We performed first-principles density functional theory (DFT) and GW calculations to investigate electronic structures of bulk and few-layer PdSe2. We obtained the quasiparticle band structure of bulk PdSe2, and the obtained energy gap agrees excellently with the reported experimental value. For monolayer and bilayer PdSe2, we obtained quasiparticle band structures with respect to the vacuum level. We analyzed DFT and GW band structures in detail, finding k-space positions of valence band maxima and conduction band minima, effective masses, the quasiparticle density of states, work functions, ionization potentials, electron affinities, and k-space shapes of electron and hole pockets. These results provide a foundation for development of basic studies and device applications.
We compute the shift current bulk photovoltaic effect (BPVE) in bulk BaTiO$_3$ and two-dimensional monochalcogenide SnSe considering quasi-particle corrections and exciton effects. We explore changes in shift current peak position and magnitude reduction due to band renormalization. For BaTiO$_3$, we demonstrate that shift current is reduced near the band edge due to exciton effects. Comparison of these results with experiments on BaTiO$_3$ indicate that mechanisms other than shift current may be contributing to BPVE. Additionally, we reveal that the shift current near the band gap shows only …
In this work we employ the $GW$ approximation in the framework of the SternheimerGW method to investigate the effects of many-body corrections to the band structures and Fermi surfaces of bulk and monolayer NbS$_2$. For the bulk system, we find that the inclusion of these many-body effects leads to important changes in the band structure, especially in the low-energy regime around the Fermi level, and that our calculations are in good agreement with recent ARPES measurements. In the case of a free-standing monolayer NbS$_2$, we observe a strong increase of the screened Coulomb interaction and …
Group-IV monochalcogenides have attracted much attention due to their potential of ferroelectric and multiferroic properties. Recently, centrosymmetric gamma-phase GeSe in a double-layer honeycomb lattice has been theoretically predicted, but the synthesized gamma-phase GeSe showed a noncentrosymmetric atomic structure, leading to the possibility of ferroelectricity and spin-splitting. Here, we study the quasiparticle band structures, spontaneous polarization, and spin-splitting in noncentrosymmetric gamma-GeSe using density functional theory and GW calculations. Our results show that noncentr…
Layered van der Waals (vdW) magnetic materials have attracted significant research interest to date. In this work, we employ the first-principles many-body perturbation theory to calculate excited-state properties of a prototype vdW magnet, chromium trichloride (CrCl3), covering monolayer, bilayer, and bulk structures. Unlike usual non-magnetic vdW semiconductors, in which many-electron interactions and excited states are sensitive to dimensionality, many-electron interactions are always enhanced and dominate quasiparticle energies and optical responses of both two-dimensional and bulk CrCl3. …
Drumhead surface states that link together loops of nodal lines arise in Dirac nodal-line semimetals as a consequence of the topologically non-trivial band crossings. We used low-temperature scanning tunneling microscopy and Fourier-transformed scanning tunneling spectroscopy to investigate the quasiparticle interference (QPI) properties of ZrSiTe. Our results show two scattering signals across the drumhead state resolving the energy-momentum relationship through the occupied and unoccupied energy ranges it is predicted to span. Observation of this drumhead state is in contrast to previous stu…
We present a combined theoretical and experimental study of the electronic structure of stoichiometric Bi$_4$Te$_3$, a natural superlattice of alternating Bi$_2$Te$_3$ quintuple layers and Bi bilayers. In contrast to the related semiconducting compounds Bi$_2$Te$_3$ and Bi$_1$Te$_1$, density functional theory predicts Bi$_4$Te$_3$ to be a semimetal. In this work, we compute the quasiparticle electronic structure of Bi$_4$Te$_3$ in the framework of the $GW$ approximation within many-body perturbation theory. The quasiparticle corrections are found to modify the dispersion of the valence and con…
Effects of van der Waals interactions and quasiparticle corrections on the electronic and transport properties of Bi2Te3
采用密度泛函理论，考虑范德华相互作用（vdW）和准粒子自能修正，对Bi 2 Te 3体材料的结构、电子和输运性质进行了理论研究.发现optB 86 b-vdW泛函能很好地再现Bi 2 Te 3的实验晶格常数和层间距。在优化结构的基础上，采用GW近似和Wannier函数插值的方法，通过第一性原理计算计算得到了Bi 2 Te 3的能带结构.整体能带极值偏离高对称线，计算的实际能带与实验测量的结果符合得很好。结合Boltzmann理论，GW计算也给出了输运性质的准确预测，计算的Bi 2 Te 3的热电系数与实验数据基本吻合。
Dirac nodal line semimetals represent a new state of quantum matters in which the electronic bands touch to form a closed loop with linear dispersion. Here, we report a combined study on ZrSiS by density functional theory calculation, scanning tunneling microscope (STM) and magneto-transport measurements. Our STM measurements reveal the spectroscopic signatures of a diamond-shaped Dirac bulk band and a surface band on two types of cleaved surfaces as well as a spin polarized surface band at ${\barΓ}$ at E~0.6eV on S-surface, consistent with our band calculation. Furthermore, we find the surfac…
Dispersion Corrected Structural Properties and Quasiparticle Band Gaps of Several Organic Energetic Solids
我们对一系列含能固体进行了{ \it ab initio}计算，探索了它们的结构和电子性质。为了计算这些分子固体的基态体积，在DFT中采用了不同的色散校正方法，即Tkartchenko-Scheffler方法（有和没有自洽屏蔽），Grimme方法（D2，D3（BJ））和vdw-DF方法。我们的研究结果表明，色散校正方法是必不可少的，在理解这些复杂的结构与范德华相互作用和氢键。计算的基态体积和体积模量表明，每种方法的性能是不是唯一的，因此，仔细检查是强制性的解释理论预测。这项工作还强调了准粒子计算的重要性，在预测的带隙，这是在这里得到的GW近似。我们发现，所获得的带隙的范围从4至7 eV的不同的化合物，表明它们的绝缘性质。此外，我们显示准粒子能带结构计算的重要作用，相关的间隙与能量特性。
