Ultra-low temperatures to understand quantum criticality The recent discovery of a new energy scale which vanishes at the quantum critical point of a heavy fermion material calls for entirely new theoretical approaches, and for advanced experimental studies of matter at extremely low temperatures. Professor Silke BühlerPaschen of the QuantumPuzzle project explains how her research will advance the field At absolute zero in temperature matter can reach a highly exotic state where the system itself is uncertain about the next stage in its development, and undergoes strong collective quantum fluctuations as a result. These fluctuations are described by quantum criticality, an area that forms the primary research focus of the QuantumPuzzle project, an ERC-backed initiative based at the Institute of Solid State Physics at Vienna University of Technology. “We aim to advance the field of quantum criticality in strongly correlated electron systems, which are characterized by low-lying and frequently competing energy scales,” says Professor Silke Bühler-Paschen, the project’s Principal Investigator. The recent discovery of a new energy scale, which vanishes at the quantum critical point of a heavy fermion material, cannot be explained by the standard theory of quantum phase transitions; therefore entirely new experimental and theoretical approaches are needed to advance the field, a goal which QuantumPuzzle is working towards. The five-year project started in 2009 and is run by an international team of more than 10 Post-Docs and PhD students, supported by experienced scientists at Vienna University of Technology and from abroad. QuantumPuzzle’s primary focus is on pursuing experiments into the nature of the new low-lying energy scale that has been identified; the project is looking mainly at heavy fermion compounds, the properties of which have made it an attractive field of investigation for numerous groups worldwide. “The physical 62
properties of heavy fermion compounds are governed by very low energy scales,” explains Professor Bühler-Paschen. “This allows researchers to deliberately induce changes between different ground states by varying external parameters such as pressure or magnetic field. This has led to impressive progress being made in the field in recent years.”
Experimental challenges “In order to study zero-temperature phase transitions – called quantum phase transitions – other, non-thermal parameters are needed in addition to low temperatures,” explains Professor BühlerPaschen. “Physical properties must then be studied as a function of at least two parameters. In QuantumPuzzle we are working on several experiments that have
not been used before to characterize quantum criticality. For instance, in the newly founded Vienna Micro-Kelvin Laboratory, which is equipped with a powerful nuclear de-magnetization cryostat, samples shall be cooled down to the micro-Kelvin regime, which is about two orders of magnitude lower than used in most state-of-the-art dilution refrigerators.” The project is using several other highly sophisticated techniques to characterize quantum criticality, which could lead to significant advances in the field. For instance, microwave experiments shall be used to study the dynamic behaviour near the quantum critical point, which Professor Bühler-Paschen believes will reveal the microscopic meaning of the mysterious new energy scale. “Such experiments are extremely challenging. In order to keep the sample at very low temperatures during a measurement, only very low excitations (e.g., electrical currents) may be used,” she explains. “The tiny measurement signals (e.g., electrical voltages) must then be detected to a very high level of precision.”
Quantum critical behaviour in different materials Across the new energy scale (red line) conduction electrons and spins get successively entangled, leading to full screening of the spins at high fields. At zero temperature, at the quantum critical point, the crossover is abrupt and is experimentally seen as a jump in the Fermi volume
Studying quantum critical behaviour usually goes hand in hand with identifying the types of systems in which it occurs. As such Professor Bühler-Paschen says it is important to study various different materials, and ultimately even different classes of materials, to gain an accurate picture. “Quantum critical behaviour has
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At a glance Full Project Title Quantum Criticality - The Puzzle of Multiple Energy scales Project Funding €2,100,000 Project Duration 60 months Project Partners Single PI project.
The QuantumPuzzle team been observed in systems as diverse as high-temperature superconductors, metamagnets, organic compounds, and heavy fermion compounds. From classical (finite temperature) phase transitions the phenomenon of universality is well known. We want to find out the extent to which quantum phase transitions are also universal,” she outlines. “One important aspect in our search for new materials is their dimensionality. The form of quantum criticality we recently observed in a cubic material was unexpected and calls for verification with other three-dimensional materials. Synthesising adequate compounds is just as challenging as
quantum criticality is that quantum critical fluctuations can stabilize new phases. Unconventional superconductivity, for instance, occurs in numerous heavy fermion systems in the immediate vicinity of a quantum critical point. Quantum critical fluctuations are also discussed as ‘glue’ between the Cooper pairs in the high-temperature cuprate superconductors. In some materials ‘nonFermi liquid phases’ with still unknown microscopic origins have been observed,” says Professor Bühler-Paschen. The nature of the field offers infinite scope for further research, and the project is still pursuing further investigations into low-lying
We hope that such experiments will not only broaden the range in which we can test quantum
Standard theory and beyond While the ‘standard theory’ of quantum criticality is an extension of the theory of classical continuous phase transitions to zero temperature, a number of materials have been identified which appear to require completely new methods of theoretical description. Analysis of quantum fluctuations in these materials has already led to some interesting results. “One of the most interesting aspects of
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Professor Silke Bühler-Paschen
critical scaling
relations but also lead to the discovery of new phases. Ultimately we hope this will help to solve the ‘puzzle’ of quantum criticality beyond the standard scenario measuring them – the slightest amount of disorder may alter the properties severely and thus hide the intrinsic properties.”
Contact Details Project Coordinator, Professor Silke Bühler-Paschen Institute of Solid State Physics Vienna University of Technology Wiedner Hauptstraße 8-10 1040 Vienna Austria T: +43 1 58801 13716 F: +43 1 58801 13899 E: paschen@ifp.tuwien.ac.at W: http://erc.tuwien.ac.at/paschen
energy scales. The results generated are likely to advance not only the fields of heavy fermion systems and quantum criticality, but also the current understanding of phase transitions in general, something which holds relevance beyond the condensed matter physics field. “We hope that our experiments will not only broaden the range in which we can test quantum critical scaling relations, but also lead to the discovery of new phases,” says Professor Bühler-Paschen. “Ultimately we hope this will help to solve the ‘puzzle’ of quantum criticality beyond the standard scenario.”
Silke Bühler-Paschen is a Full Professor of Solid State Physics at the Institute of Solid State Physics, part of Vienna University of Technology. She gained a PhD in physics from EPF Lausanne in 1995 and worked in Germany and Japan before assuming her current role.
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