arXiv subjects

Nathan Sitaraman

Publications and source records attributed to Nathan Sitaraman.

9 recordsLinked to original sources

arXiv2026

A Three-Fluid Model of Dissipation at Surfaces in Superconducting Radiofrequency Cavities

Experiments on superconducting cavities have found that under large RF fields the quality factor can improve with increasing field amplitude, a so-called ``anti-Q slope.'' We numerically solve the Bogoliubov-de Gennes equations at a superconducting surface in a parallel magnetic field, finding at large fields there are surface quasiparticle states with energies below the bulk superconducting gap that emerge and disappear as the field cycles. Modifying the standard two-fluid model, we introduce a ``three''-fluid model where we partition the normal fluid to consider continuum and surface quasiparticle states separately. We compute dissipation in a semi-classical theory of conductivity, where we provide physical estimates of elastic scattering times of Bogoliubov quasiparticles with point-like impurities having potential strengths informed from complementary \textit{ab initio} calculations of impurities in bulk niobium. We show, in this simple yet effective framework, how the relative scattering rates of surface and continuum quasiparticle states can play a role in producing an anti-Q slope while demonstrating how this model naturally includes a mechanism for turning the anti-Q slope on and off.

cond-mat.supr-con↗

arXiv2026

Synthesis and Characterization of Atomically-Sharp Superconductor-Dielectric Interface

Modification of superconductor-dielectric interfaces is known to strongly impact coherence times of superconducting quantum devices. This relationship is thought to arise from differences in the concentration of "two-level system" defects in the disordered dielectrics and superconductor-dielectric interfaces; these defects couple to electromagnetic modes in the device and cause dissipation. Zirconium oxide barrier layers on niobium have emerged as a promising pathway to low-loss interfaces in recent years, evidently due to the crystalline nature of these layers in comparison to the amorphous niobium native oxide. We explain the unique ability of zirconium oxide to form a crystalline layer, to maintain a sharp interface with metallic niobium, and to prevent niobium oxide re-growth in terms of the chemical properties of ZrO$_2$ and the Nb-Zr-O ternary system. We demonstrate a new method to grow air-stable zirconium oxide layers on niobium with a higher level of crystallinity and a sharper oxide-metal interface than previously shown, and provide the first comprehensive microscopic analysis of ZrO$_2$ capping layer properties. These developments pave the way toward vital performance advances in superconducting quantum devices.

cond-mat.mtrl-sci↗

arXiv2025

A Computational Picture of Hydride Formation and Dissipation In Nb SRF Cavities

Research 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.

cond-mat.supr-con↗

arXiv2025

$\textit{Ab initio}$ Theory of Eliminating Surface Oxides of Superconductors with Noble-Metal Encapsulation

Nanometer-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.

cond-mat.supr-con↗

arXiv2023

ZrNb(CO) RF superconducting thin film with high critical temperature in the theoretical limit

Superconducting radio-frequency (SRF) resonators are critical components for particle accelerator applications, such as free-electron lasers, and for emerging technologies in quantum computing. Developing advanced materials and their deposition processes to produce RF superconductors that yield nanoohms surface resistances is a key metric for the wider adoption of SRF technology. Here we report ZrNb(CO) RF superconducting films with high critical temperatures (Tc) achieved for the first time under ambient pressure. The attainment of a Tc near the theoretical limit for this material without applied pressure is promising for its use in practical applications. A range of Tc, likely arising from Zr doping variation, may allow a tunable superconducting coherence length that lowers the sensitivity to material defects when an ultra-low surface resistance is required. Our ZrNb(CO) films are synthesized using a low-temperature (100 - 200 C) electrochemical recipe combined with thermal annealing. The phase transformation as a function of annealing temperature and time is optimized by the evaporated Zr-Nb diffusion couples. Through phase control, we avoid hexagonal Zr phases that are equilibrium-stable but degrade Tc. X-ray and electron diffraction combined with photoelectron spectroscopy reveal a system containing cubic ZrNb mixed with rocksalt NbC and low-dielectric-loss ZrO2. We demonstrate proof-of-concept RF performance of ZrNb(CO) on an SRF sample test system. BCS resistance trends lower than reference Nb, while quench fields occur at approximately 35 mT. Our results demonstrate the potential of ZrNb(CO) thin films for particle accelerator and other SRF applications.

