Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B2: Novel Metal-like States in Two-Dimensions |
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Sponsoring Units: DCMP Chair: Sankar Das Sarma, Univ. of Maryland Room: LACC 151 |
Monday, March 21, 2005 11:15AM - 11:51AM |
B2.00001: Intermediate phases of the two dimensional electron fluid between the Fermi liquid and the Wigner crystal Invited Speaker: We study the consequences of Coulomb interactions in a 2D system undergoing a putative first order phase transition as a function of density. In two dimensions (2D), near the critical density, the system is universally unstable to the formation of new intermediate phases, which we call ``microemulsion phases''. They consist of an intermediate-scale mixture of regions of the two competing phases. A corollary is that there can be no direct transition as a function of density from a 2D Wigner crystal to a uniform electron liquid. Rather, there must always exist intermediate electronic micro-emulsion phases, and an accompanying sequence of continuous phase transitions. Among the intermediate electronic phases which we find are a variety of bubble phases and liquid crystalline phases. The existence of these phases can be established in the neighborhood of the phase boundaries on the basis of an asymptotically exact analysis. They likely occur in clean Si MOSFETs and p-GaAs heterojunctions in the range of densities in which an ``apparent metal to insulator transition'' has been observed in existing experiments. We also point out that, in analogy with the Pomaranchuk effect in $^{3}$He, the Wigner crystal phase has higher spin entropy than the Fermi liquid phase, leading to an increasing tendency towards electronic crystallization with increasing temperature and with magnetic field parallel to the film. Finally we discuss the consequences of this effect for the temperature and the magnetic field dependences of the resistance of strongly correlated electron liquids and for the drag effect in p-GaAs double layers. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:27PM |
B2.00002: Incipient Superconductivity in Metallic and Insulating Phases of Indium Oxide Near the Superconductor-Insulator Transition Invited Speaker: Disordered thin films of indium oxide undergo a magnetic field-tuned superconductor-to-insulator transition at low temperatures. Concentrating on the nature of the insulating phase, our study reveals a wide range of insulator strength above the transition field Hc, depending on the disorder. Isotherms of the resistivity cross at a temperature-independent transition, then peak at a higher field and decay slowly to very high magnetic fields. We suggest that at this peak the film crosses over from a boson-dominated insulating phase at lower fields to a Fermi-dominated insulating phase at higher fields, a result of an increased depairing rate which destroys the Bose-insulator. Despite the increased depairing rate, pairing susceptibility persists to the highest accessible fields, approximately 32T, as the normal state does not appear to be fully recovered and a vestige of superconductivity remains. The behaviour of the films in some regime of parameters is similar to reported behaviour of the high-temperature superconductor LaSrCuO, suggesting a common underlying mechanism. In particular, we suggest that upon increasing the magnetic field in the cuprate, a tendency towards a Bose-insulating phase appears before a true ``normal state'' is recovered at much higher fields. We shall explore the possibility that disordered films like InOx can be used as model systems for further study of high-Tc phenomena. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 1:03PM |
B2.00003: Effect of in-plane Magnetic Field on Conductivity of Two Dimensional Electrons on Silicon Surface Invited Speaker: Sergey Vitkalov The two dimensional system of electrons in silicon inversion layers is one of the most interesting low dimensional systems, where the effects of externally controlled electron-electron interactions are observed directly. Among several fascinating phenomena discovered in the past few years, one of the most intriguing and dramatic is the strong effect of in- plane magnetic field on the 2D conductivity. It was found$^a$ that the energy scale associated with this magnetic field response goes to zero at a density $n_0=0.8 \times 10^{11}$ cm$^{-2}$, indicating the approach to a quantum phase transition.