Effect of laser intensity on the semiconductor–metal transition in a doped Quantum Well

We demonstrate laser induced semiconductor–metal transition through an abrupt change in diamagnetic susceptibility of a donor at critical concentration in a GaAs/Al_xGa_1−xAs Quantum Well for finite barrier model in the effective mass approximation using variational principle. We have considered Anderson‘s localization due to the random distribution of impurities in our calculation. The nonparabolicity of the conduction band is also considered. Our results without laser field agree with the earlier theoretical results and also with the recent experimental results.     

Metal–insulator transition and colossal magnetoresistance: relevance of electron–lattice coupling and electronic phase separation

In this article, we review the insulator–metal transition and the colossal magnetoresistance effect in manganites. The relevance of electron–lattice coupling and the resulting Jahn–Teller polaron is elaborated. The general features of electronic phase separation, which results from disorder and strain effects, are discussed along with electron–lattice coupling effects. Although a comprehensive theory is still lacking that can account for all the intricate features of manganite physics, electronic-phase separation and electron–lattice coupling appear to capture the essence of the colossal magnetoresistance effect in manganites.     

Semiconductor to metal transition in strictly two dimension–a comparative study on GaAs and carbon nanotube

Choosing GaAs, donor binding energies in two dimension are obtained using Thomas–Fermi and Hartree screening functions within the effective mass approximation. Binding energies are computed both in the hydrogen atom model and D ^? ion model. The results show the non-feasibility of semiconductor-to-metal transition (SMT) in two-dimensional system of GaAs. For curiosity we have obtained the donor binding energies in carbon nanotube (CNT), a highly correlated system by considering it as two-dimensional structure. We could observe the feasibility of SMT in CNT. This indicates the strong confinement in nanostructures and the results will be useful for nanodevice fabrication.     

Analysis of interface states and series resistance for Al/PVA:n-CdS nanocomposite metal–semiconductor and metal–insulator–semiconductor diode structures

This paper presents the fabrication and characterization of Al/PVA:n-CdS (MS) and Al/Al₂O₃/PVA:n-CdS (MIS) diode. The effects of interfacial insulator layer, interface states (N _ss ) and series resistance (R _s ) on the electrical characteristics of Al/PVA:n-CdS structures have been investigated using forward and reverse bias I–V, C–V, and G/w–V characteristics at room temperature. Al/PVA:n-CdS diode is fabricated with and without insulator Al₂O₃ layer to explain the effect of insulator layer on main electrical parameters. The values of the ideality factor (n), series resistance (R _s ) and barrier height (? _b ) are calculated from ln(I) vs. V plots, by the Cheung and Norde methods. The energy density distribution profile of the interface states is obtained from the forward bias I–V data by taking into account the bias dependence ideality factor (n(V)) and effective barrier height (? _e ) for MS and MIS diode. The N _ss values increase from mid-gap energy of CdS to the bottom of the conductance band edge for both MS and MIS diode.     

Comparison of α CH and CF activation in alkyl transition metal complexes: a DFT and CASSCF study

DFT (B3PW91) and CASSCF calculations have been carried out to study the relative α migratory abilities of H and F in alkyl transition metal complexes. It is shown that the activation energy is considerably lower to migrate H than F, whereas the energies of reaction are similar for the two reactions. A study of the electron configurations and the orbitals describing these configurations shows that the high activation energy for F is due to a 4-electron repulsion between an F lone pair and the occupied Ru=C π orbital.     

XPS study of the chemical bonding in hydrogenated amorphous germanium–carbon alloys

A systematic study of the chemical bonding in hydrogenated amorphous germanium–carbon (a-Ge_1-xC_x:H)alloys using X-ray photoelectron spectroscopy (XPS) is presented. The films, with carbon content ranging from 0 at. % to 100 at. %, were prepared by the rf co-sputtering technique. Raman spectroscopy was used to investigate the carbon hybridization. Rutherford backscattering spectroscopy (RBS) and XPS were used to determine the film stoichiometry. The Ge 3d and C 1s core levels were used for investigating the bonding properties of germanium and carbon atoms, respectively. The relative concentrations of C–Ge, C–C, and C–H bonds were calculated using the intensities of the chemically shifted C 1s components. It was observed that the carbon atoms enter the germanium network with different hybridization, which depends on the carbon concentration. For concentrations lower than 20 at. %, the carbon atoms are preferentially sp³ hybridized, and approximately randomly distributed. As the carbon content increases the concentration of sp² sites also increases and the films are more graphitic-like. Received: 4 May 1999 / Accepted: 24 November 1999 / Published online: 24 March 2000     

Metal–insulator phase transition and topology in a three-component system

Due to the topology, insulators become non-trivial, particularly those with large Chern numbers which support multiple edge channels, catching our attention. In the framework of the tight binding approximation, we study a non-interacting Chern insulator model on the three-component dice lattice with real nearest-neighbor and complex next-nearest-neighbor hopping subjected to Λ-or V-type sublattice potentials. By analyzing the dispersions of corresponding energy bands, we find that the system undergoes a metal–insulator transition which can be modulated not only by the Fermi energy but also the tunable extra parameters. Furthermore, rich topological phases, including the ones with high Hall plateau, are uncovered by calculating the associated band’s Chern number. Besides, we also analyze the edge-state spectra and discuss the correspondence between Chern numbers and the edge states by the principle of bulk-edge correspondence. In general, our results suggest that there are large Chern number phases with C = ±3 and the work enriches the research about large Chern numbers in multiband systems.     

