Pseudogap in novel superconductors at high electric fields

The novel superconductors with high critical temperatures not only have stimulated world-wide research activities but also have triggered many new commercial applications. The fact that these materials can transport electric energy with no or negligible dissipative losses enable an excellent performance of superconducting narrow-band filters for telecommunication electronics, ultra-sensitive magnetic field sensors, fault-current limiters, power transmission cables, and many other applications. Such devices currently reach the market and are expected to be of large technical importance in a few years. On the other hand, there exists no consensus on the fundamental mechanisms evoking high-Tc superconductivity and hence further progress in elevating the critical temperature has been delayed. The fundamental parameter for superconductivity is the energy gap between the superconducting carrier pairs and the normal-conducting charge carriers that are responsible for charge transport in the superconducting and the normal state of the material, respectively. The theoretical understanding of the conventional superconductors was based on the energy gap and it can be expected that the novel superconductors equally need a thorough investigation of the gap. Recently, evidence is emerging that in high-Tc superconductors with a reduced number of charge carriers, energy gap-like phenomena can extend to room temperature and even higher, although the conventional experimental indications of superconductivity, zero resistance and strong diamagnetism, are only observed at rather low temperature. This puzzling observation of a not fully developed energy gap is commonly addressed as "pseudogap". Many observations indicate that the pseudogap is tightly connected with superconductivity which has provoked the speculation that high-Tc superconductivity may have its origin near room temperature. Mainly two schools of theoretical models are competing to explain the pseudogap. One is based on antiferromagnetic spin fluctuations the other assumes a precursor state of superconduc-tivity at high temperatures that attains its phase coherence, the prerequisite for zero resistance, at lower temperatures only. In this project we will investigate the electrical properties of underdoped high-Tc superconductors in high electric fields to decide between these models. We will use and improve our state-of-the-art apparatus for measurements with intense pulsed current densities to explore a new regime of experimental parameters. Our research will be conducted in tight collaboration with research groups in Austria, France, Italy, and Japan.

A novel effect among the large variety of puzzling and still inexplicable observations in the cuprate superconductors with high critical temperature has been uncovered. These exciting materials not only have stimulated world-wide research activities but also have triggered many new commercial applications due to their ability to transport electric energy with no or negligible dissipative losses below a critical temperature. Despite of intense research efforts, there exists no consensus on the fundamental mechanisms evoking the so-called high-temperature superconductivity and hence further progress in elevating the critical temperature has been delayed. The fundamental parameter for superconductivity and its theoretical description is an energy gap between the superconducting carrier pairs and the normal-conducting charge carriers that are responsible for charge transport in the superconducting and the normal state of the material, respectively. Recently, evidence is emerging that in high-temperature superconductors with a reduced number of charge carriers, phenomena that appear to be associated with an energy gap can extend to room temperature and even higher, although the conventional experimental indications of superconductivity, zero resistance and strong diamagnetism, are only observed at rather low temperature. This puzzling observation of a not fully developed energy gap is commonly addressed as "pseudogap". Many observations indicate that the pseudogap is tightly connected with superconductivity which has provoked the speculation that some sort of a precursor of superconductivity may exist up to room temperature. We have developed a unique apparatus that allows to measure electrical properties of superconductors at unprecedented dissipation power levels and to explore a new regime of experimental parameters. In very high electric fields, we have investigated the Hall effect, a transverse voltage that results from the deflection of moving charge carriers in a magnetic field. Contrary to the observations in metals and many other materials, the Hall effect does not increase linearly with the electric field in cuprate superconductors in a narrow temperature range above the critical temperature. The temperature regime of the non-linear behaviour increases significantly with a rising of the pseudogap temperature and appears to be connected to the process that evokes the pseudogap. Our novel findings can help to decide between several competing theories to explain the origin of the pseudogap in high- temperature superconductors.