arXiv subjects

Mahroo Shiranzaei

Publications and source records attributed to Mahroo Shiranzaei.

8 recordsLinked to original sources

arXiv2023

Temperature-anisotropy conjugate magnon squeezing in antiferromagnets

Quantum squeezing is an essential asset in the field of quantum science and technology. In this study, we investigate the impact of temperature and anisotropy on squeezing of quantum fluctuations in two-mode magnon states within uniaxial antiferromagnetic materials. Through our analysis, we discover that the inherent nonlinearity in these bipartite magnon systems gives rise to a conjugate magnon squeezing effect across all energy eigenbasis states, driven by temperature and anisotropy. We show that temperature induces amplitude squeezing, whereas anisotropy leads to phase squeezing. In addition, we observe that the two-mode squeezing characteristic of magnon eigenenergy states is associated with amplitude squeezing. This highlights the constructive impact of temperature and the destructive impact of anisotropy on two-mode magnon squeezing. Nonetheless, our analysis shows that the destructive effect of anisotropy is bounded. We demonstrate this by showing that, at a given temperature, the squeezing of the momentum (phase) quadrature (or equivalently, the stretching of the position (amplitude) quadrature) approaches a constant function of anisotropy after a finite value of anisotropy. Moreover, our study demonstrates that higher magnon squeeze factors can be achieved at higher temperatures, smaller levels of anisotropy, and closer to the Brillouin zone center. All these characteristics are specific to low-energy magnons in the uniaxial antiferromagnetic materials that we examine here.

quant-ph↗

arXiv2023

The impact of defects on excitations in two-dimensional bipartite uniaxial antiferromagnet insulators

We address scatterings of spin waves off uncorrelated defects in two-dimensional (2D) easy-axis antiferromagnet (AFM) insulators. Although an onsite magnetic anisotropy leads gapped Goldstone modes, such that a long range (AFM) order can be established in 2D, lattice imperfections tend to weaken, and eventually destroy the magnetic ordering. Here, the impact of defects is considered within two limits, single and multiple defects. Using Green's function, we perform self consistent simulations to study magnon properties such as density of states and lifetime of induced resonances. Our findings show that repulsive defects decrease the magnon density of states while attractive ones may enhance it. We provide a comprehensive analysis of how defects can result in a reduction and even closing, the anisotropy induced gap, which weakens the long range (AFM) order parameter in the 2D state. We conclude that a small concentration of random defects can fill the gap of magnon spectrum.

cond-mat.mtrl-sci↗

arXiv2022

Thermal squeezing and nonlinear spectral shift of magnons in antiferromagnetic insulators

We investigate the effect of magnon-magnon interactions on the dispersion and polarization of magnons in collinear antiferromagnetic (AF) insulators at finite temperatures. In two-sublattice AF systems with either uniaxial or biaxial magnetocrystalline anisotropies, we implement a self-consistent Hartree-Fock mean-field approximation to explore the nonlinear thermal interactions. The resulting nonlinear magnon interactions separate into two-magnon intra- and interband scattering processes. Furthermore, we compute the temperature dependence of the magnon bandgap and AF resonance modes due to nonlinear magnon interactions for square and hexagonal lattices. In addition, we study the effect of magnon interactions on the polarization of magnon modes. We find that although the noninteracting eigenmodes in the uniaxial case are circularly polarized, but in the presence of nonlinear thermal interactions the local U(1) symmetry of the Hamiltonian is broken. The attractive nonlinear interactions squeeze the low-energy magnon modes and make them elliptical. In the biaxial case, on the other hand, the bare eigenmodes of low energy magnons are elliptically polarized but thermal nonlinear interactions squeeze them further. Direct measurements of the predicted temperature-dependent AF resonance modes and their polarization can be used as a tool to probe the nonlinear interactions. Our findings establish a framework for exploring the effect of thermal magnon interactions in technologically important magnetic systems, such as magnetic stability of recently discovered two-dimensional magnetic materials, coherent transport of magnons, Bose-Einstein condensation of magnons, and magnonic topological insulators.

cond-mat.mtrl-sci↗

arXiv2021

Monolayer CrCl$_3$ as an ideal Test Bed for the Universality Classes of 2D Magnetism

