arXiv subjects

I. J. Hamad

Publications and source records attributed to I. J. Hamad.

12 recordsLinked to original sources

arXiv2024

Singlet polaron theory of low-energy optical excitations in NiPS$_3$

Light-matter interactions can be used as a tool to realize novel many-body states of matter and to study the interplay between electronic and magnetic degrees of freedom. In particular, tightly bound many-body states that behave as coherent quasi-particles are rare, and may lead to unconventional technological applications beyond the semi-conductor paradigm, particularly if these excitations are bosonic and can condense. Two-dimensional magnetic systems present a pristine platform to realize and study such states. We construct a theory that explains the low-energy optical excitations at 1.476 eV and 1.498 eV observed by photoluminiscence, optical absorption, and RIXS in the van der Waals antiferromagnet NiPS$_3$. Using \textit{ab initio} methods, we construct a two-band Hubbard model for two \textit{effective} Ni orbitals of the original lattice. The dominant effective hopping corresponds to third-nearest neighbours. This model exhibits two triplet-singlet excitations of energy near two times the Hund exchange. From perturbation theory, we obtain an effective model for the movement of the singlets in an antiferromagnetic background, that we solve using a generalized self-consistent Born approximation. These singlet excitations, dressed by a cloud of magnons, move coherently as polaronic-like quasi-particles, "singlet polarons". Our theory explains the main features of the observed spectra.

cond-mat.str-el↗

arXiv2021

Magnon-assisted dynamics of a hole doped in a cuprate superconductor

We calculate the quasiparticle dispersion and spectral weight of the quasiparticle that results when a hole is added to an antiferromagnetically ordered CuO$_2$ plane of a cuprate superconductor. We also calculate the magnon contribution to the quasiparticle spectral function. We start from a multiband model for the cuprates considered previously [Nat. Phys. \textbf{10}, 951 (2014)]. We map this model and the operator for creation of an O hole to an effective one-band generalized $t-J$ model, without free parameters. The effective model is solved using the state of the art self-consistent Born approximation. Our results reproduce all the main features of experiments. They also reproduce qualitatively the dispersion of the multiband model, giving better results for the intensity near wave vector $(π,π)$, in comparison with the experiments. In contrast to what was claimed in [Nat. Phys. \textbf{10}, 951 (2014)], we find that spin fluctuations play an essential role in the dynamics of the quasiparticle, and hence in both its weight and dispersion.

cond-mat.supr-con↗

arXiv2018

Generalized one-band model based on Zhang-Rice singlets for Tetragonal CuO

Tetragonal CuO (T-CuO) has attracted attention because of its structure similar to that of the cuprates. It has been recently proposed as a compound whose study can give an end to the long debate about the proper microscopic modeling for cuprates. In this work, we rigorously derive an effective one-band generalized $t\!-\!J$ model for T-CuO, based on orthogonalized Zhang-Rice singlets, and make an estimative calculation of its parameters, based on previous \textit{ab initio} calculations. By means of the self-consistent Born approximation, we then evaluate the spectral function and the quasiparticle dispersion for a single hole doped in antiferromagnetically ordered half-filled T-CuO. Our predictions show very good agreement with angle-resolved photoemission spectra and with theoretical multiband results. We conclude that a generalized $t\!-\!J$ model remains the minimal Hamiltonian for a correct description of single-hole dynamics in cuprates.

