About
Matthias Liepe is a researcher associated with CLASSE at Cornell University, where the focus is on accelerator physics, photon science, and particle physics and astrophysics. His work involves the design, construction, and operation of particle accelerators, including optimizing the Cornell Electron Storage Ring for photon production and developing advanced technologies for the Energy Recovery Linac (ERL). The ERL is a cutting-edge X-ray source that upgrades the CHESS National X-ray Facility, supporting multidisciplinary research across physics, chemistry, biology, environmental and materials sciences, and engineering. In addition to his contributions to photon science, Matthias Liepe is involved in research related to particle accelerators, which serve as powerful microscopes for probing the fundamental building blocks of matter. His work encompasses non-linear dynamics, computational physics, engineering, and material sciences. He is part of the broader efforts at CLASSE to advance accelerator technology and particle physics, including participation in experiments at the Large Hadron Collider, the g-2 experiment, and the development of detectors for cosmic microwave background measurements. His research supports the discovery of new phenomena in the universe and the development of innovative accelerator-based scientific tools.
Research topics
- Materials science
- Condensed matter physics
- Nanotechnology
- Thermodynamics
- Composite material
- Metallurgy
- Physics
- Quantum mechanics
Selected publications
A Computational Picture of Hydride Formation and Dissipation In Nb SRF Cavities
ArXiv.org · 2025-09-16
preprintOpen accessSenior authorResearch linking surface hydrides to Q-disease, and the subsequent development of methods to eliminate surface hydrides, is one of the great successes of SRF cavity R&D. We use time-dependent Ginzburg-Landau to extend the theory of hydride dissipation to sub-surface hydrides. Just as surface hydrides cause Q-disease behavior, we show that sub-surface hydrides cause high-field Q-slope (HFQS) behavior. We find that the abrupt onset of HFQS is due to a transition from a vortex-free state to a vortex-penetration state. We show that controlling hydride size and depth through impurity doping can eliminate HFQS.
arXiv (Cornell University) · 2025-09-03
preprintOpen accessNanometer-scale surface chemistry limits the performance of SRF cavities and quantum circuits. We present an ab initio framework connecting DFT interfacial energetics with strong-coupling Eliashberg theory for capped Nb and Ta surfaces. This approach identifies Au and Au-based alloys (AuPd, AuPt) as effective passivation layers. Our model further predicts that combining a noble-metal capping layer with an appropriate wetting/adhesion layer (WAL) yields far more robust adhesion than a capping layer alone under realistic conditions, enabling thinner caps, and thereby addressing a central challenge in superconducting surface passivation.
Physical Review Research · 2024-11-13 · 3 citations
articleOpen access<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi mathvariant="normal">Nb</a:mi><a:mn>3</a:mn></a:msub><a:mi>Sn</a:mi></a:mrow></a:math> film coatings have the potential to drastically improve the accelerating performance of Nb superconducting radiofrequency (SRF) cavities in next-generation linear particle accelerators. Unfortunately, persistent <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:msub><c:mi mathvariant="normal">Nb</c:mi><c:mn>3</c:mn></c:msub><c:mi>Sn</c:mi></c:mrow></c:math> stoichiometric material defects formed during fabrication limit the cryogenic operating temperature and accelerating gradient by nucleating magnetic vortices that lead to premature cavity quenching. The SRF community currently lacks a predictive model that can explain the impact of chemical and morphological properties of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:msub><e:mi mathvariant="normal">Nb</e:mi><e:mn>3</e:mn></e:msub><e:mi>Sn</e:mi></e:mrow></e:math> defects on vortex nucleation and maximum accelerating gradients. Both experimental and theoretical studies of the material and superconducting properties of the first 100 nm of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mrow><g:msub><g:mi mathvariant="normal">Nb</g:mi><g:mn>3</g:mn></g:msub><g:mi>Sn</g:mi></g:mrow></g:math> surfaces are complicated by significant variations in the volume distribution and topography of stoichiometric defects. This work contains a coordinated experimental study with supporting simulations to identify how the observed chemical composition and morphology of certain Sn-rich and Sn-deficient surface defects can impact the SRF performance. <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:msub><i:mi mathvariant="normal">Nb</i:mi><i:mn>3</i:mn></i:msub><i:mi>Sn</i:mi></i:mrow></i:math> films were prepared with varying degrees of stoichiometric defects, and the film surface morphologies were characterized. Both Sn-rich and Sn-deficient regions were identified in these samples. For Sn-rich defects, we focus on elemental Sn islands that are partially embedded into the <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:mrow><k:msub><k:mi mathvariant="normal">Nb</k:mi><k:mn>3</k:mn></k:msub><k:mi>Sn</k:mi></k:mrow></k:math> film. Using finite element simulations of the time-dependent Ginzburg-Landau equations, we estimate vortex nucleation field thresholds at Sn islands of varying size, geometry, and embedment. We find that these islands can lead to significant SRF performance degradation that could not have been predicted from the ensemble stoichiometry