Coherent Spin-flip Research Articles

Efficiency enhancement of the coherent electron spin-flip Raman scattering through thermal phonons in (In,Ga)As/GaAs quantum dots

The spin-flip Raman scattering efficiency of the resident electron is thermally enhanced in singly charged (In,Ga)As/GaAs quantum dots, for probing the $s$- or $p$-shell trions. The Raman shift, polarization characteristics, and spectral position of the resonant scattering profile are insensitive to the sample temperature up to 50 K. This indicates a thermally robust mechanism of the coherent electron spin-flip based on exchange interaction. The background of the scattering spectra, whose intensity increases also by about one order of magnitude with temperature, is associated with acoustic phonon scattering. We propose that acoustic phonons enhance the spin-flip probability of the resident electron with growing temperature. The coherent spin-flip Raman scattering is ultimately suppressed at temperatures, which are a few times lower than that required for thermal trion dissociation.

Complete Monitoring of Coherent and Incoherent Spin Flip Domains in the Recombination of Charge-Separated States of Donor-Iridium Complex-Acceptor Triads.

The spin chemistry of photoinduced charge-separated (CS) states of three triads comprising one or two triarylamine donors, a cyclometalated iridium complex sensitizer and a naphthalene diimide (NDI) acceptor, was investigated by transient absorption spectroscopy in the ns-μs time regime. Strong magnetic-field effects (MFE) were observed for two triads with a phenylene bridge between iridium complex sensitizer and NDI acceptor. For these triads, the lifetimes of the CS states increased from 0.6 μs at zero field to 40 μs at about 2 T. Substituting the phenylene by a biphenyl bridge causes the lifetime of the CS state at zero field to increase by more than 2 orders of magnitude (τ = 79 μs) and the MFE to disappear almost completely. The kinetic MFE was analyzed in the framework of a generalized Hayashi-Nagakura scheme describing coherent (S, T0 ↔ T±) as well as incoherent (S, T0 ⇌ T±) processes by a single rate constant k±. The magnetic-field dependence of k± of the triads with phenylene bridge spans 2 orders of magnitude and exhibits a biphasic behavior characterized by a superposition of two Lorentzians. This biphasic MFE is observed for the first time and is clearly attributable to the coherent (B < 10 mT) and incoherent (10 mT < B < 2 T) domains of spin motion induced by isotropic and anisotropic hyperfine coupling. The parameters of both domains are well understood in terms of the structural properties of the two triads, including the effect of electron hopping in the triad with two donor moieties. The kinetic model also accounts for the reduction of the MFE on reducing the rate constant of charge recombination in the triad with the biphenyl bridge.

A new spin on neutrino quantum kinetics

Recent studies have demonstrated that in anisotropic environments a coherent spin-flip term arises in the Quantum Kinetic Equations (QKEs) which govern the evolution of neutrino flavor and spin in hot and dense media. This term can mediate neutrino–antineutrino transformation for Majorana neutrinos and active-sterile transformation for Dirac neutrinos. We discuss the physical origin of the coherent spin-flip term and provide explicit expressions for the QKEs in a two-flavor model with spherical geometry. In this context, we demonstrate that coherent neutrino spin transformation depends on the absolute neutrino mass and Majorana phases.

Generation and manipulation of spin current via a hybrid four-terminal single-molecule junction

We present a new device which consists of a molecular quantum dot (MQD) attached to a normal-metal, two ferromagnetic (FM), and a superconducting leads. The spin-related Andreev reflection (AR) current and the spin-dependent single-particle tunneling current through the normal-metal terminal are obtained, and it is found that the spin current exhibits the transistor-like behavior. The joint effects of the coherent spin flip and the angle between magnetic moments of the two FM leads on the spin current are also studied, these results provide the possibility to manipulate the spin current with the system parameters.

Tunable spin current through a T-shaped double quantum dot molecule

We present a spin current generator based on a T-shaped double quantum dot (TDQD) molecule connected with two leads, and the coherent spin-flip effect is taken into account within the TDQD. The spin current from the right output terminal is obtained, more importantly, the properties of the spin current are investigated in detail, these results offer us a way to manipulate the spin current with the system parameters.

Andreev reflection current through a molecule quantum dot in the presence of the electron–phonon interaction and the spin-flip scattering

With the aid of the nonequilibrium Green's function and the Lang–Firsov canonical transformation, we investigate the joint effects of a phononic environment and the spin-flip scattering on the Andreev reflection (AR) in a ferromagnet/single-molecular quantum dot/superconductor (FM/MQD/SC) system. In the presence of the strong electron–phonon interaction (EPI), it is found that the EPI strongly suppresses the AR current (called the Franck–Condon blockade). When the coherent spin-flip (similar to a transverse magnetic field) is taken into account within the MQD, the AR current is significantly enhanced, furthermore, the spin-polarized AR current or even the pure spin-polarized AR current can be generated. By tuning the system parameters, the amplitude and direction of the AR current can be changed, this provides an efficient mechanism for controlling the AR process.

