Showing papers on "Exchange interaction published in 2003"
TL;DR: The unusual temperature dependence of the magnetization coercivity is discussed in terms of a temperature-dependent exchange interaction involving paramagnetic Ni2+ ions.
Abstract: Ferromagnetism with T c > 350 K is observed in the diluted magnetic semiconductor Ni 2 + :ZnO synthesized from solution. Whereas colloidal Ni 2 + :ZnO nanocrystals are paramagnetic, their aggregation gives rise to robust ferromagnetism. The appearance of ferromagnetism is attributed to the increase in domain volumes and the generation of lattice defects upon aggregation. The unusual temperature dependence of the magnetization coercivity is discussed in terms of a temperature-dependent exchange interaction involving paramagnetic Ni 2 + ions.
414 citations
TL;DR: In this paper, the authors focused on such exchange coupled polymetallic, both homo- and hetero-metallic systems containing the bridging core MNOM′ with an aim to delineate the exchange mechanism through bridging oximates, and confirmed the essentially σ nature of the exchange interaction transmitted through the diatomic NO-bridging ligand.
346 citations
TL;DR: The nature and origin of ferromagnetism in magnetic semiconductors is investigated by means of highly precise electronic and magnetic property calculations on MnxGe1-x as a function of the location of Mn sites in a large supercell, finding that the coupling is not always ferromagnetic (FM), even for large Mn-Mn distances.
Abstract: The nature and origin of ferromagnetism in magnetic semiconductors is investigated by means of highly precise electronic and magnetic property calculations on MnxGe1-x as a function of the location of Mn sites in a large supercell. Surprisingly, the coupling is not always ferromagnetic (FM), even for large Mn-Mn distances. The exchange interaction between Mn ions oscillates as a function of the distance between them and obeys the Ruderman-Kittel-Kasuya-Yosida analytic formula. The estimated Curie temperature is in good agreement with recent experiments, and the estimated effective magnetic moment is about 1.7mu(B)/Mn, in excellent agreement with the experimental values, (1.4-1.9)mu(B)/Mn.
143 citations
TL;DR: In this article, the effect of varying Mn concentration on the electronic and magnetic properties of Mn-doped GaN was studied by a first-principles plane-wave method.
Abstract: Mn-doped semiconductors are extremely interesting systems due to their novel magnetic properties suitable for the spintronics applications. It has been shown recently by both theory and experiment that Mn-doped GaN systems have a very high Curie temperature compared to that of Mn-doped GaAs systems. To understand the electronic and magnetic properties, we have studied Mn-doped GaN system in detail by a first-principles plane-wave method. We show here the effect of varying Mn concentration on the electronic and magnetic properties. In agreement with previous studies, d states of Mn form an impurity band completely separated from the valence-band states of the host GaN for dilute Mn concentration. This is in contrast to the Mn-doped GaAs system where Mn d states in the gap lie very close to the valence-band edge and hybridize strongly with the delocalized valence-band states. To study the effects of electron correlation, LSDA+U calculations have been performed. Calculated exchange interaction in (Mn,Ga)N is short ranged contrary to that in (Mn,Ga)As where the strength of the ferromagnetic coupling between Mn spins is not decreased substantially for large Mn-Mn separation. Also, the exchange interactions are anisotropic in different crystallographic directions due to the presence or absence of connectivity between Mn atoms through As bonds.
129 citations
TL;DR: In this paper, the levels of the 4fn−15d configuration of Ce3+, Pr3+ and Tb3+ in compounds are compared with each other and two main sources of deviations from the shift model are identified.
