arXiv subjects

M. A. Continentino

Publications and source records attributed to M. A. Continentino.

32 records · Page 2Linked to original sources

arXiv2006

Experimental observation of quantum entanglement in low dimensional spin systems

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 MgMnB_2O_5 and the and the warwickite MgTiOBO_3, systems with spin 5/2 and 1/2 respectively. By using the magnetic susceptibility as an entanglement witness we are able to quantify entanglement as a function of temperature and magnetic field. In addition, we experimentally distinguish for the first time a random singlet phase from a Griffiths phase. This analysis opens the possibility of a more detailed characterization of low dimensional materials.

cond-mat.str-el↗

arXiv2005

Quantum corrections to the phase diagram of heavy-fermion superconductors

The competition between magnetism and Kondo effect is the main effect determining the phase diagram of heavy fermion systems. It gives rise to a quantum critical point which governs the low temperature properties of these materials. However, experimental results made it clear that a fundamental ingredient is missing in this description, namely superconductivity. In this paper we make a step forward in the direction of incorporating superconductivity and study the mutual effects of this phase and antiferromagnetism in the phase diagram of heavy fermion metals. Our approach is based on a Ginzburg-Landau theory describing superconductivity and antiferromagnetism in a metal with quantum corrections taken into account through an effective potential. The proximity of an antiferromagnetic instability extends the region of superconductivity in the phase diagram and drives this transition into a first order one. On the other hand superconducting quantum fluctuations near a metallic antiferromagnetic quantum critical point gives rise to a first order transition from a low moment to a high moment state in the antiferromagnet. Antiferromagnetism and superconductivity may both collapse at a quantum bicritical point whose properties we calculate.

cond-mat.str-el↗

arXiv2004

Intergranular pinning potential and critical current in the magnetic superconductor RuSr2Gd1.5Ce0.5Cu2O10

The intergranular pinning potential U and the critical current density Jc for polycrystalline RuSr2Gd1.5Ce0.5Cu2O10 ruthenate-cuprate were determined at zero magnetic field and temperature through the frequency shift in the peak of the imaginary part of the ac magnetic susceptibility, X". A critical state model, including a flux creep term, was found to accurately describe the X" behavior. The obtained values, U(H=0,T=0) ~ 30 meV and Jc(H=0,T=0) ~ 110 A/cm^2 are about two orders of magnitude and four times lower, respectively, in comparison with the high-Tc cuprate YBa2Cu3O7. These results were ascribed to the effects of the Ru magnetization on the connectivity of the weak-linked network, giving an intrinsic local field at the junctions of ~ 15 Oe. The impact on Jc is less intense because of the small average grain radius (~1 micrometer). The intragranular London penetration length at T = 0, [lambda_L(0) ~ 2 micrometer], was derived using a Kim-type expression for the field dependence of Jc. A possible source for the large value of lambda_L in comparison to the high-Tc cuprates is suggested to come from a strong intragrain granularity, due to structural domains of coherent rotated RuO6 octahedra separated by antiphase boundaries.

cond-mat.supr-con↗

arXiv2004

Solid state Pomeranchuk effect

Recently we have shown that $YbInCu_4$ and related compounds present a solid state Pomeranchuk effect. These systems have a first order volume transition where a local moment phase coexists with a renormalized Fermi liquid in analogy with $^3He$ at its melting curve. We demonstrate here experimentally that the solid state Pomeranchuk effect, controlled by a magnetic field, can be used to produce cooling.

cond-mat.str-el↗

arXiv2004

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 step-like transitions below 5 K 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.

cond-mat.mtrl-sci↗

arXiv2003

Spin-3/2 random quantum antiferromagnetic chains

We use a modified perturbative renormalization group approach to study the random quantum antiferromagnetic spin-3/2 chain. We find that in the case of rectangular distributions there is a quantum Griffiths phase and we obtain the dynamical critical exponent $Z$ as a function of disorder. Only in the case of extreme disorder, characterized by a power law distribution of exchange couplings, we find evidence that a random singlet phase could be reached. We discuss the differences between our results and those obtained by other approaches.

cond-mat.dis-nn↗

arXiv2002

Phase diagram of the random Heisenberg antiferromagnetic spin-1 chain

We present a new perturbative real space renormalization group (RG) to study random quantum spin chains and other one-dimensional disordered quantum systems. The method overcomes problems of the original approach which fails for quantum random chains with spins larger than S=1/2. Since it works even for weak disorder we are able to obtain the zero temperature phase diagram of the random antiferromagnetic Heisenberg spin-1 chain as a function of disorder. We find a random singlet phase for strong disorder and as disorder decreases, the system shows a crossover from a Griffiths to a disordered Haldane phase.

