arXiv subjects

Yee Sin Ang

Publications and source records attributed to Yee Sin Ang.

At least 37 records · Page 2Linked to original sources

arXiv2025

Achieving fully-compensated ferrimagnetism through two-dimensional heterojunctions

In addition to altermagnets, fully-compensated ferrimagnets are another category of collinear magnetic materials that possess zero-net total magnetic moment and exhibit spin-splitting, making them promising for low-energy spintronics, high-density data storage and high-sensitivity sensors. Although many methods, such as alloying, external electric field, Janus engineering, ferroelectric field and spin ordering, have been proposed to achieve fully-compensated ferrimagnetism, these approaches either face experimental difficulties or produce a small spin-splitting or are volatile. Here, we propose to form vertical heterostructures by stacking two different but equally magnetized two-dimensional ferromagnetic materials. If an A-type antiferromagnetic ordering is satisfied, a fully compensated ferrimagnet can be formed. This vertical heterostructure approach is insensitive to lattice matching and stacking manner, thus being more conducive to experimental realization. Through first-principles calculations, we verify our proposal with several examples, focusing in particular on $\mathrm{CrI_3}$/$\mathrm{CrGeTe_3}$ heterojunction composed of experimentally synthesized $\mathrm{CrI_3}$ and $\mathrm{CrGeTe_3}$ monolayers. The calculations show that $\mathrm{CrI_3}$/$\mathrm{CrGeTe_3}$ is a fully-compensated ferrimagnet, with pronounced spin-splitting, and that tensile strain is more favorable for achieving fully-compensated ferrimagnetism. Our work provides an experimentally feasible strategy for realizing fully-compensated ferrimagnetism, thereby further advancing the development of this field.

cond-mat.mtrl-sci↗

arXiv2025

Ferroelastic Altermagnetism

Synergizing altermagnetism and other ferroic orders, such as ferroelectric switchable altermagnetism [Phys. Rev. Lett. 134, 106801 (2025) and ibid. 106802 (2025)], offers an effective route to achieve nonvolatile switching of altermagnetic spin splitting. In this work, by synergizing altermagnetism and ferroelasticity, we propose the concept of ferroelastic altermagnets in which the ferroelastic crystal reorientation can drive multistate nonvolatile switching of the altermagnetic spin splitting via altermagnetoelastic effect. Using monolayers RuF4 and CuF2 as material candidates, we demonstrate 2-state and 3-state altermagnetic spin splitting switching as driven by ferroelastic strain states. Transport calculation shows that multistate spin conductivities can be ferroelastically encoded in an ferroelastic altermagnet, thus suggesting the potential of ferroelastic altermagnetic as nonvolatile nanomechanical spin switches. The proposed concept of ferroelastic altermagnetism enriches the emerging landscape of multiferroic altermagnetism, paving a way towards altermagnetic-based straintronic device applications.

cond-mat.mtrl-sci↗

arXiv2025

Computational Design of Two-Dimensional MoSi$_2$N$_4$ Family Field-Effect Transistor for Future Ångström-Scale CMOS Technology Nodes

Advancing complementary metal-oxide-semiconductor (CMOS) technology into the sub-1-nm angström-scale technology nodes is expected to involve alternative semiconductor channel materials, as silicon transistors encounter severe performance degradation at physical gate lengths below 10 nm. Two-dimensional (2D) semiconductors have emerged as strong candidates for overcoming short-channel effects due to their atomically thin bodies, which inherently suppress electrostatic leakage and improve gate control in aggressively scaled field-effect transistors (FETs). Among the growing library of 2D materials, the MoSi$_2$N$_4$ family -- a synthetic septuple-layered materials -- has attracted increasing attention for its remarkable ambient stability, suitable bandgaps, and favorable carrier transport characteristics, making it a promising platform for next-generation transistors. While experimental realization of sub-10-nm 2D FETs remains technologically demanding, computational device simulation using first-principles density functional theory combined with nonequilibrium Green's function transport simulations provide a powerful and cost-effective route for exploring the performance limits and optimal design of ultrascaled FET. This review consolidates the current progress in the computational design of MoSi$_2$N$_4$ family FETs. We review the physical properties of MoSi$_2$N$_4$ that makes them compelling candidates for transistor applications, as well as the simulated device performance and optimization strategy of MoSi$_2$N$_4$ family FETs. Finally, we identify key challenges and research gaps, and outline future directions that could accelerate the practical deployment of MoSi$_2$N$_4$ family FET in the angström-scale CMOS era.

