Electrical and optical properties of CuIn5S8 single crystals

Summary Several transport and optical properties have been studied onn-type CuIn₅S₈ single crystals. The energy gap at 0 K was determined from the electrical measurements to be 1.4 eV. An anisotropy of the magnetoresistance effect was found and it was suggested that the minima of the conduction band were located at points along the [100] directions ink-space. An optical-absorption band was found in an infrared region of (1÷1.6) μm and was attributed to the transitions from the lowest conduction band situated along the [100] directions to an upper conduction band. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

On the generalized diffusion theory of current transport in isotype heterojunctions

Summary The generalized diffusion theory of current transport inn-n isotype heterojunctions enunciated previously has been extended to the realistic case of there being two different electron mobilities in the two semiconductors forming then-n heterojunction. This extended theory too reduces to the diffusion theory of current in metal-semiconductor contacts under the appropriate limiting conditions in the same way as the previous theory does. The current-voltage characteristics forn-n heterojunctions according to this extended theory would be different from those according to the previous theory if the values of all the parameters involved such as Λ _nn C, ψ_D1, ψ_D2,V ₁,V ₂, etc. could be calculated rigorously. However, utilizing the values of some parameters such as Λ _nn andC calculated by means of approximate expressions derived under these limiting conditions, one would evaluate current-voltage characteristics that are the same as in the previous theory. A single curve representing the current-voltage characteristic of any particularn-n isotype heterojunction can also be plotted by using a formula derived previously under zero-current conditions, in conjunction with the appropriate expressions derived in the present paper. In appendix B an alternative expression for the current density in an-n isotype heterojunction is also derived in the framework of the generalized diffusion theory.

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Riassunto La teoria della diffusione generalizzata del transporto di corrente nelle eterogiunzioni di isotropin-n enunciata in precedenza è stata estesa al caso realistico in cui vi sono due diverse mobilità di elettroni nei due semiconduttori che formano l'eterogiunzionen-n, Anche questa teoria estesa si riduce alla teoria di diffusione della corrente nei contatti dei semiconduttori metallici nelle condizioni limitanti appropriate nello stesso modo della teoria precedente. Le caratteristiche del voltaggio di corrente per le eterogiunzioni n-n secondo questa teoria estesa sarebbero differenti da quelle secondo la teoria precedente se i valori di tutti i parametri coinvolti come Λ _nn ,C, ψ_D1, ψ_D2,V ₁,V ₂, ecc. potessero essere calcolati rigorosamente. Comunque, utilizzando i valori di alcuni parametri come Λ _nn eC calcolati per mezzo di espressioni approssimate dedotte in queste condizioni limitanti, si valuterebbero le caratteristiche del voltaggio di corrente uguali a quelle delle teoria precedente. Si può anche tracciare una singola curva rappresentante la caratteristica del voltaggio di corrente di ogni particolare eterogiunzione di isotopin-n usando una formula dedotta precedentemente in condizioni di corrente zero, assieme alle espressioni appropriate dedotte in questo lavoro. Nell'appendice B si deduce anche un'espressione alternativa per la densità di corrente in un'eterogiunzione di isotopin-n nel modello della teoria della diffusione generalizzata.

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Рэзюме Обобщенная теория диффузэии для переноса тока в n-n гетеропереходе, развитая ранее, обобщается на реалистический случай, когде в двук полупроводниках, образующих n-n гетеропереход, электроны имеют две различные подвижности. Предложенная обобщенная теииая теория сводится к теории диффузии для тока в контакте металл-полупроводник при определенных ограничивающих условиях, как и в случае предыдущей теории. Вольт-амперная характеристика для n-n гетероперереходов, согласно предлагаемой обобщенной теории, будет отличаться от вольт-амперной характеристики предыдущей теории, если все парамеры теории Λ _nn , С, ψ_D1, ψ_D2,V ₁,V ₂ и т.д, могут быть вычислены строго. Однако, используя для некоторых параметров, таких как Λ _nn и С, приближенные выражения, вычисленные в рамках вышеуказанных ограничивающих условий, получаются вольт-амперные характеристики, совпадающие с результатами предыдущей теории. Приводятся кривые для вольт-амперных характеристик для любого n-n гетероперехода. В Приложении П в рамках обобщенной теории диффузии выводится альтернативное выражение для плотности тока в n-n гетеропереходе.