alone. For Sn-deficient <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow><m:msub><m:mi mathvariant="normal">Nb</m:mi><m:mn>3</m:mn></m:msub><m:mi>Sn</m:mi></m:mrow></m:math> surfaces, we experimentally identify a periodic nanoscale surface corrugation that likely forms because of extensive Sn loss from the surface. Simulation results show that the surface corrugations contribute to the already substantial drop in the vortex nucleation field of Sn-deficient <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mrow><o:msub><o:mi mathvariant="normal">Nb</o:mi><o:mn>3</o:mn></o:msub><o:mi>Sn</o:mi></o:mrow></o:math> surfaces. This work provides a systematic approach for future studies to further detail the relationship between experimental <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:mrow><q:msub><q:mi mathvariant="normal">Nb</q:mi><q:mn>3</q:mn></q:msub><q:mi>Sn</q:mi></q:mrow></q:math> growth conditions, stoichiometric defects, geometry, and vortex nucleation. These findings have technical implications that will help guide improvements to <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"><s:mrow><s:msub><s:mi mathvariant="normal">Nb</s:mi><s:mn>3</s:mn></s:msub><s:mi>Sn</s:mi></s:mrow></s:math> fabrication procedures. Our outlined experiment-informed theoretical methods can assist future studies in making additional key insights about <u:math xmlns:u="http://www.w3.org/1998/Math/MathML"><u:mrow><u:msub><u:mi mathvariant="normal">Nb</u:mi><u:mn>3</u:mn></u:msub><u:mi>Sn</u:mi></u:mrow></u:math> stoichiometric defects that will help build the next generation of SRF cavities and support related superconducting materials development efforts. Published by the American Physical Society 2024
Microscopy and Microanalysis · 2024-07-01 · 1 citations
articleOpen accessSuperconducting radio frequency linear collider HELEN
Journal of Instrumentation · 2023-09-01 · 4 citations
articleOpen accessAbstract This article discusses a proposed Higgs-Energy LEptoN (HELEN) e + e - linear collider based on advanced traveling wave superconducting radio frequency technology. The proposed collider offers cost and AC power savings, smaller footprint (relative to the ILC), and could be built at Fermilab. After the initial physics run at 250 GeV, the collider could be upgraded either to higher luminosity or to higher, up to 500 GeV, energies.
Journal of Applied Physics · 2023-12-11 · 6 citations
articleOpen accessNb 3 Sn and V3Si thin films are promising candidates for the next generation of superconducting radio-frequency (SRF) cavities. However, sputtered films often suffer from stoichiometry and strain issues. This exploratory study investigates the structural and chemical effects of thermal annealing, both in−situ and post-sputtering, on DC-sputtered Nb3Sn and V3Si films with varying thicknesses, deposited on Nb or Cu substrates. Building upon our initial studies [Howard et al., Proceedings of the SRF’21, East Lansing, MI (JACoW, 2021), p. 82.], we provide fundamental insights into recrystallization, phase changes, and the issues of stoichiometry and strain. Through annealing at 950 °C, we have successfully enabled the recrystallization of 100 nm thin Nb3Sn films on Nb substrates, yielding stoichiometric and strain-free grains. For 2 μm thick films, elevated annealing temperatures led to the removal of internal strain and a slight increase in grain size. Moreover, annealing enabled a phase transformation from an unstable to a stable structure in V3Si films, while we observed significant Sn loss in 2 μm thick Nb3Sn films after high-temperature annealing. Similarly, annealing films atop Cu substrates resulted in notable Sn and Si loss due to the generation of Cu–Sn and Cu–Si phases, followed by evaporation. These results encourage us to refine our process to obtain high-quality sputtered films for SRF use.
RF and thermal studies on conduction cooled Nb<sub>3</sub>Sn SRF cavity
Engineering Research Express · 2023-06-01 · 9 citations
articleOpen accessAbstract Advancements in the development of Nb 3 Sn coatings for superconducting radio-frequency (SRF) cavities have enabled efficient RF operation at 4.2 K. This has made the use of new cooling methods possible, namely those based on conduction cooling from commercial cryocoolers. Using cryocoolers in place of liquid helium as a cooling source eliminates the need for expensive and complex cryogenic infrastructure, making SRF technology accessible to small-scale applications in fields such as medicine, industry, environmental sustainability and more. At Cornell University, we have developed a new cavity testing assembly which uses a Cryomech PT420-RM cryocooler to cool a 2.6 GHz Nb 3 Sn cavity. We have performed several rounds of RF and diagnostic testing using this new assembly. Our best results demonstrated stable CW operation at 10 MV/m, with the cavity remaining at 4.2 K or lower. This represents breakthrough performance for a conduction cooled cavity, in which accelerating gradients relevant to some industrial applications were achieved. Our analysis highlights the importance of reducing thermal gradients across the cavity during cooldown; different methods for achieving this were successfully developed and demonstrated. We also found close agreement regarding thermal behavior between experimental measurements and numerical simulations, validating our chosen conduction cooling methods and providing guidance for future improvements. These findings will serve as a foundation for designing a new cryocooler-based cryomodule which will provide beam energy gains on the order of 1 MeV for beam currents up to 100 mA.
