Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X17: Quasi-1D Transport and Quantum Dot Spectroscopy |
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Sponsoring Units: DCMP Chair: Jean Heremans, Ohio University Room: LACC 404B |
Friday, March 25, 2005 8:00AM - 8:12AM |
X17.00001: Charge Configuration of a Quantum Point Contact near pinchoff Lindsay Moore, S. Luescher, D. Goldhaber-Gordon, H. Shtrikman Over the last decade, there has been great interest in how electrons flow through a quantum point contact (QPC) as it is just opened up, before a fully transmitting 1D channel is available. Remarkably, there does not seem to be a smooth transition from tunneling to ballistic transport. Instead, a shoulder appears in the conductance versus channel width, at a conductance of roughly 0.7 times that of an open spin-degenerate channel. Experiments have built a consensus that this so-called ``0.7 structure'' is related to electron spin and electron-electron interaction, but the detailed description remains controversial. To study this system, we have made devices with two QPC's which share a thin central gate, fabricated on a high mobility GaAs/AlGaAs heterostructure. In this experiment, one QPC acts as sensitive charge detector for the other, neighboring quantum point contact. Measurements of the changing capacitive coupling between the detector QPC and other gates on the device have provided new insight into the charge configuration of the second QPC as it is opened from pinchoff through several conductance plateaux. We acknowledge support from the ONR Young Investigator Program, Award No. N00014-01-1-0569 and a Research Corporation Research Innovation Award, No. RI1260. [Preview Abstract] |
Friday, March 25, 2005 8:12AM - 8:24AM |
X17.00002: High-quality quantum point contacts in GaN/AlGaN heterostructures H. T. Chou, S. Luscher, D. Goldhaber-Gordon, M.J. Manfra, R.J. Molnar We have fabricated quantum point contacts on a high-mobility GaN/AlGaN heterostructure using the split-gate technique. The conductance of our devices shows well-quantized plateaus, which spin-split in high perpendicular magnetic field. The g factor is derived from the point contact subband splitting versus perpendicular magnetic field. In addition to the well-resolved plateaus, we also observe evidence of ``0.7 structure'' which has been mainly investigated in the GaAs system. The work at Stanford was sponsored by the Office of Naval Research Young Investigator Program under award no. N00014-01-1-0569, and by a Seed Grant from Stanford University's Center for Integrated Systems. [Preview Abstract] |
Friday, March 25, 2005 8:24AM - 8:36AM |
X17.00003: Non-Fermi liquid resonances in cleaved-edge overgrown AlAs quantum wires J. Moser, M. Grayson, F. Ertl, T. Zibold, D. Schuh, M. Bichler, G. Abstreiter, S. Roddaro, V. Pelligrini We report conductance measurements in cleaved-edge overgrown quantum wires made of aluminum arsenide, a heavy-mass, multi- valley system. 1D conductance steps as a function of gate bias are observed. The step heights are substantially reduced from the anticipated value for a spin- and valley-degenerate 1D system, possibly as a result of disorder-induced backscattering in the wire itself. We report on a temperature T and dc bias V study of tunneling resonances in low-density AlAs quantum wires near pinchoff. Both the peak conductance and the area of resonances exhibit power laws in T and V, suggesting the existence of a non-Fermi liquid. A quantitative comparison with the sequential resonant tunneling model in a Luttinger liquid is presented, leading to the surprising result that the power law exponent shows differing values for high- and low-temperatures. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X17.00004: Zero bias conductance peak in a gated quantum wire R.W. Giannetta, T. Olheiser, M. Hannan, I. Adesida, M.R. Melloch Conductance measurements are reported for an 0.4 micron wide GaAs/AlGaAs quantum wire with 7 cross-channel gates. The device exhibited integral conductance steps, magnetoconductance plateaus in agreement with the multiprobe formula and a conductance feature at 0.65 2e$^{2}$/h. Differential conductance measurements down to 50 mK revealed a zero bias conductance peak that vanished with an in-plane field of 1 Tesla. The width of this peak was comparable to that reported in high mobility quantum point contacts.[1] At low conductances this device also exhibited single electron charging characteristic of a multiple quantum dot. Work at UIUC was supported by NSF ECS02-10447, ARO grant DAAH04-95-1-0618, NSF grant ECS 92-02294 and of JSEP grant N00014-90-J-1270.[1] S.M. Cronenwett, et. al., Phys. Rev. Lett \textbf{88}, 226805 (2002) [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X17.00005: Quantum Conductance and Magnetic Focusing in InSb Heterostructures David Deen, A.R. Dedigama, S.Q. Murphy, N. Goel, J. Keay, M.B. Santos, K. Suzuki, S. Miyashita, Y. Hirayama Both single and double quantum point contact devices have been fabricated using in-plane gating in symmetrically doped InSb/InAlSb heterostructures. The devices were designed with typical dimensions of 0.5micron which preserves ballistic transport up to 185K as measured in separate experiments. Quantized conductance was observed at 4.2K in the single quantum point contact devices. With the application of a small perpendicular magnetic field, the double quantum point contact devices should act as current focusers. Preliminary measurements display such current focusing peaks. It is predicted that the large spin-orbit effect in InSb may lead to spin split focusing peaks. This work is supported by the National Science Foundation under grants No.DMR-0209371 and DMR-0080054. