O. G. Symko

Spin glass behavior of Cd1−xMnxTe below the nearest‐neighbor percolation limit

Magnetization measurements down to 10 mK of Cd1−xMnxTe for Mn concentrations 0.01≤x≤0.15 show spin glass behavior. Such behavior is attibuted to short‐range exchange and dipolar interactions. Both interactions are used to explain the concentration dependence of the spin freezing temperatures for 0.01≤x≤0.6, the short‐range exchange dominating at high concentrations, and the dipolar interaction at low concentrations.

Period doubling in a perturbed sine-Gordon system, a long Josephson junction

Abstract A route to chaos, by period-doubling bifurcation, has been found in a perturbed sine-Gordon system which describes the dynamics of long Josephson junctions in a magnetic field. The chaotic motion is within Fiske steps and it is caused by irregular motion of solitons. Negative dynamic resistance regions are found in the chaotic regime and they are compared with experimental results.

Magnetic shielding and relaxation characteristics of superconducting YBa2Cu3O7 tubes

High‐Tc superconducting tubes have been developed for magnetic shielding of SQUIDs at 77 K. The characteristics of such tubes show that adequate shielding exists for magnetic fields up to a critical field determined by the current density of the tube. Relaxation phenomena with two different time dependencies are observed when the external magnetic field enters the tubes.

Discrete spatial filtering with SQUID gradiometers in biomagnetism

First‐, second‐, and third‐order gradiometers used in detecting biomagnetic signals are analyzed as spatial filters. Their transfer functions independent of the source to be measured are presented and both the magnitude and phase characteristics of the transfer functions are analyzed. The distortion introduced by the gradiometer can be estimated from these characteristics. In order to treat the signal in that approach, the spatial Fourier transform of a magnetic signal produced by a current dipole at a given distance is discussed.

Characteristics of YBaCuO magnetic shields

Magnetic shielding properties of YBaCuO tubes have been studied at temperatures of 77 K and 4 K. Shielding effectiveness was determined by measuring the magnetic field inside the tube in the presence of external magnetic fields. A high degree of shielding was achieved up to a critical external field determined by the critical current density of the material. Typically, this critical field is 23 Oe at 77 K extending up to 105 Oe at 4 K. Tubes made with material containing 10% silver exhibited much stronger flux pinning behavior when the external magnetic field started to penetrate the tubes. The tubes studied were effective in shielding a RF-biased SQUID (superconducting quantum interference device) for operation at 77 K.

Spatial Fourier transform method for evaluating SQUID gradiometers

A simple method of measuring the spatial transfer function of a gradiometer, consisting of a flux transformer coupled to a SQUID, is presented and it is compared with theoretical predictions. Based, on this approach, a new method of reporting a gradiometer’s performance is proposed; the rejection factor is expressed in decibels obtained directly from the transfer function plot.

Absorption at radio frequencies in superconducting Y1Ba2Cu3Oy

The large magnetic‐field‐dependent ac absorption in superconducting Y1 Ba2 Cu3 Oy ceramics and powders decreases slowly with increasing frequency in the range 2–16 MHz. The magnetic‐field‐dependent ac absorption is observed below Tc in superconducting Y1 Ba2 Cu3 Oy ceramics, powders and twinned crystals and in (La,Sr)2 CuO4 and BaPb0.75 Bi 0.25 CuO3 powders and appears to contain distinct contributions from trapped flux and/or intragrain tunnel junctions and from ordinary superconductor surface impedance. The contribution from trapped flux and/or intragrain tunnel junctions is important in dc magnetic fields of up to approximately 20 G at all temperatures below Tc. There is an identifiable modulated ac absorption which changes slowly over more than 12 kG at low temperatures and which we propose is primarily caused by the ac surface impedance with possible contributions from the critical state. A twinned crystal of Y1 Ba2 Cu3 Oy exhibits the same ac absorption except that it has an anisotropic dependence o...The large magnetic-field-dependent ac absorption in superconducting Y/sub 1/ Ba/sub 2/ Cu/sub 3/ O/sub /ital y// ceramics and powders decreases slowly with increasing frequency in the range 2--16 MHz. The magnetic-field-dependent ac absorption is observed below /ital T//sub /ital c// in superconducting Y/sub 1/ Ba/sub 2/ Cu/sub 3/ O/sub /ital y// ceramics, powders and twinned crystals and in (La,Sr)/sub 2/ CuO/sub 4/ and BaPb/sub 0.75/ Bi /sub 0.25/ CuO/sub 3/ powders and appears to contain distinct contributions from trapped flux and/or intragrain tunnel junctions and from ordinary superconductor surface impedance. The contribution from trapped flux and/or intragrain tunnel junctions is important in dc magnetic fields of up to approximately 20 G at all temperatures below /ital T//sub /ital c//. There is an identifiable modulated ac absorption which changes slowly over more than 12 kG at low temperatures and which we propose is primarily caused by the ac surface impedance with possible contributions from the critical state. A twinned crystal of Y/sub 1/ Ba/sub 2/ Cu/sub 3/ O/sub /ital y// exhibits the same ac absorption except that it has an anisotropic dependence on the applied dc magnetic field.

Femtosecond dynamics of quasi-particles in YBa/sub 2/Cu/sub 3/O/sub 7- delta / superconductor films

The transient electronic response of epitaxially grown YBa/sub 2/Cu/sub 3/O/sub 7- delta / superconductor thin films in the femtosecond time domain was investigated using transient photoinduced reflectivity ( Delta R) with 60-fs time resolution. For temperatures T>T/sub c/, only a bolometric signal was observed with Delta R>0. For T 0. The results for T<T/sub c/ are explained in terms of quasi-particle (QP) electronic response giving Delta R<0. Thus, the femtosecond rise time is interpreted as avalanche multiplication of QP across the gap 2 Delta , and the subsequent picosecond relaxation is interpreted as QP recombination. The QP optical response is explained within the framework of the two-fluid model.

Noise characteristics and instabilities of long Josephson junctions

The noise characteristics and chaotic behavior of long Josephson junctions in a magnetic field are investigated. In a magnetic field, current-biased junctions exhibit the dynamics of fluxon motion affected by fluctuations. These consist of telegraph noise at voltage steps and instabilities due to chaotic behavior. Results on long junctions with McCumber number ranging from 10 to 100 show such behavior. The telegraph noise is driven by thermal fluctuations. Modeling of the junctions using a perturbed sine-Gordon equation shows chaotic and periodic regions. Some of the chaotic regions are between Fiske steps, while others are at the edge of the Fiske steps. These observations are confirmed by numerical calculations. >

Magnetic phase diagram of Cd1−xMnxSe below the nearest neighbor percolation limit

Abstract Magnetization measurements down to 10 mK of Cd1−xMnxSe at Mn concentrations below the nearest-neighbor percolation limit show spin glass behavior. The previously accepted phase diagram is revised, showing a x2 dependence at low x. Such behavior is attributed to neighbors further than nearest-neighbors and Bloembergen-Rowland type of indirect exchange.