Session U18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Spin Seebeck and Magneto-optics

Electric Field-Driven Coherent Spin Reorientation of Optically Generated Electron Spin Packets in InGaAs

Full electric-field control of spin orientations is one of the key tasks in semiconductor spintronics. We demonstrate that electric field pulses can be utilized for phase-coherent 2-pi spin rotation of optically generated electron spin packets in InGaAs epilayers using time-resolved Faraday rotation. Through spin-orbit interaction, the electric-field pulses act as local magnetic field pulses. By the temporal control of these pulses, we can turn on and off electron spin precession and thereby rotate the spin direction into arbitrary orientations in a 2-dimensional plane [1]. Moreover, we apply two subsequent electric field pulses of opposite field polarity to perform spin echo studies of the diffusing spin packet by reversing both the spin precession and the drift direction. In this spin-echo type spin drift experiment we find an unexpected spin rephasing, which is evident by a doubling of the spin dephasing time.\\[4pt] [1] S. Kuhlen et al., Phys. Rev. Lett. 109, 146603 (2012)   

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 such an elusive quantity as the electron-hole drag resistivity $\rho_{eh}$, which depends on the Coulomb interaction between the carriers. In this paper we develop the interacting drift-diffusion theory, from which $\rho_{eh}$ can be determined, given the measured mobility of an electron-hole grating. From this theory we predict a cross-over 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.   

Carrier and Spin Dynamics in InAsP Ternary Alloys

The recent rapid progress in the field of spintronics involves extensive measurements of carrier and spin relaxation dynamics in III-V semiconductors. In addition, as the switching rates in electronic and optoelectronic devices are pushed to higher frequencies, it is important to understand carrier dynamic phenomena in semiconductors on femtosecond time-scales. In this work, we employed time and polarization-resolved differential transmission measurements in near and mid-infrared, to probe carrier and spin relaxation times in several InAsP ternary alloys. Our results demonstrate the unique and complex dynamics in this material system that can be important for electronic and optoelectronic devices. We present our experimental observations and compare them with the observations in InAs and InP.   

Time resolved Magneto-Photoluminescence in $InAs_{x}P_{1-x}$ alloys

Recently, g-factor engineering has attracted much attention for potential applications in spintronics. In the case of $InAs_{x}P_{1-x}$ alloys, a wide range of g-factors, including g=0, can be achieved. In order to probe the band-structure and the dynamics of photo-excited carriers in $InAs_{x}P_{1-x}$ epitaxial films with x=0.13, 0.4, we measured NIR absorption spectra at 4K and 300K, as well as magneto- photoluminescence spectra in both the time and frequency domain for magnetic fields in the range of 0-15T and temperatures in the range of 4-90K. From the temporal measurements, we observed strong tunability in the relaxation dynamics as a function of excitation wavelength. We present these experimental observations and compare them with theoretical calculations.   

Probing of the Nature of Carrier Recombination in GaInNAs epilayers using Optical Spin Injection

Optical pumping experiments have been performed on as-grown and p-type MBE grown GaInNAs epilayers. The PL peak of the nominally undoped as-grown sample exhibits the characteristic S-shaped dependence of dilute nitride material for T \textless\ 60 K [1]. This is associated with carrier recombination via localized states at low temperatures. The reflectance spectra on the other hand map the band-to-band free carrier transition, displaying a Varshni-type behavior. In the p-type material the S-dependence of the PL disappears, and the PL peak coincides with the reflectance spectrum at all temperatures. This indicates band-to-band, rather than localized exciton recombination, in the p-type GaInNAs at all temperatures. This picture was verified by optical pumping experiments. In the undoped sample a large degree of circular polarization was evident only at T \textgreater\ 60 K: below 60 K the polarization is small, and coincident with the reflectance peak. In the p-type samples, on the other hand, non-zero circular polarization, whose maximum matches the peak PL energy, was evident at all temperatures.\\[4pt] [1] A. Polimeni \textit{et al}. Phys. Rev. B. 63, 195320 (2001)   

Dynamic spin Seebeck coefficient and thermo-spin Hall conductivity in systems with Rashba and Dresselhaus spin-orbit coupling

The generation of spin currents by thermal gradients is a central issue of spin caloritronics. In addition to the recently observed spin Seebeck effect, a transverse thermoelectric effect has been proposed. This is the generation of a spin Hall current by a temperature gradient in a two-dimensional electron gas (2DEG) with Rashba spin-orbit interaction (SOI). We calculate the spin Seebeck coefficient and the thermo-spin Hall conductivity tensor of the spin current response induced by a frequency dependent temperature gradient in a 2DEG with Rashba and Dresselhaus SOI. We consider quantum wells grown in the main crystallographic directions. The spin splitting caused by SOI opens the possibility of resonant effects due to transitions between the spin-split subbands in response to alternating thermoelectric fields and temperature gradients in the THz regime. The spin current response shows characteristic spectral features in notable contrast to the pure Rashba coupling case. Such behavior is caused by the reduced symmetry of the momentum space available for transitions and the presence of critical points. This anisotropic dynamic response could be useful for spin manipulation via thermal means.   

