Dissipative two-level systems (TLS) have been a long-standing problem in glassy solids over the last fifty years, and have recently gained new relevance as sources of decoherence in quantum computing. Resonant absorption by TLSs in the dielectric poses a serious limitation to the performance of superconducting qubits; however, the microscopic nature of these systems has yet to be established. Based on first-principles calculations, we propose that hydrogen impurities in Al2O3 are the main source of TLS resonant absorption. Hydrogen is an ubiquitous impurity and can easily incorporate in Al2O3. We find that interstitial H in Al2O3 forms a hydrogen bond (O-H…O). At specific O-O distances, consistent with bond lengths found in amorphous Al2O3 or near Al2O3 surfaces or interfaces, the H atom feels a double well. Tunneling between two symmetric positions gives rise to resonant absorption in the range of 10 GHz, explaining the experimental observations. We also calculate the expected qubit-TLS coupling and find it to lie between 16 and 20 MHz, consistent with experimental measurements.

The maximally localized Wannier functions ͑MLWFs͒ of 32 A N B 8−N compounds, ranging from elemental group-IV solids to I-VII compounds, have been constructed according to the method of Marzari and Vanderbilt ͓Phys. Rev. B 56, 12847 ͑1997͔͒. The considered systems crystallize in the diamond, zinc-blende, or rocksalt structures. A different bond ionicity scale has been introduced based on the deviation of the centers of the MLWFs from the corresponding bond centers, which involves only physical constants. The present bond ionicity of the considered compounds can be considered, to a very good approximation, as the best fit to the previous empirical and self-consistent ionicity scales. The critical value of the bond ionicity that separates the fourfold and sixfold coordinate structures is found to be of about 0.7, which is smaller than previous theoretical values of about 0.8. The volume variation of the bond ionicity is found to be much smaller than previously thought, except for SiC.

In the present work, an inclusive study was carried out about the structural, optoelectronic, and thermoelectric nature of ternary type novel NaLiX (X = S, Se, Te) Chalcogenides. The WC-GGA...

We analyze the atomistic mechanisms driving the compositional correlation of In and N in the quaternary Inx Ga1-xAs1-yNy alloys combining atomic scale chemical analysis in transmission electron microscopy and density-functional theory calculations. Our results show that for typical growth conditions surface kinetics prevail over bulk thermodynamics resulting in a hitherto unexpected compositional anticorrelation between In and N.