Abdus Salam International Centre for Theoretical Physics
Recent publications
Thermal diffusion results in spatiotemporally evolving temperature distribution in material mediums. In dielectric mediums it will also lead to change in local refractive index. Digital holographic interferometry can be used for high contrast quantitative phase imaging of spatially and temporally evolving refractive index variations in dielectric mediums, arising due to temperature changes. Phase retrieved from numerically processed digital holograms recorded at different time instances can be compared to obtain this information. Since the thermal conductivity varies for materials, the thermal diffusion and hence the refractive index across them will also vary. Time varying optical path length distributions of the test sample under thermal stress, obtained from the phase maps retrieved from the recorded holograms, can be used to obtain this information. This leads to detection of regions having different thermal conductivities and hence identification of inhomogeneities in the test sample. Lens less Fourier transform digital holographic interferometry is a perfect tool to quantify optical path length distributions with nanometre accuracy to quantify spatiotemporally evolving refractive index distributions. In this technique, a single Fourier transform of the recorded hologram yields the spatial distribution of object phase, making it ideal for real time imaging and applications. Here we describe our efforts in the development lens less Fourier transform digital holographic interferometric technique for the imaging and quantification of spatiotemporally evolving refractive index distributions in test samples under thermal stress and its application in detection of surface and sub-surface inhomogeneities.
This chapter provides an overview of the characteristics of the climate over the Mediterranean region and of the processes that determine its evolution. It describes how numerical models are used for simulating the climate at the regional Mediterranean scale, the heat, and moisture balance at Mediterranean regional scale, their relation to surface climate. The evolution of the regional climate conditions is described since the times when the Mediterranean present morphology emerged as a remnant of the Tethys Ocean. Finally, the impacts of the ongoing anthropogenic climate change are described.
Generalizing previous work of Iwaniec, Luo, and Sarnak (2000), we use information from one-level density theorems to estimate the proportion of non-vanishing of L-functions in a family at a low-lying height on the critical line (measured by the analytic conductor). To solve the Fourier optimization problems that arise, we provide a unified framework based on the theory of reproducing kernel Hilbert spaces of entire functions (there is one such space associated to each symmetry type). Explicit expressions for the reproducing kernels are given. We also revisit the problem of estimating the height of the first low-lying zero in a family, considered by Hughes and Rudnick (2003) and Bernard (2015). We solve the associated Fourier optimization problem in this setting by establishing a connection to the theory of de Branges spaces of entire functions and using the explicit reproducing kernels. In an appendix, we study the related problem of determining the sharp embeddings between the Hilbert spaces associated to the five symmetry types and the classical Paley-Wiener space.
The study of non-equilibrium dynamics of many-body systems after a quantum quench received a considerable boost and a deep theoretical understanding from the path integral formulation in imaginary time. However, the celebrated problem of a quench in the Luttinger parameter of a one dimensional quantum critical system (massless quench) has so far only been solved in the real-time Heisenberg picture. In order to bridge this theoretical gap and to understand on the same ground massive and massless quenches, we study the problem of a gaussian field characterized by a coupling parameter K K within a strip and a different one K_0 K 0 in the remaining two semi-infinite planes. We give a fully analytical solution using the electrostatic analogy with the problem of a dielectric material within a strip surrounded by an infinite medium of different dielectric constant, and exploiting the method of charge images. After analytic continuation, this solution allows us to obtain all the correlation functions after the quench within a path integral approach in imaginary time, thus recovering and generalizing the results in real time. Furthermore, this imaginary-time approach establishes a remarkable connection between the quench and the famous problem of the conductivity of a Tomonaga-Luttinger liquid coupled to two semi-infinite leads: the two are in fact related by a rotation of the spacetime coordinates.
We have presented a self bound quark star model that possesses some net electric charge. The quarks considered here are deconfined non-interacting Fermi gas. The solution has been found by solving Einstein-Maxwell field equations through MIT-bag equation of state and a metric potential. The obtained solution is further use to present a comparative studies of four compact stars 4U 1820-30, PSR J1903+327, Vela X-1 and PSR J1614-2230. The models are free from singularity, fulfil energy conditions, stability criteria and far within Buchdahl-Andreasson limit. Finally, we have predicted radii of these four compact objects.
