Fe2-homogenization of micromorphic elasto-plastic materials

In this work, a homogenization strategy for a micromorphic–type inelastic material is presented. In the spirit of FE2, a representative volume element is attached to each macroscopic quadrature point. Due to the inherent length scale of the micromorphic continuum, size effects for inelastic behavior are obtained on RVE–level. A focus is placed on the computation of the homogenized algorithmic tangent. It is determined via sensitivity analyses with respect to the boundary conditions imposed on the RVE. Following this procedure, costly single–scale computations with dense meshes can be replaced by a robust homogenization approach with optimal convergence rates

Computational modelling of a multifield single-crystal gradient plasticity formulation

A model of higher-order single crystal plasticity is presented and reviewed in order to develop a corresponding finite-element framework. Contrary to the underlying model of Gurtin [Int. J. Plast. 24:702-725, 2008], here rather than the slip rate, the slip and its gradient constitute primary micro state variables. The resulting rate-dependent formulation accounts for size effects through the free energy depending on density of geometrically necessary dislocations. The relationship to multifield theories of continua with microstructure is pointed out. With the presented finite-element approach, the corresponding fully coupled initial-boundary value problem is solved monolithically, and features of the model are illustrated in two preliminary numerical example

Computational and theoretical aspects of a grain-boundary model that accounts for grain misorientation and grain-boundary orientation

A detailed theoretical and numerical investigation of the infinitesimal single-crystal gradient plasticity and grain-boundary theory of Gurtin (2008) "A theory of grain boundaries that accounts automatically for grain misorientation and grain-boundary orientation". Journal of the Mechanics and Physics of Solids 56 (2), 640-662, is performed. The governing equations and flow laws are recast in variational form. The associated incremental problem is formulated in minimization form and provides the basis for the subsequent finite element formulation. Various choices of the kinematic measure used to characterize the ability of the grain boundary to impede the flow of dislocations are compared. An alternative measure is also suggested. A series of three-dimensional numerical examples serve to elucidate the theory

Quantum Tricritical Points in NbFe2_2

Quantum critical points (QCPs) emerge when a 2nd order phase transition is suppressed to zero temperature. In metals the quantum fluctuations at such a QCP can give rise to new phases including unconventional superconductivity. Whereas antiferromagnetic QCPs have been studied in considerable detail ferromagnetic (FM) QCPs are much harder to access. In almost all metals FM QCPs are avoided through either a change to 1st order transitions or through an intervening spin-density-wave (SDW) phase. Here, we study the prototype of the second case, NbFe$_2$. We demonstrate that the phase diagram can be modelled using a two-order-parameter theory in which the putative FM QCP is buried within a SDW phase. We establish the presence of quantum tricritical points (QTCPs) at which both the uniform and finite $q$ susceptibility diverge. The universal nature of our model suggests that such QTCPs arise naturally from the interplay between SDW and FM order and exist generally near a buried FM QCP of this type. Our results promote NbFe$_2$ as the first example of a QTCP, which has been proposed as a key concept in a range of narrow-band metals, including the prominent heavy-fermion compound YbRh$_2$Si$_2$.Comment: 21 pages including S

Ultrasmall Moment Incommensurate Spin Density Wave Order Masking a Ferromagnetic Quantum Critical Point in NbFe₂

In the metallic magnet Nb1−yFe2þy, the low temperature threshold of ferromagnetism can be investigatedby varying the Fe excessywithin a narrow homogeneity range. We use elastic neutron scattering to trackthe evolution of magnetic order from Fe-rich, ferromagnetic Nb0.981Fe2.019to approximately stoichiometricNbFe2, in which we can, for the first time, characterize a long-wavelength spin density wave state burying aferromagnetic quantum critical point. The associated ordering wave vectorqSDW¼ð0;0;lSDWÞis found todepend significantly onyandT, staying finite but decreasing as the ferromagnetic state is approached. Thephase diagram follows a two-order-parameter Landau theory, for which all of the coefficients can now bedetermined. Our findings suggest that the emergence of spin density wave order cannot be attributed toband structure effects alone. They indicate a common microscopic origin of both types of magnetic orderand provide strong constraints on related theoretical scenarios based on, e.g., quantum order by disorder

