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D. Y. Sun

Publications and source records attributed to D. Y. Sun.

At least 19 recordsLinked to original sources
arXiv2025

Temporal Renormalization and the Critical-like Behavior in Supercooled Liquids

Inspired by the Kadanoff transformation in the standard renormalization group theory, we propose a temporal renormalization scheme. A Boltzmann factor that explicitly depends on the renormalized timescale is constructed, permitting thermodynamic quantities to be evaluated self-consistently across different timescales. By applying the scheme to the long-time dynamics of supercooled liquids, we uncover critical-like behaviors of supercooled liquid with three characteristic renormalization timescales: At the first timescale s_α, the system appears to be "thermodynamically frozen", i.e., the energy fluctuation becomes temperature-independent throughout the supercooled regime. At the second timescale s_β, the third-order moment of energy distribution reaches a maximum, and s_β is nearly temperature-independent. At the third timescale s_γ, the third-order moment of energy distribution passes through a minimum, and s_γ diverges as a power law s_γ=(T-T_{c})^(-γ). The scaling relations may reveal an intrinsic behavior in supercooled liquids, highlighting their unique feature. The current findings also demonstrate that temporal renormalization provides a powerful lens for investigating the timescale-specific dynamics.

cond-mat.stat-mech↗
arXiv2025

Crossover between Solid-like and Liquid-like Behavior in Supercooled Liquids

In supercooled liquids, at a temperature between the glass transition temperature Tg and the melting point Tm, thermodynamic properties remain continuous, while dynamic behavior exhibits anomalies. The origin of such thermodynamics-dynamic decoupling has long been a puzzle in the field of glass researches. In this study, we show that the ratio of the alpha-relaxation time associated with the relative and center-of-mass coordinate of nearest-neighbor atomic pairs can effectively characterize the dynamic features of supercooled liquids. With this approach, supercooled liquids can be categorized into two distinct 'states' based on their dynamics: solid-like and liquid-like behaviors. We further propose four possible paths from the liquid to the final glass state, each exhibiting unique thermodynamic and dynamic behaviors. Two of these paths predict a characteristic temperature Tx between Tm and Tg, where a crossover between solid-like and liquid-like behaviors occurs in supercooled liquids. The molecular dynamics simulations of several supercooled liquids reveal that the actual path followed by all these systems undergo the crossover between solid-like and liquid-like behaviors. Tx is found to reside in a similar temperature range as the critical temperature Tc in the mode-coupling theory and the breakdown temperature Tb of the Stokes-Einstein relation. This crossover provides a new microscopic perspective for explaining macroscopic dynamic anomalies, and the absence of a typical thermodynamic phase transition at Tg.

cond-mat.soft↗
arXiv2025

Dynamical Classification of Supercooled Liquids: Critical Cooling Rates and Entropic Signatures

Using molecular dynamics simulations, we systematically investigate supercooled liquids formed at cooling rates below and above the critical cooling rate (CCR). By analyzing the distribution of short-time averaged potential energies (DoPE) and crystallization behaviors, we identify two distinct dynamical regimes in supercooled liquids: the glass-forming regime (GFR) and the crystal-forming regime (CFR). For systems cooled below CCR (CFR), the DoPE exhibits a sharp peak, indicative of reduced configurational entropy. In contrast, liquids cooled above CCR (GFR) display a broad DoPE distribution, reflecting higher configurational entropy. These findings establish a robust classification framework for supercooled liquids. Further analysis reveals a crossover temperature (T_x) in both regimes, consistent with the freezing temperature (T_f). Near T_x, crystallization barrier-temperature relationships exhibit abrupt changes. Below T_x, CFR crystallizes marginally faster than GFR, whereas above T_x, the influence of cooling rates on crystallization rates diminishes. These results further categorize GFR and CFR into high and low-temperature sub-regimes, highlighting the interplay between thermodynamics and kinetics in supercooled liquids.

