Propagating coherent acoustic phonon wave packets inInxMn1−xAs∕GaSb

Abstract

We observe pronounced oscillations in the differential reflectivity of a ferromagnetic InMnAs/GaSb heterostructure using two-color pump-probe spectroscopy. Although originally thought to be associated with the ferromagnetism, our studies show that the oscillations instead result from changes in the position and frequency-dependent dielectric function due to the generation of coherent acoustic phonons in the ferromagnetic InMnAs layer and their subsequent propagation into the GaSb. Our theory accurately predicts the experimentally measured oscillation period and decay time as a function of probe wavelength.Recently, there has been much interest in (III,Mn)V dilute magnetic semiconductors (DMS) with carriermediated ferromagnetism, a promising system for the realization of future semiconductor spintronic devices capable of performing information processing, data storage, and communication functions simultaneously [1,2,3]. InMnAs is the prototypical III-V DMS, being the first DMS to exhibit ferromagnetism, and the first DMS in which cyclotron resonance was observed, evidence that at least some of the carriers are itinerant [4,5,6].Despite the fact that time-domain studies of the dynamic aspects of (III,Mn)Vs are more informative than static magnetization or electrical transport measurements, relatively few time-dependent studies have been attempted. In this paper, we report on our timedependent femtosecond transient reflectivity measurements on an InMnAs/GaSb heterostructure using twocolor pump-probe spectroscopy. In addition to changes in the reflectivity associated with utlrashort carrier lifetimes (∼ 2 ps) and multi-level carrier decay dynamics which we attribute to a large density of bound states and a high concentration of Mn p-type dopants, we observe pronounced oscillations in the differential reflectivity signal. While originally the oscillations were believed to originate from the ferromagnetism (since they were not observed in similiar systems without Mn doping), a systematic study shows that they instead result from * Permanent address: Graduate School of Science and Technology, Chiba University, Chiba, Japan † Author to whom correspondence should be addressed; Electronic address: kono@rice.edu ‡ Present address: Department of Applied Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JAPAN the generation of coherent phonons in the InMnAs layer which then propagate into the GaSb. The period and the characteristic decay time of the transient reflectivity oscillations are consistent with a propagating coherent acoustic phonon wavepacket model [7].While femtosecond spectroscopy has previously been used for generating and detecting coherent phonons in many materials (e.g, optical phonons in bulk semiconductors, metals (Bi, Sb) and superconductors; acoustic phonons in InGaN/GaN-based semiconductor heterostructures [7,8,9,10,11,12,13]), only recently has there been interest in the application of this technique to probing coherent excitations (e.g., phonons and magnons) in strongly-correlated ...

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