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BIT has made Important progress in the study of interface reflection

  A few days ago, Yu Zhiming and his partner, from Professor Yao Yugui's research team of Beijing Institute of Technology, made important progress in the study of interface reflection. They found that the anomalous displacement circulation in interface reflection can be quantized. Took a new step towards important but very rare quantized physical quantities in physics.

  Interface reflection is a common and well-known physical process, but research in this field is still booming. In interfacial reflection, an interesting physical phenomenon is anomalous displacement phenomenon, which means that when particles scatter at the interface, their ejection points and incident points are separated at the interface, and there is a displacement difference. Abnormal displacement was first found in optics, and then introduced into electronic systems because of the rise of electron optics, even in Andreev reflection at metal superconducting interfaces. Therefore, anomalous displacement is a ubiquitous physical phenomenon. studying and revealing novel anomalous displacement behavior is not only a basic physical research problem, but also can provide a new potential means to detect the physical properties of materials.

  In recent years, Yu Zhiming, professor Yao Yugui and professor Yang Shengyuan have made a series of important progress in the field of anomalous displacement research. Yu Zhiming and his coworkers first pointed out that transverse displacement can exist in Andreev reflection at the superconducting interface of metal and s- waves in 2017[ Physical Review B,96,121101(2017)], and later in 2018, it was found that unconventional superconductors can independently lead to anomalous transverse displacement without the need for spin-orbit coupling. The existence conditions of anomalous transverse displacement are greatly expanded.[Physical Review Letters 121, 176602 (2018)]  The above work lays a very important research foundation for Yu Zhiming and his coworkers to find the quantized anomalous displacement circulation in the near future.

Figure 1: (a)Schematic illustration of anomalous displacement in interfacial reflection  The blue line segment represents the incident and reflected electron flow, and the red line segment represents the anomalous displacement. (b) Anomalous displacement is a vector that varies with the motion of the interface  C represents the loop integration of anomalous displacement in the momentum space at the interface

Figure 2: (a) Schematic illustration of the model of common metal and outer metal (b)Vector field of anomalous displacement in interfacial momentum space  The arrow represents the direction of the vector and the color represents the absolute value of the vector. (c)The phase of the reflected amplitude varies with the interfacial momentum

Figure 3:  The behavior of anomalous displacements in Andreev reflection under (a) s-wave superconductivity ,(b) chiral p- wave and (d) d wave superconductivity  (c) The behavior of the phase angle of Andreev reflected amplitude in the interfacial momentum space in the case of chiral p- waves

  A general model for studying anomalous displacement phenomena can be divided into two parts, one is the incident region, the other is the transmission or target region, as shown in figure 1(a) below. When the incident particle beam (such as light or electron) moves from the incident region to the target region, it can produce abnormal displacement when reflected at the interface.  Anomalous displacement is a vector defined in the interfacial momentum space, which is defined as the projection of the incident region fermi surface on the interfacial momentum space, as shown in Figure 1(b), respectively. Yu Zhiming and others first point out that when the incident region has some specific symmetry, the circulation of anomalous displacement must be quantized.  It is particularly important that the symmetry requirements of the quantization are not harsh  Many experimental conventional systems, such as ordinary metals, meet the symmetry requirements of quantization. The work then calculated two specific models:  anomalous displacements of common metal and outer semimetal models, and common metal and superconductor models.  The results show that the vector field of anomalous displacement will form quantum vortex in the momentum space of the interface, which directly proves that the anomalous displacement circulation has the characteristics of quantization, as shown in Figure 2 and Figure 3.  In addition, the calculation also shows that the value of quantization circulation depends on the topological charge of outer point and superconductor.
 This work shows that if the target region has a nonzero topological charge, it will lead to a nontrivial quantization of the loop integration.  This in turn indicates that the loop integral of the anomalous displacement can be used to detect the topological properties of the target region.  In view of the importance of quantized physical quantities in physics, this work will greatly promote the study of anomalous displacements and novel topological materials.

  The article is published in the top physics journal Physical Review Letters, Author Liu Ying, Yu Zhiming (correspondent author), Xiao Cong and Yang Shengyuan (Ying Liu,Zhi-Ming Yu*,Cong Xiao,and Shengyuan A.Yang), Physical Review Letters 125,076801(2020). Paper link: https: //doi. org/10.1103/ PhysRevLett. 125.076801