Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R2: Fractional Quantum Hall Systems |
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Sponsoring Units: DCMP Chair: Horst Stormer, Columbia University Room: Baltimore Convention Center Ballroom III |
Wednesday, March 15, 2006 2:30PM - 3:06PM |
R2.00001: New physics in the second Landau Level Invited Speaker: Clean two-dimensional electron systems subjected to perpendicular magnetic fields have a long history of revealing exotic ground states. A large number of these states are only possible in the presence of electron-electron interactions. Two notable examples of such collective ground states are the fractional quantum Hall liquids of the first two Landau levels and the electronic solids called the stripe and bubble phases forming beyond the first Landau level. These two classes of phases are simultaneously present and hence compete in the second Landau level. This talk will highlight our latest results on the evolution of the states of the second Landau level as the magnetic field is tilted away from the direction perpendicular to the sample. The challenging task of cooling to millidegree temperatures and in-situ tilting in this low temperature environment is achieved with a hydraulically driven rotator equipped with sintered Silver heat exchangers mounted onto the nuclear demagnetization stage of a dilution refrigerator. We found that the bubble states are rapidly destroyed with tilt and argue that such a behavior is consistent with the formation of an electronic solid. Furthermore, the well developed $\nu$ = 2+1/5 and 2+4/5 liquids are found to be driven insulating while the 2+1/3 and 2+2/3 states survive to the largest tilt angles accessible. The simplest interpretation of the rapid evolution of these states is that bubble phases melt into a classical Hall gas and the $\nu$ = 2+1/5 and 2+4/5 liquids solidify with tilt. Our data suggest that spin interaction plays an important role in the formation of these phases. In particular, we surmise that the bubble phases are not fully spin-polarized but most likely have a substantial antiferromagnetic order. These bubble phases could be first examples of antiferromagnetically ordered solids in a single layer two-dimensional electron system. This work was done in collaboration with J.S. Xia, C.L. Vicente, E.D. Adams, N.S. Sullivan, D.C. Tsui, H.L. Stormer, L.N. Pfeiffer, and K.W. West. [Preview Abstract] |
Wednesday, March 15, 2006 3:06PM - 3:42PM |
R2.00002: How free are composite fermions? An inelastic light scattering perpective. Invited Speaker: The presentation is focused on the study of spin excitations in the regime of the Fractional Quantum Hall Effect (FQHE) to probe composite fermion interactions. The spin excitations are observed by inelastic light scattering methods. The composite fermion picture is appealing because the FQHE of strongly interacting electrons in 2D becomes the Integer Quantum Hall Effect of nearly-free composite fermions. Recent inelastic light scattering experiments show that the study of spin excitations of composite fermion quasiparticles offers unique venues to address the question: how free are composite fermions? Our experiments are conducted in the filling factor range 2/7<$\nu$<2/5. We measured low energy spin-flip excitations of composite fermions in which orientation of spin and composite fermion Landau level quantum number change simultaneously. The filling factor dependence of the spin-flip excitation intensity displays a striking non-monotonic behavior as the occupation of composite fermion Landau levels are changed. This occurs when the filling factor moves above and below $\nu=1/3$. We find remarkable behaviors that are attributed to the strong impact of residual composite fermion interactions. The breakdown of the non-interacting composite fermion picture seen in our experiments seems to manifest quasiparticle condensations that in magneto-transport experiments are interpreted as the FQHE of composite fermions. (*) The work was done in collaboration with Jun Yan, T.H. Kirschenmann, C.F. Hirjibehedin, I. Dujovne, A. Pinczuk , B.S. Dennis, L.N. Pfeiffer and K.W. West. [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 4:18PM |
