At high enough input power in stacks of ${\text{Bi}}_2{\text{Sr}}_2{\text{CaCu}}_2{\text{O}}_8$ intrinsic Josephson junctions a hot spot (a region heated to above the superconducting transition temperature) coexists with regions being still in the superconducting state. In the “cold” regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong and stable coherent terahertz (THz) emission. We investigate the interplay of hot spots and standing electromagnetic waves by low-temperature scanning laser microscopy and THz emission measurements, using stacks of various geometries. Standing electromagnetic wave patterns and THz emission are observed for a disk-shaped sample. The growth of a hot spot with increasing input power is monitored by small detector junctions surrounding a large rectangular mesa. For two rectangular mesas equipped with two current injectors and one arrow-shaped structure we show that the standing wave can be turned on and off in various regions of the stack structure, depending on the hot-spot position. The results support the picture of the hot spot acting as a reflective termination of the cavity, formed by the cold part of the mesa.

A device made from a layered superconductor emits electromagnetic waves in a frequency range for which good radiation sources had been lacking.

We experimentally demonstrate the occurrence of negative absolute resistance (NAR) up to about $-1\Omega$ in response to an externally applied dc current for a shunted $\mathrm{Nb}\mathrm{\text{−}}\mathrm{Al}/{\mathrm{AlO}}_x\mathrm{\text{−}}\mathrm{Nb}$ Josephson junction, exposed to a microwave current at frequencies in the GHz range. The realization (or not) of NAR depends crucially on the amplitude of the applied microwave current. Theoretically, the system is described by means of the resistively and capacitively shunted junction model in terms of a moderately damped, classical Brownian particle dynamics in a one-dimensional potential. We find excellent agreement of the experimental results with numerical simulations of the model.

We report the levitation of a superconducting lead-tin sphere with $100\mathrm{\mu }\mathrm{m}$ diameter (corresponding to a mass of $5.6\mathrm{\mu }\mathrm{g}$) in a static magnetic trap formed by two coils in an anti-Helmholtz configuration, with adjustable resonance frequencies up to 240 Hz. The center-of-mass motion of the sphere is monitored magnetically using a dc superconducting quantum interference device as well as optically and exhibits quality factors of up to $2.6\times {10}^7$. We also demonstrate 3D magnetic feedback control of the motion of the sphere. The setup is housed in a dilution refrigerator operating at 15 mK. By implementing a cryogenic vibration isolation system, we can attenuate environmental vibrations at 200 Hz by approximately 7 orders of magnitude. The combination of low temperature, large mass, and high quality factor provides a promising platform for testing quantum physics in previously unexplored regimes with high mass and long coherence times.

We have studied experimentally and numerically temperature profiles and the formation of hot spots in intrinsic Josephson junction stacks in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$ (BSCCO). The superconducting stacks are biased in a state where all junctions are resistive. The 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 $T_c$. The numerical simulations are in good agreement with the experimental observations.

Electromagnetic radiation at frequencies in the terahertz (THz) range is highly interesting for potential applications in biology or medical diagnostics, high-bandwidth communication, security and defence, nondestructive evaluation, and other fields. However, particularly, the frequency range between 0.5 and 1.5 THz has turned out to be difficult to fill with powerful coherent solid-state sources. Suitably patterned structures made of the high critical temperature superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ can operate in this frequency range, and the best devices presently reach an output power around $100\mu \mathrm{W}$ . In this tutorial, we will give an introduction into the basic physics of this type of oscillator and into the challenges that appear. We will explain how the generators are fabricated, which measurement techniques are used for characterization, and which models are used to describe their dynamics. We will also give some examples demonstrating potential applications in the fields of THz imaging and THz spectroscopy.