Ahana Chakraborty - Academia.edu (original) (raw)

Papers by Ahana Chakraborty

Physical Review B

We study the transition from a many-body localized phase to an ergodic phase in spin chain with c... more We study the transition from a many-body localized phase to an ergodic phase in spin chain with correlated random magnetic fields. Using multiple statistical measures like gap statistics and extremal entanglement spectrum distributions, we find the phase diagram in the disorder-correlation plane, where the transition happens at progressively larger values of the correlation with increasing values of disorder. We then show that one can use the average of sample variance of magnetic fields as a single parameter which encodes the effects of the correlated disorder. The distributions and averages of various statistics collapse into a single curve as a function of this parameter. This also allows us to analytically calculate the phase diagram in the disorder-correlation plane.

arXiv: Superconductivity, Nov 14, 2021

Recently, the possibility of inducing superconductivity for electrons in two dimensional material... more Recently, the possibility of inducing superconductivity for electrons in two dimensional materials has been proposed via cavity-mediated pairing. The cavity-mediated electron-electron interactions are long range, which has two main effects: firstly, within the standard BCS-type pairing mediated by adiabatic photons, the superconducting critical temperature depends polynomially on the coupling strength, instead of the exponential dependence characterizing the phonon-mediated pairing; secondly, as we show here, the effect of photon fluctuations is significantly enhanced. These mediate novel non-BCS-type pairing processes, via non-adiabatic photons, which are not sensitive to the electron occupation but rather to the electron dispersion and lifetime at the Fermi surface. Therefore, while the leading temperature dependence of BCS pairing comes from the smoothening of the Fermi-Dirac distribution, the temperature dependence of the fluctuation-induced pairing comes from the electron lifetime. For realistic parameters, also including cavity loss, this results into a critical temperature which can be more than one order of magnitude larger than the BCS prediction. Moreover, a finite average number photons (as can be achieved by incoherently pumping the cavity) adds to the fluctuations and leads to a further enhancement of the critical temperature.

Nature Communications, 2022

Physical Review B, 2020

We study the dynamics of one and two dimensional disordered lattice bosons/fermions initialized t... more We study the dynamics of one and two dimensional disordered lattice bosons/fermions initialized to a Fock state with a pattern of 1 and 0 particles on A andĀ sites. For non-interacting systems we establish a universal relation between the long time density imbalance between A andĀ site, I(∞), the localization length ξ l , and the geometry of the initial pattern. For alternating initial pattern of 1 and 0 particles in 1 dimension, I(∞) = tanh[a/ξ l ], where a is the lattice spacing. For systems with mobility edge, we find analytic relations between I(∞), the effective localization lengthξ l and the fraction of localized states f l. The imbalance as a function of disorder shows non-analytic behaviour when the mobility edge passes through a band edge. For interacting bosonic systems, we show that dissipative processes lead to a decay of the memory of initial conditions. However, the excitations created in the process act as a bath, whose noise correlators retain information of the initial pattern. This sustains a finite imbalance at long times in strongly disordered interacting systems.

Bulletin of the American Physical Society, 2019

arXiv: Statistical Mechanics, 2018

Schwinger Keldysh field theory is a widely used paradigm to study non-equilibrium dynamics of qua... more Schwinger Keldysh field theory is a widely used paradigm to study non-equilibrium dynamics of quantum many-body systems starting from a thermal state. We extend this formalism to describe non-equilibrium dynamics of quantum systems starting from arbitrary initial many-body density matrices. We show how this can be done for both bosons and fermions, and for both closed and open quantum systems, using additional sources coupled to bilinears of the fields at the initial time, calculating Greens' functions in a theory with these sources, and taking appropriate set of derivatives of these Greens' functions w.r.t. initial sources to obtain physical observables. The set of derivatives depend on the initial density matrix. The physical correlators in a dynamics with arbitrary initial conditions do not satisfy Wick' theorem, even for non-interacting systems. However our formalism constructs intermediate "n-particle Greens' functions" which obey Wick's theorem an...

