Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A15: Focus Session: Spins in Semiconductors - Spin Dynamics |
Hide Abstracts |
Sponsoring Units: DMP GMAG FIAP Chair: Craig Pryor, University of Iowa Room: D171 |
Monday, March 21, 2011 8:00AM - 8:36AM |
A15.00001: ``Listening" to the spin noise of electrons and holes in semiconductor quantum dots Invited Speaker: The coherence and dynamical properties of spins in semiconductors are usually studied with powerful techniques based on optical pump-probe or spin resonance methods. Such methods are necessarily perturbative, in that one measures the (dissipative) response of the spins resulting from an external drive or excitation field (\emph{eg}, free-induction decays). However, in accord with the fluctuation-dissipation theorem, the intrinsic fluctuations of the spin system - if experimentally measurable - can also reveal the same dynamical properties (such as $g$-factors and decoherence times) without ever perturbing the spin ensemble from thermal equilibrium. This talk describes how we measure electron and hole spin dynamics in semiconductors by passively ``listening'' to these small spin noise signals [1]. We employ a spin noise spectrometer based on a sensitive optical Faraday rotation magnetometer that is coupled to a digitizer and field-programmable gate array (FPGA), to acquire noise spectra from 0-1 GHz in real time with picoradian/root-Hz sensitivity. In doped (In,Ga)As/GaAs quantum dots, both electron and hole spin fluctuations generate distinct noise peaks whose shift and broadening with magnetic field directly reveal their $g$-factors and dephasing rates. A large, energy-dependent anisotropy of in-plane hole $g$-factors is clearly exposed, reflecting systematic variations in the average confinement potential. In contrast with conventional pump-probe studies, noise signals increase as the probed volume shrinks, suggesting possible routes towards non-perturbative, sourceless magnetic resonance of few-spin systems.\\[4pt][1] PRL \textbf{104}, 036601 (2010); PRB \textbf{79}, 035208 (2009). [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A15.00002: Understanding the modulation frequency dependence of continuous wave optically/electrically detected magnetic resonance Sang-Yun Lee, Seoyoung Paik, Dane R. McCamey, Christoph Boehme Continuous wave optically and electrically detected magnetic resonance spectroscopy (cwODMR/cwEDMR) are powerful methods which allow the investigation of the microscopic nature of paramagnetic states involved in spin-dependent transitions, like recombination and transport. Although experimentally similar to conventional electron spin resonance (ESR), there exist limitations when applying conventional theoretical models originally developed for ESR to explain how the observables (luminescence and electric current) of cwODMR and cwEDMR behave under the influences of various experimental parameters. Here we present closed-form solutions for the modulation frequency dependence of cwODMR and cw EDMR based on an intermediate pair recombination model [1] and discuss ambiguities which arise when attempting to distinguishing the dominant spin-dependent processes underlying experimental data. These include: 1) a large number of quantitatively different models cannot be differentiated, 2) signs of signal are determined not only by recombination, but also by other processes like dissociation, intersystem-crossing, pair generation, and even an experimental parameter, modulation frequency. \\[0pt] [1] D. Kaplan, I. Solomon, and N. Mott, Journal de Physique Lettres 39, 51 (1978). [Preview Abstract] |
Monday, March 21, 2011 8:48AM - 9:00AM |
A15.00003: Observation of Long Spin Coherence Times in CdSe/CdS Colloidal Nanostructures K.J. van Schooten, J. Huang, D.V. Talapin, W.J. Baker, C. Boehme, J.M. Lupton Spin states in colloidal quantum dots have been intensively studied over the past decade, usually through various all optical time-resolved pump-probe techniques of excitonic fine-structure. Coherence times measured in this manner, which are usually limited to T$_{2}^{\ast }$, have ranged in order from 1ps to 1ns, thus limiting the potential to use these types of quantum dots in quantum memory schemes. Here, we describe coherence times (T$_{2})$ on the order of 100ns for optical excitations in ensembles of CdSe/CdS heterostructure colloidal nanocrystals at 10K. In contrast to the more conventional pump-probe techniques, we employ a time-correlated optically-detected magnetic resonance scheme to measure the true T$_{2}$ of optically generated excitations via a Hahn echo sequence. A strong temperature dependence of the spin-dependent luminescence rate is observed, demonstrating that longitudinal spin-relaxation in these strongly spin-orbit coupled semiconductors is thermally activated. