arXiv subjects

J. P. Eisenstein

Publications and source records attributed to J. P. Eisenstein.

At least 37 records · Page 2Linked to original sources

arXiv2010

Tilt-Induced Anisotropic to Isotropic Phase Transition at $ν= 5/2$

A modest in-plane magnetic field \Bpar\ is sufficient to destroy the fractional quantized Hall states at $ν= 5/2$ and 7/2 and replace them with anisotropic compressible phases. Remarkably, we find that at larger \Bpar\ these anisotropic phases can themselves be replaced by isotropic compressible phases reminiscent of the composite fermion fluid at $ν= 1/2$. We present strong evidence that this transition is a consequence of the mixing of Landau levels from different electric subbands. We also report surprising dependences of the energy gaps at $ν= 5/2$ and 7/3 on the width of the confinement potential.

cond-mat.str-el↗

arXiv2010

Measurement of the electronic compressibility of bilayer graphene

We present measurements of the electronic compressibility, $K$, of bilayer graphene in both zero and finite magnetic fields up to 14 T, and as a function of both the carrier density and electric field perpendicular to the graphene sheet. The low energy hyperbolic band structure of bilayer graphene is clearly revealed in the data, as well as a sizable asymmetry between the conduction and valence bands. A sharp increase in $K^{-1}$ near zero density is observed for increasing electric field strength, signaling the controlled opening of a gap between these bands. At high magnetic fields, broad Landau level (LL) oscillations are observed, directly revealing the doubled degeneracy of the lowest LL and allowing for a determination of the disorder broadening of the levels.

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arXiv2010

Clausius-Clapeyron Relations For First Order Phase Transitions In Bilayer Quantum Hall Systems

A bilayer system of two-dimensional electron gases in a perpendicular magnetic field exhibits rich phenomena. At total filling factor $ν_{tot} = 1$, as one increases the layer separation, the bilayer system goes from an interlayer coherent exciton condensed state to an incoherent phase of, most likely, two decoupled composite-fermion Fermi liquids. Many questions still remain as to the nature of the transition between these two phases. Recent experiments have demonstrated that spin plays an important role in this transition. Assuming that there is a direct first order transition between the spin-polarized interlayer-coherent quantum Hall state and spin-partially-polarized composite Fermi liquid state, we calculate the phase boundary $(d/l)_c$ as a function of parallel magnetic field, NMR/heat pulse, temperature, and density imbalance, and compare with experimental results. Remarkably good agreement is found between theory and various experiments.

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arXiv2010

Thermopower of Two-Dimensional Electrons at $ν$ = 3/2 and 5/2

The longitudinal thermopower of ultra-high mobility two-dimensional electrons has been measured at both zero magnetic field and at high fields in the compressible metallic state at filling factor $ν= 3/2$ and the incompressible fractional quantized Hall state at $ν= 5/2$. At zero field our results demonstrate that the thermopower is dominated by electron diffusion for temperatures below about $T = 150$ mK. A diffusion dominated thermopower is also observed at $ν= 3/2$ and allows us to extract an estimate of the composite fermion effective mass. At $ν= 5/2$ both the temperature and magnetic field dependence of the observed thermopower clearly signal the presence of the energy gap of this fractional quantized Hall state. We find that the thermopower in the vicinity of $ν= 5/2$ exceeds that recently predicted under the assumption that the entropy of the 2D system is dominated by non-abelian quasiparticle exchange statistics.

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arXiv2009

Quantum Hall Exciton Condensation at Full Spin Polarization

Using Coulomb drag as a probe, we explore the excitonic phase transition in quantum Hall bilayers at nu=1 as a function of Zeeman energy, E_Z. The critical layer separation d/l for exciton condensation initially increases rapidly with E_Z, but then reaches a maximum and begins a gentle decline. At high E_Z, where both the excitonic phase at small d/l and the compressible phase at large d/l are fully spin polarized, we find that the width of the transition, as a function of d/l, is much larger than at small E_Z and persists in the limit of zero temperature. We discuss these results in the context of two models in which the system contains a mixture of the two fluids.

