Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q23: Quantum Hall Effect: Tunneling, High-Frequency |
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Sponsoring Units: FIAP DCMP Chair: Ramesh Mani, Georgia State University Room: 325 |
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q23.00001: Landau Level Spectrum in In$_{0.53}$Ga$_{0.47}$As/InP Heterostructures Chi Zhang, Yanhua Dai, Kristjan Stone, Rui-Rui Du We report on magnetotransport results from a high-quality Rashba two-dimensional electron gas (2DEG) formed in undoped In$_{0.53}$Ga$_{0.47}$As/InP heterostructures, which were MOCVD grown on (100) InP substrate. The 2DEG has a density of n$_{e}\sim $ 1.1 x 10$^{11}$/cm$^{2}$ and a mobility of $\mu \sim $ 2 x 10$^{5}$ cm$^{2}$/Vs at T = 300 mK, and with illumination from a light-emitting diode, the n$_{e}$ can be tuned to 1.5 x 10$^{11}$/cm$^{2}$. A systematic pattern in R$_{xx}$ and R$_{xy}$, corresponding to the opening and closing of the integer quantum Hall gaps, was observed as a function of n$_{e}$, as well as a function of tilt angle in tilted field experiments. We are interested in the possible observation of resonant spin Hall conductance in this system. Experimental data and a brief discussion will be presented. The work at Rice was supported by NSF DMR-0706634. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q23.00002: The quenching of scattering-enhanced tunneling in the Quantum Hall Regime Kasey Russell, Federico Capasso, Venkatesh Narayanamurti, Hong Lu, Joshua Zide, Arthur Gossard Using capacitance-voltage spectroscopy, we are able to probe the magnetic field dependence of the quasi-bound state lifetime of a quantum well. Our measurements are done on a special InGaAs/InAlAs heterostructure that is designed to promote scattering-enhanced tunneling out of the quantum well. The bound state lifetime shows large oscillations as the magnetic field is varied, which result from the quenching of electron scattering as the Fermi level crosses a Landau Level. The results are interpreted in terms of the edge states and cyclotron orbitals of the Integer Quantum Hall Effect. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q23.00003: Microwave modes of a two dimensional electron systems in the presence of a perpendicular magnetic anisotropy provided by a macroscopic ferromagnet Brenden Magill, L. W. Engel, M. P. Lilly, J. A. Simmons, J. L. Reno We report on a resonance in the microwave absorption spectrum of a high mobility two dimensional electron system (2DES) in a spatially varying magnetic field from a ferromagnet placed near the sample. The ferromagnet, made of Dy, cylinder or plates, with a hole through it. Microwave transmission between contacts capacitively coupled to the 2DES near the hole or cylinder shows a resonant absorption peak f$_{pk}$ for holes and cylinders with varying radii, r$_{m}$, from 0.5 mm to .125 mm. The resonance absorption is present for a uniform external field B$_{o }$less than 1 T and only for specific magnetization of the ferromagnet with respect to B$_{0}$ with f$_{pk}$ decreasing as either B$_{o}$ or r$_{m}$ increase. We will interpret the data in terms of plasma excitations similar to edge magnetoplasmons [1] confined along the magnetic field inhomogeneity by the large magnetic field gradients there. [1] See, for example, V. A. Volkov and S. A. Mikhailov, Sov. Phys.-JETP \textbf{67}, 1639(1988). [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q23.00004: Itinerant electron-driven chiral magnetic ordering and spontaneous quantum Hall effect in triangular lattice models Ivar Martin, C.D. Batista We study the Kondo Lattice and the Hubbard models on a triangular lattice. We find that at the mean field level, these rotationally invariant models naturally support a non-coplanar chiral magnetic ordering. It appears as a weak-coupling instability at the band filling factor 3/4 due to the perfect nesting of the itinerant electron Fermi surface. This ordering is a triangular-lattice counterpart of the collinear Neel ordering that occurs on the half-filled square lattice. While the long-range magnetic ordering is destroyed by thermal fluctuations, the chirality can persist up to a finite temperature, causing a spontaneous quantum Hall effect in the absence of any externally applied magnetic field. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q23.00005: Comparative study of radiation-induced transport in Wegscheider's and Umansky's GaAs/AlGaAs material R.G. Mani, W. Wegscheider, V. Umansky Transport studies of GaAs/AlGaAs specimens have shown radiation-induced, periodic-in-the-inverse-magnetic-field, magnetoresistance oscillations that saturate into novel radiation-induced zero- resistance states (RIZRS) at the deepest oscillatory minima.