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Yee Sin Ang

Publications and source records attributed to Yee Sin Ang.

At least 55 records · Page 3Linked to original sources
arXiv2024

Field-Effect Josephson Diode via Asymmetric Spin-Momentum-Locking States

Recent breakthroughs in Josephson diodes dangle the possibility of extending conventional non-reciprocal electronics into the realm of superconductivity. While a strong magnetic field is recognized for enhancing diode efficiency, it concurrently poses a risk of undermining the essential superconductivity required for non-dissipative devices. To circumvent the need for magnetic-based tuning, we propose a field-effect Josephson diode based on the electrostatic gate control of finite momentum Cooper pairs in asymmetric spin-momentum-locking states. We propose two possible implementations of our gate-controlled mechanism: (i) a topological field-effect Josephson diode in time-reversal-broken quantum spin Hall insulators; and (ii) semiconductor-based field-effect Josephson diodes attainable in current experimental setups involving a Zeeman field and spin-orbit coupling. Notably, the diode efficiency is highly enhanced in the topological field-effect Josephson diode because the current carried by the asymmetric helical edge states is topologically protected and can be tuned by local gates. In the proposed Josephson diode, the combination of gates and asymmetric spin-momentum-locking nature is equivalent to that of a magnetic field, thus providing an alternative electrical operation in designing nonreciprocal superconducting devices.

cond-mat.mes-hall↗
arXiv2024

Nano-ironing van der Waals Heterostructures Towards Electrically Controlled Quantum Dots

Assembling two-dimensional van der Waals layered materials into heterostructures is an exciting development that sparked the discovery of rich correlated electronic phenomena and offers possibilities for designer device applications. However, resist residue from fabrication processes is a major limitation. Resulting disordered interfaces degrade device performance and mask underlying transport physics. Conventional cleaning processes are inefficient and can cause material and device damage. Here, we show that thermal scanning probe based cleaning can effectively eliminate resist residue to recover pristine material surfaces. Our technique is compatible at both the material- and device-level, and we demonstrate the significant improvement in the electrical performance of 2D WS2 transistors. We also demonstrate the cleaning of van der Waals heterostructures to achieve interfaces with low disorder. This enables the electrical formation and control of quantum dots that can be tuned from macroscopic current flow to the single-electron tunnelling regime. Such material processing advances are crucial for constructing high-quality vdW heterostructures that are important platforms for fundamental studies and building blocks for quantum and nano-electronics applications.

cond-mat.mes-hall↗
arXiv2023

Electric-filed tuned anomalous valley Hall effect in A-type hexagonal antiferromagnetic monolayer

The combination of antiferromagnetic (AFM) spintronics and anomalous valley Hall effect (AVHE) is of great significance for potential applications in valleytronics. Here, we propose a design principle for achieving AVHE in A-type hexagonal AFM monolayer. The design principle involves the introduction of layer-dependent electrostatic potential caused by out-of-plane external electric field, which can break the combined symmetry ($PT$ symmetry) of spatial inversion ($P$) and time reversal ($T$), producing spin splitting. The spin order of spin splitting can be reversed by regulating the direction of electric field. Based on first-principles calculations, the design principle can be verified in AFM $\mathrm{Cr_2CH_2}$. The layer-locked hidden Berry curvature can give rise to layer-Hall effect, including valley layer-spin Hall effect and layer-locked AVHE. Our works provide an experimentally feasible way to realize AVHE in AFM monolayer.

cond-mat.mtrl-sci↗
arXiv2023

Interplay between Haldane and modified Haldane models in $α$-$T_{3}$ lattice: Band structures, phase diagrams and edge states