cond-mat.supr-con↗

arXiv2015

Scaling of conductance through quantum dots with magnetic field

Using different techniques, and Fermi-liquid relationships, we calculate the variation with applied magnetic field (up to second order) of the zero-temperature equilibrium conductance through a quantum dot described by the impurity Anderson model. We focus on the strong-coupling limit $U \gg Δ$ where $U$ is the Coulomb repulsion and $Δ$ is half the resonant-level width, and consider several values of the dot level energy $E_d$, ranging from the Kondo regime $ε_F-E_d \gg Δ$ to the intermediate-valence regime $ε_F-E_d \sim Δ$, where $ε_F$ is the Fermi energy. We have mainly used density-matrix renormalization group (DMRG) and numerical renormalization group (NRG) combined with renormalized perturbation theory (RPT). Results for the dot occupancy and magnetic susceptibility from DMRG and NRG+RPT are compared with the corresponding Bethe ansatz results for $U \rightarrow \infty$, showing an excellent agreement once $E_d$ is renormalized by a constant Haldane shift. For $U < 3 Δ$ a simple perturbative approach in $U$ agrees very well with the other methods. The conductance decreases with applied magnetic field for dot occupancies $n_d \sim 1$ and increases for $n_d \sim 0.5$ or $n_d \sim 1.5$ regardless of the value of $U$. We also relate the energy scale for the magnetic-field dependence of the conductance with the width of low energy peak in the spectral density of the dot.

cond-mat.mes-hall↗

arXiv2015

Selfconsistent hybridization expansions for static properties of the Anderson impurity model

By means of a projector-operator formalism we derive an approximation based on a self consistent hybridization expansion to study the ground state properties of the Anderson Impurity model. We applied the approximation to the general case of finite Coulomb repulsion $U$, extending previous work with the same formalism in the infinite-$U$ case. The treatment provides a very accurate calculation of the ground state energy and their related zero temperature properties in the case in which $U$ is large enough, but still finite, as compared with the rest of energy scales involved in the model. The results for the valence of the impurity are compared with exact results that we obtain from equations derived using the Bethe ansatz and with a perturbative approach. The magnetization and magnetic susceptibility is also compared with Bethe ansatz results. In order to do this comparison, we also show how to regularize the Bethe ansatz integral equations necessary to calculate the impurity valence, for arbitrary values of the parameters.

cond-mat.str-el↗

arXiv2013

Transport Regimes in a Double Quantum Dot Device

We analyze the transport properties of a double quantum dot device with both dots coupled to perfect conducting leads and to a finite chain of N non-interacting sites connecting both of them. The inter-dot chain strongly influences the transport across the system and the Local Density of States of the dots. We study the case of small number of sites, so that Kondo box effects are present, varying the coupling between the dots and the chain. For odd N and small coupling between the inter-dot chain and the dots, a state with two coexisting Kondo regimes develops: the bulk Kondo due to the quantum dots connected to leads and the one produced by the screening of the quantum dots spins by the spin in the finite chain at the Fermi level. As the coupling to the inter-dot chain increases, there is a crossover to a molecular Kondo effect, due to the screening of the molecule (formed by the finite chain and the quantum dots) spin by the leads. For even N the two-Kondo temperatures regime does not develop and the physics is dominated by the usual competition between Kondo and antiferromagnetism between the quantum dots. We finally study how the transport properties are affected as N is increased. For the study we used exact multi-configurational Lanczos calculations and finite U slave-boson mean-field theory at T = 0. The results obtained with both methods describe qualitatively and also quantitatively the same physics.

cond-mat.mes-hall↗

arXiv2013

Transport properties of a two impurity system: a theoretical approach

A system of two interacting cobalt atoms, at varying distances, was studied in a recent scanning tunneling microscope experiment by Bork et. al.[Nature Phys. 7, 901 (2011)]. We propose a microscopic model that explains, for all experimentally analyzed interatomic distances, the physics observed in these experiments. Our proposal is based on the two-impurity Anderson model, with the inclusion of a two-path geometry for charge transport. This many-body system is treated in the finite-U slave boson mean-field approximation and the logarithmic-discretization embedded-cluster approximation. We physically characterize the different charge transport regimes of this system at various interatomic distances and show that, as in the experiments, the features observed in the transport properties depend on the presence of two impurities but also on the existence of two conducting channels for electron transport. We interpret the splitting observed in the conductance as the result of the hybridization of the two Kondo resonances associated with each impurity.

