Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Quantum Dots |
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Sponsoring Units: GMAG DMP FIAP Chair: Avadh B. Saxena, Los Alamos National Laboratory Room: 320 |
Friday, March 22, 2013 8:00AM - 8:12AM |
Y18.00001: Unconventional Nodal Wavefunctions in Quantum Dots Jeongsu Lee, Karel V\'{y}born\'{y}, Igor \v{Z}uti\'{c}, Jong Han In a single band model such as one electron in a box, it is well known that the ground state wavefunction has no node maximizing its spatial symmetry. However, the ordering of eigenstates in a multiband system e.g., p-doped semiconductor quantum dots (QDs) can be very different due to spin-orbit interaction, symmetry of the underlying lattice and geometry of the confinement. Such unconventional ordering of states has appeared in the literature {[}1, 2{]} but it is often ignored or merely considered a shortcoming of $k\cdot p$ model {[}3{]}. We investigate spatial structure of hole envelope-wavefunctions in QDs with a focus on its symmetry. Our calculation shows a counter-intuitive ordering of eigenstates where a single hole ``ground-state'' has a node at the center. For simplicity, we start with a 2D QD tight-binding model and extend the discussion to 3D QD tight-binding and $k\cdot p$ models. We also discuss experimental implications of the wavefunction ordering described above. {[}1{]} K. V\'{y}born\'{y} et al., PRB \textbf{85}, 155312 (2012) {[}2{]} A. Bagga et al., PRB \textbf{71}, 115327 (2005); P. Horodysk\'{a} et al., PRB \textbf{81}, 045301 (2010); J. Xia and J. Li, PRB \textbf{60}, 11540 (1999) {[}3{]} L. W. Wang et al., APL \textbf{76}, 339 (2000) [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y18.00002: Spin Wigner molecules in quantum dots Igor Zutic, Rafal Oszwaldowski, Peter Stano, A. G. Petukhov The interplay of confinement and Coulomb interactions in quantum dots can lead to strongly correlated phases differing qualitatively from the Fermi liquid behavior. While in three dimensions the correlation-induced Wigner crystal is elusive and expected only in the limit of an extremely low carrier density, its nanoscale analog, the Wigner molecule, has been observed in quantum dots at much higher densities [1]. We explore how the presence of magnetic impurities in quantum dots can provide additional opportunities to study correlation effects and the resulting ordering in carrier and impurity spins [2]. By employing exact diagonalization we reveal that seemingly simple two-carrier quantum dots lead to a rich phase diagram [2,3]. We propose experiments to verify our predictions; in particular, we discuss interband optical transitions as a function of temperature and magnetic field. [1] C. Ellenberger et al., Phys. Rev. Lett. {\bf 96}, 126806 (2006); A. Singha et al., Phys. Rev. Lett. {\bf 104}, 246802 (2010). [2] R. Oszwaldowski, P. Stano, A. G. Petukhov, and I. Zutic, Phys. Rev. B (Rapid Comm.), in press, arXiv:1210.6422. [3] R. Oszwaldowski, I. Zutic, and A. G. Petukhov, Phys. Rev. Lett. {\bf 106}, 177201 (2011). [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y18.00003: Spin polarized current through a quantum shuttle Jorge Villavicencio, Irene Maldonado, Ernesto Cota, Gloria Platero We study spin current through a vibrating triple quantum dot system in a linear arrangement, as a function of detuning across the device, in the presence of a magnetic field, taking into account non-spin-conserving tunneling processes induced by spin-orbit interaction (SOI). Using the density matrix master equation approach, we calculate the current and polarization for both the static and dynamic cases. In the former case the central dot is at rest, while in the latter it is oscillating (triple quantum dot shuttle, TQDS). In both cases, we find new resonances in the current with a definite spin polarization, for both symmetric and asymmetric Zeeman splitting. These resonances are shown to correspond to anticrossings in the energy spectrum reflecting coupling between states due to SOI. For the asymmetric TQDS we obtain a spin filter behavior in the weak coupling regime. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y18.00004: Nonmagnetic spin current generation as nonequilibrium Kondo effect in a spin-orbit nano interferometer Nobuhiko Taniguchi We investigate electric generation of spin-dependent transport through a single-level quantum dot embedded in a ring by help of the Rashba spin-orbit coupling\footnote{N. Taniguchi and K. Isozaki, arXiv:1210.6428 (2012).