Epitaxial Nb thin films deposited onto the same crystalline insulating surface can evolve in very different fashions depending on specific deposition conditions, thereby affecting their microstructure, surface morphology and superconducting properties. Here, we examine and compare the microstructure and ensuing surface morphology from two distinct Nb/MgO series each with its own epitaxial registry—namely Nb(001)/MgO(001) and Nb(110)/MgO(001)—leading to distinct surface anisotropy and we closely examine the dynamical scaling of the surface features during growth. We compare our findings with those in other metal/MgO epitaxial systems and for the first time, general scaling formalism is applied to analyze anisotropic surfaces exhibiting biaxial symmetry. Further, Power Spectral Density is applied to the specific problem of thin film growth and surface evolution to qualify the set of deposition conditions leading to smoother surfaces. We find good correlation between the surface morphology and microstructure of the various Nb films with superconducting properties such as their residual resistance ratio and lower critical field.

Thin film MgB2 is a promising material for technical improvements in superconducting radio frequency (SRF) technology and applications. At present, bulk niobium SRF accelerating cavities suffer from a fundamental upper limit in maximally sustained accelerating gradients; however, a scheme involving multilayered superstructures consisting of superconducting-insulating-superconducting (SIS) layers has been proposed to overcome this fundamental material limit of 50 MV/m. The SIS multilayer paradigm is reliant upon implementing a thin shielding material with a suitably high HC1, which may prevent early field penetration in a bulk material layer and consequently delay the high field breakdown. It has been predicted that for thin superconducting films - thickness less than the London penetration depth ( ~ 120 nm in the case of MgB2) - the lower critical field HC1 can be enhanced with decreasing thickness. Thus, MgB2, with a high TC and relatively low HC1 value, as compared with Nb, is a prime candidate for such SIS structures. Here we present our study on the microstructure, surface morphology, and superconducting properties on a thickness series of MgB2 thin films and correlate the effects of film thickness and surface morphology on HC1.

Thin film MgB 2 is a promising material for technical improvements in superconducting radio frequency (SRF) technology and applications. At present, bulk niobium SRF accelerating cavities suffer from a fundamental upper limit in maximally sustained accelerating gradients; however, a scheme involving multilayered superstructures consisting of superconducting-insulating-superconducting (SIS) layers has been proposed to overcome this fundamental material limit of 50 MV/m. The SIS multilayer paradigm is reliant upon implementing a thin shielding material with a suitably high HC1, which may prevent early field penetration in a bulk material layer and consequently delay the high field breakdown. It has been predicted that for thin superconducting films - thickness less than the London penetration depth ( ~ 120 nm in the case of MgB 2 ) - the lower critical field H C1 can be enhanced with decreasing thickness. Thus, MgB 2 , with a high T C and relatively low H C1 value, as compared with Nb, is a prime candidate for such SIS structures. Here we present our study on the microstructure, surface morphology, and superconducting properties on a thickness series of MgB 2 thin films and correlate the effects of film thickness and surface morphology on H C1 .

This paper describes surface studies to address roughness issues inherent to thin film coatings deposited onto superconducting radio frequency (SRF) cavities. This is particularly relevant for multilayered thin film coatings that are being considered as a possible scheme to overcome technical issues and to surpass the fundamental limit of similar to 50 MV/m accelerating gradient achievable with bulk niobium. In 2006, a model by Gurevich [Appl. Phys. Lett. 88, 012511 (2006)] was proposed to overcome this limit that involves coating superconducting layers separated by insulating ones onto the inner walls of the cavities. Thus, we have undertaken a systematic effort to understand the dynamic evolution of the Nb surface under specific deposition thin film conditions onto an insulating surface in order to explore the feasibility of the proposed model. We examine and compare the morphology from two distinct Nb/MgO series, each with its own epitaxial registry, at very low growth rates and closely examine the dynamical scaling of the surface features during growth. Further, we apply analysis techniques such as power spectral density to the specific problem of thin film growth and roughness evolution to qualify the set of deposition conditions that lead to successful SRF coatings. DOI: 10.1103/PhysRevSTAB.16.022001

The current technology in superconducting radio frequency linear accelerators is based on the use of bulk niobium cavities. However, optimization of bulk technology is approaching the accelerating gradient limit set by the thermodynamic critical field of niobium, H-C = 200 mT. In order to surpass niobium's ultimately achievable accelerating gradient, it has been proposed to use multilayer coatings to shield bulk niobium from higher fields. These multilayer coatings involve alternating superconducting and insulating layers. The superconductor used in this multilayer structure must have a higher H-C than that of niobium. NbN is one such superconductor that has potential application in these multilayer coatings. Recently, it has been shown that NbN can sufficiently shield an underlying niobium layer. However, this reported shielding has never been shown to be above the lower critical field of niobium. In this work, we present NbN multilayers that for the first time are shown to be capable of shielding an underlying niobium layer beyond the lower critical field of bulk niobium.

Thin film coatings have the potential to increase both the thermal efficiency and accelerating gradient in superconducting radio frequency accelerator cavities. However, before this potential can be realized, systematic studies on structure-property correlations in these thin films need to be carried out since the reduced geometry, combined with specific growth parameters, can modify the physical properties of the materials when compared to their bulk form. Here, we present our systematic studies of Nb thin films deposited onto Cu surfaces to clarify possible reasons for the limited success that this process exhibited in previous attempts. We compare these films with Nb grown on other surfaces. In particular, we study the crystal structure and surface morphology and their effect on superconducting properties, such as critical temperature and lower critical field. We found that higher deposition temperature leads to a sharper critical temperature transition, but also to increased roughness indicating that there are competing mechanisms that must be considered for further optimization.

