In this work, we find by means of first principle calculations a new physical mechanism to generate a two dimensional electron gas, namely, the breaking of charge ordering at the surface of a charge ordered semiconductor due to the incomplete oxygen environment of the surface ions. The emergence of the 2D gas is independent of the presence of oxygen vacancies or polar discontinuities; this is a self-doping effect. This mechanism might apply to many charge ordered systems, in particular, we study the case of BaBiO3(001). In bulk, this material is a prototype of a "forbidden valence" compound in which the formal "metallic" Bi4+ state is skipped exhibiting a charge disproportionated Bi3+ - Bi5+ ordered structure. At room temperature, this charge disproportionation together with the breathing distortions gives rise to a Peierls semiconductor with monoclinic crystal structure. At higher temperature (T > 750 K) or upon doping, it turns cubic and metallic. Interestingly, doped BaBiO3 was one of the first non-cuprate high-Tc superconductors discovered. The outer layer of the Bi-terminated simulated surface turns more cubic- like and metallic while the inner layers remain in the insulating monoclinic state. On the other hand, the metallization does not occur for the Ba termination, a fact that makes this system appealing for nanostructuring. Finally, this finding sets another possible route for future exploration: the potential scenario of 2D superconductivity at the BaBiO3 surface.
Comments on "Giant Dielectric Response in the One-Dimensional Charge-Ordered Semiconductor (NbSe_{4})_{3}I" and "Colossal Magnetocapacitance and Colossal Magnetoresistance in HgCr_{2}S_{4}"
对“一维电荷有序半导体（NbSe_{4}）_{3}I中的巨介电响应”（D. Staresinic等人，物理修订版信函96，046402（2006））和“Colossal Magnetocapacitance and Colossal Magnetoresistance in HgCr_{2} S_{4}“（S. Weber等人，物理修订信函157202（2006））
The factors that affect the thermal conductivity of semiconductors is a topic of great scientific interest, especially in relation to thermoelectrics. Key developments have been the concept of the phonon-glass-electron-crystal (PGEC) and the related idea of rattling to achieve this. We use first principles phonon and thermal conductivity calculations in order to explore the concept of rattling for stoichiometric ordered half-Heusler compounds. These compounds can be regarded as filled zinc blende materials, and the filling atom could be viewed as a rattler if it is weakly bound. We use two sim…
REPLY to Comments on "Giant Dielectric Response in the One-Dimensional Charge-Ordered Semiconductor (NbSe4)3I" and "Colossal Magnetocapacitance and Colossal Magnetoresistance in HgCr2S4" (cond-mat/0607500)
We present a simple model of the local order in amorphous organic semiconductors which naturally produces a spatially correlated exponential density of states (DOS). The dominant contribution to the random energy landscape is provided by electrostatic contributions from dipoles or quadrupoles. An assumption of the preferable parallel orientation of neighbor quadrupoles or antiparallel orientation of dipoles directly leads to the formation of the exponential tails of the DOS even for a moderate size of the ordered domains. The insensitivity of the exponential tail formation to the details of th…
A highly conductive metallic gas that is quantum mechanically confined at a solid-state interface is an ideal platform to explore nontrivial electronic states that are otherwise inaccessible in bulk materials. Although two-dimensional electron gas (2DEG) has been realized in conventional semiconductor interfaces, examples of two-dimensional hole gas (2DHG), which is the counter analogue of 2DEG, are still limited. Here, we report the observation of a 2DHG in solution-processed organic semiconductors in conjunction with an electric double-layer using ionic liquids. A molecularly flat single cry…
Big pores, small pores, ordered pores, random pores, they all have a function and as is often found, show behaviour in new materials that is not always predicted or obvious at the outset. I started my research journey trying to put extremely thin films onto near-perfect III-V crystals to control (opto)electronic properties and when the first TEM on our campus showed remarkable pore growth and structure in InP almost 21 years ago, the electrochemical modification of the InP made more sense. In this paper, I will summarise a few aspects of research into porous materials and semiconductors, from …
In recent experiments, high Curie temperatures Tc above room temperature were reported in ferromagnetic semiconductors Fe-doped GaSb and InSb, while low Tc between 20 K to 90 K were observed in some other semiconductors with the same crystal structure, including Fe-doped InAs and Mn-doped GaSb, InSb, and InAs. Here we study systematically the origin of high temperature ferromagnetism in Fe, Mn, Cr-doped GaSb, InSb, and InAs magnetic semiconductors by combining the methods of density functional theory and quantum Monte Carlo. In the diluted impurity limit, the calculations show that the impurit…
Ideal Spin Filters: Theoretical Study of Electron Transmission Through Ordered and Disordered Interfaces Between Ferromagnetic Metals and Semiconductors
Heuslers are a prominent family of multi-functional materials that includes semiconductors, half metals, topological semimetals, and magnetic superconductors. Owing to their same crystalline structure, yet quite different electronic properties and flexibility in chemical composition, Heusler-based heterostructures can be designed to show intriguing properties at the interface. Using electronic structure calculations, we show that two dimensional electron or hole gases (2DEG or 2DHG) form at the interface of half-Heusler (HH) semiconductors without any chemical doping. We use CoTiSb/NiTiSn as a…