We study the topological properties of the Haldane and modified Haldane models in $α$-$T_{3}$ lattice. The band structures and phase diagrams of the system are investigated. Individually, each model undergoes a distinct phase transition: (i) the Haldane-only model experiences a topological phase transition from the Chern insulator ($\mathcal{C} = 1$) phase to the higher Chern insulator ($\mathcal{C} = 2$) phase; while (ii) the modified-Haldane-only model experiences a phase transition from the topological metal ($\mathcal{C} = 2$) phase to the higher Chern insulator ($\mathcal{C} = 2$) phase and we show that $\mathcal{C}$ is insufficient to characterize this system because $\mathcal{C}$ remains unchanged before and after the phase transition. By plotting the Chern number and $\mathcal{C}$ phase diagram, we show that in the presence of both Haldane and modified Haldane models in the $α$-$T_{3}$ lattice, the interplay between the two models manifests three distinct topological phases, namely the $\mathcal{C} = 1$ Chern insulator (CI) phase, $\mathcal{C} = 2$ higher Chern insulator (HCI) phase and $\mathcal{C} = 2$ topological metal (TM) phase. These results are further supported by the $α$-$T_{3}$ zigzag edge states calculations. Our work elucidates the rich phase evolution of Haldane and modified Haldane models as $α$ varies continuously from $0$ to $1$ in an $α$-$T_3$ model.

cond-mat.str-el↗
arXiv2023

Layer-Locked Anomalous Valley Hall Effect in Two-Dimensional A-Type Tetragonal Antiferromagnetic Insulator

Antiferromagnetic (AFM) spintronics provides a route towards energy-efficient and ultrafast device applications. Achieving anomalous valley Hall effect (AVHE) in AFM monolayers is thus of considerable interest for both fundamental condensed matter physics and device enginering. Here we propose a route to achieve AVHE in A-type AFM insulator composed of vertically-stacked monolayer quantum anomalous Hall insulator (QAHI) with strain and electric field modulations. Uniaxial strain and electric field generate valley polarization and spin splitting, respectively. Using first-principles calculations, $\mathrm{Fe_2BrMgP}$ monolayer is predicted to be a prototype A-type AFM hosting \emph{valley-polarized quantum spin Hall insulator} (VQSHI) in which AVHE and quantum spin Hall effect (QSHE) are synergized in a single system. Our findings reveal a route to achieve multiple Hall effects in 2D tetragonal AFM monolayers.

cond-mat.mes-hall↗
arXiv2023

Ultrathick MA$_2$N$_4$(M'N) Intercalated Monolayers with Sublayer-Protected Fermi Surface Conduction States: Interconnect and Metal Contact Applications

Recent discovery of ultrathick $\mathrm{MoSi_2N_4(MoN)_n}$ monolayers open up an exciting platform to engineer 2D material properties via intercalation architecture. Here we computationally investigate a series of ultrathick MA$_2$N$_4$(M'N) monolayers (M, M' = Mo, W; A = Si, Ge) under both homolayer and heterolayer intercalation architectures in which the same and different species of transition metal nitride inner core layers are intercalated by outer passivating nitride sublayers, respectively. The MA$_2$N$_4$(M'N) monolayers are thermally, dynamically and mechanically stable with excellent mechanical strength and metallic properties. Intriguingly, the metallic states around Fermi level are localized within the inner core layers. Carrier conduction mediated by electronic states around the Fermi level is thus spatially insulated from the external environment by the native outer nitride sublayers, suggesting the potential of MA$_2$N$_4$(M'N) in back-end-of-line (BEOL) metal interconnect applications. Nitrogen vacancy defect at the outer sublayers creates `punch through' states around the Fermi level that bridges the carrier conduction in the inner core layers and the outer environment, forming a electrical contact akin to the `vias' structures of metal interconnects. We further show that MoSi$_2$N$_4$(MoN) can serve as a quasi-Ohmic contact to 2D WSe$_2$. These findings reveal the promising potential of ultrathick MA$_2$N$_4$(MN) monolayers as metal electrodes and BEOL interconnect applications.

cond-mat.mtrl-sci↗
arXiv2023

Sub-5-nm Ultra-thin In$_2$O$_3$ Transistors for High-Performance and Low-Power Electronic Applications