In this paper, computation of the Effective Drought Index (EDI) is revisited and the performance of the derived model (mEDI) is compared to Standardized Precipitation Index (SPI) using Self-Calibrating Palmer Drought Severity Index (scPDSI) as the benchmark. Four monthly data sets over Central-Africa were used: precipitation from ten observation stations, gridded precipitation from Global Precipitation Climatology Centre (GPCC) and Climatic Research Unit (CRU), and gridded scPDSI. Station data span the periods 1951–2005 and 1971–2013 for Cameroon and Democratic Republic of Congo (DRC), respectively, and 1950–2019 for all gridded data. SPI was computed based on gamma fitting function and EDI was revised so that mEDI uses monthly precipitation as input data and can quantify multi-scalar droughts. As results, the performances of both indices generally increase with the time scale (TS) and decrease as total annual precipitation increases. The mEDI model outperforms SPI for precipitation above 2288.91 mm/2444.02 mm at TS < 12-month, and below 3233.87 mm/2366.42 mm at 12-month TS, for GPCC/CRU data, while SPI performs better for precipitation out of these intervals, except at the particular TSs of 12, 24, 36 and 48 months where mEDI regains the advantage. At 15-month TS and over, both indices show substantially equal performances. If spatial average of precipitation is used as input for each of the four defined climatic zones, the performances of both indices are improved. SPI best describes drought on short TSs while on medium and long TSs (> 9-month TS), mEDI shows the best performance.
We here inter-compare four different tracking algorithms by applying them onto the precipitation fields of an ensemble of convection-permitting regional climate models (cpRCMs) and on high-resolution observational datasets of precipitation. The domain covers the Alps and the northern Mediterranean and thus we here analyse heavy precipitation events, that are renowned for causing hydrological hazards. In this way, this study is both, an inter-comparison of tracking algorithms as well as an evaluation study of cpRCMs in the Lagrangian frame of reference. The tracker inter-comparison is performed by comparison of two case studies as well as of climatologies of cpRCMs and observations. We find that that all of the trackers produce qualitatively equal results concerning characteristic track properties. This means that, despite of quantitative differences, equivalent scientific conclusions would be drawn. This result suggests that all trackers investigated are reliable analysis tools of atmospheric research. With respect to the model ensemble evaluation, we find an encouraging performance of cpRCMs in comparison to radar-based observations. In particular prominent hotspots of heavy precipitation events are well-reproduced by the models. In general most characteristic properties of precipitation events have positive biases. Assuming the under-catchment of precipitation in observations in a domain of such complex orography, this result is to be expected. Only the mean area of tracks is underestimated, while their duration is overestimated. Mean precipitation rate is estimated well, while maximum precipitation rate is overestimated. Furthermore, geometrical and rain volume are overestimated. We find that models overestimate the occurrence of precipitation events over all mountain chains, whereas over plain terrain in summer precipitation events are seen underestimated. This suggests that, despite the convection-permitting resolution, thermally driven thunderstorms are either not triggered or their dynamics still under-resolved. Eventually we find that biases in the spatio-temporal properties of precipitation events appear reduced when evaluating cpRCMs against Doppler radar-based and rain gauge-adjusted observational datasets of comparable spatial resolution, strengthening their role in evaluation studies.
Despite the necessity of Global Climate Models (GCMs) sub‐selection in downscaling studies, an objective approach for their selection is currently lacking. Building on the previously established concepts in GCMs evaluation frameworks, we develop a weighted averaging technique to remove the redundancy in the evaluation criteria and rank 37 GCMs from the sixth phase of the Coupled Models Intercomparison Project over the contiguous United States. GCMs are rated based on their average performance across 66 evaluation measures in the historical period (1981–2014) after each metric is weighted between zero and one, depending on its uniqueness. The robustness of the outcome is tested by repeating the process with the empirical orthogonal function analysis in which each GCM is ranked based on its sum of distances from the reference in the principal component space. The two methodologies work in contrasting ways to remove the metrics redundancy but eventually develop similar GCMs rankings. A disparity in GCMs' behavior related to their sensitivity to the size of the evaluation suite is observed, highlighting the need for comprehensive multi‐variable GCMs evaluation at varying timescales for determining their skillfulness over a region. The sub‐selection goal is to use a representative set of skillful models over the region of interest without substantial overlap in their future climate responses and modeling errors in representing historical climate. Additional analyses of GCMs' independence and spread in their future projections provide the necessary information to objectively select GCMs while keeping all aspects of necessity in view.
