arXiv subjects

Jiwon Chang

Publications and source records attributed to Jiwon Chang.

14 recordsLinked to original sources

arXiv2026

ListK: Semantic ORDER BY and LIMIT K with Listwise Prompting

Semantic operators abstract large language model (LLM) calls in SQL clauses. It is gaining traction as an easy method to analyze semi-structured, unstructured, and multimodal datasets. While a plethora of recent works optimize various semantic operators, existing methods for semantic ORDER BY (full sort) and LIMIT K (top-K) remain lackluster. Our ListK framework improves the latency of semantic ORDER BY ... LIMIT K at no cost to accuracy. Motivated by the recent advance in fine-tuned listwise rankers, we study several sorting algorithms that best combine partial listwise rankings. These include: 1) deterministic listwise tournament (LTTopK), 2) Las Vegas and embarrassingly parallel listwise multi-pivot quickselect/sort (LMPQSelect, LMPQSort), and 3) a basic Monte Carlo listwise tournament filter (LTFilter). Of these, listwise multi-pivot quickselect/sort are studied here for the first time. The full framework provides a query optimizer for combining the above physical operators based on the target recall to minimize latency. We provide theoretical analysis to easily tune parameters and provide cost estimates for query optimizers. ListK empirically dominates the Pareto frontier, halving latency at virtually no cost to recall and NDCG compared to prior art.

cs.DB↗

arXiv2025

Approximating Opaque Top-k Queries

Combining query answering and data science workloads has become prevalent. An important class of such workloads is top-k queries with a scoring function implemented as an opaque UDF - a black box whose internal structure and scores on the search domain are unavailable. Some typical examples include costly calls to fuzzy classification and regression models. The models may also be changed in an ad-hoc manner. Since the algorithm does not know the scoring function's behavior on the input data, opaque top-k queries become expensive to evaluate exactly or speed up by indexing. Hence, we propose an approximation algorithm for opaque top-k query answering. Our proposed solution is a task-independent hierarchical index and a novel bandit algorithm. The index clusters elements by some cheap vector representation then builds a tree of the clusters. Our bandit is a diminishing returns submodular epsilon-greedy bandit algorithm that maximizes the sum of the solution set's scores. Our bandit models the distribution of scores in each arm using a histogram, then targets arms with fat tails. We prove that our bandit algorithm approaches a constant factor of the optimal algorithm. We evaluate our standalone library on large synthetic, image, and tabular datasets over a variety of scoring functions. Our method accelerates the time required to achieve nearly optimal scores by up to an order of magnitude compared to exhaustive scan while consistently outperforming baseline sampling algorithms.

cs.DB↗

arXiv2024

CoolerSpace: A Language for Physically Correct and Computationally Efficient Color Programming

Color programmers manipulate lights, materials, and the resulting colors from light-material interactions. Existing libraries for color programming provide only a thin layer of abstraction around matrix operations. Color programs are, thus, vulnerable to bugs arising from mathematically permissible but physically meaningless matrix computations. Correct implementations are difficult to write and optimize. We introduce CoolerSpace to facilitate physically correct and computationally efficient color programming. CoolerSpace raises the level of abstraction of color programming by allowing programmers to focus on describing the logic of color physics. Correctness and efficiency are handled by CoolerSpace. The type system in CoolerSpace assigns physical meaning and dimensions to user-defined objects. The typing rules permit only legal computations informed by color physics and perception. Along with type checking, CoolerSpace also generates performance-optimized programs using equality saturation. CoolerSpace is implemented as a Python library and compiles to ONNX, a common intermediate representation for tensor computations. CoolerSpace not only prevents common errors in color programming, but also does so without run-time overhead: even unoptimized CoolerSpace programs out-perform existing Python-based color programming systems by up to 5.7 times; our optimizations provide up to an additional 1.4 times speed-up.

cs.PL↗

arXiv2016

Theoretical and experimental investigation of vacancy-based doping of monolayer MoS$_2$ on oxide

