D.J. Zheng

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.

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.

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. >

Dissipative flow of Josephson and Abrikosov fluxons in high Tc superconductors

Abstract A model is presented for the field dependent microwave absorption in high T c superconductors; fluxon motion in Josephson juctions and in bulk material causes the absorption. Junction dissipation is demonstrated by microwave induced dc voltage across YBaCuO samples which has a magnetic field dependence.

Absorption of microwaves in superconducting YBa 2 Cu 3 O 7 and field-cooling effects

Measurements of magnetic-field-dependent microwave absorption as a function of both the temperature (4 K to Tc) and cooling field (0–10 kG) are reported. Below a certain characteristic temperature, T* < Tc, the absorption strongly depends on the magnetic field in which the samples are cooled. The characteristic temperature T* scales with Tc.

Microwave-induced voltage in superconducting YBaCuO

Abstract Dc voltage is induced across bulk samples of YBa 2 Cu 3 O 7 exposed to microwave radiation. This voltage is a function of sample temperature and microwave power level with a strong dependence on the external magnetic field, the maximum being at zero field. This is attributed to Josephson junctions in the material.

Scaling behavior of a dilute spin glass of Ag-Mn

Abstract The magnetization of a dilute Ag-Mn alloy (150 ppm) was studied from 4 K down to 10 mK in magnetic fields ranging from 932 to 1 Oe. The non-linear susceptibility is analyzed in terms of a scaling theory. Scaling exponents agree with those in much higher concentration spin glasses.

High T/sub c/ RF-biased SQUID

Results are presented on the behaviour of a simple RF-biased SQUID (superconducting quantum interference device) made out of YBa/sub 2/Cu/sub 3/O/sub 7/. It consists of a fractured ceramic pellet stuck together at room temperature. This simple device is shielded by a high-T/sub c/ tube and shows the characteristic flux quantization behaviour up to 77 K. At 4.2 K its magnetic flux resolution is less than 2*10/sup -4/ phi /sub 0// square root Hz, and it shows sensitivity degradation as the temperature is raised. >