So you have a superconductor, and it works great! But you want to know exactly why and how it works on the atomic level. What do you do?
NC State physicist Lex Kemper was part of team that decided the best answer to the above question is: hit it with laser pulses. Time-resolved, angle-resolved photoelectron spectroscopy (tr-ARPES) is the name for the technique they chose, which enables scientists to observe the ways in which electrons within a material interact with other electrons and with the atoms that make up the material. The technique has two steps: an initial laser pulse infuses the material with a lot of energy, which gets the electrons excited. The second pulse acts as a probe which measures the electrons’ quantum states and how they move back to their initial state.
“It’s like ringing a bell,” Kemper says. “You strike the bell and it vibrates, but eventually the vibrations stop. We are interested in how those vibrations stop – like watching a movie in reverse.”
肯珀（Kemper）是一组研究人员的成员，其中包括来自布鲁克黑文国家实验室（BNL）和德国杜伊斯堡大学的科学家。科学家认为超快光谱可能是揭示超导体奥秘的一种方式。具体而言，它们内部的电子相互作用如何促进其具有的属性。例如，超导体通常仅在非常寒冷的温度下效果很好。为了扩大这些材料的用途，研究人员需要了解其最基本的特性。
So the team set out to match theory to practice. Kemper designed computer simulations that predicted how the experiments might actually turn out, while BNL colleague Jonathan Rameau ran the physical side of the experiment in the lab of Uwe Bovensiepen in Germany. They tested the technique on a superconducting material, Bi2Sr2CaCu2O8+x(Bi2212), a complex layered material that has puzzled scientists for decades.
The combination of the simulations and the tr-ARPES measurements allowed the researchers to isolate unique signatures that corresponded to different types of electron interactions within the material, such as interactions between electrons and atoms or electrons and electrons.
“In normal experiments all of these electron interactions are mixed together, but this technique gave us the ability to pull them apart and understand how each one of these complex interactions work within the material,” Kemper says.
“We’re really building the theoretical underpinnings of this field – creating tools and experiments that will enable us to understand not just one material, but also fundamentals of nature.”
The researchers’ next steps include using the technique to study superconductors while they’re in the superconducting state, since their initial experiments were done at ambient, or room, temperature.
The work appears inNature Communications并得到了能源部和国家科学基金会的赠款的部分支持。
Below: Movie indicating how many electrons live in the quantum states. Typically, before the system is hit with a laser pulse, all the electrons live in the quantum states below the line at y=0. The laser pulse kicks them above it, and researchers observe how they return to their initial states.Credit: Lex Kemper.
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