arXiv subjects

Chin Shen Ong

Publications and source records attributed to Chin Shen Ong.

15 recordsLinked to original sources

arXiv2026

Ultrafast electron crystallography reveals the atomic pathway of a light-driven correlated insulator-to-metal transition

Ultrafast phase transitions in correlated materials are often inferred from selected diffraction peak intensities or diffraction peak displacements, leaving the underlying three-dimensional atomic trajectories elusive. Resolving these trajectories is essential for identifying which atomic motions drive changes in electronic properties and how they couple to electronic degrees of freedom. We address this challenge in vanadium dioxide (VO2), a correlated oxide with a near-room-temperature transition between the insulating monoclinic (M1) phase and metallic rutile (R) phase. For this purpose, we introduce ultrafast three-dimensional electron diffraction, which enables refinement of the transient unit cell and internal V and O coordinates, revealing the V-V dimerization and zigzag motion during the phase transition. The refined atomic coordinates follow a linear trajectory in real space during the transition, in contrast to nonlinear or sequential transformation pathways inferred from more indirect observables in earlier work. Quantum many-body calculations treating each V-V pair as a correlated unit show that dimerization creates the level splitting responsible for the electronic gap, which is further enhanced by nonlocal electronic interactions between the paired V atoms. The gap collapses when dimerization is lost. This work turns time-resolved diffraction from order-parameter tracking into transient crystallography, directly connecting atomic trajectories to electronic mechanisms in correlated materials.

cond-mat.str-el↗

arXiv2026

Unraveling the symmetry of Al5C3N

The high-temperature ceramic compound Al5C3N with promising application usage belongs to the scarcely studied Al-C-N system. It was originally reported as an ordered compound in the non-centrosymmetric space group P63mc and described as a nanolaminate with an -Al2C-AlN-Al2C2- stacking sequence. The recently reported structural disorder in the related compound Al4SiC4 led us to question this proposed structure for Al5C3N and investigate the possibility of a disordered structure in the centrosymmetric space group P63/mmc. In the present work, we employed different synthesis routes to maximize the yield and quality of the desired phase, and applied a variety of techniques to probe the Al5C3N crystal structure. Our single-crystal X-ray diffraction analysis clearly indicates that the non-centrosymmetric space group P63mc must be rejected. From a joint refinement of single-crystal X-ray and powder neutron diffraction data, the occupancies of C and N were refined at two sites in P63/mmc resulting in the stacking sequence -Al2C-Al(C/N)-Al2(C/N)2-. Furthermore, DFT calculations show that a centrosymmetric disordered structure described in a supercell has the lowest energy, 0.2 eV per formula unit, relative to the previously reported P63mc structure. The calculated band structure shows both direct and indirect band gaps which lead to implications for the physical properties. Finally, STEM analysis provides additional evidence that the crystal structure of Al5C3N is better described in the centrosymmetric space group P63/mmc.

cond-mat.mtrl-sci↗

arXiv2025

Topological Metal-Insulator Transition within the Ferromagnetic state

A major challenge in condensed matter physics is integrating topological phenomena with correlated electron physics to leverage both types of states for next-generation quantum devices. Metal-insulator transitions (MITs) are central to bridging these two domains while simultaneously serving as 'on-off' switches for electronic states. Here, we demonstrate how the prototypical material of K2Cr8O16 undergoes a ferromagnetic MIT accompanied by a change in band topology. Through inelastic x-ray and neutron scattering experiments combined with first-principles theoretical calculations, we demonstrate that this transition is not driven by a Peierls mechanism, given the lack of phonon softening. Instead, we establish the transition as a topological MIT within the ferromagnetic phase (topological-FM-MIT) with potential axionic properties, where electron correlations play a key role in stabilizing the insulating state. This work pioneers the discovery of a topological-FM-MIT and represents a fundamentally new class of topological phase transitions, revealing a unique pathway through which magnetism, topology, and electronic correlations interact.

