Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W31: Focus Session: SpinDependent Phenomena in Semiconductors: Spins in Quantum Dots and Impurities 
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Sponsoring Units: GMAG DMP FIAP Chair: Jason Petta, Princeton University Room: 207A 
Thursday, March 5, 2015 2:30PM  3:06PM 
W31.00001: Extreme Harmonic Generation in Electrically Driven Spin Resonance Invited Speaker: Jiri Stehlik InAs nanowire double quantum dots offer a rich platform for studying single spin physics in a material with large spinorbit (SO) coupling. The large SO coupling allows all electrical control of the electron spin through electric dipole spin resonance (EDSR).\footnote{V. N. Golovach, M. Borhani, and D. Loss, Phys. Rev. B \textbf{74}, 165319 (2006).} Here an oscillating electric field of frequency $f$ displaces the electron wave function, while a magnetic field with strength $B$ is applied. Spin rotations occur when the resonance condition $hf = g \mu_{\rm B} B$ is met. Here $g$ is the electron $g$factor, $h$ is Planck's constant, and $\mu_{\rm B}$ is the Bohr magneton. We find that near zero interdot detuning efficient spin rotations also occur when $hf = n g \mu_{\rm B} B$, with $n$ being an integer as large as 8 in our system.\footnote{J. Stehlik, M. D. Schroer, M. Z. Maialle, M. H. Degani, and J. R. Petta, Phys. Rev. Lett. \textbf{112}, 227601 (2014).} The harmonics feature a striking odd/even dependence. While the odd harmonics show an enhancement of the leakage current, the even harmonics show a reduction. In contrast, we do not observe any measurable harmonics at large detuning. We link the presence of harmonics with additional anticrossings present in the level diagram. This implies that harmonics are the result of LandauZener transitions occurring at multiple anticrossings. Recent theoretical work supports this conclusion.\footnote{J. Danon and M. S. Rudner, arXiv:1407.2097.} [Preview Abstract] 
Thursday, March 5, 2015 3:06PM  3:18PM 
W31.00002: Spin relaxation of conduction electrons by inelastic scattering with neutral donors Lan Qing, Hanan Dery, Jing Li, Ian Appelbaum At low temperatures in ndoped semiconductors, a significant fraction of shallow donor sites are occupied by electrons, neutralizing the impurity core charge in equilibrium. Inelastic scattering by externallyinjected conduction electrons accelerated by electric fields can excite transitions within the manifold of these localized states. Promotion into highly spinmixed excited states results in spin relaxation that couples strongly to the conduction electrons by exchange interaction. Through experiments with silicon spin transport devices and complementary theory, we reveal the consequences of this previously unknown depolarization mechanism both below and above the impact ionization threshold and into the ``deep inelastic'' regime. [Preview Abstract] 
Thursday, March 5, 2015 3:18PM  3:30PM 
W31.00003: Singleshot readout and relaxation measurements in exchange coupled $^{31}P$ electron spins in silicon Juan Pablo Dehollain, Juha Muhonen, Kuan Tan, Andre Saraiva, David Jamieson, Andrew Dzurak, Andrea Morello We present the experimental observation of a large exchange coupling $J\approx300$ $\mu$eV between two $^{31}P$ electron spin qubits in silicon (Dehollain, \textbf{PRL} 112, 236801). The singlet and triplet states of the coupled spins are monitored in real time by a singleelectron transistor, which detects ionization from tunnelratedependent processes in the coupled spin system, yielding singleshot readout fidelities above 95\%. The triplet to singlet relaxation time $T_1\approx4$ ms at zero magnetic field agrees with the theoretical prediction for the observed $J$coupling energy in $^{31}P$ dimers in silicon. The three order of magnitude increase in relaxation rate compared to single donors, is caused by a hyperfine interaction mediated mixing of the singlet and triplet states. Additionally, the time evolution of the twoelectron state populations reveals an inversion in the energetic hierarchy of the valleyorbit excited states, which had been theoretically predicted for donor pairs with $<6$ nm separation. These results pave the way to the realization of twoqubit quantum logic gates with spins in silicon and highlight the necessity to adopt gating schemes compatible with weak $J$coupling strengths. [Preview Abstract] 
Thursday, March 5, 2015 3:30PM  3:42PM 