\\ In my talk the effects of in-plane magnetic field on the conductivity of two dimensional electron systems in Si-MOSFETs will be discussed. We will report recent experimental results demonstrating that the application of an in-plane magnetic field significantly reduces the metallic ($d\sigma/dT<0$) temperature dependence of the conductivity over a broad range of electron densities, extending deep into the metallic regime, where the high field conductivity is of the order of $10 e^2/h$. The strong suppression (or ``quenching'') of the metallic behavior by the magnetic field indicates that the spin degrees of freedom play an important role in the transport properties of these two-dimensional systems. \\ Work done in collaboration with Yeekin Tsui, M. P. Sarachik and Teun M. Klapwijk.\\ $^a$ S. A. Vitkalov, Hairong Zheng, K. M. Mertes, M. P. Sarachik, T. M. Klapwijk Phys. Rev. Lett. 87, 086401, 2001 [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:39PM |
B2.00004: Counterflow Conductivity of Bilayer 2D Hole Systems at Filling Factor 1 Invited Speaker: Interacting bilayers with no inter-layer tunneling exhibit a peculiar superfluid in perpendicular magnetic fields at total filling factor $\nu=1$ (layer filling 1/2), when equal and opposite currents are passed in the two layers (counterflow geometry). This phenomenon can be explained by the formation of electron-hole pairs (excitons) in the two, half-filled layers, and the resulting excitonic condensation at the lowest temperatures ($T$). In this talk we present recent experimental results in strongly interacting GaAs hole bilayers in the limit of zero inter-layer tunneling. Using bilayer samples with independent contacts to each layer, we explore the counterflow transport properties. At the lowest temperatures both Hall and longitudinal counterflow resistivities ($\rho_{xx}$ and $\rho_{xy}$) vanish at $\nu=1$, a finding which demonstrates the existence of a counterflow superfluid in the limit of $T=0$ at this filling factor. A marked feature of our data is that the counterflow $\rho_{xy}$ remains much smaller than the counterflow $\rho_{xx}$ when the temperature is increased. This property becomes more prominent as the bilayer density is reduced, thus placing the bilayer in a stronger inter-layer interaction regime. The counterflow $\rho_{xx}$ at $\nu=1$ however, shows little dependence on total bilayer density, but can be greatly affected by small changes in the layer charge distribution. Much like the case of superfluid helium, our results can be explained by considering the existence of mobile vortices which move across the superfluid current, thus creating dissipation and finite counterflow $\rho_ {xx}$. The counterflow $\rho_{xx}$ dependence on total bilayer density, as well as layer density imbalance suggests that the vortices are caused by the disorder present in our samples. Work performed in collaboration with M. Shayergan and D. Huse, and with support from the NSF and DOE. [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 2:15PM |
B2.00005: Spin Dependent Onset of Exciton Condensation in Bilayer Quantum Hall Systems Invited Speaker: At total filling factor $\nu_{tot} = 1$, a bilayer two- dimensional electron system at high perpendicular magnetic fields and small interlayer spacing can enter an exotic new collective phase displaying spontaneous interlayer quantum phase coherence. This phase can be viewed in various ways, including as a Bose condensate of interlayer excitons. Using a combination of heat pulse and NMR techniques we show that the location of the phase boundary between this phase and the weakly coupled, compressible phase at larger layer spacing is influenced by the spin polarization of the nuclei in the host semiconductor. Due to the hyperfine interaction between the electrons and nuclei, a reduction in the nuclear polarization increases the electronic Zeeman energy. By depolarizing the nuclei via thermal pulses or RF radiation tuned to a nuclear resonance frequency, we are able to temporarily create the excitonic phase at larger layer spacing than it exists in equilibrium. This result demonstrates that, contrary to the usual assumption, the transition from the compressible, weakly coupled bilayer phase to the excitonic phase is accompanied by a change in the electronic spin polarization.$\\$$\\$This work was done in collaboration with I.B. Spielman, J.P. Eisenstein (Caltech) , L.N. Pfeiffer, and K.W. West (Bell Laboratories, Lucent Technologies), and with support from the DOE, NSF, and NDSEG. [Preview Abstract] |
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