arXiv subjects

L. K. Ang

Publications and source records attributed to L. K. Ang.

At least 37 records · Page 2Linked to original sources

arXiv2021

Absence of Space-Charge-Limited Current in Unconventional Field Emission

For field emission (FE), it is widely expected that its emitting current density $J$ will become space-charge-limited current (SCLC) due the built-up of charge in-transit within a gap spacing $D$ biased at sufficiently large voltage $V$. In this paper, we reveal a peculiar finding in which this expected two-stage transition (from FE to SCLC) is no longer valid for FE not obeying the traditional Fowler-Nordheim (FN) law. %Such effect arises when the non-FN based emitters fails to inject sufficient %charge current at high voltage to sustain the SCLC. By employing a generalized FN scaling of $\ln\left(J/V^k\right) \propto - 1/V$, we show the existence of a \emph{critical exponent} $k_c \equiv 3/2$ where unusual behaviours occur for $k < k_c$: (a) Only FE at small $D$ (no transition to SCLC even at infinitely large $V$), and (b) Three-stage transition from FE first to SCLC then back to FE at large $D$. For any $k > k_c$, the conventional two-stage transition from FE to SCLC will always occur for all $D$, which also includes the conventional FN law at $k$ = 2. Using various unconventional FE models with $k \neq 2$, we specifically demonstrate these peculiar transitions. Under a normalized model, our findings uncover the rich interplay between the source-limited FE and bulk-limited SCLC over a wide range of operating conditions.

physics.plasm-ph↗

arXiv2021

Thermal-Field Electron Emission from Three-Dimensional Topological Semimetals

A model is constructed to describe the thermal-field emission of electrons from a three-dimensional ($3$D) topological semimetal hosting Dirac/Weyl node(s). The traditional thermal-field electron emission model is generalised to accommodate the $3$D non-parabolic energy band structures in the topological Dirac/Weyl semimetals, such as cadmium arsenide (\ch{Cd3As2}), sodium bismuthide (\ch{Na3Bi}), tantalum arsenide (\ch{TaAs}) and tantalum phosphide (\ch{TaP}). Due to the unique Dirac cone band structure, an unusual dual-peak feature is observed in the total energy distribution (TED) spectrum. This non-trivial dual-peak feature, absent from traditional materials, plays a critical role in manipulating the TED spectrum and the magnitude of the emission current. At zero temperature limit, a new scaling law for pure field emission is derived and it is different from the well-known Fowler-Nordheim (FN) law. This model expands the recent understandings of electron emission studied for the Dirac $2$D materials into the $3$D regime, and thus offers a theoretical foundation for the exploration in using topological semimetals as novel electrodes.

cond-mat.mes-hall↗

arXiv2020

Two-dimensional van der Waals electrical contact to monolayer MoSi$_2$N$_4$

Two-dimensional (2D) MoSi$_2$N$_4$ monolayer is an emerging class of air-stable 2D semiconductor possessing exceptional electrical and mechanical properties. Despite intensive recent research efforts devoted to uncover the material properties of MoSi$_2$N$_4$, the physics of electrical contacts to MoSi$_2$N$_4$ remains largely unexplored thus far. In this work, we study the van der Waals heterostructures composed of MoSi$_2$N$_4$ contacted by graphene and NbS$_2$ monolayers using first-principle density functional theory calculations. We show that the MoSi$_2$N$_4$/NbS$_2$ contact exhibits an ultralow Schottky barrier height (SBH), which is beneficial for nanoelectronics applications. For MoSi$_2$N$_4$/graphene contact, the SBH can be modulated via interlayer distance or via external electric fields, thus opening up an opportunity for reconfigurable and tunable nanoelectronic devices. Our findings provide insights on the physics of 2D electrical contact to MoSi$_2$N$_4$, and shall offer a critical first step towards the design of high-performance electrical contacts to MoSi$_2$N$_4$-based 2D nanodevices.

