arXiv subjects

James S. Lord

Publications and source records attributed to James S. Lord.

9 recordsLinked to original sources

arXiv2026

Radiation damage and recovery of SiPMs exposed to low-momentum muon beams for innovative scintillation-based muon beam monitors

Silicon PhotoMultipliers (SiPMs) are among the standard devices for light detection in scintillator detectors. Some detector designs require these devices to be placed in radioactive areas or directly exposed to particle beams. Specifically, the damage from exposure to low-momentum muons is particularly important, as they have a high energy deposit and they have several applications in material science and particle physics. Eight SiPM models from different manufacturers were exposed to 29 MeV/c positive muons, with an intensity of O(10^6) muons/s for 13 hours, maximising the charge deposited in the depletion region. I-V curves were measured before/after irradiation and annealing; DCR and crosstalk analyses are ongoing, to extract dark count rate and optical crosstalk. Despite showing a measurable increase in dark current, the devices preserved operational stability. Thermal annealing has also been tested by heating the devices at 125°C, showing a recovery of the optimal SiPM features. This work is part of a collaboration between the University of Pavia and ISIS, to develop detectors for low-momentum muons (< 40 MeV/c), based on thin scintillators.

physics.ins-det↗

arXiv2026

The scintillating fibre beam monitor for the CHNet-MAXI experiment at ISIS: characterisation of the light collection and detection system

The CHNet-MAXI experiment at ISIS aims at a non-destructive isotopic characterisation of lead samples, based on muonic atom formation. A custom scintillation-based detector is used for the reconstruction of the time and space distributions of the low-energy muon beam employed in the experiment. The beam monitor consists of two orthogonal planes of 10 cm-long, 3 mm $\times$ 3 mm square scintillating fibres, totalling 64 fibres, read out by four arrays of 16 Hamamatsu S14160-3015PS SiPMs each. The optical performance of the detector has been characterised, focusing on the SiPM behaviour, and also checking and correcting the uniformity of the light response across channels with a $^{60}$ Co source. A first detector test with protons at the CNAO-XPR has been carried out. The results of the Geant4 simulation of the detector, fundamental for the detector operation and data analysis, are also presented. All these activities are preliminary to the final detector operation in the CHNet-MAXI experiment at the RIKEN-RAL Port4 decay muon beamline at ISIS.

physics.ins-det↗

arXiv2026

Discovery of Quasi One Dimensional Superconductivity in PtPb3Bi

Quasi one dimensional materials provide a compelling platform where reduced dimensionality stabilizes intertwined topological and superconducting phases. Here we report superconductivity in a new Bi based quasi 1D compound, PtPb3Bi, which hosts a nontrivial electronic structure. It exhibits type II superconductivity below 3.01(1) K. Heat capacity and transverse field muon spin rotation relaxation (muSR) measurements demonstrate a fully gapped isotropic s wave state with moderate electron phonon coupling, while zero field muSR confirms the preservation of time reversal symmetry (TRS). Transport measurements reveal low carrier mobility with diffusive normal state transport. Electronic structure calculations show strong dispersion along the quasi 1D direction and relatively flatter bands in the transverse plane, giving rise to pronounced Fermi surface nesting in the kx-ky plane. Consistent with this, the compound undergoes a charge density wave transition at 280(1) K. The flow of Wannier charge centers, together with surface state dispersion, establishes nontrivial band topology. These results identify PtPb3Bi as a new quasi 1D superconductor with nontrivial electronic structure and a promising candidate for topological superconductivity.

cond-mat.supr-con↗

arXiv2024

Multi-condensate lengths with degenerate excitation gaps in BaNi$_2$As$_2$ revealed by muon spin relaxation study

The recently discovered (Ba,Sr)Ni$_2$As$_2$ family provides an ideal platform for investigating the interaction between electronic nematicity and superconductivity. Here we report the muon spin relaxation ($μ$SR) measurements on BaNi$_2$As$_2$. Transverse-field $μ$SR experiments indicate that the temperature dependence of superfluid density is best fitted with a single-band $s$-wave model. On the other hand, the magnetic penetration depth $λ$ shows magnetic field dependence, which contradicts with the single-band fully-gapped scenario. Zero-field $μ$SR experiments indicate the absence of spontaneous magnetic field in the superconducting state, showing the preservation of time-reversal symmetry in the superconducting state. Our $μ$SR experiments suggest that BaNi$_2$As$_2$ is a fully-gapped multiband superconductor. The superconducting gap amplitudes of each band are nearly the same while different bands exhibit different coherence lengths. The present work helps to elucidate the controversial superconducting property of this parent compound, paving the way for further research on doping the system with Sr to enhance superconductivity.

