Layered compounds with T-Pn tertrahedra are intensely explored, given the discovery of unconventional superconductivity in such systems with T = Fe. The "1111" and "122" are probably the best known Fe pnictide structures. We discovered new TPns with T = Cu and Ni, compounds with a structure consisting of a combination of the "1111" and the "122" structures. The physical properties of these R3T4As4O2 compounds vary from superconductivity for R = La to long range magnetic order and spin glass in the R = Ce - Nd members of these series.
Transition Metal Dichalcogenides and Pnictides
Layered transition metals - chalcogenides TCs (containing X = S, Se, Te) or pnictides TPns (Pn = P, As, Sb, Bi) - display a wealth of physical properties, from charge and spin density wave (CDW, SDW) to magnetism and superconductivity, with subsets of these often in competition. We are interested in finding links between the crystal structure and the physical properties as a path towards making the design of materials with targeted properties more controllable. Some of the materials we are currently studying include the layered TCs TiSe2 and TaS2, and the checkerboard compounds AFe2OX2, and the pnictides R3T4As4O2. In most of the layered TCs, CDW an superconducting states compete. The oxypnictides and oxychalcogenides we study in my group are mostly systems close to Mott instabilities, where unconventional superconductivity can often be unveiled.
Chih-Wei Chen, Jiakui K. Wang, E. Morosan, "Enhanced ferromagnetism induced by structural phase transitions in Co2As1−xPx" Physica B 481, 236 (2016)
W. J. Hardy, C. W. Chen, A. Marcinkova, H. Ji, J. Sinova, D. Natelson, and E. Morosan, "Very large magnetoresistance in Fe 0.28 TaS 2 single crystals" Phys. Rev. B 91, 054426 (2015)
J. S. Chen, J. K. Wang, S. V. Carr, S. C. Vogel, O. Gourdon, P. Dai, and E. Morosan, "Chemical tuning of electrical transport in Ti1−xPtxSe2−y" Phys. Rev. B 91, 045125 (2015)
Jiakui K Wang, A Marcinkova, Chih-Wei Chen, Hua He, Meigan Aronson, E Morosan, "Magnetic and transport properties of the layered transition-metal pnictides R3T4As4O2−δ (R = La, Ce, Pr, Nd, and Sm, T= Ni, Cu)" Phys. Rev. B 89, 094405 (2014)
Liang L. Zhao, Shan Wu, Jiakui K. Wang, J. P. Hodges, C. Broholm, and E. Morosan "Quasi-two-dimensional non-collinear magnetism in the Mott insulator Sr2F2Fe2OS2" Phys. Rev. B 87, 020406(R) (2013)
Jiakui K. Wang, Liang L. Zhao, Quan Yin, G. Kotliar, Moosung Kim, Meigan Aronson and E. Morosan "A layered transition metal pnictide SrMnBi2 with metallic blocking layer" Phys. Rev. B 84, 064428 (2011) - Editor's suggestion
Pt and Se doping reveals highly tunable electrical properties in PtxTi1-xSe2-y, spanning nearly ten orders of magnitude in scaled resistivity. In the absence of Pt doping (for x = 0), Se deficiency increases the metallic character of TiSe2, while a large increase of the low-temperature resistivity is favored by a lack of Se deficiency (y = 0) and increasing amounts of doped Pt (x > 0). Simultaneous Pt doping and Se deficiency (x,y > 0) confirms the competition between the two opposing trends in electrical transport. A combination of Pt doping (in place of Ti) and partial Se deficiency results yields a metallic system with nearly unperturbed CDW state.
There is great interest in understanding the physics of magnetic ordering and electronic transport in materials of reduced dimensionality with strong spin-orbit coupling. This paper presents magnetotransport measurements of Fe0.28TaS2 single crystals, which are found to exhibit very large magnetoresistance (MR) for magnetic fields along the easy axis. The authors believe that such a large MR arises from spin disorder scattering and propose to use this mechanism as a design principle for materials with large MR. Further tests are needed to fully rule out contributions from a more conventional anisotropic MR mechanism.
Very Large Magnetoresistance in Fe0.28TaS2
Of the new R3T4As4O2 compounds we discovered, the R = Ce ones proved to sum up very complex physics. Neutron scattering and physical properties measurements on Ce3T4As4O2 reveal a complex magnetic structure: layers of CeCu4As4 alternate with single square Ce-lattice and Ce2O2 bi-layer. Three magnetic phase transitions occur in this compound below its antiferromagnetic ordering, a likely result of competing interactions and anisotropies for the two rare earth sites.
These compounds are often close to a Mott instability. We seek to understand how unconventional superconductivity may occur at the local-to-itinerant moment crossover by tuning the checkerboard compounds between the two magnetic limits. In Sr2FeFe2OS2, we discovered a remarkable magnetic structure consisting of two orthogonal ferro- and anti-ferromagnetic stripes in the Fe2OX2 plane, intersecting at superexchange-mediating O sites. Together with band structure calculation we confirmed the Mott insulator state (AFM order + finite calculated DOS at EF but experimental gap Eg).
Checkerboard compounds A2Fe2OX2 (A = LaO, SrF)
Remarkable electrical transport induced by doping in PtxTi1-xSe2-y
Enhanced ferromagnetism induced by structural phase transitions in Co2As1−xPx
P doping in Co2As induces two structural transitions, resulting in an enhanced ferromagnetic state at intermediate P compositions. In Co2As1−xPx, doping induces a room temperature α-to-β structural distortion around x = 0.04, similar to what temperature (T = 725 K) does in the parent compound (x = 0). The resulting β phase displays an enhanced ferromagnetic ground state. Close to x = 0.85, a hexagonal-to-orthorhombic phase transition occurs, concomitant with the quenching of the magnetic order. Band structure calculations for the three different phases confirm the experimental observations while revealing remarkably high charge carrier polarization rate for the β phase.
Correlations of crystallographic defects and anisotropy with magnetotransport properties in FexTaS2 single crystals (0.23 ≤ x ≤ 0.35)
Following the discovery of the large enhancement of MR in Fe0.28TaS2, a systematic studey of the magneto-transport properties across the ferromagnetic Fe-intercalated TaS2 series revealed that disorder maximizes not only the magnetoresistance
Chih-Wei's paper "Correlations of crystallographic defects and anisotropy with magnetotransport properties in FexTaS2 single crystals (0.23 ≤ x ≤ 0.35)" appeared in Phys. Rev. B.