arXiv subjects

Nicholas Ng

Publications and source records attributed to Nicholas Ng.

11 recordsLinked to original sources

arXiv2026

Building real-time digital twin instances with Function+Data Flow: user evaluation and extension for iterative pipelines

Digital twins (DTs) increasingly leverage artificial intelligence (AI) and machine learning (ML) pipelines, both to build real-time DTs from high-fidelity simulations and to instantiate them with historical data. However, engineering these pipelines remains largely ad-hoc: pipelines are hard to specify, validate, and reuse, with scarce dedicated tooling. Function+Data Flow (FDF) addresses this by defining a visual domain-specific language (DSL) that represents functions (ML models) explicitly, enabling their composition and reuse. We implemented FDF in DesCartes Builder, an integrated modeling environment supporting FDF-based DT synthesis and validation. In this paper, we report on an empirical user study evaluating whether FDF and DesCartes Builder can make AI-based DT development more accessible and reliable. Participants implemented a representative real-time DT prototype within DesCartes Builder, and we measured perceived usability and feature adequacy through quantitative and qualitative measures. Our results indicate that DesCartes Builder and FDF achieve a good level of usability across a broad range of potential users, and particularly for the intended audience of domain experts. The study additionally surfaces concrete strengths and areas for improvement of both the tool and the underlying FDF framework. Informed by these findings, we propose H-FDF, a Hierarchical extension of FDF supporting iterative and modular pipelines, enabling the formal specification of more complex DT pipelines such as dual training. Our findings suggest that integrated, model-driven platforms are a promising direction to transform AI-based DT engineering into a disciplined modeling practice.

cs.SE↗

arXiv2026

Comparison Geometry on Manifolds with Density via Modified Hessians

Comparison geometry for Bakry-\'Emery Ricci curvature has been extensively developed by Wei-Wylie and others. Motivated by the weighted sectional curvature framework introduced by Wylie and further developed by Kennard-Wylie-Yeroshkin, we study radial comparison geometry on manifolds with density through a modified Hessian arising from this framework. Under nonnegative weighted sectional curvature together with suitable density control assumptions, we obtain a modified Hessian estimate for the radial function $u = \frac{1}{2}r^2$. From this estimate, we derive Hessian comparison, shape operator comparison, weighted Laplacian comparison, asymptotic radial volume density estimates, and polynomial weighted volume growth bounds. We introduce a normalized weighted radial volume density satisfying a monotonicity property analogous to the radial volume density monotonicity underlying Bishop-Gromov comparison. We also study rigidity phenomena associated with these comparison estimates. Equality in the Hessian comparison theorem yields radial conformal rigidity, while equality in the modified Hessian estimate forces the metric to have an exact metric cone structure.

math.DG↗

arXiv2025

Competing magnetic phases in Cr$_{3+\delta}$Te$_4$ are spatially segregated

Cr$_{1+x}$Te$_2$ is a self-intercalated vdW system that is of current interest for its room-temperature FM phases and tunable topological properties. Early NPD measurements on the monoclinic phase Cr$_3$Te$_4$ ($x=0.5$) presented evidence for competing FM and AFM phases. Here we apply neutron diffraction to a single crystal of Cr$_{3+\delta}$Te$_4$ with $\delta=-0.10$ and discover that it consists of two distinct monoclinic phases, one with FM order below $T_{\rm C} \approx 321$ K and another that develops AFM order below $T_{\rm N} \approx 86$ K. In contrast, we find that a crystal with $\delta=-0.26$ exhibits only FM order. The single-crystal analysis is complemented by results obtained with NPD, XPD, and TEM measurements on the $\delta=-0.10$ composition. From observations of spontaneous magnetostriction of opposite sign at $T_{\rm C}$ and $T_{\rm N}$, along with the TEM evidence for both monoclinic phases in a single thin ($\approx$ 100 nm) grain, we conclude that the two phases must have a fine-grained ($\lesssim$ 100 nm) intergrowth character, as might occur from high-temperature spinodal decomposition during the growth process. Calculations of the relaxed lattice structures for the FM and AFM phases with DFT provide a rationalization of the observed spontaneous magnetostrictions. Correlations between the magnitude and orientation of the magnetic moments with lattice parameter variation demonstrate that the magnetic orders are sensitive to strain, thus explaining why magnetic ordering temperatures and anisotropies can be different between bulk and thin-film samples, when the latter are subject to epitaxial strain. Our results point to the need to investigate the supposed coexistence FM and AFM phases reported elsewhere in the Cr$_{1+x}$Te$_2$ system, such as in the Cr$_5$Te$_8$ phase ($x=0.25$).

