Computational Condensed Matter Physics

Roxana Margine Research Group

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 SnSe2
J. Ying, H. Paudyal, C. Heil, X.-J. Chen, V. V. Struzhkin, and E. R. Margine,
Phys. Rev. Lett. 121, 027003 (2018)


Pressure-Induced Periodic 
            Lattice Distortion in SnSe<sub>2</sub>


We performed high-pressure x-ray diffraction (XRD), Raman, and transport measurements combined with first-principles calculations to investigate the behavior of tin diselenide (SnSe2) 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)


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


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 NbS2
C. Heil, S. Poncé, H. Lambert, M. Schlipf, E. R. Margine, and F. Giustino,
Phys. Rev. Lett. 119, 087003 (2017)


Origin of Superconductivity 
			and Latent Charge Density Wave in NbS<sub>2</sub


We elucidate the origin of the phonon-mediated superconductivity in 2H-NbS2 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-NbS2, 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)


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


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)


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


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 C6CaC6 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)


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


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)
Two-gap superconductivity in n-doped graphene


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 MgB2. Read more


Thermal stability of graphene and carbon nanotubes functionalization
E. R. Margine, M.-L. Bocquet, and X. Blase, Nano. Lett. 8, 3315 (2008)


Desorption of phenyl pairs from graphene


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

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