cond-mat.dis-nn↗

arXiv2001

Role of Disorder on the Quantum Critical Point of a Model for Heavy Fermions

A zero temperature real space renormalization group (RG) approach is used to investigate the role of disorder near the quantum critical point (QCP) of a Kondo necklace (XY-KN) model. In the pure case this approach yields $J_{c}=0$ implying that any coupling $J \not = 0 $ between the local moments and the conduction electrons leads to a non-magnetic phase. We also consider an anisotropic version of the model ($X-KN$), for which there is a quantum phase transition at a finite value of the ratio between the coupling and the bandwidth, $(J/W)$. Disorder is introduced either in the on-site interactions or in the hopping terms. We find that in both cases randomness is irrelevant in the $X-KN$ model, i.e., the disorder induced magnetic-non-magnetic quantum phase transition is controlled by the same exponents of the pure case. Finally, we show the fixed point distributions $P_{J}(J/W)$ at the atractors of the disordered, non-magnetic phases.

cond-mat.dis-nn↗

arXiv2001

Phase diagram of the Kondo necklace: a mean-field renormalization group approach

In this paper we investigate the magnetic properties of heavy fermions in the antiferromagnetic and dense Kondo phases in the framework of the Kondo necklace model. We use a mean field renormalization group approach to obtain a temperature versus Kondo coupling $(T-J)$ phase diagram for this model in qualitative agreement with Doniach's diagram, proposed on physical grounds. We further analyze the magnetically disordered phase using a two-sites approach. We calculate the correlation functions and the magnetic susceptibility that allow to identify the crossover between the spin-liquid and the local moment regimes, which occurs at a {\em coherence} temperature.

cond-mat.str-el↗

arXiv2000

Breakdown of the perturbative renormalization group for S >= 1 random antiferromagnetic spin chains

We investigate the application of a perturbative renormalization group (RG) method to random antiferromagnetic Heisenberg chains with arbitrary spin size. At zero temperature we observe that initial arbitrary probability distributions develop a singularity at J=0, for all values of spin S. When the RG method is extended to finite temperatures, without any additional assumptions, we find anomalous results for S >= 1. These results lead us to conclude that the perturbative scheme is not adequate to study random chains with S >= 1. Therefore a random singlet phase in its more restrictive definition is only assured for spin-1/2 chains.

cond-mat.dis-nn↗

arXiv2000

Short-range antiferromagnetic correlations in Kondo insulators

We study the influence of short range antiferromagnetic correlations between local $f$-electrons on the transport and thermodynamic properties of Kondo insulators, as first proposed by Coqblin et al. for metallic heavy fermions. The inter-site magnetic correlations produce an effective bandwidth for the $f$-electrons. They are treated on the same footing as the local Kondo correlations such that two energy scales appear in our approach. We discuss the competition between these two scales on the physical properties.

cond-mat.str-el↗

arXiv1995

Excitonic Phase Transition in Electronic Systems

Although predicted theoretically excitonic phase transitions have never been observed. Recently it has been claimed that they can occur in some strongly correlated materials. We study the possibility of an excitonic transition in a two-band model and show that a true phase transition never occurs in the presence of hybridization. We suggest an alternative interpretation for these experiments based on the opening or closing of a hybridization gap at a critical pressure.

cond-mat↗

arXiv1993

Ising Spin Glass in a Transverse Magnetic Field

We study the three-dimensional quantum Ising spin glass in a transverse magnetic field following the evolution of the bond probability distribution under Renormalisation Group transformations. The phase diagram (critical temperature $T_c$ {\em vs} transverse field $Γ$) we obtain shows a finite slope near $T=0$, in contrast with the infinite slope for the pure case. Our results compare very well with the experimental data recently obtained for the dipolar Ising spin glass LiHo$_{0.167}$Y$_{0.833}$F$_4$, in a transverse field. This indicates that this system is more apropriately described by a model with short range interactions than by an equivalent Sherrington-Kirkpatrick model in a transverse field.

cond-mat↗

arXiv1993

Charging Effects and Quantum Crossover in Granular Superconductors

The effects of the charging energy in the superconducting transition of granular materials or Josephson junction arrays is investigated using a pseudospin one model. Within a mean-field renormalization-group approach, we obtain the phase diagram as a function of temperature and charging energy. In contrast to early treatments, we find no sign of a reentrant transition in agreement with more recent studies. A crossover line is identified in the non-superconducting side of the phase diagram and along which we expect to observe anomalies in the transport and thermodynamic properties. We also study a charge ordering phase, which can appear for large nearest neighbor Coulomb interaction, and show that it leads to first-order transitions at low temperatures. We argue that, in the presence of charge ordering, a non monotonic behavior with decreasing temperature is possible with a maximum in the resistance just before entering the superconducting phase.

cond-mat↗