Insulator–metal transition in Mn-doped NaNbO3 induced by chemical and thermal treatment

The transition between insulator and metal conductor states, induced by oxygen non-stoichiometry, was studied for NaNbO₃ : Mn crystals. Conditions for an optimal reduction were determined on the basis of TGA tests. The temperature dependencies of the resistance measured on the macroscale showed that the transition from thermally activated to metallic features depends on the level of oxygen deficiency. The LC-AFM measurement exhibited non-homogeneous electric resistance on the nanoscale. We ascribed the local insulator–metal transition to changeover in the electronic state of the Nb ions occurring in filaments. The Mn dopant stabilised the induced oxygen non-stoichiometry and the metallic conduction down to room temperature.     

Initial dynamic mechanical and dielectric studies on a systematic series of γ glutamate polypeptides

The complementary dynamic mechanical and dielectric methods have been extensively employed in the characterization of organic polymers. While the former method is sensitive to molecular motions that affect the mechanical properties, the latter has the advantage of detecting molecular motions that involve dipole oscillation. We have used both methods simultaneously within this study in order to help understand a portion of the molecular transitions that occur within a series of synthetic polypeptides.     

First-principles calculation of the electronic structure,chemical bonding,and thermodynamic properties of β-US_2

The electronic structure, magnetic states, chemical bonding, and thermodynamic properties of β-US2 are investigated by using first-principles calculation through the density functional theory(DFT) +U approach. The obtained band structure exhibits a direct band gap semiconductor at Γ point with a band gap of 0.9 e V for β-US2, which is in good agreement with the recent experimental data. The charge-density differences, the Bader charge analysis, and the Born effective charges suggest that the U–S bonds of the β-US2 have a mixture of covalent and ionic characters, but the ionic character is stronger than covalent character. The Raman-active, infrared-active, and silent modes at the Γ point are further assigned and discussed. The obtained optical-mode frequencies indicate that the three apparent LO–TO(longitudinal optical–transverse optical) splittings occur in B1 u, B2 u, and B3 umodes, respectively. Furthermore, the Helmholtz free energy ?F, the specific heat ?E, vibrational entropy S, and constant volume CVare studied over a range from 0 K~100 K. We expect that our work can provide some valuable information for further experimental investigation of the dielectric properties and the infrared reflectivity spectrum of uranium chalcogenide.     

The structures and electronic properties of ferrous nitride–fulminic acid clusters

The structures and electronic properties of ferrous nitride–fulminic acid (FeN–HCNO) clusters are investigated by using PW91 functional. The results reveal that the Fe atom of an FeN molecule prefers to be absorbed on the N–O of an HCNO molecule and it shows the highest kinetic stability. The Fe–N–C–N ring structure has the highest kinetic stability, while the FeN–HCNO chain configuration shows the highest kinetic activity. The N–Fe–ONCH chain structure displays the maximum dipole magnitude (DM) value (8.246 Debye). The Fe atom of the FeN molecule adsorbed on the C atom of the HCNO molecule has the minimum DM value (0.641 Debye). The linear FeN–HCNO configuration displays the largest total spin (2.945?μ_B/atom). The hybridization of sp orbital of the FeN–HCNO clusters is very important.     

Charge state of a transition metal impurity in II–VI semiconductors

The results of calculating the electronic structure of semiconductor compounds A^IIB^VI: 3d(A = Zn; B = S, Se, Te; 3d = Sc-Cu) at a low content of 3d impurities are discussed. The excess charge of an impurity ion with respect to the charge of the zinc ion is determined for the whole series of 3d impurities. It is found that the excess charge gradually varies from +0.6|e| for the scandium impurity to ?0.2|e| for the copper impurity. Photoionization of an impurity ion is simulated by adding a hole or an electron to the impurity center. The added charge is redistributed between the impurity ion and its nearest neighbors, thus decreasing or increasing the total excess charge of the impurity center by a magnitude of ～ 0.2|e|.     

The CASPT2 method in inorganic electronic spectroscopy: from ionic transition metal to covalent actinide complexes∗

During the past ten years, the CASSCF/CASPT2 method has been applied with considerable success to a substantial number of problems related to the electronic spectroscopy of transition metal complexes, thus providing a definite breakthrough of ab initio quantum chemistry in this domain. This will be illustrated in the present contribution by means of a few representative cases from the field of inorganic, organometallic and bio-chemistry. Furthermore, CASPT2 results obtained for the excitation energies of the ions UO₂ ²⁺ and UO₂Cl₄ ^2- will be presented, indicating that the method is also applicable with comparable accuracy for molecules with very heavy metals (provided that relativistic effects are accounted for). We will also show that the success of the method is critically dependent upon its judicious application, in particular upon the choice of the orbitals to be included in the reference active space. A link will be made between the latter choice and the specific electronic structure of the metal—ligand interactions.     

