Quantum and classical criticality in a dimerized quantum antiferromagnet

A quantum critical point (QCP) is a singularity in the phase diagram arising because of quantum mechanical fluctuations. The exotic properties of some of the most enigmatic physical systems, including unconventional metals and superconductors, quantum magnets and ultracold atomic condensates, have been related to the importance of critical quantum and thermal fluctuations near such a point. However, direct and continuous control of these fluctuations has been difficult to realize, and complete thermodynamic and spectroscopic information is required to disentangle the effects of quantum and classical physics around a QCP. Here we achieve this control in a high-pressure, high-resolution neutron scattering experiment on the quantum dimer material TlCuCl3. By measuring the magnetic excitation spectrum across the entire quantum critical phase diagram, we illustrate the similarities between quantum and thermal melting of magnetic order. We prove the critical nature of the unconventional longitudinal (Higgs) mode of the ordered phase by damping it thermally. We demonstrate the development of two types of criticality, quantum and classical, and use their static and dynamic scaling properties to conclude that quantum and thermal fluctuations can behave largely independently near a QCP.

The Exploitation of Versatile Building Blocks for the Self-Assembly of Novel Molecular Magnets

Using molecular building blocks to self-assemble lattices supporting long-range magnetic order is currently an active area of solid-state chemistry. Consequently, it is the realm of supramolecular chemistry that synthetic chemists are turning to in order to develop techniques for the synthesis of structurally well-defined supramolecular materials. In recent years we have investigated the versatility and usefulness of two classes of molecular building blocks, namely, tris-oxalato transition-metal (M. Pilkington and S. Decurtins, in “Magnetoscience—From Molecules to Materials,” Wiley–VCH, 2000), and octacyanometalate complexes (Pilkington and Decurtins, Chimia 54, 593 (2001)), for applications in the field of molecule-based magnets. Anionic, tris-chelated oxalato building blocks are able to build up two-dimensional honeycomb-layered structural motifs as well as three-dimensional decagon frameworks. The discrimination between the crystallization of the two- or three-dimensional structures relies on the choice of the templating counterions (Decurtins, Chimia 52, 539 (1998); Decurtins et al. Mol. Cryst. Liq. Cryst. 273, 167 (1995); New J. Chem. 117 (1998)). These structural types display a range of ferro, ferri, and antiferromagnetic properties (Pilkington and Decurtins, in “Magnetoscience—From Molecules to Materials”). Octacyanometalate building blocks self-assemble to afford two new classes of cyano-bridged compounds namely, molecular clusters and extended three dimensional networks (J. Larionova et al., Angew. Chem. Int. Ed. 39, 1605 (2000); Pilkington et al., in preparation). The molecular cluster with a MnII9MoV6 core has the highest ground state spin value, S=51/2, reported to-date (Larionova et al., Angew. Chem. Int. Ed. 39, 1605 (2000)). In the high-temperature regime, the magnetic properties are characterized by ferromagnetic intracluster coupling. In the magnetic range below 44 K, the magnetic cluster signature is lost as possibly a bulk behavior starts to emerge. The three-dimensional networks exhibit both paramagnetic and ferromagnetic behavior, since the magnetic properties of these materials directly reflect the electronic configuration of the metal ion incorporated into the octacyanometalate building blocks (Pilkington et al., in preparation). For both the oxalate- and cyanide-bridged materials, we are able to manipulate the magnetic properties of the supramolecular assemblies by tuning the electronic configurations of the metal ions incorporated into the appropriate molecular building blocks (Pilkington and Decurtins, in “Magnetoscience—From Molecules to Materials,” Chimia 54, 593 (2000)).

Automatic Scan Planning for Magnetic Resonance Imaging of the Knee Joint

Automatic scan planning for magnetic resonance imaging of the knee aims at defining an oriented bounding box around the knee joint from sparse scout images in order to choose the optimal field of view for the diagnostic images and limit acquisition time. We propose a fast and fully automatic method to perform this task based on the standard clinical scout imaging protocol. The method is based on sequential Chamfer matching of 2D scout feature images with a three-dimensional mean model of femur and tibia. Subsequently, the joint plane separating femur and tibia, which contains both menisci, can be automatically detected using an information-augmented active shape model on the diagnostic images. This can assist the clinicians in quickly defining slices with standardized and reproducible orientation, thus increasing diagnostic accuracy and also comparability of serial examinations. The method has been evaluated on 42 knee MR images. It has the potential to be incorporated into existing systems because it does not change the current acquisition protocol.

