P. Graham Pritchard | Searcharxiv

arXiv2026

Transition-state lattice modes and the breakdown of adiabatic tunneling for hydrogen and deuterium in bcc Nb

Interstitial hydrogen and deuterium in body-centered-cubic metals constitute archetypal quantum tunneling systems. Their relevance has been renewed by the connection between hydrogenic tunneling in Nb and defect-induced decoherence in superconducting qubits, motivating a predictive microscopic theory. Existing theoretical treatments invoke an adiabatic separation between the light interstitial and the host lattice, an assumption whose validity has not been rigorously established for hydrogenic species. Here, we show that the experimentally measured tunnel splittings of O-trapped H and D in bcc Nb are quantitatively reproduced only within a five-dimensional (5D) Lattice-Renormalized Born-Oppenheimer (LRBO) framework. This approach treats three interstitial modes and two judiciously selected lattice modes, which includes a transition-state mode, on equal quantum footing. By recasting nested Born-Oppenheimer hierarchies within this same formalism and benchmarking against modern \textit{ab initio} potential energy surfaces, we show that adiabatic separation of the light particle from lattice dynamics is satisfied only in the positive-muon ($μ^{+}$) mass limit. In contrast, tunneling for H and D is fundamentally a collective, nonadiabatic process mediated by anharmonic lattice couplings. Finally, we show that the breakdown of adiabaticity can be anticipated from simple energy estimates involving the ground-state light-particle energy evaluated at a small number of fixed lattice configurations, providing a practical criterion for assessing the validity of adiabatic tunneling theories in other systems.

quant-ph↗

arXiv2024

Suppressed paramagnetism in amorphous Ta$_2$O$_{5-x}$ oxides and its link to superconducting qubit performance

Reduced transmon qubit $T_1$ coherence times have been linked to the amorphous oxide layers formed by thin film capacitors during processing. Because Ta or Ta capped Nb capacitors exhibit overall superior qubit performance to those fabricated with Nb capacitors, it has been hypothesized that the amorphous, non-stoichiometric Ta$_2$O$_{5-x}$ oxide is less lossy than its Nb$_2$O$_{5-x}$ counterpart. The origins of what makes amorphous Ta$_2$O$_{5-x}$ less susceptible to accepted decoherence channels is unknown. Here we establish the microscopic features of amorphous Nb$_2$O$_{5-x}$ and Ta$_2$O$_{5-x}$ using a combination of \textit{ab initio} molecular dynamics and density functional theory calculations. Our simulations establish that oxygen deficiency is less likely to occur in amorphous Ta$_2$O$_{5-x}$ than in Nb$_2$O$_{5-x}$ for $0\le x \le 0.25$ and that at a given level of oxygen deficiency the formation of metal Ta-Ta bonds is enhanced. These bonds, which are accommodated by structural flaws in the amorphous network, capture electrons better than in amorphous Nb$_2$O$_{5-x}$. These thermochemical differences quench or highly suppress magnetic moments in amorphous Ta$_2$O$_{5-x}$ and eliminate a potential source of quasiparticles and magnetic flux noise. Our calculations also show that hyperfine couplings between Nb nuclei and local magnetic moments in Nb$_2$O$_{5-x}$ could form "two-level systems" (TLS) or "two-level fluctuators" (TLF) with energy splittings of 100-1000 MHz or higher. This reveals a new TLS mechanism in amorphous Nb$_2$O$_{5-x}$ oxide layers that is unlikely in Ta$_2$O$_{5-x}$. Our work provides fundamental understanding of the materials chemistry and limitations imposed by native oxides of superconducting qubits, which can be used to guide materials selection and processing.

cond-mat.mtrl-sci↗