Search

We revisit the fundamentals of two different methods for calculating classical observables: the eikonal method, which is a scattering amplitude-based method, and the worldline quantum field theory (WQFT) method. The latter has been considered an extension of the worldline effective field theory. We show that the eikonal and WQFT methods are equivalent and that calculations can be translated freely between them. Concretely, we focus on 2-into-2 scattering processes mediated by massless force carriers. On the one hand, taking the classical limit of the QFT scattering amplitude leads to the eikonal method. On the other hand, since in the classical limit the scattering particles are almost on-shell throughout the scattering process, the worldline, a first quantized formalism, is the most efficient framework to study the scattering amplitude. This is an alternate but equivalent formalism to the quantum field theoretic (QFT) framework. By taking the classical limit of the scattering amplitude computed in the worldline, we can derive the WQFT rules of Mogull, Plefka and Steinhoff. In WQFT, the Feynman diagrams are reorganized into a new set of diagrams that facilitate the $\hbar$ expansion. Unlike the QFT eikonal method, which works recursively in identifying the eikonal phase, the worldline-based computation allows to target and systematically extract the classical contributions directly through a specific set of WQFT diagrams. In worldline formalism the perturbative expansion of the scattering amplitude is naturally organized in diagrams which factorize (reducible) and diagrams which are new to that order (irreducible), in a one-to-one map with the structure of the amplitude in the eikonal method. This opened up the possibility to investigate and prove the conjectured exponentiation of the eikonal phase in arXiv: 2409.12895., Comment: 75 pages

In this paper, working in the eikonal approximation, we present a proof for the exponentiation of the 2-body eikonal phase to {\it all orders in the eikonal expansion}, for scalar particles interacting electromagnetically or gravitationally. The proof is based on the worldline formalism, which is an alternative, first quantized method to the standard QFT calculation of the scattering amplitude. We show that in the worldline formalism the 2-body scattering amplitude written in impact parameter space naturally factorizes at each loop order. This factorization is responsible for the exponentiation of the eikonal phase, a result which was anticipated in the work of Mogull, Plefka, and Steinhoff [2010.02865 [hep-th]]., Comment: 40 pages

This article presents a comprehensive sentiment analysis (SA) of comments on YouTube videos related to Sidewalk Delivery Robots (SDRs). We manually annotated the collected YouTube comments with three sentiment labels: negative (0), positive (1), and neutral (2). We then constructed models for text sentiment classification and tested the models' performance on both binary and ternary classification tasks in terms of accuracy, precision, recall, and F1 score. Our results indicate that, in binary classification tasks, the Support Vector Machine (SVM) model using Term Frequency-Inverse Document Frequency (TF-IDF) and N-gram get the highest accuracy. In ternary classification tasks, the model using Bidirectional Encoder Representations from Transformers (BERT), Long Short-Term Memory Networks (LSTM) and Gated Recurrent Unit (GRU) significantly outperforms other machine learning models, achieving an accuracy, precision, recall, and F1 score of 0.78. Additionally, we employ the Latent Dirichlet Allocation model to generate 10 topics from the comments to explore the public's underlying views on SDRs. Drawing from these findings, we propose targeted recommendations for shaping future policies concerning SDRs. This work provides valuable insights for stakeholders in the SDR sector regarding social perception, interaction, and safety., Comment: 34 pages, 7 figures, 2 tables

With the rise in demand for local deliveries and e-commerce, robotic deliveries are being considered as efficient and sustainable solutions. However, the deployment of such systems can be highly complex due to numerous factors involving stochastic demand, stochastic charging and maintenance needs, complex routing, etc. We propose a model that uses continuous approximation methods for evaluating service trade-offs that consider the unique characteristics of large-scale sidewalk delivery robot systems used to serve online food deliveries. The model captures both the initial cost and the operation cost of the delivery system and evaluates the impact of constraints and operation strategies on the deployment. By minimizing the system cost, variables related to the system design can be determined. First, the minimization problem is formulated based on a homogeneous area, and the optimal system cost can be derived as a closed-form expression. By evaluating the expression, relationships between variables and the system cost can be directly obtained. We then apply the model in neighborhoods in New York City to evaluate the cost of deploying the sidewalk delivery robot system in a real-world scenario. The results shed light on the potential of deploying such a system in the future.

