Showing papers by "Anthony A. Amato published in 2010"

Immune-mediated necrotizing myopathy associated with statins.

Abstract: We report patients from two neuromuscular centers who were evaluated between the years 2000 and 2008 and met the following criteria: (1) proximal muscle weakness occurring during or after treatment with statins; (2) elevated serum creatine kinase (CK); (3) persistence of weakness and elevated CK despite discontinuation of the statin; (4) improvement with immunosuppressive agents; and (5) muscle biopsy showing necrotizing myopathy without significant inflammation. Twenty-five patients fulfilled our inclusion criteria. Twenty-four patients required multiple immunosuppressive agents. Fifteen patients relapsed after being tapered off immunosuppressive therapy. Exposure to statins prior to onset was significantly higher in patients with necrotizing myopathy (82%) as compared to those with dermatomyositis (18%), polymyositis (24%), and inclusion-body myositis (38%) seen in the same time period. The lack of improvement following discontinuation of statins, the need for immunosuppressive therapy, and frequent relapse when treatment was tapered suggest an immune-mediated etiology for this rare, statin-associated necrotizing myopathy.

The non-dystrophic myotonias: molecular pathogenesis, diagnosis and treatment

Abstract: The non-dystrophic myotonias are an important group of skeletal muscle channelopathies electrophysiologically characterized by altered membrane excitability. Many distinct clinical phenotypes are now recognized and range in severity from severe neonatal myotonia with respiratory compromise through to milder late-onset myotonic muscle stiffness. Specific genetic mutations in the major skeletal muscle voltage gated chloride channel gene and in the voltage gated sodium channel gene are causative in most patients. Recent work has allowed more precise correlations between the genotype and the electrophysiological and clinical phenotype. The majority of patients with myotonia have either a primary or secondary loss of membrane chloride conductance predicted to result in reduction of the resting membrane potential. Causative mutations in the sodium channel gene result in an abnormal gain of sodium channel function that may show marked temperature dependence. Despite significant advances in the clinical, genetic and molecular pathophysiological understanding of these disorders, which we review here, there are important unresolved issues we address: (i) recent work suggests that specialized clinical neurophysiology can identify channel specific patterns and aid genetic diagnosis in many cases however, it is not yet clear if such techniques can be refined to predict the causative gene in all cases or even predict the precise genotype; (ii) although clinical experience indicates these patients can have significant progressive morbidity, the detailed natural history and determinants of morbidity have not been specifically studied in a prospective fashion; (iii) some patients develop myopathy, but its frequency, severity and possible response to treatment remains undetermined, furthermore, the pathophysiogical link between ion channel dysfunction and muscle degeneration is unknown; (iv) there is currently insufficient clinical trial evidence to recommend a standard treatment. Limited data suggest that sodium channel blocking agents have some efficacy. However, establishing the effectiveness of a therapy requires completion of multi-centre randomized controlled trials employing accurate outcome measures including reliable quantitation of myotonia. More specific pharmacological approaches are required and could include those which might preferentially reduce persistent muscle sodium currents or enhance the conductance of mutant chloride channels. Alternative strategies may be directed at preventing premature mutant channel degradation or correcting the mis-targeting of the mutant channels.

Synthesis and crystal growth of Cs0.8(FeSe0.98)2: a new iron-based superconductor with Tc=27K

Abstract: We report on the synthesis of large single crystals of a new FeSe-layer superconductor Cs0.8(FeSe0.98)2. X-ray powder diffraction, neutron powder-diffraction and magnetization measurements have been used to compare the crystal structure and the magnetic properties of Cs0.8(FeSe0.98)2 with those of the recently discovered potassium intercalated system KxFe2Se2. The new compound Cs0.8(FeSe0.98)2 shows a slightly lower superconducting transition temperature (Tc=27.4 K) in comparison to 29.5 in K0.8(FeSe0.98)2). The volume of the crystal unit cell increases by replacing K by Cs - the c-parameter grows from 14.1353(13) A to 15.2846(11) A. For the so far known alkali metal intercalated layered compounds (K0.8Fe2Se2 and Cs0.8(FeSe0.98)2) the Tc dependence on the anion height (distance between Fe-layers and Se-layers) was found to be analogous to those reported for As-containing Fe-superconductors and Fe(Se1-xChx), where Ch=Te, S.

Pressure induced static magnetic order in superconducting FeSe1-x.

