arXiv subjects

S. A. Owerre

Publications and source records attributed to S. A. Owerre.

48 records · Page 3Linked to original sources

arXiv2016

A Large-$S$ Study of Quantum Kagome Ice

We present a large-$S$ study of a quantum spin ice Hamiltonian, introduced by Huang, et al., [PRL 112, 167203 (2014)], on the kagome lattice. This model involves a competition between the frustrating Ising term of classical kagome ice, a Zeeman magnetic field $h$,and a nearest-neighbor transverse spin-flip term $S^x_iS^x_j - S^y_iS^y_j$. Recent Quantum Monte Carlo (QMC) simulations by Carrasquilla, et al., [Nature Communications, 6, 7421, (2015)], uncovered lobes of a disordered phase for large Ising interaction and $h\neq 0$ -- a putative quantum spin liquid phase. Here, we examine the nature of this model using large-$S$ expansion. We show that the ground state properties generally have the same trends with those observed in QMC simulations. In particular, the large-$S$ ground state phase diagram captures the existence of the disordered lobes.

cond-mat.str-el↗

arXiv2016

Weyl semimetal in ultra-thin film of topological insulator multilayer

In ultra-thin film of topological insulator, the hybridization between the top and bottom surfaces opens an energy gap and forms two degenerate quantum anomalous Hall states, which give rise to a quantum spin Hall state. In this paper, we demonstrate that a Weyl semimetal can be realized in an ultra-thin film of topological insulator heterostructure in a similar way to that of the surface state of a strong three-dimensional (3D) topological insulator studied by Burkov and Balents . We find that the system realizes both 3D quantum anomalous Hall phase and 3D quantum spin Hall phase, and the Weyl nodes occur at zero energy when both time-reversal symmetry and inversion symmetry are explicitly broken by the magnetic field and the structure inversion asymmetry of the thin film.

cond-mat.str-el↗

arXiv2016

The phase-diagram of the Blume-Capel-Haldane-Ising spin chain

We consider the one-dimensional spin chain for arbitrary spin $s$ on a periodic chain with $N$ sites, the generalization of the chain that was studied by Blume and Capel \cite{bc}: $$H=\sum_{i=1}^N \left(a (S^z_i)^2+ b S^z_iS^z_{i+1}\right).$$ The Hamiltonian only involves the $z$ component of the spin thus it is essentially an Ising \cite{Ising} model. The Hamiltonian also figures exactly as the anisotropic term in the famous model studied by Haldane \cite{haldane} of the large spin Heisenberg spin chain \cite{bethe}. Therefore we call the model the Blume-Capel-Haldane-Ising model. Although the Hamiltonian is trivially diagonal, it is actually not always obvious which eigenstate is the ground state. In this paper we establish which state is the ground state for all regions of the parameter space and thus determine the phase diagram of the model. We observe the existence of solitons-like excitations and we show that the size of the solitons depends only on the ratio $a/b$ and not on the number of sites $N$. Therefore the size of the soliton is an intrinsic property of the soliton not determined by boundary conditions.

cond-mat.str-el↗

arXiv2015

Quantum spin Hall effect on the porphyrin lattice

We study the physics of quantum spin Hall (QSH) effect and topological quantum phase transition on the porphyrin lattice. We show that in a special limit the pristine model on this lattice reduces to the usual topological insulator (TI) thin film model. By incorporating explicit time reversal symmetry (TRS) breaking interactions, we explore the topological properties of this model and show that it exhibits an ordinary insulator phase, a quantum anomalous Hall (QAH) phase, and a quantum spin Hall (QSH) phase.

cond-mat.str-el↗

arXiv2015

Theoretical Studies of Dimeric Exchange-Coupled Single-Molecule Magnets

We study a molecular dimer model that takes higher order interaction term as well as antisymmetric term into account. We mainly focus on the theoretical quantum properties that are of interest. We numerically diagonalize the system and investigate different phenomena which include oscillations of ground state energy splitting, magnetization steps, Schottky anomaly, torque oscillations, and magnetocaloric effects. Indeed, we show that these interactions can exotically influence these phenomena, which should be of interest.

cond-mat.mes-hall↗

arXiv2015

Topological quantum phase transitions in topological insulator thin film

Topological quantum phase transition in electron gas systems is an enthralling phenomena. This phase transition has a unique property in that it is associated with a quantum phase transition point, which separates different regions with gapped phases, characterized by an immutable topological quantity called the Chern number. In this paper, we study a tight binding model for a three-dimensional topological insulator $(TI)$ thin film that captures different topological quantum phase transitions in the entire Brillouin zone. We investigate the effects of a staggered magnetic field, an off-diagonal coupling term, and a Rashba spin-orbit coupling $(SOC)$ on the topological insulator thin film model. We give a lucid exposition of the topological phases by exploring all the topological properties of this system in the entire Brillouin zone, as a function of the competing interactions. We find that the system exhibits quantum spin Hall $(QSH)$ phase, quantum anomalous Hall $(QAH)$, semi-metallic phase, and an ordinary insulator phase. The transition from one phase to another is invariably associated with a gap closing point. We further corroborate our results by solving for the edge states modes of the system.

