Metamagnetism of itinerant electrons in multi-layer ruthenates

The problem of quantum criticality in the context of itinerant ferro- or metamagnetism has received considerable attention [S. A. Grigera et. al., Science 294, 329 (2001); C. Pfleiderer et. al., Nature, 414, 427 (2001)]. It has been proposed that a new kind of quantum criticality is realised in materials such as MnSi or Sr_3Ru_2O_7. We show based on a mean-field theory that the low-temperature behaviour of the n-layer ruthenates Sr_{n+1}Ru_nO_{3n+1} can be understood as a result of a Van Hove singularity (VHS). We consider a single band whose Fermi energy, E_F, is close to the VHS and deduce a complex phase diagram for the magnetism as a function of temperature, magnetic field and E_F. The location of E_F with respect to the VHS depends on the number of layers or can be tuned by pressure. We find that the ferromagnetic quantum phase transition in this case is not of second but of first order, with a metamagnetic quantum critical endpoint at high magnetic field. Despite its simplicity this model describes well the properties of the uniform magnetism in the single, double and triple layer ruthenates. We would like to emphasise that the origin of this behaviour lies in the band structure.Comment: 7 pages, 3 figures, typos corrected and acknowledgement added, to appear in the Europhysics Letter

Chirality induced anomalous-Hall effect in helical spin crystals

Under pressure, the itinerant helimagnet MnSi displays unusual magnetic properties. We have previously discussed a BCC helical spin crystal as a promising starting point for describing the high pressure phenomenology. This state has topologically nontrivial configurations of the magnetization field. Here we note the consequences for magneto-transport that arise generally from such spin textures. In particular a skyrmion density induced `topological' Hall effect, with unusual field dependence, is described.Comment: 4 pages, 3 figures, to appear in the proceedings of SCES 07 (the international conference on strongly correlated electron systems 2007 in Houston, USA

Theoretical proposal predicting anomalous magnetoresistance and quadratic Hall effect in the partially ordered state of MnSi

In [B. Binz, A. Vishwanath and V. Aji, Phys. Rev. Lett. 96, 207202 (2006)], a magnetic structure that breaks time reversal symmetry in the absence of net magnetization was proposed as an explanation for the high pressure "partially ordered" state of MnSi. Here we make explicit the anomalous magneto-transport properties of such a state: a magnetoresistivity which is linear and a Hall conductance which is quadratic in the applied magnetic field. Field cooling procedures for obtaining single domain samples are discussed. The anomalous effects are elaborated in the case of three geometries chosen to produce experimentally unambiguous signals of this unusual magnetic state; e.g., it is predicted that a field in z-direction induces an anisotropy in the x-y plane. Another geometry leads to a Hall voltage parallel to the magnetic field.Comment: 2 pages, 2 figures, International Conference on Magnetism 2006 in Kyot

Thermodynamics of itinerant metamagnetic transitions

Theoretical studies of the metamagnetism and anomalous phase of Sr3Ru2O7 have focused on the role of van Hove singularities, although much experimental evidence points towards quantum criticality having a large effect. We investigate the magnetic and thermodynamic properties of systems where magnetic field tunes through such a peak in the electronic density of states. We study the generic case of a van Hove singularity in 2D. We see that in combination with the requirement of number conservation and interaction effects the peak in the density of states produces several interesting phenomena including raising the critical field of the transition above naive estimates, altering the relationship between temperature and field scales and creating a distinctive double-peak structure in the electronic specific heat. We show that this apparent non-Fermi liquid behaviour can be caused at mean-field level by a peak in the density of states.Comment: 6 pages, 4 figure

Phase-synchronous undersampling in nonlinear spectroscopy

We introduce the concept of phase-synchronous undersampling in nonlinear spectroscopy. The respective theory is presented and validated experimentally in a phase-modulated quantum beat experiment by sampling high phase modulation frequencies with low laser repetition rates. The advantage of undersampling in terms of signal quality and reduced acquisition time is demonstrated and breakdown conditions are identified. The presented method is particularly beneficial for experimental setups with limited signal/detection rates.Comment: 4 pages, 5 figure

Are Local Milieus the Key to Innovation Performance?

This study investigates how local milieus foster innovation success of firms. We complement the common practice of linking firm performance indicators to regional characteristics with survey evidence on the perceived importance of locational factors. While the former approach assumes that location characteristics affect all firms in the same way, the survey allows us to model firms judging the attractiveness of locations by a heterogeneous set of criteria. It turns out that the availability of highly skilled labor and the proximity to suppliers matters for firms? innovation performance. Interestingly, location factors obtained from the survey provide a more accurate explanation on how local milieus facilitate innovation. --Innovation performance,R&D,location factors,Flanders

Financial Constraints: Routine Versus Cutting Edge R&D Investment

We analyze financial constraints for R&D, where we account for heterogeneity among investments which has been neglected in previous literature. According to economic theory, investments should be distinguished by their degree of uncertainty, e.g. routine R&D versus cutting-edge R&D. Financial constraints should be more binding for cutting-edge R&D than for routine R&D. Using panel data we find that R&D spending of firms devoting a significant fraction of R&D to cutting-edge projects is curtailed by credit constraints while routine R&D investments are not. This has important policy implications with respect to the distribution of R&D subsidies in the economy. --R&D,Financial Constraints,Panel Data

Emulating Human Developmental Stages with Bayesian Neural Networks

We compare the acquisition of knowledge in humans and machines. Research from the field of developmental psychology indicates, that human-employed hypothesis are initially guided by simple rules, before evolving into more complex theories. This observation is shared across many tasks and domains. We investigate whether stages of development in artificial learning systems are based on the same characteristics. We operationalize developmental stages as the size of the data-set, on which the artificial system is trained. For our analysis we look at the developmental progress of Bayesian Neural Networks on three different data-sets, including occlusion, support and quantity comparison tasks. We compare the results with prior research from developmental psychology and find agreement between the family of optimized models and pattern of development observed in infants and children on all three tasks, indicating common principles for the acquisition of knowledge