arXiv: Superconductivity, 2020

Recently, the possibility of inducing superconductivity for electrons in two dimensional material... more Recently, the possibility of inducing superconductivity for electrons in two dimensional materials has been proposed via cavity-mediated pairing. The cavity-mediated electron-electron interactions are long range, which has two main effects: firstly, within the standard BCS-type pairing mediated by adiabatic photons, the superconducting critical temperature depends polynomially on the coupling strength, instead of the exponential dependence characterizing the phonon-mediated pairing; secondly, as we show here, the effect of photon fluctuations is significantly enhanced. These mediate novel non-BCS-type pairing processes, via non-adiabatic photons, which are not sensitive to the electron occupation but rather to the electron dispersion and lifetime at the Fermi surface. Therefore, while the leading temperature dependence of BCS pairing comes from the smoothening of the Fermi-Dirac distribution, the temperature dependence of the fluctuation-induced pairing comes from the electron lifet...

arXiv: Statistical Mechanics, 2018

We propose a new method of calculating entanglement entropy of a many-body interacting Bosonic sy... more We propose a new method of calculating entanglement entropy of a many-body interacting Bosonic system (open or closed) in a field theoretic approach without replica methods. The Wigner function and Renyi entropy of a Bosonic system undergoing arbitrary non-equilibrium dynamics can be obtained from its Wigner characteristic function, which we identify with the Schwinger Keldysh partition function in presence of quantum sources turned on at the time of measurement. For non-interacting many body systems, starting from arbitrary density matrices, we provide exact analytic formulae for Wigner function and entanglement entropy in terms of the single particle Green's functions. For interacting systems, we relate the Wigner characteristic to the connected multi-particle correlators of the system. We use this formalism to study the evolution of an open quantum system from a Fock state with negative Wigner function and zero entropy, to a thermal state with positive Wigner function and fin...

Bulletin of the American Physical Society, 2018

Physical Review Letters, 2021

We propose a new field theoretic method for calculating Renyi entropy of a subsystem of many inte... more We propose a new field theoretic method for calculating Renyi entropy of a subsystem of many interacting Bosons without using replica methods. This method is applicable to dynamics of both open and closed quantum systems starting from arbitrary initial conditions. Our method identifies the Wigner characteristic of a reduced density matrix with the partition function of the whole system with a set of linear sources turned on only in the subsystem and uses this to calculate the subsystem's Renyi entropy. We use this method to study evolution of Renyi entropy in a non-interacting open quantum system starting from an initial Fock state. We find a relation between the initial state and final density matrix which determines whether the entropy shows non-monotonic behaviour in time. For non-Markovian dynamics, we show that the entropy approaches its steady state value as a power law with exponents governed by non-analyticities of the bath. We illustrate that this field-theoretic method can be used to study large bosonic open quantum systems.

Physical Review Letters, 2021

Recently, the possibility of inducing superconductivity for electrons in two dimensional material... more Recently, the possibility of inducing superconductivity for electrons in two dimensional materials has been proposed via cavity-mediated pairing. The cavity-mediated electron-electron interactions are long range, which has two main effects: firstly, within the standard BCS-type pairing mediated by adiabatic photons, the superconducting critical temperature depends polynomially on the coupling strength, instead of the exponential dependence characterizing the phonon-mediated pairing; secondly, as we show here, the effect of photon fluctuations is significantly enhanced. These mediate novel non-BCS-type pairing processes, via non-adiabatic photons, which are not sensitive to the electron occupation but rather to the electron dispersion and lifetime at the Fermi surface. Therefore, while the leading temperature dependence of BCS pairing comes from the smoothening of the Fermi-Dirac distribution, the temperature dependence of the fluctuation-induced pairing comes from the electron lifetime. For realistic parameters, also including cavity loss, this results into a critical temperature which can be more than one order of magnitude larger than the BCS prediction. Moreover, a finite average number photons (as can be achieved by incoherently pumping the cavity) adds to the fluctuations and leads to a further enhancement of the critical temperature.