cond-mat.mtrl-sci↗

arXiv2025

Pressure-Driven Metallicity in Ångström-Thickness 2D Bismuth and Layer-Selective Ohmic Contact to MoS2

Recent fabrication of two-dimensional (2D) metallic bismuth (Bi) via van der Waals (vdW) squeezing method opens a new avenue to ultrascaling metallic materials into the ångström-thickness regime [Nature 639, 354 (2025)]. However, freestanding 2D Bi is typically known to exhibit a semiconducting phase [Nature 617, 67 (2023), Phys. Rev. Lett. 131, 236801 (2023)], which contradicts with the experimentally observed metallicity in vdW-squeezed 2D Bi. Here we show that such discrepancy originates from the pressure-induced buckled-to-flat structural transition in 2D Bi, which changes the electronic structure from semiconducting to metallic phases. Based on the experimentally fabricated MoS2-Bi-MoS2 trilayer heterostructure, we demonstrate the concept of layer-selective Ohmic contact in which one MoS2 layer forms Ohmic contact to the sandwiched Bi monolayer while the opposite MoS2 layer exhibits a Schottky barrier. The Ohmic contact can be switched between the two sandwiching MoS2 monolayers by changing the polarity of an external gate field, thus enabling charge to be spatially injected into different MoS2 layers. The layer-selective Ohmic contact proposed here represents a layertronic generalization of metal/semiconductor contact, paving a way towards layertronic device application.

cond-mat.mtrl-sci↗

arXiv2025

STI-SNN: A 0.14 GOPS/W/PE Single-Timestep Inference FPGA-based SNN Accelerator with Algorithm and Hardware Co-Design

Brain-inspired Spiking Neural Networks (SNNs) have attracted attention for their event-driven characteristics and high energy efficiency. However, the temporal dependency and irregularity of spikes present significant challenges for hardware parallel processing and data reuse, leading to some existing accelerators falling short in processing latency and energy efficiency. To overcome these challenges, we introduce the STI-SNN accelerator, designed for resource-constrained applications with high energy efficiency, flexibility, and low latency. The accelerator is designed through algorithm and hardware co-design. Firstly, STI-SNN can perform inference in a single timestep. At the algorithm level, we introduce a temporal pruning approach based on the temporal efficient training (TET) loss function. This approach alleviates spike disappearance during timestep reduction, maintains inference accuracy, and expands TET's application. In hardware design, we analyze data access patterns and adopt the output stationary (OS) dataflow, eliminating the need to store membrane potentials and access memory operations. Furthermore, based on the OS dataflow, we propose a compressed and sorted representation of spikes, then cached in the line buffer to reduce the memory access cost and improve reuse efficiency. Secondly, STI-SNN supports different convolution methods. By adjusting the computation mode of processing elements (PEs) and parameterizing the computation array, STI-SNN can accommodate lightweight models based on depthwise separable convolutions (DSCs), further enhancing hardware flexibility. Lastly, STI-SNN also supports both inter-layer and intra-layer parallel processing. For inter-layer parallelism, we ...

