Dynamic Spin Polarization Research Articles

Characteristic features of the singlet–triplet mechanism of the electron spin polarization

Characteristic features of net chemically induced dynamic electron spin polarization (CIDEP) P n in triplet–radical (TR) quenching are analyzed in detail within the framework of a general model that enables one to analyze CIDEP both numerically and analytically. This model also makes it possible to accurately describe the nonadiabatic transitions between the terms of the TR-pair spin Hamiltonian that lead to CIDEP generation. The proposed theory yields a simple analytical dependence of P n on the parameters of the model. In particular, it is shown that, within a wide region of parameters, the dependence of P n on the coefficient of relative TR diffusion D r is described by a simple linear relation: $$P_n^{ - 1}\left( {{D_r}} \right) = {Q_0} + \overline {{q_n}} {D_r}$$ (with Q 0 and $${{q_n}}$$ independent of D r ). It is also demonstrated that the numerical and analytical results obtained are very useful in analysis of experimental data, as demonstrated by analyzing the experimental dependence of P n on D r .

Dynamics of nuclear spin polarization induced and detected by coherently precessing electron spins in fluorine-doped ZnSe

We study the dynamics of optically-induced nuclear spin polarization in a fluorine-doped ZnSe epilayer via time-resolved Kerr rotation. The nuclear polarization in the vicinity of a fluorine donor is induced by interaction with coherently precessing electron spins in a magnetic field applied in the Voigt geometry. It is detected by nuclei-induced changes in the electron spin coherence signal. This all-optical technique allows us to measure the longitudinal spin relaxation time $T_{1}$ of the $^{77}\text{Se}$ isotope in a magnetic field range from 10 to 130~mT under illumination. We combine the optical technique with radio frequency methods to address the coherent spin dynamics of the nuclei and measure Rabi oscillations, Ramsey fringes and the nuclear spin echo. The inhomogeneous spin dephasing time $T_{2}^{*}$ and the spin coherence time $T_{2}$ of the $^{77}\text{Se}$ isotope are measured. While the $T_{1}$ time is on the order of several milliseconds, the $T_{2}$ time is several hundred microseconds. The experimentally determined condition $T_{1}\gg T_{2}$ verifies the validity of the classical model of nuclear spin cooling for describing the optically-induced nuclear spin polarization.

Influence of temperature on spin polarization dynamics in dilute nitride semiconductors—Role of nonparamagnetic centers

We report theoretical studies of spin polarization dynamics in dilute nitride semiconductors. We develop a commonly used rate equation model [Lagarde et al., Phys. Status Solidi A 204, 208 (2007) and Kunold et al. Phys. Rev. B 83, 165202 (2011)] to take into account the influence of shallow localizing states on the temperature dependence of spin polarization dynamics and a spin filtering effect. Presented investigations show that the experimentally observed temperature dependence of a spin polarization lifetime in dilute nitrides can be related to the electron capture process by shallow localizing states without paramagnetic properties. This process reduces the efficiency of spin filtering effect by deep paramagnetic centers, especially at low temperatures.

Spin-dependent tunneling in semiconductor heterostructures with a magnetic layer

We present a theory that describes the appearance of circular polarization of the photoluminescence (PL) in ferromagnet-semiconductor hybrid heterostructures due to spin-dependent tunneling of photoexcited carriers from a quantum well into a magnetic layer. The theory succeeds in explaining the experimental data on time-resolved PL for heterostructures consisting of InGaAs-based quantum well (QW) and a spatially separated Mn $\delta$-layer. We show that the circular polarization of the PL originates from dynamic spin polarization of electrons due to spin-dependent leakage from the QW onto Mn donor states split by the exchange field of the ferromagnetic Mn delta-layer.

Theoretical model of dynamic spin polarization of nuclei coupled to paramagnetic point defects in diamond and silicon carbide

Dynamic nuclear spin polarization (DNP) mediated by paramagnetic point defects in semiconductors is a key resource for both initializing nuclear quantum memories and producing nuclear hyperpolarization. DNP is therefore an important process in the field of quantum-information processing, sensitivity-enhanced nuclear magnetic resonance, and nuclear-spin-based spintronics. DNP based on optical pumping of point defects has been demonstrated by using the electron spin of nitrogen-vacancy (NV) center in diamond, and more recently, by using divacancy and related defect spins in hexagonal silicon carbide (SiC). Here, we describe a general model for these optical DNP processes that allows the effects of many microscopic processes to be integrated. Applying this theory, we gain a deeper insight into dynamic nuclear spin polarization and the physics of diamond and SiC defects. Our results are in good agreement with experimental observations and provide a detailed and unified understanding. In particular, our findings show that the defects' electron spin coherence times and excited state lifetimes are crucial factors in the entire DNP process.

