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Xia-ji Liu - One of the best experts on this subject based on the ideXlab platform.
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low momentum Dynamic Structure factor of a strongly interacting fermi gas at finite temperature the goldstone phonon and its landau damping
Physical Review A, 2018Co-Authors: Peng Zou, Xia-ji LiuAbstract:We develop a microscopic theory of the Dynamic Structure factor to describe the Bogoliubov-Anderson-Goldstone phonon mode and its damping rate in a strongly interacting Fermi gas at finite temperature. It is based on a density functional approach---the so-called superfluid local density approximation. The accuracy of the theory is quantitatively examined by comparing the theoretical predictions with recent experimental measurements for the local Dynamic Structure factor of a nearly homogeneous unitary Fermi gas at low transferred momentum [S. Hoinka et al., Nat. Phys. 13, 943 (2017)], without any free parameters. We calculate the Dynamic Structure factor as functions of temperature and transferred momentum, and determine the temperature evolution of the phonon damping rate, by considering the dominant decay process of the phonon mode via scatterings off fermionic quasiparticles. These predictions can be confronted with future Bragg scattering experiments on a unitary Fermi gas near the superfluid transition.
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low momentum Dynamic Structure factor of a strongly interacting fermi gas at finite temperature a two fluid hydroDynamic description
Physical Review A, 2018Co-Authors: Peng Zou, Xia-ji LiuAbstract:We provide a description of the Dynamic Structure factor of a homogeneous unitary Fermi gas at low momentum and low frequency, based on the dissipative two-fluid hydroDynamic theory. The viscous relaxation time is estimated and is used to determine the regime where the hydroDynamic theory is applicable and to understand the nature of sound waves in the density response near the superfluid phase transition. By collecting the best knowledge on the shear viscosity and thermal conductivity known so far, we calculate the various diffusion coefficients and obtain the damping width of the (first and second) sounds. We find that the damping width of the first sound is greatly enhanced across the superfluid transition and very close to the transition the second sound might be resolved in the density response for the transferred momentum up to the half of Fermi momentum. Our work is motivated by the recent measurement of the local Dynamic Structure factor at low momentum at Swinburne University of Technology and the on-going experiment on sound attenuation of a homogeneous unitary Fermi gas at Massachusetts Institute of Technology. We discuss how the measurement of the velocity and damping width of the sound modes in low-momentum Dynamic Structure factor may lead to an improved determination of the universal superfluid density, shear viscosity and thermal conductivity of a unitary Fermi gas.
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Dynamic Structure factor of a strongly correlated fermi superfluid within a density functional theory approach
New Journal of Physics, 2016Co-Authors: Peng Zou, F Dalfovo, Rishi Sharma, Xia-ji LiuAbstract:We theoretically investigate the Dynamic Structure factor of a strongly interacting Fermi gas at the crossover from Bardeen–Cooper–Schrieffer superfluids to Bose–Einstein condensates, by developing an improved random phase approximation within the framework of a density functional theory (DFT)—the so-called superfluid local density approximation. Compared with the previous random-phase-approximation studies based on the standard Bogoliubov–de Gennes equations, the use of the DFT greatly improves the accuracy of the equation of state at the crossover, and leads to a better description of both collective Bogoliubov-Anderson-Goldstone phonon mode and single-particle fermionic excitations at small transferred momentum. Near unitarity, where the s-wave scattering length diverges, we show that the single-particle excitations start to significantly contribute to the spectrum of Dynamic Structure factor once the frequency is above a threshold of the energy gap at . The sharp rise in the spectrum at this threshold can be utilized to measure the pairing gap Δ. Together with the sound velocity determined from the phonon branch, the Dynamic Structure factor provides us some key information of the crossover Fermi superfluid. Our predictions could be examined in experiments with 6Li or 40K atoms using Bragg spectroscopy.
