关注：

1) 镧系及锕系元素及其化合物高压相变的理论及实验研究

2) 镧系及锕系元素及其化合物电子结构的理论及实验研究

3) 超导体的分类及超导转变温度的理论计算： MgB2 vs AlB2 MgB2 type structure

The coexistence of 2D covalent in-plane and 3D metallic-type
interlayer conducting bands is a peculiar feature of MgB2.

PuCoGa5

4) 表面与界面相关的理论计算

参考网址：

http://en.wikipedia.org/wiki/Cubic_crystal_system

http://en.wikipedia.org/wiki/Magnesium_diboride

摘录学习：

http://hoffmann.chem.cornell.edu/guoying-gao/

  I am very interested in the crystal structure, chemical bonding, phase transition, electronic structure, lattice dynamics and superconductivity of condensed matter under high pressure. I am concentrated on the following two projects:

1.      Nb-H system

Searching for metallic hydrogen has been the topic for more than a half century. However, the realization of metallic hydrogen within the reach of diamond anveil cell still remains unsolved.

In 1983, Carlsson and Ashcroft proposed that impurities could induce the insulator-metal transition in solid hydrogen at a lower external pressure required. They suggested that transition metals might be good choice due to that transition metals are known to effectively dissociate hydrogen molecules and some transition metals has formed hydrogen-rich compounds, such as YH3.

  We choose Nb as an example due to that pure Nb holds the record for the highest superconducting transition temperature Tc (9.3 K) of an element at normal pressure; And pure H2 itself is predicted to be a very good high-temperature superconductor due to its very high phonon frequency, if it could be metalized.

So how about if we put a little bit of Nb into the dense hydrogen?

2.      Ca/Si/C system CaSi2 with AlB2-type structure is a superconductor and the highest Tc can reach up to 14 K.

  CaC2 is theoretically predicted to take AlB2-type structure at high pressure.

It is well known that MgB2 with AlB2-type structure has the highest Tc of ~ 40 K in the simple compounds.

As the same main group IV elements, C and Si, together, might form ternary compound with Ca.

And considering that CaSi2 and CaC2 being of the layered structure at low pressure, Ca/Si/C ternary compounds might also take the layered structure, which further might be a good candidate to be a superconductor.

Moreover, on our way to study this ternary phase diagram, we are also interested in studying the binary phase diagram for every two elements. We will explore the new C and Si networks in these carbides and silicides under pressure, which will be very helpful for us the understanding the chemistry of C and Si.

1. Guoying Gao, Roald Hoffmann, Neil W. Ashcroft, AUG 7 2013

The unusual and the expected in the Si/C phase diagram, Journal of the American Chemical Society, 137, 064502 (2013)

Nature of bonding and electronic structure in MgB2, a boron intercalation
superconductor

Chemical bonding and electronic structure of MgB2, a boron-based newly discovered superconductor,
is studied using self-consistent band structure techniques.

Analysis of the transformation of the
band structure for the hypothetical series of graphite – primitive graphite – primitive graphite-like
boron – intercalated boron, shows that the band structure of MgB2 is graphite-like, with  bands
falling deeper than in ordinary graphite.

These bands possess a typically delocalized and metallic, as
opposed to covalent, character.

The in-plane  bands retain their 2D covalent character, but exhibit
a metallic hole-type conductivity.

The coexistence of 2D covalent in-plane and 3D metallic-type
interlayer conducting bands is a peculiar feature of MgB2.

We analyze the 2D and 3D features of
the band structure of MgB2 and related compounds, and their contributions to conductivity.

Two Types of Multistack Structures in MgB₂-Type Superconductor CaAlSi

http://journals.jps.jp/doi/abs/10.1143/JPSJ.75.043713

Crystal structure analysis of an AlB ₂ -type superconductor, CaAlSi, has been carried out using synchrotron X-rays and neutron diffraction measurements. Two different stackings along the  c -axis–5-fold and 6-fold structures–with different values of  T _c 6.0 and 8.2 K, respectively, are confirmed. Our results show that Al and Si atoms are ordered alternately in honeycomb layers, and Ca atoms are intercalated at regular intervals between the (Al,Si) layers in both the structures. The 5-fold and 6-fold structures are determined by the arrangement of Al and Si along the  c -axis. The relation between  T _c and the structure is discussed based on our structural analysis.

High-Pressure Study of the 40 K Superconductor MgB2

http://www.esrf.eu/UsersAndScience/Publications/Highlights/2001/materials/MAT5.html

The report by the team of Akimitsu [1] of a superconducting transition temperature as high as 40 K in MgB₂, a compound which has been known for a long time, was probably one of the most unexpected discoveries of year 2001 in the field of solid-state physics.