cond-mat.str-el↗

arXiv2015

Antiferromagnetic molecular nanomagnets with odd-numbered coupled spins

In recent years, studies on cyclic molecular nanomagnets have captivated the attention of researchers. These magnets are finite in size and contain very large spins. They are interesting because they possess macroscopic quantum tunneling of Néel vectors. For antiferromagnetic molecular nanomagnets with finite number of even-numbered coupled spins, tunneling involves two classical localized Néel ground states separated by a magnetic energy barrier. The question is: can such phenomena be observed in nano magnets with odd number of magnetic ions? The answer is not directly obvious because cyclic chains with odd-numbered coupled spins are frustrated as one cannot obtain a perfect Néel order. These frustrated spins can indeed be observed experimentally, so they are of interest. In this Letter, we theoretically investigate macroscopic quantum tunneling in these odd spin systems with arbitrary spins $s$, in the presence of a magnetic field applied along the plane of the magnet. In contrast to systems with an even-numbered coupled spins, the ground state of the cyclic odd-spin system contains a topological soliton due to spin frustration. Thus the classical ground state is $2N$-fold degenerate as the soliton can be placed anywhere along the ring with total $S_z=\pm s$. Small quantum fluctuations delocalize the soliton with a formation of an energy band. We obtain this energy band using degenerate perturbation theory at order $2s$. We show that the soliton ground state is chiral for half-odd integer spins and non-chiral for integer spins. From the structure of the energy band we infer that as the value of the spin increases the inelastic polarized neutron-scattering intensity may increase or decrease depending on the strengths of the parameters of the Hamiltonian.

cond-mat.mes-hall↗

arXiv2015

A toy model for quantum spin Hall effect

In this Communication, we investigate a toy model of three-dimensional topological insulator surface, coupled homogeneously to a fictitious pseudo spin-$\frac{1}{2}$ particle. We show that this toy model captures the interesting features of topological insulator surface states, which include topological quantum phase transition and quantum spin hall effect. We further incorporate an out-of-plane magnetic field and obtain the Landau levels.

cond-mat.mes-hall↗

arXiv2014

Macroscopic quantum tunneling and quantum-classical phase transitions of the escape rate in large spin systems

This article presents a review on the theoretical and the experimental developments on macroscopic quantum tunneling and phase transition of the escape rate in spin systems. We present the basic ideas with simplified calculations so that it is readable to both specialists and nonspecialists in this area of research. A brief derivation of the path integral formulation of quantum mechanics in its original form using the orthonormal position and momentum basis is reviewed. For spin systems such as single molecule magnets, the formulation of path integral requires the use of non-orthonormal spin coherent state in $(2s+1)$ dimensional Hilbert space, the coordinate independent and the coordinate dependent form of the spin coherent state path integral is derived. These two forms of spin coherent state path integral are applied to the tunneling of single molecule magnets through its magnetic anisotropy barrier. Most experimental and numerical results are presented. The suppression of tunneling for half-odd integer spin (spin-parity effect) at zero magnetic field is derived from both forms, which shows that this result (spin-parity effect) is independent of the coordinate. At nonzero magnetic field we present both the experimental and the theoretical results of the oscillation of tunneling splitting as a function of the applied magnetic field applied along the spin hard anisotropy axis direction. The experimental and the theoretical results of the tunneling in antiferromagnetic exchange coupled dimer model are also reviewed. As the spin coherent state path integral formalism is a semi-classical method, an alternative exact mapping of a spin system to a particle in a potential field (effective potential method) is derived. This effective potential method allows for the investigation of phase transition of the escape rate in spin systems.

cond-mat.other↗

arXiv2014

Haldane-like antiferromagnetic spin chain in the large anisotropy limit

We consider the one dimensional, periodic spin chain with $N$ sites, similar to the one studied by Haldane \cite{hal}, however in the opposite limit of very large anisotropy and small nearest neighbour, anti-ferromagnetic exchange coupling between the spins, which are of large magnitude $s$. For a chain with an even number of sites we show that actually the ground state is non degenerate and given by a superposition of the two Néel states, due to quantum spin tunnelling. With an odd number of sites, the Néel state must necessarily contain a soliton. The position of the soliton is arbitrary thus the ground state is $N$-fold degenerate. This set of states reorganizes into a band. We show that this occurs at order $2s$ in perturbation theory. The ground state is non-degenerate for integer spin, but degenerate for half-odd integer spin as is required by Kramer's theorem \cite{kram}.

cond-mat.str-el↗

arXiv2014

Rotating Entangled States of an Exchange-Coupled Dimer of Single-Molecule Magnets

An antiferromagnetically exchange-coupled dimer of single molecule magnets which possesses a large spin tunneling has been investigated. For this system the ground and first excited states are entangled states and the Hamiltonian is effectively similar to that of a two-state system at $2s^{\text{th}}$ order in perturbation theory, thus this system can be mapped to an entangled pseudospin 1/2 particles. We study the effects of interaction and rotation of this system about its staggered easy-axis direction . The corresponding Hamiltonian of a rotated two-state entangled spin system is derived with its exact low-energy eigenstates and eigenvalues. We briefly discuss the effect of a dissipative environment on this rotated two-state system.

cond-mat.mes-hall↗

arXiv2014

Quantum-classical transition of the escape rate of a biaxial ferromagnetic spin with an external magnetic field

We study the model of a biaxial single ferromagnetic spin Hamiltonian with an external magnetic field applied along the medium axis. The phase transition of the escape rate is investigated. Two different but equivalent methods are implemented. Firstly, we derive the semi-classical description of the model which yields a potential and a coordinate dependent mass. Secondly, we employ the method of spin-particle mapping which yields a similar potential to that of semi-classical description but with a constant mass. The exact instanton trajectory and its corresponding action, which have not been reported in any literature is being derived. Also, the analytical expressions for the first- and second-order crossover temperatures at the phase boundary are derived. We show that the boundary between the first-and the second-order phase transitions is greatly influenced by the magnetic field.

cond-mat.mes-hall↗