Designing thermoelectric materials with high figure of merit ZT = S 2 GT /κ requires fulfilling three often irreconcilable conditions, i.e., the high electrical conductance G, small thermal conductance κ and high Seebeck coefficient S. Nanostructuring is one of the promising ways to achieve this goal as it can substantially suppress lattice contribution to κ. However, it may also unfavorably influence the electronic transport in an uncontrollable way. Here we theoretically demonstrate that this issue can be ideally solved by fabricating graphene nanoribbons with heavy adatoms and nanopores. These systems, acting as a two-dimensional topological insulator with robust helical edge states carrying electrical current, yield a highly optimized power factor S 2 G per helical conducting channel. Concurrently, their array of nanopores impedes the lattice thermal conduction through the bulk. Using quantum transport simulations coupled with first-principles electronic and phononic band structure calculations, the thermoelectric figure of merit is found to reach its maximum ZT 3 at T 40 K. This paves a way to design high-ZT materials by exploiting the nontrivial topology of electronic states through nanostructuring.PACS numbers: 73.50. Lw, 03.65.Vf, 85.80.Fi Thermoelectrics [1-3] transform temperature gradients into electric voltage and vice versa. Although a plethora of thermoelectric energy harvesting and cooling applications has been envisioned, their usage is presently limited by their small efficiency. This is due to the fact that increasing thermoelectric figure of meritrequires careful trade-off between electrical conductance G, the Seebeck coefficient S, and the thermal conductance κ el + κ ph . The total thermal conductance has contributions from both electrons κ el and phonons (i.e., lattice vibrations) κ ph . ZT quantifies the maximum efficiency of a thermoelectric cycle conversion in the linearresponse regime where a small voltage ∆V = −S∆T exactly cancels the current induced by the small temperature difference ∆T = T H − T C at average operating temperature T = (T H + T C )/2. The values approaching ZT → ∞ would ensure Carnot efficiency as the theoretical limit for a heat engine operating between a hot T H and a cold T C temperature. However, ZT of realistic devices is limited by irreversible energy losses via Joule heat and thermal conduction, so that a pragmatic goal is to achieve ZT 3 with low parasitic losses and stability over a broad temperature range [2, 3]. The major directions to increase ZT have been focused on either materials with high power factor S 2 G, such as doped narrow-gap semiconductors; or on minimizing κ ph by enhanced phonon scattering in different frequency ranges, such as through nanostructuring [1,2]. Although nanostructuring has progressed rapidly over the past decade [1,2], it typically affects bulk electronic states of conventional materials in unfavorable way for thermoelectricity. Thus, the recently discovered topological insulator (TI) materials [4,5] are of particular intere...

We analyze electronic and phononic quantum transport through zigzag or chiral graphene nanoribbons (GNRs) perforated with an array of nanopores. Since local charge current profiles in these GNRs are peaked around their edges, drilling nanopores in their interior does not affect such edge charge currents while drastically reducing heat current carried by phonons in sufficiently long wires. The combination of these two effects can yield highly efficient thermoelectric devices with maximum ZT 11 at liquid nitrogen temperature and ZT 4 at room temperature achieved in ∼ 1 µm long zigzag GNRs with nanopores of variable diameter and spacing between them. Our analysis is based on the π-orbital tight-binding Hamiltonian with up to third nearest-neighbor hopping for electronic subsystem, the empirical fourth-nearest-neighbor model for phononic subsystem, and nonequilibrium Green function formalism to study quantum transport in both of these models.

