arXiv subjects

Stephen Wilson

Publications and source records attributed to Stephen Wilson.

13 recordsLinked to original sources

arXiv2025

$^{51}$V NMR evidence for interlayer-modulated charge order and a first-order low-temperature transition in CsV$_3$Sb$_5$

Charge order in the kagome superconductor CsV$_3$Sb$_5$ exhibits a complex three-dimensional organization and intermediate-temperature anomalies whose bulk character has remained unsettled. We use orientation-dependent $^{51}$V NMR as a site-selective probe to determine the stacking of the charge density wave (CDW) state and its thermal evolution. Below $T_{\mathrm{CDW}}\!\approx\!94$~K, the field-linear splitting of the $^{51}$V central transition together with the anisotropy of the Knight-shift tensor identify an interlayer-modulated $3\mathbf{q}$ CDW whose local environments are consistent with a four-layer $2\times2\times4$ stacking with mixed trihexagonal/Star-of-David distortions, in agreement with synchrotron x-ray determinations. For comparison, RbV$_3$Sb$_5$ serves as a reference exhibiting a uniform trihexagonal $2\times2\times2$ stacking, allowing us to isolate features unique to the $2\times2\times4$ state in CsV$_3$Sb$_5$. With $\mathbf{H}_0\!\parallel\!c$, the $^{51}$V quadrupolar satellites through the intermediate temperature scale near $T_{\mathrm{CO}}\!\approx\!65$ K reorganize into two well-resolved electric-field-gradient manifolds that coexist over a finite interval; their relative spectral weights interchange on cooling while the total integrated satellite intensity remains conserved and $\nu_Q$ within each manifold is nearly temperature independent. The coexistence without critical broadening, together with conserved intensity, provides bulk evidence consistent with a first-order charge-order transition near $T_{\mathrm{CO}}$. Our measurements do not resolve whether this lower-temperature transition corresponds to a distinct in-plane order or a reorganization of the $3\mathbf{q}$ state; rather, they delimit this window and provide bulk, site-resolved constraints that connect prior reported anomalies to a thermodynamic first-order transition.

cond-mat.str-el↗

arXiv2025

Orbits of consistent walk in dart-transitive maps

In a simple graph, a shunt is a symmetry which sends an edge to an incident edge (without fixing their shared vertex). The orbit of this edge under the shunt forms a consistent cycle. The important theorem of Biggs and Conway says that in a dart-transitive graph of valence q, there are exactly q-1 orbits of consistent cycles. These ideas have become a useful tool in the area of graphs symmetries, and generalize easily to consistent walks in graphs which are not simple. These walks are not necessarily cycles, or even circuits. This paper considers these walks and their orbits in the venue of dart-transitive maps and classifies them geometrically.

math.CO↗

arXiv2024

End-to-end multi-modal product matching in fashion e-commerce

Product matching, the task of identifying different representations of the same product for better discoverability, curation, and pricing, is a key capability for online marketplace and e-commerce companies. We present a robust multi-modal product matching system in an industry setting, where large datasets, data distribution shifts and unseen domains pose challenges. We compare different approaches and conclude that a relatively straightforward projection of pretrained image and text encoders, trained through contrastive learning, yields state-of-the-art results, while balancing cost and performance. Our solution outperforms single modality matching systems and large pretrained models, such as CLIP. Furthermore we show how a human-in-the-loop process can be combined with model-based predictions to achieve near perfect precision in a production system.

cs.CV↗

arXiv2023

Transitive cornerations in maps

A corner in a map is an edge-vertex-edge triple consisting of two distinct edges incident to the same vertex. A corneration is a set of corners that covers every arc of the map exactly once. Cornerations in a dart-transitive map generalize the notion of a cycle structure in a symmetric graph. In this paper, we study the cornerations (and associated structures) that are preserved by a vertex-transitive group of automorphisms of the map.

