arXiv subjects

Kin Fai Mak

Publications and source records attributed to Kin Fai Mak.

At least 37 records · Page 2Linked to original sources

arXiv2025

Emergence of Chern metal in a moiré Kondo lattice

A Chern metal is a two-dimensional metallic state of matter carrying chiral edge states. It can emerge as a doped Chern insulator, but theoretical studies have also predicted its emergence near a Kondo breakdown separating a metallic chiral spin liquid and a heavy Fermi liquid in a frustrated lattice. To date, the latter exotic scenario has not been realized. Here, we report the observation of a Chern metal at the onset of the magnetic Kondo breakdown in a frustrated moiré Kondo lattice--angle-aligned MoTe2/WSe2 bilayers. The state is compressible and is manifested by a nearly quantized Hall resistance but a finite longitudinal resistance that arises from a bad metallic bulk. The state also separates an itinerant and a heavy Fermi liquid and appears far away from the band inversion critical point of the material, thus ruling out its origin from simply doping a Chern insulator. We demonstrate the presence of a chiral edge state by nonlocal transport measurements and current-induced quantum anomalous Hall breakdown. Magnetic circular dichroism measurements further reveal a magnetization plateau for the Chern metal before a metamagnetic transition at the Kondo breakdown. Our results open an opportunity for moiré engineering of exotic quantum phases of matter through the close interplay between band topology and Kondo interactions.

cond-mat.str-el↗

arXiv2025

Bright hybrid excitons in molecularly tunable bilayer crystals

Bilayer crystals, built by stacking crystalline monolayers, generate interlayer potentials that govern excitonic phenomena but are constrained by fixed covalent lattices and orientations. Replacing one layer with an atomically thin molecular crystal overcomes this limitation, as diverse functional groups enable tunable molecular lattices and interlayer potentials, tailoring a wide range of excitonic properties. Here, we report hybrid excitons in four-atom-thick hybrid bilayer crystals (HBCs), directly synthesized with single-crystalline perylene diimide (PDI) molecular crystal atop WS2 monolayers. These excitons arise from a hybridized bilayer band structure, revealed by lattice-scale first-principles calculations, inheriting properties from both monolayers. They exhibit bright photoluminescence with near-unity polarization above and below the WS2 bandgap, along with spectral signatures of exciton delocalization, supported by theory, while their energies and intensities are tuned by modifying the HBC composition by synthesis. Our work introduces a molecule-based 2D quantum materials platform for bottom-up design and control of optoelectronic properties.

cond-mat.mtrl-sci↗

arXiv2025

Molecular tuning of excitons in four-atom-thick hybrid bilayer crystals

Bilayer crystals, formed by stacking monolayers of two-dimensional (2D) crystals, create interlayer potentials that govern excitonic phenomena but are constrained by their fixed covalent lattices. Replacing one layer with an atomically thin molecular crystal overcomes this limitation, as precise control of functional groups enables tunable 2D molecular lattices and, consequently, electronic structures. Here, we report molecular tuning of lattices and excitons in four-atom-thick hybrid bilayer crystals (HBCs), synthesized as monolayers of perylene-based molecular and transition metal dichalcogenide (TMD) single crystals. In HBCs, we observe an anisotropic photoluminescence signal exhibiting characteristics of both molecular and TMD excitons, directly tuned by molecular geometry and HBC composition. Ab initio calculations reveal that this anisotropic emission arises from hybrid excitons, which inherit properties from both layers through a hybridized bilayer band structure. Our work establishes a synthetically derived, molecule-based 2D quantum materials platform with the potential for engineering interlayer potentials.

cond-mat.mtrl-sci↗

arXiv2025

Quantum oscillations in a dipolar excitonic insulator

Quantum oscillations in magnetization or resistivity are a defining feature of metals subject to an external magnetic field. The phenomenon is generally not expected in insulators without a Fermi surface. The observations of quantum oscillations in Kondo insulating materials have provided a rare counterexample and attracted much theoretical interest. However, the magnetic oscillations in correlated insulators remain poorly understood. Here we report the observations of resistivity quantum oscillations in an excitonic insulator realized in Coulomb-coupled electron-hole double layers with gate-tunability that allows the phenomenon to be explored in a more controllable fashion than in bulk materials. When the cyclotron energy of the electrons or holes is tuned to be comparable to or larger than the exciton binding energy, recurring transitions between excitonic insulators and electron-hole decoupled quantum Hall states are observed. Compressibility measurements show an oscillatory exciton binding energy as a function of magnetic field and electron-hole pair density. Coulomb drag measurements further reveal the formation of excitons with finite angular momentum. Our results are qualitatively captured by mean-field theory calculations. The study demonstrates a new platform for studying quantum oscillations in correlated insulators.

