Ideal maximum strengths and defect-induced softening in nanocrystalline-nanotwinned metals

Strengthening of metals through nanoscale grain boundaries and coherent twin boundaries is manifested by a maximum strength—a phenomenon known as Hall–Petch breakdown. Different softening mechanisms are considered to occur for nanocrystalline and nanotwinned materials. Here, we report nanocrystalline-nanotwinned Ag materials that exhibit two strength transitions dissimilar from the above mechanisms. Atomistic simulations show three distinct strength regions as twin spacing decreases, delineated by positive Hall–Petch strengthening to grain-boundary-dictated (near-zero Hall–Petch slope) mechanisms and to softening (negative Hall–Petch slope) induced by twin-boundary defects. An ideal maximum strength is reached for a range of twin spacings below 7 nm. We synthesized nanocrystalline-nanotwinned Ag with hardness 3.05 GPa—42% higher than the current record, by segregating trace concentrations of Cu impurity (\u3c1.0 weight (wt)%). The microalloy retains excellent electrical conductivity and remains stable up to 653 K; 215 K better than for pure nanotwinned Ag. This breaks the existing trade-off between strength and electrical conductivity, and demonstrates the potential for creating interface-dominated materials with unprecedented mechanical and physical properties

Observation of a new type of THz resonance of surface plasmons propagating on metal-film hole arrays

Highly conducting metal-film subwavelength hole arrays, lithographically fabricated on high-resistivity silicon wafers in optical contact with thick silicon plates, have been characterized by terahertz time-domain spectroscopy with subpicosecond resolution and over a frequency range from 0.5 to 3 THz with 5 GHz resolution. A well-defined ringing structure extending to more than 250 psec is observed on the trailing edge of the transmitted THz pulse. In the frequency domain this ringing structure corresponds to a new type of extremely sharp resonant line structure between the fundamental surface plasmon modes of the hole array. A simple theoretical model is presented and shows good agreement with the experimental data.Peer reviewedElectrical and Computer Engineerin

Quantum Oscillations in Magnetothermopower Measurements of the Topological Insulator Bi₂Te₃

We report the magnetothermopower measurements of the nonmetallic topological insulator Bi2Te3 in magnetic fields up to 35 T. Quantum oscillations arising from surface states are observed in both thermoelectric and conductivity tensors. The inferred surface thermopower has a peak magnitude ~1mV/K possibly as a result of surface electron and bulk phonon interaction. At the n=1 Landau level, we resolve additional quantum oscillations signaling Landau sublevels

Low Temperature Magnetothermoelectric Effect and Magnetoresistance in Te Vapor Annealed Bi₂Te₃

The electrical properties of single crystals of p-type Bi2Te3 are shown to be tuned by annealing as-grown crystals in elemental Te vapor at temperatures in the range of 400-420°C. While as-grown nominally stoichiometric Bi2Te3 has p-type conductivity below room temperature, Te vapor annealed Bi2Te3 shows a cross over from p- to n-type behavior. The temperature dependent resistivity of the Te annealed crystals shows a characteristic broad peak near 100 K. Applied magnetic fields give rise to a large low temperature magnetothermoelectric effect in the Te annealed samples and enhance the low temperature peak in the resistivity. Further, Te annealed Bi2Te3 shows a large positive magnetoresistance, ~ 200% at 2 K, and ~ 15% at room temperature. The annealing procedure described can be employed to optimize the properties of Bi2Te3 for study as a topological insulator

Quantum Oscillations and Hall Anomaly of Surface States in the Topological Insulator Bi₂Te₃

Topological insulators are insulating materials that display massless, Dirac-like surface states in which the electrons have only one spin degree of freedom on each surface. These states have been imaged by photoemission, but little information on their transport parameters, for example, mobility, is available. We report the observation of Shubnikov - de Haas oscillations arising from the surface states in nonmetallic crystals of Bi2Te3. In addition, we uncovered a Hall anomaly in weak fields, which enables the surface current to be seen directly. Both experiments yield a surface mobility (9000 to 10,000 centimeter2 per volt-second) that is substantially higher than in the bulk. The Fermi velocity of 4 x 10 5 meters per second obtained from these transport experiments agrees with angle-resolved photoemission experiments

Quantum oscillations in a topological insulator Bi2Te2Se with large bulk resistivity (6 Omega cm)

We report the observation of prominent Shubnikov-de Haas oscillations in a Topological Insulator, Bi2Te2Se, with large bulk resistivity (6 Omega cm at 4 K). By fitting the SdH oscillations, we infer a large metallicity parameter k(F)l=41, with a surface mobility (mu(s) similar to 2800 cm(2)/V s) much larger than the bulk mobility (mu(b) similar to 50 cm(2)/V s). The plot of the index fields B-v vs. filling factor v shows a 1/2-shift, consistent with massless, Dirac states. (C) 2011 Elsevier B.V. All rights reserved

Quantum Oscillations in a Topological Insulator Bi₂Te₂Se with Large Bulk Resistivity (\u3csup\u3e6 Ω cm\u3c/sup\u3e)

We report the observation of prominent Shubnikovde Haas oscillations in a Topological Insulator, Bi2Te2Se, with large bulk resistivity (6Ωcm at 4 K). By fitting the SdH oscillations, we infer a large metallicity parameter kF?=41, with a surface mobility ( μs~2800 cm2/Vs) much larger than the bulk mobility ( μb~50 cm2/Vs). The plot of the index fields Bν vs. filling factor ν shows a 12-shift, consistent with massless, Dirac states

Quantum Interference in Macroscopic Crystals of Nonmetallic Bi₂Se₃

Photoemission experiments have shown that Bi2Se3 is a topological insulator. By controlled doping, we have obtained crystals of Bi2Se3 with nonmetallic conduction. At low temperatures, we uncover a novel type of magnetofingerprint signal which involves the spin degrees of freedom. Given the mm-sized crystals, the observed amplitude is 200-500x larger than expected from universal conductance fluctuations. The results point to very long phase-breaking lengths in an unusual conductance channel in these nonmetallic samples. We discuss the nature of the in-gap conducting states and their relation to the topological surface states