Electrical,Thermal and Spectroscopic Characterization of Bulk Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Topological Insulator

We report the electrical (angular magneto-resistance 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. Magneto-transport 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 (R _H) is seen nearly invariant with a negative carrier sign down to 5 K albeit having near-periodic oscillations above 100 K. The heat capacity (C _p) versus temperature plot is seen without any phase transitions down to 5 K and is well fitted (C _p = γ T + β T ³) at low temperature with a calculated Debye temperature (?? _D) value of 105.5 K. Clear Raman peaks are seen at 72, 131 and 177 cm^?1 corresponding to A\(_{\mathrm {1g}}^{1}\), E\(_{\mathrm {g}}^{2}\) and A\(_{1\mathrm {g}}^{2}\), respectively. Though two distinct asymmetric characteristic peak shapes are seen for Bi 4f_7/2 and Bi 4f_5/2, 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-grown Bi₂Se₃ crystal are reported here.     

Enhanced magnetoelectric coupling in La-modified Bi<Subscript>5</Subscript>Co<Subscript>0.5</Subscript>Fe<Subscript>0.5</Subscript>Ti<Subscript>3</Subscript>O<Subscript>15</Subscript> multiferroic ceramics

Multiferroic properties of La-modified four-layered perovskite Bi_5?x La _x Fe_0.5Co_0.5Ti₃O₁₅ (0 ≤ x ≤ 1) ceramics were investigated, by analyzing the magnetodielectric effect, magneto-polarization response and magnetoelectric conversion. X-ray diffraction indicated the formation of pure Aurivillius ceramics, and Raman spectroscopy revealed the Bi ions displacement and the crystal structure variation. The enhancement of ferromagnetic and ferroelectric properties was observed in Bi_5?x La _x Fe_0.5Co_0.5Ti₃O₁₅ after La modification. The evidence for enhanced ME coupling was determined by magnetic field-induced marked variations in the dielectric constant and polarization. A maximum ME coefficient of 1.15 mV/cm·Oe was achieved in Bi_4.25La_0.75Fe_0.5Co_0.5Ti₃O₁₅ ceramic, which provides the possible promise for novel magnetoelectric device application.     

Influence of isovalent and heterovalent substitution for Bi<Superscript>3+</Superscript> and Fe<Superscript>3+</Superscript> on the properties of Bi<Subscript>2</Subscript>Fe<Subscript>4</Subscript>O<Subscript>9</Subscript>-based solid solutions

Bi_2–хLa_хFe₄O₉ and Bi₂Fe_4–2xTi_xCo_xO₉ ferrites have been prepared by solid-state reactions at a temperature of 1073 K. X-ray diffraction data indicate that, in the Bi_2–хLa_хFe₄O₉ system, the limiting degree of La³⁺ substitution for Bi³⁺ ions in Bi₂Fe₄O₉ does not exceed 0.05 and that the limiting degree of substitution in the Bi₂Fe_4–2xTi_xCo_xO₉ system lies in the range 0.05 < x < 0.1. The specific magnetization and specific magnetic susceptibility of the samples have been measured at temperatures from 5 to 300 K in a magnetic field of 0.86 T. The field dependences of magnetization obtained for the Bi_2–хLa_хFe₄O₉ and Bi₂Fe_4–2xTi_xCo_xO₉ ferrites at temperatures of 300 and 5 K demonstrate that partial isovalent substitution of La³⁺ for Bi³⁺ ions in Bi₂Fe₄O₉ and heterovalent substitution of Ti⁴⁺ and Co²⁺ ions for two Fe³⁺ ions leads to partial breakdown of the antiferromagnetic state and nucleation of a ferromagnetic state.     

High-Field Magneto-Conductivity Analysis of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystal

