Modification of the Bloch law in ferromagnetic nanostructures

The temperature dependence of magnetization in ferromagnetic nanostructures (e.g., nanoparticles or nanoclusters) is usually analyzed by means of an empirical extension of the Bloch law sufficiently flexible for a good fitting to the observed data and indicates a strong softening of magnetic coupling compared to the bulk material. We analytically derive a microscopic generalization of the Bloch law for the Heisenberg spin model which takes into account the effects of size, shape and various surface boundary conditions. The result establishes explicit connection to the microscopic parameters and differs significantly from the existing description. In particular, we show with a specific example that the latter may be misleading and grossly overestimates magnetic softening in nanoparticles. It becomes clear why the usual $T^{3/2}$ dependence appears to be valid in some nanostructures, while large deviations are a general rule. We demonstrate that combination of geometrical characteristics and coupling to environment can be used to efficiently control magnetization and, in particular, to reach a magnetization higher than in the bulk material.Comment: 7 pages, 4 figure

Breathers and Raman scattering in a two-leg ladder with staggered Dzialoshinskii-Moriya interaction

Recent experiments have revealed the role of staggered Dzialoshinskii-Moriya interaction in the magnetized phase of an antiferromagnetic spin 1/2 two-leg ladder compound under a uniform magnetic field. We derive a low energy effective field theory describing a magnetized two-leg ladder with a weak staggered Dzialoshinskii-Moriya interaction. This theory predicts the persistence of the spin gap in the magnetized phase, in contrast to standard two-leg ladders, and the presence of bound states in the excitation spectrum. Such bound states are observable in Raman scattering measurements. These results are then extended to intermediate Dzialoshinskii-Moriya interaction using Exact Diagonalizations.Comment: RevTeX 4, 14 pages, 11 EPS figure

Spin-torque generation by dc or ac voltages in magnetic layered structures

A general expression of the current induced spin torque in a magnetic layered structure in the presence of external dc or ac voltages is derived in the framework of the scattering matrix approach. A detailed analysis is performed for a magnetic-nonmagnetic-magnetic trilayer connected to external leads in the presence of dc voltage bias in the ballistic regime. Alternatively, the possibility of producing spin torque by means of the adiabatic ac modulation of external gate voltages (quantum pumping) is proposed and discussed

Critical behaviour of a spin-tube model in a magnetic field

We show that the low-energy physics of the spin-tube model in presence of a critical magnetic field can be described by a broken SU(3) spin chain. Using the Lieb-Schultz-Mattis Theorem we characterize the possible magnetization plateaus and study the critical behavior in the region of transition between the plateaus m=1/2 and m=3/2 by means of renormalization group calculations performed on the bosonized effective continuum field theory. We show that in certain regions of the parameter space of the effective theory the system remains gapless, and we compute the spin-spin correlation functions in these regions. We also discuss the possibility of a plateau at m=1, and show that although there exists in the continuum theory a term that might cause the appearance of a plateau there, such term is unlikely to be relevant. This conjecture is proved by DMRG techniques. The modifications of the three-leg ladder Hamiltonian that might show plateaus at m =1,5/6,7/6 are discussed, and we give the expected form of correlation functions on the m=1 plateau.Comment: RevTeX, 43 pages, 5 EPS figure

An efficient consistency management algorithm for real-time mobile collaboration

Real time mobile collaboration involves two or more co-workers operating concurrently on a shared document using independent mobile devices. The replicated architecture is attractive for such applications since it does not rely on a central server and a user can continue to work on his or her own local document replica even during disconnection period. Several consistency management algorithms have been proposed, however the resource usage of such algorithms, which is critical in a mobile environment, has not been formally studied. Mobile devices are constrained in terms of memory and processing power, and operate in networking environments with limited bandwidth and transient connectivity. Therefore, algorithms that use resources more effectively improve the quality of the user experience in a mobile environment. ISO 9126-1 considers software to be efficient if it provides a balance between performance, and resource utilisation while performing its function. Therefore, this paper evaluates the efficiency of existing techniques, and proposes a more efficient consistency management algorithm. The new algorithm leverages existing techniques which are shown to be efficient and incorporates a novel history management strategy called partial history copy. Different combinations of these techniques are tested and compared to determine which one is most efficient and thus suitable for mobile usage

Collective excitations of trapped one-dimensional dipolar quantum gases

We calculate the excitation modes of a 1D dipolar quantum gas confined in a harmonic trap with frequency $\omega_0$ and predict how the frequency of the breathing n=2 mode characterizes the interaction strength evolving from the Tonks-Girardeau value $\omega_2=2\omega_0$ to the quasi-ordered, super-strongly interacting value $\omega_2=\sqrt{5}\omega_0$. Our predictions are obtained within a hydrodynamic Luttinger-Liquid theory after applying the Local Density Approximation to the equation of state for the homogeneous dipolar gas, which are in turn determined from Reptation Quantum Monte Carlo simulations. They are shown to be in quite accurate agreement with the results of a sum-rule approach. These effects can be observed in current experiments, revealing the Luttinger-liquid nature of 1D dipolar Bose gases.Comment: 5 pages, 2 EPS figures, RevTeX

