Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D21: Semiconductors: Transport |
Hide Abstracts |
Sponsoring Units: FIAP Chair: Sergey Vitkalov, City University of New York Room: 323 |
Monday, March 16, 2009 2:30PM - 2:42PM |
D21.00001: Unified Theory of Charge Transport in Wide-Band and Narrow-Band Semiconductors Frank Ortmann, Friedhelm Bechstedt, Karsten Hannewald The charge carrier mobility is often calculated within one of the two limiting cases: wide bands or narrow bands. In the case of wide-band systems, usually pure band transport is assumed along with a calculated relaxation time. In contrast, for narrow-band materials, hopping is usually considered prevalent and the interaction with lattice vibrations is described within the polaron concept. In this talk, we will present a unified approach to the description of charge transport based upon the Kubo formalism applied to a Holstein Hamiltonian. As a result, we obtain an analytical formula for the temperature dependence and anisotropy of the mobility describing a seamless transition from band transport at low temperatures to hopping transport at high temperatures. The results are illustrated for naphthalene crystals and a comparison to previous approaches [1,2] is made. \\[3pt] [1] V.M. Kenkre, Phys. Lett. A 305, 443 (2002)\\[0pt] [2] K. Hannewald and P.A. Bobbert, Phys. Rev. B 69, 075212 (2004) [Preview Abstract] |
Monday, March 16, 2009 2:42PM - 2:54PM |
D21.00002: Carrier transport in nanodevices : a competitive playground for the Boltzmann and the Wigner distribution functions? Fons Brosens, Wim Magnus In principle, transport of charged carriers in nanometer sized solid-state devices can be fully characterized once the non- equilibrium distribution function describing the carrier ensemble is known. In this light, we have revisited the Boltzmann and the Wigner distribution functions and the framework in which they emerge from the classical respectively quantum mechanical Liouville equation. We have assessed the method of the characteristic curves as a potential workhorse to solve the time dependent Boltzmann equation for carriers propagating through spatially non-uniform systems, such as nanodevices. In order to validate the proposed solution strategy, we numerically solve the Boltzmann equation for a one- dimensional conductor mimicking the basic features of a biased low-dimensional transistor operating in the on-state. Finally, we propose a computational scheme capable of extending the benefits of the above mentioned solution strategy when it comes to solve the Wigner-Liouville equation. [Preview Abstract] |
Monday, March 16, 2009 2:54PM - 3:06PM |
D21.00003: Measurement and analysis of extraordinary electroconductance in Ti-GaAs hybrid structures S.A. Solin, A.K.M. Newaz, Y. Wang, J. Wu, W.-J. Chang, V.R. Kavasseri, I.S. Ahmad, I. Adesida, R. Bashir We present a comprehensive study of a new phenomenon, extraordinary electroconductance (EEC), in microscopic and mesoscopic metal-semiconductor hybrid structures (MSH) at room temperature with different geometrical characteristics. Our artificially designed MSH structures show highly efficient external electric field sensing properties not exhibited by bare semiconductor structures. The EEC device has been fabricated from a GaAs epitaxial layer with a Ti/Au shunt. When subject to an external electric field it gives a maximum 5.2$\%$ EEC effect corresponding to an external electric field resolution of 3.05V/cm at a bias field of 2.5 KV/cm. Moreover, the study reveals a strong dependence of the transport properties on the geometry of the MSH. An analytical 2-layer model is developed which provides good agreement with the experimentally observed data.\footnote{Y. Wang, {\textit{et. al.}}, {\bf{App. Phys. Lett.}}, 92, 262106 (2008)} We propose that scaled down nanoscopic EEC sensor arrays can be used as a novel technique for imaging the charge distribution on a single cell surface in real time. