Recently, Professor Wang Jian's team from the School of Physics and Optoelectronic Engineering at Shenzhen University published a research paper titled "Quantifying the photocurrent fluctuation in quantum materials by shot noise" in the comprehensive journal "Nature Communications" (Impact Factor 16.6, TOP journal in the first division of the Chinese Academy of Sciences) [Nat. Commun. 15, 2012 (2024)]. The paper theoretically proposes that photocurrent shot noise can be used to probe the quantum geometric properties of centrosymmetric quantum materials.

The bulk photovoltaic effect plays a crucial role in the design of optoelectronic devices and solar energy conversion. Originating from intrinsic second-order nonlinear photocurrent responses such as shift photocurrent and injection photocurrent, the bulk photovoltaic effect has been shown in recent research to be applicable for detecting the topological and quantum geometric properties of quantum materials. For instance, experimental studies have demonstrated that second-order nonlinear injection photocurrent can measure the topological charge of Weyl points in topological Weyl semimetals. However, as a second-order nonlinear response, second-order nonlinear photocurrents can only exist in quantum materials with broken centrosymmetric symmetry. Additionally, the observed photocurrent in experiments represents only the statistical average of the photocurrent operator. Besides the statistical average, the quantum fluctuations of the photocurrent also carry important information, yet there is currently no theory regarding the quantum fluctuations of photocurrents.

Professor Wang Jian and colleagues employed nonlinear response theory to develop a quantum theory of second-order DC shot noise in insulators. They decomposed the second-order photocurrent operator into non-diagonal and diagonal parts, obtaining shift and injection DC shot noise, respectively. Through symmetry analysis, they found that both types of second-order DC shot noise are not restricted by spatial inversion symmetry and can occur in centrosymmetric quantum materials. This research revealed a close relationship between second-order DC shot noise and the quantum geometric tensor of quantum materials, including Berry curvature and quantum metric. Finally, the developed shot noise theory was combined with first-principles calculations to predict the second-order DC shot noise in two-dimensional materials GeS and MoS2. The computational results showed that the second-order DC shot noise in the centrosymmetric phase (mmm) of GeS is stronger than that in its non-centrosymmetric phase (mm2), as shown in Figure 1.

This study demonstrates that the quantum fluctuations of photocurrents and the second-order photocurrent itself can serve as complementary tools to characterize the band geometry of quantum materials. The proposed shot noise theory for insulators, along with the thermal Hall noise theory published by the team in January 2023 [Phys. Rev. Lett. 130, 036202 (2023)], collectively establishes a quantum noise theory framework based on the language of quantum geometric tensors.

Figure 1: Second-order DC scattering noise in GeS

Dr. Xiang Longjun, a postdoctoral researcher in Professor Wang Jian's team, is the first author of the paper, with Professor Wang Jian as the corresponding author, and Associate Professor Jin Hao participating in the research. Shenzhen University is the first and corresponding unit. The research was supported by the National Natural Science Key Project. Paper link: https://www.nature.com/articles/s41467-024-46264-1.

In addition to the research work on Hall effect theory and quantum spin transport published by Professor Wang Jian's team in Physical Review B in 2024, it also includes:

(1) Theoretical prediction of the intrinsic in-plane magnetic field-induced Hall effect in Weyl semimetal materials, discussing the influence of the Zeeman effect on this Hall effect, and proposing an experimental verification approach. Dr. Xiang Longjun is the first author, and Professor Wang Jian is the corresponding author. Paper link: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.109.075419

(2) Theoretical prediction of the first-order displacement current Hall effect induced by quantum metric, which is a first-order intrinsic Hall effect that does not break time reversal symmetry, corresponding to the first-order Hall effect caused by Berry curvature, which breaks time reversal symmetry. This completes the linear response theory of Bloch electrons. Dr. Xiang Longjun is the first author, and Professors Wang Bin and Wang Jian are the co-corresponding authors. Paper link: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.109.115121

(3) The influence of spin transport characteristics in disordered metal/ferromagnetic insulator composite structures using full counting statistics combined with coherent potential approximation methods was studied, and the inhibitory effect of strong anisotropic hopping energy in staggered magnets on spin transport was discussed. Dr. Li Gaoyang is the first author, and Professor Wang Jian is the corresponding author. Paper link: https://journals.aps.org/prb/abstract/10.1103/PhysRevB.109.125403