cond-mat.mtrl-sci↗

arXiv2020

Generalized scaling law for exciton binding energy in two-dimensional materials

Binding energy calculation in two-dimensional (2D) materials is crucial in determining their electronic and optical properties pertaining to enhanced Coulomb interactions between charge carriers due to quantum confinement and reduced dielectric screening. Based on full solutions of the Schrödinger equation in screened hydrogen model with a modified Coulomb potential ($1/r^{β-2}$), we present a generalized and analytical scaling law for exciton binding energy, $E_β = E_{0}\times \big (\,aβ^{b}+c\big )\, (μ/ε^{2})$, where $β$ is a fractional-dimension parameter accounted for the reduced dielectric screening. The model is able to provide accurate binding energies, benchmarked with the reported Bethe-Salpeter Equation (BSE) and experimental data, for 58 mono-layer 2D and 8 bulk materials respectively through $β$. For a given material, $β$ is varied from $β$ = 3 for bulk 3D materials to a value lying in the range 2.55$-$2.7 for 2D mono-layer materials. With $β_{\text{mean}}$ = 2.625, our model improves the average relative mean square error by 3 times in comparison to existing models. The results can be used for Coulomb engineering of exciton binding energies in the optimal design of 2D materials.

physics.app-ph↗

arXiv2020

Electrical Contact between an Ultrathin Topological Dirac Semimetal and a Two-Dimensional Material

Ultrathin films of topological Dirac semimetal, Na$_3$Bi, has recently been revealed as an unusual electronic materials with field-tunable topological phases. Here we investigate the electronic and transport properties of ultrathin Na$_3$Bi as an electrical contact to two-dimensional (2D) metal, i.e. graphene, and 2D semiconductor, i.e. MoS$_2$ and WS$_2$ monolayers. Using combined first-principle density functional theory and nonequilibrium Green's function simulation, we show that the electrical coupling between Na$_3$Bi bilayer thin film and graphene results in a notable interlayer charge transfer, thus inducing sizable $n$-type doping in the Na$_3$Bi/graphene heterostructures. In the case of MoS$_2$ and WS$_2$ monolayers, the lateral Schottky transport barrier is significantly lower than many commonly studied bulk metals, thus unraveling Na$_3$Bi bilayer as a high-efficiency electrical contact material for 2D semiconductors. These findings opens up an avenue of utilizing topological semimetal thin film as electrical contact to 2D materials, and further expands the family of 2D heterostructure devices into the realm of topological materials.

cond-mat.mes-hall↗

arXiv2020

Broadband strong optical dichroism in topological Dirac semimetals with Fermi velocity anisotropy

Prototypical three-dimensional (3D) topological Dirac semimetals (DSMs), such as Cd$_3$As$_2$ and Na$_3$Bi, contain electrons that obey a linear momentum-energy dispersion with different Fermi velocities along the three orthogonal momentum dimensions. Despite being extensively studied in recent years, the inherent \emph{Fermi velocity anisotropy} has often been neglected in the theoretical and numerical studies of 3D DSMs. Although this omission does not qualitatively alter the physics of light-driven massless quasiparticles in 3D DSMs, it does \emph{quantitatively} change the optical coefficients which can lead to nontrivial implications in terms of nanophotonics and plasmonics applications. Here we study the linear optical response of 3D DSMs for general Fermi velocity values along each direction. Although the signature conductivity-frequency scaling, $σ(ω) \propto ω$, of 3D Dirac fermion is well-protected from Fermi velocity anisotropy, the linear optical response exhibits strong linear dichroism as captured by the \emph{universal} extinction ratio scaling law, $Λ_{ij} = (v_i/v_j)^2$ (where $i\neq j$ denotes the three spatial coordinates $x,y,z$, and $v_i$ is the $i$-direction Fermi velocity), which is independent of frequency, temperature, doping, and carrier scattering lifetime. For Cd$_3$As$_2$ and Na$_3$Bi$_3$, an exceptionally strong extinction ratio larger than 15 and covering broad terahertz window is revealed. Our findings shed new light on the role of Fermi velocity anisotropy in the optical response of Dirac semimetals and open up novel polarization-sensitive functionalities, such as photodetection and light modulation.

