arXiv subjects

Xianghua Kong

Publications and source records attributed to Xianghua Kong.

15 recordsLinked to original sources

arXiv2026

Josephson energy of superconducting junctions: amorphous versus crystalline tunnel barriers

The Josephson energy $E_J$ is a key parameter governing the properties of transmon superconducting qubits. In Al/AlO$_x$/Al junctions, $E_J$ is set by electron tunneling through an ultrathin oxide barrier and therefore depends exponentially on the atomic structure of the barrier. We compute $E_J$ by first-principles device modeling based on the NEGF-DFT quantum-transport method, comparing a junction with a crystalline Al$_2$O$_3$ barrier against ten junctions with melt--quenched amorphous Al$_2$O$_3$ barriers of the same thickness. From the Fermi-level transmission and the Ambegaokar--Baratoff relation, we obtain a mean $E_J/h$ of $2.78$ GHz for the amorphous ensemble, with a standard deviation of $4.67$ GHz, compared with $0.73$ GHz for the crystalline reference; individual amorphous values span nearly two orders of magnitude. Scattering-state analysis shows that transport is quantum tunneling and that the variability originates from stoichiometric inhomogeneity of the amorphous oxide: Al-rich, low-barrier regions can connect into percolation-like tunneling pathways that strongly enhance the conductance. A realistic $200\times200$ nm$^2$ junction self-averages over more than $2\times10^4$ such microscopic regions. These results establish a quantitative atomistic route from oxide microstructure to the superconducting-circuit energy scale $E_J$.

quant-ph↗

arXiv2026

Moiré Phonon Condensation in Magic-Angle Twisted Bilayer Graphene

Twisted bilayer graphene reconstructs from weak breathing corrugation to large common bending near the magic angle, but the origin of this collective crossover has remained unclear. Here we show that the crossover is a soft-mode condensation of layer-symmetric $A_1$ moiré flexural phonons: these modes soften on the breathing branch, lose stiffness near the magic angle, and freeze into the bending morphology. We call this mechanism Moiré Phonon Condensation (MPC). At $θ=1.08^\circ$, it is extremely surprising that displacements of all 11164 atoms in the moiré supercell, with a maximum atomic position shift of 2.30 Angstrom, is captured by only two $A_1$ phonon modes at more than $99.5\%$ spectral weight. A first-harmonic continuum theory identifies a dimensionless control parameter of the phenomenon, showing that as the twist approaches the magic angle, the growing moiré length scale amplifies a smooth stress-bending competition until the flexural stiffness changes sign. Mode-resolved tight-binding calculations further show that the condensed phonon coordinates are electronically active. This work identifies MPC as a twist-controlled structural order parameter for moiré reconstruction.

cond-mat.mes-hall↗

arXiv2026

Primitive-cell-resolved Crystallography for Moiré Bilayers from Imaging

Accurate geometric decoding of moiré bilayers from imaging is essential for engineering quantum systems. Existing schemes, limited by identity or aligned assumptions requiring diagonal beating-to-moiré transformations, do not apply to general non-aligned geometries and become underdetermined when buried layers are unresolved. We establish a primitive-cell-resolved moiré crystallography framework that treats the beating-to-moiré relation in full generality and introduces a complete descriptor set $\{θ_r,\boldsymbol{\varepsilon},(T_{Mt},T_{Mb}),N_B\}$, where the integer moiré--layer matrices $(T_{Mt},T_{Mb})$ and the beating number $N_B$ determine the commensurate unit cell. A hybrid analytical--numerical workflow reconstructs buried-layer lattices, solves Diophantine constraints to obtain $(T_{Mt},T_{Mb})$ and $N_B$, and extracts $(θ_r,\varepsilon_b,θ_u,\varepsilon_u)$ with Poisson effects and tensile/compressive branches treated on equal footing. Reanalyzing twisted bilayer graphene, we identify a $N_B=3$ primitive cell rather than a $N_B=9$ aligned supercell, reducing the atomistic basis threefold and correcting the moiré Brillouin-zone construction. The framework provides a crystallographically consistent route from imaging to primitive-cell-resolved atomistic and many-body models.

cond-mat.mes-hall↗

arXiv2026

Cold source field-effect transistor with type-III band-aligned HfS$_2$/WTe$_2$ heterostructure