cs.AR↗

arXiv2025

Many-body critical non-Hermitian skin effect

Criticality in non-Hermitian systems unveils unique phase transitions and scaling behaviors beyond Hermitian paradigms, offering new insights into the interplay between gain/loss, non-reciprocity, and complex energy spectra. In this paper, we uncover a new class of many-body critical non-Hermitian skin effect (CSE) originating from the interplay between multiple non-Hermitian pumping channels and Hubbard interactions. In particular, criticality in the real-to-complex transitions can selectively emerge within the subspace of bound states or scattering states, as well as their interacting admixtures. These mechanisms possess no single-particle analog and can be diagnosed through a specially defined correlation function. As more particles are involved, higher-order CSEs naturally arise, with greatly enhanced effective coupling strengths and hence greater experimental accessibility. Our results reveal an enriched landscape of non-Hermitian critical phenomena in interacting many-body systems, and pave the way for investigating unconventional non-Hermitian criticality in the context of various interaction-induced particle clustering configurations.

quant-ph↗

arXiv2025

Valley Gapless Semiconductor: Models and Applications

The emerging field of valleytronics harnesses the valley degree of freedom of electrons, akin to how electronic and spintronic devices utilize the charge and spin degrees of freedom of electrons respectively. The engineering of valleytronic devices typically relies on the coupling between valley and other degrees of freedom such as spin, giving rise to valley-spintronics where an external magnetic field manipulates the information stored in valleys. Here, the valley gapless semiconductor is proposed as a potential electrically controlled valleytronic platform because the valley degree of freedom is coupled to the carrier type, i.e., electrons and holes. The valley degree of freedom can be electrically controlled by tuning the carrier type via the device gate voltage. We demonstrate the proposal for realizing a valley gapless semiconductor in the honeycomb lattice with the Haldane and modified Haldane models. The system's valley-carrier coupling is further studied for its transport properties in an all-electrically controlled valley filter device setting. Our work highlights the significance of the valley gapless semiconductor for valleytronic devices.

cond-mat.mes-hall↗

arXiv2024

Floquet engineering of topological phase transitions in quantum spin Hall $α$-$T_{3}$ system

Floquet engineering of topological phase transitions driven by a high-frequency time-periodic field is a promising approach to realizing new topological phases of matter distinct from static states. Here, we theoretically investigate Floquet engineering topological phase transitions in the quantum spin Hall $α$-$T_{3}$ system driven by an off-resonant circularly polarized light. In addition to the quantum spin (anomalous) Hall insulator phase with multiple helical (chiral) edge states, spin-polarized topological metallic phases are observed, where the bulk topological band gap of one spin sub-band overlaps with the other gapless spin sub-band. Moreover, with a staggered potential, the topological invariants of the system depend on whether the middle band is occupied because of the breaking of symmetry with respect to the center of energy-momentum plane. Our work highlights the significance of Floquet engineering in realizing new topological phases in $α$-$T_{3}$ lattices.

cond-mat.mes-hall↗

arXiv2024

Zero-Dipole Schottky Contact: Homologous Metal Contact to 2D Semiconductor

Band alignment of metal contacts to 2D semiconductors often deviate from the ideal Shottky-Mott (SM) rule due to the non-ideal factors such as the formation of interface dipole and metal-induced gap states (MIGS). Although MIGS can be strongly suppressed using van der Waals (vdW) contact engineering, the interface dipole is hard to eliminate due to the electronegativity difference of the two contacting materials. Here we show that interface dipole can be practically eliminated in 2D semiconducting MoSi$_2$N$_4$ when contacted by its homologous metallic counterpart MoSi$_2$N$_4$(MoN)$_n$ ($n = 1-4$). The SiN outer sublayers, simultaneously present in both MoSi$_2$N$_4$ and MoSi$_2$N$_4$(MoN)$_n$, creates nearly equal charge `push-back' effect at the contact interface. This nearly symmetrical charge redistribution leads to zero net electron transfer across the interface, resulting in a \emph{zero-dipole} contact. Intriguingly, we show that even in the extreme close-contact case where MoSi$_2$N$_4$(MoN) is arbitrarily pushed towards MoSi$_2$N$_4$ with extremely small interlayer distance, the interface dipole remains practically zero. Such \emph{zero-dipole} Schottky contact represents a peculiar case where the SM rule, usually expected to occur only in the non-interacting regime, manifests in MoSi$_2$N$_4$/MoSi$_2$N$_4$(MoN)$_n$ vdWH even though the constituent monolayers interact strongly. A model for pressure sensing is then proposed based on changing the interlayer distance in MoSi$_2$N$_4$/MoSi$_2$N$_4$(MoN) vdWH.

