Sharif Digital Repository

We study an S = 1/2 Heisenberg model on the honeycomb lattice with first and second neighbor antiferromagnetic exchange (J1-J2 model), employing exact diagonalization in both the Sz = 0 basis and nearest neighbor singlet valence bond (NNVB) basis. We find that for 0.2 < J2/J1 < 0.3, the NNVB basis gives a proper description of the ground state in comparison with the exact results. By analyzing the dimer-dimer as well as the plaquette-plaquette correlations and also defining appropriate structure factors, we investigate possible symmetry breaking states as the candidates for the ground state in the frustrated region. We provide a body of evidence in favor of plaquette valence bond ordering... 

We present a nonlocal spin transport theory for the coupled dynamics of magnetization and lattice vibrations in antiferromagnetic insulators. We find that magnon-polaron formation, i.e., coherently hybridized magnon and acoustic phonon modes, not only leads to anomalous features in the nonlocal spin current but also renormalizes the spin-flop transition field of the antiferromagnets. A length scale for the magnon-polaron formation below which the spin current is not affected by the lattice is also extracted from this nonlocal setup. © 2021 American Physical Society  

We have investigated the zero-and finite-temperature behaviors of the anisotropic antiferromagnetic Heisenberg XXZ spin-1/2 chain in the presence of a transverse magnetic field (h). The attention is concentrated on an interval of magnetic field between the factorizing field (hf) and the critical one (hc). The model presents a spin-flop phase for 0 < h < hf with an energy scale which is defined by the long range antiferromagnetic order while it undergoes an entanglement phase transition at h = hf. The entanglement estimators clearly show that the entanglement is lost exactly at h = hf, which justifies different quantum correlations on both sides of the factorizing field. As a consequence of... 

We study the effect of thermal fluctuations in a recently proposed protocol for transmission of unknown quantum states through quantum spin chains. We develop a low-temperature expansion for general spin chains. We then apply this formalism to study exactly thermal effects on short spin chains of four spins. We show that optimal times for extraction of output states are almost independent of the temperature, which lowers only the fidelity of the channel. Moreover we show that thermal effects are smaller in the antiferromagnetic chains than the ferromagnetic ones. © 2005 The American Physical Society  

We have numerically studied the thermodynamic properties of the spin 1 2 XXZ chain in the presence of a transverse (noncommuting) magnetic field. The thermal, field dependence of specific heat and correlation functions for chains up to 20 sites has been calculated. The area where the specific heat decays exponentially is considered as a measure of the energy gap. We have also obtained the exchange interaction between chains in a bulk material using the random phase approximation and derived the phase diagram of the three-dimensional material with this approximation. The behavior of the structure factor at different momenta verifies the antiferromagnetic long-range order in the y direction... 

We determine thermal entanglement in mean-field clusters of N spin one-half particles interacting via the anisotropic Heisenberg interaction, with and without external magnetic field. For the xxx cluster in the absence of magnetic field we prove that only the N=2 ferromagnetic cluster shows entanglement. An external magnetic field B can only entangle xxx antiferromagnetic clusters in certain regions of the B-T plane. On the other hand, the xxz clusters of size N>2 are entangled only when the interaction is ferromagnetic. Detailed dependence of the entanglement on various parameters is investigated in each case. © 2006 The American Physical Society  

The emergence of phase separation is investigated in the framework of a 2D t-J model by means of a variational product ansatz, which covers the infinite lattice by two types of L × L clusters. Clusters of the first type are completely occupied with electrons, i.e. they carry maximal charge Qe = L2 and total spin 0, and thereby form the antiferromagnetic background. Holes occur in the second type of clusters - called 'hole clusters'. They carry a charge Qh0.5) it turns out that hole clusters are occupied with an even... 

