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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.


Related in: MedlinePlus

Deviations from cubic  structure.(a) CuIr2S4 data at 10 K (open blue symbols), the best fit cubic model (solid red line), and the difference curve (green solid line) offset for clarity. Dashed lines are experimental uncertainties on the 2σ level. (b) Same as (a) but for 40% Cr sample. The grey area marks the r-region where the crossover from local to average behavior occurs. The temperature dependencies of the lattice parameter and Ir isotropic atomic displacement parameter (Uiso) for the 40% Cr sample are shown in (c) and (d), respectively. Light red solid lines represent fits to the high temperature region of (c) (fit with a linear function) and (d) (fit with the Debye model). Deviations of the data from these trends are clearly observed at low temperature. (d) The Debye model with static offset (represented by the double arrow) set to zero (dark solid red line). The black dashed line represents the Debye model fit to Ir Uiso for CuIr2S4 data. An appreciably larger static offset for the 40% Cr doped sample reflects the higher level of quenched disorder as compared to CuIr2S4. The inset to (b) shows the doping dependence of the lower r-boundary where deviations from the cubic model become apparent at low temperature.
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f5: Deviations from cubic structure.(a) CuIr2S4 data at 10 K (open blue symbols), the best fit cubic model (solid red line), and the difference curve (green solid line) offset for clarity. Dashed lines are experimental uncertainties on the 2σ level. (b) Same as (a) but for 40% Cr sample. The grey area marks the r-region where the crossover from local to average behavior occurs. The temperature dependencies of the lattice parameter and Ir isotropic atomic displacement parameter (Uiso) for the 40% Cr sample are shown in (c) and (d), respectively. Light red solid lines represent fits to the high temperature region of (c) (fit with a linear function) and (d) (fit with the Debye model). Deviations of the data from these trends are clearly observed at low temperature. (d) The Debye model with static offset (represented by the double arrow) set to zero (dark solid red line). The black dashed line represents the Debye model fit to Ir Uiso for CuIr2S4 data. An appreciably larger static offset for the 40% Cr doped sample reflects the higher level of quenched disorder as compared to CuIr2S4. The inset to (b) shows the doping dependence of the lower r-boundary where deviations from the cubic model become apparent at low temperature.

Mentions: Here, we explore the correlation length of local dimer order across the phase diagram. Since the PDF provides structural information on different lengthscales, this may be done by carrying out a variable r-range fit to the PDF of carefully chosen models23. Here, we utilize the fact that the cubic spinel model, which describes the global structure well, fails for the dimer case. The observation of local dimers implies that the global cubic symmetry comes from an average over incoherent domains of locally ordered dimers where there is a lower symmetry within the domain. We expect the cubic model to work well on lengthscales much larger than the local domain size, but to fail for short lengthscales dominated by the intra-domain signal. In Fig. 5(a) and (b) fits of the cubic model to the low temperature data of 0% Cr (exhibiting long range ordered dimers) and 40% Cr (no long range ordered dimers, dimer signal relatively weak) samples are shown, respectively. The model fails for CuIr2S4 on all lengthscales, as expected. On the other hand, for the 40% Cr sample with global symmetry the cubic model fails only on lengthscales shorter than ~ 1 nm, as evident from the difference curve extending out of the 2σ uncertainty parapet in Fig. 5(b). The correlation length determined in this way for all the samples is plotted in the inset to Fig. 5(b), where the error bars indicate a range of uncertainty associated with determining this crossover marked as the grey band in Fig. 5(b), with further discussion of how this was obtained provided in the Supplementary Material. No significant temperature variation of this bound could be established within the accuracy of our measurement. Although our analysis does not explicitly address the nature of the dimer order, it is interesting to contemplate what such short range dimer order may include. While the observed correlation length bound extends only to near neighbor dimers along the (110)-chains, it encompasses both near neighbor and next near neighbor dimers spanning the octamers, suggesting a three dimensional character of the dimer correlations.


