Skyrmion Lattice in a Chiral Magnet

Skyrmions represent topologically stable field configurations with particle-like properties. We used neutron scattering to observe the spontaneous formation of a two-dimensional lattice of skyrmion lines, a type of magnetic vortices, in the chiral itinerant-electron magnet MnSi. The skyrmion lattice stabilizes at the border between paramagnetism and long-range helimagnetic order perpendicular to a small applied magnetic field regardless of the direction of the magnetic field relative to the atomic lattice. Our study experimentally establishes magnetic materials lacking inversion symmetry as an arena for new forms of crystalline order composed of topologically stable spin states

Magnetic phase diagram of MnSi inferred from magnetization and ac susceptibility

We report simultaneous measurements of the magnetization and the ac susceptibility across the magnetic phase diagram of single-crystal MnSi. In our study we explore the importance of the excitation frequency, excitation amplitude, sample shape, and crystallographic orientation. The susceptibility, dM/dH, calculated from the magnetization, is dominated by pronounced maxima at the transition from the helical to the conical and the conical to the skyrmion lattice phase. The maxima in dM/dH are not tracked by the ac susceptibility, which in addition varies sensitively with the excitation amplitude and frequency at the transition from the conical to the skyrmion lattice phase. The same differences between dM/dH and the ac susceptibility exist for Mn1-xFexSi (x=0.04) and Fe1-xCoxSi (x=0.20). Taken together our study establishes consistently for all major crystallographic directions the existence of a single pocket of the skyrmion lattice phase in MnSi, suggestive of a universal characteristic of all B20 transition metal compounds with helimagnetic order.Comment: 19 pages, 20 figure

Functional connectivity profile of the human inferior frontal junction: involvement in a cognitive control network

Background: The human inferior frontal junction area (IFJ) is critically involved in three main component processes of cognitive control (working memory, task switching and inhibitory control). As it overlaps with several areas in established anatomical labeling schemes, it is considered to be underreported as a functionally distinct location in the neuroimaging literature. While recent studies explicitly focused on the IFJ´s anatomical organization and functional role as a single brain area, it is usually not explicitly denominated in studies on cognitive networks. However based on few analyses in small datasets constrained by specific a priori assumptions on its functional specialization, the IFJ has been postulated to be part of a cognitive control network. Goal of this meta-analysis was to establish the IFJ’s connectivity profile on a high formal level of evidence by aggregating published implicit knowledge about its co-activations. We applied meta-analytical connectivity modeling (MACM) based on the activation likelihood estimation (ALE) method without specific assumptions regarding functional specialization on 180 (reporting left IFJ activity) and 131 (right IFJ) published functional neuroimaging experiments derived from the BrainMap database. This method is based on coordinates in stereotaxic space, not on anatomical descriptors. Results: The IFJ is significantly co-activated with areas in the dorsolateral and ventrolateral prefrontal cortex, anterior insula, medial frontal gyrus / pre-SMA, posterior parietal cortex, occipitotemporal junction / cerebellum, thalamus and putamen as well as language and motor areas. Results are corroborated by an independent resting-state fMRI analysis. Conclusions: These results support the assumption that the IFJ is part of a previously described cognitive control network. They also highlight the involvement of subcortical structures in this system. A direct line is drawn from works on the functional significance of brain activity located at the IFJ and its anatomical definition to published results related to distributed cognitive brain systems. The IFJ is therefore introduced as a convenient starting point to investigate the cognitive control network in further studies

Towards literature-based feature selection for diagnostic classification: A meta-analysis of resting-state fMRI in depression

Information derived from functional magnetic resonance imaging (fMRI) during wakeful rest has been introduced as a candidate diagnostic biomarker in unipolar major depressive disorder (MDD). Multiple reports of resting state fMRI in MDD describe group effects. Such prior knowledge can be adopted to pre-select potentially discriminating features for diagnostic classification models with the aim to improve diagnostic accuracy. Purpose of this analysis was to consolidate spatial information about alterations of spontaneous brain activity in MDD, primarily to serve as feature selection for multivariate pattern analysis techniques (MVPA). Thirty two studies were included in final analyses. Coordinates extracted from the original reports were assigned to two categories based on directionality of findings. Meta-analyses were calculated using the non-additive activation likelihood estimation approach with coordinates organized by subject group to account for non-independent samples. Converging evidence revealed a distributed pattern of brain regions with increased or decreased spontaneous activity in MDD. The most distinct finding was hyperactivity/hyperconnectivity presumably reflecting the interaction of cortical midline structures (posterior default mode network components including the precuneus and neighboring posterior cingulate cortices associated with self-referential processing and the subgenual anterior cingulate and neighboring medial frontal cortices) with lateral prefrontal areas related to externally-directed cognition. Other areas of hyperactivity/hyperconnectivity include the left lateral parietal cortex, right hippocampus and right cerebellum whereas hypoactivity/hypoconnectivity was observed mainly in the left temporal cortex, the insula, precuneus, superior frontal gyrus, lentiform nucleus and thalamus. Results are made available in two different data formats to be used as spatial hypotheses in future studies, particularly for diagnostic classification by MVPA