cond-mat.mes-hall↗

arXiv2025

Time-reversal symmetry breaking fractional quantum spin Hall insulator in moiré MoTe2

Twisted bilayer transition metal dichalcogenide semiconductors, which support flat Chern bands with enhanced interaction effects, realize a platform for fractional Chern insulators and fractional quantum spin Hall (FQSH) insulators. A recent experiment has reported the emergence of a FQSH insulator protected by spin-Sz conservation at a moiré lattice filling factor ν=3 in 2.1-degree twisted bilayer MoTe2. Theoretical studies have proposed both time-reversal symmetric and asymmetric ground states as possible candidates for the observed FQSH insulator, but the nature of the state remains unexplored. Here we report the observation of spontaneous time-reversal symmetry breaking at generic fillings in 2.1-degree twisted bilayer MoTe2 from ν<1 all the way to ν>6 except at ν=2, 4, and 6. Although zero Hall response is observed at ν=3 for magnetic fields higher than 20 mT, a finite anomalous Hall response accompanied by a magnetic hysteresis is observed at lower magnetic fields, demonstrating spontaneous time-reversal symmetry breaking. Our work shows the tendency towards ferromagnetism by doping the first three pairs of conjugate Chern bands in the material; it also sheds light on the nature of the FQSH insulator at ν=3.

cond-mat.mes-hall↗

arXiv2024

Relativistic Mott transition in strongly correlated artificial graphene

The realization of graphene has provided a bench-top laboratory for quantum electrodynamics. The low-energy excitations of graphene are two-dimensional massless Dirac fermions with opposite chiralities at the $\pm$K valleys of the graphene Brillouin zone. It has been speculated that the electron-electron interactions in graphene could spontaneously break the chiral symmetry to induce a finite mass for Dirac fermions, in analogue to dynamical mass generation in elementary particles. The phenomenon is also known as the relativistic Mott transition and has not been observed in pristine graphene because the interaction strength is insufficient. Here, we report the realization of strongly correlated artificial graphene and the observation of the relativistic Mott transition in twisted WSe2 tetralayers. Using magneto transport, we show that the first $Γ$-valley moiré valence band mimics the low-energy graphene band structure. At half-band filling, the system exhibits hallmarks of massless Dirac fermions, including an anomalous Landau fan originated from a $π$-Berry phase and a square-root density dependence of the cyclotron mass. We tune the interaction across the semimetal-insulator transition by reducing the twist angle below about 2.7 degrees. The emergent insulator is compatible with an antiferromagnetic Mott insulator. Our results open the possibility of studying strongly correlated Dirac fermions in a condensed matter system.

cond-mat.mes-hall↗

arXiv2024

A New Moiré Platform Based on M-Point Twisting

We introduce a new class of moiré systems and materials based on monolayers with triangular lattices and low-energy states at the M points of the Brillouin zone. These M-point moiré materials are fundamentally distinct from those derived from $Γ$- or K-point monolayers, featuring three time-reversal-preserving valleys related by three-fold rotational symmetry. We propose twisted bilayers of experimentally exfoliable 1T-SnSe$_2$ and 1T-ZrS$_2$ as realizations of this new class. Using extensive ab initio simulations, we develop quantitative continuum models and analytically show that the corresponding M-point moiré Hamiltonians exhibit emergent momentum-space non-symmorphic symmetries and a kagome plane-wave lattice in momentum space. This represents the first experimentally viable realization of a projective representation of crystalline space groups in a non-magnetic system. With interactions, these materials represent six-flavor Hubbard simulators with Mott physics, as can be seen by their flat Wilson loops. Furthermore, the presence of a non-symmorphic momentum-space in-plane mirror symmetry makes some of the M-point moiré Hamiltonians quasi-one-dimensional in each valley, suggesting the possibility of realizing Luttinger liquid physics. We predict the twist angles at which a series of (conduction) flat bands appear, provide a faithful continuum Hamiltonian, analyze its topology and charge density and briefly discuss several aspects of the physics of this new platform.