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)

Deviations from cubic  structure.(a) CuIr2S4 data at 10 K (open blue symbols), the best fit cubic model (solid red line), and the difference curve (green solid line) offset for clarity. Dashed lines are experimental uncertainties on the 2σ level. (b) Same as (a) but for 40% Cr sample. The grey area marks the r-region where the crossover from local to average behavior occurs. The temperature dependencies of the lattice parameter and Ir isotropic atomic displacement parameter (Uiso) for the 40% Cr sample are shown in (c) and (d), respectively. Light red solid lines represent fits to the high temperature region of (c) (fit with a linear function) and (d) (fit with the Debye model). Deviations of the data from these trends are clearly observed at low temperature. (d) The Debye model with static offset (represented by the double arrow) set to zero (dark solid red line). The black dashed line represents the Debye model fit to Ir Uiso for CuIr2S4 data. An appreciably larger static offset for the 40% Cr doped sample reflects the higher level of quenched disorder as compared to CuIr2S4. The inset to (b) shows the doping dependence of the lower r-boundary where deviations from the cubic model become apparent at low temperature.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f5: Deviations from cubic structure.(a) CuIr2S4 data at 10 K (open blue symbols), the best fit cubic model (solid red line), and the difference curve (green solid line) offset for clarity. Dashed lines are experimental uncertainties on the 2σ level. (b) Same as (a) but for 40% Cr sample. The grey area marks the r-region where the crossover from local to average behavior occurs. The temperature dependencies of the lattice parameter and Ir isotropic atomic displacement parameter (Uiso) for the 40% Cr sample are shown in (c) and (d), respectively. Light red solid lines represent fits to the high temperature region of (c) (fit with a linear function) and (d) (fit with the Debye model). Deviations of the data from these trends are clearly observed at low temperature. (d) The Debye model with static offset (represented by the double arrow) set to zero (dark solid red line). The black dashed line represents the Debye model fit to Ir Uiso for CuIr2S4 data. An appreciably larger static offset for the 40% Cr doped sample reflects the higher level of quenched disorder as compared to CuIr2S4. The inset to (b) shows the doping dependence of the lower r-boundary where deviations from the cubic model become apparent at low temperature.
Mentions: Here, we explore the correlation length of local dimer order across the phase diagram. Since the PDF provides structural information on different lengthscales, this may be done by carrying out a variable r-range fit to the PDF of carefully chosen models23. Here, we utilize the fact that the cubic spinel model, which describes the global structure well, fails for the dimer case. The observation of local dimers implies that the global cubic symmetry comes from an average over incoherent domains of locally ordered dimers where there is a lower symmetry within the domain. We expect the cubic model to work well on lengthscales much larger than the local domain size, but to fail for short lengthscales dominated by the intra-domain signal. In Fig. 5(a) and (b) fits of the cubic model to the low temperature data of 0% Cr (exhibiting long range ordered dimers) and 40% Cr (no long range ordered dimers, dimer signal relatively weak) samples are shown, respectively. The model fails for CuIr2S4 on all lengthscales, as expected. On the other hand, for the 40% Cr sample with global symmetry the cubic model fails only on lengthscales shorter than ~ 1 nm, as evident from the difference curve extending out of the 2σ uncertainty parapet in Fig. 5(b). The correlation length determined in this way for all the samples is plotted in the inset to Fig. 5(b), where the error bars indicate a range of uncertainty associated with determining this crossover marked as the grey band in Fig. 5(b), with further discussion of how this was obtained provided in the Supplementary Material. No significant temperature variation of this bound could be established within the accuracy of our measurement. Although our analysis does not explicitly address the nature of the dimer order, it is interesting to contemplate what such short range dimer order may include. While the observed correlation length bound extends only to near neighbor dimers along the (110)-chains, it encompasses both near neighbor and next near neighbor dimers spanning the octamers, suggesting a three dimensional character of the dimer correlations.

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