We present a systematic investigation of muon-stopping states in superconductors that reportedly exhibit spontaneous magnetic fields below their transition temperatures due to time-reversal symmetry ...breaking. These materials include elemental rhenium, several intermetallic systems, and Sr_{2}RuO_{4}. We demonstrate that the presence of the muon leads to only a limited and relatively localized perturbation to the local crystal structure, while any small changes to the electronic structure occur several electron volts below the Fermi energy, leading to only minimal changes in the charge density on ions close to the muon. Our results imply that the muon-induced perturbation alone is unlikely to lead to the observed spontaneous fields in these materials, whose origin is more likely intrinsic to the time-reversal symmetry-broken superconducting state.
We present the results of muon-spin relaxation (SR) measurements on antiferromagnetic and ferromagnetic spin chains. In antiferromagnetic CuF2(pyz) we identify a transition to long range magnetic ...order taking place at K, allowing us to estimate a ratio with the intrachain exchange of and the ratio of interchain to intrachain exchange coupling as . The ferromagnetic chain Sm(hfac)3(boaDTDA)n undergoes an ordering transition at K, seen via a broad freezing of dynamic fluctuations on the muon (microsecond) timescale and implying . The ordered radical moment continues to fluctuate on this timescale down to 0.3 K, while the Sm moments remain disordered. In contrast, the radical spins in La(hfac)3(boaDTDA)n remain magnetically disordered down to T = 0.1 K suggesting .
We present the results of muon-spin spectroscopy ( +SR) measurements on the molecular spin ladder system (Hpip)2CuBr4(1−x)Cl4x, Hpip = (C5H12N). Using transverse field +SR we are able to identify ...characteristic behaviour in each of the regions of the phase diagram of the x = 0 strong-rung spin ladder system (Hpip)2CuBr4. Comparison of our results to those of the dimer-based molecular magnet Cu(pyz)(gly)(ClO4) shows several common features. We locate the crossovers in partially disordered (Hpip)2CuBr4(1−x)Cl4x (x = 0.05), where a region of behaviour intermediate between quantum disordered and Luttinger liquid-like is identified. Our interpretation of the results incorporates an analysis of the probable muon stopping states in (Hpip)2CuBr4 based on density functional calculations and suggests how the muon plus its local distortion can lead to a local probe unit with good sensitivity to the magnetic state. Using longitudinal field +SR we compare the dynamic response of the x = 1 strong-rung material (Hpip)2CuCl4 to that of the strong-leg material (C7H10N)2CuBr4 (known as DIMPY) and demonstrate that our results are in agreement with predictions based on interacting fermionic quasiparticle excitations in these materials.
We present the results of x-ray scattering and muon-spin relaxation (SR) measurements on the iron-pnictide compound FeCrAs. Polarized non-resonant magnetic x-ray scattering results reveal the 120° ...periodicity expected from the suggested three-fold symmetric, non-collinear antiferromagnetic structure. SR measurements indicate a magnetically ordered phase throughout the bulk of the material below K. There are signs of fluctuating magnetism in a narrow range of temperatures above involving low-energy excitations, while at temperatures well below behaviour characteristic of freezing of dynamics is observed, likely reflecting the effect of disorder in our polycrystalline sample. Using density functional theory we propose a distinct muon stopping site in this compound and assess the degree of distortion induced by the implanted muon.
We report the results of muon-spin spectroscopy (μ+ SR) measurements on the staggered molecular spin chain pym- Cu(NO3)2 (H2O)2 (pym = pyrimidine), a material previously described using sine-Gordon ...field theory. Zero-field μ+ SR reveals a long range magnetically ordered ground state below a transition temperature TN = 0.23 (1) K. Using longitudinal-field (LF) μ+ SR we investigate the dynamic response in applied magnetic fields 0 < B < 500 mT and find evidence for ballistic spin transport. Our LF μ+ SR measurements on the chiral spin chain Cu (pym) (H2O)4 SiF6 ⋅ H2O instead demonstrate one-dimensional spin diffusion, and the distinct spin transport in these two systems suggests that additional anisotropic interactions play an important role in determining the nature of spin transport in S = 1/2 antiferromagnetic chains.
Significant progress has recently been made in calculating muon stopping sites using density functional theory. The technique aims to address two of the most common criticisms of the muon-spin ...spectroscopy (μ+SR) technique, namely, where in the sample does the muon stop, and what is its effect on its local environment. We have designed and developed a program called MuFinder that enables users to carry out these calculations through a simple graphical user interface (GUI). The procedure for calculating muon sites by generating initial muon positions, relaxing the structures, and then clustering and analysing the resulting candidate sites, can be done entirely within the GUI. The local magnetic field at the muon site can also be computed, allowing the connection between the muon sites obtained and experiment to be made. MuFinder will make these computations significantly more accessible to non-experts and help to establish muon site calculations as a routine part of μ+SR experiments.
