arXiv subjects

Ya-Ning Ren

Publications and source records attributed to Ya-Ning Ren.

At least 19 recordsLinked to original sources

arXiv2026

Controlled chemical vapor deposition for synthesis of emerging Mo(W)Te2 systems

The Group-VI transition metal ditellurides offer a rich platform for correlated and topological phenomena, yet their structural polymorphism and instability complicate the creation of single crystals and heterointerfaces. Here, we introduce a confined-space chemical vapor deposition (CVD) strategy that lowers the growth temperature window and, when combined with tailored precursor configurations and stepwise thermal ramps, enables the deterministic synthesis of high-quality single crystals, alloys, and lateral/vertical heterostructures. High-resolution aberration-corrected STEM provides atomic characterization of lattice-matched Mo(W)Te2 lateral heterostructure, revealing nearly atomically sharp, compositionally well-defined seamless boundaries. This approach avoids the thickness nonuniformity and structural limitations commonly associated with exfoliated samples, enabling reproducible fabrication of clean heterointerfaces and establishing a nearly ideal in-situ experimental system. Furthermore, scanning tunneling microscopy and spectroscopy (STM and STS) enable direct imaging of the seamless boundaries in Mo(W)Te2 lateral heterostructures, while uncovering their distinct real-space distributions of the local density of states. Our results establish a scalable pathway for engineering crystalline Te-based structures with controlled geometry and stacking, providing an essential step toward quantum and topological device platforms based on the transition metal ditellurides family.

cond-mat.mtrl-sci↗

arXiv2025

Microscopic signatures of Chern number sign reversal in twisted bilayer WSe2

The discovery of quantized Chern numbers in twisted transition metal dichalcogenide (TMD) homobilayers,including 3.7{deg} twisted MoTe2 and 1.23{deg} twisted WSe2,has emerged as a defining breakthrough in physics. A striking and unresolved puzzle from these studies is the unexpected opposite sign of the observed Chern numbers between the two systems. Recent theory has proposed a twist-angle-dependent Chern number sign reversal in both twisted MoTe2 and WSe2, offering a potential explanation for the disparate experimental observations6. However, a direct experimental verification of the twist-angle-dependent Chern number sign reversal in a specific twisted TMD homobilayer is still elusive. Here, we report the first experimental demonstration that the Chern numbers of the moire frontier bands undergo sign reversal at a critical twist angle 1.42{deg} in twisted WSe2 bilayers (tWSe2). Using scanning tunnelling microscopy and spectroscopy, we direct measure layer-pseudospin skyrmion textures of tWSe2 and our results reveal that tWSe2 in the vicinity of the 1.42{deg} exhibits a twist-angle-dependent layer-pseudospin polarization, an effect that serves as the fundamental origin of the observed Chern number sigh reversal6-12.

cond-mat.mes-hall↗

arXiv2025

Orbital hybridization in graphene-based artificial atoms

Intraatomic orbital hybridization and interatomic bond formation are the two fundamental processes when real atoms are condensed to form matter. Artificial atoms mimic real atoms by demonstrating discrete energy levels attributable to quantum confinement. As such, they offer a solid-state analogue for simulating intraatomic orbital hybridization and interatomic bond formation. Signatures of interatomic bond formation has been extensively observed in various artificial atoms. However, direct evidence of the intraatomic orbital hybridization in the artificial atoms remains to be experimentally demonstrated. Here we, for the first time, realize the orbital hybridization in artificial atoms by altering the shape of the artificial atoms. The anisotropy of the confining potential gives rise to the hybridization between quasibound states with different orbital quantum numbers within the artificial atom. These hybridized orbits are directly visualized in real space in our experiment and are well reproduced by both numerical calculations and analytical derivations. Our study opens an avenue for designing artificial matter that cannot be accessed on real atoms through experiments. Moreover, the results obtained inspire the progressive control of quantum states in diverse systems.

cond-mat.mes-hall↗

arXiv2024

Layer-dependent evolution of electronic structures and correlations in rhombohedral multilayer graphene

