Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F48: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spins in 2D Systems and II-VI Quantum Dots |
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Sponsoring Units: GMAG DMP FIAP Chair: Aubrey Hanbicki, Naval Research Laboratory Room: Mile High Ballroom 1A |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F48.00001: Spin and Valley Noise in Two-dimensional Transition Metal Dichalcogenides Wang-Kong Tse, D.L. Smith, N.A. Sinitsyn We develop a theory for the spin dynamics and optical spin noise spectroscopy in two-dimensional transition metal dichalcogenides. Different from spin noise in conventional semiconductors, we find that the Faraday rotation fluctuations consist of intrinsic noise not only from spins but also the valley degrees of freedom in the presence of inter-valley scattering processes. When inter-valley scattering is fast compared to spin flip scattering, we find that the spin relaxation time is renormalized by spin-orbit coupling and the valley relaxation time. When spin flip scattering is, on the other hand, fast compared to inter-valley scattering, spin relaxation and valley relaxation processes decouple. The Faraday rotation noise power spectrum displays distinctive signatures in both cases. We propose optical spin noise spectroscopy as a useful nonperturbative technique for probing the spin and valley relaxation processes in transition metal dichalcogenides. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F48.00002: Suppression of Coulomb exchange energy in quasi-2D spin-3/2 hole systems R. Winkler, T. Kernreiter, M. Governale, U. Z\"ulicke We have calculated the exchange-energy contribution to the total energy of quasi-2D spin-3/2 hole systems in typical semiconductors [1]. The magnitude of the exchange energy turns out to be suppressed from the value expected for analogous spin-1/2 conduction electron systems whenever the mixing between heavy-hole and light-hole components is strong. Our results are obtained using a general formalism for calculating the exchange energy of many-particle systems where single-particle states are multicomponent spinors. We have applied this approach to obtain analytical results for spin-3/2 hole systems in limiting cases.\\{} [1] Kernreiter et al., Phys.\ Rev.\ B \textbf{88}, 125309 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F48.00003: Layered magnetic dichalcogenide in the nanoscale thickness regime W.J. Hardy, H. Ji, A. Marcinkova, E. Morosan, D. Natelson We report results of transport measurements on FexTaS2, with x approximately 0.1. This layered dichalcogenide material is a ferromagnet with a Curie temperature of about 80 K and very large magnetocrystalline anisotropy. Our study includes anisotropic magnetoresistance (AMR) and anomalous Hall effect (AHE) measurements on single crystals of nanoscale thickness, produced by tape exfoliation. We find an out-of-plane magnetoresistance effect considerably larger than was previously reported on bulk samples with x $=$ 0.25, as well as a less pronounced change with temperature of the Hall resistance hysteresis loops. These marked differences may help further elucidate the material's transport mechanisms and the nature of the ferromagnetic state. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F48.00004: Mn-doped monolayer MoS$_2$: An atomically thin dilute magnetic semiconductor Doron Naveh, Ashwin Ramasubramaniam We investigate the electronic and magnetic properties of Mn-doped monolayer MoS$_{2}$ using a combination of first-principles density functional theory (DFT) calculations and Monte Carlo simulations. Mn dopants that are substitutionally inserted at Mo sites are shown to couple ferromagnetically via a double-exchange mechanism. This interaction is relatively short ranged, making percolation a key factor in controlling long-range magnetic order. The DFT results are parameterized using an empirical model to facilitate Monte Carlo studies of concentration- and temperature-dependent ordering in these systems, through which we obtain Curie temperatures in excess of room temperature for Mn doping in the range of 10--15{\%}. Our studies demonstrate the potential for engineering a new class of atomically thin dilute magnetic semiconductors based on Mn-doped MoS$_{2}$ monolayers. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F48.00005: Current induced spin orbit torque in 2D ferromagnetic and anti-ferromagnetic system Huawei Gao, Tomas Jungwirth, Jairo Sinova Due to space inversion asymmetry in 2D ferromagnetic or anti-ferromagnetic system with spin orbit coupling, unpolarized electric current can induce non equilibrium spin polarization from carriers and act as torques on the magnetic order. We'll report analytical calculation of this effect in a simple 2D ferromagnetic model and numerical results for the 2D anti-ferromagnetic model using Kubo linear response formula. In ferromagnetic case, there is a disorder independent out-of-plane spin polarization component. This out-of-plane component has different signs for the sub-lattices in the anti-ferromagnetic case, which will exert a torque on the in-plane Neel order. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F48.00006: Probing the coupling between holes and nuclear spins in a 2D GaAs Hole System using a Landau level spin diode Alex Hamilton, Oleh Klochan, I. Farrer, D.A. Ritchie Hole spins have recently attracted significant interest due to their potential applications for quantum computing applications due to reduced coupling to the nuclear spin bath, which is the main source of decoherence in GaAs electron spin qubit systems. Here we report a study of the interaction between nuclear and hole spins in a two-dimensional hole system in the Quantum Hall regime, using the Landau level diode technique to make separate electrical contact to two edge states with different spin polarizations. Experiments on similar electron systems show hysteretic I-V traces and electrically detected nuclear magnetic resonance revealing coupling of electron and nuclear spins. For 2D hole systems however, although non-linear I-V characteristics are observed, we were unable to detect coupling between hole and nuclear spins over a wide range of hole densities, consistent with a greatly reduced hyperfine interaction. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F48.00007: Direct observation of an out-of-plane spin polarization caused by an in-plane magnetic field in a GaAs 2D hole system LaReine Yeoh, Ashwin Srinivasan, Oleh Klochan, Adam Micolich, Roland Winkler, Michelle Simmons, David Ritchie, Michael Pepper, Alexander Hamilton Recent interest in spin-orbit coupling has led to studies of quantum confined, hole based semiconductor devices, which naturally possess strong spin-orbit interaction due to the intrinsic spin-3/2 nature of holes. In general both crystal anisotropies and quantum confinement will affect the spin properties of holes. In high symmetry crystals such anisotropies can be ignored, however in low symmetry crystals this complex interplay between the crystal and the confining potential gives rise to intriguing spin behavior, which has no counterpart in spin-1/2 electron-based systems. Here I will present the first direct observations of an unusual effect where a magnetic field applied in the plane of the 2D hole system generates a spin polarization perpendicular to the 2D plane. This out-of-plane spin polarisation is detected in transport measurements of a symmetrically doped, GaAs 2D hole quantum well in tilted magnetic fields. We are able to extract the sign of this off-diagonal component of the Land\'{e} g-factor and show that it is consistent with theory. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F48.00008: Electric field control of cubic-Rashba spin orbit interaction in two-dimentional hole gas confined in Ge/SiGe quantum well Rai Moriya, Yusuke Hoshi, Kentarou Sawano, Yasuhiro Shiraki, Noritaka Usami, Satoru Masubuchi, Tomoki Machida Recently, control of hole spins in the semiconductor heterostructure have received considerable attention. Due to the strong spin orbit interaction (SOI) of the holes, efficient electrical control of hole spins can be demonstrated. However due to the difficulty in fabrication of high quality samples, the studies of the SOI in the holes are still limited. The high mobility two-dimentional hole gass (2DHG) can be obtained in Ge/SiGe quntum well structure due to its small hole effective mass. Moreover since Ge is inversion symmetric crystal, only the Rashba-type SOI due to the structural inversion asymmetry is allowed. Thus 2DHG in Ge is a good platform to study the SOI of the holes and to demonstrate its electric field control. We fabricated gated Hall bar device from Ge/SiGe quantum well and studied low magnetic field transport of the 2DHG. At low temperature, a weak anti-localization is observed. From the comparison with analytical model, we attribute this is due to the cubic-Rashba spin orbit interaction. The electric field applied with gate voltage significantly alter the weak anti-localization peak, thus enable us to control SOI with electric field. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F48.00009: Magneto-optical studies of (Zn,Mn)Se/ZnTe quantum dots B. Barman, Y. Tsai, T. Scrace, I. Zutic, B.D. McCombe, A. Petrou, W.C. Chou, M.H. Tsou, C.S. Yang, I.R. Sellers, R. Oszwaldowski, A.G. Petukhov We have recorded the circular polarization $P$ of photoluminescence from (Zn,Mn)Se/ZnTe quantum dots (QDs) as function of magnetic field $B$. The polarization at a fixed temperature increases monotonically with $B$ and saturates for B \textgreater 3 tesla at $P_{sat}$. The value of $P_{sat}$ depends strongly on the laser photon energy. When we excite above (below) the ZnMnSe gap with photons of energy of 3.81 eV (2.54 eV), we measure $P_{sat}=$55$\% (P_{sat}=$20$\% )$. We interpret these results as due to the difference in the Zeeman band splitting between the magnetic (Zn,Mn)Se matrix and the non-magnetic ZnTe QDs. For 3.81 eV excitation, electron-hole pairs are generated mainly in the (Zn,Mn)Se matrix. The majority of the holes relax to the $+$3/2 state before capture by the ZnTe QDs. With 2.54 eV excitation, all electron-hole pairs are excited in the QDs where the Zeeman splitting is negligible. Thus, $P_{sat}$ is determined in this case by the relatively small Zeeman splitting of ZnMnSe conduction band. We relate these findings to our previous results for magnetic type-II QDs, where $P_{sat}$ does not depend on the exciting photon energy. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F48.00010: Magneto-optical properties of core/shell quantum dots doped with radial position controlled magnetic impurities Gary Sanders, Chris Stanton We present a theory for the electronic and magneto-optical properties of spherical quantum dots consisting of an inner core surrounded by an outer shell. This core/shell quantum dot is doped by magnetic Mn impurities all of which are implanted at a preselected radius on a spherical surface withinthe dot. The spherical symmetry of the dot is broken by the application of an external magnetic field. The electronic states in the presence of a magnetic field are treated in an effective mass model which includes the s-d and p-d exchange interaction with localized Mn d electrons. The strain in the quantum dot due to lattice mismatch between core and shell regions is assumed to be pseudomorphic and the effect of this strain field on the electronic states is also included. The optical properties of the quantum dot are computed using the effective mass electronic states and Fermi's golden rule. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F48.00011: Magnetic polarons in type-II (Zn,Mn)Se/ZnTe quantum dots J.R. Murphy, B. Barman, Y. Tsai, T. Scrace, J.M. Pientka, I. Zutic, B.D. McCombe, A. Petrou, A.N. Cartwright, W.C. Chou, M.H. Tsou, C.S. Yang, I.R. Sellers, R. Oszwaldowski, A.G. Petukhov We have studied magnetic polaron formation dynamics in (Zn,Mn)Se/ZnTe quantum dots$^{\mathrm{2}}$ (QDs) using time-resolved photoluminescence (TRPL) spectroscopy. The emitted light was spectrally and temporally analyzed; the emission spectra were recorded as function of time delay ($\Delta t)$ from the exciting laser pulse. The recombination time at $T=$10 K in our samples is 2.3 ns. The peak energy of the emission red shifts with increasing $\Delta t $due to the lowering of the hole-Mn spin complex (magnetic polaron) energy. From this shift we determined the magnetic polaron formation energy ($E_{MP})$ at $T=$10 K to be 20 meV, which is half the value observed in the ZnSe/(Zn,Mn)Te system studied previously.$^{\mathrm{3}} \quad E_{MP\thinspace }$decreases with increasing temperature, in contrast to the behavior of the ZnSe/(Zn,Mn)Te system$^{\mathrm{3}}$ in which $E_{MP}$ is temperature independent. These results are discussed in terms of a theoretical model. [2] L. Lee, et al., J. Cryst. Growth \textbf{378}, 222 (2013). [3] I. R. Sellers, et al., Phys. Rev. B \textbf{82}, 195320 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F48.00012: Time Resolved Kerr Rotation Studies on Sub-monolayer Type-II ZnTe/ZnSe Quantum Dots Vasilios Deligiannakis, Siddharth Dhomkar, Haojie Ji, Bidisha Roy, Daniela Pagliero, Igor L. Kuskovsky, Maria C. Tamargo, Carlos A. Meriles Semiconductor quantum dot (QD) systems have been proposed as possible candidates to store and transport quantum information. Systems with a type-II band alignment are of particular interest due to the spatial separation of electrons and holes. Yet, there is very little work that has been reported on the spin dynamics of type-II QDs. Here we report time resolved Kerr rotation (TRKR) measurements on sub-monolayer type-II ZnTe/ZnSe QDs. The TRKR results for three samples indicate that there is an increase spin lifetime with higher QD density. The spin relaxation rates increased with decreasing temperature. This behavior has been reported for undoped II-VI materials. In the low carrier density region, electron-hole exchange interaction is dominant at low temperatures. However, this enhanced relaxation rate with decreasing temperature is suppressed in samples with the highest quantum dot density, suggesting that the presence of type-II nanoislands modify the spin relaxation behavior in these materials. [Preview Abstract] |
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