cond-mat.str-el↗

arXiv2024

Correlated states controlled by tunable van Hove singularity in moiré WSe2

Twisted bilayers of transition metal dichalcogenide semiconductors have enabled the discovery of superconductivity, ferromagnetism, correlated insulators and a series of new topological phases of matter. However, the connection between these electronic phases and the underlying band structure singularities in these materials has remained largely unexplored. Here, combining the magnetic circular dichroism and electronic compressibility measurements, we investigate the influence of a van Hove singularity on the correlated phases in bilayer WSe2 with twist angle between 2-3 degrees. We demonstrate stabilizing the Stoner ferromagnetism below moiré lattice filling one and Chern insulators at filling one by tuning the van Hove singularity cross the Fermi level using the electric and magnetic fields. The experimental observations are supported by the continuum model band structure calculations. Our results highlight the prospect of engineering the electronic phases by tunable van Hove singularities.

cond-mat.str-el↗

arXiv2024

Interlayer Fermi polarons of excited exciton states in quantizing magnetic fields

The study of exciton-polarons has offered profound insights into the many-body interactions between bosonic excitations and their immersed Fermi sea within layered heterostructures. However, little is known about the properties of exciton polarons with interlayer interactions. Here through magneto-optical reflectance contrast measurements, we experimentally investigate interlayer Fermi polarons for 2s excitons in WSe$_2$/graphene heterostructures, where the excited exciton states (2s) in the WSe$_2$ layer are dressed by free charge carriers of the adjacent graphene layer in the Landau quantization regime. First, such a system enables an optical detection of integer and fractional quantum Hall states (e.g. $ν=\pm1/3$, $\pm$2/3) of monolayer graphene. Furthermore, we observe that the 2s state evolves into two distinct branches, denoted as attractive and repulsive polarons, when graphene is doped out of the incompressible quantum Hall gaps. Our work paves the way for the understanding of the excited composite quasiparticles and Bose-Fermi mixtures.

cond-mat.mes-hall↗

arXiv2024

Unconventional superconductivity in twisted bilayer WSe2

Moiré materials have enabled the realization of flat electron bands and quantum phases that are driven by strong correlations associated with flat bands. Superconductivity has been observed, but solely, in graphene moiré materials. The absence of robust superconductivity in moiré materials beyond graphene, such as semiconductor moiré materials, has remained a mystery and challenged our current understanding of superconductivity in flat bands. Here, we report the observation of robust superconductivity in 3.65-degree twisted bilayer WSe2 which hosts a honeycomb moiré lattice. Superconductivity emerges at half-band filling and under small sublattice potential differences, where the moiré band is a flat Chern band. The optimal superconducting transition temperature is about 220 mK and constitutes 2% of the effective Fermi temperature; the latter is comparable to the value in high-temperature cuprate superconductors and suggests strong pairing. The superconductor borders on two distinct metals below and above half-band filling; it undergoes a continuous transition to a correlated insulator by tuning the sublattice potential difference. The observed superconductivity on the verge of Coulomb-induced charge localization suggests roots in strong electron correlations.

cond-mat.mes-hall↗

arXiv2024

Tuning exciton emission via ferroelectric polarization at a heterogeneous interface between a monolayer transition metal dichalcogenide and a perovskite oxide membrane

We demonstrate the integration of a thin BaTiO$_3$ (BTO) membrane with monolayer MoSe$_2$ in a dual gate device that enables in-situ manipulation of the BTO ferroelectric polarization with a voltage pulse. While two-dimensional (2D) transition metal dichalcogenides (TMDs) offer remarkable adaptability, their hybrid integration with other families of functional materials beyond the realm of 2D materials has been challenging. Released functional oxide membranes offer a solution for 2D/3D integration via stacking. 2D TMD excitons can serve as a local probe of the ferroelectric polarization in BTO at a heterogeneous interface. Using photoluminescence (PL) of MoSe$_2$ excitons to optically readout the doping level, we find that the relative population of charge carriers in MoSe$_2$ depends sensitively on the ferroelectric polarization. This finding points to a promising avenue for future-generations versatile sensing devices with high sensitivity, fast read-out, and diverse applicability for advanced signal processing.

