The Superheating Field of Niobium: Theory and Experiment
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Proceedings of SRF2011, Chicago, IL USA TUIOA05 THE SUPERHEATING FIELD OF NIOBIUM: THEORY AND EXPERIMENT N. Valles† and M. Liepe, Cornell University, CLASSE, Ithaca, NY 14853, USA Abstract new research being done at Cornell to probe this phenom- ena. Finally, the implications of the work presented here This study discusses the superheating field of Niobium, are discussed and regions for further study are explored. a metastable state, which sets the upper limit of sustainable magnetic fields on the surface of a superconductors before flux starts to penetrate into the material. Current models Critical Fields of Superconductors for the superheating field are discussed, and experimental When a superconductor in a constant magnetic field is results are presented for niobium obtained through pulsed, cooled below its critical temperature, it expels the mag- high power measurements performed at Cornell. Material netic field from the bulk of the superconductor. This is preparation is also shown to be an important parameter in accomplished by superconducting electrons establishing a exploring other regions of the superheating field, and fun- magnetization cancelling the applied field in the bulk of damental limits are presented based upon these experimen- the material. The magnetic is field limited to a small region tal and theoretical results. close to the surface, characterized by a penetration depth λ, which is the region in which supercurrents flow. Another INTRODUCTION important length scale in superconductors is the coherence length, ξ0 which is related to the spatial variation of the An important property of superconductors that has posed superconducting electron density. The ratio of these char- both theoretical and experimental challenges is the mag- acteristic length scales yields the Ginsburg-Landau (GL) netic superheating field, a metastable state wherein the ma- parameter, κ ≡ λ/ξ. terial remains fully superconducting above the respective Pippard improved upon the superconductor model that critical fields in steady state. Obtaining accurate measure- only assumed local electron interaction to take into account ments of this metastable state is essential to expand the the- non-local effects. He argued that superconducting wave- oretical understanding of this phenomena and determine functions should have a characteristic dimension ξ 0. If only the ultimate limit of radio-frequency superconductors. In electrons around kB Tc of the Fermi energy can be involved this study, niobium is used to probe this limiting field. in the dynamics around the critical temperature, and they Studying the superheating field is important on sev- have a momentum range Δp ≈ kBTc/vF , where vF is eral counts. First, current theoretical models can imme- the Fermi velocity, then the approximate coherence length diately be tested against measurements. Second, the re- should, by the uncertainty principle be gions in which empirical methods accurately describe the field tend to be in limiting cases, such as the low- or high-κ ¯h ¯hvF Δx ∼ → ξ0 ∼ . (1) limit,[1, 2] or in small temperature ranges around the criti- Δp kBTc cal temperature.[3] Measuring the superheating field in the intermediate range, where κ ∼ 1, provides an important re- Assuming a pure material has a coherence length ξ 0, Pi- sult that can guide development of new models covering the pard showed that if impurities introduce scattering centers entire range of κ. Third, most models are based upon a phe- giving an electron mean free path of , then the coherence nomenological model of the superheating field, only valid length of the impure material, ξ, is modified to be near Tc, and so say nothing about the behaviour as a func- 1 1 1 tion of temperature. Newer, non-phenomenological calcu- = + , (2) lations taking temperature dependence into account need ξ ξ0 accurate data to test against. Finally, the accurate determi- and the penetration depth becomes nation of the superheating field also is of particular interest T in application. While theory and experiment agree near c, ξ niobium microwave cavities for particle accelerators oper- λ = λ 1+ 0 , L (3) ate at temperatures <Tc/4, where theory and experiment are disparate, so measurements can set stringent upper lim- where λL is the mean free-path of the material with no scat- its for what is possible with these cavities. tering sites.