Abstract: The levels of the 4fn−15d configuration of Ce3+(n = 1), Pr3+(n = 2), and Tb3+(n = 8) in compounds are compared with each other. A model is presented that, by means of energy shift operations performed on the five 5d levels of Ce3+, reproduces the energies of those for Pr3+ and Tb3+. Using Tb3+ data, two main sources of deviations from the shift model are identified. One is related to the size difference between Tb3+ and Ce3+ which affects the lattice relaxation and the crystal field splitting of the 5d configuration. The other is related to the isotropic exchange interaction between the 5d electron spin and the total spin of the 4f7 electrons in Tb3+. The exchange splitting is about 1 eV in fluorides, sulfates, and phosphates. In oxides with less strongly bonded oxygen 2p electrons, the exchange splitting decreases to 0.6 eV. The effects of the two deviations on the predictability of the 4fn−15d energy levels of Tb3+ and other lanthanides are discussed.
128 citations
TL;DR: In this article, the authors derived the dissipative torque produced by the $s\ensuremath{-}d$ exchange interaction at the ferromagnet--normal-metal interface.
Abstract: We study the enhancement of the ferromagnetic relaxation rate in thin films due to the adjacent normal-metal layers. Using linear-response theory, we derive the dissipative torque produced by the $s\ensuremath{-}d$ exchange interaction at the ferromagnet--normal-metal interface. For a slow precession, the enhancement of the Gilbert damping constant is proportional to the square of the $s\ensuremath{-}d$ exchange constant times the zero-frequency limit of the frequency derivative of the local dynamic spin susceptibility of the normal metal at the interface. Electron-electron interactions increase the relaxation rate by the Stoner factor squared. We attribute the large anisotropic enhancements of the relaxation rate observed recently in multilayers containing palladium to this mechanism. For free electrons, the present theory compares favorably with recent spin-pumping results of Tserkovnyak et al. [Phys. Rev. Lett. $88,$ 117601 (2002)].
121 citations
TL;DR: By measuring splittings from the singly charged and doubly charged biexcitons, all on the same quantum dot, it is shown how the various electron-hole exchange energies can be measured without having to break the symmetry of the dot.
Abstract: An exciton in a symmetric semiconductor quantum dot has two possible states, one dark and one bright, split in energy by the electron-hole exchange interaction. We demonstrate that for a doubly charged exciton, there are also two states split by the electron-hole exchange, but both states are now bright. We also uncover a fine structure in the emission from the triply charged exciton. By measuring these splittings, and also those from the singly charged and doubly charged biexcitons, all on the same quantum dot, we show how the various electron-hole exchange energies can be measured without having to break the symmetry of the dot.
108 citations
TL;DR: In this article, the optical conductivity of LuMnO3 was measured at temperatures between 4 and 300 K, and it was shown that a symmetry allowed on-site Mn d-d transition near 1.7 eV is observed to blueshift (approximately 0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions.
Abstract: We have measured the optical conductivity of single crystal LuMnO3 from 10 to 45000 cm(-1) at temperatures between 4 and 300 K. A symmetry allowed on-site Mn d-d transition near 1.7 eV is observed to blueshift ( approximately 0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in the temperature dependence of the quasistatic dielectric constant epsilon(0) below T(N) approximately 90 K is overwhelmingly dominated by the phonon contributions.
101 citations
TL;DR: A "renormalized magnetic force theorem" is presented that allows one to obtain results from a non-self-consistent calculation of the (single-electron) band energy only, which greatly reduces the computational effort.
Abstract: The ``magnetic force theorem'' is frequently used to compute exchange interaction parameters and adiabatic spin-wave spectra of ferromagnets. The interest of this approach is that it allows one to obtain these results from a non-self-consistent calculation of the (single-electron) band energy only, which greatly reduces the computational effort. However, as discussed by various authors, this approach entails some systematic error. Here, a ``renormalized magnetic force theorem'' allows us to remove this systematic error without significantly increasing the computational effort involved. For systems with one atom per unit cell, it amounts to a simple renormalization of the spin-wave spectrum.
98 citations
TL;DR: In this paper, the donor electron exchange coupling as a function of donor position incorporating the full Bloch structure of the Kohn-Luttinger electron wave functions was calculated and the expected exchange coupling between neighboring donors that have been implanted into the silicon substrate using an 15 keV ion beam in the so-called top-down fabrication scheme for a Kane solid-state quantum computer.