Ultrasound has been used to non-invasively manipulate neuronal functions in humans and other anim... more Ultrasound has been used to non-invasively manipulate neuronal functions in humans and other animals1–4. However, this approach is limited as it has been challenging to target specific cells within the brain or body5–8. Here, we identify human Transient Receptor Potential A1 (hsTRPA1) as a candidate that confers ultrasound sensitivity to mammalian cells. Ultrasound-evoked gating of hsTRPA1 specifically requires its N-terminal tip region and cholesterol interactions; and target cells with an intact actin cytoskeleton, revealing elements of the sonogenetic mechanism. Next, we use calcium imaging and electrophysiology to show that hsTRPA1 potentiates ultrasound-evoked responses in primary neurons. Furthermore, unilateral expression of hsTRPA1 in mouse layer V motor cortical neurons leads to c-fos expression and contralateral limb responses in response to ultrasound delivered through an intact skull. Collectively, we demonstrate that hsTRPA1-based sonogenetics can effectively manipulate...

Physical Review B, 2018

The Born-Markov approximation is widely used to study dynamics of open quantum systems coupled to... more The Born-Markov approximation is widely used to study dynamics of open quantum systems coupled to external baths. Using Keldysh formalism, we show that the dynamics of a system of bosons (fermions) linearly coupled to non-interacting bosonic (fermionic) bath falls outside this paradigm if the bath spectral function has non-analyticities as a function of frequency. In this case, we show that the dissipative and noise kernels governing the dynamics have distinct power law tails. The Green's functions show a short time "quasi" Markovian exponential decay before crossing over to a power law tail governed by the non-analyticity of the spectral function. We study a system of bosons (fermions) hopping on a one dimensional lattice, where each site is coupled linearly to an independent bath of non-interacting bosons (fermions). We obtain exact expressions for the Green's functions of this system which show power law decay ∼ |t − t | −3/2. We use these to calculate density and current profile, as well as unequal time current-current correlators. While the density and current profiles show interesting quantitative deviations from Markovian results, the current-current correlators show qualitatively distinct long time power law tails |t − t | −3 characteristic of non-Markovian dynamics. We show that the power law decays survive in presence of inter-particle interaction in the system, but the cross-over time scale is shifted to larger values with increasing interaction strength. I. INTRODUCTION

Animals integrate changes in external and internal environments to generate behavior. While neura... more Animals integrate changes in external and internal environments to generate behavior. While neural circuits detecting external cues have been mapped, less is known about how internal states like hunger are integrated into behavioral outputs. We use the nematode C. elegans to decode how changes in internal nutritional status affects chemosensory behaviors. We show that acute food deprivation leads to a reversible decline in repellent, but not attractant, sensitivity. This behavioral change requires two conserved transcription factors MML-1 (Mondo A) and HLH-30 (TFEB), both of which translocate from the intestinal nuclei to the cytoplasm upon food deprivation. Next, we identify insulin-like peptides INS-23 and INS-31 as candidate ligands relaying food-status signals from the intestine to other tissues. Furthermore, we show that ASI chemosensory neurons use the DAF-2 insulin receptor, PI-3 Kinase, and the mTOR complex to integrate these intestine-released peptides. Together, our study ...

Physical Review B, 2019

field theory is a widely used paradigm to study non-equilibrium dynamics of quantum many-body sys... more field theory is a widely used paradigm to study non-equilibrium dynamics of quantum many-body systems starting from a thermal state. We extend this formalism to describe non-equilibrium dynamics of quantum systems starting from arbitrary initial many-body density matrices. We show how this can be done for both Bosons and Fermions, and for both closed and open quantum systems, using additional sources coupled to bilinears of the fields at the initial time, calculating Green's functions in a theory with these sources, and then taking appropriate set of derivatives of these Green's functions w.r.t. initial sources to obtain physical observables. The set of derivatives depend on the initial density matrix. The physical correlators in a dynamics with arbitrary initial conditions do not satisfy Wick's theorem, even for non-interacting systems. However our formalism constructs intermediate "n-particle Green's functions" which obey Wick's theorem and provide a prescription to obtain physical correlation functions from them. This allows us to obtain analytic answers for all physical many body correlation functions of a non-interacting system even when it is initialized to an arbitrary density matrix. We use these exact expressions to obtain an estimate of the violation of Wick's theorem, and relate it to presence of connected multi-particle initial correlations in the system. We illustrate this new formalism by calculating density and current profiles in many body Fermionic and Bosonic open quantum systems initialized to non-trivial density matrices. We have also shown how this formalism can be extended to interacting many body systems.

