arXiv subjects

A. D. Hillier

Publications and source records attributed to A. D. Hillier.

At least 19 recordsLinked to original sources

arXiv2026

Absolute intensity measurement of pulsed muon beams using in-beam activation

The absolute number of negative muons contained in a beam is essential for many experiments at accelerator facilities, but determining it in pulsed beams has been difficult, particularly at high intensities. The method utilizing the yield of the $β$ delayed $γ$ rays from the residual nuclei after the muon nuclear capture reaction has recently been developed to determine the muon number in the pulsed muon beam. In particular, the in-beam activation method employs isotopes with short lifetimes, enabling the beam intensity to be measured over a short period with irradiating muon beams. However, only a limited number of isotopes have reliable measurements of production branching ratios (BRs), which are required to determine the absolute muon number in the pulsed beam. To search for new candidate isotopes that are suitable for in-beam activation method, the production branching ratio after the muon nuclear capture reaction was measured for natural abundance Cu, Zn, and Ag. Considering the strength of the BR, the muon capture probability, the practical detection efficiency of the detector, and rarity of the target material in the surrounding structures, the reaction $^\mathrm{nat}$Ag ($μ^-, ν_μx$) $^{107m}$Pd is found to be a useful reference for the muon number calibration.

physics.ins-det↗

arXiv2026

Molecular effects in low-energy muon transfer from muonic hydrogen to oxygen

In the present study we determine from the available experimental data the cross section of muon transfer to molecular oxygen at low energies with account of the oxygen molecule structure. Building on an earlier work, the results highlight the role of the molecular structure effects and signifcantly improve the agreement with theoretical calculations of the muon transfer rate. An effcient computational model of the kinetics of processes involving muonic hydrogen atoms in gaseous mixture of H2 and O2 is developed and analyzed. The model is applied in the description of the FAMU experiment for the measurement of the hyperfine splitting in muonic hydrogen and the Zemach radius of the proton.

physics.atom-ph↗

arXiv2025

First operation of the FAMU experiment at the RIKEN-RAL high intensity muon beam facility

The FAMU experiment, supported and funded by the Italian Institute of Nuclear Physics (INFN) and by the Science and Technology Facilities Council (STFC), aims to perform the first measurement of the ground-state hyperfine splitting (1S-hfs) of muonic hydrogen ($μH$). This quantity is highly sensitive to the proton's Zemach radius $R_Z$. An experimental determination of $R_Z$ provides significant constraints on the parametrization of the proton form factors as well as on theoretical models describing the proton's electromagnetic structure. Following years of technological and methodological development, the FAMU experiment began operations in 2023 at Port 1 of the RIKEN-RAL muon beam line at the ISIS Neutron and Muon Source facility (Didcot, UK). In this paper, we first describe the unique detection technique employed by FAMU to determine the 1S-hfs of muonic hydrogen, followed by a detailed presentation of the final experimental layout. Finally, we report the first outcome from the 2023 commissioning run and from the initial physics runs performed in 2023 and 2024.

physics.atom-ph↗

arXiv2025

Unconventional superconductivity in a non-centrosymmetric $α$-Mn alloy NbTaOs$_{2}$

Non-centrosymmetric superconductors have emerged as a fascinating avenue for exploring unconventional superconductivity. Their broken inversion and time-reversal symmetries make them prime candidates for realizing the intrinsic superconducting diode effect (SDE). In this work, we synthesize the ternary non-centrosymmetric $α$-Mn alloy NbTaOs$_{2}$ and conduct a comprehensive investigation of its superconducting properties through resistivity, magnetization, specific heat and muon spin rotation/relaxation ($μ$SR) techniques. Our transverse field-$μ$SR and specific heat results provide evidence of a moderately coupled, fully-gaped superconducting state. Zero field-$μ$SR measurements reveal a subtle increase in the relaxation rate below the transition temperature, suggesting time reversal symmetry breaking in the superconducting ground state of NbTaOs$_{2}$.

cond-mat.supr-con↗

arXiv2025

Time reversal symmetry breaking and s-wave superconductivity in CaPd2Ge2: A $μ$SR study

