arXiv subjects

Jimin Bai

Publications and source records attributed to Jimin Bai.

3 recordsLinked to original sources

arXiv2026

Three-stage melting of a macroscopic continuous spacetime crystal

A spacetime crystal is a phase of matter that spontaneously develops periodic order in both space and time. Spacetime crystals have been experimentally observed in microscopic quantum many-body systems and, very recently, in a mesoscopic nematic liquid crystal. However, the melting process of a spacetime crystal and its underlying physical mechanisms have not yet been experimentally reported. Here, we present a direct observation of a classical continuous spacetime crystal melting in a table-top experiment with macroscopic active granular disks in 2+1 spacetime dimensions. The spacetime crystal is characterized by the spontaneous formation of a coherent, rigid-body rotation of a 2D triangular lattice that persists for almost a day and remains remarkably robust to noise. By tuning the disk packing fraction, we observe a complex three-stage melting process involving a spatially hexatic phase and multiple coexistence regions. Importantly, we show that spatial and temporal crystalline orders melt separately through distinct mechanisms: spatial order is destroyed by the proliferation of topological defects, while temporal order is lost through the decay of directional persistence caused by the progressive weakening of many-body interactions. Our results demonstrate that the spontaneous breaking of spatial and temporal translational symmetries can be decoupled, leading to the emergence of exotic out-of-equilibrium classical phases of matter.

cond-mat.soft↗

arXiv2025

Tracking shear mode dynamics across the glass transition in a 2D colloidal system

Long-wavelength collective shear dynamics are profoundly different in solids and liquids. According to the theoretical framework developed by Maxwell and Frenkel, collective shear waves vanish upon melting by acquiring a characteristic wave-vector gap, known as the $k$-gap. While this prediction has been supported by numerous simulations, experimental validation remains limited. Moreover, this phenomenon has been never tested across a continuous glass transition between a liquid phase and a glassy state with large but finite viscosity. In this work, we track the dispersion relation of collective shear modes in a two-dimensional colloidal system and provide direct experimental evidence for the emergence of a $k$-gap. This gap opens continuously at an effective temperature consistent with the onset of the glass transition and the vanishing of the static shear modulus. By extracting the instantaneous shear velocity from the experimental data, we uncover a shear relaxation time exhibiting a super-Arrhenius temperature dependence characteristic of glass-forming materials, accurately described by the Vogel-Fulcher-Tammann (VFT) relation. Our results confirm the predictions of the Maxwell-Frenkel framework and highlight their relevance across continuous melting processes originating from low-temperature amorphous solid phases.

cond-mat.soft↗

arXiv2025

Stress-stress correlations in two-dimensional amorphous and crystalline solids

Stress-stress correlations in crystalline solids with long-range order can be straightforwardly derived using elasticity theory. In contrast, the `emergent elasticity' of amorphous solids, rigid materials characterized by an underlying disordered structure, defies direct explanation within traditional theoretical frameworks. To address this challenge, tensor gauge theories have been recently proposed as a promising approach to describe the emergent elasticity of disordered solids and predict their stress-stress correlations. In this work, we revisit this problem in two-dimensional amorphous and crystalline solids by employing a canonical elasticity theory approach, supported by experimental and simulation data. We demonstrate that, with respect to static stress-stress correlations, the response of a 2D disordered solid is indistinguishable from that of a 2D isotropic crystalline solid and it is well predicted by vanilla elasticity theory. Moreover, we show that the presence of pinch-point singularities in the stress response is not an exclusive feature of amorphous solids. Our results confirm previous observations about the universal character of static stress-stress correlations in crystalline and amorphous packings.

cond-mat.soft↗