arXiv subjects

S. J. Blundell

Publications and source records attributed to S. J. Blundell.

At least 55 records · Page 3Linked to original sources

arXiv2017

Coexistence of magnetism and superconductivity in separate layers of the iron-based superconductor Li_{1-x}Fe_{x}(OH)Fe_{1-y}Se

The magnetic properties attributed to the hydroxide layer of Li1-xFex(OH)Fe1-ySe have been elucidated by the study of superconducting and nonsuperconducting members of this family. Both ac magnetometry and muon spin relaxation measurements of nonsuperconductors find a magnetic state existing below approximately 10 K which exhibits slow relaxation of magnetization. This magnetic state is accompanied by a low-temperature heat capacity anomaly present in both superconducting and nonsuperconducting variants suggesting that the magnetism persists into the superconducting state. The estimated value of magnetic moment present within the hydroxide layer supports a picture of a glassy magnetic state, probably comprising clusters of iron ions of varying cluster sizes distributed within the lithium hydroxide layer.

cond-mat.supr-con↗

arXiv2017

Quantum-critical spin dynamics in a Tomonaga-Luttinger liquid studied with muon-spin relaxation

We demonstrate that quantum-critical spin dynamics can be probed in high magnetic fields using muon-spin relaxation ($μ^{+}$SR). Our model system is the strong-leg spin ladder bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY). In the gapless Tomonaga-Luttinger liquid phase we observe finite-temperature scaling of the $μ^{+}$SR $1/T_1$ relaxation rate which allows us to determine the Luttinger parameter $K$. We discuss the benefits and limitations of local probes compared with inelastic neutron scattering.

cond-mat.str-el↗

arXiv2016

Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi

We present the results of transverse-field muon-spin rotation measurements on an epitaxially grown 40 nm-thick film of MnSi on Si(111) in the region of the field-temperature phase diagram where a skyrmion phase has been observed in the bulk. We identify changes in the quasistatic magnetic field distribution sampled by the muon, along with evidence for magnetic transitions around $T\approx 40$ K and 30 K. Our results suggest that the cone phase is not the only magnetic texture realized in film samples for out-of-plane fields.

cond-mat.str-el↗

arXiv2016

Emergence, causation and storytelling: condensed matter physics and the limitations of the human mind

The physics of matter in the condensed state is concerned with problems in which the number of constituent particles is vastly greater than can be easily comprehended. The inherent physical limitations of the human mind are fundamental and restrict the way in which we can interact with and learn about the universe. This presents challenges for developing scientific explanations that are met by emergent narratives, concepts and arguments that have a non-trivial relationship to the underlying microphysics. By examining examples within condensed matter physics, and also from cellular automata, I show how such emergent narratives efficiently describe elements of reality.

physics.hist-ph↗

arXiv2016

Novel magnetism on a honeycomb lattice in a-RuCl3 studied by muon spin rotation

Muon spin rotation measurements have been performed on a powder sample of a-RuCl3, a layered material which previously has been proposed to be a quantum magnet on a honeycomb lattice close to a quantum spin liquid ground state. Our data reveal two distinct phase transitions at 11 K and 14 K which we interpret as originating from the onset of three-dimensional order and in-plane magnetic order, respectively. We identify, with the help of density functional theory calculations, likely muon stopping sites and combine these with dipolar field calculations to show that the two measured muon rotation frequencies are consistent with two inequivalent muon sites within a zig-zag antiferromagnetic structure proposed previously.

cond-mat.str-el↗

arXiv2016

Muon-spin relaxation study of the double perovskite insulators Sr$_{2}B$OsO$_6$ ($B={\rm Fe, Y, In}$)

We present the results of zero-field muon-spin relaxation measurements made on the double perovskite insulators Sr$_{2}B$OsO$_6$ ($B={\rm Fe, Y, In}$). Spontaneous muon-spin precession indicative of quasistatic long range magnetic ordering is observed in Sr$_{2}$FeOsO$_6$ within the AF1 antiferromagnetic phase for temperatures below $T_{\rm N}=135 \pm 2~{\rm K}$. Upon cooling below $T_2 \approx 67~{\rm K}$ the oscillations cease to be resolvable owing to the coexistence of the AF1 and AF2 phases, which leads to a broader range of internal magnetic fields. Using density functional calculations we identify a candidate muon stopping site within the unit cell, which dipole field simulations show to be consistent with the proposed magnetic structure. The possibility of incommensurate magnetic ordering is discussed for temperatures below $T_{\rm N}=53~{\rm K}$ and 25~K for Sr$_{2}$YOsO$_6$ and Sr$_{2}$InOsO$_6$, respectively.