${\rm CaPd_2Ge_2}$ which crystallizes in ${\rm ThCr_2Si_2}$-type body-centered tetragonal structure exhibits superconductivity below the critical temperature $T_{\rm c} = 1.69$~K\@. We have investigated the superconducting gap structure and time reversal symmetry of the ground state in ${\rm CaPd_2Ge_2}$ by means of muon spin relaxation and rotation ($μ$SR) measurements. Our analysis of $μ$SR data collected in transverse magnetic field reveals BCS superconductivity with a single-band $s$-wave singlet pairing and an isotropic energy gap having the value $2Δ(0)/k_{\rm B}T_{\rm c} = 3.50(1)$. Further, an increased relaxation rate in zero field $μ$SR asymmetry spectra below $T_{\rm c} $ provides evidence for the presence of a spontaneous magnetic field in the superconducting state revealing that the time-reversal symmetry is broken in ${\rm CaPd_2Ge_2}$.

cond-mat.supr-con↗

arXiv2025

Spin dynamics and magnetic excitations of quasi-1D spin chain Ca$_3$ZnMnO$_6$

To reveal the structure-property relationship in quasi-one-dimensional (1D) spin-chain system Ca$_3$ZnMnO$_6$, we present comprehensive results, combining basic physical characterizations such as muon spin relaxation/rotation ($μ$SR), neutron powder diffraction (NPD), inelastic neutron scattering (INS), and theoretical calculations. Ca$_3$ZnMnO$_6$ features a dominant intrachain coupling $J_1$ and two distinct interchain interactions $J_2$ and $J_3$, and it undergoes antiferromagnetic ordering below $T_{\mathrm{N}}=25$~K, as revealed by dc magnetic susceptibility and specific-heat measurements. Zero-field $μ$SR shows persistent spin dynamics below $T_{\mathrm{N}}$, suggesting unconventional magnetic excitations in the ordered state. NPD results indicate a commensurate magnetic ground state with a propagation vector $\mathbf{k}=0$, where the Mn spins lie in the $ab$-plane. INS spectra display dispersive magnetic excitations extending up to about 5~meV, with an energy gap smaller than 0.5~meV. Notably, these spectra exhibit three-dimensional (3D) gapped features rather than the expected 1D behavior, yet spin-wave dispersion analysis confirms an underlying quasi-1D energy hierarchy. We discuss this apparent paradox of 3D-like magnetic excitations in a quasi-1D system in terms of the energy hierarchy modified by nonmagnetic-ion substitution and finite-temperature first-principles calculations. We also suggest that Ca$_3$ZnMnO$_6$ could be a potential candidate for an M-type altermagnet.

cond-mat.str-el↗

arXiv2024

$μ$SR study on noncentrosymmetric superconductor NbGe$_{\mathbf{2}}$

We report a muon spin relaxation ($μ$SR) study on polycrystalline noncentrosymmetric superconductor NbGe$_2$~with the superconducting transition temperature $T_c=2.0\sim2.1$~K. Zero-field $μ$SR~experiment indicates the absence of spontaneous magnetic field in the superconducting state, showing the preservation of time-reversal symmetry in the superconducting state. Transverse-field $μ$SR~experiment is performed to map the phase diagram of NbGe$_2$, from which clear evidence of both type-I and type-II superconductivity is obtained. More importantly, we clearly delineate the region in the phase diagram where type-I and type-II superconductivity coexist.

cond-mat.supr-con↗

arXiv2024

Magnetic ground state and excitations in mixed 3$d$-4$d$ quasi-1D spin-chain oxide Sr$_3$NiRhO$_6$

