arXiv subjects

Michael S. Fuhrer

Publications and source records attributed to Michael S. Fuhrer.

At least 19 recordsLinked to original sources

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↗

arXiv2026

Imaging topological polar structures in marginally twisted 2D semiconductors

Moire superlattices formed in van der Waals heterostructures due to twisting, lattice mismatch and strain present an opportunity for creating novel metamaterials with unique properties not present in the individual layers themselves. Ferroelectricity for example, arises due to broken inversion symmetry in twisted and strained bilayers of 2D semiconductors with stacking domains of alternating out-of-plane polarization. However, understanding the individual contributions of twist and strain to the formation of topological polar nanostructures remains to be established and has proven to be experimentally challenging. Inversion symmetry breaking has been predicted to give rise to an in-plane component of polarization along the domain walls, leading to the formation of a network of topologically non-trivial merons (half-skyrmions) that are Bloch-type for twisted and Neel-type for strained systems. Here we utilise angle-resolved, high-resolution vector piezoresponse force microscopy (PFM) to spatially resolve polarization components and topological polar nanostructures in marginally twisted bilayer WSe2, and provide experimental proof for the existence of topologically non-trivial meron/antimeron structures. We observe both Bloch-type and Neel-type merons, allowing us to differentiate between moire superlattices formed due to twist or heterogeneous strain. This first demonstration of non-trivial real-space topology in a twisted van der Waals heterostructure opens pathways for exploring the connection between twist and topology in engineered nano-devices.

cond-mat.mes-hall↗

arXiv2026

Efficient and Robust p-type Transistor based on Ultra-wide-bandgap Semiconductor

The p-type transistor is an indispensable component of semiconductor technology, enabling complementary operation with n-channel transistors for computation, storage, and communication. Achieving both high robustness and high efficiency is highly desirable but challenging for p-type transistors due to limited semiconductors with reliable hole transport and their high activation energies. Here, we achieved a robust yet efficient p-type transistor by heterogeneously integrating an ultra-wide-bandgap semiconductor and a high-k dielectric layer through van der Waals integration. The p-type transistor employs a two-dimensional hole channel on hydrogenated diamond (bandgap 5.6 eV) combined with a high-k (30.5) SrTiO3 perovskite membrane. At room temperature, the transistor exhibits stable operation with a high on-current (~200 mA/mm), low subthreshold swing (70 mV/dec), high hole mobility (566 cm^2/Vs to 572 cm^2/Vs) and high on-off ratio (~10^9). Furthermore, tuning annealing temperature allows operation in either enhancement or depletion mode. The robust p-type transistor with high efficiency holds great potential for future power electronics, UV optoelectronics, and harsh-environment electronic applications.

cond-mat.mtrl-sci↗

arXiv2026

Disorder-induced symmetry breaking in moiré bands of marginally twisted bilayer MoS$_2$

Twisted transition-metal dichalcogenides host highly tunable moiré potentials, flat bands, and correlated electronic phases, yet the role of disorder in shaping these emergent properties remains largely unresolved. Using scanning tunneling spectroscopy, we investigate the impact of electrostatic disorder on the electronic structure of marginally twisted ($θ\approx 0.95^\circ$) bilayer MoS$_2$. Differences of 15 meV in the onset energies of the valence and conduction bands between MX- and XM-stacked regions are observed and are unexpected based on symmetry considerations. We further observe spatially correlated disorder in the band onset energy that is consistent with a background random charge density of a few $10^{11}\,\mathrm{cm}^{-2}$. Continuum model calculations for twisted MoS$_2$ reveal dramatic changes in the low-energy moiré bands in response to an electric displacement field, in quantitative agreement with experiment. Moreover, the calculated local density of states including disorder broadening reproduces the experimental observations only when structural relaxation is taken into account. These results highlight the critical role of electrostatic disorder in determining the electronic structure of moiré materials at the nanoscale.

cond-mat.mtrl-sci↗

arXiv2026

Contact resistance and interfacial engineering: Advances in high-performance 2D-TMD based devices

