2012-2013 JSA/JLab Graduate Fellowship Report Department of Physics, College of William and Mary

Zhaozhu Li

Summary The JSA fellowship helped my participation in two technical projects during the 2012-2013

academic year. The first project pertains to superconducting thin films and multilayers for

superconducting radio frequency (SRF) cavity applications. The second project is about potential

new materials for novel photocathodes able to sustain high currents especially useful for next

generation electron-ion colliders (EIC). The research accomplishments on both projects are

described below.

Research Accomplishments 1) Superconducting thin films and multilayers for applications on SRF cavities

The multilayer thin film structures studied were based on the

superconductor-insulator-superconductor (SIS) model proposed by A. Gurevich[1] to enhance

the breakdown field (Hc) of SRF cavities. A SIS multilayer stack over bulk Nb has been

predicted to be able to shield and prevent magnetic field penetration and thus hence enhance

the caviity’s field breakdown. However, despite experimental proof of the effectiveness of the

Gurevich’s model, further choice and characterizaqtion of superconducting materials as thin

films is necessary tor further understand practical limitations and future application of this

approach. In 2012, I participated and contributed to the study and characterization of adequate

superconducting materials such as (NbN, etc) for the SIS structures in collaboration with two

other senior graduate students, William Roach(now Dr Roach) and Douglas Beringer, as well

as my advisor professor R.A. Lukaszew. Based on our previous results showing a lower

critical field(Hc1) for NbN thin films higher than that of bulk Nb(see paper 1), we investigated

multilayered NbN-MgO-Nb thin films on MgO substrates. Using DC reactive sputtering as

thin film deposition technique, we achieved a series of thin film multilayered samples and

characterized them with X-ray diffraction (XRD) measurements to establish their

microstructure. We also measured the surface morphology using atomic force microscopy

(AFM). We also measured the superconducting properties of the produced thin films using a

superconducting quantum interference device (SQUID) and determined a higher critical field

than that of bulk Nb in all cases. The NbN-MgO-Nb-MgO multilayers were experimentally

shown to achieve a higher Hc1 than bulk Nb under dc superconducting measurements using

SQUID. More detailed results are shown in reference [2].

We also measured the superconducting property of MgB2 thin films provided by our

collaborator professor X.X. Xi, et al from Temple University. Similarly as in the NbN case,

we measured the critical field for MgB2 films of different thickness using SQUID. We also

determined their microstructure with XRD. Oure results show a thickness dependence on the

dc superconducting performance of MgB2 thin films following equation [1] below. This result

was presented at the AVS 59th International Symposium and Exhibitions in 2012.

]1[)07.0(ln2

20

1 equationdd

Hc −=ξπ

φ

where Hc1 is lower critical field of type-II superconductor, 0φ is the quantum flux,   d the film

thickness and ξthe superconductor coherence length

Fig 1 The blue line represents the simulated curve for ξ=5nm using equation [1]; the red line

represents simulated curve for ξ=7nm.[4] We note remarkable correlation for films thicker

than 40nm. We also note that our surface roughness studies indicate that below 60nm the

roughness of the films compares with the actual thickness.

2) Novel photocathode approaches for EIC The second project is related to potential metallic photocathodes able to sustain high currents. The

ultimate goal is to find and develop appropriate metal photocathodes that have enhanced quantum

efficiency (QE) especially for the needs of future EIC. To achieve the enhancement of QE, we

have designed a system using diffraction grating patterns, so that we can excite the surface

plasmon resonance (SPR) on the metallic photocathode to enhance light absorption. We also

deposit appropriate oxide thin film over layers on top of metal photocathode to lower the work

function of the photocathode to achieve higher quantum efficiency.[3]

I started this project with mathematical simulations to help identify the relationship between the

thin film thickness, diffraction grating spacing, light wavelength and the corresponding SPR angle

in order to satisfy our experimental geometry, as shown in Fig 2. Our setup shows that the incident

angle in our system is set to be at around 45 degrees and hence this determines the required SPR

resonance angle for any prospective sample investigated. Thus we needed the proper grating

spacing as well as thickness of metal and oxide layers to satisfy this requirement. In collaboration

with graduate student Kaida Yang, Dr Jose Riso and my advisor Dr Lukaszew we set up our

experiment system to form a dc electrical circuit to measure small photocurrent (pA to uA) under

ultra-high vacuum (E-9 torr) and light incident at 45 degrees. The photocathode was prepared

using a sputtering deposition system onto a diffraction pattern and under the guidance of the

simulation results, and afterward checking the SPR angle experimentally. The final experimental

setup was accomplished on October 2013 and we were able to achieve a ~256 pA (Fig 3)

photocurrent for MgO/Ag/grating samples under red light (638nm). Subsequent studies to further

improve on these results are in progress.

Fig 2

Fig 3

Research Presentations 10/28/2012 - 11/02/2012: Oral presentation at AVS 59th International Symposium and Exhibitions.

“Thickness dependence of superconducting properties in MgB2 thin films” Paid by travel grant

from JSA fellowship.

Publications 1 W. Roach, J. Skuza, D. Beringer, Z. Li, C. Clavero and R. Lukaszew, “NbN thin films for

superconducting radio frequency cavities”, 2012 Supercond. Sci. Technol. 25 125016

2 W. M. Roach, D. B. Beringer, Z. Li, C. Clavero, and R. A. Lukaszew, “Magnetic Shielding

Larger Than the Lower Critical Field of Niobium in Multilayers”, IEEE Trans. Appl. Supercond.

23, 8600203 (2013)

3 K. Yang and Z. Li, “Film Roughness Analysis and Magnetic Properties in Ultrathin

Sample (Grounded)

Faraday Cup (+180V)

Co/Pd Multilayers”, Journal of the Magnetics Society of Japan, May 23, 2013

References [1] A. Gurevich, Appl. Phys. Lett. 88, 012511(2006)

[2] C. Bohmer, G. Brandstatter and H.W. Weber, Supercond Sci Techno 10, A1-A10(1997)

[3] L. Giordano et al, Phs Rev B 73, 045414 (2005)