arXiv subjects

Di Zhou

Publications and source records attributed to Di Zhou.

83 records · Page 5Linked to original sources

arXiv2017

Assembly and Phase Transitions within Colloidal Crystals

Micrometre sized colloidal particles can be viewed as large atoms with tailorable size, shape and interactions. These building blocks can assemble into extremely rich structures and phases, in which the thermal motions of particles can be directly imaged and tracked using optical microscopy. Hence, colloidal particles are excellent model systems for studying phase transitions, especially for poorly understood kinetic and nonequilibrium microscale processes. Advances in colloid fabrication, assembly and computer simulations have opened up numerous possibilities for such research. In this Review, we describe recent progress in the study of colloidal crystals composed of tunable isotropic spheres, anisotropic particles and active particles. We focus on advances in crystallization, melting and solid solid transitions, and highlight challenges and future perspectives in phase transition studies within colloidal crystals.

cond-mat.soft↗

arXiv2017

Topological edge floppy modes in disordered fiber networks

Disordered fiber networks are ubiquitous in a broad range of natural (e.g., cytoskeleton) and manmade (e.g., aerogels) materials. In this paper, we discuss the emergence of topological floppy edge modes in these fiber networks as a result of deformation or active driving. It is known that a network of straight fibers exhibits bulk floppy modes which only bend the fibers without stretching them. We find that, interestingly, with a perturbation in geometry, these bulk modes evolve into edge modes. We introduce a topological index for these edge modes and discuss their implications in biology.

cond-mat.soft↗

arXiv2017

Reply to "Failure to replicate long-range tunable attractions in colloidal system"

An arxiv paper, ref. [1] by Cao et al., claimed that the tunable attraction reported in our ref. [2] could not be detected. Ref. [1] was submitted to Nature in Apr. 2016 as a Comment on our ref. [2]. Our reply in May 2016 responded to ref. [1] and was reviewed by the editor of Nature and an external referee. Ref. [1] was rejected by Nature in Aug. 2016. We provide our reply, which answers all of the criticisms in ref. [1], in Appendix I. The main text contains a brief reply to the main criticisms.

cond-mat.soft↗

arXiv2016

Anomalous Sound Velocity and Dielectric Shift in Glass: a Renormalization Technique for Mechanical and Dielectric Susceptibilities from Generic Coupled Block Model

Glass sound velocity shift was observed to be longarithmically temperature dependent in both relaxation and resonance regimes: $Δc/c=\mathcal{C}\ln T$. It does not monotonically increase with temperature from $T=0$K, but to reach a maximum around a few Kelvin. Different from tunneling-two-level-system (TTLS) which gives the slope ratio between relaxation and resonance regimes $\mathcal{C}^{\rm rel }:\mathcal{C}^{\rm res }=-\frac{1}{2}:1$, we develop a generic coupled block model to give $\mathcal{C}^{\rm rel }:\mathcal{C}^{\rm res }=-1:1$, which agrees well with the majority of experimental measurements. We use electric dipole-dipole interaction to carry out a similar behavior for glass dielectric constant shift $Δε/ε=\mathcal{C}\ln T$. The slope ratio between relaxation and resonance regimes is $\mathcal{C}^{\rm rel}:\mathcal{C}^{\rm res}=1:-1$ which agrees with dielectric measurements quite well. By developing a renormalization procedure for non-elastic stress-stress and dielectric susceptibilities, we prove these universalities essentially come from $1/r^3$ long range interactions, independent of materials' microscopic properties.

cond-mat.dis-nn↗

arXiv2016

A Microscopic Field Theory for the Universal Shift of Sound Velocity and Dielectric Constant in Low-Temperature Glasses

In low-temperature glasses, the sound velocity changes as the logarithmic function of temperature below $10$K: $[c(T) - c(T_0)]/c(T_0) = \mathcal{C}\ln(T/T_0)$. With increasing temperature starting from $T=0$K, the sound velocity does not increase monotonically, but reaches a maximum at a few Kelvin and decreases at higher temperatures. Tunneling-two-level-system (TTLS) model explained the $\ln T$ dependence of sound velocity shift. In TTLS model the slope ratio of $\ln T$ dependence of sound velocity shift between lower temperature increasing regime (resonance regime) and higher temperature decreasing regime (relaxation regime) is $\mathcal{C}^{\rm res }:\mathcal{C}^{\rm rel }=1:-\frac{1}{2}$. In this paper we develop the generic coupled block model to prove the slope ratio of sound velocity shift between two regimes is $\mathcal{C}^{\rm res }:\mathcal{C}^{\rm rel }=1:-1$ rather than $1:-\frac{1}{2}$, which agrees with the majority of the measurements. The dielectric constant shift in low-temperature glasses, $[ε_r(T)-ε_r(T_0)]/ε_r(T_0)$, has a similar logarithmic temperature dependence below $10$K: $[ε(T)-ε(T_0)]/ε(T_0) = \mathcal{C}\ln(T/T_0)$. In TTLS model the slope ratio of dielectric constant shift between resonance and relaxation regimes is $\mathcal{C}^{\rm res}:\mathcal{C}^{\rm rel}=-1:\frac{1}{2}$. In this paper we apply the electric dipole-dipole interaction, to prove that the slope ratio between two regimes is $\mathcal{C}^{\rm res}:\mathcal{C}^{\rm rel} = -1:1$ rather than $-1:\frac{1}{2}$. Our result agrees with the dielectric constant measurements. By developing a real space renormalization technique for glass non-elastic and dielectric susceptibilities, we show that these universal properties essentially come from the $1/r^3$ long range interactions, independent of the materials' microscopic properties.

