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.

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.

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.

Controlled aggregation of magnetic ions in a semiconductor: an experimental demonstration.

The control on the distribution of magnetic ions into a semiconducting host is crucial for the functionality of magnetically doped semiconductors. Through a structural analysis at the nanoscale, we give experimental evidence that the aggregation of Fe ions in (Ga,Fe)N and consequently the magnetic response of the material are affected by the growth rate and doping with shallow impurities.

Observation of strong-coupling effects in a diluted magnetic semiconductor Ga1-xFexN.

The giant Zeeman splitting of free excitons is measured in Ga(1-x)Fe(x)N. Magneto-optical and magnetization data imply the ferromagnetic sign and a reduced magnitude of the effective p-d exchange energy governing the interaction between Fe(3+) ions and holes in GaN, N_{0}beta(app)=+0.5+/-0.2 eV. This finding corroborates the recent suggestion that the strong p-d hybridization specific to nitrides and oxides leads to significant renormalization of the valence band exchange splitting.

Sensory gating of auditory evoked and induced gamma band activity in intracranial recordings.

Oscillatory activity in the gamma band range (30-50 Hz) and its functional relation to auditory evoked potentials (AEPs) is yet poorly understood. In the current study, we capitalized on the advantage of intracranial recordings and studied gamma band activity (GBA) in an auditory sensory gating experiment. Recordings were obtained from the lateral surface of the temporal lobe in 34 epileptic patients undergoing presurgical evaluation. Two kinds of activity were differentiated: evoked (phase locked) and induced (not phase locked) GBA. In 18 patients, an intracranial P50 was observed. At electrodes with maximal P50, evoked GBA occurred with a similar peak latency as the P50. However, the intensities of P50 and evoked GBA were only modestly correlated, suggesting that the intracranial P50 does not represent a subset of evoked GBA. The peak frequency of the intracranial evoked GBA was on average relatively low (approximately 25 Hz) and is, therefore, probably not equivalent to extracranially recorded GBA which has normally a peak frequency of approximately 40 Hz. Induced GBA was detected in 10 subjects, nearly exclusively in the region of the superior temporal lobe. The induced GBA was increased after stimulation for several hundred milliseconds and encompassed frequencies up to 200 Hz. Single-trial analysis revealed that induced GBA occurred in relatively short bursts (mostly <<100 ms), indicating that the duration of the induced GBA in the averages originates from summation effects. Both types of gamma band activity showed a clear attenuation with stimulus repetition.

Velocity of domain-wall motion induced by electrical current in the ferromagnetic semiconductor (Ga,Mn)As.

Current-induced domain-wall motion with velocity spanning over 5 orders of magnitude up to 22 m/s has been observed by the magneto-optical Kerr effect in (Ga,Mn)As with perpendicular magnetic anisotropy. The data are employed to verify theories of spin transfer by the Slonczewski-like mechanism as well as by the torque resulting from spin-flip transitions in the domain-wall region. Evidence for domain-wall creep at low currents is found.

Domain-wall resistance in ferromagnetic (Ga,Mn)As.

A series of microstructures designed to pin domain walls (DWs) in (Ga,Mn)As with perpendicular magnetic anisotropy has been employed to determine extrinsic and intrinsic contributions to DW resistance. The former is explained quantitatively as resulting from a polarity change in the Hall electric field at DW. The latter is 1 order of magnitude greater than a term brought about by anisotropic magnetoresistance and is shown to be consistent with disorder-induced mistracking of the carrier spins subject to spatially varying magnetization.

Spin reorientation transition in single-domain.

We demonstrate that the interplay of in-plane biaxial and uniaxial anisotropy fields in results in a spin reorientation transition and an anisotropic ac susceptibility which is fully consistent with a simple single-domain model. The uniaxial and biaxial anisotropy constants vary, respectively, as the square and fourth power of the spontaneous magnetization across the whole temperature range up to . The weakening of the anisotropy at the transition may be of technological importance for applications involving thermally assisted magnetization switching.

Monte Carlo simulations of ferromagnetism in p-Cd1-xMnxTe quantum wells.

Monte Carlo simulations, in which the Schrödinger equation is solved at each Monte Carlo sweep, are employed to assess the influence of magnetization fluctuations, short-range antiferromagnetic interactions, disorder, magnetic polaron formation, and spin-Peierls instability on the carrier-mediated Ising ferromagnetism in two-dimensional electronic systems. The determined critical temperature and hysteresis are affected in a nontrivial way by the antiferromagnetic interactions. The findings explain striking experimental results for modulation-doped p-Cd1-xMnxTe quantum wells.

Processing of famous faces and medial temporal lobe event-related potentials: a depth electrode study.