[4] This paper begins with a brief review of the the criti- These expressions can be used to give κ as a function of cal fields of superconductors, discusses the current state mean free path: of knowledge on the superheating field and then presents 3/2 ∗ Work supported by DOE award ER41628 λL ξ0 + † κ()= . (4) *[email protected] ξ0 04 Material studies 293 TUIOA05 Proceedings of SRF2011, Chicago, IL USA The ratio κ separates superconductors√ into two broad the behaviour near the surfaces must be taken into account. categories; those with κ<1/ 2 are√ called Type-I super- For fields parallel to a superconducting surface, above H c2 conductors and those with κ>1/ 2 are called Type-II superconductivity can nucleate at a metal-insulator inter- superconductors. The implications of this categorization faces, though the bulk remains normal conducting. will be elaborated upon later in this paper. Both theory and experiment have shown that for fields of The superconducting state is more ordered than the nor- magnitude mal conducting state because of the Cooper pairing of elec- Hc3 =1.695Hc2 (11) trons near the Fermi energy. Subjecting a material to a a superconducting surface sheath of thickness ξ persists, DC magnetic field causes supercurrents to flow in the layer while the bulk is normal conducting. This field dominates a within a penetration depth of the superconducting surface different type of phenomena compared with the other crit- to cancel out interior fields, which raises the free-energy of ical fields, because of the depth dependence of the state. the superconductor, Fs, which depends on the applied field. The other fields have constant depth profiles, either being When Fs is equal to the free energy of the normal conduct- in the Meissner state or normal conducting state, where as ing state, Fn, flux enters the superconductor, of volume V s, Hc3 transitions from superconducting to normal as depth and a phase transition occurs. increases. Finally, the field orientation for the other fields Mathematically, one can write an expression for the crit- is normal to the surface, whereas for Hc3 the field is paral- ical field that causes this phase transition, Hc, lel to the surface. Hc Fn = Fs(H =0)+μ0Vs HdH, (5) Metastability So far, these critical fields mentioned 0 are in equilibrium conditions. Before a transition takes which is applicable for Type-I superconductors in steady- place, there is an energy cost to nucleate a fluxoid, which state conditions. leaves open the possibility of a metastable state in which Supposing the energy density of superconductors is sup- the energetically favorable transition has not occurred due pressed over a coherence length ξ0, the free energy per unit to the activation energy barrier. This barrier vanishes at the area would be increased by superheating field, Hsh. Both Type-I and Type-II superconductors can persist in μ0 2 H ξ . the Meissner state above Hc1. The precise relationship be- 2 c 0 (6) tween Hsh and Hc is still a field of active experimental and If magnetic field, He is admitted to penetrate the material a theoretical research, but a phenomenological result shows distance λ, the free energy is lowered by that near Tc μ 2 0 2 T − He λ, (7) 2 Hsh = c(κ)Hc 1 − , (12) Tc giving a net boundary energy per unit area of c(κ) μ where is the ratio of the superheating field and the 0 ξ H2 − λH2 2 0 c e (8) thermodynamic critical field. The determination of this fac- tor is one of the central focuses of this work. Type-II superconductors, (having ξ 0 <λ) can benefit from a negative surface energy gain by allowing flux tubes SURVEY OF PREVIOUS WORK into the bulk of a superconductor at a field H c1, which by definition is below the field Hc. The superconductor is then Theoretical Survey in a mixed state having normal conducting regions inter- The superheating field has been studied in sev- H penetrating the bulk material, up to a field c2 when the eral regimes, mostly with phenomenological treatments entire material transitions into the normal state. through Ginzburg-Landau theory and is a challenging field H H An expression relating c1 and c, was given by to determine theoretically. Because it has been the most Merrill[5] as frequently used tools to study superconductivity, here we 0.08 outline the basic tenets of the GL Model. Hc1 = ln κ + √ Hc (9) Ginzburg and Landau approached superconductivity 2κ λ phenomenologically by introducing a parameter κ = ξ , The thermodynamic critical field and Hc2 are related ac- that is the ratio of the two characteristic lengths in a super- cording to conductor which can be used to broadly characterize su- 1 Hc2 perconductors into two classes, Type-I, with κ< √ and Hc = √ . (10) 2 2κ √1 <κ Type-II having 2 . There is one more type of critical field, denoted H c3 The basic postulate of GL theory is that if ψ, a pseudo- that is a surface effect first predicted by Saint-James and wavefunction that is the order parameter corresponding to de Gennes. Because real superconductors are finite in size, the number of superconducting electrons, varies slowly in 294 04 Material studies Proceedings of SRF2011, Chicago, IL USA TUIOA05 space then the free energy density f can be expanded in a superheating field as a function of temperature. Eliash- series of the form berg theory[7] requires the full information about the elec- tronic structure of the superconductor and is very difficult 2 2 2 β 4 1 ¯h ∗ μ0h to solve.