Giant spin Hall Effect in two-dimensional monochalcogenides

One of the most exciting properties of two dimensional materials is their sensitivity to external tuning of the electronic properties, for example via electric field or strain. Recently discovered analogues of phosphorene, group-IV monochalcogenides (MX with M = Ge, Sn and X = S, Se, Te), display several interesting phenomena intimately related to the in-plane strain, such as giant piezoelectricity and multiferroicity, which combine ferroelastic and ferroelectric properties. Here, using calculations from first principles, we reveal for the first time giant intrinsic spin Hall conductivities (SHC) in these materials. In particular, we show that the SHC resonances can be easily tuned by combination of strain and doping and, in some cases, strain can be used to induce semiconductor to metal transitions that make a giant spin Hall effect possible even in absence of doping. Our results indicate a new route for the design of highly tunable spintronics devices based on two-dimensional materials

Advanced modeling of materials with PAOFLOW 2.0:New features and software design

Recent research in materials science opens exciting perspectives to design novel quantum materials and devices, but it calls for quantitative predictions of properties which are not accessible in standard first principles packages. PAOFLOW, is a software tool that constructs tight-binding Hamiltonians from self consistent electronic wavefunctions by projecting onto a set of atomic orbitals. The electronic structure provides numerous materials properties that otherwise would have to be calculated via phenomenological models. In this paper, we describe recent re-design of the code as well as the new features and improvements in performance. In particular, we have implemented symmetry operations for unfolding equivalent k-points, which drastically reduces the runtime requirements of first principles calculations, and we have provided internal routines of projections onto atomic orbitals enabling generation of real space atomic orbitals. Moreover, we have included models for non-constant relaxation time in electronic transport calculations, doubling the real space dimensions of the Hamiltonian as well as the construction of Hamiltonians directly from analytical models. Importantly, PAOFLOW has been now converted into a Python package, and is streamlined for use directly within other Python codes. The new object oriented design treats PAOFLOW's computational routines as class methods, providing an API for explicit control of each calculation.</p

Origin of low thermal conductivity in In4Se3

In4Se3 is an attractive n-type thermoelectric material for mid-range waste heat recovery, owing to its low thermal conductivity (~ 0.9 W∙m- 1 K- 1 at 300 K). Here, we explore the relationship between the elastic properties, thermal conductivity and structure of In4Se3. The experimentally-determined average sound velocity (2010 m s-1), Young’s modulus (47 GPa), and Debye temperature (198 K) of In4Se3 are rather low, indicating considerable lattice softening. This behavior, which is consistent with low thermal conductivity, can be related to the complex bonding found in this material, in which strong covalent In-In and In-Se bonds coexist with weaker electrostatic interactions. Phonon dispersion calculations show that Einstein-like modes occur at ~ 30 cm-1. These Einstein-like modes can be ascribed to weakly bonded In+ cations located between strongly-bonded [(In3)5+(Se2-)3]- layers. The Grüneisen parameter for the soft-bonded In+ at the frequencies of the Einstein-like modes is large, indicating a high degree of bond anharmonicity and hence increased phonon scattering. The calculated thermal conductivity and elastic properties are in good agreement with experimental results

Jahn-Teller driven electronic instability in thermoelectric tetrahedrite

Tetrahedrite, Cu12Sb4S13, is an abundant mineral with excellent thermoelectric properties owing to its low thermal conductivity. The electronic and structural origin of the intriguing physical properties of tetrahedrite, including its metal-to-semiconductor transition, remains largely unknown. This work presents the first determination of the low-temperature structure of tetrahedrite that accounts for its unique properties. Contrary to prior conjectures, our results show that the trigonal-planar copper cations remain in planar coordination below the metal-to-semiconductor transition. The atomic displacement parameters of the trigonal-planar copper cations, which have been linked to low thermal conductivity, increase by 200% above the metal-to-semiconductor transition. The phase transition is consequence of the orbital degeneracy of the highest occupied 3d cluster orbitals of the copper clusters found inside the sodalite cages in the cubic phase. This study reveals that a Jahn-Teller electronic instability leads to the formation of “molecular-like” Cu57+ clusters and suppresses copper rattling vibrations due to the strengthening of direct copper-copper interactions. Our first-principles calculations demonstrate that the structural phase transition opens a small band gap in the electronic density of states and eliminates the unstable phonon modes. The present results provide insights on the interplay between phonon transport, electronic properties and crystal structure in mixed-valence compounds

Transport properties and electronic density-of-states of Zn-doped colusite Cu26Cr2Ge6S32

International audienceThermoelectric colusites, one of the most recently identified and most promising family of complex Cu-S materials, have quickly attracted significant attention based on their outstanding performance. Herein, we investigate the effect of zinc for copper substitution on the thermoelectric properties of the high-performance Cr-Ge colusite, Cu26Cr2Ge6S32. We discuss the striking impact of the aliovalent Zn/Cu substitution on the charge carrier mobility and effective mass and the consequences on the electrical and thermal transport properties. The investigation is supported by first-principles calculations of the electronic density-of-states of doped colusites. The theoretical study reveals the removal of the sharp features at the top of the valence manifold with the incorporation of Zn in the conductive network, with a strong reduction in the estimated relaxation time. These theoretical and experimental observations confirm the importance of disorder within the conductive network and the high sensitivity of the Cu-S tetrahedral framework toward defects in high-performance thermoelectric colusites

Copper‐Rich Thermoelectric Sulfides: Size‐Mismatch Effect and Chemical Disorder in the [TS4]Cu6 Complexes of Cu26T2Ge6S32 (T=Cr, Mo, W) Colusites

