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Evidence of frustrated magnetic interactions in a Wigner-Mott insulator

Cornell Affiliated Author(s)
Author
Yanhao Tang
Kaixiang Su
Lizhong Li
Yang Xu
Song Liu
Kenji Watanabe
Takashi Taniguchi
James Hone
Chao-Ming Jian
Cenke Xu
Kin Mak
Jie Shan
Abstract

Electrons in two-dimensional semiconductor moiré materials are more delocalized around the lattice sites than those in conventional solids1,2. The non-local contributions to the magnetic interactions can therefore be as important as the Anderson superexchange3, which makes the materials a unique platform to study the effects of competing magnetic interactions3,4. Here we report evidence of strongly frustrated magnetic interactions in a Wigner–Mott insulator at a two-thirds (2/3) filling of the moiré lattice in angle-aligned WSe2/WS2 bilayers.

Journal
Nature Nanotechnology
Date Published
Funding Source
DMR-1807810
FA9550-20-1-0219
DE-SC0019481
DMR-1719875
NNCI-2025233
12274365
DMR-1920434
JPMJCR15F3
2022YFA1402403
2022YFA1405400
Group (Lab)
Chao-Ming Jian Group
Jie Shan Group
Kin Fai Mak Group

Exciton density waves in Coulomb-coupled dual moiré lattices

Cornell Affiliated Author(s)
Author
Y. Zeng
Z. Xia
R. Dery
K. Watanabe
T. Taniguchi
J. Shan
K.F. Mak
Abstract

Strongly correlated bosons in a lattice are a platform that can realize rich bosonic states of matter and quantum phase transitions1. While strongly correlated bosons in a lattice have been studied in cold-atom experiments2–4, their realization in a solid-state system has remained challenging5. Here we trap interlayer excitons–bosons composed of bound electron–hole pairs, in a lattice provided by an angle-aligned WS2/bilayer WSe2/WS2 multilayer. The heterostructure supports Coulomb-coupled triangular moiré lattices of nearly identical period at the top and bottom interfaces.

Journal
Nature Materials
Date Published
Funding Source
DMR-2114535
N00014-21-1-2471
NNCI-2025233
DE-SC0019481
JPMJCR15F3
Group (Lab)
Jie Shan Group
Kin Fai Mak Group

A tunable bilayer Hubbard model in twisted WSe2

Cornell Affiliated Author(s)
Author
Y. Xu
K. Kang
K. Watanabe
T. Taniguchi
K.F. Mak
J. Shan
Abstract

Moiré materials with flat electronic bands provide a highly controllable quantum system for studies of strong-correlation physics and topology. In particular, angle-aligned heterobilayers of semiconducting transition metal dichalcogenides with large band offset realize the single-band Hubbard model. Introduction of a new layer degree of freedom is expected to foster richer interactions, enabling Hund’s physics, interlayer exciton condensation and new superconducting pairing mechanisms to name a few.

Journal
Nature Nanotechnology
Date Published
Funding Source
DMR-2114535
N00014-21-1-2471
DE-SC0019481
DE-SC0022058
NNCI-2025233
JPMJCR15F3
2021YFA1401300
Group (Lab)
Jie Shan Group
Kin Fai Mak Group

Dielectric catastrophe at the Wigner-Mott transition in a moiré superlattice

Cornell Affiliated Author(s)
Author
Y. Tang
J. Gu
S. Liu
K. Watanabe
T. Taniguchi
J.C. Hone
K.F. Mak
J. Shan
Abstract

The bandwidth-tuned Wigner-Mott transition is an interaction-driven phase transition from a generalized Wigner crystal to a Fermi liquid. Because the transition is generally accompanied by both magnetic and charge-order instabilities, it remains unclear if a continuous Wigner-Mott transition exists. Here, we demonstrate bandwidth-tuned metal-insulator transitions at fixed fractional fillings of a MoSe2/WS2 moiré superlattice. The bandwidth is controlled by an out-of-plane electric field. The dielectric response is probed optically with the 2s exciton in a remote WSe2 sensor layer.

