Hyuk Chan Kwon

Improvement of the balance of the single-layer second-order high Tc SQUID gradiometer

Abstract We have studied detailed balancing of the directly coupled second-order YBa 2 Cu 3 O 7 superconducting quantum interference device gradiometer by adjusting the line width of the center pickup loop with all other parameters fixed. We obtained a balancing factor of 1.7×10 3 for the best-balanced device. Measured gradient noise level was 2 pT/cm 2 /√Hz at 10 Hz and the corresponding field noise was 200 fT/√Hz.

'Is tinnitus accompanied by hemifacial spasm in normal-hearing patients also a type of hyperactive neurovascular compression syndrome? : A magnetoencephalography study.

BackgroundTraditionally, tinnitus accompanied by hemifacial spasm has been considered a type of hyperactive neurovascular compression syndrome that is similar to hemifacial spasm alone because of the anatomically close relationship between the facial nerve and cochlear nerve as well as the hyperactive clinical nature.MethodsParticipants were 29 subjects who presented with hemifacial spasm and neuroradiological evidence of vascular compression of the cranial (facial/cochlear) nerve. We used magnetoencephalography (MEG) to estimate the activity of the cochlear nerve in patients with and without tinnitus on the ipsilateral side. We compared the difference in the latency and the ratio of the equivalent current dipole (ECD) strength between the ipsilateral and contralateral sides of the spasm and tinnitus.ResultsCochlear nerve activity in patients with tinnitus was increased with a shorter latency (p = 0.016) and stronger ECD strength (p = 0.028) compared with patients without tinnitus.ConclusionThe MEG results from normal-hearing patients who had tinnitus accompanied by hemifacial spasm suggest that the hyperactivity of the auditory central nervous system may be a crucial pathophysiological factor in the generation of tinnitus in these patients. The neurovascular compression that causes sensory input from the pathologic facial nerve activity may contribute to this hyperactivity of the central auditory nervous system.

DC SQUID magnetometers with the directly coupled pick-up loop

DC SQUID magnetometers with a directly coupled square pick-up loop have been made from single layer YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films using step edge junctions or bicrystal grain boundary junctions on SrTiO/sub 3/ substrates. YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films were deposited by off-axis RF sputtering or laser ablation and patterned by photolithography and Ar ion milling to form the SQUID, pick-up loop and modulation coil. The SQUIDs showed RSJ like current vs. voltage characteristics and voltage modulation in response to external fields at 77 K. The SQUID inductance was 55-140 pH, pick-up loop size was 47 mm/sup 2/ and the effective area of magnetometers estimated from the measured modulation period was 0.2-0.4 mm/sup 2/. The magnetic field noise was 7.4/spl times/10/sup -13/ T//spl radic/(Hz) at 10 Hz and 77 K.<<ETX>>

Accuracy of MEG in localizing irritative zone and seizure onset zone: Quantitative comparison between MEG and intracranial EEG.

BACKGROUND We conducted the study to examine accuracy of the magnetoencephalography (MEG) spike source localization in presurgical evaluation of patients with medically refractory focal epilepsy. METHODS Ten consecutive patients with refractory focal epilepsy who were candidates for two-stage surgery with long-term intracranial electroencephalography (ICEEG) monitoring were enrolled. Interictal MEG recordings with simultaneous scalp EEG were obtained within 7days before the ICEEG electrode implantation. The location of each MEG spike source was quantitatively compared with ICEEG spike foci (focal area of interictal spikes) and ICEEG ictal foci (earliest cortical origin of seizures). Gyral-width concordance and sublobar concordance were also determined for all MEG spike sources. Gyral-width concordance was defined by distance of 15mm or less between MEG spike sources and ICEEG spike foci or ICEEG ictal foci. RESULTS Visual analyses of the MEG traces of all 10 patients revealed 292 spikes (29.2±24.0 per patient). Spike yield of the MEG was similar to the simultaneously recorded scalp EEG. MEG spike sources were closely located with ICEEG spike foci (distance: 9.3±10.8mm). Clustered MEG spike sources were even closer to ICEEG spike foci (distance: 7.3±6.4mm). MEG spike sources, even clustered ones, were less concordant with ICEEG ictal foci and had significant longer distance from ICEEG ictal foci (distance: 21.5±15.6mm for all sources, 19.7±13.7mm for clustered sources). Gyral-width concordance rate and sublobar concordance rate were also higher with ICEEG interictal spike foci than with ICEEG ictal foci. On the other hand, 53.4% of interictal spike foci from ICEEG were not detected by interictal MEG recordings. CONCLUSIONS MEG spike sources, especially clustered ones, from interictal recording could localize the irritative zone of ICEEG with a high accuracy. However, MEG spike sources have relatively poor correlation with seizure onset zone and lower sensitivity in identifying all irritative zones of ICEEG. This limitation should be considered in the interpretation of MEG results.

Electron diffraction due to a reflection grating in a conducting wire

We report on quantum transport in the presence of an electron reflection grating fabricated within a high electron mobility transistor structure. The grating was composed of a periodically corrugated potential wall by which the electron waves are diffracted. The low temperature conductance shows a number of peaks with respect to the gate voltage, which are consistent with the electron diffraction effect and are predicted by the Fraunhofer diffraction condition.

Assessment of non-motor hearing symptoms in hemifacial spasm using magnetoencephalography.

