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- Published: 2017-11-20

**Ya-Wen Sun （ 孙雅文）**

**Position**

Assistant Professor (2017 - current)

**Research Interest**

The Anti-de Sitter/Conformal field theory (AdS/CFT) correspondence is a conjecture that was first proposed in 1997 by Juan Maldacena in the framework of string theory, which has been developed as a consistent theory to combine gravity and quantum mechanics. The conjecture equates two seemingly very different theories to each other: a 4+1 dimensional weakly coupled gravity theory and a 3+1 dimensional strongly coupled conformal field theory. Soon after string theorists found more and more evidence to support this conjecture, they started to discover that AdS/CFT correspondence has a lot of implications and applications in various areas of physics and connects many unrelated theories together, e.g. quantum gravity, quantum field theory, hydrodynamics, nuclear physics, QCD, quantum information etc. In 2007, physicists (both high energy and condensed matter theorists) started to use AdS/CFT correspondence to solve strongly coupled condensed matter problems. AdS/CFT correspondence is a strong-weak duality, i.e. weakly coupled gravity theory in the classical limit corresponds to strongly coupled quantum field theory at the boundary. This allows us to look at difficult strongly coupled problems using the language of weakly coupled gravity theory. AdS/CFT correspondence also provides a useful tool to study the real time propagators compared to the conventional methods in condensed matter physics. AdS/CMT (applications of AdS/CFT to condensed matter theories) has been a very fruitful research area during the last ten years and has become more closely related to experiments in condensed matter physics.

My research includes the following several topics:

1 The dual of topologically nontrivial condensed matter systems. In the condensed matter community, there has been a lot of interest in various topological condensed matter systems recently, for example, quantum hall systems and topological insulators which are gapped topological systems, and Weyl semimetals which are gapless topological systems. The concept of topology in condensed matter systems has been developed in the weakly coupled limit and an immediate question that arises is whether nontrivial topological systems still exist at strong coupling. Does there exist a duality between a certain geometry in weakly coupled gravity theory and topologically nontrivial state? If it exists, is there any novel prediction or hint from the gravity side for the dual topological system? As a first step to answer these question we build a holographic model for a topological gapless Weyl semimetal state and study possible implications of the holographic model to the transport behavior of a topological Weyl semimetal system.

2 Hydrodynamics and transport properties. The first remarkable application of AdS/CFT to strongly coupled systems (and also its first confrontation with experiments) was the discovery of KSS (Kovtun, Son, Starinets) bound in hydrodynamics using AdS/CFT correspondence, which conjectures that the ratio of the shear viscosity over the entropy density is always larger than or equal to 1/(4Pi). This has been confirmed in various experiments, e.g. for helium, for the quark gluon plasma in RHIC, etc. This suggests that AdS/CFT can be used to study hydrodynamic properties of strongly coupled systems. A related attempt is to study various transport properties of strongly coupled condensed matter systems using AdS/CFT correspondence. In this direction, we have studied the property of negative magneto-resistivity of chiral systems using both hydrodynamics and AdS/CFT correspondence.

3 Fermionic holographic systems. Due to the sign problems, strongly coupled fermionic systems are difficult to be solved even using Monte-Carlo methods. Holography provides an efficient tool to study strongly coupled strange metal systems of condensed matter physics. We studied the lattice effect in strongly coupled holographic fermionic systems and the quantum phase transition in a strongly coupled Bose-Fermi competition system.

I am also interested in/have worked on some other research topics, including black hole physics, quantum gravity, etc.

- Karl Landsteiner, Yan Liu,
**Ya-Wen Sun**, Odd ciscosity in the quantum critical region of a holographic Weyl semimetal, Phys. Rev. Lett. 117(2016) no. 8, 081604 **Ya-Wen Sun**, Qing Yang, Negative magnetoresistivity in holography, JHEP 1609 (2016) 122- Karl Landsteiner, Yan Liu,
**Ya-Wen Sun**, Quantum phase transition between a topological and a trivial semimetal from holography, Phys. Rev. Lett. 116 (2016) no. 8, 081602 - Jan Zaanen,
**Ya-Wen Sun**, Yan Liu, Koenraad Schalm, Holographic duality in condensed matter physics, Apr 24, 2015 44 pp. - Amadeo Jimenez-Alba, Karl Landsteiner, Yan Liu,
**Ya-Wen Sun**, Anomalous magnetoconductivity and relaxation times in holography, JHEP 1507 (2015) 117

Email: yawen.sun@ucas.ac.cn

Office: Rm. S401-1, UCAS Zhong-Guan-Cun Teaching Building, No. 3 Nan-Yi-Tiao Road, Haidian District, Beijing, P. R. China (view map)

通讯地址：北京市海淀区中关村南一条3号 中国科学院大学中关村教学楼S401-1室