Recent atomic physics experiments and numerical works have reported complementary signatures of the emergence of a topological quantum spin liquid in models with blockade interactions. However, the specific mechanism stabilizing such a phase remains unclear. Here, we introduce an exact relation between an Ising-Higgs lattice gauge theory on the kagome lattice and blockaded models on Ruby lattices. This relation elucidates the origin of previously observed topological spin liquids by directly linking the latter to a deconfined phase of a solvable gauge theory. By means of exact diagonalization and unbiased quantum Monte Carlo simulations, we show that the deconfined phases extend in a broad region of the parameter space; these states are characterized by a large ground state overlap with resonating valence bond wave functions. These blockaded models include both creation or annihilation and hopping dynamics, and can be experimentally realized with Rydberg-dressed atoms, offering novel and controllable platforms for the engineering and characterization of spin liquid states.
Based on the deformed Heisenberg algebra, we propose a nonlinear Tavis–Cummings model (TCM) which is obtained from the usual TCM by using deformed operators of a two-mode field. Such a generalization of the TCM considers the interaction of a two-atom system with a correlated two-mode field in the presence of time-dependent coupling. We show the effect of the deformation and interaction process between the two modes and atoms on the time variation of current interest quantum phenomena, such as entanglement and parameter estimation. The obtained results provide a new approach to manipulate the interaction between the atoms and fields that can be useful for considerable applications in quantum optics and information.
Association rule mining has gained much popularity in facilitating disease diagnosis and the healthcare industry's decision-making process. The cases of Drug Resistance Tuberculosis (DR-TB) have substantially increased in recent times and the factors driving its rapid rise have not been clearly identified. This research explores the Frequent Pattern (FP) Growth algorithm to identify recurring relationships, disease co-occurrences, and generate some very interesting diagnostics rules for DR-TB. This is achieved through data extraction from patients' TB health database, transforming raw data into a knowledge discovery system that provides a good idea for further exploration through medical research to clarify the unknown patterns from the obtained result. The FP growth algorithm efficiently generates the frequent symptoms associated with pulmonary tuberculosis and drug-resistance tuberculosis by generating the association rules at minimum confidence of 80%. The analysis performed is knowledgeable and conforms to the clinical practices and design science research processes.
In the framework of time-varying coupling and power-law potentials, we investigate quantum entanglement and quantum Fisher information in a system that consists of a three-level atom interacting with a quantized field. The results illustrate that the quantum entanglement and quantum Fisher information's temporal evolution depend critically on the physical properties of the field and its coupling to the atom. It is noteworthy to notice that the quantifiers are sensitive to the exponent parameter of the potential with and without time-varying coupling. Considerable nonlocal correlation with the accuracy of parameter estimation can be achieved through the proper control of physical parameters.
Understanding the interfacial properties between an atomic layer and its substrate is of key interest at both the fundamental and technological levels. From Fermi level pinning to strain engineering and superlubricity, the interaction between a single atomic layer and its substrate governs electronic, mechanical and chemical properties. Here, we measure the hardly accessible interfacial transverse shear modulus of an atomic layer on a substrate. By performing measurements on bulk graphite, and on epitaxial graphene films on SiC with different stacking orders and twisting, as well as in the presence of intercalated hydrogen, we find that the interfacial transverse shear modulus is critically controlled by the stacking order and the atomic layer–substrate interaction. Importantly, we demonstrate that this modulus is a pivotal measurable property to control and predict sliding friction in supported two-dimensional materials. The experiments demonstrate a reciprocal relationship between friction force per unit contact area and interfacial shear modulus. The same relationship emerges from simulations with simple friction models, where the atomic layer–substrate interaction controls the shear stiffness and therefore the resulting friction dissipation.
In this paper, the original SIR model is improved by considering a new compartment, representing the hospitalization of critical cases. A system of differential equations with four blocks is developed to analyze the treatment of severe cases in an Intensive Care Unit (ICU). The outgoing rate of the infected individuals who survive is divided into nI and [Formula: see text] where the second term represents the transition rate of critical cases that are hospitalized in ICU. The findings demonstrate the existence of forward, backward and Hopf bifurcations in various ranges of parameters.