math.CO↗

arXiv2023

MatKB: Semantic Search for Polycrystalline Materials Synthesis Procedures

In this paper, we present a novel approach to knowledge extraction and retrieval using Natural Language Processing (NLP) techniques for material science. Our goal is to automatically mine structured knowledge from millions of research articles in the field of polycrystalline materials and make it easily accessible to the broader community. The proposed method leverages NLP techniques such as entity recognition and document classification to extract relevant information and build an extensive knowledge base, from a collection of 9.5 Million publications. The resulting knowledge base is integrated into a search engine, which enables users to search for information about specific materials, properties, and experiments with greater precision than traditional search engines like Google. We hope our results can enable material scientists quickly locate desired experimental procedures, compare their differences, and even inspire them to design new experiments. Our website will be available at Github \footnote{https://github.com/Xianjun-Yang/PcMSP.git} soon.

cs.CL↗

arXiv2022

PcMSP: A Dataset for Scientific Action Graphs Extraction from Polycrystalline Materials Synthesis Procedure Text

Scientific action graphs extraction from materials synthesis procedures is important for reproducible research, machine automation, and material prediction. But the lack of annotated data has hindered progress in this field. We demonstrate an effort to annotate Polycrystalline Materials Synthesis Procedures (PcMSP) from 305 open access scientific articles for the construction of synthesis action graphs. This is a new dataset for material science information extraction that simultaneously contains the synthesis sentences extracted from the experimental paragraphs, as well as the entity mentions and intra-sentence relations. A two-step human annotation and inter-annotator agreement study guarantee the high quality of the PcMSP corpus. We introduce four natural language processing tasks: sentence classification, named entity recognition, relation classification, and joint extraction of entities and relations. Comprehensive experiments validate the effectiveness of several state-of-the-art models for these challenges while leaving large space for improvement. We also perform the error analysis and point out some unique challenges that require further investigation. We will release our annotation scheme, the corpus, and codes to the research community to alleviate the scarcity of labeled data in this domain.

cs.CL↗

arXiv2022

Evidence for a robust sign-changing s-wave order parameter in monolayer films of superconducting Fe (Se,Te)/Bi2Te3

The Fe-based superconductor Fe (Se,Te) combines non-trivial topology with unconventional superconductivity and may be an ideal platform to realize exotic states such as high-order topological corner modes and Majorana modes. Thin films of Fe (Se,Te) are important for device fabrication, phase sensitive transport measurements and for realizing proposals to engineer higher-order modes. However, while bulk Fe (Se,Te) has been extensively studied with a variety of techniques, the nature of the superconducting order parameter in the monolayer limit has not yet been explored. In this work, we study monolayer films of Fe (Se,Te) on Bi2Te3 with scanning tunneling spectroscopy and Bogoliubov quasiparticle interference (BQPI). We discover that the monolayer Fe (Se,Te)/Bi2Te3 heterostructures host a robust, multigap superconducting state that strongly resembles the bulk. BQPI maps at the gap energies show a strong spatial modulation, oriented 45 degrees to the Fe-Se bond direction. Analysis of the phase-referenced quasiparticle interference signal reveals a sign-changing s-wave order parameter similar to the bulk. Moreover, we observe a unique pattern of sign changes in the BQPI signal which have not been observed in the bulk. Our work establishes monolayer Fe (Se,Te)/Bi2Te3 as a robust multi-band unconventional superconductor and sets the stage for explorations of non-trivial topology in this highly-tunable system.

cond-mat.supr-con↗

arXiv2017

Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal bismuth telluride

We use a magnetothermal resistance method to measure the lattice thermal conductivity of a single crystal of Bi$_2$Te$_3$ from 5 to 60 K. We apply a large transverse magnetic field to suppress the electronic thermal conduction while measuring thermal conductivity and electrical resistivity. The lattice thermal conductivity is then calculated by extrapolating the thermal conductivity versus electrical conductivity curve to a zero electrical conductivity value. Our results show that the measured phonon thermal conductivity follows the $e^{({\Delta}_{min}/T)}$ temperature dependence and the Lorenz ratio corresponds to the modified Sommerfeld value in the intermediate temperature range. Our low-temperature experimental data and analysis on Bi$_2$Te$_3$ are an important compliment to previous measurements of Goldsmid [14] and theoretical calculations by Broido $\textit{et al.}$ [21] at higher temperature 100 - 300 K.

cond-mat.mtrl-sci↗

arXiv2015

The Separated Box Product of Two Digraphs

A new product construction of graphs and digraphs, based on the standard box product of graphs and called the separated box product, is presented, and several of its properties are discussed. Questions about the symmetries of the product and their relations to symmetries of the factor graphs are considered. An application of this construction to the case of tetravalent edge-transitive graphs is discussed in detail.