We investigate systematically the spin-Nernst effect in Néel and zigzag ordered honeycomb antiferromagnets. Monolayers of transition-metal trichalcogenides, MnPSe3, MnPS3, and VPS3 show an antiferromagnetic Néel order while CrSiTe3, NiPS3, and NiPSe3 show an antiferromagnetic zigzag order. We extract the exchange and Dzyaloshinskii-Moriya interaction parameters from ab initio calculations. Using these parameters, we predict that the spin-Nernst coefficient is at least two orders of magnitude larger in zigzag compared to the Néel ordered antiferromagnets. We find that this enhancement relies on the large band splitting due to the symmetry of magnetic configuration in the zigzag order. Our... 

We have studied the effect of both axial and transverse anisotropy on the critical field and thermodynamic properties of the field induced three dimensional antiferromagnetic Heisenberg model on the frustrated hexagonal lattice for Cs 2CuCl 4 compound. The spin model is mapped to a bosonic one with the hard core repulsion constraint and the Green's function approach has been implemented to get the low energy spectrum and the corresponding thermodynamic properties. To find the critical field (B c ) we have looked for the Bose-Einstein condensation of quasi-particles (magnons) which takes place when the magnon spectrum vanishes at the ordering spiral wave vector. We have also obtained the... 

In this paper, we have studied the single-and two-channel Kondo lattice model consisting of localized spins interacting anti-ferromagnetically with itinerant electrons. We have employed the dynamical mean field theory and have used the exact diagonalization method in our impurity solver. This method allowed us to access low temperatures and large values of exchange couplings. Our results for the single-channel case confirm and extend the recent investigations. In the two-channel case, at absolute zero temperature for half-filling and for the exchange couplings J/t>0.8, we have found a spontaneous symmetry breaking quantum phase transition, with one band showing a metallic behavior and the... 

We have studied the electrical and magnetic properties of RuGd 2-xCexSr2Cu2O10-δ (Ru-1222) with content of Ce = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, prepared by the standard solid-state reaction technique. To determine how the magnetic and superconducting properties of this layered cuprate system can be affected by Ce substitution, the resistivity and magnetoresistivity have been measured at various temperatures. Resistivity curves reveal the presence of four regions for various contents of x. In high temperatures, system is paramagnetic. Magnetic transition from paramagnetic state to antiferromagnetic state occurs at 140 K for x = 0, which varies very slowly with increasing Ce substitution for Gd.... 

Using tight binding band picture for 2D graphite, and the Hubbard interaction, recently we obtained a gapless, neutral spin-1 collective mode branch in graphite [Phys. Rev. Lett. 89, 016402], In this paper we present a detailed RPA analysis of the Neutron Scattering cross section for this collective mode. Near K-point and very close to Γ-point, the intensity of neutron scattering peaks vanishes as q3. This is shown using a simple Dirac cone model for the graphite band structure, which captures the small-q behavior of the system. As we move away from the Γ- and Κ-points in the Brillouin zone of the collective mode momenta, we can identify our collective mode quanta with spin triplet excitons... 

We present experimental data for artificial metaconductors exhibiting skin effect suppression at microwave frequencies. The metaconductor consists of a stack comprising twelve periods of alternating ferromagnetic (Permalloy) and normal metal (Cu) layers. Near the effective antiferromagnetic resonant frequency the average in-plane magnetic permeability of the stack approaches zero, leading to an increase in the skin depth. Compared to a Cu-based device, up to 70% loss reduction has been achieved by a metaconductor based coplanar wave guide at ∼ 10 GHz without changing the propagation wavelength. Moreover, unlike conventional magnetic devices, no external magnetic bias is required due to the... 

We have calculated the electronic structure of SrRu1-xMn xO3 using the full potential linearized augmented plane wave method by LSDA and LSDAU. The antiparallel alignment between the Mn and Ru ions are consistent with the competition between ferromagnetism and antiferromagnetism in the low Mn-doped polycrystalline samples. This is in contrast to the appearance of quantum critical point and FM and AFM transitions in the single crystal measurement. Our results show that the discrepancy between different experimental phase diagrams is related to the conditions of sample preparation and also the difference between the degree of magnetic interactions between the Mn and Ru moments. The DOS and the... 

