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Register a new userMagnetic properties of Mn-doped Bi$_2$Se$_3$ compound: temperature dependence and pressure effects
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Added by
Gennadiy Grechnev E
Publication date
2015
fields
Physics
and research's language is
English
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Magnetic susceptibility $chi$ of Bi$_{2-x}$Mn$_{x}$Se$_3$ ($x = 0.01-0.2$) was measured in the temperature range $4.2-300$ K. For all the samples, a Curie-Weiss behaviour of $chi(T)$ was revealed with effective magnetic moments of Mn ions corresponding to the spin value S=5/2, which couple antiferromagnetically with the paramagnetic Curie temperature $Thetasim -50$ K. In addition, for the samples of nominal composition $x$ = 0.1 and 0.2 the effect of a hydrostatic pressure $P$ up to 2 kbar on $chi$ has been measured at fixed temperatures 78 and 300 K that allowed to estimate the pressure derivative of $Theta$ to be d$Theta$/d$Psim-0.8$ K/kbar. Based on the observed behaviour of $Theta$ with varied Mn concentration and pressure, a possible mechanism of interaction between localized Mn moments is discussed.
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Doping Bi$_2$Se$_3$ by magnetic ions represents an interesting problem since it may break the time reversal symmetry needed to maintain the topological insulator character. Mn dopants in Bi$_2$Se$_3$ represent one of the most studied examples here. However, there is a lot of open questions regarding their magnetic ordering. In the experimental literature different Curie temperatures or no ferromagnetic order at all are reported for comparable Mn concentrations. This suggests that magnetic ordering phenomena are complex and highly susceptible to different growth parameters, which are known to affect material defect concentrations. So far theory focused on Mn dopants in one possible position, and neglected relaxation effects as well as native defects. We have used ab initio methods to calculate the Bi$_2$Se$_3$ electronic structure influenced by magnetic Mn dopants, and exchange interactions between them. We have considered two possible Mn positions, the substitutional and interstitial one, and also native defects. We have found a sizable relaxation of atoms around Mn, which affects significantly magnetic interactions. Surprisingly, very strong interactions correspond to a specific position of Mn atoms separated by van der Waals gap. Based on the calculated data we performed spin dynamics simulations to examine systematically the resulting magnetic order for various defect contents. We have found under which conditions the experimentally measured Curie temperatures ${T_{rm{C}}}$ can be reproduced, noticing that interstitial Mn atoms appear to be important here. Our theory predicts the change of ${T_{rm{C}}}$ with a shift of Fermi level, which opens the way to tune the system magnetic properties by selective doping.
We investigate the properties of a single substitutional Mn impurity and its associated acceptor state on the (111) surface of Bi$_2$Se$_3$ topological insulator. Combining ab initio calculations with microscopic tight-binding modeling, we identify the effects of inversion-symmetry and time-reversal-symmetry breaking on the electronic states in the vicinity of the Dirac point. In agreement with experiments, we find evidence that the Mn ion is in the ${+2}$-valence state and introduces an acceptor in the bulk band gap. The Mn-acceptor has predominantly $p$-character, and is localized mainly around the Mn impurity and its nearest-neighbor Se atoms. Its electronic structure and spin-polarization are determined by the hybridization between the Mn $d$-levels and the $p$-levels of surrounding Se atoms, which is strongly affected by electronic correlations at the Mn site. The opening of the gap at the Dirac point depends crucially on the quasi-resonant coupling and the strong real-space overlap between the spin-chiral surface states and the mid-gap spin-polarized Mn-acceptor states.
We report the preparation of high-quality single crystal of Bi$_2$Se$_3$, a well-known topological insulator and its Ti-doped compositions using Bridgeman technique. Prepared single crystals were characterized by x-ray diffraction (XRD) to check the crystalline structure and energy dispersive analysis of x-rays for composition analysis. The XRD data of Ti-doped compounds show a small shift with respect to normal Bi$_2$Se$_3$ indicating changes in the lattice parameters while the structure type remained unchanged; this also establishes that Ti goes to the intended substitution sites. All the above analysis establishes successful preparation of these crystals with high quality using Bridgman technique. We carried out x-ray photo-emission spectroscopy to study the composition via investigating the core level spectra. Bi$_2$Se$_3$ spectra exhibit sharp and distinct features for the core levels and absence of impurity features. The core level spectra of the Ti-doped sample exhibit distinct signal due to Ti core levels. The analysis of the spectral features reveal signature of plasmon excitation and final state satellites; a signature of finite electron correlation effect in the electronic structure.
Structural, magnetic and magnetotransport properties of (Bi$_{1-x}$Eu$_x$)$_2$Se$_3$ thin films have been studied experimentally as a function of Eu content. The films were synthesized by MBE. It is demonstrated that Eu distribution is not uniform, it enter quint-layers forming inside them plain (pancake-like) areas containing Eu atoms, which sizes and concentration increase with the growth of Eu content. Positive magnetoresistance related to the weak antilocalization was observed up to 15K. The antilocalization was not followed by weak localization as theory predicts for nontrivial topological states. Surprisingly, the features of antilocalization were seen even at Eu content $x$ $=$ 0.21. With the increase of Eu content the transition to ferromagnetic state occurs at $x$ about 0.1 and with the Curie temperature $approx$ 8K, that rises up to 64K for $x$ $=$ 0.21. At temperatures above 1-2 K, the dephasing length is proportional to $T^{-1/2}$ indicating the dominant contribution of inelastic $e-e$ scattering into electron phase breaking. However, at low temperatures the dephasing length saturates, that could be due to the scattering on magnetic ions.
Three-dimensional topological insulators (3D-TIs) possess a specific topological order of electronic bands, resulting in gapless surface states via bulk-edge correspondence. Exotic phenomena have been realized in ferromagnetic TIs, such as the quantum anomalous Hall (QAH) effect with a chiral edge conduction and a quantized value of the Hall resistance ${R_{yx}}$. Here, we report on the emergence of distinct topological phases in paramagnetic Fe-doped (Bi,Sb)${_2}$Se${_3}$ heterostructures with varying structure architecture, doping, and magnetic and electric fields. Starting from a 3D-TI, a two-dimensional insulator appears at layer thicknesses below a critical value, which turns into an Anderson insulator for Fe concentrations sufficiently large to produce localization by magnetic disorder. With applying a magnetic field, a topological transition from the Anderson insulator to the QAH state occurs, which is driven by the formation of an exchange gap owing to a giant Zeeman splitting and reduced magnetic disorder. Topological phase diagram of (Bi,Sb)${_2}$Se${_3}$ allows exploration of intricate interplay of topological protection, magnetic disorder, and exchange splitting.
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