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arXiv:2311.12383 · Physical Review Applied

Magnonic spin-current shot noise in an itinerant Fermi gas

Tingyu Zhang, Hiroyuki Tajima, Haozhao Liang

Published Mar 1, 2024• 6 citations
arXivHTMLPDF

Abstract

While spin transport plays an important role in fundamental physics as well as spintronics applications, the spin-flip tunneling process associated with magnon transport remains elusive in ultracold Fermi gases. This study propose a method for detecting spin-flip tunneling via the Fano factor, which is defined as the noise-to-current ratio and is proportional to the effective charge of carriers. The authors show that the tunneling channel can be altered by tuning either the interaction strength or the potential barrier. This result could strongly impact the study of spin-transport phenomena, and our understanding of quantum transport in strongly interacting systems.

DOI
10.1103/physrevapplied.21.l031001

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The Goldstino, which is a fermionic Nambu-Goldstone mode, has been predicted in a Bose-Fermi mixture when the supersymmetry is broken. To detect this excitation mode, we theoretically investigate the shot noise of the supersymmetry-like tunneling current in a weakly interacting ultracold Bose-Fermi mixture. The Fano factor, which is defined by the noise-to-current ratio, reflect the elementary carriers of the tunneling process. The change of the Fano factor microscopically as the density changes evinces a crossover from the quasiparticle transport to multiparticle (Goldstino) transport. The tunneling channel can also be changed by tuning the potential barrier.
The detection of elementary carriers in transport phenomena is one of the most important keys to understand nontrivial properties of strongly correlated quantum matter. Here, we propose a method to identify the tunneling current carrier in strongly interacting fermions from nonequilibrium noise in the Bardeen-Cooper-Schrieffer to Bose-Einstein condensate crossover. The noise-to-current ratio, the Fano factor, can be a crucial probe for the current carrier. Bringing strongly correlated fermions into contact with a dilute reservoir produces a tunneling current in between. The associated Fano factor increases from one to two as the interaction becomes stronger, reflecting the fact that the dominant conduction channel changes from the quasiparticle tunneling to the pair tunneling.
The Goldstino, which is a fermionic Nambu-Goldstone mode, has been predicted in a Bose-Fermi mixture when the supersymmetry is broken. To detect this excitation mode, we theoretically investigate the shot noise of the supersymmetry-like tunneling current in a weakly interacting ultracold Bose-Fermi mixture. The Fano factor, which is defined by the noise-to-current ratio, reflects the elementary carriers of the tunneling process. The change of the Fano factor microscopically as the density changes evinces a crossover from the quasiparticle transport to multiparticle (Goldstino) transport. The tunneling channel can also be changed by tuning the potential barrier.
We theoretically study the spin current between two polarized Fermi gases with repulsive interactions near the itinerant ferromagnetic phase transition. We consider a two-terminal model where the left reservoir is fixed to be fully polarized while the polarization of the right reservoir is tuned through a fictitious magnetic field defined by the chemical-potential difference between different atomic hyperfine states. We calculate the spectra of the spin-flip susceptibility function, which displays a magnon dispersion emerging from the Stoner continuum at low momentum in the ferromagnetic phase. Based on the spin-flip susceptibility and using Keldysh Green's function formalism, we investigate the spin current induced by quasiparticle and spin-flip tunneling processes, respectively, and show their dependence on the polarization bias between two reservoirs. The one-body (quasiparticle) tunneling demonstrates a linear dependence with respect to the polarization bias. In contrast, the spin-flip process manifests a predominantly cubic dependence on the bias. While indicating an enhanced magnon tunneling in the strong-coupling regime, our results also demonstrate a characteristic behavior around the critical repulsive strength for ferromagnetic phase transition at low temperatures.
Transport measurement is one of the most important ways to study the properties of matter. In this article, we discuss recent experiments in ultracold atomic gases where the analog of Landauer transport in mesoscopic devices is realized and spin dynamics in a strongly interacting Fermi gas is probed. In the latter case, we pay special attention to the peculiarity of spin dynamics due to identical spin rotation effect which leads to a novel form of spin diffusion current. This modifies the usual diffusion equation into a more complicated form and leads to important consequence for, in particular, transverse spin diffusion in ultracold Fermi gases.
We derive tunneling Hamiltonians from a microscopic model of ultracold atoms separated by an external potential barrier. Not only usual one-body tunneling but also pair- and spin-tunneling processes naturally arise from a well-known microscopic model without any empirical parameters, where the tunneling process occurs via local transfer in the overlapped region of the many-body wave functions around the potential barrier. Applying the Schwinger-Keldysh formalism to the effective model in the momentum space, we demonstrate how anomalous tunneling currents occur via the tunneling Hamiltonians in strongly interacting systems. Our formulation is useful for various junction systems as well as sheds light on a clear connection between atomtronics and spintronics.