The cold source field-effect transistor (CSFET) is promising for reducing power dissipation in integrated circuits by engineering the density of states at the injecting source. Existing CSFET designs utilizing Dirac-source metals or p-Metal-n stacks are challenged by Schottky barriers at the metal-semiconductor interface. In this work, a 2D WTe$_2$/HfS$_2$ heterojunction with type-III band alignment is proposed to be an excellent design of cold source and CSFET. The architecture has a high band-to-band transport mechanism by removing the detrimental Schottky barrier issues. Importantly, the proposed CSFET has the same channel barrier modulation principle as conventional MOSFET to enable a high on-state current. Using first-principles-based quantum transport modeling, we predict a very high $I_{\rm on}$/$I_{\rm off}$ ratio at $\sim$ 10$^{10}$, a low subthreshold swing below the thermal limit for a wide range of gate voltages, reaching as small as 41.3 mV/dec, at low source-drain bias $V_{DS}=0.3$ $\rm V$. These findings establish a design principles for next-generation low-power nanoelectronic switches leveraging 2D van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2026

Electromechanical Switching and Momentum-Selective Transport in Geometry-Defined Blue Phosphorus Homojunctions

Developing intrinsic homojunctions without chemical heterogeneity remains a key challenge in future two - dimensional devices. Here, we report a geometry - defined metal--semiconductor--metal homojunction in bilayer blue phosphorus (BlueP) created by a localized bubble corrugation, without chemical doping or foreign - material interfaces. First - principles calculations show that enlarging the interlayer separation in the metallic A$_1$B$_1$ - stacked BlueP bilayer opens a band gap, enabling a semiconducting barrier embedded between metallic segments. First - principles quantum - transport simulations reveal a crossover from ballistic to tunneling transport upon bubble formation. In the tunneling regime, transmission decreases exponentially with bubble width while remaining weakly sensitive to bubble height and bulging direction. The junction acts as an orientation - dependent $k$ - space filter, producing transport anisotropy and momentum selectivity. Orbital - resolved scattering analysis shows that intralayer - bonding channels persist under deformation whereas interlayer - hybridized channels are quenched, and that σ- type bonding yields higher conductance than π- type bonding. These insights motivate two electromechanical device concepts: a mechanically switchable memory element with ON/OFF ratios up to 30 and a nanoscale sliding rheostat with reproducible exponential resistance tuning for Å- scale displacement sensing.

cond-mat.mes-hall↗

arXiv2025

Kinetically accessible 1D magnetic chains of transition-metal chalcogenides and halides on van der Waals surfaces

One-dimensional (1D) chains offer unique opportunities for nanoelectronics and spintronics, yet their experimental realization remains challenging because 1D motifs are often thermodynamically disfavored relative to higher-dimensional phases. Here we present a high-throughput first-principles exploration of 1D single-atomic transition-metal chalcogenide and halide chains, screening 6,832 candidates constructed from binary combinations of 28 metals and 8 non-metals. To assess kinetic accessibility, we compare the formation energetics of 1D chains with competing two-dimensional polymorphs at the nucleation stage across relevant chemical-potential windows, using nucleation-stage thermodynamic selectivity as a proxy. This workflow identifies 183 kinetically accessible 1D chains. Interpretable machine-learning analysis reveals two simple stability descriptors as key drivers of 1D stabilization. The accessible chains exhibit diverse magnetic configurations with different magnetic characters. We further uncover their pronounced magnetoelastic couplings, exemplified by CrTe with giant magnetostriction reaching 5.93%. Finally, we show that selected metallic ferromagnetic chains retain robust edge magnetism on superconducting substrates, laying the groundwork for proximity-induced topological superconductivity and Majorana zero modes.

cond-mat.mtrl-sci↗

arXiv2025

Semiregular tessellation of electronic lattices in untwisted bilayer graphene under anisotropic strain gradients

Two-dimensional (2D) moiré superlattices have emerged as a versatile platform for uncovering exotic quantum phases, many of which arise in bilayer systems exhibiting Archimedean tessellation patterns such as triangular, hexagonal, and kagome lattices. Here, we propose a strategy to engineer semiregular tessellation patterns in untwisted bilayer graphene by applying anisotropic epitaxial tensile strain (AETS) along crystallographic directions. Through force-field and first-principles calculations, we demonstrate that AETS can induce a rich variety of semiregular tessellation geometries, including truncated hextille, prismatic pentagon, and brick-phase arrangements. The characteristic electronic bands (Dirac and flat bands) of the lattice models associated with these semiregular tessellations are observed near the Fermi level, arising from interlayer interactions generated by the redistribution of specific stacking registries (AB, BA, and SP). Furthermore, the electronic kagome, distorted Lieb, brick-like, and one-dimensional stripe lattices captured in real-space confirm the tunable nature of the semiregular tessellation lattices enabled by AETS. Our study identifies AETS as a promising new degree of freedom in moiré engineering, offering a reproducible and scalable platform for exploring exotic electronic lattices in moiré systems.

