Electrical, Thermal and Spectroscopic Characterization of Bulk Bi2Se3 Topological Insulator

We report electrical (angular magneto-resistance, and Hall), thermal (heat capacity) and spectroscopic (Raman, x-ray photo electron, angle resolved photo electron) characterization of bulk Bi2Se3 topological insulator, which is being is grown by self flux method through solid state reaction from high temperature (950C) melt and slow cooling (2C/hour) of constituent elements. Bi2Se3 exhibited metallic behaviour down to 5K. Magneto transport measurements revealed linear up to 400% and 30% MR at 5K under 14 Tesla field in perpendicular and parallel field direction respectively. We noticed that the magneto-resistance (MR) of Bi2Se3 is very sensitive to the angle of applied field. MR is maximum when the field is normal to the sample surface, while it is minimum when the field is parallel. Hall coefficient (RH) is seen nearly invariant with negative carrier sign down to 5K albeit having near periodic oscillations above 100K. Heat capacity (Cp) versus temperature plot is seen without any phase transitions down to 5K and is well fitted (Cp = gammaT + betaT3) at low temperature with calculated Debye temperature (ThetaD) value of 105.5K. Clear Raman peaks are seen at 72, 131 and 177 cm-1 corresponding to A1g1, Eg2 and A1g2 respectively. Though, two distinct asymmetric characteristic peak shapes are seen for Bi 4f7/2 and Bi 4f5/2, the Se 3d region is found to be broad displaying the overlapping of spin - orbit components of the same. Angle-resolved photoemission spectroscopy (ARPES) data of Bi2Se3 revealed distinctly the bulk conduction bands (BCB), surface state (SS), Dirac point (DP) and bulk valence bands (BVB) and 3D bulk conduction signatures are clearly seen. Summarily, host of physical properties for as grown Bi2Se3 crystal are reported here.Comment: 6 Pages Text + Figs; Comments Suggestions welcom

Unexplored photoluminescence from bulk and mechanically exfoliated few layers of Bi2Te3

We report the exotic photoluminescence (PL) behaviour of 3D topological insulator Bi2Te3 single crystals grown by customized self-flux method and mechanically exfoliated few layers (18 plus minus 2 nm)/thin flakes obtained by standard scotch tape method from as grown Bi2Te3 crystals.The experimental PL studies on bulk single crystal and mechanically exfoliated few layers of Bi2Te3 evidenced a broad red emission in the visible region. These findings are in good agreement with our theoretical results obtained using the ab initio density functional theory framework.Comment: Main MS (17 Pages text including 4 Figs): Suppl. info. (4 pages); Accepted Scientific Report

Crystal growth and magneto-transport of Bi2Se3 single crystals

In this letter, we report on the growth and characterization of bulk Bi2Se3 single crystals. The studied Bi2Se3 crystals are grown by the self-flux method through the solid-state reaction from high-temperature (950 °C) melt of constituent elements and slow cooling (2 °C/h). The resultant crystals are shiny and grown in the [00l] direction, as evidenced from surface XRD. Detailed Reitveld analysis of powder X-ray diffraction (PXRD) of the crystals showed that these are crystallized in the rhombohedral crystal structure with a space group of R3m (D5), and the lattice parameters are a = 4.14 (2), b = 4.14 (2), and c = 28.7010 (7) A° . Temperature versus resistivity (ρ − T ) plots revealed metallic conduction down to 2 K, with typical room temperature resistivity (ρ300 K) of around 0.53 mΩ- cm and residual resistivity (ρ0 K) of 0.12 mΩ-cm. Resistivity under magnetic field [ρ(T )H] measurements exhibited large+ve magneto-resistance right from 2 to 200 K. Isothermal magneto-resistance [ρH] measurements at 2, 100, and 200 K exhibited magneto-resistance (MR) of up to 240 %, 130 %, and 60 %, respectively, at 14 T. Further, the MR plots are nonsaturating and linear with the field at all temperatures. At 2 K, the MR plots showed clear quantum oscillations at above say 10 T applied field. Also, the Kohler plots, i.e., Δρ/ρo versus B/ρ, were seen consolidating on one plot. Interestingly, the studied Bi2Se3 single crystal exhibited the Shubnikov-de Haas (SdH) oscillations at 2 K under different applied magnetic fields ranging from 4 to 14 T

Electrical, thermal and spectroscopic characterization of bulk Bi2Se3 topological insulator

We report the electrical (angular magnetoresistance and Hall), thermal (heat capacity) and spectroscopic (Raman, X-ray photoelectron, angle-resolved photoelectron) characterization of a bulk Bi2Se3 topological insulator, which was grown by self-flux method through solid-state reaction from high-temperature (950 ◦C) melt and slow cooling (2 ◦C/h) of constituent elements. Bi2Se3 exhibited metallic behaviour down to 5 K. Magnetotransport measurements revealed linear up to 400 and 30% magneto-resistance (MR) at 5 K under a 14-T field in perpendicular and parallel field directions, respectively. We noticed that the MR of Bi2Se3 is very sensitive to the angle of the applied field. The MR is maximum when the field is normal to the sample surface, while it is minimum when the field is parallel. The Hall coefficient (RH) is seen nearly invariant with a negative carrier sign down to 5 K albeit having near-periodic oscillations above 100 K. The heat capacity (Cp) versus temperature plot is seen without any phase transitions down to 5 K and is well fitted (Cp = γT + βT 3) at low temperature with a calculated Debye temperature (θD) value of 105.5 K. Clear Raman peaks are seen at 72, 131 and 177 cm−1 corresponding to A1 1g, E2g and A2 1g, respectively. Though two distinct asymmetric characteristic peak shapes are seen for Bi 4f7/2 and Bi 4f5/2, the Se 3d region is found to be broad, displaying the overlapping of spin-orbit components of the same. Angle-resolved photoemission spectroscopy (ARPES) data of Bi2Se3 revealed distinctly the bulk conduction bands (BCB), surface state (SS), Dirac point (DP) and bulk valence bands (BVB), and 3D bulk conduction signatures are clearly seen. Summarily, a host of physical properties for the as-grownBi2Se3 crystal are reported here

