Pressure effects in ferromagnetic manganites and carrier scattering by disordered magnetic rare earths

The influence of hydrostatic pressure on the transport properties of (La_1-xR_x)_0.67Ca_0.33MnO₃ (,Tb) ferromagnetic manganites is investigated. The enhancement of the Curie temperature T_C under pressure agrees with previous data. In the paramagnetic range, the resistivity can be represented by a Mott localisation law, with a characteristic temperature T₀ decreasing with pressure. The variation of T_C with pressure is compared to the effect induced by replacing La by a magnetic rare earth in (La_1-xR_x)_0.67Sr_0.33MnO₃ manganites (, ..., Tm). The main effect is not related to the decrease of the mean radius of the cation, but to an additional scattering by the magnetic moment of the rare earth. Received: 15 May 1998 / Revised: 6 July 1998 / Accepted: 16 July 1998     

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.     

Spin textures of strongly correlated spin Hall quantum dots

We study the spin ordering of a quantum dot defined via magnetic barriers in an interacting quantum spin Hall edge. The spin‐resolved density–density correlation functions are computed. We show that strong electron interactions induce a ground state with a highly correlated spin pattern. The crossover from the liquid‐type correlations at weak interactions to the ground state spin texture found at strong interactions parallels the formation of a one‐dimensional Wigner molecule in an ordinary strongly interacting quantum dot.

     

Topological Insulators on the Ruby Lattice with Rashba Spin-Orbit Coupling

We investigate a tight-binding model of the ruby lattice with Rashba spin-orbit coupling. We calculate the band structure of the lattice and evaluate the Z2 topological indices. According to the Z2 topological indices and the band structure, we present the phase diagrams of the lattice with different filling fractions. We find that topological insulators occur in some range of parameters at 1/6, 1/3, 1/2, 2/3 and 5/6 filling fractions. We analyze and discuss the characteristics of these topological insulators and their edge states.     

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.     

NMR probe of bulk electronic structures in H+ beam irradiated Bi2Te3 topological insulator

We report a nuclear magnetic resonance (NMR) study on H⁺ beam irradiated Bi₂Te₃ powdered single crystals. In this work, we demonstrate that the beam creates defects within its penetration range giving rise to delocalized charge carriers, thereby making further ¹²⁵Te NMR Knight shift and line broadening. Upon increasing temperature, the NMR line narrowing manifests the activated motions of thermally excited charge carriers in the irradiated sample. In contrast, it reveals that in the unirradiated sample the free-charge carriers at the Fermi level dominantly contribute to the Knight shift. Our results show that the orbital contribution to the Knight shift in the bulk state of Bi₂Te₃ becomes predominant in the system with the higher density of defects, as evidenced by modified electronic structures induced by the beam irradiation.