Physical Review B

We study the transition from a many-body localized phase to an ergodic phase in spin chain with c... more We study the transition from a many-body localized phase to an ergodic phase in spin chain with correlated random magnetic fields. Using multiple statistical measures like gap statistics and extremal entanglement spectrum distributions, we find the phase diagram in the disorder-correlation plane, where the transition happens at progressively larger values of the correlation with increasing values of disorder. We then show that one can use the average of sample variance of magnetic fields as a single parameter which encodes the effects of the correlated disorder. The distributions and averages of various statistics collapse into a single curve as a function of this parameter. This also allows us to analytically calculate the phase diagram in the disorder-correlation plane.

arXiv: Superconductivity, Nov 14, 2021

Recently, the possibility of inducing superconductivity for electrons in two dimensional material... more Recently, the possibility of inducing superconductivity for electrons in two dimensional materials has been proposed via cavity-mediated pairing. The cavity-mediated electron-electron interactions are long range, which has two main effects: firstly, within the standard BCS-type pairing mediated by adiabatic photons, the superconducting critical temperature depends polynomially on the coupling strength, instead of the exponential dependence characterizing the phonon-mediated pairing; secondly, as we show here, the effect of photon fluctuations is significantly enhanced. These mediate novel non-BCS-type pairing processes, via non-adiabatic photons, which are not sensitive to the electron occupation but rather to the electron dispersion and lifetime at the Fermi surface. Therefore, while the leading temperature dependence of BCS pairing comes from the smoothening of the Fermi-Dirac distribution, the temperature dependence of the fluctuation-induced pairing comes from the electron lifetime. For realistic parameters, also including cavity loss, this results into a critical temperature which can be more than one order of magnitude larger than the BCS prediction. Moreover, a finite average number photons (as can be achieved by incoherently pumping the cavity) adds to the fluctuations and leads to a further enhancement of the critical temperature.

Nature Communications, 2022

Physical Review B, 2020

We study the dynamics of one and two dimensional disordered lattice bosons/fermions initialized t... more We study the dynamics of one and two dimensional disordered lattice bosons/fermions initialized to a Fock state with a pattern of 1 and 0 particles on A andĀ sites. For non-interacting systems we establish a universal relation between the long time density imbalance between A andĀ site, I(∞), the localization length ξ l , and the geometry of the initial pattern. For alternating initial pattern of 1 and 0 particles in 1 dimension, I(∞) = tanh[a/ξ l ], where a is the lattice spacing. For systems with mobility edge, we find analytic relations between I(∞), the effective localization lengthξ l and the fraction of localized states f l. The imbalance as a function of disorder shows non-analytic behaviour when the mobility edge passes through a band edge. For interacting bosonic systems, we show that dissipative processes lead to a decay of the memory of initial conditions. However, the excitations created in the process act as a bath, whose noise correlators retain information of the initial pattern. This sustains a finite imbalance at long times in strongly disordered interacting systems.

Bulletin of the American Physical Society, 2019

arXiv: Statistical Mechanics, 2018

Schwinger Keldysh field theory is a widely used paradigm to study non-equilibrium dynamics of qua... more Schwinger Keldysh field theory is a widely used paradigm to study non-equilibrium dynamics of quantum many-body systems starting from a thermal state. We extend this formalism to describe non-equilibrium dynamics of quantum systems starting from arbitrary initial many-body density matrices. We show how this can be done for both bosons and fermions, and for both closed and open quantum systems, using additional sources coupled to bilinears of the fields at the initial time, calculating Greens' functions in a theory with these sources, and taking appropriate set of derivatives of these Greens' functions w.r.t. initial sources to obtain physical observables. The set of derivatives depend on the initial density matrix. The physical correlators in a dynamics with arbitrary initial conditions do not satisfy Wick' theorem, even for non-interacting systems. However our formalism constructs intermediate "n-particle Greens' functions" which obey Wick's theorem an...

