Matthias Liepe



Radio Frequency Superconductivity, Accelerator Physics and Particle Accelerators

My research focuses on studying the science and technology of bulk and thin-film superconductors in high electromagnetic fields at ultra-high frequencies and the application of these superconductors, e.g. in superconducting RF cavities for particle accelerators and quantum computing.

Research Focus

Superconducting radio frequency (RF) cavities are feet-long structures, providing extremely high electric field gradients (tens of MV/m) for the acceleration of charged particle beams. The electric field inside these cavities oscillates at GHz frequencies, with exceptional high quality factors of 1E10 to 1E11. By using superconducting materials operated at temperatures between 1.5K and 4K for the walls of the cavities, we can achieve such high efficiency. The evolution in the performance of superconducting cavities has revolutionized the performance and scientific reach of particle accelerators for a variety of science applications, including high-energy physics, nuclear physics, synchrotron radiation-based research, and high-power lasers. Future particle accelerators for science, industry, energy generation, and medical applications all rely on the performance we hope to achieve in next-generation superconducting RF cavities.

I am the head of Cornell’s Superconducting Radio Frequency (SRF) group, which is a world leader in the science of microwave superconductivity and its application to accelerating cavities for particle accelerators. We have an extensive, state of the art infrastructure for the design, fabrication, preparation and test of superconducting cavities. Our research program is multi-faceted with high-impact and visibility. We have established a wide range of collaborations with international and national accelerator labs as well as with industry. We are part of the NSF Center for Bright Beams (CBB), which gives our graduate students exciting opportunities for interdisciplinary collaborative research.

SRF cavities not only enable accelerator-based sciences, but they also allow measuring superconducting response under extreme conditions with very high sensitivity. They allow the study of surface resistance, critical fields, superconducting magnetic microwave shielding, and the metastability of the superheating-field barrier. They are a testing ground for the science of disorder and defects, coupling superconductivity and high fields to grain boundaries, surface anisotropy, surface oxides, and crystal orientation. My current research concentrates on the following areas:

Synthesis, characterization, and microwave surface resistance of new superconducting compound materials for SRF cavity applications

Current SRF cavities exclusively use niobium as superconductor and are approaching theoretical limits. However, new potentially game-changing materials (e.g. Nb3Sn, MgB2, NbN) have the potential for fields and cavity quality factors far above the niobium limit. Moving to higher-kappa, compound superconductors brings new questions: What alternative superconductors with critical temperatures higher than that of niobium can open the path towards a new generation of SRF cavities with even lower RF surface resistance and higher accelerating fields? Does the small coherence length of these superconductors limit their usefulness due to grain boundary losses or defects? How can these more complex compound superconductors be synthesized with ideal stoichiometry and defect free? Our research program on Nb3Sn is world leading and has resulted in the first ever alternative material SRF cavity to clearly outperform traditional niobium cavities.

Superheating fields in superconductors

The highest gradient niobium SRF cavities are operated with peak magnetic fields beyond Hc1, the field where vortices (that would cause massive losses) would penetrate in equilibrium. Operation is possible until a higher superheating field Hsh because of a surface barrier to flux penetration. Our group has done a first measurement of the full temperature dependence of the superheating field of niobium using SRF cavities and has shown that it depends strongly on the preparation of the niobium surface. Much remains unknown. How does the superheating field depend on the Fermi surface anisotropy, i.e. could optimally oriented superconducting surfaces offer higher fields in SRF cavities? How do strong-coupling effects, as present in many of the higher-temperature traditional superconductors, impact Hsh? Can defects bypass the metastable superheating-field barrier for large-kappa materials?

Processing, characterization and microwave surface resistance of the RF surface penetration layer of superconductors

The surface resistance of a superconductor in microwave fields is determined by a highly complex surface layer of a few 100 nm thickness (roughly the penetration depth of the field), with oxides, grain boundaries, impurities, and defects present. This surface resistance, and is observed strong field dependence, strikingly depend on the surface treatment protocol. For example, we have shown the doping of niobium with impurities can drastically reduce RF surface resistance by optimizing the electron mean free path and reducing overheating of the quasiparticles. Open questions include: What is the physics underlying the residual surface resistance at the lowest temperatures? What are the effects of surface oxides on the surface electronic structure of materials and their impact on RF cavity performance characteristics? What surface morphology and (likely) mixed metallic phases arise from electrochemical polishing of niobium? What are the sources of the observed strong field dependence of the microwave surface resistance, and how does it depend on impurity doping?

Developing the superconducting linac technology for future high-power, high energy-efficiency particle accelerators

In addition to designing and optimizing the cavities for future superconducting accelerators, we are developing related and technologically challenging components like RF input couplers, Higher-Order-Mode dampers and frequency tuners. We are designing, building and testing entire, complex SRF cryomodules. This work relates to a wide breath of scientific and engineering questions. A particular focus of our work is on making future particle accelerators much more energy efficient.

Current Graduate Students

Gabriel Gaitan is developing a Chemical Vapor Deposition system for coating of superconducting RF cavities with multi-layer superconductors. He will use these cavities to study novel superconducting surfaces under extreme conditions. 

Pete Koufalis is working on ultra-high efficient SRF cavities, exploring impurity doping of the RF penetration layer via low and high temperature heat treatments. Impurity doping has been shown to strongly impact the RF surface resistance and critical fields of the superconductor, and we are exploring the physics behind these observed effects to determine optimal dopants and doping levels for SRF applications.