cond-mat.mtrl-sci↗

arXiv2020

Efficient generation of extreme terahertz harmonics in 3D Dirac semimetals

Frequency multiplication of terahertz signals on a solid state platform is highly sought-after for the next generation of high-speed electronics and the creation of frequency combs. Solutions to efficiently generate extreme harmonics (up to the $31^{\rm{st}}$ harmonic and beyond) of a terahertz signal with modest input intensities, however, remain elusive. Using fully nonperturbative simulations and complementary analytical theory, we show that 3D Dirac semimetals (DSMs) have enormous potential as compact sources of extreme terahertz harmonics, achieving energy conversion efficiencies beyond $10^{-5}$ at the $31^{\rm{st}}$ harmonic with input intensities on the order of $10$ MW/cm$^2$, over $10^5$ times lower than in conventional THz high harmonic generation systems. Our theory also reveals a fundamental feature in the nonlinear optics of 3D DSMs: a distinctive regime where higher-order optical nonlinearity vanishes, arising as a direct result of the extra dimensionality in 3D DSMs compared to 2D DSMs. Our findings should pave the way to the development of efficient platforms for high-frequency terahertz light sources and optoelectronics based on 3D DSMs.

physics.optics↗

arXiv2019

Super-Andreev reflection and longitudinal shift of pseudospin-one fermions

Novel fermions with a pseudospin-1 structure can be realized as emergent quasiparticles in condensed matter systems. Here, we investigate its unusual properties during the Andreev reflection at a normal-metal/superconductor (NS) interface. We show that distinct from the previously studied pseudospin-1/2 and two dimensional electron gas models, the pseudospin-1 fermions exhibit a strongly enhanced Andreev reflection probability, and remarkably, can be further tuned to approach perfect Andreev reflection with unit efficiency for all incident angles, exhibiting a previously unknown {super-Andreev reflection effect}. The super-Andreev reflection leads to perfect transparency of the NS interface that strongly promotes charge injection into the superconductor, and directly manifests as a differential conductance peak which can be readily probed in experiment. Additionally, we find that sizable longitudinal shifts exist in the normal and Andreev reflections of pseudospin-1 fermions. Distinct from the pseudospin-1/2 case, the shift is always in the forward direction in the subgap regime, regardless of whether the reflection is of retro- or specular type.

cond-mat.mes-hall↗

arXiv2019

Generalized high-energy thermionic electron injection at graphene interface

Graphene thermionic electron emission across high-interface-barrier involves energetic electrons residing far away from the Dirac point where the Dirac cone approximation of the band structure breaks down. Here we construct a full-band model beyond the simple Dirac cone approximation for the thermionic injection of high-energy electrons in graphene. We show that the thermionic emission model based on the Dirac cone approximation is valid only in the graphene/semiconductor Schottky interface operating near room temperature, but breaks down in the cases involving high-energy electrons, such as graphene/vacuum interface or heterojunction in the presence of photon absorption, where the full-band model is required to account for the band structure nonlinearity at high electron energy. We identify a critical barrier height, $Φ_B^{(\text{c})} \approx 3.5$ eV, beyond which the Dirac cone approximation crosses over from underestimation to overestimation. In the high-temperature thermionic emission regime at graphene/vacuum interface, the Dirac cone approximation severely overestimates the electrical and heat current densities by more than 50\% compared to the more accurate full-band model. The large discrepancies between the two models are demonstrated using a graphene-based thermionic cooler. These findings reveal the fallacy of Dirac cone approximation in the thermionic injection of high-energy electrons in graphene. The full-band model developed here can be readily generalized to other 2D materials, and shall provide an improved theoretical avenue for the accurate analysis, modeling and design of graphene-based thermionic energy devices.

cond-mat.mes-hall↗

arXiv2018

Surface exciton polaritons: a promising mechanism for sensing applications

The possibility of constructing a surface exciton polariton (SEP) based sensor at room temperature is explored. The proposed SEP sensor is based on the Kretschmann-Raether configuration of conventional surface plasmon resonance (SPR) sensor, where the metal thin film was replaced by the J-aggregate cyanine dye film. The excitation of SEP results in a strong electric field at the interface of TDBC and analyte, and exponentially decaying into the analyte, which is sensitive to the refractive index variations of analyte. The sensitivity of 118.1818 $^\circ/\text{RIU}$ (140.4286 $^\circ/\text{RIU}$) was achieved for the proposed SEP sensor within the refractive index range 1.0-1.001 (1.33-1.36) of gaseous analyte (aqueous solutions), which is about 2 (3.5) times higher than that of conventional gold-based SPR sensor. The significant superiority of the SEP sensor in sensitivity revealing SEP as a new promising mechanism for sensing applications.

