• A 'zigzag' blueprint for topological ele

    From ScienceDaily@1:317/3 to All on Wednesday, March 09, 2022 21:30:48
    A 'zigzag' blueprint for topological electronics

    Date:
    March 9, 2022
    Source:
    ARC Centre of Excellence in Future Low-Energy Electronics
    Technologies
    Summary:
    A collaborative study confirms a potential new switching mechanism
    for a proposed generation of ultra-low energy topological
    electronics. Based on novel, quantum nanoribbons terminating on
    'zigzag' edges, such devices would switch from non-conducting
    to conducting state, whereby electrical current could flow along
    topological edge states without wasted dissipation of energy.



    FULL STORY ==========================================================================
    A collaborative study led by the University of Wollongong confirms
    switching mechanism for a new, proposed generation of ultra-low energy 'topological electronics'.


    ========================================================================== Based on novel quantum topological materials, such devices would
    'switch' a topological insulator from non-conducting (conventional
    electrical insulator) to a conducting (topological insulator) state,
    whereby electrical current could flow along its edge states without
    wasted dissipation of energy.

    Such topological electronics could radically reduce the energy consumed
    in computing and electronics, which is estimated to consume 8% of global electricity, and doubling every decade.

    Led by Dr Muhammad Nadeem at the University of Wollongong (UOW), the
    study also brought in expertise from FLEET Centre collaborators at UNSW
    and Monash University.

    Resolving the Switching Challenge, and Introducing the Tqfet
    Two-dimensional topological insulators are promising materials for
    topological quantum electronic devices where edge state transport can
    be controlled by a gate-induced electric field.



    ========================================================================== However, a major challenge with such electric-field-induced topological switching has been the requirement for an unrealistically large electric
    field to close the topological bandgap.

    The cross-node and interdisciplinary FLEET research team studied the
    width- dependence of electronic properties to confirm that a class of
    material known as zigzag-Xene nanoribbons would fulfil the necessary
    conditions for operation, namely:
    1. Spin-filtered chiral edge states in zigzag-Xene nanoribbons remain
    gapless and protected against backward scattering
    2. The threshold voltage required for switching between gapless
    and gapped
    edge states reduces as the width of the material decreases,
    without any fundamental lower bound
    3. Topological switching between edge states can be achieved without
    the
    bulk (ie, interior) bandgap closing and reopening
    4. Quantum confined zigzag-Xene nanoribbons may prompt the progress of
    ultra-low energy topological computing technologies.

    Zigzag Xenes Could Be Key Graphene was the first confirmed atomically-thin material, a 2D sheet of carbon atoms (group IV) arranged in a honeycomb lattice. Now, topological and electronic properties are being investigated
    for similar honeycomb sheets of group-IV and group-V materials,
    collectively called 2D-Xenes.

    2D-Xenes are topological insulators -- ie, electrically insulating
    in their interior but conductive along their edges, where electrons
    are transmitted without dissipating any energy (similar to a
    superconductor). When a 2D-Xene sheet is cut into a narrow ribbon
    terminated on 'zigzag' edges, known as zigzag-Xene-nanoribbons, it retains
    the conducting edge modes characteristic of a topological insulator, which
    are thought to retain their ability to carry current without dissipation.



    ==========================================================================
    It has recently been shown that zigzag-Xene-nanoribbons have potential
    to make a topological transistor which can reduce switching energy by
    a factor of four.

    The new research led by UOW found the following: Maintaining edge
    states Measurements indicated that spin-filtered chiral edge states
    in zigzag-Xene nanoribbons remain gapless and protected against the
    backward scattering that causes resistance, even with finite inter-edge overlapping in ultra-narrow ribbons (Meaning that a 2D quantum spin Hall material undergoes a phase transition to a 1D topological metal.) This is driven by the edge states intertwining with intrinsic band topology-driven energy-zero modes.

    "Quantum confined zigzag-Xene-nanoribbons are a special class of
    topological insulating materials where the energy gap of the bulk sample increases with a decrease in width, while the edge state conduction
    remains robust against dissipation even if the width is reduced to a quasi-one-dimension," says FLEET researcher and collaborator on the new
    study A/Prof Dmitrie Culcer (UNSW).

