Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H18: Spins in Reduced Dimensional SemiconductorsFocus Industry
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Sponsoring Units: GMAG DMP FIAP Chair: Scott Crooker, Los Alamos National Laboratory Room: 317 |
Tuesday, March 15, 2016 2:30PM - 2:42PM |
H18.00001: ABSTRACT WITHDRAWN |
Tuesday, March 15, 2016 2:42PM - 2:54PM |
H18.00002: Spin-orbit interaction in monolayer (group-III) metal-monochalcogenides pengke li, Ian Appelbaum Beginning with an analysis of the fundamental symmetries of monolayer (group-III) metal-monochalcogenides (such as GaSe), we examine various spin-dependent properties of this new series of 2D semiconductors. Interesting features resulting from spin-orbit interaction include broken valence band degeneracy, cubic Dresselhaus spin splitting, and eigenstate spin-mixing. The latter two control the type and magnitude of dominant spin relaxation pathways and influence the `caldera' shape valence band edge. Further phenomena endowed by spin-orbit interaction include a modest orbital contribution to the Lande g-factors and the possibility of optical orientation via band-edge photoexcitation spectroscopy, which shows an energy-dependent reversal of conduction electron spin polarization. Based on this analysis, we propose an experiment to use optically-driven spin dynamics to quantify different spin lifetimes for electron and holes. Reference: arXiv:1508.06963 [Preview Abstract] |
Tuesday, March 15, 2016 2:54PM - 3:06PM |
H18.00003: Controlling spin lifetime with Dresselhaus and Rashba fields in the 2D semiconductor $MX$ Ian Appelbaum, Pengke Li It is widely believed that whenever spin encodes logic state in a semiconductor device, transport channel materials with the longest spin lifetime are the most suitable choice. However, once a logic operation is completed, residual spins can and will interfere with those involved in future operations. We propose to solve this problem by utilizing the unique properties of spin-orbit effects in the electronic structure of monolayer of group-III metal-monochalcogenide ($MX$) semiconductors. The interplay of Dresselhaus and Rashba effective magnetic fields in these materials will be shown to provide effective external control over spin polarization lifetime, potentially useful for future spin-enabled digital devices. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:42PM |
H18.00004: Long-lived Spin Relaxation and Spin Coherence of Electrons in Monolayer MoS$_{\mathrm{2}}$ Invited Speaker: Luyi Yang Monolayer MoS$_{\mathrm{2}}$ and related transition metal dichalcogenides (TMDs) are direct-gap semiconductors in which strong spin-orbit coupling and a lack of structural inversion symmetry give rise to new coupled spin--valley physics. Although robust spin and valley degrees of freedom have been inferred from polarized photoluminescence (PL) studies of \textit{excitons}, PL timescales are necessarily constrained by short (3--100 ps) electron--hole recombination. Direct probes of spin/valley dynamics of resident carriers in electron (or hole)-doped TMDs, which may persist long after recombination ceases, are still at an early stage. Here we directly measure the coupled spin-valley dynamics of \textit{resident} electrons in $n$-type monolayer MoS$_{\mathrm{2}}$ using optical Kerr-rotation spectroscopy [1], and reveal very long spin lifetimes exceeding 3ns at 5K (orders of magnitude longer than typical exciton lifetimes). In contrast with conventional III-V or II-VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields. This suggests a novel mechanism of electron spin dephasing in monolayer TMDs, driven by rapidly-fluctuating internal spin-orbit fields due to fast intervalley scattering. Additionally, a small but very long-lived oscillatory signal is observed, indicating spin coherence of localized states [2]. These studies provide direct insight into the physics underpinning the spin and valley dynamics of electrons in monolayer TMDs. [1] L. Yang \textit{et al}., \textit{Nature Physics} \textbf{11}, 830 (2015). [2] L. Yang \textit{et al}., \textit{submitted.} [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 3:54PM |