}. Although it is known for some time that applying finite bias to this type of the spin-orbit interferometer induces finite spin polarization on the dot\footnote{M. Crisan et al. Phys. Rev. B \textbf{79} 125319 (2009).}, the mechanism of driving such spin polarization to flow has not fully been understood. For instance, in spite of finite spin polarization on a noninteracting single-level dot, no spin current is found to appear. We show theoretically that it is possible to generate electrically large spin-dependent current through an interacting single-level dot, as a combined effect of the Kondo effect and finite bias as well as the Rashba spin-orbit interaction. In contrast to earlier work\footnote{H.-F. L\"{u} and Y. Guo, Phys. Rev. B \textbf{76}, 045120 (2007).}, we argue the emergent spin-dependent transport in the present model is viewed as a new type of nonequilibrium Kondo effect; it appears in the middle of the Kondo valley and is suppressed by bias voltage larger than the Kondo energy properly renormalized by the Rashba spin-orbit coupling. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y18.00005: The Optimization of Magnetic Ordering in Quantum Dots James Pientka, Rafal Oszwaldowski, Igor Zutic, Jong Han, Andre Petukhov Lately, there have been several theoretical studies that demonstrate how carrier-mediated magnetic ordering is influenced by multiple occupancies in quantum dots (QD) [1,2]. Experimentally, multiple-occupancy can be reached by high photo-excitation intensity. It was observed in type-II QDs that magnetic polaron (MP) formation persists at large temperatures [3]. We show that varying QD occupancy has important consequences, including thermally enhanced magnetic ordering in QDs [4]. We extend our method to take into account the formation of magnetic bipolarons (MBP) [1,2]. We show that a standard mean-field treatment of MBP leads to unphysical phase transitions, removed when fluctuations are taken into account. Finally, we demonstrate that for a single MP, the shrinking of the carrier wave function due to the exchange with magnetic impurities is a small effect. [1] R. Oszwaldowski, I. Zutic, and A. G. Petukhov, Phys. Rev. Lett. 106, 177201 (2011). [2] R. Oszwaldowski, P. Stano, A. G. Petukhov, and I. Zutic, accepted to Phys. Rev. B. (Rapid Communications), arxiv:1210.6422. [3] I. R. Sellers, R. Oszwaldowski, et al., Phys. Rev. B 82, 195320 (2010). [4] J. M. Pientka, R. Oszwaldowski, A. G. Petukhov, J. E. Han, and I. Zutic, Phys. Rev. B. 86, 161403(R) (2012). [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y18.00006: Single-electron transport in a magnetic quantum-dot molecule Javier Romero, Eduardo Mucciolo We study single-electron transport in a magnetic quantum-dot molecule by using a stationary rate equation approach. In the molecule, two quantum dots play the roles of magnetic ions and are connected to each other through a third quantum dot which plays the role of a nonmagnetic ion. The magnetic quantum dots are coupled to ideal metallic leads and a back gate voltage is applied to the molecule, forming a field-effect transistor setup. A hopping Hamiltonian, which includes on-site repulsion and magnetic anisotropies, is employed to describe this molecule, resulting in an energy spectrum similar to that of single molecule magnets in the giant spin approximation. An external, in-plane magnetic field is then used to drive the molecule to a diabolical point, where states with maximum total spin with opposite directions are degenerated. Both linear and nonlinear transport are evaluated near the diabolical point, showing features that can be attributed to Berry-phase interference of spin tunneling paths. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:48AM |
Y18.00007: Optical control and coherence of electron or hole spins in coupled quantum dots Invited Speaker: Samuel Carter The spin of an electron or hole in an InAs quantum dot is an attractive qubit because it combines the advantages of a semiconductor platform with the power of ultrafast optical coherent control techniques. In the last few years, basic quantum operations such as initialization, rotation, and readout have become possible using single spins, but now improvements in spin coherence and demonstrations of multi-qubit systems are needed. In this work, we combine advances in the design and growth of coupled quantum dots with optical coherent control techniques to demonstrate ultrafast manipulation and coherence improvements for one or two interacting