Superconducting microwave resonators are critical to quantum computing and sensing technologies. Additionally, they are common proxies for superconducting qubits when determining the effects of performance-limiting loss mechanisms such as from two-level systems (TLSs). The extraction of these loss mechanisms is often performed by measuring the internal quality factor Qi as a function of power or temperature. In this work, we investigate large temporal fluctuations of Qi at low powers over periods of 12–16 h (relative standard deviation σQi/Qi=13%). These fluctuations are ubiquitous across multiple resonators, chips, and cooldowns. We are able to attribute these fluctuations to variations in the TLS loss tangent due to two main indicators. First, measured fluctuations decrease as power and temperature increase. Second, for interleaved measurements, we observe correlations between low- and medium-power Qi fluctuations and an absence of correlations with high-power fluctuations. Agreement with the TLS loss tangent mean is obtained by performing measurements over a time span of a few hours. We hypothesize that, in addition to decoherence, due to coupling to individual near-resonant TLS, superconducting qubits are affected by these observed TLS loss tangent fluctuations.

Superconducting niobium (Nb) thin films have recently attracted significant attention due to their utility for quantum information technologies. In the processing of Nb thin films, fluoride-based chemical etchants are commonly used to remove surface oxides that are known to affect superconducting quantum devices adversely. However, these same etchants can also introduce hydrogen to form Nb hydrides, potentially negatively impacting microwave loss performance. Here, comprehensive materials characterization of Nb hydrides formed in Nb thin films as a function of fluoride chemical treatments is presented. In particular, secondary-ion mass spectrometry, X-ray scattering, and transmission electron microscopy reveal the spatial distribution and phase transformation of Nb hydrides. The rate of hydride formation is determined by the fluoride solution acidity and the etch rate of Nb2O5, which acts as a diffusion barrier for hydrogen into Nb. The resulting Nb hydrides are detrimental to Nb superconducting properties and lead to increased power-independent microwave loss in coplanar waveguide resonators. However, Nb hydrides do not correlate with two-level system loss or device aging mechanisms. Overall, this work provides insight into the formation of Nb hydrides and their role in microwave loss, thus guiding ongoing efforts to maximize coherence time in superconducting quantum devices. This study provides comprehensive materials characterization of niobium (Nb) hydrides formed in Nb thin films as a function of fluoride chemical treatments that are commonly employed in the fabrication of superconducting qubits. This work provides insight into the formation of Nb hydrides and their role in microwave loss, thus guiding ongoing efforts to maximize coherence time in superconducting quantum devices. image

A combination of depth-resolved cathodoluminescence spectroscopy (DRCLS) and X-ray photoemission depth profiling (XPS) measured the pronounced changes in both the electronic density of states and lithium composition near the nanoscale LixV2O5/LiPON interface. DRCLS studies of electrochemically lithiated bare V2O5 and the sputter deposited V2O5 plus LiPON overlayer electrochemically lithiated in stages both showed that in the bulk the luminescence intensity of the "split-off" hybridized bonding density of states was anticorrelated with XPSmeasured Li content, decreasing as the Li content increased. However, the LiPON overlayer was found to modify the band structure of the underlying LixV2O5 (LVO) to a depth of at least 30 nm beneath the V2O5 interface. DRCLS spectra near the electrochemically lithiated LiPON/ LVO interface showed a significant intensity of the split-off band, implying a low Li content. However, XPS depth profiling revealed a pronounced negative gradient of Li extending from a maximum Li content at the intimate LiPON boundary to its lowest content of similar to 30 nm into the V2O5 in the same region, indicating a strong interaction between band structure and Li electrochemical potential near this heterojunction. These results provide evidence for substantial effects on the local band structure near an electrolyte/cathode interface and insights into the electrochemical interface behavior of solid-state batteries in general.

