Thermal and Quantum Routes to Melting

An international team of researchers including a group from the London Centre for Nanotechnology (LCN) at UCL have uncovered vital information regarding the “quantum melting” of the magnetic structure of Thallium Copper Trichloride (TlCuCl3), reports Nature Physics.

The findings of team from the UK, Switzerland, France and China establish for the first time that thermal and quantum routes to melting behave largely independently of one another.

At sufficiently high temperature the particles contained within a material will move increasingly vigorously until the forces between them are eventually no longer strong enough to hold them together, causing it to melt. 

This familiar idea suggests that freezing should occur as solids are cooled towards absolute zero where all thermal motion ceases. Various important classes of materials exist, however, whose low-temperature properties are also dominated by the effects of quantum mechanics.

In such quantum matter, melting may proceed at zero temperature due to quantum rather than thermal fluctuations. Understanding the role of quantum melting, an example of a quantum phase transition, is believed to lie at the heart of many of the outstanding problems in physics, including high-temperature superconductivity.

In this study, TlCuCl3 was selected as the interactions between its magnetic ions – and hence the strength of the quantum fluctuations – can be varied with exquisite precision by applying external pressure. Using neutron scattering the researchers were then able to follow the destruction of order induced either by quantum fluctuations (varying the pressure) or induced by thermal fluctuations (varying the temperature).  

A key feature of their work was the determination of the specific nature of the critical fluctuations driving the thermal and quantum melting. In both cases the relevant fluctuation, or excitation, is longitudinal in nature, and detailed calculations show that it is closely analogous to the Higgs particle recently discovered in high-energy physics experiments at the Large Hadron Collider (LHC).

Professor Christian Ruegg, lead scientist on the project, commented, “We were absolutely astonished that these excitations play a key role, irrespective of whether the order is destroyed by quantum-mechanical or classical fluctuations – a fascinating feature of quantum phase transitions.” 

Journal reference: Nature Physics, May 2014

Figure: Thermal and quantum melting of the magnetic order in TlCuCl3. In the limit of zero temperature T and zero pressure P the quantum fluctuations in  TlCuCl3 are severe enough to prevent the onset of any magnetic order. Increasing the pressure at zero temperature acts to reduce quantum fluctuations. The system eventually passes through a quantum critical point (QCP) and becomes magnetic. The effects of thermal and quantum fluctuations can then be disentangled by varying T and P. The coloured panels show neutron scattering data at fixed pressure where the critical fluctuations driving the melting of the order can be studied as a function of energy and temperature.

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Des McMorrow, Phil Merchant, Christian Ruegg, Bruce Normand, Karl Kramer, and Martin Boehm
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