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SUMMARY:Experiments\, quantum mechanics and molecular dynamics of high pre
 ssure hydrogen - Graeme Ackland\, School of Physics and Astronomy\, Univer
 sity of Edinburgh
DTSTART:20180621T110000Z
DTEND:20180621T120000Z
UID:TALK107563@talks.cam.ac.uk
CONTACT:Georg Schusteritsch
DESCRIPTION:In this talk\, I will discuss recent work on high pressure hyd
 rogen at an accessible level\, in particular relating density functional c
 alculations to spectroscopic measurements.\n\nIn 1935\, Wigner and Hunting
 ton used the recently-derived quantum theory of nearly free electrons to p
 redict that hydrogen would become metallic under high pressure.  Their cal
 culation of lattice energy is in good agreement with modern methods.   A m
 assive underestimate of the required pressure has inspired generations of 
 experimentalists to claim synthesis of solid metallic hydrogen: the aptly 
 named "Holy Grail" of high pressure physics.   Further theoretical predict
 ions of zero-temperature melting and room temperature superconductivity ha
 s excited the search.  At high temperatures\, a negative-sloped melting cu
 rve and a transition to a metallic liquid provides further intrigue.\n\nTo
  date\, five solid phases of hydrogen are reported\, and in none of them a
 re the atomic positions known.  In part this is because the crystal struct
 ures are complex\, but also because for highly quantum nuclei even the con
 cept of "atomic position" is moot. Despite the obviously quantum nature of
  the system\,  and the large zero-point energies\, treating the atoms clas
 sically in density functional calculation yields a phase diagram in remark
 ably good agreement with the experiment.  It fails quantitatively in two c
 ases: the phase I-II transition where classical rotors are arrested faster
  than quantum ones\, and in the liquid-liquid transition where quantum vib
 rations break covalent bonds more effectively than classical one.   Elsewh
 ere\, the correspondence principle holds good.\n\nThe seminal work of Pick
 ard and Needs mapped out the low temperature structures\, showing a series
  of phase transformations due (in my interpretation) to quadrupole interac
 tions\, efficient molecular packing\, molecular metallisation and atomisat
 ion.  At higher temperatures\, the atoms are far more mobile and their ave
 rage position does not correspond to minima of the energy surface.  We use
 d molecular dynamics to study the phases I\, IV and V and the melt\, findi
 ng good agreement with the experiments and providing insights into the str
 ucture and dynamics of the atoms. 
LOCATION:Goldsmiths 1\, Lecture Theatre\, Department of Materials Science 
 &amp\; Metallurgy
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