We use the worldline formalism to study tree-level scattering processes involving gravitons. A massless spin 2 particle is described by an $N=4$ supersymmetric worldline action which is also $O(4)$ symmetric. More generally, $N=2S$ supersymmetric worldline actions exhibiting $O(N)$ symmetry describe free spin $S$ particles. Recently a BRST approach was used to construct the on-shell background graviton emission vertex from a graviton worldline. Nonetheless, an action describing the coupling of higher spin ($S\geq 2$) particles with generic background gravity is unknown. In this paper, we found that in order to reproduce Einstein's general relativity 3-point graviton vertex, interpreted as the emission of an off-shell graviton from the worldline, the coupling to background gravity must break the $O(4)$ symmetry to $O(2) \times O(2)$. In addition to this symmetry-breaking feature, we also found that the coefficient $\beta$ of the worldline action counterterm $\beta R$ differs from previous results in the literature. By comparing the linearized graviton and photon emission vertex operators from different worldlines, we noticed that they obey a squaring relation. For MHV (Maximal Helicity Violating) amplitudes, these squaring relations among the linearized vertex operators directly result in double-copy-like relations between the scattering amplitudes., Comment: 63 pages

Allowing for the possibility of extra dimensions, there are two paradigms: either the extra dimensions are hidden from observations by being compact and small as in Kaluza-Klein scenarios, or the extra dimensions are large/non-compact and undetectable due to a large warping as in the Randall-Sundrum scenario. In the latter case, the five-dimensional background has a large curvature, and Isaacson's construction of the gravitational energy-momentum tensor, which relies on the assumption that the wavelength of the metric fluctuations is much smaller than the curvature length of the background spacetime, cannot be used. In this paper, we construct the gravitational energy-momentum tensor in a strongly curved background such as Randall-Sundrum. We perform a scalar-vector-tensor decomposition of the metric fluctuations with respect to the $SO(1,3)$ background isometry and construct the covariantly-conserved gravitational energy-momentum tensor out of the gauge-invariant metric fluctuations. We give a formula for the power radiated by gravitational waves and verify it in known cases. In using the gauge-invariant metric fluctuations to construct the gravitational energy-momentum tensor we follow previous work done in cosmology. Our framework has applicability beyond the Randall-Sundrum model., Comment: 54 pages, minor typos fixed, section 4.A expanded, conclusions added

We study the effect of compact extra dimensions on the gravitational wave luminosity and waveform. We consider a toy model, with a compactified fifth dimension, and matter confined on a brane. We work in the context of five dimensional ($5d$) general relativity, though we do make connections with the corresponding Kaluza-Klein effective $4d$ theory. We show that the luminosity of gravitational waves emitted in $5d$ gravity by a binary with the same characteristics (same masses and separation distance) as a $4d$ binary is 20.8\% less relative to the $4d$ case, to leading post-Newtonian order. The phase of the gravitational waveform differs by 26\% relative to the $4d$ case, to leading post-Newtonian order. Such a correction arises mainly due to the coupling between matter and dilaton field in the effective $4d$ picture and agrees with previous calculations when we set black holes' scalar charges to be those computed from the Kaluza-Klein reduction. The above corrections to the waveform and the luminosity are inconsistent with the gravitational-wave and binary pulsar observations and thus they effectively rule out the possibility of such a simple compactified higher dimensions scenario. We also comment on how our results change if there are several compactified extra dimensions, and show that the discrepancy with $4d$ general relativity only increases., Comment: 33 pages, 1 figure, updated references, expanded discussion

