Research

I am broadly interested in various topics of modern physics. Current interests include: entanglement in many-body systems, critical systems in and out equilibrium, renormalization group flows in generic space of QFTs, impurity in static ground states and non-equilibrium dynamics, matrix product states, holography, e.tc.

Why numerics?

Perhaps my research record suggests that I like to working with field theories. HOWEVER, I do not consider field theories as the answer to everything. In fact, I consider numerical simulation is more reliable for accessing the physics of our real world. This is also the reason why I always work in a combination of theory and numerics: 

propose a theory or theoretical expectation —> 

 check it numerically on a corresponding lattice construction

or

construct a lattice model with numerical simulation on it —> 

  possible theoretical understanding of numerical observations

I find that this process is very helpful for building a faithful picture of target physics. 

Why entanglement?

In recent years, the study on (long-range) entanglement in many-body systems has brought novel insights to some long-standing problems in many-body physics. I believe there is a chance to find a new paradigm from such crossing, and the study on entanglement in many-body systems could be a trigger of the revolution in modern theoretical physics.

Research Projects       

A publication list can be found on My Google Scholar Page.

Entanglement in (conformal) field theory

How to access the entanglement properties of many-body systems and extract universal information from them is an important and challenging question.

• Qicheng Tang, Zixia Wei, Yin Tang, Xueda Wen and W. Zhu, Universal entanglement signatures of interface conformal field theories, Phys. Rev. B 109, L041104 (2024), arXiv: 2308.03646 [cond-mat.stat-mech].

We explore possible universal signatures about an interface gluing two distinct CFTs, by combing holographic calculation and lattice simulation. The results are potentially useful for characterizing the underlying universality of generic critical interface theories.

• Yin Tang, Qicheng Tang, and W. Zhu, Thermal correction to entanglement spectrum for conformal field theories, JHEP08(2022)239, arXiv: 2206.10285 [hep-th].

We analytically derive the thermal correction to the entanglement spectrum for general two-dimensional conformal field theories.

• Qicheng Tang and W. Zhu, Entanglement entropy in (2+1)D quantum critical points with quenched disorder: A dimensional reduction approach, Phys. Rev. B 106, 195152 (2022), arXiv: 2201.05035 [cond-mat.str-el].

We develop an analytic method to calculate the entanglement entropy for (2+1)-dimensional theories with quench disorders.

• Qi-Cheng Tang and Wei Zhu, Critical Scaling Behaviors of Entanglement Spectra, Chin. Phys. Lett., 2020, 37 (1): 010301, arXiv: 1911.03125 [cond-mat.stat-mech].

We analytically solve the entanglement spectrum during global quench that is driven by a critical Hamiltonian. It is found that the conformal tower structure of the critical Hamiltonian can be encoded into the entanglement spectrum of a pure state via global quench.

Emergent criticality in non-unitary dynamics

Exploring novel critical systems from non-unitary dynamics is potentially helpful to build up the underlying paradigm in statistical physics.

• Qicheng Tang, Xiao Chen and W. Zhu, Quantum criticality in the nonunitary dynamics of (2+1)-dimensional free fermions, Phys. Rev. B 103, 174303 (2021) [editor’s suggestion], arXiv: 2101.04320 [cond-mat.stat-mech] .

We design a dynamical steady state with critical signatures of entanglement via nonunitary dynamics of free fermions. The emergent criticality, observed by large-scale numerical simulations, can be phenomenologically described by a master equation of two-point correlation and a quasi-particle picture of producing entanglement. Analytic results indicate the possible extension into general spatial dimensions.

• Qicheng Tang and W. Zhu, Measurement-induced phase transition: A case study in the non-integrable model by density-matrix renormalization group calculations, Phys. Rev. Research 2, 013022 (2020), arXiv: 1908.11253 [cond-mat.stat-mech].

We provide one more example of the measurement-induced phase transition by density-matrix renormalization group calculations. 

Phase transition from electron structure

Traditional condensed matter physics (or said solid state physics) provides a way to access the properties of solid states from their band structure.

• Qicheng Tang, L. A. Svyatkin, and I. P. Chernov, Influence of hydrogen on electron-phonon coupling and intrinsic electrical resistivity in zirconium: A first-principles study, Phys. Rev. B 99, 205152 (2019), arXiv: 1806.00847 [cond-mat.mtrl-sci].

We calculate the electron and phonon structure of zirconium with hydrogen atoms. By estimating the dependence of electron-phonon coupling with the concentration of hydrogen, we provide a link to the change of electrical resistivity and structural transition in zirconinum.