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D21.00004: Universal oscillations in counting statistics Christian Flindt, Christian Fricke, Frank Hohls, Tomas Novotny, Karel Netocny, Tobias Brandes, Rolf. J. Haug Noise and fluctuations are results of stochastic processes that originate from quantum or classical sources. Higher-order cumulants of the probability distribution underlying the stochastic events are believed to contain detailed information about the stochastic process, but they are often difficult to measure. In this talk we report the first measurements of the transient cumulants of the number of electrons passing through a quantum dot to very high orders (up to order 15) [1]. The cumulants grow factorially in magnitude with the cumulant order and show surprising oscillations as functions of measurement time. Based on theory for high-order derivatives in the complex plane we show that the oscillations in fact constitute a universal phenomenon, appearing as a function of almost any system parameter for a large class of stochastic systems. Our theory provides a unified interpretation of previous theoretical studies of high-order cumulants [2] as well as our new experimental data. [1] C. Flindt, C. Fricke, F. Hohls, T. Novotny, K. Netocny, T. Brandes {\&} R. J. Haug, submitted (2008). [2] C. Flindt, T. Novotny, A. Braggio, M. Sassetti {\&} A.-P. Jauho, \textit{Phys. Rev. Lett. }\textbf{100}, 150601 (2008). [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D21.00005: Bloch Oscillations of Electrons in a Quantum-Dot Superlattice Danhong Huang, S.K. Lyo Numerical results for both the transient and steady-state currents in a strong DC electric field are presented for electrons in a quantum-dot superlattice. A microscopic scattering model is applied to study the dynamics of electrons scattered by impurities and phonons based on the Boltzmann equation. Good agreement is found between the numerical results and a recent analytic solution under a relaxation-time approximation for electron-phonon scattering [S. K. Lyo, Phys. Rev. B 77, 195306 (2008)]. Different roles played by elastic and inelastic scattering on the damped Bloch oscillations and the nonlinear steady-state are demonstrated from our numerical results. We will also briefly discuss suppression of the dynamical localization by strong Bloch oscillations under an additional nonlinear AC field and opposite roles played by elastic and inelastic scattering on the damped dynamical localization. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D21.00006: Full Counting Statistics of a Quantum Point Contact with Time-dependent Transparency Jin Zhang, Yury Sherkunov, Nicholas d'Ambrumenil, Boris Muzykantskii Controlled injection of single electrons into a ballistic conductor is essential for ``moving-electron-based'' quantum computation. We consider an electron system in two 1D ballistic conductors separated by a tunneling barrier at zero temperature. We present numerical results for the full counting statistics (FCS) for the case when the barrier potential is modulated in the presence of a time-dependent bias voltage applied between the leads. The calculation is based on Abanov and Ivanov's formula[1]. For the case of a periodic input with a large number of cycles, we perform the calculation in discretized energy space and obtain the characteristic function $\chi(\lambda)$ as well as physical observables such as the current, noise and entanglement entropy directly. We show how to optimize the gate potential of a quantum point contact to generate a single electron excitation with minimal noise. [1]A.G.~Abanov and D.A.~Ivanov, {\em Phys.Rev.Lett.}, 100, 086602 [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D21.00007: Optimal electron entangler at low temperatures Yury Sherkunov, Jin Zhang, Nicholas d'Ambrumenil, Boris Muzykantskii Electron transport in mesoscopic contacts at low temperatures is accompanied by logarithmically divergent equilibrium noise when a tunneling barrier transparency is changed abruptly. We show that the equilibrium noise can be dramatically suppressed in the case of tunneling junction with smoothly tunable transparency. We study analytically the full counting statistics of the transparency modulated quantum contact and identify the minimal excitation states generated by the gate voltage. The proposed scheme could be used as an optimal electron entangler at low temperatures. