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)
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