# Browsing by Author "Chandrasekharan, S"

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Item Open Access Conductance of quantum impurity models from quantum monte carlo(Physical Review B - Condensed Matter and Materials Physics, 2010-10-28) Liu, D; Chandrasekharan, S; Baranger, HUThe conductance of two Anderson impurity models, one with twofold and another with fourfold degeneracy, representing two types of quantum dots, is calculated using a world-line quantum Monte Carlo (QMC) method. Extrapolation of the imaginary time QMC data to zero frequency yields the linear conductance, which is then compared to numerical renormalization-group results in order to assess its accuracy. We find that the method gives excellent results at low temperature (T TK) throughout the mixed-valence and Kondo regimes but it is unreliable for higher temperature. © 2010 The American Physical Society.Item Open Access Fermion bag approach to lattice field theories(Physical Review D - Particles, Fields, Gravitation and Cosmology, 2010-07-14) Chandrasekharan, SWe propose a new approach to the fermion sign problem in systems where there is a coupling U such that when it is infinite the fermions are paired into bosons, and there is no fermion permutation sign to worry about. We argue that as U becomes finite, fermions are liberated but are naturally confined to regions which we refer to as fermion bags. The fermion sign problem is then confined to these bags and may be solved using the determinantal trick. In the parameter regime where the fermion bags are small and their typical size does not grow with the system size, construction of Monte Carlo methods that are far more efficient than conventional algorithms should be possible. In the region where the fermion bags grow with system size, the fermion bag approach continues to provide an alternative approach to the problem but may lose its main advantage in terms of efficiency. The fermion bag approach also provides new insights and solutions to sign problems. A natural solution to the "silver blaze problem" also emerges. Using the three-dimensional massless lattice Thirring model as an example, we introduce the fermion bag approach and demonstrate some of these features. We compute the critical exponents at the quantum phase transition and find ν=0.87(2) and η=0.62(2). © 2010 The American Physical Society.Item Open Access Fermion Bags and A New Origin for a Fermion MassChandrasekharan, SThe fermion bag is a powerful idea that helps to solve fermion lattice field theories using Monte Carlo methods. Some sign problems that had remained unsolved earlier can be solved within this framework. In this work we argue that the fermion bag also gives insight into a new mechanism of fermion mass generation, especially at strong couplings where fermion masses are related to the fermion bag size. On the other hand, chiral condensates arise due to zero modes in the Dirac operator within a fermion bag. Although in traditional four-fermion models the two quantities seem to be related, we show that they can be decoupled. While fermion bags become small at strong couplings, the ability of zero modes of the Dirac operator within fermion bags to produce a chiral condensate, can be suppressed by the presence of additional zero modes from other fermions. Thus, fermions can become massive even without a chiral condensate. This new mechanism of mass generation was discovered long ago in lattice field theory, but has remained unappreciated. Recent work suggests that it may be of interest even in continuum quantum field theory.Item Open Access Finite size effects in the presence of a chemical potential: A study in the classical nonlinear O(2) sigma model(Physical Review D - Particles, Fields, Gravitation and Cosmology, 2010-06-08) Banerjee, D; Chandrasekharan, SIn the presence of a chemical potential, the physics of level crossings leads to singularities at zero temperature, even when the spatial volume is finite. These singularities are smoothed out at a finite temperature but leave behind nontrivial finite size effects which must be understood in order to extract thermodynamic quantities using Monte Carlo methods, particularly close to critical points. We illustrate some of these issues using the classical nonlinear O(2) sigma model with a coupling β and chemical potential μ on a 2+1-dimensional Euclidean lattice. In the conventional formulation this model suffers from a sign problem at nonzero chemical potential and hence cannot be studied with the Wolff cluster algorithm. However, when formulated in terms of the worldline of particles, the sign problem is absent, and the model can be studied efficiently with the "worm algorithm." Using this method we study the finite size effects that arise due to the chemical potential and develop an effective quantum mechanical approach to capture the effects. As a side result we obtain energy levels of up to four particles as a function of the box size and uncover a part of the phase diagram in the (β,μ) plane. © 2010 The American Physical Society.Item Open Access Perspectives of women in prostitution diversion program on DNA collection for a high-risk DNA database(International Journal of Criminal Justice Sciences, 2017-01-01) Katsanis, SH; Felini, M; Kim, J; Minear, M; Chandrasekharan, S; Wagner, JK© 2017 International Journal of Criminal Justice Sciences. DNA can be collected from women at high risk of violent crime as a pre-emptive biometric for post-mortem identification. We conducted focus groups with women in a prostitution diversion program that offers preemptive DNA collection. In general, the women supported the program but voiced concern for law enforcement collecting DNA. These data provide insights into the challenges of collecting DNA from women whom law enforcement might consider alternately to be victims or vilified. Hearing the voices of these women provides the forensic community an opportunity to design programs to minimize harm and maximize utility of DNA for victim identification.