This study examines the occurrences rate of geomagnetic storms during the solar cycles (SCs) 20-24. It also investigates the solar sources at SCs 23 and 24. The Disturbed storm time (Dst) and Sunspot Number (SSN) data were used in the study. The study establishes that the magnitude of the rate of occurrences of geomagnetic storms is higher (lower) at the descending phases (minimum phases) of solar cycle. It as well reveals that severe and extreme geomagnetic storms (Dst < -250 nT) seldom occur at low solar activity but at very high solar activity and are mostly associated with coronal mass ejections (CMEs) when occurred. Storms caused by CME+CH-HSSW are more prominent during the descending phase than any other phase of the solar cycle. Solar minimum features more CH-HSSW- associated storms than any other phase. It was also revealed that all high intensity geomagnetic storms (strong, severe and extreme) are mostly associated with CMEs. However, CH-HSSW can occasionally generate strong storms during solar minimum. The results have proven that CMEs are the leading cause of geomagnetic storms at the ascending, maximum and the descending phases of the cycles 23 and 24 followed by CME+CH-HSSW. The results from this study indicate that the rate of occurrence of geomagnetic storms could be predicted in SC phases.
The Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) has adopted the FAIR Guiding Principles. We present the Atlas chapter of Working Group I (WGI) as a test case. We describe the application of the FaIR principles in the atlas, the challenges faced during its implementation, and those that remain for the future. We introduce the open source repository resulting from this process, including coding (e.g., annotated Jupyter notebooks), data provenance, and some aggregated datasets used in some figures in the Atlas chapter and its interactive companion (the Interactive Atlas), open to scrutiny by the scientific community and the general public. We describe the informal pilot review conducted on this repository to gather recommendations that led to significant improvements. Finally, a working example illustrates the re-use of the repository resources to produce customized regional information, extending the Interactive atlas products and running the code interactively in a web browser using Jupyter notebooks.
The dramatic mass loss of Tropical Andean glaciers under the influence of climate change has caused alterations in regional hydrological regimes, including development and expansion of glacial lakes, especially moraine-dammed lakes, supraglacial lakes and ice-dammed lakes. There is a broad consensus on Moraine-Dammed Glacial Lakes (MDGLs) to be commonly understood as potentially most dangerous lakes that can trigger Glacial Lake Outburst Floods (GLOFs). The GLOF event in that process is expected to negatively impact the downstream communities, agricultural assets and infrastructure. In this study, we have prepared an updated and detailed inventory of MDGLs in the Cordillera Blanca region of the Peruvian Andes. The multi-temporal satellite data (TM, ETM, OLI and Sentinel-2A) was used to analyze the changes in lake area over a period of 40 years from 1980 to 2020. A total of 38 MDGLs (size > 0.05 km2) covering an area of 10.30 km2, and located in the altitudinal zone ranging from 4155 to 4960 masl were identified and mapped. From 1980 (6.59 km2) to 2020 (10.3 km2), an expansion of 3.7 km2 (35%) at an annual rate of 0.09 km2/year was observed in the lake area. This study also contributes in terms of developing a database of past GLOF events from an extensive literature survey to understand the hazard and disaster profile of the region for the period 1702–2020. A total of 28 GLOF events have been reported in the region which brought devastation to the surrounding communities. We conclude that the region is highly prone to GLOFs as understood from the occurrence of GLOFs in the past as well as from the current scenario of MDGLs.
A bstract We explore the notion of c -functions in renormalization group flows between theories in different spacetime dimensions. We discuss functions connecting central charges of the UV and IR fixed point theories on the one hand, and functions which are monotonic along the flow on the other. First, using the geometric properties of the holographic dual RG flows across dimensions and the constraints from the null energy condition, we construct a monotonic holographic c -function and thereby establish a holographic c -theorem across dimensions. Second, we use entanglement entropies for two different types of entangling regions in a field theory along the RG flow across dimensions to construct candidate c -functions which satisfy one of the two criteria but not both. In due process we also discuss an interesting connection between corner contributions to the entanglement entropy and the topology of the compact internal space. As concrete examples for both approaches, we holographically study twisted compactifications of 4d $$ \mathcal{N} $$ N = 4 SYM and compactifications of 6d $$ \mathcal{N} $$ N = (2, 0) theories.
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271 members
Édgar Roldán
  • Section of Quantitative Life Sciences
Humberto Cabrera
  • Section of Applied Physics
Marco Zennaro
  • Telecommunications/ICT for Development Laboratory
Arturo Rodolfo Sanchez
  • Section of High Energy, Cosmology and Astroparticle Physics
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