arXiv subjects

Md Asaduzzaman

Publications and source records attributed to Md Asaduzzaman.

13 recordsLinked to original sources

arXiv2026

Identifiability and Information-Based Inference for Epidemic Transmission Models Under Partial Observation

Inference for epidemic transmission on dynamic networks is fundamentally limited by latent infection times, incomplete contact histories, imperfect observation, and external sources of infection. Although coherent likelihood formulations are available for partially observed epidemic processes, considerably less is known about the theoretical limits of statistical inference under such observation mechanisms. This paper develops a unified framework for studying identifiability and Fisher information in epidemic transmission models observed on dynamic contact networks. We establish conditions for structural and local identifiability, derive observed and complete-data information matrices, and quantify information loss arising from unobserved transmission events and missing network information through a missing-information decomposition. We further investigate how observation frequency, network coverage, and measurement accuracy influence parameter estimability and statistical efficiency, providing a principled basis for evaluating surveillance strategies. Simulation studies demonstrate that the proposed framework accurately characterises the relationship between observation design, statistical information, and parameter estimation, with theoretical predictions closely matching finite-sample performance. The proposed framework clarifies the relationship between observation design, identifiability, and inferential precision, and provides a theoretical foundation for statistical inference in partially observed epidemic transmission models.

math.ST↗

arXiv2026

A Complete-Data Likelihood for Epidemic Processes on Partially Observed Dynamic Networks

Inference for infectious disease transmission on dynamic contact networks is complicated by latent infection times, partially observed network evolution, measurement error in contact data, and infection originating from outside the observed population. Existing likelihood-based approaches typically address these challenges separately and often rely on restrictive assumptions such as fully observed networks, closed populations, or symptom onset as a surrogate for infection time. We develop a unified complete-data likelihood framework for epidemic processes evolving on partially observed dynamic networks. The proposed formulation represents disease progression, network evolution, and observation mechanisms as interacting continuous-time stochastic processes within a common probabilistic framework. Specifically, we couple a susceptible-exposed-infectious-removed (SEIR) epidemic process with a status-dependent dynamic contact network and explicit observation models for symptoms and contacts. The resulting framework accommodates latent incubation periods, intermittent network observation, contact measurement error, and external infection pressure while preserving a coherent likelihood structure. Our principal contribution is the derivation of a complete-data event-history likelihood for the joint epidemic-network process under partial observation. The likelihood provides a rigorous foundation for likelihood-based and Bayesian inference through data augmentation, clarifies how information from disease progression and contact dynamics jointly determines parameter estimability, and reveals a broad class of existing epidemic network models as special cases. More generally, the framework contributes to statistical inference for partially observed interacting stochastic systems on evolving networks and establishes a foundation for uncertainty-aware analysis of complex transmission processes.

stat.ME↗

arXiv2025

Superconducting properties of thin film $\mathrm{Nb_{1-x}Ti_xN}$ studied via the NMR of implanted $^8$Li

We report measurements of the normal-state and superconducting properties of thin-film $\mathrm{Nb_{1-x}Ti_xN}$ using $^{8}$Li $\beta$-detected nuclear magnetic resonance ($\beta$-NMR). In these experiments, radioactive $^{8}$Li$^{+}$ probes were implanted $\sim21$ nm below the surface of a $\mathrm{Nb_{1-x}Ti_xN}$(91 nm) film in $\mathrm{Nb_{0.75}Ti_{0.25}N}$(91 nm)/AlN(4 nm)/Nb and its NMR response recorded (via $^{8}$Li's $\beta$-emissions) between 4.6 K and 270 K in a 4.1 T field applied normal to its surface. Resonance measurements reveal wide, symmetric lineshapes at all temperatures, with significant additional broadening below the film's superconducting transition temperature $T_\mathrm{c}(0 \; \mathrm{T}) = 15.4 \pm 0.7$ K due to vortex lattice formation. Fits to a broadening model find a magnetic penetration depth $\lambda(0 \; \mathrm{K})= 180.57 \pm 0.30$ nm and upper critical field $B_\mathrm{c2}(0 \; \mathrm{K})= 18 \pm 4$ T, consistent with literature estimates. Spin-lattice relaxation (SLR) measurements find a Korringa response at low temperatures, with dynamic (i.e., thermally activated) contributions dominating above $\sim100$ K. Below $T_\mathrm{c}$, we observe a small Hebel-Slichter coherence peak characterized by a superconducting energy gap $\Delta(0 \; \mathrm{K}) = 2.60 \pm 0.12$ meV and modest Dynes-like broadening. Our measurements suggest a gap ratio $2\Delta(0 \; \mathrm{K})/k_\mathrm{B}T_\mathrm{c}(0 \; \mathrm{T}) = 3.92 \pm 0.25$, consistent with strong-coupling behavior. Sources for the dynamic high-$T$ relaxation are suggested.

cond-mat.supr-con↗

arXiv2024

Measurements of the first-flux-penetration field in surface-treated and coated Nb: Distinguishing between surface pinning and an interface energy barrier