Entanglement of spin and orbital degrees of freedom, via relativistic spin-orbit coupling, in 4$d$ transition metal oxides can give rise to a variety of novel quantum phases. A previous study of mixed 3$d$-4$d$ quasi-1D spin-chain oxide Sr$_3$NiRhO$_6$ using the magnetization measurements by Mohapatra et al. [Phys. Rev. B 75, 214422 (2007)] revealed a partially disordered antiferromagnetic (PDA) structure below 50 K [Mohapatra et al, Phys. Rev. B 75, 214422 (2007)]. We here report the magnetic ground state and spin-wave excitations in Sr$_3$NiRhO$_6$, using muon spin rotation and relaxation ($μ$SR), and neutron (elastic and inelastic) scattering techniques. Our neutron diffraction study reveals that in the magnetic structure of Sr$_3$NiRhO$_6$, Rh$^{4+}$ and Ni$^{2+}$ spins are aligned ferromagnetically in a spin-chain, with moments along the crystallographic $c$-axis. However, spin-chains are coupled antiferromanetically in the $ab$-plane. $μ$SR reveals the presence of oscillations in the asymmetry-time spectra below 50 K, supporting the long-range magnetically ordered ground state. Our inelastic neutron scattering study reveals gapped quasi-1D magnetic excitations with a large ratio of gap to exchange interaction. The observed spin-wave spectrum could be well fitted with a ferromagnetic isotropic exchange model (with $J = 3.7 $ meV) and single ion anisotropy ($D=10$ meV) on the Ni$^{2+}$ site. The magnetic excitations survive up to 85 K, well above the magnetic ordering temperature of $\sim 50$ K, also indicating a quasi-1D nature of the magnetic interactions in Sr$_3$NiRhO$_6$.

cond-mat.str-el↗

arXiv2024

Magnetic structure and crystal field states of antiferromagnetic CeNiGe$_3$: Neutron scattering and $μ$SR investigations

We present the results of microscopic investigations of antiferromagnetic CeNiGe$_3$, using neutron powder diffraction (NPD), inelastic neutron scattering (INS), and muon spin relaxation ($μ$SR) measurements. CeNiGe$_3$ crystallizes in a centrosymmetric orthorhombic crystal structure (space group: $Cmmm$) and undergoes antiferromagnetic (AFM) ordering. The occurrence of long-range AFM ordering at $T_{\rm N} \approx 5.2$~K is confirmed by magnetic susceptibility, heat capacity, neutron diffraction, and $μ$SR measurements. The NPD data characterize the AFM state with an incommensurate helical magnetic structure having a propagation vector $k$ = (0, 0.41, 1/2). In addition, INS measurements at 10~K identified two crystal electric field (CEF) excitations at 9.17~meV and 18.42~meV. We analyzed the INS data using a CEF model for an orthorhombic environment of Ce$^{3+}$ ($J=5/2$) and determined the CEF parameters and ground state wavefunctions of CeNiGe$_3$. Moreover, zero-field $μ$SR data for CeNiGe$_3$ at $T< T_{\rm N}$ show long-range AFM ordering with three distinct oscillation frequencies corresponding to three different internal fields at the muon sites. The internal fields at the muon-stopping sites have been further investigated using density functional theory calculations.

cond-mat.str-el↗

arXiv2024

The muon beam monitor for the FAMU experiment: design, simulation, test and operation

FAMU is an INFN-led muonic atom physics experiment based at the RIKEN-RAL muon facility at the ISIS Neutron and Muon Source (United Kingdom). The aim of FAMU is to measure the hyperfine splitting in muonic hydrogen to determine the value of the proton Zemach radius with accuracy better than 1%.The experiment has a scintillating-fibre hodoscope for beam monitoring and data normalisation. In order to carry out muon flux estimation, low-rate measurements were performed to extract the single-muon average deposited charge. Then, detector simulation in Geant4 and FLUKA allowed a thorough understanding of the single-muon response function, crucial for determining the muon flux. This work presents the design features of the FAMU beam monitor, along with the simulation and absolute calibration measurements in order to enable flux determination and enable data normalisation.

physics.ins-det↗

arXiv2024

Time-Reversal Symmetry Breaking in Re-Based Kagome Lattice Superconductor

We investigated the Re-based kagome superconductor Re$_2$Zr through various measurements, including resistivity, magnetization, specific heat, and muon spin rotation and relaxation spectroscopy. These results suggest that Re$_2$Zr is a moderately coupled potential two-gap superconductor. Zero-field muon relaxation data indicate the possible presence of a time-reversal symmetry-breaking state in the superconducting ground state. Our investigation identifies Re$_{2}$Zr as a new unconventional superconductor with a potential complex order parameter that warrants considerable experimental and theoretical interest.

