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52 CHAPTER 3. T-J MODEL ON THE TRIANGULAR LATTICE with the Haldane gap of S=1 chains or S=1/2 two-leg ladders. The theoretical understanding of these properties is at best very primitive, and reliable numerical results are called for. The recent discovery of superconductivity at low temperatures in the triangular Na x CoO 2 .yH 2 O layered compounds [74] is a very interesting event, since it may be the long sought low-temperature resonating valence bond superconductor, on a lattice which was at the basis of Anderson’s original ideas on a possible quantum spin liquid state. Na x CoO 2 .yH 2 O consists of two dimensional CoO 2 layers separated by thick insulating layers of Na + 2 ions and H 2O molecules. It is a triangular net of edge sharing oxygen octahedra; Co atoms are at the center of the octahedra forming a 2D triangular lattice. Many interesting behaviors occur in Na x CoO 2 .yH 2 O . In the high doping limit where charge conduction is present, the magnetic susceptibility shows a strange behavior. In a usual metal, the fraction of spins that can be field-aligned is small and shrinks to zero with decreasing temperature. In contrast, in Na x CoO 2 ,the spin population that contributes to the susceptibility just equals the population of holes and stays the same when temperature falls. The susceptibility is therefore like the one of a Mott insulator. This ambivalence between the metallic behavior in the charge conduction and the insulator behavior in the spin susceptibility has led to call the high-doping phase a spin-density wave metal. Equally puzzling at high doping is the fact that Na x CoO 2 has a large thermopower. In metals, an electrical (or charge) current involves the flow of electrons and is usually accompanied by an electronic heat current. The thermo-power measures the ratio of the heat to the charge current. In most metals the thermopower is very small. A clue to understand the large thermo-power is that at low temperature T =2K, the thermo-power is suppressed by a magnetic field. Measurements confirm that the vanishing of the thermo-power coincides with the complete alignment of the spins by the field. Consequently, this implies that the thermo-power is mostly coming from the spin entropy carried by the holes in the spin-density wave metallic phase. Very interestingly, it was also observed that when the compound is cooled below T =4K [74], it becomes a superconductor when the doping is lying between 1/4
3.2. SUPERCONDUCTIVITY IN THE COBALTITES 53 Figure 3.1: The figure was reproduced from Ref. [73]. Phase diagram of Na x CoO 2 .yH 2 O . The compound is electronically doped. At half-filling (x =0) the compound is unfortunately not stable and could consequently not be studied. The compound becomes superconducting for x = 1 − 1 . At high doping, the 4 3 compound is a spin-density wave metal.
Page 1: VARIATIONAL STUDY OF STRONGLY CORRE
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Page 12 and 13: 12 CONTENTS 3.3.5 Order parameters
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112 CHAPTER 5. THE FLUX PHASE FOR T
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176 CHAPTER 7. CONCLUSION is the st
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178 CHAPTER 7. CONCLUSION although
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180 BIBLIOGRAPHY [17] E. Dagotto. R
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182 BIBLIOGRAPHY [59] F. F. Assad.
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184 BIBLIOGRAPHY [98] M. Posternak,
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186 BIBLIOGRAPHY [136] B. Fauqué,
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188 BIBLIOGRAPHY
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190 APPENDIX A. DETERMINANTS AND PF
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192 APPENDIX B. A PAIR OF PARTICLES
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Education • Reviewer activity at
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2. Model of strongly correlated ele
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List of publications 1. Ising Trans
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Schools, workshops and conferences
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