Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Z46: 2D Multi-Valley Systems: QHE Experiment |
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Sponsoring Units: FIAP Chair: Bruce Kane, University of Maryland Room: Baltimore Convention Center 349 |
Friday, March 17, 2006 11:15AM - 11:27AM |
Z46.00001: Observation of Inter-valley Gap Anomaly in Two Dimensional Electrons in Si K. Lai, T.M. Lu, D.C. Tsui, S.A. Lyon, W. Pan, M. Muhlberger, F. Schaffler, J. Liu, Y.H. Xie We report a systematic study of the energy gaps at the odd-integer quantum Hall states $\nu$=3 and 5 under tilted magnetic (B) fields in the two-dimensional electron system (2DES) in Si/Si$_{1-x}$Ge$_x$ heterostructures [1]. Consistent with previous studies, we find that out of the coincidence region, the valley splitting is independent of the in-plane B-field. However, the $\nu$=3 valley gap appears to be highly asymmetric and differs significantly on different sides of the coincidence. Similar behaviors were observed in both high (20m$^2$/Vs) and low (6m$^2$/Vs) mobility samples. More surprisingly, instead of reducing to zero at coincidence, as expected in the independent-electron model, the inter-valley gaps at $\nu$=3 and 5 rise rapidly towards the coincidence angles. We will discuss our results in the framework of two known models, level coupling with random-matrix elements and quantum Hall ferromagnetism, and show that the anomaly is related to the strong couplings of the Landau levels close in energy in the coincidence region. [1] K. Lai $et$ $al$., cond-mat/0510599. [Preview Abstract] |
Friday, March 17, 2006 11:27AM - 11:39AM |
Z46.00002: Integer quantum Hall effect and valley splitting on a H-Si(111) surface Kevin Eng, Robert N. McFarland, Bruce E. Kane We have recently developed a high mobility two-dimensional electron system, where an electric field is applied through an encapsulated vacuum cavity and induces electrons on a clean and atomically flat hydrogen-passivated Si surface. Low temperature magneto-transport measurements (0$<$B$<$12T) made on such an inversion layer has led us to the first observation of the integer quantum Hall effect on a Si(111) surface. With a mobility of $\sim $10,000 cm$^{2}$/Vs at T=150 mK and n$_{s}$=6.75 x 10$^{11}$ cm$^{-2}$, we observed filling factors $\nu $=6, 4, 3, and 2. This is particularly interesting, because Si(111) is expected to have a six-fold valley degeneracy. Activation energy measurements within filling factors 6 and 2 coupled with analysis of Shubnikov-de Haas oscillations suggests that the six-fold valley degeneracy is broken asymmetrically into 2-fold and 4-fold at B=0. In addition, the observation of $\nu $=3 indicates that individual valleys are splitting at higher fields. We will discuss these results along with the possible origins of the breaking of the six-fold valley degeneracy at B=0. [Preview Abstract] |
Friday, March 17, 2006 11:39AM - 11:51AM |
Z46.00003: Valley Dynamics on H-Si(111) in Tilted Magnetic Fields Robert N. McFarland, Kevin Eng, Bruce E. Kane Because of the predicted sixfold valley degeneracy of the Si(111) surface and the numerous mechanisms that can lift this degeneracy, 2D electron transport on this surface displays complex and interesting behavior. To explore this behavior, we have performed low temperature ($\sim $150 mK) tilted-field magneto-transport measurements (0$<$B$_{tot}<$12 T) on H-passivated Si(111) surfaces encapsulated in a vacuum cavity. We find a strong dependence of the longitudinal magneto-resistance on the in-plane B field. At low fields ($<$2 T) this manifests as an amplification of the Shubnikov-de Hass amplitudes without a significant effect on the phase or frequency. At high fields ($>$6 T, $\nu <$6) the valley degeneracy is lifted, and we observe energy gaps at $\nu $=4 and $\nu $=3 which are strongly affected by small in-plane fields. Since in neither case are the observed effects linear in total B field, the results are inconsistent with Zeeman-like effects. We present measurements of the activation energies for the gaps at $\nu $=4 and 3 as a function of tilt angle. Finally, we consider possible mechanisms for this field-dependent splitting and propose future experiments that may shed further light on the matter. [Preview Abstract] |
Friday, March 17, 2006 11:51AM - 12:03PM |
