We report on magnetization M (H), dc/ac magnetic susceptibility χ(T ), specific heat Cm(T ) and muon spin relaxation (µSR) measurements of the Kitaev honeycomb iridate Cu2IrO2 with quenched disorder. In spite of the chemical disorders, we find no indication of spin glass down to 260 mK from the Cm(T ) and µSR data. Furthermore, a persistent spin dynamics observed by the zero-field muon spin relaxation evidences an absence of static magnetism. The remarkable observation is a scaling relation of χ[H, T ] and M [H, T ] in H/T with the scaling exponent α = 0.26 − 0.28, expected from bond randomness. However, Cm[H, T ]/T disobeys the predicted universal scaling law, pointing towards the presence of low-lying excitations in addition to random singlets. Our results signify an intriguing role of quenched disorder in a Kitaev spin system in creating low-energy excitations possibly pertaining to Z2 fluxes.The exactly solvable Kitaev honeycomb model provides a novel route to achieve elusive topological and quantum spin liquids [1,2].Exchange frustration of bond-dependent Ising interactions fractionalizes the j eff = 1 2 spin into itinerant Majorana fermion and static Z 2 gauge flux [3][4][5]. Edge-sharing of octahedrally coordinated metal ions subject to strong spin-orbit coupling supports the realization of Kitaev-type interactions [6][7][8].In the quest for a Kitaev honeycomb magnet, the family of A 2 IrO 3 (A = Na, Li) and α-RuCl 3 are considered prime candidate materials [9][10][11][12][13][14][15][16]. In these compounds, however, the theoretically predicted spin-liquid state is preempted by long-range magnetic order due to structural imperfections. As the real materials are vulnerable to a monoclinic stacking of honeycomb layers, non-Kitaev terms seem inevitable. A related issue is to engineer local crystal environments towards an optimal geometry to maximize the Kitaev interactions.Very recently, the new Kitaev honeycomb iridates H 3 LiIr 2 O 6 and Cu 2 IrO 3 have been derived from their ancestors A 2 IrO 3 through soft structural modifications [17,18]. H 3 LiIr 2 O 6 is obtained by replacing the interlayer Li + ions with H + from α-Li 2 IrO 3 , while the honeycomb layer remains intact. A scaling of the specific heat and NMR relaxation rate gives evidence for the presence of fermionic excitations [17]. In stabilizing a Kitaev-like spin liquid, hydrogen disorders turn out to a key ingredient by enhancing Kitaev exchange interactions and promoting spin disordering [19,20]. In case of Cu 2 IrO 3 , all of the A-site cations of Na 2 IrO 3 are permuted by Cu + ions. Consequently, in-plane bond disorders become significant in determining magnetic behavior. Figure 1(a) presents the crystal structure of Cu 2 IrO 3

Considerable efforts have been devoted to enhancing thermoelectric performance, by employing phonon scattering from nanostructural architecture, and material design using phonon-glass and electron-crystal concepts. The nanostructural approach helps to lower thermal conductivity but has limited effect on the power factor. Here, we demonstrate selective charge Anderson localization as a route to maximize the Seebeck coefficient while simultaneously preserving high electrical conductivity and lowering the lattice thermal conductivity. We confirm the viability of interface potential modification in an n-type Bi-doped PbTe/Ag 2 Te nanocomposite and the resulting enhancement in thermoelectric figure-of-merit ZT. The introduction of random potentials via Ag 2 Te nanoparticle distribution using extrinsic phase mixing was determined using scanning tunneling spectroscopy measurements. When the Ag 2 Te undergoes a structural phase transition (T > 420 K) from monoclinic β-Ag 2 Te to cubic α-Ag 2 Te, the band gap in the α-Ag 2 Te increases due to the p−d hybridization. This results in a decrease in the potential barrier height, which gives rise to partial delocalization of the electrons, while wave packets of the holes are still in a localized state. Using this strategic approach, we achieved an exceptionally high thermoelectric figure-of-merit in n-type PbTe materials, a ZT greater than 2.0, suitable for waste heat power generation.

Topological insulators generally share commonalities with good thermoelectric (TE) materials because of their narrow band gaps and heavy constituent elements. Here, we propose that a topological crystalline insulator (TCI) could exhibit a high TE performance by breaking its crystalline symmetry and tuning the chemical potential by elemental doping. As a candidate material, we investigate the TE properties of the Cl-doped TCI PbSnSe. The infrared absorption spectra reveal that the band gap is increased from 0.055 eV for PbSnSe to 0.075 eV for PbSnSeCl, confirming that the Cl doping can break the crystalline mirror symmetry of a TCI PbSnSe and thereby enlarge its bulk electronic band gap. The topological band inversion is confirmed by the extended X-ray absorption fine structure spectroscopy, which shows that the TCI state is weakened in a chlorine x = 0.05-doped compound. The small gap opening and partial linear band dispersion with massless and massive bands may have a high power factor (PF) for high electrical conductivity with an enhancement of the Seebeck coefficient. As a result, PbSnSeCl shows a considerably enhanced ZT of 0.64 at 823 K, which is about 1200% enhancement in ZT compared with that of the undoped PbSnSe. This work demonstrates that the optimal n-type Cl doping tunes the chemical potential together with breaking the state of the TCI, suppresses the bipolar conduction at high temperatures, and thereby enables the Seebeck coefficient to increase up to 823 K, resulting in a significantly enhanced PF at high temperatures. In addition, the bipolar contribution to thermal conductivity is effectively suppressed for the Cl-doped samples of PbSnSeCl ( x ≥ 0.01). We propose that breaking the crystalline mirror symmetry in TCIs could be a new research direction for exploring high-performance TE materials.

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Bismuth-Telluride-based compounds are unique materials for thermoelectric cooling applications. Because Bi2Te3 is a narrow gap semiconductor, the bipolar diffusion effect is a critical issue to enhance thermoelectric performance. Here, we report the significant reduction of thermal conductivity by decreasing lattice and bipolar thermal conductivity in extrinsic phase mixing of MgO and VO2 nanoparticles in Bi0.5Sb1.5Te3 (BST) bulk matrix. When we separate the thermal conductivity by electronic κel, lattice κlat, and bipolar κbi thermal conductivities, all the contributions in thermal conductivities are decreased with increasing the concentration of oxide particle distribution, indicating the effective phonon scattering with an asymmetric scattering of carriers. The reduction of thermal conductivity affects the improvement of the ZT values. Even though significant carrier filtering effect is not observed in the oxide bulk composites due to micro-meter size agglomeration of particles, the interface between oxide and bulk matrix scatters carriers giving rise to the increase of the Seebeck coefficient and electrical resistivity. Therefore, we suggest the extrinsic phase mixing of nanoparticles decreases lattice and bipolar thermal conductivity, resulting in the enhancement of thermoelectric performance over a wide temperature range.