arXiv subjects

Cunyuan Jiang

Publications and source records attributed to Cunyuan Jiang.

At least 19 recordsLinked to original sources

arXiv2026

Vibrational inhomogeneity in amorphous solids and its geometric features

The dynamical response of amorphous solids is expected to be inhomogeneous due to the absence of translational symmetry, unlike in crystals. In this work, we use Green's function method combined with the Hessian matrix from simulation data to characterize the vibrational inhomogeneity of a two-dimensional amorphous solid. We define an order parameter for vibrational inhomogeneity, the frequency-resolved participation ratio, which equals a constant value of one over all frequencies for a crystal, while being around $\sim 0.7$ for almost the entire frequency range in an amorphous solid, thus indicating a similar level of vibrational inhomogeneity across most of the frequency range. The spatial distribution of vibrational inhomogeneity in the amorphous solid is presented in three ways: by frequency, at the particle level, and via percolation analysis, all showing that the vibrational inhomogeneity is fragmented and occurs at the particle scale. The results suggest that the Green's function method is a convenient tool for probing vibrational inhomogeneity, and provide an alternative perspective on the role of vibrational inhomogeneity in amorphous solids.

cond-mat.dis-nn↗

arXiv2026

Microscopic origin of Boson peak in amorphous solids

We proposed a non-analytic model to explain the microscopic origin of the anomalous vibrational density of states (DOS), the Boson peak (BP), in amorphous solids based on the scalar dynamical matrix of a network with springs and nodes. We argue that disorder can be classified into two factors: fluctuation of spring strength and fluctuation of coordination numbers (the number of springs connected to a node). The results suggest that BP originates solely from fluctuation of coordination numbers, while the fluctuation of spring strength only contributes to the effect of damping and has very limited effect on low frequency DOS. This work converts complexity into simplicity and provides a direct answer to the puzzle of the microscopic origin of BP in amorphous solids.

cond-mat.dis-nn↗

arXiv2026

Absence of solid phase in dense amorphous active granular matter

Solid phase of dense granular matter is inevitable because of jamming transition when the packing fraction or the pressure suffered is high enough. The experiment suggests that active Brownian granular matter will keep fluid phase even under the highest packing fraction (higher than the packing fraction of crystallization) if crystallization is prevented by mixing granular particles of different sizes. The findings encourage us to reconsider the role of activity in affecting the global dynamical properties of matter.

cond-mat.soft↗

arXiv2026

A flat-band perspective on the boson peak in amorphous solids

The boson peak is a characteristic anomaly of amorphous solids broadly defined as a low-energy excess in the density of states and heat capacity compared to the textbook predictions of Debye theory. The origin of this anomaly has long been the subject of ongoing debate and remains a topic of active controversy. We propose that the boson peak may have a defining dynamical feature: the accumulation of vibrational spectral weight within a narrow frequency window that is only weakly dependent on wavevector. In this perspective, the boson peak reflects a flat or weakly dispersive band in the dynamical structure factor rather than a propagating excitation. We revisit both experimental and simulation data from the literature through this lens and conduct further simulations in 2D and 3D amorphous systems. Taken together, these analyses provide compelling converging evidence for this interpretation and sharply constrain the space of viable theoretical descriptions of the boson peak.

cond-mat.soft↗

arXiv2025

Impact of elastic inhomogeneity on collective dynamical properties investigated by field theoretical description in real space

Interpreting the vibrational properties of amorphous solids beyond Debye's theory is challenging due to the presence of inhomogeneity on the mesoscopic scale. In this work, we model this inhomogeneity by real-space fluctuating elasticity with a spatially correlated distribution and calculate the dynamical properties using an exact real-space field theoretical approach. Our results clarify that the excess low-frequency density of states (DOS) originates from a selective scattering effect (stronger scattering of short wavelengths) induced by elastic inhomogeneity. The visualization of the local DOS in real space reveals the existence of anomalous modes, highly excited spots, at low frequencies. The findings regarding these highly excited spots and the selectivity of the correlation length were missed in previous perturbative field approaches in wave-vector space, and they align with recent progress from particle-level simulations and experiments. These results provide concrete insights into the low-frequency vibrational anomaly of amorphous solids from the perspective of simple elastic inhomogeneity.