We report the high-field (up to 14 Tesla) magneto-conductivity analysis of Bi₂Se₃ topological insulator grown via the self-flux method. The detailed experimental investigations including crystal growth as well as the electrical, thermal, and spectroscopic characterizations of the resultant Bi₂Se₃ single crystal are already reported by some of us. The current letter deals with high-field magneto-conductivity analysis in terms of Hikami Larkin Nagaoka (HLN) model, which revealed that the electronic conduction is dominated by both surface state-driven weak anti-localization (WAL), as well as the bulk weak localization (WL) states. Further, by applying the HLN equation, we have extracted the fitting parameters, i.e., phase coherence length (l_φ) and the pre-factor (α). Here, the magneto-conductivity data is fitted up to ± 5 Tesla, but in order to extract reliable fitting parameters, the same is fitted at much lower magnetic fields, i.e., up to ± 1 Tesla. The value of the HLN coefficient (α), extracted from the HLN equation exhibited values close to ? 1.0, indicating both WAL and WL contributions. On the other hand, the extracted \(l_{\varphi }\) is seen to decrease from 11.125 to 5.576 nm as the temperature is increased from 5 to 200 K, respectively. Summarily, the short letter discusses primarily the temperature-dependent magneto-conductivity analysis of pristine Bi₂Se₃ single crystal by the HLN model.     

Ultralow thermal conductivity of cerium-doped Nd<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> over a wide doping range

In this paper, we report an ultralow thermal conductivity and a high-temperature phase stability of the (Nd_1?x Ce _x )₂Zr₂O_7+x system over the temperature range from room temperature to 1600 °C and over a wide composition range (0.2 ≤ x ≤ 0.8), and the (Nd_1?x Ce _x )₂Zr₂O_7+x system is therefore considered a strong candidate material for the fabrication of next-generation high-temperature thermal barrier coatings. The observed thermal conductivities (0.65–1.0 W/mK) are about 60–40% lower than those of undoped Nd₂Zr₂O₇ over the same temperature range (100–700 °C) and indicate a glass-like behavior. For comparison, the variation in the thermal conductivity with the temperature of the (Gd_1?x Ce _x )₂Zr₂O_7+x system with similar point defects was also measured, and the observed behavior was almost the same as that of undoped Gd₂Zr₂O₇ and was mostly determined by phonon–phonon scattering (λ ∝ 1/T). The effect of point defect scattering and strong phonon scattering sources (rattlers) on the thermal conductivity is also discussed in this paper. The results of this study suggest that the ultralow thermal conductivity of (Nd_1?x Ce _x )₂Zr₂O_7+x can be attributed to the presence of rattlers because of the large difference between the ionic radii of the Nd³⁺ and Ce⁴⁺ ions.     

Effect of Bi and Sr doping on morphological and magnetic properties of LaCo<Subscript>0.6</Subscript>Fe<Subscript>0.4</Subscript>O<Subscript>3</Subscript> nanosized perovskites

Nanopowders of La _1?x Bi _x Co_0.6Fe_0.4O₃ (x = 0, 0.1, 0.2) and La _1?2x Bi _x Sr _x Co_0.6Fe_0.4O₃ (x = 0.1) multinary perovskites were synthesized by citrate sol–gel autocombustion method. Crystalline phase and the lattice parameters were obtained from X-ray diffraction pattern. The XRD result shows that all compounds have rhombhohedral crystal structure with \(\bar {\mathbf {R}\mathbf {3}}\)c space group and Bi (x = 0.2) have the presence of secondary peaks. Crystallite size, dislocation density, specific area and strain were calculated from XRD. The elemental composition and micrographs of grain were obtained from EDAX (energy dispersive X-ray analysis) and SEM (scanning electron microscopy), with an average grain size below 400 nm. Surface morphological studies using XPS (X-ray photoelectron spectroscopy) were used to find out the chemical states and surface proportion of oxygen present in samples. Finally, using the vibrating sample magnetometer the room temperature magnetic behaviour of compounds was studied and it was observed that the ferromagnetic behaviour of LaCo_0.6Fe_0.4O₃ was reduced by Bi and Sr doping.     

The H-T and P-T Phase Diagram of the Superconducting Phase in Pd:Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>

We study the magnetic field vs. temperature (H – T) and pressure vs. temperature (P – T) phase diagrams of the T _c ≈ 5.5 K superconducting phase in Pd _x Bi₂Te₃ (x ≈ 1) using electrical resistivity versus temperature measurements at various applied magnetic fields (H) and magnetic susceptibility versus temperature measurements at various applied magnetic fields (H) and pressure (P). The H – T phase diagram has an initial upward curvature as observed in some unconventional superconductors. The critical field extrapolated to T = 0 K is H _c (0) ≈ 6–10 kOe. The T _c is suppressed approximately linearly with pressure at a rate d T _c /d P ≈ ?0.28 K/GPa.     