Monolayer transition metal dichalcogenides are novel, gapped two-dimensional materials. Toward device applications, we consider MoS$_2$ layers on dielectrics, in particular in this work, the effect of vacancies on the electronic structure. In density-functional based simulations, we consider the effects of near-interface O vacancies in the oxide slab, and Mo or S vacancies in the MoS$_2$ layer. Band structures and atom-projected densities of states for each system and with differing oxide terminations were calculated, as well as those for the defect-free MoS$_2$-dielectrics system and for isolated dielectric layers for reference. Among our results, we find that with O vacancies, both the Hf-terminated HfO$_2$-MoS$_2$ system, and the O-terminated and H-passivated Al$_2$O$_3$-MoS$_2$ systems appear metallic due to doping of the oxide slab followed by electron transfer into the MoS$_2$, in manner analogous to modulation doping. The n-type doping of monolayer MoS$_2$ by high-k oxides with oxygen vacancies then is experimentally demonstrated by electrically and spectroscopically characterizing back-gated monolayer MoS$_2$ field effect transistors encapsulated by oxygen deficient alumina and hafnia.

cond-mat.mes-hall↗

arXiv2016

Air Stable Doping and Intrinsic Mobility Enhancement in Monolayer $MoS_{2}$ by Amorphous $TiO_{x}$ Encapsulation

To reduce Schottky-barrier-induced contact and access resistance, and the impact of charged impurity and phonon scattering on mobility in devices based on 2D transition metal dichalcogenides (TMDs), considerable effort has been put into exploring various doping techniques and dielectric engineering using $high-κ$ oxides, respectively. The goal of this work is to demonstrate a $high-κ$ dielectric that serves as an effective n-type charge transfer dopant on monolayer (ML) molybdenum disulfide ($MoS_{2}$). Utilizing amorphous titanium suboxide (ATO) as the '$high-κ$ dopant', we achieved a contact resistance of ~ $180$ $Ω.μm$ which is the lowest reported value for ML $MoS_{2}$. An ON current as high as $240$ $μA/μm$ and field effect mobility as high as $83$ $cm^2/V-s$ were realized using this doping technique. Moreover, intrinsic mobility as high as $102$ $cm^2/V-s$ at $300$ $K$ and $501$ $cm^2/V-s$ at $77$ $K$ were achieved after ATO encapsulation which are among the highest mobility values reported on ML $MoS_{2}$. We also analyzed the doping effect of ATO films on ML $MoS_{2}$, a phenomenon which is absent when stoichiometric $TiO_{2}$ is used, using ab initio density functional theory (DFT) calculations which shows excellent agreement with our experimental findings. Based on the interfacial-oxygen-vacancy mediated doping as seen in the case of $high-κ$ ATO - ML $MoS_{2}$, we propose a mechanism for the mobility enhancement effect observed in TMD-based devices after encapsulation in a $high-κ$ dielectric environment.

cond-mat.mes-hall↗

arXiv2015

Modeling of Anisotropic Two-Dimensional Materials Monolayer HfS2 and Phosphorene MOSFETs

Ballistic transport characteristics of metal-oxide semiconductor field effect transistors (MOSFETs) based on anisotropic two-dimensional (2-D) materials monolayer HfS2 and phosphorene are explored through quantum transport simulations. We focus on the effects of the channel crystal orientation and the channel length scaling on device performances. Especially, the role of degenerate conduction band (CB) valleys in monolayer HfS2 is comprehensively analyzed. Benchmarking monolayer HfS2 with phosphorene MOSFETs, we predict that the effect of channel orientation on device performances is much weaker in monolayer HfS2 than in phosphorene due to the degenerate CB valleys of monolayer HfS2. Our simulations also reveal that, at 10 nm channel length scale, phosphorene MOSFETs outperform monolayer HfS2 MOSFETs in terms of the onstate current. However, it is observed that monolayer HfS2 MOSFETs may offer comparable, but a little bit degraded, device performances as compared with phosphorene MOSFETs at 5 nm channel length.