Abstract: We report on a detailed investigation of the electronic phase diagram of FeSe1-x under pressures up to 1.4 GPa by means of ac magnetization and muon-spin rotation. At a pressure ≃0.8 GPa the nonmagnetic and superconducting FeSe1-x enters a region where static magnetic order is realized above Tc and bulk superconductivity coexists and competes on short length scales with the magnetic order below Tc. For even higher pressures an enhancement of both the magnetic and the superconducting transition temperatures as well as of the corresponding order parameters is observed. These exceptional properties make FeSe1-x to be one of the most interesting superconducting systems investigated extensively at present. © 2010 The American Physical Society

Clinical findings in MuSK-antibody positive myasthenia gravis: a U.S. experience.

Abstract: We performed a retrospective chart review on 53 muscle-specific kinase antibody (MuSK-Ab)-positive myasthenia gravis (MG) patients at nine university-based centers in the U.S. Of these, 66% were Caucasian, 85% were women, and age of onset was 9-79 years. Twenty-seven patients were nonresponsive to anticholinesterase therapy. Myasthenia Gravis Foundation of America improvement status was achieved in 53% patients on corticosteroids, 51% with plasma exchange, and in 20% on intravenous immunoglobulin (IVIG). Thymectomy was beneficial in 7/18 patients at 3 years. Long-term (> or =3 years) outcome was very favorable in 58% of patients who achieved remission and/or minimal manifestation status. Overall, 73% improved. There was one MG-related death. This survey reinforces several cardinal features of MuSK-Ab-positive MG, including prominent bulbar involvement and anticholinesterase nonresponsiveness. Facial or tongue atrophy was rare. Most patients respond favorably to immunotherapy. The best clinical response was to corticosteroids and plasma exchange, and the poorest response was to IVIG. Long-term outcome is favorable in about 60% of cases.

Interferon-stimulated gene 15 (ISG15) conjugates proteins in dermatomyositis muscle with perifascicular atrophy.

Abstract: Dermatomyositis (DM) is an autoimmune disease of unknown cause primarily affecting skeletal muscle and skin. The characteristic muscle pathology of DM is notable for capillary abnormalities and small, abnormal appearing muscle fibers around the periphery of some muscle fascicles bordering the perimysial connective tissue, a lesion called perifascicular atrophy (PFA). Recent microarray and pathological studies have pointed towards a mechanism of tissue injury in DM associated with the overexpression of type 1 interferon-inducible genes. Over 85% of the highest expressed transcripts in muscle from DM compared to normal and other inflammatory myopathies are from genes known to be induced by type 1 interferons.1 Plasmacytoid dendritic cells (pDCs), cells that produce high levels of type 1 interferons, have infiltrated DM muscle1 and skin.2 Expression of the type 1 interferon-inducible protein MxA occurs in the characteristic sites of DM pathology, the perifascicular myofibers and capillaries. These findings suggest that injury to capillaries and myofibers may result directly from the intracellular overproduction of one or more type 1 interferon-inducible transcripts or proteins. Interferon-stimulated gene 15 (ISG15), a type 1 interferon-inducible protein, is an ubiquitin-like modifier with poorly understood function. The enzymatic pathway for ISG15 conjugation to a target protein involves three conjugating enzymes, the E1 Ube1L,3 the E2 Ube2L6,4 and the E3 HERC5,5, 6 and a deconjugating enzyme USP18.7 Immunoprecipitates of ISG15 from interferon-β stimulated HeLa cells contain many putative ISG15-conjugated proteins.8 The conjugation of ISG15 to proteins in a human tissue sample has only been reported in 3 endometrial and 2 colon cancer biopsy specimens;9 the identities of any such proteins are unknown. We previously reported that ISG15 is the single most overexpressed gene in DM muscle compared to both normal muscle and muscle from patients with other inflammatory myopathies.1 It is also highly expressed in circulating mononuclear blood cells of patients with DM.10 In this study, we have examined the ISG15 pathway in DM and other inflammatory myopathy muscle biopsy specimens and in cultured human skeletal muscle exposed to type 1 interferons.

