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Quantum Critical Point (quantum + critical_point)

Distribution by Scientific Domains
Physics and Astronomy71%

Selected Abstracts

HTSC cuprate phase diagram using a modified Boson,Fermion,Gossamer model describing competing orders, a quantum critical point and possible resonance complex

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2009
Richard H. Squire
Abstract There has been considerable effort expended toward understanding high temperature superconductors (HTSC), and more specifically the cuprate phase diagram as a function of doping level. Yet, the only agreement seems to be that HTSC is an example of a strongly correlated material where Coulomb repulsion plays a major role. This manuscript proposes a model based on a Feshbach resonance pairing mechanism and competing orders. An initial BCS-type superconductivity at high doping is suppressed in the two particle channel by a localized preformed pair (PP) (Nozieres and Schmitt-Rink, J Low Temp Phys, 1985, 59, 980) (circular density wave) creating a quantum critical point. As doping continues to diminish, the PP then participates in a Feshbach resonance complex that creates a new electron (hole) pair that delocalizes and constitutes HTSC and the characteristic dome (Squire and March, Int J Quantum Chem, 2007, 107, 3013; 2008, 108, 2819). The resonant nature of the new pair contributes to its short coherence length. The model we propose also suggests an explanation (and necessity) for an experimentally observed correlated lattice that could restrict energy dissipation to enable the resonant Cooper pair to move over several correlation lengths, or essentially free. The PP density wave is responsible for the pseudogap as it appears as a "localized superconductor" since its density of states and quasiparticle spectrum are similar to those of a superconductor (Peierls,Fröhlich theory), but with no phase coherence between the PP. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source]

Low frequency spin dynamics in the quantum magnet copper pyrazine dinitrate

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2010
H. Kühne
Abstract The S,=,1/2 antiferromagnetic Heisenberg chain exhibits a magnetic field driven quantum critical point. We study the low frequency spin dynamics in copper pyrazine dinitrate (CuPzN), a realization of this model system of quantum magnetism, by means of 13C-NMR spectroscopy. Measurements of the nuclear spin,lattice relaxation rate in the vicinity of the saturation field are compared with quantum Monte Carlo calculations of the dynamic structure factor. Both show a strong divergence of low energy excitations at temperatures in the quantum regime. The analysis of the anisotropic -rates and frequency shifts allows one to disentangle the contributions from transverse and longitudinal spin fluctuations for a selective study and to determine the transfer of delocalized spin moments from copper to the neighboring nitrogen atoms. [source]

Quantum criticality in CePt1,xNix due to hydrostatic and chemical pressure

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2010
Nadezda Bagrets
Abstract We have investigated the possibility of tuning ferromagnetic CePt1,xNix for and 0.9 to a quantum critical point (QCP) by measuring the magnetization between 2 and 300,K on polycrystals under pressures up to 13,kbar. Thus we constructed a TC,p phase diagram for CePt1,xNix with . For the alloy with we show that the ferromagnetic hysteresis loop at 2.5,K is completely suppressed by a hydrostatic pressure of 10.2,kbar. Our findings suggest that Ce(Pt:Ni) can be tuned to the QCP with the application of hydrostatic pressure at considerably lower pressures than pure CePt. [source]

Entanglement of spin chains with general boundaries and of dissipative systems

ANNALEN DER PHYSIK, Issue 7-8 2009
T. Stauber
Abstract We analyze the entanglement properties of spins (qubits) close to the boundary of spin chains in the vicinity of a quantum critical point and show that the concurrence at the boundary is significantly different from the one of bulk spins. We also discuss the von Neumann entropy of dissipative environments in the vicinity of a (boundary) critical point, such as two Ising-coupled Kondo-impurities or the dissipative two-level system. Our results indicate that the entanglement (concurrence and/or von Neumann entropy) changes abruptly at the point where coherent quantum oscillations cease to exist. The phase transition modifies significantly less the entanglement if no symmetry breaking field is applied and we argue that this might be a general property of the entanglement of dissipative systems. We finally analyze the entanglement of an harmonic chain between the two ends as function of the system size. [source]

Entanglement of spin chains with general boundaries and of dissipative systems

ANNALEN DER PHYSIK, Issue 7-8 2009
T. Stauber
Abstract We analyze the entanglement properties of spins (qubits) close to the boundary of spin chains in the vicinity of a quantum critical point and show that the concurrence at the boundary is significantly different from the one of bulk spins. We also discuss the von Neumann entropy of dissipative environments in the vicinity of a (boundary) critical point, such as two Ising-coupled Kondo-impurities or the dissipative two-level system. Our results indicate that the entanglement (concurrence and/or von Neumann entropy) changes abruptly at the point where coherent quantum oscillations cease to exist. The phase transition modifies significantly less the entanglement if no symmetry breaking field is applied and we argue that this might be a general property of the entanglement of dissipative systems. We finally analyze the entanglement of an harmonic chain between the two ends as function of the system size. [source]

Quantum criticality and novel phases: Summary and outlook

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2010
A. J. Schofield
Abstract This conference summary and outlook provides a personal overview of the topics and themes of the August 2009 Dresden meeting on quantum criticality and novel phases. The dichotomy between the local moment and the itinerant views of magnetism is revisited and refreshed in new materials, new probes, and new theoretical ideas. New universality and apparent zero temperature phases of matter move us beyond the old ideas of quantum criticality. This is accompanied by alternative pairing interactions and as yet unidentified phases developing in the vicinity of quantum critical points. In discussing novel order, the magnetic analogs of superconductivity are considered as candidate states for the hidden order that sometimes develops in the vicinity of quantum critical points in metallic systems. These analogs can be thought of as "pairing" in the particle,hole channel and are tabulated. This analogy is used to outline a framework to study the relation between ferromagnetic fluctuations and the propensity of a metal to nematic type phases , which at weak coupling correspond to Pomeranchuk instabilities. This question can be related to the fundamental relations of Fermi liquid theory. [source]