April 29 (Thu.) – May 2 (Sun.), 2010
at the Hirosaki University 50th Anniversary Auditorium
The theme of “Emission of Continuous THz Waves through Use of Superconductors and Its Application” that is currently being researched as one of the areas within the CREST Program of “Photonics and Quantum Optics for the Creation of Innovative Functions” was taken up at the seventh international symposium held on the high-temperature superconducting cuprate intrinsic Josephson junction system discussed at “The 7th International Symposium on Intrinsic Josephson Effects and Plasma Oscillations in High-Tc Superconductors (PLASMA 2010)” over the four days of April 29 to May 2, 2010 at Hirosaki University, which was mainly hosted by CREST-related persons.
This international symposium presented some results that had been accumulated over two and half years since the global first discovery of powerful electromagnetic wave oscillation phenomenon at the THz bandwidth in a high-temperature superconducting intrinsic Josephson junction in Japan, 2007, to interested researchers and thus the world, and which then proved to be an excellent opportunity to floor multilateral opinions and criticisms.
The innovative results mentioned above were initiated through our global-first observation of the Josephson plasma (superconducting plasma) of superconductors with the intrinsic junction of a high-temperature superconductor in the microwave range that took place in 1995. The energy necessary for plasma excitation falls within the energy scale of up to eV due to high electron density while the binding energy of normal superconductive electrons falls within up to meV. Exciting the plasma oscillation automatically destroys the superconducting pairing and shifts it to a quasi-particle state. The consideration used to be that the plasma phenomenon of superconductive electrons would not therefore be observable in a superconductor. However, the collective excitation of superconductive electrons was predicted to in theory at least be possible in Goldstone mode: P.W. Anderson pointed out that a gap existed when k=0 resulting from the long-range interaction of electrons. It is well known that this then led to the idea of the Anderson-Higgs-Kibble mechanism that has played such an important role in elementary particle theory. It was also theoretically pointed out that collective excitation could be conducted using layered superconductors and the Josephson junction in experimentally verifying it; however, no reported experimental observations were then made until 1995. The first observation was successfully made using a high-temperature superconducting intrinsic Josephson junction as the Josephson plasma absorption of the microwaves.
A very interesting point here is that the excitation energy of the Josephson plasma exists in a sharply excited state as it is actually much lower than the superconducting gap energy. The above fact then gave rise to the simple idea of the Josephson plasma being removed as electromagnetic waves with a powerful and sharp spectrum if it could be excited in some form. This phenomenon was as a consequence then actively pursued in Japan from around 2000, although it did prove very difficult to attain any meaningful results. It was however eventually realized in 2007.
As seen above a series of symposia has played an important role in compiling the results and presenting them globally every two years as the research area of the symposium has since developed into being very large with regard to the phenomenon of the superconducting plasma that was first discovered in Japan. Over the last 15 years the phenomenon first discovered in this new field has also always attracted a number of researchers. The symposium is therefore somewhere the passion of the researchers that have been making efforts in the relentless pursuit of their goals both day and night can be united. The seventh symposium was an important place for the integration of the results of observation of the long-sought THz wave oscillation in particular to be presented. We also cannot overlook the existence of the large single crystal of a high-temperature superconductor that is of the highest quality in the world which lies as background. Clarification took place that indeed “new material sciences begin with new materials”.
With this as background we therefore invited about 20 lecturers that included Professor Paul Muller from the University of Erlangen-Nürnberg, who is the leading authority on and discoverer of the intrinsic Josephson junction, and Professor Reinhold Kleiner from the University of Tübingen, etc. from overseas: Intensely deep research presentations and discussions took place over the four days and it proved to be an important opportunity to strengthen mutual understanding. At first the THz oscillation could not have been observed by any other groups; however, multiple research organizations had arrived at independently observing it from 2010 on, which has thus accelerated activities in the field.
The content of this conference can be seen on the website (http://kadowaki.ims.tsukuba.ac.jp/~plasma2010/) among others. We will now introduce a few of the themes that were of particular interest. The first can be considered to have virtually singlehandedly answered the fundamental question of why the powerful THz wave is oscillated, and involves the following: A number of intrinsic Josephson junctions of equivalent atomic layer level are nonlinearly combined and the superconducting oscillation current that is excited by AC Josephson effect creates the phenomenon of the resonance of all the intrinsic Josephson junctions being synchronized. However, it would appear to need a little more time before it can be established as a rigorous theory. The reason for that is the Josephson junction system is mathematically difficult as it is nonlinear and results in coherent synchronization phenomenon under certain conditions: To make the matter even more complicated the superconductor must not be isolated and instead integrated into peripheral space.
In experiments the joint team of Professor Kleiner and Dr. Wang (NIMS) demonstrated that (1) partial superconductor destruction is caused inside the mesa as a hot spot due to heat on the higher current side, and (2) stripe patterns can be observed in the rest of the part that can be considered to be superconductive, and can be taken to be the standing wave of the electromagnetic wave by means of a laser microscope. However, the phenomenon that actually occurs in that non-equilibrium state has not been clearly revealed yet and is thus a subject of future research.
The Kadowaki group conducted THz wave spectroscopic experiments with a number of mesas involving the following: Clarification of relationship between oscillation frequency and form of the superconducting mesa, establishment of the necessary conditions for oscillating phenomenon to occur, identification of the plasma mode that is excited inside the mesa through verifying the results of measurement of the spatial distribution of the THz oscillation intensity against model calculations incorporating antenna theory, the pursuit of the necessary conditions for increasing oscillation intensity for both experimental and theoretical reasons, etc. Complicated phenomenon was realized inside the mesa, and which cannot be explained using the cavity resonator model: It was pointed out that more detailed experiments would be necessary to clarify this mysterious phenomenon.
In the present situation the maximum intensity is 30μW: The largest issue is how to increase it to around 1mW.
The symposium included the following: a total of 41 lectures (4 plenary talks), 20 poster presentations, and in all 74 participants (including 29 foreign researchers).