A joint effort of Dr. Hui Xing's group from Shanghai Jiaotong University, Prof. Zhu-An Xu's group from Zhejiang University, and Prof. Fuqiang Huang’s group from Shanghai Institute of Ceramics have led to the discovery of the anomalous enhancement of the Nernst effect in the strange-metal state in a novel transition metal chalcogenide material 2M-WS2. It was the first time people obtained a quantitative description of the entropic behavior of the elusive charge carriers in strange metals. The results place experimental constraints on the physical mechanism of strange metals and shed new light on the understanding of high-temperature superconductivity and strongly correlated systems. The paper, entitled "Anomalous enhancement of the Nernst effect at the crossover between a Fermi liquid and a strange metal." was published in Nature Physics on Jan 16, 2013.
Strange-metal states are ubiquitous in almost all major strongly correlated electron systems, including high-temperature superconducting cuprates, iron-based superconductors, and heavy-fermion systems. Its understanding could provide important insight into high-Tc superconductivity and quantum criticality. However, with the Fermi liquid theory failing in strange metals, understanding the highly unconventional behaviors has been a long-standing challenge. Fundamental aspects of strange metals remain elusive, including the nature of their charge carriers.
Fig. 1: (a) Schematic of the thermoelectric transport measurement; (b) The phase diagram of the carrier scattering rate in 2M-WS2 as a function of temperature; (c) Temperature dependence of the Nernst coefficient in 2M-WS2. The blue line is the quasiparticle contribution of the Fermi liquid. It can be seen that the Nernst coefficient is significantly enhanced relative to the contribution of the Fermi liquid quasiparticles.
To tackle this problem, the research team used thermoelectric transport measurements to probe the anomalous behavior of charge carriers in strange metals. The Nernst effect is a specific thermoelectric effect referring to the transverse electric field generated in a conductor by a longitudinal thermal gradient under an orthogonal magnetic field. It is extremely sensitive to the entropy change of the charge carrier. Utilizing a crossover from a low-temperature Fermi liquid state to a high-temperature strange-metal state in 2M-WS2, the research team tracked closely the entropy change that accompanies the crossover using the Nernst measurements. The experiments showed that at the crossover between the Fermi liquid state and the strange-metal state (i.e., around the Fermi liquid quasiparticle coherence temperature), a large Nernst response emerges with a magnitude more than an order of magnitude larger than that expected for Fermi liquid quasiparticle contribution. The anomalous enhancement of the Nernst effect uncovered a large entropy change of the charge carriers at the crossover.
Fig. 2: (a) The phase diagram of the carrier scattering rate in 2M-WS2. (b) The phase diagram of anomalously enhanced Nernst coefficient in 2M-WS2. (c) Anomalously enhanced Nernst effect in independent strange metals.
Further investigation found that the anomalously enhanced Nernst effect exists in other independent strange metals, demonstrating the universality of the effect. The large entropy change at the crossover between quasiparticles in Fermi liquids and the carriers in strange metals provides a stringent experimental constraint on the mechanism of strange metals. Quasiparticle fractionalization and particle-hole asymmetric excitations are among the most attractive possibilities.
Shanghai Jiao Tong University is the first-author affiliation of this work. Yusen Yang, a graduate student in the School of Physics and Astronomy at Shanghai Jiao Tong University, Tao Qian, an assistant researcher in the Department of Physics at Zhejiang University, and Fang Yuqiang, a postdoctoral fellow at the Shanghai Institute of Ceramics, are the co-first authors of the paper. Associate researcher Dr. Hui Xing, Prof. Fuqiang Huang, and Prof. Zhu-An Xu are the co-corresponding authors. Prof. Wenxin Ding from Anhui University provided theoretical support; Prof. Zhiqiang Mao and Dr. Yu Wang from Pennsylvania State University, Prof. Xiangfan Xu from Tongji University, graduate students Guoxiong Tang, Chao Yao, and Xiaoxian Yan from the School of Physics and Astronomy at Shanghai Jiao Tong University, and graduate student Chenxi Jiang from the School of Physics at Zhejiang University participated in the experiments. Dr. Hui Xing led the research and wrote the research paper. He also wrote a "Behind the paper" essay in Springer Nature Communities to introduce the background and research experience related to the paper.
This work was supported by the National Key R&D Program of China, the General Project of the National Natural Science Foundation of China, the General Project of the Shanghai Natural Science Foundation, and the Key R&D Project of Zhejiang Province.
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