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1. Influence of doping and thickness on domain avalanches in lead zirconate titanate thin films

   https://doi.org/10.1063/5.0149457  

   Peters, Travis; Zhu, Wanlin; Checa, Marti; Collins, Liam; Trolier-McKinstry, Susan (March 2023, Applied Physics Letters)

   In undoped lead zirconate titanate films of 1–2  μm thick, domain walls move in clusters with a correlation length of approximately 0.5–2  μm. Band excitation piezoresponse force microscopy mapping of the piezoelectric nonlinearity revealed that niobium (Nb) doping increases the average concentration or mobility of domain walls without changing the cluster area of correlated domain wall motion. In contrast, manganese (Mn) doping reduces the contribution of mobile domain walls to the dielectric and piezoelectric responses without changing the cluster area for correlated motion. In both Nb and Mn doped films, the cluster area increases and the cluster density drops as the film thickness increases from 250 to 1250 nm. This is evident in spatial maps generated from the analysis of irreversible to reversible ratios of the Rayleigh coefficients. 

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2. Surface oxides, carbides, and impurities on RF superconducting Nb and Nb ₃ Sn: a comprehensive analysis

   https://doi.org/10.1088/1361-6668/acff23  

   Sun, Zeming; Baraissov, Zhaslan; Dukes, Catherine A.; Dare, Darrah K.; Oseroff, Thomas; Thompson, Michael O.; Muller, David A.; Liepe, Matthias U. (October 2023, Superconductor Science and Technology)

   Abstract Surface structures on radio-frequency (RF) superconductors are crucially important in determining their interaction with the RF field. Here we investigate the surface compositions, structural profiles, and valence distributions of oxides, carbides, and impurities on niobium (Nb) and niobium–tin (Nb₃Sn)in situunder different processing conditions. We establish the underlying mechanisms of vacuum baking and nitrogen processing in Nb and demonstrate that carbide formation induced during high-temperature baking, regardless of gas environment, determines subsequent oxide formation upon air exposure or low-temperature baking, leading to modifications of the electron population profile. Our findings support the combined contribution of surface oxides and second-phase formation to the outcome of ultra-high vacuum baking (oxygen processing) and nitrogen processing. Also, we observe that vapor-diffused Nb₃Sn contains thick metastable oxides, while electrochemically synthesized Nb₃Sn only has a thin oxide layer. Our findings reveal fundamental mechanisms of baking and processing Nb and Nb₃Sn surface structures for high-performance superconducting RF and quantum applications. 

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3. Luminescence and Scintillation in the Niobium Doped Oxyfluoride Rb4Ge5O9F6:Nb

   https://doi.org/10.3390/inorganics10060083  

   Carone, Darren; Klepov, Vladislav V.; Misture, Scott T.; Schaeperkoetter, Joseph C.; Jacobsohn, Luiz G.; Aziziha, Mina; Schorne-Pinto, Juliano; Thomson, Stuart A.; Hines, Adrian T.; Besmann, Theodore M.; et al (June 2022, Inorganics)

   A new niobium-doped inorganic scintillating oxyfluoride, Rb4Ge5O9F6:Nb, was synthesized in single crystal form by high-temperature flux growth. The host structure, Rb4Ge5O9F6, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.98430(10) Å, b = 11.7265(2) Å, and c = 19.2732(3) Å, consisting of germanium oxyfluoride layers made up of Ge3O9 units connected by GeO3F3 octahedra. In its pure form, Rb4Ge5O9F6 shows neither luminescence nor scintillation but when doped with niobium, Rb4Ge5O9F6:Nb exhibits bright blue luminescence and scintillation. The isostructural doped structure, Rb4Ge5O9F6:Nb, crystallizes in the orthorhombic space group Pbcn with lattice parameters a = 6.9960(3) Å, b = 11.7464(6) Å, and c = 19.3341(9) Å. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements suggest that the niobium is located in an octahedral coordination environment. Optical measurements inform us that the niobium dopant acts as the activator. The synthesis, structure, and optical properties are reported, including radioluminescence (RL) measurements under X-ray irradiation. 

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4. Spectroscopic evidence of flat bands in breathing kagome semiconductor Nb3I8

   https://doi.org/10.1038/s43246-022-00318-3  

   Regmi, Sabin; Fernando, Tharindu; Zhao, Yuzhou; Sakhya, Anup Pradhan; Dhakal, Gyanendra; Bin Elius, Iftakhar; Vazquez, Hector; Denlinger, Jonathan D.; Yang, Jihui; Chu, Jiun-Haw; et al (December 2022, Communications Materials)

   Abstract Kagome materials have become solid grounds to study the interplay among geometry, topology, correlation, and magnetism. Recently, niobium halide semiconductors Nb 3 X 8 ( X  = Cl, Br, I) have been predicted to be two-dimensional magnets and these materials are also interesting for their breathing kagome geometry. However, experimental electronic structure studies of these promising materials are still lacking. Here, we report the spectroscopic evidence of flat and weakly dispersing bands in breathing-kagome semiconductor Nb 3 I 8 around 500 meV binding energy, which is well supported by our first-principles calculations. These bands originate from the breathing kagome lattice of niobium atoms and have niobium d -orbital character. They are found to be sensitive to the polarization of the incident photon beam. Our study provides insight into the electronic structure and flat band topology in an exfoliable kagome semiconductor, thereby providing an important platform to understand the interaction of geometry and electron correlations in two-dimensional materials. 

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5. MAGNETIC FIELD PENETRATION TECHNIQUE TO STUDY FIELD SHIELDING OF MULTILAYERED SUPERCONDUCTORS

   https://doi.org/10.18429/JACoW-SRF2021-SUPTEV001  

   I. H. Senevirathne; A. Gurevich; J. R. Delayen; A-M Valente-Feliciano (July 2021, SRF)

   The SIS structure which consists of alternative thin lay- ers of superconductors and insulators on a bulk niobium has been proposed to shield niobium cavity surface from high magnetic field and hence increase the accelerating gradient. The study of the behavior of multilayer supercon- ductors in an external magnetic field is essential to opti- mize their SRF performance. In this work we report the de- velopment of a simple and efficient technique to measure penetration of magnetic field into bulk, thin film and mul- tilayer superconductors. Experimental setup contains a small superconducting solenoid which can produce a par- allel surface magnetic field up to 0.5 T and Hall probes to detect penetrated magnetic field across the superconduct- ing sample. This system was calibrated and used to study the effect of niobium sample thickness on the field of full magnetic flux penetration. We determined the optimum thickness of the niobium substrate to fabricate the multi- layer structure for the measurements in our setup. This technique was used to measure penetration fields of Nb 3 Sn thin films and Nb 3 Sn/Al 2 O 3 multilayers deposited on Al 2 O 3 wafers. The system was optimized to mitigate thermo- magnetic flux jumps at low temperatures. 

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