Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of "medium purity" Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range 800 -1400 • C was done in a newly designed vacuum induction furnace. Q0 values of the order of 2 × 10 10 at 2.0 K and peak surface magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of (5 ± 1) × 10 10 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 • C. This is the highest value ever reported at this temperature, frequency and field. Samples heat treated with the cavity at 1400 • C were analyzed by secondary ion mass spectrometry, secondary electron microscopy, energy dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a complex surface composition which includes titanium oxide, increased carbon and nitrogen content but reduced hydrogen concentration compared to a non heat-treated sample.

The minority carrier diffusion length, LD, was directly measured in individual ZnO nanowires by a near-field scanning photocurrent microscopy technique. The diameter dependence of LD suggests a diameter-dependent surface electronic structure, particularly an increase in the density of mid-band-gap surface states with the decreasing diameter. This diameter dependence of the surface electronic structure might be a universal phenomenon in wurtzite-type nanostructures, and is critical in interpreting and understanding the effects of surfaces on various material properties.

We report the rf performance of a single-cell superconducting radiofrequency cavity after low temperature baking in a nitrogen environment. A significant increase in quality factor has been observed when the cavity was heat treated in the temperature range of 120-160 °C with a nitrogen partial pressure of ~25 mTorr. This increase in quality factor as well as the Q-rise phenomenon ("anti-Q-slope") is similar to those previously obtained with high temperature nitrogen doping as well as titanium doping. In this study, a cavity N 2 -treated at 120 °C and at140 °C, showed no degradation in accelerating gradient, however the accelerating gradient was degraded by ~25% with a 160 °C N 2 treatment. Sample coupons treated in the same conditions as the cavity were analyzed by scanning electron microscope, x-ray photoelectron spectroscopy and secondary ion mass spectroscopy revealed a complex surface composition of Nb 2 O 5 , NbO and NbN (1-x) O x within the rf penetration depth. Furthermore, magnetization measurements showed no significant change on bulk superconducting properties.

In a recent comment Romanenko and Grassellino 1 made unsubstantiated statements about our work [Appl. Phys. Lett. 104, 092601 (2014)] and ascribed to us wrong points which we had not made. Here we show that the claims of Romanenko and Grassellino are based on misinterpretation of Ref. 2, and inadequate data analysis in their earlier work 3 .The goal of Ref. 2 was to reveal mechanisms of the microwave enhancement of the quality factor Q(H) observed on Ti-alloyed Nb cavities. This was done using the standard Arrhenius method to deconvolute a thermally-activated and residual contributions to the surface resistance 2 ,The Arrhenius method was used in Ref. 4 to separate the temperature-independent residual resistance R i from the conventional BCS contribution R BCS (T ) Ae −U/kTs measured in Nb cavities at low H = 4 mT. This procedure was based on the Mattis-Bardeen expression 5 for R BCS (T ), which enables one to unambiguously separate the quasiparticle contribution R BCS from additional temperature-independent contributions to R i at T T c which are not described by the simplest version of the BCS model. In Refs. 2 and 3 the Arrhenius method was adopted to analyze R s (T, H) at strong rf fields in the region of nonlinear electromagnetic response. In this case no simple theoretical expression for the quasiparticle contribution is available so the physical meaning of the phenomenological parameters in Eq. (1) becomes far from obvious. This is because the thermally-activated contribution A(H) exp[−U (H)/kT s (H)] becomes highly nonlinear in H and gets intertwined with complex nonequilibrium kinetics of quasiparticles 6 , and with such extrinsic mechanisms as trapped vortices, proximity coupled normal oxide regions, etc. One of the manifestations of nonequilibrium effects is rf heating which makes the local temperature of quasiparticles T s higher than the bath temperature T 0 , the electron temperature can be higher than the lattice temperature at kT << U ( Ref. 6).In our work we stated two facts: 1. Alloying Nb cavities with Ti or N significantly extends the field region where Q(H) exhibits a remarkable increase with H, 2. Heating effects were disregarded in the analysis of Ref. 3. Contrary to the assertion of Romanenko and Grassellino, we did not make any comments on the validity of Ref. 3,

We report on the design, fabrication, and performance of a nanoporous, coaxial array capacitive detector for highly sensitive chemical detection. Composed of an array of vertically aligned nanoscale coaxial electrodes constructed with porous dielectric coax annuli around carbon nanotube cores, this sensor is shown to achieve parts per billion level detection sensitivity, at room temperature, to a broad class of organic molecules. The nanoscale, 3D architecture and microscale array pitch of the sensor enable rapid access of target molecules and chip-based multiplexing capabilities, respectively.

