New understanding of liquid thermodynamics, viscosity and its lower bounds – Kostya Trachenko ( Queen Mary, University of London)

November 11, 2021 @ 2:00 pm – 3:00 pm
Details of video conferencing will be distributed nearer the time.
Edgar Engel

Understanding most basic thermodynamic properties of the liquid state
such as energy and heat capacity turned out to be a long-standing
problem in physics [1]. Landau&Lifshitz textbook states that no general
formulas can be derived for liquid thermodynamic functions because the
interactions are both strong and system-specific. Phrased differently,
liquids have no small parameter. Recent experimental and theoretical
results open a new way to understand liquid thermodynamics on the basis
of collective modes (phonons) as is done in the solid state theory.
There are important differences between phonons in solids and liquids,
and we have recently started to understand and quantify this difference.
I will review collective modes in liquids including high-frequency
solid-like transverse modes and will discuss how a gap in the reciprocal
space emerges and develops in their spectrum [2,3]. This reduces the
number of phonons with temperature, consistent with the experimental
decrease of constant-volume specific heat with temperature [1]. I will
discuss the implication of the above theory for fundamental
understanding of liquids. I will also mention how this picture can be
extended above the critical point where the recently proposed Frenkel
line on the phase diagram separates liquid-like and gas-like states of
supercritical dynamics [1,4]. I will subsequently describe how this
leads to the theory of minimal quantum viscosity in terms of fundamental
physical constants and will compare this minimum to the holographic
bound [5]. The minimum of thermal diffusivity can be equally written as
the same combination of fundamental constants, in agreement with
experiments. I also mention an upper bound on the speed of sound in
terms of fundamental constants following from a similar approach [6].
Finally, I will note that the kinematic viscosity of the quark-gluon
plasma is surprisingly close to the kinematic viscosity of liquids at
their minimum [7].

1. K Trachenko and V Brazhkin, Collective modes and thermodynamics of
the liquid state, Reports on Progress in Physics 79, 016502 (2016)
2. C Yang, M T Dove, V Brazhkin and K Trachenko, Physical Review Letters
118, 215502 (2017)
3. M Baggioli, M Vasin, V Brazhkin and K Trachenko, Physics Reports 865,
1 (2020)
4. C Cockrell, V Brazhkin and K Trachenko, arXiv:2104.10619
5. K Trachenko and V Brazhkin, Minimal quantum viscosity from
fundamental physical constants, Science Adv. 6, eaba3747 (2020)
6. K Trachenko, B Monserrat, C Pickard and V Brazhkin, Science Adv. 6,
eabc8662 (2020)
7. K Trachenko, V Brazhkin and M Baggioli, Similarity between the
kinematic viscosity of quark-gluon plasma and liquids at the viscosity
minimum, SciPost Phys 10, 118 (2021)

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