Electron correlation in semiconductors and insulators via symbolic regression
Oden Institute for Computational Engineering and Sciences, The University of Texas at Austin（德克萨斯大学奥斯汀分校奥登计算工程与科学研究所）；Department of Physics, The University of Texas at Austin（德克萨斯大学奥斯汀分校物理系）
Sculpting the shape of semiconductor heteroepitaxial islands: from dots to rods
在Ge on Si模型异质外延系统中，硅表面上的金属图案对高度有序的Ge岛的形态提供了前所未有的控制。岛的形状包括纳米棒和截棱锥由金属种类和衬底取向设置。岛刻面的分析阐明了突出的作用，在促进生长的优选面取向的金属，而岛的组成和结构的调查揭示了在岛的演变中的Si-Ge混合的重要性。这些效应反映了金属介导的生长现象的显著组合，其可以被利用来定制异质外延系统中的岛阵列的功能。
Electronic structures and magnetic orders of Fe-vacancies ordered ternary iron selenides TlFe$_{1.5}$Se$_2$ and AFe$_{1.5}$Se$_2$ (A=K, Rb, or Cs)
通过第一性原理电子结构计算，我们发现Fe空位有序TlFe $_{1.5}$ Se $_2$的基态是一个准二维共线反铁磁半导体，其能隙为94 meV，与实验测量相一致.这种反铁磁有序是由Fe磁矩之间的Se桥反铁磁超交换相互作用驱动的。同样地，我们发现晶体AFe $_{1.5}$ Se $_2$（A=K，Rb，或Cs）也是反铁磁半导体，但具有零能隙半导体态或与基态近似简并的半金属态。因此，丰富的物理性质和相图的预期。
Crystals grow by gathering and bonding of atoms to form an ordered structure. Typically, the growth is equally probable in all crystalline directions, but sometimes, it is not, as is the case of nanowire growth. Nanowire growth is explained, in most cases, by the presence of liquid metal droplets that mediate between an incoming flux of atoms and a substrate or an existing crystal nucleus, while defining the lateral dimension. Here, we report and explain a previously unknown mode of non-isotropic crystal growth observed in two wurtzite semiconductors, InN and ZnO. Being of polar structure, wur…
The long-missing polarization-induced two-dimensional hole gas is finally observed in undoped gallium nitride quantum wells. Experimental results provide unambiguous proof that a 2D hole gas in GaN grown on AlN does not need acceptor doping, and can be formed entirely by the difference in the internal polarization fields across the semiconductor heterojunction. The measured 2D hole gas densities, about $4 \times 10^{13}$ cm$^{-2}$, are among the highest among all known semiconductors and remain unchanged down to cryogenic temperatures. Some of the lowest sheet resistances of all wide-bandgap s…
Semiconductor interfaces, such as these existing in multilayer structures (e.g., quantum wells (QWs)), are interesting because of their ability to form 2D electron gases (2DEGs), in which charge carriers behave completely differently than they do in the bulk. As an example, in the presence of a strong magnetic field, the Landau quantization of electronic levels in the 2DEG results in the quantum Hall effect (QHE), in which Hall conductance is quantized. This chapter is devoted to the properties of such 2DEGs in multilayer structures made of compound semiconductors belonging to the class of Se-…
Ion beams are used routinely for processing of semiconductors, particularly sputtering, ion implantation and direct-write fabrication of nanostructures. However, the utility of ion beam techniques is limited by crystal damage and surface roughening. Damage can be reduced or eliminated by performing irradiation at elevated temperatures. However, at these conditions, surface roughening is highly problematic due to thermal mobility of adatoms and surface vacancies. Here we solve this problem using hydrogen gas, which we use to stabilize surface mass flow and suppress roughening during ion bombard…
Tunable two-dimensional electron gas at the surface of thermoelectric material In$_4$Se$_3$
本文报道了用角分辨光电子能谱在热电材料In $_4$ Se $_3$表面发现二维电子气。所观察到的2DEG表现出几乎各向同性的能带色散，具有相当小的有效质量m* = 0.16 m_0，并且其载流子密度显示出显著的温度依赖性，导致在表面处的非常规的金属-半导体转变。在In $_4$ Se $_3$中观察到的2DEG的宽范围热可调谐性产生额外的自由度以更好地控制半导体的表面载流子。
A new class of strongly excited plasmonic modes that open access to unprecedented Petavolts per meter electromagnetic fields promise wide-ranging, transformative impact. These modes are constituted by large amplitude oscillations of the ultradense, delocalized free electron Fermi gas which is inherent in conductive media. Here structured semiconductors with appropriate concentration of n-type dopant are introduced to tune the properties of the Fermi gas for matched excitation of an electrostatic, surface "crunch-in" plasmon using readily available electron beams of ten micron overall dimension…
Large-area single-crystal monolayers of two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) can be grown by chemical vapour deposition (CVD). However, the high temperatures and fast timescales at which the conversion from a gas-phase precursor to the 2D material appear, make it extremely challenging to simultaneously follow the atomic arrangements. We utilise helium atom scattering to discover and control the growth of novel 2D h-BN nanoporous phases during the CVD process. We find that prior to the formation of h-BN from the gas-phase precursor, a metastable $(3…
We present a hybrid CCSD(T)+PBE-D3 approach to calculating the vibrational signatures for gas phase benzene and benzene adsorbed on an ordered water-ice surface. We compare the results of our method against experimentally recorded spectra and calculations performed using PBE-D3-only approaches (harmonic and anharmonic). Calculations use a proton ordered XIh water-ice surface consisting of 288 water molecules, and results are compared against experimental spectra recorded for an ASW ice surface. We show the importance of including a water ice surface into spectroscopic calculations, owing to th…
