Bose polarons near quantum criticality

Abstract

The emergence of quasiparticles in strongly interacting matter represents one of the cornerstones of modern physics. However, when different phases of matter compete near a quantum critical point, the very existence of quasiparticles comes under question. Here we create Bose polarons near quantum criticality by immersing atomic impurities in a Bose-Einstein condensate (BEC) with near-resonant interactions. Using locally-resolved radiofrequency spectroscopy, we probe the energy, spectral width, and short-range correlations of the impurities as a function of temperature. Far below the superfluid critical temperature, the impurities form well-defined quasiparticles. However, their inverse lifetime, given by their spectral width, is observed to increase linearly with temperature at the Planckian scale k B T , a hallmark of quantum critical behavior. Close to the BEC critical temperature, the spectral width exceeds the binding energy of the impurities, signaling a breakdown of the quasiparticle picture. arXiv:1904.02685v3 [cond-mat.quant-gas]

Fate of the Bose polaron at finite temperature

“…We also investigate how the impurity spectral function is modified in the presence of boson-boson interactions, and in particular we argue that the width of the polaron peak will scale linearly with T at low T and for strong bosonimpurity interactions. This is consistent with recent experiments [22].…”

“…We can recover the single-channel (broad resonance) model by taking the limit R * → 0. Indeed, the recent experiments [20][21][22] on the Bose polaron have all been in the broad-resonance regime, where R * |a|. However, we choose to work with a two-channel model since we require an additional short-distance parameter to set the scale of Efimov physics [48], as we discuss in Sec.…”

“…where we suppress the explicit time dependence, noting that throughout this paper we will be concerned with the stationary operators introduced in Eq. (22). This impurity operator explicitly contains up to three-body correlations (the impurity plus two Bogoliubov excitations), and the general form of the operator can be obtained by commuting the bare impurity operator four times with the Hamiltonian, while keeping only excitations of the condensate.…”

“…In the limit of weak boson-impurity interactions, the Bose polaron maps onto the Fröhlich model [15,16] and thus realizes the original concept of a polaron, where an electron couples to phonons in an ionic lattice [17][18][19]. However, the regime of strong boson-impurity coupling is far less understood and has only recently been accessed in cold-atom experiments, where long-lived quasiparticles were recently observed [20][21][22]. Moreover, unlike the Fermi polaron, the medium for the Bose polaron exhibits a phase transition from a BEC to a thermal Bose gas at finite temperature, which can dramatically change the character of the impurity quasiparticle [23][24][25].…”

Induced pairing of fermionic impurities in a one-dimensional strongly correlated Bose gas

“…The Bose polaron problem for a single impurity, stemming originally from the physics of electronphonon interactions 45 , has been extensively studied in recent years with ultra-cold atoms in one dimension [46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65] , and above [66][67][68][69][70][71][72] . Recent experiments on impurities in a one-dimensional Bose gas [73][74][75][76] and Bose-Einstein condensates in three dimensions [77][78][79][80] have also been performed, with successful measurements of the polaron selfenergy, which can be extracted from spectroscopy measurements 78 , or the effective polaronic mass, from monitoring dipole oscillations of the impurity in a harmonic trap 74 .…”