Showing 1–50 of 62 results for author: Chung, M

Boundary Free Energies, Quenched Mixing, and Gibbs-State Selection in Disordered Ising Models

Authors: Mauris Chueng, Hexiang Wang, Keheng Zhu

Abstract: We study boundary free energies and half-space responses in nearest-neighbor disordered Ising models. First, we prove the existence of fixed-depth tangential pressure and identify its derivative almost everywhere with the limiting boundary response. Second, under quenched exponential boundary-response mixing, we prove Gibbs-state selection independence and exponential convergence of finite-depth p… ▽ More We study boundary free energies and half-space responses in nearest-neighbor disordered Ising models. First, we prove the existence of fixed-depth tangential pressure and identify its derivative almost everywhere with the limiting boundary response. Second, under quenched exponential boundary-response mixing, we prove Gibbs-state selection independence and exponential convergence of finite-depth pressures. We further show that uniform one-spin mixing yields DLR uniqueness and Gaussian normal localization, and verify the required mixing conditions whenever $(2d-1)\mathbb E\tanh(β\vert{}J\vert{})<1$. Finally, we establish an all-face surface limit in the bounded Dobrushin regime and provide counterexamples that delimit general surface and stiffness claims. △ Less

Submitted 25 August, 2026; originally announced August 2026.

Comments: 33 pages, 2 figures

MSC Class: 82B44; 82B20; 60K35

Multiple Softening Q-vectors Driving a Cascade of CDW Phases in $\mathrm{1T-VSe}_{2}$

Authors: Zheng-Hong Li, Yung-Ting Lee, Yu-Chan Tai, Cheng-Tien Chiang, Chien-Cheng Kuo, Meng-Kai Lin, Chun-Liang Lin, Hung-Chung Hsueh, Ming-Chiang Chung, Po-Tuan Chen, Chi-Cheng Lee

Abstract: Charge density wave (CDW) formation in two-dimensional materials is governed by complex competing lattice instabilities that remain incompletely understood. Here, we investigate the structural evolution of monolayer $\mathrm{1T-VSe}_{2}$ using first-principles electronic and phonon calculations. The pristine phase exhibits several imaginary-frequency phonon modes associated with dominant instabili… ▽ More Charge density wave (CDW) formation in two-dimensional materials is governed by complex competing lattice instabilities that remain incompletely understood. Here, we investigate the structural evolution of monolayer $\mathrm{1T-VSe}_{2}$ using first-principles electronic and phonon calculations. The pristine phase exhibits several imaginary-frequency phonon modes associated with dominant instability wave vectors $\mathrm{Q}_{CDW}$, which generate the first-generation CDW phases. Subsequent phonon analyses reveal that several of these intermediate structures remain dynamically unstable and undergo further symmetry-lowering distortions into larger superstructures. Through iterative phonon-driven relaxations, we identify multiple transformation pathways that converge toward the same low-energy $2\sqrt{3}\times4$ CDW configuration. Although these pathways originate from distinct intermediate CDW states, they ultimately reach nearly degenerate energetically stable phases, demonstrating that different phonon-driven routes can lead to the same ground-state configuration. The results establish a unified phonon-driven cascade mechanism for hierarchical CDW formation in monolayer $\mathrm{1T-VSe}_{2}$ and provide a systematic framework for understanding competing ordered phases in low-dimensional quantum materials. △ Less

Submitted 13 May, 2026; originally announced May 2026.

Comments: 21 pages, 6 figures

Entanglement Properties of the One-Dimensional Dimerized Fermi-Hubbard Model

Authors: Min-Chul Cha, Hoon Beom Kwon, Ji-Woo Lee, Myung-Hoon Chung

Abstract: We study the entanglement properties of the one-dimensional dimerized Fermi-Hubbard model. Using a matrix-product-state approach, we compute the ground state and identify two insulating phases at 1/2- and 3/4-filling, along with a metallic phase, whose mechanisms can be characterized by their entanglement spectra. Our findings indicate that the two insulating phases are distinct, implying that the… ▽ More We study the entanglement properties of the one-dimensional dimerized Fermi-Hubbard model. Using a matrix-product-state approach, we compute the ground state and identify two insulating phases at 1/2- and 3/4-filling, along with a metallic phase, whose mechanisms can be characterized by their entanglement spectra. Our findings indicate that the two insulating phases are distinct, implying that the phase at 1/2-filling has a charge gap arising from the band gap, which is enhanced by repulsive interactions, while the phase at 3/4-filling exhibits a Mott gap resulting from particle interactions. This difference between the two insulating phases is reflected in the scaling properties of the half-chain entanglement entropy and the distribution of the entanglement spectrum. △ Less

Submitted 26 July, 2026; v1 submitted 24 February, 2026; originally announced February 2026.

Comments: 6 pages and 5 figures

High-Throughput GW Calculations via Machine Learning

Authors: Ragab. A. Abdelghany, Chih-En Hsu, Hung-Chung Hsueh, Yuan-Hong Tsai, Ming-Chiang Chung

Abstract: We present a machine learning (ML) framework that predicts $G_0W_0$ quasiparticle energies across molecular dynamics (MD) trajectories with high accuracy and efficiency. Using only DFT-derived mean-field eigenvalues and exchange-correlation potentials, the model is trained on 25\% of MD snapshots and achieves RMSEs below 0.1 eV. It accurately reproduces k-resolved quasiparticle band structures and… ▽ More We present a machine learning (ML) framework that predicts $G_0W_0$ quasiparticle energies across molecular dynamics (MD) trajectories with high accuracy and efficiency. Using only DFT-derived mean-field eigenvalues and exchange-correlation potentials, the model is trained on 25\% of MD snapshots and achieves RMSEs below 0.1 eV. It accurately reproduces k-resolved quasiparticle band structures and density of states, even for BN polymorphs excluded from the training data. This approach bypasses the computational bottlenecks of $G_0W_0$ simulations over dynamic configurations, offering a scalable route to excited-state electronic structure simulations with many-body accuracy. △ Less

Submitted 5 May, 2025; originally announced May 2025.

Comments: 7 pages, 5 figures, 1 supplementary material

Tensor network method for solving the Ising model with a magnetic field

Authors: Myung-Hoon Chung

Abstract: We study the two-dimensional square lattice Ising ferromagnet and antiferromagnet with a magnetic field by using tensor network method. Focusing on the role of guage fixing, we present the partition function in terms of a tensor network. The tensor has a different symmetry property for ferromagnets and antiferromagnets. The tensor network of the partition function is interpreted as a multiple prod… ▽ More We study the two-dimensional square lattice Ising ferromagnet and antiferromagnet with a magnetic field by using tensor network method. Focusing on the role of guage fixing, we present the partition function in terms of a tensor network. The tensor has a different symmetry property for ferromagnets and antiferromagnets. The tensor network of the partition function is interpreted as a multiple product of the one-dimensional quantum Hamiltonian. We perform infinite density matrix renormalization group to contract the two-dimensional tensor network. We present the numerical result of magnetization and entanglement entropy for the Ising ferromagnet and antiferromagnet side by side. In order to determine the critical line in the parameter space of temperature and magnetic field, we use the half-chain entanglement entropy of the one-dimensional quantum state. The entanglement entropy precisely indicates the critical line forming the parabolic shape for the antiferromagnetic case, but shows the critical point for the ferromagnetic case. △ Less

Submitted 2 January, 2025; originally announced January 2025.

Comments: 6 pages, 7 figures

Thermal Conductivity Measurement Using Modulated Photothermal Radiometry for Nitrate and Chloride Molten Salts

Authors: Ka Man Chung, Tianshi Feng, Jian Zeng, Sarath Reddy Adapa, Xintong Zhang, Andrew Z. Zhao, Ye Zhang, Peiwen Li, Youyang Zhao, Javier E. Garay, Renkun Chen

Abstract: Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corros… ▽ More Molten salts are being used or explored for thermal energy storage and conversion systems in concentrating solar power and nuclear power plants. Thermal conductivity of molten salts is an important thermophysical property dictating the performance and cost of these systems, but its accurate measurement has been challenging, as evidenced by wide scattering of existing data in literature. The corrosive and conducting nature of these fluids also leads to time consuming sample preparation processes of many contact-based measurements. Here, we report the measurement of thermal conductivity of molten salts using a modulated photothermal radiometry (MPR) technique, which is a laser-based, non-contact, frequency-domain method adopted for molten salts for the first time. By unitizing the advantages of front side sensing of frequency-domain measurements and the vertical holder orientation, the technique can minimize the natural convection and salt creeping effects, thus yielding accurate molten salt thermal conductivity. The MPR technique is first calibrated using standard molten materials including paraffin wax and sulfur. It is then applied on measuring pure nitrate salts ($NaNO_3$ and $KNO_3$), solar salt ($NaNO_3-KNO_3$ mixture), and chloride salt ($NaCl-KCl-MgCl_2$). The measurement results are compared with data from literature, especially those obtained from laser flash analysis (LFA). Our results demonstrate that the MPR is a convenient and reliable technique of measuring thermal conductivity of molten salts. Accurate thermal conductivity data of molten salts will be valuable in developing the next-generation high-temperature thermal energy storage and conversion systems. △ Less

Submitted 31 August, 2023; originally announced September 2023.