Magnetic flux trapped during the cooldown of superconducting radio-frequency cavities through the transition temperature due to incomplete Meissner state is known to be a significant source of radio-frequency losses. The sensitivity of flux trapping depends on the distribution and the type of defects and impurities which pin vortices, as well as the cooldown dynamics when the cavity transitions from a normal to superconducting state. Here we present the results of measurements of the flux trapping sensitivity on 1.3 GHz elliptical cavities made from large-grain niobium with different purity for different cooldown dynamics and surface treatments. The results show that lower purity material results in a higher fraction of trapped flux. We present an overview of published data on the mean free path and frequency dependence of the trapped flux sensitivity which shows a significant scatter which highlights the complexity of the pinning phenomenon on a bulk superconductor with a large curved surface. We discuss contributions of different physical mechanisms to rf losses resulting from oscillations of flexible vortex segments driven by weak rf fields. In particular, we address the dependence of the rf losses on the mean free path in the cases of sparse strong pinning defects and collective pinning by many weak defects for different orientations of the vortex with respect to the inner cavity surface. This analysis shows that the effect of the line tension of vortices is instrumental in the physics of flux trapping and rf losses, and theoretical models taking into account different pinning strength and geometry of flexible pinned vortex segments can provide a good qualitative description of the experimental data.

Magnetization, AC susceptibility and µSR measurements have been performed in neutral phthalocyaninato lanthanide ([LnPc2] 0 ) single molecule magnets in order to determine the low-energy levels structure and to compare the low-frequency spin excitations probed by means of macroscopic techniques, such as AC susceptibility, with the ones explored by means of techniques of microscopic character, such as µSR. Both techniques show a high temperature thermally activated regime for the spin dynamics and a low temperature tunneling one. While in the activated regime the correlation times for the spin fluctuations estimated by AC susceptibility and µSR basically agree, clear discrepancies are found in the tunneling regime. In particular, µSR probes a faster dynamics with respect to AC susceptibility. It is argued that the tunneling dynamics probed by µSR involves fluctuations which do not yield a net change in the macroscopic magnetization probed by AC susceptibiliy. Finally resistivity measurements in [TbPc2] 0 crystals show a high temperature nearly metallic behaviour and a low temperature activated behaviour.

As a result of a collaboration between Jefferson Lab and niobium manufacturer CBMM, ingot niobium was explored as a possible material for superconducting radiofrequency (SRF) cavity fabrication. The first single cell cavity from large grain high purity niobium was fabricated and successfully tested at Jefferson Lab in 2004. This pioneering work triggered research activities in other SRF laboratories around the world. Large grain niobium became not only an interesting alternative material for cavity builders, but also material scientists and surface scientists were eager to participate in the development of this material. Most of the original expectations for this material of being less costly and allowing less expensive fabrication and treatment procedures at the same performance levels in cavities have been met. Many single cell cavities made from material of different suppliers have been tested successfully and several multi-cell cavities have shown the performances comparable to the best cavities made from standard polycrystalline niobium. Several 9-cell cavities fabricated by Research Instruments and tested at DESY exceeded the best performing fine grain cavities with a record accelerating gradient of E acc = 45.6 MV/m. Recently-at JLab-by using a new furnace treatment procedure a single cell cavity made of ingot niobium performed at a remarkably high Q 0value (~5×10 10 ) at an accelerating gradient of ~20 MV/m, at 2K. Such performance levels push the state-of-the art of SRF technology to new limits and are of great interest for future accelerators. This contribution reviews the development of ingot niobium technology and attempts to make a case for this material being the choice for future accelerators. PACS numbers: 84.40.-x, 84.71.-b 2