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Cu(Ir₁ - xCrx)₂S₄: a model system for studying nanoscale phase coexistence at the metal-insulator transition.

Božin ES, Knox KR, Juhás P, Hor YS, Mitchell JF, Billinge SJ - Sci Rep (2014)

Bottom Line: Increasingly, nanoscale phase coexistence and hidden broken symmetry states are being found in the vicinity of metal-insulator transitions (MIT), for example, in high temperature superconductors, heavy fermion and colossal magnetoresistive materials, but their importance and possible role in the MIT and related emergent behaviors is not understood.We demonstrate a hitherto unobserved coexistence of an Ir(4+) charge-localized dimer phase and Cr-ferromagnetism.The resulting phase diagram that takes into account the short range dimer order is highly reminiscent of a generic MIT phase diagram similar to the cuprates.

View Article: PubMed Central - PubMed

Affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973.

ABSTRACT
Increasingly, nanoscale phase coexistence and hidden broken symmetry states are being found in the vicinity of metal-insulator transitions (MIT), for example, in high temperature superconductors, heavy fermion and colossal magnetoresistive materials, but their importance and possible role in the MIT and related emergent behaviors is not understood. Despite their ubiquity, they are hard to study because they produce weak diffuse signals in most measurements. Here we propose Cu(Ir₁ - xCrx)₂S₄ as a model system, where robust local structural signals lead to key new insights. We demonstrate a hitherto unobserved coexistence of an Ir(4+) charge-localized dimer phase and Cr-ferromagnetism. The resulting phase diagram that takes into account the short range dimer order is highly reminiscent of a generic MIT phase diagram similar to the cuprates. We suggest that the presence of quenched strain from dopant ions acts as an arbiter deciding between the competing ground states.

No MeSH data available.


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Structural effect of dimerization in spinels.(a) Regular pyrochlore sublattice in the high temperature cubic phase of transition metal spinels. All near neighbor transition metal distances (grey) are of equal average length (t2g are degenerate). The metal-metal bonds are shown, indicating degenerate interpenetrating 1D chains of ions. (b) Structural distortion (for example tetragonal along c-axis) lifts the degeneracy (making the (110)-type directions (green) special). (c) Dimerization along the (110)-type chains occurring in the low temperature insulating phases results in a redistribution of bond-lengths: each dimer converts two average distances to a pair of short (red) and long (blue) distances. Details of the three dimensional ordering of the dimers then depend on the specifics of the transitional metal spinel family.
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f1: Structural effect of dimerization in spinels.(a) Regular pyrochlore sublattice in the high temperature cubic phase of transition metal spinels. All near neighbor transition metal distances (grey) are of equal average length (t2g are degenerate). The metal-metal bonds are shown, indicating degenerate interpenetrating 1D chains of ions. (b) Structural distortion (for example tetragonal along c-axis) lifts the degeneracy (making the (110)-type directions (green) special). (c) Dimerization along the (110)-type chains occurring in the low temperature insulating phases results in a redistribution of bond-lengths: each dimer converts two average distances to a pair of short (red) and long (blue) distances. Details of the three dimensional ordering of the dimers then depend on the specifics of the transitional metal spinel family.

Mentions: Among the wide variety of materials displaying MITs is a class of transition metal spinels with partially filled t2g orbitals31. Exotic types of orbital and charge ordering, spin dimerization, and associated superstructures emerge in the low temperature insulating phases, such as octamers in CuIr2S432, helices in MgTi2O433, and heptamers in AlV2O434. Appearance of these complex motifs is accompanied by a decrease in the magnetic susceptibility. This behavior is understood first in CuIr2S4 as the formation of Ir4+-Ir4+ structural dimers which results in the spins on the dimerized iridum ions forming a singlet (S = 0) state. The interesting isomeric patterns that form in the different spinels depend on details of the chemistry and structure of the specific system and come about from different orderings of the dimers on the pyrochlore sublattice of corner shared tetrahedra of transition metal ions (Fig. 1(a))32, but through a careful analysis of the local structure and properties in CuIr2S4 we show that the important physics driving the MIT is the charge localization and dimer formation itself rather than the dimer ordering and isomerization.