cond-mat.supr-con↗

arXiv2024

Magnetic skin effect in Pb(Fe$_{1/2}$Nb$_{1/2}$)O$_3$

Relaxor-ferroelectrics display exceptional dielectric properties resulting from the underlying random dipolar fields induced by strong chemical inhomogeneity. An unusual structural aspect of relaxors is a skin-effect where the near-surface region in single crystals exhibit structures and critical phenomena that differ from the bulk. Relaxors are unique in that this skin effect extends over a macroscopic lengthscale of $\sim$ 100$μ$m whereas usual surface layers only extend over a few unit cells (or $\sim$ nm). We present a muon spectroscopy study of Pb(Fe$_{1/2}$Nb$_{1/2}$)O$_{3}$ (PFN) which displays ferroelectric order, including many relaxor-like dielectric properties such as a frequency broadened dielectric response, and antiferromagnetism with spatially short-range polar correlations and hence can be termed a multiferroic. In terms of the magnetic behavior determined by the Fe$^{3+}$ ($S=5/2$, $L\approx0$) ions, PFN has been characterized as a unique example of a "cluster spin-glass". We use variable momentum muon spectroscopy to study the depth dependence of the slow magnetic relaxations in a large 1 cm$^{3}$ crystal of PFN. Zero-field positive muon spin relaxation is parameterized using a stretched exponential, indicative of a distribution of relaxation rates of the Fe$^{3+}$ spins. This bandwidth of frequencies changes as a function of muon momentum, indicative of a change in the Fe$^{3+}$ relaxation rates as a function of muon implantation depth in our single crystal. Using negative muon elemental analysis, we find small-to-no measurable change in the Fe$^{3+}$/Nb$^{5+}$ concentration with depth implying that chemical concentration alone cannot account for the change in the relaxational dynamics. PFN displays an analogous magnetic skin effect reported to exist in the structural properties of relaxor-ferroelectrics.

cond-mat.str-el↗

arXiv2024

Evidence of quantum spin liquid state in a Cu$^{2+}$-based $S = 1/2$ triangular lattice antiferromagnet

The layered triangular lattice owing to $1:2$ order of $B$ and $B'$ sites in the triple perovskite $A_3 B B'_2$O$_9$ family provides an enticing domain for exploring the complex phenomena of quantum spin liquids (QSLs). We report a comprehensive investigation of the ground state properties of Sr$_3$CuTa$_2$O$_9$ that belongs to the above family, by employing magnetization, specific heat, and muon spin relaxation ($μ$SR) experiments down to the lowest temperature of 0.1~K. Analysis of the magnetic susceptibility indicates that the spin-lattice is a nearly isotropic $S = 1/2$ triangular lattice. We illustrate the observation of a gapless QSL, in which conventional spin ordering or freezing effects are absent, even at temperatures more than two orders of magnitude smaller than the exchange energy ($J_{\rm CW}/k_{\rm B} \simeq -5.04$~K). Magnetic specific heat in zero-field follows a power law, $C_{\rm m} \sim T^η$, below 1.2~K with $η\approx 2/3$, which is consistent with a theoretical proposal of the presence of spinon Fermi surface. Below 1.2~K, the $μ$SR relaxation rate shows no temperature dependence, suggesting persistent spin dynamics as expected for a QSL state. Delving deeper, we also analyze longitudinal field $μ$SR spectra revealing strong dynamical correlations in the spin-disordered ground state. All of these highlight the characteristics of spin entanglement in the QSL state.

cond-mat.str-el↗

arXiv2024

Probing the superconducting gap structure of ScRuSi via $μ$SR and first-principles calculations

