Report on the progress of the Finnish Centre of Excellence "The Low Temperature Laboratory at Helsinki Technical University"
By Hans Rudolph Ott
The evaluation of the Centre is based on the site visit of the evaluator assigned by the Finnish Academy of Finland, on May 6, 2004. The report aims at addressing the individual items which, in a memorandum of February 28, 2002, were indicated to be of particular interest to the Finnish Academy of Finland. No other particular items were requested to be addressed in the present evaluation.
The evaluation is based on the following information:
Both these documents were provided prior to the site visit.
During the site visit, further information was provided by
1. Overall presentation of the Centre
The first presentation covered an overview on different general aspects of the Centre, which includes programs in low-temperature physics and brain research. The report provided an overview of past achievements in the form of publications, awards and honours, the development of the personnel, some statistical data on PhD education and awards and, finally, the development of the funding situation in the past and its expected trend in the near future. Comments and recommendations concerning the entire Centre are given in section 3 of this report.
2. Individual Research Projects
a) Electronic Coolers and Pumps
This project is a new activity for the centre and started with Academy Professor J. Pekola joining the LTL in 2002. The main goal of the project is to provide refrigerators that are of such small size that they can be accommodated on a chip of the type that is commonly used in microelectronics. It represents a relatively new approach to cryogenics, the traditional backbone of the laboratory, and is based on forced changes of the distribution function of conduction electrons in suitable materials. The method is very useful for experiments in the sub-Kelvin regime, involving electronic systems on the micro- or nanosize scale, a topic of research which is of high current interest worldwide. The group is among the world leaders which develop this new approach to refrigeration. The plans for the future, in particular to develop solid state refrigerators which span the temperature regime up to room temperature, seem very ambitious but not completely unfeasible. In parallel, the group has also developed a device which is capable of a controlled transfer or pumping of Cooper pairs. Previous work by other authors has demonstrated the possibility of pumping single electrons. The new approach has the potential of pumping controlled amounts of charge at higher speed, The presentation also revealed a number of new ideas which are all very promising but require the solution of many technical problems. In this sense, the future plans (2004-07) are ambitious but confirm the high originality of the group' s thinking. There is little doubt that it will continue to visibly contribute to research and applications of mesoscopic electronic systems at the currently high international level. The group is also well positioned with respect to collaborations with outside partners in the academic, as well as in the industrial sector.
b) Quantum amplifiers and noise
This project, already well under way at the time of the last site visit has, under the leadership of Prof. P. Hakonen, continued its remarkable work on developing low-noise single electron transistors (SET) by adding an inductive device (L-SET) without interference of shot-noise and small back action. The related work on quantum amplifiers has met some difficulties with the practical realization of Bloch oscillator transistor (BOT) structures. Nevertheless, the potential of these new devices and combinations thereof is promising enough, to continue with this type of work, in particular in collaboration with the VTT. The development of this type of devices and related applications offers a wide range of experiments of more fundamental interest and the group has also made remarkable achievements in this respect. The group has obviously managed to transfer the high level technical know-how to solving difficult problems in the area of mesoscopic electronic systems and there is little doubt that, as the future plans reveal, it will continue to do so in the future. Somewhat less prominent than the rest of the program are the achievements in the experiments with multiwalled carbon nanotubes (MWNT), and this activity might be an item for reduction, if necessary. A particularly valuable asset is the now installed theoretical support. It would be nice, if this subgroup could support the planned experimental efforts towards realizing qubits.
