Vacancy defects and monopole dynamics in oxygen-deficient pyrochlores.

The idea of magnetic monopoles in spin ice has enjoyed much success at intermediate temperatures, but at low temperatures a description in terms of monopole dynamics alone is insufficient. Recently, numerical simulations were used to argue that magnetic impurities account for this discrepancy by introducing a magnetic equivalent of residual resistance in the system. Here we propose that oxygen deficiency is the leading cause of magnetic impurities in as-grown samples, and we determine the defect structure and magnetism in Y2Ti2O7-δ using diffuse neutron scattering and magnetization measurements. These defects are eliminated by oxygen annealing. The introduction of oxygen vacancies causes Ti(4+) to transform to magnetic Ti(3+) with quenched orbital magnetism, but the concentration is anomalously low. In the spin-ice material Dy2Ti2O7 we find that the same oxygen-vacancy defects suppress moments on neighbouring rare-earth sites, and that these magnetic distortions markedly slow down the long-time monopole dynamics at sub-Kelvin temperatures.

Magnetic monopoles in spin ice.

Electrically charged particles, such as the electron, are ubiquitous. In contrast, no elementary particles with a net magnetic charge have ever been observed, despite intensive and prolonged searches (see ref. 1 for example). We pursue an alternative strategy, namely that of realizing them not as elementary but rather as emergent particles-that is, as manifestations of the correlations present in a strongly interacting many-body system. The most prominent examples of emergent quasiparticles are the ones with fractional electric charge e/3 in quantum Hall physics. Here we propose that magnetic monopoles emerge in a class of exotic magnets known collectively as spin ice: the dipole moment of the underlying electronic degrees of freedom fractionalises into monopoles. This would account for a mysterious phase transition observed experimentally in spin ice in a magnetic field, which is a liquid-gas transition of the magnetic monopoles. These monopoles can also be detected by other means, for example, in an experiment modelled after the Stanford magnetic monopole search.

Magnetic Coulomb fields of monopoles in spin ice and their signatures in the internal field distribution.

Fractionalization-the breaking up of an apparently indivisible microscopic degree of freedom-is one of the most counterintuitive phenomena in many-body physics. Here we study its most fundamental manifestation in spin ice, the only known fractionalized magnetic compound in 3D: we directly visualize the 1/r(2) magnetic Coulomb field of monopoles that emerge as the atomic magnetic dipoles fractionalize. We analyze the internal magnetic field distribution, relevant for local experimental probes. In particular, we present new zero-field NMR measurements that exhibit excellent agreement with the calculated line shapes, noting that this experimental technique can in principle measure directly the monopole density in spin ice. The distribution of field strengths is captured by a simple analytical form that exhibits a low density of low-field sites-in apparent disagreement with reported muon spin rotation results. Counterintuitively, the density of low-field locations decreases as the local ferromagnetic correlations imposed by the ice rules weaken.

Magnetic monopole density and antiferromagnetic domain control in spin-ice iridates.

Magnetically frustrated systems provide fertile ground for complex behaviour, including unconventional ground states with emergent symmetries, topological properties, and exotic excitations. A canonical example is the emergence of magnetic-charge-carrying quasiparticles in spin-ice compounds. Despite extensive work, a reliable experimental indicator of the density of these magnetic monopoles is yet to be found. Using measurements on single crystals of Ho2Ir2O7 combined with dipolar Monte Carlo simulations, we show that the isothermal magnetoresistance is highly sensitive to the monopole density. Moreover, we uncover an unexpected and strong coupling between the monopoles on the holmium sublattice and the antiferromagnetically ordered iridium ions. These results pave the way towards a quantitative experimental measure of monopole density and demonstrate the ability to control antiferromagnetic domain walls using a uniform external magnetic field, a key goal in the design of next-generation spintronic devices.

Thermal quenches in spin ice.

We study the diffusion-annihilation process which occurs when spin ice is quenched from a high temperature paramagnetic phase deep into the spin-ice regime, where the excitations--magnetic monopoles--are sparse. We find that due to the Coulomb interaction between the monopoles, a dynamical arrest occurs, in which nonuniversal lattice-scale constraints impede the complete decay of charge fluctuations. This phenomenon is outside the reach of conventional mean-field theory for a two-component Coulomb liquid. We identify the relevant time scales for the dynamical arrest and propose an experiment for detecting monopoles and their dynamics in spin ice based on this nonequilibrium phenomenon.

Unconventional magnetization processes and thermal runaway in spin-ice Dy2Ti2O7.

