Single-crystalline KFe 2 As 2 and CaT 2 As 2 (T = Fe, Co, Ni, Cu) are synthesized and investigated by resistivity, susceptibility and optical spectroscopy. It is found that CaCu 2 As 2 exhibits a similar transition to the lattice abrupt collapse transitions discovered in CaFe 2 (As 1−x P x ) 2 and Ca 1−x Re x Fe 2 As 2 (Re = rare-earth element). The resistivity of KFe 2 As 2 and CaT 2 As 2 (T = Fe, Co, Ni, Cu) approximately follows the similar T 2 dependence at low temperature, but the magnetic behaviors vary with different samples. Optical measurement reveals the optical response of CaCu 2 As 2 is not sensitive to the transition at 50 K, with no indication of development of a new energy gap below the transition temperature. Using Drude-Lorentz model, We find that two Drude terms, a coherent one and an incoherent one, can fit the low-energy optical conductivity of KFe 2 As 2 and CaT 2 As 2 (T = Fe, Co, Ni) very well. However, in CaCu 2 As 2 , which is a sp-band metal, the low-energy optical conductivity can be well described by a coherent Drude term. Lack of the incoherent Drude term in CaCu 2 As 2 may be attributed to the weaker electronic correlation than KFe 2 As 2 and CaT 2 As 2 (T = Fe, Co, Ni). Spectral weight analysis of these samples indicates that the unconventional spectral weight transfer, which is related to Hund's coupling energy J H , is only observed in iron pnictides, supporting the viewpoint that J H may be a key clue to seek the mechanism of magnetism and superconductivity in pnictides.

Rare-earth tri-tellurium RTe 3 is a typical quasi-two dimensional system which exhibits obvious charge density wave (CDW) orders. So far, RTe 3 with heavier R ions (Dy, Ho, Er and Tm) are believed to experience two CDW phase transitions, while the lighter ones only hold one. TbTe 3 is claimed to belong to the latter. However in this work we present evidences that TbTe 3 also possesses more than one CDW order. Aside from the one at 336 K, which was extensively studied and reported to be driven by imperfect Fermi surface nesting with a wave vector q = (2/7c * ), a new CDW energy gap (260 meV) develops at around 165 K, revealed by both infrared reflectivity spectroscopy and ultrafast pump-probe spectroscopy. More intriguingly, the origin of this energy gap is different from the second CDW order in the heavier R ions-based compounds RTe 3 (R=Dy, Ho, Er and Tm).

We report an optical spectroscopy study on the single crystal of Na 2 Ti 2 As 2 O, a sister compound of superconductor BaTi 2 Sb 2 O. The study reveals unexpectedly two density wave phase transitions. The first transition at 320 K results in the formation of a large energy gap and removes most part of the Fermi surfaces. But the compound remains metallic with residual itinerant carriers. Below 42 K, another density wave phase transition with smaller energy gap scale occurs and drives the compound into semiconducting ground state. These experiments thus enable us to shed light on the complex electronic structure in the titanium oxypnictides. PACS numbers: 71.45.Lr, 75.30.Fv, 74.25.Gz, The low temperature broken-symmetry states in lowdimension materials, such as superconductivity, charge/spin density wave (CDW/SDW), are among the most fascinating collective phenomena in solids and the interplay between them has been a subject of considerable interest in condensed matter physics [1][2][3][4][5][6]. Most density wave (DW) instabilities (either CDW or SDW) are driven by the nesting topology of Fermi surfaces (FSs), that is, the matching of sections of FS to others by a wave vector 2k F , where the electronic susceptibility has a divergence. Brought about by electron-phonon or electron-electron interactions, a single particle energy gap opens in the nested regions of FSs, resulting in the lowering of the electronic energy of the system. In one-dimensional (1D) system the DW phase transition usually causes an semiconducting ground state due to the opening of a full energy gap arising from the perfect nesting of Fermi surfaces. However, for two-dimensional (2D) or three-dimensional (3D) materials, the CDW or SDW ground states mostly remain metallic due to the formation of partial energy gap induced by the imperfect nesting of FSs. To date, there seems no reported example of a truly semiconducting ground state caused by the DW phase transition in a 2D or 3D system.Recently, a new superconducting system Ba 1−x Na x Ti 2 Sb 2 O (T c ∼2-5 K) has attracted much attention [7][8][9]. The system belongs to a two-dimensional (2D) titanium oxypnictide family, consisting of alternate stacking of conducting octahedral layers Ti 2 Pn 2 O (Pn=As, Sb) and other insulating layers (e.g. Na 2 , Ba, (SrF) 2 , (SmO) 2 ) [9][10][11][12][13][14][15][16][17][18] , as characterized by the sharp jumps in resistivity and drops in magnetic susceptibility. First principle band structure calculations indicate that the phase transitions are driven by the DW instabilities arising from the nested electron and hole FSs [19][20][21][22][23]. As superconductivity emerges only in compound with low phase transition temperature and T c is further enhanced when the phase transition temperature was suppressed by doping, the family offers a new playground to study the interplay between superconductivity and DW instabilities. Understanding the electronic properties in the undoped compounds is an essential step towards understanding the superconductivity in this...