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A15.00004: Universal scheme for optically-detected T$_{1}$ measurements John Colton, Ken Clark, Tyler Park, Dallas Smith, Scott Thalman A two laser pump-probe scheme for measuring spin flip (T$_{1})$ lifetimes in GaAs-related materials has been developed. The pump and probe beams are switched on and off electronically, with pulse widths and delays controlled by a two-channel pulse generator. The effect of the pump beam on the probe beam is seen by monitoring the Kerr rotation of the reflected probe beam. The technique has broad applicability, and should work for any material in which Kerr rotation spin measurement can be employed. The authors have applied this technique to a lightly-doped GaAs layer (n=3E14 cm$^{-3})$, to compare it with two other samples (at slightly higher\footnote{Colton et al., Phys. Rev. B \textbf{75}, 205201 (2007).} and slightly lower\footnote{Fu, et al., Phys. Rev. B \textbf{74}, 121304(R) (2006).} doping levels) whose T$_{1}$ dependence on field had substantial qualitative and quantitative differences from each other. Results for this sample will be presented. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A15.00005: Ultrafast Measurement of Critical Slowing Down of Hole-Spin Relaxation in Ferromagnetic GaMnAs Aaron Patz, Tianqi Li, Ilias Perakis, Xinyu Liu, Jacek Furdyna, Jigang Wang We have studied ultrafast photoinduced hole spin relaxation in GaMnAs via degenerate ultrafast magneto-optical Kerr spectroscopy. Near-infrared pump pulses strongly excite the sample, and probe pulses at the same photon energy reveal subpicosecond demagnetization accompanied by energy and spin relaxation of holes manifesting themselves as a fast ($\sim $200fs) and a slow (ps) recovery of transient MOKE signals. By carefully analyzing the temporal profiles at different temperatures, we are able to isolate femtosecond hole spin relaxation processes, which are subject to a critical slowing down near the critical temperature of 77K. These results demonstrate a new spectroscopy tool to study the highly elusive hole spin relaxation processes in heavily-doped, correlated spin systems, and have important implications for future applications of these materials in spintronics and magnetic-photonic-electronic multifunctional devices. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A15.00006: A Model Study of Photomagnetization in Diluted Magnetic Semiconductors S.N. Behera, S.M. Bose, J.T. Schick In the context of application to spintronics, photon induced magnetization or photomagnetization (PM) of diluted magnetic semiconductors (DMS) like Hg$_{1-x}$Mn$_{x}$Te [1] has been the subject of many recent investigations. We present results of a model calculation of the dependence of the PM on the photon power in a DMS for different temperatures and different magnetic impurity concentrations. The model which includes kinetic energies of the charge carriers created by the incident light, the attractive Coulomb interaction between the electrons and the holes treated in the mean field approximation, the coupling of the photon with the exciton density, and the magnetic interaction between the spins of the charge carriers and the magnetic moments of the magnetic impurity atoms in the semiconductor is solved exactly using the equation of motion of the Green's functions method. Expressions for the densities of spin up and spin down charge carriers, and their magnetization and that of the magnetic impurities obtained in the form of a set of coupled equations are solved self consistently to determine the PM. Interestingly there is a temperature dependent threshold in photon power for the appearance of the PM. A detailed study of the dependence of the PM on different parameters will be presented. \\[4pt] [1] H. Krenn et al., PRL \textbf{55}, 1510 (1985). [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A15.00007: Dynamic Magnetic Polarization in Semi-magnetic II-VI Quantum Dots via Electrical/Optical Carrier Injection Bahman Roustai, Ramin Abolfath, Thomas Brabec, Pawel Hawrylak Theory of Dynamic Magnetic Polarization (DMP), the enhancement of collective spin polarization of magnetic impurities (MI) in semi-magnetic II-VI quantum dots is presented. DMP, known for nuclear spins, is the result of the transfer of electron's spin to MI's spin polarization as a function of time. DMP has been recently observed in various opto-electronic experiments [1]. We study the interplay of optically/electrically pumped electrons from the leads to the quantum dot and their effects on DMP in the dot. The interaction of MI's with electron spin and orbital degrees of freedom is modeled. In the weak coupling (t$>>$J), the DMP is the result of electron tunneling followed by the exchange interaction J with MI. In the strong coupling (J$>>$t) the electrons in the lead and the magnetic impurity in the dot form a Kondo-type bound state resulting in even stronger DMP.\\[4pt] [1] Ochsenbein et al. Nature Nanotechnology 4, 681 (2009). [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A15.00008: Carrier spin polarization and magneto-polaron formation in colloidal quantum dots Savas Delikanli, Andreas Russ, Lars Schweidenback, Sungjin Kim, Joseph Murphy, Alexander Cartwright, Athos Petrou, Hao Zeng We present a magneto-optical study of magnetic polarons in Mn-doped II-VI colloidal quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. The long lifetime of the excitons allows the observation of the complete formation process of the magneto polaron. The spin alignment occurs at the time scale of hundreds of ps. The extra energy is dissipated through spin lattice interactions, during the next hundreds of nanoseconds. The dependence of these effects on quantum confinement are studied in different systems. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A15.00009: Non-equilibrium Magnetic Ordering in Quantum Dots James Pientka, Rafal Oszwaldowski, Igor Zutic, Jong Han, Andre Petukhov We study semiconductor Quantum Dots (QDs) with magnetic impurities. The magnetism in these systems can be controlled in ways not possible in bulk semiconductors [1]. Robust magnetic effects have been observed recently in both colloidal and self-assembled QDs [2,3]. Here, we develop a rate-equations approach to describe the carrier-mediated magnetic ordering in QDs. In this situation, the magnetic properties are different from the steady-state scenario, due to different carrier spin density, which affects the magnetic-impurity alignment. We focus on a type-II QD band profile, where the electrons reside in the barrier, while the holes are localized in the QD interior, which contains the magnetic impurities. Supported by DOE-BES, US ONR, AFOSR, NSF-DMR and NSF-ECCS CAREER. \\[4pt] [1] R. M. Abolfath, A. G. Petukhov, and I. Zutic, Phys. Rev. Lett. \textbf{101}, 207202 (2008); I. Zutic and A. G. Petukhov, Nature Mater.\textbf{4}, 623 (2009).\\[0pt] [2] R. Beaulac et al., Science \textbf{325}, 973 (2009).\\[0pt] [3] I. R. Sellers, R. Oszwaldowski, et al., Phys. Rev. B \textbf{82}, 195320 (2010). [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A15.00010: Magneto-optical studies of magnetic polarons in type-II (Zn,Mn)Te/ZnSe quantum dots Biplob Barman, Andreas Russ, Lars Schweidenback, Joseph Murphy, Rafal Oszwaldowski, Ian Sellers, Athos Petrou, Igor Zutic, Bruce McCombe, Alexander Cartwright, Andre Petukhov, Wu-Ching Chou, Wen Chung Fan We have recorded time-resolved emission spectra from a series of MBE grown (Zn,Mn)Te/ZnSe quantum dots (QDs) at 7 K in the 0 - 4 tesla magnetic field range. The photoluminescence (PL) spectra were analyzed into their $\sigma $+ and $\sigma -$ circularly-polarized components. The holes in this type-II system are confined in the (Zn,Mn)Te QDs, while the electrons reside in the surrounding ZnSe matrix. The PL intensity, peak energy, and circular polarization were recorded as a function of time and magnetic field. These studies show evidence of exchange coupling between the holes and Mn spins in the (Zn,Mn)Te QDs, which leads to the formation of magnetic polarons. The time scale of polaron formation is shorter than the recombination time in this type-II system. We discuss our results within the framework of a model that describes the magnetic polaron formation in this system. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A15.00011: Prediction of extremely long mobile electron spin lifetimes at room temperature in low-Z wurtzite semiconductor quantum wells Nicholas Harmon, William Putikka, Robert Joynt Many proposed spintronics devices require mobile electrons at room temperature with very long spin lifetimes. One route to achieving this is to use quantum wells with tunable spin-orbit (SO) parameters. Research has focused on materials with the zincblende structure such as GaAs, which however, do not have long spin lifetimes at room temperature. We show that low-Z materials with the wurtzite structure are much better suited for spintronics applications. Their hexagonal symmetry implies that SO couplings can be completely canceled over a very wide range of electron momenta at zero temperature. Low-Z materials possess smaller SO couplings resulting in long spin lifetimes at room temperature. This leads to predictions of spin lifetimes exceeding 2 ms at helium temperatures in wurtzite AlN and, most relevant to spintronic devices, spin lifetimes up to 0.5 $\mu s$ are predicted for tuned AlN wells at room temperature. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A15.00012: Conductance signatures of spin correlations and quantum phase transitions in parallel quantum dots Arturo Wong, William Lane, Luis Dias, Kevin Ingersent, Nancy Sandler, Sergio Ulloa Semiconductor quantum dots provide a highly controllable environment to study strongly correlated phenomena and quantum phase transitions (QPT). A parallel double-quantum-dot system, in which dot 1 is in the Kondo regime and dot 2 behaves as a non-interacting resonant level, shows a QPT separating Kondo-screened and local-moment phases [1]. In this work, we use the numerical renormalization-group approach to explore the effect of a nonzero Coulomb interaction $U_{2}$ in dot 2. When dot-2 level is fixed at the Fermi energy, a critical value of $U_{2}$ separates local-moment and Kondo-screened phases. By contrast, if $U_{2}$ is increased keeping particle-hole symmetry in dot 2, the system evolves from a local-moment regime to an underscreened spin-1 regime. Signatures of these behaviors can be experimentally identified through the conductance of the system. We also calculated the spin-spin correlations between the dots and between each dot and the leads to identify how the spin-spin interactions are distributed throughout the structure.\\[0pt] [1] L. G. G. V. Dias da Silva, N. P. Sandler, K. Ingersent, and S. E. Ulloa, Phys. Rev. Lett. 97, 096603 (2006). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700