cond-mat.str-el↗

arXiv2025

Driving collective current excitations using light: The two-time $GW$ approach

We identify a distinct transverse collective excitation, which we name the "curron", arising from current-current interactions in a driven quantum metal. Unlike plasmons, which involve longitudinal charge oscillations, currons are transverse current-density oscillations resulting from the interplay between the vector potential generated by the current and the external driving field. We demonstrate the emergence of this excitation in sodium metal by solving the Kadanoff-Baym equations on a complex time contour within the non-equilibrium two-time (TT) $GW$ formalism, marking, to our knowledge, the first TT-$GW$ calculation on a realistic material. We further show that two-time quantum memory effects leave measurable signatures: a pump-induced elevation in the baseline of the current-to-field response, potentially observable in polarization- and momentum-resolved conductivity experiments. By extracting effective resistive and memory coefficients from the TT-$GW$ dynamics, we introduce a generalized d'Alembert wave equation that captures the many-body damping and retardation inherent to driven quantum systems. These results establish current-current response functions as a platform to harness qualitatively new collective dynamics in correlated matter, opening new avenues for probing light-matter interactions beyond charge-density dynamics.

cond-mat.mtrl-sci↗

arXiv2025

How quantum selection rules influence the magneto-optical effects of driven, ultrafast magnetization dynamics

Ultrafast magnetization dynamics driven by ultrashort pump lasers is typically explained by changes in electronic populations and scattering pathways of excited conduction electrons. This conventional approach overlooks the fundamental role of quantum mechanical selection rules, governing transitions from core states to the conduction band, that forms the key method of the probing step in these experiments. By employing fully ab initio time-dependent density functional theory, we reveal that these selection rules profoundly influence the interpretation of ultrafast spin dynamics at specific probe energies. Our analysis for hcp Co and fcc Ni at the M edge demonstrates that the transient dynamics, as revealed in pump-probe experiments, arise from a complex interplay of optical excitations of the M shell. Taking into account the selection rules and conduction electron spin flips, this leads to highly energy-dependent dynamics. These findings address longstanding discrepancies in experimental TMOKE measurements and show that only through meticulous consideration of matrix elements at the probe stage, can one ensure that magnetization dynamics is revealed in its true nature, instead of being muddled by artifacts arising from the choice of probe energy.

cond-mat.mtrl-sci↗

arXiv2023

Optically controlled single-valley exciton doublet states with tunable internal spin structures and spin magnetization generation

Manipulating quantum states through light-matter interactions has been actively pursued in two-dimensional (2D) materials research. Significant progress has been made towards the optical control of the valley degrees of freedom in semiconducting monolayer transition-metal dichalcogenides (TMD), based on doubly degenerate excitons from their two distinct valleys in reciprocal space. Here, we introduce a novel kind of optically controllable doubly degenerate exciton states that come from a single valley, dubbed as single-valley exciton doublet (SVXD) states. They are unique in that their constituent holes originate from the same valence band, making possible the direct optical control of the spin structure of the excited constituent electrons. Combining ab initio GW plus Bethe-Salpeter equation (GW-BSE) calculations and a newly developed theoretical analysis method, we demonstrate such novel SVXD in substrate-supported monolayer bismuthene -- which has been successfully grown using molecular beam epitaxy. In each of the two distinct valleys in the Brillouin zone, strong spin-orbit coupling and $C_{3v}$ symmetry lead to a pair of degenerate 1s exciton states (the SVXD states) with opposite spin configurations. Any coherent linear combinations of the SVXD in a single valley can be excited by light with a specific polarization, enabling full manipulation of their internal spin configurations. In particular, a controllable net spin magnetization can be generated through light excitation. Our findings open new routes to control quantum degrees of freedom, paving the way for applications in spintronics and quantum information science.

cond-mat.mtrl-sci↗

arXiv2022

sp$^{2}$/sp$^{3}$ bonding controlling mechanism at the $α$-Al$_{2}$O$_{3}|$graphene interface

First-principles calculations reported here illuminate the effects of the interfacial properties of $α$-Al$_{2}$O$_{3}$ and graphene, with emphasis on the structural and electronic properties. Various contact interfaces and different $α$-Al$_{2}$O$_{3}$ surface terminations are considered with on and slightly-off stoichiometric aluminium oxide. We show that depending on whether aluminium or oxygen is in contact with graphene, an $sp^{3}$ structural deformation and spontaneous spin-polarization may occur next to the interface contact. Interestingly, some cases cause a $p$-type doping in the graphene band structure, depending on the initial $α$-Al$_{2}$O$_{3}$ geometry placed on graphene. The importance of leaving the surface dangling bonds of alumina saturated or not is also highlighted, and we show that it might be a control mechanism for opening a gap in graphene by the influence of the $sp^{3}$ bond between oxygen and carbon atoms at the interface. We discuss the potential of utilizing this sensitivity for practical applications.