We report measurements of the first-flux-penetration field in surface-treated and coated Nb samples using muon spin rotation ($\mu$SR). Using thin Ag foils as energy moderators for the implanted muon spin-probes, we "profile" the vortex penetration field $\mu_{0} H_{\mathrm{vp}}$ at sub-surface depths on the order of $\sim 10$ $\mathrm{\mu}$m to $\sim 100$ $\mathrm{\mu}$m. In a coated sample [Nb$_3$Sn(2 $\mathrm{\mu}$m)/Nb], we find that $\mu_{0} H_{\mathrm{vp}}$ is depth-independent with a value of 234.5(35) mT, consistent with Nb's metastable superheating field and suggestive of surface energy barrier for flux penetration. Conversely, in a surface-treated sample [Nb baked in vacuum at 120 $^{\circ}$C for 48 h], vortex penetration onsets close to pure Nb's lower critical field $\mu_{0}H_\mathrm{c1} \approx 170$ mT, but increases with increasing implantation depth, consistent with flux-pinning localized at the surface. The implication of these results for technical applications of superconducting Nb, such as superconducting radio frequency (SRF) cavities, is discussed.

cond-mat.supr-con↗

arXiv2023

Depth-resolved measurement of the Meissner screening profile in a niobium thin film from spin-lattice relaxation of the implanted $β$-emitter $^{8}$Li

We report measurements of the Meissner screening profile in a Nb(300 nm)/Al$_{2}$O$_{3}$ thin film using $^{8}$Li $β$-detected nuclear magnetic resonance ($β$-NMR). The NMR probe $^{8}$Li was ion-implanted into the Nb film at energies $\leq$ 20 keV, corresponding to mean stopping depths comparable to Nb's magnetic penetration depth $λ$. $^{8}$Li's strong dipole-dipole coupling with the host $^{93}$Nb nuclei provided a "cross-relaxation" channel that dominated in low magnetic fields, which conferred indirect sensitivity to the local magnetic field via the spin-lattice relaxation (SLR) rate $1/T_{1}$. From a fit of the $1/T_{1}$ data to a model accounting for its dependence on temperature, magnetic field, and $^{8}$Li$^{+}$ implantation energy, we obtained a magnetic penetration depth $λ_{0}$ = 51.5(22) nm, consistent with a relatively short carrier mean-free-path $\ell$ = 18.7(29) nm typical of similarly prepared Nb films. The results presented here constitute an important step towards using $^{8}$Li $β$-NMR to characterize bulk Nb samples with engineered surfaces, which are often used in the fabrication of particle accelerators.

cond-mat.mtrl-sci↗

arXiv2023

Comment on "Strong Meissner screening change in superconducting radio frequency cavities due to mild baking" [Appl. Phys. Lett. 104, 072601 (2014)]

In a recent Letter by Romanenko et al., the authors used low-energy muon spin rotation (LE-$\mu$SR) to measure the Meissner screening profile in cutouts from Nb superconducting radio frequency (SRF) cavities, systematically comparing how different surface treatments affect the screening properties of the elemental type-II superconductor. They reported a "strong" modification to the character of the screening profile upon mild baking at 120 $^{\circ}$C for 48 h, which was interpreted as a depth-dependent carrier mean-free-path resulting from a "gradient in vacancy concentration" near the surface. While this observation led to speculation that this surface treatment yields an "effective" superconducting bilayer, we suggest that its likeness to such is accidental and that the behavior is an artifact from the analysis.

cond-mat.supr-con↗

arXiv2023

Evidence for current suppression in superconductor-superconductor bilayers

Superconducting radio frequency (SRF) cavities, which are critical components in many particle accelerators, need to be operated in the Meissner state to avoid strong dissipation from magnetic vortices. For a defect-free superconductor, the maximum attainable magnetic field for operation is set by the superheating field, $B_{\mathrm{sh}}$, which directly depends on the surface current. In heterostructures composed of different superconductors, the current in each layer depends not only on the properties of the individual material, but also on the electromagnetic response of the adjacent layers through boundary conditions at the interfaces. Three prototypical bilayers [$\mathrm{Nb_{1-x}Ti_xN}$(50 nm)/Nb, $\mathrm{Nb_{1-x}Ti_xN}$(80 nm)/Nb, and $\mathrm{Nb_{1-x}Ti_xN}$(160 nm)/Nb] are investigated here by depth-resolved measurements of their Meissner screening profiles using low-energy muon spin rotation (LE-$\mu$SR). From fits to a model based on London theory (with appropriate boundary and continuity conditions), a magnetic penetration depth for the thin $\mathrm{Nb_{1-x}Ti_xN}$ layers of $\lambda_\mathrm{Nb_{1-x}Ti_xN} =$ 182.5(31) nm is found, in good agreement with literature values for the bulk alloy. Using the measured $\lambda_\mathrm{Nb_{1-x}Ti_xN}$, the maximum vortex-free field, $B_{\mathrm{max}}$, of the superconductor-superconductor (SS) bilayer structure was estimated to be 610(40) mT. The strong suppression of the surface current in the $\mathrm{Nb_{1-x}Ti_xN}$ layer suggests an optimal thickness of $\sim 1.4 \lambda_{\mathrm{Nb_{1-x}Ti_xN}} =$ 261(14) nm.

