Hot Spots and Waves in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathbf{O}}_8$ Intrinsic Josephson Junction Stacks: A Study by Low Temperature Scanning Laser Microscopy

Recently, it has been shown that large stacks of intrinsic Josephson junctions in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ 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. We verify the appearance of cavity modes at low bias and in the high input-power regime we find that 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.

Figure 1

Principle of low temperature scanning laser microscopy (LTSLM). The beam of a diode laser is deflected by a scanning unit and focused onto the sample surface (spot size $1–2\mu \mathrm{m}$). In order to provide a load line for stable operation, the mesa covered with Au is biased using a current source and variable resistor parallel to the mesa. Local laser heating by a few K (affecting an in-plane area of a few $\mu \mathrm{m}$ and about half of the mesa thickness, cf. Fig. S1 in Ref. 31), causes a response $\Delta V$ serving as the contrast for the LTSLM image. In order to improve the signal-to-noise ratio the beam is modulated at 10–80 kHz and $\Delta V$ is detected by a lock-in amplifier.

Figure 2

$40\times 330\mu {\mathrm{m}}^2$ BSCCO mesa, at $T=25\mathrm{K}$. Left: current-voltage characteristic (IVC) on large current and voltage scales. Solid arrows denote jumps in the IVC; inset shows the geometry of the device. Right: enlargement of the IVC (bottom) in the region where LTSLM images ($A$–$C$; shown above) were recorded. Upper graph shows line scans from images $A$–$C$, along the long side of the mesa at half width, cf. dashed line in image $A$. For a more complete set of LTSLM images, see Fig. S2 in Ref. 31.

Figure 3

Current-voltage characteristic and LTSLM data of a $30\times 330\mu {\mathrm{m}}^2$ large BSCCO mesa at 50 K. Red solid arrows in the IVC denote switching processes, black arrows indicate bias points where LTSLM images $A$–$K$ have been taken. Displayed on the right are line scans from images $A$, $B$, $D$–$G$ along the long side of the mesa at half width, cf. white dashed line in image $A$.