cond-mat.soft↗
arXiv2025

Scaling in Kinetics of Supercooled Liquids

The present study introduces a renormalization based approach to investigate the relaxation dynamics within supercooled liquids. By applying a numerical scale transformation to potential energies along the temporal axis, we have established a novel framework that elucidates the underlying kinetics of supercooled liquids. Our findings indicate that the skewness of the potential energy distribution attains its maximum at a characteristic time scale, D, which exhibits a Curie like scaling relationship with temperature. This scaling relationship is characterized by an exponent, g, that experiences a discontinuous transition at a critical cooling rate, signifying a kinetic like phase transition.We further demonstrate that g maintains an approximate scaling relationship with the cooling rate, where the product of g and the logarithm of the cooling rate is approximately constant.This constant, however, varies depending on whether the cooling rate is above or below the critical value, effectively classifying supercooled liquids into two distinct categories: the glass transition as the destiny of supercooled liquid, GDL, and the crystallization as the destiny of supercooled liquid, CDL. Furthermore, we identify that Ts corresponds to the glass transition temperature for GDL and the crystallization temperature for CDL, respectively. We have successfully developed a theoretical model,which not only derives the Curie like power law but also provides profound insights into the physical implications of D, g and Ts. This research delineates the differences between GDL and CDL, and offers a fresh perspective for exploring the nature of glasses. The findings contribute to the broader understanding of the dynamics of supercooled liquids and the mechanisms of glass formation.

cond-mat.soft↗
arXiv2022

A Generic Solution of Fermion Sign Problem

The fermion sign problem, the biggest obstacle in quantum Monte Carlo calculations, is completely solved in this paper. Here, we find a strategy, in which the contribution from those negative-weighted paths is thoroughly cancelled or replaced by some positive-weighted paths. The crucial point lies on the Feynman path integral formula proposed in our group, which allows us to deeply analyze the Boltzmann weight of each path. Through mathematical proof, we demonstrate that physical quantities can be exactly calculated within a specific kind of paths, which have positive the Boltzmann weight. With this finding, a new Monte Carlo method is proposed, in which the fermion sign problem is absent. As an example, the current method is applied to the two-dimensional Hubbard model, and the results do manifest the correctness.

physics.comp-ph↗
arXiv2022

The Angell Plot from the Potential Energy Landscape Perspective

Within the scenario of the potential energy landscape (PEL), a thermodynamic model has been developed to uncover the physics behind the Angell plot. In our model, by separating the barrier distribution in PELs into a Gaussian-like and a power-law form, we obtain a general relationship between the relaxation time and the temperature. The wide range of the experimental data in the Angell plot, as well as the molecular-dynamics data, can be excellently fitted by two characteristic parameters, the effective barrier (ω) and the effective width (σ) of a Gaussian-like distribution. More importantly, the fitted ω and σ^2 for all glasses are found to have a simple linear relationship within a very narrow band, and fragile and strong glasses are well separated in the ω-σ^2 plot, which indicates that glassy states only appear in a specific region of the PEL.

cond-mat.mtrl-sci↗
arXiv2022

Efficient World-line-based Quantum Monte Carlo Method Without Hubbard-Stratonovich Transformation

By precisely writing down the matrix element of the local Boltzmann operator, we have proposed a new path integral formulation for quantum field theory and developed a corresponding Monte Carlo algorithm. With current formula, the Hubbard-Stratonovich transformation is not necessary, and is not based on the determinant approach, which can improve the computational efficiency. The results show that, the simulation time has the square-law scaling with system sizes, which is comparable with the usual first-principle calculation. The current formula also improves the accuracy of the Suzuki-Trotter decomposition. As an example, we have studied the one-dimensional half-filled Hubbard model at finite temperature. The obtained results are in excellent agreement with the known solutions. The new formula and Monte Carlo algorithm can be used in various studies.

