Pairing and radio-frequency spectroscopy in two-dimensional Fermi gases

We investigate strong-coupling properties of a trapped two-dimensional normal Fermi gas.Within the framework of a combined T -matrix theory with the local density approximation, we calculate the local density of states, as well as the photoemission spectrum, to see how two-dimensional pairing fluctuations affect these single-particle quantities. In the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region, we show that the local density of states exhibits a dip structure in the trap center, which is more remarkable than the three-dimensional case. This pseudogap phenomenon is found to naturally lead to a double peak structure in the photoemission spectrum. The peak-to-peak energy of the spectrum at p = 0 agrees well with the recent experiment on a two-dimensional 40 K Fermi gas [M. Feld, et al., Nature 480, 75 (2011)]. Since pairing fluctuations are sensitive to the dimensionality of a system, our results would be useful for the study of many-body physics in the BCS-BEC crossover regime of a two-dimensional Fermi gas.

“…This can be achieved by introducing the two-dimensional s-wave scattering length a s , which is related to the pairing interaction U as [46,49] …”

Section: Formulationmentioning

confidence: 99%

“…(3) describes fluctuation corrections to single-particle excitations. In the ordinary T -matrix approximation, it is given by [27,28,36,46],…”

Section: Formulationmentioning

confidence: 99%

See 1 more Smart Citation

Low-dimensional pairing fluctuations and pseudogapped photoemission spectrum in a trapped two-dimensional Fermi gas

Watanabe¹,

Tsuchiya²,

Ōhashi³

2013

“…The observed temperature dependence of the RF spectra indicates the existence of a nontrivial sharp transition between quantum and classical regimes, in addition to a precise determination of the polaron energy. Obviously, an explanation of the MIT experiment demands self-consistent theoretical frameworks to take the strong correlations into account [51][52][53][54][55][56][57][58][59].In this Letter, motivated by the MIT experiment [50], we investigate thermal evolution of RF spectra of a strongly-interacting polarized Fermi gas by means of selfconsistent many-body calculations. By analyzing typical schemes of the RF spectroscopy, we demonstrate that a transfer from Fermi polarons to Boltzmann gas becomes rich due to correlation effects and thermal broadening.…”

mentioning

confidence: 99%

Thermal crossover, transition, and coexistence in Fermi polaronic spectroscopies

Tajima

Uchino

2019

We investigate thermal evolution of radio-frequency (RF) spectra of a spin-imbalanced Fermi gas near a Feshbach resonance in which degenerate Fermi-polaron and classical Boltzmann-gas regimes emerge in the low-temperature and high-temperature limits, respectively. By using self-consistent frameworks of strong-coupling diagrammatic approaches, both of the ejection and reserve RF spectra available in cold-atom experiments are analyzed. We find a variety of transfers from Fermi polarons to the Boltzmann gas such that a thermal crossover expected in the weak-coupling regime is shifted to a sharp transition near unitarity and to double-peak coexistence of attractive and repulsive branches in the strong-coupling regime. Our theory provides semiquantitative descriptions for a recent experiment on the ejection RF spectroscopy at unitarity [Z. Yan et al., arXiv:1811.00481v1] and suggests the importance of beyond-two-body correlations in the high-temperature regime due to the absence of Pauli-blocking effects.A spectroscopic method is one of central themes in physics including hadron-mass spectroscopy in nuclear physics [1-3] and gravitational wave detection in astrophysics [4,5]. In condensed matter, a spectroscopic technique is of importance to examine low-energy excitations in quantum many-body systems, which led to discoveries of pseudogap in high-T c superconductors [6-8] and of topological states of matter [9]. In an ultracold atomic gas that is an ideal platform to realize nontrivial quantum states of matter and yet has limited probes due to electrical charge neutrality, a quantum many-body spectroscopy is an inevitable tool to extract fundamental properties of the system [10]. For instance, the Bragg spectroscopy allows to measure Nambu-Goldstone modes in superfluid gases [11,12], and the lattice modulation spectroscopy to measure the Mott gap in an optical lattice system [13]. In addition, the radio-frequency (RF) spectroscopy in cold atoms provides an alternative route to probe interacting atomic gases, and revealed the essential properties such as pseudogap in normal Fermi gases [14-17], superfluid gap [18] and Higgs mode [19] in superfluid Fermi gases, and Efimov effect in threecomponent Fermi gases [20,21]. Since the RF spectroscopy is sensitive to excitation properties in quantum many-body systems, of current interest in the RF spectrum measurements is a polaron which is a prototype on how a strong interaction affects quasiparticle properties.In cold atoms, polaron physics can be achieved simply by considering a polarized mixture. When such a mixture consists of a two-component Fermi gas, the system at a low-temperature reduces to a Fermi polaron, which is a mobile impurity surrounded by the Fermi sea. The RF spectral results as well as the experimental realizations [22][23][24][25][26][27] triggered a number of theoretical works mostly under the condition of a single impurity at absolute zero . The current consensus is that such single-impurity calculations agree well with low-temperature spectral data of th...

“…It has even been suggested that finite temperature plays a crucial role here -see Ref. [21] for an alternative explanation based on fermionic polarons. However, the shift due to confinement appears to be substantial at T = 0, with a direction that is consistent with experiment, and therefore it cannot be disregarded.…”

Section: B Experimental Probesmentioning

confidence: 99%

BCS-BEC crossover in a quasi-two-dimensional Fermi gas

Fischer

Parish

2013

We consider a two-component gas of fermionic atoms confined to a quasi-two-dimensional (quasi-2D) geometry by a harmonic trapping potential in the transverse direction. We construct a mean field theory of the BCS-BEC crossover at zero temperature that allows us to extrapolate to an infinite number of transverse harmonic oscillator levels. In the extreme BEC limit, where the confinement length exceeds the dimer size, we recover 3D dimers confined to 2D with weak repulsive interactions. However, even when the interactions are weak and the Fermi energy is less than the confinement frequency, we find that the higher transverse levels can substantially modify fermion pairing. We argue that recent experiments on pairing in quasi-2D Fermi gases [Y. Zhang et al., Phys. Rev. Lett. 108, 235302 (2012)] have already observed the effects of higher transverse levels.