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Register a new userExperimental Proof of Universal Conductance Fluctuation in Quasi-1D Epitaxial Bi$_{2}$Se$_{3}$ Wires
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We report on conductance fluctuation in quasi-one-dimensional wires made of epitaxial Bi$_{2}$Se$_{3}$ thin film. We found that this type of fluctuation decreases as the wire length becomes longer and that the amplitude of the fluctuation is well scaled to the coherence, thermal diffusion, and wire lengths, as predicted by conventional universal conductance fluctuation (UCF) theory. Additionally, the amplitude of the fluctuation can be understood to be equivalent to the UCF amplitude of a system with strong spin-orbit interaction and no time-reversal symmetry. These results indicate that the conductance fluctuation in Bi$_{2}$Se$_{3}$ wires is explainable through UCF theory. This work is the first to verify the scaling relationship of UCF in a system with strong spin-orbit interaction.
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Transport and torque magnetometry measurements are performed at high magnetic fields and low temperatures in a series of p-type (Ca-doped) Bi$_{2}$Se$_{3}$ crystals. The angular dependence of the Shubnikov-de Haas and de Haas-van Alphen quantum oscillations enables us to determine the Fermi surface of the bulk valence band states as a function of the carrier density. At low density, the angular dependence exhibits a downturn in the oscillations frequency between $0^circ$ and $90^circ$, reflecting a bag-shaped hole Fermi surface. The detection of a single frequency for all tilt angles rules out the existence of a Fermi surface with different extremal cross-sections down to $24$~meV. There is therefore no signature of a camel-back in the valence band of our bulk samples, in accordance with the direct band gap predicted by $GW$ calculations.
We calculate the distribution of the conductance G in a one-dimensional disordered wire at finite temperature T and bias voltage V in a independent-electron picture and assuming full coherent transport. At high enough temperature and bias voltage, where several resonances of the system contribute to the conductance, the distribution P(G(T,V)) can be represented with good accuracy by autoconvolutions of the distribution of the conductance at zero temperature and zero bias voltage. The number of convolutions depends on T and V. In the regime of very low T and V, where only one resonance is relevant to G(T,V), the conductance distribution is analyzed by a resonant tunneling conductance model. Strong effects of finite T and V on the conductance distribution are observed and well described by our theoretical analysis, as we verify by performing a number of numerical simulations of a one-dimensional disordered wire at different temperatures, voltages, and lengths of the wire. Analytical estimates for the first moments of P(G(T,V)) at high temperature and bias voltage are also provided.
Helical spin textures with the marked spin polarizations of topological surface states have been firstly unveiled by the state-of-the-art spin- and angle-resolved photoemission spectroscopy for two promising topological insulators Bi$_2$Te$_2$Se and Bi$_2$Se$_2$Te. The highly spin-polarized natures are found to be persistent across the Dirac point in both compounds. This novel finding paves a pathway to extending their utilization of topological surface state for future spintronic applications.
Mesoscopic transport measurements reveal a large effective phase coherence length in epitaxial GaMnAs ferromagnets, contrary to usual 3d-metal ferromagnets. Universal conductance fluctuations of single nanowires are compared for epilayers with a tailored anisotropy. At large magnetic fields, quantum interferences are due to structural disorder only, and an unusual behavior related to hole-induced ferromagnetism is evidenced, for both quantum interferences and decoherence. At small fields, phase coherence is shown to persist down to zero field, even in presence of magnons, and an additional spin disorder contribution to quantum interferences is observed under domain walls nucleation.
In this study, we address the phase coherent transport in a sub-micrometer-sized Hall bar made of epitaxial Bi2Se3 thin film by probing the weak antilocalization (WAL) and the magnetoresistance fluctuation below 22 K. The WAL effect is well described by the Hikami-Larkin-Nagaoka model, where the temperature dependence of the coherence length indicates that electron conduction occurs quasi-one-dimensionally in the narrow Hall bar. The temperature-dependent magnetoresistance fluctuation is analyzed in terms of the universal conductance fluctuation, which gives a coherence length consistent with that derived from the WAL effect.
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