Ultra-thin (UT) oxide semiconductors are promising candidates for back-end-of-line (BEOL) compatible transistors and monolithic three-dimensional integration. Experimentally, UT indium oxide (In$_2$O$_3$) field-effect transistors (FETs) with thicknesses down to 0.4 nm exhibits extremely high drain current (10000 $μ$A/$μ$m) and transconductance (4000 $μ$S/$μ$m). Here, we employ the ab initio quantum transport simulation to investigate the performance limit of sub-5-nm gate length (Lg) UT In$_2$O$_3$ FET. Based on the International Technology Roadmap for Semiconductors (ITRS) criteria for high-performance (HP) devices, the scaling limit of UT In$_2$O$_3$ FETs can reach 2 nm in terms of on-state current, delay time, and power dissipation. The wide bandgap nature of UT In$_2$O$_3$ (3.15 eV) renders it a suitable candidate for ITRS low-power (LP) electronics with Lg down to 3 nm. Both the HP and LP UT In$_2$O$_3$ FETs exhibit superior energy-delay products as compared to other common 2D semiconductors such as monolayer MoS2 and MoTe2. Our study unveils the immense promise of UT In$_2$O$_3$ for both HP and LP device applications.

physics.app-ph↗
arXiv2023

Exceptional points in non-Hermitian Photonics: Applications and Recent Developments

Exceptional points are complex branching singularities of non-Hermitian bands that have lately attracted considerable interest, particularly in non-Hermitian photonics. In this article, we review some recent developments in non-Hermitian photonic platforms such as waveguides, photonic crystals, Fabry-Perot resonators and plasmonic systems, and suggest how optical non-linearities and exceptional bound states can significantly impact the development of non-Hermitian photonics in the near future.

physics.optics↗
arXiv2023

MA$_2$Z$_4$ Family Heteorstructures: Promises and Prospects

Recent experimental synthesis of ambient-stable MoSi2N4 monolayer have garnered enormous research interests. The intercalation morphology of MoSi2N4 - composed of a transition metal nitride (Mo-N) inner sub-monolayer sandwiched by two silicon nitride (Si-N) outer sub-monolayers - have motivated the computational discovery of an expansive family of synthetic MA2Z4 monolayers with no bulk (3D) material counterpart (where M = transition metals or alkaline earth metals; A = Si, Ge; and N = N, P, As). MA2Z4 monolayers exhibit interesting electronic, magnetic, optical, spintronic, valleytronic and topological properties, making them a compelling material platform for next-generation device technologies. Furthermore, heterostructure engineering enormously expands the opportunities of MA2Z4. In this review, we summarize the recent rapid progress in the computational design of MA2Z4-based heterostructures based on first-principle density functional theory (DFT) simulations - a central \emph{work horse} widely used to understand the physics, chemistry and general design rules for specific targeted functions. We systematically classify the MA2Z4-based heterostructures based on their contact types, and review their physical properties, with a focus on their performances in electronics, optoelectronics and energy conversion applications. We review the performance and promises of MA2Z4-based heterostructures for device applications that include electrical contacts, transistors, spintronic devices, photodetectors, solar cells, and photocatalytic water splitting. This review unveils the vast device application potential of MA2Z4-based heterostructures, and paves a roadmap for the future experimental and theoretical development of MA2Z4-based functional heterostructures and devices.

cond-mat.mtrl-sci↗
arXiv2023

Towards Sustainable Ultrawide Bandgap Van der Waals Materials: An ab initio Screening Effort

The sustainable development of next-generation device technology is paramount in the face of climate change and the looming energy crisis. Tremendous efforts have been made in the discovery and design of nanomaterials that achieve device-level sustainability, where high performance and low operational energy cost are prioritized. However, many of such materials are composed of elements that are under threat of depletion and pose elevated risks to the environment. The role of material-level sustainability in computational screening efforts remains an open question thus far. Here we develop a general van der Waals materials screening framework imbued with sustainability-motivated search criteria. Using ultrawide bandgap (UWBG) materials as a backdrop -- an emerging materials class with great prospects in dielectric, power electronics, and ultraviolet device applications, we demonstrate how this screening framework results in 25 sustainable UWBG layered materials comprising only of low-risks elements. Our findings constitute a critical first-step towards reinventing a more sustainable electronics landscape beyond silicon, with the framework established in this work serving as a harbinger of sustainable 2D materials discovery.

cond-mat.mtrl-sci↗
arXiv2023

Sub-5 nm Gate-All-Around InP Nanowire Transistors Towards High-Performance Devices

