Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D1: Phase Transitions and Transport in Quantum Hall Superfluids |
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Sponsoring Units: DCMP Chair: Allan MacDonald, University of Texas at Austin Room: Spirit of Pittsburgh Ballroom A |
Monday, March 16, 2009 2:30PM - 3:06PM |
D1.00001: Phase Diagram of Bilayer 2D Electron Systems at $\nu _{T}$ = 1 Invited Speaker: Alexandre Champagne Bilayer 2D electron systems at total filling fraction $\nu _{T}$ = 1 and small interlayer spacing can support a strongly correlated phase which exhibits spontaneous interlayer phase coherence and may be described as an excitonic Bose condensate. We use electron interlayer tunnelling and transport to explore the phase diagram of bilayer 2D electron systems at $\nu _{T}$ = 1, and find that phase transitions between the excitonic $\nu _{T}$ = 1 phase and bilayer states which lack significant interlayer correlations can be induced in three different ways: by increasing the effective interlayer spacing, d/$\ell $, the temperature, T, or the charge imbalance, $\Delta \nu =\nu _{1}-\nu _{2}$. First, for the balanced ($\Delta \nu $ = 0) system we find that the amplitude of the resonant tunneling in the coherent $\nu _{T}$ = 1 phase obeys an empirical power law scaling versus d/$\ell $ at various T, and the layer separation where the tunneling disappears scales linearly with T. Our results [1] offer strong evidence that a finite temperature phase transition separates the balanced interlayer coherent phase from incoherent phases which lack strong interlayer correlations. Secondly, we observe [2] that close to the phase boundary the coherent $\nu _{T}$ = 1 phase can be absent at $\Delta \nu $ = 0, present at intermediate $\Delta \nu $, and absent again at large $\Delta \nu $, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. Lastly, at $\Delta \nu $ = 1/3 we report [2] the observation of a direct phase transition between the coherent $\nu _{T}$ = 1 bilayer integer quantum Hall phase and the pair of single layer fractional quantized Hall states at $\nu _{1}$ = 2/3 and $\nu _{2}$ = 1/3.\\[4pt] [1] A.R. Champagne, \textit{et al.,} \textit{Phys. Rev. Lett}. \textbf{100}, 096801 (2008).\\[0pt] [2] A.R. Champagne, \textit{et al}, \textit{Phys. Rev. B }\textbf{78}, 205310 (2008) [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D1.00002: $\nu=1/2+1/2$ Quantum Hall Bilayers Invited Speaker: Steven H. Simon Quantum Hall bilayer systems at filling fractions near $\nu=1/2+1/2$ undergo a transition from a compressible phase with strong intralayer correlation to an incompressible phase with strong interlayer correlations as the layer separation $d$ is reduced below some critical value. Deep in the intralayer phase (large separation) the system can be interpreted as a fluid of composite fermions (CFs), whereas deep in the interlayer phase (small separation) the system can be interpreted as a fluid of composite bosons (CBs). The focus of this paper is to understand the states that occur for intermediate layer separation by using trial variational wavefunctions. We consider two main classes of wavefunctions. In the first class, previously introduced in we consider interlayer BCS pairing of two independent CF liquids. We find that these wavefunctions are exceedingly good for $d \agt \ell_0$ with $\ell_0$ the magnetic length. The second class of wavefunctions naturally follows the reasoning of [2] and generalizes the idea of pairing wavefunctions by allowing the CFs also to be replaced continuously by CBs. This generalization allows us to construct exceedingly good wavefunctions for interlayer spacings of $d \alt \ell_0$, as well. The accuracy of the wavefunctions discussed in this work, compared with exact diagonalization, approaches that of the celebrated Laughlin wavefunction. More details can be found online in [3]. \\[4pt] [1] G. Moller, S. H. Simon, and E. Rezayi PRL {\bf 101}, 176803 (2008). \\[0pt] [2] S. H. Simon, E. Rezayi, and M. Milovanovic PRL {\bf 91}, 046803 (2003) \\[0pt] [3] G. Moller, S. H. Simon, and E. Rezayi, arXiv:0811.4116 [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D1.00003: Optical probes of excitonic phases in quantum Hall bilayers at $\nu_T$=1* Invited Speaker: Vittorio Pellegrini In this talk we discuss our recent inelastic light scattering results that shed light on the interplay between incompressible and compressible quantum phases of electron bilayers at total filling factor $\nu_T$ = 1. In the regime of finite values of tunneling gaps, we observe a quantum phase transformation between composite fermion (CF) metal and incompressible excitonic states as the tunneling gap is reduced. We show that the transition becomes discontinuous (first-order) by impacts of different terms of the electron-electron interactions that prevail on weak residual disorder [1]. The evidence is based on precise determinations of the excitonic order parameter and of measurements of CF spin excitations by resonant inelastic light scattering close to the phase boundary [2,3]. While there is marked softening of low-lying excitations, our experiments underpin the roles of competing order parameters linked to quasi-particle correlations in removing the divergence of quantum fluctuations [4]. In the regime of vanishingly small tunneling gaps we show that the abrupt disappearing of CF spin excitations below the spin-wave indicates the emergence of the inter-layer correlated quantum Hall state in the vicinity of $\nu _T$ = 1 and when the temperature is lowered below a critical value [5]. Finally, the evolution of the spin-wave mode as a function of the Zeeman energy suggests the occurrence of a spin transition [5]. * Work done in collaboration with: B. Karmakar, A. Pinczuk, L.N. Pfeiffer, K.W. West.\\[4pt] [1] B. Karmakar, submitted; [2] S. Luin, et al. Phys. Rev. Lett. {\bf 94}, 146804 (2005); [3] B. Karmakar et al. Solid State Communications {\bf 143}, 499 (2007); [4] J. Schliemann, S. M. Girvin and A. H. MacDonald, Phys. Rev. Lett. {\bf 86}, 1849 (2001); [5] B. Karmakar et al. unpublished. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D1.00004: Theory of Activated Transport in Bilayer Quantum Hall Systems Invited Speaker: H.A. Fertig We analyze the transport properties of bilayer quantum Hall systems at total filling factor ? = 1 in drag geometries as a function of interlayer bias, in the limit where the disorder is sufficiently strong to unbind meron-antimeron pairs, the charged topological defects of the system. We compute the typical energy barrier for these objects to cross incompressible regions within the disordered system using a Hartree-Fock approach, and show how this leads to multiple activation energies when the system is biased. We then demonstrate using a bosonic Chern-Simons theory that in drag geometries, current in a single layer directly leads to forces on only two of the four types of merons, inducing dissipation only in the drive layer. Dissipation in the drag layer results from interactions among the merons, resulting in very different temperature dependences for the drag and drive layers. Connections with recent experiments will be discussed. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D1.00005: Spin-dependent phase diagram in bilayer 2D electron systems Invited Speaker: Koji Muraki Bilayer electron systems with total filling $\nu=1$ involve rich physics arising from the interplay between the intralayer and interlayer interactions parameterized by the ratio between the interlayer distance $d$ and the magnetic length $\ell_{B}$. One key issue in this system is the nature of the phase transition that occurs when exploring the system between the two limits of weak and strong interlayer interactions, i.e., compressible Fermi-liquid states of composite fermions and an incompressible quantum Hall state. Here we report tilted-field experiments on a double quantum well with negligible tunneling that demonstrate that the spin degree of freedom plays a decisive role in the ground-state phase diagram of this system [1]. When the ratio $\eta$ of the Zeeman to Coulomb energies is enhanced by tilting the sample in a field by an angle $\theta$, we observe that the phase boundary located at $d/\ell_{B}=1.90$ for $\theta=0$ shifts to higher densities until it saturates at $d/\ell_{B}=2.33$ for $\theta \geq 60$ degree. The data thus establish a spin-dependent phase diagram as a function of $\eta$ and $d/\ell_{B}$. We model the energies of the competing phases treating the compressible state as nearly independent Fermi liquids of composite fermions. The excellent agreement between the model and experiment indicates that at small $\theta$ the compressible state is only partially polarized and its Zeeman-dependent energy is responsible for the observed shift of the phase boundary, with the saturation at large $\theta$ signaling the full polarization. This in turn implies that the intrinsic transition, expected for the ideal system without spin and intensively studied in theory, is preempted by a transition to a partially polarized compressible state in the standard experimental conditions and can only be revealed by suppressing the spin degree of freedom. Our results thus shed new light on previous experiments and show a way to investigate the intrinsic properties of the system. [1] P. Giudici \textit {et al}., Phys. Rev. Lett. \textbf{100}, 106803 (2008). [Preview Abstract] |
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