Abstract: Intervalley interference between degenerate conduction band minima has been shown to lead to oscillations in the exchange energy between neighboring phosphorus donor electron states in silicon [B. Koiller, X. Hu, and S. Das Sarma, Phys. Rev. Lett. 88, 027903 (2002); Phys. Rev. B 66, 115201 (2002)]. These same effects lead to an extreme sensitivity of the exchange energy on the relative orientation of the donor atoms, an issue of crucial importance in the construction of silicon-based spin quantum computers. In this article we calculate the donor electron exchange coupling as a function of donor position incorporating the full Bloch structure of the Kohn-Luttinger electron wave functions. It is found that due to the rapidly oscillating nature of the terms they produce, the periodic part of the Bloch functions can be safely ignored in the Heitler-London integrals as was done by Koiller, Hu, and Das Sarma, significantly reducing the complexity of calculations. We address issues of fabrication and calculate the expected exchange coupling between neighboring donors that have been implanted into the silicon substrate using an 15 keV ion beam in the so-called top down fabrication scheme for a Kane solid-state quantum computer. In addition, we calculate the exchange coupling as a function of the voltage bias on control gates used to manipulate the electron wave functions and implement quantum logic operations in the Kane proposal, and find that these gate biases can be used to both increase and decrease the magnitude of the exchange coupling between neighboring donor electrons. The zero-bias results reconfirm those previously obtained by Koiller, Hu, and Das Sarma.
84 citations
TL;DR: In this article, the bias shift and coercivity are controlled by domain wall formation between exchange-coupled grains in the antiferromagnetic bilayers, which leads naturally to a training effect during magnetization loop cycling.
Abstract: A mechanism for exchange bias and training for antiferromagnet/ferromagnet bilayers with fully compensated interfaces is proposed. In this model, the bias shift and coercivity are controlled by domain-wall formation between exchange-coupled grains in the antiferromagnet. A finite element micromagnetic calculation is used to show that a weak exchange interaction between randomly oriented antiferromagnetic grains and spin-flop coupling at a perfectly compensated interface are sufficient to create shifted hysteresis loops characteristic of exchange bias. Unlike previous partial wall models, the energy associated with the unidirectional anisotropy is stored in lateral domain walls located between antiferromagnetic grains. We also show that the mechanism leads naturally to a training effect during magnetization loop cycling.
TL;DR: The microscopic theory of the magnetically anisotropic effective pseudo-spin-(1)/(2) Hamiltonian for a pair of Co(2+) ions is reported and analytical expressions are found for the components of the tensor of the exchange interaction, g-tensor, and the factor in the temperature independent paramagnetic contribution.
Abstract: The microscopic theory of the magnetically anisotropic effective pseudo-spin-1/2 Hamiltonian for a pair of Co2+ ions is reported. In the framework of the second-order perturbation approach, the analytical expressions are found for the components of the tensor of the exchange interaction, g-tensor, and the factor in the temperature independent paramagnetic contribution. The parameters of the Hamiltonian are expressed in terms of the basic intra- and intercenter parameters of the pair, namely, the spin−orbit coupling constant, orbital reduction factor, exchange integral, and low symmetry crystal field parameters including axial and rhombic terms.
TL;DR: In this article, the long-range ferromagnetic ordering induces a drastic expansion of the lattice and the coexistence of the large and small volume phases near the Curie temperature.
Abstract: Crystallographic and magnetic structures of the cubic NaZn13-type intermetallic compound LaFe11.4Si1.6 have been studied by means of powder neutron diffraction. Rietveld analysis indicates that Si atoms substitute for Fe atoms randomly on two different Fe sites. All spins in the unit cell are aligned ferromagnetically with the FeI?(8b) moment smaller than the FeII?(96i) one. The long-range ferromagnetic ordering induces a drastic expansion of the lattice and the coexistence of the large and small volume phases near the Curie temperature. Even in the ferromagnetic state, the lattice expansion still correlates strongly with the spontaneous magnetic moment, marked by a large positive magnetovolume coupling constant kC = 1.14 ? 10?8 cm6?emu?2. From the temperature dependence of Fe?Fe bond lengths, we suggest that the Fe?Fe exchange interaction between the clusters (each formed by a central FeI atom and 12 surrounding FeII atoms) plays an important role in the magnetic properties of La(Fe1?xAl/Six)13, as does that within the clusters.