We investigate the nonequilibrium behavior of the spin-ice Dy2Ti2O7 by studying its magnetization as a function of the field sweep rate. Below the enigmatic ''freezing'' temperature T(equil)≈600 mK, we find that even the slowest sweeps fail to yield the equilibrium magnetization curve and instead give an initially much flatter curve. For higher sweep rates, the magnetization develops sharp steps accompanied by similarly sharp peaks in the temperature of the sample. We ascribe the former behavior to the energy barriers encountered in the magnetization process, which proceeds via flipping of spins on filaments traced out by the field-driven motion of the gapped, long-range interacting magnetic monopole excitations. The peaks in temperature result from the released Zeeman energy not being carried away efficiently; the resulting heating triggers a chain reaction.

Role of defects in determining the magnetic ground state of ytterbium titanate.

Pyrochlore systems are ideally suited to the exploration of geometrical frustration in three dimensions, and their rich phenomenology encompasses topological order and fractional excitations. Classical spin ices provide the first context in which it is possible to control emergent magnetic monopoles, and anisotropic exchange leads to even richer behaviour associated with large quantum fluctuations. Whether the magnetic ground state of Yb2Ti2O7 is a quantum spin liquid or a ferromagnetic phase induced by a Higgs transition appears to be sample dependent. Here we have determined the role of structural defects on the magnetic ground state via the diffuse scattering of neutrons. We find that oxygen vacancies stabilise the spin liquid phase and the stuffing of Ti sites by Yb suppresses it. Samples in which the oxygen vacancies have been eliminated by annealing in oxygen exhibit a transition to a ferromagnetic phase, and this is the true magnetic ground state.

Peritoneal dialysis in nondiabetic patients older than 70 years: comparison with patients aged 40 to 60 years.

In all industrial countries, the number of elderly patients who need dialysis has increased in recent years. In the present study, we retrospectively analyzed two different age groups of nondiabetic peritoneal dialysis patients treated at the same unit by the same team of physicians and nurses with the same protocols. However, our purpose was to study possible differences in technique and survival rates, causes of dropout, complications, hospitalization rate, and everyday needs between the two groups. The results of 63 consecutive nondiabetic patients older than 70 years treated with continuous ambulatory peritoneal dialysis (CAPD) were compared with those of 86 nondiabetic patients aged 40 to 60 years treated during the same period. Patient survival was significantly worse in the elderly patients, but the observed to expected survival ratio with respect to age was similar. Technique survival was comparable in the two groups. Total hospitalization was 5,501 days (32 d/yr) in the elderly patients and 4,511 days (18 d/yr; P < 0.05) in the younger group. The peritonitis rate was 0.52 episodes/patient-year in the elderly patients and 0.37 episodes/patient-year in the younger patients (P < 0.002). The exit site infection rate was similar in the two groups (0.30 episodes/yr v0.29 episodes/yr). Other complications related to CAPD did not differ between the elderly and younger patients. Rehabilitation and biochemical data after 1 year of CAPD were similar in the two groups of patients. After 1 year of treatment, 12% of the younger patients and 43% of the elderly patients (P < 0.005) needed a partner for dialysis. Twenty-nine of 39 (74%) of the elderly patients and 30 of 53 (57%) of the younger patients considered their lifestyle acceptable after 1 year of dialysis. Thirty-four of 39 (87%) of the elderly patients and 32 of 53 (60%) of the younger patients (P < 0.02) rated their physical and social state after rehabilitation as better or comparable to that they had before terminal uremia.

Symptomatic accidental introduction of disinfectant electrolytic chloroxidizer solution into the peritoneal cavity of CAPD patients. Incidence and long-term effects on ultrafiltration.

In recent years the Y-set with disinfectant [Buoncristiani et al. 1983] has been used in several continuous ambulatory peritoneal dialysis (CAPD) centers. This system has considerably improved the short- and long-term results of CAPD [Maiorca et al. 1983, Scalamogna et al. 1990]. Accidental introduction of disinfectant during the exchange is a risk associated with this system, but at the present time we have little data pertaining to incidence complication and its possible effects on peritoneal function. The aim of the present study was to assess the incidence of acute symptomatic accidental introduction of disinfectant in CAPD patients on the Y system and to establish whether this complication is followed by any long-term changes in peritoneal ultrafiltration.

Fractal analysis of sampled profiles: systematic study.