Experimental demonstrations of 1D van der Waals material tellurium have been presented by Raman spectroscopy under strain and magneto-transport. Raman spectroscopy measurements have been performed under strains along different principle axes. Pronounced strain response along c-axis is observed due to the strong intra-chain covalent bonds, while no strain response is obtained along a-axis due to the weak inter-chain van der Waals interaction. Magneto-transport results further verify its anisotropic property, resulting in dramatically distinct magneto-resistance behaviors in terms of three different magnetic field directions. Specifically, phase coherence length extracted from weak antilocalization effect, L$_{\Phi}$ ~ T$^{-0.5}$, claims its 2D transport characteristics when an applied magnetic field is perpendicular to the thin film. In contrast, L$_{\Phi}$ ~ T$^{-0.33}$ is obtained from universal conductance fluctuations once the magnetic field is along c-axis of Te, indicating its nature of 1D transport along the helical atomic chains. Our studies, which are obtained on high quality single crystal tellurium thin film, appear to serve as strong evidences of its 1D van der Waals structure from experimental perspectives. It is the aim of this paper to address this special concept that differs from the previous well-studied 1D nanowires or 2D van der Waals materials.

The reliable production of two-dimensional crystals is essential for the development of new technologies based on 2D materials. However, current synthesis methods suffer from a variety of drawbacks, including limitations in crystal size and stability. Here, we report the fabrication of large-area, high-quality 2D tellurium (tellurene) using a substrate-free solution process. Our approach can create crystals with a process-tunable thickness, from monolayer to tens of nanometres, and with lateral sizes of up to 100 um. The chiral-chain van der Waals structure of tellurene gives rise to strong in-plane anisotropic properties and large thickness dependent shifts in Raman vibrational modes, which is not observed in other 2D layered materials. We also fabricate tellurene field-effect transistors, which exhibit air-stable performance at room temperature for over two months, on off ratios on the order of 106 and field-effect mobilities of around 700 cm2 per Vs. Furthermore, by scaling down the channel length and integrating with high-k dielectrics, transistors with a significant on-state current density of 1 A mm-1 are demonstrated.

In this letter, a new approach to chemically dope black phosphorus (BP) is presented, which significantly enhances the device performance of BP field-effect transistors for an initial period of 18 h, before degrading to previously reported levels. By applying 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), low ON-state resistance of 3.2 ohm.mm and high field-effect mobility of 229 cm2/Vs are achieved with a record high drain current of 532 mA/mm at a moderate channel length of 1.5 {\mu}m., Comment: Published in IEEE Electron Device Letters, VOL. 37, NO. 4, APRIL 2016

The Poisson's ratio of a material characterizes its response to uniaxial strain. Materials normally possess a positive Poisson's ratio - they contract laterally when stretched, and expand laterally when compressed. A negative Poisson's ratio is theoretically permissible but has not, with few exceptions of man-made bulk structures, been experimentally observed in any natural materials. Here, we show that the negative Poisson's ratio exists in the low-dimensional natural material black phosphorus, and that our experimental observations are consistent with first principles simulations. Through application of uniaxial strain along zigzag and armchair directions, we find that both interlayer and intralayer negative Poisson's ratios can be obtained in black phosphorus. The phenomenon originates from the puckered structure of its in-plane lattice, together with coupled hinge-like bonding configurations.

Transition metal pentatelluride ZrTe5 is a versatile material in condensed-matter physics and has been intensively studied since the 1980s. The most fascinating feature of ZrTe5 is that it is a 3D Dirac semimetal which has linear energy dispersion in all three dimensions in momentum space. Structure-wise, ZrTe5 is a layered material held together by weak interlayer van der Waals force. The combination of its unique band structure and 2D atomic structure provides a fertile ground for more potential exotic physical phenomena in ZrTe5 related to 3D Dirac semimentals. However the physical properties of its few-layer form have yet to be thoroughly explored. Here we report strong optical and electrical in-plane anisotropy of mechanically exfoliated few-layer ZrTe5. Raman spectroscopy shows significant intensity change with sample orientations, and the behavior of angle-resolved phonon modes at the gamma point is explained by theoretical calculation. DC conductance measurement indicates a 50% of difference along different in-plane directions. The diminishing of resistivity anomaly in few-layer samples indicates the evolution of band structure with reduced thickness. Low-temperature Hall experiment sheds lights on more intrinsic anisotropic electrical transport, with hole mobility of 3,000 and 1,500 cm2/Vs along a-axis and c-axis respectively. Pronounced quantum oscillations in magneto-resistance are observed at low temperatures with highest electron mobility up to 44,000 cm2/Vs.

