Research - Assistant Professor

Bingyu Cui

Title:

Assistant Professor

Education Background:

PhD (University of Cambridge)

MS (University of Cambridge)

BSc (Imperial College London)

bycui@cuhk.edu.cn

Office Address

TC411

Phone

+86 755-235-19734

Teaching Area

Mathematics, Physics

Other

There are a few PhD and postdoc positions in the group. I am seeking graduate students/scholars with a strong theoretical background in Mathematics, Physics, Chemistry or Materials. A good programming/computational skill is also desired.

Research

Amorphous materials, nonequilibrium statistic mechanics, light-matter interaction theory, exciton dynamics, quantum measurement theory, Bohmian mechanics

Personal Website

https://myweb.cuhk.edu.cn/bycui

Research List

Dielectric and mechanical relaxation in supercooled liquids

We have contributed to the development of a unifying approach to link relaxation and vibrational properties of lattices, supercooled liquids and glasses. Once the relative initial positions of atoms are known, it is a simple task to add all contributing interactions to elastic constants for homogeneous (affine) deformations. However, even at zero temperature, particles (atoms) do not always follow homogeneous displacement fields. They instead attempt to minimize the potential energy of the system. In some cases, this requires additional local nonaffine displacements, no matter how small the deformation the system is strained to. In the framework of nonaffine lattice dynamics, the response to external strain is called affine if the interparticle displacements are just the old positions transformed by the macroscopic strain tensor. In a disordered or a non-centrosymmetric lattice where local inversion symmetry is absent, the situation becomes different since forces from the surrounding environment acting on every particle no longer cancel by symmetry. However, they have to be relaxed with additional particle displacements such that the whole system remains in mechanical equilibrium at every step in the deformation. These additional atomic displacements are called nonaffine displacements. The approach is illustrated first in the example of a granular network with and without internal stresses. Nonaffine deformations are sometimes dominant in crystals, as shown in their application to a non-centrosymmetric crystal such as alpha-quartz. Also, in the case of metallic glasses, the nonaffine description of alpha- and beta-relaxations is provided using the vibrational density of states as an input. Furthermore, the memory kernel for friction can be linked with the two-step time decay in the intermediate scattering function.

Phonon transport in amorphous materials

We have reviewed and compared the Born-Huang and the Lemaitre-Maloney theories that lead to analytical expressions for elastic constants, accounting for affine and nonaffine deformations in a lattice (or in a disordered solid). The Born-Huang method is based on Helmholtz free energy, while the Lemaitre-Maloney formalism focuses on the Gibbs ensemble with a focus on the local force. Although starting from different perspectives, material elastic constants are shown to be equivalent in all these methods in the linear elastic limit and equilibrium. The damping or attenuation coefficient of sound waves in solids due to impurities scales with the wavevector to the fourth power, also known as Rayleigh scattering. In amorphous solids, Rayleigh scattering may be enhanced by a logarithmic factor, although computer simulations offer conflicting conclusions regarding this enhancement and its microscopic origin. We have first presented a tensorial replica field-theoretic derivation based on heterogeneous or fluctuating elasticity, which shows that long-range (power-law) spatial correlations of the elastic constants are the origin of the logarithmic enhancement to Rayleigh scattering of phonons in amorphous solids. Based on this, we also derived a theory of vibrational excitations based on power-law spatial correlations in the elastic constants (or equivalently in the internal stress) to determine the vibrational density of states of disordered solids. The results provide the first prediction of a boson peak in amorphous materials where spatial correlations in the internal stresses (or elastic constants) are of the power-law form, leading to a logarithmic enhancement of (Rayleigh) phonon attenuation.

Chemical dynamics in strong light-matter interactions

Chemical manifestations of strong light-matter coupling have recently been the subject of intense experimental and theoretical studies. We developed a model to mimic the implications of the collective optical response of molecular ensembles in optical cavities on molecular vibronic dynamics. Strong molecule-radiation field coupling is often reached when a large number of of molecules respond collectively to the radiation field. In electronic strong coupling, molecular nuclear dynamics following polariton excitation reflects (a) the timescale separation between the fast electronic and photonic dynamics and the slow nuclear motion on the one hand and (b) the interplay between the collective nature of the molecule-field coupling and the local nature of the molecules nuclear response on the other. The first implies that the electronic excitation takes place, in the spirit of the Frank-Condon approximation, at an approximately fixed nuclear configuration. The second can be rephrased as the intriguing question: Can the optical excitation's collective nature lead to collective nuclear motion following polariton formation, resulting in so-called polaron decoupled dynamics? We addressed this issue by studying the dynamical properties of a simplified Holstein-Tavis-Cummings type model, in which boson modes representing molecular vibrations are replaced by two-level systems while the boson frequency and the vibronic coupling are represented by the coupling between these levels (that induces Rabi oscillations between them) and electronic state dependence of this coupling. We find that, while some aspects of the dynamical behavior appear to adhere to the polaron decoupling picture, the observed dynamics mostly reflect the local nature of the nuclear configuration of the electronic polariton rather than this picture.

Bohmian mechanics and quantum measurement theory

Bohmian mechanics, also known as pilot-wave theory or de Broglie-Bohm theory, describes a quantum world without observers. It is an alternative way (from the orthodox quantum mechanics, or Copenhagen interpretation) to formulate quantum mechanics, taking axioms that all elementary particles have a definite position at each instant and move according to an equation of motion involving a wave function that evolves under the guidance of the usual Schrodinger equation, forming the Bohmian trajectory. The particle trajectory can be written in Newton’s law, but the force is governed by an additional part known as the quantum potential. Numerical simulations under the framework of Bohmian mechanics constitute a consolidated standard tool to approach quantum many-body systems, which is an intriguing area receiving much attention in recent years. Since the Bohmian interpretation is free of external observers, it describes the measurement process as the interaction between the detected system and apparatus. Our paper discussing quantum-to-classical correspondence is potentially useful in interpreting measurement results.