A. Saguia

Spin-3/2 random quantum antiferromagnetic chains

We use a modified perturbative renormalization group approach to study the random quantum antiferromagnetic spin- 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.

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. As the disorder decreases, the system shows a crossover from a Griffiths to a disordered Haldane phase.

Random spin-1 quantum chains

We study disordered spin-1 quantum chains with random exchange and biquadratic interactions using a real space renormalization group approach. We find that the dimerized phase of the pure biquadratic model is unstable and gives rise to a random singlet phase in the presence of weak disorder. In the Haldane region of the phase diagram we obtain a quite different behavior.

Entanglement entropy in random quantum spin-S chains

We discuss the scaling of entanglement entropy in the random singlet phase (RSP) of disordered quantum magnetic chains of general spin S. Through an analysis of the general structure of the RSP, we show that the entanglement entropy scales logarithmically with the size of a block, and we provide a closed expression for this scaling. This result is applicable for arbitrary quantum spin chains in the RSP, being dependent only on the magnitude S of the spin. Remarkably, the logarithmic scaling holds for the disordered chain even if the pure chain with no disorder does not exhibit conformal invariance, as is the case for Heisenberg integer-spin chains. Our conclusions are supported by explicit evaluations of the entanglement entropy for random spin-1 and spin-3/2 chains using an asymptotically exact real-space renormalization group approach.

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

Griffiths phases in the strongly disordered Kondo necklace model

(XY\ensuremath{-}KN)

Scaling of the local quantum uncertainty at quantum phase transitions

model. In the pure case this approach yields

Phase transitions in the two-dimensional superantiferromagnetic Ising model with next-nearest-neighbor interactions

{J}_{c}=0

Thermodynamics of the random antiferromagnetic spin-1 chain

implying that any coupling

Dynamics of the quantum search and quench-induced first-order phase transitions

J\ensuremath{\ne}0