cond-mat.soft↗

arXiv2025

Revealing the Geometrical and Vibrational Properties of the Defects Driving the Boson Peak

In amorphous solids, the vibrational density of states shows an excess of modes over the Debye model, known as the boson peak, whose origin remains unclear. Studies suggest a link to quasi-localized nonphononic vibrations or 'defects,' but identifying them is challenging due to hybridization with phonons that renders methods based on localization properties, such as the participation ratio, unreliable. We introduce a practical method to separate hybridized phonons from localized vibrations and find that boson peak phonons hybridize with compact, two-dimensional defects exhibiting oscillatory pure shear deformations. These two-dimensional defects are also exposed by the procedure recently employed to identify stringlets (Nature Physics volume 18, pages 669-677 (2022)), suggesting that these may not be one-dimensional objects as speculated. Our work demonstrates the presence of localized defects at the boson peak frequency and provides a comprehensive characterization of their vibrational and geometric properties, resolving the tension between the concepts of quasi-localized quadrupolar defects and stringlets.

cond-mat.soft↗

arXiv2025

Topological signatures of collective dynamics and turbulent-like energy cascades in apolar active granular

Active matter refers to a broad class of non-equilibrium systems where energy is continuously injected at the level of individual ``particles". These systems exhibit emergent collective behaviors that have no direct thermal-equilibrium counterpart. Their scale ranges from micrometer-sized swarms of bacteria to meter-scale human crowds. In recent years, the role of topology and self-propelled topological defects in active systems has garnered significant attention, particularly in polar and nematic active matter. Building on these ideas, we investigate emergent collective dynamics in apolar active granular fluids. Using isotropic granular vibrators as a model experimental system of apolar active Ornstein-Uhlenbeck particles in a dry environment, we uncover a distinctive three-stage time evolution arising from the intricate interplay between activity and inelastic interactions. By analyzing the statistics, spatial correlations, and dynamics of vortex-like topological defects in the displacement vector field, we demonstrate their ability to describe this intrinsic collective motion. Furthermore, associated to these topological defects, we reveal the onset of a turbulent-like inverse energy cascade, where kinetic energy transfers across different length scales over time. As the system evolves, the power scaling of the energy transfer increases with the duration of observation. Our findings show that topological concepts can be extended to the nonequilibrium dynamics of apolar active matter, revealing a direct link between microscopic topological processes and emergent large-scale behaviors in active granular fluids that lack both a well-defined direction of motion and an intrinsic axis of orientation at the particle scale.

cond-mat.soft↗

arXiv2025

Can we build a transistor using vacancy-induced bound states in a topological insulator

Topological insulators (TIs) have been considered as promising candidates for next generation of electronic devices due to their topologically protected quantum transport phenomena. In this work, a scheme for atomic-scale field effect transistor (FET) based on vacancy-induced edge states in TIs is promoted. By designing the positions of vacancies, the closed channel between source and drain terminals provided by vacancy-induced edge states can have the energy spectra with a gap between edge and bulk states. When gate terminal receive the signal, electric field applied by gate terminal can shift quasi Fermi energy of the closed channel from edge states into the gap, and hence open the channel between source and drain terminals. The energy spectra and the effect of electric field are demonstrated using Haldane model and density functional theory (DFT) respectively. This work suggest possible revolutionary applicational potentials of vacancy-induced edge states in topological insulators for atomic-scale electronics.

cond-mat.mes-hall↗

arXiv2025

Designing atomic-scale resistive circuits in topological insulators through vacancy-induced localized modes

