Monopole ordered phases in dipolar and nearest-neighbours Ising pyrochlore: from spin ice to the "all-in--all-out" antiferromagnet

We study Ising pyrochlores by means of Monte Carlo simulations. We cover a set of exchange constants ranging from the frustrated ferromagnetic case (spin-ice) to the fully-ordered "all-in--all-out" antiferromagnet in the dipolar model, reinterpreting the results --as in an ionic system-- in terms of a temperature vs. magnetic charge density phase diagram. In spite of its spin nature and the presence of both double and single non-conserved magnetic charges, the dipolar model gives place to a phase diagram which is quite comparable with those previously obtained for on-lattice systems of electric charges, and on spin ice models with conserved number of single magnetic charges. The contrast between these systems, to which we add results from the nearest-neighbours model, put forward other features of our phase diagram --notably, a monopole fluid with charge order at high monopole densities that persists up to arbitrarily high temperatures-- that can only be explained taking into account construction constraints forced by the underlying spin degrees of freedom.Comment: 9 pages, 10 figure

Scaling behavior of jamming fluctuations upon random sequential adsorption

It is shown that the fluctuations of the jamming coverage upon Random Sequential Adsorption ($\sigma_{\theta_J}$), decay with the lattice size according to the power-law $\sigma_{\theta_J} \propto L^{-1 / \nu_J}$, with $\nu_{J} = 2 / (2D - d_f)$, where $D$ is the dimension of the substrate and $d_{\rm f}$ is the fractal dimension of the set of sites belonging to the substrate where the RSA process actually takes place. This result is in excellent agreement with the figure recently reported by Vandewalle {\it et al} ({\it Eur. Phys. J.} B. {\bf 14}, 407 (2000)), namely $\nu_J = 1.0 (1)$ for the RSA of needles with $D = 2$ and $d_f = 2$, that gives $\nu_J = 1$. Furthermore, our prediction is in excellent agreement with different previous numerical results. The derived relationships are also confirmed by means of extensive numerical simulations applied to the RSA of dimers on both stochastic and deterministic fractal substrates.Comment: 8 pages, 2 figures. To appear in Eur. Phys. J. B (Rapid note) (2003

Thermodynamics of the classical spin-ice model with nearest neighbour interactions using the Wang-Landau algorithm

In this article we study the classical nearest-neighbour spin-ice model (nnSI) by means of Monte Carlo simulations, using the Wang-Landau algorithm. The nnSI describes several of the salient features of the spin-ice materials. Despite its simplicity it exhibits a remarkably rich behaviour. The model has been studied using a variety of techniques, thus it serves as an ideal benchmark to test the capabilities of the Wang Landau algorithm in magnetically frustrated systems. We study in detail the residual entropy of the nnSI and, by introducing an applied magnetic field in two different crystallographic directions ([111] and [100],) we explore the physics of the kagome-ice phase, the transition to full polarisation, and the three dimensional Kasteleyn transition. In the latter case, we discuss how additional constraints can be added to the Hamiltonian, by taking into account a selective choice of states in the partition function and, then, show how this choice leads to the realization of the ideal Kasteleyn transition in the system.Comment: 9 pages, 9 figure

Rigid-Band Shift of the Fermi Level in a Strongly Correlated Metal: Sr(2-y)La(y)RuO(4)

We report a systematic study of electron doping of Sr2RuO4 by non-isovalent substitution of La^(3+) for Sr^(2+). Using a combination of de Haas-van Alphen oscillations, specific heat, and resistivity measurements, we show that electron doping leads to a rigid-band shift of the Fermi level corresponding to one doped electron per La ion, with constant many-body quasiparticle mass enhancement over the band mass. The susceptibility spectrum is substantially altered and enhanced by the doping but this has surprisingly little effect on the strength of the unconventional superconducting pairing.Comment: 4 pages, 3 figure

An intermediate state between the kagome-ice and the fully polarized state in Dy2_2Ti2_2O7_7