cond-mat.mtrl-sci↗

arXiv2022

Dissipation by surface states in superconducting RF cavities

Recent experiments on superconducting cavities have found that under large radio-frequency (RF) electromagnetic fields the quality factor can improve with increasing field amplitude, a so-called "anti-Q slope." Linear theories of dissipation break down under these extreme conditions and are unable to explain this behavior. We numerically solve the Bogoliubov-de Gennes equations at the surface of a superconductor in a parallel AC magnetic field, finding that at large fields there are quasiparticle surface states with energies below the bulk value of the superconducting gap. As the field oscillates, such states emerge and disappear with every cycle. We consider the dissipation resulting from inelastic quasiparticle-phonon scattering into these states and investigate the ability of this mechanism to explain features of the experimental observations, including the field dependence of the quality factor. We find that this mechanism is likely not the dominant source of dissipation and does not produce an anti-Q slope by itself; however, we demonstrate in a modified two-fluid model how these bound states can play a role in producing an anti-Q slope.

cond-mat.supr-con↗

arXiv2020

Analysis of Magnetic Vortex Dissipation in Sn-Segregated Boundaries in Nb$_3$Sn Superconducting RF Cavities

We study mechanisms of vortex nucleation in Nb$_3$Sn Superconducting RF (SRF) cavities using a combination of experimental, theoretical, and computational methods. Scanning transmission electron microscopy (STEM) image and energy dispersive spectroscopy (EDS) of some Nb$_3$Sn cavities show Sn segregation at grain boundaries in Nb$_3$Sn with Sn concentration as high as $\sim$35 at.\% and widths $\sim$3 nm in chemical composition. Using ab initio calculations, we estimate the effect excess tin has on the local superconducting properties of the material. We model Sn segregation as a lowering of the local critical temperature. We then use time-dependent Ginzburg-Landau theory to understand the role of segregation on magnetic vortex nucleation. Our simulations indicate that the grain boundaries act as both nucleation sites for vortex penetration and pinning sites for vortices after nucleation. Depending on the magnitude of the applied field, vortices may remain pinned in the grain boundary or penetrate the grain itself. We estimate the superconducting losses due to vortices filling grain boundaries and compare with observed performance degradation with higher magnetic fields. We estimate that the quality factor may decrease by an order of magnitude ($10^{10}$ to $10^9$) at typical operating fields if 0.03\% of the grain boundaries actively nucleate vortices. We additionally estimate the volume that would need to be filled with vortices to match experimental observations of cavity heating.

cond-mat.supr-con↗

arXiv2017

SRF Theory Developments from the Center for Bright Beams

We present theoretical studies of SRF materials from the Center for Bright Beams. First, we discuss the effects of disorder, inhomogeneities, and materials anisotropy on the maximum parallel surface field that a superconductor can sustain in an SRF cavity, using linear stability in conjunction with Ginzburg-Landau and Eilenberger theory. We connect our disorder mediated vortex nucleation model to current experimental developments of Nb$_3$Sn and other cavity materials. Second, we use time-dependent Ginzburg-Landau simulations to explore the role of inhomogeneities in nucleating vortices, and discuss the effects of trapped magnetic flux on the residual resistance of weakly- pinned Nb$_3$Sn cavities. Third, we present first-principles density-functional theory (DFT) calculations to uncover and characterize the key fundamental materials processes underlying the growth of Nb$_3$Sn. Our calculations give us key information about how, where, and when the observed tin-depletedregions form. Based on this we plan to develop new coating protocols to mitigate the formation of tin depleted regions.

physics.acc-ph↗

arXiv2016

In-vacuum scattered light reduction with cupric oxide surfaces for sensitive fluorescence detection

We demonstrate a simple and easy method for producing low-reflectivity surfaces that are ultra-high vacuum compatible, may be baked to high temperatures, and are easily applied even on complex surface geometries. Black cupric oxide (CuO) surfaces are chemically grown in minutes on any copper surface, allowing for low-cost, rapid prototyping and production. The reflective properties are measured to be comparable to commercially available products for creating optically black surfaces. We describe a vacuum apparatus which uses multiple blackened copper surfaces for sensitive, low-background detection of molecules using laser-induced fluorescence.

physics.ins-det↗