A quantitative evaluation of the influence of sampling on the numerical fractal analysis of experimental profiles is of critical importance. Although this aspect has been widely recognized, a systematic analysis of the sampling influence is still lacking. Here we present the results of a systematic analysis of synthetic self-affine profiles in order to clarify the consequences of the application of a poor sampling (up to 1000 points) typical of scanning probe microscopy for the characterization of real interfaces and surfaces. We interpret our results in terms of a deviation and a dispersion of the measured exponent with respect to the "true" one. Both the deviation and the dispersion have always been disregarded in the experimental literature, and this can be very misleading if results obtained from poorly sampled images are presented. We provide reasonable arguments to assess the universality of these effects and propose an empirical method to take them into account. We show that it is possible to correct the deviation of the measured Hurst exponent from the "true" one and give a reasonable estimate of the dispersion error. The last estimate is particularly important in the experimental results since it is an intrinsic error that depends only on the number of sampling points and can easily overwhelm the statistical error. Finally, we test our empirical method calculating the Hurst exponent for the well-known 1+1 dimensional directed percolation profiles, with a 512-point sampling.

Phosphatidylcholine administration in continuous ambulatory peritoneal dialysis (CAPD) patients with reduced ultrafiltration.

To evaluate the possible role of oral phosphatidylcholine administration in improving peritoneal ultrafiltration sixteen continuous ambulatory peritoneal dialysis (CAPD) patients with a reduced ultrafiltration rate (less than 500 mL/4 h after a standard 3.86% glucose exchange) were studied. Three patients spontaneously stopped phosphatidylcholine due to gastric side effects. Three out of the remaining 13 patients showed a mild increase of standard ultrafiltration, which was not followed by any increase in daily dialysate output. No differences in glucose, sodium, potassium, urea and creatinine equilibration curves were seen. In conclusion, in our series oral phosphatidylcholine is not free of side effects, and its efficacy in improving ultrafiltration is scanty, with no clinical relevance in increasing daily ultrafiltration.

Beta-2 microglobulin in patients on peritoneal dialysis and hemodialysis.

Serum beta 2 microglobulin (beta 2 mu) levels were determined in 62 patients on chronic dialysis, divided according to the type of dialysis--cuprophane hemodialysis, chronic ambulatory peritoneal dialysis (CAPD), or CAPD started after 76 +/- 47 months on cuprophane hemodialysis--and to residual urine output greater than 400 mL/day or less than 10 mL/day. In addition, for patients on CAPD, peritoneal excretion, peritoneal clearance, and urinary excretion of the protein were determined. In anuric patients serum beta 2 mu levels were significantly higher in HD than in CAPD. In patients with residual urine output, serum concentrations of the microprotein were similar in HD and in CAPD. Significant differences were observed in beta 2 mu serum levels and peritoneal clearances in patients switched to CAPD from hemodialysis as compared to those starting with CAPD. Peritoneal clearances of the microprotein was slightly and non-significantly greater in patients with urine output than in anuric patients.

Comparison of three different tests for assessment of hepatitis C virus in dialysis patients.

OBJECTIVE: To evaluate the relationship between hepatitis C virus antibodies (HCV-Ab) and viremia and to compare the prevalence of HCV-Ab and HCV viremia in hemodialysis (HD) and continuous ambulatory peritoneal dialysis (CAPD) patients. DESIGN: Cross-sectional study. SETTING: Dialysis unit of a nephrology division in a public university hospital. PATIENTS: All dialysis patients who came for routine clinic visits during the study period. None denied informed consent. Forty-eight patients on HD and 79 on CAPD were examined. INTERVENTION: Blood samples were tested by second-generation enzyme-linked immunosorbent assay (ELISA II) and recombinant immunoblot assay (RIBA II) to look for HCV-Ab and by the polymerase chain reaction (PCR) to look for HCV viremia. RESULTS: ELISA II was positive in 52% of HD patients and in 14% of CAPD patients. RIBA II was positive in 48% of HD patients and in 11% of CAPD patients. HCV viremia was positive by PCR in 41.6% of HD patients and in 12% of CAPD patients. Two of these PCR-positive patients did not show HCV-Ab by ELISA II and RIBA II. The sensitivity and specificity of ELISA II were 93% and 92%, the sensitivity and specificity of RIBA II were 86% and 94%. CONCLUSIONS: Our data confirm a higher prevalence of HCV viremia in HD than in CAPD patients. The absence of Ab against virus C in 2 patients positive with PCR might be due to recent HCV infection or to weak virus replication or to a poor immune response.

Incidence, possible causes and social aspects of the symptomatic introduction of disinfectant into the peritoneal cavity in CAPD.