Cu(Ir₁ - xCrx)₂S₄: a model system for studying nanoscale phase coexistence at the metal-insulator transition.

Božin ES, Knox KR, Juhás P, Hor YS, Mitchell JF, Billinge SJ - Sci Rep (2014)

Structural effect of dimerization in spinels.(a) Regular pyrochlore sublattice in the high temperature cubic phase of transition metal spinels. All near neighbor transition metal distances (grey) are of equal average length (t2g are degenerate). The metal-metal bonds are shown, indicating degenerate interpenetrating 1D chains of ions. (b) Structural distortion (for example tetragonal along c-axis) lifts the degeneracy (making the (110)-type directions (green) special). (c) Dimerization along the (110)-type chains occurring in the low temperature insulating phases results in a redistribution of bond-lengths: each dimer converts two average distances to a pair of short (red) and long (blue) distances. Details of the three dimensional ordering of the dimers then depend on the specifics of the transitional metal spinel family.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3921632&req=5

f1: Structural effect of dimerization in spinels.(a) Regular pyrochlore sublattice in the high temperature cubic phase of transition metal spinels. All near neighbor transition metal distances (grey) are of equal average length (t2g are degenerate). The metal-metal bonds are shown, indicating degenerate interpenetrating 1D chains of ions. (b) Structural distortion (for example tetragonal along c-axis) lifts the degeneracy (making the (110)-type directions (green) special). (c) Dimerization along the (110)-type chains occurring in the low temperature insulating phases results in a redistribution of bond-lengths: each dimer converts two average distances to a pair of short (red) and long (blue) distances. Details of the three dimensional ordering of the dimers then depend on the specifics of the transitional metal spinel family.
Mentions: Among the wide variety of materials displaying MITs is a class of transition metal spinels with partially filled t2g orbitals31. Exotic types of orbital and charge ordering, spin dimerization, and associated superstructures emerge in the low temperature insulating phases, such as octamers in CuIr2S432, helices in MgTi2O433, and heptamers in AlV2O434. Appearance of these complex motifs is accompanied by a decrease in the magnetic susceptibility. This behavior is understood first in CuIr2S4 as the formation of Ir4+-Ir4+ structural dimers which results in the spins on the dimerized iridum ions forming a singlet (S = 0) state. The interesting isomeric patterns that form in the different spinels depend on details of the chemistry and structure of the specific system and come about from different orderings of the dimers on the pyrochlore sublattice of corner shared tetrahedra of transition metal ions (Fig. 1(a))32, but through a careful analysis of the local structure and properties in CuIr2S4 we show that the important physics driving the MIT is the charge localization and dimer formation itself rather than the dimer ordering and isomerization.

Bottom Line: Increasingly, nanoscale phase coexistence and hidden broken symmetry states are being found in the vicinity of metal-insulator transitions (MIT), for example, in high temperature superconductors, heavy fermion and colossal magnetoresistive materials, but their importance and possible role in the MIT and related emergent behaviors is not understood.We demonstrate a hitherto unobserved coexistence of an Ir(4+) charge-localized dimer phase and Cr-ferromagnetism.The resulting phase diagram that takes into account the short range dimer order is highly reminiscent of a generic MIT phase diagram similar to the cuprates.

View Article: PubMed Central - PubMed

Affiliation: Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973.

ABSTRACT
Increasingly, nanoscale phase coexistence and hidden broken symmetry states are being found in the vicinity of metal-insulator transitions (MIT), for example, in high temperature superconductors, heavy fermion and colossal magnetoresistive materials, but their importance and possible role in the MIT and related emergent behaviors is not understood. Despite their ubiquity, they are hard to study because they produce weak diffuse signals in most measurements. Here we propose Cu(Ir₁ - xCrx)₂S₄ as a model system, where robust local structural signals lead to key new insights. We demonstrate a hitherto unobserved coexistence of an Ir(4+) charge-localized dimer phase and Cr-ferromagnetism. The resulting phase diagram that takes into account the short range dimer order is highly reminiscent of a generic MIT phase diagram similar to the cuprates. We suggest that the presence of quenched strain from dopant ions acts as an arbiter deciding between the competing ground states.

No MeSH data available.


Related in: MedlinePlus