High-quality large 1T phase of TiX 2 (X = Te, Se, and S) single crystals have been grown by chemical vapor transport using iodine as a transport agent. The samples are characterized by compositional and structural analyses, and their properties are investigated by Raman spectroscopy. Several phonon modes have been observed, including the widely reported A 1g and Eg modes, the rarely reported Eu mode (∼183 cm−1 for TiTe2, and ∼185 cm−1 for TiS2), and even the unexpected K mode (∼85 cm−1) of TiTe2. Most phonons harden with the decrease of temperature, except that the K mode of TiTe2 and the Eu and “A 2u /Sh” modes of TiS2 soften with the decrease of temperature. In addition, we also found phonon changes in TiSe2 that may be related to charge density wave phase transition. Our results on TiX 2 phonons will help to understand their charge density wave and superconductivity.

Planar heterojunction perovskite solar cells (PHJ-PSCs) based on CH3NH3PbI3/TiO2 with high-quality CH3NH3PbI3 films were fabricated by two-step interdiffusion technology. The photovoltaic performance of PHJ-PSCs, including short-circuit-current (Jsc) and open-circuit-voltage (Voc), was found to be strongly dependent on the thickness of TiO2 layer. The largest Jsc was obtained at an optimized TiO2 thickness of 65 nm, resulting in a moderate power conversion efficiency (PCE) of 8.40% with a device structure of fluorine doped SnO2/compact TiO2/CH3NH3PbI3/Spiro-MeOTAD/Ag. A thicker compact TiO2 film could lead to a lower Voc, and the physical mechanism between the Voc and the thickness of compact TiO2 layer was disclosed.

We use high-resolution angle-resolved photoemission spectroscopy to investigate the electronic structure of the antiferromagnetic heavy fermion compound CePt2In7, which is a member of the CeIn3-derived heavy fermion material family. Weak hybridization among 4f electron states and conduction bands was identified in CePt2In7 at low temperature much weaker than that in the other heavy fermion compounds like CeIrIn5 and CeRhIn5. The Ce 4f spectrum shows fine structures near the Fermi energy, reflecting the crystal electric field splitting of the 4f 1 5/2 and 4f 1 7/2 states. Also, we find that the Fermi surface has a strongly three-dimensional topology, in agreement with density-functional theory calculations. PACS numbers: 74.25.Jb,71.18.+y,74.70.Tx, The physics underlying the formation of superconductivity from a coherent heavy fermion (HF) state has persisted as a central mystery despite more than four decades of intensive experimental and theoretical study [1,2]. HF superconductivity, similar to traditional Bardeen-Cooper-Schrieffer (BCS) superconductivity, is more likely to occur in compounds with particular crystal structures [3,4], and for magnetically mediated superconductivity, quasi-two-dimensional (2D) compounds are likely to have a higher transition temperature than a three-dimensional counterpart [5]. This expectation appears to be borne out in a family of tetragonal HF compounds Ce m M n In 3m+2n (M = Co, Rh, and Ir) in which the quasi-2D members with m=1, n=1 have T c 's [6] up to an order of magnitude higher than the maximum pressure-induced T c of 0.25 K found in cubic building block CeIn 3 [7]. Recent polarized soft x-ray absorption and nonresonant inelastic x-ray scattering experiments find that, in addition to the crystal structure, details of anisotropic hybridization of f and itinerant electrons play a nontrivial role in determining the ground state of the m=1, n=1 family members [8,9]. Understanding how superconductivity originates from a strongly correlated ground state which must be associated with structure, dimensionality, and orbital anisotropy requires detailed high-resolution techniques that allow studying quantitatively a system's electronic structure.Low-dimensional Ce-based systems hold a promise for the successful angle-resolved photoemission spectroscopy (ARPES) exploration of the HF electronic structure with high momentum and energy resolution. Most prior ARPES investigations of Ce-based HFs have been done on 3D systems but significant k z broadening [10-14] limits the intrinsic accuracy of 3D band measurement. Low photon energies produce high-energy and in-plane momentum resolutions; however, the 4f signal is quite low and sits atop a relatively large background. The part of the reciprocal-space spectrum where the nearly flat 4f bands, split by spin-orbit interaction, intersect with conduction bands is the site of interest for detailed inquiry. A Ce-based HF system, as close to 2D as possible, is needed to extract this information and that would provide a level of detail comparabl...

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