We use ultra-high resolution, tunable, VUV laser-based, angle-resolved photoemission spectroscopy (ARPES) and temperature and field dependent resistivity and thermoelectric power (TEP) measurements to study the electronic properties of WTe2, a compound that manifests exceptionally large, temperature dependent magnetoresistance. The temperature dependence of the TEP shows a change of slope at T=175 K and the Kohler rule breaks down above 70-140 K range. The Fermi surface consists of two electron pockets and two pairs of hole pockets along the X-Γ-X direction. Upon increase of temperature from 40K, the hole pockets gradually sink below the chemical potential. Like BaFe2As2, WTe2 has clear and substantial changes in its Fermi surface driven by modest changes in temperature. In WTe2, this leads to a rare example of temperature induced Lifshitz transition, associated with the complete disappearance of the hole pockets. These dramatic changes of the electronic structure naturally explain unusual features of the transport data.The discovery of giant magnetoresistance (GMR) in Fe/Cr superlattice [1,2], opened a new era of applications in magnetic field sensors, read heads in high density hard disks, random access memories, and galvanic isolators [3]. In the quest of achieving large MR in crystalline materials, a class of manganese oxides colossal magnetoresistance (CMR) materials was discovered that exhibits a large change in resistance with applied magnetic fields but only at low temperatures [4][5][6] The temperature dependent electrical resistivity, Hall coefficient and thermoelectric power of tungstenditelluride have been known for several decades [13] and a three-carrier semi-metal band model [13,14] was proposed to explain the electrical resistivity. Later on, density-functional based augmented spherical wave (ASW) electronic structure calculations and relatively low resolution ARPES [15] were used to study the electronic properties of WTe 2 and further supported the semimetallic nature of this material. However due to the low resolution of these early experiments, no details about the Fermi surface or band dispersion were obtained. Recent ARPES [11] and quantum oscillation [16] results revealed presence of small electron and hole pockets of roughly similar size. These findings were consistent with carrier compensation mechanism as the primary source of the MR effect [10,12]. Furthermore, this study also reported a change of the size of the Fermi pockets between 20K and 100K. More recently, Jiang et al.[17] claimed observation of strong spin-orbital coupling effect and proposed that the backscattering protection mechanism also may play a role in the large nonsaturating MR of WTe 2 .In this letter, we present the results from temperature dependent transport and ultra-high resolution, tunable VUV laser based ARPES[18] measurements. Our data show that there are two electron pockets and four hole pockets along the X-Γ-X direction and a fully occupied light "hole" band at the center of the Brillouin Zone (BZ) located j...

Recently, another consequence of the size of the Caions has been discovered. Iyo et al. 15 have found that a family of ordered CaAFe 4 As 4 (1144) compounds can be formed for A = K, Rb, Cs where the key to the formation is the difference in ionic size between the Ca and the A ion. This family is not a (Ca 1−x A x )Fe 2 As 2 solid-solution, where the Ca and A ions randomly occupy a single crystallographic site, 16 but rather is a distinct, quaternary, line compound in which the Ca and A sites form alternating planes along the crystallographic c-axis, separated by FeAs slabs 15 . In essence, the CaAFe 4 As 4 structure is identical to the CaFe 2 As 2 structure, just with layer by layer segregation of the Ca and A ions. The 1144 structure was also found for SrAFe 4 As 4 (A = Rb, Cs). Solid-solutions of Ca (Sr) 122 structures were found for arXiv:1605.05617v2 [cond-mat.supr-con]

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