We demonstrate that vacancies can induce topologically protected localized electronic excitations within the bulk of a topological insulator, and when sufficiently close, give rise to one-dimensional propagating chiral bulk modes. We show that the dynamics of these modes can be effectively described by a tight-binding Hamiltonian, with the hopping parameter determined by the overlap of electronic wave functions between adjacent vacancies, accurately predicting the low-energy spectrum. Building on this phenomenon, we propose that vacancies in topological materials can be utilized to design atomic-scale resistive circuits, and estimate the associated resistance as a function of the vacancy distribution's geometric properties.

cond-mat.mes-hall↗

arXiv2025

Experimental observation of gapped shear waves and liquid-like to gas-like dynamical crossover in active granular matter

Unlike crystalline solids, liquids lack long-range order, resulting in diffusive shear fluctuations rather than propagating waves. Simulations predict that liquids exhibit a $k$-gap in wave-vector space, where solid-like transverse waves reappear above this gap. Experimental evidence in classical liquids has been limited, observed only in 2D dusty plasmas. Here, we investigate this phenomenon using active Brownian vibrators and uncover distinct gas-like and liquid-like phases depending on the packing fraction. We measure key properties, including pair correlation functions, mean square displacements, velocity auto-correlation functions, and vibrational density of states. In the liquid-like phase, we confirm the $k$-gap in transverse excitations, whose size grows as the packing fraction decreases and eventually disappears in the gas phase. Our findings extend the concept of the $k$-gap to active granular systems and reveal striking parallels with supercritical fluids.

cond-mat.soft↗

arXiv2025

The anomalous density of states and quasi-localized vibration through homogeneous thermalization of an inhomogeneous elastic system

Amorphous solids are dynamically inhomogeneous due to in lack of translational symmetry and hence exhibit vibrational properties different from crystalline solids with anomalous low frequency vibrational density of states (VDOS) and related low temperature thermal properties. However, an interpretation of their origin from basic physical laws is still needed compared with rapidly progressed particle level investigations. In this work, we start with the quasi-equilibrium condition, which requires elastic potential energy to be homogeneously distributed even in an inhomogeneous elastic solid over long time observation. Analytical result shows that the anomalous low frequency VDOS behavior $D(ω) \propto ω^4$ can be obtained when the quasi-equilibrium condition is satisfied on an inhomogeneous elastic system. Under high frequency after a crossover depending on the length scale of inhomogeneity, the power law of VDOS is changed to square $D(ω) \propto ω^2$ which is Debye's law for crystalline solids. These features agree with recent particle level investigations. Our work suggest that the universal low frequency anomaly of amorphous solids can be considered as a result of homogeneous thermalization.

cond-mat.soft↗

arXiv2024

Dispersionless Flat Mode and Vibrational Anomaly in Active Brownian Vibrators Induced by String-like Dynamical Defects

In recent years, active Brownian particles have emerged as a prominent model system for comprehending the behaviors of active matter, wherein particles demonstrate self-propelled motion by harnessing energy from the surrounding environment. A fundamental objective of studying active matter is to elucidate the physical mechanisms underlying its collective behaviors. Drawing inspiration from advancements in molecular glasses, our study unveils a low-energy ``flat mode" within the transverse spectrum of active Brownian vibrators -- a nearly two-dimensional, bi-disperse granular assembly. We demonstrate that this collective excitation induces an anomalous excess in the vibrational density of states (VDOS) beyond the phononic Debye contribution. We characterize the properties of this flat mode by exploring the parameter space of our experimental system and tuning the packing fraction, the vibrational frequency, the particle size ratio and the mixture ratio. Additionally, we establish through empirical evidence that string-like dynamical defects, discerned via the spatial distribution of each particle's contribution to the reduced transverse VDOS, serve as the microscopic origin of the flat mode and its associated anomalies.