cond-mat.supr-con↗

arXiv2022

Depth-resolved measurements of the Meissner screening profile in surface-treated Nb

We report depth-resolved measurements of the Meissner screening profile in several surface-treated Nb samples using low-energy muon spin rotation (LE-$μ$SR). In these experiments, implanted positive muons, whose stopping depths below Nb's surface were adjusted between ~10 nm to ~150 nm, reveal the field distribution inside the superconducting element via their spin-precession (communicated through their radioactive decay products). We compare how the field screening is modified by different surface treatments commonly employed to prepare superconducting radio frequency (SRF) cavities used in accelerator beamlines. In contrast to an earlier report [A. Romanenko et al., Appl. Phys. Lett. 104 072601 (2014)], we find no evidence for any "anomalous" modifications to the Meissner profiles, with all data being well-described by a London model. Differences in screening properties between surface treatments can be explained by changes to the carrier mean-free-paths resulting from dopant profiles near the material's surface.

cond-mat.supr-con↗

arXiv2022

A New High Parallel-Field Spectrometer at TRIUMF's $β$-NMR Facility

A new high field spectrometer has been built to extend the capabilities of the $β$-detected nuclear magnetic resonance ($β$-NMR) facility at TRIUMF. This new beamline extension allows $β$-NMR spectroscopy to be performed with fields up to 200 mT parallel to a sample's surface (perpendicular to the ion beam), allowing depth-resolved studies of local electromagnetic fields with spin polarized probes at a much higher applied magnetic field than previously available in this configuration. The primary motivation and application is to allow studies of superconducting radio frequency (SRF) materials, close to the critical fields of Nb metal, which is extensively used to fabricate SRF cavities. The details of the design considerations and implementation of the ultra-high vacuum (UHV) system, ion optics, beam diagnostics are presented here. Commissioning of the beamline and spectrometer with radioactive ions are also reported here. Future capabilities and applications in other areas are also described.

cond-mat.mtrl-sci↗

arXiv2022

An improved method of delta summation for faster current value selection across filtered subsets of interval and temporal relational data

Aggregation in relational databases is accomplished through hashing and sorting interval data, which is computationally expensive and scales poorly as the data volumes grow. In this paper, we show how quantitative interval and time-series data in relational attributes can be represented using delta summary values rather than absolute values. The need for sorting to determine the row corresponding to some maximum timestamp is negated, reducing the time complexity from at least O(n log(n)) towards O(n) and improving query execution times. We illustrate this new method in the relational algebra, present the implementation algorithmically, and test an implementation in two leading RDBMS products against the use of normal equivalents. We found this delta summation technique to be most effective for use cases with additive, numerical data upon which it is necessary to frequently obtain the latest values, and where the row cardinalities are in the order of 10^5. Our experiments found the proposed new delta summation technique could execute faster than the equivalent standard selection method by up to 22.4%, reducing the overall query cost in some circumstances by up to 24.0%, reducing I/O load by up to 60.6%, but with increased query costs for write operations, an increase in CPU time and memory allocation, uncertain performance with very low or very high cardinalities and inconsistent results across different RDBMS platforms.

cs.DB↗

arXiv2021

Vertical Injection and Wideband Grating Coupler Based on Asymmetric Grating Trenches

A Silicon-on-insulator (SOI) perfectly vertical fibre-to-chip grating coupler is proposed and designed based on engineered subwavelength structures. The high directionality of the coupler is achieved by implementing step gratings to realize asymmetric diffraction and by applying effective index variation with auxiliary ultra-subwavelength gratings. The proposed structure is numerically analysed by using two-dimensional Finite Difference Time Domain (2D FDTD) method and achieves 76% (-1.19 dB) coupling efficiency and 39 nm 1-dB bandwidth.

physics.optics↗

arXiv2019

Wi-Fi Sensing: Applications and Challenges

Wi-Fi technology has strong potentials in indoor and outdoor sensing applications, it has several important features which makes it an appealing option compared to other sensing technologies. This paper presents a survey on different applications of Wi-Fi based sensing systems such as elderly people monitoring, activity classification, gesture recognition, people counting, through the wall sensing, behind the corner sensing, and many other applications. The challenges and interesting future directions are also highlighted.

cs.HC↗

arXiv2011

A Generalized Loss Network Model with Overflow for Capacity Planning of a Perinatal Network

We develop a generalized loss network framework for capacity planning of a perinatal network in the UK. Decomposing the network by hospitals, each unit is analyzed with a GI/G/c/0 overflow loss network model. A two-moment approximation is performed to obtain the steady state solution of the GI/G/c/0 loss systems, and expressions for rejection probability and overflow probability have been derived. Using the model framework, the number of required cots can be estimated based on the rejection probability at each level of care of the neonatal units in a network. The generalization ensures that the model can be applied to any perinatal network for renewal arrival and discharge processes.

cs.PF↗