arXiv subjects

A. Xu

Publications and source records attributed to A. Xu.

5 recordsLinked to original sources

arXiv2025

Critical Current, Lengthwise Fluctuations, and Flux Jumps in REBCO CC: A Torque Magnetometry Study up to 45 T

Rare Earth Barium Copper Oxide (REBCO) coated conductors (CCs) have emerged for future high field magnets in fields and temperatures inaccessible for Nb based superconductors. However, their exceptionally high current densities pose challenges for low temperature characterization. This paper presents the design and implementation of a simple torque magnetometer that is particularly suitable for characterizing REBCO CC. It details the construction and underlying physics, with a particular emphasis on its capability to assess the angular critical currents Ic in high magnetic fields and low temperatures. The study includes characterizations of multiple REBCO samples from different manufacturers, performed under magnetic fields up to 45 T, demonstrating the exceptional capabilities of REBCO CCs in extreme fields. The results revealed significant lengthwise Ic variations, especially in tapes cut from the edges of 12 mm-wide production tapes compared to those cut from the center. These variations are most pronounced when the field is near the ab plane. Importantly, flux jumps are observed in samples with thick REBCO layers and thin stabilizers, underscoring the potential thermal instabilities. These findings provide valuable insights into the performance of REBCO tapes under extreme magnetic fields, highlighting their relevance for high-field magnet and nuclear fusion applications.

cond-mat.supr-con↗

arXiv2019

Physics case for an LHCb Upgrade II - Opportunities in flavour physics, and beyond, in the HL-LHC era

The LHCb Upgrade II will fully exploit the flavour-physics opportunities of the HL-LHC, and study additional physics topics that take advantage of the forward acceptance of the LHCb spectrometer. The LHCb Upgrade I will begin operation in 2020. Consolidation will occur, and modest enhancements of the Upgrade I detector will be installed, in Long Shutdown 3 of the LHC (2025) and these are discussed here. The main Upgrade II detector will be installed in long shutdown 4 of the LHC (2030) and will build on the strengths of the current LHCb experiment and the Upgrade I. It will operate at a luminosity up to $ 2 \times 10^{34} \rm cm^{-2}s^{-1}$, ten times that of the Upgrade I detector. New detector components will improve the intrinsic performance of the experiment in certain key areas. An Expression Of Interest proposing Upgrade II was submitted in February 2017. The physics case for the Upgrade II is presented here in more depth. $CP$-violating phases will be measured with precisions unattainable at any other envisaged facility. The experiment will probe $b\to s \ell^+\ell^-$ and $b\to d \ell^+\ell^-$ transitions in both muon and electron decays in modes not accessible at Upgrade I. Minimal flavour violation will be tested with a precision measurement of the ratio of $B(B^0\toμ^+μ^-)/B(B_s^0\to μ^+μ^-)$. Probing charm $CP$ violation at the $10^{-5}$ level may result in its long sought discovery. Major advances in hadron spectroscopy will be possible, which will be powerful probes of low energy QCD. Upgrade II potentially will have the highest sensitivity of all the LHC experiments on the Higgs to charm-quark couplings. Generically, the new physics mass scale probed, for fixed couplings, will almost double compared with the pre-HL-LHC era; this extended reach for flavour physics is similar to that which would be achieved by the HE-LHC proposal for the energy frontier.

hep-ex↗

arXiv2017

Breakthrough Listen Follow-up of the Reported Transient Signal Observed at the Arecibo Telescope in the Direction of Ross 128

We undertook observations with the Green Bank Telescope, simultaneously with the 300m telescope in Arecibo, as a follow-up of a possible flare of radio emission from Ross 128. We report here the non-detections from the GBT observations in C band (4-8 GHz), as well as non-detections in archival data at L band (1.1-1.9 GHz). We suggest that a likely scenario is that the emission comes from one or more satellites passing through the same region of the sky.

astro-ph.SR↗

arXiv2011

On the large role of weak uncorrelated pinning introduced by BZO nanorods at low temperatures in REBCO thin films

REBa$_{2}$Cu$_{3}$O$_{x}$ films can achieve remarkably high critical current density values by the incorporation of insulating nanoparticles. A particular interesting case concerns BaZrO$_{3}$ (BZO) nanorods, whose strongly correlated effect is seen at high temperatures. Here we investigate the field, temperature and angular dependence of the critical current density over a wide temperature range from 4.2 K to 77 K, and magnetic fields up to 31 T. We show that the correlated c-axis pinning of BZO nanorods becomes progressively less obvious at lower temperature. Indeed at 4.2 K and fields up to 31 T, the only correlated pinning is for fields parallel to the film plane. We interpret the change as being due to significant contributions from dense but weak pins that thermal fluctuations render ineffective at high temperatures but which become strong at lower temperatures.

cond-mat.supr-con↗

arXiv2009

Weak-link behavior of grain boundaries in superconducting Ba(Fe1-xCox)2As2 bicrystals

We show that despite the low anisotropy, strong vortex pinning and high irreversibility field Hirr close to the upper critical field Hc2 of Ba(Fe1-xCox)2As2, the critical current density Jgb across [001] tilt grain boundaries (GBs) of thin film Ba(Fe1-xCox)2As2 bicrystals is strongly depressed, similar to high-Tc cuprates. Our results suggest that weak-linked GBs are characteristic of both cuprates and pnictides because of competing orders, low carrier density, and unconventional pairing symmetry.

cond-mat.supr-con↗