Instability, magnetic

An alternative way to clarify the nature of this state is to test its stability with respect to a metal-insulator transition. This has received a lot of theoretical attention recently. The JT singlet ground state makes these compounds free from the tendency towards a magnetic instability observed in so many Mott insulators. In fact, their ground state does not break any symmetry and Capone et al. explained [43] that it then has a zero entropy, which makes a direct connection with a metal impossible (it would violate the Luttinger theorem). These authors predict that the only way to go from the insulator to the metal would be through an exotic superconducting phase or a first-order transition. 

Systems near magnetic instabilities are subject to large spin susceptibility and thereby display a strong spin-spin interaction. The kind of AF order in cuprates has motivated researchers to introduce a model where this type of interaction is central for HTS. In the momentum space, k, the model takes the form (Moriya 1990, Pines (1990)) ... 

Compared with the situation for the 3d series, the 4d orbitals are relatively more diffuse and hence can overlap more efficiently. One result of this diffuseness is the absence of permanent magnets amongst the 4d solids the 4d wavefunctions cannot support local moments. This can be rationalized in terms of the Stoner theory of itinerant magnetism which also has a cluster or molecular analog L Briefly, a magnetic instability may occur if... 

Such qualitative arguments surrounding magnetic instability were first quantitatively formalised by Stoner [4], and his scheme was subsequently... 

Using the notions of spin-fluctuation theory, the approach to the magnetic instability can be understood as a critical slowing down of the fluctuations accompanied by a gradual decrease of TSF. The deviation from Curie-Weiss law should... 

Magnet stability is another consideration. Signal averaging requires that the magnet instability not significantly broaden the observed resonance during data acquisition. 

The review of HF compounds is divided into three subsections (Ce-, U- and other materials). At the end of each paragraph dealing with a certain compound or series of compounds we give a short summary of the main impact of pSR data. Compounds where magnetic instability leads to non-Fermi-liquid behavior are discussed in the following extra section. 

Summary U2Pt2ln is an intermetallic close to a magnetic instability. It exhibits NFL behavior below 8K. [xSR sees no evidence for static magnetic order (for r 0.3 K), even in the low moment limit. The pSR spectra are not understood in detail, especially at higher temperatures (>10K). The absence of static magnetism suggests a quantum critical point as the likely cause for NFL behavior. 

Fig. 44. Transitions across hybridization gaps of the quasiparticle band structure (a) can, depending on the position of the chemical potential (assumed inside the pseudogap for this calculation), and on the other band-structure features, give rise to pronounced structures in the non-local interaction part Y = x K of the Stoner denominator [compare eqs. (19) and (20)] at small wavevectors (b). In (b) a = y/ir with y defined in eq. (20) is varied somewhat around its proper value of (Grewe and Welslau 1988) to exhibit the strong tendency towards a magnetic instability occurring for Y(9 it> i/ = 0) = l.

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