Few-layer phosphorene MOSFETs with 0.3-um-long gate and 15-nm-thick Al2O3 gate insulator was found to exhibit a forward-current cutoff frequency of 2 GHz and a maximum oscillation frequency of 8 GHz after de-embedding for the parasitic capacitance associated mainly with the relatively large probe pads. The gate lag and drain lag of the transistor was found to be on the order of 1 us or less, which is consistent with the lack of hysteresis, carrier freeze-out or persistent photoconductivity in DC characteristics. These results confirm that the phosphorene MOSFET can be a viable microwave transistor for both small-signal and large-signal applications., Comment: Accepted for oral presentation at IMS 2016

We have conducted a comprehensive investigation into the magneto-transport properties of few-layer black phosphorus in terms of phase coherence length, phase coherence time, and mobility via weak localization measurement and Hall-effect measurement. We present magnetoresistance data showing the weak localization effect in bare p-type few-layer black phosphorus and reveal its strong dependence on temperature and carrier concentration. The measured weak localization agrees well with the Hikami-Larkin-Nagaoka model and the extracted phase coherence length of 104 nm at 350 mK, decreasing as ~T^-0.51+-0.05 with increased temperature. Weak localization measurement allows us to qualitatively probe the temperature-dependent phase coherence time {\tau}, which is in agreement with the theory of carrier interaction in the diffusive regime. We also observe the universal conductance fluctuation phenomenon in few-layer black phosphorus within moderate magnetic field and low temperature regime., Comment: To be published in 2D Materials

Black phosphorus has been revisited recently as a new two-dimensional material showing potential applications in electronics and optoelectronics. Here we report the anisotropic in-plane thermal conductivity of suspended few-layer black phosphorus measured by micro-Raman spectroscopy. The armchair and zigzag thermal conductivities are ~20 and ~40 W m$^{-1}$ K$^{-1}$ for black phosphorus films thicker than 15 nm, respectively, and decrease to ~10 and ~20 W m$^{-1}$ K$^{-1}$ as the film thickness is reduced, exhibiting significant anisotropy. The thermal conductivity anisotropic ratio is found to be ~2 for thick black phosphorus films and drops to ~1.5 for the thinnest 9.5-nm-thick film. Theoretical modeling reveals that the observed anisotropy is primarily related to the anisotropic phonon dispersion, whereas the intrinsic phonon scattering rates are found to be similar along the armchair and zigzag directions. Surface scattering in the black phosphorus films is shown to strongly suppress the contribution of long-mean-free-path acoustic phonons., Comment: Accepted by Nature Communications

Phosphorus is one of the most abundant elements preserved in earth, constructing with a fraction of ~0.1% of the earth crust. In general, phosphorus has several allotropes. The two most commonly seen allotropes, white and red phosphorus, are widely used in explosives and safety matches. In addition, black phosphorus, though rarely mentioned, is a layered semiconductor and have great potentials in optical and electronic applications. Remarkably, this layered material can be reduced to one single atomic layer in the vertical direction owing to the van der Waals structure, known as phosphorene, where the physical properties can be tremendously different from its bulk counterpart. In this review article, we trace back to the 100 years research history on black phosphorus from the synthesis to material properties, and extend the topic from black phosphorus to phosphorene. The physical and transport properties are highlighted, aiming at further applications in electronic and optoelectronics devices., Comment: Invited review article published by Chemical Society Review (2014)

