Anomalous low-energy properties in amorphous solids and the interplay of electric and elastic interactions of tunneling two-level systems

Abstract

Tunneling two-level systems (TLSs), generic to amorphous solids, dictate the low-temperature properties of amorphous solids and dominate noise and decoherence in quantum nano-devices. The properties of the TLSs are generally described by the phenomenological standard tunneling model. Yet, significant deviations from the predictions of this model found experimentally suggest the need for a more precise model in describing TLSs. Here we show that the temperature dependence of the sound velocity, dielectric constant, specific heat, and thermal conductivity, can be explained using an energy-dependent TLS density of states reduced at low energies due to TLS-TLS interactions. This reduction is determined by the ratio between the strengths of the TLS-TLS interactions and the random potential, which is enhanced in systems with dominant electric dipolar interactions.

Two-level systems in superconducting quantum devices due to trapped quasiparticles

“…A similar dependence was obtained by tuning the TLS energy by an applied electrostatic field, see [36]. In contrast, for conventional non-interacting TLS in amorphous materials we expect a DOS P (E) = const [35], while for interacting TLS we expect a weak pseudogap given by P (E) = E µ [43,44]. µ ≈ 0.2 is typically found experimentally [3,9], which would for f 0 = 3 − 6 GHz only result in a ∼ 10% variation in the TLS DOS.…”

Vibrations and Heat Transfer in Glasses: The Role Played by Disorder

“…This ω-dependence of ℓ M F P thus lowers the exponent in the temperature dependence of the thermal conductivity compared to the constant-ℓ M F P case, as a result of the kinetic expression of the heat conductivity in the propagative (ballistic) regime. Specific temperature dependence of the heat capacity may also play a role [97]. Another wellknown dependence of the mean-free path results from the Rayleigh scattering on structural disorder in the low-frequency regime [46].…”

Anomalous Loss Reduction Below Two‐Level System Saturation in Aluminum Superconducting Resonators

“…[ 20 ] A typical resonator hosts an ensemble of TLS with different values of Δ and Δ 0 with their (assumed continuous) distribution function given as $$ P(\Delta ,\Delta _0)=P_0/ \Delta _0$$, where $$P_0\approx 10^{44} \text{J}^{-1}\text{m}^{-3}$$ is the density of states for TLS. The distribution function is uniform in Δ in the conventional TLS model, but could take on a very weak dependence $$\propto \Delta ^\mu$$ with $$\mu \approx 0.3$$ for a system of very strongly interacting TLS, [ 72–74 ] such as the case assumed in GTM. For simplicity and generality, the following model uses the conventional distribution function, which is constant in Δ.…”