Varying spin state composition by the choice of capping ligand in a family of molecular chains: detailed analysis of magnetic properties of chromium(III) horseshoes

We report a detailed physical analysis on a family of isolated, antiferro-magnetically (AF) coupled, chromium(III) finite chains, of general formula (Cr(RCO(2))(2)F)(n) where the chain length n = 6 or 7. Additionally, the chains are capped with a selection of possible terminating ligands, including hfac (= 1,1,1,5,5,5-hexafluoropentane-2,4-dionate(1-)), acac (= pentane-2,4-dionate(1-)) or (F)(3). Measurements by inelastic neutron scattering (INS), magnetometery and electron paramagnetic resonance (EPR) spectroscopy have been used to study how the electronic properties are affected by n and capping ligand type. These comparisons allowed the subtle electronic effects the choice of capping ligand makes for odd member spin 3/2 ground state and even membered spin 0 ground state chains to be investigated. For this investigation full characterisation of physical properties have been performed with spin Hamiltonian parameterisation, including the determination of Heisenberg exchange coupling constants and single ion axial and rhombic anisotropy. We reveal how the quantum spin energy levels of odd or even membered chains can be modified by the type of capping ligand terminating the chain. Choice of capping ligands enables Cr-Cr exchange coupling to be adjusted by 0, 4 or 24%, relative to Cr-Cr exchange coupling within the body of the chain, by the substitution of hfac, acac or (F)(3) capping ligands to the ends of the chain, respectively. The manipulation of quantum spin levels via ligands which play no role in super-exchange, is of general interest to the practise of spin Hamilton modelling, where such second order effects are generally not considered of relevance to magnetic properties.

Biological applications of magnetic nanoparticles

In this review an overview about biological applications of magnetic colloidal nanoparticles will be given, which comprises their synthesis, characterization, and in vitro and in vivo applications. The potential future role of magnetic nanoparticles compared to other functional nanoparticles will be discussed by highlighting the possibility of integration with other nanostructures and with existing biotechnology as well as by pointing out the specific properties of magnetic colloids. Current limitations in the fabrication process and issues related with the outcome of the particles in the body will be also pointed out in order to address the remaining challenges for an extended application of magnetic nanoparticles in medicine.

The amygdala in schizophrenia: a trimodal magnetic resonance imaging study

In schizophrenic psychoses, structural and functional alterations of the amygdala have been demonstrated by several neuroimaging studies. However, postmortem examinations on the brains of schizophrenics did not confirm the volume changes reported by volumetric magnetic resonance imaging (MRI) studies. In order to address these contradictory findings and to further elucidate the possibly underlying pathophysiological process of the amygdala, we employed a trimodal MRI design including high-resolution volumetry, diffusion tensor imaging (DTI), and quantitative magnetization transfer imaging (qMTI) in a sample of 14 schizophrenic patients and 14 matched controls. Three-dimensional MRI volumetry revealed a significant reduction of amygdala raw volumes in the patient group, while amygdala volumes normalized for intracranial volume did not differ between the two groups. The regional diffusional anisotropy of the amygdala, expressed as inter-voxel coherence (COH), showed a marked and significant reduction in schizophrenics. Assessment of qMTI parameters yielded significant group differences for the T2 time of the bound proton pool and the T1 time of the free proton pool, while the semi-quantitative magnetization transfer ratio (MTR) did not differ between the groups. The application of multimodal MRI protocols is diagnostically relevant for the differentiation between schizophrenic patients and controls and provides a new strategy for the detection and characterization of subtle structural alterations in defined regions of the living brain.

Single-pulse transcranial magnetic stimulation over the frontal eye field can facilitate and inhibit saccade triggering

The aim of this study was to investigate the effect of single-pulse transcranial magnetic stimulation on the triggering of saccades. The right frontal eye field was stimulated during modified gap and overlap paradigms with flashed presentation of the lateral visual target of 80 ms. In order to examine possible facilitating or inhibitory effects on saccade triggering, three different time intervals of stimulation were chosen, i.e. simultaneously with onset of the target, during the presentation and after target end. Stimulation applied simultaneously with target onset significantly decreased the latency of contralateral saccades in the gap but not in the overlap paradigm. Stimulation after target end significantly increased saccade latency for both sides in the gap paradigm and for the contralateral side in the overlap paradigm. Stimulation during presentation had no effect in either paradigm. The results show that, depending on the time interval and the paradigm tested, a facilitation or inhibition of saccade triggering can be achieved. The results are discussed in a context of two probable transcranial magnetic stimulation effects, a direct interference with the frontal eye field on the one hand and a remote interference with the superior colliculus on the other hand.