cond-mat.soft↗

arXiv2024

Phonons in stringlet-land and the boson peak

Solid materials that deviate from the harmonic crystal paradigm exhibit characteristic anomalies in the specific heat and vibrational density of states (VDOS) with respect to Debye's theory predictions. The boson peak (BP), a low-frequency excess in the VDOS over Debye law $g(ω) \propto ω^2$, is certainly the most famous among them; nevertheless, its origin is still subject of fierce debate. Recent simulation works provided strong evidence that localized one-dimensional string-like excitations (stringlets) might be the microscopic origin of the BP. In this work, we study the dynamics of acoustic phonons interacting with a bath of vibrating 1D stringlets with exponentially distributed size, as observed in simulations. We show that stringlets strongly renormalize the phonon propagator and naturally induce a BP anomaly in the vibrational density of states, corresponding to the emergence of a dispersionless BP flat mode. Additionally, phonon-stringlet interactions produce a strong enhancement of sound attenuation and a dip in the speed of sound near the BP frequency, consistent with experimental and simulation data. The qualitative trends of the BP frequency and intensity are predicted within the model and shown to be in good agreement with previous observations. In summary, our results substantiate with a simple theoretical model the recent simulation results by Hu and Tanaka claiming the origin of the BP from stringlet dynamics.

cond-mat.soft↗

arXiv2024

Shaping the topology of twisted bilayer graphene via time-reversal symmetry breaking

Symmetry breaking is an effective tool for tuning the transport and topological properties of 2D layered materials. Among these materials, twisted bilayer graphene (TBG) has emerged as a promising platform for new physics, characterized by a rich interplay between topological features and strongly correlated electronic behavior. In this study, we utilize time-reversal symmetry breaking (TRSB) to manipulate the topological properties of TBG. By varying the strength of TRSB, we discover a topological phase transition between a topological insulating phase, which exhibits a pair of flat bands with opposite Chern numbers, and a novel insulating state where the Chern number, but not the Berry curvature, of the flat bands vanishes. We demonstrate that this topological transition is mediated by a gap closing at the $Γ$ point, and we construct a three-dimensional phase diagram as a function of the twisting angle, the symmetry-breaking parameter, and the mismatch coupling between AA and AB stacking regions. Finally, we show that this novel electronic phase can be identified in the lab by measuring, as a function of the Fermi energy, its non-quantized anomalous Hall conductivity that is induced by the Berry dipole density of the lowest flat bands.

cond-mat.mes-hall↗

arXiv2024

Stringlet Excitation Model of the Boson Peak

The boson peak (BP), a low-energy excess in the vibrational density of states over the Debye contribution, is often identified as a characteristic of amorphous solid materials. Despite decades of efforts, its microscopic origin still remains a mystery. Recently, it has been proposed, and corroborated with simulations, that the BP might stem from intrinsic localized modes involving one-dimensional (1D) string-like excitations (``stringlets"). We build on a theory originally proposed by Lund that describes the localized modes as 1D vibrating strings, but we specify the stringlet size distribution to be exponential, as observed in simulations. We provide an analytical prediction for the BP frequency $ω_{BP}$ in the temperature regime well below the observed glass transition temperature $T_g$. The prediction involves no free parameters and accords quantitatively with prior simulation observations in 2D and 3D model glasses based on inverse power law potentials. The comparison of the string model to observations is more uncertain when compared to simulations of an Al-Sm metallic glass material at temperatures well above $T_g$. Nonetheless, our stringlet model of the BP naturally reproduces the softening of the BP frequency upon heating and offers an analytical explanation for the experimentally observed scaling with the shear modulus in the glass state and changes in this scaling in simulations of glass-forming liquids. Finally, the theoretical analysis highlights the existence of a strong damping for the stringlet modes above $T_g$, which leads to a large low-frequency contribution to the 3D vibrational density of states, observed in both experiments and simulations.

cond-mat.soft↗

arXiv2024

Engineering flat bands in twisted-bilayer graphene away from the magic angle with chiral optical cavities

