Origin of bulk uniaxial anisotropy in zinc-blende dilute magnetic semiconductors.

It is demonstrated that the nearest-neighbor Mn pair on the GaAs (001) surface has a lower energy for the [110] direction compared to the [110] case. According to the group theory and Luttinger's method of invariants, this specific Mn distribution results in bulk uniaxial in-plane and out-of-plane anisotropies. The sign and magnitude of the corresponding anisotropy energies determined by a perturbation method and ab initio computations are consistent with experimental results.

Anomalous Hall effect in field-effect structures of (Ga,Mn)As.

The anomalous Hall effect in metal-insulator-semiconductor structures having thin (Ga,Mn)As layers as a channel has been studied in a wide range of Mn and hole densities changed by the gate electric field. Strong and unanticipated temperature dependence, including a change of sign, of the anomalous Hall conductance sigma(xy) has been found in samples with the highest Curie temperatures. For more disordered channels, the scaling relation between sigma(xy) and sigma(xx), similar to the one observed previously for thicker samples, is recovered.

Fermi level position, Coulomb gap, and Dresselhaus splitting in (Ga,Mn)As.

Carrier-induced nature of ferromagnetism in a ferromagnetic semiconductor, (Ga,Mn)As, offers a great opportunity to observe novel spin-related phenomena as well as to demonstrate new functionalities of spintronic devices. Here, we report on low-temperature angle-resolved photoemission studies of the valence band in this model compound. By a direct determination of the distance of the split-off band to the Fermi energy EF we conclude that EF is located within the heavy/light hole band. However, the bands are strongly perturbed by disorder and disorder-induced carrier correlations that lead to the Coulomb gap at EF, which we resolve experimentally in a series of samples, and show that its depth and width enlarge when the Curie temperature decreases. Furthermore, we have detected surprising linear magnetic dichroism in photoemission spectra of the split-off band. By a quantitative theoretical analysis we demonstrate that it arises from the Dresselhaus-type spin-orbit term in zinc-blende crystals. The spectroscopic access to the magnitude of such asymmetric part of spin-orbit coupling is worthwhile, as they account for spin-orbit torque in spintronic devices of ferromagnets without inversion symmetry.

Stretching magnetism with an electric field in a nitride semiconductor.

The significant inversion symmetry breaking specific to wurtzite semiconductors, and the associated spontaneous electrical polarization, lead to outstanding features such as high density of carriers at the GaN/(Al,Ga)N interface-exploited in high-power/high-frequency electronics-and piezoelectric capabilities serving for nanodrives, sensors and energy harvesting devices. Here we show that the multifunctionality of nitride semiconductors encompasses also a magnetoelectric effect allowing to control the magnetization by an electric field. We first demonstrate that doping of GaN by Mn results in a semi-insulating material apt to sustain electric fields as high as 5 MV cm-1. Having such a material we find experimentally that the inverse piezoelectric effect controls the magnitude of the single-ion magnetic anisotropy specific to Mn3+ ions in GaN. The corresponding changes in the magnetization can be quantitatively described by a theory developed here.

Long-term habituation of brain evoked potential responses and pituitary-adrenal secretion with repeated (placebo) testing.

To study whether changes in late auditory evoked potentials (AEPs) and/or in stress-sensitive hormones of the hypothalamic-pituitary-adrenal (HPA) system take place between a first and a second placebo experiment and if so, whether these changes are possibly related to each other, we conducted two identical placebo sessions (2 ml 0.9% saline) and one cortisol session (50 mg) with 10 subjects on three different days. Plasma cortisol concentrations were significantly higher at the beginning of the first placebo experiment than the second, with a concordant decrease of plasma adrenocorticotropin hormone (ACTH) concentrations. In the AEP domain, a consistently lower P2 amplitude was observed in the first session. Since the change in late auditory processing could not be demonstrated after exogenous administration of cortisol, a direct mediation through an elevation of plasma cortisol concentrations or indirect mediation through a decrease of plasma ACTH concentrations seems unlikely. We rather propose that other stress-sensitive mechanisms, such as CCK, might account for the novelty-induced P2 amplitude lowering.