The recent discovery of superconductivity and magnetism in trilayer rhombohedral graphene (RG) establishes an ideal, untwisted platform to study strong correlation electronic phenomena. However, the correlated effects in multilayer RG have received limited attention, and, particularly, the evolution of the correlations with increasing layer number remains an unresolved question. Here, we show the observation of layer-dependent electronic structures and correlations, under surprising liquid nitrogen temperature, in RG multilayers from 3 to 9 layers by using scanning tunneling microscopy and spectroscopy. We explicitly determine layer-enhanced low-energy flat bands and interlayer coupling strengths. The former directly demonstrates the further flattening of low-energy bands in thicker RG, and the latter indicates the presence of varying interlayer interactions in RG multilayers. Moreover, we find significant splittings of the flat bands, ranging from ~50-80 meV, at 77 K when they are partially filled, indicating the emergence of interaction-induced strongly correlated states. Particularly, the strength of the correlated states is notably enhanced in thicker RG and reaches its maximum in the six-layer, validating directly theoretical predictions and establishing abundant new candidates for strongly correlated systems. Our results provide valuable insights into the layer dependence of the electronic properties in RG and demonstrate it as a suitable system for investigating robust and highly accessible correlated phases.

cond-mat.mes-hall↗

arXiv2024

Realization of 2/3-layer transition metal dichalcogenides

Layered van der Waals transition metal dichalcogenides (TMDCs), generally composed of three atomic X-M-X planes in each layer (M = transition metal, X = chalcogen), provide versatile platforms for exploring diverse quantum phenomena. In each MX2 layer, the M-X bonds are predominantly covalent in nature, as a result, the cleavage of TMDC crystals always occurring between the layers. Here we report the controllable realization of fractional-layer WTe2 via an in-situ scanning tunnelling microscopy (STM) tip manipulation technique. By applying STM tip pulses, hundreds of the topmost Te atoms are removed to form a nanoscale monolayer Te pit in the 1T'-WTe2, thus realizing a brand-new 2/3-layer WTe2. Such a unique configuration undergoes a spontaneous atomic reconstruction, yielding an energy-dependent unidirectional charge-density-wave state with the wavevector and geometry quite distinct from that of pristine 1T'-WTe2. Our results expand the conventional understanding of the TMDCs and are expected to stimulate the research on extraordinary structures and properties based on fractional-layer TMDCs.

cond-mat.mes-hall↗

arXiv2024

Visualizing orbital angular momentum induced single wavefront dislocation in graphene

Phase singularities are phase-indeterminate points where wave amplitudes are zero, which manifest as phase vertices or wavefront dislocations. In the realm of optical and electron beams, the phase singularity has been extensively explored, demonstrating a profound connection to orbital angular momentum. Direct local imaging of the impact of orbital angular momentum on phase singularities at the nanoscale, however, remains a challenge and has yet to be achieved. Here, we study the role of orbital angular momentum in phase singularities in graphene, particularly at the atomic level, through scanning tunneling microscopy and spectroscopy. Our experiments demonstrate that the scatterings between different orbital angular momentum states, which are induced by local rotational symmetry-breaking potentials, can generate additional phase singularity, and result in robust single wavefront dislocation in real space. Our results pave the way for exploring the effects of orbital degree of freedom on quantum phases in quasiparticle interference processes.

cond-mat.mes-hall↗

arXiv2023

One-dimensional moire chains with partially-filled flat bands in two-dimensional twisted bilayer WSe2

Two-dimensional (2D) moire systems based on twisted bilayer graphene and transition metal dichalcogenides provide a promising platform to investigate emergent phenomena driven by strong electron-electron interactions in partially-filled flat bands1-11. A natural question arises: is it possible to expand the 2D correlated moire physics to one-dimensional (1D)? This requires selectively doping of 1D moire chain embedded in the 2D moire systems, which is an outstanding challenge in experiment and seems to be not within the grasp of today's technology. Therefore, an experimental demonstration of the 1D moire chain with partially-filled flat bands remains absent. Here we show that we can introduce 1D boundaries, separating two regions with different twist angles, in twisted bilayer WSe2 (tWSe2) by using scanning tunneling microscopy (STM), and demonstrate that the flat bands of moire sites along the 1D boundaries can be selectively filled. The charge and discharge states of correlated moire electrons in the 1D moire chain can be directly imaged and manipulated by combining a back-gate voltage with the STM bias. Our results open the door for realizing new correlated electronic states of the 1D moire chain in 2D systems.

