Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H19: Bilayers |
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Sponsoring Units: DCMP Chair: Michael Fogler, UCSD Room: LACC 406 B |
Tuesday, March 22, 2005 8:00AM - 8:12AM |
H19.00001: In-plane magnetodrag in dilute bilayer two-dimensional systems E.H. Hwang, S. Das Sarma We report anomalous drag resistance behavior in dilute bilayer two-dimensional (2D) hole systems in the presence of a magnetic field parallel to the 2D plane. We have carried out a many-body Fermi liquid theory calculation of bilayer magnetodrag comparing it to the corresponding single layer magnetoresistance. In qualitative agreement with experiment we find relatively similar behavior in our calculated magnetodrag and magnetoresistance arising from the physical effects of screening being similarly modified (``suppressed'') by carrier spin polarization (at ``low'' field) and the conductivity effective mass being similarly modified (``enhanced'') by strong magneto-orbital correction (at ``high'' fields) in both cases. We critically discuss agreement and disagreement between our theory and the experimental results. [Preview Abstract] |
Tuesday, March 22, 2005 8:12AM - 8:24AM |
H19.00002: Effective Drag Between Strongly Inhomogeneous Layers: Exact Results Vadym Apalkov, Mikhail Raikh We generalize Dykhne's calculation of the effective resistance of the two-component medium to the case of frictional drag between the two two-component layers. The resulting exact expression for the effective drag is analyzed for the cases when the resistances and transresistances of the constituting components are strongly different - the situatin generic for the vicinity of the percolative metal-insulator transition (MIT). On the basis of this analysis we conclude that the evolution of effective drag across the MIT is completely determined by the type of correlation between the components, constituting the 2D layers. The effective drag can change either monotonically across MIT, or experience either sharp maximum or a sharp minimum. [Preview Abstract] |
Tuesday, March 22, 2005 8:24AM - 8:36AM |
H19.00003: Probing the Extents of Negative Drag in the Quantum Hall Regime Gokul Gopalakrishnan, Sanghun An, Sarah Parks, Yuko Shiroyanagi, Thomas Gramila, Loren Pfeiffer, Kenneth West “Negative drag” has been used to describe the reversal of the measured drag voltage when relative densities of the conducting layers are varied in a drag measurement in the quantum Hall regime. Although each layer in the bilayer drag sample contains electrons, the behavior observed mimics that of a system where one layer contains electrons and the other contains holes. While this phenomenon was first observed [1,2] some years ago, we remain without a satisfactory understanding of the effect. An essential element for the elucidation of the basis of this effect remains a clear determination of the experimental conditions under which negative drag is observable, as its existence is destroyed by high fields and temperatures. We report on measurements which characterize these conditions. [1] X. G. Feng et al., Phys. Rev. Lett. 81, 3219 (1998) [2] J. G. S. Lok et al., Phys. Rev. B 63, 041305(R) (2001) [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H19.00004: Exploring Electron Drag in Intermediate Magnetic Fields Sanghun An, Gokul Gopalakrishnan, Yuko Shiroyanagi, Sarah Parks, Thomas Gramila, Loren Pfeiffer, Ken West Electron Drag between two two-dimensional electron gases has shown an unusual increase in intermediate magnetic fields ($\omega_c \tau > 1$ but $KT > \hbar \omega_c$) even before the onset of the Quantum Hall Effect. For closely spaced layers, the increase in electron drag above the zero field values shows remarkably little dependence on temperature, and is found to increase as the cube of the magnetic field. Both of these aspect of the increase are difficult to understand within our current understanding of 2-D electron system. We report on measurements which explore the experimental influences on this behavior, with a focus on the temperature and densities for which the behavior is found. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H19.00005: Vortices, instantons and deconfinement in bilayer quantum Hall systems Ziqiang Wang The physics of vortices, instantons and deconfinement is studied for layered superfluids in connection to bilayer quantum Hall excitonic superfluid at filling fraction $\nu=1$. We derive an effective U(1) gauge theory description, taking into account both vortices and instantons induced by interlayer tunneling. The renormalization group flow of the gauge charge and the instanton fugacity shows that the coupling of the gauge field to vortex matter produces a continuous transition between the confining phase dominated by free instantons and condensed vortices and a deconfined gapless exciton condensate where instantons and anti-instantons are bound into magnetic dipoles. The presence of layer imbalance leads to an inhomogeneous exciton condensate. Experimental implications will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H19.00006: Undoped GaAs bilayers for exciton condensation experiments M.P. Lilly, J.A. Seamons, E. Bielejec, J.L. Reno Experimental progress in transport studies of exciton condensation of in electron and hole bilayers at high