cond-mat.mtrl-sci↗

arXiv2022

Unconventional Excitonic States with Phonon Sidebands in Layered Silicon Diphosphide

Many-body interactions between quasiparticles (electrons, excitons, and phonons) have led to the emergence of new complex correlated states and are at the core of condensed matter physics and material science. In low-dimensional materials, unique electronic properties for these correlated states could significantly affect their optical properties. Herein, combining photoluminescence, optical reflection measurements and theoretical calculations, we demonstrate an unconventional excitonic state and its bound phonon sideband in layered silicon diphosphide (SiP$_2$), in which the bound electron-hole pair is composed of electrons confined within one-dimensional phosphorus$-$phosphorus chains and holes extended in two-dimensional SiP$_2$ layers. The excitonic state and the emergent phonon sideband show linear dichroism and large energy redshifts with increasing temperature. Within the $GW$ plus Bethe$-$Salpeter equation calculations and solving the generalized Holstein model non-perturbatively, we confirm that the observed sideband feature results from the correlated interaction between excitons and optical phonons. Such a layered material provides a new platform to study excitonic physics and many-particle effects.

cond-mat.mes-hall↗

arXiv2022

Photo-induced Hidden Phase of 1T-TaS2 with Tunable Lifetime

Phase transitions are ubiquitous, appearing at every length scale from atoms to galaxies. In condensed matter, ultrafast laser pulses drive materials to highly non-equilibrium conditions allowing transitions to new phases of matter not attainable under thermal excitation. Despite the intense scrutiny these hidden phases have received, the details of the dynamics of transition and reestablishment of the ground state remain largely unexplored. Here, we show the transition to a hidden phase of 1T-TaS2 driven by the screening of Coulombic repulsive interaction by photoexcited electrons. The temporal evolution of the coherent lattice dynamics highlights the existence of a novel phase with a laser fluence-dependent lifetime. The modeling of the dynamics reveals that the transition is caused by photo-excited carriers and it disappears at the rate of electron-phonon scattering. Our results demonstrate how femtosecond laser absorption leads to a decoupling of the electronic and lattice sub-systems, opening the way to novel states of matter, which can be controlled with light. We expect our investigation to be a starting point towards the development of novel ultrafast photonics devices, such as switches and modulators, taking advantage of fast and tunable phase transitions.

cond-mat.mtrl-sci↗

arXiv2022

Nature of novel moiré exciton states in WSe$_2$/WS$_2$ heterobilayers

Moiré patterns of transition metal dichalcogenide (TMD) heterobilayers have proven to be an ideal platform to host unusual correlated electronic phases, emerging magnetism, and correlated exciton physics. While the existence of novel moiré excitonic states is established through optical measurements, the microscopic nature of these states is still poorly understood, often relying on empirically fit models. Here, combining large-scale first-principles GW-BSE calculations and micro-reflection spectroscopy, we identify the nature of the exciton resonances in WSe$_2$/WS$_2$ moiré superlattices, discovering a surprisingly rich set of moiré excitons that cannot be even qualitatively captured by prevailing continuum models. Our calculations reveal moiré excitons with distinct characters, including modulated Wannier excitons and previously unindentified intralayer charge-transfer excitons. Signatures of these distinct excitonic characters are confirmed experimentally via the unique carrier-density and magnetic-field dependences of different moiré exciton resonances. Our study highlights the highly non-trivial exciton states that can emerge in TMD moiré superlattices, and suggests novel ways of tuning many-body physics in moiré systems by engineering excited-states with specific spatial characters.

cond-mat.mes-hall↗

arXiv2021

\textit{In-situ} pseudopotentials for electronic structure theory

We present a general method of constructing \textit{in-situ} pseodopotentials from first principles, all-electron, full-potential electronic structure calculations of a solid. The method is applied to bcc Na, at equilibrium volume. The essential steps of the method involve (i) calculating an all-electron Kohn-Sham eigenstate. (ii) Replacing the oscillating part of the wavefunction (inside the muffin-tin spheres) of this state, with a smooth function. (iii) Representing the smooth wavefunction in a Fourier series, and (iv) inverting the Kohn-Sham equation, to extract the pseudopotential that produces the state generated in steps (i)-(iii). It is shown that an \textit{in-situ} pseudopotential can reproduce an all-electron, full-potential eigenvalue up to the sixth significant digit. A comparison of the all-electron theory, \textit{in-situ} pseudopotential theory and the standard nonlocal pseudopotential theory demonstrates good agreement, e.g., in the energy dispersion of the 3$s$ band state of bcc Na.