Momentum-resolved spin dynamics of bulk and surface excited States in the topological insulator Bi(2)Se(3).

The prospect of optically inducing and controlling a spin-polarized current in spintronic devices has generated wide interest in the out-of-equilibrium electronic and spin structure of topological insulators. In this Letter we show that only measuring the spin intensity signal over several orders of magnitude by spin-, time-, and angle-resolved photoemission spectroscopy can provide a comprehensive description of the optically excited electronic states in Bi_{2}Se_{3}. Our experiments reveal the existence of a surface resonance state in the second bulk band gap that is benchmarked by fully relativistic abinitio spin-resolved photoemission calculations. We propose that the newly reported state plays a major role in the ultrafast dynamics of the system, acting as a bottleneck for the interaction between the topologically protected surface state and the bulk conduction band. In fact, the spin-polarization dynamics in momentum space show that these states display macroscopically different temperatures and, more importantly, different cooling rates over several picoseconds.

Ultrafast multi-level logic gates with spin-valley coupled polarization anisotropy in monolayer MoS2.

The inherent valley-contrasting optical selection rules for interband transitions at the K and K′ valleys in monolayer MoS2 have attracted extensive interest. Carriers in these two valleys can be selectively excited by circularly polarized optical fields. The comprehensive dynamics of spin valley coupled polarization and polarized exciton are completely resolved in this work. Here, we present a systematic study of the ultrafast dynamics of monolayer MoS2 including spin randomization, exciton dissociation, free carrier relaxation, and electron-hole recombination by helicity- and photon energy-resolved transient spectroscopy. The time constants for these processes are 60 fs, 1 ps, 25 ps, and ~300 ps, respectively. The ultrafast dynamics of spin polarization, valley population, and exciton dissociation provides the desired information about the mechanism of radiationless transitions in various applications of 2D transition metal dichalcogenides. For example, spin valley coupled polarization provides a promising way to build optically selective-driven ultrafast valleytronics at room temperature. Therefore, a full understanding of the ultrafast dynamics in MoS2 is expected to provide important fundamental and technological perspectives.

A practical approach to calculate the time evolutions of magnetic field effects on photochemical reactions in nano-structured materials.

A practical method to calculate time evolutions of magnetic field effects (MFEs) on photochemical reactions involving radical pairs is developed on the basis of the theory of the chemically induced dynamic spin polarization proposed by Pedersen and Freed. In theory, the stochastic Liouville equation (SLE), including the spin Hamiltonian, diffusion motions of the radical pair, chemical reactions, and spin relaxations, is solved by using the Laplace and the inverse Laplace transformation technique. In our practical approach, time evolutions of the MFEs are successfully calculated by applying the Miller-Guy method instead of the final value theorem to the inverse Laplace transformation process. Especially, the SLE calculations are completed in a short time when the radical pair dynamics can be described by the chemical kinetics consisting of diffusions, reactions and spin relaxations. The SLE analysis with a short calculation time enables one to examine the various parameter sets for fitting the experimental date. Our study demonstrates that simultaneous fitting of the time evolution of the MFE and of the magnetic field dependence of the MFE provides valuable information on the diffusion motions of the radical pairs in nano-structured materials such as micelles where the lifetimes of radical pairs are longer than hundreds of nano-seconds and the magnetic field dependence of the spin relaxations play a major role for the generation of the MFE.

Radical-triplet pair mechanism of electron spin polarization. Detailed theoretical treatment.

Specific features of net chemically induced dynamic electron spin polarization (CIDEP) P(n), generated in liquid-phase triplet-radical (TR) quenching, are analyzed in detail within the general model, which allows for fairly simple analysis of CIDEP both numerically and analytically. This model enables one to accurately treat nonadiabatic transitions between the terms of TR-pair spin Hamiltonian, resulting in CIDEP generation. The proposed theory predicts fairly simple analytical dependence of P(n) on parameters of the model. In particular, it is shown that within the wide region of parameters the P(n) dependence on the coefficient of relative TR diffusion D(r) is described by simple linear relation P(n)(-1)(D(r)) ≈ Q0 + q̅(n)D(r) (Q0 and q̅(n) are independent of D(r)). It is also demonstrated that obtained numerical and analytical results are very helpful for the analysis of experimental data, which is demonstrated by analyzing the experimental D(r)-dependence of P(n).