cond-mat.mes-hall↗

arXiv2025

Kagome electronic states in gradient-strained untwisted graphene bilayers

Moiré superlattices in twisted homo-bilayers have revealed exotic electronic states, including unconventional superconductivity and correlated insulating phases. However, their fabrication process often introduces moiré disorders, hindering reproducibility and experimental control. Here, we propose an alternative approach using gradient strain to construct moiré superlattices in untwisted bilayer graphene (gs-BLG). Through force-field and first-principles calculations, we show that gs-BLG exhibits kagome-like interlayer-spacing distributions and strain-tunable kagome electronic bands. The competition between interlayer coupling and in-plane strain relaxation leads to distinct structural deformations, giving rise to three forms of diatomic kagome lattices: subtle, pronounced, and distorted. Kagome electronic bands are identified near the Fermi level in their band structures. Modulating strain gradients enables tailoring bandwidths and signs of hopping parameters of these kagome bands, providing a versatile platform for studying exotic electronic phases. Our findings establish gradient strain as an alternative to twist engineering, opening an avenue for exploring emergent electronic phases in graphene-based systems.

cond-mat.mes-hall↗

arXiv2023

Discovery and construction of surface kagome electronic states induced by p-d electronic hybridization

Kagome-lattice materials possess attractive properties for quantum computing applications, but their synthesis remains challenging. Herein, we show surface kagome electronic states (SKESs) on a Sn-terminated triangular Co3Sn2S2 surface, which are imprinted by vertical p-d electronic hybridization between the surface Sn (subsurface S) atoms and the buried Co kagome lattice network in the Co3Sn layer under the surface. Owing to the subsequent lateral hybridization of the Sn and S atoms in a corner-sharing manner, the kagome symmetry and topological electronic properties of the Co3Sn layer is proximate to the Sn surface. The SKESs and both hybridizations were verified via qPlus non-contact atomic force microscopy (nc-AFM) and density functional theory calculations. The construction of SKESs with tunable properties can be achieved by the atomic substitution of surface Sn (subsurface S) with other group III-V elements (Se or Te), which was demonstrated theoretically. This work exhibits the powerful capacity of nc-AFM in characterizing localized topological states and reveals the strategy for synthesis of large-area transition-metal-based kagome lattice materials using conventional surface deposition techniques.

cond-mat.mtrl-sci↗

arXiv2023

Layer sliding and twisting induced electronic transitions in correlated magnetic 1T-NbSe2 bilayers

Correlated two-dimensional (2D) layers, like 1T-phases of TaS2, TaSe2 and NbSe2, exhibit rich tunability through varying interlayer couplings, which promotes the understanding of electron-correlation in the 2D limit. However, the coupling mechanism is, so far, poorly understood and was tentatively ascribed to interactions among the d_(z^2 ) orbitals of Ta or Nb atoms. Here, we theoretically show that the interlayer hybridization and localization strength of interfacial Se pz orbitals, rather than Nb d_(z^2 ) orbitals, govern the variation of electron-correlated properties upon interlayer sliding or twisting in correlated magnetic 1T-NbSe2 bilayers. Each of the both layers is in a star-of-David (SOD) charge-density-wave phase. Geometric and electronic structures, and magnetic properties of 28 different stacking configurations were examined and analyzed using density-functional-theory calculations. We found that the SOD contains a localized region (Reg-L), in which interlayer Se pz hybridization plays a paramount role in varying the energy levels of the two Hubbard bands. These variations lead to three electronic transitions among four insulating states, which demonstrated the effectiveness of interlayer interactions to modulate correlated magnetic properties in a prototypical correlated magnetic insulator.

cond-mat.mtrl-sci↗

arXiv2018

Charge tunable structural phase transitions in few-layer tellurium: a step toward building mono-elemental devices

Recently, a covalent-like quasi-bonding was unveiled for the inter-chain interaction in a promising semiconductor, few-layer Tellurium. Such quasi-bond offers comparable bond lengths and strengths with those of a typical Te-Te covalent bond, which may lead to much easier transformations between the quasi-bonds and the covalent bonds. Here, we show a few structural phase-transitions among four Te allotropes in few-layer Te under charge doping. In particular, the semiconducting α-phase can transform into a smaller-bandgap β-phase, even-smaller-bandgap γ-phase and then metallic δ-phase in a Te bilayer. In a tri-layer, a metallic chiral α+δ phase is more stable under initial electron doping, leading to the appearance of chirality. Variations of electronic structures aside, these transitions are accompanied by the emergence or elimination of inversion centers (α-β, α-γ, α-α+δ), structural anisotropy (α-γ, γ-δ) and chirality (α-α+δ), which could result in substantial changes in optical and other properties. In light of this, this work opens the possibility toward building mono-elemental electronic and optoelectronic heterostructures or devices. It also offers a platform for studying relations between charge doping and electric/optical properties.