The control of recently observed spintronic effects in topological-insulator/ferromagnetic-metal (TI/FM) heterostructures is thwarted by the lack of understanding of band structure and spin texture around their interfaces. Here we combine density functional theory with Green's function techniques to obtain the spectral function at any plane passing through atoms of Bi2Se3 and Co or Cu layers comprising the interface. In contrast to widely assumed but thinly tested Dirac cone gapped by the proximity exchange field, we find that the Rashba ferromagnetic model describes the spectral function on the surface of Bi2Se3 in contact with Co near the Fermi level E 0 F , where circular and snowflake-like constant energy contours coexist around which spin locks to momentum. The remnant of the Dirac cone is hybridized with evanescent wave functions injected by metallic layers and pushed, due to charge transfer from Co or Cu layers, few tenths of eV below E 0 F for both Bi2Se3/Co and Bi2Se3/Cu interfaces while hosting distorted helical spin texture wounding around a single circle. These features explain recent observation [K. Kondou et al., Nat. Phys. 12, 1027] of sensitivity of spin-to-charge conversion signal at TI/Cu interface to tuning of E 0 F . Interestingly, three monolayers of Co adjacent to Bi2Se3 host spectral functions very different from the bulk metal, as well as in-plane spin textures signifying the spin-orbit proximity effect. We predict that out-of-plane tunneling anisotropic magnetoresistance in vertical heterostructure Cu/Bi2Se3/Co, where current flowing perpendicular to its interfaces is modulated by rotating magnetization from parallel to orthogonal to current flow, can serve as a sensitive probe of spin texture residing at E 0 F .The recent experiments on spin-orbit torque (SOT) [1, 2] and spin-to-charge conversion [3,4] in topologicalinsulator/ferromagnetic-metal (TI/FM) heterostructures have ignited the field of topological spintronics. In these devices, giant non-equilibrium spin densities [5][6][7][8] are expected to be generated due to strong spin-orbit coupling (SOC) on metallic surfaces of three-dimensional (3D) TIs and the corresponding (nearly [9]) helical spinmomentum locking along a single Fermi circle for Dirac electrons hosted by those surfaces [10]. Such strong interfacial SOC-driven phenomena are also envisaged to underlie a plethora of novel spintronic technologies [11].These effects have been interpreted almost exclusively using simplistic models, such as the Dirac Hamiltonian for the TI surface with an additional Zeeman term describing coupling of magnetization of the FM layer to the surface state spins [10],Ĥ Dirac = v F (σ ×p) z − ∆m ·σ, wherep is the momentum operator,σ is the vector of the Pauli matrices, m is the magnetization unit vector and v F is the Fermi velocity. Thus, the only effect of FM layer captured byĤ Dirac is proximity effect-induced exchange coupling ∆ which opens a gap in the Dirac cone energymomentum dispersion [10], thereby making Dirac electrons massive. On the ...

We predict that unpolarized charge current injected into a ballistic thin film of prototypical topological insulator (TI) Bi2Se3 will generate a noncollinear spin texture S(r) on its surface. Furthermore, the nonequilibrium spin texture will extend into 2 nm thick layer below the TI surfaces due to penetration of evanescent wavefunctions from the metallic surfaces into the bulk of TI. Averaging S(r) over few Å along the longitudinal direction defined by the current flow reveals large component pointing in the transverse direction. In addition, we find an order of magnitude smaller out-of-plane component when the direction of injected current with respect to Bi and Se atoms probes the largest hexagonal warping of the Dirac-cone dispersion on TI surface. Our analysis is based on an extension of the nonequilibrium Green functions combined with density functional theory (NEGF+DFT) to situations involving noncollinear spins and spin-orbit coupling. We also demonstrate how DFT calculations with properly optimized local orbital basis set can precisely match putatively more accurate calculations with plane-wave basis set for the supercell of Bi2Se3.

The recent observation (Gorbachev et al 2014 Science 346 448) of nonlocal resistance R NL near the Dirac point (DP) of multiterminal graphene on aligned hexagonal-boron nitride (G/hBN) has been interpreted as the consequence of topological valley Hall currents carried by the Fermi sea states just beneath the bulk gap E g induced by inversion symmetry breaking. However, the corresponding valley Hall conductivity s xy v , quantized inside E g , is not directly measurable. Conversely, the Landauer-Büttiker formula, as a numerically exact approach to observable nonlocal transport quantities, yields R NL ≡ 0 for the same simplistic Hamiltonian of gapped graphene that generates s ¹ 0 xy v OPEN ACCESS RECEIVED