Z46.00004: The spin susceptibility in Si-MOSFETs. Mariapia Marchi, De Palo Stefania, Saverio Moroni, Gaetano Senatore We have performed for the first time DMC simulations of a symmetric two-valley electron gas with variable spin polarization, both in the strictly 2D limit and with a thickness appropriate to Si-MOSFETs. We find that valley degeneracy substantially reduces the spin susceptibility enhancement $\alpha $, with respect to the conventional 2DEG. The farther reduction of $\alpha $, caused by thickness, brings our prediction in excellent agreement with the available experimental data on Si-MOSFETs[1] up to r $_{s} \quad \approx $ 5. The agreement extends to the full experimental range (r $_{s}\le $8) when weak disorder is kept into account within a response function formalism, generalizing to the two valley system the approach previously employed for the conventional 2DEG [2]. [1] See, e.g, S.V. Kravchenko and M.P.Sarachik, Rep. Prog. Phys. \textbf{67}, 1 (2004) and references therein. [2] S. De Palo et. al., Phys. Rev. Lett. \textbf{94}, 226405 (2005).\textbf{ } [Preview Abstract] |
Friday, March 17, 2006 12:03PM - 12:15PM |
Z46.00005: Valley susceptibility of an interacting two-dimensional electron system. Yakov Shkolnikov, Oki Gunawan, Kamran Vakili, Tayfun Gokmen, Etienne De Poortere, Mansour Shayegan In a wide AlAs quantum well grown on a (001) GaAs substrate, 2D electrons occupy two elliptical conduction band minima (valleys), whose major axes lie along the two in-plane $<$100$>$ directions. Strain-induced energy splitting between these valleys results in an inter-valley charge transfer and consequently in a change of the system's valley polarization. We parameterize this strain-induced change of the valley polarization using valley susceptibility $\chi _{v}$, in analogy to the spin-susceptibility traditionally used to characterize the magnetic field induced spin-polarization. We find that $\chi _{v}$ dramatically increases relative to its band value as the electron density is reduced, reflecting the dominant role of electron-electron interaction. [Preview Abstract] |
Friday, March 17, 2006 12:15PM - 12:27PM |
Z46.00006: Selective occupation of conduction band valleys in AlAs quantum wells. Medini Padmanabhan, K. Vakili, Y.P. Shkolnikov, O. Gunawan, T. Gokmen, E. Tutuc, E.P. De Poortere, M. Shayegan We report our progress in the controlled and selective occupation of various conduction band minima in AlAs quantum wells and the corresponding magnetotransport behaviors. AlAs has three conduction band minima, or valleys, located at the X-points of the Brillouin zone. By a suitable choice of well width and in-plane stress, these minima can be occupied by two-dimensional electrons singly or in various combinations. We review the factors that determine the valley occupation and show how we have used them to produce a variety of systems with different valley occupations. In particular, we will discuss two methods that we have successfully used: (a) using a piezoelectric material to impart uniaxial strain and, (b) using a material with a different thermal expansion coefficient than our samples to impart biaxial tensile strain. [Preview Abstract] |
Friday, March 17, 2006 12:27PM - 12:39PM |
Z46.00007: Landau level crossings in imbalanced, two-valley two-dimensional electron systems Kamran Vakili, Tayfun Gokmen, Medini Padmanabhan, Oki Gunawan, Yakov P. Shkolnikov, Emanuel Tutuc, Mansour Shayegan We report results of magnetotransport measurements performed on two-dimensional electron systems in AlAs quantum wells with highly imbalanced valley occupation. We observe spikes in the diagonal resistance that signal the crossing of Landau levels originating from each of the occupied valleys. From the positions of these spikes and their dependence on tilt angle, we can extract the valley splitting as well as the ratios of the effective electron masses and g-factors for the two-valleys. We find that the mass ratio is unchanged from the band value and the g-factors are equal between the two valleys despite the high degree of density imbalance. We have also explored the interaction-induced finite gaps that persist at avoided crossings between Landau levels and discuss some of our results. [Preview Abstract] |
Friday, March 17, 2006 12:39PM - 12:51PM |
