Spin diffusion in semiconductors

Diffusion mobility increases linearly on liquid binodals above triple point

Self-diffusion in fluids has been thoroughly studied numerically, but even for simple liquids just a few scaling relationships are known. Relations between diffusion, excitation spectra, and character of the interparticle interactions remain poorly understood. Here, we show that diffusion mobility of particles in simple fluids increases linearly on the liquid branch of the liquid-gas binodal, from the triple point almost up to the critical point. With molecular dynamics simulations, we considered bulk systems of particles interacting via a generalised Lennard-Jones potential, as well as ethane. Using a two-oscillator model for the analysis of excitations, we observed that the mobility (inverse diffusion) coefficient on the liquid-gas binodal increases linearly above the triple point until the dispersion of high-frequency spectra has a solid-like (oscillating) shape. In terms of a separate mode analysis (of longitudinal and transverse modes), this corresponds to crossed modes in the intermediate range of wavenumbers q, between the hydrodynamic regime (small q) and the regime of individual particle motion (large q). The results should be interesting for a broad community in physics and chemistry of fluids, since self-diffusion is among the most fundamental transport phenomena, important for prospective chemical technologies, micro-, nanofluidics, and biotechnologies.

Optical Detection of Long Electron Spin Transport Lengths in a Monolayer Semiconductor

Using a spatially resolved optical pump-probe experiment, we measure the lateral transport of spin-valley polarized electrons over very long distances (tens of micrometers) in a single WSe_{2} monolayer. By locally pumping the Fermi sea of 2D electrons to a high degree of spin-valley polarization (up to 75%) using circularly polarized light, the lateral diffusion of the electron polarization can be mapped out via the photoluminescence induced by a spatially separated and linearly polarized probe laser. Up to 25% spin-valley polarization is observed at pump-probe separations up to 20  μm. Characteristic spin-valley diffusion lengths of 18±3  μm are revealed at low temperatures. The dependence on temperature, pump helicity, pump intensity, and electron density highlight the key roles played by spin relaxation time and pumping efficiency on polarized electron transport in monolayer semiconductors possessing spin-valley locking.

Unexpected Outstanding Room Temperature Spin Transport Verified in Organic-Inorganic Hybrid Perovskite Film

Fundamental understanding on the spin transport properties of semiconducting organic-inorganic hybrid perovskites (OIHP) is of great importance for advancing their applications for spin-optoelectronic devices. Herein, the study of spin-pumping induced inverse spin Hall effect in Ni₈₀Fe₂₀(Py)/CH₃NH₃PbCl_3-xI_x/Pt trilayers with different OIHP spacer thicknesses concludes the spin diffusion length in CH₃NH₃PbCl_3-xI_x as large as 61 ± 7 nm at room temperature. In addition, spin-valves with a structure of Ni₈₀Fe₂₀(Py)/CH₃NH₃PbCl_3-xI_x/AlO_x/Co was fabricated as well. Using a ∼160 nm-thick CH₃NH₃PbCl_3-xI_x spacer, the present spin valve exhibits a positive magnetoresistance (MR) of 0.57% at 10 K. Thus, the present spin related results demonstrate that electrical spin-polarized carrier injection, transport, and detection, which are essential in spintronic devices, can be successfully established in OIHP films. The outstanding spin transport in the present CH₃NH₃PbCl_3-xI_x should be owing to its highly out-of-plane oriented crystalline texture and Rashba spin splitting at domain boundaries between crystallographic orientations. These results demonstrate OIHP as very attractive materials for spintronics.

First Principles Study on the Effect of Pressure on the Structure, Elasticity, and Magnetic Properties of Cubic GaFe(CN)6 Prussian Blue Analogue

The structure, elasticity, and magnetic properties of Prussian blue analogue GaFe(CN)6 under external pressure ranges from 0 to 40 GPa were studied by first principles calculations. In the range of pressure from 0 to 35 GPa, GaFe(CN)6 not only has the half-metallic characteristics of 100% spin polarization, but also has stable mechanical properties. The external pressure has no obvious effect on the crystal structure and anisotropy of GaFe(CN)6, but when the pressure exceeds 35 GPa, the half-metallicity of GaFe(CN)6 disappears, the mechanical properties are no longer stable, and total magnetic moments per formula unit are no longer integer values.

Distinguishing spin relaxation mechanisms in organic semiconductors

A theory is introduced for spin relaxation and spin diffusion of hopping carriers in a disordered system. For disorder described by a distribution of waiting times between hops (e.g., from multiple traps, site-energy disorder, and/or positional disorder) the dominant spin relaxation mechanisms in organic semiconductors (hyperfine, hopping-induced spin-orbit, and intrasite spin relaxation) each produce different characteristic spin relaxation and spin diffusion dependences on temperature. The resulting unique experimental signatures predicted by the theory for each mechanism in organic semiconductors provide a prescription for determining the dominant spin relaxation mechanism.

Interacting drift-diffusion theory for photoexcited electron-hole gratings in semiconductor quantum wells

Phase-resolved transient grating spectroscopy in semiconductor quantum wells has been shown to be a powerful technique for measuring the electron-hole drag resistivity ρ(eh), which depends on the Coulomb interaction between the carriers. In this Letter we develop the interacting drift-diffusion theory, from which ρ(eh) can be determined, given the measured mobility of an electron-hole grating. From this theory we predict a crossover from a high-excitation-density regime, in which the mobility has the "normal" positive value, to a low-density regime, in which Coulomb drag dominates and the mobility becomes negative. At the crossover point, the mobility of the grating vanishes.

