The spin-helical Dirac fermion topological surface states in a topological insulator nanowire or nanoribbon promise novel topological devices and exotic physics such as Majorana fermions. Here, we report local and non-local transport measurements in Bi2Te3 topological insulator nanoribbons that exhibit quasi-ballistic transport over ∼2 μm. The conductance versus axial magnetic flux Φ exhibits Aharonov–Bohm oscillations with maxima occurring alternately at half-integer or integer flux quanta (Φ0 = h/e, where h is Planck's constant and e is the electron charge) depending periodically on the gate-tuned Fermi wavevector (kF) with period 2π/C (where C is the nanoribbon circumference). The conductance versus gate voltage also exhibits kF-periodic oscillations, anti-correlated between Φ = 0 and Φ0/2. These oscillations enable us to probe the Bi2Te3 band structure, and are consistent with the circumferentially quantized topological surface states forming a series of one-dimensional subbands, which undergo periodic magnetic field-induced topological transitions with the disappearance/appearance of the gapless Dirac point with a one-dimensional spin helical mode.
Higher manganese silicides (HMS) are promising thermoelectric materials owing to the abundance of the constituent elements in the earth crust, environmental friendliness and good chemical stability at high temperatures. However, the metallic MnSi layers with a lateral size as large as [similar]50 [small mu ]m are formed in the melt-grown HMS samples. These large MnSi layers are characterized with relatively high electrical and thermal conductivities and low Seebeck coefficient, which can degrade the thermoelectric performance of the melt-grown samples. Here, we report the synthesis and thermoelectric properties of Re-substituted HMS with relatively small-size MnSi platelets via melt-quenching, followed by ball-milling, and consolidated by spark plasma sintering. As compared to the samples prepared by either solid-state reaction or mechanical alloying, the reduced lateral size of MnSi in the quenched sample leads to an increased carrier concentration without a reduction in the carrier mobility according to our electrical transport measurements. As a result, the thermoelectric power factor is increased to 1.9 +/- 0.2 [times] 10-3 W m-1 K-2 at 860 K, which is about 20% higher than that of the sample prepared by solid-state reaction. In addition, the lattice thermal conductivity of the quenched sample remains nearly the same as the samples prepared by two other synthesis methods. Therefore, a figure-of-merit ZT of 0.64 +/- 0.08 at 823 K is obtained for the quenched sample, compared to 0.57 +/- 0.07 and 0.26 +/- 0.03 obtained from the two other samples prepared by different methods.
Abstract Bulk nanostructured higher manganese silicide (HMS) samples with different grain size are prepared by melting, subsequent ball milling (BM), and followed by spark plasma sintering (SPS). The effects of \BM\ time on the microstructures and thermoelectric properties of these samples are investigated. It is found that \BM\ effectively reduces the grain size to about 90 nm in the sample after SPS, which leads to a decrease in both the thermal conductivity and electrical conductivity. By prolonging the \BM\ time, MnSi and tungsten/carbon-rich impurity phases are formed due to the impact-induced decomposition of \HMS\ and contamination from the tungsten carbide jar and balls during the BM, respectively. These impurities result in a reduced Seebeck coefficient and increased thermal conductivity above room temperature. The measured size-dependent lattice thermal conductivities agree qualitatively with the reported calculation results based on a combined phonon and diffuson model. The size effects are found to be increasingly significant as temperature decreases. Because of the formation of the impurity phases and a relatively large grain size, the ŻT\} values are not improved in the ball-milled \HMS\ samples. These findings suggest the need of alternative approaches for the synthesis of pure \HMS\ with further reduced grain size and controlled impurity doping in order to enhance the thermoelectric figure-of-merit of \HMS\ via nanostructuring.
A variety of crystals contain quasi-one-dimensional substructures, which yield distinctive electronic, spintronic, optical and thermoelectric properties. There is a lack of understanding of the lattice dynamics that influences the properties of such complex crystals. Here we employ inelastic neutron scatting measurements and density functional theory calculations to show that numerous low-energy optical vibrational modes exist in higher manganese silicides, an example of such crystals. These optical modes, including unusually low-frequency twisting motions of the Si ladders inside the Mn chimneys, provide a large phase space for scattering acoustic phonons. A hybrid phonon and diffuson model is proposed to explain the low and anisotropic thermal conductivity of higher manganese silicides and to evaluate nanostructuring as an approach to further suppress the thermal conductivity and enhance the thermoelectric energy conversion efficiency. This discovery offers new insights into the structure-property relationships of a broad class of materials with quasi-one-dimensional substructures for various applications.