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Dynamic Structure factor of a strongly correlated fermi superfluid within a density functional theory approach
arXiv: Quantum Gases, 2016Co-Authors: Peng Zou, F Dalfovo, Rishi Sharma, Xia-ji LiuAbstract:We theoretically investigate the Dynamic Structure factor of a strongly interacting Fermi gas at the crossover from Bardeen-Cooper-Schrieffer superfluids to Bose-Einstein condensates, by developing an improved random phase approximation within the framework of a density functional theory - the so-called superfluid local density approximation. Compared with the previous random-phase-approximation studies based on the standard Bogoliubov-de Gennes equations, the use of the density functional theory greatly improves the accuracy of the equation of state at the crossover, and leads to a better description of both collective Bogoliubov-Anderson-Goldstone phonon mode and single-particle fermionic excitations at small transferred momentum. Near unitarity, where the s-wave scattering length diverges, we show that the single-particle excitations start to significantly contribute to the spectrum of Dynamic Structure factor once the frequency is above a threshold of the energy gap at $2\Delta$. The sharp rise in the spectrum at this threshold can be utilized to measure the pairing gap $\Delta$. Together with the sound velocity determined from the phonon branch, the Dynamic Structure factor provides us some key information of the crossover Fermi superfluid. Our predictions could be examined in experiments with $^{6}$Li or $^{40}$K atoms using Bragg spectroscopy.
Peng Zou - One of the best experts on this subject based on the ideXlab platform.
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low momentum Dynamic Structure factor of a strongly interacting fermi gas at finite temperature the goldstone phonon and its landau damping
Physical Review A, 2018Co-Authors: Peng Zou, Xia-ji LiuAbstract:We develop a microscopic theory of the Dynamic Structure factor to describe the Bogoliubov-Anderson-Goldstone phonon mode and its damping rate in a strongly interacting Fermi gas at finite temperature. It is based on a density functional approach---the so-called superfluid local density approximation. The accuracy of the theory is quantitatively examined by comparing the theoretical predictions with recent experimental measurements for the local Dynamic Structure factor of a nearly homogeneous unitary Fermi gas at low transferred momentum [S. Hoinka et al., Nat. Phys. 13, 943 (2017)], without any free parameters. We calculate the Dynamic Structure factor as functions of temperature and transferred momentum, and determine the temperature evolution of the phonon damping rate, by considering the dominant decay process of the phonon mode via scatterings off fermionic quasiparticles. These predictions can be confronted with future Bragg scattering experiments on a unitary Fermi gas near the superfluid transition.
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low momentum Dynamic Structure factor of a strongly interacting fermi gas at finite temperature a two fluid hydroDynamic description
Physical Review A, 2018Co-Authors: Peng Zou, Xia-ji LiuAbstract:We provide a description of the Dynamic Structure factor of a homogeneous unitary Fermi gas at low momentum and low frequency, based on the dissipative two-fluid hydroDynamic theory. The viscous relaxation time is estimated and is used to determine the regime where the hydroDynamic theory is applicable and to understand the nature of sound waves in the density response near the superfluid phase transition. By collecting the best knowledge on the shear viscosity and thermal conductivity known so far, we calculate the various diffusion coefficients and obtain the damping width of the (first and second) sounds. We find that the damping width of the first sound is greatly enhanced across the superfluid transition and very close to the transition the second sound might be resolved in the density response for the transferred momentum up to the half of Fermi momentum. Our work is motivated by the recent measurement of the local Dynamic Structure factor at low momentum at Swinburne University of Technology and the on-going experiment on sound attenuation of a homogeneous unitary Fermi gas at Massachusetts Institute of Technology. We discuss how the measurement of the velocity and damping width of the sound modes in low-momentum Dynamic Structure factor may lead to an improved determination of the universal superfluid density, shear viscosity and thermal conductivity of a unitary Fermi gas.