magnetic fields [1,2] has shown this novel physics can be observed. Fabrication of the bipolar electron-hole bilayers for zero field studies of exciton condensation still remains elusive. We describe a series of experiments on undoped GaAs/AlGaAs heterostructures with the motivation of making electron-hole bilayers. In these undoped devices, external electric fields induce carriers rather than the traditional doping techniques. Single layer electron (or hole) devices demonstrate a high mobility over a wide range of density. More recently, fully undoped bilayers have been made where the density in each layer is independently controlled with gates on the top and bottom of the bilayer. In this talk we present high field transport of undoped electron-electron bilayers, and describe recent progress towards extending the fabrication techniques to creating electron-hole bilayers for exciton condensation studies at zero magnetic field. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. 1. M. Kellogg, J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 93 036801 (2004). 2. E. Tutoc, M. Shayegan, and D. A. Huse, Phys. Rev. Lett. 93, 036802 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:24AM |
H19.00007: Dipolar superfluidity in electron-hole bilayers Yogesh Joglekar, Alexander Balatsky, Peter Littlewood Bilayer electron-hole systems, where the electrons and holes are confined to separate layers and have a very low recombination rate, undergo excitonic condensation when the distance between the layers is smaller than typical distance between particles within a layer. We argue that the excitonic condensate is a novel dipolar superfluid in which the phase of the condensate couples to the {\it gradient} of the vector potential. We predict the existence of dipolar supercurrent which can be tuned by an in-plane magnetic field and detected by independent contacts to the layers. Thus the dipolar superfluid offers an example of excitonic condensate in which the {\it composite} nature of its constituent excitons is manifest in the macroscopic superfluid state. We also discuss various properties of this superfluid including the role of vortices, response to time-dependent fields, and the evolution of noise in the in- plane currents (A.V. Balatsky {\it et al.}, cond-mat.0404033). [Preview Abstract] |
Tuesday, March 22, 2005 9:24AM - 9:36AM |
H19.00008: Coherence Network in a Quantum Hall Bilayer H.A. Fertig, Ganpathy Murthy We develop a model for conduction properties of a disordered quantum Hall bilayer near total filling factor $\nu=1$. For single layers, fluctuations in the {\it local} filling factor due to quenched disorder are known to induce a network quasi-one-dimensional channels of quantum Hall liquid known as ``Efros strips''. In a bilayer system, a similar mechanism should be operative, and we make the further assumption that these strips seperate quasihole- and quasielectron-rich regions where interlayer coherence is effectively suppressed. The narrow strips seperating these regions potentially support the interlayer coherence; however, these regions will have solitons running across them that represent strings of overturned phase connecting charged vortices just inside the incoherent regions. When these strings are free to move, we demonstrate that the resulting system very naturally produces properties which are found in experiment but have so far eluded explanation: the apparent divergence in the ``Josephson length'' for current injected in a tunneling geometry, and the activated behavior in both diagonal and Hall conductivities in a counterflow geometry. In addition, the network displays interesting behavior at finite frequency that should allow experimental testing of this explanation for bilayer quantum Hall superfluidity. Effects of temperature on pinning and dephasing of the system will be discussed, as well as the possibility that the system is truly superfluid at zero temperature. [Preview Abstract] |
Tuesday, March 22, 2005 9:36AM - 9:48AM |
H19.00009: Influence of Disorder on Spontaneous Coherence and Collective Transport in Bilayer Quantum Hall Systems Enrico Rossi, Allan H. MacDonald Disorder-free bilayer quantum Hall systems have spontaneous interlayer phase coherence. The degree of phase coherence in these systems and the associated anomalies in inter-layer and counter-flow transport are strongly influenced by disorder. We discuss a realistic model of a disordered $\nu=1$ quantum Hall bilayer which is based on a mean-field treatment of intra-layer and inter-layer Hartree and exchange interactions and a smooth disorder potential that models randomness in remote ionized donor layers. The charge density is approximated as being proportional to the local curvature of the pseudospin that describes bilayer coherence and layer polarization. This model leads to a picture in which electrons are nearly fully polarized toward one of the two-layers in large parts of the sample, with areas of strong coherence separating regions that are polarized toward opposite layers. Weak coherence in large parts of the sample explains the surprisingly small inter-layer tunneling conductance. The relationship between charge density and pseudospin curvature leads to a ground state that contains many vortices. Sufficiently strong disorder can drive the KT transition temperature to