cond-mat.mtrl-sci↗

arXiv2023

Tunable interfacial chemisorption with atomic-level precision in a graphene WSe2 heterostructure

It has long been an ultimate goal to introduce chemical doping at the atomic level to precisely tune properties of materials. Two-dimensional materials have natural advantage because of its highly-exposed surface atoms, however, it is still a grand challenge to achieve this goal experimentally. Here, we demonstrate the ability to introduce chemical doping in graphene with atomic-level precision by controlling chemical adsorption of individual Se atoms, which are extracted from the underneath WSe2, at the interface of graphene-WSe2 heterostructures. Our scanning tunneling microscopy (STM) measurements, combined with first-principles calculations, reveal that individual Se atoms can chemisorbed on three possible positions in graphene, which generate distinct pseudospin-mediated atomic-scale vortices in graphene. We demonstrate that the chemisorbed positions of individual Se atoms can be manipulated by STM tip, which enables us to achieve atomic-scale controlling quantum interference of the pseudospin-mediated vortices in graphene. This result offers the promise of controlling properties of materials through chemical doping with atomic-level precision.

cond-mat.mtrl-sci↗

arXiv2023

Tunable Atomically Wide Electrostatic Barriers Embedded in a Graphene WSe2 Heterostructure

Inducing and controlling electrostatic barriers in two-dimensional (2D) quantum materials has shown extraordinary promise to enable control of charge carriers, and is key for the realization of nanoscale electronic and optoelectronic devices1-10. Because of their atomically thin nature, the 2D materials have a congenital advantage to construct the thinnest possible p-n junctions1,3,7,9,10. To realize the ultimate functional unit for future nanoscale devices, creating atomically wide electrostatic barriers embedded in 2D materials is desired and remains an extremely challenge. Here we report the creation and manipulation of atomically wide electrostatic barriers embedded in graphene WSe2 heterostructures. By using a STM tip, we demonstrate the ability to generate a one-dimensional (1D) atomically wide boundary between 1T-WSe2 domains and continuously tune positions of the boundary because of ferroelasticity of the 1T-WSe2. Our experiment indicates that the 1D boundary introduces atomically wide electrostatic barriers in graphene above it. Then the 1D electrostatic barrier changes a single graphene WSe2 heterostructure quantum dot from a relativistic artificial atom to a relativistic artificial molecule.

cond-mat.mes-hall↗

arXiv2022

Tailoring interfacial nanostructures at graphene/transition metal dichalcogenide heterostructures

Integration of two-dimensional (2D) van der Waals (vdWs) materials with non-2D materials to realize mixed-dimensional heterostructures has potential for creating functional devices beyond the reach of existing materials and has long been a pursuit of the material science community. Here we report the patterning of monolayer/bilayer transition metal dichalcogenide (TMD) nanostructures with nanometer-precision, tunable sizes and sites at interfaces of graphene/TMD heterostructures. Our experiments demonstrate that the TMD nanostructures can be created at selected positions of the interface using scanning tunneling microscope lithography and, more importantly, the sizes of the interfacial TMD nanostructures can be tuned with nanoscale precise by merging adjacent TMD nanostructures or by further tailoring. A structural phase transition from hexagonal phase in bulk TMD to monoclinic phase in the interfacial nanostructures is explicitly observed, which locally introduce well-defined electrostatic potentials on graphene. These results not only enable the creation of high-quality patterned p-n junctions in vdWs heterostructures, but also provide a new route to realize custom-designed mixed-dimensional heterostructures.

cond-mat.mtrl-sci↗

arXiv2022

Experimental evidence for electron-electron interaction and spin-charge separation in graphene quantum dots