math.CO↗

arXiv2014

Anomalous CDW ground state in Cu$_2$Se: a wave-like fluctuation of $\it{dc}$ I-V curve near 50 K

A charge density wave (CDW) ground state is observed in polycrystalline Cu$_2$Se below 125 K, which corresponds to an energy gap of 40.9 meV and an electron-phonon coupling constant of 0.6. Due to the polycrystalline structure, the Peierls transition process has been expanded to a wide temperature range from 90 to 160 K. The Hall carrier concentration shows a continuous decrease from 2.1$\times$10$^{20}$ to 1.6$\times$10$^{20}$ cm$^{-3}$ in the temperature range from 160 K to 80 K, while almost unchanged above 160 K and below 90 K. After entering the CDW ground state, a wave-like fluctuation was observed in the I-V curve near 50 K, which exhibits as a periodic negative differential resistivity in an applied electric field due to the current. We also investigated the doping effect of Zn, Ni, and Te on the CDW ground state. Both Zn and Ni doped Cu$_2$Se show a CDW character with increased energy gap and electron-phonon coupling constant, but no notable Peierls transition was observed in Te doped Cu$_2$Se. Similar wave-like I-V curve was also seen in Cu$_{1.98}$Zn$_{0.02}$Se near 40 K. The regular fluctuation in $\it{dc}$ I-V curve was not magnetic field sensitive, but temperature and sample size sensitive.

cond-mat.mtrl-sci↗

arXiv2013

Imaging the evolution of metallic states in a spin-orbit interaction driven correlated iridate

The Ruddlesden-Popper (RP) series of iridates (Srn+1IrnO3n+1) have been the subject of much recent attention due to the anticipation of emergent physics arising from the cooperative action of spin-orbit (SO) driven band splitting and Coulomb interactions[1-3]. However an ongoing debate over the role of correlations in the formation of the charge gap and a lack of understanding of the effects of doping on the low energy electronic structure have hindered experimental progress in realizing many of the predicted states[4-8] including possible high-Tc superconductivity[7,9]. Using scanning tunneling spectroscopy we map out the spatially resolved density of states in the n=2 RP member, Sr3Ir2O7 (Ir327). We show that the Ir327 parent compound, argued to exist only as a weakly correlated band insulator in fact possesses a substantial ~130meV charge excitation gap driven by an interplay between structure, SO coupling and correlations. A critical component in distinguishing the intrinsic electronic character within the inhomogeneous textured electronic structure is our identification of the signature of missing apical oxygen defects, which play a critical role in many of the layered oxides. Our measurements combined with insights from calculations reveal how apical oxygen vacancies transfer spectral weight from higher energies to the gap energies thereby revealing a path toward obtaining metallic electronic states from the parent-insulating states in the iridates.

cond-mat.mtrl-sci↗

arXiv2011

Distributed Uplink Resource Allocation in Cognitive Radio Networks -- Part I: Equilibria and Algorithms for Power Allocation

Spectrum management has been identified as a crucial step towards enabling the technology of a cognitive radio network (CRN). Most of the current works dealing with spectrum management in the CRN focus on a single task of the problem, e.g., spectrum sensing, spectrum decision, spectrum sharing or spectrum mobility. In this two-part paper, we argue that for certain network configurations, jointly performing several tasks of the spectrum management improves the spectrum efficiency. Specifically, our aim is to study the uplink resource management problem in a CRN where there exist multiple cognitive users (CUs) and access points (APs). The CUs, in order to maximize their uplink transmission rates, have to associate to a suitable AP (spectrum decision), and to share the channels used by this AP with other CUs (spectrum sharing). These tasks are clearly interdependent, and the problem of how they should be carried out efficiently and in a distributed manner is still open in the literature.

cs.IT↗

arXiv2005

Perfect Pairs of Ideals and Duals in Numerical Semigroups

This paper considers numerical semigroups $S$ that have a non-principal relative ideal $I$ such that $μ_S(I)μ_S(S-I)=μ_S(I+(S-I)) $. We show the existence of an infinite family of such which $I+(S-I)=S\backslash\{0\}$. We also show examples of such pairs that are not members of this family. We discuss the computational process used to find these examples and present some open questions pertaining to them.

math.AC↗