3,641 research outputs found
Evidence for defect-mediated tunneling in hexagonal boron nitride-based junctions
We investigate tunneling in metal-insulator-metal junctions employing few
atomic layers of hexagonal boron nitride (hBN) as the insulating barrier. While
the low-bias tunnel resistance increases nearly exponentially with barrier
thickness, subtle features are seen in the current-voltage curves, indicating
marked influence of the intrinsic defects present in the hBN insulator on the
tunneling transport. In particular, single electron charging events are
observed, which are more evident in thicker-barrier devices where direct
tunneling is substantially low. Furthermore, we find that annealing the devices
modifies the defect states and hence the tunneling signatures.Comment: 5 pages, 5 figure
Coulomb Drag in the Extreme Quantum Limit
Coulomb drag resulting from interlayer electron-electron scattering in double
layer 2D electron systems at high magnetic field has been measured. Within the
lowest Landau level the observed drag resistance exceeds its zero magnetic
value by factors of typically 1000. At half-filling of the lowest Landau level
in each layer (nu = 1/2) the data suggest that our bilayer systems are much
more strongly correlated than recent theoretical models based on perturbatively
coupled composite fermion metals.Comment: 4 pages, 4 figure
Spin and the Coulomb Gap in the Half-Filled Lowest Landau Level
The Coulomb gap observed in tunneling between parallel two-dimensional
electron systems, each at half filling of the lowest Landau level, is found to
depend sensitively on the presence of an in-plane magnetic field. Especially at
low electron density, the width of the Coulomb gap at first increases sharply
with in-plane field, but then abruptly levels off. This behavior appears to
coincide with the known transition from partial to complete spin polarization
of the half-filled lowest Landau level. The tunneling gap therefore opens a new
window onto the spin configuration of two-dimensional electron systems at high
magnetic field.Comment: 6 pages, 4 postscript figures. Minor changes. To appear in Physical
Review
Evidence for a Goldstone Mode in a Double Layer Quantum Hall System
The tunneling conductance between two parallel 2D electron systems has been
measured in a regime of strong interlayer Coulomb correlations. At total Landau
level filling the tunnel spectrum changes qualitatively when the
boundary separating the compressible phase from the ferromagnetic quantized
Hall state is crossed. A huge resonant enhancement replaces the strongly
suppressed equilibrium tunneling characteristic of weakly coupled layers. The
possible relationship of this enhancement to the Goldstone mode of the broken
symmetry ground state is discussed.Comment: 4 pages, 3 figures, 2 minor typeos fixe
Interlayer tunneling in double-layer quantum Hall pseudo-ferromagnets
We show that the interlayer tunneling I--V in double-layer quantum Hall
states displays a rich behavior which depends on the relative magnitude of
sample size, voltage length scale, current screening, disorder and thermal
lengths. For weak tunneling, we predict a negative differential conductance of
a power-law shape crossing over to a sharp zero-bias peak. An in-plane magnetic
field splits this zero-bias peak, leading instead to a ``derivative'' feature
at , which gives a direct measure of
the dispersion of the Goldstone mode corresponding to the spontaneous symmetry
breaking of the double-layer Hall state.Comment: 4 pgs. RevTex, submitted to Phys. Rev. Let
Separately contacted electron-hole double layer in a GaAs/AlxGa1−xAs heterostructure
We describe a method for creating closely spaced parallel two-dimensional electron and hole gases confined in 200 Å GaAs wells separated by a 200 Å wide AlxGa1−xAs barrier. Low-temperature ohmic contacts are made to both the electrons and holes, whose densities are individually adjustable between 10^(10)/cm^2 to greater than 10^(11)/cm^2
Evidence for a fractional quantum Hall state with anisotropic longitudinal transport
At high magnetic fields, where the Fermi level lies in the N=0 lowest Landau
level (LL), a clean two-dimensional electron system (2DES) exhibits numerous
incompressible liquid phases which display the fractional quantized Hall effect
(FQHE) (Das Sarma and Pinczuk, 1997). These liquid phases do not break
rotational symmetry, exhibiting resistivities which are isotropic in the plane.
In contrast, at lower fields, when the Fermi level lies in the third
and several higher LLs, the 2DES displays a distinctly different class of
collective states. In particular, near half filling of these high LLs the 2DES
exhibits a strongly anisotropic longitudinal resistance at low temperatures
(Lilly et al., 1999; Du et al., 1999). These "stripe" phases, which do not
exhibit the quantized Hall effect, resemble nematic liquid crystals, possessing
broken rotational symmetry and orientational order (Koulakov et al., 1996;
Fogler et al., 1996; Moessner and Chalker, 1996; Fradkin and Kivelson, 1999;
Fradkin et al, 2010). Here we report a surprising new observation: An
electronic configuration in the N=1 second LL whose resistivity tensor
simultaneously displays a robust fractionally quantized Hall plateau and a
strongly anisotropic longitudinal resistance resembling that of the stripe
phases.Comment: Nature Physics, (2011
Numerical Investigation on Asymmetric Bilayer System at Integer Filling Factor
Deformation of the easy-axis ferromagnetic state in asymmetric bilayer
systems are investigated numerically. Using the exact diagonalization the
easy-axis to easy-plane ferromagnetic transition at total filling factor 3 or 4
is investigated. At still higher filling, novel stripe state in which stripes
are aligned in the vertical direction occurs. The Hartree-Fock energies of
relevant ordered states are calculated and compared.Comment: 4 pages, 6 figures, Proceedings of EP2DS-15, to appear in Physica
The Onset of Anisotropic Transport of Two-Dimensional Electrons in High Landau Levels: An Isotropic-to-Nematic Liquid Crystal Phase Transition?
The recently discovered anisotropy of the longitudinal resistance of
two-dimensional electrons near half filling of high Landau levels is found to
persist to much higher temperatures T when a large in-plane magnetic field B||
is applied. Under these conditions we find that the longitudinal resistivity
scales quasi-linearly with B||/T. These observations support the notion that
the onset of anisotropy at B||=0 does not reflect the spontaneous development
of charge density modulations but may instead signal an isotropic-to-nematic
liquid crystal phase transition.Comment: 5 pages, 4 figure
Electron Correlations in Partially Filled Lowest and Excited Landau Levels
The electron correlations near the half-filling of the lowest and excited
Landau levels (LL's) are studied using numerical diagonalization. It is shown
that in the low lying states electrons avoid pair states with relative angular
momenta corresponding to positive anharmonicity of the interaction
pseudopotential . In the lowest LL, the super-harmonic behavior of
causes Laughlin correlations (avoiding pairs with )
and the Laughlin-Jain series of incompressible ground states. In the first
excited LL, is harmonic at short range and a different series of
incompressible states results. Similar correlations occur in the paired
Moore-Read state and in the and
states, all having small total parentage from and 3 and large
parentage from . The and states are
different from Laughlin and states and, in finite
systems, occur at a different LL degeneracy (flux). The series of Laughlin
correlated states of electron pairs at ,
, , and is proposed, although only in the
state pairing has been confirmed numerically. In the second
excited LL, is sub-harmonic at short range and (near the
half-filling) the electrons group into spatially separated larger
droplets to minimize the number of strongly repulsive pair states at and 5.Comment: 10 pages, 8 figures, submitted to PR
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