In-situ Thermophysical Measurement of Flowing Molten Chloride Salt Using Modulated Photothermal Radiometry

Authors: Ka Man Chung, Ye Zhang, Jian Zeng, Fouad Haddad, Sarath Reddy Adapa, Tianshi Feng, Peiwen Li, Renkun Chen

Abstract: Molten salts are a leading candidate for high-temperature heat transfer fluids (HTFs) for thermal energy storage and conversion systems in concentrated solar power (CSP) and nuclear energy power plants. The ability to probe molten salt thermal transport properties in both stationary and flowing status is important for the evaluation of their heat transfer performance under realistic operational co… ▽ More Molten salts are a leading candidate for high-temperature heat transfer fluids (HTFs) for thermal energy storage and conversion systems in concentrated solar power (CSP) and nuclear energy power plants. The ability to probe molten salt thermal transport properties in both stationary and flowing status is important for the evaluation of their heat transfer performance under realistic operational conditions, including the temperature range and potential degradation due to corrosion and contamination. However, accurate thermal transport properties are usually challenging to obtain even for stagnant molten salts due to different sources of errors from convection, radiation, and corrosion, let alone flowing ones. To the best of authors' knowledge, there is no available in-situ technique for measuring flowing molten salt thermal conductivity. Here, we report the first in-situ flowing molten salt thermal conductivity measurement using modulated photothermal radiometry (MPR). We could successfully perform the first in-situ thermal conductivity measurement of flowing molten $NaCl-KCl-MgCl_2$ in the typical operating temperature (520 and 580 $^oC$) with flow velocities ranging from around 0.3 to 1.0 $m$$s^-1$. The relative change of the molten salt thermal conductivity was measured. Gnielinski's correlation was also used to estimate the heat transfer coefficient h of the flowing $NaCl-KCl-MgCl_2$ in the given experimental condition. The work showed the potential of the MPR technique serving as an in-situ diagnostics tool to evaluate the heat transfer performance of flowing molten salts and other high-temperature HTFs. △ Less

Submitted 31 August, 2023; originally announced September 2023.

Deep Learning of Phase Transitions for Quantum Spin Chains from Correlation Aspects

Authors: Ming-Chiang Chung, Guang-Yu Huang, Ian P. McCulloch, Yuan-Hong Tsai

Abstract: Using machine learning (ML) to recognize different phases of matter and to infer the entire phase diagram has proven to be an effective tool given a large dataset. In our previous proposals, we have successfully explored phase transitions for topological phases of matter at low dimensions either in a supervised or an unsupervised learning protocol with the assistance of quantum information related… ▽ More Using machine learning (ML) to recognize different phases of matter and to infer the entire phase diagram has proven to be an effective tool given a large dataset. In our previous proposals, we have successfully explored phase transitions for topological phases of matter at low dimensions either in a supervised or an unsupervised learning protocol with the assistance of quantum information related quantities. In this work, we adopt our previous ML procedures to study quantum phase transitions of magnetism systems such as the XY and XXZ spin chains by using spin-spin correlation functions as the input data. We find that our proposed approach not only maps out the phase diagrams with accurate phase boundaries, but also indicates some new features that have not observed before. In particular, we define so-called relevant correlation functions to some corresponding phases that can always distinguish between those and their neighbors. Based on the unsupervised learning protocol we proposed [Phys. Rev. B 104, 165108 (2021)], the reduced latent representations of the inputs combined with the clustering algorithm show the connectedness or disconnectedness between neighboring clusters (phases), just corresponding to the continuous or disrupt quantum phase transition, respectively. △ Less

Submitted 9 May, 2023; v1 submitted 16 January, 2023; originally announced January 2023.

Comments: 18 pages, 21 figures

Revealing the Charge Density Wave caused by Peierls instability in two-dimensional NbSe$_{2}$

Authors: Yung-Ting Lee, Po-Tuan Chen, Zheng-Hong Li, Jyun-Yu Wu, Chia-Nung Kuo, Chin-Shan Lue, Chien-Te Wu, Chien-Cheng Kuo, Cheng-Tien Chiang, Chun-Liang Lin, Chi-Cheng Lee, Hung-Chung Hsueh, Ming-Chiang Chung

Abstract: The formation of a charge density wave (CDW) in two-dimensional (2D) materials caused by Peierls instability is a controversial topic. This study investigates the extensively debated role of Fermi surface nesting in causing the CDW state in 2H-NbSe$_{2}$ materials. Four NbSe$_{2}$ structures (i.e., normal, stripe, filled, and hollow structures) are identified on the basis of the characteristics in… ▽ More The formation of a charge density wave (CDW) in two-dimensional (2D) materials caused by Peierls instability is a controversial topic. This study investigates the extensively debated role of Fermi surface nesting in causing the CDW state in 2H-NbSe$_{2}$ materials. Four NbSe$_{2}$ structures (i.e., normal, stripe, filled, and hollow structures) are identified on the basis of the characteristics in scanning tunneling microscopy images and first-principles simulations. The calculations reveal that the filled phase corresponds to Peierls' description; that is, it exhibits fully opened gaps at the CDW Brillouin zone boundary, resulting in a drop at the Fermi level in the density of states and the scanning tunneling spectroscopy spectra. The electronic susceptibility and phonon instability in the normal phase indicate that the Fermi surface nesting is triggered by two nesting vectors, whereas the involvement of only one nesting vector leads to the stripe phase. This comprehensive study demonstrates that the filled phase of NbSe$_{2}$ can be categorized as a Peierls-instability-induced CDW in 2D systems. △ Less

Submitted 14 November, 2022; v1 submitted 2 November, 2022; originally announced November 2022.

Comments: 4 figures

Thermal Conductivity Modeling of Monodispersed Microspheres using Discrete Element Method

Authors: Jian Zeng, Ka Man Chung, Xintong Zhang, Sarath Adapa, Tianshi Feng, Yu Pei, Renkun Chen

Abstract: Particle beds are widely used in various systems and processes, such as particle heat exchangers, granular flow reactors, and additive manufacturing. Accurate modeling of thermal conductivity of particle beds and understanding of their heat transfer mechanisms are important. However, previous models were based on a simple cubic packing of particles which could not accurately represent the actual h… ▽ More Particle beds are widely used in various systems and processes, such as particle heat exchangers, granular flow reactors, and additive manufacturing. Accurate modeling of thermal conductivity of particle beds and understanding of their heat transfer mechanisms are important. However, previous models were based on a simple cubic packing of particles which could not accurately represent the actual heat transfer processes under certain conditions. Here, we examine the effect of the packing structure on thermal conductivity of particle beds. We use monodispersed silica microspheres with average article sizes ranging from 23 to 330 um as a model material. We employ a transient hot-wire technique to measure the thermal conductivity of the particle beds with packing density of 43 to 57% within a temperature range of room temperature to 500 deg. C and under N2 gaseous pressures of 20 to 760 Torr. We then use a discrete element method (DEM) to obtain the realistic packing structure of the particles, which is then fed into a finite-element model (FEM) to calculate the thermal conductivity, with the consideration of solid conduction, gas conduction, and radiation heat transfer. Our results show that the thermal conductivity model based on the more realistic random packing structure derived from the DEM shows better agreement with the experimental data compared to that based on the simple cubic packing structure. The combined DEM and FEM methodology can serve as a useful tool to predict effective thermal conductivity of particle beds and to quantify different heat transfer mechanisms under various conditions. △ Less

Submitted 15 September, 2021; v1 submitted 17 June, 2021; originally announced June 2021.