cond-mat.mtrl-sci↗

arXiv2020

Bright excitons with negative-mass electrons

Bound electron-hole excitonic states are generally not expected to form with charges of negative effective mass. We identify such excitons in a single layer of the semiconductor WSe2, where they give rise to narrow-band upconverted photoluminescence in the UV, at an energy of 1.66 eV above the first band-edge excitonic transition. Negative band curvature and strong electron-phonon coupling result in a cascaded phonon progression with equidistant peaks in the photoluminescence spectrum, resolvable to ninth order. Ab initio GW-BSE calculations with full electron-hole correlations unmask and explain the admixture of upper conduction-band states to this complex many-body excitation: an optically bright, bound exciton in resonance with the semiconductor continuum. This exciton is responsible for atomic-like quantum-interference phenomena such as electromagnetically induced transparency. Since band curvature can be tuned by pressure or strain, synthesis of exotic quasiparticles such as flat-band excitons with infinite reduced mass becomes feasible.

cond-mat.mes-hall↗

arXiv2018

Valley-dependent Exciton Fine Structure and Autler-Townes Doublets from Berry Phases in Monolayer Molybdenum Diselenide

The Berry phase of Bloch states can have profound effects on electron dynamics lead to novel transport phenomena, such as the anomalous Hall effect and the valley Hall effect. Recently, it was predicted that the Berry phase effect can also modify the exciton states in transition metal dichalcogenide monolayers, and lift the energy degeneracy of exciton states with opposite angular momentum through an effective valley-orbital coupling. Here, we report the first observation and control of the Berry-phase induced splitting of the 2p-exciton states in monolayer molybdenum diselenide using the intraexciton optical Stark spectroscopy. We observe the time-reversal-symmetric analog of the orbital Zeeman effect resulting from the valley-dependent Berry phase, which leads to energy difference of +14 (-14) meV between the $2p^+$ and $2p^-$ exciton states in +K (-K) valley, consistent with the ordering from our ab initio GW-BSE results. In addition, we show that the light-matter coupling between intraexciton states are remarkably strong, leading to prominent valley-dependent Autler-Townes doublet under resonant driving. Our study opens up new pathways to coherently manipulate the quantum states and excitonic excitation with infrared radiation in two-dimensional semiconductors.

cond-mat.mes-hall↗

arXiv2017

Dynamics of Symmetry-Breaking Stacking Boundaries in Bilayer MoS2

Crystal symmetry of two-dimensional (2D) materials plays an important role in their electronic and optical properties. Engineering symmetry in 2D materials has recently emerged as a promising way to achieve novel properties and functions. The noncentrosymmetric structure of monolayer transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS2), has allowed for valley control via circularly polarized optical excitation. In bilayer TMDCs, inversion symmetry can be controlled by varying the stacking sequence, thus providing a pathway to engineer valley selectivity. Here, we report the in situ integration of AA' and AB stacked bilayer MoS2 with different inversion symmetries by creating atomically sharp stacking boundaries between the differently stacked domains, via thermal stimulation and electron irradiation, inside an atomic-resolution scanning transmission electron microscopy. The setup enables us to track the formation and atomic motion of the stacking boundaries in real time and with ultrahigh resolution which enables in-depth analysis on the atomic structure at the boundaries. In conjunction with density functional theory calculations, we establish the dynamics of the boundary nucleation and expansion and further identify metallic boundary states. Our approach provides a means to synthesize domain boundaries with intriguing transport properties and opens up a new avenue for controlling valleytronics in nanoscale domains via real-time patterning of domains with different symmetry properties.

cond-mat.mtrl-sci↗

arXiv2017

Real-space study of the optical absorption in alternative phases of silicon

We introduce a real-space approach to understand the relationship between optical absorption and crystal structure. We apply this approach to alternative phases of silicon, with a focus on the Si$_{20}$ crystal phase as a case study. We find that about 83% of the changes in the calculated low-energy absorption in Si$_{20}$ as compared to Si in the diamond structure can be attributed to reducing the differences between the on-site energies of the bonding and anti-bonding orbitals as well as increasing the hopping integrals for specific Si-Si bonds.

cond-mat.mtrl-sci↗