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D21.00008: Observation of quantum phase shift in an Aharonov-Bohm ring with a fully controlled flying charge qubit Michihisa Yamamoto, Christopher Bauerle, Seigo Tarucha Aharonov-Bohm effect is one of the most typical interference phenomena of electrons. Although a number of experiments have been performed to date, observation of phase shift at each path has been rather difficult due to the phase rigidity in the two-terminal setup. In this study, we employed a hybrid device consisting of a parallel coupled-wire and an AB ring, in which each coherently propagating electron acts as a flying qubit. In this device, phase rigidity no longer exists as there are two output contacts. The qubit is defined as superposition of two quantum states: an electron exists in one of the two wires. Then, the inter-wire tunnel coupling gives flipping between the two quantum states, and the evolution of the phase in the AB ring is translated into rotation about the z-axis of the Bloch sphere. In the experiment, we defined the initial qubit state by injecting electrons into only one of the two wires, and obtained the arbitrary output state by tuning gate voltages. The output state also oscillates as a function of perpendicular magnetic field $B$ with the AB oscillation period. We observed the shift of k-vector in one of the two wires works equivalently as the shift of$ B$. This is the direct observation of the phase shift \textit{$\Delta \theta =\Delta $k$\cdot $L}. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D21.00009: Current dependent dephasing in an Aharonov-Bohm Interferometer Kuan-Ting Lin, Yiping Lin, J. C. Chen, T. Ueda, S. Komiyama We have studied the temperature dependence of the current induced dephasing rate in a ballistic GaAs/Al$_{x}$Ga$_{1-x}$As ring. The dephasing rate is linearly proportional to the temperature regardless of the current applied. The AB oscillations are suppressed by the increase of the excitation current; however, the dephasing becomes less temperature dependent. Our observations cannot be interpreted by Joule heating effect. Possible decoherence mechanisms caused by the excess current will be discussed. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D21.00010: Tunable Channel Interference in an Aharonov-Bohm Ring Yiping Lin, Pei-Jung Wu, Kuan-Ting Lin, J. C. Chen, T. Ueda, S. Komiyama We have investigated the Aharonov-Bohm effect in a quasi one-dimensional ring on a GaAs/Al$_{0.3}$Ga$_{0.7}$As heterostructure, which is defined by two metallic arc gates coupled to each branch of the ring. Each gate can be separately biased to uniformly squeeze the channel width of electrons, thereby externally tuning the transverse modes in the interference paths. The oscillatory magnetoconductance of the device is systematically studied by varying the number of channels in each path. We have observed the evidence of phase shifts in the magnetoconductance oscillations due to the suppression of the mode numbers on the ring path. Though the periodicity is not well resolved, qualitatively our data support the random phase shifts between the successive modes. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D21.00011: Transport and noise in 90nm n-GaAs Epilayers A. Gilbertson, J.D. Moore, G. Perkins, J. Gallop, L.F. Cohen, A.K.M. Newaz, S.A. Solin Extraordinary Magnetoresistance (EMR) belongs to the family of
EXX effects which form the basis for a number of devices that
offer the potential for high sensitivity applications. Such
devices would benefit from minimising the active volume of the
sensor. To reduce that volume and minimize wafer fabrication
complexity it is desirable to employ unltra-thin GaAs epilayers.