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X17.00006: Experimental investigation of localization in antidot lattices Hong Chen, J.A. Peters, J.J. Heremans, N. Goel, S.J. Chung, M.B. Santos We investigate magnetotransport properties of both square and hexagonal antidot lattices fabricated on high-mobility InSb/InAlSb heterostructures. The magnetoresistance shows a strong localization peak at zero magnetic field as well as ballistic peaks due to the antidot lattice. The strength of the localization peak decreases exponentially, with a characteristic temperature of $\sim $ 25 K, as temperature increases from 0.4 K to 50 K. The exponential behavior and high characteristic temperature can be explained by classical scattering in chaotic systems. The localization and ballistic features are compared with results of antidot lattices made on other semiconductor heterostructures. (NSF DMR-0094055, DMR- 0080054, DMR-0209371) [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X17.00007: Quasi-continuous Charge Transfer via 2D Hopping Yusuf Kinkhabwala, Viktor Sverdlov, Konstantin Likharev We have extended our Monte Carlo simulations of hopping transport in completely disordered 2D conductors to the process of external charge relaxation. In this situation, the conductor shunts an external capacitor $C$ with initial charge $Q_i \sim e$. As the charge relaxes due to random hops of electrons through the conductor, so does the electric field $E =Q_R \left( t \right) / C L$ applied to it. At $T \rightarrow 0$, the charge relaxation process stops at some ``residual" charge value $Q_R < e$ corresponding to the effective Coulomb blockade of hopping. We have calculated the r.m.s. value of $Q_R$ (for the statistical ensemble of conductors with random distribution of localized sites) as a function of parameters of the system, and have found that for conductors with sufficiently large area $L \times W \gg a^2$ (where $a$ is the localization radius) it is a universal function of the ratio $(LW/a^2)/C$ for negligible electron- electron interaction and of the ratio $(LW/a^2)/(\chi C)^{2}$ for substantial interaction. (Here $\chi = e^{2} \nu_0 a/ \kappa$ is the dimensionless strength of the Coulomb interaction with $\nu_0$ the density of states and $\kappa$ the dielectric constant.) [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X17.00008: Thermal Transport in Semiconductor Quantum Dot Superlattices Alexander Khitun, Jianlin Liu, Kang Wang We present a theoretical model allowing us to predict thermal properties in semiconductor quantum dot superlattices (QDS). The model is based on the relaxation time approximation taking into account phonon relaxation due to the scattering on quantum dots. According to the model, the thermal properties of semiconductor QDS can be effectively controlled by quantum dot composition, size and arrangement. Numerical calculations were carried out for a structure that consists of multiple layers of Si with randomly distributed Ge quantum dots separated by spacer layers. The results of numerical simulations are in a good agreement with experimental data obtained for Si/Ge QDS in a wide temperature range (100-300K). The model explains the artificial anisotropy of the thermal conductivity experimentally observed in Si/Ge QDSs. The obtained results are important for the most recently suggested applications of quantum dot superlattices for thermoelectric devices. [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X17.00009: Si-based single-electron devices with tunable barriers Neil M. Zimmerman, Akira Fujiwara, Hiroshi Inokawa, Kenji Yamazaki, Hideo Namatsu, Yasuo Takahashi, Stuart B. Martin Although single-electron devices have been pursued for metrology, nano-electronics and other applications, it has been difficult to find a way to make reproducible, reliable devices. We have recently made and demonstrated Si-based SET transistors with high yield, good reliability, and good uniformity as well. These devices are similar to Si MOSFETs, with the addition of tunable barriers formed by two layers of gates; the lower gates form the tunnel barriers, and the upper gate provides the inversion necessary for conduction. Having measured four devices to date, we have discovered that the nonuniformity of various device capacitances is less than 10 % (previous best was about 50 %) and that these devices have Ec ~ 20 K. The tunability of the barriers also provides an interesting playground for a wide variety of studies; we will report on some of these, possibly including the systematic evolution from single to double islands, the new concept of a "barrier capacitance", etc. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X17.00010: Rate equations for Coulomb blockade with ferromagnetic leads Stephan Braig, Piet W. Brouwer We derive a density matrix rate-equation approach to sequential tunneling through a metal particle weakly coupled to ferromagnetic leads. Our formalism is valid for an arbitrary number of electrons on the dot, as well as for an arbitrary angle between the polarization directions of the leads. The effects of spin-orbit scattering can be included straightforwardly. One of our major results is that, if the electron-electron interactions inside the dot are described by the ``universal interaction Hamiltonian'', the standard scalar rate equations fail even for collinear or unpolarized leads once occupation numbers larger than two are taken into account. We calculate the linear conductance for the case that transport occurs through one electronic level only, and we show numerical results for