The development of advanced electronic devices is contingent upon sustainable material development and pioneering research breakthroughs. Traditional semiconductor-based electronic technology faces constraints in material thickness scaling and energy efficiency. Atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as promising candidates for next-generation nanoelectronics and optoelectronic applications, boasting high electron mobility, mechanical strength, and a customizable band gap. Despite these merits, the Fermi level pinning effect introduces uncontrollable Schottky barriers at metal-2D-TMD contacts, challenging prediction through the Schottky-Mott rule. These barriers fundamentally lead to elevated contact resistance and limited current-delivery capability, impeding the enhancement of 2D-TMD transistor and integrated circuit properties. In this review, we succinctly outline the Fermi pinning effect mechanism and peculiar contact resistance behavior at metal/2D-TMD interfaces. Subsequently, highlights on the recent advances in overcoming contact resistance in 2D-TMDs devices, encompassing interface interaction and hybridization, van der Waals (vdW) contacts, prefabricated metal transfer and charge-transfer doping will be addressed. Finally, the discussion extends to challenges and offers insights into future developmental prospects.

cond-mat.mtrl-sci↗

arXiv2025

Spin-polarised surface fermiology of ohmic WSe$_2$/NbSe$_2$ interfaces

Discovering and engineering spin-polarised surface states in the electronic structures of condensed matter systems is a crucial first step in development of spintronic devices, wherein spin-polarised bands crossing the Fermi level can facilitate information transfer. Here, we show how the spin-orbit split K-point valleys of monolayer WSe$_2$ can be made potentially suitable for this purpose, despite the semiconducting ground state. By interfacing with metallic 2H-NbSe$_2$, these valence band extrema are shifted by $\sim$800~meV to produce a surface-localised Fermi surface populated only by spin-polarised carriers. By increasing the WSe$_2$ thickness, the Fermi pockets can be moved from K to $Γ$, demonstrating tunability of novel semi-metallic phases that exist atop a substrate additionally possessing charge density wave and superconducting transitions. Together, this study provides spectroscopic understanding into $p$-type, Schottky barrier-free interfaces, which are of urgent interest for bypassing the limitations of current-generation vertical field effect transistors, in addition to longer-term spintronics development.

cond-mat.mtrl-sci↗

arXiv2025

Designer three-dimensional electronic bands in asymmetric transition metal dichalcogenide heterostructures

Van der Waals materials enable the construction of atomically sharp interfaces between compounds with distinct crystal and electronic properties. This is dramatically exploited in moiré systems, where a lattice mismatch or twist between monolayers generates an emergent in-plane periodicity, giving rise to electronic properties absent in the constituent materials. In contrast, vertical superlattices, formed by stacking dissimilar materials in the out-of-plane direction on the nanometer scale, have received far less attention despite their potential to realize analogous emergent phenomena in three dimensions. Through angle-resolved photoemission spectroscopy and density functional theory, we investigate six-to-eight-layer transition metal dichalcogenide (TMD) heterostructures constructed from pairs of stacked few-layer materials. Counterintuitively, we find that even these single superlattice units can host fully-delocalised bands, evidencing a robust coherent interlayer coupling across lattice-mismatched interfaces over extended spatial scales. We show how uncompensated semimetallic phases and energetically-mismatched topological surface states are readily and exclusively stabilized within such asymmetrical architectures. These findings establish two-component heterostructures in the intermediate layer-regime as platforms to invoke and control unprecedented combinations and instances of the diverse quantum phases native to many-layer TMDs.

cond-mat.mtrl-sci↗

arXiv2025

Visualization of Strain-Induced Landau Levels in a Graphene - Black Phosphorus Heterostructure