cond-mat.str-el↗

arXiv2015

Robustness of superconductivity to structural disorder in Sr$_{0.3}$(NH$_2$)$_{y}$(NH$_3$)$_{1-y}$Fe$_2$Se$_2$

The superconducting properties of a recently discovered high $T_{\rm c}$ superconductor, Sr/ammonia-intercalated FeSe, have been measured using pulsed magnetic fields down to $4.\,\rm {K}$ and muon spin spectroscopy down to $1.5\,\rm K$. This compound exhibits intrinsic disorder resulting from random stacking of the FeSe layers along the $c$-axis that is not present in other intercalates of the same family. This arises because the coordination requirements of the intercalated Sr and ammonia moieties imply that the interlayer stacking (along $c$) involves a translation of either ${\bf a}/2$ or ${\bf b}/2$ which locally breaks tetragonal symmetry. The result of this stacking arrangement is that the Fe ions in this compound describe a body-centred tetragonal lattice in contrast to the primitive arrangement of Fe ions described in all other Fe-based superconductors. In pulsed magnetic fields the upper critical field $H_{\text{c2}}$ was found to increase on cooling with an upwards curvature that is commonly seen in type-II superconductors of a multi-band nature. Fitting the data to a two-band model and extrapolation to absolute zero gave a maximum upper critical field ${μ_0H_{\rm c2}(0)}$ of ${33(2)\,\rm T}$. A clear superconducting transition with a diamagnetic shift was also observed in transverse-field muon measurements at ${T_{\text c}\approx36.3(2)\,\rm K}$. These results demonstrate that robust superconductivity in these intercalated FeSe systems does not rely on perfect structural coherence along the $c$-axis.

cond-mat.supr-con↗

arXiv2015

Magnetic phase diagram of La$_{2-x}$Sr$_{x}$CoO$_{4}$ revised using muon-spin relaxation

We report the results of a muon-spin relaxation ($μ$SR) investigation of La$_{2-x}$Sr$_{x}$CoO$_{4}$, an antiferromagnetic insulating series which has been shown to support charge ordered and magnetic stripe phases and an hourglass magnetic excitation spectrum. We present a revised magnetic phase diagram, which shows that the suppression of the magnetic ordering temperature is highly sensitive to small concentrations of holes. Distinct behavior within an intermediate $x$ range ($0.2 \leq x \lesssim 0.6$) suggests that the putative stripe ordered phase extends to lower $x$ than previously thought. Further charge doping ($0.67 \leq x \leq 0.9$) prevents magnetic ordering for $T \gtrsim 1.5~{\rm K}$

cond-mat.str-el↗

arXiv2015

Magnetism in a family of $S = 1$ square lattice antiferromagnets Ni$X_2$(pyz)$_2$ ($X = $ Cl, Br, I, NCS; pyz = pyrazine)

The crystal structures of Ni$X_2$(pyz)$_2$ ($X$ = Cl (\textbf{1}), Br (\textbf{2}), I (\textbf{3}) and NCS (\textbf{4})) were determined at 298~K by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN$_4$$X_2$ units that are bridged by pyz ligands. The 2D layered motifs displayed by \textbf{1}-\textbf{4} are relevant to bifluoride-bridged [Ni(HF$_2$)(pyz)$_2$]$Z$F$_6$ ($Z$ = P, Sb) which also possess the same 2D layers. In contrast, terminal $X$ ligands occupy axial positions in \textbf{1}-\textbf{4} and cause a staggering of adjacent layers. Long-range antiferromagnetic order occurs below 1.5 (Cl), 1.9 (Br and NCS) and 2.5~K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and $g$ factor of \textbf{2}, \textbf{3} and \textbf{4} are measured by electron spin resonance where no zero--field splitting was found. The magnetism of \textbf{1}-\textbf{4} crosses a spectrum from quasi-two-dimensional to three-dimensional antiferromagnetism. An excellent agreement was found between the pulsed-field magnetization, magnetic susceptibility and $T_\textrm{N}$ of \textbf{2} and \textbf{4}. Magnetization curves for \textbf{2} and \textbf{4} calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. \textbf{3} is characterized as a three-dimensional antiferromagnet with the interlayer interaction ($J_\perp$) slightly stronger than the interaction within the two-dimensional [Ni(pyz)$_2$]$^{2+}$ square planes ($J_\textrm{pyz}$).