Vanadium oxides are widely seen as strong candidates for next-generation energy-saving electrochemical devices, ranging from their use as cathode materials in inherently safe high energy all-solid-state batteries to smart windows that employ their wide color range of electrochromic response. However, critical questions about these materials remain largely unanswered: interfacial reactions and the evolution of the electrode material as delithiation takes place. Distinguishing between topotactic (i.e., reversible) intercalation, conversion, and alloying reactions in ion tunable vanadium oxide devices, in operando, at a resolution that matches the size of structural building units, is a particularly challenging task. In this work, we investigated the effects of lithiation on the structural and optical characteristics of a model thin film system - LixV2O5 - as a function of depth, using several highly sensitive and nondestructive spectroscopic methods with different depth sensitivities. We exploit (1) LixV2O5 electrochromic properties to utilize in operando optical response, (2) depth-resolved cathodoluminescence spectroscopy (DRCLS), and (3) Raman spectroscopy to monitor the changes in LixV2O5 electronic structure from the surface to the bulk of the thin film with nanoscale resolution. We find that the degradation of electrochemical performance with deep discharge of LixV2O5 is associated with drastic band structure changes that accompany octahedral distortion, rather than with a chemical conversion reaction. Elongation along the c axis and charge redistribution induced by varying levels of V(3d)-O(2p) hybridization in the presence of the Li considerably affect the electronic band structure. The coexistence of multiple metastable phases, strong electron correlation, and deviation from an ideal cubic symmetry results in lower structural reversibility with a higher bandgap. Beyond these specific inferences, these results suggest that these optical techniques-Raman, optical absorption/reflection, and cathodoluminescence-can be a powerful combination to reveal electrochemical behavior of ion-tunable transition metal oxides materials and associated reaction mechanisms.

Vanadium pentoxide (V2O5) is a very well-known cathode material that has attracted considerable interest for its potential use in solid-state lithium-ion batteries. We pioneer the use of depth-resolved cathodoluminescence spectroscopy (DRCLS) to monitor the changes in the electronic structure of lithiated V(2)O(5)from the free surface to the thin film bulk several hundred nm below as a function of lithiation. DRCLS measurements of V(2)O(5)interband transitions are in excellent agreement with density functional theory (DFT) calculations. The direct measure of V2O5's electronic band structure as a function of lithiation level provided by DRCLS can help inform solid-state battery designs to further withstand degradation and increase efficiency. In particular, these unique electrode measurements may reveal physical mechanisms of lithiation that change V(2)O(5)irreversibly, as well as methods to mitigate them in solid-state batteries.

Renewable sources of energy are becoming a more preferred energy source as the environmental impact of non-renewable fossil fuels becomes apparent. Lithium-ion batteries have been an attractive option for portable reliable energy sources as they possess high energy density, reliability and show little self-discharge. Most lithium-ion batteries utilise an extremely toxic electrolyte composed of a mixture of organic compounds. Aqueous batteries have been a promising research topic in recent times, as they can provide energy storage without the dangerous environmental and health impacts of traditional lithium-ion batteries. A well-studied cathode material suitable in aqueous electrolytes is LiMn2O4. This paper aims to examine the electrochemical effects of nickel and cobalt doping in LiMn2O4 and the effect of synthesis temperature on the crystal structure. To accomplish this, LiMn2O4, LiMn1.8 Co0.2O4 and LiNi0.2Mn1.8 O4 were synthesised using a sol-gel method. Each sample was calcined at 750, 850 and 950 °C before being examined by XRD analysis, cyclic voltammetry scan and a charge-discharge test. The results of this study show the calcination temperature has a large impact on the structure and electrochemical performance of the battery. There was no clear trend in reversibility with the LiMn2O4, however, Li Co0.2Mn1.8O4 demonstrated increased reversibility with increasing calcination temperature, and in general, showed superior reversibility compared to LiMn2O4. Introduction of nickel showed no significant change in reversibility compared to the undoped version, however reversibility of LiMn1.8 Ni0.2O4 decreased with increased calcination temperature. The discharge capacity test showed 750 °C to be the optimal temperature for synthesis of LiMn2O4 and LiCo0.2Mn1.8O4 whilst 950 °C was the best performing nickel doped variant. Overall, an introduction of cobalt was observed to increase discharge capacity by 5.0 % (0.589 mAh compared to 0.560 mAh), whilst nickel reduced discharge performance by 2.8 % (0.544 mAh vs 0.590 mAh).