# Welcome

In our group we are interested in developing and applying cutting-edge computational methods for description and rational design of emerging materials. We are particularly interested in superconductors and thermoelectrics which have strategic applications in energy transport, energy harvesting, and electronics.

We are part of the developing team of the EPW package that offers cutting-edge
functionalities which are unique in the current landscape
of electron structure codes.
EPW is an open-source F90/MPI code
which provides unprecedented levels of accuracy and efficiency
in calculations of materials properties defined by the electron-phonon
interactions that are critical for understanding and designing
electronic materials and devices. Since April 2016, the EPW code has been distributed
as part of the
Quantum ESPRESSO suite. Prof. Margine has been awarded
a $500,000 3-year NSF grant to continue the on-going effort
in developing new functionalities in EPW.
The group aims to expand the current capabilities of the code to modeling
a wider range of materials with complex electronic and
magnetic properties.

# Opportunities

**There are openings for highly motivated graduate and undergraduate students.**

# Selected Publications

Unusual Pressure-Induced Periodic Lattice Distortion in SnSe

_{2}

J. Ying, H. Paudyal, C. Heil, X.-J. Chen, V. V. Struzhkin, and E. R. Margine,

Phys. Rev. Lett. 121, 027003 (2018)

We performed high-pressure x-ray diffraction (XRD), Raman, and transport measurements combined
with first-principles calculations to investigate the behavior of tin diselenide (SnSe_{2})
under compression. The obtained single-crystal XRD data indicate the formation of a (1/3,1/3,0)-type
superlattice above 17 GPa. According to our density functional theory results, the pressure-induced
transition to the commensurate periodic lattice distortion (PLD) phase is due to the combined effect
of strong Fermi surface nesting and electron-phonon coupling at a momentum wave vector q=(1/3,1/3,0).
In contrast, similar PLD transitions associated with charge density wave (CDW) orderings in transition
metal dichalcogenides (TMDs) do not involve significant Fermi surface nesting. The discovered
pressure-induced PLD is quite remarkable, as pressure usually suppresses CDW phases in related
materials. Our findings, therefore, provide new playgrounds to study the intricate mechanisms governing
the emergence of PLD in TMD-related materials.
Read more

Towards predictive many-body calculations of phonon-limited carrier mobilities in semiconductors

S. Poncé, E. R. Margine, and F. Giustino, Phys. Rev. B 97, 121201(R) (2018)

We probe the accuracy limit of ab initio calculations of carrier mobilities in semiconductors, within
the framework of the Boltzmann transport equation. By focusing on the paradigmatic case of silicon,
we show that fully predictive calculations of electron and hole mobilities require many-body quasiparticle
corrections to band structures and electron-phonon matrix elements, the inclusion of spin-orbit coupling,
and an extremely fine sampling of inelastic scattering processes in momentum space. By considering all
these factors we obtain excellent agreement with experiment, and we identify the band effective masses
as the most critical parameters to achieve predictive accuracy. Our findings set a blueprint for future
calculations of carrier mobilities, and pave the way to engineering transport properties in semiconductors
by design.
Read more

Origin of Superconductivity and Latent Charge Density Wave in NbS

_{2}

C. Heil, S. Poncé, H. Lambert, M. Schlipf, E. R. Margine, and F. Giustino,

Phys. Rev. Lett. 119, 087003 (2017)

We elucidate the origin of the phonon-mediated superconductivity in
2H-NbS_{2} using the ab initio anisotropic Migdal-Eliashberg theory
including Coulomb interactions. We demonstrate that superconductivity is
associated with Fermi surface hot spots exhibiting an unusually strong
electron-phonon interaction. The electron-lattice coupling is dominated
by low-energy anharmonic phonons, which place the system on the verge of
a charge density wave instability. We also provide definitive evidence for
two-gap superconductivity in 2H-NbS_{2}, and show that the low- and
high-energy peaks observed in tunneling spectra correspond to the Γ- and
K-centered Fermi surface pockets, respectively. The present findings call
for further efforts to determine whether our proposed mechanism underpins
superconductivity in the whole family of metallic transition metal dichalcogenides.
Read more

EPW: Electron–phonon coupling, transport and superconducting properties using maximally localized Wannier functions