Gate-all-around (GAA) nanowire (NW) field-effect transistor (FET) is a promising device architecture due to its superior gate controllability than that of the conventional FinFET architecture. The significantly higher electron mobility of indium phosphide (InP) NW than silicon NW makes it particularly well-suited for high-performance (HP) electronics applications. In this work, we perform an ab initio quantum transport simulation to investigate the performance limit of sub-5-nm gate length (Lg) GAA InP NW FETs. The GAA InP NW FETs with Lg of 4 nm can meet the International Technology Roadmap for Semiconductors (ITRS) requirements for HP devices from the perspective of on-state current, delay time, and power dissipation. We also investigate the impact of strain on 3-nm-Lg GAA InP NW FETs. The application of tensile strain results in a remarkable increase of over 60% in the on-state current. These results highlight the potential of GAA InP NW FETs for HP applications in the sub-5-nm Lg region.

cond-mat.mtrl-sci↗
arXiv2023

Altermagnetic Schottky Contact

Altermagnet is an emerging antiferromagnetic material subclass that exhibits spin-splitting in momentum space without net global magnetization and spin-orbit-coupling effect. In this work, we develop a model of thermal charge injection across an altermagnet/semiconductor (AM/S) Schottky contact. We obtain analytical expressions describing the spin-dependent thermionic current injection across the AM/S contact under any arbitrary interface orientation angles. Interestingly, the spin-contrasting Fermi surface of an altermagnetic electrode enables spin-polarized current to be injected into a nonmagnetic semiconductor even though the system has net-zero magnetization. Our findings thus reveal an altermagntic mechanism to achieve spin injection without involving ferromagnetic ordering. The AM/S Schottky contact proposed here shall provide a potential altermagnetic building block of spintronic devices that are resilient against stray magnetic field perturbation and inherently compatible with ultracompact integration.

cond-mat.mes-hall↗
arXiv2023

How to produce spin-splitting in antiferromagnetic materials

Antiferromagnetic (AFM) materials have potential advantages for spintronics due to their robustness, ultrafast dynamics, and magnetotransport effects. However, the missing spontaneous polarization and magnetization hinders the efficient utilization of electronic spin in these AFM materials. Here, we propose a simple way to produce spin-splitting in AFM materials by making the magnetic atoms with opposite spin polarization locating in the different environment (surrounding atomic arrangement), which does not necessarily require the presence of spin-orbital coupling (SOC). We confirm our proposal by four different types of two-dimensional (2D) AFM materials within the first-principles calculations. Our works provide a intuitional design principle to find or produce spin-splitting in AFM materials.

cond-mat.mtrl-sci↗
arXiv2023

Half-Valley Ohmic Contact and Contact-Limited Valley-Contrasting Current Injection

Two-dimensional (2D) ferrovalley semiconductor (FVSC) with spontaneous valley polarization offers an exciting material platform for probing Berry phase physics. How FVSC can be incorporated in valleytronic device applications, however, remain an open question. Here we generalize the concept of metal/semiconductor (MS) contact into the realm of valleytronics. We propose a half-valley Ohmic contact based on FVSC/graphene heterostructure where the two valleys of FVSC separately forms Ohmic and Schottky contacts with those of graphene, thus allowing current to be valley-selectively injected through the `Ohmic' valley while being blocked in the `Schottky' valley. We develop a theory of contact-limited valley-contrasting current injection and demonstrate that such transport mechanism can produce gate-tunable valley-polarized injection current. Using RuCl$_2$/graphene heterostructure as an example, we illustrate a device concept of valleytronic barristor where high valley polarization efficiency and sizable current on/off ratio, can be achieved under experimentally feasible electrostatic gating conditions. These findings uncover contact-limited valley-contrasting current injection as an efficient mechanism for valley polarization manipulation, and reveals the potential of valleytronic MS contact as a functional building block of valleytronic device technology.

