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1.
Phase transitions of the layered antiferromagnetic Ising model on a cubic lattice with allowance for intralayer next nearest neighbor interactions in the range of 0.0 ? r < 1.0 have been studied on the basis of the replica algorithm by the Monte Carlo method. The character of phase transitions has been analyzed on the basis of the histogram method and the Binder cumulant method. The phase diagram of the dependence of the critical temperature on the next nearest neighbor interaction has been constructed. It has been shown that a secondorder phase transition is observed in the interval of 0.0 ? r ? 0.5 and for the value r = 0.9 and a first-order phase transition is observed in the interval of 0.6 ? r < 0.8.  相似文献   

2.
The spins of Ru5+ ions in Sr2YRuO6 form a face-centered cubic lattice with antiferromagnetic nearest neighbor interaction J≈25 meV. The antiferromagnetic structure of the first type experimentally observed below the Néel temperature T N =26 K corresponds to four frustrated spins of 12 nearest neighbors. In the Heisenberg model in the spin-wave approximation, the frustrations already cause instability of the antiferromagnetic state at T=0 K. This state is stabilized by weak anisotropy D or exchange interaction I with the next-nearest neighbors. Low D/JI/J~10?3 values correspond to the experimental T N and sublattice magnetic moment values.  相似文献   

3.
The replica Monte Carlo method has been used to investigate the critical behavior of a threedimensional antiferromagnetic Ising model on a body-centered cubic lattice, taking into account interactions of the adjacent behind neighbors. Investigations are carried out for the ratios of the values of exchange interactions behind the nearest and next nearest neighbors k = J 2/J 1 in the range of k ∈ [0.0, 1.0] with the step Δk = 0.1. In the framework of the theory of finite-dimensional scaling the static critical indices of heat capacity α, susceptibility γ, of the order parameter β, correlation radius ν, and also the Fisher index η are calculated. It is shown that the universality class of the critical behavior of this model is kept in the interval of k ∈ [0.0, 0.6]. It is established that a nonuniversal critical behavior is observed in the range k ∈ [0.8, 1.0].  相似文献   

4.
The theory of magnetization in a longitudinal magnetic field is developed for an easy-plane multisublattice antiferromagnet with a singlet ground state and a strong single-ion anisotropy exceeding the magnitude of exchange interaction. The magnetic-field-induced phase transition from the singlet (magnetically dis-ordered) state to a multisublattice antiferromagnetic state represents a displacive magnetic phase transition. At T=0, this transition proceeds continuously and belongs to second-order phase transitions, while at T ≠0, the behavior changes to jumplike and the process becomes the first-order phase transition.  相似文献   

5.
We investigate the pairing symmetry of the Kondo-Heisenberg model on triangular lattice, which is believed to capture the core competition of Kondo screening and local magnetic exchange interaction in heavy electron compounds. On the dominant background of the heavy fermion state, the introduction of the Heisenberg antiferromagnetic interaction (J H ) leads to superconducting pairing instability. Depending on the strength of the interactions, it is found that the pairing symmetry favours an extended s-wave for small J H and high conduction electron density but a chiral \(d_{x^2 - y^2 } + id_{xy}\)-wave for large J H and low conduction electron density, which provides a phase diagram of pairing symmetry from the calculations of the ground-state energy. The transition between these two pairing symmetries is found to be first-order. Furthermore, we also analyze the phase diagram from the pairing strengths and find that the phase diagram obtained is qualitatively consistent with that based on the ground-state energy. In addition, we propose an effective single-band BCS Hamiltonian, which is able to describe the low-energy thermodynamic behaviors of the heavy fermion superconducting states. These results further deepen the understanding of the antiferromagnetic interaction which results in a geometric frustration for the model studied. Our work may provide a possible scenario to understand the pairing symmetry of the heavy fermion superconductivity, which is one of active issues in very recent years.  相似文献   

6.
Phase transitions (PTs) and frustrations in two-dimensional structures described by a three-vertex antiferromagnetic Potts model on a triangular lattice are investigated by the Monte Carlo method with regard to nearest and next-nearest neighbors with interaction constants J1 and J2, respectively. PTs in these models are analyzed for the ratio r = J2/J1 of next-nearest to nearest exchange interaction constants in the interval |r| = 0–1.0. On the basis of the analysis of the low-temperature entropy, the density of states function of the system, and the fourth-order Binder cumulants, it is shown that a Potts model with interaction constants J1 < 0 and J2 < 0 exhibits a first-order PT in the range of 0 ? r < 0.2, whereas, in the interval 0.2 ? r ? 1.0, frustrations arise in the system. At the same time, for J1 > 0 and J2 < 0, frustrations arise in the range 0.5 < |r| < 1.0, while, in the interval 0 ? |r| ? 1/3, the model exhibits a second-order PT.  相似文献   