arXiv: Superconductivity, 2020

Recently, the possibility of inducing superconductivity for electrons in two dimensional material... more Recently, the possibility of inducing superconductivity for electrons in two dimensional materials has been proposed via cavity-mediated pairing. The cavity-mediated electron-electron interactions are long range, which has two main effects: firstly, within the standard BCS-type pairing mediated by adiabatic photons, the superconducting critical temperature depends polynomially on the coupling strength, instead of the exponential dependence characterizing the phonon-mediated pairing; secondly, as we show here, the effect of photon fluctuations is significantly enhanced. These mediate novel non-BCS-type pairing processes, via non-adiabatic photons, which are not sensitive to the electron occupation but rather to the electron dispersion and lifetime at the Fermi surface. Therefore, while the leading temperature dependence of BCS pairing comes from the smoothening of the Fermi-Dirac distribution, the temperature dependence of the fluctuation-induced pairing comes from the electron lifet...

arXiv: Statistical Mechanics, 2018

We propose a new method of calculating entanglement entropy of a many-body interacting Bosonic sy... more We propose a new method of calculating entanglement entropy of a many-body interacting Bosonic system (open or closed) in a field theoretic approach without replica methods. The Wigner function and Renyi entropy of a Bosonic system undergoing arbitrary non-equilibrium dynamics can be obtained from its Wigner characteristic function, which we identify with the Schwinger Keldysh partition function in presence of quantum sources turned on at the time of measurement. For non-interacting many body systems, starting from arbitrary density matrices, we provide exact analytic formulae for Wigner function and entanglement entropy in terms of the single particle Green's functions. For interacting systems, we relate the Wigner characteristic to the connected multi-particle correlators of the system. We use this formalism to study the evolution of an open quantum system from a Fock state with negative Wigner function and zero entropy, to a thermal state with positive Wigner function and fin...

Bulletin of the American Physical Society, 2018

Physical Review Letters, 2021

We propose a new field theoretic method for calculating Renyi entropy of a subsystem of many inte... more We propose a new field theoretic method for calculating Renyi entropy of a subsystem of many interacting Bosons without using replica methods. This method is applicable to dynamics of both open and closed quantum systems starting from arbitrary initial conditions. Our method identifies the Wigner characteristic of a reduced density matrix with the partition function of the whole system with a set of linear sources turned on only in the subsystem and uses this to calculate the subsystem's Renyi entropy. We use this method to study evolution of Renyi entropy in a non-interacting open quantum system starting from an initial Fock state. We find a relation between the initial state and final density matrix which determines whether the entropy shows non-monotonic behaviour in time. For non-Markovian dynamics, we show that the entropy approaches its steady state value as a power law with exponents governed by non-analyticities of the bath. We illustrate that this field-theoretic method can be used to study large bosonic open quantum systems.

Physical Review Letters, 2021

Recently, the possibility of inducing superconductivity for electrons in two dimensional material... more Recently, the possibility of inducing superconductivity for electrons in two dimensional materials has been proposed via cavity-mediated pairing. The cavity-mediated electron-electron interactions are long range, which has two main effects: firstly, within the standard BCS-type pairing mediated by adiabatic photons, the superconducting critical temperature depends polynomially on the coupling strength, instead of the exponential dependence characterizing the phonon-mediated pairing; secondly, as we show here, the effect of photon fluctuations is significantly enhanced. These mediate novel non-BCS-type pairing processes, via non-adiabatic photons, which are not sensitive to the electron occupation but rather to the electron dispersion and lifetime at the Fermi surface. Therefore, while the leading temperature dependence of BCS pairing comes from the smoothening of the Fermi-Dirac distribution, the temperature dependence of the fluctuation-induced pairing comes from the electron lifetime. For realistic parameters, also including cavity loss, this results into a critical temperature which can be more than one order of magnitude larger than the BCS prediction. Moreover, a finite average number photons (as can be achieved by incoherently pumping the cavity) adds to the fluctuations and leads to a further enhancement of the critical temperature.