cond-mat.str-el↗
arXiv2022

Discovery of a paired Gaussian and long-tailed distribution of potential energies in nanoglasses

It is generally believed that the intrinsic properties of glasses are intimately related to potential-energy landscapes (PELs). However, little is known about the PELs of glasses below the glass transition temperature (Tg). Taking advantage of lower potential-energy barriers in nanosystems, we have systematically investigated the dynamics behavior of two nanoglasses, Al43 and Al46. Structure transformation is identified in our pure molecular-dynamics simulation far below Tg, which manifests the existence of metabasins in PELs, at least for nanoglasses. Surprisingly, we find that the distribution of potential energies shows a paired Gaussian and long-tailed distribution at temperatures below and approaching Tg; correspondingly, the distribution of the α-relaxation time exhibits an exponential-like decay. In contrast to the Gaussian distribution of energy in typical liquids and solids, the unexpected distribution may reflect the intrinsic feature of nanoglasses. Associated with the exponential-like distribution of the α-relaxation time, the stretched-exponential structural relaxation is found, and the maximum stretched behavior appears around Tg. Despite our studies focused on nanoglasses, the current finding may shed light on future studies of bulk glasses.

cond-mat.soft↗
arXiv2020

Manipulating the Glass Transition in Nanoscale

The intrinsic nature of glass states or glass transitions has been a mystery for a long time. Recently, more and more studies tend to show that a glass locates at a specific potential energy landscape (PEL). To explore how the flatness of the PEL related to glass transition, we develop a method to adjust the PEL in a controllable manner. We demonstrate that a relatively flat PEL is not only necessary but also sufficient for the formation of a nanoscale glass. We show that: (1) as long as a nanocluster is located in a region of PEL with local minimum deep enough, it can undergo a first-order solid-liquid phase transition; and (2) if a nanocluster is located in a relatively flat PEL, it can undergo a glass transition. All these transitions are independent of its structure symmetry, order or disorder. Our simulations also uncover the direct transition from one potential energy minimum to another below the glass transition temperature, which is the consequence of flat PELs.

cond-mat.soft↗
arXiv2017

Intrinsic features of an ideal glass

In order to understand the long-standing problem of the nature of glass states, we performed intensive simulations on the thermodynamic properties and potential energy surface of an ideal glass. We found that the atoms of an ideal glass manifest cooperative diffusion, and show clearly different behavior from the liquid state. By determining the potential energy surface, we demonstrated that the glass state has a flat potential landscape, which is the critical intrinsic feature of ideal glasses. When this potential region is accessible through any thermal or kinetic process, the glass state can be formed and a glass transition will occur, regardless of any special structural character. With this picture, the glass transition can be interpreted by the emergence of configurational entropies, as a consequence of flat potential landscapes.

cond-mat.dis-nn↗
arXiv2012

The structure of Lennard-Jones nanowires encapsulated by carbon nanotubes

Molecular dynamics simulations have been used to investigate the structures of Lennard-Jones (LJ) nanowires (NWs) encapsulated by carbon nanotubes (CNTs). We found that, when the radius of CNTs is small, the structures of NWs are quite simple, {\it i.e,} only multishell NWs are formed. For CNTs with larger radius, the structure of NWs becomes much richer. In addition to crystal and multishell NWs, three new kinds of NWs are found, {\it i.e,} a hybrid NW with a crystal core coated by a few shells, a crystal NW with droplet-like pits on the side of NW, and a multishell NW with droplet-like pits on the side of NW. The 'phase' diagram, which describes the structure change with the number of LJ atoms and the interaction between NWs and CNTs, is also obtained.

cond-mat.mes-hall↗
arXiv2012

Sealing off a carbon nanotube with a self-assembled aqueous valve for the storage of hydrogen in GPa pressure