Quantum coherence and ground-state phase transition in a four-chain Bose–Hubbard model

We study the quantum coherence and ground-state phase transition of a four-chain Bose–Hubbard model with the long-range interaction. In a special four-chain Bose–Hubbard model,i.e., each chain only has one optical potential, four types of the ground-state phases are discovered. The effects of the disorder, the on-site interaction and the long-range interaction on the quantum coherence are studied. For the system without the long-range interaction, the quantum coherence changes from one periodic oscillation to two periodic oscillations as the onsite interaction increases. By considering the long-range interaction, the quantum coherence goes back to one periodic oscillation again. The on-site interaction itself suppresses the quantum coherence, both the on-site interaction and long-range interaction together enhance the quantum coherence with the weak disorder. If the disorder strength is increased beyond a critical value,they start to suppress the quantum coherence. In a regular four-chain Bose–Hubbard model, i.e.,each chain has many optical potentials, the ground-state phase transitions are obtained by using the cluster Gutzwiller mean-field method. Exotic ground-state phases are found, i.e., superfluid phase, integer Mott insulator phase, supersolid phase and loophole insulator phase. The combination of the loophole insulator phase and the supersolid phase expands the lobes with the half-integer filling per site for the small ratio β = t_■/t_⊥.     

Charge order–superfluidity transition in a two-dimensional system of hard-core bosons and emerging domain structures

We have used high-performance parallel computations by NVIDIA graphics cards applying the method of nonlinear conjugate gradients and Monte Carlo method to observe directly the developing ground state configuration of a two-dimensional hard-core boson system with decrease in temperature, and its evolution with deviation from a half-filling. This has allowed us to explore unconventional features of a charge order—superfluidity phase transition, specifically, formation of an irregular domain structure, emergence of a filamentary superfluid structure that condenses within of the charge-ordered phase domain antiphase boundaries, and formation and evolution of various topological structures.     

In situ electronic structural study of VO_2 thin film across the metal–insulator transition

The in situ valence band photoemission spectrum （PES） and X-ray absorption spectrum （XAS） at V LⅡ-LⅢ edges of the VO2 thin film, which is prepared by pulsed laser deposition, are measured across the metal–insulator transition （MIT） temperature （TMIT=67 ℃）. The spectra show evidence for changes in the electronic structure depending on temperature. Across the TMIT, pure V 3d characteristic d‖ and O 2p-V 3d hybridization characteristic πpd, σpd bands vary in binding energy position and density of state distributions. The XAS reveals a temperature-dependent reversible energy shift at the V LⅢ-edge. The PES and XAS results imply a synergetic energy position shift of occupied valence bands and unoccupied conduction band states across the phase transition. A joint inspection of the PES and XAS results shows that the MIT is not a one-step process, instead it is a process in which a semiconductor phase appears as an intermediate state. The final metallic phase from insulating state is reached through insulator–semiconductor, semiconductor–metal processes, and vice versa. The conventional MIT at around the TMIT=67 ℃ is actually a semiconductor–insulator transformation point.     

Density functional theory of transition metal phthalocyanines,I: electronic structure of NiPc and CoPc—self-interaction effects

We present a two-part systematic density functional theory study of the electronic structure of selected transition metal phthalocyanines. We use a semi-local generalized gradient approximation (GGA) functional, as well as several hybrid exchange-correlation functionals, and compare the results to experimental photoemission data. Here, we study the low-spin systems NiPc and CoPc. We show that hybrid functionals provide computed photoemission spectra in excellent agreement with experimental data, whereas the GGA functional fails qualitatively. This failure is primarily because of under-binding of localized orbitals due to self-interaction errors.     

Density functional theory of transition metal phthalocyanines,II: electronic structure of MnPc and FePc—symmetry and symmetry breaking

We present a two-part systematic density functional theory (DFT) study of the electronic structure of selected transition metal phthalocyanines. We use a semi-local generalized gradient approximation (GGA) functional, as well as several hybrid exchange-correlation functionals, and compare the results to experimental photoemission data. Here, we study the intermediate spin systems MnPc and FePc. We show that DFT calculations of these systems are extremely sensitive to the choice of functional and basis set with respect to the obtained electronic configuration and to symmetry breaking. Interestingly, all simulated spectra are in good agreement with experiment despite the differences in the underlying electronic configurations.     

Features of the Crystal Structure of Tm1–xYbxB12 Dodecaborides near a Quantum Critical Point and at a Metal–Insulator Transition

JETP Letters - The magnetic phase diagram of Tm1–xYbxB12 antiferromagnets is determined from the specific heat measurements performed at low and ultralow temperatures (0.07–10 K)....