TL;DR: In this paper, the spectral properties of hexagonal HoMnO$_3$ single crystals in the temperature range 10-300 K were analyzed and scattering selection rules for the second order scattering processes were presented.
Abstract: Polarized Raman scattering and infrared reflection spectra of hexagonal HoMnO$_3$ single crystals in the temperature range 10-300 K are reported. Group-theoretical analysis is performed and scattering selection rules for the second order scattering processes are presented. Based on the results of lattice dynamics calculations, performed within the shell model, the observed lines in the spectra are assigned to definite lattice vibrations. The magnetic ordering of Mn ions, which occurs below T$_N$=76 K, is shown to effect both Raman- and infrared-active phonons, which modulate Mn-O-Mn bonds and, consequently, Mn exchange interaction.
TL;DR: In this article, a theory of the fine structure of the excited state of the quantum-dot trion, which is due to spin-spin interactions, is presented, and it is shown that the fine structures are universal for both positive (two holes, one electron) and negative (two electrons, one hole) trions.
Abstract: A theory of the fine structure of the excited state of the quantum-dot trion, which is due to spin-spin interactions, is presented. In particular, it is shown that the fine structure is universal for both positive (two holes, one electron) and negative (two electrons, one hole) trions. It consists of four doublets, three of them forming a compact group of levels separated from the remaining doublet by the energy of the exchange interaction between identical particles (i.e. between two electrons or two holes). The energy separation between the adjacent doublets in the six-level group is of the order of the much smaller energy of the exchange interaction between particles belonging to different bands, i.e. of the exciton exchange energy.
TL;DR: In this article, the integrability of one-dimensional classical continuum inhomogeneous biquadratic Heisenberg spin chain and the effect of nonlinear inhomogeneity on the soliton of an underlying completely integrable spin model are studied.
Abstract: The integrability of one-dimensional classical continuum inhomogeneous biquadratic Heisenberg spin chain and the effect of nonlinear inhomogeneity on the soliton of an underlying completely integrable spin model are studied. The dynamics of the spin system is expressed in terms of a higher order generalized nonlinear Schrodinger equation through a differential geometric approach which becomes integrable for a particular choice of the biquadratic exchange interaction and for linear inhomogeneity. The effect of nonlinear inhomogeneity on the spin soliton is studied by carrying out a multiple scale perturbation analysis.
TL;DR: In this article, the authors present theoretical results for a Kondo-lattice model with spin-$1/2$ localized moments, including both the intrasite Kondo coupling and an inter-site antiferromagnetic exchange interaction, treated within an extended mean-field approximation.
Abstract: We present theoretical results for a Kondo-lattice model with spin-$1/2$ localized moments, including both the intrasite Kondo coupling and an intersite antiferromagnetic exchange interaction, treated within an extended mean-field approximation. We describe here the case of a noninteger conduction-band filling for which an ``exhaustion'' problem arises when the number of conduction electrons is not large enough to screen all the lattice spins. This is best seen in the computed magnetic susceptibility. The Kondo temperature so obtained is different from the single-impurity one, and increases for small values of the intersite interaction, but the Kondo-effect disappears abruptly for low band filling and/or strong intersite coupling; a phase diagram is presented as a function of both parameters. A discussion of experimental results on cerium Kondo compounds is also given.
TL;DR: In this paper, the authors studied the relaxation of the exciton spin in single asymmetrical quantum dots due to an interplay of the short-range exchange interaction and acoustic-phonon deformation.