We apply the quasiparticle self-consistent GW method (QSGW) to slab models of ionic materials, LiF, KF, NaCl, MgO, and CaO, under electric field. Then we obtain the optical dielectric constants E(Slab) from the differences of the slopes of the electrostatic potential in the bulk and vacuum regions. Calculated E(Slab) show very good agreements with experiments. For example, we have E(Slab)=2.91 for MgO, in agreement with the experimental value E(Experiment)=2.96. This is in contrast to E(RPA)=2.37, which is calculated in the random-phase approximation for the bulk MgO in QSGW. After we explain …
Temperature-Dependent nonlinear optics from first-principles: Second-Harmonic Generation in few-layers MoS$_2$
Queen’s University Belfast（贝尔法斯特女王大学）；European Theoretical Spectroscopy Facilities (ETSF)（欧洲理论光谱设施）；CNRS/Aix-Marseille Université, Centre Interdisciplinaire de Nanoscience de Marseille UMR 7325（法国国家科学研究中心/艾克斯-马赛大学，马赛跨学科纳米科学中心 UMR 7325）
Chiral fermions in solid state feature "Fermi arc" states, connecting the surface projections of the bulk chiral nodes. The surface Fermi arc is a signature of nontrivial bulk topology. Unconventional chiral fermions with an extensive Fermi arc traversing the whole Brillouin zone have been theoretically proposed in CoSi. Here, we use scanning tunneling microscopy / spectroscopy to investigate quasiparticle interference at various terminations of a CoSi single crystal. The observed surface states exhibit chiral fermion-originated characteristics. These reside on (001) and (011) but not (111) su…
We report diffusion quantum Monte Carlo (DMC) and many-body $GW$ calculations of the electronic band gaps of monolayer and bulk hexagonal boron nitride (hBN). We find the monolayer band gap to be indirect. $GW$ predicts much smaller quasiparticle gaps at both the single-shot $G_0W_0$ and the partially self-consistent $GW_0$ levels. In contrast, solving the Bethe-Salpeter equation on top of the $GW_0$ calculation yields an exciton binding energy for the direct exciton at the $K$ point in close agreement with the DMC value. Vibrational renormalization of the electronic band gap is found to be si…
Quasiparticle interference (QPI) of the electronic states has been widely applied in scanning tunneling microscopy (STM) to analyze the electronic band structure of materials. Single-defect induced QPI reveals defect-dependent interaction between a single atomic defect and electronic states, which deserves special attention. Due to the weak signal of single-defect-induced QPI, the signal-to-noise ratio (SNR) is relatively low in a standard two-dimensional QPI measurement. In this paper, we introduce a projective quasiparticle interference (PQPI) method, in which a one-dimensional measurement i…
Material's geometrical structure is a fundamental part of their properties. The honeycomb geometry of graphene is responsible for the arising of its Dirac cone, while the kagome and Lieb lattice hosts flat bands and pseudospin-1 Dirac dispersion. These features seem to be particular for few 2D systems rather than a common occurrence. Given this correlation between structure and properties, exploring new geometries can lead to unexplored states and phenomena. Kepler is the pioneer of the mathematical tiling theory, describing ways of filing the euclidean plane with geometrical forms in its book…
We describe the ground- and excited-state electronic structure of bulk MnO and NiO, two prototypical correlated electron materials, using coupled cluster theory with single and double excitations (CCSD). As a corollary, this work also reports the first implementation of unrestricted periodic ab initio equation-of motion CCSD. Starting from a Hartree-Fock reference, we find fundamental gaps of 3.46 eV and 4.83 eV for MnO and NiO respectively for the 16 unit supercell, slightly overestimated compared to experiment, although finite-size scaling suggests that the gap is more severely overestimated…
The performance of organic semiconductor devices is linked to highly-ordered nanostructures of self-assembled molecules and polymers. We employ many-body perturbation theory and study the excited states in bulk compolymers. We discover that acceptors in the polymer scaffold introduce a, hitherto unrecognized, conduction impurity band. The donor units are surrounded by conjugated bands which are only mildly perturbed by the presence of acceptors. Along the polymer axis, mutual interactions among copolymer strands hinder efficient band transport, which is, however, strongly enhanced across indiv…
Fully self-consistent GW (sc-GW) methods are now available to evaluate quasiparticle and spectral properties of various molecular and bulk systems. However, such techniques based on the full matrix of G and W are computationally demanding. The routinely used single-shot GW-approximation (G0W0) has an undesirable dependency on the choice of initial exchange-correlation functional. In the literature, many so-called self-consistent GW methods are based on diagonal approximation of G and low-ranking approximation of W. It is thus worth to check how good such approximations are in comparison with t…
We report the exotic photoluminescence (PL) behaviour of 3D topological insulator Bi2Te3 single crystals grown by customized self-flux method and mechanically exfoliated few layers (18 plus minus 2 nm)/thin flakes obtained by standard scotch tape method from as grown Bi2Te3 crystals.The experimental PL studies on bulk single crystal and mechanically exfoliated few layers of Bi2Te3 evidenced a broad red emission in the visible region. These findings are in good agreement with our theoretical results obtained using the ab initio density functional theory framework.