Strain-induced pseudo magnetic fields offer the possibility of realizing zero magnetic field Quantum Hall effect in graphene, possibly up to room temperature, representing a promising avenue for lossless charge transport applications. Strain engineering on graphene has been achieved via random nanobubbles or artificial nanostructures on the substrate, but the highly localized and non-uniform pseudomagnetic fields can make spectroscopic probes of electronic structure difficult. Heterostructure engineering offers an alternative approach: By stacking graphene on top of another van der Waals material with large lattice mismatch at a desired twist angle, it is possible to generate large strain-induced pseudo magnetic fields uniformly over the entire heterostructure. Here, we report using nano-angle resolved photoemission spectroscopy (nano-ARPES) to probe the electronic bandstructure of a graphene/black phosphorus heterostructure (G/BP). By directly measuring the iso-energy contours of graphene and black phosphorus we determine a twist angle of 20-degrees in our heterostructure. High-resolution nano-ARPES of the graphene bands near the Fermi level reveals the emergence of flat bands located within the Dirac cone. The spacing of the flat bands is consistent with Landau level formation in graphene, and corresponds to a pseudo-field of 11.36 T. Our work provides a new way to study quantum Hall phases induced by strain in 2D materials and heterostructures.

cond-mat.mes-hall↗

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↗

arXiv2025

Valley optoelectronics based on meta-waveguide photodetectors

In transition metal dichalcogenides, the valley degree of freedom directly couples valley-polarised excitons - excited by circularly polarised light - to valley-dependent chiral photons, enabling ultrafast light-driven valleytronics. However, achieving fully integrated valley optoelectronics - incorporating on-chip generation, selective routing, and electrical readout of valley-dependent chiral photons - remains an unresolved challenge. We present a valley-driven hybrid nanophotonic-optoelectronic circuit that integrates chirality-selective meta-waveguide photodetectors with transition metal dichalcogenides. At room temperature, our purposely designed meta-waveguide device generates near-unity valley-dependent chiral photons in the second harmonic generation from an encapsulated tungsten disulfide monolayer and selectively couples them to unidirectional waveguide modes, achieving an exceptional polarisation selectivity of 0.97. These valley-dependent waveguide modes were subsequently detected by atomically thin few-layer tungsten diselenide photodetectors, exclusively responsive to the above-bandgap upconverted photons, thereby enabling all-on-chip processing of valley-multiplexed images. Our demonstration bridges a critical gap in lightwave valleytronics, paving the way for compact, scalable valley information processing and fostering the development of light-based valleytronic quantum technologies.

physics.optics↗

arXiv2025

Efficient light upconversion via resonant exciton-exciton annihilation of dark excitons in few-layer transition metal dichalcogenides

In this work, we report a pronounced light upconversion in few-layer transition metal dichalcogenides. Our joint theory-experiment study attributes the upconversion photoluminescence to a resonant exciton-exciton annihilation involving a pair of dark excitons with opposite momenta, followed by the spontaneous emission of upconverted bright excitons, which can have a high upconversion efficiency. Additionally, the upconversion photoluminescence is generic in MoS2, MoSe2, WS2, and WSe2, showing a high tuneability from green to ultraviolet light.

cond-mat.mtrl-sci↗

arXiv2025

Three-dimensional flat band in ultra-thin Kagome metal Mn3Sn film

Flat bands with small energy dispersion can give rise to strongly correlated electronic and topological phases, especially when located at the Fermi level. Whilst flat bands have been experimentally realized in two-dimensional (2D) twisted van der Waals heterostructures, they are highly sensitive to twist angle, necessitating complex fabrication techniques. Geometrically frustrated kagome lattices have emerged as an attractive platform as they natively host flat bands that have been observed experimentally in quasi-2D bulk-crystal kagome metals. An outstanding experimental question is whether flat bands can be realized in atomically thin metals, with opportunities for stronger electron-electron interactions through tuning of the surrounding dielectric environment. Here we use angle-resolved photoelectron spectroscopy, scanning tunnelling microscopy and band structure calculations to show that ultra-thin films of the kagome metal Mn3Sn host a robust dispersionless flat band with a bandwidth of 50 meV. Furthermore, we demonstrate chemical tuning of the flat band to near the Fermi level via manganese defect engineering. The realization of tunable kagome-derived flat bands in an ultra-thin kagome metal, represents a promising platform to study strongly correlated and topological phenomena, with applications in quantum computing, spintronics and low-energy electronics.

