Emergent Quantum Phenomena
At the beginning of the last century, Einstein and Indian physicist Bose had predicted that identical bosons without interactions would condense at low temperatures, and that many particles were in the same quantum state. However, in practical systems, particles usually have strong interactions, so this ideal Bose-Einstein condensation phenomenon has not been directly proved by experiments. At the end of the last century, with the rapid development of laser cooling technology, atoms could be cooled to very low temperatures, and thus the Bose-Einstein condensation phenomenon was directly observed in many alkali metal gases.
Bose-Einstein condensate is a statistical correlation effect. Atoms in the condensed state have super fluidity, being similar to liquid helium superfluids, they also have a quantized vortex structure. The micro-mechanism of Bose-Einstein condensation is now clear, so the ideal study of Bose-Einstein condensation is not very challenging in theory. However, research in this area has greatly promoted the interdisciplinary research of quantum optics and condensed matter physics. In 2002, German scientists put cold atoms into the optical grid field caused by multiple laser interference in space. For the first time, they observed the phenomenon of superfluid-insulator phase transition in solids, which caused a sensation and was named the top ten science news of the year. Due to the high controllability and purity of the light lattice and the tunability of the cold atom interaction intensity, a controllable strong correlation system becomes possible, opening up a new field for the theoretical and experimental research of the strong correlation effect.
Recently, American scientists cooled Fermi-type atoms 40K to very low temperature by laser, and adjusted the magnetic field to control the interaction between atoms. Under the conditions close to the Feshbach resonance, a phenomenon of fermion pair condensation similar to superconductivity was observed. Their work provides a new way to study the transition and connection between BCS superconductivity and atomic Bose-Einstein condensation, which is a development direction worthy of attention.
An exciton is a bound state formed by electrons and holes in a semiconductor, similar to a hydrogen atom formed by an electron and a proton. Like an atom, a Bose-Einstein condensation can also occur in an exciton, but usually because the exciton has a short lifespan, the exciton decays compositely before it cools down to the temperature at which the condensation occurs. In recent years, two experimental groups in the United States have used semiconductor coupled quantum wells and the spatial separation of the electrons and holes that make up the exciton in this system, reducing the electron and electron hole recombination rate. Moreover, the life of the exciton was increased by more than two orders of magnitude, the exciton was successfully cooled to a low temperature, and the macroscopic quantum state of the high degenerate exciton at the edge of condensation was observed. They also further found that when coupled quantum wells is irradiated with a strong laser beam, an exciton luminous ring with a radius that increases linearly with the light intensity appears around the irradiation spot, and the luminescence on the ring is not uniform, showing periodic spotted structure (Figure 1). The appearance of this kind of ring, especially the periodic spot structure on the ring, is the result of the correlation and competition of electrons, holes and excitons.
Compared with the atomic Bose-Einstein condensation, the exciton Bose-Einstein condensate has its particularity and broader application prospects. First, in semiconductors, the effective mass of excitons is very small. Under the same density conditions, exciton condensation can occur at a temperature much higher than the atomic Bose-Einstein condensation transition temperature, which can reach the amount of 1K. It can be achieved by using liquid helium cooling, without the complicated laser cooling process, and it is much easier than the atomic Bose-Einstein condensation. Second, the condensed state of the rarefied exciton gas is similar to the atomic Bose-Einstein condensation, and the condensed state of high-density exciton is similar to the BCS superconducting state. Therefore, the study of the exciton Bose-Einstein condensation also provides a means to study the transition from Bose-Einstein condensation to BCS superconductivity. Third, the carrier of the exciton Bose-Einstein condensation is a semiconductor quantum well, and the semiconductor has the characteristics of artificial physical cutting. Therefore, it is possible for the exciton Bose-Einstein condensation to be applied to high power and low consumption devices.