cond-mat.mes-hall↗

arXiv2018

Fractional Fresnel coefficients for optical absorption in femtosecond laser-induced rough metal surfaces

The surface morphology of metal influences its optical absorptivity. Recent experiments have demonstrated that the femtosecond laser induced surface structures on metals could be dynamically controlled by the fluence of laser and the number of pulses. In this paper, we formulate an analytical model to calculate the optical absorption of a rough metallic surface by modeling the roughness as a fractal slab. For a given experimental image of the surface roughness, we characterize the roughness with a fractal parameter by using box-counting method. With this parameter as input, we calculate the absorption of 800 nm laser pulse impinging on gold, copper and platinum, and the calculated results show excellent agreements. In terms of physics, our model can be viewed as a fractional version of the Fresnel coefficients, and it will be useful for designing suitable surface structures to tune the light absorption on metals from purely reflective to highly absorptive based on different applications.

physics.app-ph↗

arXiv2018

Universal Scaling Laws in Schottky Heterostructures Based on Two-Dimensional Materials

We identify a new universality in the carrier transport of two-dimensional(2D)-material-based Schottky heterostructures. We show that the reversed saturation current ($\mathcal{J}$) scales universally with temperature ($T$) as $ \log(\mathcal{J}/T^β) \propto -1/T$, with $β= 3/2$ for lateral Schottky heterostructures and $β= 1$ for vertical Schottky heterostructures, over a wide range of 2D systems including nonrelativistic electron gas, Rashba spintronic system, single and few-layer graphene, transition metal dichalcogenides and thin-films of topological solids. Such universalities originate from the strong coupling between the thermionic process and the in-plane carrier dynamics. Our model resolves some of the conflicting results from prior works and is in agreement with recent experiments. The universal scaling laws signal the breakdown of $β=2$ scaling in the classic diode equation widely-used over the past 60 years. Our findings shall provide a simple analytical scaling for the extraction of the Schottky barrier height in 2D-material-based heterostructure, thus paving way for both fundamental understanding of nanoscale interface physics and applied device engineering.

cond-mat.mes-hall↗

arXiv2017

Fractional Fowler-Nordheim Law for Field Emission from Rough Surface with Nonparabolic Energy Dispersion

The theories of field electron emission from perfectly planar and smooth canonical surfaces are well understood, but they are not suitable for describing emission from rough, irregular surfaces arising in modern nanoscale electron sources. Moreover, the existing models rely on Sommerfeld's free-electron theory for the description of electronic distribution which is not a valid assumption for modern materials with nonparabolic energy dispersion. In this paper, we derive analytically a generalized Fowler-Nordheim (FN) type equation that takes into account the reduced space-dimensionality seen by the quantum mechanically tunneling electron at a rough, irregular emission surface. We also consider the effects of non-parabolic energy dispersion on field-emission from narrow-gap semiconductors and few-layer graphene using Kane's band model. The traditional FN equation is shown to be a limiting case of our model in the limit of a perfectly flat surface of a material with parabolic dispersion. The fractional-dimension parameter used in this model can be experimentally calculated from appropriate current-voltage data plot. By applying this model to experimental data, the standard field-emission parameters can be deduced with better accuracy than by using the conventional FN equation.

physics.acc-ph↗

arXiv2017

Valleytronics in merging Dirac cones: All-electric-controlled valley filter, valve and universal reversible logic gate

Despite much anticipation of valleytronics as a candidate to replace the ageing CMOS-based information processing, its progress is severely hindered by the lack of practical ways to manipulate valley polarization all-electrically in an electrostatic setting. Here we propose a class of all-electric-controlled valley filter, valve and logic gate based on the valley-contrasting transport in a merging Dirac cones system. The central mechanism of these devices lies on the pseudospin-assisted quantum tunneling which effectively quenches the transport of one valley when its pseudospin configuration mismatches that of a gate-controlled scattering region. The valley polarization can be abruptly switched into different states and remains stable over semi-infinite gate-voltage windows. Colossal tunneling valley-pseudo-magnetoresistance ratio of over 10,000\% can be achieved in a valley-valve setup. We further propose a valleytronic-based logic gate capable of covering all 16 types of two-input Boolean logics. Remarkably, the valley degree of freedom can be harnessed to resurrect logical-reversibility in two-input universal Boolean gate. The (2+1) polarization states -- two distinct valleys plus a null polarization -- re-establish one-to-one input-to-output mapping, a crucial requirement for logical-reversibility, and significantly reduce the complexity of reversible circuits due to the built-in nature of valley degree of freedom. Our results suggest that the synergy of valleytronics and digital logics may provide new paradigms for valleytronic-based information processing and reversible computing.

