Ordering of Magnetic Impurities and Tunable Electronic Properties of Topological Insulators

<p>The three-dimensional topological insulators Bi₂Se₃ and Bi₂Te₃ are model systems of a new class of materials with an insulating bulk and gapless surface states. Their small band gaps and the heavy elements are essential for the topologically non-trivial band structure, but these features are similarly responsible for other remarkable properties, such as their high thermoelectric performance. This thesis investigates the electronic properties of the topological insulators Bi₂Se₃ and Bi₂Te₃ with a broad range of experimental methods. Ferromagnetism in Mn doped Bi₂Te₃ is shown to disappear under sample sintering. A surprisingly large magnetoresistance and a charge carrier independent change in the sign of the thermopower with increasing Mn content are discussed.¹²⁵Te nuclear magnetic resonance (NMR) of Bi₂Te₃ single crystals suggest an unusual electronic spin susceptibility and complex NMR shifts. The quadrupole interaction of ²⁰⁹Bi nuclei in Bi₂Se₃ single crystals is shown to be a signature of the band inversion in quantitative agreement with first-principle calculations. Furthermore, it is proposed that the strong spin-orbit coupling of conduction electrons causes a non-trivial orientation dependent quadrupole splitting of the ²⁰⁹Bi resonance.</p>

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Magnetic and electronic properties of iron-based superconducting systems

10.26686/wgtn.17013278.v1 ◽

2021 ◽

Author(s):

◽

Sebastian Sambale

Keyword(s):

Electronic Properties ◽

Carrier Transport ◽

High Temperature Superconductors ◽

Creep Model ◽

Hole Doping ◽

Magnetic Impurities ◽

Superconducting Transition ◽

Superconducting Layer ◽

Fluctuation Conductivity ◽

Strong Localization

<p>This thesis is motivated by the large variety of high-temperature superconductors that contain iron in the superconducting layer. This number has grown rapidly since the discovery in 2008 of the iron-pnictides (and chalcogenides), where iron and arsenic form the superconducting layer. Also of interest are the iron-cuprate hybrid materials, where one out of three copper atoms is replaced by iron. The aim is to understand the superconducting, magnetic and electronic properties of these materials in respect to their iron content. This thesis describes some of these properties for the iron-pnictide compounds of CeFeAsO₁₋xFx and AFe₂As₂ (A=Ba, Sr), and for the ironcuprate hybrids of FeSr₂YCu₂O₆₊y and FeSr₂Y₂₋xCexCu₂O₁₀₋y. Here it has been found that CeFeAsO₁₋xFx follows a 3D fluctuation conductivity above the superconducting transition and the thermal activation energy is correlated to the critical current density within a two fluid-flux creep model below the superconducting transition. NMR measurements show that there is considerable charge disorder within the superconducting doping region. The AFe₂As₂ show a positive magnetoresistance, which could be interpreted through three-carrier transport. Superconducting samples of SrFe₂As₂ display a large enhancement in the magnetoresistance below the superconducting transition up to 1600 %, which is due to three-carrier transport through metallic and superconducting regions in an inhomogeneous state. The superconducting properties of the iron-cuprate FeSr₂YCu₂O₆₊y in respect to the location of iron was studied under the influence of electron and hole doping and with additional magnetic impurities. FeSr₂Y₂₋xCexCu₂O₁₀₋y shows a disorder induced spin-glass state and strong localization depending on the doping.</p>

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Non-magnetic impurities and in-gap bound states in topological insulators

New Journal of Physics ◽

10.1088/1367-2630/13/10/103016 ◽

2011 ◽

Vol 13 (10) ◽

pp. 103016 ◽

Cited By ~ 29

Author(s):

Jie Lu ◽

Wen-Yu Shan ◽

Hai-Zhou Lu ◽

Shun-Qing Shen

Keyword(s):

Bound States ◽

Topological Insulators ◽

Magnetic Impurities

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Theoretical studies of single magnetic impurities on the surface of semiconductors and topological insulators

MRS Proceedings ◽

10.1557/opl.2013.1088 ◽

2013 ◽

Vol 1564 ◽

Author(s):

M. R. Mahani ◽

A. Pertsova ◽

C.M. Canali ◽

M. F. Islam ◽

A.H. MacDonald

Keyword(s):

Topological Insulators ◽

Tight Binding ◽

Three Dimensional ◽

Magnetic Coupling ◽

Magnetic Impurities ◽

Theoretical Studies ◽

Acceptor State ◽

Binding Model ◽

Deep Acceptor ◽

Antisite Defects

ABSTRACTWe present results of theoretical studies of transition metal dopants in GaAs, based on microscopic tight-binding model and ab-initio calculations. We focus in particular on how the vicinity of surface affects the properties of the hole-acceptor state, its magnetic anisotropy and its magnetic coupling to the magnetic dopant. In agreement with STM experiments, Mn substitutional dopants on the (110) GaAs surface give rise to a deep acceptor state, whose wavefunction is localized around the Mn center. We discuss a refinement of the theory that introduces explicitly the d-levels for the TM dopant. The explicit inclusion of d-levels is particularly important for addressing recent STM experiments on substitutional Fe in GaAs. In the second part of the paper we discuss an analogous investigation of single dopants in Bi2Se3 three-dimensional topological insulators, focusing in particular on how substitutional impurities positioned on the surface affect the electronic structure in the gap. We present explicit results for BiSe antisite defects and compare with STM experiments.

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