Abstract: We study the relaxation of the exciton spin (longitudinal relaxation time ${T}_{1})$ in single asymmetrical quantum dots due to an interplay of the short--range exchange interaction and acoustic-phonon deformation. The calculated relaxation rates are found to depend strongly on the dot size, magnetic field, and temperature. For typical quantum dots and temperatures below 100 K, the zero-magnetic field relaxation times are long compared to the exciton lifetime, yet they are strongly reduced in high magnetic fields. We discuss explicitly quantum dots based on (In,Ga)As and (Cd,Zn)Se semiconductor compounds.
TL;DR: It is demonstrated that for a weak exchange interaction the new structure, the quasiferromagnetic decagonal configuration, corresponds to the minimum of the free energy.
Abstract: The stable magnetization configurations of a ferromagnet on a quasiperiodic tiling have been derived theoretically. The magnetization configuration is investigated as a function of the ratio of the exchange to the dipolar energy. The exchange coupling is assumed to decrease exponentially with the distance between magnetic moments. It is demonstrated that for a weak exchange interaction the new structure, the quasiferromagnetic decagonal configuration, corresponds to the minimum of the free energy. The decagonal state represents a new class of frustrated systems where the degenerated ground state is aperiodic and consists of two parts: ordered decagon rings and disordered spin-glass-like phase inside the decagons.
TL;DR: In this article, the spin interaction and the effective g factor of a magnetic exciton (ME) were investigated theoretically in a diluted magnetic semiconductor (DMS) quantum dot (QD), including the Coulomb interaction and sp-d exchange interaction.
Abstract: The spin interaction and the effective g factor of a magnetic exciton (ME) are investigated theoretically in a diluted magnetic semiconductor (DMS) quantum dot (QD), including the Coulomb interaction and the sp-d exchange interaction. At low magnetic field, the ME energy decreases rapidly with increasing magnetic field and saturates at high magnetic field for high Mn concentration. The ground state of the ME exhibits an interesting crossing behavior between sigma(+)-ME and sigma(-)-ME for low Mn concentration. The g(ex) factor of the ME in a DMS QD displays a monotonic decrease with increasing magnetic field and can be tuned to zero by an external magnetic field. (C) 2003 American Institute of Physics.
TL;DR: In this paper, the authors examined the magnetic properties of silicon quantum dots and found that exchange interaction hardly works between electrons in different valleys, and that low-spin states are easily realized by applying a small magnetic field.
Abstract: Electronic states in silicon quantum dots are examined theoretically, taking into account a multivalley structure of the conduction band. We find that (i) exchange interaction hardly works between electrons in different valleys. In consequence electrons occupy the lowest level in different valleys in the absence of Hund's coupling when the dot size is less than 10 nm. High-spin states are easily realized by applying a small magnetic field. (ii) When the dot size is much larger, the electron-electron interaction becomes relevant in determining the electronic states. Electrons are accommodated in a valley, making the highest spin, to gain the exchange energy. (iii) In the presence of intervalley scattering, degenerate levels in different valleys are split. This could result in low-spin states. These spin states in multivalley artificial atoms can be observed by looking at the magnetic-field dependence of peak positions in the Coulomb oscillation.
TL;DR: In this article, a modified kinetic exchange theory is adapted to a realistic description of the electronic structure of lanthanide ions in solids, and the general procedure of the calculation of the 4f-3d anisotropic exchange spin Hamiltonian is presented and applied to the YbcrBr 9 3-dimer.