cond-mat.soft↗

arXiv2016

Real Space Renormalization: A Generic Microscopic Theory for Low-Temperature Avalanches in Static Strained Insulating Glass

We propose a microscopic model to study the avalanche problem of insulating glass deformed by external static uniform strain below $T=60$K. We use three-dimensional real-space renormalization procedure to carry out the glass mechanical susceptibility at macroscopic length scale. We prove the existence of irreversible stress drops in amorphous materials, corresponding to the steep positive-negative transitions in glass mechanical susceptibility. We also obtain the strain directions in which the glass system is brittle. The irreversible stress drops in glass essentially come from non-elastic stress-stress interaction which is generated by virtual phonon exchange process.

cond-mat.dis-nn↗

arXiv2016

A Generic Microscopic Theory for the Universality of TTLS Model Meissner-Berret Ratio in Low-Temperature Glasses

Tunneling-two-level-system (TTLS) model has successfully explained several low-temperature glass universal properties which do not exist in their crystalline counterparts. The coupling constants between longitudinal and transverse phonon strain fields and two-level-systems are denoted as $γ_l$ and $γ_t$. The ratio $γ_l/γ_t$ was observed to lie between $1.44$ and $1.84$ for 18 different kinds of glasses. Such universal property cannot be explained within TTLS model. In this paper by developing a microscopic generic coupled block model, we show that the ratio $γ_l/γ_t$ is proportinal to the ratio of sound velocity $c_l/c_t$. We prove that the universality of $γ_l/γ_t$ essentially comes from the mutual interaction between different glass blocks, independent of the microscopic structure and chemical compound of the amorphous materials. In the appendix we also give a detailed correction on the coefficient of non-elastic stress-stress interaction $Λ_{ijkl}^{(ss')}$ which was obtained by Joffrin and Levelut\cite{Joffrin1976}.

cond-mat.stat-mech↗

arXiv2016

Multiple Unpinned Dirac Points in Group-Va Single-layers with Phosphorene Structure

Emergent Dirac fermion states underlie many intriguing properties of graphene, and the search for them constitute one strong motivation to explore two-dimensional (2D) allotropes of other elements. Phosphorene, the ultrathin layers of black phosphorous, has been a subject of intense investigations recently, and it was found that other group-Va elements could also form 2D layers with similar puckered lattice structure. Here, by a close examination of their electronic band structure evolution, we discover two types of Dirac fermion states emerging in the low-energy spectrum. One pair of (type-I) Dirac points is sitting on high-symmetry lines, while two pairs of (type-II) Dirac points are located at generic $k$-points, with different anisotropic dispersions determined by the reduced symmetries at their locations. Such fully-unpinned (type-II) 2D Dirac points are discovered for the first time. In the absence of spin-orbit coupling, we find that each Dirac node is protected by the sublattice symmetry from gap opening, which is in turn ensured by any one of three point group symmetries. The spin-orbit coupling generally gaps the Dirac nodes, and for the type-I case, this drives the system into a quantum spin Hall insulator phase. We suggest possible ways to realize the unpinned Dirac points in strained phosphorene.

cond-mat.mtrl-sci↗

arXiv2016

Random Nonlinear Infinite-Level-System Model for Amorphous Solid Phonon Echo and Saturation Phenomena

The first two successful predictions for amorphous solid experiments by tunneling-two-level-system (TTLS) was phonon echo and saturation phenomena. In this paper by generalizing TTLS to infinite-level-system model with certain randomness and nonlinearity, we prove that phonon echo is a stimulated emission process. The classical infinite-level-system (taking $\hbar\to 0$ limit) cannot find saturation effect, while quantum infinite-level-system can find it with certain nonlinearity assumed. We also prove that without randomness or nonlinearity neither phonon echo nor saturation exists in arbitrary infinite-level-system.

cond-mat.other↗

arXiv2013

Percolation of Partially Interdependent Scale-free Networks

We study the percolation behavior of two interdependent scale-free (SF) networks under random failure of 1-$p$ fraction of nodes. Our results are based on numerical solutions of analytical expressions and simulations. We find that as the coupling strength between the two networks $q$ reduces from 1 (fully coupled) to 0 (no coupling), there exist two critical coupling strengths $q_1$ and $q_2$, which separate three different regions with different behavior of the giant component as a function of $p$. (i) For $q \geq q_1$, an abrupt collapse transition occurs at $p=p_c$. (ii) For $q_2 3$, $q_1$ decreases with increasing $λ$. $(b)$ In the hybrid transition, at the $q_2 < q < q_1$ region, the mutual giant component $P_{\infty}$ jumps discontinuously at $p=p^{jump}_c$ to a very small but non-zero value, and when reducing $p$, $P_{\infty}$ continuously approaches to 0 at $p_c = 0$ for $λ< 3$ and at $p_c > 0$ for $λ> 3$. Thus, the known theoretical $p_c=0$ for a single network with $λ\leqslant 3$ is expected to be valid also for strictly partial interdependent networks.

physics.data-an↗

arXiv2012

Assortativity Decreases the Robustness of Interdependent Networks

It was recently recognized that interdependencies among different networks can play a crucial role in triggering cascading failures and hence system-wide disasters. A recent model shows how pairs of interdependent networks can exhibit an abrupt percolation transition as failures accumulate. We report on the effects of topology on failure propagation for a model system consisting of two interdependent networks. We find that the internal node correlations in each of the two interdependent networks significantly changes the critical density of failures that triggers the total disruption of the two-network system. Specifically, we find that the assortativity (i.e. the likelihood of nodes with similar degree to be connected) within a single network decreases the robustness of the entire system. The results of this study on the influence of assortativity may provide insights into ways of improving the robustness of network architecture, and thus enhances the level of protection of critical infrastructures.

physics.soc-ph↗