Changes in late auditory evoked potentials induced by growth hormone-releasing hormone (GHRH) but not somatostatin (SRIF) after peripheral administration in male controls.

To investigate possible influences of growth hormone-releasing hormone (GHRH) and somatostatin (SRIF) on auditory perceptional processes, 12 subjects received either placebo (sodium chloride 0.9%), GHRH (50 micrograms), or SRIF (100 micrograms) on different days. Late auditory evoked potentials (AEP) were computed and further analyzed by using the brain electric source analysis (BESA) method. Reduced late AEP latencies were observed following GHRH administration. In contrast, SRIF had no significant effects on the AEP. The changes in late auditory processing seen after administration of GHRH were most likely induced by a direct central nervous action.

Enhanced long-latency somatosensory potentials in major depressive disorder.

Bodily misperceptions are a frequent symptom in major depressive disorder. A reduced ability to deflect attention from somatosensory stimuli may contribute to the generation of unpleasant bodily sensations and co-occur with altered habituation of the brain electric reactions to somatosensory stimuli. The aim of the present study was to explore whether attention-related components of somatosensory evoked potentials (SSEP) and the habituation of these components are altered in major depression. Fifteen patients with major depressive disorder were compared to an age- and gender-matched group of 15 healthy controls. A series of identical, intrusive but not painful electric stimuli were applied to the left index finger for 48 min. Averaged SSEP were computed from multichannel EEG recordings for consecutive recording blocks of the experiment, each block containing 162 stimuli. Based on these data the habituation process of late components of the SSEP was analysed in two latency intervals (50-150, 170-370 ms). Patients showed significantly enhanced reactions throughout the entire experiment. The persistence of enhanced SSEP components throughout the habituation process may be caused by a deficit in reducing the activity of attention-related brain processes concerned with intrusive, yet behaviourally irrelevant, continued stimulation in the state of major depression.

Orienting response and frontal midline theta activity: a somatosensory spectral perturbation study.

OBJECTIVE: Brain electric activity in the theta frequency band has been associated with the encoding of new, and the retrieving of previously stored, information. We studied the time course of stimulus-to-stimulus changes of theta activity under repetitive somatosensory stimulation. MATERIALS AND METHODS: Twelve healthy subjects participated in the study. Repetitive electric stimuli, grouped into 48 stimulus trains, were applied to the left index finger. The stimulus trains contained 27 stimuli (0.9 Hz, 2.5 times sensory threshold). Each stimulus train of 30 s was followed by a stimulus-free break of 30 s. This stimulation paradigm allowed the separate estimation of effects for each position of the stimulus in the train and an analysis of stimulus-to-stimulus changes. Multichannel EEG recordings allowed a topographic analysis of the event-related spectral perturbation effects in the theta frequency band. The brain electric novelty response triggered by the stimulus train onset was analyzed by 3 methods: (1) event-related potentials; (2) event-related power spectra for the investigation of spectral perturbation effects on theta activity; and (3) an approach to break down the stimulus-induced theta activity into phase-locked activity and effects on the spontaneous, ongoing theta activity using digital filtering. RESULTS: The main findings are a frontal midline activation in the theta band with the beginning of the stimulus train, which habituates during the subsequent stimulation cycles, as well as evidence that distinct effects of the first stimulus on the ongoing, non-phase-locked, theta activity exist.

Involuntary vocalisations and a complex hyperkinetic movement disorder following left side thalamic haemorrhage.

A variety of involuntary speech phenomena as for example palilalia have been described as consequences of neurological disorders. Palilalia is the involuntary repetition of syllabels, words and phrases in ongoing speech. We describe a 73 year old woman who suffered from a hypertensive thalamic haemorrhage. MRI revealed that the lesion was predominantly located within the pulvinar, extending to the lateroposterior thalamic nuclei and to the pretectal area with possible involvement of the medial geniculate body. Few months after the event she developed involuntary vocalisations with whole words and meaningless syllables being rapidly reiterated. In contrast to typical palilalia these vocalisations were not meaningfully related to the ongoing speech of the patient. In addition, the patient developed a complex hyperkinetic movement disorder with right-sided painful hemidystonia and bilateral clonic jerks and a right-sided postural tremor.