Comments: 23 pages, 6 figures

Deep learning of topological phase transitions from entanglement aspects: An unsupervised way

Authors: Yuan-Hong Tsai, Kuo-Feng Chiu, Yong-Cheng Lai, Kuan-Jung Su, Tzu-Pei Yang, Tsung-Pao Cheng, Guang-Yu Huang, Ming-Chiang Chung

Abstract: Machine learning techniques have been shown to be effective to recognize different phases of matter and produce phase diagrams in the parameter space interested, while they usually require prior labeled data to perform well. Here, we propose a machine learning procedure, mainly in an unsupervised manner, which can first identify topological/non-topological phases and then refine the locations of p… ▽ More Machine learning techniques have been shown to be effective to recognize different phases of matter and produce phase diagrams in the parameter space interested, while they usually require prior labeled data to perform well. Here, we propose a machine learning procedure, mainly in an unsupervised manner, which can first identify topological/non-topological phases and then refine the locations of phase boundaries. By following this proposed procedure, we expand our previous work on the one-dimensional $p$-wave superconductor [Phys. Rev. B 102, 054512 (2020)] and further on the Su-Schrieffer-Heeger model, with an emphasis on using the quantum entanglement-based quantities as the input features. We find that our method not only reproduces similar results to the previous work with sharp phase boundaries but importantly it also does not rely on prior knowledge of the phase space, e.g., the number of phases present. We conclude with a few remarks about its potential, limitations, and explainabilities. △ Less

Submitted 3 June, 2021; v1 submitted 9 May, 2021; originally announced May 2021.

Comments: 10 pages, 13 figues, 9 tables

Journal ref: Phys. Rev. B 104, 165108 (2021)

Deep learning of topological phase transitions from entanglement aspects for two-dimensional chiral p-wave superconductors

Authors: Ming-Chiang Chung, Tsung-Pao Cheng, Guang-Yu Huang, Yuan-Hong Tsai

Abstract: Applying deep learning to investigate topological phase transitions (TPTs) becomes a useful method due to not only its ability to recognize patterns but also its statistical excellency to examine the amount of information carried by different types of data inputs. Among possible data types, entanglement-related quantities, such as Majorana correlation matrices (MCMs), one-particle entanglement spe… ▽ More Applying deep learning to investigate topological phase transitions (TPTs) becomes a useful method due to not only its ability to recognize patterns but also its statistical excellency to examine the amount of information carried by different types of data inputs. Among possible data types, entanglement-related quantities, such as Majorana correlation matrices (MCMs), one-particle entanglement spectra (OPES), and entanglement eigenvectors (OPEEs), have been proved effective, however, are to date mostly restricted to one dimension. Here, we propose practical input data forms based on those quantities to study TPTs and to compare the efficiency of each form on classic two-dimensional chiral $p$-wave superconductors via the deep learning approach. First, we find that different input forms, either matrices or tensors both originated from real MCMs, can affect the precise locations of the predicted transition points. Next, due to the complex nature of OPEEs, we extract three spatially dependent quantities from OPEEs, one related to the "intensity", and the other two related to "phases" of particle and hole components. We show that similar to taking OPES directly as inputs, solely using "intensity" quantity can only distinguish topological phases from trivial ones, whereas using either whole MCMs or complete OPEE-extracted quantities can provide sufficient information for deep learning to distinguish between phases of matter with different $U(1)$ gauges or Chern numbers. Finally, we discuss certain characteristic features in the deep learning approach and, in particular, they reveal that our trained models indeed learn physically meaningful features, which confirms the potential use even at high dimensions. △ Less

Submitted 23 July, 2021; v1 submitted 8 April, 2021; originally announced April 2021.

Comments: 10 pages, 7 figures

Journal ref: Phys. Rev. B 104, 024506 (2021)

Phase transitions in the one-dimensional ionic Hubbard model

Authors: Myung-Hoon Chung

Abstract: We study quantum phase transitions by measuring the bond energy, the number density, and the half-chain entanglement entropy in the one-dimensional ionic Hubbard model. By performing the infinite density matrix renormalization group with matrix product operator, we obtain ground states as the canonical form of matrix product states. Depending on the chemical potential and the staggered potential,… ▽ More We study quantum phase transitions by measuring the bond energy, the number density, and the half-chain entanglement entropy in the one-dimensional ionic Hubbard model. By performing the infinite density matrix renormalization group with matrix product operator, we obtain ground states as the canonical form of matrix product states. Depending on the chemical potential and the staggered potential, the number density and the half-chain entanglement entropy shows clear signatures of the Mott transition. Our results confirm the success of the matrix product operator method for investigation of itinerant fermion systems. △ Less

Submitted 11 November, 2020; originally announced November 2020.

Comments: 5 pages, 5 figures

Measurement of High-temperature Thermophysical Properties of Bulk and Coatings Using Modulated Photothermal Radiometry

Authors: Jian Zeng, Ka Man Chung, Qingyang Wang, Xiaoxin Wang, Yu Pei, Peiwen Li, Renkun Chen

Abstract: This paper presents the development of instrumentation for the measurement of high-temperature thermal conductivity of bulk and coatings using a modulated photothermal radiometry (MPR) method, where a sample is heated by an intensity-modulated laser to probe into different layers of the sample. While MPR has been previously established, most of the previous studies only focus on the measurement at… ▽ More This paper presents the development of instrumentation for the measurement of high-temperature thermal conductivity of bulk and coatings using a modulated photothermal radiometry (MPR) method, where a sample is heated by an intensity-modulated laser to probe into different layers of the sample. While MPR has been previously established, most of the previous studies only focus on the measurement at room temperature. The MPR has not been well studied for measurements of bulk and coating materials at high temperatures, which are increasingly important for a multitude of applications, such as materials used in the concentrating solar power (CSP) plants and the nuclear reactors. MPR is a non-contact technique that utilizes the intrinsic thermal emission from the specimens for thermometry, which is favorable for measurements at high temperatures in harsh environment. The authors designed and utilized a sample holder suitable for high temperature measurement up to 973 K with good temperature uniformity within the sample. The high-temperature MPR setup was validated by measuring bulk materials with known thermal conductivity. The setup and technique were then extended to the measurement of black solar-absorbing coatings of 10 to 50 μm thick on various substrates by modulating the frequency of the laser heating beam and the thermal penetration depth. The studies showed that thermal conductivities of typical solar-absorbing coatings are 0.4 ~ 0.8 W m-1 K-1, indicating a possibly large temperature drop within the coating under high solar irradiation flux, such as over 1000-sun for central solar towers in CSP plants. △ Less

Submitted 20 November, 2020; v1 submitted 30 August, 2020; originally announced August 2020.

Size Disorder as a Descriptor for Predicting Reduced Thermal Conductivity in Medium- and High-Entropy Pyrochlores

Authors: Andrew J. Wright, Qingyang Wang, Shu-Ting Ko, Ka Man Chung, Renkun Chen, Jian Luo

Abstract: High-entropy ceramics generally exhibit reduced thermal conductivity, but little is known about what controls this suppression and which descriptor can predict it. Herein, 18 medium- and high-entropy pyrochlores were synthesized to measure their thermal conductivity and Young's modulus. Up to 35% reductions in thermal conductivity were achieved with retained moduli, thereby attaining insulative ye… ▽ More High-entropy ceramics generally exhibit reduced thermal conductivity, but little is known about what controls this suppression and which descriptor can predict it. Herein, 18 medium- and high-entropy pyrochlores were synthesized to measure their thermal conductivity and Young's modulus. Up to 35% reductions in thermal conductivity were achieved with retained moduli, thereby attaining insulative yet stiff properties for potential thermal barrier coating applications. Notably, the measured thermal conductivity correlates well with a modified size disorder parameter. Thus, this modified size disorder parameter is suggested as a useful descriptor for designing thermally-insulative medium- and high-entropy ceramics (broadly defined as "compositionally-complex ceramics"). △ Less

Submitted 25 December, 2019; originally announced December 2019.

Ground-state properties of the one-dimensional Hubbard model with pairing potential

Authors: Myung-Hoon Chung, Edmond Orignac, Didier Poilblanc, Sylvain Capponi

Abstract: We consider a modification of the one-dimensional Hubbard model by including an external pairing potential. Guided by analytic bosonization results, we quantitatively determine the grand-canonical zero-temperature phase diagram using both finite and infinite density matrix renormalization group algorithm based on the formalism of matrix product states and matrix product operator, respectively. By… ▽ More We consider a modification of the one-dimensional Hubbard model by including an external pairing potential. Guided by analytic bosonization results, we quantitatively determine the grand-canonical zero-temperature phase diagram using both finite and infinite density matrix renormalization group algorithm based on the formalism of matrix product states and matrix product operator, respectively. By computing various local quantities as well as the half-system entanglement, we are able to distinguish between Mott, metallic and superconducting phases. We point out the compressible nature of the Mott phase and the fully gapped nature of the many-body spectrum of the superconducting phase, in the presence of explicit U(1)-charge symmetry breaking. △ Less

Submitted 26 September, 2020; v1 submitted 21 December, 2019; originally announced December 2019.