[1] The origin of these RIZRS remains a topic for further experimental investigation, as does the dependence of these phenomena on the impurity configuration and the material quality. On the latter point, it remains to be understood if similar material prepared in different laboratories yield a similar response under the same conditions. In addressing this issue, we examine here the radiation-induced transport in GaAs/AlGaAs material prepared by W. Wegscheider and co-workers. In a previous study, Simovic et al.[2] have reported the observation of B-periodic radiation- induced oscillations and the strong suppression of the inverse- B periodic oscillations in Wegscheider's GaAs/AlGaAs material. Here, we compare our experimental results to their study and also to our own previous results obtained on specimens prepared by V. Umansky and co-workers. 1) R. G. Mani, Appl. Phys. Lett., 91, 132103 (2007). 2) B. Simovic et al., Phys. Rev. B 71, 233303 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q23.00006: Transport measurements and simulations of GaAs/AlGaAs ``anti-Hall-bar within a Hall bar'' devices Annika Kriisa, Ramesh G. Mani Hall effect measurements are often carried out in the Hall geometry, which is a thin rectangular plate with current and Hall voltage contacts at the external boundary. The motivation of this study is to further understand the impact on Hall effect when a hole is inserted inside Hall geometry. One way on conducting this investigation is to superimpose an ``anti-Hall bar'' inside the standard Hall bar, where the anti Hall bar is actually the hole inside the Hall device with contacts on the inside boundary of this hole. This configuration is thought to generate an ordinary Hall effect within the interior boundary such that the interior Hall voltage divided by the interior injected current equals the Hall resistance. One believes that it might also be possible to simultaneously realize multiple independent Hall effects by injecting multiple currents into the multiply connected device [1]. We have studied Hall effect in the doubly connected ``anti-Hall bar within a Hall bar'' geometry fabricated out of the GaAs/AlGaAs semiconductor system. Also the simulations of the distribution of the Hall current and potential profile within the specimen are conducted. To attain understanding of how the Hall effect arises in this geometry, the simulation plots are compared to the experimental results. [1] R. G. Mani and K. von Klitzing, App. Phys. Lett. 1993, 64, 1262-1264. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q23.00007: Effect of strain on nematic phases of two-dimensional hole gases Sunanda Koduvayur, Leonid Rokhinson, Michael Manfra We study the effect of uniaxial strain on high Landau levels(LL), $N\geq2$ (N is the LL index), in two dimensional hole gases(2DHG). The presence of anisotropic magnetotransport at certain half-integer filling factors in these systems has been understood as a signature of stripe or nematic phases. Recent studies on 2DHG in a perpendicular field have shown anisotropic transport at filling factors $\nu=7/2$ and $11/2$ accompanied by an isotropic $9/2$ state. These results differ from those of 2D electrons where anisotropy is only observed for LLs with $N\geq3$. While this difference has been attributed to stronger spin-orbit interactions in holes, the origin and conditions necessary for the stabilization of these states are still open questions. We study samples fabricated in the Van der Pauw geometry from C-doped GaAs/AlGaAs 2DHG grown on (001) substrate. We apply uniaxial strain along $[110]$ and study the transport properties in a perpendicular field at $10$mK. We introduce nematic states earlier on for $2\leq N\leq7$ with large enough strain. Furthermore, we demonstrate reversal in direction of anisotropy at filling factors $\nu=7/2$ and $5/2$ with strain modulation. We also see a difference in strain response of the resistances along $[110]$ and $[1\overline{1}0]$. We try to understand the observed effects using an electrostatic model which incorporates the anisotropy of the elastic moduli of GaAs. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q23.00008: Dependence of Effective Mass on Spin and Valley Degrees of Freedom Mansour Shayegan, Tayfun Gokmen, Medini Padmanabhan We measure the effective mass ($m^*$) of interacting two- dimensional electrons confined to an AlAs quantum well at a fixed density while we change the conduction-band valley occupation and the spin polarization via the application of strain and magnetic field, respectively [1]. Compared to its band value, $m^*$ is enhanced when the electrons are valley or spin unpolarized, and the largest enhancement is observed for the case where both spin and valley are unpolarized. Consistent with the study of M. Padmanabhan et al., in the fully spin- and valley-polarized regime, the measured $m^*$ is suppressed compared to the band value. Incidentally, in the fully spin- and valley-polarized regime, the electron system exhibits an insulating behavior. [1] T. Gokmen \textit{et al.