New understanding of liquid and supercritical states of matter

Research Group: 
Centre for Condensed Matter and Material Physics
Number of Students: 
Length of Study in Years: 
Full-time Project: 
QM Scholarship
Project Description: 
Traditionally, liquids have been much harder to understand in comparison to solids and gases. Recent new understanding is beginning to change this picture: contrary to the pessimistic and vague picture often drawn about them, liquids are emerging as exciting and unique systems amenable to theoretical understanding in a consistent picture [1]. This understanding includes the supercritical matter. According to current understanding, no differences can be made between a gas and a liquid above the critical point, an abrupt terminus of the liquid-gas coexisting line. Recently, we have discovered that this is not the case, and that the phase diagram of matter should be modified [1-3]. We have proposed that a new line ("Frenkel line") exists on the phase diagram above the critical point at arbitrarily high pressure and temperature, and which separates two physically distinct states of matter. Crossing the line corresponds to qualitative changes of system's physical properties.

In this project, we will focus on dynamic and thermodynamic properties of both subcritical liquids and supercritical fluids. Our study will include:
(a) collective modes (phonons) and their evolution with pressure and temperature including the recently discovered phonon gap in the reciprocal space [1]. We will study how the phonon gap emerges, evolves and crosses over at the Frenkel line;
(b) structural changes across the Frenkel line and
(c) thermodynamic properties of liquid and supercritical states and their relationship to dynamics and structure. We will pay particular attention to supercritical fluids such as H2O and CO2 that have recently started to be widely deployed in important environmental and industrial applications.

Depending on candidate's skills and preferences, we will use any or all of the three lines of enquiry: computer modelling, theory and experiments. We will benefit from access to central neutron scattering and X-ray facilities in the UK and abroad as well as from using massive parallel computing facilities in the UK. In the modelling component of this project, the student will learn how to perform record-breaking atomistic simulations using the UK’s national supercomputing resources. This will involve understanding the basic simulation algorithms, together with understanding approaches to modelling the forces between atoms. It will also require engagement in the process associated with access to these resources, including writing proposals and reports on a regular basis and engagement with the UK modelling community. The project will require development of specific computer programs for generating the original atomic configurations and for data analysis, and the student will be trained in modern programming methods. In the experimental component of the project, the student will benefit from broad training in performing neutron and X-ray scattering expriments at major national and international facilities, including writing and defending experimental proposals and analyzing the data. Finally, the student will be trained in relevant theoretical approaches, including gaining deeper knowledge of phonons, phase transitions and other basic condensed matter concepts.

1. K. Trachenko and V. Brazhkin, Collective modes and thermodynamics of the liquid state, Reports on Progress in Physics 79, 016502 (2016).
2. V. Brazhkin and K. Trachenko, What separates a liquid from a gas? Physics Today, 65(11), 68 (2012).
3. V. Brazhkin, Yu.D. Fomin, A.G. Lyapin, V. N. Ryzhov, E.N. Tsiok and K. Trachenko, "Liquid-gas" transition in the supercritical region: Fundamental changes in the particle dynamics, Physical Review Letters 111, 145901 (2013).
SPA Academics: 
Kostya Trachenko
Prof Martin Dove

See for more details and references.