cond-mat.mtrl-sci↗

arXiv2024

Observation of the fractional quantum spin Hall effect in moiré MoTe2

Quantum spin Hall (QSH) insulators are two-dimensional electronic materials that have a bulk band gap like an ordinary insulator but have topologically protected pairs of edge modes of opposite chiralities. To date, experimental studies have found only integer QSH insulators with counter-propagating up-spins and down-spins at each edge leading to a quantized conductance G0=e^2/h. Here we report transport evidence of a fractional QSH insulator in 2.1-degree-twisted bilayer MoTe2, which supports spin-Sz conservation and flat spin-contrasting Chern bands. At filling factor v = 3 of the moiré valence bands, each edge contributes a conductance 3/2 G0 with zero anomalous Hall conductivity. The state is likely a time-reversal pair of the even-denominator 3/2-fractional Chern insulators. Further, at v = 2, 4 and 6, we observe a single, double and triple QSH insulator with each edge contributing a conductance G0, 2G0 and 3G0, respectively. Our results open up the possibility of realizing time reversal symmetric non-abelian anyons and other unexpected topological phases in highly tunable moiré materials.

cond-mat.mes-hall↗

arXiv2024

Observation of the double quantum spin Hall phase in moiré WSe2

Quantum spin Hall (QSH) insulators are a topologically protected phase of matter in two dimensions that can support non-dissipative spin transport. A hallmark of the phase is a pair of helical edge states surrounding an insulating bulk. A higher (even) number of helical edge state pairs is usually not possible in real materials because spin mixing would gap out the edge states. Multiple pairs of helical edge states have been proposed in materials with spin conservation symmetry and high spin Chern bands, but remained experimentally elusive. Here, we demonstrate a QSH phase with one and two pairs of helical edge states in twisted bilayer WSe2 at moiré hole filling factor ν= 2 and 4, respectively. We observe nearly quantized conductance or resistance plateaus of h/(νe^2 ) at ν = 2 and 4 while the bulk is insulating. The conductance is nearly independent of out-of-plane magnetic field and decreases under an in-plane magnetic field. We also observe nonlocal transport, which is sensitive only to the in-plane magnetic field. The results agree with quantum transport of helical edge states protected by Ising spin conservation symmetry and open a promising platform for low-power spintronics.

cond-mat.mes-hall↗

arXiv2023

A degenerate trion liquid in atomic double layers

Trions are a three-particle bound state of electrons and holes. Experimental realization of a trion liquid in the degenerate quantum limit would open a wide range of phenomena in quantum many-body physics. However, trions have been observed only as optically excited states in doped semiconductors to date. Here we report the emergence of a degenerate trion liquid in a Bose-Fermi mixture of holes and excitons in Coulomb-coupled MoSe2/WSe2 monolayers. By electrically tuning the hole density in WSe2 to be two times the electron density in MoSe2, we generate equilibrium interlayer trions with binding energy about 1 meV at temperatures two orders of magnitude below the Fermi temperature. We further demonstrate a density-tuned phase transition to an electron-hole plasma, spin-singlet correlations for the constituent holes and Zeeman-field-induced dissociation of trions. The results pave the way for exploration of the correlated phases of composite particles in solids.

cond-mat.mes-hall↗

arXiv2023

Emergence of ferromagnetism at the onset of moiré Kondo breakdown

The interaction of a lattice of localized magnetic moments with a sea of conduction electrons in Kondo lattice models induces rich quantum phases of matter, such as Fermi liquids with heavily renormalized electronic quasiparticles, quantum critical non-Fermi liquid metals and unconventional superconductors, among others. The recent demonstration of moiré Kondo lattices has opened the door to investigate the Kondo problem with continuously tunable parameters. Although a heavy Fermi liquid phase has been identified in moiré Kondo lattices, the magnetic phases and Kondo breakdown transitions remain unexplored. Here we report a density-tuned Kondo destruction in AB-stacked MoTe2/WSe2 moiré bilayers by combining magneto transport and optical studies. As the itinerant carrier density decreases, the Kondo temperature decreases. At a critical density, we observe a heavy Fermi liquid to insulator transition, and a nearly concomitant emergence of ferromagnetic order. The observation is consistent with the scenario of a ferromagnetic Anderson insulator and suppression of the Kondo screening effect. Our results pave the path for inducing other exotic quantum phase transitions in moiré Kondo lattices.