    "This feature of confined zigzag-Xene-nanoribbons is in stark contrast
    to other 2D topological insulating materials in which confinement effects
    also induce an energy gap in the edge states." Low threshold voltage Due
    to width- and momentum-dependent tunability of gate-induced inter-edge coupling, the threshold-voltage required for switching between gapless
    and gapped edge states reduces as the width of the material decreases,
    without any fundamental lower limit.

    "An ultra-narrow zigzag-Xene-nanoribbon can 'toggle' between a quasi-one- dimensional topological metal with conducting gapless edge states and
    an ordinary insulator with gapped edge states with a little tweaking
    of a voltage knob, says lead author Dr Muhammad Nadeem (UOW). "The
    desired tweaking of a voltage knob decreases with decrease in width of zigzag-Xene-nanoribbons, and lower operating voltage means the device
    can use less energy. The reduction in voltage knob tweaking comes about
    due to a relativistic quantum effect called spin-orbit coupling and
    is highly contrasting from pristine zigzag-Xene- nanoribbons which
    are ordinary insulators and in which desired voltage knob tweaking
    increases with decrease in width." Topological switching without bulk
    bandgap closing When the width of zigzag-Xene nanoribbons is smaller
    than a critical limit, topological switching between edge states can be attained without bulk bandgap closing and reopening. This is primarily
    due to the quantum confinement effect on the bulk band spectrum, which increases the nontrivial bulk bandgap with decrease in width.

    "This behaviour is new and distinct from 2D topological insulators,
    where bandgap closing and re-opening is always required to change
    the topological state" says Prof Michael Fuhrer (Monash). "Wide zigzag-Xene-nanoribbons act more like the 2D case, where gate
    electric field switches edge state conductance while simultaneously
    closing and reopening bulk bandgap." "In the presence of spin-orbit
    coupling, topological switching mechanism in large-gap confined zigzag-Xene-nanoribbons overturns the general wisdom of utilizing narrow
    gap and wide channel materials for reducing threshold-voltage in a
    standard field effect transistor analysis," says Prof Xiaolin Wang (UOW).

    "In addition, topological quantum field effect transistor utilizing
    zigzag- Xene-nanoribbons as a channel material has several advantages
    of engineering intricacies involved in design and fabrication," says
    Prof Alex Hamilton (UNSW).

    Unlike MOSFET technology, in which size dependence of threshold-voltage is tangled with isolation techniques, the reduction of threshold-voltage in
    a topological quantum field effect transistor is an intrinsic property
    of zigzag- Xene-nanoribbons associated with topological and quantum
    mechanical functionalities.

    Along with vastly different conduction and switching mechanisms, the technological aspects required for fabricating a topological quantum
    field effect transistor with zigzag-Xene-nanoribbons also radically
    differ from those of MOSFETs: There is no fundamental requirement of specialized technological/ isolation techniques for a low-voltage TQFET
    with an energy-efficient switching mechanism.

    With preserved ON-state topological robustness and minimal threshold
    voltage, channel width can be reduced to a quasi-one-dimension. This
    allows optimized geometry for a topological quantum field effect
    transistor with enhanced signal-to-noise ratio via multiple edge state channels.


    ========================================================================== Story Source: Materials provided by ARC_Centre_of_Excellence_in_Future_Low-Energy_Electronics
    Technologies. Note: Content may be edited for style and length.


    ========================================================================== Journal Reference:
    1. Muhammad Nadeem, Chao Zhang, Dimitrie Culcer, Alex R. Hamilton,
    Michael
    S. Fuhrer, Xiaolin Wang. Optimizing topological switching in
    confined 2D- Xene nanoribbons via finite-size effects. Applied
    Physics Reviews, 2022; 9 (1): 011411 DOI: 10.1063/5.0076625 ==========================================================================

    Link to news story: https://www.sciencedaily.com/releases/2022/03/220309104511.htm

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