}, Phys. Rev. Lett. {\bf 101}, 146405 (2008). [2] M. Padmanabhan, \textit{et al.}, Phys. Rev. Lett. {\bf 101}, 026402 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q23.00009: Anomalous Effective Mass of Two-dimensional Holes in a Strong Parallel Magnetic Field YenTing Chiu, Medini Padmanabhan, Javad Shabani, Mansour Shayegan, Roland Winkler We report effective hole mass (m*) measurements through analyzing the temperature dependence of the Shubnikov-de Haas oscillations in dilute (density $\sim $ 5x10$^{10}$cm$^{-2})$ two-dimensional (2D) hole systems confined to a 20nm-wide, (311)A GaAs quantum well. In this system the 2D holes occupy two spin-subbands whose m* we measure to be $\sim $ 0.2 (in units of free electron mass), in good agreement with the theoretical band calculations. We then apply a sufficiently strong ($>$10T) parallel magnetic field to fully depopulate one of the spin subbands, and measure m* for the populated subband. We find that this latter m* is close in magnitude to the m* we measure in the absence of the parallel field. This is a surprising observation as it is in stark disagreement with the results of our band calculations which take into account the spin-orbit interaction and the holes' finite layer thickness, and predict a large enhancement of m* in a strong parallel magnetic field. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q23.00010: Quantum Hall Effect and Field Dependent Valley Splitting on High Mobility Silicon-(111) Surfaces Tomasz M. Kott, Robert N. McFarland, Luyan Sun, Bruce E. Kane, Kevin Eng We have developed a method for fabricating field effect transistors, using vacuum as the dielectric, in order to study electron transport on a clean, flat, chemically prepared hydrogen-terminated surface. Resulting devices display high mobilities (110,000 cm$^2$/V s at 70 mK), enabling us to probe field dependent transport dynamics of this six-fold valley degenerate surface. I will present evidence that a low oxygen environment during sample preparation is necessary to achieve high mobilities. To support the correlations between surface chemistry and electronic properties, I will show AFM images of the surface for various preparation techniques. Finally, I will describe high field magneto-transport measurements (up to 12 T) that indicate field-dependent valley splitting. In particular, we find easily resolvable filling factors of, amongst others, 3, 5, and 7; an indication that the six-fold degeneracy is possibly broken by many-body effects. I will also show preliminary data with hints of the FQHE at $\nu = \frac{4}{3}$ and $\frac{8}{5}$. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q23.00011: Interaction Effects in Conductivity of a Two-Valley Electron System in High-Mobility Si Inversion Layers Nikolai N. Klimov, Dmitry A. Knyazev, Oleg E. Omel'yanovskii, Vladimir M. Pudalov, Harry Kojima, Michael E. Gershenson We have measured the conductivity of high-mobility (001)~Si metal-oxide-semiconductor field-effect transistors over wide ranges of electron densities $n = (1.8-15)\times 10^{11}$\,cm$^{-2}$, temperatures $T = 30$\,mK$-4.2$\,K, and in-plane magnetic fields $B_\parallel= 0-5$\,T [1]. The experimental data have been analyzed using the theory of interaction effects [2] in the conductivity $\sigma$ of disordered two-dimensional (2D) systems. The parameters essential for comparison with the theory, such as the intervalley scattering time and valley splitting, have been measured or evaluated in independent experiments [1,3]. The observed behavior of $\sigma$, including its quasi-linear increase with decreasing $T$ down to $\sim 0.4$\,K and its downturn at lower temperatures, is in agreement with the theory. The values of the Fermi-liquid parameter obtained from the comparison agree with the corresponding values extracted from the analysis of Shubnikov--de Haas oscillations based on the theory of magneto-oscillations in interacting 2D systems [4]. [1] N.\,N.\,Klimov {\em et. al.}, PRB {\bf 78}, 195308 (2008). [2] G.\,Zala {\em et. al.}, PRB {\bf 64}, 214204 (2001); {\bf 65}, 020201(R) (2001). [3] A.\,Yu.\,Kuntsevich {\em et. al.}, PRB {\bf 75}, 195330 (2007). [4] Y.\,Adamov {\em et. al.