. The charge fluctuations are corroborated by the distinct intensity dependence of the effective spin relaxation rate.

Tunablegfactor and phonon-mediated hole spin relaxation in Ge/Si nanowire quantum dots

We theoretically consider g factor and spin lifetimes of holes in a longitudinal Ge/Si core/shell nanowire quantum dot that is exposed to external magnetic and electric fields. For the ground states, we find a large anisotropy of the g factor which is highly tunable by applying electric fields. This tunability depends strongly on the direction of the electric field with respect to the magnetic field. We calculate the single-phonon hole spin relaxation times T1 for zero and small electric fields and propose an optimal setup in which very large T1 of the order of tens of milliseconds can be reached. Increasing the relative shell thickness or the longitudinal confinement length further prolongs T1. In the absence of electric fields, the dephasing vanishes and the decoherence time T2 is determined by T2 = 2 T1.

Long spin relaxation time of holes in InGaAs/GaAs quantum wells probed by cyclotron resonance spectroscopy

In this paper, we report on a long, ms range, hole spin relaxation time in InGaAs/GaAs quantum wells probed by cyclotron resonance spectroscopy in pulsed magnetic fields. In our experiments, we found strong hysteresis in the spectral weights of cyclotron resonance absorption lines when rapidly changing magnetic field is used for the experiment. The hysteresis vanishes when a much slower changing magnetic field is used. We attribute this behavior to a long energy relaxation time between the two lowest spin-split hole Landau levels, i.e., a long hole spin relaxation time. We also present transition frequencies calculated using a $4\ifmmode\times\else\texttimes\fi{}4$ Luttinger Hamiltonian, which confirm our findings.

Two-phonon process and hyperfine interaction limiting slow hole-spin relaxation time in InAs/GaAs quantum dots

We study the hole-spin relaxation in p-doped InAs quantum dots. Two relaxation mechanisms are evidenced, at low magnetic field ($0\ensuremath{\le}B\ensuremath{\le}2T$) and low temperature ($2\ensuremath{\le}T\ensuremath{\le}50K$), by using a pump-probe configuration and a recent experimental technique working in the frequency domain. At $T=2K$, the coupling to nuclear spins and the hole wave-function inhomogeneity fix the hole-spin relaxation rate value, ${\ensuremath{\Gamma}}_{1}^{h}\ensuremath{\approx}1\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}{\mathrm{s}}^{\ensuremath{-}1}$. It decreases with increasing magnetic field and reaches a plateau at 0.4 $\ensuremath{\mu}{\mathrm{s}}^{\ensuremath{-}1}$. At $T\ensuremath{\ge}7K$, two-phonon spin-orbit process dominates and leads to a quadratic temperature dependence of ${\ensuremath{\Gamma}}_{1}^{h}$, in good agreement with theory.

Spin band-gap renormalization and hole spin dynamics in Ge/SiGe quantum wells

We experimentally determine a spin-dependent band-gap renormalization of 60 $\ensuremath{\mu}$eV, and the hole spin relaxation dynamics in highly excited, strained Ge/SiGe quantum wells using a differential pump-probe technique which exploits the indirect nature of the band structure. Compressive strain lifts the degeneracy between heavy- and light-hole subbands, and both are addressed individually with our spectrally and polarization-resolved approach. We calculate that in our quantum wells, contrary to bulk Ge, we excite almost pure spin eigenstates so that a hole spin polarization exceeding 80$%$ is obtained. A hole spin relaxation time of 2.1 ps is observed at 10 K.

Hole spin relaxation and coefficients in Landau-Lifshitz-Gilbert equation in ferromagnetic (Ga,Mn)As

We investigate the temperature dependence of the coefficients in the Landau-Lifshitz-Gilbert equation in ferromagnetic GaMnAs by employing the Zener model. We first calculate the hole spin relaxation time based on the microscopic kinetic equation. We find that the hole spin relaxation time is typically several tens of femtoseconds and can present a nonmonotonic temperature dependence due to the variation of the interband spin mixing, influenced by the temperature-related Zeeman splitting. With the hole spin relaxation time, we are able to calculate the coefficients in the Landau-Lifshitz-Gilbert equation, such as the Gilbert damping, nonadiabatic spin torque, spin stiffness, and vertical spin stiffness coefficients. We find that the nonadiabatic spin torque coefficient $\ensuremath{\beta}$ is around $0.1\ensuremath{-}0.3$ at low temperature, which is consistent with the experiment [J.-P. Adam et al., Phys. Rev. B 80, 193204 (2009)]. As the temperature increases, $\ensuremath{\beta}$ monotonically increases. We show that the Gilbert damping coefficient $\ensuremath{\alpha}$ increases with temperature below the Curie temperature, showing good agreement with the experiments [J. Sinova et al., Phys. Rev. B 69, 085209 (2004); Kh. Khazen et al., Phys. Rev. B 78, 195210 (2008)]. Moreover, we also calculate the temperature dependences of the spin stiffness and vertical spin stiffness.

Atomistic pseudopotential theory of spin relaxation in self-assembled In1−xGaxAs/GaAs quantum dots at zero magnetic field

An atomistic pseudopotential calculation of the spin-flip time (${T}_{1}$) of electrons and holes mediated by acoustic phonons in self-assembled In${}_{1\ensuremath{-}x}$Ga${}_{x}$As/GaAs quantum dots at zero magnetic field is presented. At low magnetic field, the first-order process is suppressed, and the second-order process becomes dominant. It is found that the spin-phonon-interaction-induced spin relaxation time is 100--221 s for electrons, and 4.4--60 ms for holes at 4.2 K. The calculated hole-spin relaxation times are in good agreement with recent experiments, which suggests that the two-phonon process is one of the main relaxation mechanisms for hole-spin relaxation in the self-assembled quantum dots at zero field. The structural and alloy composition effects on the spin relaxation in the quantum dots are clarified.

Hole spin relaxation and intervalley electron scattering in germanium

Hole spin relaxation and intervalley electron scattering in bulk Ge at 300 K are distinguished and selectively investigated using a spectrally, temporally, and polarization-resolved pump-probe differential transmission technique that takes advantage of the indirect band gap nature of Ge and of the differing circular selection rules for the direct split-off and heavy-hole valence-to-conduction band transitions. Spin-polarized carriers are injected across the direct gap by a circularly polarized pump. Subsequently, the circular dichroisms associated with direct across-gap interband transitions from the split-off and heavy-hole valence bands are measured by a probe pulse tuned to interrogate both transitions. We demonstrate that the Pauli-blocking of the conduction states needed for the direct transition from the split-off valence band by electrons dominate the dichroism in the differential transmission for early times, but once the electrons scatter to the side valleys, the dichroism reverses sign and is caused by the holes blocking the initial valence states needed for the direct heave-hole transition, thus, allowing us to study the hole dynamics. From these measurements, we estimate an intervalley scattering time of \ensuremath{\sim}200 fs and an effective hole spin relaxation time of \ensuremath{\sim}700 fs.