     

Interfacial barrier contribution in Na2B4O7 glass system

Summary Frequency domain measurements on Na₂B₄O₇ and Na₂B₄O₇(95%)-V₂O₅(5%) glass systems have been carried out in the frequency range 10⁻² to 10⁴ Hz and temperature range 300 to 450 K. The complex capacitance has shown fractional power law dependences on frequency which obey the generalized Maxwell-Wagner relationships. The activation energy is found to increase with an increase in V₂O₅ composition.     

Topological insulators and C-algebras: Theory and numerical practice

We apply ideas from C^∗-algebra to the study of disordered topological insulators. We extract certain almost commuting matrices from the free Fermi Hamiltonian, describing band projected coordinate matrices. By considering topological obstructions to approximating these matrices by exactly commuting matrices, we are able to compute invariants quantifying different topological phases. We generalize previous two dimensional results to higher dimensions; we give a general expression for the topological invariants for arbitrary dimension and several symmetry classes, including chiral symmetry classes, and we present a detailed K-theory treatment of this expression for time reversal invariant three dimensional systems. We can use these results to show non-existence of localized Wannier functions for these systems.We use this approach to calculate the index for time-reversal invariant systems with spin–orbit scattering in three dimensions, on sizes up to 12³, averaging over a large number of samples. The results show an interesting separation between the localization transition and the point at which the average index (which can be viewed as an “order parameter” for the topological insulator) begins to fluctuate from sample to sample, implying the existence of an unsuspected quantum phase transition separating two different delocalized phases in this system. One of the particular advantages of the C^∗-algebraic technique that we present is that it is significantly faster in practice than other methods of computing the index, allowing the study of larger systems. In this paper, we present a detailed discussion of numerical implementation of our method.     

The bulk-edge correspondence for continuous honeycomb lattices

Abstract

We study bulk/edge aspects of continuous honeycomb lattices in a magnetic field. We compute the bulk index of Bloch eigenbundles: it equals 2 or –2, with sign depending on nearby Dirac points and on the magnetic field. We then prove the existence of two topologically protected unidirectional waves propagating along line defects. This shows the bulk/edge correspondence for our class of Hamiltonians.     

Two-dimensional topological insulators with large bulk energy gap

Two-dimensional(2D) topological insulators(TTs,or quantum spin Hall insulators) are special insulators that possess bulk 2D electronic energy gap and time-reversal symmetry protected one-dimensional(1D) edge state.Carriers in the edge state have the property of spin-momentum locking,enabling dissipation-free conduction along the 1D edge.The existence of 2D TIs was confirmed by experiments in semiconductor quantum wells.However,the 2D bulk gaps in those quantum wells are extremely small,greatly limiting potential application in future electronics and spintronics.Despite this limitation,2D TIs with a large bulk gap attracted plenty of interest.In this paper,recent progress in searching for TIs with a large bulk gap is reviewed briefly.We start by introducing some theoretical predictions of these new materials and then discuss some recent important achievements in crystal growth and characterization.     

Semimetal behavior of bilayer stanene

Stanene is a two-dimensional (2D) buckled honeycomb structure which has been studied recently owing to its promising electronic properties for potential electronic and spintronic applications in nanodevices. In this article we present a first-principles study of electronic properties of fluorinated bilayer stanene. The effect of tensile strain, intrinsic spin-orbit and van der Waals interactions are considered within the framework of density functional theory. The electronic band structure shows a very small overlap between valence and conduction bands at the Γ point which is a characteristic of semimetal in fluorinated bilayer stanene. A relatively high value of tensile strain is needed to open an energy band gap in the electronic band structure and the parity analysis reveals that the strained nanostructure is a trivial insulator. According to our results, despite the monolayer fluorinated stanene, the bilayer one is not an appropriate candidate for topological insulator.