c) Dynamics in coherent quantum systems
This activity is, in its field of research, unmatched in the world. This is primarily due to the availability of a home built experimental infrastructure that, after completion of the current substantial renovation work, will not be available in another laboratory. Although this monopoly also bears some obvious risks, the work that has been done and is foreseen for the near future, is of such high quality and interest to physics in general, that it would be absolutely foolish to consider a discontinuation. The oral presentation of the group's leader, Prof. M. Krusius, concentrated on the presentation and discussion of results in connection with turbulence in superfluid 3He-B. Even this relatively narrow aspect, if considering all the other possibilities, revealed the richness in the behaviour of this seemingly simple fluid. Apart from serving as a model system for investigations of fluid dynamics and structures, the experimental installation will continue to serve as the testing ground for theoretical ideas that were presented under d). The mere possibility of realizing, in a controlled way, situations which, in view of very general arguments, correspond to certain configurations in the universe, such as black holes, is too tempting to not at least be tried out.
d) Cosmological constant and vacuum energy
The presentation of this topic by Prof. G. Volovik started from the claim that for many body systems, such as quantum liquids and solids, the ground state energy corresponds to the vacuum energy. By discussing a thermodynamic analysis of the quantum vacuum, based on the knowledge of the ground state properties of condensed matter systems, such as the superfluid varieties of both He isotopes, and relating the vacuum energy to the cosmological constant, he emphasized the gross disagreement between theoretical and experimental evaluations of the magnitude of the cosmological constant in all cases. The entire program, embracing lot of other basic issues of relativistic quantum field theory and gravity, is original albeit speculative to some extent. The possible interaction, listed under future work, with experimental activities using the available infrastructure mentioned under c), is clearly a positive asset.
e) YKI - Mikrokelvin Experiments
This extremely powerful refrigeration system has, as before, been used for experiments on mixtures of 3He and 4He. The indirect cooling of these mixtures with a Cu nuclear demagnetization stage has resulted in reaching temperatures of the liquid of between 10 and 100 mK. According to the group leader, Dr. J. Tuoriniemi, these efforts are planned to being terminated. They will be replaced by testing a new method of cooling Helium mixtures by adiabatic melting. Calculations indicate that higher cooling powers at very low temperatures might be achieved. The success of this enterprise is difficult to predict. While testing Li metal for superconductivity, evidence for some type of cooperative phenomenon was found by 7Li NMR in ultralow magnetic fields and below 100 mK. It is not quite clear, in what other way the characteristics of this new phenomenon may be clarified. In this sense, most of this activity is explorative in character and requires a long term involvement. The decision of the future of this project is clearly with the management of the Centre. The scientifically most efficient use of this unique facility (YKI cryostat) in the future might be the topic of a dedicated workshop involving external experts with relevant ideas.
f) lnterfaces of quantum crystals
Again, this project requires experimental skills and instrumental infrastructure which are hardly available at any other comparable institution. As the group leader, Dr. H. AlIes explained, Helium crystals are, first of all, good model systems for studying crystal growth aspects in general, and they are crystals that are influenced by quantum effects and exhibit some unusual properties that are not common to other crystals. As a highlight, the observation of the growth of a 3He crystal by optical interferometry at sub-mK temperature should be mentioned. The analysis of available data indicates that the surface behaviour of a quantum solid turns to classical at temperatures very close to 0 K, a counterintuitive result, to say the least. Particularly this examples shows that surprises are often met where they are completely unexpected. The future of this enterprise is still bright. For example, the predicted occurrence of crystallization waves in 3He at very low temperatures or the influence of magnetic order onthe growth of 3He crystals should definitely be examined in the future, as planned.
g) Dynamics of superfluid 3 He and superconductors
During the past two years, theoretical calculations and simulations in relation to the measurements of turbulence in superfluid 3He-B, mentioned under c), were carried out in this project. Prof. N. Kopnin worked out a model for describing the onset of turbulence. An analysis of the stability of vortices seems to provide an explanation of the experimentally established phase diagram of turbulence, i.e., the distinct separation into regions of turbulent and regular superflow, separated by a narrow boundary region between 0.55 and 0.6 Tc. Further work covered aspects of single electron transport, especially heat transport, in Andreev wires. In the near future, ongoing work, again related with superfluid turbulence is planned. This effort will clearly enhance the impact and significance of the experimental work, mentioned above under c). Further planned work, related to non-equilibrium situations in mesoscopic superconductors will likely be beneficial to the experimental efforts mentioned under a) and b).
