Quantum critical point

The zero-temperature calculations of the previous section Section, the XY model with impurities, represent a highly idealized situation however, it is unclear whether they have any relevance to the system at nonzero temperature. Since the properties of a quantum system for low temperatures are strongly influenced by nearby quantum critical points, it is tempting to attribute the effect of nearby critical points to persistent mixed-state entanglement in the thermal state. 

A. del Campo, M. M. Rams, and W. H. Zurek. Assisted finite-rate adiabatic passage across a quantum critical point exact solution for the quantum Ising model. Phys. Rev. Lett., 109(11) 115703-115707(2012). 

This behavior matches quite well to the scenario that has recently been developed for 2D spin systems in the proximity to a quantum critical point [16] and is a direct fingerprint of the tendency to form local correlations. It has to be mentioned, however, that the lowest energy excitation still has triplet character. For compounds with closer proximity to quantum criticality, such as the 1/6 depleted triangular lattice (Kagome) [6] and the 1/5 depleted square lattice, also exists a dense singlet spectrum within the singlet triplet gap [17-20], Unfortunately no Raman scattering data for the related compounds are available up to now. 

FIHFM field-induced heavy-fermion metal QCP quantum critical point... 

These observations justify the conjecture that there is a better chance to find a quantum critical point in the U(Pt,Pd)3 system with the Pd concentration as the control parameter. In order to verify these suggestions, accurate neutron diffraction and pSR experiments have been performed in the composition range close to x = 0.006 where superconductivity disappears and long-range magnetic order starts to develop. We subdivide the discussion in compounds with 0 < x < 0.005 and with 0.006 < x < 0.01. 

We like to mention that at a quantum critical point where antiferromagnetic order is on the verge to disappear, the value of a is expected to be 3/2, whereas a value of 5/3 hints to the presence of ferromagnetic fluctuations. The actual experimental value for a for x> 0.006 is equal to 1.6, i.e. in between the two above given values. Specific-heat... 

The MIT point in Fig. 6 is a quantum critical point (QCP), and there have been various theoretical approaches addressing the existence of such a QCP. Such approaches often consider different mechanisms for the SC pairing and for the driving force of the PG state, and consequently predict (in difference with the present approach) the existence different regimes of different symmetries, within the SC phase [47], This has not been confirmed by experiment, in agreement with the present approach. The existence of an intrinsic nanoscale heterogeneity of SC and PG regions [48] will be addressed below. 

FIRST ORDER SUPERCONDUCTING TRANSITION NEAR A FERROMAGNETIC QUANTUM CRITICAL POINT... 

Keywords ferromagnetic superconductors, ferromagnetic quantum critical point, first order... 

First Order Superconducting Transition near a Ferromagnetic Quantum Critical Point 219... 

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