Phase relations and thermoelectric properties of alloys in the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

Using differential thermal analysis and x-ray diffraction, we have shown that the Bi₂Te₃-Bi₂Se₃ system contains a continuous series of solid solutions in a narrow temperature range and a compound of composition Bi₂Te₂Se below the solidus line. The liquidus and solidus lines determined using zone-melted samples differ little from those reported in the literature for equilibrium samples. The Bi₂Te_3?x Se _x solid-solution phase extends to ～-14 mol % Bi₂Se₃ (Bi₂Te_2.58Se_0.42). The thermoelectric power of the alloys drops sharply near the boundary of the two-phase region. Within the homogeneity range of Bi₂Te₂Se (33.3 mol % Bi₂Se₃), the thermoelectric power factor has a minimum, while the thermoelectric power has a small maximum.     

Crystallization from high-temperature solutions in the K<Subscript>2</Subscript>O-P<Subscript>2</Subscript>O<Subscript>5</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> system

We have studied general trends of crystallization from high-temperature solutions in the K₂O-P₂O₅-V₂O₅-Bi₂O₃ system at P/V = 0.5?2.0, K/(P + V) = 0.7?1.4, and Bi₂O₃ contents from 25 to 50 wt % and identified the stability regions of BiPO₄, K₃Bi₅(PO₄)₆, K₂Bi₃O(PO₄)₃, and K₃Bi₂(PO₄)_{3 ? x} (VO₄) _x (x = 0?3) solid solutions. The synthesized compounds have been characterized by X-ray powder diffraction and IR spectroscopy, and the structure of two solid solutions has been determined by single-crystal X-ray diffraction (sp. gr. C ₂/c): K₃Bi₂(PO₄)₂(VO₄), a = 13.8857(8), b = 13.5432(5), c = 6.8679(4) Å, β = 114.031(7)°; K₃Bi₂(PO₄)_1.25(VO₄)_1.75, a = 13.907(4), b = 13.615(2), c = 6.956(2) Å, β = 113.52(4)°.     

Influence of Nd doping on microwave dielectric properties of SrTiO<Subscript>3</Subscript> ceramics

Sr_1?x Nd _x TiO₃ (x?=?0.08–0.14) ceramics were prepared by conventional solid-state methods. The analysis of crystal structure suggested Sr_1?x Nd _x TiO₃ ceramics appeared to form tetragonal perovskite structure. The relationship between charge compensation mechanism, microstructure feature and microwave dielectric properties were investigated. Trivalent Nd³⁺ substituting Sr²⁺ could effectively decrease oxygen vacancies. This reduction and relative density were critical to improve Q?×?f values of Sr_1?x Nd _x TiO₃ ceramics. For ε _r values, incorporation of Nd could restrain the rattling of Ti⁴⁺ cations and led to the reduction of dielectric constant. The τ _f values were strongly influenced by tilting of oxygen octahedral. The τ _f values decreased from 883 to 650 ppm/°C with x increasing from 0.08 to 0.14. A better microwave dielectric property was achieved for composition Sr_0.92Nd_0.08TiO₃ at 1460 °C: ε _r ?=?160, Q?×?f?=?6602 GHz, τ _f ?=?883 ppm/°C.     

Influence of silver addition on microstructures and transport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>:Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> composites

In order to obtain high temperature coefficient of resistance (TCR) value of La_0.67Ca_0.33MnO₃:Ag _x (LCMO:Ag _x ) composites, samples with different Ag contents (x?=?0, 0.1, 0.2, 0.25, 0.3, and 0.5) were prepared by sol–gel method. X-ray diffraction analyses indicated that all samples had orthorhombic perovskite structures. As x increased, lattice parameters (a, b, c) and cell volumes underwent slight expansions. Interestingly, the addition of Ag dramatically affected TCR and magneto-resistance (MR) values. Elevated TCR value up to 53.46%·K^?1 at 277 K was observed for LCMO:Ag _x composites with added Ag at the composition of x?=?0.1. Meanwhile, MR value at 263 K reached 71% at the magnetic field of 1 T for samples with Ag composition of x?=?0.25. The increase in Mn⁴⁺/Mn³⁺ ratio and improvement in crystallization caused by added Ag was found responsible for the elevated values of TCR, MR, and T_p. These findings may have practical use in high-performance magneto-resistive manganites.     

