Identification of Strong and Weak Interacting Two-Level Systems in KBr:CN

Gaita‐Ariño, Alejandro; Schechter, Moshe

doi:10.1103/physrevlett.107.105504

Cited by 21 publications

(51 citation statements)

References 26 publications

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“…Thus, the TLS-DOS in disordered lattices is not small, but the relevant TLSs are τ -TLSs, between which the interaction is small and can therefore be neglected, and the experimentally observed smallness of the TLSs DOS is related to the fact that only TLSs with appreciable tunneling amplitude can be detected. Note that in KBr:CN the TLS is formed by each CN low energy state and the state related to it by a CN flip [12,14,23,29], and is thus a τ -TLS, where all other single particle states of a given CN impurity are much higher in energy [27], as they constitute an S excitation with respect to the CN low energy state [23].…”

Section: Introductionmentioning

confidence: 99%

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Acoustic interactions between inversion symmetric and asymmetric two-level systems

Churkin

Barash

Schechter

2014

J. Phys.: Condens. Matter

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Amorphous solids, as well as many disordered lattices, display remarkable universality in their low temperature acoustic properties. This universality is attributed to the attenuation of phonons by tunneling two-level systems (TLSs), facilitated by the interaction of the TLSs with the phonon field. TLS-phonon interaction also mediates effective TLS-TLS interactions, which dictates the existence of a glassy phase and its low energy properties. Here we consider KBr:CN, the archetypal disordered lattice showing universality. We calculate numerically, using conjugate gradients method, the effective TLS-TLS interactions for inversion symmetric (CN flips) and asymmetric (CN rotations) TLSs, in the absence and presence of disorder, in two and three dimensions. The observed dependence of the magnitude and spatial power law of the interaction on TLS symmetry, and its change with disorder, characterizes TLS-TLS interactions in disordered lattices in both extreme and moderate dilutions. Our results are in good agreement with the two-TLS model, recently introduced to explain long-standing questions regarding the quantitative universality of phonon attenuation and the energy scale of ≈ 1 − 3 K below which universality is observed.

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Section: Introductionmentioning

confidence: 99%

“…These results support both the two-TLS model and the notion that it is indeed CN flips that constitute the TLSs for KBr:CN. However, the ab-initio and DFT calculations [29] required the use of very small samples, not allowing the study of the effective TLS-TLS interactions and their distance dependence.…”

Section: Introductionmentioning

confidence: 99%

Acoustic interactions between inversion symmetric and asymmetric two-level systems

Churkin

Barash

Schechter

2014

J. Phys.: Condens. Matter

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“…Experimentally, TLSs can be directly probed by phonon echo experiments [47][48][49]. However, because of the scarcity of S-TLSs [34] at energies probed by these experiments so far (of order 10GHz and below), experiments find only one type of TLSs, with a coupling constant shown to agree with that of τ-TLSs [36]. However, under certain conditions the S-TLSs can dominate spectral diffusion and the acoustic response.…”

Section: Spectral Diffusion and Nonlinear Acoustic Absorptionmentioning

confidence: 99%

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

Schechter¹,

Nalbach²,

Burin³

2018

New J. Phys.

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Recent experimental results showing atypical nonlinear absorption and marked deviations from well known universality in the low temperature acoustic and dielectric losses in amorphous solids prove the need for improving the understanding of the nature of two-level systems (TLSs) in these materials.Here we suggest the study of TLSs focused on their properties which are nonuniversal. Our theoretical analysis shows that the standard tunneling model and the recently suggested two-TLS model provide markedly different predictions for the experimental outcome of these studies. Our results may be directly tested in disordered lattices, e.g KBr:CN, where there is ample theoretical support for the validity of the two-TLS model, as well as in amorphous solids. Verification of our results in the latter will significantly enhance understanding of the nature of TLSs in amorphous solids, and the ability to manipulate them and reduce their destructive effect in various cutting edge applications including superconducting qubits.

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“…example C in figure 1 of [48]) states. Since defect-phonon coupling in amorphous solids is expected to be an order of magnitude larger than in disordered crystals [26,[50][51][52], our results suggest that the tunneling amplitude of inversion asymmetric TLSs, such as in example C in figure 1 of [48] for KCl (from [56]) and the geometry factor [23] is = f 1 5 t . With these parameters the Debye-Waller exponent equation (A.1) can be determined keeping in mind that the longitudinal speed of sound is larger and the transversal one and thus contributions from longitudinal phonon branches are negligible in equation (A.1).…”

Section: Experimental Relevancementioning

confidence: 99%

“…Most experimental data, however, is acquired in samples with defect concentrations  n 100 ppm, where many body effects complicate the situation. From a first-principle argumentation one actually expects for tunneling defects with no special symmetry, rather strong defect-phonon couplings [24,26].…”

Section: Introductionmentioning

confidence: 99%

Symmetry reduction for tunneling defects due to strong couplings to phonons

Nalbach

Schechter

2017

New J. Phys.

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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.

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Identification of Strong and Weak Interacting Two-Level Systems in KBr:CN

Cited by 21 publications

References 26 publications

Acoustic interactions between inversion symmetric and asymmetric two-level systems

Acoustic interactions between inversion symmetric and asymmetric two-level systems

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

Symmetry reduction for tunneling defects due to strong couplings to phonons

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