Luogo: Aula Newton Plesso Fisico

Relatore: Prof. Matteo Calandra   CNRS and Université P. et M. Curie, Paris, France

E-mail organizzatore: raffaella.burioni@unipr.it

 

Abstact: 

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

could

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

that

RSMG has a unique Raman fingerprint that can be used to identify it in

natural graphite samples. Finally, I will show that the comparison

between

theory, Raman, transport and angular resolved photoemission provide

strong

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

the

light mass of the hydrogen atom, anharmonicity and proton quantum

effects

(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

the

hydrogen bond and the critical pressure for the structural transition

between the so-called ice X and ice VIII crystal structures [4].

 

I will show that, in the recently discovered sulfur hydride

superconductor

(H3S) [5], 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].

 

References:

[1] B. Pamuk et al.  arXiv:1610.03445

[2] D. Pierucci et al. ACS nano   9,  5432 (2015)

[3] Younes et al. Nano Letters 16 , 3710 (2016)

[4] M. Benoit, D. Marx and M. Parrinello, Nature 392, 258 (1998)

[5] Drozdov, A. P. et al., Nature  525, 7567 (2015)

[6] Errea, I. , Calandra, M et al. Nature 532, 7597 (2016)

[7] Errea, I.  Calandra, M. et al. Phys. Rev. Lett. 114, 157004 (2015)