Comparative Magnetic and Structural Properties Study of Micro- and Nanopowders of Nd<Subscript>2</Subscript>Fe<Subscript>14</Subscript>B Doped with Ni

Micropowders of melted and heat-treated Nd₁₆(Fe_76?x Ni _x )B₈ alloys system, with x = 0, 10, 20, and 25 (size distribution under 20 μm), were studied and compared with the study of nanopowders obtained, from the previous ones, by surfactant-assisted ball-milling process during 2 h. By XRD, a majority of Nd₂Fe₁₄B hard phase and a minority of α-Fe, Nd_1.1Fe₄B₄ and NdNi₂ phases were detected. The last one increases with Ni content. The crystallite size of the hard phase, in both types of samples, is not affected by the Ni content; however, the grains in micropowders are oblate, with a mean size of 37 nm, while those of the nanopowders are symmetric, with a mean size of 35 nm. Mössbauer spectra were fitted with seven sextets, which correspond to the six ferromagnetic sites of the hard phase and that of the α-Fe, and a doublet corresponding to the paramagnetic Nd_1.1Fe₄B₄ phase. The mean hyperfine magnetic field, for both types of samples, decreases with Ni content. The hysteresis loops of both types of samples show a hard magnetic character, however, the coercive field and the M _r/M _s values for nanopowders are greater than those obtained for micropowders for all the Ni contents. Values of H _c = 2 kOe and M _r/ M _s = 0.54 were obtained for nanopowders with 10 at.% Ni. From the hysteresis loops, which include the initial magnetization curve, Henkel plots for all the samples were obtained. These plots show that for micropowders, the predominant magnetic interaction is of dipolar type, while for nanopowders, the ferromagnetic exchange is the predominant one, which favored the magnetization.     

Magnetic and Electrical Properties Induced by the Substitution of Divalent by Monovalent in the La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> Compound

The La_0.6Ca_0.4?x Ag _x MnO₃ samples with x = 0 and 0.10 were prepared by sol–gel methods. Structural and electrical measurements were performed to examine the effect of the silver substitution in the calcium sites on the physical properties. Magnetization versus temperature studies have shown that all samples exhibit a magnetic transition from ferromagnetic to paramagnetic phase when temperature is increased. The second transition in the resistivity in the La_0.6Ca_0.4MnO₃ compound can be attributed to an abnormality characteristic of charge ordering (CO) effect. The electrical resistivity was described by a phenomenological percolation model. Ten percent of Ag substitution in the Ca site exhibits a magnetoresistance value about 75 % near room temperature at the applied magnetic field of 8 T.     

Inhomogeneous State of the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Doped with Manganese

Basing on electron spin resonance (ESR) data for Bi₂Te₃ doped by Mn ions we argue that this compound can be inhomogeneous and consists of two components with the different structures. Its main phase Bi _2?x Mn _x Te ₃ is intertwined with the microscopical inclusions of MnBi phase. The integral volume of these intermetal clusters is less than 1 % but nevertheless they exert the serious impact on the dynamic magnetic properties of the entire system. These inclusions are ferromagnetic with the Curie temperature of 630 K, while the main bulk phase Bi _2?x Mn _x Te ₃ has x= 0.05 orders at T _c= 10 K (qualitatively this twophase picture is valid not only for this given x). Below this temperature two ferromagnetic phases coexist. Since the integral spontaneous polarization in MnBi phase is averaged out due to its random orientations in different clusters the time-reversal symmetry of Bi ₂Te ₃ doped by Mn ions is violated only at the low-temperature ferromagnetic transition.     

Effects of Iron Contents on the Vortex State in Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>0.5</Subscript>Te<Subscript>0.5</Subscript>

We investigate the effects of iron content on the upper critical field (H _c2) and the activation energy U(T) in thermally activated flux flow in Fe _x Se_0.5Te_0.5 near the superconducting transition temperature T _c . The variations in H _c2(T) with temperature are analyzed using Ginzburg-Landau (GL), Werthamer-Helfand-Hohenberg (WHH) models along with the empirical relation (ER). The obtained values of H _c2(0) depend strongly on the model and the criteria used to determine the transition temperature. However, the general trend is that that H _c2(0) increases with the increasing Fe content. The activation energy U(T) is maximum for x =? 1 and rapidly suppressed by excess or deficiency of iron. The low values of U(T) (～10 meV) reflect the low vortex-pinning nature (due to defects, vacancies, etc.) in the Fe _x Se_0.5Te_0.5 superconductor.     

