### Session W43: Coupled 2D and Multiple Quantum Dot Systems

 Thursday, March 8, 2007 2:30PM - 2:42PM W43.00001: A study of interminiband Rabi oscillations in biased semiconductor superlattices Pavel Abumov , Donald Sprung Semiconductor superlattices can be a flexible source of coherent electrons, with possible application as sources of terahertz radiation and in quantum computing. A better understanding of the underlying quantum transport phenomena is essential for making further progress in these fields. We have studied interminiband Rabi oscillations of an electron in biased semiconductor superlattices, specifically the conditions for their occurrence and their variation with bias tuning at energy level anticrossings. Our simulations were based on direct solution of the time-dependent Schroedinger equation, using transparent boundary conditions. It has been explicitly demonstrated that interminiband Rabi oscillations result from constructive interference between Bloch and intrawell oscillations, and the conditions for resonant bias values have been investigated. We also report a simulation of interminiband Rabi oscillations directly across three minibands at high bias, which show interaction between three strongly coupled minibands. Thursday, March 8, 2007 2:42PM - 2:54PM W43.00002: Microwave Switching in Amorphous-Carbon Quantum Wells Somnath Bhattacharyya , Luis Gomez Rojas , S. Ravi. P. Silva Demonstration of long phase coherence length showing resonant tunnelling and fast switching in amorphous carbon quantum well structures has recently been established [1]. Here we show a bias controlled reversible switching of the complex impedance by transmitting a microwave signal up to 110\textit{GHz} through amorphous carbon resonant tunnel diodes. By employing a coplanar waveguide technique and through the analysis of the return loss ($S_{11})$ microwave enhanced mobility greater than 30\textit{cm}$^{2}$\textit{(Vs)}$^{-1}$ in the delocalized regime of (filamentary) conduction in these devices is demonstrated. Also a switching behaviour at about 85\textit{GHz} can also be observed. We suggest a new model for the microscopic origin of the increased mobility and show routes to achieve longer coherence lengths. In addition microwave conductance of carbon quantum wells parallel to their plane and across a channel length larger than 100 nm determines the momentum scattering time of electrons in carbon. These results exhibit a potential for pure amorphous carbon-based fast memory devices. [1] S. Bhattacharyya, S.J. Henley, E. Mendoza, L. Gomez Rojas, J. Allam and S.R.P. Silva, Nature Mater. \textbf{5}, 19 (2006). Thursday, March 8, 2007 2:54PM - 3:06PM W43.00003: Transient response and electric field domain relocation in semiconductor superlattices Huidong Xu , Stephen Teitsworth Numerical simulation results are presented for a discrete drift-diffusion model that describes electronic transport in weakly-coupled semiconductor superlattices under voltage bias [1]. Sequential resonant tunneling between adjacent quantum wells is the primary conduction mechanism for this model which also incorporates an effective conductivity $\sigma$ associated with the injecting contact. We study time-averaged current-voltage characteristics and the transient current response associated with electric field domain relocation when the applied voltage is abruptly shifted by an amount $V_{step}$. For intermediate values of $\sigma$ and a range of $V_{step}$ values, two types of complex transient response are observed: 1) the \textbf{tripole/dipole} mechanism in which a charge depletion and a charge accumulation layer move together from the contact, and 2) the \textbf{injected monopole} mechanism, in which a small amplitude accumulation layer moves rapidly from the contact. Generally, the injected monopole relocation mechanism is much faster than the tripole/dipole mechanism. At moderately larger values of $\sigma$, the tripole/dipole mechanism is not observed for any value of $V_{step}$ because the higher levels of injected charge suppress formation of a moving depletion layer. Thus, a relatively small increase in $\sigma$ can result in significantly shorter domain relocation times. [1] L. L. Bonilla and H. T. Grahn, Rep. Prog. Phys. \textbf{68}, pp. 577 - 683, and references therein. Thursday, March 8, 2007 3:06PM - 3:18PM W43.00004: Exchange-driven re-entrant layer-occupancy transitions in biased bilayer systems J.R. Rodriguez , C.B. Hanna Hamilton \textit{et al.} showed experimentally that an externally biased double-quantum-well system in zero magnetic field could exhibit an exchange-driven bilayer-to-monolayer (2-1'') transition as the total carrier density was increased. This transition is due to the combined effects of the negative compressibility of the low-density carriers and the layer imbalance produced by external gate biases. We give an approximate criterion for observing a re-entrant 2-1-2'' transition that repopulates the emptied layer as the total carrier density is further increased. The gate voltages required for repopulation are shown to be impractically high for $p$-type GaAs bilayer devices with hole carriers. We show, however, that it may be possible to observe a 2-1-2'' transition in low-density $n$-type electron bilayer systems with very small layer separations. Thursday, March 8, 2007 3:18PM - 3:30PM W43.00005: Using the angular dependence of a quantizing magnetic field to probe the Bloch states in room temperature superlattice devices. Ross McDonald , Shigeki Kobayashi , S. Jim Allen , Susan Cox , John Singleton The prospect of designing Bloch-oscillator superlattice structures that operate at room temperature has both intrigued and eluded the scientific community since its conception over 35 years ago. Advances in band structure architecture and engineering continuously address this issue, improving the fabrication of devices designed to operate as room temperature THz frequency oscillators. Here we report room-temperature pulsed-IV measurements in tilted magnetic fields of up to 30 Tesla, designed to probe the coherence of superlattice Bloch states. Biasing these devices beyond Ohmic conduction reveals differential conductance features with a 1/cos($\theta )$ dependence upon the field angle. The voltages at which these features occur is determined by the condition that the ratio of the Bloch to cyclotron frequencies be an integer. This behavior is consistent with resonant de-localization of Bloch oscillations due to nonlinear coupling to the cyclotron motion in tipped field. Thursday, March 8, 2007 3:30PM - 3:42PM W43.00006: Effective Hamiltonian Approach for the Magnetic Band Structure and Novel Oscillations in the Magnetization Manfred Taut The one-electron Schroedinger equation in a 2D periodic effective potential and an homogeneous magnetic field B has been solved numerically in the framework of magnetic band structure theory. Alternatively,the spectrum around a given rational flux quantum number p0/q0 can also been obtained by semi-classical quantization of the exact magnetic band structure (MBS) at p0/q0. To implement the latter procedure, a generalized effective Hamiltonian theory based on the MBS at finite magnetic fields has been established. The total energy has been calculated numerically as a function of magnetic field B and of band filling. The magnetization M is the derivative of the total energy with irespect to the magnetic field. The total energy as a function of B shows series of kinks, which produce series of oscillations in the magnetization. One of these series, the de Haas-van Alphen oscillation, contains information about the (zero magnetic field) band structure. The other series provide the corresponding information about certain MBSs. In order to obtain the information about the MBS at field B0, we have to plot the magnetization as a function of 1/(B-B0). The asymptotic period of the oscillations in M(1/(B-B0)) provides the Fermi surface cross sections for the MBS at B0. Thursday, March 8, 2007 3:42PM - 3:54PM W43.00007: Novel Flux Matching Effects in Potentially Type-I Superconducting Au/Pb Bilayers Patterned with Antidot Lattices Lance De Long , Sergiy Kryukov , Vitali Metlushko We report AC and DC SQUID magnetometer data for Au(25nm)/Pb(x) bilayers (x = 50, 100 nm) patterned with square antidot (AD) lattices having AD diameter D = 600 nm and AD separation d = 1 micron, in DC magnetic fields applied perpendicular to the film plane. Both AC and DC data for x = 100 nm samples exhibit a two-horned'' magnetization m(H) well below T$_{C}$, with small, sharp cusps having DC field spacings near 3 Oe. Just below T$_{C}$ = 6.2 K, m(H) is highly reversible, and exhibits at least two matching fields H$_{n}$ = (20 Oe)n. This striking behavior is compared with recent theoretical models for flux matching in patterned films in the Type-I intermediate state, for which formation of giant vortices'' or pinning of normal domains by AD are possible. In contrast, data for x = 50 nm samples exhibit smooth (no small cusps) m(H) behavior with sharp matching peaks and highly irreversible behavior just below T$_{C}$, typical of extensively studied, Type-II patterned films. Thursday, March 8, 2007 3:54PM - 4:06PM W43.00008: Variational Monte Carlo Method for Coupled Quantum Dots in Magnetic Fields Jihan Kim , Dmitriy Melnikov , Michele Casula , Jean-Pierre Leburton The electronic properties of two-dimensional coupled quantum dots (QD) in presence of an external magnetic field are investigated using a variational Monte Carlo (VMC) method. The many-body Schr\"{o}dinger Equation with fixed model potential for coupled QDs is solved by using two-electron trial wavefunctions made of a product of two-body Jastrow term and single-particle orbitals for both singlet and triplet states. We use the steepest descent (SD) method to optimize the expectation value of energy by iteratively updating the variational parameters. In co-linear triple QDs, we show that the exchange energy between two electrons can be tuned by varying the confinement of the central dot. We also find that the electron separation in the singlet and triplet states evolve differently upon increasing the magnetic field. Thursday, March 8, 2007 4:06PM - 4:18PM W43.00009: Topological Hunds rules and the electronic properties of a triple lateral quantum dot molecule P. Hawrylak , M. Korkusinski , F. Delgado , L. Gaudreau , S. Studenikin , A. Kam , A. Sachrajda We analyze theoretically and experimentally the electronic structure and charging diagram of three coupled lateral quantum dots in a magnetic field filled with electrons. Using the Hubbard model and real-space exact diagonalization techniques we show that the electronic properties of this artificial molecule can be understood using a set of topological Hunds rules[1]. These rules relate the multi-electron energy levels to spin and the inter-dot tunneling t, and control charging energies. We map out the charging diagram for up to N=6 electrons and predict a spin singlet for two electrons, spin-polarized phase for two holes, and a magnetically frustrated ground state for three electrons. We show that spin polarization can be tuned by magnetic field perpendicular to the triple dot device. The theoretical charging diagram is compared with the measured charging diagram of the gated triple-dot device[1]. [1] P. Hawrylak and M. Korkusinski, Solid State Commun. 