Comments: 26 pages, 8 figures

Journal ref: Physica B: Condensed Matter, Volume 604, 412665 (2021)

Deep learning of topological phase transitions from entanglement aspects

Authors: Yuan-Hong Tsai, Meng-Zhe Yu, Yu-Hao Hsu, Ming-Chiang Chung

Abstract: The one-dimensional $p$-wave superconductor proposed by Kitaev has long been a classic example for understanding topological phase transitions through various methods, such as examining Berry phase, edge states of open chains and, in particular, aspects from quantum entanglement of ground states. In order to understand the amount of information carried in the entanglement-related quantities, here… ▽ More The one-dimensional $p$-wave superconductor proposed by Kitaev has long been a classic example for understanding topological phase transitions through various methods, such as examining Berry phase, edge states of open chains and, in particular, aspects from quantum entanglement of ground states. In order to understand the amount of information carried in the entanglement-related quantities, here we study topological phase transitions of the model with emphasis of using the deep learning approach. We feed different quantities, including Majorana correlation matrices (MCMs), entanglement spectra (ES) or entanglement eigenvectors (EE) originated from Block correlation matrices (BCMs), into the deep neural networks for training, and investigate which one could be the most useful input format in this approach. We find that ES is indeed too compressed information compared to MCM or EE. MCM and EE can provide us abundant information to recognize not only the topological phase transitions in the model but also phases of matter with different $U$(1) gauges, which is not reachable by using ES only. △ Less

Submitted 26 December, 2019; v1 submitted 10 September, 2019; originally announced September 2019.

Comments: 5 pages, 4 figures

Journal ref: Phys. Rev. B 102, 054512 (2020)

Half-Chain Entanglement Entropy in the One-Dimensional Spinless Fermion Model

Authors: Myung-Hoon Chung

Abstract: We calculate the half-chain entanglement entropy of the ground state in the one-dimensional spinless fermion model. Considering a tiny corner of the Hilbert space represented by matrix product states, we efficiently find the ground state by the infinite time-evolving block decimation. The Schmidt coefficients are used to determine the half-chain entanglement entropy. Using the bond dimension scali… ▽ More We calculate the half-chain entanglement entropy of the ground state in the one-dimensional spinless fermion model. Considering a tiny corner of the Hilbert space represented by matrix product states, we efficiently find the ground state by the infinite time-evolving block decimation. The Schmidt coefficients are used to determine the half-chain entanglement entropy. Using the bond dimension scaling of the half-chain entanglement entropy, we find the critical region, which is consistent with the previous results. △ Less

Submitted 17 October, 2016; originally announced October 2016.

Comments: 12 pages, 4 figures

Relaxation of the entanglement spectrum in quench dynamics of topological systems

Authors: Yi-Hao Jhu, Pochung Chen, Ming-Chiang Chung

Abstract: We study how the entanglement spectrum relaxes to its steady state in one-dimensional quadratic systems after a quantum quench. In particular we apply the saddle point expansion to the dimerized chains and 1-D p-wave superconductors. We find that the entanglement spectrum always exhibits a power-law relaxation superimposed with oscillations at certain characteristic angular frequencies. For the di… ▽ More We study how the entanglement spectrum relaxes to its steady state in one-dimensional quadratic systems after a quantum quench. In particular we apply the saddle point expansion to the dimerized chains and 1-D p-wave superconductors. We find that the entanglement spectrum always exhibits a power-law relaxation superimposed with oscillations at certain characteristic angular frequencies. For the dimerized chains, we find that the exponent $ν$ of the power-law decay is always $3/2$. For 1-D p-wave superconductors, however, we find that depending on the initial and final Hamiltonian, the exponent $ν$ can take value from a limited list of values. The smallest possible value is $ν=1/2$, which leads to a very slow convergence to its steady state value. △ Less

Submitted 8 March, 2017; v1 submitted 4 October, 2016; originally announced October 2016.

Comments: 26 pages, 14 figures

Quantum Quenches in the Luttinger model and its close relatives

Authors: M. A. Cazalilla, M. -C. Chung

Abstract: A number of results on quantum quenches in the Luttinger and related models are surveyed with emphasis on post-quench correlations. For the Luttinger model and initial gaussian states, we discuss both sudden and smooth quenches of the interaction and the emergence of a steady state described by a generalized Gibbs ensemble. Comparisons between analytics and numerics, and the question of universali… ▽ More A number of results on quantum quenches in the Luttinger and related models are surveyed with emphasis on post-quench correlations. For the Luttinger model and initial gaussian states, we discuss both sudden and smooth quenches of the interaction and the emergence of a steady state described by a generalized Gibbs ensemble. Comparisons between analytics and numerics, and the question of universality or lack thereof are also discussed. The relevance of the theoretical results to current and future experiments in the fields of ultracold atomic gases and mesoscopic systems of electrons is also briefly touched upon. Wherever possible, our approach is pedagogical and self-contained. This work is dedicated to the memory of our colleague Alejandro Muramatsu. △ Less

Submitted 14 March, 2016; originally announced March 2016.

Comments: 51+epsilon pages. Review article for or special issue of JSTAT on non-equilibrium dynamics in integrable systems, Feedback is welcome

Journal ref: Journal of Statistical Mechanics: Theory and Experiment (2016) 064004

Entanglement convertibility by sweeping through the quantum phases of the alternating bonds $XXZ$ chain

Authors: Yu-Chin Tzeng, Li Dai, M. -C. Chung, Luigi Amico, Leong-Chuan Kwek

Abstract: We study the entanglement structure and the topological edge states of the ground state of the spin-1/2 XXZ model with bond alternation. We employ parity-density matrix renormalization group with periodic boundary conditions. The finite-size scaling of Rényi entropies $S_2$ and $S_\infty$ are used to construct the phase diagram of the system. The phase diagram displays three possible phases: Halda… ▽ More We study the entanglement structure and the topological edge states of the ground state of the spin-1/2 XXZ model with bond alternation. We employ parity-density matrix renormalization group with periodic boundary conditions. The finite-size scaling of Rényi entropies $S_2$ and $S_\infty$ are used to construct the phase diagram of the system. The phase diagram displays three possible phases: Haldane type (an example of symmetry protected topological ordered phases), Classical Dimer and Néel phases, the latter bounded by two continuous quantum phase transitions. The entanglement and non-locality in the ground state are studied and quantified by the entanglement convertibility. We found that, at small spatial scales, the ground state is not convertible within the topological Haldane dimer phase. The phenomenology we observe can be described in terms of correlations between edge states. We found that the entanglement spectrum also exhibits a distinctive response in the topological phase: the effective rank of the reduced density matrix displays a specifically large "susceptibility" in the topological phase. These findings support the idea that although the topological order in the ground state cannot be detected by local inspection, the ground state response at local scale can tell the topological phases apart from the non-topological phases. △ Less

Submitted 24 May, 2016; v1 submitted 17 December, 2015; originally announced December 2015.

Comments: Final version

Journal ref: Sci. Rep. 6, 26453 (2016)

Two-species Bose-Einstein condensate in an optical lattice: analytical approximate formulæ

Authors: R. Cipolatti, L. Villegas-Lelovsky, M. C. Chung, C. Trallero-Giner

Abstract: Employing a general variational method and perturbation theory, we derived explicit solutions for the description of one-dimensional two species Bose-Einstein condensates confined by a harmonic trap potential in an optical lattice. We consider the system of two coupled Gross-Pitaevkii equations (GPE) and derive explicit expressions for the chemical potentials and wavefunctions in terms of the atom… ▽ More Employing a general variational method and perturbation theory, we derived explicit solutions for the description of one-dimensional two species Bose-Einstein condensates confined by a harmonic trap potential in an optical lattice. We consider the system of two coupled Gross-Pitaevkii equations (GPE) and derive explicit expressions for the chemical potentials and wavefunctions in terms of the atom-atom interaction parameters and laser intensity. We have compared our results with the numerical solutions of the GPE and performed a quantitative analysis for the both considered methods. We underline the importance of the obtained explicit solutions to characterize the density profile or degree of miscibility of the two components. △ Less

Submitted 18 August, 2015; originally announced August 2015.

Comments: 10 pages and 5 figures

Network topology transition at criticality

Authors: Chung-Pin Chou, Yi-Hua Wang, Ming-Chiang Chung

Abstract: Many-body systems when continuous phase transition occurs are mainly built in the interrelationship between particles, implemented through many-body correlations. Some of them may exhibit so-called topological order hardly measured by experiments. Therefore we need, beyond mean-field theory, the complex-systems approach that stresses the systemic complexity of many-body network at criticality. Acc… ▽ More Many-body systems when continuous phase transition occurs are mainly built in the interrelationship between particles, implemented through many-body correlations. Some of them may exhibit so-called topological order hardly measured by experiments. Therefore we need, beyond mean-field theory, the complex-systems approach that stresses the systemic complexity of many-body network at criticality. According to our previous study, network space experiences the homogeneous-heterogeneous transition invisible in traditional phase transitions. The network robustness can be a useful indicator to capture the critical phenomena of phase transitions with/without symmetry breaking. In this work, we demonstrate the idea of the change of robust networks is successfully applied to the well-known 1D quantum and 2D classical XY models. △ Less

Submitted 16 June, 2015; originally announced June 2015.

