Superconductor Superconductor CobaltitesCobaltitesSaban M HUSSaban M HUSSaban M. HUSSaban M. HUS

UTKUTKSolid State IISolid State II

OutlineOutlineOutlineOutlineIntroductionIntroductionIntroductionIntroduction

Superconductivity in Layered OxidesSuperconductivity in Layered OxidesSuperconductivity inCobalt OxidesSuperconductivity inCobalt Oxides

Structure and Phase Diagrams ofStructure and Phase Diagrams ofStructure and Phase Diagrams ofStructure and Phase Diagrams of•• NaNaXXCoOCoO22

•• NaNa CoOCoO ·1 3H·1 3H OO•• NaNaxxCoOCoO22 ·1.3H·1.3H22OOSuperconductivity in 2Superconductivity in 2--D CoO LayersD CoO Layers

Summary Summary

Superconductivity inSuperconductivity inCopper Oxides Copper Oxides

Discovered in 1986* Discovered in 1986* byby

J. G. Bednorz J. G. Bednorz K. A. MüllerK. A. MüllerK. A. MüllerK. A. Müller

inin5 5 5(3 )yx xBa La Cu O −−

*In 1987 Müller and Bednorz were *In 1987 Müller and Bednorz were jointly awarded the Nobel Prize in jointly awarded the Nobel Prize in

h i th h t t ti b t thh i th h t t ti b t th

5 5 5(3 )yx x −

physics the shortest time between the physics the shortest time between the discovery and the prize award for any discovery and the prize award for any Nobel. Nobel.

Superconductivity inSuperconductivity inCopper Oxides Copper Oxides

19871987YBCO ( YBaYBCO ( YBa22CuCu33OO77 ))92 K92 K

19931993HgHg0.80.8TlTl0.20.2BaBa22CaCa22CuCu33OO8+8+δδ

138 K138 K20082008

(Sn(Sn PbPb InIn )Ba)Ba TmTm(Sn(Sn1.01.0PbPb0.50.5InIn0.50.5)Ba)Ba44TmTm55CuCu77OO20+20+

185 K185 K ??????185 K 185 K ??????

Superconductivity inSuperconductivity inCobalt OxidesCobalt Oxides

20032003Takada et. al.Takada et. al.NaNa0.350.35CoOCoO22 ·1.3H·1.3H22OO5K5K

Structure of Layered NaStructure of Layered Na CoOCoOStructure of Layered NaStructure of Layered NaXXCoOCoO22

structure consist of triangular CoO22 layers

ith N i di t ib t d iwith Na ions distributed in intervening charge reservoir layersreservoir layers.

Phase Diagram of Layered Phase Diagram of Layered NaNaXXCoOCoO22

0.3 < x < 0.50.3 < x < 0.5Paramagnetic MetalParamagnetic Metal

x = ½x = ½ChargeCharge--Ordered InsulatorOrdered Insulator

0.5 < x < 0.750.5 < x < 0.75CurieCurie--Weiss MetalWeiss Metal

0 75 < x0 75 < x0.75 < x0.75 < xSpin Density Wave MetalSpin Density Wave Metal

CurieCurie--Weiss MetalWeiss MetalSpin Density Wave MetalSpin Density Wave Metal

X 2/3X 2/3X~2/3 X~2/3 metallicmetallic--like in charge like in charge cond ctioncond ctionconduction conduction insulatorinsulator--like in spin like in spin alignmentalignmentalignment alignment

χχ=C / (T+70)=C / (T+70)X 3/4X 3/4X=3/4X=3/4

Very weak Very weak magnetizationmagnetizationmagnetizationmagnetization

M~0.03µM~0.03µB B per Coper CoC t ( B l 20 K)C t ( B l 20 K)χχ~Cont ( Below 20 K)~Cont ( Below 20 K)

ChargeCharge Ordered InsulatorOrdered InsulatorChargeCharge--Ordered InsulatorOrdered InsulatorX=1/2X=1/2X=1/2X=1/2

inin--plane resistivity ρ plane resistivity ρ shows a dramaticshows a dramaticshows a dramatic shows a dramatic changechangeCurieCurie--Weiss behavior inWeiss behavior inCurieCurie Weiss behavior in Weiss behavior in χχ vanishesvanishesNa superstructure Na superstructure pp

Bragg spots appearBragg spots appearˆ ˆ3 2a x ay

Bragg spots appearBragg spots appeara strong interaction a strong interaction between the Na ions between the Na ions and holes requiredand holes required

Preparation of NaPreparation of Na CoOCoO ·1 3H·1 3H OOPreparation of NaPreparation of NaxxCoOCoO22 ·1.3H·1.3H22OO

Parent oxideParent oxideNaNa0.70.7CoOCoO22

Na + ions are deintercalated in Na + ions are deintercalated in BrBr22 solution. solution. HH O l l i t l tO l l i t l tHH22O molecules intercalate O molecules intercalate between Na and CoObetween Na and CoO22 layers. layers. Distance between succeedingDistance between succeedingDistance between succeeding Distance between succeeding CoOCoO22 layers increased from layers increased from 10.96 Ǻ to 19.62 Ǻ10.96 Ǻ to 19.62 Ǻ10.96 Ǻ to 19.62 Ǻ10.96 Ǻ to 19.62 ǺOptimum water content is Optimum water content is found to be 1.3 per Cofound to be 1.3 per Copp