H18.00005: Spin Transport in Single Layer Transition Metal Dichalcogenides Michael Phillips, Vivek Aji Inversion symmetry breaking and strong spin orbit coupling in two dimensional transition metal dichalcogenides leads to interesting new phenomena such as the valley hall and spin hall effects. The nontrivial Berry curvature of the bands yields transverse spin currents in applied field. In this talk we characterize the spin transport in hole-doped systems. Due to the large spin-splitting, time-reversal invariance, and the large separation of hole pockets in momentum space, spin flip scattering involves inter-valley processes with large momentum. As such, one expects large spin life times and a large spin hall angle. We analyze the robustness of the phenomena to various scattering processes and explore the viability of transition metal dichalcogenides for spintronic applications. [Preview Abstract] |
Tuesday, March 15, 2016 3:54PM - 4:06PM |
H18.00006: Spin polarized transport in MoS2 Andr\'{e} Dankert, Parham Pashaei, Venkata Kamalakar Mutta, Saroj Prasad Dash The two-dimensional (2D) semiconductor MoS2 possesses a high potential for spintronic devices due to a rich spin-valley physics and large spin-orbit coupling. While there have been significant advances in studying the spin and valley dynamics in MoS2 using optical spectroscopy techniques, electronic spin transport in semiconducting MoS2 or its heterostructures have not yet been demonstrated. Here we report the electronic and spin transport properties in MoS2 employing ferromagnetic electrodes in a vertical device geometry. Such vertical devices with MoS2 channel length defined by the thickness of the 2D layer allow to investigate the spin injection, transport and detection. We observe a magnetoresistance effect over a large temperature range up to 300 K and investigate the temperature and bias dependence behavior. Using magnetotransport data and calculations we extract spin parameters in the MoS2 spin valve devices. These findings can open new avenues for exploring spin functionalities in 2D semiconductor heterostructures for spin logic applications. [Preview Abstract] |
Tuesday, March 15, 2016 4:06PM - 4:18PM |
H18.00007: Novel valley depolarization dynamics and valley Hall effect of exciton in mono- and bilayer MoS$_2$ T. Yu, M. W. Wu We investigate the valley depolarization dynamics and valley Hall effect of exciton due to the electron-hole exchange interaction in mono- and bilayer MoS$_2$. For the valley depolarization dynamics, in the monolayer MoS$_2$, it is found that in the strong scattering regime, the conventional motional narrowing picture is no longer valid, and a novel valley depolarization channel is opened. For the valley Hall effect of exciton, in both the mono- and bilayer MoS$_2$, with the exciton equally pumped in the K and K' valleys, the system can evolve into the equilibrium state with the valley polarization parallel to the effective magnetic field due to the exchange interaction. With the drift of this equilibrium state by applied uniaxial strain, the {\it momentum-dependent} valley/photoluminesence polarization is induced by the exchange interaction, which leads to the valley/photoluminesence Hall current. Specifically, the disorder strength dependence of the valley Hall conductivity is revealed. In the strong scattering regime, the valley Hall conductivity decreases with the increase of the disorder strength; whereas in the weak scattering regime, it saturates to a constant, which can be much larger than the one in Fermi system due to the absence of the Pauli blocking. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H18.00008: Spin-orbit coupling and spin relaxation in phosphorene Marcin Kurpas, Martin Gmitra, Jaroslav Fabian We employ first principles density functional theory calculations to study intrinsic and extrinsic spin-orbit coupling in monolayer phosphorene. We also extract the spin-mixing amplitudes of the Bloch wave functions to give realistic estimates of the Elliott-Yafet spin relaxation rate. The most remarkable result is the striking anisotropy in both spin-orbit coupling and spin relaxation rates, which could be tested experimentally in spin injection experiments. We also identify spin hot spots in the electronic structure of phosphorene at accidental bands anticrossings. We compare the Elliott-Yafet with Dyakonov-Perel spin relaxation times, obtained from extrinsic couplings in an applied electric field. We also compare the results in phosphorene with those of black phosphorous. This work is supported by the DFG SPP 1538, SFB 689, and by the EU Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H18.00009: Probing Hole Spins in an InAs/GaAs Quantum Dot Molecule subject to Lateral Electric Fields Xiangyu Ma, Garnett Bryant, Matthew Doty Quantum dot molecules (QDMs) are structures in which coherent interactions between two or more adjacent quantum dots (QDs) can lead to unique, tunable electronic and spin properties. We explore computationally spin-mixing interactions in the molecular states of single holes confined in vertically-stacked InAs/GaAs self-assembled QDMs. We consider the spin properties of the hole states subject to electric fields that have components both parallel and perpendicular to the molecular stacking axis. We compute the energies of the QDM hole states under various electric and magnetic fields with a combination of full tight binding atomistic calculations and approximate atomistic results using eigenstates found at particular fields as a basis to extrapolate to other fields. We observe a relatively large Stark shift in hole states with the application of lateral electric fields, as well as a quenching of the Zeeman splitting. Most importantly, we observe that lateral electric fields induce hole spin mixing with a magnitude that increases with increasing lateral electric field over a moderate range. These results suggest that applied lateral electric fields provide an opportunity to fine-tune and manipulate, in situ, the energy levels and spin properties of single holes confined in QDMs. [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H18.00010: Spin Dynamics of Tellurium Isoelectronic Centers Bound Excitons in Zn-Se-Te Nanostructures Vasilios Deligiannakis, Siddharth Dhomkar, Daniela Pagliero, Haojie Ji, Maria Tamargo, Igor Kuskovsky, Carlos Meriles Three-dimensionally confined structures such as quantum dots (QDs) have been of considerable interest due to their ability to closely imitate isolated atoms on mesoscopic length scales. Recently, single impurity states in bulk semiconductors have also attracted attention due to their ability to optically address quantum states. Here we show results pertaining to the optical and spin properties of Te isoelectronic centers present in type-II sub-monolayer QDs within a ZnSe matrix. Time resolved Kerr rotation (TRKR) measurements were performed using a degenerate pump-and-probe setup. Attempts to probe the QDs by direct optical excitation did not show any results most likely due to the weak oscillator strength of this transition resulting from their type-II nature. Centering the pump and probe pulses around the band edge of ZnSe and performing TRKR vs energy measurements we were able to address the spin dynamics of Te-isoelectronic centers present in the spacer layer. Results show that the $\tau $*2 lifetimes exhibit a bi-exponential decay and persist up to 1 ns. Further measurements will be done on samples with varying Te concentration, as well as a function of the applied magnetic-field to understand the spin properties of this defect. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H18.00011: Self-consistent Theory of Magnetic Polarons in Semiconductor Quantum Dots. Dan Rederth, Rafal Oszwaldowski, A. G. Petukhov Nanostructures based on dilute magnetic semiconductors indicate paths towards novel devices that could employ carrier spin [1-2]. Magnetic quantum dots (QDs) are an example of such structures. We use a robust numerical method, based on the Luttinger-Kohn Hamiltonian and suitable for realistic self-assembled QD geometries [3], to study electronic structure and magnetism of p-type II-VI quantum dots doped with Mn magnetic ions. Our method relies on self-consistent treatment of exchange coupling of holes and magnetic ions within