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arXiv2009

Breakdown of Particle-Hole Symmetry in the Lowest Landau Level Revealed by Tunneling Spectroscopy

Tunneling measurements on 2D electron gases at high magnetic field reveal a qualitative difference between the two spin sublevels of the lowest Landau level. While the tunneling current-voltage characteristic at filling factor $ν= 1/2$ is a single peak shifted from zero bias by a Coulomb pseudogap, the spectrum at $ν=3/2$ shows a well-resolved double peak structure. This difference is present regardless of whether $ν=1/2$ and $ν= 3/2$ occur at the same or different magnetic fields. No analogous effect is seen at $ν= 5/2$ and 7/2 in the first excited Landau level.

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arXiv2009

A Hot Electron Thermocouple and the Diffusion Thermopower of Two-Dimensional Electrons in GaAs

A simple hot electron thermocouple is realized in a two-dimensional electron system (2DES) and used to measure the diffusion thermopower of the 2DES at zero magnetic field. This hot electron technique, which requires no micron-scale patterning of the 2DES, is much less sensitive than conventional methods to phonon-drag effects. Our thermopower results are in good agreement with the Mott formula for diffusion thermopower for temperatures up to T ~ 2 K.

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arXiv2008

Charge Imbalance and Bilayer 2D Electron Systems at $ν_T = 1$

We use interlayer tunneling to study bilayer 2D electron systems at $ν_T = 1$ over a wide range of charge density imbalance, $Δν=ν_1-ν_2$, between the two layers. We find that the strongly enhanced tunneling associated with the coherent excitonic $ν_T = 1$ phase at small layer separation can survive at least up to an imbalance of $Δν$ = 0.5, i.e $(ν_1, ν_2)$ = (3/4, 1/4). Phase transitions between the excitonic $ν_T = 1$ state 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, $Δν$. We observe that close to the phase boundary the coherent $ν_T = 1$ phase can be absent at $Δν$ = 0, present at intermediate $Δν$, but then absent again at large $Δν$, thus indicating an intricate phase competition between it and incoherent quasi-independent layer states. At zero imbalance, the critical $d/\ell$ shifts linearly with temperature, while at $Δν$ = 1/3 the critical $d/\ell$ is only weakly dependent on $T$. At $Δν$ = 1/3 we report the first observation of a direct phase transition between the coherent excitonic $ν_T = 1$ bilayer integer quantum Hall phase and the pair of single layer fractional quantized Hall states at $ν_1$ = 2/3 and $ν_2=1/3$.

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arXiv2008

Observation of Chiral Heat Transport in the Quantum Hall Regime

Heat transport in the quantum Hall regime is investigated using micron-scale heaters and thermometers positioned along the edge of a millimeter-scale two dimensional electron system (2DES). The heaters rely on localized current injection into the 2DES, while the thermometers are based on the thermoelectric effect. In the $ν= 1$ integer quantized Hall state, a thermoelectric signal appears at an edge thermometer only when it is "downstream," in the sense of electronic edge transport, from the heater. When the distance between the heater and the thermometer is increased, the thermoelectric signal is reduced, showing that the electrons cool as they propagate along the edge.

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arXiv2008

Area dependence of interlayer tunneling in strongly correlated bilayer 2D electron systems at $ν_T=1$

The area and perimeter dependence of the Josephson-like interlayer tunneling signature of the coherent $ν_T=1$ quantum Hall phase in bilayer two-dimensional electron systems is examined. Electrostatic top gates of various sizes and shapes are used to locally define distinct $ν_T=1$ regions in the same sample. Near the phase boundary with the incoherent $ν_T=1$ state at large layer separation, our results demonstrate that the tunneling conductance in the coherent phase is closely proportional to the total area of the tunneling region. This implies that tunneling at $ν_T=1$ is a bulk phenomenon in this regime.

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arXiv2008

Evidence for a finite temperature phase transition in a bilayer quantum Hall system

We study the Josephson-like interlayer tunneling signature of the strongly correlated $ν_T = 1$ quantum Hall phase in bilayer two-dimensional electron systems as a function of the layer separation, temperature and interlayer charge imbalance. Our results offer strong evidence that a finite temperature phase transition separates the interlayer coherent phase from incoherent phases which lack strong interlayer correlations. The transition temperature is dependent on both the layer spacing and charge imbalance between the layers.