multiple levels with up to triple occupation on the dot. For certain configurations, one may observe negative differential conductance. [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X17.00011: Correlation of Conductance Measurements from a Quantum Dot with Three Terminals Ryan Toonen, Marta Prada, Hua Qin, Andreas Huettel, Srijit Goswami, Mark Eriksson, Robert Blick, Daniel van der Weide, Karl Eberl We have measured the differential conductance of a quantum dot coupled by three tunable tunneling barriers to three terminals. The quantum dot is formed by laterally constricting a two-dimensional electron gas (2DEG) in an Al$_{x}$Ga$_{1-x}$As/GaAs heterostructure with Schottky split-gates. The advantage to performing conductance measurements on a quantum dot with three leads is that we are able to directly measure information about the individual tunneling barriers and determine how the states interact with the leads. At a base temperature of 250mK, we have observed new phenomena not previously reported from three-terminal, mesoscopic experiments. These effects include conductance peak suppression in the nonlinear bias regime and the simultaneous coupling of two different states to two separate leads. [Preview Abstract] |
Friday, March 25, 2005 10:12AM - 10:24AM |
X17.00012: Nonlinear properties of ballistic quantum dot under influence of microwave radiation Jingqiao Zhang, Sergey Vitkalov, Ze Don Kvon, J. C. Portal , A. D. Wieck Microwave rectification and effects of microwave radiation (1-40 GHz) on electron DC transport through a ballistic quantum dot are studied experimentally at electron temperature down to 0.3K. The experiments demonstrate significant contribution of the ballistic electron motion to the observed nonlinearities. [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 10:36AM |
X17.00013: Triple Quantum Dots Andy Vidan, Michael Stopa, Robert M. Westervelt, Micah Hanson, Art C. Gossard We report on studies of three coupled lateral quantum dots fabricated in a GaAs/AlGaAs heterostructure containing a two-dimensional electron gas, with the three dots arranged in a ring geometry. We discuss the design of the triple dots and present electron transport measurements. Recently, we have realized a triple quantum dot single-electron rectifier [1]. Triple quantum dots can be used to study the exchange interaction in the presence of three spins and may also serve as building blocks for spin-qubit circuits. This work was supported at Harvard by DARPA DAAD19-01-1-0659 and at UCSB by iQuest. 1. A. Vidan, R.M. Westervelt, M. Stopa, M. Hanson, A.C. Gossard "Triple Quantum Dot Charging Rectifier", Applied Physics Letters 85, 3602 (2004). [Preview Abstract] |
Friday, March 25, 2005 10:36AM - 10:48AM |
X17.00014: Quantum Dot Spectroscopy D.M. Zumb\"uhl, C.M. Marcus, M.P. Hanson, A.C. Gossard Spectroscopy experiments of a GaAs lateral double dot system are presented. A large dot is weakly coupled to an adjacent few electron dot, which is used as a tunable energy spectrometer probing the density of states locally in the big dot, revealing quasistable periodic orbits embedded in a sea of chaotic electron dynamics in the big dot. A comparison with wave-packet simulations gives good agreement. Further, the tunneling current from the big dot into the spectrometer follows a power law, demonstrating a mesoscopic Fermi edge singularity which is caused by a quasiresonance of electrons with the holes left behind after tunneling from the big dot into the spectrometer. The measured power law exponents depend on mesoscopic parameters such as shape of the big dot, spectrometer energy and magnetic flux, as recently predicted by theory. Finally, the electron distribution functions in the big dot can be directly measured using the spectrometer. For short dwell times with several modes coupling the big dot to the reservoirs, a double stepped non- equilibrium distribution function is observed. When the dwell time is increased by reducing the number of modes, a single fermi-like distribution function is seen. Partially supported by DARPA SPIN (MDA972-01-1-0024) and by the NSF (DMR-0072777). [Preview Abstract] |
Friday, March 25, 2005 10:48AM - 11:00AM |
X17.00015: Capacitance lineshape in a two-channel quantum dot design C. J. Bolech, Nayana Shah We propose a set-up and discuss how the charge fluctuations on a small dot can be experimentally accessed by using a system of two single electron transistors arranged in parallel. We derive a microscopic Hamiltonian description of the set-up that allows us to make connection with the two-channel Anderson model (extensively used in heavy-Fermion systems) and make detailed predictions for the differential capacitance of the dot. [Preview Abstract] |
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X17.00016: Fermionic Mach-Zehnder interferometer subject to a Quantum Bath Florian Marquardt The Mach-Zehnder interferometer represents the simplest possible two-way interference setup. In this talk I will present a theoretical analysis of decoherence in a fermionic Mach-Zehnder interferometer coupled to any quantum-mechanical environment, employing an equations-of-motion approach [cond-mat/0410333]. I will discuss the energy-resolved dephasing rate, the connection to the theory of dephasing in weak-localization, the importance of Pauli blocking, and the shot noise correction due to the environment. The results will be compared with simpler models of dephasing, including classical noise sources, and I will comment on their relevance for experiments [cf. Yi et al., Nature 422, 415 (2003)]. [Preview Abstract] |
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