Comments: 15 pages, 4 figures. arXiv admin note: text overlap with arXiv:1308.0255

Breakdown of local convertibility through Majorana modes in a quantum quench

Authors: Li Dai, Ming-Chiang Chung

Abstract: The local convertibility of quantum states, measured by the Rényi entropy, is concerned with whether or not a state can be transformed into another state, using only local operations and classical communications. We found that in the one-dimensional Kitaev chain with quenched chemical potential $μ$, the convertibility between the state for $μ$ and that for $μ+δμ$, depends on the quantum phases of… ▽ More The local convertibility of quantum states, measured by the Rényi entropy, is concerned with whether or not a state can be transformed into another state, using only local operations and classical communications. We found that in the one-dimensional Kitaev chain with quenched chemical potential $μ$, the convertibility between the state for $μ$ and that for $μ+δμ$, depends on the quantum phases of the system ($δμ$ is a perturbation). This is similar to the adiabatic case where the ground state is considered. Specifically, when the quenched system has edge modes and the subsystem size for the partition is much larger than the correlation length of the Majorana fermions which forms the edge modes, the quenched state is locally inconvertible. We give a physical interpretation for the result, based on analyzing the interactions between the two subsystems for various partitions. Our work should help to better understand the many-body phenomena in topological systems and also the entanglement properties in the Majorana fermionic quantum computation. △ Less

Submitted 27 May, 2015; v1 submitted 1 April, 2015; originally announced April 2015.

Comments: 8 pages, 5 figures, accepted by Physical Review A

Journal ref: Phys. Rev. A 91, 062319 (2015)

Entanglement in composite free-fermion systems

Authors: Viktor Eisler, Ming-Chiang Chung, Ingo Peschel

Abstract: We consider fermionic chains where the two halves are either metals with different bandwidths or a metal and an insulator. Both are coupled together by a special bond. We study the ground-state entanglement entropy between the two pieces, its dependence on the parameters and its asymptotic form. We also discuss the features of the entanglement Hamiltonians in both subsystems and the evolution of t… ▽ More We consider fermionic chains where the two halves are either metals with different bandwidths or a metal and an insulator. Both are coupled together by a special bond. We study the ground-state entanglement entropy between the two pieces, its dependence on the parameters and its asymptotic form. We also discuss the features of the entanglement Hamiltonians in both subsystems and the evolution of the entanglement entropy after joining the two parts of the system. △ Less

Submitted 8 July, 2015; v1 submitted 31 March, 2015; originally announced March 2015.

Comments: 20 pages, 13 figures, published version, minor corrections, references added

Journal ref: J. Stat. Mech. (2015) P07011

Damping of Confined Excitations Modes of 1D Condensates in an Optical Lattice

Authors: C. Trallero-Giner, Darío G. Santiago-Pérez, Ming-Chiang Chung, G. E. Marques, R. Cipolatti

Abstract: We study the damping of the collective excitations of Bose-Einstein condensates in a harmonic trap potential loaded in an optical lattice. In the presence of a confining potential the system is non-homogeneous and the collective excitations are characterized by a set of discrete confined phonon-like excitations. We derive a general convenient analytical description for the damping rate, which take… ▽ More We study the damping of the collective excitations of Bose-Einstein condensates in a harmonic trap potential loaded in an optical lattice. In the presence of a confining potential the system is non-homogeneous and the collective excitations are characterized by a set of discrete confined phonon-like excitations. We derive a general convenient analytical description for the damping rate, which takes into account, the trapping potential and the optical lattice, for the Landau and Beliaev processes at any temperature, $T$. At high temperature or weak spatial confinement, we show that both mechanisms display linear dependence on $T$. In the quantum limit, we found that the Landau damping is exponentially suppressed at low temperatures and the total damping is independent of $T$. Our theoretical predictions for the damping rate under thermal regime is in completely correspondence with the experimental values reported for 1D condensate of sodium atoms. We show that the laser intensity can tune the collision process, allowing a \textit{resonant effect} for the condensate lifetime. Also, we study the influence of the attractive or repulsive non-linear terms on the decay rate of the collective excitations. A general expression of the renormalized Goldstone frequency has been obtained as a function of the 1D non-linear self-interaction parameter, laser intensity and temperature. △ Less

Submitted 24 July, 2015; v1 submitted 30 March, 2015; originally announced March 2015.

Comments: 9 pages, 9 figures

Journal ref: Phys. Rev. A 92, 042502 (2015)

Extracting entangled qubits from Majorana fermions in quantum dot chains through the measurement of parity

Authors: Li Dai, Watson Kuo, Ming-Chiang Chung

Abstract: We propose a scheme for extracting entangled charge qubits from quantum-dot chains that support zero-energy edge modes. The edge mode is composed of Majorana fermions localized at the ends of each chain. The qubit, logically encoded in double quantum dots, can be manipulated through tunneling and pairing interactions between them. The detailed form of the entangled state depends on both the parity… ▽ More We propose a scheme for extracting entangled charge qubits from quantum-dot chains that support zero-energy edge modes. The edge mode is composed of Majorana fermions localized at the ends of each chain. The qubit, logically encoded in double quantum dots, can be manipulated through tunneling and pairing interactions between them. The detailed form of the entangled state depends on both the parity measurement (an even or odd number) of the boundary-site electrons in each chain and the teleportation between the chains. The parity measurement is realized through the dispersive coupling of coherent-state microwave photons to the boundary sites, while the teleportation is performed via Bell measurements. Our scheme illustrates \emph{localizable entanglement} in a fermionic system, which serves feasibly as a quantum repeater under realistic experimental conditions, as it allows for finite temperature effect and is robust against disorders, decoherence and quasi-particle poisoning. △ Less

Submitted 27 May, 2015; v1 submitted 30 March, 2014; originally announced March 2014.

Comments: Accepted by Scientific Reports

Journal ref: Scientific Reports (2015) 5, Article number: 11188

Quantum Critical Spin-2 Chain with Emergent SU(3) Symmetry

Authors: Pochung Chen, Zhi-Long Xue, I. P. McCulloch, Ming-Chiang Chung, Chao-Chun Huang, S. -K. Yip

Abstract: We study the quantum critical phase of a SU(2) symmetric spin-2 chain obtained from spin-2 bosons in a one-dimensional lattice. We obtain the scaling of the entanglement entropy and finite-size energies by exact diagonalization and density-matrix renormalization group methods. From the numerical results of the energy spectrum, central charge, and scaling dimension we identify the conformal field t… ▽ More We study the quantum critical phase of a SU(2) symmetric spin-2 chain obtained from spin-2 bosons in a one-dimensional lattice. We obtain the scaling of the entanglement entropy and finite-size energies by exact diagonalization and density-matrix renormalization group methods. From the numerical results of the energy spectrum, central charge, and scaling dimension we identify the conformal field theory describing the whole critical phase to be the SU(3)$_1$ Wess-Zumino-Witten model. We find that while in the whole critical phase the Hamiltonian is only SU(2) invariant, there is an emergent SU(3) symmetry in the thermodynamic limit. △ Less

Submitted 17 March, 2015; v1 submitted 23 February, 2014; originally announced February 2014.

Journal ref: Phys. Rev. Lett. 114, 145301 (2015)

A Memory of Majorana Fermions through Quantum Quench

Authors: Ming-Chiang Chung, Yi-Hao Jhu, Pochung Chen, Chung-Yu Mou, Xin Wan

Abstract: We study the sudden quench of a one-dimensional p-wave superconductor through its topological signature in the entanglement spectrum. The long-time evolution of the system and its topological characterization depend on a pseudomagnetic field ${\bs R}_{\mbox{\text eff}}(k)$, which connects both the initial and the final Hamiltonians, hence exhibiting a memory effect. In particular, we explore the r… ▽ More We study the sudden quench of a one-dimensional p-wave superconductor through its topological signature in the entanglement spectrum. The long-time evolution of the system and its topological characterization depend on a pseudomagnetic field ${\bs R}_{\mbox{\text eff}}(k)$, which connects both the initial and the final Hamiltonians, hence exhibiting a memory effect. In particular, we explore the robustness of the Majorana zero-mode associated with the entanglement cut in the topologically nontrivial phase and identify the parameter space in which the mode can survive in the infinite-time limit. △ Less

Submitted 19 January, 2014; v1 submitted 2 January, 2014; originally announced January 2014.