Phonon Drag in InSb: Experiment

A thermoelectric power is reported in a thermocouple in which both arms are made of the same material (n-type InSb) with the same electron concentration, but the phonons have different mean free paths at cryogenic temperatures. This experiment, inspired by [1], isolates the phonon-drag contribution to the thermopower from the diffusion thermopower. The experiment decouples the behavior of the subthermal phonons that drag the electrons, and the thermal phonons that carry most heat. We add data on the contributions of both to the thermal conductivity. This sheds new light on the details of the physical mechanism behind the giant spin-Seebeck effect (GSSE) recently observed [2] on the same material. The GSSE signal was attributed to a combination of electron-phonon drag that pushes the electrons, which are spin-polarized by Zeeman splitting, far from thermal equilibrium, and strong spin-orbit interactions that make the Zeeman splitting sensitive to the electron momentum. Furthermore, we may have found experimental clues about the nature of the phonon force [3]. 1. T. H. Geballe and G. W. Hull, Conference de physique des basses temperatures, p 460, Paris, 1955 2. C.M. Jaworski et al. Nature 487, 210 (2012) 3. S. E. Barnes and S. Maekawa, Phys. Rev. Lett. 98 246601 (2007)   

Phonon Drag in InSb: Theory and ``spin''-motive force

The phonon number operator $\hat n \to \sin^2 \frac{\theta}{2}$ defines the Euler angle $\theta$ and with the phase $\phi$ this maps to a precessing spin. Defined are a ``spin" Berry phase and a ``spin''-motive force (smf)[1]. Unlike an emf, an smf can act upon neutral phonons. Tradition[2] has sub-thermal phonons as central to the thermopower of semi-conductors. The momentum given to these phonons, by the temperature gradient, is transferred to the electrons by ``drag'' where it cancels a Seebeck effect electric field $\vec E$. Here, for InSb at low temperatures, thermal phonons actually relax momentum via boundary and umklapp scattering and energy conservation involves sub-thermal phonons, created by anharmonic effects, with a frequency $\hbar \omega_{\vec q} \sim k_B (dT/dx) \ell$ where $\ell$ is the phonon mean-free-path (mfp). The resulting smf acting upon the thermal phonons produces a ``spin'' voltage $\sim (k_B/e) \Delta T \sim 100\mu$V/K. Via the electron-phonon interaction, the smf, multiplied by the ratio $\ell_{ep}/\ell$, where $\ell_{ep}$ is the electron-phonon mfp, are detected, but not created by the few electrons in our InSb samples. [1] S. E. Barnes and S. Maekawa, Phys. Rev. Lett. {\bf 98}, 246601 (2007) [2] C. Herring, Phys. Rev. {\bf 95}, 954 (1954).   

Planar Nernst effect and Spin dependent Seebeck effect on Py/Ag thin films

We are reporting a systematic study of planar Nernst effect (PNE) and Spin dependent Seebeck effect (SDSE) measurements and their relation to the Anisotropic Magneto Resistance (AMR) on Py thin films grown on SiOx substrates by magnetron sputtering. A 30nm thick Py film was followed by a 15nm of Ag cross electrodes. An in-situ mask exchanging system was allowed the Py and Ag to grow without breaking the vacuum. The sample was placed on top of two thermal baths which were independently controlled by a PID controller. A constant temperature gradient of 15K/cm was applied along the sample and the resultant voltages across the Ag electrodes were measured by nanovoltmeters as the field was swept. In measuring AMR no thermal gradient was applied, and a constant current was applied using a function generator. Both PNE and SDSE showed an AMR like field dependence and angular dependence. SDSE showed a Cos$^{2}$ ($\theta$) angular dependence and PNE showed a Sin (2$\theta$) angular dependence. AMR showed the same angular dependence along the Py film and across the Py film respectively. This suggests both PNE and SDSE behave similar to the AMR in thin films.   

Theory of thermal spin-charge coupling in electronic systems

The interplay between spin transport and thermoelectricity offers several novel ways of generating, manipulating, and detecting nonequilibrium spin in a wide range of materials. Here, we formulate a phenomenological model in the spirit of the standard model of electrical spin injection to describe the electronic mechanism coupling charge, spin, and heat transport and employ the model to analyze several different geometries containing ferromagnetic (F) and nonmagnetic (N) regions: F, F/N, and F/N/F junctions which are subject to thermal gradients (i.e., the spin-dependent Seebeck effect). Furthermore, we study the Peltier and spin-dependent Peltier effects in F/N and F/N/F junctions.