Crystal Growth and Magneto-transport of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystals

In this letter, we report on the growth and characterization of bulk Bi ₂Se ₃ single crystals. The studied Bi ₂Se ₃ 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 ℃/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) Å. Temperature versus resistivity (ρ?T) plots revealed metallic conduction down to 2 K, with typical room temperature resistivity (ρ _{300 K}) of around 0.53 m Ω-cm and residual resistivity (ρ _{0 K}) of 0.12 m Ω-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/ ρ _oversus B/ ρ, were seen consolidating on one plot. Interestingly, the studied Bi ₂Se ₃ single crystal exhibited the Shubnikov-de Haas (SdH) oscillations at 2 K under different applied magnetic fields ranging from 4 to 14 T.     

In-field Conductivity Fluctuations in Ba<Subscript>0.72</Subscript>K<Subscript>0.28</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Fluctuations in the conductivity of Ba_0.72K_0.28Fe₂As₂ single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki?Thompson and Aslamakov?Larkin (MT–AL) two- to three-dimensional (2D–3D) crossover is found on the excess conductivity (Δσ) curves as the temperature approaches the superconducting critical temperature, T _c. 3D fluctuations in superconductivity are realized near T _c that are well fitted to experimental data by the 3D Aslamazov–Larkin theory. The Maki–Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition, T ₀, which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(ε ₀), as functions of magnetic field were used to determine the coherence length and the lifetime, τ _φ , of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba_0.72K_0.28Fe₂As₂ single crystal gives a value of 3.76 × 10^??12 s for the τ _φ in the absence of magnetic field and it decreases to 2.4 × 10^??12 s in magnetic field of 13 T.     

Ferromagnetism in Fe-doped BaTiO<Subscript>3</Subscript> Ceramics

Polycrystalline samples of BaTi_1?xFe_xO₃ (x = 0.00–0.30) are prepared by solid-state reaction method and their structural and magnetic properties are studied. Detailed investigation of XRD patterns reveal the coexistence of tetragonal (space group P4mm) and hexagonal phases (space group P6 ₃/mmc) for x ≥ 0.1. Magnetic measurements reveal room-temperature ferromagnetism in x = 0.15–0.3 samples, and their ferromagnetic transition temperature increases from 397 K for x = 0.15 to 464 K for x = 0.3. The initial magnetization curves for x = 0.15–0.3 are analyzed in terms of bound magnetic polaron (BMP) model. The analysis of susceptibility data in the paramagnetic region by Curie-Weiss law confirms the ferromagnetic transition for x ≥ 0.15 and the effective magnetic moment systematically increases with increase in Fe concentration.     

Electric-field effect on crystal growth in the Li<Subscript>3</Subscript>PO<Subscript>4</Subscript>-Li<Subscript>4</Subscript>GeO<Subscript>4</Subscript>-Li<Subscript>2</Subscript>MoO<Subscript>4</Subscript>-LiF system

We have studied the electric-field effect on crystallization processes in the Li₃PO₄-Li₄GeO₄-Li₂MoO₄-LiF system. In zero field, Li_3+x P_1?x Ge _x O₄ (x = 0.31) crystals were grown on the cathode under the conditions of this study. At low applied voltages (≤ 0.5 V), we obtained Li₂MoO₄, Li₂GeO₃, and Li_1.3Mo₃O₈. In the range V = 0.5–1 V, crystals of Li_3+x P_1?x Ge _x O₄ solid solutions with x = 0.17, 0.25, 0.28, 0.29, and 0.36 were obtained. An applied electric field was shown to reduce the melting temperature of the starting mixtures and the crystallization onset temperature.     

Electrical conductivity of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Tm<Subscript>2</Subscript>O<Subscript>3</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> solid solutions

New solid solutions, Bi_2?x?y Tm _x Nb _y O_3+δ, with tetragonal and cubic structures have been synthesized in the Bi₂O₃-Tm₂O₃-Nb₂O₅ system, and their electrical conductivity has been measured at temperatures from 670 to 1020 K. The 1020-K conductivity of the tetragonal solid solution Bi_1.8Tm_0.15Nb_0.05O_3+δ is comparable to that of Bi_1.75Tm_0.25O₃, the best conductor in the Bi₂O₃-Tm₂O₃ system.