136, 508 (2005). [2] L. Gaudreau, S. A. Studenikin, A. S. Sachrajda, P. Zawadzki, A. Kam, J. Lapointe, M. Korkusinski, and P. Hawrylak, Phys. Rev. Lett. 97, 036807 (2006). Thursday, March 8, 2007 4:18PM - 4:30PM W43.00010: Tunable Noise Cross-Correlations in a Double Quantum Dot Douglas McClure , Leonardo DiCarlo , Yiming Zhang , Hans-Andreas Engel , Charles Marcus , Micah Hanson , Art Gossard We report measurements of the cross-correlation between temporal current fluctuations in two quantum dots in the Coulomb blockade regime, with purely capacitive inter-dot coupling. The dots act as a pair of tunable interacting localized states, enabling a systematic study of Coulomb-induced correlation. The sign of the cross-spectral density is found to be tunable by gate voltage and source-drain bias. We find good agreement between the experimental results and a sequential-tunneling model of transport through capacitively coupled single-level dots. Thursday, March 8, 2007 4:30PM - 4:42PM W43.00011: Phonon decoherence in a double dot qubit embedded inside a suspended phonon cavity Ying-Yen Liao , Yueh-Nan Chen , Der-San Chuu The phonon-induced decoherence in a double dot charge qubit embedded inside a semiconductor slab is investigated theoretically. We employ the Redfield formalism to solve the density matrix in the Born-Markov approximation. Our calculations show some interesting results in the presence of slab cavity. In particular, the decoherence behaves significantly due to particular phonon couplings such as the van Hove singularity and vanishing deformation potential. Thursday, March 8, 2007 4:42PM - 4:54PM W43.00012: Investigation of chaos-assisted tunneling in a weakly coupled, double quantum dot Dong Ho Wu , Bernard Matis It is known that chaos-assisted dynamical tunneling may occur in nonintegrable (chaotic) systems. In our previous experiments we measured the tunneling rate in a weakly coupled, 2D microwave double cavity. The results seemed to indicate that the presence of chaotic modes changes not only the dynamical tunneling rate but also the spatial tunneling rate, as the electromagnetic-field leakage (wave tunneling) rate between the weakly coupled, 2D double cavities increases significantly, if one of the cavities is nonintegrable. We have now investigated these phenomena with gate-defined quantum dots, fabricated on a GaAs$\backslash$AlGaAs 2DEG substrate. The experiments were performed on quantum dots of various shapes. In this presentation we will discuss these recent results on tunneling rate and quantum conductance. Thursday, March 8, 2007 4:54PM - 5:06PM W43.00013: Fermi-liquid versus non-Fermi-liquid behavior in triple quantum dots Janez Bonca , Rok Zitko We study the effect of electron hopping in triple quantum dots modelled by the three-impurity Anderson model. In a wide interval around the particle-hole symmetric point, the triple quantum dot system has a FL ground state with high conductance at $T=0$. The different regimes exhibit different approaches to this fixed point. The most likely candidate for observing non-Fermi-liquid (NFL) behavior is the cross-over regime with competing magnetic ordering and Kondo screening, $J \sim T_K$. In this regime the NFL behavior occurs in a wide temperature range and it is fairly robust against various perturbations that do not additionally increase the channel asymmetry. As the crossover regime is entered from the above, the conductance through the side dots increases to a half of the conductance quantum, while the conductance through the system remains small. At lower temperatures the conductance through the system increases to the unitary limit as the system crosses over to the Fermi-liquid ground state. The signature of the NFL behavior can be detected by measuring different conductances in a three terminal configuration. Our findings suggest, that properly choosing parameters of the triple quantum dot system that set it into the crossover regime, represents a road map for observation of NFL behavior. Thursday, March 8, 2007 5:06PM - 5:18PM W43.00014: Signatures and Implementations of Adiabatic Quantum Pumping Kunal Das We explore the mechanism of adiabatic quantum pumping from a fundamental quantum mechanical perspective, and consider analogies with other adiabatic processes. We discuss ideas for generalizing the mechanism to alternate entities other than charge and spin and in alternative physical systems. Our study is motivated by and grounded in possibilities of easier experimental realizations to get around the difficulties encountered in previous mesoscopic experiments. In addition we wish to have an unambiguous definition and signature for the phenomenon of quantum pumping.