We study the evolution of the band gap structure in few-layer MoTe2 crystals, by means of low-temperature microreflectance (MR) and temperature-dependent photoluminescence (PL) measurements. The analysis of the measurements indicate that in complete analogy with other semiconducting transition metal dichalchogenides (TMDs) the dominant PL emission peaks originate from direct transitions associated with recombination of excitons and trions. When we follow the evolution of the PL intensity as a function of layer thickness, however, we observe that MoTe2 behaves differently from other semiconducting TMDs investigated earlier. Specifically, the exciton PL yield (integrated PL intensity) is identical for mono and bilayer, decreases slightly for trilayer, and it is significantly lower in the tetralayer. The analysis of this behavior and of all our experimental observations is fully consistent with mono and bilayer MoTe2 being direct band gap semiconductors with tetralayer MoTe2 being an indirect gap semiconductor and with trilayers having nearly identical direct and indirect gaps. This conclusion is different from the one reached for other recently investigated semiconducting transition metal dichalcogenides for which monolayers are found to be direct band gap semiconductors, and thicker layers have indirect band gaps that are significantly smaller (by hundreds of meV) than the direct gap. We discuss the relevance of our findings for experiments of fundamental interest and possible future device applications.

BiCuSeO system is achieved via heavily doping with Ba and refining grain sizes (200-400 nm), which is higher than any thermoelectric oxide. Excellent thermal and chemical stabilities up to 923 K and high thermoelectric performance confirm that the BiCuSeO system is promising for thermoelectric power generation applications.The thermoelectric (TE) energy conversion technology, which can be used to convert wasted heat into electricity, has received much attention in the past decade. The efficiency of TE devices is characterized by the dimensionless figure of merit, ZT ¼ (S 2 s/k)T, where S, s, k, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the absolute temperature, respectively. Until now, several classes of bulk materials with high ZT values have been discovered, 1,2 including nanostructured BiSbTe alloys, 3 filled skutterudites, 4 zinc antimonide, 5 AgPb 18+x SbTe 20 , 6 Tl doped PbTe 7 or (AgSbTe 2 ) 1Àx (GeTe) x alloys, 8 but they lack thermal and chemical stabilities in air. Therefore, TE oxides are expected to play an important role in extensive applications for waste heat recovery, on the basis of their potential advantage over heavy metallic alloys of chemical and thermal robustness. To date, several families of oxides have been developed as promising TE materials. Typical TE oxides include Ca 2.8 Ag 0.15 Lu 0.05 Co 4 O 9+d

We report on the high thermoelectric performance of p-type polycrystalline BiCuSeO, a layered oxyselenide composed of alternating conductive (Cu 2 Se 2 ) 2 À and insulating (Bi 2 O 2 ) 2 þ layers. The electrical transport properties of BiCuSeO materials can be significantly improved by substituting Bi 3 þ with Ca 2 þ . The resulting materials exhibit a large positive Seebeck coefficient of B þ 330 lV K À1 at 300 K, which may be due to the 'natural superlattice' layered structure and the moderate effective mass suggested by both electronic density of states and carrier concentration calculations. After doping with Ca, enhanced electrical conductivity coupled with a moderate Seebeck coefficient leads to a power factor of B4.74 lW cm À1 K À2 at 923 K. Moreover, BiCuSeO shows very low thermal conductivity in the temperature range of 300 (B0.9 W m À1 K À1 ) to 923 K (B0.45 W m À1 K À1 ). Such low thermal conductivity values are most likely a result of the weak chemical bonds (Young's modulus, EB76.5 GPa) and the strong anharmonicity of the bonding arrangement (Gruneisen parameter, cB1.5). In addition to increasing the power factor, Ca doping reduces the thermal conductivity of the lattice, as confirmed by both experimental results and Callaway model calculations. The combination of optimized power factor and intrinsically low thermal conductivity results in a high ZT of B0.9 at 923 K for Bi 0.925 Ca 0.075 CuSeO.

We report a combined experimental and theoretical study of the candidate type-II Weyl semimetal MoTe 2 . Using laser-based angle-resolved photoemission, we resolve multiple distinct Fermi arcs on the inequivalent top and bottom (001) surfaces. All surface states observed experimentally are reproduced by an electronic structure calculation for the experimental crystal structure that predicts a topological Weyl semimetal state with eight type-II Weyl points. We further use systematic electronic structure calculations simulating different Weyl point arrangements to discuss the robustness of the identified Weyl semimetal state and the topological character of Fermi arcs in MoTe 2 .

Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides continuously reveal new phenomena. The latest example is 1T'-WTe2, a semimetal found to exhibit the largest known magnetoresistance in the bulk, and predicted to become a topological insulator in strained monolayers. Here we show that reducing the thickness through exfoliation enables the electronic properties of WTe2 to be tuned, which allows us to identify the mechanisms responsible for the observed magnetotransport down to the atomic scale. The longitudinal resistance and the unconventional magnetic field dependence of the Hall resistance are reproduced quantitatively by a classical two-band model for crystals as thin as six monolayers, whereas a crossover to an Anderson insulator occurs for thinner crystals. Besides establishing the origin of the magnetoresistance of WTe2, our results represent a complete validation of the classical theory for two-band electron-hole transport, and indicate that atomically thin WTe2 layers remain gapless semimetals.

We report angle-resolved photoemission experiments resolving the distinct electronic structure of the inequivalent top and bottom (001) surfaces of WTe 2 . On both surfaces, we identify a surface state that forms a large Fermi arc emerging out of the bulk electron pocket. Using surface electronic structure calculations, we show that these Fermi arcs are topologically trivial and that their existence is independent of the presence of type-II Weyl points in the bulk band structure. This implies that the observation of surface Fermi arcs alone does not allow the identification of WTe 2 as a topological Weyl semimetal. We further use the identification of the two different surfaces to clarify the number of Fermi surface sheets in WTe 2 . DOI: 10.1103/PhysRevB.94.121112 Transition-metal dichalcogenides (TMDs) have long been studied in many-body physics as model systems for metalinsulator transitions, multiband superconductivity, and charge density waves [1][2][3]. In recent years, the interest in TMDs intensified because of the promising optoelectronic properties of monolayer or few-layer devices based on hexagonal semiconducting MX 2 compounds with M = W,Mo and X = Se,S [4,5]. Unlike these materials, WTe 2 crystallizes in the orthorhombic, noncentrosymmetric 1T structure (P mn2 1 space group) and is semimetallic due to a small overlap of valence and conduction bands at the Fermi level [6,7]. Recent theoretical work [8] predicts that WTe 2 is an example of a new type of Weyl semimetal with strongly tilted Weyl cones that arise from topologically protected crossings of valence and conduction bands causing touching points between electron and hole pockets near the Fermi level. In type-I Weyl semimetals, realized for example in TaAs [9][10][11][12][13], the projections of opposite chirality Weyl points onto a surface are isolated from the bulk continuum and must be connected by well-defined Fermi arcs. This is not generally the case for type-II Weyl points, which are necessarily accompanied by bulk carrier pockets. The surface Fermi arcs corresponding to the bulk Weyl points in these materials can emerge within the projection of the bulk carrier pockets, rendering the robust identification of their topological nature challenging. Indeed, very recent angle-resolved photoemission spectroscopy (ARPES) experiments on the related Mo x W 1-x Te 2 and MoTe 2 systems report conflicting interpretations of the topological character of potential surface states [14][15][16]. ARPES studies on pure WTe 2 have to date not reported any surface states [17][18][19].In addition, WTe 2 is attracting interest because of its nonsaturating magnetoresistance [7] and the recent discovery of pressure induced superconductivity [20,21]. A possible relation between these phenomena and the topological nature of the low-energy surface excitations in WTe 2 is an intriguing prospect but has not yet been established. To date, even the basic bulk electronic structure underlying these phenomena remains controversial. The complex magnetotransport proper...

We report on the structural and electronic transport properties of BiCuSeO based compounds, that have recently been reported as promising thermoelectric materials with figure of merit ZT > 0.8 at 923 K, and share the same crystal structure as the high-Tc iron based 1111 oxypnictides. We show that the substitution of Bi 3+ by Sr 2+ induces a strong decrease of the electrical resistivity up to the solubility limit reached for x = 0.35, which originates from the strong increase of the carriers concentration. Two anomalies in the resistivity curves have been observed, one for the undoped compound near 260 K and the other for the doped samples at very low temperature. However, structural and magnetic measurements do not provide indications of structural or magnetic phase transition or superconductivity as it had been previously suggested in BiCu 1−x OS. We show that the thermoelectric properties of Bi 1−x Sr x CuSeO materials can be well understood through the analysis of the electronic band structure and the density of states close to the Fermi level and we provide possible directions toward the enhancement of the thermoelectric figure of merit of these materials.

We report the influence on the superconducting critical temperature Tc in doped SrTiO3 of the substitution of the natural 16O atoms by the heavier isotope 18O. We observe that for a wide range of doping this substitution causes a strong (~50%) enhancement of Tc. Also the magnetic critical field Hc2 is increased by a factor ~2. Such a strong impact on Tc and Hc2, with a sign opposite to conventional superconductors, is unprecedented. The observed effect could be the consequence of strong coupling of the doped electrons to lattice vibrations (phonons), a notion which finds support in numerous optical and photo-emission studies. The unusually large size of the observed isotope effect supports a recent model for superconductivity in these materials based on strong coupling to the ferroelectric soft modes of SrTiO3.