Program Title: MuFinder
CPC Library link to program files:https://doi.org/10.17632/pwwt7p9hv8.1
Developer's repository link:https://gitlab.com/BenHuddart/mufinder
Licensing provisions: GPLv3
Programming language: Python
Nature of problem: To automate the process of calculating muon stopping sites using density functional theory, thereby making these calculations accessible to non-experts.
Solution method: A Python-based graphical user interface (GUI) through which users can calculate muon stopping sites using the structural relaxation method. The program makes use of newly-developed algorithms for generating candidate initial muon positions and for clustering muon positions obtained from the structural relaxations. Analysis of the muon sites, including calculation of the local dipolar magnetic field, is also possible within the GUI.
We present longitudinal-field muon-spin relaxation (LF μ SR) measurements on two systems that stabilize a skyrmion lattice (SkL): Cu2 OSeO3 , and Cox Zny Mn20−x−y for (x , y) = (10 , 10) , (8, 9), ...and (8, 8). We find that the SkL phase of Cu2 OSeO3 exhibits emergent dynamic behavior at megahertz frequencies, likely due to collective excitations, allowing the SkL to be identified from the μ SR response. From measurements following different cooling protocols and calculations of the muon stopping site, we suggest that the metastable SkL is not the majority phase throughout the bulk of this material at the fields and temperatures where it is often observed. The dynamics of bulk Co8 Zn9 Mn3 are well described by ≃ 2 GHz excitations that reduce in frequency near the critical temperature, while in Co8 Zn8 Mn4 we observe similar behavior over a wide range of temperatures, implying that dynamics of this kind persist beyond the SkL phase.
We present the results of muon-spin relaxation (Formula: see textSR) measurements on antiferromagnetic and ferromagnetic spin chains. In antiferromagnetic CuF
(pyz) we identify a transition to long ...range magnetic order taking place at Formula: see text K, allowing us to estimate a ratio with the intrachain exchange of Formula: see text and the ratio of interchain to intrachain exchange coupling as Formula: see text. The ferromagnetic chain Sm(hfac)
(boaDTDA)
undergoes an ordering transition at Formula: see text K, seen via a broad freezing of dynamic fluctuations on the muon (microsecond) timescale and implying Formula: see text. The ordered radical moment continues to fluctuate on this timescale down to 0.3 K, while the Sm moments remain disordered. In contrast, the radical spins in La(hfac)
(boaDTDA)
remain magnetically disordered down to T = 0.1 K suggesting Formula: see text.
We present the results of a muon-spin relaxation (μ+SR) investigation of the crystalline organic radical compound 4-(2-benzimidazolyl)-1,2,3,5-dithiadiazolyl (HbimDTDA), in which we demonstrate the ...hysteretic magnetic switching of the system that takes place at T=(274 ± 11)K caused by a structural phase transition. Muon-site analysis using electronic structure calculations suggests a range of candidate muon stopping sites. The sites are numerous and similar in energy but, significantly, differ between the two structural phases of the material. Despite the difference in the sites, the muon remains a faithful probe of the transition, revealing a dynamically-fluctuating magnetically disordered state in the low-temperature structural phase. In contrast, in the high temperature phase the relaxation is caused by static nuclear moments, with rapid electronic dynamics being motionally narrowed from the muon spectra.
We report spatially resolved small-angle neutron-scattering measurements of the conical and skyrmion states of a bulk single crystal of nickel-substituted Cu2OSeO3, with a nominal concentration of Ni ...of 14%. We observe a significant spatial dependence of the structure of these magnetic states, characterized by increased disorder and misalignment with respect to the applied field as we approach the edge of the sample. Remarkably, the edge skyrmion state is also characterized by an extended stability towards lower temperatures. Surprisingly, in the same region of the sample, the metastable skyrmion state did not show simple decay. Instead, only a fraction of the scattered intensity appeared to decay, and the intensity therefore did not approach zero during our measurements. We suggest that the increased local disorder and the coexistence of conical and skyrmion states, induced by demagnetization effects at the edge of the sample, are responsible for the increased stability of this skyrmion state. We also infer that the unclear metastable behavior of the skyrmion lattice at the edge of the sample is due to the local geometry of the sample, which induces coexistence of different skyrmion states whose lifetimes are superimposed and difficult to separate in the data.
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