A model that can predict the exact quasiparticle properties of heavy fermipolarons

A model that can predict the exact quasiparticle properties of heavy fermipolarons

Two-component BCS superfluid single-particle spectrum occupancy and structural sketch. (A) shows the spectrum when impurities are in a non-interacting state (black arrow is up). (B) and (c) show the spectra when the impurity interaction is on (downward black arrow) at zero and finite temperatures, respectively. The absorption spectra of the parameters $ T = 0.1E_F $ and $ k_Fa = -2 $ in (d) show the characteristics of the universal polaron. In addition, at finite temperatures, there are additional decay channels (green and purple arrows in (c)) through the Yu-Shiba-Rusinov in the gap, producing additional resonant peaks (YSR function). Credits: Wang, Liu, Hu.

Physicists studying quantum many-body physics rarely come up with accurate solutions and conclusions, especially in multiple dimensions. This also applies to the fermipolaron problem, which explains the case of fermigus with many-body quantum backgrounds that do not interact with each other.

The Fermipolaron problem has been extensively studied over the last decade or so. However, so far, it has proven very difficult to predict the quasiparticle properties of fermipolarons with high reliability.

Researchers at Swinburne University of Technology recently introduced a model that can be used to predict the exact quasiparticle properties of heavy polarons in the Bardeen-Cooper-Schrieffer (BCS) femtometre superfluidity. Their dissertation is Physical review letterIntroduces a many-body theoretical and accurate solution. This can ultimately be tested and achieved in an experimental environment.

Recent research is based on one of the team’s previous papers published in Physical Review A.. This past study has specifically focused on crossover polarons, which contain mobile impurities.

“Our previous work and many other theoretical studies of polarons using various approximations give some universal features, such as the presence of attractive / repulsive polarons and dark continuums.” Jia Wang, one of the researchers who did the research, said Phys.org. “We believe that suppression of multiple quasiparticle excitations in the background medium is the underlying mechanism of these functions.”

Wang and his colleagues believe that the mechanism that underpins the universal characteristics of fermipolarons may be either the recoil energy of mobile impurities or the presence of an energy gap in superfluidity. But in order for their hypothesis to be testable in an experimental setting, they had to first express it theoretically.

“I came across a fascinating paper studying immovable impurities in non-interacting fermigus,” Wang said. “This model can be solved accurately using the” functional determinant approach (FDA) “method. However, due to the famous “Anderson Orthogonal Catastrophe”, Polaron does not exist in such a system. In essence, this is because immovable impurities have no rebound energy and the presence of multiple particle-hole excitations destroys the polaron resonance. “

In the many-body system described by Wang and his colleagues, the presence of superfluid gaps can suppress multiple particle-hole excitations of polarons. Therefore, they set out to extend the FDA method, which is not normally applicable to Fermipolaron, to the BCS superfluid system.

“I also wanted to experimentally investigate Fermi superfluid excitation, a long-standing research topic,” Wang explained. “Recently, some experiments have realized the introduction of another type of atom into the BCS superfluidity that can act as an impurity. These accessible systems use the polaron spectrum of the impurity. It is predicted that the characteristics of the background superfluid excitation can be measured. Spectrum (such as the Yu-Shiba-Rusinov state of the superfluid gap and subgap). “

A model that can predict the exact quasiparticle properties of heavy fermipolarons

Polaron spectrum as a function of interaction strength (1 / a) and frequency. The additional function that appears at finite temperature is due to the presence of the Yu-Shiba-Rusinov state in the gap. The location of these new features (red dashed and dotted curves) is quantitatively determined by the polaron energy, the superfluid gap, and the Yu-Shiba-Rusinov state energy. Credits: Wang, Liu, Hu.

The calculations performed by Wang and his colleagues technically assume immovable impurities in the system, but they also provide a good approximation of heavy impurities. Alternatively, in an experimental environment, physicists need to be able to identify impurities using a deep optical lattice.

“Our research was theoretical,” Wang explained. “Our model considers a system of immovable impurities in a two-component Fermi superfluid. Impurities have two internal states (ultrafine spin states), one of which interacts strongly with superfluidity. , The other is assumed to be non-interactive. “

Using an FDA-based theoretical model, researchers were able to uncover all universal polaron functions with simple, principled and accurate calculations. This is a remarkable achievement, as previous studies have not been able to accurately prove all the accurate and universal quasiparticle properties of the fermipolaron system.

“We first prepared the impurities in a non-interacting state and calculated the probability that the impurities would absorb the photons and switch to a state of strong interaction as a function of the photon frequency. This is referred to as A (ω).” Wang said. “Assuming this absorption probability shows a sharp peak around a certain frequency ω, this indicates the presence of quasiparticles of energy ℏω. This is called a heavy crossover polaron.”

In the future, theoretical work done by this team of researchers may pave the way for laboratory experiments with cold atoms to test their hypothesis. In addition, physicists can take inspiration from the paper and perform slightly different tests called “ramsey-interfering experiments” that include some of the processes and technical details outlined in the paper.

The theory presented by the King and his colleagues is fairly general, so it can be applied to several different experimentally feasible systems. For example, the team proposes an experimental implementation of the proposed system using the 6Li atom BCS Fermi superfluid and heavy 133Cs impurities, which had already been achieved in several previous studies.

“There are two contributions to our work,” Wang said. “First, we investigated a model that could be solved accurately and gave all the universal features of fermion polarons. These features have only been estimated in various studies so far, but analysis shows. These universal features are fermion media. Next, we discover an interesting finite temperature phenomenon of magnetic impurities (interacting with the two components of superfluidity of different intensities) in the two-component fermion superfluid. “

When they performed their calculations, researchers discovered that the polaron spectrum showed additional augmented peaks at finite temperatures. This corresponds to the Yu-Shiba-Rusinov bound state of the subgap. Their interesting theoretical predictions could soon be tested in various physics laboratories around the world.

“As far as we know, this is the first study to apply polaron-related theories to investigate the subgap Yu-Shiba-Rusinov bound state of cryogenic gases,” Wang added. “In the next study, we will investigate the heavy polarons of other superfluid systems, such as topological superfluids. This method will help us understand the topological phase transitions of background media through accurate principles and calculations. I hope that. ”

To kill quasiparticles: Quantum Whodunit

For more information:
Jia Wang et al, BCS Fermi Accurate quasiparticle properties of heavy polarons in superfluidity, Physical review letter (2022). DOI: 10.1103 / PhysRevLett.128.175301

Jia Wang et al, Heavy Polaron in Ultracold Atomic Fermi Superfluidity at BEC-BCS Crossover: Form and Application, Physical Review A (2022). DOI: 10.1103 / PhysRevA.105.043320

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Quote: Https: //phys.org/news/2022-06-exact-quasi-particle-properties-heavy The exact quasiparticle of the heavy Fermi Polaron (June 14, 2022) obtained on June 14, 2022. Model that can predict particle properties-fermi.html

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