1:30pm - 2:00pmInvited
Unusually Stronger Quantum Fluctuation with Larger Spins: Novel Phenomena Revealed by Emergent Magnetism in Pressurized High-temperature Superconductor FeSe
1Tsung Dao Lee Institute and Shanghai Jiao Tong University, China; 2Sun Yat-Sen University, China; 3Tulane University, United States; 4Michigan State University, United States
A counter-intuitive enhancement of quantum fluctuation with larger spins, together with a few novel physical phenomena, is discovered in studying the recently observed emergent magnetism in high-temperature superconductor FeSe under pressure. Starting with experimental crystalline structure from our high-pressure X-ray refinement, we analyze theoretically the stability of the magnetically ordered state with a realistic spin-fermion model. We find surprisingly that in comparison with the magnetically ordered Fe-pnictides, the larger spins in FeSe suffer even stronger long-range quantum fluctuation that diminishes their ordering at ambient pressure. This "fail-to-order" quantum spin liquid state then develops into an ordered state above 1GPa due to weakened fluctuation accompanying the reduction of anion height and carrier density. The ordering further benefits from the ferro-orbital order and shows the observed enhancement around 1GPa. We further clarify the controversial nature of magnetism and its interplay with nematicity in FeSe in the same unified picture for all Fe-based superconductors. In addition, the versatile itinerant carriers produce interesting correlated metal behavior in a large region of phase space. Our study establishes a generic exceptional paradigm of stronger quantum fluctuation with larger spins that complements the standard knowledge of insulating magnetism.
2:00pm - 2:30pmInvited
The Modeling of Nano-material Graphene Based Photonic Devices
Zhejiang University, China
Graphene, which is a kind of two-dimensional material made of carbon atoms in a honeycomb lattice, has attracted a lot of attention due to its exceptional electrical and optical properties. To enable the simulation of graphene-based devices, we have studied the modeling method of graphene for both the numerical simulation and the equivalent circuit simulation. For the numerical simulation, it is found that graphene can be considered as either a two-dimensional sheet model or a three-dimensional volumetric model. In the sheet model, graphene has zero-thickness and its surface impedance is the reciprocal of its sheet conductivity calculated with the Kubo formula. In the volumetric model, graphene is treated as an anisotropic dielectric, whose in-plane relative permittivity is the function of its sheet conductivity while its out-of-plane relative permittivity is a constant of 2.5. For the equivalent circuit simulation, graphene is modeled as an impedance which is equal to the reciprocal of graphene’s sheet conductivity. By using above models, we have conducted some simulations of graphene-based devices, e.g. graphene-based tunable frequency selective surface, graphene-based nonvolatile active absorber and graphene-based optical modulators. Moreover, based on the equivalent circuit model of graphene, a non-contact method, which is used to characterize graphene at microwave frequency, is also proposed. The measurement results of this method can be in agreement with those measured by electrodes and the deteriorating of graphene in the measurements with electrodes can avoided by using the proposed non-contact method. Our works may benefit the modeling and design of graphene-based devices.
2:30pm - 3:00pmInvited
Ab initio Explorations of Elemental 2D Materials beyond Graphene
Dalian University of Technology, China
In recent years, elemental 2D materials beyond graphene emerge, such as silicene, germanene, and phosphorene, and they show great promise for future devices. The presence of defects, substrates, molecular adsorption, etc. have appreciable effects on the growth and physical properties of these 2D materials, which are imperative to be understood. In this presentation, I will briefly introduce the recent progress of theoretical simulation of post-graphene elemental 2D materials in our group. First, we explored grain boundaries (GB) in phosphorene. Their structures, thermodynamic stability and effects on the electronic properties of phosphorene were addressed. Second, we exploited noncovalent functionalization for band gap engineering of germanene on transition metal dichalcogenides (TMD) substrates. Third, we investigated the interactions of silicene, germanene, stanene with various metal substrates. An optimal interaction strength was proposed for the epitaxial growth of these group IV monolayers. Last, we proposed to use selected transition metal substrates to synthesize graphyne under carbon poor conditions.
