arXiv subjects

Guoying Gao

Publications and source records attributed to Guoying Gao.

22 records · Page 2Linked to original sources

arXiv2018

Fermi Surface and Carriers Compensation of pyrite-type PtBi$_{2}$ Revealed by Quantum Oscillations

Large non-saturating magnetoresistance has been observed in various materials and electron-hole compensation has been regarded as one of the main mechanisms. Here we present a detailed study of the angle-dependent Shubnikov -de Haas effect on large magnetoresistance material pyrite-type PtBi$_{2}$, which allows us to experimentally reconstruct its Fermi-surface structure and extract the physical properties of each pocket. We find its Fermi surface contains four types of pockets in the Brillouin zone: three ellipsoid-like hole pockets $α$ with C$_4$ symmetry located on the edges (M points), one intricate electron pocket $β$ merged from four ellipsoids along [111] located on the corners (R points), two smooth and cambered octahedrons $γ$ (electron) and $δ$ (hole) on the center ($Γ$ point). The deduced carrier densities of electrons and holes from the volume of pockets prove carrier compensation. This compensation at low temperatures is also supported by fitting the field dependence of Hall and magnetoresistance at different temperatures. We conclude that the compensation is the main mechanism for the large non-saturating magnetoresistance in pyrite-type PtBi$_{2}$. We found the hole pockets α may contribute major mobility because of their light masses and anisotropy to relatively avoid large-angle scattering at low temperature. This may be a common feature of semimetals with large magnetoresistance. The found sub-quadratic magnetoresistance in high field is probably due to field-dependent mobilities, another feature of semimetals under high magnetic fields.

cond-mat.mtrl-sci↗

arXiv2016

Thermoelectric transport in monolayer SnSe from first-principles calculations

Electron-crystal and phonon-glass are regarded as two essential factors for ideal thermoelectric materials, which require both an enhanced electronic transport and a depressed phononic transport. These two characteristics usually can not coexist in complete bulk semiconductors because of the strong interaction between electrons and phonons. Introducing nanostructures into bulk thermoelectric materials has long been applying to allow for diverse phonon scattering mechanisms to reduce lattice thermal conductivity while keeping the electrical conductivity intact. However, some two-dimensional materials do the opposite. Here, we propose an example of monolayer SnSe introduced from recent newly researched bulk thermoelectric materials SnSe. Through first-principles electronic structure calculations and Boltzmann transport theory, we show that the lattice thermal conductivity of monolayer SnSe is instead higher than that of bulk SnSe, leading to a depressed thermoelectric figure of merit compared with bulk one. Besides, we also find the anisotropic level in monolayer SnSe is reduced, and better thermoelectric performance can be seen in electron doping.

cond-mat.mtrl-sci↗

arXiv2016

Monolayer MXenes: promising half-metals and spin gapless semiconductors

Half-metals and spin gapless semiconductors are promising candidates for spintronic applications due to the complete (100%) spin polarization of electrons around the Fermi level. Based on recent experimental and theoretical findings of graphene-like monolayer transition metal carbides and nitrides (also known as MXenes), we demonstrate from first-principles calculations that monolayer Ti2C and Ti2N exhibit nearly half-metallic ferromagnetism with the magnetic moments of 1.91 and 1.00 UB per formula unit, respectively, while monolayer V2C is a metal with instable antiferromagnetism, and monolayer V2N is a nonmagnetic metal. Interestingly, under a biaxial strain, there is a phase transition from nearly half-metal to truly half-metal, spin gapless semiconductor, and metal for monolayer Ti2C. Monolayer Ti2N is still a nearly half-metal under a proper biaxial strain. Large magnetic moments can be induced by the biaxial tensile and compressive strains for monolayer V2C and V2N, respectively. We also show that the structures of these four monolayer MXenes are stable according to the calculated formation energy and phonon spectrum. Our investigations suggest that, unlike monolayer graphene, monolayer MXenes Ti2C and Ti2N without vacancy, doping or external electric field exhibit intrinsic magnetism, especially the half-metallic ferromagnetism and spin gapless semiconductivity, which will stimulate further studies on possible spintronic applications for new two-dimensional materials of MXenes.

cond-mat.mes-hall↗

arXiv2015

High-efficient thermoelectric materials: The case of orthorhombic IV-VI compounds

Improving the thermoelectric efficiency is one of the greatest challenges in materials science. The recent discovery of excellent thermoelectric performance in simple orthorhombic SnSe crystal offers new promise in this prospect [Zhao et al. Nature 508, 373 (2014)]. By calculating the thermoelectric properties of orthorhombic IV-VI compounds GeS,GeSe,SnS,and SnSe based on the first-principles combined with the Boltzmann transport theory, we show that the Seebeck coefficient, electrical conductivity, and thermal conductivity of orthorhombic SnSe are in agreement with the recent experiment. Importantly, GeS,GeSe,and SnS exhibit comparative thermoelectric performance compared to SnSe. Especially, the Seebeck coefficients of GeS,GeSe,and SnS are even larger than that of SnSe under the studied carrier concentration and temperature region. We also use the Cahill's model to estimate the lattice thermal conductivities at the room temperature. The large Seebeck coefficients, high power factors, and low thermal conductivities make these four orthorhombic IV-VI compounds promising candidates for high-efficient thermoelectric materials.

cond-mat.mtrl-sci↗