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Register a new userCorrelated States of Electrons in Wide Quantum Wells at Low Fillings: The Role of Charge Distribution Symmetry
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Magneto-transport measurements on electrons confined to a 57 nm-wide, GaAs quantum well reveal that the correlated electron states at low Landau level fillings ($ u$) display a remarkable dependence on the symmetry of the electron charge distribution. At a density of $1.93 times 10^{11}$ cm$^{-2}$, a developing fractional quantum Hall state is observed at the even-denominator filling $ u = 1/4$ when the distribution is symmetric, but it quickly vanishes when the distribution is made asymmetric. At lower densities, as we make the charge distribution asymmetric, we observe a rapid strengthening of the insulating phases that surround the $ u = 1/5$ fractional quantum Hall state.
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We report the observation of developing fractional quantum Hall states at Landau level filling factors $ u = 1/2$ and 1/4 in electron systems confined to wide GaAs quantum wells with significantly $asymmetric$ charge distributions. The very large electric subband separation and the highly asymmetric charge distribution at which we observe these quantum Hall states, together with the fact that they disappear when the charge distribution is made symmetric, suggest that these are one-component states, possibly described by the Moore-Read Pfaffian wavefunction.
Moire superlattices of van der Waals materials, such as twisted graphene and transitional metal dichalcogenides, have recently emerged as a fascinating platform to study strongly correlated states in two dimensions, thanks to the strong electron interaction in the moire minibands. In most systems, the correlated states appear when the moire lattice is filled by integer number of electrons per moire unit cell. Recently, correlated states at fractional fillings of 1/3 and 2/3 holes per moire unit cell has been reported in the WS2/WSe2 heterobilayer, hinting the long range nature of the electron interaction. In this work, employing a scanning microwave impedance microscopy technique that is sensitive to local electrical properties, we observe a series of correlated insulating states at fractional fillings of the moire minibands on both electron- and hole-doped sides in angle-aligned WS2/WSe2 hetero-bilayers, with certain states persisting at temperatures up to 120 K. Monte Carlo simulations reveal that these insulating states correspond to ordering of electrons in the moire lattice with a periodicity much larger than the moire unit cell, indicating a surprisingly strong and long-range interaction beyond the nearest neighbors. Our findings usher in unprecedented opportunities in the study of strongly correlated states in two dimensions.
The dominance of Coulomb interactions over kinetic energy of electrons in narrow, non-trivial moir{e} bands of magic-angle twisted bilayer graphene (TBG) gives rise to a variety of correlated phases such as correlated insulators, superconductivity, orbital ferromagnetism, Chern insulators and nematicity. Most of these phases occur at or near an integer number of carriers per moir{e} unit cell. Experimental demonstration of ordered states at fractional moir{e} band-fillings at zero applied magnetic field $B$, is a challenging pursuit. In this letter, we report the observation of states at half-integer band-fillings of $ u = 0.5$ and $3.5$ at $Bapprox 0$ in a TBG proximitized by a layer of tungsten diselenide (WSe$_2$). The magnetotransport data enables us to deduce features in the underlying band structure consistent with a spontaneously broken translational symmetry supercell with twice the area of the original TBG moir{e} cell. A series of Lifshitz transitions due to the changes in the topology of the Fermi surface implies the evolution of van Hove singularities (VHS) of the diverging density of states at a discrete set of partial fillings of flat bands. Further, we observe reset of charge carriers at $ u = 2, 3$. In addition to magnetotransport, we employ thermoelectricity as a tool to probe the system at $B=0$. Band structure calculations for a TBG moir{e} pattern, together with a commensurate density wave potential and spin-orbit coupling (SOC) terms, allow to obtain degeneracy-lifted, zone-folded moir{e} bands with spin-valley isospin ordering anisotropy that describe the states at half-integer fillings observed experimentally. Our results suggest the emergence of a spin-charge density wave ground state in TBG in the zero $B-$ field limit.
Moire superlattices are excellent platforms to realize strongly correlated quantum phenomena, such as Mott insulation and superconductivity. In particular, recent research has revealed stripe phases and generalized Wigner crystals at fractional fillings of moire superlattices. But these experiments have not focused on the influence of electronic crystallization on the excitonic and valleytronic properties of the superlattices. Here we report excitonic and valleytronic signatures of correlated states at fractional fillings in a WSe$_2$/WS$_2$ moire superlattice. We observe reflection spectral modulation of three intralayer moire excitons at filling factors $ u$ = 1/3 and 2/3. We also observe luminescence spectral modulation of interlayer trions at around a dozen fractional filling factors, including $ u$ = -3/2, 1/4, 1/3, 2/5, 2/3, 6/7, 5/3. In addition, the valley polarization of interlayer trions is noticeably suppressed at some fractional fillings. These results demonstrate a new regime of light-matter interactions, in which electron crystallization significantly modulates the absorption, emission, and valley dynamics of the excitonic states in a moire superlattice.
We propose a universal quantum computing scheme in which the orthogonal qubit states $|0>$ and $|1>$ are identical in their single-particle spin and charge properties. Each qubit is contained in a single quantum dot and gate operations are induced all-electrically by changes in the confinement potential. Within the computational space, these qubits are robust against environmental influences that couple to the system through single-particle channels. Due to the identical spin and charge properties of the $|0>$, $|1>$ states, the lowest-order relaxation and decoherence rates $1/T_1$ and $1/T_2$, within the Born-Markov approximation, both vanish for a large class of environmental couplings. We give explicit pulse sequences for a universal set of gates (phase, $pi/8$, Hadamard, textsc{cnot}) and discuss state preparation, manipulation, and detection.
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