Abstract: The superexchange interaction between Yb 3 + and Cr 3 + ions in the mixed YbCrBr 9 3- bioctahedral face-sharing dimer is quantitatively analyzed using a modified kinetic exchange theory, which is adapted to a realistic description of the electronic structure of lanthanide ions in solids. The general procedure of the calculation of the 4f-3d anisotropic exchange spin Hamiltonian is presented and applied to the YbcrBr 9 3-dimer. The spin-Hamiltonian of the Yb 3 + -Cr 3 + exchange interaction is found to be extremely anisotropic, H=J z S z Y b S z C r + J⊥(S x Y b S x C r +S y Y b S y C r ), with the antiferromagnetic J z and ferromagnetic J⊥parameters, where S μ Y b and S μ C r (μ = x,y,z) are the components of the effective spin S Y b = ½ of the Yb 3 + ion (corresponding to the ground Γ 6 Kramers doublet) and the true spin S C r = 3/2 of the Cr 3 + ion, respectively. The calculated exchange parameters are quite consistent with the experimental data (J z = -5.16 cm - 1 and J⊥ = +4.19 cm - 1 ) at reasonable values of the Yb→Cr and Yb→Cr charge transfer energies. The contributions to the J z and J⊥ exchange parameters from the individual states of the 4f 1 2 -3d 4 and 4 f 1 4 -3d 2 charge transfer configurations are analyzed in detail and general regularities are established. Our results indicate that a very strong 4f-3d exchange anisotropy can appear even in the absence of the crystal-field anisotropy on the lanthanide ion.
TL;DR: In this paper, the authors studied transport through a lateral quantum dot in the vicinity of the singlet-triplet transition in its ground state and found that the linear conductance has a maximum in the crossover region.
Abstract: We study transport through a lateral quantum dot in the vicinity of the singlet-triplet transition in its ground state. This transition, being sharp in an isolated dot, is broadened to a crossover by the exchange interaction of the dot electrons with the conduction electrons in the leads. For a generic set of system parameters, the linear conductance has a maximum in the crossover region. At zero temperature and magnetic field, the maximum is the strongest. It becomes less pronounced at finite Zeeman splitting, which leads to an increase of the background conductance and a decrease of the conductance in the maximum.
TL;DR: In this paper, a microscopic model for electron scattering on both neutral and charged excitons in quantum wells is presented, taking into account both elastic and dissociating scattering. And the model is based on calculating the exciton-electron direct and exchange interaction matrix elements.
Abstract: Electron scattering on both neutral (X) and charged ${(X}^{\ensuremath{-}})$ excitons in quantum wells is studied theoretically. A microscopic model is presented, taking into account both elastic and dissociating scattering. The model is based on calculating the exciton-electron direct and exchange interaction matrix elements, from which we derive the exciton scattering rates. Scattering by electrons is found to be an efficient process even for very low electron densities. In particular, the charged exciton linewidth due to electron scattering is larger than that of the neutral exciton, partially because of the larger contribution from the dissociating process. Calculated reflection spectra are then obtained by considering the three electronic excitations of the system, namely, the heavy-hole and light-hole $1S$ neutral excitons, and the heavy-hole $1S$ charged exciton, with the appropriate oscillator strengths.
TL;DR: In this paper, the low energy effective Hamiltonian (LEH) was derived for a typical quasi-one-dimensional Ising-like antiferromagnet CsCoCl 3, where second-order processes via crystal-field excited states were also taken into account.
Abstract: The low-energy effective Hamiltonian is derived for a typical quasi-one-dimensional Ising-like antiferromagnet CsCoCl 3 . In contrast to previous treatments, in which only the processes within the crystal-field ground Kramers doublet are considered, second-order processes via crystal-field excited states are also taken into account in the present paper. They result in a new term, which is essentially a next-nearest neighbor ferromagnetic exchange whose anisotropy is different from the nearest-neighbor exchange. By including the new term the magnetization curve and ESR in CsCoCl 3 are analyzed to show that those experiments can be explained in a way consistent with other experiments. Related theories on CsCoCl 3 are discussed in this connection.
TL;DR: In this paper, the local density of states (LDOS) of electron systems belonging to the bulk conduction band of InAs was investigated at T = 6 K and B = 6 T.