Magnetic properties of epitaxial topological insulator Bi₂Te₃/MnTe bilayer and pulsed laser deposited polycrystalline CoFeB thin films

Most electronic devices are made of tiny transistors and capacitors. But there is a limitation on how small these devices can be made up to. This limitation leads to consequences such as super magnetism and overheating of the device. This problem can be avoided by exploring the spin rather than the charge property of an electron giving rise to a new field called spintronics. The thesis gives an outline about the characterization of magnetic thin films like Cobalt-Iron-Boron and magnetic topological insulator films that can be potentially used in magnetic spintronics memory-based devices. The first section includes the theory behind spintronics and topological insulators. The second section describes the experimental techniques used behind achieving the results. And last but not least are the results and their analysis. </p

Crystal Growth and Magneto-transport of Bi2Se3 Single Crystals

In this letter, we report on the growth and characterization of bulk Bi2Se3 single crystals. The studied Bi2Se3 crystals are grown by the self-flux method through the solid-state reaction from high-temperature (950 degrees C) melt of constituent elements and slow cooling (2 degrees C/h). The resultant crystals are shiny and grown in the [00l] direction, as evidenced from surface XRD. Detailed Reitveld analysis of powder X-ray diffraction (PXRD) of the crystals showed that these are crystallized in the rhombohedral crystal structure with a space group of R3m (D5), and the lattice parameters are a = 4.14 (2), b = 4.14 (2), and c = 28.7010 (7) (A) over circle. Temperature versus resistivity (rho-T) plots revealed metallic conduction down to 2 K, with typical room temperature resistivity (rho(300 K)) of around 0.53 m Omega-cm and residual resistivity (rho(0 K)) of 0.12 m Omega-cm. Resistivity under magnetic field [rho(T) H] measurements exhibited large+ve magneto-resistance right from 2 to 200 K. Isothermal magneto-resistance [rho H] measurements at 2, 100, and 200 K exhibited magneto-resistance (MR) of up to 240 %, 130 %, and 60 %, respectively, at 14 T. Further, the MR plots are nonsaturating and linear with the field at all temperatures. At 2 K, the MR plots showed clear quantum oscillations at above say 10 T applied field. Also, the Kohler plots, i.e., Delta rho/rho o versus B/rho, were seen consolidating on one plot. Interestingly, the studied Bi2Se3 single crystal exhibited the Shubnikov-de Haas (SdH) oscillations at 2 K under different applied magnetic fields ranging from 4 to 14 T

Unexplored photoluminescence from bulk and mechanically exfoliated few layers of Bi2Te3

We report the exotic photoluminescence (PL) behaviour of 3D topological insulator Bi2Te3 single crystals grown by customized self-flux method and mechanically exfoliated few layers (18 +/- 2 nm)/thin flakes obtained by standard scotch tape method from as grown Bi2Te3 crystals. The experimental PL studies on bulk single crystal and mechanically exfoliated few layers of Bi2Te3 evidenced a broad red emission in the visible region from 600-690 nm upon 375 nm excitation wavelength corresponding to optical band gap of 2 eV. These findings are in good agreement with our theoretical results obtained using the ab initio density functional theory framework. Interestingly, the observed optical band gap is several times larger than the known electronic band gap of similar to 0.15 eV. The experimentally observed 2 eV optical band gap in the visible region for bulk as well as for mechanically exfoliated few layers Bi2Te3 single crystals clearly rules out the quantum confinement effects in the investigated samples which are well known in the 2D systems like MoS2, WS2, WSe2, and MoSe2 for 1-3 layers

Magnetic and structural properties of CoFeB thin films grown by pulsed laser deposition

The emergence of thin film CoFeB has driven research and industrial applications in the past decades, with the magnetic random access memory (MRAM) the most prominent example. Because of its beneficial properties, it fulfills multiple functionalities as information-storing, spin-filtering, and reference layer in magnetic tunnel junctions. In future, this versatility can be exploited beyond the traditional applications of spintronics by combining with advanced materials, such as oxide-based materials. Pulsed laser deposition (PLD) is their predominant growth-method, and thus the compatibility of CoFeB with this growth technique will be tested here. This encompasses a comprehensive investigation of the structural and magnetic propoperties. In particular, we find a substantial 'dead' magnetic layer and confirm that it is caused by oxidation employing the x-ray magnetic circular dichroism (XMCD) effect. The low damping encountered in vector network analyzer-based ferromagnetic resonance (VNA-FMR) renders them suitable for magnonics applications. These findings demonstrate that CoFeB thin films are compatible with emergent, PLD-grown materials, ensuring their relevance for future applications