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تسجيل مستخدم جديدHot spots and waves in Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks -a study by Low Temperature Scanning Laser Microscopy
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Recently, it has been shown that large stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization presumably triggered by a cavity resonance. To investigate this effect we use Low Temperature Scanning Laser Microscopy to image electric field distributions. Apart from verifying the appearance of cavity modes at low bias we find that, in a high input power regime, standing-wave patterns are created through interactions with a hot spot, possibly pointing to a new mode of generating synchronized radiation in intrinsic Josephson junction stacks.
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We have studied experimentally and numerically temperature profiles and the formation of hot spots in intrinsic Josephson junction stacks in Bi2Sr2CaCu2O8 (BSCCO). The superconducting stacks are biased in a state where all junctions are resistive. Th
e formation of hot spots in this system is shown to arise mainly from the strongly negative temperature coefficient of the c-axis resistivity of BSCCO at low temperatures. This leads to situations where the maximum temperature in the hot spot can be below or above the superconducting transition temperature Tc. The numerical simulations are in good agreement with the experimental observations.
We report on THz emission measurements and low temperature scanning laser imaging of Bi_2Sr_2CaCu_2O_8 intrinsic Josephson junction stacks. Coherent emission is observed at large dc input power, where a hot spot and a standing wave, formed in the col
d part of the stack, coexist. By varying the hot spot size the cavity resonance frequency and the emitted radiation can be tuned. The linewidth of radiation is much smaller than expected from the quality factor of the cavity mode excited. Thus, an additional mechanism of synchronization seems to play a role, possibly arising from nonequilibrium processes at the hot spot edge.
Recently it has been found that, when operated at large input power, the linewidth of terahertz radiation emitted from intrinsic Josephson junction stacks can be as narrow as some megahertz. In this high-bias regime a hot spot coexists with regions w
hich are still superconducting. Surprisingly, the linewidth was found to decrease with increasing bath temperature. We present a simple model describing the dynamics of the stack in the presence of a hot spot by two parallel arrays of pointlike Josephson junctions and an additional shunt resistor in parallel. Heat diffusion is taken into account by thermally coupling all elements to a bath at temperature T_b. We present current-voltage characteristics of the coupled system and calculations of the linewidth of the radiation as a function of T_b. In the presence of a spatial gradient of the junction parameters critical current and resistance, the linewidth deceases with increasing T_b, similar to the experimental observation.
We report on measurements of the linewidth {Delta}f of THz radiation emitted from intrinsic Josephson junction stacks, using a Nb/AlN/NbN integrated receiver for detection. Previous resolution limited measurements indicated that {Delta}f may be below
1 GHz - much smaller than expected from a purely cavity-induced synchronization. While at low bias we found {Delta}f to be not smaller than ? 500 MHz, at high bias, where a hotspot coexists with regions which are still superconducting, {Delta}f turned out to be as narrow as 23 MHz. We attribute this to the hotspot acting as a synchronizing element. {Delta}f decreases with increasing bath temperature, a behavior reminiscent of motional narrowing in NMR or ESR, but hard to explain in standard electrodynamic models of Josephson junctions.
Tunneling magnetoresistance (TMR) in a vertical manganite junction was investigated by low-temperature scanning laser microscopy (LTSLM) allowing to determine the local relative magnetization M orientation of the two electrodes as a function of magni
tude and orientation of the external magnetic field H. Sweeping the field amplitude at fixed orientation revealed magnetic domain nucleation and propagation in the junction electrodes. For the high-resistance state an almost single-domain antiparallel magnetization configuration was achieved, while in the low-resistance state the junction remained in a multidomain state. Calculated resistance $R_mathrm{calc}(H)$ based on the local M configuration obtained by LTSLM is in quantitative agreement with R(H) measured by magnetotransport.
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