Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spin and Noise in Quantum Dots |
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Sponsoring Units: GMAG DMP FIAP Chair: George Kioseoglou, University of Crete, Greece Room: 104 |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y8.00001: Nonequilibrium Spin Noise Spectroscopy Nikolai Sinitsyn, Yuri Pershin, Valery Slipko, Fuxiang Li Spin Noise Spectroscopy (SNS) is an experimental approach to obtain correlators of mesoscopic spin fluctuations in time by purely optical means. We explore the information that this technique can provide when it is applied to a weakly non-equilibrium regime when an electric current is driven through a sample by an electric field. We find that the noise power spectrum of conducting electrons experiences a shift, which is proportional to the strength of the spin-orbit coupling for electrons moving along the electric field direction. We propose applications of this effect to measurements of spin orbit coupling anisotropy and separation of spin noise of conducting and localized electrons. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y8.00002: Using spin fluctuation correlations to reveal homogeneous linewidths within (In,Ga)As quantum dot ensembles: Two-color spin noise spectroscopy Luyi Yang, Scott Crooker, Philipp Glasenapp, Alex Greilich, Manfred Bayer, Dmitri Yakovlev ``Spin noise spectroscopy'' is a powerful optical technique for passively probing the spin dynamics of electrons and holes that is based on measuring their intrinsic spin fluctuations while in thermal equilibrium. This approach is guaranteed by the fluctuation-dissipation theorem. Here, we use the \textit{correlation properties} of spin fluctuations to reveal the underlying homogeneous linewidth of (In,Ga)As quantum dots (QDs) in an otherwise strongly inhomogeneously-broadened ensemble. When two narrowband probe lasers are tuned in wavelength far from each other, each is sensitive only to spin fluctuations from those QDs that are spectrally close to that laser. Therefore the detected spin noise signals from each laser are uncorrelated. In contrast, when the two lasers have exactly the same wavelength, then they are sensitive to the same QDs and the spin noise signals are perfectly correlated. By measuring the degree of correlation as a function of laser detuning, we reveal the homogeneous linewidth of the QDs even in the presence of a strong inhomogeneous broadening. This information is otherwise inaccessible by conventional linear optical spectroscopic techniques. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y8.00003: Nodal ``ground states'' and orbital textures in semiconductor quantum dots Jeongsu Lee, Karel Vyborny, Jong Han, Igor Zutic Unlike the common expectation, theoretical calculations in quantum wires and quantum dots have predicted hole ground state wavefunctions with a node [1-2] that are often associated with the formation of dark excitons [3]. The inversion of the energy level ordering between nodeless (S-like) and nodal (P-like) wavefunction states occurs due to various factors, e.g., confinement size and strength, choice of a material, and spin-orbit interaction. However, the existence of the nodal ground states has been debated and even viewed merely as an artifact of a k\textbullet p model [4]. Using complementary approaches of both k\textbullet p and tight-binding models, further supported by an effective Hamiltonian for a continuum model, we reveal that the nodal ground states in quantum dots are not limited to a specific theoretical model. Remarkably, the emergence of the nodal ground states can be attributed to the formation of the orbital vortex textures that minimizes ``divergence''. We discuss how our findings and the studies of orbital textures could be also relevant for different materials systems. [1] K. V\'{y}born\'{y} et al., PRB 85, 155312 (2012) [2] A. Bagga et al., PRB 74, 035341 (2006); P. Horodysk\'{a} et al., PRB 81, 045301 (2010); J. Xia and J. Li, PRB 60, 11540 (1999); M. P. Persson and H. Q. Xu, PRB 73, 125346 (2006). [3] M. Nirmal, et al., PRL 75, 3728 (1995); Al. L. Efros, et al., PRB 54, 4843 (1996). [4] L. W. Wang et al., APL 76, 339 (2000) [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y8.00004: Charge and spin noise from semiconductor quantum dots Invited Speaker: Richard J. Warburton Improving the quantum coherence of solid-state systems that mimic