arXiv subjects

Abhishek Joshi

Publications and source records attributed to Abhishek Joshi.

23 records · Page 2Linked to original sources

arXiv2019

Unsupervised Synthesis of Anomalies in Videos: Transforming the Normal

Abnormal activity recognition requires detection of occurrence of anomalous events that suffer from a severe imbalance in data. In a video, normal is used to describe activities that conform to usual events while the irregular events which do not conform to the normal are referred to as abnormal. It is far more common to observe normal data than to obtain abnormal data in visual surveillance. In this paper, we propose an approach where we can obtain abnormal data by transforming normal data. This is a challenging task that is solved through a multi-stage pipeline approach. We utilize a number of techniques from unsupervised segmentation in order to synthesize new samples of data that are transformed from an existing set of normal examples. Further, this synthesis approach has useful applications as a data augmentation technique. An incrementally trained Bayesian convolutional neural network (CNN) is used to carefully select the set of abnormal samples that can be added. Finally through this synthesis approach we obtain a comparable set of abnormal samples that can be used for training the CNN for the classification of normal vs abnormal samples. We show that this method generalizes to multiple settings by evaluating it on two real world datasets and achieves improved performance over other probabilistic techniques that have been used in the past for this task.

cs.CV↗

arXiv2019

Thermal transitions of the modulated superfluid for spin-orbit coupled correlated bosons in an optical lattice

We investigate the thermal physics of a Bose-Hubbard model with Rashba spin-orbit coupling starting from a strong coupling mean-field ground state. The essential role of the spin-orbit coupling $\left(γ\right)$ is to promote condensation of the bosons at a finite wavevector ${k}_{0}$. We find that the bosons display either homogeneous or phase-twisted or orbital ordered superfluid phases, depending on $γ$ and the inter-species interaction strength $λ$. We show that an increase of $γ$ leads to suppression of the critical interaction $U_c$ for the superfluid to Mott insulator transition in the ground state, and a reduction of the $T_c$ for superfluid to Bose-liquid transition at a fixed interaction. We capture the thermal broadening in the momentum distribution function, and the real space profiles of the thermally disordered magnetic textures, including their homogenization for $T \gtrsim T_{c}$. We provide a Landau theory based description of the ground state phase boundaries and thermal transition scales, and discuss experiments which can test our theory.

cond-mat.str-el↗

arXiv2018

Numerical Evaluation of the Relativistic Magnetized Plasma Susceptibility Tensor and Faraday Rotation Coefficients

Polarized models of relativistically hot astrophysical plasmas require transport coefficients as input: synchrotron absorption and emission coefficients in each of the four Stokes parameters, as well as three Faraday rotation coefficients. Approximations are known for all coefficients for a small set of electron distribution functions, such as the Maxwell-Juttner relativistic thermal distribution, and a general procedure has been obtained by Huang & Shcherbakov for an isotropic distribution function. Here we provide an alternative general procedure, with a full derivation, for calculating absorption and rotation coefficients for an arbitrary isotropic distribution function. Our method involves the computation of the full plasma susceptibility tensor, which in addition to absorption and rotation coefficients may be used to determine plasma modes and the dispersion relation. We implement the scheme in a publicly available library with a simple interface, thus allowing for easy incorporation into radiation transport codes. We also provide a comprehensive survey of the literature and comparison with earlier results.

astro-ph.IM↗

arXiv2018

Thermal phases of correlated lattice boson: a classical fluctuation theory

We present a method that generalises the standard mean field theory of correlated lattice bosons to include amplitude and phase fluctuations of the $U(1)$ field that induces onsite particle number mixing. This arises formally from an auxiliary field decomposition of the kinetic term in a Bose Hubbard model. We solve the resulting problem, initially, by using a classical approximation for the particle number mixing field and a Monte Carlo treatment of the resulting bosonic model. In two dimensions we obtain $T_c$ scales that dramatically improve on mean field theory and are within about 20% of full quantum Monte Carlo estimates. The `classical approximation' ground state, however, is still mean field, with an overestimate of the critical interaction, $U_c$, for the superfluid to Mott transition. By further including low order quantum fluctuations in the free energy functional we improve significantly on the $U_c$, and the overall thermal phase diagram. The classical approximation based method has a computational cost linear in system size. The methods readily generalise to multispecies bosons and the presence of traps.

cond-mat.str-el↗

arXiv2017

Thermal decoherence in a strongly correlated Bose liquid

We compute the single particle spectral function of a Bose liquid on a lattice, at integer filling, close to the superfluid-Mott transition. We use a `static path approximation' that retains all the classical thermal fluctuations in the problem, and a real space implementation of the random phase approximation (RPA) for the Green's functions on the thermally fluctuating backgrounds. This leads to the standard RPA answers in the ground state but captures the progressive damping of the excitations with increasing temperature. We focus on the momentum resolved lineshape across the superfluid to Bose liquid thermal transition. In the superfluid regime we observe a gapped `amplitude' mode, and gapless `phase' modes of positive and negative energy. The dispersion and weight of these modes changes with interaction but are almost temperature independent, even into the normal state, except near critical coupling. The damping of the modes varies roughly as $T^α f_{\bf k}$, where $T$ is the temperature and ${\bf k}$ the momentum, with $α\sim 0.5$ and $f_{\bf k}$ having non trivial momentum dependence. The Mott phase has gapped dispersive spectra. Near critical coupling the thermal Bose `liquid' is gapped, with progressive widening of the gap with increasing temperature, a feature that it shares with the Mott insulator.

cond-mat.str-el↗