cond-mat.str-el↗

arXiv2015

Transverse field muon-spin rotation signature of the skyrmion lattice phase in Cu2OSeO3

We present the results of transverse field (TF) muon-spin rotation (muSR) measurements on Cu2OSeO3, which has a skyrmion lattice (SL) phase. We measure the response of the TF muSR signal in that phase along with the surrounding ones, and suggest how the phases might be distinguished using the results of these measurements. Dipole field simulations support the conclusion that the muon is sensitive to the SL via the TF lineshape and, based on this interpretation, our measurements suggest that the SL is quasistatic on a timescale tau > 100 ns.

cond-mat.str-el↗

arXiv2015

Spin diffusion in the low-dimensional molecular quantum Heisenberg antiferromagnet Cu(pyz)(NO$_{3}$)$_{2}$ detected with implanted muons

We present the results of muon-spin relaxation measurements of spin excitations in the one-dimensional quantum Heisenberg antiferromagnet Cu(pyz)(NO$_{3}$)$_{2}$. Using density-functional theory we propose muon sites and assess the degree of perturbation the muon probe causes on the system. We identify a site involving the muon forming a hydroxyl-type bond with an oxygen on the nitrate group that is sensitive to the characteristic spin dynamics of the system. Our measurements of the spin dynamics show that in the temperature range $T_{\mathrm{N}} J$ and that in the related two-dimensional system Cu(pyz)$_2$(ClO$_4$)$_{2}$.

cond-mat.str-el↗

arXiv2015

Spin waves and revised crystal structure of honeycomb iridate Na2IrO3

We report inelastic neutron scattering measurements on Na2IrO3, a candidate for the Kitaev spin model on the honeycomb lattice. We observe spin-wave excitations below 5 meV with a dispersion that can be accounted for by including substantial further-neighbor exchanges that stabilize zig-zag magnetic order. The onset of long-range magnetic order below 15.3 K is confirmed via the observation of oscillations in zero-field muon-spin rotation experiments. Combining single-crystal diffraction and density functional calculations we propose a revised crystal structure model with significant departures from the ideal 90 deg Ir-O-Ir bonds required for dominant Kitaev exchange.

cond-mat.str-el↗

arXiv2014

Probing the magnetic phases in the Ni-V alloy close to the disordered ferromagnetic quantum critical point with muSR

Zero (ZF) and longitudinal field (LF) muon spin relaxation data of the {\it d}-metal alloy Ni$_{1-x}$V$_{x}$ are presented at several vanadium concentrations $x$ below and above the critical $x_c \approx 11$% where long-range ferromagnetic (FM) order is suppressed. The clear single precession frequency observed for Ni, as expected for a homogeneous FM, changes to rather damped oscillation with small V substitution at $x=4$%, confirming magnetic inhomogeneities caused by the less magnetic V environments in the magnetic Ni matrix. Furthermore, local fields and spatial field distributions can be estimated to characterize different inhomogeneous regimes developing with $x$ in the FM phase of Ni$_{1-x}$V$_{x}$. In the regime of $x=7-10$% a Kubo Toyabe function well describes the low temperature ZF and LF asymmetry data supporting a static Gaussian field distribution. Closer to the quantum critical concentration a single scale static Kubo Toyabe function with one field distribution is not sufficient to describe the muon relaxation. These data indicate that further changes in spatial distributions and dynamics are evolving as expected within the critical regime of a disordered quantum critical point.

cond-mat.str-el↗

arXiv2014

The magneto-structural relationship in the tetrahedral spin chain oxide CsCoO$_2$

We have investigated the structural and magnetic transitions in CsCoO$_2$ using calorimetric measurements, neutron powder diffraction (NPD), density functional theory (DFT) calculations and muon-spin relaxation ($μ$SR) measurements. CsCoO$_2$ exhibits three-dimensional long-range antiferromagnetic (AFM) order at 424~K, resulting in antiferromagnetic alignment of chains of ferromagnetically ordered Co-Co spin dimers. Although there is no change in magnetic structure around a structural transition at $T^{*}=100$~K, the resulting bifurcation of corner-shared Co--O--Co bond angles causes a weakening of the AFM interaction for one set of bonds along the chains. Consequently, the system undergoes a complex freezing out of relaxation processes on cooling.