Journal
Nature Communications
Date Published
Funding Source
DMR-2114535
W911NF-17-1-0605
DE-SC0019481
NNCI-2025233
JPMJCR15F3
Group (Lab)
Jie Shan Group
Kin Fai Mak Group

Van der Waals πjosephson Junctions

Cornell Affiliated Author(s)
Author
K. Kang
H. Berger
K. Watanabe
T. Taniguchi
L. Forró
J. Shan
K.F. Mak
Abstract

Proximity-induced superconductivity in a ferromagnet can induce Cooper pairs with a finite center-of-mass momentum and stabilize Josephson junctions (JJs) with πphase difference in superconductor-ferromagnet-superconductor heterostructures. The emergence of two-dimensional layered superconducting and magnetic materials promises a new platform for realizing πJJs with atomically sharp interfaces. Here we demonstrate a thickness-driven 0-πtransition in JJs made of NbSe2(an Ising superconductor) and Cr2Ge2Te6(a ferromagnetic semiconductor).

Journal
Nano Letters
Date Published
Funding Source
DMR-1807810
NNCI-2025233
FA9550-18-1-0480
W911NF-17-1-0605
DMR-1719875
JPMJCR15F3
Group (Lab)
Jie Shan Group
Kin Fai Mak Group

Semiconductor moiré materials

Cornell Affiliated Author(s)
Author
K.F. Mak
J. Shan
Abstract

Moiré materials have emerged as a platform for exploring the physics of strong electronic correlations and non-trivial band topology. Here we review the recent progress in semiconductor moiré materials, with a particular focus on transition metal dichalcogenides. Following a brief overview of the general features in this class of materials, we discuss recent theoretical and experimental studies on Hubbard physics, Kane–Mele–Hubbard physics and equilibrium moiré excitons.

Journal
Nature Nanotechnology
Date Published
Funding Source
DMR- 2114535
N00014-21-1-2471
Group (Lab)
Jie Shan Group
Kin Fai Mak Group

Dipolar excitonic insulator in a moiré lattice

Cornell Affiliated Author(s)
Author
J. Gu
L. Ma
S. Liu
K. Watanabe
T. Taniguchi
J.C. Hone
J. Shan
K.F. Mak
Abstract

Two-dimensional moiré materials provide a highly controllable solid-state platform for studies of quantum phenomena1–3. To date, experimental studies have focused on correlated electronic states, whereas correlated bosonic states in moiré materials have received less attention. Here we report the observation of a correlated dipolar excitonic insulator—a charge-insulating state driven by exciton formation4—in a device where a WSe2 monolayer and WSe2/WS2 moiré bilayer are coupled via Coulomb interactions. The system is a Mott insulator when all the holes reside in the moiré layer.

Journal
Nature Physics
Date Published
Funding Source
DMR-2004451
N00014-21-1-2471
FA9550-18-1-0480
DE-SC0019481
DE-SC0022058
NNCI-1542081
JPMJCR15F3
Group (Lab)
Jie Shan Group
Kin Fai Mak Group

Strong interlayer interactions in bilayer and trilayer moire superlattices

Author
S. Xie
B.D. Faeth
Y. Tang
L. Li
E. Gerber
C.T. Parzyck
Debanjan Chowdhury
Y.-H. Zhang
C. Jozwiak
A. Bostwick
E. Rotenberg
Eun-Ah Kim
J. Shan
K.F. Mak
K.M. Shen
Abstract

Moire superlattices constructed from transition metal dichalcogenides have demonstrated a series of emergent phenomena, including moire excitons, flat bands, and correlated insulating states. All of these phenomena depend crucially on the presence of strong moire potentials, yet the properties of these moire potentials, and the mechanisms by which they can be generated, remain largely open questions. Here, we use angle-resolved photoemission spectroscopy with submicron spatial resolution to investigate an aligned WS2/WSe2moire superlattice and graphene/WS2/WSe2trilayer heterostructure.

Journal
Science Advances
Date Published
Group (Lab)
Debanjan Chowdhury Group
Jie Shan Group
Kin Fai Mak Group
Kyle Shen Group

Reproducibility in the fabrication and physics of moiré materials

Cornell Affiliated Author(s)
Author
C.N. Lau
M.W. Bockrath
K.F. Mak
F. Zhang
Abstract

Overlaying two atomic layers with a slight lattice mismatch or at a small rotation angle creates a moiré superlattice, which has properties that are markedly modified from (and at times entirely absent in) the ‘parent’ materials. Such moiré materials have progressed the study and engineering of strongly correlated phenomena and topological systems in reduced dimensions.

Journal
Nature
Date Published
Funding Source
DMR 2004801
DMR-2105028
DOE DE-SC0020187
FA9550-20-1-0219
DMR-1921581
DMR-1945351
DMR-2105139
W911NF-18-1-0416
Group (Lab)
Kin Fai Mak Group