ECNU Scientists discover new ferroic-family member


On December 16, 2016, Nature Communications published “Concepts of ferrovalley material and anomalous valley Hall effect” online. Tong Wenyi, a Ph.D candidate from Key Laboratory of Polar Materials and Devices, Ministry of Education, ECNU, is the first author of the paper. Prof. Duan Chungang, the corresponding author of the paper, and Prof. Wan Xiangang from Nanjing University co-supervised Tong in this study. They, for the first time, unveil the ferrovalley material, a new member of ferroic-family. A new form of Hall effect, namely anomalous valley Hall effect in ferrovalley materials, is introduced by them as well.

Tong Wenyi (Right) and her supervisor Prof. Duan Chungang.

Conventional electronics is based on the intrinsic charge degree of freedom of an electron, which gives birth to the very large scale integration (VLSI). Spintronics, utilizing electrons’ spin degree of freedom, further improves the information processing speed and storage capacity to a great extent. Undoubtedly, the exploration of novel electronic degrees of freedom has important implications in both basic quantum physics and advanced information technology. 

With the celebrated discovery of graphene, the concept of valleytronics based on the valley degree of freedom has attracted immense attention. Since the valley index constitutes the binary states, similar to charge and spin of electrons, it offers additional opportunities for information storage, electronic, magnetic and optical switches.

From the point of view on long-term information storage, the valley polarization, i.e. to break the degeneracy between the two prominent K+ and K- valleys, is highly desirable in valleytronics. Enormous attempts including optical pumping, strain engineering, magnetic doping, applying magnetic or oscillating electric field have been adopted. However, the strategies relying on external means are volatile in most cases, and suffer from relatively small strength of the valley polarization. 

Through years of experience in multiferroics, the researchers from Key Laboratory of Polar Materials and Devices, Ministry of Education, realized at once that in analogy to ferroelectric materials with spontaneous charge polarization, or ferromagnetic materials with spontaneous spin polarization, there must be a new ferroic-family member with spontaneous valley polarization in valleytronics. It can be named as a ferrovalley material and is of great significance in the utilization and manipulation of the valley degree of freedom effectively.

Sketch of data storage utilizing hole-doped ferrovalley materials based on anomalous valley Hall effect.

By rewriting the total Hamiltonian based on the two-band k·p model, they establish the general rule to hunt for such a material in monolayers of transition metal dichalcogenides, that is the coexistence of the spin-orbit coupling (SOC) with the intrinsic exchange interaction. Following the tactic, they firstly unveil a certain ferrovalley material: 2H-VSe2 monolayer. Its up to ~ 590 K Curie temperature reveals that it is indeed an ideal candidate for practical applications even in room temperature. 

Combining first-principles calculations with group theory and optical analysis programs they own developed, they predict that an unusual chirality-dependent optical band gap exists in such ferrovalley materials, indicating the possibility to judge the occurrence of valley polarization and its polarity using noncontact and nondestructive circularly polarized optical means. More interestingly, they introduce a new form of Hall effect, namely anomalous valley Hall effect in ferrovalley materials. Except for the combination of spin and valley Hall current in normal one, an additional charge Hall current deriving from the spontaneous valley polarization is present in the anomalous valley Hall effect. The long-lived charge, spin and valley accumulations on sample sides brings charming phenomena, such as emission of photons with opposite circular polarizations on the two boundaries, and more importantly provides a really feasible route toward data storage utilizing ferrovalley materials. As shown below, based on the anomalous valley Hall effect, the binary information is stored by the valley polarization of the ferrovalley material, which could be controlled by the magnetic moment through an external magnetic field. And it can be easily “read out” utilizing the sign of the transverse Hall voltage. Besides the nonvolatile electrically reading and magnetically writing memory devices, valley filter, valley valve and other promising valleytronic devices are anticipated. 

This work can serve as a bridge between valleytronics and multiferroics, which are hot fields in condensed matter physics. In addition to ferroelectricity, ferromagnetism, ferroelasticity, and ferrotoroidicity, ferrovalley is now introduced to the ferroic-family. Its potential coupling with others may provide novel physics in both valleytronic and multiferroic fields and greatly promote their developments. 

The research was supported by the National Key Project for Basic Research of China, the National Natural Science Foundation of China, and ECNU Outstanding Doctoral Dissertation Cultivation Plan of Action. Computations were performed at the ECNU computing center. 

Read more about the discovery:ncomms13612.pdf


East China Normal University