Dy$_2$Ti$_2$O$_7$ is at present the cleanest example of a spin-ice material. Previous theoretical and experimental work on the first-order transition between the kagome-ice and the fully polarized state has been taken as a validation for the dipolar spin-ice model. Here we investigate in further depth this phase transition using ac-susceptibility and dc-magnetization, and compare this results with Monte-Carlo simulations and previous magnetization and specific heat measurements. We find signatures of an intermediate state between the kagome-ice and full polarization. This signatures are absent in current theoretical models used to describe spin-ice materials.Comment: 7 pages, 4 figure

Multiple first-order metamagnetic transitions and quantum oscillations in ultrapure

We present measurements on ultra clean single crystals of the bilayered ruthenate metal Sr3Ru2O7, which has a magnetic-field-tuned quantum critical point. Quantum oscillations of differing frequencies can be seen in the resistivity both below and above its metamagnetic transition. This frequency shift corresponds to a small change in the Fermi surface volume that is qualitatively consistent with the small moment change in the magnetisation across the metamagnetic transition. Very near the metamagnetic field, unusual behaviour is seen. There is a strong enhancement of the resistivity in a narrow field window, with a minimum in the resistivity as a function of temperature below 1 K that becomes more pronounced as the disorder level decreases. The region of anomalous behaviour is bounded at low temperatures by two first-order phase transitions. The implications of the results are discussed. PACS: 68.35.Rh, 71.27.+a, 72.15.-v, 74.70.PqComment: 12 pages 4 figures, submitte

Formation of a Nematic Fluid at High Fields in Sr3Ru2O7

In principle, a complex assembly of strongly interacting electrons can self-organise into a wide variety of collective states, but relatively few such states have been identified in practice. We report that, in the close vicinity of a metamagnetic quantum critical point, high purity Sr3Ru2O7 possesses a large magnetoresistive anisotropy, consistent with the existence of an electronic nematic fluid. We discuss a striking phenomenological similarity between our observations and those made in high purity two-dimensional electron fluids in GaAs devices.Comment: 14 pages, 3 figures, 11 extra pages of supplementary informatio

Correlation between wearable inertial sensor data and standardised Parkinson's disease axial impairment measures using machine learning

Wearable sensors represent a valuable means for monitoring motion signs and symptoms of Parkinson’s disease (PD). In this paper, we explore the potential of a single inertial sensor to yield information correlated to the patient’s subjective perception of axial motion impairment during daily activities. This latter is expressed using as a relevant metric the sum of MDS-UPDRS items 2.11-2.13. Methods: thirty-one patients with PD were enrolled in this study, and asked to perform a timed-up-and-go test while wearing an inertial sensor on their thigh. Several time- and frequency-domain features were extracted from the inertial signals. They were fed to a random forest regression model for the prediction of the axial impairment metric. The model was optimized using 10-fold cross-validation and performance were assessed using leave-one-subject-out test. Results: Pearson correlation coefficient with the addressed metric of 0.76 (0.86) and mean absolute error of 1.70 (1.52) were obtained in patients under (not under) dopaminergic therapy. Moreover, moderate to strong correlations were found between the predicted score and some important disease progression, axial impairment, and motor performance metrics. Conclusion: a single wearable inertial sensor may be used for assessing motor disabilities of patients with PD

The de Haas-van Alphen effect across the metamagnetic transition in Sr3_3Ru2_2O7_7

We report a study of the de Haas-van Alphen (dHvA) effect on the itinerant metamagnet Sr$_3$Ru$_2$O$_7$. Extremely high sample purity allows the observation of dHvA oscillations both above and below the metamagnetic transition field of 7.9 T. The quasiparticle masses are fairly large away from the transition, and are enhanced by up to an extra factor of three as the transition is approached, but the Fermi surface topography change is quite small. The results are qualitatively consistent with a field-induced Stoner transition in which the mass enhancement is the result of critical fluctuations.Comment: 4 pages, 3 figure