cond-mat.str-el↗

arXiv2024

Reversal of charge transfer doping on the negative electronic compressibility surface of MoS2

The strong electron-electron interaction in transition metal dichalcogenides (TMDs) gives rise to phenomena such as strong exciton and trion binding and excitonic condensation, as well as large negative exchange and correlation contributions to the electron energies, resulting in negative electronic compressibility. Here we use angle-resolved photoemission spectroscopy to demonstrate a striking effect of negative electronic compressibility in semiconducting TMD MoS2 on the charge transfer to and from a partial overlayer of monolayer semimetallic WTe2. We track the changes in binding energy of the valence bands of both WTe2 and MoS2 as a function of surface transfer doping with donor (K) and acceptor (F4-TCNQ) species. Donor doping increases the binding energy of the MoS2 valence band, as expected, while counterintuitively reducing the binding energy of the WTe2 valence bands and core levels. The inverse effect is observed for acceptor doping, where a typical reduction in the MoS2 binding energies is accopanied by an unexpected increase in those of WTe2. The observations imply a reversal of the expected charge transfer; donor (acceptor) deposition decreases (increases) the carrier density in the WTe2 adlayer. The charge transfer reversal is a direct consequence of the negative electronic compressibility of the MoS2 surface layer, for which addition (subtraction) of charge leads to attraction (repulsion) of further charge from neighbouring layers. These findings highlight the importance of many-body interactions for the electrons in transition metal dichalcogenides and underscore the potential for exploring strongly correlated quantum states in two-dimensional semiconductors.

cond-mat.str-el↗

arXiv2024

Sub-100-fs formation of dark excitons in monolayer WS2

Two-dimensional semiconducting transition metal dichalcogenides (TMDs) are promising for optoelectronic applications due to their strongly bound excitons. While bright excitons have been thoroughly scrutinized, dark excitons are much less investigated as they are not observable with far-field spectroscopy. However, with their non-zero momenta, dark excitons are significant for applications requiring long-range transport or coupling to external fields. We access such dark excitons in WS2 monolayer using transient photoemission electron microscopy with sub-diffraction limited spatial resolution (75 nm) and exceptionally high temporal resolution (13 fs). Image time series of TMD flakes are recorded at several different fluences. We directly observe the ultrafast formation of dark K-Q excitons in monolayer WS2 occurring within 14-50 fs and follow their subsequent picosecond decay. We distinguish exciton dynamics between the interior and edges of the monolayer TMD and conclude that the long-term evolution of dark excitations is defect-mediated while intervalley scattering is not affected.

cond-mat.mes-hall↗

arXiv2024

Reactivity of ultra-thin Kagome Metal FeSn towards Oxygen and Water

The kagome metal FeSn, consists of alternating layers of kagome-lattice Fe3Sn and honeycomb Sn2, and exhibits great potential for applications in future low energy electronics and spintronics because of an ideal combination of novel topological phases and high-temperature magnetic ordering. Robust synthesis methods for ultra-thin FeSn films, as well as an understanding of their air stability is crucial for its development and long-term operation in future devices. In this work, we realize large area, sub-10 nm epitaxial FeSn thin films, and explore the oxidation process via synchrotron-based photoelectron spectroscopy using in-situ oxygen and water dosing, as well as ex-situ air exposure. Upon exposure to atmosphere the FeSn films are shown to be highly reactive, with a stable ~3 nm thick oxide layer forming at the surface within 10 minutes. Notably the surface Fe remains largely unoxidized when compared to Sn, which undergoes near-complete oxidation. This is further confirmed with controlled in-situ dosing of O2 and H2O where only the Sn2 (stanene) inter-layers within the FeSn lattice oxidize, suggesting the Fe3Sn kagome layers remain almost pristine. These results are in excellent agreement with first principles calculations, which show Fe-O bonds to the Fe3Sn layer are energetically unfavorable, and furthermore, a large formation energy preference of 1.37 eV for Sn-O bonds in the stanene Sn2 layer over Sn-O bonds in the kagome Fe3Sn layer. The demonstration that oxidation only occurs within the stanene layers may provide new avenues in how to engineer, handle and prepare future kagome metal devices.