cond-mat.str-el↗

arXiv2025

DesCartes Builder: A Tool to Develop Machine-Learning Based Digital Twins

Digital twins (DTs) are increasingly utilized to monitor, manage, and optimize complex systems across various domains, including civil engineering. A core requirement for an effective DT is to act as a fast, accurate, and maintainable surrogate of its physical counterpart, the physical twin (PT). To this end, machine learning (ML) is frequently employed to (i) construct real-time DT prototypes using efficient reduced-order models (ROMs) derived from high-fidelity simulations of the PT's nominal behavior, and (ii) specialize these prototypes into DT instances by leveraging historical sensor data from the target PT. Despite the broad applicability of ML, its use in DT engineering remains largely ad hoc. Indeed, while conventional ML pipelines often train a single model for a specific task, DTs typically require multiple, task- and domain-dependent models. Thus, a more structured approach is required to design DTs. In this paper, we introduce DesCartes Builder, an open-source tool to enable the systematic engineering of ML-based pipelines for real-time DT prototypes and DT instances. The tool leverages an open and flexible visual data flow paradigm to facilitate the specification, composition, and reuse of ML models. It also integrates a library of parameterizable core operations and ML algorithms tailored for DT design. We demonstrate the effectiveness and usability of DesCartes Builder through a civil engineering use case involving the design of a real-time DT prototype to predict the plastic strain of a structure.

cs.SE↗

arXiv2023

High magnetic anisotropy and magnetocaloric effects in single crystal Cr$_2$Te$_3$

We report a systematic investigation of anisotropic magnetocaloric effects in single crystal Cr$_2$Te$_3$. Single crystal samples are synthesized by chemical vapor transport and characterized by x-ray and Laue diffraction methods. The maximum magnetic entropy change $-\Delta S_{\text M}^{\text{max}}$ is 4.50 J kg$^{-1}$ K$^{-1}$ for the easy c-axis (3.36 J kg$^{-1}$ K$^{-1}$ for the hard ab-plane) and the relative cooling power RCP is 296.7 J kg$^{-1}$ for the easy c-axis (183.84 J kg$^{-1}$ for the hard axis ab-plane) near the Curie temperature for a magnetic field change of 9 T. The magneto-crystalline anisotropy constant K$_u$ is estimated to be 486.92 kJ m$^{-3}$ at 146 K, decreasing to 148.60 kJ m$^{-3}$ at 168 K. Meanwhile, the maximum of the rotational magnetic entropy change $\Delta S_{\text M}^{\text R}(T, H)$ between the c-axis and the ab-plane is about 1.14 J kg$^{-1}$K$^{-1}$ for magnetic-field change of 9 T. The critical exponents are estimated by analyzing magnetocaloric effects, which indicate 2D-Ising type magnetic system. The accuracy of estimated critical exponents is verified by scaling analysis. The maximum magnetic entropy change $-\Delta S_{\text M}^{\text{max}}$ $\approx$5.25 J kg$^{-1}$ K$^{-1}$ (along the c-axis) and the corresponding adiabatic temperature change $\Delta T_{\text{ad}}$ $\approx$3.31 K (along the c-axis) are estimated by analyzing heat capacity measurements with a magnetic field up to 9 Tesla.

cond-mat.mtrl-sci↗

arXiv2023

On homogeneous closed gradient Laplacian solitons

We prove a structure theorem for homogeneous closed gradient Laplacian solitons and use it to show some examples of closed Laplacian solitons cannot be made gradient. More specifically, we show that the Laplacian solitons on nilpotent Lie groups found by Nicolini are not gradient up to homothetic $G_2$-structures except for $N_1$, where $f$ must be a Gaussian. We also show that the closed $G_2$-structure $\varphi_{12}$ on $N_{12}$ constructed by Fern\'andez-Fino-Manero cannot be a gradient soliton. We further show that closed non-torsion-free gradient Laplacian solitons on almost abelian solvmanifolds are isometric to products $N \times \mathbb R^k$ with $f$ constant on $N$.

math.DG↗

arXiv2022

Misfit Layered Compounds: Unique, Tunable Heterostructured Materials with Untapped Properties