Ultrasound has been used to non-invasively manipulate neuronal functions in humans and other anim... more Ultrasound has been used to non-invasively manipulate neuronal functions in humans and other animals1–4. However, this approach is limited as it has been challenging to target specific cells within the brain or body5–8. Here, we identify human Transient Receptor Potential A1 (hsTRPA1) as a candidate that confers ultrasound sensitivity to mammalian cells. Ultrasound-evoked gating of hsTRPA1 specifically requires its N-terminal tip region and cholesterol interactions; and target cells with an intact actin cytoskeleton, revealing elements of the sonogenetic mechanism. Next, we use calcium imaging and electrophysiology to show that hsTRPA1 potentiates ultrasound-evoked responses in primary neurons. Furthermore, unilateral expression of hsTRPA1 in mouse layer V motor cortical neurons leads to c-fos expression and contralateral limb responses in response to ultrasound delivered through an intact skull. Collectively, we demonstrate that hsTRPA1-based sonogenetics can effectively manipulate...

Physical Review B, 2018

The Born-Markov approximation is widely used to study dynamics of open quantum systems coupled to... more The Born-Markov approximation is widely used to study dynamics of open quantum systems coupled to external baths. Using Keldysh formalism, we show that the dynamics of a system of bosons (fermions) linearly coupled to non-interacting bosonic (fermionic) bath falls outside this paradigm if the bath spectral function has non-analyticities as a function of frequency. In this case, we show that the dissipative and noise kernels governing the dynamics have distinct power law tails. The Green's functions show a short time "quasi" Markovian exponential decay before crossing over to a power law tail governed by the non-analyticity of the spectral function. We study a system of bosons (fermions) hopping on a one dimensional lattice, where each site is coupled linearly to an independent bath of non-interacting bosons (fermions). We obtain exact expressions for the Green's functions of this system which show power law decay ∼ |t − t | −3/2. We use these to calculate density and current profile, as well as unequal time current-current correlators. While the density and current profiles show interesting quantitative deviations from Markovian results, the current-current correlators show qualitatively distinct long time power law tails |t − t | −3 characteristic of non-Markovian dynamics. We show that the power law decays survive in presence of inter-particle interaction in the system, but the cross-over time scale is shifted to larger values with increasing interaction strength. I. INTRODUCTION

Animals integrate changes in external and internal environments to generate behavior. While neura... more Animals integrate changes in external and internal environments to generate behavior. While neural circuits detecting external cues have been mapped, less is known about how internal states like hunger are integrated into behavioral outputs. We use the nematode C. elegans to decode how changes in internal nutritional status affects chemosensory behaviors. We show that acute food deprivation leads to a reversible decline in repellent, but not attractant, sensitivity. This behavioral change requires two conserved transcription factors MML-1 (Mondo A) and HLH-30 (TFEB), both of which translocate from the intestinal nuclei to the cytoplasm upon food deprivation. Next, we identify insulin-like peptides INS-23 and INS-31 as candidate ligands relaying food-status signals from the intestine to other tissues. Furthermore, we show that ASI chemosensory neurons use the DAF-2 insulin receptor, PI-3 Kinase, and the mTOR complex to integrate these intestine-released peptides. Together, our study ...

Physical Review B, 2019

field theory is a widely used paradigm to study non-equilibrium dynamics of quantum many-body sys... more field theory is a widely used paradigm to study non-equilibrium dynamics of quantum many-body systems starting from a thermal state. We extend this formalism to describe non-equilibrium dynamics of quantum systems starting from arbitrary initial many-body density matrices. We show how this can be done for both Bosons and Fermions, and for both closed and open quantum systems, using additional sources coupled to bilinears of the fields at the initial time, calculating Green's functions in a theory with these sources, and then taking appropriate set of derivatives of these Green's functions w.r.t. initial sources to obtain physical observables. The set of derivatives depend on the initial density matrix. The physical correlators in a dynamics with arbitrary initial conditions do not satisfy Wick's theorem, even for non-interacting systems. However our formalism constructs intermediate "n-particle Green's functions" which obey Wick's theorem and provide a prescription to obtain physical correlation functions from them. This allows us to obtain analytic answers for all physical many body correlation functions of a non-interacting system even when it is initialized to an arbitrary density matrix. We use these exact expressions to obtain an estimate of the violation of Wick's theorem, and relate it to presence of connected multi-particle initial correlations in the system. We illustrate this new formalism by calculating density and current profiles in many body Fermionic and Bosonic open quantum systems initialized to non-trivial density matrices. We have also shown how this formalism can be extended to interacting many body systems.