Quantum tricritical points in NbFe2

Quantum critical points (QCPs) emerge when a 2nd order phase transition is suppressed to zero temperature. In metals the quantum fluctuations at such a QCP can give rise to new phases including unconventional superconductivity. Whereas antiferromagnetic QCPs have been studied in considerable detail ferromagnetic (FM) QCPs are much harder to access. In almost all metals FM QCPs are avoided through either a change to 1st order transitions or through an intervening spin-density-wave (SDW) phase. Here, we study the prototype of the second case, NbFe$_2$. We demonstrate that the phase diagram can be modelled using a two-order-parameter theory in which the putative FM QCP is buried within a SDW phase. We establish the presence of quantum tricritical points (QTCPs) at which both the uniform and finite $q$ susceptibility diverge. The universal nature of our model suggests that such QTCPs arise naturally from the interplay between SDW and FM order and exist generally near a buried FM QCP of this type. Our results promote NbFe$_2$ as the first example of a QTCP, which has been proposed as a key concept in a range of narrow-band metals, including the prominent heavy-fermion compound YbRh$_2$Si$_2$

Experimental signatures of the mixed axial-gravitational anomaly in the Weyl semimetal NbP

Weyl semimetals are materials where electrons behave effectively as a kind of massless relativistic particles known asWeyl fermions. These particles occur in two flavours, or chiralities, and are subject to quantum anomalies, the breaking of a conservation law by quantum fluctuations. For instance, the number of Weyl fermions of each chirality is not independently conserved in parallel electric and magnetic field, a phenomenon known as the chiral anomaly. In addition, an underlying curved spacetime provides a distinct contribution to a chiral imbalance, an effect known as the mixed axial-gravitational anomaly, which remains experimentally elusive. However, the presence of a mixed gauge-gravitational anomaly has recently been tied to thermoelectrical transport in a magnetic field, even in flat spacetime, opening the door to experimentally probe such type of anomalies in Weyl semimetals. Using a temperature gradient, we experimentally observe a positive longitudinal magnetothermoelectric conductance (PMTC) in the Weyl semimetal NbP for collinear temperature gradients and magnetic fields (DT || B) that vanishes in the ultra quantum limit. This observation is consistent with the presence of a mixed axial-gravitational anomaly. Our work provides clear experimental evidence for the existence of a mixed axial-gravitational anomaly of Weyl fermions, an outstanding theoretical concept that has so far eluded experimental detection

Dynamic Diagnosis of Familial Prion Diseases Supports the β2-α2 Loop as a Universal Interference Target

[Background] Mutations in the cellular prion protein associated to familial prion disorders severely increase the likelihood of its misfolding into pathogenic conformers. Despite their postulation as incompatible elements with the native fold, these mutations rarely modify the native state structure. However they variably have impact on the thermodynamic stability and metabolism of PrPC and on the properties of PrPSc aggregates. To investigate whether the pathogenic mutations affect the dynamic properties of the HuPrP(125-229) α-fold and find possible common patterns of effects that could help in prophylaxis we performed a dynamic diagnosis of ten point substitutions.[Methodology/Principal Findings] Using all-atom molecular dynamics simulations and novel analytical tools we have explored the effect of D178N, V180I, T183A, T188K, E196K, F198S, E200K, R208H, V210I and E211Q mutations on the dynamics of HuPrP(125-228) α-fold. We have found that while preserving the native state, all mutations produce dynamic changes which perturb the coordination of the α2-α3 hairpin to the rest of the molecule and cause the reorganization of the patches for intermolecular recognition, as the disappearance of those for conversion inhibitors and the emergence of an interaction site at the β2-α2 loop region.[Conclusions/Significance] Our results suggest that pathogenic mutations share a common pattern of dynamical alterations that converge to the conversion of the β2-α2 loop into an interacting region that can be used as target for interference treatments in genetic diseases.This work was supported in parts by grants BFU2009-07971 from the MICINN (MG), FundaciÃ3n Cien (MG); Fondazione Cariplo (GC) and AIRC (GC). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. No additional external funding received for this study.Peer reviewe