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Dynamic Structure factor of a strongly correlated fermi superfluid within a density functional theory approach
New Journal of Physics, 2016Co-Authors: Peng Zou, F Dalfovo, Rishi Sharma, Xia-ji LiuAbstract:We theoretically investigate the Dynamic Structure factor of a strongly interacting Fermi gas at the crossover from Bardeen–Cooper–Schrieffer superfluids to Bose–Einstein condensates, by developing an improved random phase approximation within the framework of a density functional theory (DFT)—the so-called superfluid local density approximation. Compared with the previous random-phase-approximation studies based on the standard Bogoliubov–de Gennes equations, the use of the DFT greatly improves the accuracy of the equation of state at the crossover, and leads to a better description of both collective Bogoliubov-Anderson-Goldstone phonon mode and single-particle fermionic excitations at small transferred momentum. Near unitarity, where the s-wave scattering length diverges, we show that the single-particle excitations start to significantly contribute to the spectrum of Dynamic Structure factor once the frequency is above a threshold of the energy gap at . The sharp rise in the spectrum at this threshold can be utilized to measure the pairing gap Δ. Together with the sound velocity determined from the phonon branch, the Dynamic Structure factor provides us some key information of the crossover Fermi superfluid. Our predictions could be examined in experiments with 6Li or 40K atoms using Bragg spectroscopy.
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Dynamic Structure factor of a strongly correlated fermi superfluid within a density functional theory approach
arXiv: Quantum Gases, 2016Co-Authors: Peng Zou, F Dalfovo, Rishi Sharma, Xia-ji LiuAbstract:We theoretically investigate the Dynamic Structure factor of a strongly interacting Fermi gas at the crossover from Bardeen-Cooper-Schrieffer superfluids to Bose-Einstein condensates, by developing an improved random phase approximation within the framework of a density functional theory - the so-called superfluid local density approximation. Compared with the previous random-phase-approximation studies based on the standard Bogoliubov-de Gennes equations, the use of the density functional theory greatly improves the accuracy of the equation of state at the crossover, and leads to a better description of both collective Bogoliubov-Anderson-Goldstone phonon mode and single-particle fermionic excitations at small transferred momentum. Near unitarity, where the s-wave scattering length diverges, we show that the single-particle excitations start to significantly contribute to the spectrum of Dynamic Structure factor once the frequency is above a threshold of the energy gap at $2\Delta$. The sharp rise in the spectrum at this threshold can be utilized to measure the pairing gap $\Delta$. Together with the sound velocity determined from the phonon branch, the Dynamic Structure factor provides us some key information of the crossover Fermi superfluid. Our predictions could be examined in experiments with $^{6}$Li or $^{40}$K atoms using Bragg spectroscopy.
J Colmenero - One of the best experts on this subject based on the ideXlab platform.
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single chain Dynamic Structure factor of linear polymers in an all polymer nano composite
Macromolecules, 2016Co-Authors: Arantxa Arbe, Jose A Pomposo, Isabel Asenjosanz, Debsindhu Bhowmik, Oxana Ivanova, J Kohlbrecher, J ColmeneroAbstract:We present neutron spin echo (NSE) experiments on the single chain Dynamic Structure factor of long poly(ethylene oxide) (PEO) linear chains in the presence of poly(methyl methacrylate)-based single-chain nanoparticles (SCNPs). A complementary structural characterization of the system discards a significant interpenetration of the components and reveals a close to globular conformation of the SCNPs when surrounded by PEO chains. Analogous NSE measurements on blends of PEO with the linear precursor chains of the SCNPs are taken as a reference for the Dynamics study. For short times (below approximately 5 ns) PEO in both mixtures exhibits slowed down Rouse Dynamics with respect to bulk PEO behavior. The similar deceleration observed in both environments suggests this effect to be due to the large Dynamic asymmetry in the mixtures as evidenced by DSC experiments. More interestingly, the NSE results at longer times reveal a spectacular increase of the explored volume of PEO chains in the all-polymer nanocompo...
