Ge Wei pattern is a staggered triangular network, which is famous among Japanese traditional basket weavers and condensed matter physicists. The unusual geometric shapes of metal atoms in Ge Wei lattice and the resulting electronic behavior make it a paradise for exploring strange and wonderful quantum phenomena, which form the basis of the next generation of equipment research.
A key example is unconventional-such as high-temperature superconductivity, which does not follow the traditional laws of superconductivity. Most superconducting materials show their seemingly magical zero-resistance characteristics at temperatures of several Kelvin: these temperatures are simply unrealistic for most applications. This is an attractive prospect for materials showing so-called "high temperature" superconductivity. The temperature of this material can be achieved by cooling with liquid nitrogen (even at room temperature). Finding and synthesizing new materials with unconventional superconductivity has become the holy grail of condensed matter physicists-but to achieve this goal, we need to have a deeper understanding of the singularity and topological electronic behavior in materials.
For a long time, people have been arguing about an unusual electron transport behavior, which leads to the spontaneous flow of circulating charges, which is the precursor of high temperature superconductivity and the mechanism behind another mysterious phenomenon: quantum anomalous Hall effect. Duncan Haldane won the 20 16 Nobel Prize in Physics for his theoretical discovery of topological phase transition and topological phase of matter. This topological effect appears in some two-dimensional electronic materials, which is related to the fact that current can be generated even without external magnetic field. Understanding quantum anomalous Hall effect is not only of great significance to basic physics, but also to the potential application of new electrons and devices. Now, an international cooperation organization led by the Swiss Paul Scheler Institute (PSI) has found strong evidence to support this elusive electronic transmission behavior.
The team led by researchers from PSI Meson Spin Spectroscopy Laboratory discovered a weak internal magnetic field, which indicated that there was a strange charge ordering in a related Kagome superconductor. These magnetic fields break the so-called time reversal symmetry, which is a kind of symmetry, that is to say, whether you look forward or backward at a system, the laws of physics are the same.
A natural explanation for breaking the symmetric magnetic field of time reversal is a new charge ordering. Charge ordering can be understood as the periodic modulation of electron density through lattice and the rearrangement of atoms into higher-order (superlattice) structures. The research team focused their research on Kagome lattice, KV3Sb5, which is superconducting below 2.5 Kelvin. Below the high critical temperature of about 80 Kelvin, a huge quantum anomalous Hall effect was observed in this material, which could not be explained before. Below this critical temperature of about 80 Kelvin, there is a strange charge sorting, which is called "charge sorting temperature".
It is found that the magnetic field that breaks the symmetry of time reversal means an unusual charge order, and the current moves around the cell of Kagome lattice, which is called "orbital current". These generated magnetism are dominated by the extended orbital motion of electrons in the atomic lattice.
Zurab Guguchia, a correspondent who led the team, explained: "The experimental realization of this phenomenon is extremely challenging, because the materials showing the orbital current are very rare, and the characteristic signals are often too weak to be detected."
Although previous studies have shown that the symmetry of time reversal below superconducting temperature is broken, this is the first example that the symmetry of time reversal is broken by charge order. This means that this assumed external charge order belongs to the new quantum stage of matter.
Very convincing evidence
In order to find the controversial "orbital current" for a long time, physicists use highly sensitive μSR(μSR) to detect the weak and suggestive magnetic signals they will produce. The meson injected into the sample can be used as a local high-sensitivity magnetic probe for the internal magnetic field of the material, so that the magnetic field as small as 0.00 1 mW can be detected. In the presence of internal magnetic field, the spin of muon will be depolarized. Muons decay into high-energy positrons, which emit along the direction of muon spin, carrying the information of muon spin polarization in local environment.
Researchers have observed that when the temperature drops below 80K (charge sorting temperature), the magnetic signal will change systematically. Using PSI, the world's most advanced μSR facility, a magnetic field as high as 9.5 Tesla can be applied, and the research team can use external high magnetic field to enhance the transformation of tiny internal magnetic field, providing stronger evidence that the magnetic field is caused by internal orbital current.
"We first conducted experiments without an external magnetic field," Dr. Guguchiya explained. "When we saw that the transformation of the system appeared below the charge sorting temperature, we felt very motivated to continue. But then we applied a high field to promote this electronic reaction, and we were very happy. This is a very convincing evidence, which has been elusive for a long time. "
Deep understanding of unconventional superconductivity and quantum anomalous Hall effect
This study can be said to provide the strongest evidence that the long-debated "orbital current" really exists in Kagome material KV3Sb5. Theory shows that quantum anomalous Hall effect originates from "orbital current". Therefore, "orbital current" is proposed in some unconventional superconductors that show amazing quantum anomalous Hall effect. Namely graphene, cuprate and Ge Wei lattice, but until now there is no actual evidence to prove their existence.
The discovery of a magnetic field that breaks the symmetry of time reversal means that orbital currents-and the strange charge ordering that produces them-have opened a strange road for the study of physics and the next generation of equipment. Rail flow is considered to play a fundamental role in the mechanism of various unconventional transmission phenomena, including high temperature superconductivity, and its application scope includes power transmission and maglev trains. The concept of orbital current also forms the basis of orbital electronics-a field that uses orbital freedom as the information carrier of solid-state equipment.