BackgroundHemifacial spasm patients often suffer from non-motor symptoms such as tinnitus. These non-motor symptoms are known to be associated with changes in cortical activity. Magnetoencephalography (MEG) is a technique that can record brain activity noninvasively. To determine the usefulness of MEG in assessing changes in cortical activity associated with non-motor hearing symptoms in hemifacial spasm patients.MethodsWe used MEG to evaluate the reactivity of the auditory cortex in 26 hemifacial spasm patients. We divided patients into a subjective tinnitus group (n = 10) and a non-tinnitus group (n = 16). The latency and amplitude of the most prominent deflection, N100m, was compared between the two groups.ResultsThere was a significant difference in the pure tone audiogram on the spasm side compared with the non-spasm side. After stimulation on the spasm side, the amplitude of the N100m peak in the contralateral hemisphere was lower in the subjective tinnitus group than in the non-tinnitus group.ConclusionsOur results indicate that MEG can detect differences in cortical activity between hemifacial spasm patients with and without tinnitus. This suggests that MEG can identify changes in cortical activity associated with non-motor symptoms.

Magnetic field dependence of the resistance anomaly in superconducting mesoscopic aluminum structures

A mesoscopic aluminum wire of 0.28 μm width was prepared to investigate the resistance anomaly found in the superconducting transitions. We found that the radio-frequency (rf) radiation is an important factor in the anomaly, which strongly supports the recent work of Strunk et al. [Strunk, C., Bruyndoncx, V., Van Haesendonck, C., Moshchalkov, V. V., Bruynseraede, Y., Burk, B., Chien, C. -J. and Chandrasekhar, V., Phys. Rev. B, 1996, 53, 11332]. We performed a systematic study of the magnetic field dependence of the anomaly, which revealed the “mesa” type magnetoresistance near H=0 for 1.13 K<T<1.16 K. The boundary of the anomalous region in H–T space coincides with the upper critical field curve of the 2 μm wide leads connected to the wire, implying a crucial role of the wide leads for the anomaly, having different Tc and dimensionality from those of the narrow wire. As soon as the leads become superconducting, the undissipated rf field transmits to the wire and induces the phase-slip centers to manifest the anomalous behavior.

Direct-coupled high T/sub c/ DC SQUID gradiometer with single layer YBa/sub 2/Cu/sub 3/O/sub 7/

We have studied first derivative high T/sub c/ dc SQUID (Superconducting QUantum Interference Device) gradiometer from a single layer of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ thin film using pulsed laser deposition (PLD) and argon ion beam etching method. The planar gradiometer consists of two pickup loops coupled directly to dc SQUID with an inductance of about 20 pH in the center of the structure. The gradiometer contains step-edge type junctions of 3 /spl mu/m width and is made on the SrTiO/sub 3/ substrate of 12/spl times/25 mm/sup 2/ in size. The SQUID showed RSJ type junction I-V characteristics with T/sub c/ of 86 K and swing voltage of /spl sim/4 /spl mu/V at 79.3 K. The magnetometer was sensitive only to the gradient of the magnetic field with well balanced null response to uniform fields.

High T/sub c/ SNS step-edge junctions and SQUIDs by using; YBa/sub 2/Cu/sub 3/O/sub 7//CaRuO/sub 3/ double layer

Superconductor-normal-superconductor (SNS) step-edge junction dc SQUIDs were fabricated from YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films with CaRuO/sub 3/ (CRO) on top as the normal metal. The films were made by pulsed laser deposition (PLD) and patterned by photolithography and ion milling. The junctions and SQUIDs had to be annealed in oxygen environment for a much longer period of time (5-48 hours) compared with standard YBCO film preparation (as deposited or 1 hour), indicating that the top CRO film blocks oxygen in and out of the YBCO film. Before annealing T/sub c/s of CRO/YBCO double layers and junctions made from them were <70 X and <50 K, respectively, and increased to >88 K and >77 K, respectively after long time annealing. Current-voltage curves of the junctions showed well-behaved resistively-shunted-junction (RSJ) properties and the temperature dependence of the junction critical current was roughly linear. SQUIDs made of those junctions showed voltage modulation in response to applied fields at 77 K.<<ETX>>

Symptom-associated change of motor-related neuromagnetic fields in a patient with multiple sclerosis: A case report

The objective of this study was to investigate functional abnormalities of the brain in a patient with multiple sclerosis (MS) by using magnetoencephalography (MEG) and a finger-tapping task. A 46-year-old woman that presented with motor weakness of left hand and was diagnosed with MS. Conventional magnetic resonance imaging demonstrated a white matter lesion with hyperintensity on T2-weighted images in the right motor area. MEG recordings were performed during the period of motor weakness and after clinical improvement. Neuromagnetic brain activation was elicited by a simple, visually cued finger movement. The Equivalent current dipole (ECD) strength of the movement-evoked field (MEF) in the affected hemisphere was significantly decreased relative to the unaffected hemisphere. After improvement in motor weakness, we found that the lower amplitude of the readiness field and decreased ECD strength of the MEF observed in affected hemisphere during motor weakness had recovered. Analysis of motor-related neuromagnetic fields revealed that MEG may be used to detect diffuse changes in the brain that are not observable by conventional imaging of white matter regions in MS. We further found that brain activities can change after improvement in motor weakness.