W31.00004: gFactors of Electrons, Holes and Excitons in TypeII ZnTe/ZnSe Submonolayer Quantum Dots Haojie Ji, Siddharth Dhomkar, Jonathan Ludwig, Dmitry Smirnov, Maria Tamargo, Igor Kuskovsky In recent years there has been intense interest in manipulating exciton spin states in semiconductor quantum dots (QDs) for application in spin electronics and quantum information processing. In these applications, the ability to enhance and control Zeeman spin splitting, which can be characterized by gfactors, plays a key role. Here we report our study of the gfactors of electrons, holes and excitons in typeII ZnTe/ZnSe submonolayer QD superlattices. Via analysis of left and right circularly polarized photoluminescence spectra, we determine the gfactor of typeII excitons. We obtain the gfactor of electrons by fitting the temperature dependence of degree of circular polarization. Thus, we find out the gfactor of holes confined in ZnTe QDs. This gfactor of confined holes is larger than those reported for bulk ZnTe. We propose that the enhancement of gfactor of holes is due to quantum confinement which leads to the admixture of the subband states. [Preview Abstract] 
Thursday, March 5, 2015 3:42PM  3:54PM 
W31.00005: Competition between applied and exchange magnetic fields in (Zn,Mn)Se/ZnTe quantum dots Biplob Barman, Y. Tsai, T. Scrace, I. Zutic, B.D. McCombe, A. Petrou, WC Chou, MH Tsou, CS Yang, I.R. Sellers, R. Oszwaldowski We have measured the peak energy of the photoluminescence (PL) emission and its circular polarization from type II (Zn,Mn)Se/ZnTe Quantum Dot structures in the Faraday and Voigt geometries. In the Faraday geometry the PL energy shows a 6 meV red shift at B$=$6 tesla. This result verifies that the holes are confined in the nonmagnetic ZnTe QDs, while the electrons move in the magnetic (Zn,Mn)Se matrix. The PL circular polarization saturates at 45{\%}. In the Voigt geometry, the circular polarization is nearzero and the red shift is 2 meV. These results are discussed using a model that takes into account that electrons are influenced by the combination of the externally applied magnetic field and the exchange field due to the interaction between the Mnspins and the carriers. [Preview Abstract] 
Thursday, March 5, 2015 3:54PM  4:06PM 
W31.00006: Spin structure of germanene quantum dot as a function of normal electric field Apalkov Vadym, Venkata Chaganti Germanene quantum dot consisting of 13 germanium atoms is studied numerically within the nearest neighbor tightbinding model. Both the energy spectra and the spin structure of the corresponding Eigenfunctions are obtained. Due to strong spinorbit interaction in germanene the spin polarization of the germanene quantum dot strongly depends on the energy of the corresponding Eigenstate and on the external electric field, $E_{\mathrm{z}}$. There are two states with energies close to zero, for which the direction of the spin is along zaxis, where zaxis is perpendicular of the quantum dot layer. For the higher energy levels the spin deviates from the zaxis with maximum angle $\theta _{\mathrm{max}}=$3.9$^{\mathrm{0}}$ for the levels with energy 1128 meV (for electron channel) and 1128 meV (for hole channel) and zero electric field, $E_{\mathrm{z}}=$0. The angle $\theta_{\mathrm{max}}$ increases almost linearly with $E_{\mathrm{z\thinspace }}$and takes the value of 4.2$^{\mathrm{0}}$ at $E_{\mathrm{z}}=$100 meV/{\AA}. The inplane direction of spin is also sensitive to external electric field. With increasing electric field, the inplane spin rotates in the anticlockwise and clockwise directions for the 1128 meV and 1128 meV levels, respectively. Due to such sensitivity of spin polarization to external electric field, applying a bias voltage can control the spin current through germanene quantum dot. [Preview Abstract] 
Thursday, March 5, 2015 4:06PM  4:18PM 
W31.00007: Structure determination of individual electronnuclear spin complexes in a solidstate matrix Abdelghani Laraoui, Daniela Pagliero, Carlos Meriles A spinbased quantum computer will store and process information via ``spin complexes'' formed by a small number of interacting electronic and nuclear spins within a solidstate host. Unlike present electronic circuits, differences in the atomic composition and local geometry make each of these spin clusters distinct from the rest. Integration of these units into a working network thus builds on our ability to determine the cluster atomic structure, a problem we tackle herein with the aid of a magnetic resonance protocol. Using the nitrogenvacancy (NV) center in diamond as a model system, we show analytically and numerically that the spatial coordinates of weakly coupled 13C spins can be determined by selectively transferring and retrieving spin polarization. The technique's spatial resolution can reach up to 0.1 nm, limited by the NV spin coherence lifetime. No external magnetic field gradient is required, which makes this imaging scheme applicable to NV13C complexes buried deep inside the crystal host. Further, this approach can be adapted to nuclear spins other than 13C, and thus applied to the characterization of individual molecules anchored to the diamond surface. [Preview Abstract] 
Thursday, March 5, 2015 4:18PM  4:30PM 
