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.

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.

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 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.

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.

Dirac strings and magnetic monopoles in the spin ice Dy2Ti2O7.

Sources of magnetic fields-magnetic monopoles-have so far proven elusive as elementary particles. Condensed-matter physicists have recently proposed several scenarios of emergent quasiparticles resembling monopoles. A particularly simple proposition pertains to spin ice on the highly frustrated pyrochlore lattice. The spin-ice state is argued to be well described by networks of aligned dipoles resembling solenoidal tubes-classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defects look like magnetic monopoles. We demonstrated, by diffuse neutron scattering, the presence of such strings in the spin ice dysprosium titanate (Dy2Ti2O7). This is achieved by applying a symmetry-breaking magnetic field with which we can manipulate the density and orientation of the strings. In turn, heat capacity is described by a gas of magnetic monopoles interacting via a magnetic Coulomb interaction.

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.

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.

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.

Diffusion of HCV through peritoneal membrane in HCV positive patients treated with continuous ambulatory peritoneal dialysis.

PURPOSE OF THE STUDY: We evaluated the presence of HCV in the peritoneal effluents of viraemic patients treated with continuous ambulatory peritoneal dialysis (CAPD) to evaluate the risk of transmitting the infection with this procedure. PROCEDURE: Fifteen of 81 CAPD patients (18.5%) had anti-HCV antibodies and eight were viraemic. At the beginning of CAPD two of the viraemic patients had ascites with a clinical picture of chronic active hepatitis and cirrhosis. Peritoneal dialysates were collected after an overnight exchange with 1.36% glucose and after a 4-h exchange with 3.86% glucose. Fluids from the overnight exchange were spun to obtain a cellular pellet and the supernatant 100-fold concentrated. RESULTS: No viral genome could be detected in unconcentrated samples and in cellular pellets, while HCV-RNA at low titre was detected in concentrated dialysates from the two patients with active liver disease. CONCLUSIONS: Our findings confirm that HCV may be present in the CAPD effluent of some patients; however, the titre of virus in the effluent was extremely low, at the limit of detection of the PCR assay. Peritoneal fluids originating from patients with HCV associated severe liver disease may be a potential source of infection.

Continuous ambulatory peritoneal dialysis in patients after intra-abdominal prosthetic vascular graft surgery.

The purpose of this study was to assess the feasibility of continuous ambulatory peritoneal dialysis (CAPD) after intra-abdominal prosthetic vascular graft surgery. We report 8 consecutive patients with end-stage renal disease, who previously underwent intra-abdominal prosthetic aortic graft replacement, treated by CAPD between November 1983 and November 1994. All patients received a peritoneal dialysis catheter without technical problems and were dialyzed for a total of 208 months. Six episodes of peritonitis occurred in 4 patients without clinical evidence of any abdominal aortic graft infection. Three patients developed intermittent claudication and 2 died of myocardial infarct. A similar peritonitis and cardiovascular complication rate was observed in a control group of age- and sex-matched CAPD patients with no aortic prosthesis. We conclude that CAPD is feasible in patients with abdominal aortic prosthesis.

Nine patients treated for more than 10 years with continuous ambulatory peritoneal dialysis.

We have retrospectively examined the clinical outcomes of the 9 patients who survived for more than 10 years in our continuous ambulatory peritoneal dialysis (CAPD) program. Six were men and 3 women aged 50.8 +/- (SD) 11.5 years. Three had been previously treated by hemodialysis. None of them had diabetes or neoplasms, 1 had liver cirrhosis, 3 had ischemic cardiopathy, 1 had peripheral artery disease, and all were hypertensive. The hospitalization rate ranged from 0 to 4.5 days/patient/year, the peritonitis rate was one episode every 57 months. Six patients had no peritonitis during the first 10 years of treatment. Exit-site episodes were one every 46.7 patient months. Six peritoneal catheters were removed from 4 patients. KT/V and peritoneal permeability, assessed by the peritoneal equilibration test, were within the normal range in the majority of the patients. Five patients died between the 121st and the 149th month, and 4 are still alive. Three of them are working. These results show that CAPD can be effective, peritoneal catheters can survive, and some patients can be free from peritonitis episodes for more than 10 years. After the 10-year on CAPD, the survival is poor, and the morbidity is high.

Cuff-shaving procedure. A rescue treatment for exit-site infection unresponsive to medical therapy.

We performed 41 cuff-shaving procedures in 38 patients on continuous ambulatory peritoneal dialysis (CAPD) with exit-site infection unresponsive to medical treatment. Cuff shaving was performed on three patients with two catheters each. This procedure was effective in eliminating 50% of S. aureus exit-site infection and all S. epidermidis exit-site infection, but was ineffective in Gram-negative exit-site infection. After cuff-shaving procedure, 20 catheters (49%) were removed; 11 for persistent tunnel infection and nine because of development of secondary peritonitis. The probability of catheter survival at 1 year was 50% and remained stable thereafter. Cuff-shaving procedure may be a valuable mode of therapy for treating patients with S. aureus and/or S. epidermidis exit-site infection unresponsive to medical treatment.

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.

High prevalence of silent gallstone disease in dialysis patients.

Gallstone disease was detected in 28% of 119 patients on regular dialysis treatment. The disease was silent in 82% of the patients. Stones were radiolucent in 88% of the cases, radioopaque in 8% and mixed in 4%. Among 49 variables considered, increasing age was the only variable that correlated significantly with increasing prevalence of gallstone disease. Since no relationships were found between gallstones and age or modes of dialysis, it is conceivable that some mechanism(s) linked with the preexisting chronic nephropathy might have been involved in the development of cholelithiasis. End-stage renal disease could be another so far unrecognized risk factor for cholelithiasis.