Evidence for time-reversal symmetry breaking in superconducting PrPt4Ge12

Abstract: Zero- and longitudinal-field muon-spin-rotation $(\ensuremath{\mu}\text{SR})$ experiments were performed on the superconductors ${\text{PrPt}}_{4}{\text{Ge}}_{12}$ and ${\text{LaPt}}_{4}{\text{Ge}}_{12}$. In ${\text{PrPt}}_{4}{\text{Ge}}_{12}$ below ${T}_{c}$ a spontaneous magnetization with a temperature variation resembling that of the superfluid density appears. This observation implies time-reversal symmetry (TRS) breaking in ${\text{PrPt}}_{4}{\text{Ge}}_{12}$ below ${T}_{c}=7.9\text{ }\text{K}$. This remarkably high ${T}_{c}$ for an anomalous superconductor and the weak and gradual change in ${T}_{c}$ and of the related specific-heat anomaly upon La substitution in ${\text{La}}_{1\ensuremath{-}x}{\text{Pr}}_{x}{\text{Pt}}_{4}{\text{Ge}}_{12}$ suggests that the TRS breaking is due to orbital degrees of freedom of the Cooper pairs.

Evolution of two-gap behavior of the superconductor FeSe1-x.

Abstract: The superfluid density, rho{s}, of the iron chalcogenide superconductor, FeSe1-x, was studied as a function of pressure by means of muon-spin rotation. The analysis of rho{s}(T) within the two-gap scheme reveals that the effect on both, the transition temperature T{c} and rho{s}(0), is entirely determined by the band(s) where the large superconducting gap develops, while the band(s) with the small gap become practically unaffected.

Statin myopathy: a review of recent progress.

Abstract: Purpose of reviewStatins are commonly prescribed lipid-lowering medications that significantly reduce the risk of cardiovascular events. However, they can have myotoxic effects ranging in severity from myalgias to rhabdomyolysis. This review focuses on recent progress in defining the clinical featur

Permissive environment for B-cell maturation in myositis muscle in the absence of B-cell follicles.

Abstract: Myositis muscle contains antigen-matured B-cells and plasma cells. Myositis muscle biopsy specimens were examined for nodular collections of T-cells, B-cells, myeloid dendritic cells, plasma cells, and follicular dendritic cells. Immunoglobulin and B-cell-activating factor (BAFF) transcripts were quantitated. Laser-capture microdissection was used to isolate single plasma cells, and their immunoglobulin transcripts were sequenced. Dense inflammatory infiltrates contained histological elements of ectopic lymphoid tissue but not B-cell follicles. Immunoglobulin transcript sequence analysis demonstrated spatially distributed, clonally related B-cells and plasma cells, suggesting local maturation of B-cells into plasma cells in myositis muscle. Regions of dense cellular infiltrates in myositis muscle are sometimes areas of B-cell maturation into antibody-producing plasma cells. An atypical lymphoid histology, lacking concentrated collections of germinal-center-like B-cell follicles, is capable of antigen-stimulated clonal maturation of antibody-producing plasma cells.

The dorsal root ganglion under attack: the acquired sensory ganglionopathies.

Abstract: Acquired sensory ganglionopathies--or sensory neuronopathies--are a rare type of peripheral neuropathy characterised by damage to the sensory nerve cell bodies in the dorsal root ganglia. Subacute or chronic in onset, sensory ganglionopathies typically present with a non-length dependent pattern of large fibre sensory loss. The causes of this distinct clinical picture include paraneoplastic syndromes, immune mediated diseases, infections, as well as drug, toxin and excess vitamin exposure. Here we discuss the clinical and pathological features of acquired sensory ganglionopathies and focus on a practical approach to their diagnosis and management.

What determines quality of life in inclusion body myositis

Abstract: Background Quality of life (QoL) assessment allows healthcare professionals to appreciate the patient perspective of their disease. This can help us make a better choice from among the various ways we currently measure the severity of a muscle disease such as inclusion body myositis (IBM). However, we cannot assume that QoL in IBM is just related to disease severity as psychosocial factors may play an important role in determining QoL. Methods Sixty subjects with IBM had assessments of disease severity and concurrent assessment of mood and QoL using the Short-Form 36 (SF-36). Results There were significant reductions in Physical functioning, Role physical, General health and Social functioning domains of the SF-36. Functional disability was more indicative of the broader effects of IBM on SF-36 than was the muscle strength sum score. Mood was relatively independent of disease severity and had a different profile of effects on SF-36 domains. Up to 14% of the effect of functional disability on some aspects of QoL was mediated through mood. Conclusions The functional disability caused by IBM reduces QoL, but psychosocial factors such as mood affect QoL directly and by influencing the degree to which disease severity reduces QoL. Further study should follow the effects of IBM on QoL over time and look at the influence of other psychosocial factors. Such studies may point to psychosocial interventions that may help improve QoL in IBM even if the disease itself cannot be treated.