Accordingly, we report here the transport and noise properties
of 90nm Si-doped GaAs films grown by molecular beam epitaxy which
have been fabricated into both microscopic EMR devices and
macroscopic van der Pauw geometries. These films exhibit a room
temperature electron mobility and density of 3225
$cm^2V^{-1}s^{-1}$ and1.45x$10^{17}cm^{-3}$, respectively, and
show only a 6\% variation over the temperature range $2K |
Monday, March 16, 2009 4:42PM - 4:54PM |
D21.00012: New Measurement of Diffusion Thermopower of 2D Electron Systems W.E. Chickering, J.P. Eisenstein, J.L. Reno The thermoelectric properties of low-dimensional electronic systems provide information about carrier transport that is complementary to that obtained from ordinary charge transport. In conventional measurements of the thermopower $S$ of two-dimensional electron systems (2DESs), phonon drag overwhelms the diffusion thermopower $S_d$ of the electron gas for temperatures $T > \sim 0.1$ K. We introduce a new hot electron thermocouple technique which vastly reduces the importance of phonon drag and allows us to accurately determine $S_d$ in a 2DES in a GaAs/AlGaAs heterostructure. Differentially gated 2DES channels provide the analogs of the dissimilar metals used in a conventional thermocouple. The device is calibrated via the temperature dependence of the longitudinal resistance of the 2DES at the thermocouple junction. Our results are in good quantitative agreement with the Mott formula for the temperature and density dependence of $S_d$ for temperatures up to $T \sim 2$ K. This work is supported by DOE grant DE-FG03-99ER45766 and Microsoft Project Q. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D21.00013: Thermopower of n- and p-type InN Nate Miller, Joel Ager, Rebecca Jones, Holland Smith, Kin Man Yu, Eugene Haller, Wladek Walukiewicz, William Schaff, Chad Gallinat, Gregor Koblmuller, Jim Speck The exceptionally large ($>$ 5.5 eV) electron affinity of InN leads to unique electronic properties such as surface electron accumulation and an extreme propensity for n-type conduction. This, combined with a small energy gap and strongly energy dependent effective mass, makes an analysis of charge transport and determination of band structure parameters an arduous task. In this work we show that thermopower (Seebeck coefficient) measurements can address some of the issues by providing a new tool to study the unique charge transport properties of InN and In-rich group III-nitride alloys. Our thermopower experiments are used to demonstrate the presence of mobile holes in Mg-doped InN providing the first direct, quantitative measurement of hole transport in InN. We also report modeling of the thermopower of n-type InN considering the various scattering mechanisms. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D21.00014: Measurement of [N] and T-dependence of electron effective mass in GaAsN Tassilo Dannecker, Yu Jin, John Buckeridge, Ctirad Uher, Cagliyan Kurdak, Stephen Fahy, Rachel S. Goldman The electron effective mass of GaAs$_{1-x}$N$_{x}$ is predicted to be dependent on N-composition, x, and temperature, T; however, conflicting results have been observed using cyclotron resonance and thermomagnetic measurements. Using thermopower and Hall measurements, in conjunction with assumptions of parabolic bands and Fermi-Dirac statistics, we determined the T-dependence of the electron effective mass of GaAs$_{1-x}$N$_{x}$, in comparison with that of GaAs. Measurements of the T-dependent Seebeck coefficient, S, for N compositions ranging from x=0 to 0.0100, reveal a decrease in dS/dT with increasing x. For GaAs, the free carrier concentration, [n], is independent of T. In all other cases, [n] increases (decreases) with T (x). For GaAs, the effective mass decreases from 0.06m$_{0}$ at 140K to 0.052m$_{0}$ at 300K, similar to literature reports.$^{4}$ For GaAsN, the effective mass apparently increases (decreases) with x (T), ranging from 0.1m$_{0}$ to 0.16m$_{0}$ at 140K, with values 14{\%} (40{\%}) lower for x=0.0075 (x=0.0100) at 300K. [Preview Abstract] |
Monday, March 16, 2009 5:18PM - 5:30PM |
D21.00015: Temperature dependence of electron mobilities in InN Leonardo Hsu, Wladek Walukiewicz InN allows the possibility of engineering nitride materials with bandgaps as small as 0.7 eV. We have calculated electron mobilities in InN taking into account the non-parabolicity of the conduction band, as well as the standard scattering mechanisms of acoustic and optical phonons and Coulomb scattering from charged impurities. Although our calculations explain well the experimentally measured mobilities at temperatures higher than about 200 K, the measured mobilities in lightly doped InN at low temperatures decrease in a way that cannot be accounted for by the standard theory. We discuss the characteristics and possible origins of the additional mechanism that must be included in the calculations in order to fit the experimental results. [Preview Abstract] |
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