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Unconventional Superconductivity

Updated: Mar 30, 2023


A team of scientists, including physicist Eugene Demler from ETH Zurich, for the first time, closely observed how magnetic correlations play a role in mediating hole pairing.


Superconductivity only occurs in pairs by the way.


Therefore, in order for conductance without electrical resistance to take place in specific materials, the charge carriers must pair up.


In traditional superconductors, the current is made up of electrons and pairing is facilitated by the collective movements of the crystal lattice, referred to as phonons.


This mechanism is well understood by serious physics and chemistry geeks.


However, in recent decades, a growing number of materials have been found that don’t fit within this conventional theoretical framework as understood by said army of geek boffins.


The leading theories of said geeks at large for unconventional superconductors suggest that magnetic fluctuations, not phonons, lead to pairing in these systems, — and surprisingly, magnetic interactions arise from the repulsive Coulomb interaction between electrons.


However, verifying these models in experiments is extremely difficult.


Hence the excitement as a team of scientists led by Sarah Hirthe, Prof. Immanuel Bloch, and Dr. Timon Hilker at the Max Planck Institute of Quantum Optics in Garching (Germany), Dr. Annabelle Bohrdt at Harvard University (US), Prof. Fabian Grusdt at the Ludwig Maximilian University of Munich (Germany), and Prof. Eugene Demler in the Departement of Physics at ETH Zurich (Switzerland) now report experiments that confirm central predictions of these theories.


Writing in Nature, Das Geeks show that in a synthetic crystal so-called holes — in essence empty sites in a lattice filled with fermions — can form pairs mediated by magnetic correlations alone.


Pairing up for exciting physics

The synthetic crystal that the uber geek team created consists of atoms trapped in complex optical structures formed by intersecting laser beams.


In such crystals, the key parameters defining the properties and behavior of the system can be controlled with a degree of precision and flexibility that is typically out of reach in real materials.


Moreover, in the setup at Garching, individual atoms can be traced while also probing their interactions with the other atoms, thereby offering microscopic insight into the quantum many-body system at hand.


Theoretical Physicists Devise New Path toward High-Energy ‘Quantum Light’


Physicists have developed a novel theory describing a new state of light, which has controllable quantum properties over a broad range of frequencies, up as high as X-ray frequencies.


The world we observe around us can be described according to the laws of classical physics, but once we observe things at an atomic scale, the strange world of quantum physics takes over.


Imagine a basketball: observing it with the naked eye, the basketball behaves according to the laws of classical physics. But the atoms that make up the basketball behave according to quantum physics instead.


“Light is no exception: from sunlight to radio waves, it can mostly be described using classical physics,” said Dr. Andrea Pizzi, a physicist in the Department of Physics at Harvard University and the Cavendish Laboratory at the University of Cambridge.


“But at the micro and nanoscale so-called quantum fluctuations start playing a role and classical physics cannot account for them.”


In their research, Dr. Pizzi and colleagues aimed to develop a theory that predicts a new way of controlling the quantum nature of light.


“Quantum fluctuations make quantum light harder to study, but also more interesting: if correctly engineered, quantum fluctuations can be a resource,” Dr. Pizzi said.


“Controlling the state of quantum light could enable new techniques in microscopy and quantum computation.”


One of the main techniques for generating light uses strong lasers.


When a strong enough laser is pointed at a collection of emitters, it can rip some electrons away from the emitters and energize them.


Eventually, some of these electrons recombine with the emitters they were extracted from, and the excess energy they absorbed is released as light.


This process turns the low-frequency input light into a high-frequency output radiation.


“The assumption has been that all these emitters are independent from one another, resulting in output light in which quantum fluctuations are pretty featureless,” Dr. Pizzi said.


“We wanted to study a system where the emitters are not independent, but correlated: the state of one particle tells you something about the state of another.”


“In this case, the output light starts behaving very differently, and its quantum fluctuations become highly structured, and potentially more useful.”


To solve this type of problem, known as a many body problem, the researchers used a combination of theoretical analysis and computer simulations, where the output light from a group of correlated emitters could be described using quantum physics.


The theory demonstrates that controllable quantum light can be generated by correlated emitters with a strong laser.


The method generates high-energy output light, and could be used to engineer the quantum-optical structure of X-rays.


“We worked for months to get the equations cleaner and cleaner, until we got to the point where we could describe the connection between the output light and the input correlations with just one compact equation. As a physicist, I find this beautiful,” Dr. Pizzi said.


“Looking forward, we would like to collaborate with experimentalists to provide a validation of our predictions.”


“On the theory side of things, our work suggests many-body systems as a resource for generating quantum light, a concept that we want to investigate more broadly, beyond the setup considered in this work.”


I got dibs on the first Quantum light bulb by the way...


The team’s work appears today in the journal Nature Physics.


A. Pizzi et al. Light emission from strongly driven many-body systems. Nat. Phys, published online February 2, 2023; doi: 10.1038/s41567-022-01910-7



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