The end section of a carbon nanotube, cut by acid treatment, contains hydrophillic oxygen groups. Water molecules can self-assemble around these groups to seal off a carbon nanotube and form an "aqueous valve". Molecular dynamics simulations on single-wall (12,12) and (15,15) tubes with dangling carboxyl groups show that the formation of aqueous valves can be achieved both in the absence of and in the presence of high pressure hydrogen. Furthermore, significant diffusion barriers through aqueous valves are identified. It indicates that such valves could hold hydrogen inside the tube with GPa pressure. Releasing hydrogen is easily achieved by melting the "aqueous valve". Such a design provides a recyclable and non- destructive way to store hydrogen in GPa pressure. Under the storage conditions dictated by sealing off the container in liquid water, the hydrogen density inside the container is higher than that for solid hydrogen, which promises excellent weight storage efficiency.

cond-mat.mes-hall↗
arXiv2011

Quantum Effects of Strain Influence on the Doping Energy in Semiconductors

Applying external strain is an efficient way to manipulate the site preference of dopants in semiconductors, however, the validity of the previous continuum elastic model for the strain influence on the doping forma- tion energy is still under debate. In this paper, by combining quantum mechanical theoretical analysis and first-principles calculations, we show that if the occupation change of different orbitals caused by the strain is negligible, the continuum elastic model is valid, otherwise it will fail. Our theory is confirmed by first-principles calculation of Mn-doped GaAs system. Moreover, we show that under compressive strain the hole density, thus the Curie temperature TC can increase in Mn-doped spintronic materials.

cond-mat.mtrl-sci↗
arXiv2010

Sieving hydrogen based on its high compressibility

A molecular sieve for hydrogen is presented based on a carbon nanotube intramolecular junction and a $C_{60}$. The small interspace formed between $C_{60}$ and junction provides a size changeable channel for the permselectivity of hydrogen while blocking $Ne$ and $Ar$. The sieving mechanism is due to the high compressibility of hydrogen.

cond-mat.mes-hall↗
arXiv2009

Capillary force-induced structural instability in liquid infiltrated elastic circular tubes

The capillary-induced structural instability of an elastic circular tube partially filled by a liquid is studied by combining theoretical analysis and molecular dynamics simulations. The analysis shows that, associated with the instability, there is a well-defined length scale (elasto-capillary length), which exhibits a scaling relationship with the characteristic length of the tube, regardless of the interaction details. We validate this scaling relationship for a carbon nanotube partially filled by liquid iron. The capillary-induced structural transformation could have potential applications for nano-devices.

cond-mat.soft↗
arXiv2009

Local Coarse-grained Approximation to Path Integral Monte Carlo Integration for Fermion Systems

An approximate treatment of exchange in finite-temperature path integral Monte Carlo simulations for fermions has been proposed. In this method, some of the fine details of density matrix due to permutations have been smoothed over or averaged out by using the coarse-grained approximation. The practical usefulness of the method is tested for interacting fermions in a three dimensional harmonic well. The results show that, the present method not only reduces the sign fluctuation of the density matrix, but also avoid the fermion system collapsing into boson system at low temperatures. The method is substantiated to be exact when applied to free particles.

cond-mat.stat-mech↗
arXiv2008

Effect of the equilibrium pair separation on cluster structures

A simple pair potential, which equilibrium pair separation can be varied under a fixed interaction range, has been proposed. The new potential can make both face-centered-cubic(fcc) and body-centered-cubic(bcc) structure stable by simply changing one parameter. To investigate the general effect of the potential shape on cluster structures, the evolution of cluster structures is calculated for different equilibrium pair separations. The small size clusters($N<25$), which adopt the polytetrahedra, are almost independent on the details of the potential. For the large size clusters($25<N<150$), the potential with large equilibrium pair separation trends to stable decahedra and close-packed structure, disordered clusters appear for the potential with small equilibrium pair separation, while for the middle range of equilibrium pair separation, the icosahedra are dominated.

cond-mat.mtrl-sci↗