In this study, we present a thorough investigation into the superconducting state of the ruthenium-based ternary equiatomic compound ScRuSi. Our analysis combines experimental techniques, including muon spin rotation/relaxation ($μ$SR) and low-temperature resistivity measurements, with theoretical insights derived from first-principles calculations. The low-temperature resistivity measurements reveal a distinct superconducting phase transition in the orthorhombic structure of ScRuSi at a critical temperature ($T_\text{C}$) of $2.5$ K. Further, the TF-$μ$SR analysis yields a gap-to-critical-temperature ratio of $2Δ/k_\mathrm{B}T_\mathrm{C} = 2.71$, a value consistent with results obtained from previous heat capacity measurements. The temperature dependence of the superconducting normalized depolarization rate is fully described by the isotropic $s$-wave gap model. Additionally, zero-field $μ$SR measurements indicate that the relaxation rate remains nearly identical below and above $T_\text{C}$. This observation strongly suggests the preservation of time-reversal symmetry within the superconducting state. By employing the McMillan-Allen-Dynes equation, we calculate a $T_\text{C}$ of $2.11$ K from first-principles calculations within the density functional theory framework. This calculated value aligns closely with the experimentally determined critical temperature. The coupling between the low-frequency phonon modes and the transition metal d-orbital states play an important role in governing the superconducting pairing in ScRuSi. The combination of experimental and theoretical approaches provides a comprehensive microscopic understanding of the superconducting nature of ScRuSi, offering insights into its critical temperature, pairing symmetry, and the underlying electron-phonon coupling mechanism.

cond-mat.supr-con↗

arXiv2024

Inelastic neutron scattering and muon spin relaxation investigations of the deuterated Kondo lattices CeNiSnD$ _x $

CeNiSn is a Kondo semimetal where a gap opens at low temperatures due to hybridization between 4$f$ and conduction electrons, but a full insulating state fails to develop. Upon the insertion of hydrogen, long range magnetic order is induced. Here we report zero-field muon-spin relaxation and inelastic neutron scattering measurements of polycrystalline samples of the deuterides CeNiSnD$_x$ ($x$=1.0, 1.8). The muon-spin relaxation results confirm magnetic ordering in the whole sample of CeNiSnD below around 4.7 K, while inelastic neutron scattering reveals two well-defined crystalline-electric field (CEF) excitations at around 13 meV and 34 meV in CeNiSnD, and 5 meV and 27 meV for CeNiSnD$_{1.8}$. These results suggest that hydrogenation leads to the localization of the Ce-4$f$ electrons, giving rise to long-range magnetic order. We propose CEF level schemes for both systems, which predict a ground state moment of 0.96$μ_{\rm B}$/Ce within the $ab$-plane for CeNiSnD$_{1.8}$ and a saturated moment of 1.26$μ_{\rm B}$/Ce along the easy $c$ axis for CeNiSnD, that account for the observed magnetic properties.

cond-mat.str-el↗

arXiv2024

Investigating the Proton Structure: The FAMU experiment

The article gives the motivations for the measurement of the hyperfine splitting (hfs) in the ground state of muonic hydrogen to explore the properties of the proton at low momentum transfer. It summarizes these proposed measurement methods and finally describes the FAMU experiment in more detail.

physics.atom-ph↗

arXiv2024

Broken time-reversal symmetry in a new non-centrosymmetric superconductor Re8NbTa

Re-based superconductors provide a rich platform for the study of unconventional superconductivity. We have investigated the superconducting properties of Re$_{8}$NbTa, a new noncentrosymmetric cubic ($α$-Mn structure) rhenium-based ternary superconductor using transport, magnetization, specific heat, and muon spin rotation/relaxation ($μ$SR) measurements. Specific heat and transverse field $μ$SR measurements suggest moderately coupled fully gapped superconductivity, well described by BCS theory. However, our zero-field $μ$SR measurements reveal a small internal field onsetting around the superconducting T$_c$, indicating that the superconducting order parameter breaks the time-reversal symmetry.

cond-mat.supr-con↗

arXiv2023

Status of the detector setup for the FAMU experiment at RIKEN-RAL for a precision measurement of the Zemach radius of the proton in muonic hydrogen

The FAMU experiment at RIKEN-RAL is a muonic atom experiment with the aim to determine the Zemach radius of the proton by measuring the 1s hyperfine splitting in muonic hydrogen. The activity of the FAMU Collaboration in the years 2015-2023 enabled the final optimisation of the detector-target setup as well as the gas working condition in terms of temperature, pressure and gas mixture composition. The experiment has started its data taking in July 2023. The status of the detector setup for the 2023 experimental runs, for the beam characterisation and muonic X-ray detection in the 100-200 keV energy range, is presented and discussed.

physics.atom-ph↗