4,980 research outputs found
High-precision torsional magnetometer: Application to two-dimensional electron systems
A dc torsional magnetometer for use in high magnetic fields is described. With a resolution of 10^–12 J/T at 5 T and excellent rejection of background moments, this device has been used to study the de Haas–van Alphen effect in two-dimensional electron systems. This resolution is about 100 times that obtained with a commercially available superconducting quantum interference device magnetometer. The device is useful over a wide temperature range including that below 1 K
Measurement of the electronic compressibility of bilayer graphene
We present measurements of the electronic compressibility, , 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 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.Comment: 5 pages, 3 figures; final version for publicatio
Quantum phase transitions of polar molecules in bilayer systems
We investigate the quantum phase transitions of bosonic polar molecules in a
two-dimensional double layer system. We show that an interlayer bound state of
dipoles (dimers) can be formed when the dipole strength is above a critical
value, leading to a zero-energy resonance in the interlayer s-wave scattering
channel. In the positive detuning side of the resonance, the strong repulsive
interlayer pseudopotential can drive the system into a maximally entangled
state, where the wave function is a superposition of two states that have all
molecules in one layer and none in the other. We discuss how the zero-energy
resonance, dimer states, and the maximally entangled state can be measured in
time-of-flight experiments.Comment: Minor correction
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 v=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
Tunnel transport and interlayer excitons in bilayer fractional quantum Hall systems
In a bilayer system consisting of a composite-fermion Fermi sea in each
layer, the tunnel current is exponentially suppressed at zero bias, followed by
a strong peak at a finite bias voltage . This behavior, which is
qualitatively different from that observed for the electron Fermi sea, provides
fundamental insight into the strongly correlated non-Fermi liquid nature of the
CF Fermi sea and, in particular, offers a window into the short-distance
high-energy physics of this state. We identify the exciton responsible for the
peak current and provide a quantitative account of the value of .
The excitonic attraction is shown to be quantitatively significant, and its
variation accounts for the increase of with the application of an
in-plane magnetic field. We also estimate the critical Zeeman energy where
transition occurs from a fully spin polarized composite fermion Fermi sea to a
partially spin polarized one, carefully incorporating corrections due to finite
width and Landau level mixing, and find it to be in satisfactory agreement with
the Zeeman energy where a qualitative change has been observed for the onset
bias voltage [Eisenstein et al., Phys. Rev. B 94, 125409 (2016)]. For
fractional quantum Hall states, we predict a substantial discontinuous jump in
when the system undergoes a transition from a fully spin
polarized state to a spin singlet or a partially spin polarized state.Comment: 14 pages, 14 figure
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
Vanishing Hall Resistance at High Magnetic Field in a Double Layer Two-Dimensional Electron System
At total Landau level filling factor a double layer
two-dimensional electron system with small interlayer separation supports a
collective state possessing spontaneous interlayer phase coherence. This state
exhibits the quantized Hall effect when equal electrical currents flow in
parallel through the two layers. In contrast, if the currents in the two layers
are equal, but oppositely directed, both the longitudinal and Hall resistances
of each layer vanish in the low temperature limit. This finding supports the
prediction that the ground state at is an excitonic superfluid.Comment: 4 pages, 4 figure
Quantum Hall Effect and Semimetallic Behavior of Dual-Gated ABA-Stacked Trilayer Graphene
The electronic structure of multilayer graphenes depends strongly on the
number of layers as well as the stacking order. Here we explore the electronic
transport of purely ABA-stacked trilayer graphenes in a dual-gated field-effect
device configuration. We find that both the zero-magnetic-field transport and
the quantum Hall effect at high magnetic fields are distinctly different from
the monolayer and bilayer graphenes, and that they show electron-hole
asymmetries that are strongly suggestive of a semimetallic band overlap. When
the ABA trilayers are subjected to an electric field perpendicular to the
sheet, Landau level splittings due to a lifting of the valley degeneracy are
clearly observed.Comment: 5 figure
Exciton Condensation in Bilayer Quantum Hall Systems
The condensation of excitons, bound electron-hole pairs in a solid, into a coherent collective electronic state was predicted more than 50 years ago. Perhaps surprisingly, the phenomenon was first observed in a system consisting of two closely spaced parallel two-dimensional electron gases in a semiconductor double quantum well. At an appropriate high magnetic field and low temperature, the bilayer electron system condenses into a state resembling a superconductor, only with the Cooper pairs replaced by excitons consisting of electrons in one layer bound to holes in the other. In spite of being charge neutral, the transport of excitons within the condensate gives rise to several spectacular electrical effects. This article describes these phenomena and examines how they inform our understanding of this unique phase of quantum electronic matter
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