Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J40: QHE |
Hide Abstracts |
Sponsoring Units: FIAP Chair: Mansour Shayegan, Princeton University Room: Colorado Convention Center 503 |
Tuesday, March 6, 2007 11:15AM - 11:27AM |
J40.00001: Experimental observation of six valleys and an anisotropic integer quantum Hall effect on H-Si(111) surfaces Kevin Eng, Robert N. McFarland, Bruce E. Kane We have recently developed a new high mobility 2DES on a clean and atomically flat hydrogen-passivated Si(111) surface, where electrons are gated through an encapsulated vacuum dielectric. Our devices have exhibited peak scattering times, $\tau \sim $5ps, which exceed even the best Si(100) MOSFETs. We will discuss magneto-transport measurements made as a function of tilted magnetic fields in such a 2DES. The Si(111) surface is unique in that it has been calculated to have six valleys of equal energy in its ground state. Measurements at T=150mK and n$_{s}$= 6.5 x 10$^{11}$cm$^{-2 }$show clear signatures of the integer quantum Hall effect where contrary to predictions filling factors less than 6 are observed (i.e. $\nu $=6, 5, 4, 3, and 2). In addition, we have observed anisotropy in R$_{xx}$ with respect to the crystal orientation in a magnetic field range of 0$\le $B$\le $12T, which is also unexpected for an equally occupied six-fold degenerate system. As a result, an application of a non-interacting model whereby two valleys have a greater population than the remaining four has general agreement with the Landau level crossings at specific tilt orientations and with the anisotropy of R$_{xx}$ at low fields. At high fields, the presence of $\nu $=3,4 and 5 indicates that individual valleys are splitting but what is more interesting is the anisotropy of R$_{xx}$ at $\nu $=3 {\&} 4 toward in-plane magnetic fields. [Preview Abstract] |
Tuesday, March 6, 2007 11:27AM - 11:39AM |
J40.00002: Anomalous Resistance Fluctuations in a Macroscopic 2DEG on a H-Si(111) Surface Robert N. McFarland, Kevin Eng, Bruce E. Kane We report the experimental observation of large ($\sigma (R)/\left\langle R \right\rangle \sim $15{\%}) resistance fluctuations as a function of electron density for a high mobility 2DES induced on a free H-passivated Si(111) surface in the strongly `metallic' regime. The observed fluctuations are reproducible and two orders of magnitude larger than the time-dependent noise. As the contact spacing ($\sim $1mm) is four orders of magnitude larger than the mean free path length ($\sim $100 nm), an explanation in terms of universal conductance fluctuations seems implausible. Because the dielectric is vacuum, the dominant scattering centers are located right at the surface. As discussed in [1], this 2DES has 6 unequally occupied valleys, which leads to an anisotropic longitudinal resistance. Interestingly, we note a strong anti-correlation between the fluctuations observed for orthogonal current directions. Furthermore, the fluctuations appear largely insensitive to small magnetic fields ($\vert $B$\vert <$ 2T). We present a systematic experimental characterization of this phenomenon, including temperature dependence (0.15 to 14K), I-V characteristics, and the response to perpendicular and parallel magnetic fields up to 12 T. [1] See talk ``Experimental observation of six valleys and an anisotropic IQHE on H-Si(111) surfaces'' K. Eng \textit{et. al.} [Preview Abstract] |
Tuesday, March 6, 2007 11:39AM - 11:51AM |
J40.00003: Spin Susceptibility of an Anisotropic 2D Electron System in an AlAs Quantum Well T. Gokmen, M. Padmanabhan, E. P. De Poortere, E. Tutuc, M. Shayegan We report measurements of the spin susceptibility in dilute 2D electrons confined to a 150A wide AlAs quantum well. In the absence of in-plane strain, the electrons in this well occupy two degenerate in-plane valleys with an anisotropic mass. We lift this degeneracy by applying symmetry breaking strain in the plane, thus obtaining a 2D electron system in a single, anisotropic, in-plane valley. In this system we observe an enhancement of the spin susceptibility over the band value that increases as the density is decreased. Yet, the spin susceptibility is suppressed compared to the results of quantum Monte Carlo calculations. We attribute the suppression to the finite layer thickness of electrons and the anisotropic in-plane Fermi contour. Our measurements also show that the effective mass remains nearly constant and close to its value in bulk AlAs down to the lowest densities (r$_{s}$ = 10), in contrast to Si-MOSFET data. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J40.00004: Effetcs of thickness and mass anisotropy on the spin susceptibility of the 2DEG in AlAs QWs Stefania De Palo, Saverio Moroni, Gaetano Senatore It has been demonstrated that device details, such as the transverse thickness, may affect in a substanial manner the spin susceptibility of the two dimensional electron gas (2DEG) which is realized in semiconducting heterostructures [1]. An important device detail in AlAs quantum wells(QW) is an in-plane mass anisotropy [2], which even in the regime with only one valley occupied is combined with a sizeable transverse thickness. For selected values of the well width appropriate to the experiments, we evaluate the effect of such thickness and, partly, of the mass anisotropy through a mapping of the 2DEG with mass anisotropy onto an `equivalent' isotropic 2DEG with effective mass $m^*=\sqrt{m_tm_l}$. We then critically compare our results with experimental meausurent and assess the importance of anisotropy effects that go beyond this simple minded mapping. \newline [1] S. De Palo