Spin-flip induced magnetoresistance in positionally disordered organic solids

A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Faster spin-flip transitions open up "spin-blocked" pathways to become viable conduction channels and hence produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with previous measurements, including the sensitive dependence of the magnetic-field dependence of the magnetoresistance on the ratio of the carrier hopping time to the hyperfine-induced carrier spin precession time. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory.

Electrical measurement of the direct spin hall effect in Fe/InxGa(1-x)As heterostructures

We report on an all-electrical measurement of the spin Hall effect in epitaxial Fe/InxGa(1-x)As heterostructures with n-type (Si) channel doping and highly doped Schottky tunnel barriers. A transverse spin current generated by an ordinary charge current flowing in the InxGa(1-x)As is detected by measuring the spin accumulation at the edges of the channel. The spin accumulation is identified through the observation of a Hanle effect in the voltage measured by pairs of ferromagnetic Hall contacts. We investigate the bias and temperature dependence of the resulting Hanle signal and determine the skew and side-jump contributions to the total spin Hall conductivity.

Optical measurement and control of spin diffusion in n-doped GaAs quantum wells

Transient spin gratings are used to study spin diffusion in lightly n-doped GaAs quantum wells at low temperatures. The spin grating is shown to form in the excess electrons from doping, providing spin relaxation and transport properties of the carriers most relevant to spintronic applications. We demonstrate that spin diffusion of the these carriers is accelerated by increasing the density or energy of the optically excited carriers. These results can be used to better understand and even control spin transport in experiments that optically excite spin-polarized carriers.

Electrical detection of spin accumulation at a ferromagnet-semiconductor interface

We show that the accumulation of spin-polarized electrons at a forward-biased Schottky tunnel barrier between Fe and -GaAs can be detected electrically. The spin accumulation leads to an additional voltage drop across the barrier that is suppressed by a small transverse magnetic field, which depolarizes the spins in the semiconductor. The dependence of the electrical accumulation signal on magnetic field, bias current, and temperature is in good agreement with the predictions of a drift-diffusion model for spin-polarized transport.

Electrical spin injection from an n-type ferromagnetic semiconductor into a III-V device heterostructure

The use of carrier spin in semiconductors is a promising route towards new device functionality and performance. Ferromagnetic semiconductors (FMSs) are promising materials in this effort. An n-type FMS that can be epitaxially grown on a common device substrate is especially attractive. Here, we report electrical injection of spin-polarized electrons from an n-type FMS, CdCr(2)Se(4), into an AlGaAs/GaAs-based light-emitting diode structure. An analysis of the electroluminescence polarization based on quantum selection rules provides a direct measure of the sign and magnitude of the injected electron spin polarization. The sign reflects minority rather than majority spin injection, consistent with our density-functional-theory calculations of the CdCr(2)Se(4) conduction-band edge. This approach confirms the exchange-split band structure and spin-polarized carrier population of an FMS, and demonstrates a litmus test for these FMS hallmarks that discriminates against spurious contributions from magnetic precipitates.

Dynamic ferromagnetic proximity effect in photoexcited semiconductors

The spin dynamics of photoexcited carriers in semiconductors in contact with a ferromagnet is treated theoretically and compared with time-dependent Faraday rotation experiments. The long-time response of the system is found to be governed by the first tens of picoseconds in which the excited plasma interacts strongly with the intrinsic interface between the semiconductor and the ferromagnet in spite of the existence of a Schottky barrier in equilibrium.

Coherent spin manipulation without magnetic fields in strained semiconductors

A consequence of relativity is that in the presence of an electric field, the spin and momentum states of an electron can be coupled; this is known as spin-orbit coupling. Such an interaction opens a pathway to the manipulation of electron spins within non-magnetic semiconductors, in the absence of applied magnetic fields. This interaction has implications for spin-based quantum information processing and spintronics, forming the basis of various device proposals. For example, the concept of spin field-effect transistors is based on spin precession due to the spin-orbit coupling. Most studies, however, focus on non-spin-selective electrical measurements in quantum structures. Here we report the direct measurement of coherent electron spin precession in zero magnetic field as the electrons drift in response to an applied electric field. We use ultrafast optical techniques to spatiotemporally resolve spin dynamics in strained gallium arsenide and indium gallium arsenide epitaxial layers. Unexpectedly, we observe spin splitting in these simple structures arising from strain in the semiconductor films. The observed effect provides a flexible approach for enabling electrical control over electron spins using strain engineering. Moreover, we exploit this strain-induced field to electrically drive spin resonance with Rabi frequencies of up to approximately 30 MHz.

Spin diffusion in double-exchange manganites

The theoretical study of spin diffusion in double-exchange magnets by means of dynamical mean-field theory is presented. We demonstrate that the spin-diffusion coefficient becomes independent of the Hund's coupling J(H) in the range of parameters J(H)S>>W>>T, W being the bandwidth, relevant to colossal magnetoresistive manganites in the metallic part of their phase diagram. Our study reveals a close correspondence as well as some counterintuitive differences between the results on Bethe and hypercubic lattices. Our results are in accord with neutron-scattering data and with previous theoretical work for high temperatures.

Spintronics: a spin-based electronics vision for the future

This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.

Gateable suppression of spin relaxation in semiconductors

The decay of spin memory in a 2D electron gas is found to be suppressed close to the metal-insulator transition. By dynamically probing the device using ultrafast spectroscopy, relaxation of optically excited electron spin is directly measured as a function of the carrier density. Motional narrowing favors spin preservation in the maximally scattered but nonlocalized electronic states. This implies that the spin-relaxation rate can be both tuned in situ and specifically engineered in appropriate device geometries.