Comments: 5 pages, 4 figures

A Solution of the Hubbard Model

Authors: Myung-Hoon Chung

Abstract: We report a ground-state solution for the two-dimensional fermionic Hubbard model, which is obtained via a numerical variational method. The two ingredients in this approach are tensor network states and the time-evolving block decimation. We easily handle the horizontal hopping in the Hamiltonian, and we proceed further to observe the fermion-exchange effect caused by the vertical hopping. By req… ▽ More We report a ground-state solution for the two-dimensional fermionic Hubbard model, which is obtained via a numerical variational method. The two ingredients in this approach are tensor network states and the time-evolving block decimation. We easily handle the horizontal hopping in the Hamiltonian, and we proceed further to observe the fermion-exchange effect caused by the vertical hopping. By requiring no divergence and no convergence to zero for the ground state, we successively determine the ground-state energy per site as a function of the chemical potential and the lattice length. In addition, we observe saturation in the behavior of the ground-state energy as the lattice length increases. △ Less

Submitted 27 April, 2014; v1 submitted 20 November, 2013; originally announced November 2013.

Comments: 20 pages, 4 figures

Network topology: detecting topological phase transitions in the Kitaev chain and the rotor plane

Authors: Chung-Pin Chou, Ming-Chiang Chung

Abstract: We propose a novel network measure of topological invariants, called small-worldness, for identifying topological phase transitions of quantum and classical spin models. Small-worldness is usually defined in the study of social networks based on the best known discovery that one can find a short chain of acquaintances connecting almost any two people on the planet. Here we demonstrate that the sma… ▽ More We propose a novel network measure of topological invariants, called small-worldness, for identifying topological phase transitions of quantum and classical spin models. Small-worldness is usually defined in the study of social networks based on the best known discovery that one can find a short chain of acquaintances connecting almost any two people on the planet. Here we demonstrate that the small-world effect provides a useful description to distinguish topologically trivial and non-trivial phases in the Kitaev chain and accurately capture the Kosterlitz-Thouless transition in the rotor plane. Our results further suggest that the small-worldness containing both locality and non-locality of the network topology can be a practical approach to extract characteristic quantities of topological states of matter. △ Less

Submitted 8 May, 2014; v1 submitted 1 August, 2013; originally announced August 2013.

Comments: 7 pages, 5 figures

Entanglement entropy scaling of the XXZ chain

Authors: Pochung Chen, Zhi-long Xue, I. P. McCulloch, Ming-Chiang Chung, Miguel Cazalilla, S. -K. Yip

Abstract: We study the entanglement entropy scaling of the XXZ chain. While in the critical XY phase of the XXZ chain the entanglement entropy scales logarithmically with a coefficient that is determined by the associated conformal field theory, at the ferromagnetic point, however, the system is not conformally invariant yet the entanglement entropy still scales logarithmically albeit with a different coeff… ▽ More We study the entanglement entropy scaling of the XXZ chain. While in the critical XY phase of the XXZ chain the entanglement entropy scales logarithmically with a coefficient that is determined by the associated conformal field theory, at the ferromagnetic point, however, the system is not conformally invariant yet the entanglement entropy still scales logarithmically albeit with a different coefficient. We investigate how such an nontrivial scaling at the ferromagnetic point influences the estimation of the central charge $c$ in the critical XY phase. In particular we use the entanglement scaling of the finite or infinite system, as well as the finite-size scaling of the ground state energy to estimate the value of $c$. In addition, the spin-wave velocity and the scaling dimension are also estimated. We show that in all methods the evaluations are influenced by the nearby ferromagnetic point and result in crossover behavior. Finally we discuss how to determine whether the central charge estimation is strongly influenced by the crossover behavior and how to properly evaluate the central charge. △ Less

Submitted 14 September, 2013; v1 submitted 24 June, 2013; originally announced June 2013.

Journal ref: J. Stat. Mech. (2013) P10007

Topological entanglement entropy in bilayer quantum Hall systems

Authors: Myung-Hoon Chung

Abstract: We calculate the topological entanglement entropy in bilayer quantum Hall systems, dividing the set of quantum numbers into four parts. This topological entanglement entropy allows us to draw a phase diagram in the parameter space of layer separation and tunneling amplitude. We perform the finite size scaling analysis of the topological entanglement entropy in order to see the quantum phase transi… ▽ More We calculate the topological entanglement entropy in bilayer quantum Hall systems, dividing the set of quantum numbers into four parts. This topological entanglement entropy allows us to draw a phase diagram in the parameter space of layer separation and tunneling amplitude. We perform the finite size scaling analysis of the topological entanglement entropy in order to see the quantum phase transition clearly. △ Less

Submitted 20 June, 2013; originally announced June 2013.

Comments: 14 pages

Matrix Product States for Quantum Many-Fermion Systems

Authors: Myung-Hoon Chung

Abstract: We describe a simple method to find the ground state energy without calculating the expectation value of the Hamiltonian in the time-evolving block decimation algorithm with tensor network states. For example, we consider quantum many-fermion systems with matrix product states, which are updated consistently in a way that accounts for fermion exchange effects. This method can be applied to a wide… ▽ More We describe a simple method to find the ground state energy without calculating the expectation value of the Hamiltonian in the time-evolving block decimation algorithm with tensor network states. For example, we consider quantum many-fermion systems with matrix product states, which are updated consistently in a way that accounts for fermion exchange effects. This method can be applied to a wide class of fermion systems. We test this method in spinless fermion system where the exact ground state energy is known. We analyze finite size effects to determine the ground state energy in the thermodynamic limit that is compared to the exact value. △ Less

Submitted 30 May, 2013; originally announced May 2013.

Comments: 8 pages

Quench Dynamics of Topological Maximally-Entangled States

Authors: Ming-Chiang Chung, Yi-Hao Jhu, Pochung Chen, Chung-Yu Mou

Abstract: We investigate the quench dynamics of the one-particle entanglement spectra (OPES) for systems with topologically nontrivial phases. By using dimerized chains as an example, it is demonstrated that the evolution of OPES for the quenched bi-partite systems is governed by an effective Hamiltonian which is characterized by a pseudo spin in a time-dependent pseudo magnetic field $\vec{S}(k,t)$. The ex… ▽ More We investigate the quench dynamics of the one-particle entanglement spectra (OPES) for systems with topologically nontrivial phases. By using dimerized chains as an example, it is demonstrated that the evolution of OPES for the quenched bi-partite systems is governed by an effective Hamiltonian which is characterized by a pseudo spin in a time-dependent pseudo magnetic field $\vec{S}(k,t)$. The existence and evolution of the topological maximally-entangled edge states are determined by the winding number of $\vec{S}(k,t)$ in the $k$-space. In particular, the maximally-entangled edge states survive only if nontrivial Berry phases are induced by the winding of $\vec{S}(k,t)$. In the infinite time limit the equilibrium OPES can be determined by an effective time-independent pseudo magnetic field $\vec{S}_{\mb{eff}}(k)$. Furthermore, when maximally-entangled edge states are unstable, they are destroyed by quasiparticles within a characteristic timescale in proportional to the system size. △ Less

Submitted 7 June, 2012; v1 submitted 31 May, 2012; originally announced May 2012.

Comments: 5 pages, 3 figures

Thermalization in Systems with Bipartite Eigenmode Entanglement

Authors: Ming-Chiang Chung, Anibal Iucci, Miguel. A. Cazalilla

Abstract: It is analytically shown that the asymptotic correlations in exactly solvable models following a quantum quench can behave essentially as thermal correlations provided the entanglement between two eigenmodes is sufficiently strong. We provide one example and one counter example of this observation. The example illustrates the fact that the thermal correlations arise from initial states where the e… ▽ More It is analytically shown that the asymptotic correlations in exactly solvable models following a quantum quench can behave essentially as thermal correlations provided the entanglement between two eigenmodes is sufficiently strong. We provide one example and one counter example of this observation. The example illustrates the fact that the thermal correlations arise from initial states where the entanglement between the eigenmodes stems from the existence of a large energy gap in the initial state. On the other hand, the counter-example shows that when the bi-partite entanglement of the eigenmodes stems from interactions that do not open a gap, the correlations at asymptotically long times are non-thermal. We also show that the thermal behavior concerns only the asymptotic correlation functions, as the difference with an actual thermal ensemble can be observed measuring the energy fluctuations of the system. The latter observation implies a breakdown of the fluctuation-dissipation theorem. △ Less

Submitted 1 March, 2012; originally announced March 2012.