zero. We will discuss attempts to quantitatively relate disorder strength, KT temperatures, and transport anomalies. [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H19.00010: Real Spin in Pseudospin Quasiparticles of Bilayer Quantum Hall systems Bahman Roostaei, H. A. Fertig, Kieran Mullen Recent experiments have observed enhanced nuclear spin relaxation in double layer quantum Hall systems near the phase boundary between compressible and incompressible states(1). We investigate the electronic spin structure of such systems by calculating the groundstate close to $\nu = 1$ using the Hartree-Fock approximation. This state is a quasiparticle lattice, and we examine the possibility of optimizing its energy by allowing the real spin to tilt away from the majority direction in the quasiparticle cores, analogous to what has been suggested in field theoretic studies of single quasiparticles(2). A broken symmetry of these states introduces low energy spin modes which may couple to the nuclear spins. We calculate both the spin and pseudospin textures for the system near the transition and discuss whether they can account for the observed relaxation rates.\newline 1) I.B. Spielman et al., cond-mat/0410092; N. Kumada et al., cond-mat/0410495\newline 2) S. Ghosh and R. Rajaraman, Phys. Rev. B63, 035304 (2001); Z.F. Izawa and G. Tsitsishvili, cond- mat/0311406.\newline Grants: NSF MRSEC DMR-0080054, NSF EPS-9720651 and NSF DMR- 0454699 [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H19.00011: Tunneling and the Collective Mode Spectrum of Strongly Correlated Bilayer 2D Electron Systems X. Lopez-Yglesias, I.B. Spielman, J.P. Eisenstein, L.N. Pfeiffer, K.W. West Bilayer 2D electron systems at $\nu_T = 1$ show a huge and sharply resonant peak in the interlayer tunneling conductance at zero bias if the layer separation is sufficiently small[1]. This Josephson-like effect is a signature of the long wavelength Goldstone collective mode characteristic of the pseudo-ferromagnetic (or excitonic) ground state of the system. Application of an in-plane magnetic field has allowed for verification of the linear wave-vector dependence of this mode[2]. In this talk we will report measurements of the various low energy features of the tunnel spectrum as functions of temperature, energy, and in-plane magnetic field. In particular, we will discuss the results of a search for the expected magneto-roton minimum in the collective mode spectrum when the system is near the critical layer separation. This work was supported by the NSF and the DOE. [1] I.B. Spielman, et al., Phys. Rev. Lett. {\bf 84}, 5808 (2000). [2] I.B. Spielman, et al., {\it ibid.}, {\bf 87}, 036803 (2001). [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H19.00012: Tunneling Under Microwave Illumination in Bilayer Two Dimensional Electron Systems J.A. Bonetti, L.C. Sinclair, J.P. Eisenstein, L.N. Pfeiffer, K.W. West The striking Josephson-like effect recently observed [1] in bilayer 2D electron systems at $\nu_{t}=1$ raises important questions about the nature of photon-assisted tunneling in this system. For instance, it is unknown whether Shapiro steps will arise, or whether the presence of radiation will lead to photon- assisted sidebands [2]. In order to address these questions, we have examined the effect of microwave radiation on tunneling in bilayer electron systems. Several aspects of coupling radiation into the sample will be presented, including issues of heating and gating. Preliminary results demonstrate a conduction enhancement near gate voltages corresponding to top and bottom layer depletion. The frequency and power dependence of this enhancement will be presented. This work is supported by the NSF and DOE. $\\$ [1] I.B. Spielman, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West, Phys. Rev. Lett. 84, 5808 (2000).$\\$ [2]Ady Stern, S. M. Girvin, A. H. MacDonald, and Ning Ma, Phys. Rev. Lett. 86, 1829 (2001) [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H19.00013: Resonance in Microwave Spectra of Bilayer Hole Sample Zhihai Wang, Y. Chen, G. Sambandamurthy, L. W. Engel, D. C. Tsui, E. Tutuc, M. Shayegan We report microwave spectra of a bilayer hole sample, with nominal interlayer distance d $\approx $ 230 {\AA} and balanced total density p$\approx $7.46 $\times $10$^{10}$ cm$^{-2}$. The wafer was designed to suppress interlayer tunneling; dc transport studies [1] of other pieces of the wafer exhibited the interlayer-coherent quantum Hall effect at total Landau filling factor $\nu $=1, with an insulating phase reentrant around it. In the present study, a clearly identifiable peak in the spectrum (Re($\sigma _{xx})$ vs frequency) appears at 3.2 GHz as $\nu $ is decreased below $\sim $0.9. The peak frequency increases with decreasing $\nu $ to $\sim $ 5 GHz at $\nu $ =0.2. Though quite broad (Q$\sim $1) for $\nu >$0.5, the resonance sharpens drastically as $\nu $ is decreased below that value, reaching Q$\approx $10 at $\nu $=0.2. Particularly for $\nu <$0.5, the resonance can be interpreted analogously to that observed in single layer 2D hole systems [2], as a pinning mode of a Wigner crystal. [1] E. Tutuc et al. , Phys. Rev. Lett. \textbf{91}, 076802 (2003). [2] C. C. Li et al., Phys. Rev. Lett. \textbf{79}, 1353 (1997). [Preview Abstract] |
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