3:00pm - 3:15pmOral
Tailoring Structural and Electronic Properties of Graphene on Metals: Towards an Atomistic High-accuracy Description of Large Systems
1Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid, Spain; 2Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, Spain; 3International Center for Materials Nanoarchitectonics, National Institute for Materials Science (NIMS), Tsukuba, Japan
In this work we demonstrate the possibility of modeling complex graphene-metal (G-M) systems by means of Density Functional Theory (DFT) calculations. Since different modification techniques are currently applied on G -—and related materials— to get new functionalities, our work does not restrict to the mere description of the G-M interface but we must reproduce all these surface modifications. More precisely, we reveal the multi-domain structure of G grown on Rh(111), an archetypical strongly-interacting substrate in which G adopts a rippled structure with corrugations larger than 1 Å . Additionally, we will present some recent examples of surface modification like the evolution of G properties as a function of the oxygen coverage in the interface , with special emphasis in the atomistic mechanisms involved on this process  or the alteration of electronic properties after nitrogen doping [4,5].
However, the ultimate purpose of our work is to find a satisfactory quantum approach to deal with very large systems. Since standard plane-wave DFT calculations are limited to relatively small systems containing a few hundreds of atoms, a myriad of interesting phenomena falls outside the scope of these quantum simulations, i.e. grain boundary or edge effects, G-covered metallic steps, or interfase chemical reactions. Therefore, we present a new approach based on highly optimized localized orbital basis set to reach high-accuracy calculations on systems with thousands of atoms. As a result we present some calculations on a metallic step fully covered by G whose structure unveils new features not explored so far.
 A. Martín-Recio, C. Romero-Muñiz, et al. Nanoscale 7 (2015) 11300
 C. Romero-Muñiz, A. Martín-Recio, et al. Carbon 101 (2016) 129
 C. Romero-Muñiz, A. Martín-Recio, et al. JACS (submitted)
 A. Martín-Recio, C. Romero-Muñiz et al. Nanoscale 8 (2016) 17686
 A. Martín-Recio, C. Romero-Muñiz, et al. 2DMater. (submitted)
3:15pm - 3:30pmOral
Modulation of Bandgap in Bilayer Armchair Graphene Ribbons by Tuning Vertical and Transverse Electric Fields
1Department of Physics, School of Education, Can Tho University, Vietnam; 2School of Graduate, College of Natural Sciences, Can Tho University, Vietnam; 3Molecular and Macroscopic Energetics and Combustion Laboratory (EM2C), CentraleSupélec, Université Paris Saclay, Centre Nationnal de la Recherche Scientifique (CNRS), France; 4Center for Nanosciences and Nanotechnologies (C2N), Université Paris-sud, Université Paris Saclay, Centre Nationnal de la Recherche Scientifique (CNRS), France
We investigate the effects of external electric fields on the electronic properties of bilayer armchair graphene nano-ribbons. Using atomistic simulations with Tight Binding calculations and the Non-equilibrium Green’s function formalism, we demonstrate that (i) in semi-metallic structures, vertical fields impact more effectively than transverse fields in terms of opening larger bandgap, showing a contrary phenomenon compared to that demonstrated in previous studies in bilayer zigzag graphene nano-ribbons; (ii) in some semiconducting structures, if transverse fields just show usual effects as in single layer armchair graphene nano-ribbons where the bandgap is suppressed when varying the applied potential, vertical fields exhibit an anomalous phenomenon that the bandgap can be enlarged, i.e., for a structure of width of 16 dimer lines, the bandgap increases from 0.255 eV to the maximum value of 0.40 eV when a vertical bias equates 0.96 V applied. Although the combined effect of two fields does not enlarge the bandgap as found in bilayer zigzag graphene nano-ribbons, it shows that the mutual effect can be useful to reduce faster the bandgap in semiconducting bilayer armchair graphene nano-ribbons. These results are important to fully understand the effects of electric fields on bilayer graphene nano-ribbons (AB stacking) and also suggest appropriate uses of electric gates with different edge orientations.