Orientation of electron spins in hybrid ferromagnet–semiconductor nanostructures

The spin orientation of electrons is studied in ferromagnet (FM)–semiconductor (SC) hybrid structures composed of a (Ga,Mn)As ferromagnetic layer, which is placed in the direct vicinity of a non‐magnetic SC quantum well (QW). It is shown that the polarization of carriers in the SC QW is achieved by spin‐dependent tunnelling into the magnetized ferromagnetic layer. This leads to dynamical spin polarization of the electrons, which can be directly observed by means of time‐resolved photoluminescence. We find that the electron spin polarization grows in time after excitation with an optical pulse and may reach values as large as 30%. The rate of spin‐dependent capture grows exponentially steeply with decreasing thickness of the spacer between ferromagnetic layer and QW, and it persists up to the Curie temperature of the (Ga,Mn)As layer. From time‐resolved pump–probe Kerr rotation data, we evaluate a value of only a few eV for the energy splitting between the electron Zeeman sublevels due to interaction with the ferromagnetic (Ga,Mn)As layer, indicating that the equilibrium spin polarization is negligible.Schematic presentation of electron spin orientation in a semiconductor quantum well (QW) under linearly polarized excitation due to spin‐dependent capture of electrons in the ferromagnetic layer (FM). The arrows in the FM box indicate the orientation of the magnetization . The effect is detected by appearance of a circular polarization degree of photoluminescence after pulsed optical excitation (right). The data are shown for a spacer thickness of 5 nm.

Spin-dependent phenomena in semiconductors in strong electric fields

We develop a theory of spin-dependent phenomena in the streaming regime characterized by ballistic acceleration of electrons in the moderate electric field until they achieve the optical phonon energy and abruptly emit phonons. It is shown that the Dyakonov–Perel spin relaxation is drastically modified in this regime, the current-induced spin orientation remarkably increases, reaches a high value ≈2% in the electric field ∼1 kV cm−1 and falls with further increase in the field. The spin polarization enhancement is caused by squeezing of the electron momentum distribution in the direction of drift. We also predict field-induced oscillatory dynamics of spin polarization of the photocarriers excited into the conduction band by a short circularly polarized optical pulse.

Transient Current in the Spin Blockade Region of a Double Quantum Dot

We investigate the slow transient buildup of leakage current influenced by dynamic spin polarization (DNP) in the spin blockade region of a double quantum dot (DQD). We focused on slightly off-resonant conditions, where the current is extremely small with nominally unpolarized nuclear spins. Leaving the system in such a situation causes the current to increase stepwise twice. Such transient current steps can be understood by considering that at the first step inhomogeneous nuclear spin polarization significantly increases while at the second step stable polarization is attained. We believe that the DNP produces a feedback effect.

Electronic and Exchange Coupling in a Cross-Conjugated D–B–A Biradical: Mechanistic Implications for Quantum Interference Effects

A combination of variable-temperature EPR spectroscopy, electronic absorption spectroscopy, and magnetic susceptibility measurements have been performed on Tp(Cum,Me)Zn(SQ-m-Ph-NN) (1-meta) a donor-bridge-acceptor (D-B-A) biradical that possesses a cross-conjugated meta-phenylene (m-Ph) bridge and a spin singlet ground state. The experimental results have been interpreted in the context of detailed bonding and excited-state computations in order to understand the excited-state electronic structure of 1-meta. The results reveal important excited-state contributions to the ground-state singlet-triplet splitting in this cross-conjugated D-B-A biradical that contribute to our understanding of electronic coupling in cross-conjugated molecules and specifically to quantum interference effects. In contrast to the conjugated isomer, which is a D-B-A biradical possessing a para-phenylene bridge, admixture of a single low-lying singly excited D → A type configuration into the cross-conjugated D-B-A biradical ground state makes a negligible contribution to the ground-state magnetic exchange interaction. Instead, an excited state formed by a Ph-NN (HOMO) → Ph-NN (LUMO) one-electron promotion configurationally mixes into the ground state of the m-Ph bridged D-A biradical. This results in a double (dynamic) spin polarization mechanism as the dominant contributor to ground-state antiferromagnetic exchange coupling between the SQ and NN spins. Thus, the dominant exchange mechanism is one that activates the bridge moiety via the spin polarization of a doubly occupied orbital with phenylene bridge character. This mechanism is important, as it enhances the electronic and magnetic communication in cross-conjugated D-B-A molecules where, in the case of 1-meta, the magnetic exchange in the active electron approximation is expected to be J ~ 0 cm(-1). We hypothesize that similar superexchange mechanisms are common to all cross-conjugated D-B-A triads. Our results are compared to quantum interference effects on electron transfer/transport when cross-conjugated molecules are employed as the bridge or molecular wire component and suggest a mechanism by which electronic coupling (and therefore electron transfer/transport) can be modulated.