Comments: 8 pages

Journal ref: New J. Phys. 14 (2012) 075013

Superfluidity and collective oscillations of trapped Bose-Einstein condensates in a periodical potential

Authors: C. Trallero-Giner, V. Lopez-Richard, Y. Núñez-Fernández, Maurice Oliva, Ming-Chiang Chung

Abstract: Based on a unified theoretical treatment of the 1D Bogoliubov-de Genes equations, the superfluidity phenomenon of the Bose-Einstein condensates (BEC) loaded into trapped optical lattice is studied. Within the perturbation regime, an all-analytical framework is presented enabling a straightforward phenomenological mapping of the collective excitation and oscillation character of a trapped BEC where… ▽ More Based on a unified theoretical treatment of the 1D Bogoliubov-de Genes equations, the superfluidity phenomenon of the Bose-Einstein condensates (BEC) loaded into trapped optical lattice is studied. Within the perturbation regime, an all-analytical framework is presented enabling a straightforward phenomenological mapping of the collective excitation and oscillation character of a trapped BEC where the available experimental configurations also fit. △ Less

Submitted 16 December, 2011; v1 submitted 25 July, 2011; originally announced July 2011.

Comments: 5 pages and 2 Figs, some errors have been corrected in versions 2

Thermalization and Quantum Correlations in Exactly Solvable Models

Authors: Miguel A. Cazalilla, A. Iucci, Ming-Chiang Chung

Abstract: The generalized Gibbs ensemble introduced for describing few body correlations in exactly solvable systems following a quantum quench is related to the nonergodic way in which operators sample, in the limit of infinite time after the quench, the quantum correlations present in the initial state. The nonergodicity of the correlations is thus shown \emph{analytically} to imply the equivalence with t… ▽ More The generalized Gibbs ensemble introduced for describing few body correlations in exactly solvable systems following a quantum quench is related to the nonergodic way in which operators sample, in the limit of infinite time after the quench, the quantum correlations present in the initial state. The nonergodicity of the correlations is thus shown \emph{analytically} to imply the equivalence with the generalized Gibbs ensemble for quantum Ising and XX spin chains as well as for the Luttinger model the thermodynamic limit, and for a broad class of initial states and correlation functions of both local and nonlocal operators. △ Less

Submitted 5 December, 2011; v1 submitted 26 June, 2011; originally announced June 2011.

Comments: 12 pages, 4 figures. Expanded in response to Referee criticism

Journal ref: Phys. Rev. E 85, 011133 (2012)

On the relation between entanglement and subsystem Hamiltonians

Authors: Ingo Peschel, Ming-Chiang Chung

Abstract: We show that a proportionality between the entanglement Hamiltonian and the Hamiltonian of a subsystem exists near the limit of maximal entanglement under certain conditions. Away from that limit, solvable models show that the coupling range differs in both quantities and allow to investigate the effect. We show that a proportionality between the entanglement Hamiltonian and the Hamiltonian of a subsystem exists near the limit of maximal entanglement under certain conditions. Away from that limit, solvable models show that the coupling range differs in both quantities and allow to investigate the effect. △ Less

Submitted 25 May, 2011; v1 submitted 19 May, 2011; originally announced May 2011.

Comments: 7 pages, 2 figures version2: minor changes, typos corrected

Journal ref: EPL 96, 50006 (2011)

Dimerized and trimerized phases for spin-2 Bosons in a one-dimensional optical lattice

Authors: Pochung Chen, Zhi-Long Xue, I. P. McCulloch, Ming-Chiang Chung, S. -K. Yip

Abstract: We study the phase diagram for spin-2 bosons loaded in a one-dimensional optical lattice. By using non-Abelian density matrix renormalization group (DMRG) method we identify three possible phases: ferromagnetic, dimerized, and trimerized phases. We sketch the phase boundaries based on DMRG. We illustrate two methods for identifying the phases. The first method is based on the spin-spin correlation… ▽ More We study the phase diagram for spin-2 bosons loaded in a one-dimensional optical lattice. By using non-Abelian density matrix renormalization group (DMRG) method we identify three possible phases: ferromagnetic, dimerized, and trimerized phases. We sketch the phase boundaries based on DMRG. We illustrate two methods for identifying the phases. The first method is based on the spin-spin correlation function while in the second method one observes the excitation gap as a dimerization or a trimerization superlattice is imposed. The advantage of the second method is that it can also be easily implemented in experiments. By using the scattering lengths in the literature we estimate that $^{83}$Rb, $^{23}$Na, and $^{87}$Rb be ferromagnetic, dimerized, and trimerized respectively. △ Less

Submitted 20 May, 2011; v1 submitted 18 May, 2011; originally announced May 2011.

Comments: 4 pages, 3 figures. Add acknowledgement

Journal ref: Phys. Rev. A 85, 011601(R) (2012)

Edge State, Entanglement Entropy Spectra and Critical Hopping Coupling of Anisotropic Honeycomb Lattice

Authors: Ming-Chiang Chung, Yi-Hao Jhu, Pochung Chen, Sungkit Yip

Abstract: For a bipartite honeycomb lattice, we show that the Berry phase depends not only on the shape of the system but also on the hopping couplings. Using the entanglement entropy spectra obtained by diagonalizing the block Green's function matrices, the maximal entangled state with the eigenvalue $λ_m=1/2$ of the reduced density matrix is shown to have one-to-one correspondence to the zero energy state… ▽ More For a bipartite honeycomb lattice, we show that the Berry phase depends not only on the shape of the system but also on the hopping couplings. Using the entanglement entropy spectra obtained by diagonalizing the block Green's function matrices, the maximal entangled state with the eigenvalue $λ_m=1/2$ of the reduced density matrix is shown to have one-to-one correspondence to the zero energy states of the lattice with open boundaries, which depends on the Berry phase. For the systems with finite bearded edges along $x$-direction we find critical hopping couplings: the maximal entangled states (zero-energy states) appear pair by pair if one increases the hopping coupling $h$ over the critical couplings $h_c$s. △ Less

Submitted 4 May, 2011; v1 submitted 21 April, 2010; originally announced April 2010.

Comments: 4 pages, 4 figures

Phase Diagrams for Spin-1 Bosons in an Optical Lattice

Authors: Ming-Chiang Chung, Sungkit Yip

Abstract: In this paper, the phase diagrams of a polar spin-1 Bose gas in a three-dimensional optical lattice with linear and quadratic Zeeman effects both at zero and finite temperatures are obtained within mean-field theory. The phase diagrams can be regrouped to two different parameter regimes depending on the magnitude of the quadratic Zeeman effect $Q$. For large $Q$, only a first-order phase transit… ▽ More In this paper, the phase diagrams of a polar spin-1 Bose gas in a three-dimensional optical lattice with linear and quadratic Zeeman effects both at zero and finite temperatures are obtained within mean-field theory. The phase diagrams can be regrouped to two different parameter regimes depending on the magnitude of the quadratic Zeeman effect $Q$. For large $Q$, only a first-order phase transition from the nematic (NM) phase to the fully magnetic (FM) phase is found, while in the case of small $Q$, a first-order phase transition from the nematic phase to the partially magnetic (PM) phase, plus a second-order phase transition from the PM phase to the FM phase is obtained. If a net magnetization in the system exists, the first-order phase transition causes a coexistence of two phases and phase separation: for large $Q$, NM and FM phases and for small $Q$, NM and PM phases. The phase diagrams in terms of net magnetization are also obtained. △ Less

Submitted 27 April, 2009; originally announced April 2009.

Journal ref: Phys.Rev.A 80, 053615 (2009)

Cooling into the Spin-Nematic State for a Spin-1 Bose gase in an optical lattice

Authors: Ming-Chiang Chung, Sungkit Yip

Abstract: The possibility of adiabatically cooling a spin-1 polar Bose gas to a spin-nematic phase is theoretically discussed. The relation between the order parameter of the final spin-nematic phase and the starting temperature of the spinor Bose gas is obtained both using the mean-field approach for the high temperature and spin-wave approach for the low temperature. We find that there exists a good pos… ▽ More The possibility of adiabatically cooling a spin-1 polar Bose gas to a spin-nematic phase is theoretically discussed. The relation between the order parameter of the final spin-nematic phase and the starting temperature of the spinor Bose gas is obtained both using the mean-field approach for the high temperature and spin-wave approach for the low temperature. We find that there exists a good possibility to reach the spin-nematic ordering starting with spinor antiferromagnetic Bose gases. △ Less

Submitted 13 November, 2008; originally announced November 2008.