cond-mat.mes-hall↗

arXiv2017

Generalized Fowler-Nordheim field-induced vertical electron emission model for two-dimensional materials

Current theoretical description of field-induced electron emission remains mostly bounded by the classic Fowler-Nordheim (FN) framework developed nearly one century ago. For the emerging class of two-dimensional (2D) materials, many basic assumptions of FN model become invalid due to their reduced dimensionality and exotic electronic properties. In this work, we develop analytical and semi-analytical models of field-induced vertical electron emission from the surface of 2D materials by explicitly taking into account the reduced dimensionality, non-parabolic energy spectrum, non-conserving in-plane electron momentum, finite-temperature and space-charge-limited effects. We show that the traditional FN law is no longer valid for 2D materials. The modified vertical field emission model developed here provides better agreement with experimental results. Intriguingly, a new high-field regime of \emph{saturated surface field emission} emerges due to the reduced dimensionality of 2D materials. A remarkable consequence of this saturated field emission effect is the absence of space-charge-limited current normally expected at high field in three-dimensional bulk material.

cond-mat.mes-hall↗

arXiv2017

Shortcut to adiabatic light transfer in waveguide couplers with a sign flip in the phase mismatch

Employing counterdiabatic shortcut to adiabaticity (STA), we design shorter and robust achromatic two- and three- waveguide couplers. We assume that the phase mismatch between the waveguides has a sign flip at maximum coupling, while the coupling between the waveguides has a smooth spatial shape. We show that the presented coupler operates as a complete achromatic optical switch for two coupled waveguides and as an equal superposition beam splitter for three coupled waveguides. An important feature of our devices is that they do not require larger coupling strength as compared to previous designs, which make them easier to realize in an experimental setting. Additionally, we show that the presented waveguide couplers operate at a shorter device length and are robust against variations in the coupling strength and the phase mismatch.

quant-ph↗

arXiv2017

Ultrasensitive and highly accurate long-range surface plasmon resonance biosensors based on two-dimensional transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (TMDCs), as promising alternative plasmonics supporting materials to graphene, exhibit potential applications in sensing. Here, we propose an ultrasensitive, accurate long-range surface plasmon resonance (LRSPR) imaging biosensor with two-dimensional TMDC layers, which shows higher detection accuracy than that of conventional SPR biosensor. It is found that the imaging sensitivity of the proposed LRSPR biosensor can be enhanced by the integration of TMDC layers, which is different from the previous graphene-based LRSPR or SPR imaging sensor, whose imaging sensitivity usually decreases with the number of graphene layers. The sensitivity enhancement or degradation effect for the proposed chalcogenide-cytop-gold-TMDCs based biosensor depends on the thickness of gold thin film and cytop layer. Imaging sensitivity of more than 4000 $\text{RIU}^{-1}$ can be obtained with a high detection accuracy of more than 120 $\text{deg}^{-1}$. We expect that the proposed TMDCs mediated LRSPR imaging sensor could provide potential applications in chemical sensing and biosensing for a highly sensitive and accurate simultaneous detection of multiple biomolecular interactions.

physics.app-ph↗

arXiv2017

Electrical transport and persistent photoconductivity in monolayer MoS2 phototransistors

We study electrical transport properties in exfoliated molybdenum disulfide (MoS2) back-gated field effect transistors at low drain bias and under different illumination intensities. It is found that photoconductive and photogating effect as well as space charge limited conduction can simultaneously occur. We point out that the photoconductivity increases logarithmically with the light intensity and can persist with a decay time longer than 10^4 s, due to photo-charge trapping at the MoS2/SiO2 interface and in MoS2 defects. The transfer characteristics present hysteresis that is enhanced by illumination. At low drain bias, the devices feature low contact resistance of 1.4 kΩ/μm, ON current as high as 1.25 nA/μm, 10^5 ON-OFF ratio, mobility of 1 cm^2/Vs and photoresponsivity R=1 A/W.

cond-mat.mes-hall↗