Measurements of the amplitude-dependent microwave surface resistance of an Au/Nb bilayer
arXiv (Cornell University) · 2023-05-19
preprintOpen accessSenior authorSurface properties are critical to the capabilities of superconducting microwave devices. The native oxide of niobium-based devices is thought to consist of a thin normal conducting layer. To improve understanding on the importance of this layer, an attempt was made to replace it with a more easily controlled gold film. A niobium sample host microwave cavity was used to measure the surface resistance in continuous wave operation at 4.0 GHz and 5.2 GHz. Sample conditions studied include temperatures ranging from 1.6 K to 4.2 K with RF magnetic fields on the sample surface ranging from 1 mT to the maximum field before the superconducting properties were lost (quench field). The nominal film thickness of the gold layer was increased from 0.1 nm to 2.0 nm in five steps to study the impact of the normal layer thickness on surface resistance on a single niobium substrate. The 0.1 nm film was found to reduce the surface resistance of the sample and to enhance the quench field. With the exception of the final step from a 1.5 nm gold film to 2.0 nm, the magnitude of the surface resistance increased substantially with gold film thickness. The nature of the surface resistance field-dependence appeared to be roughly independent from the gold layer thickness. This initial study provides new perspectives and suggests avenues for optimizing and designing surfaces for resonant cavities in particle accelerators and quantum information applications.
arXiv (Cornell University) · 2023-05-04 · 1 citations
preprintOpen accessSenior authorSurface structures on radio-frequency (RF) superconductors are crucially important in determining their interaction with the RF field. Here we investigate the surface compositions, structural profiles, and valence distributions of oxides, carbides, and impurities on niobium (Nb) and niobium-tin (Nb3Sn) in situ under different processing conditions. We establish the underlying mechanisms of vacuum baking and nitrogen processing in Nb and demonstrate that carbide formation induced during high-temperature baking, regardless of gas environment, determines subsequent oxide formation upon air exposure or low-temperature baking, leading to modifications of the electron population profile. Our findings support the combined contribution of surface oxides and second-phase formation to the outcome of ultra-high vacuum baking (oxygen processing) and nitrogen processing. Also, we observe that vapor-diffused Nb3Sn contains thick metastable oxides, while electrochemically synthesized Nb3Sn only has a thin oxide layer. Our findings reveal fundamental mechanisms of baking and processing Nb and Nb3Sn surface structures for high-performance superconducting RF and quantum applications
Enhanced Surface Superconductivity of Niobium by Zirconium Doping
Physical Review Applied · 2023-07-28 · 7 citations
articleOpen accessSuperconducting radio-frequency (SRF) cavities currently rely on niobium (Nb), and could benefit from a higher-${T}_{c}$ surface, which would enable a higher operating temperature, lower surface resistance, and higher maximum fields. Surface zirconium (Zr) doping is one option for improvement, which has not previously been explored, likely because bulk alloy experiments showed only mild ${T}_{c}$ enhancements of 1--2 K relative to Nb. Our ab initio results reveal a more nuanced picture: an ideal bcc Nb-Zr alloy would have ${T}_{c}$ over twice that of niobium, but displacements of atoms away from the high-symmetry bcc positions due to the Jahn-Teller-Peierls effect almost completely eliminates this enhancement in typical disordered alloy structures. Ordered Nb-Zr alloy structures, in contrast, are able to avoid these atomic displacements and achieve higher calculated ${T}_{c}$ up to a theoretical limit of 17.7 K. Encouraged by this, we tested two deposition methods: a physical-vapor Zr deposition method, which produced Nb-Zr surfaces with ${T}_{c}$ values of 13.5 K, and an electrochemical deposition method, which produced surfaces with a possible 16-K ${T}_{c}$. An rf test of the highest-${T}_{c}$ surface showed a mild reduction in BCS surface resistance relative to Nb, demonstrating the potential value of this material for RF devices. Finally, our Ginzburg-Landau theory calculations show that realistic surface doping profiles should be able to reach the maximum rf fields necessary for next-generation applications, such as the ground-breaking LCLS-II accelerator. Considering the advantages of Nb-Zr compared to other candidate materials such as ${\mathrm{Nb}}_{3}\mathrm{Sn}$ and Nb-Ti-N, including a simple phase diagram with relatively little sensitivity to composition, and a stable, insulating ${\mathrm{Zr}\mathrm{O}}_{2}$ native oxide, we conclude that Nb-Zr alloy is an excellent candidate for next-generation, high-quality-factor superconducting rf devices.
Recent grants
Frequent coauthors
- 91 shared
Georg Hoffstaetter
Cornell University
- 76 shared
V. Veshcherevich
- 71 shared
Fumio Furuta
Fermi National Accelerator Laboratory
- 67 shared
Peter Quigley
- 67 shared
Daniel Hall
- 62 shared
Sam Posen
Fermi National Accelerator Laboratory
- 59 shared
James Maniscalco
- 45 shared
Daniel Gonnella
Labs
SRFPI
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Awards & honors
- Alfred P. Sloan Research Fellow, 2008-2012
- Graduate and Professional Student Assembly (GPSA) Faculty Aw…
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