Mucio A. Continentino

Quantum scaling in many-body systems

Abstract The theory of quantum critical phenomena is introduced to study some current many-body problems in condensed matter physics. Renormalization group concepts are applied to strongly correlated electronic materials which are close to a zero-temperature instability. These systems have enhanced effective masses and susceptibility. Scaling arguments yield the exponents which govern the critical behavior of these quantities in terms of the usual critical exponents associated with a zero-temperature phase transition. We show the existence of a new energy scale, related to the quantum nature of the many-body instability, which can be generally associated with the setting of Fermi-liquid behavior with decreasing temperature in three-dimensional strongly interacting electronic systems. The theory of quantum critical phenomena is used to investigate the Kondo lattice problem, which provides a model to describe heavy-fermion systems and to introduce a scaling theory of the Mott transition with special emphasis on charge fluctuation effects. However, this report is not a review on heavy fermions and Mott insulators. The microscopic theories of these systems are still controversial and present some of the most challenging and instigating problems in condensed matter physics. This state of affairs stimulated the author to review and extend the scaling approach. The scaling theory we develop provides a powerful tool, based on the notion of universality, to understand the physical properties of correlated systems beyond the mean-field level. This is illustrated by our treatment of the one-dimensional Hubbard model, where, although the Fermi-liquid fixed point does not survive the fluctuations, the scaling approach is still useful. Finally, we discuss briefly how disorder affect our results.

Abrupt field-induced transition triggered by magnetocaloric effect in phase-separated manganites

The occurrence at low temperatures of an ultrasharp field-induced transition in phase-separated manganites is analyzed. Experimental results show that magnetization and specific-heat steplike transitions below

Pressure study of the paraconductivity of high Tc superconductors

5\phantom{\rule{0.3em}{0ex}}\mathrm{K}

Experimental observation of quantum entanglement in low-dimensional spin systems

are correlated with an abrupt change of the sample temperature, which happens at a certain critical field. This temperature rise, a magnetocaloric effect, is interpreted as produced by the released energy at the transition point and is the key to understand the existence of the abrupt field-induced transition. A qualitative analysis of the results suggests the existence of a critical growing rate of the ferromagnetic phase, beyond which an avalanche effect is triggered.

Thermodynamic quantum critical behavior of the Kondo necklace model

Abstract We measure the resistivity of several high T c ceramic superconductors ( REBa 2 Cu 3 O 7− δ , RE = Y, Yb, Gd and Er) under applied pressure. An analysis of the temperature derivative of the resistivity close to the critical temperature shows that the transition to the superconducting state occurs in two steps. We discuss the origin of this double transition and obtain the exponents associated with the paraconductivity which give information on the dimensionality of the critical fluctuations.

Bose-Einstein condensation in antiferromagnets close to the saturation field

We report macroscopic magnetic measurements carried out in order to detect and characterize field-induced quantum entanglement in low-dimensional spin systems. We analyze the pyroborate

Magnetism and charge ordering in Fe 3 O 2 BO 3 studied by Fe 57 Mössbauer spectroscopy

\mathrm{Mg}\mathrm{Mn}{\mathrm{B}}_{2}{\mathrm{O}}_{5}

Spin-3/2 random quantum antiferromagnetic chains

and the warwickite

Phase diagram of the random Heisenberg antiferromagnetic spin-1 chain.

\mathrm{Mg}\mathrm{Ti}\mathrm{O}\mathrm{B}{\mathrm{O}}_{3}

Asymmetric superconductivity in metallic?systems

, systems with spin