arXiv subjects

Yande Que

Publications and source records attributed to Yande Que.

13 recordsLinked to original sources

arXiv2026

Excitonic order in quantum materials: fingerprints, platforms and opportunities

The exciton insulator (EI) is a unique many-body ground state of condensed, spontaneously formed excitons (electron-hole pairs) in equilibrium, distinct from conventional band or Mott insulators. Originally proposed over half a century ago, the concept has recently gained renewed experimental traction thanks to advances in spectroscopic resolution, ultrafast probes, and materials synthesis. In this Review, we outline the essential theoretical ingredients underpinning excitonic order and discuss how dimensionality, disorder and screening affect stability. We then examine the diverse experimental fingerprints of the excitonic state, with central focus on strategies to disentangle excitonic order from competing phases such as charge density waves, Mott insulating states, and hybridization-driven insulators, particularly in systems where non-trivial band topology plays a role. We survey the rapidly expanding family of candidate materials, from layered chalcogenides and correlated rare-earth compounds to artificial excitonic platforms and optically driven non-equilibrium condensates. Finally, we discuss the key challenges and emerging opportunities in the field, identifying the theoretical and experimental frontiers that promise to shape the next decade of research.

cond-mat.str-el↗

arXiv2026

Orbitally resolved single-photon emission from an individual atomic vacancy center in a semiconductor

Atomically confined spins are emerging as active components in quantum optoelectronic devices such as quantum bits and sensors. However, interrogating single spins at atomic length-scales remains a sizeable challenge, limited by diffraction in conventional optics. Here we show that the highly-local excitation provided by injecting energetic charge carriers from the atomically sharp probe of a scanning tunneling microscope can trigger single-photon emission from individual atomic vacancy centers in a layered semiconductor. With an effective spatial resolution of <1 nm, we show that the captured light closely mirrors the orbital symmetry of the bound-state wavefunction of the vacancy center while photon correlation measurements confirm single-photon emission, as reflected in clear photon anti-bunching signatures. Our results constitute an important step toward the realization of an electrically addressable single-atom quantum light source and solid-state spinphoton interface, addressed at the atomic-scale.

cond-mat.mes-hall↗

arXiv2025

Controlling an altermagnetic spin density wave in the kagome magnet CsCr3Sb5

The interplay of charge and spin orders lies at the heart of correlated electron physics and plays a critical role in the emergence of unconventional quantum phases. Kagome magnets provide a particularly promising platform to investigate these phenomena, owing to their geometrically frustrated lattice structure. However, resolving spin and charge orders microscopically and establishing ways to control them remain fundamental challenges. Here, we demonstrate magnetic field control of an altermagnetic spin density wave order intertwined with charge density wave order in kagome magnet CsCr3Sb5. Scanning tunneling microscopy down to deep cryogenic temperature of 50 mK reveals two previously unreported charge density wave orders in the Sb surface. Density functional theory confirms that one of them is coupled to a spin density wave with an altermagnetic ground state. The charge density waves can be tuned in both amplitude and phase by an external magnetic field, reflected in domain switching and stripe sliding of the charge density wave. Our findings deepen the understanding of symmetry-breaking in kagome systems, providing a tunable platform to explore the interplay of electronic correlation with emergent quantum magnetism.

cond-mat.str-el↗

arXiv2025

Observation of charge density wave excitonic order parameter in topological insulator monolayer WTe2

Strong electron-hole interactions in a semimetal or narrow-gap semiconductor may drive a ground state of condensed excitons. Monolayer WTe2 has been proposed as a host material for such an exciton condensate, but the order parameter - the key signature of a macroscopic quantum-coherent condensate - has not been observed. Here we use Fourier-transform scanning tunnelling spectroscopy (FT-STS) to study quasi-particle interference (QPI) and periodic modulations of the local density of states (LDOS) in monolayer WTe2. In WTe2 on graphene, in which the carrier density can be varied via back-gating, FT-STS shows QPI features in the 2D bulk bands, confirming the interacting nature of the bandgap in neutral WTe2 and the semi-metallic nature of highly n- and p-doped WTe2. We observe additional non-dispersive spatial modulations in the LDOS imprinted on the topological edge mode of neutral WTe2 on metallic substrates (graphene and graphite), which we interpret as the interaction of the topological edge mode with the expected charge density wave order parameter of the excitonic condensate in WTe2 at low interaction strength due to screening by the metallic substrates.