cond-mat.supr-con↗

arXiv2011

Behavior of hydrogen ions, atoms, and molecules in alpha-boron studied using density functional calculations

We examine the behaviour of hydrogen ions, atoms and molecules in alpha-boron using density functional calculations. Hydrogen behaves as a negative-U centre, with positive H ions preferring to sit off-center on inter-layer bonds and negative H ions sitting preferably at in-plane sites between three B12 icosahedra. Hydrogen atoms inside B12 icosahedral cages are unstable, drifting off-center and leaving the cage with only a 0.09 eV barrier. While H0 is extremely mobile (diffusion barrier 0.25 eV), H+ and H- have higher diffusion barriers of 0.9 eV. Once mobile these defects will combine, forming H2 in the interstitial void space, which will remain trapped in the lattice until high temperatures. Based on these results we discuss potential differences for hydrogen behaviour in beta-boron, and compare with experimental muon-implantation data.

cond-mat.mtrl-sci↗

arXiv2006

A magnetic analog of the isotope effect in cuprates

We present extensive magnetic measurements of the (Ca_xLa_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_{3}O_{y} (CLBLCO) system with its four different families (x) having a Tc^max(x) variation of 28% and minimal structural changes. For each family we measured the Neel temperature, the anisotropies of the magnetic interactions, and the spin glass temperature. Our results exhibit a universal relation Tc=c*J*n_s for all families, where c~1, J is the in plane Heisenberg exchange, and n_s is the carrier density. This relates cuprate superconductivity to magnetism in the same sense that phonon mediated superconductivity is related to atomic mass.

cond-mat.supr-con↗

arXiv2002

Common energy scale for magnetism and superconductivity in underdoped cuprates: a MuSR investigation of (CaLa)(BaLa)CuO

We characterize the spontaneous magnetic field, and determine the associated temperature Tg, in the superconducting state of Ca(x)La(1-x)Ba(1.75-x)La(0.25+x)Cu(3)O(y) using zero and longitudinal field MuSR measurements for various values of x and y. Our major findings are: (I) Tg and Tc are controlled by the same energy scale, (II) the phase separation between hole poor and hole rich regions is a microscopic one, and (III) spontaneous magnetic fields appear gradually with no moment size evolution.

cond-mat.supr-con↗

arXiv2002

Dynamics at T -> 0 in the half-integer isotropic high spin molecules

We investigate the dynamical spin-spin auto-correlation function of the isotropic high spin molecules CrCu$_{6}$ (S=9/2), CrNi$_{6}$ (S=15/2) and CrMn$_{6}$ (S=27/2), using magnetization, $μ$SR and NMR measurements. We find that the field autocorrelation time $τ$ of the molecule's spin at zero and low fields is nearly temperature independent as $T\to 50$ mK. The high temperatures $τ$ is very different between the molecules. Surprisingly, it is identical ($\sim$ 10 nsec) at low temperature. This suggests that $τ$ is governed by hyperfine interactions.

cond-mat.mes-hall↗

arXiv2001

Probing exotic spin correlations by Muon Spin depolarization measurements with applications to spin glass dynamics

We develop a new method to probe the local spin dynamic autocorrelation function, using magnetic field dependent muon depolarization measurements. We apply this method to muSR experiments in the dilute Heisenberg spin glass AgMn(p at. %) at T>T_g where the correlations of the Mn local magnetic moment are strongly non-exponential. Our results clearly indicate that the dynamics of this spin glass cannot be described by a distribution of correlation times. Therefore, we analyze the data assuming a local spin correlation function which is the product of a power law times a cutoff function. The concentration and temperature dependence of the parameters of this function are determined. Our major conclusion is that in the temperature region close to Tg the correlation function is dominated by an algebraic relaxation term.

cond-mat.str-el↗