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Register a new userCorrelated Insulating States at Fractional Fillings of the WS2/WSe2 Moire Lattice
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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.
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Stripe phases, in which the rotational symmetry of charge density is spontaneously broken, occur in many strongly correlated systems with competing interactions. One representative example is the copper-oxide superconductors, where stripe order is thought to be relevant to the mechanism of high-temperature superconductivity. Identifying and studying the stripe phases in conventional strongly correlated systems are, however, challenging due to the complexity and limited tunability of these materials. Here we uncover stripe phases in WSe2/WS2 moire superlattices with continuously gate-tunable charge densities by combining optical anisotropy and electronic compressibility measurements. We find strong electronic anisotropy over a large doping range peaked at 1/2 filling of the moire superlattice. The 1/2-state is incompressible and assigned to a (insulating) stripe crystal phase. It can be continuously melted by thermal fluctuations around 35 K. The domain configuration revealed by wide-field imaging shows a preferential alignment along the high-symmetry axes of the moire superlattice. Away from 1/2 filling, we observe additional stripe crystals at commensurate filling 1/4, 2/5 and 3/5. The anisotropy also extends into the compressible regime of the system at incommensurate fillings, indicating the presence of electronic liquid crystal states. The observed filling-dependent stripe phases agree with the theoretical phase diagram of the extended Hubbard model on a triangular lattice in the flat band limit. Our results demonstrate that two-dimensional semiconductor moire superlattices are a highly tunable platform to study the stripe phases and their interplay with other symmetry breaking ground states.
We report the nanoscale conductivity imaging of correlated electronic states in angle-aligned WSe2/WS2 heterostructures using microwave impedance microscopy. The noncontact microwave probe allows us to observe the Mott insulating state with one hole per moire unit cell that persists for temperatures up to 150 K, consistent with other characterization techniques. In addition, we identify for the first time a Mott insulating state at one electron per moire unit cell. Appreciable inhomogeneity of the correlated states is directly visualized in the hetero-bilayer region, indicative of local disorders in the moire superlattice potential or electrostatic doping. Our work provides important insights on 2D moire systems down to the microscopic level.
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.
Moire superlattices in van der Waals heterostructures have emerged as a powerful tool for engineering novel quantum phenomena. Here we report the observation of a correlated interlayer exciton insulator in a double-layer heterostructure composed of a WSe2 monolayer and a WS2/WSe2 moire bilayer that are separated by an ultrathin hexagonal boron nitride (hBN). The moire WS2/WSe2 bilayer features a Mott insulator state at hole density p/p0 = 1, where p0 corresponds to one hole per moire lattice site. When electrons are added to the Mott insulator in the WS2/WSe2 moire bilayer and an equal number of holes are injected into the WSe2 monolayer, a new interlayer exciton insulator emerges with the holes in the WSe2 monolayer and the electrons in the doped Mott insulator bound together through interlayer Coulomb interactions. The excitonic insulator is stable up to a critical hole density of ~ 0.5p0 in the WSe2 monolayer, beyond which the system becomes metallic. Our study highlights the opportunities for realizing novel quantum phases in double-layer moire systems due to the interplay between the moire flat band and strong interlayer electron interactions.
Transition metal dichalcogenide (TMD) moire heterostructures provide an ideal platform to explore the extended Hubbard model1 where long-range Coulomb interactions play a critical role in determining strongly correlated electron states. This has led to experimental observations of Mott insulator states at half filling2-4 as well as a variety of extended Wigner crystal states at different fractional fillings5-9. Microscopic understanding of these emerging quantum phases, however, is still lacking. Here we describe a novel scanning tunneling microscopy (STM) technique for local sensing and manipulation of correlated electrons in a gated WS2/WSe2 moire superlattice that enables experimental extraction of fundamental extended Hubbard model parameters. We demonstrate that the charge state of local moire sites can be imaged by their influence on STM tunneling current, analogous to the charge-sensing mechanism in a single-electron transistor. In addition to imaging, we are also able to manipulate the charge state of correlated electrons. Discharge cascades of correlated electrons in the moire superlattice are locally induced by ramping the STM bias, thus enabling the nearest-neighbor Coulomb interaction (UNN) to be estimated. 2D mapping of the moire electron charge states also enables us to determine onsite energy fluctuations at different moire sites. Our technique should be broadly applicable to many semiconductor moire systems, offering a powerful new tool for microscopic characterization and control of strongly correlated states in moire superlattices.
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