cond-mat.str-el↗

arXiv2023

A gate-tunable quantum phase transition in a topological excitonic insulator

Coulomb interactions among electrons and holes in two-dimensional (2D) semimetals with overlapping valence and conduction bands can give rise to a correlated insulating ground state via exciton formation and condensation. One candidate material in which such excitonic state uniquely combines with non-trivial band topology are atomic monolayers of tungsten ditelluride (WTe2), in which a 2D topological excitonic insulator (2D TEI) forms. However, the detailed mechanism of the 2D bulk gap formation in WTe2, in particular with regard to the role of Coulomb interactions, has remained a subject of ongoing debate. Here, we show that WTe2 is susceptible to a gate-tunable quantum phase transition, evident from an abrupt collapse of its 2D bulk energy gap upon ambipolar field-effect doping. Such gate tunability of a 2D TEI, into either n- and p-type semimetals, promises novel handles of control over non-trivial 2D superconductivity with excitonic pairing.

cond-mat.str-el↗

arXiv2023

Performance benchmarking of an ultra-low vibration laboratory to host a commercial millikelvin scanning tunnelling microscope

Ultra-low temperature scanning tunnelling microscopy and spectroscopy (STM/STS) achieved by dilution refrigeration can provide unrivalled insight into the local electronic structure of quantum materials and atomic-scale quantum systems. Effective isolation from mechanical vibration and acoustic noise is critical in order to achieve ultimate spatial and energy resolution. Here, we report on the design and performance of an ultra-low vibration (ULV) laboratory hosting a customized but otherwise commercially available 40mK STM. The design of the vibration isolation consists of a T-shaped concrete mass block (55t), suspended by actively controlled pneumatic springs, and placed on a foundation separated from the surrounding building in a "room-within-a-room" design. Vibration levels achieved are meeting the VC-M vibration standard at >3 Hz, reached only in a limited number of laboratories worldwide. Measurement of the STM's junction noise confirms effective vibration isolation on par with custom built STMs in ULV laboratories. In this tailored low-vibration environment, the STM achieves an energy resolution of 43ueV (144 mK), promising for the investigation and control of quantum matter at atomic length scales.

cond-mat.supr-con↗

arXiv2021

Reversible Structural Transition of Two-Dimensional Copper Selenide on Cu(111)

Structural engineering opens a door to manipulating the structures and thus tuning the properties of two-dimensional materials. Here, we report a reversible structural transition in honeycomb CuSe monolayer on Cu(111) through scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Direct selenization of Cu(111) gives rise to the formation of honeycomb CuSe monolayers with 1D moir\'e structures (stripe-CuSe), due to the asymmetric lattice distortions in CuSe induced by the lattice mismatch. Additional deposition of Se combined with post annealing results in the formation of honeycomb CuSe with quasi-ordered arrays of triangular holes (hole-CuSe), namely, the structural transition from stripe-CuSe to hole-CuSe. Further, annealing the hole-CuSe at higher temperature leads to the reverse structural transition, namely from hole-CuSe to stripe-CuSe. AES measurement unravels the Se content change in the reversible structural transition. Therefore, both the Se coverage and annealing temperature play significant roles in the reversible structural transition in CuSe on Cu(111). Our work provides insights in understanding of the structural transitions in 2D materials.

cond-mat.mtrl-sci↗

arXiv2021

On-Surface Synthesis of Graphene Nanoribbons on Two-Dimensional Rare Earth-Gold Intermetallic Compounds

Here, we demonstrate two reliable routes for the fabrication of armchair-edge graphene nanoribbons (GNRs) on TbAu2/Au(111), belonging to a class of two-dimensional ferromagnetic rare earth-gold intermetallic compounds. On-surface synthesis directly on TbAu2 leads to the formation of GNRs, which are short and interconnected with each other. In contrast, the intercalation approach - on-surface synthesis of GNRs directly on Au(111) followed by rare earth intercalation - yields GNRs on TbAu2/Au(111), where both the ribbons and TbAu2 are of high quality comparable with those directly grown on clean Au(111). Besides, the as-grown ribbons retain the same band gap while changing from p-doping to weak n-doping mainly due to a change in the work function of the substrate after the rare earth intercalation. The intercalation approach might also be employed to fabricate other types of GNRs on various rare earth intermetallic compounds, providing platforms to tailor the electronic and magnetic properties of GNRs on magnetic substrates.