et. al., Phys. Rev. Lett. 94, 226405 (2005). \newline [2] M. Shayegan et. al., phys. stat. sol. (b) 243, 3629 (2006) [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J40.00005: Scanning Probe Study of Edge States in a Two-Dimensional Electron System Cemil Kayis, Stuart Tessmer, Loren Pfeiffer , Ken West The edge-state picture in two-dimensional electron systems (2DESs) in an applied perpendicular magnetic field successfully explains several properties observed in transport measurements in the quantum Hall regime. Although a handful of scanning probe experiments have resolved these states, a detailed understanding of the apparent edge-state width and resistance has yet to emerge. We apply Scannning Charge Accumulation Imaging, a cryogenic capacitance technique, to study the behavior of a 2DES in a GaAs/AlGaAs heterostructure sample. The sample contains a pattern of narrow metallic gates fabricated on the exposed surface, a 2DES 80 nm below the surface, and an underlying metallic electrode. By allowing charge to tunneling vertically from the underlying electrode our technique is especially sensitive to compressibility variations of the system. Here we report progress on resolving the edge state properties with this method. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J40.00006: Characterization and modelling of one-dimensional states in a bent quantum Hall system L. Steinke, D. Schuh, M. Bichler, G. Abstreiter, M. Grayson We study the transport properties of a one-dimensional (1D) wire state at the corner of a $90^{\circ}$ bent quantum Hall (QH) system. Such a system is formed in a corner-overgrown bent quantum well [1] by applying a tilted magnetic field $B$. The corner geometry itself serves as a sharp QH boundary and hosts strongly coupled 1D forward and reverse movers with no barrier in between. At different magnetic fields we measure a different conductance behavior of the 1D wire, depending on the QH filling factor $\nu$. In the integer QH regime, at equal filling factors $\nu = 1$ and $\nu = 2$ on both facets of the bent 2D system, we observe an insulating phase where the wire conductance decreases rapidly with decreasing temperature $T$ and DC bias Voltage $V_{DC}$. The integer filling factors $\nu>2$ show a critical behavior with only weak dependence on $T$ and $V_{DC}$. Spin-unresolved Hartree calculations of the dispersions of the corner states illustrate possible origins of the two observed phases [2]. The calculations also provide an insight into the electronic states in the bent QH system, which has no analogue in a planar structure. \noindent[1] M. Grayson, D. Schuh, M. Huber, M. Bichler, and G. Abstreiter, APL 86,); \noindent[2] M. Grayson, L. Steinke, D. Schuh, M. Bichler, L. Hoeppel, J. Smet, K. v. Klitzing, D. K. Maude, and G. Abstreiter, submitted; [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J40.00007: Temperature Limited Spectroscopy of the Quantum Hall Liquid O.E. Dial, R.C. Ashoori, L.N. Pfeiffer, K.W. West We present spectra of the tunneling density states of a two dimensional electron gas (2DEG) in GaAs over a range of 30 meV centered about the Fermi surface, revealing the beautiful structure present in these systems far from the Fermi energy. Using these measurements, we examine the dependence of the exchange-enhanced spin-gap on electron density (0-$4\times{}10^{11}\ \mathrm{cm}^{-2}$) and magnetic field, observe induced spin splittings in Landau levels away from the Fermi energy, and compare measured linewidths to the expected lifetime broadening from interactions. The measurements are performed using time domain capacitance spectroscopy which uses short pulses to drive electrons perpendicularly between the 2DEG and a bulk electrode while monitoring the induced image charge on an isolated electrode. Using a very low duty cycle maintains a 100 mK electron temperature even when injecting electrons at energies 1000 times larger than $\mathrm{k}_{\mathrm{b}} \mathrm{T}$, while the absence of in-plane current allows us to continue to measure when the 2DEG is fully depleted or has vanishing in-plane conductivity. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J40.00008: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J40.00009: Time-Dependent Conductivity in the Quantum Hall Effect Manabu Machida, Naomichi Hatano, Jun Goryo We analyze the quantum Hall effect in a 2D electron system with a periodic potential. We show that the conductivity begins to oscillate in time when an electric field is suddenly switched on. Assuming linear response, we obtain an analytical expression of the time-dependent conductivity. The time dependence comes theoretically from the Fourier components of the response function with nonzero frequencies. The amplitude of the oscillation gradually decreases as a function of time and the conductivity eventually approaches to its average, which is given by the Chern number according to the Kubo formula. We numerically calculate the temporal oscillation of the conductivity in the case of a superlattice in a semiconductor. We find that both the Hall and diagonal conductivities oscillate with a period of pico to nano seconds. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J40.00010: Simple-layered high mobility field effect heterostructured two-dimensional carrier devices. Robert Willett, Loren Pfeiffer, Kenneth West We present in this talk a two-dimensional electron heterostructure field effect device of simplistic design and ease of fabrication that displays high mobility electron transport. This is accomplished using a high efficacy contacting scheme and simple metallic overlapping gate, obviating dopant layers. The resultant devices demonstrate adjustable electron densities and mobilities larger than 8x10$^{6}$ cm$^{2}$/V-sec at the highest densities of 2.4x10$^{11}$/cm$^{2}$. This device type provides a new experimental avenue for studying electron correlations and may answer demands for routine fabrication of practical HEMTs. In one extension of this work, using the same basic heterostructure design with appropriate contacts diffused to the AlGaAs/GaAs interface, a 2D high mobility hole channel can be populated through field effect, resulting in transport with clearly resolvable quantum Hall states at high magnetic fields. Finally, we also present a method for producing mesoscopic structures in these field effect 2D electron systems, which takes advantage of the extensive electron density control available when both the bulk and mesoscopically defined electronic densities can be tuned via overlapping gate arrays. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J40.00011: 2D hole transport in GaAs MOSFETs Tzu-Ming Lu, Keji Lai, Daniel Tsui, Loren Pfeiffer, Ken West We have fabricated enhancement-mode p-channel GaAs MOSFETs on the (100) surface of undoped GaAs/Al$_{x}$Ga$_{1-x}$As heterostructures, using atomic-layer-deposited Al$_{2}$O$_{3 }$dielectric and Ti/Au gate, and measured their transport properties. The capacitively induced 2D hole density (p), determined from Shubnikov-de Haas oscillations, can be tuned from 9x10$^{9 }$to 3x10$^{11}$cm$^{-2 }$by applying a negative gate bias. Within this range, the effective capacitance is close to that of an ideal parallel plate capacitor, and the leakage current remains virtually zero. The highest possible density is limited by the heterostructure design, not by gate leakage. The 2D hole mobility at T=0.3K increases with p and saturates at 6.3x10$^{5}$ cm$^{2}$/Vs for p$>$2.3x10$^{11}$cm$^{-2}$. In this talk, we present data on transistor drain current-voltage characteristics, as well as magneto transport and the quantum Hall effects. [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J40.00012: Towards Novel Electron and Hole Structures: Characterizing n- and p-Type (110) GaAs/AlAs M. Grayson, S. Dasgupta, S. F. Roth, N. Isik, A. Fontcuberta-i-Morral, M. Bichler The (110) facet of GaAs holds promise for new devices because it is the cleave facet, allowing cleaved edge overgrowth [1] and corner overgrowth structures [2], and because recent work demonstrates that Si can also function as an acceptor for high mobility p-type structures [3]. We present characterizations of p-doped GaAs on (110) wafers and cleave facets, which show an interesting spin-orbit coupling effect, resulting in a spin-index anticrossing in the lowest Landau level. n-doped AlAs on the same (110) facet shows a strong anisotropy, suggesting that only a single anisotropic-mass valley is occupied. Initial attempts at combining n- and p-type doped structures in coplanar 2D systems will be presented. [1] M. Grayson, APL 87, 212113 (2005); [2] M. Grayson, APL 86, 032101 (2005); [3] F. Fischer, APL 86, 192106 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J40.00013: Electron correlations and single particle physics in the integer quantum Hall effect Alexander Struck, Bernhard Kramer Recently, the local inverse compressibility of an integer quantum Hall system has been measured (Ilani et. al., Nature {\bf 427}, 328(2004)) as a function of electron density $n$ and magnetic field $B$, a quantity which mimics the change of the chemical potential in the system with respect to the particle density. These compressibility patterns reveal signatures of charging in the quantum Hall system, which in general are attributed to Coulomb interaction in correlated systems and are incompatible with single-particle physics. We have developed a mean-field description for these charging patterns within a spin-unrestricted Hartree-Fock approximation, but allowing for charge rearrangement in the ground state with respect to changes in $n$ and $B$. Our results match the experimental observations at least in the localized regions and are compatible with the single-particle picture of the localization-delocalization transition. In agreement with experimental data we show that electron-electron interaction cannot be neglegted in a comprehensive theory of the integer quantum Hall effect. [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J40.00014: ABSTRACT HAS BEEN MOVED TO S40.00013 |
Tuesday, March 6, 2007 2:03PM - 2:15PM |
J40.00015: Experimental technique for the realization of multiple simultaneous Hall effects Ramesh Mani The Hall effect is examined in multiply connected specimens through transport studies of a double-boundary geometry fabricated on two-dimensional (2D) GaAs/AlGaAs heterostructures. The study begins with the identification of a complement of the Hall bar, called the ``anti Hall bar,'' which helps to generate a Hall effect within interior boundaries, when current is passed via interior contacts. Then, a double current technique is applied in an `anti-Hall bar within a Hall bar' geometry, which includes the above-mentioned `anti-Hall bar' within the usual Hall bar configuration. The double current experiments show that (i) the Hall effect on a boundary depends only on the current injected via the same boundary, while (ii) the magnetoresistive voltages are insensitive to the origin of the current within the specimen. The experimental results are compared with the recent theoretical modeling of this configuration by Oswald et al. (Phys. Rev. B 72, 035334 (2005)). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700