Z46.00008: Parallel magnetic field induced valley splitting T. Gokmen, O. Gunawan, K. Vakili, E. P. De Poortere, M. Shayegan In an 11nm-wide thick AlAs quantum well grown on a GaAs (001) substrate, the two-dimensional electrons occupy two in-plane conduction-band valleys. The Fermi contours for these two valleys are ellipses whose major axes are rotated by 90 degrees. The energy separation between these two valleys at zero magnetic field is normally controlled via the application of in-plane strain along the major axis of one of the valleys. In this talk, through a combination of experimental results and calculations, we demonstrate a novel method for tuning of the valley splitting. Namely, we show that the application of a magnetic field parallel to the major axis of one the ellipses leads to a lifting of the valley degeneracy. This is because the parallel field which, thanks to the finite thickness of the electron layer couples to the electrons' orbital motion modifies the effective masses of the two valleys (in the direction perpendicular to the field direction) by different amounts. [Preview Abstract] |
Friday, March 17, 2006 12:51PM - 1:03PM |
Z46.00009: Multiple Phases with the Same Quantized Hall Conductance in a Two-Subband System Xinchang Zhang, Donald Richard Faulhaber, Hongwen Jiang In a GaAs/AlGaAs two-dimensional electron system with two occupied subbands, the experimentally determined phase diagram exhibits rich topological features. Ring-like structures are observed at even integer filling factors. Even with the identical quantized Hall resistance values as those given rise by the ordinary integer quantum Hall effect due to the Landau level quantization; the activation energies of these states within the rings are much smaller. These ring structures cannot account for by the simple single particle picture. We argue that ferromagnetic quantum Hall states, due to the interaction of two energy levels with opposite spin and different subband indices, are responsible for these unusual structures. [Preview Abstract] |
Friday, March 17, 2006 1:03PM - 1:15PM |
Z46.00010: Anomalous magnetoresistance peaks: Evidence for Landau level spin-anticrossing in (110) GaAs two-dimensional hole systems M. Grayson, S. F. Roth, Y. Xiang, F. Fischer, M. Bichler, D. Schuh, R. Winkler We report anomalous peaks in the longitudinal resistance of (110) GaAs two-dimensional hole systems (2DHS), which we associate with an anticrossing of spin split Landau levels. GaAs hole systems are of research interest due to their heavy mass and strong spin-orbit coupling. With modulated growth conditions, Si can be used as an acceptor for high mobility 2DHS on (110) wafers with mobilities up to $7 \times 10^5 {\mathrm cm} ^2$/Vs at a hole density of $1.2\times 10^{11} \mathrm{cm}^{-2}$. Resistance peaks within the $\nu = 1$ minimum of the quantum Hall effect are observed in various samples. We propose that these anomalous peaks arise from an anticrossing of Landau levels with magnetic field. The position of the anticrossing field is independent of top-gate voltage, but can be tuned by illumination. This behaviour is in agreement with self- consistent calculations of the valence band mixing, assuming an illumination dependent background charge in the substrate. Band calculations predict a spin-flip at the anticrossing, which might be exploited to test new spin based device concepts. [Preview Abstract] |
Friday, March 17, 2006 1:15PM - 1:27PM |
Z46.00011: Monitoring the Motion of Charge Carriers in the Integer Quantum Hall Bulk with a Single Electron Transistor Lee Farina, \c{C}agliyan Kurdak, Mansour Shayegan Using an Al/AlO$_{x}$/Al single electron transistor on top of an antidot etched into a GaAs/AlGaAs heterostructure containing a two-dimensional electron gas, we studied the quantum Hall system in the middle of the $\nu$=2 plateau. In this highly insulating regime, oscillations in single electron transistor resistance indicate that the charges in the system adjust slowly ($\sim$ 1 hour) to changes in the back gate voltage or magnetic field. We find that after a small increase in magnetic field an excess of electrons leave the bulk. Moreover, the presence of the etched antidot under the SET creates a small but clear discrepancy between behavior after a change in magnetic field and change in back gate voltage indicating charging of the antidot after a change in magnetic field. When the antidot is accounted for we find the expected ratio of 2 electrons/flux quanta at $\nu$=2. We also present the time dependence of the charge equilibration for 18 mK to 280 mK. Bulk conductivities in the range of 10$^{-18}$-10$^{- 18}$ /$\Omega$ are obtained from this measurement and compared to conductivies obtained from lock-in measurements at high temperatures (1-10 K). [Preview Abstract] |
Friday, March 17, 2006 1:27PM - 1:39PM |