Building on discoveries in graphene and two-dimensional (2D) transition metal dichalcogenides, van der Waals (VdW) layered heterostructures - stacks of such 2D materials - are being extensively explored with resulting new discoveries of novel electronic and magnetic properties in the ultrathin limit. Here we review a class of naturally occurring heterostructures - so called misfits - that combine disparate VdW layers with complex stacking. Exhibiting remarkable structural complexity and diversity of phenomena, misfits provide a platform on which to systematically explore the energetics and local bonding constraints of heterostructures and how they can be used to engineer novel quantum fabrics, electronic responsiveness, and magnetic phenomena. Like traditional classes of layered materials, they are often exfoliatable and thus also incorporatable as units in manually or robotically stacked heterostructures. Here we review the known classes of misfit structures, the tools for their single crystal and thin film synthesis, the physical properties they exhibit, and computational and characterization tools that are unraveling their complexity. Directions for future research are also discussed.

cond-mat.mtrl-sci↗

arXiv2022

Single Crystal Growth Tricks and Treats

Single crystal growth is a widely explored method of synthesizing materials in the solid state. The last few decades have seen significant improvements in the techniques used to synthesize single crystals, but there has been comparatively little discussion on ways to disseminate this knowledge. We aim to change that. Here we describe the principles of known single crystal growth techniques as well as lesser-known variations that have assisted in the optimization of defect control in known materials. We offer a perspective on how to think about these synthesis methods in a grand scheme. We consider the temperature interdependence with the reaction time as well as ways to carry out synthesis to scale up and address some outstanding synthesis challenges. We hope our descriptions will aid in technological advancements as well as further developments to gain even better control over synthesis.

cond-mat.mtrl-sci↗

arXiv2019

Multiparty Session Type-safe Web Development with Static Linearity

Modern web applications can now offer desktop-like experiences from within the browser, thanks to technologies such as WebSockets, which enable low-latency duplex communication between the browser and the server. While these advances are great for the user experience, they represent a new responsibility for web developers who now need to manage and verify the correctness of more complex and potentially stateful interactions in their application. In this paper, we present a technique for developing interactive web applications that are statically guaranteed to communicate following a given protocol. First, the global interaction protocol is described in the Scribble protocol language -- based on multiparty session types. Scribble protocols are checked for well-formedness, and then each role is projected to a Finite State Machine representing the structure of communication from the perspective of the role. We use source code generation and a novel type-level encoding of FSMs using multi-parameter type classes to leverage the type system of the target language and guarantee only programs that communicate following the protocol will type check. Our work targets PureScript -- a functional language that compiles to JavaScript -- which crucially has an expressive enough type system to provide static linearity guarantees. We demonstrate the effectiveness of our approach through a web-based Battleship game where communication is performed through WebSocket connections.

cs.PL↗

arXiv2017

Fencing off Go: Liveness and Safety for Channel-based Programming (extended version)

Go is a production-level statically typed programming language whose design features explicit message-passing primitives and lightweight threads, enabling (and encouraging) programmers to develop concurrent systems where components interact through communication more so than by lock-based shared memory concurrency. Go can only detect global deadlocks at runtime, but provides no compile-time protection against all too common communication mismatches or partial deadlocks. This work develops a static verification framework for liveness and safety in Go programs, able to detect communication errors and partial deadlocks in a general class of realistic concurrent programs, including those with dynamic channel creation, unbounded thread creation and recursion. Our approach infers from a Go program a faithful representation of its communication patterns as a behavioural type. By checking a syntactic restriction on channel usage, dubbed fencing, we ensure that programs are made up of finitely many different communication patterns that may be repeated infinitely many times. This restriction allows us to implement procedures to check for liveness and safety in types which in turn approximates liveness and safety in Go programs. We have implemented a type inference and liveness and safety checks in a tool-chain and tested it against publicly available Go programs. Note: This is a revised extended version of a paper that appeared in POPL 2017, see page 13 for details.

cs.PL↗

arXiv2013

Towards deductive verification of MPI programs against session types

The Message Passing Interface (MPI) is the de facto standard message-passing infrastructure for developing parallel applications. Two decades after the first version of the library specification, MPI-based applications are nowadays routinely deployed on super and cluster computers. These applications, written in C or Fortran, exhibit intricate message passing behaviours, making it hard to statically verify important properties such as the absence of deadlocks. Our work builds on session types, a theory for describing protocols that provides for correct-by-construction guarantees in this regard. We annotate MPI primitives and C code with session type contracts, written in the language of a software verifier for C. Annotated code is then checked for correctness with the software verifier. We present preliminary results and discuss the challenges that lie ahead for verifying realistic MPI program compliance against session types.

cs.DC↗