}, PRB {\bf 73}, 045426 (2006). [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q23.00012: 2DEG effect on vibration of piezoelectric plates Alexey Suslov Resonances of a GaAs wafer with a GaAs/AlGaAs heterostructure grown on one of its sides were studied in the temperature range 0.05-10K in magnetic fields of up to 18 T. To the best of our knowledge, this is the first use of the Resonant Ultrasound Spectroscopy in a dilution refrigerator. Observed quantum oscillations of the resonance frequencies and linewidths were caused by the Quantum Hall Effect in the 2DEG. The wafer with the 2D gas can be conceived as a film with field dependent conductivity deposited on a piezoelectric plate. Being dielectric, the film does not affect properties of GaAs and, thus, the resonance frequencies are defined only by the elastic, piezoelectric, and dielectric constants of GaAs. Being metallic, the 2D sheet effectively screens the parallel electric field, so the ultrasound wave velocities and resonance frequencies decrease with the increase of the sheet conductivity. Oscillations of the resonance linewidth reflect the influence of the 2D system on the ultrasound attenuation. A metallic film as well as a dielectric one does not affect this attenuation but at some finite nonzero value of the conductivity the linewidth approaches a maximum value. The observed phenomena can be described by the relaxation type equations. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q23.00013: Quantum Hall transition on a triangular lattice; network model and analytical renormalization-group treatment Vagharsh Mkhitaryan, Mikhail Raikh Common approach to the theoretical study of the quantum Hall transition is the Chalker-Coddington network model on the square lattice. We introduce a new version of the network model formulated on the triangular lattice, where the scattering at the sites is described by $3\times 3$ matrix. Extending renormalization-group description of the classical site percolation to the quantum case, we derive a closed equation for the distribution function of conductance. Solving this equation numerically, we get for the critical exponent of the correlation radius $\nu \approx 2.3\div2.76$ in good agreement with established value $\nu=2.33$. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q23.00014: Half integer features in the quantum Hall Effect: experiment and theory Tobias Kramer, E.J. Heller, R.E. Parrott, C.-T. Liang, C.F. Huang, K. Y. Chen, L.-H. Lin, J.-Y. Wu, S.-D. Lin We discuss experimental data and a new model of the integer quantum Hall effect (IQHE), which explains an intriguing substructure within Landau levels observed at higher currents. The experiments show inflection points in the Hall resistivity around filling factors 5/2 and 7/2. The experiments require to revisit the foundations of the IQHE and to establish an injection model which incorporates the correct boundary conditions imposed by a real Hall device and the Lorentz force. We have to follow the electrons to their source: one corner of the Hall bar and its steep electric field gradients, rather than focusing on the middle of the Hall device. We find the entire Hall resistivity curve is calculable as a function of magnetic field, temperature, and current. In contrast to previous theories of the IQHE, disorder plays no fundamental role in our theory. Contrary to the standard picture of Landau levels in disorder system, we predict and observe gaps right in the middle of certain Landau levels. The Hall plateaus and half integer inflections are shown to result from the LDOS appropriate to the magnetic field and the strong electric field at the injection corner. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q23.00015: Operation of a Single Electron Transistor Placed on Stacked Integer Quantum Hall Layers as a Magnetometer Hailing Cheng, Yu Jin, Rachel Goldman, Cagliyan Kurdak A single electron transistor (SET) placed on an integer quantum Hall liquid (IQHL) can detect small time varying magnetic fields in the presence of a large constant magnetic field. To enhance the sensitivity, we placed a SET made out of Al/AlOx/Al tunnel junctions on top of a GaAs/AlGaAs heterostructure with 25 identical quantum well structures. By monitoring the conductance of the SET following a small change in magnetic field, we studied the equilibration processes in IQHLs. The equilibration times associated with small changes in magnetic field are found to be strongly dependent on the magnetic field and became unmeasurably long (many days) as we got closer to the center of the quantum Hall plateaus. We have also characterized the SET magnetometer by an ac technique where we applied 170 nT ac magnetic field and measured the response of the SET using a double lock-in technique. At T=20 mK, the SET magnetometer worked at filling fractions up to n=12 and was most sensitive at the filling fraction n=6. [Preview Abstract] |
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