two-level atoms, for instance spin qubits or single-photon emitters using semiconductor quantum dots, involves dealing with the noise inherent to the device. Charge noise results in a fluctuating electric field, spin noise in a fluctuating magnetic field at the location of the qubit, and both can lead to dephasing and decoherence of optical and spin states. We investigate noise in an ultrapure semiconductor device using a minimally invasive, ultrasensitive local probe: resonance fluorescence from a single quantum dot. We distinguish between charge noise and spin noise through a crucial difference in their optical signatures. Noise spectra for both electric and magnetic fields are derived from 0.1 Hz to 100 kHz. The charge noise dominates at low frequencies, spin noise at high frequencies. The noise falls rapidly with increasing frequency, allowing us to demonstrate transform-limited quantum-dot optical linewidths by operating the device above 50 kHz. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y8.00005: Spin-polarized conductance in double quantum dots with ferromagnetic contacts Ireneusz Weymann, Krzysztof Wojcik We study the transport properties of double quantum dots in T-shape geometry strongly coupled to external ferromagnetic contacts. The analysis is performed with the aid of the numerical renormalization group method, which allows us to study the behavior of respective spectral functions and the linear conductance through the system in the full parameter space of the model. The considered device enables a unique possibility to explore the interplay of the Fano and Kondo effects with ferromagnetic-contact induced exchange field. We show that the presence of gate-tunable exchange field leads to strong dependence of the spin polarization of conductance on the position of the dot levels. By tuning the level of the decoupled dot, the conductance may become fully spin polarized. Moreover, when changing the dot level positions, one can also tune the sign of the spin polarization. The increased spin polarization of the conductance is a consequence of a subtle interplay between the interference effects, the Kondo effect and the exchange field. Double quantum dots with ferromagnetic contacts can be thus considered as efficient spin current sources, where the degree of spin polarization can be tuned by purely electrical means, without the necessity to apply external magnetic field. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y8.00006: Single-shot readout of electron high-spin states in a quantum dot coupled to quantum Hall edge states Haruki Kiyama, Akira Oiwa, Seigo Tarucha The ability to prepare and probe an electron spin in a quantum dot (QD) is indispensable for spintronics and quantum information processing. Spin-resolved quantum Hall edge states (SRESs) are expected to be applied for such applications, since their spatial separation by two-dimensional electron gas (2DEG) edge potential provides spin-dependent tunnel coupling with QDs. However, the spin filtering efficiencies reported previously have not been high enough for spin injection and detection. In this work, we firstly enhanced the efficiency of the spin filtering by the electrical tuning of 2DEG potential landscape. Larger separation of SRESs are obtained by making the change of 2DEG potential near the tunnel barrier more gradual. Secondly, using the highly efficient spin filtering, we demonstrated single-shot readout of electron spins in a QD. The maximum visibility of two-electron spin readout reached to 94{\%}. This is the highest value among reports in GaAs-based QDs. Subsequently we applied this scheme to measure the dynamics of the multi-electron high-spin states. We find that the spin relaxation rate of the S$=$3/2 or S$=$2 high-spin excited states to the S$=$1/2 or S$=$1 ground spin states are about 10 times faster than that of S$=$0 first excited state. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y8.00007: Emerging Modified Transverse-Field Ising Model On A Hydrogenated Silicon Surface Burkhard Ritter, Kevin Beach Advances in the precise placement of dangling bonds on a hydrogenated silicon surface open the prospect of manufacturing large scale quantum dot arrays. Small clusters of specifically arranged quantum dots comprise a system of bistable, interacting cells. Starting from an extended Hubbard model and using a set of controlled Hilbert space truncations, we show that such a system of quantum dot cells can be mapped to a modified transverse-field Ising model with long-ranged interactions. Each cell is described by a pseudo-spin. Because we control cell orientation and placement, we can construct a wide range of structures, with ferromagnetic and antiferromagnetic chains as simple examples. The Ising-like model is amenable to stochastic series expansion Monte Carlo, allowing the simulation and characterization of large systems. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y8.00008: Semiconductor Hall magnetometers for magnetic measurement of (In,Cr)As quantum dots Joon-Il Kim, T. Guan, S. von Molnar, P. Xiong, S.L. Wang, H.L. Wang, J.H. Zhao Recently, SQUID magnetometry measurements of MBE-grown self-assembled (In,Cr)As QDs showed magnetic hysteresis indicating possible existence of ferromagnetic ordering above 300 K [1]. However, the temperature dependence of the remnant magnetization did not follow the standard Brillouin-like behavior, and the interpretation of the data and elucidation of the origin of the ferromagnetism in the QDs have been hindered by the large ensemble-averaged measurement. Measurements on small clusters or even individual QDs would facilitate a direct correlation of the measured magnetic properties with their structural and chemical characteristics, possibly enabling a definitive understanding of the origin of the ferromagnetism in the diluted magnetic semiconductor QDs. Towards this goal, we have fabricated integrated micro-Hall magnetometers based on high-mobility GaAs/AlGaAs 2DEG in order to facilitate static and dynamic magnetic measurements of the QDs via the Hall gradiometry technique. Integrated structures of (In,Cr)As QDs on top of a GaAs/AlGaAs heterostructure were grown entirely in situ by MBE. Micro-Hall magnetometer devices with six Hall-crosses were fabricated using photolithography and wet chemical etching. Using carefully calibrated selective chemical etching, all QDs were removed except those on three of the Hall-crosses so as to enable gradiometry measurement. Results of on-going measurements will be discussed.\\[4pt] [1] H. J. Meng et al., Euro. Phys. Lett. 84, 58007 (2008). [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y8.00009: Overhauser effect in spin blockaded double quantum dots-the case of dual hysteresis Siddharth Buddhiraju, Bhaskaran Muralidharan In the spin blockade transport regime across GaAs double quantum dots (DQD), experiments [1] revealed that the hyperfine interaction with host nuclei can have profound consequences on the electron-spin dynamics. One of which, is the observation of bistablity and flat-topped behavior in the current versus applied DC magnetic-field $I(B_{dc} )$characteristics. In this talk, we will first explain the essence of this flat-topped hysteretic behavior using a simple six-state model that captures the multiple-feedback mechanisms that are involved. We will then consider a more detailed model that elucidates the role of the physical parameter space of the DQD set up and a feedback mechanism involving the difference Overhauser field caused by the two separate nuclear spin baths of the DQD set up. [1] K. Ono and S. Tarucha, Phys Rev Lett., 92, 256803 (2004). [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y8.00010: A theory on the self-sustained current oscillations in spin-blockaded quantum dots Xiangrong Wang, Bin Hu Based on the experimental fact that the self-sustained current oscillations (SSCO) in spin-blockaded double quantum dots is closely associated with the dynamically polarized nuclear spins, we consider the possible scenario that the SSCO in the spin-blockaded double quantum dots is the manifestation of the periodic motion of dynamical nuclear spin polarization (along a limit cycle) under an external magnetic field and a spin-transfer torque. Based on the Landau-Lifshitz-Gilbert equation, it is shown that a sequence of semistable limit cycle, Hopf, and homoclinic bifurcations occur as the external field is tuned. Although the fundamental time scale is nanoseconds for electron tunneling and micro seconds for nuclear spin dynamics under the external field or Overhauser fields, the divergent period near the homoclinic bifurcation explains well why the period in experiments can be many orders of magnitude longer than all microscopic time scales. Some predictions associate with the theory may also be tested experimentally. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y8.00011: Parallel single qubit gates in distant semiconductor quantum dots using engineered optical pulses Angela Gamouras, Reuble Mathew, Sabine Freisem, Dennis Deppe, Kimberley Hall Semiconductor quantum dots are promising for the development of a scalable system of qubits as such a platform would benefit from established semiconductor fabrication capabilities. Here we report the demonstration of simultaneous high-fidelity $\pi$ and 2$\pi$ single qubit gates on excitons in two uncoupled self-assembled quantum dots within the micron-scale control laser focal spot by engineering the phase of the broad-bandwidth femtosecond control pulse. The pulse phase is engineered using optimal quantum control (OQC), which has been applied to the optimization of quantum gates in atomic and molecular systems in recent years [1] and is extended here to gate optimization in a system of solid state qubits. The deterministic control of two distant, uncoupled qubits we have achieved constitutes a step towards scaling of semiconductor-based quantum computing platforms, and may enable the development of small quantum simulators based on complex instruction set quantum computing using semiconductor quantum dots [2]. [1] Campbell et al. Phys. Rev. Lett. 105, 090502 (2010); Kirchmair et al., New J. Phys. 11, 023002 (2009); Amitay et al., Chem. Phys. Lett. 359, 8 (2002). [2] Sanders et al., Phys. Rev. A 59, 1098 (1999). [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y8.00012: Spin Ordering and Fluctuations in Magnetic Quantum Dots James Pientka, Rafal Oszwaldowski, Igor Zutic, Jong Han, Andre Petukhov The presence of magnetic impurities in quantum dots can lead to unconventional ordering in carrier and impurity spins [1-3]. While, due to its simplicity, it is tempting to use the mean field approximation for such systems, we explain critical problems of that description and an essential role of spin fluctuations. We consider two different situations of singly and doubly occupied magnetic dots that correspond to the formation of magnetic polarons [4,5] and bipolarons [1,2]. The two cases reveal qualitatively different manifestations of finite size effects on spin ordering and the removal of spurious phase transitions at the mean field level. Using both analytical and Monte Carlo methods, we elucidate the interplay of spin ordering and fluctuations in these systems. \\[4pt] [1] J. M. Pientka, R. Oszwaldowski, A. G. Petukhov, J. E. Han, and I. Zutic, Phys. Rev. B. 86, 161403(R) (2012).\\[0pt] [2] R. Oszwaldowski, I. Zutic, and A. G. Petukhov, Phys. Rev. Lett. 106, 177201 (2011).\\[0pt] [3] R. Oszwaldowski, P. Stano, A. G. Petukhov, and I. Zutic, Phys. Rev. B. 86, 201408(R) (2012).\\[0pt] [4] R. Beaulac et al., Science 325, 973 (2009).\\[0pt] [5] I. R. Sellers, et al., Phys. Rev. B 82, 195320 (2010). [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y8.00013: Ultrafast adiabatic rapid passage in single InAs quantum dots Reuble Mathew, Eric Dilcher, Angela Gamouras, Sabine Freisem, Dennis Deppe, Kimberley Hall Fast quantum gates are important in quantum information processing because operations must be performed within the coherence time of the qubit. Faster single-qubit operations are also important for specific applications such as decoherence control via dynamical decoupling [1], entanglement operations to generate cluster states [2], and probabilistic gates [3]. Here we report an experimental demonstration of optical state inversion of the p-shell exciton in a single InGaAs quantum dot via adiabatic rapid passage (ARP) using a 1.1 ps optical pulse. Population inversion via ARP using frequency swept pulses is robust against experimental instabilities associated with the control pulse and variations in the light-matter coupling parameters. In contrast to previous work [4], shorter optical control pulses provide a 13.5 fold improvement in operation speed and a 200 fold reduction in the required chirp. We find that a chirp of 55,000 fs$^2$, corresponding to a pulse width of 1.1 ps, is sufficient to achieve ARP. [1] Viola \textit{et al.}, PRA 58, 2733 (1998), [2] Barrett \textit{et al.}, PRA 71, 060310(R) (2005), [3] Olmschenk \textit{et al.}, Science 323, 486 (2009), [4] Wu \textit{et al.}, PRL 106, 067401 (2011), Simon \textit{et al.}, PRL 106, 1666801 (2011) [Preview Abstract] |
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