Structure of Layered Structure of Layered NaNaxxCoOCoO22 ·1.3H·1.3H22OO

Consists of two Consists of two dimensional CoOdimensional CoO22 layers layers separated by a thickseparated by a thickseparated by a thick separated by a thick insulating layer of Na+ insulating layer of Na+ ions and Hions and H22O molecules.O molecules.22

CoOCoO2 2 planes planes Electronically activeElectronically active

NaNax x ·1.3H·1.3H22O layers O layers Spacer Spacer ppCharge reservoirCharge reservoir

Superconductivity phase diagram Superconductivity phase diagram of Naof NaxxCoOCoO22 ·1.3H·1.3H22OO

a.c. susceptibility a.c. susceptibility measurements are requiredmeasurements are requiredmeasurements are required measurements are required for weakly superconducting for weakly superconducting samples.samples.ppSingle phase NaSingle phase NaxxCoOCoO22·1.3H·1.3H22O has maximum Na O has maximum Na 22content x~0.35content x~0.35Maximum TMaximum TC C value 4.3 K is value 4.3 K is displayed by the samples displayed by the samples with x = 0.30 with x = 0.30

Superconductivity phase diagram Superconductivity phase diagram of Naof NaxxCoOCoO22 ·1.3H·1.3H22OO

The optimal Na composition The optimal Na composition for the occurrence of for the occurrence of superconductivity is x = 0 30superconductivity is x = 0 30superconductivity is x = 0.30superconductivity is x = 0.30xx oxidation state of Cooxidation state of Co0 260 26 3 74+3 74+0.260.26 3.74+3.74+0.300.30 3.70+3.70+0 350 35 3 65+3 65+0.350.35 3.65+3.65+Superconductivity may Superconductivity may occur in a narrower xoccur in a narrower xoccur in a narrower x occur in a narrower x interval. Samples with interval. Samples with ideally uniform x should be ideally uniform x should be yyprepared to test prepared to test

Important observations forImportant observations forCobalt oxideCobalt oxide

SuperconductorsSuperconductorsSuperconductorsSuperconductorsTwo dimensional character of the structure is Two dimensional character of the structure is important important

Lower hydrates with closer CoOLower hydrates with closer CoO22 -- CoOCoO22 layers are layers are yy 22 22 yynot superconducting above 2 K. not superconducting above 2 K. TTCC decreases under pressure decreases under pressure C C

Water molecules intercalate between Na+ ions Water molecules intercalate between Na+ ions and the Coand the Co22 planes and therefore screen the planes and therefore screen the 2 2 ppconduction electrons from the Na potentialconduction electrons from the Na potential

MonoMono--hydrates fail to superconducthydrates fail to superconductMonoMono hydrates fail to superconducthydrates fail to superconduct

Similarities between the layeredSimilarities between the layeredyycopper oxide and cobalt oxidecopper oxide and cobalt oxide

S d tS d tSuperconductorsSuperconductors

Both superconductors have transition Both superconductors have transition metal oxide layers that have very weakmetal oxide layers that have very weakmetal oxide layers that have very weak metal oxide layers that have very weak interinter--planar coupling planar coupling Both superconductors is mixed valanceBoth superconductors is mixed valance

CuCu2+2+ && CuCu3+ 3+ CoCo3+3+ && CoCo4+4+CuCu & & CuCu CoCo & & CoCo

Differences between the layeredDifferences between the layeredDifferences between the layeredDifferences between the layeredcopper oxide and cobalt oxidecopper oxide and cobalt oxide

SuperconductorsSuperconductorsGeometryGeometryGeometryGeometry

nearly squarenearly square--planar coordination of copper in the layers of the planar coordination of copper in the layers of the highhigh--TTc superconductors, c superconductors, the cobalt oxide is composed of distorted edgethe cobalt oxide is composed of distorted edge--shared octahedra shared octahedra of cobalt and oxygen with cobaltof cobalt and oxygen with cobalt--oxygen bond angles of either oxygen bond angles of either 8181°° or 99or 99°°

Electronic structureElectronic structureThere is a strong mixing between the oxygen and transition There is a strong mixing between the oxygen and transition metal levels for the copper oxidesmetal levels for the copper oxidesmetal levels for the copper oxidesmetal levels for the copper oxidesthere is little hybridization between the oxygen and transition there is little hybridization between the oxygen and transition metal orbitals in the layers of Nametal orbitals in the layers of NaXXCoOCoO22

Summary & ViewsSummary & ViewsSummary & ViewsSummary & Views

NaNaxxCoOCoO22 ·1.3H·1.3H22O is (was) one of the few O is (was) one of the few examples of layered transition metal oxide examples of layered transition metal oxide p yp ysuperconductors that do not contain superconductors that do not contain coppercoppercoppercopperContrasts and similarities between it and Contrasts and similarities between it and copper oxides may help us to better copper oxides may help us to better understand the highunderstand the high-- TTCC superconductivitysuperconductivitygg CC p yp y

ReferencesReferencesReferences References

Not Yet !Not Yet !

IronIron BasedBasedIronIron--Based Based Layered SuperconductorsLayered Superconductorsy py p

2008200820082008Hosomo et. al.Hosomo et. al.La[OLa[O FF ]FeAs]FeAsLa[OLa[O11--xxFFxx]FeAs ]FeAs ((xx = 0.05= 0.05--0.12)0.12)26K26K26K26K

After 2 monthsAfter 2 monthsPr[OPr[O11--xxFFxx]FeAs]FeAs52 K52 K

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