cond-mat.mes-hall↗

arXiv2015

Theoretical Study of Phosphorene Tunneling Field Effect Transistors

In this work, device performances of tunneling field effect transistors (TFETs) based on phosphorene are explored via self-consistent atomistic quantum transport simulations. Phosphorene is an ultra-thin two-dimensional (2-D) material with a direct band gap suitable for TFETs applications. Our simulation shows that phosphorene TFETs exhibit subthreshold slope (SS) below 60 mV/dec and a wide range of on-current depending on the transport direction due to highly anisotropic band structures of phosphorene. By benchmarking with monolayer MoTe2 TFETs, we predict that phosphorene TFETs oriented in the small effective mass direction can yield much larger on-current at the same on-current/off-current ratio than monolayer MoTe2 TFETs. It is also observed that a gate underlap structure is required for scaling down phosphorene TFETs in the small effective mass direction to suppress the source-to-drain direct tunneling leakage current.

cond-mat.mes-hall↗

arXiv2014

Atomistic simulation of the electronic states of adatoms in monolayer MoS2

Using an ab initio density functional theory (DFT) based electronic structure method, we study the effects of adatoms on the electronic properties of monolayer transition metal dichalcogenide (TMD) Molybdenum-disulfide (MoS2). We consider the 1st (Li, Na, K) and 7th (F, Cl, Br) column atoms and metals (Sc, Ti, Ta, Mo, Pd, Pt, Ag, Au). Three high symmetry sites for the adatom on the surface of monolayer MoS2 are examined as starting points to search for the most energetically stable configuration for each adatom-monolayer MoS2 system, as well as the type of associated bonding. For the most stable adatom positions, we characterize the emergence of adatom-induced electronic states including any dopant states.

cond-mat.mes-hall↗

arXiv2014

Ballistic performance comparison of monolayer transition metal dichalcogenide MX2 (M = Mo, W; X = S, Se, Te) MOSFETs

We study the transport properties of monolayer MX2 (M = Mo, W; X = S, Se, Te) n- and p- channel metal-oxide-semiconductor field effect transistors (MOSFETs) using full-band ballistic non-equilibrium Green's function simulations with an atomistic tight-binding Hamiltonian with hopping potentials obtained from density functional theory. We discuss the subthreshold slope, drain-induced barrier lowering (DIBL), as well as gate-induced drain leakage (GIDL) for different monolayer MX2 MOSFETs. We also report the possibility of negative differential resistance behavior in the output characteristics of nanoscale monolayer MX2 MOSFETs.

cond-mat.mes-hall↗

arXiv2013

Atomistic Full-Band Simulations of Monolayer MoS2 Transistors

We study the transport properties of deeply scaled monolayer MoS2 n-channel metal-oxide-semiconductor field effect transistors (MOSFETs) using full-band ballistic quantum transport simulations with an atomistic tight-binding Hamiltonian obtained from density functional theory. Our simulations suggest that monolayer MoS2 MOSFETs can provide near-ideal subthreshold slope, and suppression of drain-induced barrier lowering (DIBL) and gate-induced drain leakage (GIDL). However, these full-band simulations also exhibit limited transconductance. These ballistic simulations also exhibit negative differential resistance (NDR) in the output characteristics associated with the narrow width in energy of the lowest conduction band, but this NDR may be substantially reduced or eliminated by scattering in MoS2.

cond-mat.mes-hall↗

arXiv2012

Topological insulator Bi2Se3 thin films as an alternative channel material in MOSFETs

Three-dimensional (3-D) topological insulators (TI) are characterized by the presence of metallic surface states and a bulk band gap. Recently theoretical and experimental studies have shown an induced gap in the surface state bands of TI thin films. The gap results from interaction of conduction band (CB) and valence band (VB) surface states from the opposite surfaces of a thin film, and its size is determined by the film thickness. This gap formation could open the possibility of thin-film TI-based metal-oxide-semiconductor field-effect transistors (MOSFETs). Here we explore the performance of MOSFETs based on TI thin films, specifically Bi2Se3, using quantum ballistic transport simulations with the tight-binding Hamiltonian in the atomic orbital basis. Our simulations indicate that Bi2Se3 MOSFET will be vulnerable to short-channel effects due to the high relative dielectric constant of Bi2Se3(~100)despite its expected excellent electrostatic integrity inherent in a two-dimensional system, and will have other limitations as compared to silicon-based MOSFETs. However, Bi2Se3 MOSFETs, and presumably other TI-based MOSFETs, appear to provide reasonable performance that perhaps could provide novel device opportunities when combined with novel TI properties such as spin-polarized surface states.