Microscopic study of the superconducting state of the iron pnictide RbFe 2 As 2 via muon spin rotation

Abstract: A study of the temperature and field dependence of the penetration depth λ of the superconductor RbFe2As2(Tc=2.52 K) was carried out by means of muon-spin rotation measurements. In addition to the zero-temperature value of the penetration depth λ(0)=267(5) nm, a determination of the upper critical field Bc2(0)=2.6(2) T was obtained. The temperature dependence of the superconducting carrier concentration is discussed within the framework of a multigap scenario. Compared to the other “122” systems which exhibit much higher Fermi level, a strong reduction in the large gap BCS ratio 2Δ/kBTc is observed. This is interpreted as a consequence of the absence of interband processes. Indications of possible pair-breaking effect are also discussed. © 2010 The American Physical Society

Positive muon spin rotation and relaxation measurements on the ferromagnetic superconductor UGe2 at ambient and high pressure

Abstract: Results of a detailed investigation of the ferromagnetic superconductor UGe2 using positive muon spin rotation and relaxation techniques are presented. The pressure and temperature dependences of the frequencies and related spin-spin relaxation rates show that the transition from the weakly to the strongly polarized magnetic (WP-SP) phases is still observable at TX?3?K under a pressure of 1.33(2) GPa. Thus this transition survives at higher pressures than previously believed. The temperature TX at 1.00(2) GPa corresponds to a thermodynamic phase transition rather than a crossover. No such statement can be given reliably at lower pressure. A substantial shrinking of the component along the easy axis of the diagonal hyperfine tensor, at the muon site where it is large, is observed in the SP phase relative to the WP phase. This corresponds to an appreciable decrease in the electronic density at the Fermi level in the SP phase. The investigation of the paramagnetic-ferromagnetic critical spin dynamics at ambient pressure and at 0.95(2) GPa shows that the simple one-band model is an oversimplification inconsistent with our critical spin-dynamics results. Data from specific heat, Fermi-surface studies, Hall effect, neutron form factor, and spectroscopic techniques supports this conclusion. Even at 0.95(2) GPa the conduction electrons are characterized by a small magnetic moment, relative to the bulk magnetization per uranium atom.

Magnetic properties of Ba2 HoSbO6 with a frustrated lattice geometry

Abstract: Received 8 December 2009; revised manuscript received 28 January 2010; published 23 February 2010 The study of geometrically frustrated magnets with unusual crystal field ground states offers the possibility of discovering new aspects of the physics of cooperative paramagnetic states. In the rare-earth double perovskite Ba2HoSbO6 the Ho 3+ ions occupy a face-centered cubic lattice, which frustrates near-neighbor antiferromagnetic coupling. We present a systematic experimental study of the system using dc magnetization, heat capacity, muon-spin relaxation, inelastic neutron scattering, neutron powder diffraction, and neutron polarization analysis. The bulk measurements at first point to spin liquid behavior, but the microscopic measurements indicate that this system is an example of a “nonmagnetic doublet” crystal field ground state, with low-lying excited states. Our measurements are interpreted using crystal field theory of Ho 3+ , including hyperfine coupling. We find that exchange and dipolar coupling are weak in this particular system and that the “mock spin liquid” behavior in fact reflects the unusual properties of the nonmagnetic doublet.

μSR study on antiferromagnetism of alkali-metal clusters incorporated in zeolite sodalite

Abstract: Na 4 3+ and K 4 3+ clusters arrayed in aluminosilicate sodalite are known to show antiferromagnetism below respective Neel temperatures T N =48 and ≃70 K. We newly prepared K–Rb alloy (K 3 Rb) 3+ clusters in addition to Na 4 3+ and K 4 3+ ones, and performed muon-spin relaxation (µSR) measurements. We find a large amplitude of precession signal in zero-field µSR spectra, indicating that antiferromagnetism takes place in the major volume of the samples. The T N is estimated from the temperature dependence of the precession signal to be ≃80 K for (K 3 Rb) 3+ . The internal field at the muon stopping site is estimated from the precession frequency to be ≃92, ≃142, and ≃155 Oe for Na 4 3+ , K 4 3+ , and (K 3 Rb) 3+ , respectively. Therefore, T N and the internal field are found to increase in the increasing order of atomic weight. These results are explained by the increase in the size of the s -electron wave function of clusters and the decrease in their potential depth.