7.
Influence of disorder in the form of frustration on the thermodynamic behavior of a two-dimensional three-vertex Potts model has been studied by the Monte Carlo method, taking into account the nearest and next-nearest neighbors. Systems with linear sizes of L × L = N (L = 9–48) on a triangular lattice have been considered. It has been shown that in the case of J1 > 0 and J2 < 0 frustrations appear in the spin system within the interval of 0.5 ≤ |r| ≤ 1.0. The model undergoes a phase transition outside this region.  相似文献   

8.
Majority-vote model on scale-free networks and random graphs is investigated in which a randomly chosen fraction p of agents (called anticonformists) follows an antiferromagnetic update rule, i.e., they assume, with probability governed by a parameter q (0 < q < 1∕2), the opinion opposite to that of the majority of their neighbors, while the remaining 1 ? p fraction of agents (conformists) follows the usual ferromagnetic update rule assuming, with probability governed by the same parameter q, the opinion in accordance with that of the majority of their neighbors. For p = 1 it is shown by Monte Carlo simulations and using the Binder cumulants method that for decreasing q the model undergoes second-order phase transition from a disordered (paramagnetic) state to a spin-glass-like state, characterized by a non-zero value of the spin-glass order parameter measuring the overlap of agents’ opinions in two replicas of the system, and simultaneously by the magnetization close to zero. In the case of the model on scale-free networks the critical value of the parameter q weakly depends on the details of the degree distribution. As p is decreased, the critical value of q falls quickly to zero and only the disordered phase is observed. On the other hand, for p close to zero for decreasing q the usual ferromagnetic transition is observed.  相似文献   

9.
The properties of the spin system in the FCC lattice described by the Heisenberg model (s=1/2) with antiferromagnetic interactions between the nearest neighbors were studied. It was shown within the framework of spin-wave theory that long-range antiferromagnetic order was absent because of frustration of exchange coupling and transverse quantum spin fluctuations. The system was in the quantum spin liquid state. A method for describing it within linear second-order theory with self-consistently calculated parameters was suggested. It was proved that the ground spin liquid state was singlet. The thermodynamic properties of the spin liquid in the whole temperature range and the character of spatial spin correlations, which had alternating signs and a finite correlation length, were determined. The theory was constructed based on the method of two-time Green temperature functions.  相似文献   

10.
The high-pressure magnetic states of iron borate 57FeBO3 single-crystal and powder samples have been investigated in diamond anvil cells by nuclear forward scattering (NFS) of synchrotron radiation at different temperatures. In the low-pressure (0 < P < 46 GPa) antiferromagnetic phase, an increase of the Neél temperature from 350 to 595 K induced by pressure was found. At pressures 46–49 GPa, a transition from the antiferromagnetic to a new magnetic state with a weak magnetic moment (magnetic collapse) was discovered. It is attributed to the electronic transition in Fe3+ ions from the high-spin 3d5 (S = 5/2, 6A1g) to the low-spin (S = 1/2, 2T2g) state (spin crossover) due to the insulator-semiconductor-type transition with extensive suppression of strong d-d electron correlations. At low temperatures, NFS spectra of the high-pressure phase indicate magnetic correlations in the low-spin system with a magnetic ordering temperature of about 50 K. A tentative magnetic P-T phase diagram of FeBO3 is proposed. An important feature of this diagram is the presence of two triple points where magnetic and paramagnetic phases of the high-spin and low-spin states coexist.  相似文献   

11.
The temperature dependence of the ESR linewidth in La1 ? x Ca x MnO3 single crystals with various dopant concentrations (x = 0.18, 0.2, 0.22, 0.25, and 0.3) has been studied. An abrupt decrease in the ESR linewidth has been observed in the samples with a dopant concentration of x = 0.18 and 0.2 near the respective temperatures T OO′ ≈ 260 and 240 K of the orthorhombic to pseudocubic structural phase transition. The abrupt decrease in the ESR linewidth by approximately 180 Oe has been also observed in the whole temperature range when the concentration is increased from x = 0.2 to x = 0.22. The formula for the fourth moment of the ESR line has been derived including both crystal fields and isotropic exchange interactions and taking into account the difference between the exchange coupling of a spin to its nearest in-plane and out-of-plane neighbors. The formula has been used to estimate the parameter D of the crystalline field on Mn3+ ions.  相似文献   

12.
Experimental and theoretical study of superconductor-ferromagnet-superconductor junctions (SFS junctions) showed that, in a certain range of parameters (e.g., the length of the ferromagnet dF, the exchange field Eex), the ground state of the SFS junction corresponds to a superconducting phase difference π or 0. The phase diagram of an SFS junction with respect to π and 0 states is investigated in this letter in Eex, dF, T space. It is shown that the phase diagram is very sensitive to the geometry of the system, in particular, to the amount of disorder in the junction.  相似文献   