cond-mat.str-el↗

arXiv2023

Perfect Coulomb drag in a dipolar excitonic insulator

Excitonic insulators (EIs), arising in semiconductors when the electron-hole binding energy exceeds the band gap, are a solid-state prototype for bosonic phases of matter. Unlike the charged excitations that are frozen and unable to transport current, the neutral electron-hole pairs (excitons) are free to move in EIs. However, it is intrinsically difficult to demonstrate exciton transport in bulk EI candidates. The recently emerged dipolar EIs based on Coulomb-coupled atomic double layers open the possibility to realize exciton transport across the insulator because separate electrical contacts can be made to the electron and hole layers. Here we show that the strong interlayer excitonic correlation at equal electron and hole densities in the MoSe2/WSe2 double layers separated by a 2-nm barrier gives rise to perfect Coulomb drag. A charge current in one layer induces an equal but opposite drag current in the other. The drag current ratio remains above 0.9 up to about 20 K for low exciton densities. As exciton density increases above the Mott density, the excitons dissociate into the electron-hole plasma abruptly, and only weak Fermi liquid frictional drag is observed. Our experiment moves a step closer to realizing exciton circuitry and superfluidity.

cond-mat.mes-hall↗

arXiv2023

Observation of spin polarons in a frustrated moiré Hubbard system

The electron's kinetic energy plays a pivotal role in magnetism. While virtual electron hopping promotes antiferromagnetism in an insulator, the real process usually favors ferromagnetism. But in kinetically frustrated systems, such as hole doped triangular lattice Mott insulators, real hopping has been shown to favor antiferromagnetism. Kinetic frustration has also been predicted to induce a new quasiparticle -- a bound state of the doped hole and a spin flip called a spin polaron -- at intermediate magnetic fields, which could form an unusual metallic state. However, the direct experimental observation of spin polarons has remained elusive. Here we report the observation of spin polarons in triangular lattice MoTe2/WSe2 moiré bilayers by the reflective magnetic circular dichroism measurements. We identify a spin polaron phase at lattice filling factor between 0.8-1 and magnetic field between 2-4 T; it is separated from the fully spin polarized phase by a metamagnetic transition. We determine that the spin polaron is a spin-3/2 particle and its binding energy is commensurate to the kinetic hopping energy. Our results open the door for exploring spin polaron pseudogap metals, spin polaron pairing and other new phenomena in triangular lattice moiré materials.

cond-mat.str-el↗

arXiv2023

Integer and fractional Chern insulators in twisted bilayer MoTe2

Chern insulators, which are the lattice analogs of the quantum Hall states, can potentially manifest high-temperature topological orders at zero magnetic field to enable next-generation topological quantum devices. To date, integer Chern insulators have been experimentally demonstrated in several systems at zero magnetic field, but fractional Chern insulators have been reported only in graphene-based systems under a finite magnetic field. The emergence of semiconductor moiré materials, which support tunable topological flat bands, opens a new opportunity to realize fractional Chern insulators. Here, we report the observation of both integer and fractional Chern insulators at zero magnetic field in small-angle twisted bilayer MoTe2 by combining the local electronic compressibility and magneto-optical measurements. At hole filling factor ν=1 and 2/3, the system is incompressible and spontaneously breaks time reversal symmetry. We determine the Chern number to be 1 and 2/3 for the ν=1 and ν=2/3 gaps, respectively, from their dispersion in filling factor with applied magnetic field using the Streda formula. We further demonstrate electric-field-tuned topological phase transitions involving the Chern insulators. Our findings pave the way for demonstration of quantized fractional Hall conductance and anyonic excitation and braiding in semiconductor moiré materials.

cond-mat.mes-hall↗