Spin Glass ground state in Mn$_{1-x}$Co$_{x}$Si

Abstract: We report the discovery of a new spin glass ground state in the transition metal monosilicides with the B20 crystallographic structure. Magnetic, transport, neutron and muon investigation of the solid solution Mn$_{1-x}$Co$_{x}$Si have revealed a new dome in the phase diagram with evidence of antiferromagnetic interactions. For Mn rich compounds, a sharp decrease of the Curie temperature is observed upon Co doping and neutron elastic scattering shows that helimagnetic order of MnSi persists up to $x=0.05$ with a shortening of the helix period. For higher Co ($0.05

Spin-glass ground state in Mn 1-x Co x Si

Abstract: We report the discovery of a spin-glass ground state in the transition-metal monosilicides with the B20 crystallographic structure. Magnetic, transport, neutron, and muon investigations of the solid solution ${\text{Mn}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Si}$ have revealed a new dome in the phase diagram with evidence of antiferromagnetic interactions. For Mn-rich compounds, a sharp decrease in the Curie temperature is observed upon Co doping and neutron elastic scattering shows that helimagnetic order of MnSi persists up to $x=0.05$ with a shortening of the helix period. For higher Co $(0.05lxl0.90)$ concentrations, the Curie-Weiss temperature changes sign and the system enters a spin-glass state upon cooling (${T}_{g}=9\text{ }\text{K}$ for ${x}_{\text{Co}}=0.50$), due to chemical disorder. In this doping range, a minimum appears in the resistivity, attributed to scattering of conduction electron by localized magnetic moments.

Structural and Magnetic Nature for Fully Delithiated LixNiO2: Comparative Study between Chemically and Electrochemically Prepared Samples

Abstract: Structural and magnetic properties of two LixNiO2 samples with x ≤ 0.1 have been studied by a powder X-ray diffraction, magnetic susceptibility (χ), and muon-spin rotation/relaxation (μSR) measurements. One of the two samples was prepared by an electrochemical (EC) reaction in a nonaqueous lithium cell, whereas the other sample by a chemical (C) reaction in a HNO3 aqueous solution. Although the crystal structure of both C-Li0.01NiO2 and EC-Li0.10NiO2 samples were assigned as a mixture of a cubic close-packed phase and a hexagonal close-packed phase, the effective magnetic moment (μeff) of Ni ions for C-Li0.01NiO2 was estimated as μeff = 1.43 μB and was very close to that for EC-Li0.5NiO2 (μeff = 1.39 μB). This implies that the vacant tetrahedral or octahedral sites in C-Li0.01NiO2 are partially occupied by H+ ions. Actually, the zero-field μSR time spectrum in the paramagnetic state for C-Li0.01NiO2 exhibited a large relaxation compared to those for EC-Li0.5NiO2 and EC-Li0.10NiO2. Furthermore, a pryrolysi...

Absence of magnetic phase separation in MnSi under pressure

Abstract: We report muon spin spectroscopy data $(\ensuremath{\mu}\text{SR})$ obtained under hydrostatic pressure on a large single crystal of the itinerant helimagnet MnSi and recorded down to 0.235 K and up to 15.1 kbar. Up to the critical pressure ${p}_{c}=14.9(2)\text{ }\text{kbar}$, where the magnetic order is suppressed, the $\ensuremath{\mu}\text{SR}$ data unambiguously demonstrate that the ground state of the system is magnetic with no indication of any phase separation.

Muon spin rotation study of the magnetic penetration depth in the intercalated graphite superconductor CaC 6

Abstract: We report temperature- and magnetic field-dependent bulk muon spin rotation measurements in a $c$-axis-oriented superconductor ${\text{CaC}}_{6}$ in the mixed state. Using both a simple second-moment analysis and the more precise analytical Ginzburg-Landau model, we obtained a field-independent in-plane magnetic penetration depth ${\ensuremath{\lambda}}_{ab}(0)=72(3)\text{ }\text{nm}$. The temperature dependencies of the normalized muon spin relaxation rate and of the normalized superfluid density result to be identical and both are well represented by the clean limit BCS model with $2\ensuremath{\Delta}/{\text{k}}_{B}{T}_{c}=3.6(1)$, suggesting that ${\text{CaC}}_{6}$ is a fully gapped BCS superconductor in the clean limit regime.

Are anti-ganglioside antibodies of clinical value in multifocal motor neuropathy?