Twisted bilayer graphene (TBG) is a recently discovered two-dimensional superlattice structure which exhibits strongly-correlated quantum many-body physics, including strange metallic behavior and unconventional superconductivity. Most of TBG exotic properties are connected to the emergence of a pair of isolated and topological flat electronic bands at the so-called magic angle, $θ\approx 1.05^{\circ}$, which are nevertheless very fragile. In this work, we show that, by employing chiral optical cavities, the topological flat bands can be stabilized away from the magic angle in an interval of approximately $0.8^{\circ}<θ<1.3^{\circ}$. As highlighted by a simplified theoretical model, time reversal symmetry breaking (TRSB), induced by the chiral nature of the cavity, plays a fundamental role in flattening the isolated bands and gapping out the rest of the spectrum. Additionally, TRSB suppresses the Berry curvature and induces a topological phase transition, with a gap closing at the $Γ$ point, towards a band structure with two isolated flat bands with Chern number equal to $0$. The efficiency of the cavity is discussed as a function of the twisting angle, the light-matter coupling and the optical cavity characteristic frequency. Our results demonstrate the possibility of engineering flat bands in TBG using optical devices, extending the onset of strongly-correlated topological electronic phases in moiré superlattices to a wider range in the twisting angle.

cond-mat.mes-hall↗

arXiv2023

A quantitative theoretical model of the boson peak based on stringlet excitations

The boson peak (BP), a low-energy excess in the vibrational density of states over the phonon Debye contribution, is usually identified as one of the distinguishing features between ordered crystals and amorphous solid materials. Despite decades of efforts, its microscopic origin still remains a mystery and a consensus on its theoretical derivation has not yet been achieved. Recently, it has been proposed, and corroborated with simulations, that the BP might stem from intrinsic localized modes which involve string-like excitations ("stringlets") having a one-dimensional (1D) nature. In this work, we build on a theoretical framework originally proposed by Lund that describes the localized modes as 1D vibrating strings, but we specify the stringlet size distribution to be exponential, as observed in independent simulation studies. We show that a generalization of this framework provides an analytically prediction for the BP frequency $ω_{BP}$ in the temperature regime well below the glass transition temperature in both 2D and 3D amorphous systems. The final result involves no free parameters and is in quantitative agreement with prior simulation observations. Additionally, this stringlet theory of the BP naturally reproduces the softening of the BP frequency upon heating and offers an analytical explanation for the experimentally observed scaling with the shear modulus in the glass state and changes in this scaling in cooled liquids. Finally, the theoretical analysis highlights the existence of a strong damping for the stringlet modes at finite temperature which leads to a large low-frequency contribution to the 3D vibrational density of states, as observed in both experiments and simulations.

cond-mat.soft↗

arXiv2023

Glassy heat capacity from overdamped phasons and a hypothetical phason-induced superconductivity in incommensurate structures

Phasons are collective low-energy modes that appear in disparate condensed matter systems such as quasicrystals, incommensurate structures, fluctuating charge density waves, and Moiré superlattices. They share several similarities with acoustic phonon modes, but they are not protected by any exact translational symmetry. As a consequence, they are subject to a wavevector independent damping, and they develop a finite pinning frequency, which destroy their acoustic linearly propagating dispersion. Under a few and simple well-motivated assumptions, we compute the phason density of states, and we derive the phason heat capacity as a function of the temperature. Finally, imagining a hypothetical s-wave pairing channel with electrons, we compute the critical temperature $T_c$ of the corresponding superconducting state as a function of phason damping using the Eliashberg formalism. We find that for large phason damping, the heat capacity is linear in temperature, showing a distinctive glass-like behavior. Additionally, we observe that the phason damping can strongly enhance the effective Eliashberg coupling, and we reveal a sharp non-monotonic dependence of the superconducting temperature $T_c$ on the phason damping, with a maximum located at the underdamped to overdamped crossover scale. Our simple computations confirm the potential role of overdamped modes in explaining the glassy properties of incommensurate structures, but also in possibly inducing strongly-coupled superconductivity therein, and enhancing the corresponding $T_c$.

cond-mat.supr-con↗