3. General comments and recommendations
The overall performance of the low-temperature physics part of the Centre continues to be excellent. The number of publications and their placement in high ranking journals, confirming the high quality of their content, are matching the expectations for a Research Centre of Excellence. This is clearly seen in a comparison of the respective output between LTL alone and HUT overall. As before, the main part of the scientific performance of LTL is based on outstanding technical expertise, both with respect to low-temperature equipment and the fabrication of mesoscopic electronic devices, as well as on general experimental skills of the involved persons. This combination offers opportunities for experiments that are not possible elsewhere and this should be considered in the choice of future projects. In view of the complexity and the difficulties characteristic for many of the experiments, the quantitative output in form of publications of the unit continues to be impressive.
The Centre has continued to slowly reorient its program, as requested in earlier evaluation reports. The Centre's decision to add the activity discussed in section 2a of this report to its curriculum, is judged to be very timely and well placed. The status of the programs 2a and 2b reflects a successful reorientation of the low-temperature physics part into a direction which covers a research field with potential for outstanding research in basic science but also for future applications. The combination of these two programs promises synergies that may not be available in other laboratories that are engaged in this type of research. The already existing co-operations with application oriented research organizations, such as VTT, add to the value of this part of the program. This part of the curriculum is also very well suited to contribute to the often desired technology transfer from academia to industry. In this cage this will not only happen in the form of new concepts and devices, but also in the form of well trained and technically competent Master graduates and PhD's.
As already indicated in section 2 of this report, the projects related to the physics of quantum fluids and solids continue to be of world class standard. Especially these projects require a certain continuity in human resources with exceptional experimental skills. It is recommended that this aspect is watched and handled with the necessary attention, also in times of reduced financial support.
The visibility of the Centre in the community of low-temperature physics is, as before, very high and, with the successes of the groups involved in research on mesoscopic electronic systems, it may even have been enhanced. This is reflected in new collaborations in this field of research with prominent national and international partners. As expressed already in earlier reports, it would be favourable if the theoretical activities related to superfluids and superconductors had a broader local impact, mainly in the form of PhD education.
Overall, the training of young researchers is on a good track. The projects offer attractive opportunities for students to accomplish research results that are internationally well recognized and to acquire skills that will definitely be useful in a future research career, both in academia and industry. Given the international trend, the time for PhD students to obtain their degree is at the upper end of the accepted scale. A reduction of this time span will likely influence both the depth of the education and the quality and/or the quantity of the research results. It is the task of the local academic management to set the goals in this respect.
As mentioned above, the project related with the YKI cryostat may need some longer term perspective. Experiments in the temperature range of microKelvin are highly non trivial and often time consuming by nature. An optimal use of this outstanding facility might be achieved, if a related program, based on international collaborations, is installed. This might also be a possibility to raise external funding but would, of course, limit the free access to this installation of the local group.
During this evaluation, the question concerning the continuation of the Centre after the end of the running funding period arose more than once. At this time, the low-temperature physics part of the Centre clearly fulfills all the expectations that might be attached to a Centre of Excellence and there is no reason to believe that the situation will be different in 2005. It might be worthwhile to consider an extension of the new Centre by including a similarly competent partner in the national environment which is also active in the field of mesoscopic physics and enjoys a high international recognition.
The preceding comment is based on the assumption that the Centre is not continued in its present form. Both parts of the Centre, low-temperature physics and brain research, are both very strong pillars of the enterprise. In recent years, the development of both these parts has lead to a clear separation of scientific interests and, in particular, the brain research unit should now be accommodated in a more suitable scientific environment. This measure would certainly not weaken the scientific excellence and power of both these units but, in the cage of the brain unit, even lead to new and favourable opportunities.
Zürich, May 27, 2004 H.R.Ott