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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

Reassessment of the (x, T) phase diagram of Cu(Ir1 − xCrx)2S4.(a) Average Cu(Ir1 − xCrx)2S4 phase diagram shown in Fig. 235. (b) (x, T) dependence of the cubic model fit residual, Rwp, for the  cubic model fit over the high-r 15–40 Å PDF range, reproducing the behavior in (a). (c) Same as (b) but for the PDF refinements including the low-r region, revealing the local dimers in the Rwp parameter. (d) (x, T) dependence of the differential  lattice parameter. (e) (x, T) dependence of the differential Ir Uiso. See text for definitions. (f) (x, T) dependence of Log ρ, after reference 35. In (b)–(e): Weiss temperature of our samples is shown as white solid symbols. Dashed lines reproduce phase lines discussed in Fig. 2. Vertical arrow denotes a feature discussed in the text.
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f6: Reassessment of the (x, T) phase diagram of Cu(Ir1 − xCrx)2S4.(a) Average Cu(Ir1 − xCrx)2S4 phase diagram shown in Fig. 235. (b) (x, T) dependence of the cubic model fit residual, Rwp, for the cubic model fit over the high-r 15–40 Å PDF range, reproducing the behavior in (a). (c) Same as (b) but for the PDF refinements including the low-r region, revealing the local dimers in the Rwp parameter. (d) (x, T) dependence of the differential lattice parameter. (e) (x, T) dependence of the differential Ir Uiso. See text for definitions. (f) (x, T) dependence of Log ρ, after reference 35. In (b)–(e): Weiss temperature of our samples is shown as white solid symbols. Dashed lines reproduce phase lines discussed in Fig. 2. Vertical arrow denotes a feature discussed in the text.

Mentions: Using this same approach it is then possible to determine the evolution of the dimer formation across the whole phase diagram. In Fig. 6 we plot the evolution of different system parameters vs. doping and temperature. For example, the existence of a global cubic structure may be found by excluding the low-r region of the PDF from fits of the cubic model. This is shown in Fig. 6(b) and is in good agreement with previous crystallographic results displayed in Fig. 6(a)35. However, if we include the low-r region of the PDF in the fit a very different picture emerges (Fig. 6(c)): the nanoscale phase diagram of Cu(Ir1 − xCrx)2S4 that maps out the (x,T) evolution of local dimers. A similar picture emerges from other measures of the local dimers such as the differential lattice parameter, Δa, and differential Ir isotropic atomic displacement parameter, ΔUiso (Fig. 6(d) and (e)). These quantities are obtained by subtracting the observed behavior of the parameters from the expected behavior in the absence of local dimer formation, as determined by an extrapolation to low temperature of the high-temperature behavior using standard assumptions such as as Debye behavior for the ADPs. All these measures reveal a previously undetected dome of local dimers appearing, roughly peaked at ~ 200 K at around x = 0.3. These strategies for searching for the presence of local broken symmetry states carry over, in principle, to other material systems of interest such as the cuprates and nickelates.


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)

Reassessment of the (x, T) phase diagram of Cu(Ir1 − xCrx)2S4.(a) Average Cu(Ir1 − xCrx)2S4 phase diagram shown in Fig. 235. (b) (x, T) dependence of the cubic model fit residual, Rwp, for the  cubic model fit over the high-r 15–40 Å PDF range, reproducing the behavior in (a). (c) Same as (b) but for the PDF refinements including the low-r region, revealing the local dimers in the Rwp parameter. (d) (x, T) dependence of the differential  lattice parameter. (e) (x, T) dependence of the differential Ir Uiso. See text for definitions. (f) (x, T) dependence of Log ρ, after reference 35. In (b)–(e): Weiss temperature of our samples is shown as white solid symbols. Dashed lines reproduce phase lines discussed in Fig. 2. Vertical arrow denotes a feature discussed in the text.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f6: Reassessment of the (x, T) phase diagram of Cu(Ir1 − xCrx)2S4.(a) Average Cu(Ir1 − xCrx)2S4 phase diagram shown in Fig. 235. (b) (x, T) dependence of the cubic model fit residual, Rwp, for the cubic model fit over the high-r 15–40 Å PDF range, reproducing the behavior in (a). (c) Same as (b) but for the PDF refinements including the low-r region, revealing the local dimers in the Rwp parameter. (d) (x, T) dependence of the differential lattice parameter. (e) (x, T) dependence of the differential Ir Uiso. See text for definitions. (f) (x, T) dependence of Log ρ, after reference 35. In (b)–(e): Weiss temperature of our samples is shown as white solid symbols. Dashed lines reproduce phase lines discussed in Fig. 2. Vertical arrow denotes a feature discussed in the text.
Mentions: Using this same approach it is then possible to determine the evolution of the dimer formation across the whole phase diagram. In Fig. 6 we plot the evolution of different system parameters vs. doping and temperature. For example, the existence of a global cubic structure may be found by excluding the low-r region of the PDF from fits of the cubic model. This is shown in Fig. 6(b) and is in good agreement with previous crystallographic results displayed in Fig. 6(a)35. However, if we include the low-r region of the PDF in the fit a very different picture emerges (Fig. 6(c)): the nanoscale phase diagram of Cu(Ir1 − xCrx)2S4 that maps out the (x,T) evolution of local dimers. A similar picture emerges from other measures of the local dimers such as the differential lattice parameter, Δa, and differential Ir isotropic atomic displacement parameter, ΔUiso (Fig. 6(d) and (e)). These quantities are obtained by subtracting the observed behavior of the parameters from the expected behavior in the absence of local dimer formation, as determined by an extrapolation to low temperature of the high-temperature behavior using standard assumptions such as as Debye behavior for the ADPs. All these measures reveal a previously undetected dome of local dimers appearing, roughly peaked at ~ 200 K at around x = 0.3. These strategies for searching for the presence of local broken symmetry states carry over, in principle, to other material systems of interest such as the cuprates and nickelates.

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