cond-mat.mes-hall↗

arXiv2011

Dielectric capping effects on binary and ternary topological insulator surface states

Using a density functional based electronic structure method, we study the effect of crystalline dielectrics on the metallic surface states of Bismuth- and chalcogen-based binary and ternary three dimensional topological insulator (TI) thin films. Crystalline quartz (SiO2) and boron nitride (BN) dielectrics were considered. Crystalline approximation to the amorphous quartz allows to study the effect of oxygen coverage or environmental effects on the surface states degradation which has gained attention recently in the experimental community. We considered both symmetric and asymmetric dielectric cappings to the sufaces of TI thin films. Our studies suggest that BN and quartz cappings have negligible effects on the Dirac cone surface states of both binary and ternary TIs, except in the case of an oxygen-terminated quartz surface. Dangling bond states of oxygens in oxygen-terminated quartz dominate the region close to Fermi level, thereby distorting the TI Dirac cone feature and burying the Dirac point in the quartz valence band region. Passivating the oxygen-terminated surface with atomic hydrogen removes these dangling bond states from the Fermi surface region, and consequently the clear Dirac cone is recovered. Our results are consistent with recent experimental studies of TI surface degradation in the presence of oxygen coverage.

cond-mat.mtrl-sci↗

arXiv2011

Density Functional Study of Ternary Topological Insulator Thin Films

Using an ab-initio density functional theory based electronic structure method with a semi-local density approximation, we study thin-film electronic properties of two topological insulators based on ternary compounds of Tl (Thallium) and Bi (Bismuth). We consider TlBiX$_2$ (X=Se, Te) and Bi$_2$$X$_2$Y (X,Y= Se,Te) compounds which provide better Dirac cones, compared to the model binary compounds Bi$_2$X$_3$ (X=Se, Te). With this property in combination with a structurally perfect bulk crystal, the latter ternary compound has been found to have improved surface electronic transport in recent experiments. In this article, we discuss the nature of surface states, their locations in the Brillouin zone and their interactions within the bulk region. Our calculations suggest a critical thin film thickness to maintain the Dirac cone which is significantly smaller than that in binary Bi-based compounds. Atomic relaxations or rearrangements are found to affect the Dirac cone in some of these compounds. And with the help of layer-projected surface charge densities, we discuss the penetration depth of the surface states into the bulk region. The electronic spectrum of these ternary compounds agrees very well with the available experimental results.

cond-mat.mtrl-sci↗

arXiv2010

Intrinsic and extrinsic perturbations on the topological insulator Bi2Se3 surface states

Using a density functional based electronic structure method, we study the effect of perturbations on the surface state Dirac cone of a strong topological insulator Bi$_2$Se$_3$ from both the intrinsic and extrinsic sources. We consider atomic relaxations, and film thickness as intrinsic and interfacial thin dielectric films as an extrinsic source of perturbation to the surface states. We find that atomic relaxations has no effect on the degeneracy of the Dirac cone whereas film thickness has considerable effect on the surface states inducing a gap which increases monotonically with decrease in film thickness. We consider two insulating substrates BN and quartz as dielectric films and show that surface terminations of quartz with or without passivation plays critical role in preserving Dirac cone degeneracy whereas BN is more inert to the TI surface states. The relative orbital contribution with respect to bulk is mapped out using a simple algorithm, and with the help of it we demonstrate the bulk band inversion when spin-orbit coupling is switched on. The layer projected charge density distributions of the surface states shows that these states are not strictly confined to the surface. The spatial confinement of these states extends up to two to three quintuple layers, a quintuple layer consists of five atomic layers of Bi and Se

cond-mat.mtrl-sci↗