International audienceHerein, we investigate the Mo and W substitution for Cr in synthetic colusite, Cu26Cr2Ge6S32. Primarily, we elucidate the origin of extremely low electrical resistivity which does not compromise the Seebeck coefficient and leads to outstanding power factors of 1.94 mW m−1 K−2 at 700 K in Cu26Cr2Ge6S32. We demonstrate that the abnormally long iono‐covalent T–S bonds competing with short metallic Cu–T interactions govern the electronic transport properties of the conductive “Cu26S32” framework. We address the key role of the cationic size‐mismatch at the core of the mixed tetrahedral–octahedral complex over the transport properties. Two essential effects are identified: 1) only the tetrahedra that are directly bonded to the [TS4]Cu6 complex are significantly distorted upon substitution and 2) the major contribution to the disorder is localized at the central position of the mixed tetrahedral–octahedral complex, and is maximized for x=1, i.e. for the highest cationic size‐variance, σ2

Copper‐Rich Thermoelectric Sulfides: Size Mismatch Effect and Chemical Disorder in the [TS4]Cu6 Complexes of Cu26T2Ge6S32 (T = Cr, Mo, W) Colusites

International audienceIn the present study, we investigate the Mo and W for Cr substitution in the synthetic mineral colusite, Cu26Cr2Ge6S32. Primarily, we elucidate the origin of extremely low electrical resistivity which does not compromise the Seebeck coefficient and leads to outstanding power factors of 1.94 mW m‐1 K‐2 at 700 K in Cu26Cr2Ge6S32. We demonstrate that the abnormally long iono‐covalent T‐S bonds competing with short metallic Cu‐T interactions govern the electronic transport properties of the conductive “Cu26S32” framework. Additionally, we address the key role of the cationic size‐mismatch at the core of the mixed tetrahedral‐octahedral complex, illustrated by the cation‐size variance, σ2, over the transport properties. Using a combination of experimental and theoretical tools, the explanation for the remarkable electrical and thermal transport properties of the Cu26Cr2‐xMoxGe6S32 and Cu26Cr2‐xWxGe6S32 solid solutions is discussed. In‐depth structural analysis using Rietveld refinements of XRPD data reveals two essential effects caused by the substitution of Cr in the solid solutions: (1) Only the tetrahedra that are directly bonded to the [TS4]Cu6 complex are significantly distorted upon substitution and (2) the major contribution to the disorder is localized at the central position of the mixed tetrahedral‐octahedral complex, and is maximized for x = 1, i.e. for the highest cationic size‐variance, σ2. We investigated the low‐ (5 ≤ T / K ≤ 300) and high‐ (300 ≤ T / K ≤ 700) temperature electrical and thermal transport properties, including electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall effect (low temperature only) measurements and linked the structural/chemical disorder to the radically different conduction mechanisms in the Cu26Cr2‐xMoxGe6S32 and Cu26Cr2‐xWxGe6S32 solid solutions

Copper‐rich thermoelectric sulfides: size mismatch effect and chemical disorder in the [TS4]Cu6 complexes of Cu26T2Ge6S32 (T = Cr, Mo, W) colusites

International audienceIn the present study, we investigate the Mo and W for Cr substitution in the synthetic mineral colusite, Cu26Cr2Ge6S32. Primarily, we elucidate the origin of extremely low electrical resistivity which does not compromise the Seebeck coefficient and leads to outstanding power factors of 1.94 mW m‐1 K‐2 at 700 K in Cu26Cr2Ge6S32. We demonstrate that the abnormally long iono‐covalent T‐S bonds competing with short metallic Cu‐T interactions govern the electronic transport properties of the conductive “Cu26S32” framework. Additionally, we address the key role of the cationic size‐mismatch at the core of the mixed tetrahedral‐octahedral complex, illustrated by the cation‐size variance, σ2, over the transport properties. Using a combination of experimental and theoretical tools, the explanation for the remarkable electrical and thermal transport properties of the Cu26Cr2‐xMoxGe6S32 and Cu26Cr2‐xWxGe6S32 solid solutions is discussed. In‐depth structural analysis using Rietveld refinements of XRPD data reveals two essential effects caused by the substitution of Cr in the solid solutions: (1) Only the tetrahedra that are directly bonded to the [TS4]Cu6 complex are significantly distorted upon substitution and (2) the major contribution to the disorder is localized at the central position of the mixed tetrahedral‐octahedral complex, and is maximized for x = 1, i.e. for the highest cationic size‐variance, σ2. We investigated the low‐ (5 ≤ T / K ≤ 300) and high‐ (300 ≤ T / K ≤ 700) temperature electrical and thermal transport properties, including electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall effect (low temperature only) measurements and linked the structural/chemical disorder to the radically different conduction mechanisms in the Cu26Cr2‐xMoxGe6S32 and Cu26Cr2‐xWxGe6S32 solid solutions

Toppling the Transport Properties with Cationic Overstoichiometry in Thermoelectric Colusite: [Cu 26 Cr 2 Ge 6 ] 1+δ S 32

International audienceThe excellent thermoelectric properties of colusite are known to be closely related to the nature of the cations at the core of the tetrahedral-octahedral complexes. Here, we demonstrate that cation overstoichiometry decreases the carrier concentration and also generates structural disorder, which modify the conduction mechanism in a way that resembles the effect of cation-size mismatch. This functionalization of the &quot;Cu26S32&quot; conductive network leads to a high figure of merit of 1.0 at 700 K. This study highlights the importance of the cationic arrangement and furthers our understanding on the fascinating transport properties in colusite