COLLOQUIUM 16 Febbraio 2017- Ore 16:30 - Matteo Calandra - Magnetism in graphite and high-Tc superconductivity in hydrides: success stories in condensed matter theory
Luogo: Aula Newton Plesso Fisico
Relatore: Prof. Matteo Calandra CNRS and Université P. et M. Curie, Paris, France
E-mail organizzatore: email@example.com
Nowadays first principles electronic structure calculations are a
reference in several domains of physics, chemistry, geophysics and
biophysics, as they give access to several spectroscopic properties that
can be directly compared with experimental data. Furthermore, besides
being very helpful to achieve a sound understanding of material
properties, these techniques can also be used to predict new physical
phenomena. I will illustrate some important results along these lines
obtained in my group.
I will first consider the case of magnetism in graphite. Common wisdom
tells that magnetism is related to the presence of localized electrons,
like it happens for examples in iron or in oxides with d-orbitals at the
Fermi level. In the past a big effort has been devoted to find magnetism
in graphite or diamond, however with hardly any success. Carbon is
appealing as it is cheap and easily processed. Thus, magnetic carbon
lead to low-cost fast-memory devices, logic gates, transistors, and
capacitors for computers, tablets, and hand-held devices.
By using theoretical calculations based on density functional theory, I
will show that magnetism does occur in a peculiar kind of graphite,
rhombohedral-stacked multilayer graphene (RSMG). I will demonstrate
RSMG has a unique Raman fingerprint that can be used to identify it in
natural graphite samples. Finally, I will show that the comparison
theory, Raman, transport and angular resolved photoemission provide
evidence for the occurrence of a magnetic state [1,2,3].
In the second part of my talk I will consider hydrogen-bonded solids.
These solids are very important for biophysics, chemistry and energy
applications. However, they are very difficult to simulate as, due to
light mass of the hydrogen atom, anharmonicity and proton quantum
(i.e. the fact that the hydrogen atom has a wavefunction and has to be
treated as a quantum particle) become dominant. For example, in the
high-pressure phases of ice, they determine the degrees of symmetry of
hydrogen bond and the critical pressure for the structural transition
between the so-called ice X and ice VIII crystal structures .
I will show that, in the recently discovered sulfur hydride
(H3S) , the material with the highest superconducting critical
temperature known on earth (Tc=203 K at 155 GPa), the proton quantum
effects completely determine the high-pressure structural phase diagram
and the behavior of Tc versus pressure [6,7].
 B. Pamuk et al. arXiv:1610.03445
 D. Pierucci et al. ACS nano 9, 5432 (2015)
 Younes et al. Nano Letters 16 , 3710 (2016)
 M. Benoit, D. Marx and M. Parrinello, Nature 392, 258 (1998)
 Drozdov, A. P. et al., Nature 525, 7567 (2015)
 Errea, I. , Calandra, M et al. Nature 532, 7597 (2016)
 Errea, I. Calandra, M. et al. Phys. Rev. Lett. 114, 157004 (2015)