cond-mat.str-el↗

arXiv2014

Evidence for magnetic clusters in Ni$_{1-x}$V$_{x}$ close to the quantum critical concentration

The d-metal alloy Ni$_{1-x}$V$_{x}$ undergoes a quantum phase transition from a ferromagnetic ground state to a paramagnetic ground state as the vanadium concentration $x$ is increased. We present magnetization, ac-susceptibility and muon-spin relaxation data at several vanadium concentrations near the critical concentration $x_c \approx11.6%$ at which the onset of ferromagnetic order is suppressed to zero temperature. Below $x_c$, the muon data reveal a broad magnetic field distribution indicative of long-range ordered ferromagnetic state with spatial disorder. We show evidence of magnetic clusters in the ferromagnetic phase and close to the phase boundary in this disordered itinerant system as an important generic ingredient of a disordered quantum phase transition. In contrast, the temperature dependence of the magnetic susceptibility above $x_c$ is best described in terms of a magnetic quantum Griffiths phase with a power-law distribution of fluctuation rates of dynamic magnetic clusters. At the lowest temperatures, the onset of a short-range ordered cluster-glass phase is recognized by an increase in the muon depolarization in transverse fields and maxima in ac-susceptibility.

cond-mat.str-el↗

arXiv2014

An anisotropic local modification of crystal field levels in Pr-based pyrochlores: a muon-induced effect modelled using density functional theory

Muon spin relaxation measurements on some quantum spin ice candidate materials, the insulating pyrochlores Pr2B2O7 (B = Sn, Zr, Hf), have been performed for temperatures in the range 0.05-280 K. The results are indicative of a static distribution of magnetic moments which appears to grow on cooling and whose size at low temperatures is significantly larger than that expected for Pr nuclear moments. Using density functional theory we show how this effect can be explained via a hyperfine enhancement arising from a splitting of the non-Kramers doublet ground states on Pr ions close to the muon which itself causes a highly anisotropic distortion field. We provide a quantitative relationship between this effect and the measured temperature dependence of the muon relaxation and discuss the relevance of these observations to muon experiments in other frustrated magnetic materials.

cond-mat.str-el↗

arXiv2014

Stripe disorder and dynamics in the hole-doped antiferromagnetic insulator La5/3Sr1/3CoO4

We report on an investigation into the dynamics of the stripe phase of La5/3Sr1/3CoO4, a material recently shown to have an hour-glass magnetic excitation spectrum. A combination of magnetic susceptibility, muon-spin relaxation and nuclear magnetic resonance measurements strongly suggest that the physics is determined by a disordered configuration of charge and spin stripes whose frustrated magnetic degrees of freedom are strongly dynamic at high temperature and which freeze out in a glassy manner as the temperature is lowered. Our results broadly confirm a recent theoretical prediction, but show that the charge quenching remains incomplete well below the charge ordering temperature and reveal, in detail, the manner in which the magnetic degrees of freedom are frozen.

cond-mat.str-el↗

arXiv2013

The upper critical field of NaFe$_{1-x}$Co$_x$As superconductors

The upper critical field of NaFe$_{1-x}$Co$_x$As was measured from $x=0$ to $x=0.08$, with the magnetic field applied parallel ($H_{\rm c2}^\parallel$) and normal ($H_{\rm c2}^\perp$) to the planes. The data were fitted using one-band and two-band models. The orbital and paramagnetic components of the upper critical field were extracted for $H_{\rm c2}^\parallel$, which we find to to be strongly dominated by paramagnetic pair-breaking. In the parent compound the paramagnetic limit is equal to the value expected by BCS theory. However, substitution of Fe by Co leads to an enhancement above the BCS limit by a factor of approximately 1.6. In the over-doped region, we observe a significant convex curvature in $H_{\rm c2}^\perp$ at low temperatures, which we attribute to the two-band nature of the superconductivity and the asymmetry between the two bands. Additionally, we discuss the behavior of critical field anisotropy, coherence length $ξ$, and the penetration depth $λ$.

cond-mat.supr-con↗