W31.00008: Anomalous Bfield Dependence of Spinflip Time in High Purity InP Xiayu Linpeng, Todd Karin, Russell Barbour, Mikhail Glazov, KaiMei Fu We observe an anomalous Bfield dependence of the spinflip time ($T_1$) of electrons bound to shallow donors which cannot be explained by current spinrelaxation theories. We conduct resonant pumpprobe measurements in highpurity InP from the low to high magnetic field regimes, with a maximum $T_1$ (400 $\mu$s) observed near the turning point $g \mu_B B \simeq k_B T$. At low $B$, the $T_1$ dependence on $B$ is consistent with an electron correlation time ($\tau_c$) in the tens of nanoseconds. The physical mechanism for the short $\tau_c$ in this highpurity sample ($n\simeq2\times 10^{14}$ cm$^{3}$) is unclear, but a strong temperature ($T$) dependence indicates $T_1$ can be further increased by lowering $T$ below the 1.5 K experimental temperature. At high $B$, a $B^{3}$ dependence is observed, in contrast to the expected $B^{5}$ predicted by singlephonon spinorbit mediated interactions. An understanding of the anomalous $B$field dependence is expected to elucidate the effect of electron transport (lowfield) and phonons (highfield) on $T_1$ for shallow donors, which is of interest for both ensemble and singlespin quantum information applications. [Preview Abstract] 
Thursday, March 5, 2015 4:30PM  4:42PM 
W31.00009: Spin dynamics in a quantum point contact showing the 0.7anomaly Jan von Delft, Florian Bauer, Jan Heyder, Dennis Schimmel The 0.7anomaly in the first conductance step of a quantum point contact is believed to arise from an interplay of geometry, spin dynamics and interaction effects. Various scenarios have been proposed to explain it, each evoking a different concept, including spontaneous spin polarization, or a quasilocalized state, or ferromagnetic spin fluctuations, or a van Hove ridge (a geometryinduced maximum in the densityof states). Though these scenarios differ substantially regarding numerous details, they all imply anomalous dynamics for the spins in the vicinity of the QPC. We have performed a detailed study of this spin dynamics in the central region of a parabolic quantum point contact, by using the functional renormalization group to calculate the dynamical spinspin correlation function $\chi(x,x',\omega) = \int_0^\infty \langle S_z(x,t) S_z(x',0) \rangle e^{i \omega t} $. We will discuss its behavior as function of frequency, interaction strength and gate voltage and comment on the implications of these results for each of the abovementioned scenarios. [Preview Abstract] 
Thursday, March 5, 2015 4:42PM  4:54PM 
W31.00010: Optimal quantum control via numerical pulse shape optimization for two exciton qubits confined to semiconductor quantum dots Reuble Mathew, Hong Yi Shi Yang, Kimberley Hall Optimal quantum control (OQC), which iteratively optimizes the control Hamiltonian to achieve a target quantum state, is a versatile approach for manipulating quantum systems. For opticallyactive transitions, OQC can be implemented using femtosecond pulse shaping which provides control over the amplitude and/or phase of the electric field. Optical pulse shaping has been employed to optimize physical processes such as nonlinear optical signals [1], photosynthesis [2], and has recently been applied to optimizing singlequbit gates in multiple semiconductor quantum dots [3]. In this work, we examine the use of numerical pulse shape optimization for optimal quantum control of multiple qubits confined to quantum dots as a function of their electronic structure parameters. The numerically optimized pulse shapes were found to produce high fidelity quantum gates for a range of transition frequencies, dipole moments, and arbitrary initial and final states. This work enhances the potential for scalability by reducing the laser resources required to control multiple qubits.\\[4pt] [1] Bartels, R. et al., Nature 2000, 406, 164166.\\[0pt] [2] Herek, J. L. et al., Nature 2002, 417, 533535.\\[0pt] [3] Gamouras, A. et al; Nano Lett. 2013, 13(10), 46664670. [Preview Abstract] 
Thursday, March 5, 2015 4:54PM  5:06PM 
W31.00011: Spin Manipulation through geometric phase in IIIV semiconductor quantum dots Sanjay Prbahakar, Roderick Melnik A more robust technique is proposed to flip the spin completely through geometric phase in IIIV semiconductor quantum dots (QDs). We transport the QDs adiabatically in a closed loop along the circular trajectory in the plane of two dimensional electron gas with the application of time dependent gate controlled electric fields and investigate the manipulation of Berry phase with the spinorbit couplings. Here we show that both the Rashba and the Dresselhaus couplings are present for inducing a phase necessary for spin flip. If one of them is absent, the induced phase is trivial and irrelevant for spinflip (Phys. Rev. B \textbf{89}, 245310 (2014), Applied Physics Letters 104, 142411 (2014)). [Preview Abstract] 

W31.00012: ABSTRACT WITHDRAWN 

W31.00013: ABSTRACT WITHDRAWN 
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