cond-mat.mtrl-sci↗

arXiv2024

Transverse Cooper-Pair Rectifier

Non-reciprocal devices are key components in modern electronics covering broad applications ranging from transistors to logic circuits thanks to the output rectified signal in the direction parallel to the input. In this work, we propose a transverse Cooper-pair rectifier in which a non-reciprocal current is perpendicular to the driving field, when inversion, time reversal, and mirror symmetries are broken simultaneously. The Blonder-Tinkham-Klapwijk formalism is developed to describe the transverse current-voltage relation in a normal-metal/superconductor tunneling junction, where symmetry constraints are achieved by an effective built-in supercurrent manifesting in an asymmetric and anisotropic Andreev reflection. The asymmetry in the Andreev reflection is induced when inversion and time reversal symmetry are broken by the supercurrent component parallel to the junction while the anisotropy occurs when the mirror symmetry with respect to the normal of the junction interface is broken by the perpendicular supercurrent component to the junction. Compared to the conventional longitudinal one, the transverse rectifier supports fully polarized diode efficiency and colossal nonreciprocal conductance rectification, completely decoupling the path of the input excitation from the output rectified signal. This work provides a formalism for realizing transverse non-reciprocity in superconducting junctions, which is expected to be achieved by modifying current experimental setups and may pave the way for future low-dissipation superconducting electronics.

cond-mat.mes-hall↗

arXiv2024

All-Electrical Layer-Spintronics in Altermagnetic Bilayer

Electrical manipulation of spin-polarized current is highly desirable yet tremendously challenging in developing ultracompact spintronic device technology. Here we propose a scheme to realize the all-electrical manipulation of spin-polarized current in an altermagnetic bilayer. Such a bilayer system can host layer-spin locking, in which one layer hosts a spin-polarized current while the other layer hosts a current with opposite spin polarization. An out-of-plane electric field breaks the layer degeneracy, leading to a gate-tunable spin-polarized current whose polarization can be fully reversed upon flipping the polarity of the electric field. Using first-principles calculations, we show that CrS bilayer with C-type antiferromagnetic exchange interaction exhibits a hidden layer-spin locking mechanism that enables the spin polarization of the transport current to be electrically manipulated via the layer degree of freedom. We demonstrate that sign-reversible spin polarization as high as 87% can be achieved at room temperature. This work presents the pioneering concept of layer-spintronics which synergizes altermagnetism and bilayer stacking to achieve efficient electrical control of spin.

cond-mat.mes-hall↗

arXiv2024

Toward Phonon-Limited Transport in Two-Dimensional Electronics by Oxygen-Free Fabrication

Future electronics require aggressive scaling of channel material thickness while maintaining device performance. Two-dimensional (2D) semiconductors are promising candidates, but despite over two decades of research, experimental performance still lags theoretical expectations. Here, we develop an oxygen-free approach to push the electrical transport of 2D field-effect transistors toward the theoretical phonon-limited intrinsic mobility. We achieve record carrier mobilities of 91 (132) cm2V-1s-1 for mono- (bi-) layer MoS2 transistors on SiO2 substrate. Statistics from over 60 devices confirm that oxygen-free fabrication enhances key figures of merit by more than an order of magnitude. While previous studies suggest that 2D transition metal dichalcogenides such as MoS2 and WS2 are stable in air, we show that short-term ambient exposure can degrade their device performance through irreversible oxygen chemisorption. This study emphasizes the criticality of avoiding oxygen exposure, offering guidance for device manufacturing for fundamental research and practical applications of 2D materials.