Graphene quantum dots (GQDs) can exhibit a range of spectacular phenomena such as the Klein-tunneling-induced quasibound states1-6 and Berry-phase-tuned energy spectra7-15. According to previous studies, all these interesting quantum phenomena seem to be well understood in the free electron picture1-15. However, electronic motion in the GQDs is reduced to quantized orbits by quantum confinement, which implies that the kinetic energy may be comparable to or even smaller than the Coulomb energy of the quasiparticles, possibly resulting in exotic correlated phases in the GQDs. Here we present a scanning tunneling microscopy and spectroscopy study of gate-tunable GQDs in graphene/WSe2 heterostructure devices and report for the first time the observation of electron-electron interaction and correlation-induced spin-charge separation in the GQDs. Gating allows us to precise characterize effects of the electron-electron interaction on the energy spectra of the GQDs. By measuring density of states as a function of energy and position, we explicitly uncover two density waves with different velocities in the GQDs, attributing to spin-charge separation in real space.

cond-mat.mes-hall↗

arXiv2022

Coexistence of Reconstructed and Unreconstructed Structures in Structural Transition Regime of Twisted Bilayer Graphene

In twisted bilayer graphene (TBG), a twist-angle-dependent competition between interlayer stacking energy and intralayer elastic energy results in flat rigid layers at large twist angles and lattice reconstruction at small twist angles. Despite enormous scientific interest and effort in the TBG, however, an experimental study of evolution from the rigid lattice to the reconstructed lattice as a function of twist angle is still missing. Here we present a scanning tunneling microscopy and spectroscopy study to reveal the twist-angle-dependent lattice reconstruction in the TBG. Our experiment demonstrates that there is a transition regime between the rigid regime and the relaxed regime and, unexpectedly, the reconstructed and unreconstructed structures coexist in the transition regime. The coexistence of the two distinct structures in this regime may arise from subtle balance between the interlayer stacking energy and intralayer elastic energy in the TBG with intermediate moiré sizes.

cond-mat.mes-hall↗

arXiv2022

Stabilizing sample-wide Kekulé orders in graphene/transition metal dichalcogenide heterostructures

Kekulé phases are Peierls-like lattice distortions in graphene that are predicted to host novel electronic states beyond graphene (1-8). Although the Kekulé phases are realized in graphene through introducing electron-electron interactions at high magnetic fields (9-11) or adatom superlattices (12-15), it is still an extremely challenge to obtain large-area graphene Kekulé phases in experiment. Here we demonstrate that sample-wide Kekulé distortions in graphene can be stabilized by using transition metal dichalcogenides (TMDs) as substrates and the induced Kekulé orders are quite robust in the whole graphene/TMDs heterostructures with different twist angles. The commensurate structures of the heterostructures provide periodic scattering centers that break the translational symmetry of graphene and couple electrons of the two valleys in graphene, which tips the graphene toward global Kekulé density wave phases. Unexpectedly, three distinct Kekulé bond textures stabilized at various energies are directly imaged in every graphene/TMDs heterostructure. Our results reveal an unexpected sensitivity of electronic properties in graphene to the supporting substrates and provide an attractive route toward designing novel phases in graphene/TMDs heterostructures.

cond-mat.mtrl-sci↗

arXiv2022

Molecular Collapse States in Elliptical Graphene/WSe2 Heterostructure Quantum Dots

In relativistic physics, both atomic collapse in heavy nucleus and Hawking radiation in black hole are predicted to occur through Klein tunneling process that couples particles and antiparticles. Recently, atomic collapse states (ACSs) were explicitly realized in graphene because of its relativistic Dirac excitation with large fine structure constant. However, essential role of the Klein tunneling on the ACSs remains elusive in experiment. Here we systematically study the quasibound states in elliptical graphene quantum dots (GQDs). Bonding and antibonding molecular collapse states formed by two coupled ACSs are observed in the elliptical GQDs. Our experiments, supported by theoretical calculations, indicate that the antibonding state of the ACSs will change into a Klein-tunneling-induced quasibound state, revealing deep connection between the ACSs and the Klein tunneling.