cond-mat.mtrl-sci↗

arXiv2018

Layer- and doping-tunable long range ferromagnetic order-ing in two-dimensional CrS2

Interlayer coupling is of vital importance for manipulating physical properties, e.g. electronic bandgap, in two-dimensional materials. However, tuning magnetic proper-ties in these materials is yet to be addressed. Here, we found a striped antiferromag-netic (sAFM) to ferromagnetic (FM) transition undergoing from monolayer to bilayer and thicker CrS2. This transition is attributed to charge sharing of interlayer S atoms and its resulting charge transfer from Cr d eg to t2g orbitals. The transferred charge reduces a portion of Cr4+ to Cr3+, which enhances the double-exchange mechanism favoring FM rather than sAFM ordering. Doping of electrons shares the same effect with stacking induced charge sharing. Therefore, charge doping could either manip-ulate the magnetic ordering between sAFM and FM or tune CrS2 between p- and n-doped magnetic semiconductors. We also proposed several prototype devices ena-bling to use external electric field for manipulating magnetic orderings of CrS2 layers. These results manifest the role of interlayer coupling in modifying magnetic proper-ties of layered materials

cond-mat.mtrl-sci↗

arXiv2015

Interlayer electronic hybridization leads to exceptional thickness-dependent vibrational properties in few-layer black phosphorus

Stacking two-dimensional (2D) materials into multi-layers or heterostructures, known as van der Waals (vdW) epitaxy, is an essential degree of freedom for tuning their properties on demand. Few-layer black phosphorus (FLBP), a material with high potential for nano- and optoelectronics applications, appears to have interlayer couplings much stronger than graphene and other 2D systems. Indeed, these couplings call into question whether the stacking of FLBP can be governed only by vdW interactions, which is of crucial importance for epitaxy and property refinement. Here, we perform a theoretical investigation of the vibrational properties of FLBP, which reflect directly its interlayer coupling, by discussing six Raman-observable phonons, including three optical, one breathing, and two shear modes. With increasing sample thickness, we find anomalous redshifts of the frequencies for each optical mode but a blueshift for the armchair shear mode. Our calculations also show splitting of the phonon branches, due to anomalous surface phenomena, and strong phonon-phonon coupling. By computing uniaxial stress effects, inter-atomic force constants, and electron densities, we provide a compelling demonstration that these properties are the consequence of strong and highly directional interlayer interactions arising from electronic hybridization of the lone electron-pairs of FLBP, rather than from vdW interactions. This exceptional interlayer coupling mechanism controls the stacking stability of BP layers and thus opens a new avenue beyond vdW epitaxy for understanding the design of 2D heterostructures.

cond-mat.mtrl-sci↗

arXiv2014

High-mobility transport anisotropy and linear dichroism in few-layer black phosphorus

Two-dimensional crystals are emerging materials for nanoelectronics. Development of the field requires candidate systems with both a high carrier mobility and, in contrast to graphene, a sufficiently large electronic bandgap. Here we present a detailed theoretical investigation of the atomic and electronic structure of few-layer black phosphorus (BP) in order to predict its electrical and optical properties. This system has a direct bandgap, tunable from 1.51 eV for a monolayer to 0.59 eV for a 5-layer sample. We predict that the mobilities are hole-dominated, rather high and highly anisotropic. The monolayer is exceptional in having an extremely high hole-mobility (of order 10000 cm$^{2}$ V$^{-1}$ s$^{-1}$) and anomalous elastic properties which reverse the anisotropy. Light absorption spectra indicate linear dichroism between perpendicular in-plane directions, which allows optical determination of the crystalline orientation and optical activation of the anisotropic transport properties. These results make few-layer BP a promising candidate for future electronics.

cond-mat.mtrl-sci↗

arXiv2013

Non-destructive and Rapid Evaluation of CVD Graphene by Dark Field Optical Microscopy

Non-destructive and rapid evaluation of graphene directly on the growth substrate (Cu foils) by dark field (DF) optical microscopy is demonstrated. Without any additional treatment, graphene on Cu foils with various coverages can be quickly identified by DF imaging immediately after chemical vapor deposition growth with contrast comparable to scanning electron microscopy. The improved contrast of DF imaging compared to bright field optical imaging was found to be due to Rayleigh scattering of light by the copper steps beneath graphene. Indeed, graphene adlayers are readily distinguished, due to the different height of copper steps beneath graphene regions of different thickness.

cond-mat.mtrl-sci↗