This work is supported by the US DOE/HEP and also by the ONR AppEl, and CNAM. Comparison Between Nb and High Quality MgB 2 Films for Their Mesoscopic Surface Nonlinear Electrodynamic Response Tamin Tai 1, Behnood G. Ghamsari 2, Teng Tan 3,Xiaoxing Xi 3, Steven M. Anlage 1,2 July 19, /

Motivation EH Grain Boundaries Magnify the interior surface of Nb cavities 500 x 200  m pit welds, oxidation, hydrogen poisoning A high resolution near-field magnetic field microscope can identify those defects and relate which defects results in breakdown of the cavities. 2

Nonlinear Response Setup Temperature Control RF Coil on slider Superconductor sample 3f Ratioed Measurement Unratioed Measurement 3

Finite Element Simulation of Our Design Excited RF current: 50 mA Coil turns: 10 Gap length: 200 nm Gap width: 200 nm Gap thickness: 1  m Distance between Probe and Sample : 200 nm Sample is assumed to be a perfect conductor 1  m 4 Center line

Field on Sample Surface Magnetic FieldHeight Dependence of Magnetic field From the simulation, the magnetic field produced by the near- field probe can reach the thermodynamic critical field of Nb (~200 mT) when the probe is close to the sample. 5

Nonlinear Mechanism in Superconductors 6/14 Perturbation of the superfluid density ( n s ) rf currents cause de-pairing – convert superfluid into normal fluid Nonlinearities are generally strongest near T c and weaken at lower temperatures for intrinsic nonlinear Meissner Effect (NLME) J NL (T) calculated by theory (Dahm+Scalapino) “NonlinearResponse” Temperature TcTcTcTc NLME (d-wave SC) JJ, Vortices NLME 0<<schematic>> Schematically Show What we measure ( One Fix Point Measurement )

77/14 JJ, Vortices NLME at Tc  =1*10 5 A 3 /m 2 ; T c =9.45 K (  T c =0.03K)  cutoff)=312 nm J(cutoff)=2.1*10 11 A/m 2 Nonlinear (P 3f ) measurement results on UMD sputtered Nb thin film (50 nm ) Nb thin film (50 nm)

Bulk Nb Measurement Setting Thermometer 1. Thermometer at cold head: ~ 4.2 K 2. Thermometer at cold plate: ~ 4.6 K 3. Thermometer on the top of Bulk Nb surface: ~ 5.0K 8

Bulk Nb P 3f Temperature Dependent Measurement Onset of P 3f (T) moves toward lower temperature at higher power. This implies that the magnetic probe slightly heats the bulk Nb sample. B RF approaches B c ! 9

Bulk Nb P 3f – P f dependence Power dependence measurement shows the power dependent vortex nonlinearity at different temperatures. 10

11/14 Refer to 2011 SRF Tutorial Theoretical prediction for E acc at 2 K is: Nb: 57 MV/m Nb 3 Sn: 120 MV/m MgB 2 : 200 MV/m Liu, Mazin and Kortus (2002); Choi et al, (2002) Big gap,    7.2 meV J. Akimitsu et al, Nature 410, 63 (2001) Small gap,    2.1meV MgB 2 in SRF Cavities Potential Problems: 1.Absence of weak links in high H RF ? 2.Mind its Two Gap nature !!! 1. Relatively easy to deposit high quality films 2. Similar behavior to other low temperature superconductors

Nonlinear Response of MgB 2 12/14 (APL , 2004) Calculation based on the model of Dahm and Scalapino (APL , 2004) A clear dip around 20 K indicates the complete cancellation of the nonlinear signals. Curve fitting results Experimental results experimental data simulation

13 t 0 =50 nm t 0 =100 nm t 0 =150 nm t 0 =200 nm  =8.3*10 5 A 3 /m 2 Freq=5.33GHz T c =34.6 K (  T c =1.3K) (T=0K)=100 nm cutoff =8*10 -7 m J cutoff =4.2*10 11 A/m 2 P 3f (dBm) Experimental Results for Thicker film Intrinsic NLME Simulation on MgB 2 films with different thickness (APL , 2004) Calculation based on the model of Dahm and Scalapino (APL , 2004)

How Many Nonlinear Sources Act in the Intermediate & Low Temperature Region 14 Low T High T Relative Phase Measurement of MgB 2 Measured imaginary VS real part of the complex V 3f Measured imaginary VS real part of the complex V 3f Temperature Response: Two nonlinear sources are “resonant with Temperature”

MgB 2 P 3f – P f dependence Bulk Nb P 3f – P f dependence Comparison of Power Dependence for Two Materials 15

Possible Mechanisms of Nonlinearity at Intermediate and Low Temperature for MgB 2 A.Nonlinearity Due to the Second T c B. Nonlinearity From the Leggett Mode  Josephson coupling between the  and  bands. (Leggett Mode) LL LL L JJ C. Nonlinearity From the Moving Vortices  The creation, motion and annihilation of perpendicular vortex generate this nonlinearity in our experiment.  The peak near the lower T c will be present in the de-coupled and weakly coupled band cases. D.Nonlinearity from the reported nodal gap symmetry 16 Y. D. Agassi, D. E. Oates, B. H. Moeckly, Phys. Rev. B 80 (17), , (2009). A. Gurevich et al, Phys Rev Lett 90 (4), , (2003). E. J. Nicol et al, Phys. Rev. B 73, , (2006). A. Gurevich, G. Ciovati, Phys. Rev. B 77, , (2008).

Recent Work Determine the magnitude of B RF applied to the surface The magnitude of the RF field generated by the probe can be calculated experimentally from Shapiro steps in the I-V curve measurement. 17 I+I+ I-I- V+V+ PfPfPfPf V-V- Weak-link Josephson Junction Nb film Substrate SEM image of Nb weak-link Josephson Junction Apply a localized RF magnetic field on the top

18 Scanning System of the Microscope Motor of X axis Motor of Y axis Capability of scanning mode: Contact mode Y-axis: 1 full step of the motor= 0.5  m X-axis: 1 full step of the motor=50 nm Z-axis: Under Construction R. B. Dinner, REVIEW OF SCIENTIFIC INSTRUMENTS 76, mm MgB 2 P 3f Mapping mm dBm T=15 K

Conclusions-I From the nonlinear response measurement on 50 nm Nb thin films, two nonlinear mechanisms are involved: 1 st : nonlinear mechanism near T c can be understood by the intrinsic nonlinear Meissner Effect. 2 nd : vortex entry and exit From the nonlinear response measurement on bulk Nb, only one nonlinear mechanism is involved---vortex entry and exit For Nb Materials 19

Conclusions-II The nonlinear response at T c can be well interpreted by a model of an intrinsic nonlinear Meissner effect. The nonlinear response at temperatures less than T c shows many different possible nonlinear mechanisms: 1 st : the intrinsic nonlinearity from the proximity induce second T c 2 nd : the intrinsic nonlinearity arising from Josephson coupling between the  and  bands. 3 rd : the nonlinearity due to the perpendicular vortex pairs may also be one of the nonlinear mechanisms in the high quality MgB 2 films. 4 th : the potential nonlinearity from the reported nodal gap symmetry. For high quality MgB 2 films 20

d=3 d=2 d=0.5 Gilson Carneiro P.R.B. 69, (2004) Nonlinearity from a Moving Vortex 21

Nonlinearity Due to Second Tc With Leggett Mode at Low Temperature 22