Modelling optical spin pumping of a single Mn atom in a CdTe quantum dot

We present a model for the incoherent spin dynamics of a Mn atom inside a CdTe quantum dot resonantly pumped by a laser light. The relevant quantum states of the system, 2S + 1 = 6 spin states in the optical ground state and the 24 = (2S + 1) * (2Se + 1) * (2Sh + 1) are obtained from a effective spin Hamiltonian which describes the basic features of photoluminescence. The model includes the dominant Ising exchange interactions between the spins of the hole, the Mn and the electron but neglects coherent spin-flip terms. The Mn spin dynamics is described with a master equation for the populations of the 30 levels for which dissipative spin-flip processes are permitted. The optical spin pumping observed experimentally is explained if the Mn spin relaxation time in with (T1X) exciton is much shorter than (T1G), the Mn spin relaxation time without exciton.

Ultrafast Coherent Electron Spin Flip in a Modulation-Doped CdTe Quantum Well

We report the experimental realization of coherent electron spin flip in a modulation-doped CdTe quantum well. Coherent spin rotation is realized with an off-resonant laser pulse, which induces a polarization-dependent optical Stark shift in the trion resonance. Complete electron spin flip is made possible by a laser pulse designed to avoid excessive excitations of nearby exciton resonances and minimizes detrimental many-body effects. These results demonstrate an effective approach for ultrafast optical spin control in a complex spin system.

Spin-polarized current and shot noise in the presence of spin flip in a quantum dot via nonequilibrium Green’s functions

Using nonequilibrium Green's functions we calculate the spin-polarized current and shot noise in a ferromagnet-quantum-dot-ferromagnet system. Both parallel (P) and antiparallel (AP) magnetic configurations are considered. Coulomb interaction and coherent spin flip (similar to a transverse magnetic field) are taken into account within the dot. We find that the interplay between Coulomb interaction and spin accumulation in the dot can result in a bias-dependent current polarization $\ensuremath{\wp}$. In particular, $\ensuremath{\wp}$ can be suppressed in the P alignment and enhanced in the AP case depending on the bias voltage. The coherent spin flip can also result in a switch of the current polarization from the emitter to the collector lead. Interestingly, for a particular set of parameters it is possible to have a polarized current in the collector and an unpolarized current in the emitter lead. We also found a suppression of the Fano factor to values well below 0.5.

Coulomb Promotion of Spin-Dependent Tunneling

We study transport of spin-polarized electrons through a magnetic single-electron transistor (SET) in the presence of an external magnetic field. Assuming the SET to have a nanometer size central island with a single-electron level we find that the interplay on the island between coherent spin-flip dynamics and Coulomb interactions can make the Coulomb correlations promote rather than suppress the current through the device. We find the criteria for this new phenomenon--Coulomb promotion of spin-dependent tunneling--to occur.

Unified Description of Fermi and Non-Fermi Liquid Behavior in a Conserving Slave Boson Approximation for Strongly Correlated Impurity Models

We show that the presence of Fermi or non-Fermi liquid behavior in the SU(N) x SU(M) Anderson impurity models may be read off the infrared threshold exponents governing the spinon and holon dynamics in a slave boson representation of these models. We construct a conserving T-matrix approximation which recovers the exact exponents with good numerical accuracy. Our approximation includes both coherent spin flip scattering and charge fluctuation processes. For the single-channel case the tendency to form bound states drastically modifies the low energy behavior. For the multi-channel case in the Kondo limit the bound state contributions are unimportant.

Fano resonances in the magnetoresistance of a quantum wire doped with magnetic impurities.

We calculate the scattering of conduction electrons from a magnetic impurity in an electron waveguide. A weak external magnetic field gives rise to Zeeman splitting of the waveguide modes. We find a Fano resonance in the conductance versus Fermi energy when the electrons from one Zeeman band scatter by a coherent spin-flip process into a bound state formed from the other Zeeman band. These conductance resonances should be experimentally observable in systems where the localized impurity moment is larger than the magnetic moment of the conduction electrons, such as Yb- or Er-doped GaAs quantum wires.

Dynamic nuclear spin polarization by coherent spin flip Raman scattering in InSb

Coherent spin flip Raman scattering has been applied to detect the Overhauser shift of the conduction spin resonance (CESR) in n-type InSb and to dynamically polarize the 115In, 121Sb and 123Sb nuclei. The optical method results in a very high CESR sensitivity allowing the investigation of the hyperfine interaction in narrow gap semiconductors with large g-factors not accessible by classical ESR methods.

Amorphous rare-earth alloys: neutron diffraction and correlation functions in the random anisotropy model

The authors study the longitudinal and transverse spin-correlation functions for an amorphous rare-earth alloy in the random anisotropy model within the Oguchi pair approximations. Both ferromagnetic and antiferromagnetic interactions are considered, as well as correlated and uncorrelated nearest-neighbour anisotropy axes. They relate their results to the elastic coherent spin-flip neutron scattering cross section, which should provide a test for the model.

A strong magnetic dipole excitation in 40Ca observed in high-resolution inelastic electron scattering and coherent spin-flip transitions due to ground-state correlations

An ususually strong magnetic dipole transition from the ground state to a state at Ex = 10.319 MeV in 40Ca has been observed in high-resolution inelastic electron scattering, contrary to expectations from the pure independent particle shell model. The transition strength, however, can be accounted for by coherent spin-flip transitions due to strong ground-state correlations.