S. Poncé, E. R. Margine, C. Verdi, and F. Giustino, Comput. Phys. Commun. 209, 116 (2016)

The EPW (Electron-Phonon coupling using Wannier functions) software is a Fortran90 code that
uses density-functional perturbation theory and maximally localized Wannier functions for computing
electron–phonon couplings and related properties in solids accurately and efficiently. The EPW v4 program
can be used to compute electron and phonon self-energies, linewidths, electron–phonon scattering rates,
electron–phonon coupling strengths, transport spectral functions, electronic velocities, resistivity,
anisotropic superconducting gaps and spectral functions within the Migdal–Eliashberg theory. The code now
supports spin–orbit coupling, time-reversal symmetry in non-centrosymmetric crystals, polar materials, and
k- and q-point parallelization.
Read more

Electron-phonon interaction and pairing mechanism in superconducting Ca-intercalated bilayer graphene

E. R. Margine, H. Lambert, and F. Giustino, Scientific Reports 6, 21414 (2016)

Using the ab initio anisotropic Eliashberg theory including Coulomb interactions,
we investigate the electron-phonon interaction and the pairing mechanism in the recently-reported0
superconducting Ca-intercalated bilayer graphene. We find that C_{6}CaC_{6} can support
phonon-mediated superconductivity with a critical temperature Tc = 6.8–8.1 K, in good agreement with experimental data.
Our calculations indicate that the low-energy Caxy vibrations are critical to the pairing, and that
it should be possible to resolve two distinct superconducting gaps on the electron and hole Fermi
surface pockets.
Read more

Electronic transport properties of selected carbon π-bowls with different size, curvature and solid state packing

B. T. Wang, M. A. Petrukhina, and E. R. Margine, Carbon 94, 174 (2015)

First-principles calculations combined with the Boltzmann transport theory are used to
investigate the electronic transport properties of four members of the extended family of
indenocorannulene molecular crystals. The results for the electrical conductivity suggest
that all indenocorannulene derivatives should exhibit transport characteristics significantly
improved compared to the parent corannulene. In particular, the transport properties of
1,2,4-triindenocorannulene crystal are found to be comparable for electron doping and likely
surpass for hole doping the values achievable in sumanene, assuming the same carrier lifetimes.
The findings point to a large sensitivity of the charge-carrier conductivity to the size
as well as stacking direction of the carbon-rich π-bowls and indicate that this class of
corannulene derivatives can provide new structural motifs that can be further tuned to
achieve high-performance materials for organic electronic devices.
Read more

Two-gap superconductivity in heavily

*n*-doped graphene

E. R. Margine and F. Giustino, Phys. Rev. B 90, 014518 (2014)

Graphene is the only member of the carbon family from zero- to three-dimensional materials for which
superconductivity has not been observed yet. We investigate from first principles the possibility of
inducing superconductivity in doped graphene using the fully anisotropic Migdal-Eliashberg theory powered by
Wannier-Fourier interpolation. To address a best-case scenario, we consider both electron and hole doping at
high carrier densities so as to align the Fermi level to a van Hove singularity. In these conditions, we find
superconducting gaps of *s*-wave symmetry, with a slight anisotropy induced by the trigonal warping, and, in
the case of *n*-doped graphene, an unexpected two-gap structure reminiscent of MgB_{2}.
Read more

Thermal stability of graphene and carbon nanotubes functionalization

E. R. Margine, M.-L. Bocquet, and X. Blase, Nano. Lett. 8, 3315 (2008)

We study kinetic factors governing the diffusion and desorption of covalently grafted phenyl and
dichlorocarbene radicals on graphene and carbon nanotubes. Our ab initio calculations of reaction rates show that
isolated phenyls can easily desorb and diffuse at room temperature. On the contrary, paired phenyls are expected
to remain grafted to the surface up to a few hundred degrees Celsius. In the case of dichlorocarbene, no
clustering is observed; at room temperature, the isolated radicals remain covalently attached to small-diameter
nanotubes but desorb easily from graphene. Our results on the thermal behavior of side moieties on graphitic
surfaces could be used to optimize the tradeoff between reactivity and conductance of nanotubes in the process of
covalent functionalization.
Read more