Comments: 2 figures

Journal ref: Phys.Rev.A 79, 053617 (2009)

Damping in 2D and 3D dilute Bose gases

Authors: Ming-Chiang Chung, Aranya B. Bhattacherjee

Abstract: Damping in 2D and 3D dilute gases is investigated using both the hydrodynamical approach and the Hartree-Fock-Bogoliubov (HFB) approximation . We found that the both methods are good for the Beliaev damping at zero temperature and Landau damping at very low temperature, however, at high temperature, the hydrodynamical approach overestimates the Landau damping and the HFB gives a better approxima… ▽ More Damping in 2D and 3D dilute gases is investigated using both the hydrodynamical approach and the Hartree-Fock-Bogoliubov (HFB) approximation . We found that the both methods are good for the Beliaev damping at zero temperature and Landau damping at very low temperature, however, at high temperature, the hydrodynamical approach overestimates the Landau damping and the HFB gives a better approximation. This result shows that the comparison of the theoretical calculation using the hydrodynamical approach and the experimental data for high temperature done by Vincent Liu (PRL {\bf21} 4056 (1997)) is not proper. For two-dimensional systems, we show that the Beliaev damping rate is proportional to $k^3$ and the Landau damping rate is proportional to $ T^2$ for low temperature and to $T$ for high temperature. We also show that in two dimensions the hydrodynamical approach gives the same result for zero temperature and for low temperature as HFB, but overestimates the Landau damping for high temperature. △ Less

Submitted 25 September, 2008; v1 submitted 22 September, 2008; originally announced September 2008.

Comments: 11 pages, 4 figures

Journal ref: New Journal of Phys. 11, 123012 (2009)

Bose-Einstein condensation in an optical lattice: A perturbation approach

Authors: C. Trallero-Giner, V. Lopez-Richard, Ming-Chiang Chung, Andreas Buchleitner

Abstract: We derive closed analytical expressions for the order parameter $Φ(x)$ and for the chemical potential $μ$ of a Bose-Einstein Condensate loaded into a harmonically confined, one dimensional optical lattice, for sufficiently weak, repulsive or attractive interaction, and not too strong laser intensities. Our results are compared with exact numerical calculations in order to map out the range of va… ▽ More We derive closed analytical expressions for the order parameter $Φ(x)$ and for the chemical potential $μ$ of a Bose-Einstein Condensate loaded into a harmonically confined, one dimensional optical lattice, for sufficiently weak, repulsive or attractive interaction, and not too strong laser intensities. Our results are compared with exact numerical calculations in order to map out the range of validity of the perturbative analytical approach. We identify parameter values where the optical lattice compensates the interaction-induced nonlinearity, such that the condensate ground state coincides with a simple, single particle harmonic oscillator wave function. △ Less

Submitted 31 March, 2009; v1 submitted 8 September, 2008; originally announced September 2008.

Statistical physics of cerebral embolization leading to stroke

Authors: J. P. Hague, E. M. L. Chung

Abstract: We discuss the physics of embolic stroke using a minimal model of emboli moving through the cerebral arteries. Our model of the blood flow network consists of a bifurcating tree, into which we introduce particles (emboli) that halt flow on reaching a node of similar size. Flow is weighted away from blocked arteries, inducing an effective interaction between emboli. We justify the form of the flo… ▽ More We discuss the physics of embolic stroke using a minimal model of emboli moving through the cerebral arteries. Our model of the blood flow network consists of a bifurcating tree, into which we introduce particles (emboli) that halt flow on reaching a node of similar size. Flow is weighted away from blocked arteries, inducing an effective interaction between emboli. We justify the form of the flow weighting using a steady flow (Poiseuille) analysis and a more complicated nonlinear analysis. We discuss free flowing and heavily congested limits and examine the transition from free flow to congestion using numerics. The correlation time is found to increase significantly at a critical value, and a finite size scaling is carried out. An order parameter for non-equilibrium critical behavior is identified as the overlap of blockages' flow shadows. Our work shows embolic stroke to be a feature of the cerebral blood flow network on the verge of a phase transition. △ Less

Submitted 21 October, 2009; v1 submitted 7 August, 2008; originally announced August 2008.

Comments: 11 pages, 11 figures. Major rewrite including improved justification of the model and a finite size scaling

Journal ref: Phys. Rev. E vol. 80, 051912 (2009)

Dynamical Structure Factor and Spin-Density Separation for a Weakly-Interacting Two-Component Bose Gas

Authors: M. -C. Chung, A. B. Bhattacherjee

Abstract: We show that spin-density separation in a Bose gas is not restricted to 1D but also occurs in higher dimension. The ratio ($α$) of the intra-species atom-atom interaction strength to the inter-species interaction strength, strongly influences the dynamics of spin-density separation and the elementary excitations. The density wave is phonon-like for all values of $α$. For $α<1$, spin wave is also… ▽ More We show that spin-density separation in a Bose gas is not restricted to 1D but also occurs in higher dimension. The ratio ($α$) of the intra-species atom-atom interaction strength to the inter-species interaction strength, strongly influences the dynamics of spin-density separation and the elementary excitations. The density wave is phonon-like for all values of $α$. For $α<1$, spin wave is also phonon-like. The spin waves have a quadratic dispersion in the $α=1$ coupling regime, while in the phase separated regime ($α>1$) the spin waves are found to be damped. The dynamical structure factor (DSF) reveals two distinct peaks corresponding to the density and spin waves for $α\le 1$. For $α> 1$ there is only one DSF peak corresponding to the density wave. △ Less

Submitted 4 July, 2008; v1 submitted 26 February, 2008; originally announced February 2008.

Comments: 4 pages, 2 figures

Effect of the lattice alignment on Bloch oscillations of a Bose-Einstein condensate in a square optical lattice

Authors: M. -C. Chung, A. R. Kolovsky

Abstract: We consider a Bose-Einstein condensate of ultracold atoms loaded into a square optical lattice and subject to a static force. For vanishing atom-atom interactions the atoms perform periodic Bloch oscillations for arbitrary direction of the force. We study the stability of these oscillations for non-vanishing interactions, which is shown to depend on an alignment of the force vector with respect… ▽ More We consider a Bose-Einstein condensate of ultracold atoms loaded into a square optical lattice and subject to a static force. For vanishing atom-atom interactions the atoms perform periodic Bloch oscillations for arbitrary direction of the force. We study the stability of these oscillations for non-vanishing interactions, which is shown to depend on an alignment of the force vector with respect to the lattice crystallographic axes. If the force is aligned along any of the axes, the mean field approach can be used to identify the stability conditions. On the contrary, for a misaligned force one has to employ the microscopic approach, which predicts periodic modulation of Bloch oscillations in the limit of a large forcing. △ Less

Submitted 5 November, 2007; v1 submitted 16 October, 2007; originally announced October 2007.

Comments: 4 pages, 3 figures

Entanglement scaling in critical two-dimensional fermionic and bosonic systems

Authors: Thomas Barthel, Ming-Chiang Chung, Ulrich Schollwoeck

Abstract: We relate the reduced density matrices of quadratic bosonic and fermionic models to their Green's function matrices in a unified way and calculate the scaling of bipartite entanglement of finite systems in an infinite universe exactly. For critical fermionic 2D systems at T=0, two regimes of scaling are identified: generically, we find a logarithmic correction to the area law with a prefactor de… ▽ More We relate the reduced density matrices of quadratic bosonic and fermionic models to their Green's function matrices in a unified way and calculate the scaling of bipartite entanglement of finite systems in an infinite universe exactly. For critical fermionic 2D systems at T=0, two regimes of scaling are identified: generically, we find a logarithmic correction to the area law with a prefactor dependence on the chemical potential that confirms earlier predictions based on the Widom conjecture. If, however, the Fermi surface of the critical system is zero-dimensional, we find an area law with a sublogarithmic correction. For a critical bosonic 2D array of coupled oscillators at T=0, our results show that entanglement follows the area law without corrections. △ Less

Submitted 4 February, 2006; v1 submitted 3 February, 2006; originally announced February 2006.

Comments: 4 pages, 4 figures

Journal ref: Phys. Rev. A 74, 022329 (2006)

1/f spectrum in the information transfer model for mass extinction

Authors: B. -G. Yoon, M. S. Chung, M. Y. Choi

Abstract: We study the information transfer model for biological evolution with several kinds of fitness function. The system is stimulated to volve into a stationary state, depending on the fitness function and on the dimension of the lattice formed by the species. In particular the system yields time series of the mutation rate which exhibits the $1/f$ spectrum, thus explains the power-law behavior in f… ▽ More We study the information transfer model for biological evolution with several kinds of fitness function. The system is stimulated to volve into a stationary state, depending on the fitness function and on the dimension of the lattice formed by the species. In particular the system yields time series of the mutation rate which exhibits the $1/f$ spectrum, thus explains the power-law behavior in fossil data. Effects of shortcuts introduced on the lattice are also examined and the evolution activity is usually found to increase in a well-growing system although the reduction of the overall activity may also be observed in the presence of shortcuts, depending on the initial configuration. △ Less

Submitted 5 April, 2005; originally announced April 2005.

Comments: 5 pages, 4 figures

Journal ref: J. Korean Phys. Soc. 44, 987 (2004)