cond-mat.mes-hall↗

arXiv2022

Magnetic field-tunable valley-contrasting pseudomagnetic confinement in graphene

Introducing quantum confinement has uncovered a rich set of interesting quantum phenomena and allows one to directly probe the physics of confined (quasi-)particles. In most experiments, however, electrostatic potential is the only available method to generate the quantum confinement in a continuous system. Here, we demonstrated experimentally that inhomogeneous pseudomagnetic fields in strained graphene can introduce exotic quantum confinement of massless Dirac fermions. The pseudomagnetic fields have opposite directions in the two distinct valleys of graphene. By tuning real magnetic field, the total effective magnetic fields in the two valleys are imbalanced. Then, we realized valley-contrasting spatial confinement, which lifts the valley degeneracy and results in field-tunable valley-polarized confined states in graphene. Our results provide a new avenue to manipulate the valley degree of freedom.

cond-mat.mes-hall↗

arXiv2021

Direct Imaging Strain-field Vortex Networks in Twisted Bilayer Graphene Magnified by Moiré Superlattices

In two-dimensional (2D) twisted bilayers, the van der Waals (vdW) interlayer interaction introduces atomic-scale reconstruction at interface by locally rotating lattice to form strain-field vortex networks in their moiré superlattice. However, direct imaging the tiny local lattice rotation of the strain-field vortex requires extremely high spatial resolution and is an outstanding challenge in experiment. Here, a topmost small-period graphene moiré pattern is introduced to magnify sub-Angstrom distortions of the lattice and tiny local lattice rotation in underlying twisted bilayer graphene (TBG). The local periods and low-energy van Hove singularities of the topmost graphene moiré patterns are spatially modified by the atomic-scale reconstruction of the underlying TBG, thus enabling real-space imaging of the strain-field vortex networks. Our results indicate that structure-reconstructed vdW systems can provide a unique substrate to spatially engineer supported two-dimensional materials both in structures and electronic properties.

cond-mat.mes-hall↗

arXiv2021

Imaging field-tuned quantum Hall broken-symmetry orders and quantum Hall conducting channel in charge-neutral graphene WSe2 heterostructure

The zeroth Landau level (0LL) in graphene has emerged as a flat-band platform in which distinct many-body phases can be explored with unprecedented control by simply tuning the strength and/or direction of magnetic fields1-22. A rich set of quantum Hall ferromagnetic (QHFM) phases with different lattice-scale symmetry-breaking orders are predicted to be realized in high magnetic fields when the 0LL in graphene is half filled1-8,13-16. Here we report a field-tuned continuous quantum phase transition of different valley orderings in QHFM of charge-neutral graphene on insulating tungsten diselenide (WSe2). The phase transition is clearly revealed by anomalous field-dependent energy gap in the half-filled 0LL. Via atomic resolution imaging of electronic wavefunctions during the phase transition, we observe microscopic signatures of field-tuned continuous-varied valley polarization and valley inversion, which are unexpected and beyond current theory predictions. Moreover, the topological quantum Hall conducting channel of the graphene is directly imaged when the substrate (WSe2) introduces band bending of the 0LL.

cond-mat.mes-hall↗

arXiv2021

Continuously Tunable Berry Phase and Valley-Polarized Energy Spectra in Bilayer Graphene Quantum Dots

Berry phase plays an important role in determining many physical properties of quantum systems. However, a Berry phase altering energy spectrum of a quantum system is comparatively rare. Here, we report an unusual tunable valley polarized energy spectra induced by continuously tunable Berry phase in Bernal-stacked bilayer graphene quantum dots. In our experiment, the Berry phase of electron orbital states is continuously tuned from about pi to 2pi by perpendicular magnetic fields. When the Berry phase equals pi or 2pi, the electron states in the two inequivalent valleys are energetically degenerate. By altering the Berry phase to noninteger multiples of pi, large and continuously tunable valley polarized energy spectra are detected in our experiment. The observed Berry phase-induced valley splitting, on the order of 10 meV at a magnetic field of 1 T, is about 100 times larger than Zeeman splitting for spin, shedding light on graphene-based valleytronics.

cond-mat.mes-hall↗