electron [1] or hole [2] spins in a coupled pair of InAs dots. For each of these spin systems, we use a sequence of picosecond and nanosecond pulses to initialize, manipulate, and measure the coherent spin dynamics. These dynamics include precession about a magnetic field and also entangling dynamics from the exchange interaction for coupled spins. For a single electron spin, precession dephases after only a few nanoseconds due to the hyperfine interaction with nuclear spins. For hole spins, we measure a dephasing time an order of magnitude longer due to a weaker hyperfine interaction. Coupled electron and hole spins are essential for multi-qubit systems, and they can also be used to decrease sensitivity to the environment. In these systems, we typically measure the coherent dynamics of the singlet-triplet states (m$_{\mathrm{s}}=$0), which are much less sensitive to the nuclear environment. At present, dephasing is due to fluctuations in the electrical environment. With careful sample design, we can make these systems much less sensitive to electrical fluctuations, giving a powerful combination of long coherence times and ultrafast gates. Finally, we demonstrate that these spin qubits can be incorporated into a photonic crystal cavity and manipulated with optical pulses, a major step toward a quantum interface between photons and these spin qubits.\\[4pt] [1] D. Kim \textit{et al}., Nature Phys. \textbf{7}, 223 (2011).\\[0pt] [2] A. Greilich \textit{et al}., Nature Photon. \textbf{5}, 702 (2011). [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y18.00008: Dephasing and relaxation of central hole spins by nuclear spin baths in InGaAs quantum dots: role of nuclear quadrupolar coupling Yan Li, N.A. Sinitsyn, A. Saxena, D.L. Smith, D. Reuter, A.D. Wieck, D.R. Yakovlev, B. Manfred, S.A. Crooker Single electron or hole spins in III-V semiconductor quantum dots (QDs) are promising candidates for solid-state qubits. Their coherence properties are typically governed by the hyperfine coupling between these ``central'' electronic spins and the dense surrounding bath of lattice nuclear spins. Theoretically this is a challenging problem due to its many-body and strongly-correlated nature. Here we measure the spin dynamics of holes in InGaAs quantum dots by detecting their intrinsic, random spin fluctuations while in thermal equilibrium, which reveals the spin correlation time scales $\tau_h$ and the functional form of bath-induced spin relaxation. In zero magnetic field, $\tau_h$ is very long ($\sim$400 ns) and decays exponentially, in marked contrast with recent theories. $\tau_h$ increases to $\sim$5 $\mu$s in small (100 G) longitudinal fields, and the spin dynamics evolve to a very slow $\sim$1/ln(t) decay [1]. We model the influence of nuclear quadrupolar coupling on spin dynamics in these strained QDs for both electrons and holes [2], and find a good agreement with experimental data when the quadrupolar coupling exceeds the hyperfine coupling strength. [1] Yan Li, N. Sinitsyn, et al., PRL 108, 186603 (2012). [2] N. Sinitsyn, Yan Li, et al., PRL 109, 166605 (2012). [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y18.00009: Quadrupolar spectra of nuclear spins in strained InGaAs quantum dots Ceyhun Bulutay Self-assembled quantum dots (QDs) are born out of lattice mismatched ingredients where strain plays an indispensable role. Through the electric quadrupolar coupling strain affects the nuclear spins. To guide upcoming single-QD nuclear magnetic resonance (NMR) as well as dynamic nuclear spin polarization experiments, a computational atomistic insight to the strain and quadrupolar field distributions will be presented. Among our findings, a high aspect ratio of the QD geometry enhances the quadrupolar interaction; inclined interfaces introduce biaxiality and the tilting of the major quadrupolar principal axis away from the growth axis; the alloy mixing of gallium into the QD enhances both of these features while reducing the quadrupolar energy. NMR spectra in Faraday and Voigt geometries are computed, unraveling in the first place the extend of inhomogeneous broadening and the appearance of the normally-forbidden transitions. Moreover, from the main extend of the NMR spectra the alloy mole fraction of a single QD can be inferred. In the presence of an external magnetic field, the borderlines between the quadrupolar and Zeeman regimes are extracted as 1.5 T for In and 1.1 T for As nuclei. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y18.00010: Integrability-based analysis of the hyperfine interaction induced decoherence in quantum dots Alexandre Faribault, Dirk Schuricht Using the Algebraic Bethe Ansatz in conjunction with a simple Monte Carlo sampling technique, we study the problem of the decoherence of a central spin coupled to a nuclear spin bath. We describe in detail the full crossover from strong to weak external magnetic field field, a limit where a large non-decaying coherence factor is found. This feature is explained by Bose-Einstein-condensate-like physics which also allows us to argue that the corresponding zero frequency peak would not be broadened by statistical or ensemble averaging. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y18.00011: Dynamics of carrier populations and localized spins during magnetic-polaron formation in quantum dots Biplob Barman, Rafal Oszwaldowski, Lars Schweidenback, Andreas Russ, Joseph Murphy, Alexander Cartwright, Igor Zutic, Bruce McCombe, Athos Petrou, Wu-Ching Chou, Wen Chung Fan, Ian Sellers, Andre Petukhov We have extended our previous investigation of time evolution of PL from (Zn,Mn)Te/ZnSe quantum dots in a magnetic field $B$ [1]. PL studies at $T=$5 K in these type-II dots reveal formation of magnetic polarons (MP). We find their formation time $\tau_{MP}$ to be 0.5 ns, which varies little with $B$. The circular polarization $P$ of the emission shows a surprising behavior. For all fields, the characteristic time $\tau_{P}$ is longer than $\tau_{MP}$. Furthermore, $\tau_{P}$ decreases from 10 ns to 1.9 ns as $B$ increases from 1 to 4 tesla. We attribute this effect to a low-$B$ bottleneck in the $\sigma_{+}$ recombination channel, due to the almost equal populations of the spin $\pm 1/2$ electrons participating in the interband transitions. In contrast, the $\pm 3/2$ holes in the (Zn,Mn)Te QDs, are affected mostly by the effective field due to exchange interaction between hole and Mn spins around it. This effective field is much larger than $B$. \\[4pt] [1] I.R. Sellers \textit{et al}. Phys. Rev. B. 82, 195320 (2010) [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y18.00012: Room temperature spin decoherence and dephasing in CdSe nanocrystal quantum dots Ahmad Khastehdel Fumani, Reza Sharghi-Moshtaghin, Jesse Berezovsky We combine transmission electron microscopy (TEM) and Faraday-rotation-based spin measurements to reveal the connection between coherent electron spin dynamics and the shape and size distribution of an ensemble of nanocrystal quantum dots. Optically pumped spins in CdSe nanocrystal quantum dots provide a platform for studying coherent dynamics and decoherence of spins of charge carriers in a complex, room-temperature environment. In a transverse magnetic field, decay of the ensemble spin signal is often ascribed to inhomogeneous dephasing caused by the distribution of nanocrystal sizes across the ensemble. In this work, we measure the size and shape distribution of an ensemble of nanocrystals using TEM, and compare the resulting calculated spin dynamics to those measured in a time-resolved Faraday rotation experiment. We find that the size inhomogeneity alone is insufficient to explain the measured dephasing times and decay envelopes. We propose an ensemble decoherence mechanism based on the distribution of nanocrystal shapes which can account for both the magnetic field dependence of the dephasing time and the shape of the decay envelope. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y18.00013: A phonon laser using quantum dot spin states Alexander Khaetskii, Xuedong Hu, Igor Zutic Sound analog of laser (saser) has not yet been realized experimentally, though some steps in this direction have been made recently [1]. As is known, the main reason impeding coherent generation of phonons in solid state is high density of phonon states [2]. We suggest a particular realization of saser, which consists of an ensemble of quantum dots and uses the Zeeman-split spin levels of the ground orbital state in the quantum dot. We develop a complete set of saser equations taking into account the Coulomb blockade conditions for a quantum dot, and evaluate all the parameters such as the threshold, output power and efficiency of the device. Supported by NSF-ECCS and US ONR, NSF PIF,and US ARO. [1]. R.P. Beardsley et al., PRL \textbf{104,} 085501 (2010). [2]. J. Chen and J.B. Khurgin, IEEE Journal of Quantum Electronics, \textbf{39}, 600 (2003) . [Preview Abstract] |
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