The present study aims at analyzing the modulation of two types of event-related potentials originating from the human medial temporal lobe, the rhinal AMTL-N400 and the hippocampal P600 by the processing of famous faces. Therefore, we used a face recognition paradigm in which subjects had to discriminate the faces of famous persons from the faces of non-famous persons. Eleven patients with unilateral medial temporal lobe epilepsy undergoing intrahippocampal depth electrode recording for presurgical evaluation participated in this study. Event-related potentials (ERP) were recorded while a sequence of famous and non-famous faces was presented to the patients. The presentation of each face was repeated. The faces evoked N400-like potentials (anterior medial temporal lobe N400, AMTL-N400) in the rhinal cortex and P600-like potentials in the hippocampus. ERPs elicited by famous faces were contrasted with ERPs elicited by non-famous faces. The first presentation of famous faces elicited an enhanced AMTL-N400 and an enhanced hippocampal P600 in comparison to the second presentations of the famous faces or the (first and second presentation of the) non-famous faces. This findings are discussed in terms of associative semantic memory processes and the retrieval of person-specific information from long-term memory stores triggered by the processing of famous faces.

Recognition of famous faces in the medial temporal lobe: an invasive ERP study.

OBJECTIVE: To investigate the involvement of the rhinal cortex and the hippocampus in the processing of famous faces in contrast to nonfamous faces using intracranial event-related potentials (ERPs), and to analyze repetition effects for famous and nonfamous faces. METHODS: ERPs were elicited by pictures of famous and nonfamous faces and recorded from rhinal and hippocampal sites of intracranial electrodes in 10 presurgical patients with unilateral medial temporal lobe epilepsy. Famous and nonfamous faces were presented twice and mixed with distorted faces serving as targets. There was no instruction for an overt discrimination between famous and nonfamous faces. In contrast to nonfamous faces, famous faces stimulate processes related with access and retrieval of semantic memory. RESULTS: All faces evoked anterior medial temporal lobe N400-like (AMTL-N400) potentials in the rhinal cortex and P600-like potentials in the hippocampus. The AMTL-N400 and the hippocampal P600 amplitudes were larger for famous faces than for nonfamous faces. Mean amplitudes of the first and second presentation of famous faces suggest a repetition effect for the rhinal sites; however, they are significant only in the later signal components. No repetition effect was found for nonfamous faces and for potentials from the hippocampus. CONCLUSION: The anterior medial temporal lobe N400 and the hippocampal P600 may be related to the access and retrieval of person-specific semantic memory.

Mn interstitial diffusion in (ga,mn)as.

We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low-temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, in situ Auger spectroscopy, and resistivity measurements during annealing show that the observed changes are due to out diffusion of Mn interstitials towards the surface, governed by an energy barrier of 0.7-0.8 eV. Electric fields induced by Mn acceptors have a significant effect on the diffusion.

Auditory information processing is altered in novelty stress conditions: first session effects in auditory-evoked potentials.

Novelty conditions may elicit stress responses. First session effects are systematic changes in physiological parameters, resulting from the interference of physiological processes with novelty stress. Along with endocrinological changes, these stress responses may be accompanied by alterations of sensory and attentional processes. The present study examines the impact of novelty conditions on event-related potential indicators of auditory information processing and on cortisol. Twenty-two healthy subjects participated in a series of experimental sessions. Auditory-evoked potentials were recorded, and the plasma cortisol levels were determined. The first session causes an activation of the hypothalamo-hypopituitary-adrenal axis. The auditory-evoked potentials show an additional slow negative potential component in the novelty condition. This potential component is maximal at fronto-central electrode sites and reaches its peak at about 240 ms after stimulus onset. Similarities with the processing negativity suggest alterations in attention-related auditory information-processing in the novelty condition encountered in the first session.

Spin filtering in a hybrid ferromagnetic-semiconductor microstructure.

We fabricated a hybrid structure in which cobalt and permalloy micromagnets produce a local in-plane spin-dependent potential barrier for high-mobility electrons at the GaAs/AlGaAs interface. Spin effects are observed in ballistic transport in the range of tens of mT of the external field and are attributed to switching between Zeeman and Stern-Gerlach modes--the former dominating at low electron densities.

Capgras syndrome--out of sight, out of mind?

OBJECTIVE: Recent theoretical approaches emphasize a disorder of face processing in the pathogenesis of the Capgras syndrome. We report a patient with the Capgras syndrome developing in the physical absence of the person who is believed to be replaced and thus a limited role for a disorder of face processing. METHOD: The clinical phenomenology of a case of the Capgras syndrome is explored. RESULTS: A disorder of face processing might not be a sufficient explanation of the course of the disorder in this patient. CONCLUSION: Face processing accounts, of the Capgras delusion, have to be supplemented by additional assumptions.

Probing hole-induced ferromagnetic exchange in magnetic semiconductors by inelastic neutron scattering.

The effect of hole doping on the exchange coupling of the nearest neighbor (NN) Mn pairs in Zn(1-x)MnxTe is probed by inelastic neutron scattering. The difference in the NN exchange energy DeltaJ1 in the presence and in the absence of the holes is determined. The obtained value of DeltaJ1 is in good agreement with the predictions of the Zener/RKKY model, even on the insulator side of the metal-insulator transition.

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.