R2.00003: Exotic Excitations of Composite Fermions Invited Speaker: The low-energy physics of the fractional quantum Hall effect (FQHE) is marked by the emergence of weakly correlated quasiparticles called composite fermions that are qualitatively distinct from the original strongly correlated electrons. A composite fermion (CF) is an electron bound to an even number of quantum mechanical vortices of the many-body wave function where the number of vortices characterizing the CF is called its flavor. Inelastic light scattering has played a crucial role in understanding the different types of excitations in the FQHE regime. While the lowest energy neutral excitations are accurately described as single CF particle-hole pairs, other types of interesting higher energy excitations are possible. We describe two such excitations. (i) Composite fermion flavor changing excitations (Peterson and Jain, PRL 93, 046402(2004), Wojs and Quinn, Philos. Mag. B 80, 1405(2000)): The lowest energy excitations are such that the flavor of the excited CF remains constant. We show that, in addition, there is a new class of excitations where the excited CF loses some of its vortices, i.e., it changes its flavor. Further, we suggest that these flavor changing excitations are relevant to the experimental observation of Hirjibehedin {\em{et al.}} (PRL, 91, 186802(2003)) of co-existing excitation modes of differently flavored CFs in the filling factor range $1/3>\nu\geq 1/5$. (ii) Electronlike CF excitations (Jain and Peterson, PRL 94, 186808(2005)): We show that the electron is not irretrievably lost in the FQHE regime. In particular, we find that the electron (hole) couples to a complex high energy bound state of CF quasiparticles (holes). Experimental consequences of this complex bound state will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:54PM |
R2.00004: A proposal for measuring fractional charge and statistics in fractional quantum Hall states through noise experiments Invited Speaker: Quasiparticles of fractional quantum Hall (FQH) fluids are predicted to be finite energy vortices carrying fractional charge and fractional statistics. These properties reflect the non-trivial topological order which characterizes each FQH state as a distinct phase. However, strong evidence for fractional statistics has remained an open experimental challenge. We propose a three-terminal ``T-junction'' as an experimental setup for directly detecting fractional charge and statistics of fractional quantum Hall quasiparticles via cross current noise measurements. Through a careful non-equilibrium calculation of the quantum noise in the T-junction setup for FQH Jain states, we showed that the cross current correlation (noise) can be written in a simple form with all the statistics dependence captured in a factor of $\cos\theta$ in one of two contributions, where the statistical angle $\theta$ is defined by the phase gained by a two quasiparticle wave function upon exchange. By analyzing these two contributions for different parameter ranges that are experimentally relevant, we demonstrated that the noise at finite temperature reveals signatures of generalized exclusion principles, fractional exchange statistics and fractional charge. We also predicted that the vortices of Laughlin states exhibit a ``bunching'' effect, while higher states in the Jain sequences exhibit an ``anti-bunching'' effect. [Preview Abstract] |
Wednesday, March 15, 2006 4:54PM - 5:30PM |
R2.00005: Phonon Absorption Experiments on Composite Fermions Invited Speaker: We present a time-resolved phonon absorption technique as an efficient tool to investigate the properties of the fractional quantum Hall effect (FQHE). We interpret our data within the Composite Fermion (CF) picture of the FQHE, were the CF quasiparticles are constructed by attaching two magnetic flux quanta $\phi_0$ to each electron. As a result, the CFs at filling factor $\nu$ and electron density $n$ experience only an reduced magnetic field $B^* = B - 2 \phi_0 n$, so that they behave in $B^*$ in analogy to electrons in the magnetic field $B$ and form Landau levels of CFs with effective filling factor $p = \frac{\nu}{1-2\nu}$ in particular. In a first set of experiments, we measure phonon excitation gaps in the FQH regime for various filling factors and find that the measured gaps can be well described in the framework of Landau-level transitions of CFs involving no spin flip [1]. We are able to deduce the CF mass $m^*(10~\rm{T}) = 0,5m_0$ from these transitions. We apply the same model to energy gaps which we deduce from transport experiments. These gaps are transitions from one level to another reduced by disorder. Here, spin flip are allowed. From results near the crossing of two levels, we were able to deduce the g-factor of CFs [2]. A second set of phonon absorption experiments focuses on the specific heat $C$ of the 2DES at various filling factors $\nu$ [3]. Here, we find an exponential dependence $C \propto (1/T)^2 exp(-\Delta/T)$ on temperature $T$ if the 2DES is in a state with an energy gap $\Delta$. At $\nu = 1/2$, where the external magnetic field is fully cancelled by the gauge field, we find a linear temperature dependence as expected for a Fermi sea of CFs. \begin{enumerate} \item F. Schulze-Wischeler, F. Hohls, U. Zeitler, D. Reuter, A. D. Wieck, and R. J. Haug, Phys. Rev. Lett. \textbf{93}, 026801 (2004). \item F. Schulze-Wischeler, E. Mariani, F. Hohls, and R. J. Haug, Phys. Rev. Lett. \textbf{92}, 156401 (2004). \item F. Schulze-Wischeler \emph{et al.}, to be published in 2006. \end{enumerate} [Preview Abstract] |
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