Symmetry reduction for tunneling defects due to strong couplings to phonons

Abstract

Tunneling two-level systems (TLSs) are ubiquitous in amorphous solids, and form a major source of noise in systems such as nano-mechanical oscillators, single electron transistors, and superconducting qubits. Occurance of defect tunneling despite their coupling to phonons is viewed as a hallmark of weak defect-phonon coupling. This is since strong coupling to phonons results in significant phonon dressing and suppresses tunneling in two-level tunneling defects effectively. Here we determine the dynamics of a tunneling defect in a crystal strongly coupled to phonons incorporating the full 3D geometry in our description. We find that inversion symmetric tunneling is not dressed by phonons whereas other tunneling pathways are dressed by phonons and, thus, are suppressed by strong defectphonon coupling. We provide the linear acoustic and dielectric response functions for a tunneling defect in a crystal for strong defect-phonon coupling. This allows direct experimental determination of the defect-phonon coupling. The singling out of inversion-symmetric tunneling states in single tunneling defects is complementary to their dominance of the low energy excitations in strongly disordered solids as a result of inter-defect interactions for large defect concentrations. This suggests that inversion symmetric TLSs play a unique role in the low energy properties of disordered solids.

Towards understanding two-level-systems in amorphous solids: insights from quantum circuits

“…The interaction energy scales therefore lead to a hierarchy of different responses as a function of temperature and provide a plausible explanation for much of the universal behaviour attributed to TLS. This model was motivated and tested using disordered crystals (Churkin et al, 2014;Gaita-Ariño and Schechter, 2011;Nalbach and Schechter, 2017) but direct applicability to amorphous metal-oxides is yet to be shown conclusively, although recent work suggests that nonequilibruim absorption measurements provide a method for probing such interacting TLS models (Burin and Maksymov, 2018;Schechter et al, 2018).…”

“…The canonical source of dissipation and decoherence for TLS is a coupling to phonon modes in their hosting material [77,169,253]. The physical mechanism of this coupling is the variation in groundstate energy in each well of the TLS due to the variation of the surrounding potential structure through interactions with phonons and strain, and the subsequent variation in the asymmetry energy ε of the TLS [115]:…”

Nonuniversality and strongly interacting two-level systems in glasses at low temperatures

“…CN rotations in KBr:CN, interact strongly (γ s ) with the strain, whereas (τ-TLS) excitations in which the two local states relate to each other by local inversion e.g. CN flips in KBr:CN, interact weakly (γ τ ) with the strain [33][34][35]. g ≡ γ τ /γ s ≈0.03, proportional to the strain in strongly disordered materials and quantifying the degree of deviation from local inversion symmetry, is the small parameter of the two-TLS model.…”

Identification of a telecom wavelength single photon emitter in silicon

“…On the other hand, the tunneling rates can be enhanced at elevated temperatures assisted by acoustic phonons. The interaction with phonons can be described beyond Born-Oppenheimer approximation, where the elastic distortions associated with the acoustic phonons perturb the APES [46]. Treating this electron-phonon interaction as a time-dependent perturbation of the athermal tunneling solution introduces temperature dependent direct (∝ T ) and Raman (∝ T 5 ) contributions to the rotational tunneling rates [47,48]…”