Z46.00012: Frozen Charges, Persistent Eddy Currents, and Nearly Perfect Diamagnetism in Stacked Integer Quantum Hall Layers Cagliyan Kurdak, Lee A. Farina Faraday's law of induction for an integer quantum Hall liquid dictates that after the application of a small additional magnetic field an integer number of electrons for each additional magnetic flux quanta must enter the 2DEG. Since the diagonal conductivity is close to zero, these charges are nearly frozen. The electric field associated with these charges leads to a persistent eddy current flowing in the bulk of the 2DEG. The magnetic field generated by the persistent eddy currents is much smaller than the additional magnetic field applied. However, the magnetic field generated by the persistent eddy currents can be greatly enhanced in closed stacked layers of integer quantum disks. In fact, we find that when the number of layers is much greater than inverse fine structure constant, these persistent eddy currents can perfectly screen the applied additional magnetic field at zero temperature. [Preview Abstract] |
Friday, March 17, 2006 1:39PM - 1:51PM |
Z46.00013: Spin and Valley Splitting of Two-Dimensional Electrons in Graphene in the High Magnetic Field Limit Yuanbo Zhang, Zhigang Jiang, Joshua Small, Meninder Purewal, Yan-Wen Tan, Mina Fazlollahi, Joel Chudow, John Jaszczak, Horst Stormer, Philip Kim The quantum Hall effect in high quality graphene, a sinlge atomic layer of graphite, is studied in strong magnetic fields up to 45 Tesla. The splitting of Landau levels $n$ = 0 and $\pm $1, caused by the lifting of the spin and valley degeneracies in strong magnetic fields, is observed at T $<$ 5 K. In particular, the quantum Hall state $\nu =\pm $4 is found to arise from the spin splitting of Landau level n = $\pm $1. The effective Lande g-factor measured at this state is close to 2. The spin origin of $\nu =\pm $4 is further confirmed in magnetotransport experiments performed in the presence of an in-plane magnetic field. While the exact origin of the valley degeneracy lifting is not yet clear, we propose several possibilities. [Preview Abstract] |
Friday, March 17, 2006 1:51PM - 2:03PM |
Z46.00014: Quantized conductance in an AlAs 2D electron system quantum point contact M. Shayegan, O. Gunawan, B. Habib, E.P.D. Poortere We report experimental results on a quantum point contact (QPC) device fabricated in a wide AlAs quantum well where the electrons occupy two conduction-band valleys with in-plane, elliptical Fermi contours. To probe the closely-spaced, one-dimensional, electric subbands (channels), we fabricated a QPC structure where the point contact is defined by a split-etched region covered by a global top gate. The conductance versus top gate bias trace shows a series of weak plateaus at every 2$e^{2}$/$h$, consistent with the fact that the one-dimensional confinement and the anisotropy of the Fermi contours break the valley degeneracy for the QPC channels. Moreover, we observe a ``0.7 structure'' which is much better developed than the 2$e^{2}$/$h$ plateaus, possibly reflecting the stronger electron-electron interaction in this system because of the larger electron effective mass. [Preview Abstract] |
Friday, March 17, 2006 2:03PM - 2:15PM |
Z46.00015: Nonlinearities in Strongly Coupled Lateral Quantum Hall Droplets P. Jiang, C. Chien, I. Yang, W. Kang, L.N. Pfeiffer, K.W. Baldwin, K.W. West We report on conductance characteristics of a lateral quantum Hall line junction with an aperture in a thin tunnel barrier (width = 8.8 nm). Due to the small opening inside the barrier, the charge distribution in the junction resembles that around a narrow split gate. In the presence of a magnetic field, the edge states of the two- dimensional electron systems across these particular junctions are strongly coupled compared to those with pristine barriers. In the integer quantum Hall effect (IQHE) regime, a strong resonance in conductance is detected at zero bias, while it evolves into an anomalous suppression as the system enters the fractional quantum Hall effect (FQHE) regime. The dramatic suppression of the conductance in the FQHE regime is consistent with the presence of strong repulsive interaction in the lowest Landau levels, while the enhanced conductance in the IQHE regime supports the prediction of softened Coulomb interaction. The contrasting conductance behaviors in the IQHE and the FQHE regimes clearly distinguish the effects of electron-electron interaction in the inter-edge transport in these two regimes. [Preview Abstract] |
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