The monolayer halides CrX$_3$ (X=Cl, Br, I) attract significant attention for realizing 2D magnets with genuine long-range order (LRO), challenging the Mermin-Wagner theorem. Here, we show that monolayer CrCl$_3$ has the unique benefit of exhibiting tunable magnetic anisotropy upon applying a compressive strain. This opens the possibility to use CrCl$_3$ for producing and studying both ferromagnetic and antiferromagnetic 2D Ising-type LRO as well as the Berezinskii-Kosterlitz-Thouless (BKT) regime of 2D magnetism with quasi-LRO. Using state-of-the-art density functional theory, we explain how realistic compressive strain could be used to tune the monolayer's magnetic properties so that it could exhibit any of these phases. Building on large-scale quantum Monte Carlo simulations, we compute the phase diagram of strained CrCl$_3$, as well as the magnon spectrum with spin-wave theory. Our results highlight the eminent suitability of monolayer CrCl$_3$ to achieve very high BKT transition temperatures, around 50 K, due to their singular dependence on the weak easy-plane anisotropy of the material.

cond-mat.str-el↗

arXiv2020

Highly tunable magnetic coupling in ultrathin topological insulator films due to impurity resonances

We theoretically investigate the exchange interaction between magnetic impurities in ultrathin Bi$_2$Se$_3$ topological insulator films by taking into account the low-energy states produced by the impurities. We find that the locally induced impurity resonances strongly influence the exchange interaction between magnetic moments. In particular, we find a non-collinear alignment being more favorable than the collinear ferromagnetic alignment preferred when impurity states are ignored and only the pristine topological insulator band structure is considered. Moreover, we show that by applying of an electric field perpendicular to the ultrathin film, the exchange interaction can be drastically enhanced. This opens for the possibility of highly tunable magnetism by electric field.

cond-mat.mtrl-sci↗

arXiv2018

Non-Linear Spin Susceptibility in Topological Insulators

We theoretically study the effect of impurity resonances on the indirect exchange interaction between magnetic impurities in the surface states of a three-dimensional topological insulator. The interaction is composed of an isotropic Heisenberg, and anisotropic Ising and Dzyaloshinskii-Moriya contributions. We find that all three contributions are finite at the Dirac point, which is in stark contrast to the linear response theory which predicts a vanishing Dzyaloshinskii-Moriya contribution. We show that the spin-independent component of the impurity scattering can generate large values of the DM term in comparison with the Heisenberg and Ising terms, while these latter contributions drastically reduce in magnitude and undergo sign changes. As a result, both collinear and non-collinear configurations are allowed magnetic configurations of the impurities.

cond-mat.mes-hall↗

arXiv2017

Impurity scattering on the surface of topological insulator thin films

We address the electronic structure of the surface states of topological insulator thin films with embedded local non-magnetic and magnetic impurities. Using the $T$-matrix expansion of the real space Green's function, we derive the local density of electrons states and corresponding spin resolved densities. We show that the effects of the impurities can be tuned by applying an electric field between the surface layers. The emerging magnetic states are expected to play an important role both in ferromagnetic mechanism of magnetic topological insulators as well as in its transport properties. In the case of magnetic impurities, we have categorized the possible cases for different spin-directions of the impurities as well as the spin-direction in which the spin resolved density of electron states is calculated and related this to the spin susceptibility of the system.

cond-mat.mes-hall↗

arXiv2017

Effect of Rashba splitting on RKKY interaction in topological insulator thin films

In this work we have investigated the effect of Rashba splitting on the RKKY interaction in TI thin film both at finite and zero chemical potential. We find that the spin susceptibility of Rashba materials including TI thin film is strongly dependent on the direction of distance vector. Moreover, we find another term in the off-diagonal terms of the spin-susceptibility tensor which in contrast to the well-known DM-like term is symmetric. We show how one can tune the RKKY interaction by using electric field applied perpendicularly to the surface plane and by small chemical doping giving rise to enhance the RKKY term, drastically. We have presented our results for two different situations, namely inter-surface pairing of magnetic impurities as well as intra-surface one. The behavior of these two situations is completely different which we describe it by mapping the density of states of each surface on the band dispersion.

cond-mat.mes-hall↗