Abstract: Scanning tunneling spectroscopy at T = 6 K is used to investigate the local density of states (LDOS) of electron systems belonging to the bulk conduction band of InAs. In particular, the three-dimensional electron system (3DES) of the n-doped material, an adsorbate-induced two-dimensional electron system (2DES) and the tip-induced quantum dot (0DES) are investigated at B = 0 T and B = 6 T. It is found that the 3DES at B = 0 T can be described by Bloch states weakly interacting with the potential disorder provided by ionized dopants. The 2DES at B = 0 T exhibits much stronger LDOS corrugations, stressing the tendency for weak localization in the potential disorder. In a magnetic field, 3DES and 2DES show drift states, which are expected in 2D, but are surprising in 3D, where they point to a new electron phase consisting of droplets of quasi-2D systems. The 0DES at B = 0 T reveals quantized states in accordance with Hartree calculations. At B = 6 T it exhibits Landau states with exchange enhanced spin splitting. These states are used to investigate the influence of potential disorder on the exchange enhancement, which visualizes the nonlocality of the exchange interaction.
TL;DR: This work derives a closed expression for the linear conductance through a quantum dot in the Coulomb-blockade regime in the presence of a constant exchange interaction that finds significantly better agreement with experimental data as compared with the statistics obtained in the absence of exchange.
Abstract: We derive a closed expression for the linear conductance through a quantum dot in the Coulomb-blockade regime in the presence of a constant exchange interaction. With this expression we calculate the temperature dependence of the conductance peak-height and peak-spacing statistics in chaotic quantum dots. Using a realistic value of the exchange interaction, we find significantly better agreement with experimental data as compared with the statistics obtained in the absence of exchange.
TL;DR: In this article, the dependence of the interaction energy and its components on small geometrical modifications around the minima have been examined, and the electrostatic and the exchange energies have been found to be the most important components of the overall interaction energy, although the dispersion and the induction energies also play important roles.
Abstract: Thirty-two nucleic acid hydrogen-bonded base pairs have been examined using second-order Miller-Plesset perturbation theory (MP2) and the 6-31G*(0.25) and modified aug-cc-pVDZ basis sets. Complexes of C s symmetry have been constructed from rigid monomers which allowed us to use a compact parametrization of their geometry based on the center of mass separation and Euler angles. The dependence of the interaction energy and its components on small geometrical modifications around the minima have been examined. The electrostatic and the exchange energies have been found to be the most important components of the overall interaction energy, although the dispersion and the induction energies also play important roles. The exchange energy, while typically not the largest in magnitude at the minima, is the most anisotropic component for rotations of the monomers in the plane of the complex. The analysis of the electron-correlated components reveals that the effect of the attractive dispersion energy is to a large degree canceled out by the repulsive correlation correction to the exchange energy.
TL;DR: In this article, Monte Carlo simulations have been performed to investigate the dependence of the dynamic phase behavior on the bilinear exchange anisotropy of a classical Heisenberg spin system.
Abstract: Monte Carlo simulations have been performed to investigate the dependence of the dynamic phase behavior on the bilinear exchange anisotropy of a classical Heisenberg spin system. The system under consideration is a planar thin ferromagnetic film with competing surface fields subject to a pulsed oscillatory external field. The results show that the films exhibit a single discontinuous dynamic phase transition (DPT) as a function of the anisotropy of the bilinear exchange interaction in the Hamiltonian. Furthermore, there is no evidence of stochastic resonance associated with the DPT. These results are in marked contrast to the continuous DPT observed in the same system as a function of temperature and applied field strength for a fixed bilinear exchange anisotropy.
TL;DR: In this article, the very low temperature properties of two pyrochlore compounds, Yb2Ti2O7 and Gd2Sn2O 7, were investigated using an ensemble of microscopic and bulk techniques.
Abstract: The very low temperature properties of two pyrochlore compounds, Yb2Ti2O7 and Gd2Sn2O7, were investigated using an ensemble of microscopic and bulk techniques. In both compounds, a first order transition is evidenced, as well as spin dynamics persisting down to the 20mK range. The transition however has a quite different character in the two materials: whereas that in Gd2Sn2O7$ (at 1K) is a magnetic transition towards long range order, that in Yb2Ti2O7 (at 0.24K) is reminiscent of the liquid-gas transition, in the sense that it involves a 4 orders of magnitude drop of the spin fluctuation frequency; furthermore, no long range order is observed. These unusual features we attribute to frustration of the antiferromagnetic exchange interaction in the pyrochlore lattice.