13.
We have studied the electronic and magnetic structures of the ternary iron arsenides AFe2As2 (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions bridged by As 4p orbitals. The correction from the spin-orbit interaction to the electronic band structure is given. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based on the calculations, we propose that the low energy dynamics of these materials can be described effectively by a t-J H -J 1-J 2-type model [2008, arXiv: 0806.3526v2].  相似文献   

14.
Based on the replica exchange Monte Carlo algorithm and histogram analysis of data, the phase transitions in the three-dimensional antiferromagnetic Heisenberg model on a body-centered cubic lattice with allowance for the next-nearest-neighbor interaction are studied. The study is performed for the nextnearest- neighbor exchange interaction ratio of r = 1. It is established that, for this model, the transition from the antiferromagnetic to paramagnetic phase is a first-order phase transition.  相似文献   

15.
The anion deficient cobaltite La0.5Ba0.5CoO2.8 with theformal cobalt valence state close to 3+ has been studied as function of pressure up to6.5 GPa at different temperatures by neutron powder diffraction. At ambient pressure thecrystal structure of this compound has cubic symmetry (space group Pm3?m) and is found to become antiferromagnetic withT N close to 250 K. Applied pressure inducesa gradual transition from the antiferromagnetic into a ferromagnetic state through a mixedmagnetic state. The transition is not accompanied by obvious changes in the macroscopiccrystal symmetry. It is suggested that the magnetic ground state strongly depends on theunit cell volume and that the transition is associated with a spin state crossover of thecobalt ions whereas the formal Co3+/Co4+ ratio is less importantthan expected following the double exchange scenario for the appearance offerromagnetism.  相似文献   

16.
By studying the magnetic and magnetoelastic properties, it is established that, as the temperature is lowered, Sm1?xSrxMnO3 single crystals (x=0.5, 0.55) undergo spontaneous phase transitions from the paramagnetic to a local charge-ordered state at Tco=220 K and to an A-type antiferromagnetic state at TN=175 K. It is shown that strong magnetic fields (Hcr ~ 200 kOe) break up the antiferromagnetic order and charge ordering and drive a phase transition to a conducting ferromagnetic state. H-T phase diagrams are constructed for single crystals with x=0.5 and 0.55.  相似文献   

17.
The magnetic properties of a frustrated Heisenberg antiferromagnet with the fcc lattice and exchange interaction between the nearest (J 1) and next-to-nearest (J 2) neighbors are studied in this work. For the collinear phase with the wave vector Q= (π,π,π), the equations of the self-consistent spin-wave theory are obtained and solved numerically for the sublattice magnetization and the averaged short-range order parameter. The dependence of the Néel temperatureT N on the ratio J 2/J 1 is found. It is shown that, in the case of a sufficiently strong frustration, strong short-range magnetic order persists over a wide temperature range above T N. The possible application of this result to the mechanism of structural phase transition from α-Fe to γ-Fe is considered.  相似文献   

18.
A cluster problem is analyzed as an example demonstrating that the observed three-mode behavior of spin-triplet excitations in YbB12 can be described by the asymmetric Anderson model with insulating singlet ground state. In the case of an infinite system, it is argued that the behavior of the f subsystem can be analyzed by using an effective Hamiltonian ? J with direct antiferromagnetic f-f exchange interaction. The spin excitation spectrum is shown to have a minimum at the antiferromagnetic vector, as observed experimentally. A distinctive feature of the analysis is the use of singlet and triplet basis operators.  相似文献   

19.
We consider Ising models in two and three dimensions, with short range ferromagnetic and long range, power-law decaying, antiferromagnetic interactions. We let J be the ratio between the strength of the ferromagnetic to antiferromagnetic interactions. The competition between these two kinds of interactions induces the system to form domains of minus spins in a background of plus spins, or vice versa. If the decay exponent p of the long range interaction is larger than d + 1, with d the space dimension, this happens for all values of J smaller than a critical value Jc(p), beyond which the ground state is homogeneous. In this paper, we give a characterization of the infinite volume ground states of the system, for p > 2d and J in a left neighborhood of Jc(p). In particular, we prove that the quasi-one-dimensional states consisting of infinite stripes (d = 2) or slabs (d = 3), all of the same optimal width and orientation, and alternating magnetization, are infinite volume ground states. Our proof is based on localization bounds combined with reflection positivity.  相似文献   

20.
The effect of 16O → 18O isotope substitution on the electrical and magnetic properties of the manganite system Sm1?xSrxMnO3 has been studied. It is shown that oxygen isotope substitution brings about a substantial change in the phase diagram in the intermediate region 0.4<x<0.6 between the ferromagnetic metal and antiferromagnetic insulator regions and induces phase separation and transformation of the ground metallic into insulating state for x=0.475 and 0.5. The specific features of the metal-insulator transitions in the Sm-Sr system and the nature of the low-temperature phase are discussed.  相似文献   

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