The frequency and characteristics of episodes of accidental symptomatic introduction of Amuchina in CAPD patients has been retrospectively analyzed in patients treated with three different CAPD systems with disinfectant (two reusable Y set and one disposable Y set). The rate of accidental introduction of Amuchina was 1 episode every 8981 exchanges (1/8620 with the log Baxter Y set, 1/11,060 exchanges with the short Baxter Y set and with the Bieffe L3 system). One to 15 fresh dialysis exchanges were required to relief pain due to the introduction of disinfectant. Four patients were hospitalized and one had a long lasting impairment in peritoneal ultrafiltration. There were no significant correlations between the episodes and age, sex or time when the exchange was performed.

Staphylococcus aureus exit-site and tunnel infection in CAPD.

Fifty-six S. aureus episodes of catheter exit-site tunnel infections were diagnosed in 40 out of 163 patients treated by CAPD for 30 +/- 22 months, with standard double-cuff Tenckhoff catheters. The rate of infection was 1 episode every 29 patient/months. Local care and antibiotic therapy were effective in 52% of the cases. Whereas in 29 episodes in which the medical therapy failed to eradicate the infection the entire area of granulation tissue and cellulitis was excised then the outer dacron cuff was shaved from the silicone catheter. With this treatment 13 episodes (48%) were cured while, in the remaining 14 patients the catheters were removed because of peritonitis in 10; and for failure to eradicate the infection in 4.

Clinical significance of peritoneal dialysate IgG levels in CAPD patients.

To assess the normal range and the clinical significance of peritoneal IgG, the levels of these proteins were measured in 24 hour dialysates of 69 patients. The mean + SD of IgG levels was 7.2 +/- 4.3 mg/dl. Two patients had peritoneal IgG concentrations below and 6 above the mean +/- SD range. There were no significant correlations between peritoneal IgG levels and sex, previous hemodialysis, age, time of dialysis, number of peritonitis or estimated daily protein intake. In 29 of these patients dialysate IgG levels measured again one year later did not show significant differences between the first and the second determination of IgG (from 6.3 +/- 3.3 to 6.6 +/- 3.2 mg/dl), but changes of 10% or more were observed in 14 of these patients. Our data show no relation between peritoneal IgG and peritonitis episodes. Therefore by these results dialysate IgG cannot be used as a reliable index to decide which patients are at high risk for peritonitis.

Effect of simvastatin in CAPD patients with hypercholesterolemia.

The effect of simvastatin on serum total and HDL cholesterol and total triglyceride levels in 20 hypercholesterolemic patients on CAPD treatment was studied. The drug was given at the initial dose of 10 mg/day which was doubled up to 40 mg/day. Two non-compliant patients stopped the drug in the first week of treatment. One patient had vomiting and stopped simvastatin. One patients reduced the dose from 20 to 10 mg/day because of increase in CPK level. The study was completed in 16 patients. Serum cholesterol decreased from 318 +/- 39 to 208 +/- 34 mg/dl (p < 0.001), triglyceride from 317 +/- 129 to 278 +/- 160 mg/dl and HDL cholesterol from 43 +/- 13 to 35 +/- 11 mg/dl. The effective does was 10 mg/day in 4 cases, 20 mg/dl in 7 and 40 mg/dl in 5. In CAPD patients, simvastatin is safe and effective in lowering serum cholesterol. The clinical significance of the decrease in HDL cholesterol and its possible effect on clinical outcome are still unknown.

Gemfibrozil in CAPD patients.

Gemfibrozil was given at a daily oral dose of 600 mg to 28 CAPD patients with serum triglyceride levels higher than 200 mg/dl, after 2 months of low fat low calorie diet. Eleven patients dropped out from the study due to spontaneous withdrawal of the drug or to intercurrent hospitalization. The 17 patients who completed the study showed a significant decrease in serum total (434.6 +/- 207.9 to 237.5 +/- 81.6 mg/dl, p 0.003), VLDL (268.8 +/- 17.8 to 109.2 +/- 66.9 mg/dl, p less than 0.005) and LDL (152.9 +/- 34.6 to 119.2 +/- 30.7 mg/dl, p = 0.004) triglyceride. Serum VLDL cholesterol increased from 108.7 +/- 54.1 to 59.9 +/- 29.0 mg/dl, p = 0.003. Serum HDL cholesterol decreased from 25.4 +/- 4.8 to 35.3 +/- 6.3 mg/dl (p less than 0.001). Apo A1 increased from 117.8 +/- 16.7 to 130.8 +/- 16.7 mg/dl (p less than 0.001) and Apo B increased from 173 +/- 26.7 to 137.8 +/- 21.9 mg/dl (p less than 0.001). No clinical or laboratory side effect was observed in the patients who completed the study. Therefore dropouts were due to the poor patients' compliance rather than to the severity of side effects.