cond-mat.mes-hall↗
arXiv2023

Electric-field induced half-metal in monolayer CrSBr

Two-dimensional (2D) half-metallic materials are highly desirable for nanoscale spintronic applications. Here, we propose a new mechanism that can achieve half-metallicity in 2D ferromagnetic (FM) material with two-layer magnetic atoms by electric field tuning. We use a concrete example of experimentally synthesized CrSBr monolayer to illustrate our proposal through the first-principle calculations. It is found that the half-metal can be achieved in CrSBr within appropriate electric field range, and the corresponding amplitude of electric field intensity is available in experiment. Janus monolayer $\mathrm{Cr_2S_2BrI}$ is constructed, which possesses built-in electric field due to broken horizontal mirror symmetry. However, $\mathrm{Cr_2S_2BrI}$ without and with applied external electric field is always a FM semiconductor. A possible memory device is also proposed based on CrSBr monolayer. Our works will stimulate the application of 2D FM CrSBr in future spintronic nanodevices.

cond-mat.mtrl-sci↗
arXiv2023

Experimental observation of exceptional bound states in a classical circuit network

Exceptional bound (EB) states represent an unique new class of robust bound states protected by the defectiveness of non-Hermitian exceptional points. Conceptually distinct from the more well-known topological states and non-Hermitian skin states, they were recently discovered as a novel source of negative entanglement entropy in the quantum entanglement context. Yet, EB states have been physically elusive, being originally interpreted as negative probability eigenstates of the propagator of non-Hermitian Fermi gases. In this work, we show that EB states are in fact far more ubiquitous, also arising robustly in broad classes of systems whether classical or quantum. This hinges crucially on a newly-discovered spectral flow that rigorously justifies the EB nature of small candidate lattice systems. As a highlight, we present their first experimental realization through an electrical circuit, where they manifest as prominent stable resonant voltage profiles. Our work brings a hitherto elusive but fundamentally distinctive quantum phenomenon into the realm of classical metamaterials, and provides a novel pathway for the engineering of robust modes in otherwise sensitive systems.

quant-ph↗
arXiv2023

Over-Barrier Photoelectron Emission with Rashba Spin-Orbit Coupling

We develop a theoretical model to calculate the quantum efficiency (QE) of photoelectron emission from materials with Rashba spin-orbit coupling (RSOC) effect. In the low temperature limit, an analytical scaling between QE and the RSOC strength is obtained as QE $\propto (\hbarω-W)^2+2E_R(\hbar ω-W) -E_R^2/3$, where $\hbarω$, $W$ and $E_R$ are the incident photon energy, work function and the RSOC parameter respectively. Intriguingly, the RSOC effect substantially improves the QE for strong RSOC materials. For example, the QE of Bi$_2$Se$_3$ and Bi/Si(111) increases, by 149\% and 122\%, respectively due to the presence of strong RSOC. By fitting to the photoelectron emission characteristics, the analytical scaling law can be employed to extract the RSOC strength, thus offering a useful tool to characterize the RSOC effect in materials. Importantly, when the traditional Fowler-Dubridge model is used, the extracted results may substantially deviate from the actual values by $\sim90\%$, thus highlighting the importance of employing our model to analyse the photoelectron emission especially for materials with strong RSOC. These findings provide a theoretical foundation for the design of photoemitters using Rashba spintronic materials.

cond-mat.mes-hall↗
arXiv2023

Injection-Limited and Space-Charge-Limited Conduction in Wide Bandgap Semiconductors with Velocity Saturation Effect

Carrier conduction in wide bandgap semiconductors (WBS) often exhibits velocity saturation at the high-electric field regime. How such effect influences the transition between contact-limited and space-charge-limited current in a two-terminal device remains largely unexplored thus far. Here, we develop a generalized carrier transport model that includes contact-limited field-induced carrier injection, space charge, carrier scattering and velocity saturation effect. The model reveals various transitional behaviors in the current-voltage characteristics, encompassing Fowler-Nordheim emission, trap-free Mott-Gurney (MG) SCLC and \emph{velocity-saturated SCLC}. Using GaN, 6H-SiC and 4H-SiC WBS as examples, we show that the velocity-saturated SCLC completely dominates the high-voltage ($10^2 \sim 10^4$ V) transport for typical sub-$μ$m GaN and SiC diodes, thus unravelling velocity-saturated SCLC as a central transport mechanism in WBG electronics.

physics.app-ph↗