cond-mat.mtrl-sci↗

arXiv2024

Ab Initio Device-Driven Screening of Sub-1-nm Thickness Oxide Semiconductors for Future CMOS Technology Nodes

Ultrathin oxide semiconductors with sub-1-nm thickness are promising building blocks for ultrascaled field-effect transistor (FET) applications due to their resilience against short-channel effects, high air stability, and potential for low-energy device operation. However, the n-type dominance of ultrathin oxide FET has hindered their integration into complementary metal-oxide-semiconductor (CMOS) technology, which requires both n-and p-type devices. Here we develop an ab initio device-driven computational screening workflow to identify sub-1-nm thickness oxide semiconductors for sub-5-nm FET applications. We demonstrate that ultrathin CaO2, CaO, and SrO are compatible with p-type device operations under both high-performance (HP) and low-power (LP) requirements specified by the International Technology Roadmap of Semiconductors (ITRS), thereby expanding the limited family of p-type oxide semiconductors. Notably, CaO and SrO emerge as the first-of-kind sub-1-nm thickness oxide semiconductors capable of simultaneously meeting the ITRS HP and LP criteria for both n-and p-type devices. CaO and SrO FETs outperform many existing low-dimensional semiconductors, exhibiting scalability below 5-nm gate length. Our findings offer a pioneering effort in the ab initio, device-driven screening of sub-1-nm thickness oxide semiconductors, significantly broadening the material candidate pool for future CMOS technology nodes.

cond-mat.mtrl-sci↗

arXiv2024

Symmetric n-and p-Type Sub-5-nm 1D Graphene Nanoribbon Transistors for Homogeneous CMOS Applications

Graphene nanoribbon (GNR) emerges as an exceptionally promising channel candidate due to its tunable sizable bandgap (0-3 eV), ultrahigh carrier mobility (up to 4600 cm^(2) V^(-1) s^(-1)), and excellent device performance (current on-off ratio of 10^(7)). However, the asymmetry of reported n-type and p-type GNR field-effect transistors (FETs) at ultrashort gate length (Lg) has become an obstacle to future complementary metal-oxide-semiconductor (CMOS) integration. Here, we conduct ab initio quantum transport simulations to investigate the transport properties of sub-5-nm Lg 7 armchair-edge GNR (7 AGNR) FETs. The on-state current, delay time, and power dissipation of the n-type and p-type 7 AGNR FETs fulfill the International Technology Roadmap for Semiconductors targets for high-performance devices when Lg is reduced to 3 nm. Remarkably, the 7 AGNR FETs exhibit superior n-type and p-type symmetry to the 7-9-7 AGNR FETs due to the more symmetrical electron/hole effective masses. Compared to the monolayer MoS2 and MoTe2 counterparts, the 7 AGNR FETs have better device performance, which could be further improved via gate engineering. Our results shed light on the immense potential of 7 AGNR in advancing CMOS electronics beyond silicon.

cond-mat.mes-hall↗

arXiv2024

Bilayer TeO2: The First Oxide Semiconductor with Symmetric Sub-5-nm NMOS and PMOS