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collective features in polyisobutylene a study of the static and Dynamic Structure factor by molecular Dynamics simulations
Macromolecules, 2014Co-Authors: Yasmin Khairy, F Alvarez, Arantxa Arbe, J ColmeneroAbstract:We present a study of the static and Dynamic Structure factor of polyisobutylene (PIB) by fully atomistic molecular Dynamics simulations. The reliability of the simulated cell is first assured by c...
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single chain Dynamic Structure factor of poly ethylene oxide in Dynamically asymmetric blends with poly methyl methacrylate neutron scattering and molecular Dynamics simulations
Macromolecules, 2012Co-Authors: Martin Brodeck, F Alvarez, J Colmenero, Dieter RichterAbstract:We have investigated the Dynamically asymmetric polymer blend composed of short (Mn ≈ 2 kg/mol) poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA) chains focusing on the collective Dynamics of the fast PEO component. Using neutron spin-echo (NSE) spectroscopy, the single chain Dynamic Structure factor of PEO was investigated and compared to results from molecular Dynamics simulations. After a successful validation of the simulations, a thorough analysis of the RPA approximation reveals the composition of the experimentally measured total scattering signal S(Q,t). Using the simulations, we show and calculate two contributions: (1) the relaxation of hydrogenated PEO against deuterated PEO, yielding the single chain Dynamic Structure factor of PEO, and (2) the relaxation of the PEO component against the PMMA matrix. For the short chains presented here the second contribution shows a significant decay at higher temperatures while it was previously shown that, in the case of long chains, no relaxa...
Wenyang Zhang - One of the best experts on this subject based on the ideXlab platform.
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a Dynamic Structure for high dimensional covariance matrices and its application in portfolio allocation
Journal of the American Statistical Association, 2017Co-Authors: Shaojun Guo, John Leigh Box, Wenyang ZhangAbstract:Estimation of high-dimensional covariance matrices is an interesting and important research topic. In this article, we propose a Dynamic Structure and develop an estimation procedure for high-dimensional covariance matrices. Asymptotic properties are derived to justify the estimation procedure and simulation studies are conducted to demonstrate its performance when the sample size is finite. By exploring a financial application, an empirical study shows that portfolio allocation based on Dynamic high-dimensional covariance matrices can significantly outperform the market from 1995 to 2014. Our proposed method also outperforms portfolio allocation based on the sample covariance matrix, the covariance matrix based on factor models, and the shrinkage estimator of covariance matrix. Supplementary materials for this article are available online.
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a Dynamic Structure for high dimensional covariance matrices and its application in portfolio allocation
arXiv: Methodology, 2015Co-Authors: Shaojun Guo, John Leigh Box, Wenyang ZhangAbstract:Estimation of high dimensional covariance matrices is an interesting and important research topic. In this paper, we propose a Dynamic Structure and develop an estimation procedure for high dimensional covariance matrices. Asymptotic properties are derived to justify the estimation procedure and simulation studies are conducted to demonstrate its performance when the sample size is finite. By exploring a financial application, an empirical study shows that portfolio allocation based on Dynamic high dimensional covariance matrices can significantly outperform the market from 1995 to 2014. Our proposed method also outperforms portfolio allocation based on the sample covariance matrix and the portfolio allocation proposed in Fan, Fan and Lv (2008).
V E Zakhvataev - One of the best experts on this subject based on the ideXlab platform.