cond-mat.mtrl-sci↗

arXiv2020

Two-Dimensional Rare Earth -- Gold Intermetallic Compounds on Au(111) by Surface Alloying

Surface alloying is a straightforward route to control and modify the structure and electronic properties of surfaces. Here, We present a systematical study on the structural and electronic properties of three novel rare earth-based intermetallic compounds, namely ReAu2 (Re = Tb, Ho, and Er), on Au(111) via directly depositing rare-earth metals onto the hot Au(111) surface. Scanning tunneling microscopy/spectroscopy measurements reveal the very similar atomic structures and electronic properties, e.g. electronic states, and surface work functions, for all these intermetallic compound systems due to the physical and chemical similarities between these rare earth elements. Further, these electronic properties are periodically modulated by the moiré structures caused by the lattice mismatches between ReAu2 and Au(111). These periodically modulated surfaces could serve as templates for the self-assembly of nanostructures. Besides, these two-dimensional rare earth-based intermetallic compounds provide platforms to investigate the rare earth related catalysis, magnetisms, etc., in the lower dimensions.

cond-mat.mtrl-sci↗

arXiv2020

Atomically-Precise, Custom-Design Origami Graphene Nanostructures

The construction of atomically-precise carbon nanostructures holds promise for developing novel materials for scientific study and nanotechnology applications. Here we show that graphene origami is an efficient way to convert graphene into atomically-precise, complex, and novel nanostructures. By scanning-tunneling-microscope manipulation at low temperature, we repeatedly fold and unfold graphene nanoislands (GNIs) along arbitrarily chosen direction. A bilayer graphene stack featuring a tunable twist angle and a tubular edge connection between the layers are formed. Folding single-crystal GNIs creates tubular edges with specified chirality and one-dimensional electronic features similar to those of carbon nanotubes, while folding bi-crystal GNIs creates well-defined intramolecular junctions. Both origami structural models and electronic band structures were computed to complement analysis of the experimental results. The present atomically-precise graphene origami provides a platform for constructing novel carbon nanostructures with engineered quantum properties and ultimately quantum machines.

cond-mat.mtrl-sci↗

arXiv2017

Stacking-dependent electronic property of trilayer graphene epitaxially grown on Ru(0001)

The growth, atomic structure, and electronic property of trilayer graphene (TLG) on Ru(0001) were studied by low temperature scanning tunneling microscopy and spectroscopy in combined with tight-binding approximation (TBA) calculations. TLG on Ru(0001) shows a flat surface with a hexagonal lattice due to the screening effect of the bottom two layers and the AB-stacking in the top two layers. The coexistence of AA- and AB-stacking in the bottom two layers leads to three different stacking orders of TLG, namely, ABA-, ABC-, and ABB-stacking. STS measurements combined with TBA calculations reveal that the density of states of TLG with ABC- and ABB-stacking is characterized by one and two sharp peaks near to the Fermi level, respectively, in contrast to the V-shaped feature of TLG with ABA-stacking. Our work demonstrates that TLG on Ru(0001) might be an ideal platform for exploring stacking-dependent electronic properties of graphene.

cond-mat.mtrl-sci↗

arXiv2015

Graphene-Silicon Layered Structures on Single-crystalline Ir(111) Thin Films

Single-crystalline transition metal films are ideal playing fields for the epitaxial growth of graphene and graphene-base materials. Graphene-silicon layered structures were successfully constructed on Ir(111) thin film on Si substrate with an yttria-stabilized zirconia buffer layer via intercalation approach. Such hetero-layered structures are compatible with current Si-based microelectronic technique, showing high promise for applications in future micro- and nano-electronic devices.

cond-mat.mtrl-sci↗

arXiv2014

Epitaxial Growth of Large-area Bilayer Graphene on Ru(0001)

Large-area bilayer graphene (BG) is grown epitaxially on Ru(0001) surface and characterized by low temperature scanning tunneling microscopy. The lattice of the bottom layer of BG is stretched by 1.2%, while strain is absent from the top layer. The lattice mismatch between the two layers leads to the formation of a moire pattern with a periodicity of ~21.5 nm and a mixture of AA- and AB-stacking. The root3 x root3 superstructure around atomic defects is attributed to the inter-valley scattering of the delocalized pi-electrons, demonstrating that the as-grown BG behaves like intrinsic free-standing graphene.

cond-mat.mtrl-sci↗