Neel phase

Certain two-dimensional 5 = quantum antiferromagnets can imdergo a direct continuous quantum phase transition between two ordered phases, an antiferromagnetic Neel phase and the so-called valence-bond ordered phase (where translational invariance is broken). This is in contradiction to Landau theory, which predicts phase coexistence, an intermediate phase, or a first-order transition, if any. The continuous transition is the result of topological defects that become spatially deconfined at the critical point and are not contained in an LGW description. Recently, there has been a great interest in the resulting deconfined quantum critical points. ... 

In the paramagnetic regime, the evolution of the EPR line width and g value show the presence of two transitions, observed at 142 and 61 K in the Mo salt, and at 222 and 46 K in the W salt. Based on detailed X-ray diffraction experiments performed on the Mo salt, the high temperature transition has been attributed to a structural second-order phase transition to a triclinic unit cell with apparition of a superstructure with a modulation vector q = (0,1/2, 1/2). Because of a twinning of the crystals at this transition, it has not been possible to determine the microscopic features of the transition, which is probably associated to an ordering of the anions, which are disordered at room temperature, an original feature for such centrosymmetric anions. This superstructure remains present down to the Neel... 

Gheorghe A, Van Nuijs A, Pecceu B, Bervoets L, Jorens PG, Blust R, Neels H, Covaci A (2008) Analysis of cocaine and its principal metabolites in waste and surface water using solid-phase extraction and liquid chromatography-ion trap tandem mass spectrometry. Anal Bioanal Chem 391 1309-1319... 

Dirtu AC, Roosens L, Geens T, Gheorghe A, Neels H, Covad A (2008) Simultaneous determination of bisphenol A, triclosan, and tetrabromobisphenol A in human serum using solid-phase extraction and gas chromatography-electron capture negative-ionization mass spectrometry. Anal Bioanal Chem 391 1175-1181... 

The disappearance of the magnetization above the Curie (or Neel) temperature (see Chap. 6) can be used to identify iron oxides. Thermal reactions - dehydration/dehy-droxylation, oxidation/reduction - with accompanying phase transformations may give additional information about phases present. Therefore, not only the heating cycle, but also the cooling cycle is usually recorded to give the so-called Js(T) curve. 

The addition of 2.5% Cr02 leads at intermediate temperature to a phase i n which only half the V ions are paired the others form a zig-zag chain (Marezio et al 1972, Pouget et al 1974). At low temperatures pairing takes place, and at higher temperatures the usual transition to the metallic rutile form. This intermediate phase has high susceptibility, and the zig-zag chains are interpreted as onedimensional Mott-Hubbard insulators above their Neel temperature. Since the transition temperature is little changed, this shows that U is the most important quantity in determining the gap. 

The resistivities of the mixed crystals are shown in Fig. 6.22. It can be seen that (as indeed follows from the phase diagram, Fig. 6.20) the transition to the insulating state occurs with larger x for increasing temperature, probably because of the high entropy of the AF insulator above its Neel temperature. 

In Fig. 13 the result of the field dependent calculation of the heat capacity using Landau theory for fields within the tetragonal plane is shown. In agreement with the experiment (Fig. 5), the magnetic field broadens the anomaly observed at the Neel temperature TN and lowers the transition point T of the commensurate to incommensurate phase transition. The increase of the calculated heat capacity while lowering the temperature below T, is a consequence of the approach to the temperature T of the proper instability of the ferromagnetic subsystem. As such, the same increase is observed in the experimental data too and it is possible to assume that the broad maximum at low temperature is connected with the subsequent phase transition in the magnetic subsystem of copper metaborate. 

Fig. 16 Schematic electronic phase diagram of C, compoimds, including the evolution of the Neel temperature, TN (open squares) for the family (NH3)K3 IRbIC60 (0

For infinite systems there is another type of result concerning long-range magnetic order, which is implicated in making a material magnetic in the sense that there be a phase transition at the onset, as well as hysteresis. This Neel-state type of ordering concerns the spin correlation function... 

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