A Phenomenological Description of the Non-Fermi-Liquid Phase of MnSi

In order to understand the non-Fermi-liquid behavior of MnSi under pressure we propose a scenario on the basis of the multispiral state of the magnetic moment. This state can describe the recent critical experiment of the Bragg sphere in the neutron scattering which is the key ingredient of the non-Fermi-liquid behavior.Comment: 3 page

Pfleiderer2: identification of a new globular cluster in the Galaxy

We provide evidence that indicate the star cluster Pfleiderer 2, which is projected in a rich field, as a newly identified Galactic globular cluster. Since it is located in a crowded field, core extraction and decontamination tools were applied to reveal the cluster sequences in B, V and I Color-Magnitude Diagrams (CMDs). The main CMD features of Pfleiderer 2 are a tilted Red Giant Branch, and a red Horizontal Branch, indicating a high metallicity around solar. The reddening is E(B-V)=1.01. The globular cluster is located at a distance from the Sun d$_{\odot}$ = 16$\pm$2 kpc. The cluster is located at 2.7 kpc above the Galactic plane and at a distance from the Galactic center of R$_{\rm GC}$=9.7 kpc, which is unusual for a metal-rich globular cluster.Comment: Accepted by The Astronomical Journa

Stars and gas in the very large interacting galaxy NGC 6872

The dynamical evolution of the large (> 100 kpc), barred spiral galaxy NGC 6872 and its small companion IC 4970 in the southern group Pavo is investigated. We present N-body simulations with stars and gas and 21 cm HI observations carried out with the Australia Telescope Compact Array of the large-scale distribution and kinematics of atomic gas. HI is detected toward the companion, corresponding to a gas mass of ca 1.3 10^9 Msun. NGC 6872 contains ca 1.4 10^{10} Msun of HI gas, distributed in an extended rotating disk. Massive concentrations of gas (10^9 Msun) are found at the tip of both tidal tails and towards the break seen in the optical northern arm near the companion. We detect no HI counterpart to the X-ray trail between NGC 6872 and NGC 6876, the dominant elliptical galaxy in the Pavo group located 8 arcmin to the southeast. At the sensitivity and the resolution of the observations, there is no sign in the overall HI distribution that NGC 6876 has affected the evolution of NGC 6872. There is no evidence of ram pressure stripping either. The X-ray trail could be due to gravitational focusing of the hot gas in the Pavo group behind NGC 6872 as the galaxy moves supersonically through the hot medium. The simulations of a gravitational interaction with a small nearby companion on a low-inclination prograde passage are able to reproduce most of the observed features of NGC 6872, including the general morphology of the galaxy, the inner bar, the extent of the tidal tails and the thinness of the southern tail.Comment: 12 pages, 11 figures. Accepted for publication in Astronomy & Astrophysics. The resolution of the figures has been greatly reduced. The paper with the original figures can found at http://www.oso.chalmers.se/~horellou/PAPERS/2006n6872.pd

Canted antiferromagnetism in phase-pure CuMnSb

We report the low-temperature properties of phase-pure single crystals of the half-Heusler compound CuMnSb grown by means of optical float-zoning. The magnetization, specific heat, electrical resistivity, and Hall effect of our single crystals exhibit an antiferromagnetic transition at $T_{\mathrm{N}} = 55~\mathrm{K}$ and a second anomaly at a temperature $T^{*} \approx 34~\mathrm{K}$. Powder and single-crystal neutron diffraction establish an ordered magnetic moment of $(3.9\pm0.1)~\mu_{\mathrm{B}}/\mathrm{f.u.}$, consistent with the effective moment inferred from the Curie-Weiss dependence of the susceptibility. Below $T_{\mathrm{N}}$, the Mn sublattice displays commensurate type-II antiferromagnetic order with propagation vectors and magnetic moments along $\langle111\rangle$ (magnetic space group $R[I]3c$). Surprisingly, below $T^{*}$, the moments tilt away from $\langle111\rangle$ by a finite angle $\delta \approx 11^{\circ}$, forming a canted antiferromagnetic structure without uniform magnetization consistent with magnetic space group $C[B]c$. Our results establish that type-II antiferromagnetism is not the zero-temperature magnetic ground state of CuMnSb as may be expected of the face-centered cubic Mn sublattice.Comment: 14 pages, 15 figure

Helimagnon Bands as Universal Spin Excitations of Chiral Magnets

MnSi is a cubic compound with small magnetic anisotropy, which stabilizes a helimagnetic spin spiral that reduces to a ferromagnetic and antiferromagnetic state in the long- and short-wavelength limit, respectively. We report a comprehensive inelastic neutron scattering study of the collective magnetic excitations in the helimagnetic state of MnSi. In our study we observe a rich variety of seemingly anomalous excitation spectra, as measured in well over twenty different locations in reciprocal space. Using a model based on only three parameters, namely the measured pitch of the helix, the measured ferromagnetic spin wave stiffness and the amplitude of the signal, as the only free variable, we can simultaneously account for \textit{all} of the measured spectra in excellent quantitative agreement with experiment. Our study identifies the formation of intense, strongly coupled bands of helimagnons as a universal characteristic of systems with weak chiral interactions.Comment: 8 pages, 4 figures, references updated, introduction updated, reformatte