Anomalous bond stretching phonons as a probe of charge fluctuations in perovskites

Important information on momentum resolved low energy charge response can be extracted from anomalous properties of bond stretching in plane phonons observed in inelastic neutron and X-ray scattering in cuprates and some other perovskites. We discuss a semiphenomenological model based on coupling of phonons to a single charge mode. The phonon dispersion and linewidth allow to locate the energy of the charge excitation in the mid infrared part of the spectrum and to determine some of its characteristics. New experiments on oxygen isotope substitution could allow to achieve a more detailed description. Corresponding relations following from the model can be used for the interpretation of experiments and as test of the model.Comment: presented at the M2S-HTSC-VIII conference in Dresde

Effects of magnetic field induced chiral-spin interactions on quasi-one-dimensional spin systems

It is known that in certain non-bipartite quasi-one dimensional spin systems in a magnetic field, in addition to the usual Pauli coupling of the spins to the field, new parity breaking three spin interactions, i.e. chiral spin interactions, are induced at higher order due to virtual processes involving the intrinsic electronic nature of the underlying spins. The strenght of these interactions depend strongly on the orientation of the field, a feature which can be exploited to detect chiral effects experimentally. In many spin systems, these chiral interactions are generated and should be taken into account before any comparison with experiments can be made. We study the effect of the chiral interactions on certain quasi-one-dimensional gapped spin half systems and show that they can potentially alter the physics expected from the Pauli coupling alone. In particular, we demonstrate that these terms alter the universality class of the C-IC transition in spin-tubes. More interestingly, in weakly coupled XX zig-zag ladders, we find that the field induced chiral term can close the singlet gap and drive a second order transition in the non-magnetic singlet sector, which manifests itself as a two component Luttinger liquid-like behaviour in the spin correlation functions. Finally, we discuss the relevance of our results to experiments.Comment: RevTex, 11 pages, 3 figure

Hematocrit Values Predict Carotid Intimal-Media Thickness in Obese Patients With Non-Alcoholic Fatty Liver Disease: A Cross-Sectional Study

BACKGROUND: Literature data suggest with some criticism that full-fledged cardiovascular (CV) events (acute or chronic) are likely predicted by blood components, which are reported to be associated with the presence/severity of non-alcoholic fatty liver disease (NAFLD). This study was aimed at determining which marker(s) derived from blood count, such as white blood cells, neutrophils, neutrophil/lymphocyte ratio, platelet count, hemoglobin, mean corpuscular volume, hematocrit values were associated with ear or subclinical atherosclerosis, in obese patients of various classes suffering from NAFLD. METHODS: One hundred consecutive obese patients presenting NAFLD at ultrasound, with low prevalence of co-morbidities and no history or instrumental features of CV diseases, underwent carotid intima-media thickness (IMT) assessment by Doppler ultrasonography. All of them were studied taking into account anthropometric parameters, the metabolic profile, and inflammatory markers. RESULTS: White blood cells and neutrophil count showed no statistical association with IMT, which was predicted by the amount of visceral adiposity, as appreciated by ultrasonography. After adjusting for visceral adiposity and smoking status, only age and hematocrit contextually predicted early atherosclerosis, evaluated as IMT. Visceral adiposity was a confounding factor in foreseeing IMT. CONCLUSION: Hematocrit values along with the patient's age suggest an initial atherosclerosis, evaluated as IMT, and if this finding is confirmed in larger cohorts, could be added to other canonical CV risk factors. Inferences can be enhanced by future prospective studies that aim to identify the relationships between incident cardio-metabolic cases and this hematologic parameter

Interplay between charge-lattice interaction and strong electron correlations in cuprates: phonon anomaly and spectral kinks

We investigate the interplay between strong electron correlations and charge-lattice interaction in cuprates. The coupling between half breathing bond stretching phonons and doped holes in the t-t'-J model is studied by limited phonon basis exact diagonalization method. Nonadiabatic electron-phonon interaction leads to the splitting of the phonon spectral function at half-way to the zone boundary at $\vec{q}_s=\{(\pm \pi / 2, 0), (0, \pm \pi / 2) \}$ and to low energy kink feature in the electron dispersion, in agreement with experimental observations. Another kink due to strong electron correlation effects is observed at higher energy, depending on the strength of the charge-lattice coupling.Comment: 4 pages, 3 figure