Layered two-dimensional (2D) semiconducting transition metal dichalcogenides (TMD) have been widely isolated, synthesized, and characterized recently. Numerous 2D materials are identified as the potential candidates as channel materials for future thin film technology due to their high mobility and the exhibiting bandgaps. While many TMD filed-effect transistors (FETs) have been widely demonstrated along with a significant progress to clearly understand the device physics, large contact resistance at metal/semiconductor interface still remain a challenge. From 2D device research point of view, how to minimize the Schottky barrier effects on contacts thus reduce the contact resistance of metals on 2D materials is very critical for the further development of the field. Here, we present a review of contact research on molybdenum disulfide and other TMD FETs from the fundamental understanding of metal-semiconductor interfaces on 2D materials. A clear contact research strategy on 2D semiconducting materials is developed for future high-performance 2D FETs with aggressively scaled dimensions., Comment: Published in APL Materials

Low-resistivity metal-semiconductor (M-S) contact is one of the urgent challenges in the research of 2D transition metal dichalcogenides (TMDs). Here, we report a chloride molecular doping technique which greatly reduces the contact resistance (Rc) in the few-layer WS2 and MoS2. After doping, the Rc of WS2 and MoS2 have been decreased to 0.7 kohm*um and 0.5 kohm*um, respectively. The significant reduction of the Rc is attributed to the achieved high electron doping density thus significant reduction of Schottky barrier width. As a proof-ofconcept, high-performance few-layer WS2 field-effect transistors (FETs) are demonstrated, exhibiting a high drain current of 380 uA/um, an on/off ratio of 4*106, and a peak field-effect mobility of 60 cm2/V*s. This doping technique provides a highly viable route to diminish the Rc in TMDs, paving the way for high-performance 2D nano-electronic devices., Comment: published in Nano Letters ASAP

This letter evaluates temporal and thermal stability of a state-of-the-art few-layer phosphorene MOSFET with Al2O3 surface passivation and Ti/Au top gate. As fabricated, the phosphorene MOSFET was stable in atmosphere for at least 100 h. With annealing at 200{\deg}C in dry nitrogen for 1 h, its drain current increased by an order of magnitude to approximately 100 mA/mm, which could be attributed to the reduction of trapped charge in Al2O3 and/or Schottky barrier at the source and drain contacts. Thereafter, the drain current was stable between -50{\deg}C and 150{\deg}C up to at least 1000 h. These promising results suggest that environmental protection of phosphorene should not be a major concern, and passivation of phosphorene should focus on its effect on electronic control and transport as in conventional silicon MOSFETs. With cutoff frequencies approaching the gigahertz range, the present phosphorene MOSFET, although far from being optimized, can meet the frequency and stability requirements of most flexible electronics for which phosphorene is intrinsically advantageous due to its corrugated lattice structure., Comment: To be published in IEEE Electron Device Letters

Although monolayer black phosphorus (BP) or phosphorene has been successfully exfoliated and its optical properties have been explored, most of electrical performance of the devices is demonstrated on few-layer phosphorene and ultra-thin BP films. In this paper, we study the channel length scaling of ultra-thin BP field-effect transistors (FETs), and discuss a scheme for using various contact metals to change transistor characteristics. Through studying transistor behaviors with various channel lengths, the contact resistance can be extracted from the transfer length method (TLM). With different contact metals, we find out that the metal/BP interface has different Schottky barrier heights, leading to a significant difference in contact resistance, which is quite different from previous studies of transition metal dichalcogenides (TMDs) such as MoS2 where Fermi-level is strongly pinned near conduction band edge at metal/MoS2 interface. The nature of BP transistors are Schottky barrier FETs, where the on and off states are controlled by tuning the Schottky barriers at the two contacts. We also observe the ambipolar characteristics of BP transistors with enhanced n-type drain current and demonstrate that the p-type carriers can be easily shifted to n-type or vice versus by controlling the gate bias and drain bias, showing the potential to realize BP CMOS logic circuits., Comment: Published in ACS Nano