Thomas Oseroff is investigating the RF performance of very high frequency SRF cavities, and of a range of alternative materials, i.e. NbN films produced via atomic-layer-deposition, and multi-layer superconductors.  For this research, he is using and improving a novel system for measuring the RF performance of superconducting material samples.

Ryan Porter is studying the physics and performance of high-kappa superconductors like niobium-3-tin in microwave fields. He is coating very high quality Nb3Sn surfaces in an ultra-high vacuum furnace, is measuring their RF surface impedance, and is using advanced surface analysis (e.g. FIB/TEM, XRD) to analyze Nb3Sn films to determine the sources of current performance limitations.  His research addresses the topics of metastability, disorder, and critical fields in high kappa superconductors, especially Nb3Sn. He is currently developing a novel setup to measure the maximum flux-free RF fields for a wide range of material samples.

Neil Stilin is working on very high efficiency Nb3Sn SRF cavities that will be transformational for SRF driven accelerators, and will enable new SRF accelerators in industry, medicine, environmental, & security applications. Neil is studying the different contributions to the high-field RF surface resistance of Nb3Sn, and how to further reduce these. He is also developing novel cooling concepts of high-efficiency Nb3Sn SRF cavities, leading the way to a new generation of very compact and robust SRF based accelerators.

Alumni Graduate Students

Yi Xie and Sam Posen are now scientists at Fermilab. Dan Gonnella and James Maniscalo are scientists at SLAC, working on the LCLS-II project. Daniel Hall is a Senior Design Engineer at ASML, one of the world's leading manufacturers of chip-making equipment. Nick Valles is a Systems Engineering Manager at Raytheon.

There are many opportunities for motivated undergraduate and graduate students to get involved. Contact me if you are interested in our work and would like to know more!

Group meetings:

Mondays 10:00 am: SRF group planning meeting

Fridays 10:00 am: SRF group science meeting and journal club

Wednesdays 3:30PM (biweekly): Center for Bright Beams SRF group meeting


  1. The importance of the electron mean free path for superconducting radio-frequency cavities, J. Maniscalco, D. Gonnella, M. Liepe, Journal of Applied Physics 121, 043910, (2017).
  2. Theoretical estimates of maximum fields in superconducting resonant radio frequency cavities: stability theory, disorder, and laminates, D. B Liarte, S. Posen, M. K Transtrum, G. Catelani, M. Liepe and J. P Sethna, Superconductor Science and Technology, Volume 30, Number 3, 033002, (2017).
  3. Impact of nitrogen doping of niobium superconducting cavities on the sensitivity of surface resistance to trapped magnetic flux, D. Gonnella, J. Kaufman and M. Liepe, J. Appl. Phys. 119, 073904 (2016).

  4. Shielding Superconductors with Thin Films as Applied to rf Cavities for Particle Accelerators, S. Posen, M. K. Transtrum, G. Catelani, M. Liepe, and J. P. Sethna, Phys. Rev. Applied 4, 044019 (2015).

  5. Radio Frequency Magnetic Field Limits of Nb and Nb3Sn, S. Posen, N. Valles, and M. Liepe, Phys. Rev. Lett. 115, 047001 (2015).

  6. Proof-of-principle demonstration of Nb3Sn superconducting RF cavities for high Q0 applications, S. Posen, M. Liepe. D. Hall, Applied Physics Letters 106, Issue 8 (2015).

  7. Analysis of Nb3Sn surface layers for superconducting RF cavity applications, C. Becker, S. Posen, N. Groll, R. Cook, C. M. Schlepuetz, D. Hall, M. Liepe, M. Pellin, J. Zasadzsinski, and T. Proslier, Applied Physics Letters 106, Issue 8 (2015).

  8. Nitrogen-doped 9-cell cavity performance in a test cryomodule for LCLS-II, D. Gonnella, R. Eichhorn, F. Furuta, M. Ge, D. Hall, V. Ho, G. Hoffstaetter, M. Liepe, T. O'Connell, S. Posen, P. Quigley, J. Sears, V. Veshcherevich, A. Grassellino, A. Romanenko and D. A. Sergatskov, J. Appl. Phys. 117 , 023908 (2015).

  9. Advances in development of Nb3Sn superconducting radio-frequency cavities, S. Posen and M. Liepe, Phys. Rev. ST Accel. Beams 15, 112001 (2014).

  10. The main linac cavity for Cornell's energy recovery linac: Cavity design through horizontal cryomodule prototype test, N. Valles, M. Liepe, F. Furuta, M. Gi, D. Gonnella, Y. He, K. Ho, G. Hoffstaetter, D.S. Klein, T. O'Connell, S. Posen, P. Quigley, J. Sears, G.Q. Stedman, M. Tigner, V. Veshcherevich, Nuclear Instruments Methods in Physics Research A (2013), 

  11. Record high-average current from a high-brightness photoinjector, B. Dunham, J. Barley, A. Bartnik, I. Bazarov, L. Cultrera, J. Dobbins, G. Hoffstaetter, B. Johnson, R. Kaplan, S. Karkare, V. Kostroun, Y. Li, M. Liepe, X. Liu, F. Loehl, J. Maxson, P. Quigley, J. Reilly, D. Rice, D. Sabol, E. Smith, K. Smolenski, M. Tigner, V. Vesherevich, D. Widger, and Z. Zhao, Appl. Phys. Lett. 102, 034105 (2013).

  12. Mechanical optimization of superconducting cavities in continuous wave operation, S. Posen and M. Liepe, Phys. Rev. ST Accel. Beams (2012).  

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Courses - Spring 2023