Ising models in nanosystems are studied in the presence of a magnetic field. For a one-dimensional (1-D) array of spins interacting via nearest-neighbour and next-nearest-neighbour interactions we calculate the heat capacity, the surface energy, the finite-size free energy and the bulk free energy per site. The heat capacity versus temperature exhibits a common wide peak for systems of any size. A small peak also appears at lower temperatures for small arrays when the ratio of magnetic field spin interaction energy over the nearest-neighbour spin-spin interaction energy, f, is within 0 < F ≤ 0.10 . The peak becomes smaller for longer array and eventually vanishes for long arrays,... 

We have investigated the electronic structure of Sr(Ca)Ru1-xCrxO3 using the full potential linearized augmented plane wave method by different approximation such as LSDA and LSDA+U. The LSDA calculation suggest that Cr4+-Ru4+ hybridization is responsible for the high Curie temperature TC in SrRu1-xCrxO3, but it cannot completely describe its physical behavior. Our LSDA+U DOS results for SrRu1-xCrxO3 clearly establishes renormalization of the intra-atomic exchange strength at the Ru sites, arising from the Cr-Ru hybridization. The antiferromagnetic coupling of Cr3+ with Ru4+,5+ lattice increases the screening, which is consistent with the low magnetic moment of the Ru ions. The more distorted... 

We study the thermal transport of a spin-1/2 two leg antiferromagnetic ladder in the direction of legs. The possible effect of spin-orbit coupling and crystalline electric field are investigated in terms of anisotropies in the Heisenberg interactions on both leg and rung couplings. The original spin ladder is mapped to a bosonic model via a bond-operator transformation, where an infinite hard-core repulsion is imposed to constrain one boson occupation per site. The Green's function approach is applied to obtain the energy spectrum of quasi-particle excitations responsible for thermal transport. The thermal conductivity is found to be monotonically decreasing with temperature due to increased... 

We study the zero-Temperature phase diagram of transverse field Ising model on the J 1-J 2 square lattice. In zero magnetic field, the model has a classical Néel phase for J 2/J 1 < 0.5 and an antiferromagnetic collinear phase for J 2/J 1 > 0.5. We incorporate harmonic fluctuations by using linear spin wave theory (LSWT) with single spin flip excitations above a magnetic order background and obtain the phase diagram of the model in this approximation. We find that harmonic quantum fluctuations of LSWT fail to lift the large degeneracy at J 2/J 1 = 0.5 and exhibit some inconsistent regions on the phase diagram. However, we show that anharmonic fluctuations of cluster operator approach (COA)... 

In this work, the magnetic and transport properties of micro and nanometer-sized samples of La0.9Ba0.1MnO3 manganite have been studied. The temperature variation of ac susceptibility of the nanometer-sized sample shows one transition at high temperature (265 K). On the contrary, the ac susceptibility of the micrometer-sized sample shows two transitions with a high-temperature transition occurring at 240 K and low-temperature transition around 100 K. The high-temperature transition corresponds to the paramagnetic-ferromagnetic (PM–FM) transition (Tc) and is independent of frequency, while the low-temperature transition is frequency-dependent and shifts toward high temperatures by increasing... 

An investigation has been carried out on the effects of praseodymium doping in Sm3−xPrxBa5Cu8O19 for different values of x. Results indicate that the transition temperature reduces almost linearly with x. Based on the Abrikosov–Gor’kov pair-breaking theory, it has been concluded that the exchange interaction constant is large (Jex = 110 meV) in the Sm3Ba5Cu8O19 compound. In addition, it seems that the hybridization of Pr4f states with O2p–Cu3d states has an important role in the depression of transition temperature of Sm3−xPrxBa5Cu8O19 for the Pr content < 10%. However, for the higher values of Pr, the two observed plateaus at the transition temperatures of ~ 60 K and ~ 17 K, respectively,...