Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F45: Fractional Quantum Hall Effect: First Landau Level |
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Sponsoring Units: FIAP Chair: Javad Shabani, University of California, Santa Barbara Room: Mile High Ballroom 4D |
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F45.00001: Phase Diagrams for the $\nu$ = 1/2 Fractional Quantum Hall Effect in Electron Systems Confined to Symmetric, Wide GaAs Quantum Wells L.N. Pfeiffer, J. Shabani, Y. Liu, M. Shayegan, K.W. West, K.W. Baldwin We report an experimental investigation of fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor $\nu$ = 1/2 in high quality wide GaAs quantum wells. The quasi-two-dimensional electron systems we study are confined to GaAs quantum wells with widths, W, ranging from 41 to 96 nm and have variable densities in the range of $4 \times 10^{10}$ to $4 \times 10^{11} cm^{-2}$. We present several experimental phase diagrams for the stability of the $\nu$ = 1/2 FQHE in these quantum wells. We find that the densities at which the $\nu$ = 1/2 FQHE is stable are larger for narrower quantum wells. Moreover, even a slight charge distribution asymmetry destabilizes the $\nu$ = 1/2 FQHE and turns the electron system into a compressible state. We also present a plot of the subband separation ($\Delta_{SAS}$), which characterizes the interlayer tunneling, vs density for various W. Finally, we summarize the experimental data in a diagram that takes into account the relative strengths of the inter-layer and intra-layer Coulomb interactions and $\Delta_{SAS}$. We compare this experimental phase diagram of normalized inter-layer distance vs tunneling to recent theoretical calculations which have been used to conclude a two-component origin for the $\nu$ = 1/2 FQHE. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F45.00002: Fractional Quantum Hall Effect at $\nu = 1/2$ in Hole Systems Confined to GaAs Wide Quantum Wells Sukret Hasdemir, Yang Liu, Aurelius Graninger, Mansour Shayegan, Loren Pfeiffer, Ken West, Kirk Baldwin, Roland Winkler We observe fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor $\nu = 1/2$ in two-dimensional hole systems confined to GaAs quantum wells of width 30 to 50 nm and having bilayer-like charge distributions. The $\nu = 1/2$ FQHE is stable when the charge distribution is symmetric and only in a range of intermediate densities, qualitatively similar to what is seen in two-dimensional electron systems confined to approximately twice wider GaAs quantum wells. Despite the complexity of the hole Landau level structure, originating from the coexistence and mixing of the heavy- and light-hole states, we find the hole $\nu = 1/2$ FQHE to be consistent with a two-component, Halperin-Laughlin ($\Psi_{331}$) state. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F45.00003: Even-denominator Fractional Quantum Hall Effect at a Landau Level Crossing Yang Liu, Sukret Hasdemir, Dobromir Kamburov, Aurelius Graninger, Mansour Shayegan, Loren Pfeiffer, Ken West, Kirk Baldwin, Roland Winkler The fractional quantum Hall (FQH) effect, observed in two-dimensional charged particles at high magnetic fields, occurs when the filling factor $\nu$ of the quantized Landau levels is a fraction which, with very few exceptions, has an odd denominator. Here we describe unexpected phenomena in two-dimensional hole systems confined to GaAs quantum wells. We observe an unusual crossing of the two lowest-energy Landal levels. The crossing leads to a weakening or disappearance of the commonly seen odd-denominator FQH states in the filling range $1/3 < \nu < 2/3$. But, surprisingly, a new FQH state at the even-denominator filling $\nu= 1/2$ comes to exist at the crossing. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F45.00004: Evolution of the $\nu=1/2$ Fractional Quantum Hall State in Tilted Magnetic Fields Hao Deng, Sukret Hasdemir, Yang Liu, Mansour Shayegan, Loren Pfeiffer, Ken West, Kirk Baldwin We report magneto-transport measurements of two-dimensional electron systems confined to 65-nm-wide GaAs quantum wells with density 1.4 10$^{11}$ cm$^{-2}$. We observe a remarkable evolution of the magnetoresistance around filling factor $\nu$=1/2 as we increase the tilting angle. The weak $\nu$=1/2 fractional quantum Hall (FQH) state at fully perpendicular field gets stronger as the sample is tilted, but abruptly disappears at higher tilting angles as an insulating phase moves from low fillings to higher ones near $\nu$=1/2. This insulating phase likely signals a bilayer, pinned Wigner crystal. At higher tilting angles, we observe a disappearance of the $\nu$=1 quantum Hall state and the appearance of even-numerator FQH states around $\nu$=1, which are also consistent with the interpretation that the system becomes bilayer. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F45.00005: State Counting for Excited Bands of the Fractional Quantum Hall Effect: Exclusion Rules for Bound Excitons Ajit Coimbatore Balram, Arkadiusz W\'ojs, Jainendra Jain Exact diagonalization studies have revealed that the energy spectrum of interacting electrons in the lowest Landau level splits, non-perturbatively, into bands. The theory of nearly free composite fermions (CFs) has been shown to be valid for the lowest band, and thus to capture the low temperature physics, but it over-predicts the number of states for the excited bands. We explain the state counting of higher bands in terms of composite fermions with an infinitely strong short range interaction between a CF particle and a CF hole. This interaction, the form of which we derive from the microscopic CF theory, eliminates configurations containing certain tightly bound CF excitons. With this modification, the CF theory reproduces, for all well-defined excited bands, an exact counting for $\nu>1/3$, and an almost exact counting for $\nu\leq 1/3$. The resulting insight clarifies that the corrections to the nearly free CF theory are not thermodynamically significant at sufficiently low temperatures, thus providing a microscopic explanation for why it has proved successful for the analysis of the various properties of the CF Fermi sea. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F45.00006: Integer and Fractional Quantum Hall Effect of Two-Component Bosons Yinghai Wu, Jainendra Jain We investigate integer and fractional quantum Hall states for two-component bosons in the lowest Landau level at filling factors $\nu=2/3$, 4/5, 4/3, and 2, using the generic label ``spin'' for the two components. We study ground states, excitations, edge states and entanglement spectrum for systems with up to 16 bosons, and construct explicit trial wave functions to clarify the underlying physics. For $\nu=4/3$ a ``non-Abelian spin-singlet'' state has been proposed to occur for a 2-body contact interaction; we find that it is more likely that the actual state here is a spin-singlet state of reverse-flux-attached composite fermions at filling $\nu^*=4$. The incompressible state at $\nu=2$ provides an example of bosonic integer topological states; it can be understood as the spin-singlet state of reverse-flux-attached composite fermions at $\nu^*=2$. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F45.00007: Momentum-resolved probing of the $\nu=2/3$ quantum Hall edge Hendrik Meier, Yuval Gefen, Leonid Glazman We evaluate the $I$-$V$ characteristic for momentum-resolved tunneling between the $\nu=2/3$ fractional quantum Hall state and a $\nu=1$ state in another layer of a two-dimensional electron gas (2DEG). In a version of a double-layer geometry, the momentum of tunneling electrons may be boosted by an auxiliary magnetic field parallel to the two planes of 2DEGs. The threshold behavior of the $I$-$V$ characteristic and its dependence on the boosting magnetic field yields information about the spectral function of the $\nu=2/3$ edge. It may bring insights into the nature of the various (counter)propagating modes inside the $\nu=2/3$ edge that have been discussed in the last twenty years. Effects due to in-plane disorder as well as of intralayer and interlayer Coulomb interaction are taken into account in our model. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F45.00008: Origin of composite particle ``mass'' in the fractional quantum Hall effect F.D.M. Haldane Composite particles in a partially-filled 2D Landau level are formed by ``flux attachment'' of q empty orbitals to p particles to form either a ``composite boson'' or a ``composite fermion''. The geometry of ``flux attachment'' (the shape of the q-orbital correlation hole that contains the p particles) is the principal degree of freedom of the composite particle, and can adjust to the local environment. An additional independent degree of freedom is the electric polarization of the composite particle by deformation of its inversion-symmetric charge profile to produce an electric dipole moment. In a magnetic field, the momentum is the magnetic flux density B times the electric dipole, rotated through 90 degrees. The energy increase of the composite boson as a function of its electric dipole moment is thus also its dispersion as a function of momentum. This then gives the quadratic dispersion that defines the analog of inertial ``mass'' of the composite particle. This gives both the stiffness constant of the Ginzburg-Landau term in the FQHE composite boson picture, and the dispersion of composite fermions in the $\nu$ = 1/2 composite Fermi liquid state. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F45.00009: Composite Fermion Spin Polarization Energy with Finite Layer Thickness Mansour Shayegan, Yang Liu, Sukret Hasdemir, Loren Pfeiffer, Ken West, Kirk Baldwin We study the spin polarization transitions of fractional quantum Hall (FQH) states in the filling range $1 < \nu < 2$ in symmetric quantum wells (QWs), as a function of density. Our results reveal a strong well-width dependence of the critical density $n_C$ and ratio between the Zeeman energy ($E_Z$) normalized to the Coulomb energy ($e^2/4\pi\epsilon l_B$), above which a certain FQH state becomes spin polarized. For example, the $\nu=7/5$ FQH state becomes spin polarized at about 3 times higher density or 1.7 times larger $E_Z$ in the 31-nm-wide QW than in the 65-nm-wide QW. This well-width dependence of the spin polarization stems from by the finite electron layer thickness in these QWs and the resulting softening of the Coulomb interaction. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F45.00010: Anisotropic Fermi Contour of (001) GaAs Electrons in Parallel Magnetic Fields M.A. Mueed, Dobromir Kamburov, Mansour Shayegan, L.N. Pfeiffer, K.W. West, K.W. Baldwin, J.J.D. Lee, Roland Winkler We demonstrate a severe Fermi contour anisotropy induced by the application of a parallel magnetic field to high-mobility electrons confined to a 30-nm-wide (001) GaAs quantum well. We study commensurability oscillations, namely geometrical resonances of the electron orbits with a unidirectional, surface-strain-induced, periodic potential modulation, to directly probe the size of the Fermi contours along and perpendicular to the parallel field. Their areas are obtained from the Shubnikov-de Haas oscillations. Our experimental data agree semi-quantitatively with the results of parameter-free calculations of the Fermi contours but there are significant discrepancies. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F45.00011: Fermi Contour Anisotropy of GaAs Electron-Flux Composite Fermions in Parallel Magnetic Fields Dobromir Kamburov, M.A. Mueed, Mansour Shayegan, Loren Pfeiffer, Kenneth West, Kirk Baldwin, J.J.D. Lee, Roland Winkler In high-quality two-dimensional electrons confined to GaAs quantum wells, near Landau level filling factors $\nu =$1/2 and 1/4, we observe signatures of the commensurability of the electron-flux composite fermion cyclotron orbits with a unidirectional periodic density modulation. Focusing on the data near $\nu =$1/2, we directly and quantitatively probe the shape of the composite fermions' cyclotron orbit, and therefore their Fermi contour, as a function of magnetic field (B$_{\mathrm{\vert \vert }})$ applied parallel to the sample plane. The composite fermion Fermi contour becomes severely distorted with increasing B$_{\mathrm{\vert \vert }}$ and appears to be elliptical, in sharp contrast to the electron Fermi contour which splits as the system becomes bilayer-like at high B$_{\mathrm{\vert \vert }}$. We present a simple, qualitative model to interpret our findings. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F45.00012: Spin-Polarization of $\nu =$ 3/2 Composite Fermions Insun Jo, Dobromir Kamburov, M.A. Mueed, Yang Liu, Mansour Shayegan, Loren Pfeiffer, Ken West, Kirk Baldwin, Jerry Lee We report the observation of ballistic transport commensurability minima in the magnetoresistance of $\nu =$ 3/2 composite fermions (CFs) in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to a unidirectional periodic potential modulation. The positions of the minima are consisted with the magnetic commensurability condition implying the commensurability features originate from a periodic magnetic field. Their distance away from $\nu =$ 3/2 yields the size and shape of the CF Fermi contour. At a fixed electron density of n $\approx $ 1.8 x 10$^{11}$ cm$^{-2}$, as the quantum well width increases from 30 to 60 nm, the CFs become fully spin-polarized. The application of an additional parallel magnetic field (B$_{\mathrm{\vert \vert }})$ leads to a significant distortion of the CF Fermi contour. The distortion is much more severe compared to the $\nu =$ 1/2 CF case at comparable B$_{\mathrm{\vert \vert }}$. Furthermore, the applied B$_{\mathrm{\vert \vert }}$ spin-polarizes the $\nu =$ 3/2 CFs as evinced from the size of the CF Fermi contour. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F45.00013: Observation of Incompressibility of a New Type of Fractional Quantum Hall State in the Lowest Landau Level Nodar Samkharadze, Ian Arnold, Loren Pfeiffer, Ken West, Gabor Csathy We report on an ultra-low temperature study of a quantum Hall system in the 1/3\textless $\nu $\textless 2/5 region. Due to the residual interaction of composite fermions, this region is proposed to support a new type of fractional quantum Hall states at filling factors $\nu =$4/11 and 3/8. These states are expected to belong to a different universality class from those described by the weakly interacting composite fermion model. Despite the previous observations of magnetoresistance features at $\nu =$4/11,5/13 and 3/8 [Pan et al, Phys. Rev. Lett. 90, 016801], the hallmark property of activated behavior of the fractional quantum Hall states has not yet been observed for any of these states. In our study we have achieved an electronic temperature lower by a factor 5 in comparison to the previous work, revealing for the first time, an activation gap at $\nu =$4/11 and an incipient incompressibility at $\nu =$5/13. However, despite the considerable progress in identifying the later two fractional quantum Hall ground states, at $\nu =$3/8 in our sample we observe compressible state. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F45.00014: Weakly Pinned Wigner Solid-FQHE Liquid Phase Transition in the 2-Dimensional Hole System under Ultrahigh Magnetic Fields Chi Zhang, Rui-Rui Du, Junren Shi, Xincheng Xie, Michael J. Manfra, Loren N. Pfeiffer, Ken W. West, Ju-Hyun Park In the two dimensional systems, Wigner crystals (WC) solid and fractional quantum Hall effect (FQHE) liquid phase compete under low temperature and high magnetic fields. Here, we exhibit unusual experimental results in the new developed C-doped two-dimensional hole samples. Our derivative conductivity measurements elucidate the existence of reentrant insulating phase (RIP) around the Landau level filling factor $v=$1/5 in the 2D hole system. Moreover, we report the unexpected feature in the quantum phase transition between the Wigner Solid and FQHE liquid state in the 2D hole system under ultrahigh magnetic fields. Consequently, a systematic phase diagram is obtained based on our analysis. To our surprise, the excited electric field plays an equivalent role as the temperature in our specimen. From the duality of the electric field and temperature, a characteristic length of 450 nm is derived in our Analysis, which is the mean free path of the carriers. Based on the relation between the pinning gap and electric field, we obtained a characteristic domain size of the Wigner crystal. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F45.00015: Emergence of frustrated antiferromagnet in the lowest Landau level Jun Won Rhim, Alexander C. Archer, Jainendra K. Jain, Kwon Park We investigate the spin structure of the triangular composite fermion crystals (CFCs) in the lowest Landau level (LLL). In contrast to the usual Hund's rule, our Monte-Carlo (MC) calculation finds the spin exchange energy to be antiferromagnetic in certain parameter regimes in the vicinity of $\nu=1/5$. For further physical intuition, we develop an effective two-body potential between composite fermions in the crystal phase, which provides a reasonable account of the MC results. We discuss the experimental feasibility of this physics. [Preview Abstract] |
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