Dynamical spin polarization in single-layer graphene

In the present work the dynamical behavior of π-electronic spin in graphene is investigated. The π-electron is under the influence of a normal uniform magnetic field and the Rashba spin–orbit coupling. Introducing a Casimir operator, we show that the governing Hamiltonian and, consequently, the time-evolution matrix is block-diagonal. We then proceed to calculate the temporal behavior of different spin components, when it is initially in-plane polarized. Our calculations show that the spin is dynamically polarized in a plane normal to the graphene sheet and follows the patterns of collapse-revivals. The dependence of amplitudes as well as the collapse-revivals’ periods on the external field and the Rashba spin–orbit coupling is also reported.

Ultrafast electron spin dynamics in bulk CdTe investigated by femtosecond pump-probe reflection spectroscopy

The transient spin polarization dynamics in bulk cadmium telluride (CdTe) at 70 K is investigated by the circularly polarized pump-probe reflection technique. A general expression is derived from the rate equations of a two-level system with small signal approximation to describe the light-helicity-dependent reflection spectrum. The initial degree of electron spin polarization in the excited state and the electron spin relaxation time in bulk CdTe at low temperature with different carrier density are analyzed according to this model. Our experimental results reveal that the Dʼyakonov–Perelʼ mechanism based on a fully microscopic kinetic spin Bloch equation (microscopic KSBE) approach dominates in the electron spin relaxation process in bulk CdTe crystal. • The transient spin polarization dynamics in bulk CdTe at 70 K is investigated. • A general expression is derived from the rate equations of a two-level system. • P s 0 and τ s in bulk CdTe are analyzed according to this model. • The DP mechanism based on KSBE dominates the electron spin relaxation process.

Devices with electrically tunable topological insulating phases

Solid-state topological insulating phases, characterized by spin-momentum locked edge modes, provide a powerful route for spin and charge manipulation in electronic devices. We propose to control charge and spin transport in the helical edge modes by electrically switching the topological insulating phase in a HgTe/CdTe double quantum well device. We introduce the concept of a topological field-effect-transistor and analyze possible applications to a spin battery, which also realize a set up for an all-electrical investigation of the spin-polarization dynamics in metallic islands.

Spin dynamics on photoexcited state of functionality π-radical via quantum-mixed state: Theoretical study of the spin polarized state generation using the mechanism via quantum-mixed state

A theoretical study of the generation of the dynamic spin polarization (DSP) in the phtotoexcited states utilizing the spin dynamics of the functionality π-radical is reported. The functionality π-radical can be regarded as a model compound of “trapped trion” in the molecular level. The spin dynamics from the quantum-mixed (QM) state to the strongly exchange-coupled (SC) pure quartet high-spin state of a functionality π-radical (A–D–R) was investigated theoretically by the stochastic Liouville equations (SLE). A possibility of the dynamic spin polarization utilizing the unique spin dynamics of the triad system is proposed based on the knowledge obtained by the numerical calculation of the spin dynamics and of the magnetic field dependence of the population transfer from the QM states to the SC states.

Insights into Ultrafast Demagnetization in Pseudogap Half-Metals

A series of experiments on a judicious selection of magnetic materials using ultrafast laser pulses demonstrate successful control of spin polarization and ultrafast spin dynamics in the materials through their electronic structures.

Studies on Electron Spin Relaxation in AlGaAs/GaAs Multi Quantum Wells

Time-resolved circularly polarized pump-probe spectroscopy is used to study the carrier density dependence of the electron spin polarization dynamics in AlGaAs/GaAs multi quantum wells at room temperature. Experimental results show that the spin relaxation time increases with the carrier density, which is in conformity with D'yakonov-Perel relaxation mechanism.

Kinetics of spin relaxation in quantum wires and channels: Boundary spin echo and formation of a persistent spin helix

In this paper we use a spin kinetic equation to study spin polarization dynamics in 1D wires and 2D channels. This approach is valid in both diffusive and ballistic spin transport regimes and, therefore, more general than the usual spin drift-diffusion equations. In particular, we demonstrate that in infinite 1D wires with Rashba spin-orbit interaction the exponential spin relaxation decay can be modulated by an oscillating function. In the case of spin relaxation in finite length 1D wires, it is shown that an initially homogeneous spin polarization spontaneously transforms into a persistent spin helix. An interesting sound waves echo-like behavior of initially localized spin polarization packet is found in finite length wires. We show that a propagating spin polarization profile reflects from a system boundary and returns back to its initial position similarly to the reflectance of sound waves from an obstacle. Green's function of spin kinetic equation is found for both finite and infinite 1D systems. Moreover, we demonstrate explicitly that the spin relaxation in 2D channels with Rashba and Dresselhaus spin-orbit interactions of equal strength occurs similarly to that in 1D wires of finite length. Finally, a simple transformation mapping 1D spin kinetic equation into the Klein-Gordon equation with an imaginary mass is found thus establishing an interesting connection between semiconductor spintronics and relativistic quantum mechanics.