Theoretical and experimental investigations on Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ topological insulator

Magnetic ion doping in topological insulators (TI's) has emerged to be important both from technological point of view and for experimental verification of exotic fundamental physical concepts. Magnetic ion doping not only opens up the energy gap in surface states of a TI but also changes its bulk band structure significantly. To observe the effect of magnetic ion doping on electronic band structure of TIs density functional theory based ab initio electronic structure calculations have been carried out on 2%, 4% and 6% Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ system. To validate the results single crystal of undoped and Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ have been grown and have been characterized by angle-resolved photoemission spectroscopy (ARPES) measurements with synchrotron radiation. Most of the results viz., band inversion, gapless surface states and reduction in band gap due to Mn doping, as obtained by theoretical calculations have been verified by ARPES measurements. Additionally, ARPES measurements show opening up of the gapless surface states in Mn doped samples. The samples have also been subjected to x-ray absorption spectroscopy measurements comprising of x-ray near edge structure and extended x-ray absorption fine structure measurements using synchrotron radiation which together conclusively show that Mn ions substitute at Bi sites in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ lattice.

Figure 1

(a) Crystal structure of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ hexagonal unit cell. (b) Mn atom doping in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ at Bi site.

Figure 2

Theoretically simulated electronic (a) bulk band structure showing the band inversion phenomenon (red circle: $\mathrm{Be}6p$ states, blue circle $\mathrm{Se}4p$ states) and (b) surface band structure of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$.

Figure 3

Electronic band structure, partial and total density of states of (a) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, (b) ${\mathrm{Bi}}_{1.96}{\mathrm{Mn}}_{0.04}{\mathrm{Se}}_3$, (c) ${\mathrm{Bi}}_{1.92}{\mathrm{Mn}}_{0.08}{\mathrm{Se}}_3$, and (d) ${\mathrm{Bi}}_{1.88}{\mathrm{Mn}}_{0.12}{\mathrm{Se}}_3$. The total DOS, projected Bi-p, Se-p and Mn DOS have been denoted by black, red, green, and blue lines, respectively.

Figure 4

XRD pattern of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystal.

Figure 5

ARPES data recorded with $21.2\mathrm{eV}$ photons showing clear topological surface states and energy gap for (a) ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ (b) 2% Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, and (c) 6% Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The energy gap between valence and conduction band for the undoped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ sample has been estimated from the valence band edge and the conduction band peak in energy distribution curves at $\mathrm{\Gamma }$ point in (d). The dashed line is the linear fitting to determine the valence band edge.

Figure 6

ARPES data recorded with $21.2\mathrm{eV}$ photon energy shown for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ in (a), 2% Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ in (b) and 6% Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ in (c) while the bands recorded with $83.2\mathrm{eV}$ photon energy shown for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ in (d), 2% Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ in (e) and 6% Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ in (f).

Figure 7

Topological surface state of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ collected with (a) $21.2\mathrm{eV}$ and (b) $83.2\mathrm{eV}$. The arrow in the figure shows the position of the Dirac point.

Figure 8

Normalized XANES spectra of undoped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystals along with that of ${\mathrm{Bi}}_2{\mathrm{O}}_3$ powder at Bi L3 edge.

Figure 9

The different coordination shells generated from ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystal structure which are used in EXAFS fitting at Bi ${\mathrm{L}}_3$ and Se K edge, (a) Bi-${\mathrm{Se}}_1$, (b) Bi-${\mathrm{Se}}_2$, (c) ${\mathrm{Se}}_1\text{−}{\mathrm{Se}}_1$, and (d) ${\mathrm{Se}}_{1,2}\text{−}{\mathrm{Se}}_{1,2}$.

Figure 10

Fourier transformed EXAFS spectra of undoped and Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystals measured at Bi L3 edge. The experimental data are represented by scatter points and theoretical fits are represented by solid lines.

Figure 11

XANES spectra of undoped and Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystal at Se $K$ edge along with that of Se foil.

Figure 12

Fourier transformed EXAFS spectra of Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystals measured at Se K edge along with best fit theoretical simulations. The experimental spectra are represented by scatter points and theoretical fits are represented by solid lines.

Figure 13

Normalized XANES spectra of Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystal along with ${\mathrm{MnO}}_2$ and Mn foil at Mn K edge.

Figure 14

Fourier transformed EXAFS spectra of Mn doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystal at Mn K edge along with ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ crystal data at Bi L3 edge (top panel). The experimental spectra are represented by scatter points and theoretical fits are represented by solid lines.