Neurophysiology and neuroanatomy of reflexive and volitional saccades as revealed by lesion studies with neurological patients and transcranial magnetic stimulation (TMS)

This review discusses the neurophysiology and neuroanatomy of the cortical control of reflexive and volitional saccades in humans. The main focus is on classical lesion studies and studies using the interference method of transcranial magnetic stimulation (TMS). To understand the behavioural function of a region, it is essential to assess oculomotor deficits after a focal lesion using a variety of oculomotor paradigms, and to study the oculomotor consequences of the lesion in the chronic phase. Saccades are controlled by different cortical regions, which could be partially specialised in the triggering of a specific type of saccade. The division of saccades into reflexive visually guided saccades and intentional or volitional saccades corresponds to distinct regions of the neuronal network, which are involved in the control of such saccades. TMS allows to specifically interfere with the functioning of a region within an intact oculomotor network. TMS provides advantages in terms of temporal resolution, allowing to interfere with brain functioning in the order of milliseconds, thereby allowing to define the time course of saccade planning and execution. In the first part of the paper, we present an overview of the cortical structures important for saccade control, and discuss the pro's and con's of the different methodological approaches to study the cortical oculomotor network. In the second part, the functional network involved in reflexive and volitional saccades is presented. Finally, studies concerning recovery mechanisms after a lesion of the oculomotor cortex are discussed.

Virtopsy: forensic traumatology of the subcutaneous fatty tissue; multislice computed tomography (MSCT) and magnetic resonance imaging (MRI) as diagnostic tools

Traumatic lesions of the subcutaneous fatty tissue provide important clues for forensic reconstruction. The interpretation of these patterns requires a precise description and recording of the position and extent of each lesion. During conventional autopsy, this evaluation is performed by dissecting the skin and subcutaneous tissues in successive layers. In this way, depending on the force and type of impact (right angle or tangent), several morphologically distinct stages of fatty tissue damage can be differentiated: perilobular hemorrhage (I), contusion (II), or disintegration (III) of the fat lobuli, and disintegration with development of a subcutaneous cavity (IV). In examples of virtopsy cases showing blunt trauma to the skin and fatty tissue, we analyzed whether these lesions can also be recorded and classified using multislice computed tomography (MSCT) and magnetic resonance imaging (MRI). MSCT has proven to be a valuable screening method to detect the lesions, but MRI is necessary in order to properly differentiate and classify the grade of damage. These noninvasive radiological diagnostic tools can be further developed to play an important role in forensic examinations, in particular when it comes to evaluating living trauma victims.

Mood effects of repetitive transcranial magnetic stimulation of left prefrontal cortex in healthy volunteers.

This study investigated the effect of high-frequency repetitive transcranial magnetic stimulation (HF-rTMS) of the left prefrontal cortex (LPFC) on mood in a sham-controlled crossover design. Twenty-five healthy male subjects received HF-rTMS of the LPFC in real and sham conditions. Forty trains (frequency 20 Hz, stimulation intensity 100% of individual motor threshold, train duration 2 s, intertrain interval 28 s) were applied in each session. Mood change from baseline was measured with five visual analog scales (VAS) for sadness, anxiety, happiness, tiredness and pain/discomfort. We were unable to demonstrate significant mood changes from baseline on visual analog scales after either sham or real stimulation of LPFC. There is insufficient evidence to support the general conclusion that HF-rTMS of LPFC has mood effects in healthy volunteers. Future studies should be sham-controlled, have larger sample sizes, and strictly stimulate one single region per session in order to exclude interaction effects with the previous stimulation.

Remarks on higher-order hadronic corrections to the muon g-2

Recently, it was shown that insertions of hadronic vacuum polarization at O(α4) generate non-negligible effects in the calculation of the anomalous magnetic moment of the muon. This result raises the question if other hadronic diagrams at this order might become relevant for the next round of g−2 measurements as well. In this note we show that a potentially enhanced such contribution, hadronic light-by-light scattering in combination with electron vacuum polarization, is already sufficiently suppressed.