Superconducting ultrathin films grown epitaxially onto crystalline substrates exhibit strained epitaxial growth due to lattice mismatch, which can have a significant effect on their superconducting properties. We present a complete correlation of the surface morphology, crystal growth, strain, microstructure, and superconducting properties in single-crystal Nb(110) thin films sputter deposited on a-plane sapphire substrates. Notably, we observe that the lattice mismatch between Nb and sapphire induces the formation of a hexagonal surface structure during the first three atomic layers. This is followed by a strained bcc Nb(110) phase whose in-plane lattice parameter progressively relaxes to bulk value. Similar lattice relaxation was also observed in the direction perpendicular to the interface using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Significant perpendicular strain in films up to 30 nm thick was found to ultimately affect the superconducting properties of the Nb thin films as demonstrated with AC susceptibility measurements, where dissipative effects in the lattice associated with the presence of strain and associated defects were identified.

NbN thin films have the potential to be incorporated into radio frequency cavities in a multilayer coating to overcome the fundamental field gradient limit of 50 MV m(-1) for the bulk niobium based technology that is currently implemented in particle accelerators. In addition to having a larger critical field value than bulk niobium, NbN films develop smoother surfaces which are optimal for cavity performance and lead to fewer losses. Here, we present a study on the correlation of film deposition parameters, surface morphology, microstructure, transport properties and superconducting properties of NbN thin films. We have achieved films with bulk-like lattice parameters and superconducting transition temperatures. These NbN films have a lower surface roughness than similarly grown niobium films of comparable thickness. The potential application of NbN thin films in accelerator cavities is discussed.

Surface and interface roughness are critical factors in determining the properties and technological viability of many systems. These considerations are of principle importance for the development of next-generation superconducting radio frequency (SRF) cavities where the ultimate surface determines the cavity performance. The predominant SRF technology currently used in linear accelerators is based on bulk niobium cavities, however on-going efforts to improve cavity performance have considered the possibility of multi-layered thin film coatings on SRF cavities. In particular, Superconducting / Insulating / Superconducting (SIS) multilayer structures have been proposed as a means to achieve higher field gradients in SRF cavities and overcome fundamental SRF limitations of bulk niobium [1]. Nucleation and growth kinetics influence epitaxial thin film growth on different substrates. In addition, film properties differ from those in bulk systems mainly because of limited material supply as well as stress contributions due to lattice mismatch, which can induce significant surface roughness. Rough surfaces may lead to undesirable effects for SRF applications, many of which can be minimized with suitable choices of thin film growth parameters. We have undertaken a systematic effort to understand the dynamic evolution of the Nb surface under specific deposition conditions. Here we examine the morphology of epitaxial Nb grown on MgO ceramic substrates at very low growth rates and closely examine the dynamical scaling of the surface features during growth.

Fundamental aspects of epitaxial thin film growth such as strain due to lattice mismatch can drastically affect the superconducting properties of thin films. In our studies, we show a clear correlation between the surface morphology and microstructure with the superconducting properties of single crystal Nb(110) thin films sputter deposited on a-plane sapphire substrates. We found that the lattice mismatch between Nb and sapphire induces the formation of a hexagonal surface structure during the first 3 atomic layers followed by a strained bcc Nb(110) phase whose lattice parameter progressively relaxes reaching bulk value after 14 Nb atomic layers. The influence of the properties of such initial layers on the superconducting transition process is analyzed in detail. The results reported here indicate that interfacial strain effects must also be considered when evaluating the feasibility of multilayers for SRF cavity applications

and — Interest in graphene, a single layer of carbon atoms arranged in a hexagonal lattice, has increased in recent years due to exciting characteristics such as its predicted high mobility [1]. How-ever, developing a method to produce graphene that is easily integrated into existing fabrication processes has proved difficult thus far. One promising method is high temperature annealing of 6H-SiC such that Si desorption occurs [2], although this method leads to graphene that exhibits lower mobility than predicted [3]. Thus, we have investigated the relationship between different growth conditions (i.e. annealing time and temperature), the resulting surface morphology and the transport properties of graphene films produced using this method. Raman spectroscopy, atomic force microscopy, and Van der Pauw Hall mobility measurements have been used to correlate the surface morphology to transport properties of graphene formed on SiC. Understanding the effect of growth conditions on the resulting transport properties will help optimize the fabrication of graphene for use in the next generation of electronic devices and other applications. [1] Novoselov et al. , Science 306 , 666 (2004). [2] C. Berger et al. , J. Phys. Chem. B 108 , 19912 (2004). [3] G. Gu et al. , Appl. Phys. Lett. 90 , 253507 (2007).

Due to the very shallow penetration depth of the RF fields, SRF properties are inherently a surface phenomenon involving a material thickness of less than 1 micron thus opening up the possibility of using thin film coatings to achieve a desired performance. The challenge has been to understand the dependence of the SRF properties on the detailed characteristics of real surfaces and then to employ appropriate techniques to tailor these surface properties for greatest benefit. Our aim is to achieve gradients >100 MV/m and no simple material is known to be capable of sustaining this performance. A theoretical framework has been proposed which could yield such behavior [1] and it requires creation of thin film layered structures. I will present our systematic studies on such proof-of-principle samples. Our overarching goal has been to build a basic understanding of key nano-scale film growth parameters for materials that show promise for SRF cavity multilayer coatings and to demonstrate the ability to elevate the barrier for vortex entry in such layered structures above the bulk value of Hc1 for type-II superconductors and thus to sustain higher accelerating fields.