the mean-field approximation [4]. It explicitly takes into account multi-band character of the hole kinetic energy operator. We demonstrate formation of the hole magnetic polarons, which manifests itself in self-induced splitting of the hole levels in absence of an external magnetic field [5]. Furthermore, we conduct detailed studies of the magnetically-ordered QD ground state. The structure of the ground state reveals highly anisotropic, as well as, position- and temperature-dependent self-induced magnetization. [1] Semiconductor spintronics and quantum computation, D.D. Awschalom, D. Loss, and N. Samarth eds., (Springer, Berlin, 2002). [2] J. Fabian, A. Matos-Abiague, C. Ertler, et al., Acta Phys. Slov 57, 565 (2007). [3] H. Kirmse, R. Schneider, M. Rabe, et al., Appl. Phys. Lett. 72, 1329 (1998). [4] R. Oszwaldowski, P. Stano, A. Petukhov, and I. Zutic, Phys. Rev. B 86, 201408 (2012). [5] Seufert, J., Bacher, G., Scheibner, et al., Phys. Rev. Letters 88, 27402 (2002). [Preview Abstract] |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H18.00012: Modeling of magnetic polaron properties in (Zn,Mn)Te quantum dots James Pientka, B. Barman, L. Schweidenback, A.H. Russ, Y. Tsai, J.R. Murphy, A.N. Cartwright, I. Zutic, B.D. McCombe, A. Petrou, W-C. Chou, W. C. Fan, I.R. Sellers, A.G. Petukhov, R. Oszwaldowski Magnetic polarons in (Zn,Mn)Te quantum dots (QD) show unconventional behavior [1]. These structures exhibit a small red shift of the photoluminescence peak energy in the presence of a magnetic field $B$ and they also have a weak dependence of the polaron energy $E_{\mathrm{MP}}$ on temperature $T$ and $B$. We attribute these properties to a large molecular field $B_{m}$ that is proportional to the heavy holes spin density [2]. We have calculated $B_{m}$ using the QD diameter and height as adjustable parameters. Assuming hole localization, this calculation yields values of $B_{m}$ \textgreater 20 T. The assumption that the hole localization diameter can be smaller than the QD diameter is justified due to alloy and spin disorder scattering [3]. Using the magnetic polaron free energy, we calculate $E_{\mathrm{MP}}$ as function of $T$ and $B$ for a variety of $B_{m}$ values. To get a weak dependence of $E_{\mathrm{MP}}$ on $T$ and $B $we must assume that the polaron temperature is higher than $T$. [1] B. Barman et al., Phys. Rev. B \textbf{92}, 035430 (2015). [2] J. M. Pientka et al., Phys. Rev. B \textbf{92}, 155402 (2015). [3] K. V. Kavokin et al., Phys. Rev. B \textbf{60}, 16499 (1999). [Preview Abstract] |
Tuesday, March 15, 2016 5:18PM - 5:30PM |
H18.00013: Gate-tuned spin to charge conversion in semiconducting single-walled carbon nanotubes Ei Shigematsu, Hiroshi Nagano, Sergey Dushenko, Yuichiro Ando, Tetsuya Tsuda, Susumu Kuwabata, Taishi Takenobu, Takeshi Tanaka, Hiromichi Kataura, Teruya Shinjo, Masashi Shiraishi Interconversion of spin and charge current is a hot topic in the molecular spintronics. It was achieved for the first time in a conducting conjugated polymer $^1$, and shortly followed by spin-charge conversion in graphene. However, control over carrier type has not been shown yet. In this study we focused on single-walled carbon nanotubes (SWNT). Spin injection into semiconductor from metal ferromagnet is challenging due to the presence of Schottky barrier and conductance mismatch problem. To bypass it, we used ionic liquid electric gate and ferrimagnetic insulator. We prepared SWNT layer on top of ferrimagnetic yttrium iron garnet substrate. Using spin pumping we successfully observed spin-charge conversion in metallic SWNT. As for a semiconducting SWNT, we applied a top gate using ionic liquid. The drain-source current vs. gate voltage dependence showed tuning of the Fermi level and changing of carrier type. Under gate voltage application we measured electromotive force induced by spin pumping. Detected voltage changed its sign together with carrier type. This is first evidence of spin-charge conversion in carbon nanotubes $^2$. \\$^1$ K. Ando et al., Nature Mater. 12, 622 (2013). \\$^2$ E. Shigematsu et al., submitted. [Preview Abstract] |
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