It soon led to a large variety of experimental and theoretical work aimed at understanding the reasons for such a high T_c in this simple compound. Among these, the knowledge of the high-pressure behaviour can provide insight into the origin of superconductivity, as was demonstrated in the case of the high T_c cuprates. We have therefore undertaken a study of the electrical and structural properties of MgB₂ up to 40 GPa.

Fig. 119: Experimental data and fit to the equation of state for MgB2 (the crystal structure is shown in the inset).

Magnesium diboride has the AlB₂-type hexagonal (P6/mmm) structure, with a = 3.08Å, c = 3.51 Å (inset of Figure 119). The structure can be described as the alternate stacking of planes of boron atoms forming a honeycomb lattice, and planes of magnesium atoms forming a triangular one.

The equation of state of MgB₂ was determined up to ~39 GPa by angle-resolved X-ray diffraction ( = 0.3738Å) at ID30, the high-pressure beamline, using a membrane-driven diamond anvil cell with diamond tips of diameter 300 µm and stainless-steel gaskets with holes of 120 µm. Nitrogen was used as the pressure transmitting medium in order to keep good hydrostatic conditions. The pressure was determined using the ruby fluorescence method. The diffraction patterns were recorded with a Mar345 image plate detector located at 360 mm from the sample and transformed into powder diffractograms using the Fit2D software. These data were successfully refined by the Rietveld technique with the AlB₂-type structure up to the highest pressure of ~39 GPa

http://en.wikipedia.org/wiki/Magnesium_diboride

Superconductivity[edit]

Its superconductivity was discovered by the group of Akimitsu in 2001.^[1] Its critical temperature (T_c) of 39 K (−234 °C; −389 °F) is the highest amongst conventional superconductors. This material was first synthesized and its structure confirmed in 1953,^[2] but its superconducting properties were not discovered until 2001.^[3]

Though generally believed to be a conventional (phonon-mediated) superconductor, it is a rather unusual one. Its electronic structure is such that there exist two types of electrons at the Fermi level with widely differing behaviours, one of them (sigma-bonding) being much more strongly superconducting than the other (pi-bonding). This is at odds with usual theories of phonon-mediated superconductivity which assume that all electrons behave in the same manner. Theoretical understanding of the properties of MgB₂ has almost been achieved with two energy gaps. In 2001 it was regarded as behaving more like a metallic than a cuprate superconductor.^[4]

Synthesis[edit]

Magnesium diboride can be synthesized by several routes. The simplest is by high temperature reaction between boron and magnesium powders.^[4] Formation begins at 650 °C; however, since magnesium metal melts at 652 °C, the reaction mechanism is considered to be moderated by magnesium vapor diffusion across boron grain boundaries. At conventional reaction temperatures, sintering is minimal, although enough grain recrystallization occurs to permit Josephson quantum tunnelling between grains.

Superconducting magnesium diboride wire can be produced through the powder-in-tube (PIT) process. In the in situ variant, a mixture of boron and magnesium is poured into a metal tube, which is reduced in diameter by conventional wire drawing. The wire is then heated to the reaction temperature to form MgB₂ inside. In the ex situ variant, the tube is filled with MgB₂ powder, reduced in diameter, and sintered at 800 to 1000 °C. In both cases, later hot isostatic pressing at approximately 950 °C further improves the properties.

In 2003, a new and easy in situ technique for the synthesis of MgB₂ was presented by Giunchi et al. (Edison S.p.A.).^[5] This new technique employs reactive liquid infiltration of magnesium inside a granular preform of boron powders and was called Mg-RLI technique. The method allowed to manufacture both high density (more than 90% of the theoretical density for MgB₂) bulk materials and special hollow fibers. This method is an exact of similar melt growth based methods such as the Infiltration and Growth Processing method used to fabricate bulk YBCO superconductors where the non-superconducting Y2Ba1Cu1O5 is used as granular preform inside which YBCO based liquid phases are infiltrated to make superconductive YBCO bulk. This method has been copied and adapted for MgB2 superconductor and rebranded as reactive Mg Liquid Infiltration. The process of Reactive Mg Liquid Infiltration in a boron preform to obtain MgB₂ has been a subject of patent applications by Edison S.p.A. (Italy).

Hybrid physical-chemical vapor deposition (HPCVD) has been the most effective technique for depositing magnesium diboride (MgB₂) thin films.^[6] The surfaces of MgB₂ films deposited by other technologies are usually rough and non-stoichiometric. In contrast, the HPCVD system can grow high-quality in situ pure MgB₂ films with smooth surfaces, which are required to make reproducible uniform Josephson junctions, the fundamental element of superconducting circuits.