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arXiv2007

Quantum Lifetime of Two-Dimensional Holes

The quantum lifetime of two-dimensional holes in a GaAs/AlGaAs double quantum well is determined via tunneling spectroscopy. At low temperatures the lifetime is limited by impurity scattering but at higher temperatures hole-hole Coulomb scattering dominates. Our results are consistent with Fermi liquid theory, at least up to r_s = 11. At the highest temperatures the measured width of the hole spectral function becomes comparable to the Fermi energy. A new, tunneling-spectroscopic, method for determining the in-plane effective mass of the holes is also demonstrated.

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arXiv2007

Spin Transition in the Half-Filled Landau Level

The transition from partial to complete spin polarization of two-dimensional electrons at half filling of the lowest Landau level has been studied using resistively-detected nuclear magnetic resonance (RDNMR). The nuclear spin-lattice relaxation time is observed to be density independent in the partially polarized phase but to increase sharply at the transition to full polarization. At low temperatures the RDNMR signal exhibits a strong maximum near the critical density.

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arXiv2006

Resistively-Detected NMR in a Two-Dimensional Electron System near $ν= 1$: Clues to the Origin of the Dispersive Lineshape

Resistively-detected NMR measurements on 2D electron systems near the $ν= 1$ quantum Hall state are reported. In contrast to recent results of Gervais \emph{et al.} [Phys. Rev. Lett. $\bf 94$, 196803 (2005)], a dispersive lineshape is found at all RF powers studied and Korringa-like nuclear spin-lattice relaxation is observed. The shape of the unexplained dispersive lineshape is found to invert when the temperature derivative of the longitudinal resistance changes sign. This suggests that both Zeeman and thermal effects are important to resistively-detected NMR in this regime.

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arXiv2005

Surface Acoustic Wave Propagation and Inhomogeneities in Low Density Two-Dimensional Electron Systems Near the Metal-Insulator Transition

We have measured the surface acoustic wave velocity shift in a GaAs/AlGaAs heterostructure containing a two-dimensional electron system (2DES) in a low-density regime ($<$ $10^{10}$ cm$^{-2}$) at zero magnetic field. The interaction of the surface acoustic wave with the 2DES is not well described by a simple model using low-frequency conductivity measurements. We speculate that this conflict is a result of inhomogeneities in the 2DES which become very important at low density. This has implications for the putative metal-insulator transition in two dimensions.

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arXiv2004

Spin Transition in Strongly Correlated Bilayer Two Dimensional Electron Systems

Using a combination of heat pulse and nuclear magnetic resonance techniques we demonstrate that the phase boundary separating the interlayer phase coherent quantum Hall effect at $ν_T = 1$ in bilayer electron gases from the weakly coupled compressible phase depends upon the spin polarization of the nuclei in the host semiconductor crystal. Our results strongly suggest that, contrary to the usual assumption, the transition is attended by a change in the electronic spin polarization.

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arXiv2004

Onset of Interlayer Phase Coherence in a Bilayer Two-Dimensional Electron System: Effect of Layer Density Imbalance

Tunneling and Coulomb drag are sensitive probes of spontaneous interlayer phase coherence in bilayer two-dimensional electron systems at total Landau level filling factor $ν_T = 1$. We find that the phase boundary between the interlayer phase coherent state and the weakly-coupled compressible phase moves to larger layer separations as the electron density distribution in the bilayer is imbalanced. The critical layer separation increases quadratically with layer density difference.

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arXiv2004

Bose-Einstein Condensation of Excitons in Bilayer Electron Systems

An ordered state of electrons in solids in which excitons condense was proposed many years ago as a theoretical possibility but has, until recently, never been observed. We review recent studies of semiconductor bilayer systems that provide clear evidence for this phenomenon and explain why exciton condensation in the quantum Hall regime, where these experiments were performed, is as likely to occur in electron-electron bilayers as in electron-hole bilayers. In current quantum Hall exciton condensates, disorder induces mobile vortices that flow in response to a supercurrent and limit the extremely large bilayer counterflow conductivity.

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