cond-mat.mes-hall↗

arXiv2011

Effects of semiclassical spiral fluctuations on hole dynamics

We investigate the dynamics of a single hole coupled to the spiral fluctuations related to the magnetic ground states of the antiferromagnetic J_1-J_2-J_3 Heisenberg model on a square lattice. Using exact diagonalization on finite size clusters and the self consistent Born approximation in the thermodynamic limit we find, as a general feature, a strong reduction of the quasiparticle weight along the spiral phases of the magnetic phase diagram. For an important region of the Brillouin Zone the hole spectral functions are completely incoherent, whereas at low energies the spectral weight is redistributed on several irregular peaks. We find a characteristic value of the spiral pitch, Q=(0.7,0.7)π, for which the available phase space for hole scattering is maximum. We argue that this behavior is due to the non trivial interference of the magnon assisted and the free hopping mechanism for hole motion, characteristic of a hole coupled to semiclassical spiral fluctuations.

cond-mat.str-el↗

arXiv2007

Spin polaron in the J1-J2 Heisenberg model

We have studied the validity of the spin polaron picture in the frustrated J1-J2 Heisenberg model. For this purpose, we have computed the hole spectral functions for the Neel, collinear, and disordered phases of this model, by means of the self-consistent Born approximation and Lanczos exact diagonalization on finite-size clusters. We have found that the spin polaron quasiparticle excitation is always well defined for the magnetically ordered Neel and collinear phases, even in the vicinity of the magnetic quantum critical points, where the local magnetization vanishes. As a general feature, the effect of frustration is to increase the amplitude of the multimagnon states that build up the spin polaron wave function, leading to the reduction of the quasiparticle coherence. Based on Lanczos results, we discuss the validity of the spin polaron picture in the disordered phase.

cond-mat.str-el↗

arXiv2007

Spectroscopic fingerprints of the frustrated magnetic order in Li2VOSiO4: a t-J model study

We have analyzed theoretically the photoemission spectra of the insulating compound Li2VOSiO4.Recently, this compound has been proposed as the first experimental realization of the frustrated J1-J2 Heisenberg model. Although it is well known that Li2VOSiO4 is magnetically ordered in a collinear arrangement below TN=2.8 K, there is some controversy about the coexistence of two collinear phases above TN. Using a generalized t-J model we have obtained a complex spectral structure that can be traced back to the underlying collinear magnetic structures. We discuss the possibility to use ARPES experiments as a way to discern among the different scenarios proposed in the literature.

cond-mat.str-el↗

arXiv2006

Hole dynamics in canted antiferromagnets

We have analyzed the dynamics of a single hole doped in a canted antiferromagnet using the t-J model. Within the self consistent Born approximation we have found that the hole propagates at two different energy scales along the antiferromagnetic and the ferromagnetic components of the canted order, respectively. While the many body quasiparticle excitation has its origin in the coherent coupling of the hole with the magnon excitations of the antiferromagnetic component, the ferromagnetic component gives rise to a free like hole motion at higher energies. We have found a non-trivial behavior of the hole spectral function with the canting angle. In particular, in the strong coupling regime, the quasiparticle weight strongly depends on the momenta, vanishing inside the magnetic Brillouin zone for a canting angle greater than 60 degree.

cond-mat.str-el↗

arXiv2005

1/T_1 nuclear relaxation time of κ-(BEDT-TTF)_ 2 Cu [N(CN)_2] Cl : effects of magnetic frustration

We study the role played by the magnetic frustration in the antiferromagnetic phase of the organic salt κ-(BEDT-TTF)_ 2 Cu [N(CN)_2] Cl. Using the spatially anisotropic triangular Heisenberg model we analyze previous and new performed NMR experiments. We compute the 1/T_1 relaxation time by means of the modified spin wave theory. The strong suppression of the nuclear relaxation time observed experimentally under varying pressure and magnetic field is qualitatively well reproduced by the model. Our results suggest the existence of a close relation between the effects of pressure and magnetic frustration.

cond-mat.str-el↗