Wide bandgap oxide semiconductors are very promising channel candidates for next-generation electronics due to their large-area manufacturing, high-quality dielectrics, low contact resistance, and low leakage current. However, the absence of ultra-short gate length (Lg) p-type transistors has restricted their application in future complementary metal-oxide-semiconductor (CMOS) integration. Inspired by the successfully grown high-hole mobility bilayer (BL) beta tellurium dioxide (\b{eta}-TeO2), we investigate the performance of sub-5-nm-Lg BL \b{eta}-TeO2 field-effect transistors (FETs) by utilizing first-principles quantum transport simulation. The distinctive anisotropy of BL \b{eta}-TeO2 yields different transport properties. In the y-direction, both the sub-5-nm-Lg n-type and p-type BL \b{eta}-TeO2 FETs can fulfill the International Technology Roadmap for Semiconductors (ITRS) criteria for high-performance (HP) devices, which are superior to the reported oxide FETs (only n-type). Remarkably, we for the first time demonstrate the existence of the NMOS and PMOS symmetry in sub-5-nm-Lg oxide semiconductor FETs. As to the x-direction, the n-type BL \b{eta}-TeO2 FETs satisfy both the ITRS HP and low-power (LP) requirements with Lg down to 3 nm. Consequently, our work shed light on the tremendous prospects of BL \b{eta}-TeO2 for CMOS application.

cond-mat.mes-hall↗

arXiv2024

Two-dimensional Weyl nodal-line semimetal and antihelical edge states in a modified Kane-Mele model

The Kane-Mele model has been modified to achieve versatile topological phases. Previous work [Phys. Rev. Lett. 120, 156402 (2018)] introduced a staggered intrinsic spin-orbit coupling effect to generate pseudohelical edge states, with Rashba spin-orbit coupling facilitating spin flips in alternating sublattices. Our study demonstrates that, in the absence of Rashba spin-orbit coupling, the modified Kane-Mele model with staggered intrinsic spin-orbit coupling evolves into a $Z_{2}$ class topological metal, specifically a two-dimensional Weyl nodal-line semimetal. In a nanoribbon geometry, we predict the emergence of antihelical edge states, which support spin-polarized currents flowing in the same direction along parallel boundaries. Unlike pseudohelical edge states, antihelical edge states can be viewed as a superposition of two antichiral edge states related by time-reversal symmetry. However, the spin Hall conductance from antihelical edge states is not quantized due to the presence of gapless bulk states. Additionally, we examine the robustness of helical, pseudohelical, and antihelical edge states in the presence of nonmagnetic disorders, highlighting the particular fragility of antihelical edge states. Our findings enhance the understanding of the modified Kane-Mele model, providing new insights into its topological properties.

cond-mat.mes-hall↗

arXiv2024

Small exciton effective mass in QL Bi2Se2Te: A material platform towards high-temperature excitonic condensate

Using first-principles simulations combined with many-body calculations, we show that two-dimensional free-standing quintuple-layer Bi2Se2Te is an inversion symmetric monolayer expected to achieve spatially indirect exciton with large exciton radius, small exciton effective mass and long exciton lifetime. Such system is theoretically predicted to be a promising platform for realizing excitonic Bose-Einstein condensation and superfluid due to its high phase transition temperatures of ~257 K and ~64.25 K for the BEC and excitonic superfluid, respectively. The importance of spin-orbit coupling is revealed, and the angular momentum selection rules for photon absorption are discussed. This finding suggests the potential of QL Bi2Se2Te monolayer with exotic bosonic bound states provides as a tantalizing high-temperature platform to probe excitonic physics.

cond-mat.mtrl-sci↗

arXiv2024

Strongly correlated multi-electron bunches from interaction with quantum light

Strongly correlated electron systems are a cornerstone of modern physics, being responsible for groundbreaking phenomena from superconducting magnets to quantum computing. In most cases, correlations in electrons arise exclusively due to Coulomb interactions. In this work, we reveal that free electrons interacting simultaneously with a light field can become highly correlated via mechanisms beyond Coulomb interactions. In the case of two electrons, the resulting Pearson correlation coefficient (PCC) for the joint probability distribution of the output electron energies is enhanced over 13 orders of magnitude compared to that of electrons interacting with the light field in succession (one after another). These highly correlated electrons are the result of momentum and energy exchange between the participating electrons via the external quantum light field. Our findings pave the way to the creation and control of highly correlated free electrons for applications including quantum information and ultra-fast imaging.

quant-ph↗