Dynamics of the charge-density-wave mode in (NbSe4)2I

Charge carrier localization and metal to insulator transition in cerium substituted (Bi,Pb)-2212 superconductor

“…It has been reported that intra-grain disorder induced by doping or by elements deficiency [5,6] reduces the effective coupling strength of cuprate superconductors and results in smaller carrier density whereas inter-grain disorder arising from grain boundary defects, reduces the coupling between grains and thus destroys the phase . As a consequence, the electrical properties of cuprates change from insulating (d /dT < 0) to metallic (d /dT > 0) due to the fact that all high-T C cuprates lie close to a metal-insulator transition [7,8]. A coexistence of superconductivity and localization has previously been observed for the latter case in granular superconductors in which small, metallic clusters become superconducting and are coupled through Josephson junctions [9].…”

Quasi-One-Dimensional Transition-Metal Chalcogenide Semiconductor (Nb4Se15I2)I2

“…Ta and Nb ions have alternating 4+ and 5+ valences, leaving the (TaSe 4 ) n or (NbSe 4 ) n chain with partially filled conducting d bands. (TaSe 4 ) 2 I, (NbSe 4 ) 2 I, (NbSe 4 ) 3 I, and (NbSe 4 ) 3.33 I undergo CDW transitions, and low-temperature XRD and resistivity measurements were used to demonstrate the onset of the CDW. , Traditionally, single crystals of (TaSe 4 ) 2 I and (NbSe 4 ) 3 I have been successfully synthesized by chemical vapor transport (CVT) where intermediate gaseous species deposit the target compounds at the low-temperature side of the reaction vessel at temperatures ranging from 400 to 800 °C. , (TaSe 4 ) 2 I is currently under renewed focus, particularly due to claims that it may harbor “axion insulator” behavior if the CDW wavevector is coincident with one of the Ta d bands crossing the Fermi energy, the location of which is termed a Weyl point due to the chirality of the compound. , The band crossing at the Fermi energy of (TaSe 4 ) 2 I is a clue that the material may host a CDW transition, − although it turns out that the CDW wavevector in (TaSe 4 ) 2 I is not coincident with the crossing point . Because of the complexities of these materials and the urgent need to compare and benchmark how charge-transport phenomena correlate with band structures, it is of great interest to discover new transition-metal chalcogenides with similar elemental compositions and quasi-1D structure.…”

“…Ta and Nb ions have alternating 4+ and 5+ valences, leaving the (TaSe 4 ) n or (NbSe 4 ) n chain with partially filled conducting d bands. (TaSe 4 ) 2 I, 8 (NbSe 4 ) 2 I, 25 (NbSe 4 ) 3 I, 26 and (NbSe 4 ) 3.33 I 27 undergo CDW transitions, and low-temperature XRD and resistivity measurements were used to demonstrate the onset of the CDW. 28,29 Traditionally, single crystals of (TaSe 4 ) 2 I and (NbSe 4 ) 3 I have been successfully synthesized by chemical vapor transport (CVT) 30 where intermediate gaseous species deposit the target compounds at the low-temperature side of the reaction vessel at temperatures ranging from 400 to 800 °C.…”

Exfoliable Transition Metal Chalcogenide Semiconductor NbSe2I2

“…Considering the complicated triclinic crystal structure with very low symmetry and the balanced charge without free electrons for further modulation, especially one-dimensional modulation, CDW is not expected for triclinic P 1̅ NbSe 2 I 2 , unlike materials with intermediate Ta 4+/5+ or Nb 4+/5+ valences, such as (TaSe 4 ) 2 I, (NbSe 4 ) 2 I, and (NbSe 4 ) 3.33 I …”

“…Given the fact that all iodine and selenium ions have a 1− valence, we know from charge counting that all the niobium ions have a valence of 4+, leaving one unpaired 4d electron for each Nb 4+ . Looking closer at the ab-plane structure, it can be seen that the Nb−Nb distance through two Se Considering the complicated triclinic crystal structure with very low symmetry and the balanced charge without free electrons for further modulation, especially one-dimensional modulation, CDW is not expected for triclinic P1̅ NbSe 2 I 2 , unlike materials with intermediate Ta 4+/5+ or Nb 4+/5+ valences, such as (TaSe 4 ) 2 I, 52 (NbSe 4 ) 2 I, 53 and (NbSe 4 ) 3.33 I. 20 Figure 3 shows the Rietveld-refined powder XRD pattern for ground triclinic NbSe 2 I 2 crystals, with R w = 7.955%.…”