Abstract: Pestronk and colleagues1 first reported antibodies to GMl ganglioside in a treatable form of multifocal motor neuropathy (MMN) in 1988. In subsequent years, anti-GM1 and other ganglioside antibodies were reported in a variety of other disorders, including motor neuron disease (MND), Guillain-Barre syndrome, chronic inflammatory demyelinating polyneuropathy (CIDP), monoclonal gammopathy–associated neuropathy, and, occasionally, idiopathic neuropathy, mononeuritis multiplex, radiculopathy, and plexopathy.2,–,4 In MMN, the prevalence of anti-GM1 antibodies ranges widely from 20% to 85%,2,–,6 in part because there is no consensus on the most reproducible assay for this antibody, prompting different investigators to use different ELISA techniques. The prevalence of anti-GM2 antibodies is less well-studied but is generally considerably lower than that of anti-GM1, and many of those patients with MMN having anti-GM2 antibodies also carry anti-GM1.3,7 Anti-GM2 antibodies are found in other neuromuscular disorders as well, including MND, …

Short-range spin correlations in β -LiFeO2 from bulk magnetization, neutron diffraction, and μSR experiments

Abstract: Ryota Akiyama,1 Yutaka Ikedo,2,* Martin Mansson,3 Tatsuo Goko,4 Jun Sugiyama,2 Daniel Andreica,5 Alex Amato,6 Kittiwit Matan,1 and Taku J. Sato1,† 1Neutron Science Laboratory, Institute for Solid State Physics, University of Tokyo, 106-1 Shirakata, Tokai, Ibaraki 319-1106, Japan 2Toyota Central Research and Development Laboratories Inc., Nagakute, Aichi 480-1192, Japan 3Laboratory for Neutron Scattering, ETH Zurich and Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland 4TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 2A3 5Faculty of Physics, Babes-Bolyai University, 3400 Cluj-Napoca, Romania 6Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

μ+SR study on triangular antiferromagnet LiCrO2

Abstract: In order to elucidate the magnetic nature of the lithium chromium dioxide LiCrO2, in which the Cr3+ ions form a two-dimensional triangular lattice in the CrO2 plane, we have performed a positive muon-spin rotation and relaxation (μ+SR) experiment using a powder sample at the temperatures below 155 K. Weak-transverse-field measurements indicated the existence of a bulk antiferromagnetic (AF) transition at TN=61.2 K. The ZF-μ+SR measurements indicated that LiCrO2 is a pure paramagnet for T ≥ 62.5 K, since its internal magnetic field (Hint) can be explained by solely nuclear magnetic moments. This means that, contrary to previous suggestions by susceptibility and heat capacity measurements, no short-range order exists for T ≥ 62.5 K. However, ZF-μ+SR detected the change in Hint from a low-T static state to a high-T dynamic state at 115 K, most likely connected to a change in the position/motion of the Li ions.

μSR and NMR study on quantum critical point in quantum spin ladder (CH3)2CHNH3Cu(ClxBr1−x)3

Abstract: (CH3)2CHNH3Cu(ClxBr1-x)3 abbreviated IPACu(ClxBr1-x)3 is a mixed system of spin-gap compounds IPACuCl3 and IPACuBr3. This mixed system is reported by macroscopic measurements to show a magnetic order when the value of x is between the quantum critical points (QCPs) 0.44 and 0.87. We have investigated microscopically the ground state of the samples x = 0.40, 0.45 and 0.50 which are located near the QCP of xc = 0.44, by Muon Spin Relaxation (?SR) and NMR measurements. For x = 0.40, ?SR measurements in zero and longitudinal fields have shown that the spin fluctuation shows a significant slowing down with decreasing temperature, although the magnetic order does not appear down to T = 15 mK, indicating that the ground state is an exotic one rather than a gapped one. For x = 0.45 and 0.50, ?SR and NMR measurements do not show any magnetic order at low temperatures, indicating that the QCP is larger than the reported value of xc = 0.44.

μSR study on the ground state of bond-disordered spin gap system (CH3)2CHNH3Cu(Cl1−xBrx)3 (x=0.95, 0.88)

Abstract: Solid solution of the two spin gap systems (CH3)2CHNH3CuX3 (X=Cl, Br) was reported from results of macroscopic measurements to be gapless when fraction of Cl content x is in between 0.44 and 0.87, and otherwise remains gapped. We have investigated by μSR the ground state of Cl-rich gapped region x=0.95 and 0.88 to find that they are microscopically phase separated into two components, magnetic tiny islands and singlet sea surrounding them. With decreasing temperature, the characteristic frequency of spin fluctuation in islands showed a significant slow down. However, the slow down ceased around 1 K, below which the spin fluctuation spectrum did not change down to 15 mK. These results indicate that unlike the Br-rich system x 0.87 persists to be paramagnetic without showing a long-range order. This difference is considered to be originated in the robustness of Haldane gap against disorder.