Electrons are subatomic particles with negative charges , Is one of the basic particles that make up matter , Extremely light weight , Spin very fast , Usually bound inside atoms , However, it can also 「 Form a solid 」.
Under perfect conditions , Electrons can arrange themselves into neat honeycomb patterns in some materials —— Like a solid in a solid . For the first time, physicists have studied these 「 Wigner crystal 」 Imaging was performed .
This crystal is made in Hungary - American theoretical physicist 、 Mathematician , Eugene, the founder of symmetry theory in quantum mechanics · Wigner （Eugene Wigner） Name of , He was in 1934 The existence of Wigner crystal was predicted for the first time .
One of the co authors of the study , Professor of physics at the University of California, Berkeley , Wang Feng, head of ultrafast nano optics group （Feng Wang） Express , Researchers have previously convincingly created Wigner crystals and measured their properties , But this is the first time someone has actually taken a snapshot of these substances ,「 If you say you made crystals , Seeing is believing. .」
The research paper 《Imaging two-dimensional generalized Wigner crystals》 Has been in 9 month 29 It was published in the latest issue 《 natural 》 The journal .
Thesis link ：https://www.nature.com/articles/s41586-021-03874-9
To make Wigner crystals , Wang Feng and his team have built a system consisting of two kinds of semiconductor atoms —— Structure composed of thin layers of tungsten disulfide and tungsten diselenide . And then , The researchers used an electric field to adjust the density of electrons moving freely along the interface between the two layers .
In ordinary materials , The electron velocity is too fast to be significantly affected by the repulsive force between the negative charges of the material atoms . But Wigner predicted , If the electrons move slowly enough , This repulsion will begin to dominate their behavior . In this case , Electrons will find an arrangement that minimizes their total energy , Form various patterns . therefore , Wang Feng and his colleagues slowed down the speed of electrons in the device by cooling it to a temperature only slightly higher than absolute zero .
The voids in the bilayer structure also contribute to the formation of Wigner crystals . The atomic spacing of each of the two semiconductor layers is slightly different , Therefore, when they are paired together, they will produce honeycomb 「 Moore pattern 」, Similar to what you see when you overlap two meshes . This repeated pattern produces a slightly lower energy area surface , Help the electrons stabilize .
This scanning tunneling microscope image of graphene shows , Wigner crystal —— A honeycomb arrangement of electrons has been formed in the layered structure .
After the electrons are placed in position , The team used a scanning tunneling microscope to observe Wigner crystals . When this special microscope works , The metal tip hovers over the sample surface , Voltage makes electrons jump off the tip , Generate electricity , As the tip of the needle moves , The change of current intensity reveals the position of electrons in the sample .
Wang Feng said , Initially, scanning tunneling microscopy was used to directly observe the double-layer material , Attempts to image Wigner crystals were unsuccessful . Because the current destroys the fragile arrangement of electrons . So the team added another layer of graphene on top of the semiconductor double layer , The existence of Wigner crystal slightly changes the electronic structure of graphene directly above , This phenomenon is captured by a microscope . The images obtained clearly show the neat arrangement of the underlying Wigner electrons .
As scientists expected , The continuous electrons in a Wigner crystal are about... Farther away than the actual crystal atoms on a semiconductor device 100 times .
「 The scanning tunneling microscope can be operated on such a system , I think it's a great progress ,」 Columbia University physicist Carmen Rubio Verdú Say , She said that the same graphene based method is expected to enable scanning tunneling microscopy to explore many other interesting physical phenomena .
Michael Jianhui, a physicist at Cornell University （Kin Fai Mak） Express ：「 The technology is non-invasive to the state you want to detect . It's a very clever idea for me .」
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