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Dynamic Structure factor of a lipid bilayer in the presence of a high electric field
Journal of Chemical Physics, 2019Co-Authors: V E ZakhvataevAbstract:The influence of a high average electric field (∼1 V/nm) in the hydrophobic interior of a bilayer lipid membrane on short-wavelength in-plane phononic motions of lipid chains is considered. The average electric field is assumed to be nearly constant on a picosecond time scale and a nanometer length scale. This field may be induced, for instance, by externally applied subnanosecond electric pulses or the membrane dipole potential. Using a generalized hydroDynamic approach, we derive a corresponding electrohydroDynamic model generalized to high wave numbers. In the considered approximation, all electric field effects are reduced only to a constant contribution to the generalized isothermal compressibility modulus. The corresponding Dynamic Structure factor for a lipid bilayer is derived. We show that due to polarization effects, the high field can critically impact the Dynamics of longitudinal acousticlike modes at wave numbers near the major peak of the static Structure factor. We estimate quantitatively that for typical lipid bilayers, transverse high electric fields can cause strong phonon energy softening, enhancement of phonon population, and formation of a gap in the dispersion of excitation frequency. The results obtained agree with simulations of the initiation of lipid bilayer electropores, suggesting that the proposed model reproduces the essential features of the field’s impact on atomic density fluctuations. The proposed mechanism may have significant implications for the understanding of electroporation, passive molecular transport, and spontaneous pore formation in lipid bilayers.The influence of a high average electric field (∼1 V/nm) in the hydrophobic interior of a bilayer lipid membrane on short-wavelength in-plane phononic motions of lipid chains is considered. The average electric field is assumed to be nearly constant on a picosecond time scale and a nanometer length scale. This field may be induced, for instance, by externally applied subnanosecond electric pulses or the membrane dipole potential. Using a generalized hydroDynamic approach, we derive a corresponding electrohydroDynamic model generalized to high wave numbers. In the considered approximation, all electric field effects are reduced only to a constant contribution to the generalized isothermal compressibility modulus. The corresponding Dynamic Structure factor for a lipid bilayer is derived. We show that due to polarization effects, the high field can critically impact the Dynamics of longitudinal acousticlike modes at wave numbers near the major peak of the static Structure factor. We estimate quantitatively t...
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nonequilibrium Dynamic Structure factor of a lipid bilayer in the presence of an in plane temperature gradient
Physical Review E, 2018Co-Authors: V E ZakhvataevAbstract:There is rapidly increasing evidence that nanoscale temperature heterogeneities are involved in important biological processes. Combining nanoheating and nanoscale thermosensors forms the basis of emerging unique methods of cell therapy, tissue engineering, and regenerative medicine. Understanding corresponding phenomena seems to require a mesoscopic nonequilibrium hydroDynamic theory. In this paper, a Langevin-type model of Dynamics of phonon modes propagating along a bilayer lipid membrane in the presence of an in-plane temperature gradient is proposed. Corresponding quantitative estimates for the Brillouin components of the nonequilibrium Dynamic Structure factor and the equal-time longitudinal momentum-density correlation function for a lipid bilayer are obtained. The analysis reveals that for typical values of parameters of lipid bilayer, the longitudinal temperature gradient of the order of 5qK for wave numbers q from 0.01 to 1nm^{-1} induces significant asymmetry of the Brillouin components of the Dynamic Structure factor and long-range spatial correlations in the plane of the bilayer. The corresponding membrane temperature gradients seem to be typical or achievable for cellular processes responsible for intracellular temperature variations and such external physical impacts as high-intensity electromagnetic pulses or heating of membrane-associated nanoparticles.
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refined Dynamic Structure factor of a lipid bilayer on scales comparable to its thickness
Journal of Experimental and Theoretical Physics, 2017Co-Authors: V E ZakhvataevAbstract:The structural inhomogeneity of a lipid bilayer is an obstacle to applying the classical Canham–Helfrich model to describe its Dynamics on nanometer length scales. In this paper, a refined expression for the free energy of a single-component lipid bilayer is used to describe the Dynamics of lipid density fluctuations. In particular, the expression with a term involving the gradient of the area per lipid [8] is used for the free energy per lipid. A refined expression has been derived for the Dynamic Structure factor of a free lipid bilayer in the hydroDynamic region. It leads to differences in the interpretation and values of the bilayer parameters in comparison with the standard model.