cond-mat.mtrl-sci↗

arXiv2024

Pick-and-place transfer of arbitrary-metal electrodes for van der Waals device fabrication

Van der Waals electrode integration is a promising strategy to create near-perfect interfaces between metals and two-dimensional materials, with advantages such as eliminating Fermi-level pinning and reducing contact resistance. However, the lack of a simple, generalizable pick-and-place transfer technology has greatly hampered the wide use of this technique. We demonstrate the pick-and-place transfer of pre-fabricated electrodes from reusable polished hydrogenated diamond substrates without the use of any surface treatments or sacrificial layers. The technique enables transfer of large-scale arbitrary metal electrodes, as demonstrated by successful transfer of eight different elemental metals with work functions ranging from 4.22 to 5.65 eV. The mechanical transfer of metal electrodes from diamond onto van der Waals materials creates atomically smooth interfaces with no interstitial impurities or disorder, as observed with cross-sectional high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. As a demonstration of its device application, we use the diamond-transfer technique to create metal contacts to monolayer transition metal dichalcogenide semiconductors with high-work-function Pd, low-work-function Ti, and semi metal Bi to create n- and p-type field-effect transistors with low Schottky barrier heights. We also extend this technology to other applications such as ambipolar transistor and optoelectronics, paving the way for new device architectures and high-performance devices.

physics.app-ph↗

arXiv2024

Symmetry-selective quasiparticle scattering and electric field tunability of the ZrSiS surface electronic structure

3D Dirac semimetals with square-net non-symmorphic symmetry, such as ternary ZrXY (X=Si, Ge; Y=S, Se, Te) compounds, have attracted significant attention owing to the presence of topological nodal lines, loops, or networks in their bulk. Orbital symmetry plays a profound role such materials as the different branches of the nodal dispersion can be distinguished by their distinct orbital symmetry eigenvalues. The presence of different eigenvalues suggests that scattering between states of different orbital symmetry may be strongly suppressed. Indeed, in ZrSiS, there has been no clear experimental evidence of quasiparticle scattering between states of different symmetry eigenvalue has been reported at small wave vector $q$. Here we show, using quasiparticle interference (QPI), that atomic step-edges in the ZrSiS surface facilitate quasiparticle scattering between states of different symmetry eigenvalues. This symmetry eigenvalue mixing quasiparticle scattering is the first to be reported for ZrSiS and contrasts quasiparticle scattering with no mixing of symmetry eigenvalues, where the latter occurs with scatterers preserving the glide mirror symmetry of the crystal lattice, e.g., native point defects in ZrSiS. Finally, we show that the electronic structure of the ZrSiS surface, including its unique floating band surface state (FBSS), can be tuned by a vertical electric field locally applied by the tip of a scanning tunneling microscope (STM), enabling control of a spin-orbit induced avoided crossing near the Fermi level by as much as 300%.

cond-mat.mes-hall↗

arXiv2023

Quasi-free-standing AA-stacked bilayer graphene induced by calcium intercalation of the graphene-silicon carbide interface

We study quasi-freestanding bilayer graphene on silicon carbide intercalated by calcium. The intercalation, and subsequent changes to the system, were investigated by low-energy electron diffraction, angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT). Calcium is found to intercalate only at the graphene-SiC interface, completely displacing the hydrogen terminating SiC. As a consequence, the system becomes highly n-doped. Comparison to DFT calculations shows that the band dispersion, as determined by ARPES, deviates from the band structure expected for Bernal-stacked bilayer graphene. Instead, the electronic structure closely matches AA-stacked bilayer graphene on Ca-terminated SiC, indicating a spontaneous transition from AB- to AA-stacked bilayer graphene following calcium intercalation of the underlying graphene-SiC interface.

cond-mat.mtrl-sci↗