Search

Manifestations of quantum effects in the macroscopic properties of frustrated magnets keep attracting considerable interest. We have formulated and studied a simple model of a three-sublattice mixed-spin (S = 1 , 1 / 2 , 1 / 2) S U (3) -ferrimagnet on triangular lattice in which the strong quantum fluctuations are developed due to combined effect of frustrated exchange bonds, reduced dimensionality and a single-ion easy-plane anisotropy in the spin-1 sublattice. To account correctly for the S U (3) algebra, the Hubbard operators representation of generators is used. Dependencies of the magnetic moments R and R L (for spin-1/2 and spin-1 sublattices respectively), the total magnetic moment M , as well as the quadrupole moment, on the anisotropy parameter D are calculated at zero temperature and different ratios I / J of exchange integrals from different sublattices. It is established that for I / J ≪ 1 the critical value D c , at which the system enters the quadrupole antiferromagnetic phase, can be much smaller than both I and J. Besides, with an increase in D from zero to D c the total moment M can change its direction several times via taking zero value. Classification of four branches of the spin-wave excitation spectrum of the S U (3) -ferrimagnet is carried out and modification of the spectrum with change in the single-ion anisotropy is analyzed. • Three-sublattice mixed-spin (S=1,1/2,1/2) SU(3)-ferrimagnet on triangular lattice • Single-ion easy-plane anisotropy • Quadrupole antiferromagnetic phase • Multiple compensation points [ABSTRACT FROM AUTHOR]

In the work, we investigated a generalized model of the fermionic lattice gas in the form of the extended Hubbard model with intersite Ising-like interactions (between nearest neighbors, both antiferromagnetic and ferromagnetic) at the atomic limit on the triangular lattice. In the ground state, we find the exact phase diagram as a function of μ. Within the mean-field decoupling of the intersite term and exact treatment of onsite interaction, we found also the diagrams for T ≥ 0 including metastable phases. For antiferromagnetic coupling, we find that nontrival ordered phase can exist with coexistence of charge and metamagnetic ordering. The transition between the ordered phase and the nonordered phase can be discontinuous as well continuous depending on the model parameters. Moreover, the ordered phase can coexist with the nonordered phase in phase separated states for fixed electron concentration. Additionally, the ranges of phases metastability are determined in the neighborhood of the discontinuous transitions. • The extended Hubbard model with intersite Ising-like interactions is investigated. • Orderings on the triangular lattice with geometrical frustration are analyzed. • The exact ground state for fixed chemical potential is determined. • Mean-field field solutions of the model for finite temperatures are found and discussed. • Complex phase diagrams for both signs of magnetic interactions are determined. [ABSTRACT FROM AUTHOR]

Magnetic and transport properties of triangular-lattice Ni 1.8 Ge single crystal have been studied. The results reveal that Ni 1.8 Ge is a metal with an obvious magnetic anisotropy. The compound exhibits ferrimagnetic correlations along both directions. However, the long-range magnetic order was not observed due to the geometrical frustration arising from the triangular nickel lattice. At low temperature, the electron-magnon scattering and spin fluctuation have obvious contributions to the resistivity and magnetoresistance, indicating the existence of short-range dynamic magnetic ordering. [ABSTRACT FROM AUTHOR]

The Z 2 topological invariant is defined in the chiral d -wave superconductor having a triangular lattice in the presence of the 120-degree magnetic ordering. Analyzing the Z 2 invariant, we determine the conditions of implementing topological phases in the model with regard to superconducting pairings between the nearest and next nearest neighbors. It is often supposed in such a system that the pairing parameter between the nearest neighbors should be equal to zero due to the intersite Coulomb interaction. We show that taking into account even weak pairings in the first coordination sphere leads to the disappearance of the gapless excitations of the bulk spectrum in the wide region of the parameter space. Thus, topological invariants can be defined in this region. In solving the problem of open edges it is shown that the zero energy modes are realized basically in the topologically nontrivial phases. Such zero modes are topologically protected Majorana modes. A connection between the Z 2 invariant and the integer topological invariant of the ground state of the 2D lattice is established in the presence of the electron–hole symmetry and noncollinear magnetic ordering. [ABSTRACT FROM AUTHOR]

Using the diagram technique for the Hubbard operators an integral equation that determines the order parameter of the superconducting phase Δ ( p ) was obtained in the framework of t – J – V and t – J *– V models on triangular lattice. It is shown that there are two scenarios of formation of superconducting phase with gapless spectrum at the critical concentration of carriers x c . The effect of three-center terms on implementation of this phase was also considered. [ABSTRACT FROM AUTHOR]

We discuss the conditions under which Majorana zero modes can be implemented in the coexistence phase of chiral superconductivity and 120°-type noncollinear spin structure on the triangular lattice. It is shown that the gapless elementary excitations exist on the finite region in the parameter space consisting of the effective exchange field, the chemical potential, and the superconducting order parameter. The range of parameters supporting Majorana edge states with exactly zero excitation energy has also been found. [ABSTRACT FROM AUTHOR]

Graphical abstract The system is magnetically disturbed by a substantial amount of Ti/Ru chemical disorder. The Weiss temperature and effective magnetic moment were found to be −29.5 K and 1.82 μ B /f.u. respectively, and a bifurcation in the zero field cooled and field cooled magnetic susceptibility is observed below 4.7 K, suggesting that Ba 3 TiRu 2 O 9 is a compositionally-disordered spin-glass system. Abstract The magnetic properties of Ba 3 TiRu 2 O 9 , whose crystal structure is based on stacked triangular planar lattices of M 2 O 9 dimers and single MO 6 octahedra, are reported. The system is magnetically disturbed by a substantial amount of Ti/Ru chemical disorder. The Weiss temperature and effective magnetic moment were found to be −29.5 K and 1.82 μ B /f.u. respectively, and a bifurcation in the zero field cooled and field cooled magnetic susceptibility is observed below 4.7 K, suggesting that this is a compositionally-disordered spin-glass system. The material is a semiconductor with an activation energy for charge transport of approximately 0.14 eV. [ABSTRACT FROM AUTHOR]

• The magnetic decoupling between the Cu1 ferromagnetic chain and the Cu2 antiferromagnetic chain has been revealed. • The high-field magnetization presents a metamagnetic transition at 1.7 T and a half-saturated ferromagnetic (FM)-like magnetization process. • The antiferromagnetic resonances of the antiferromagnetically coupled Cu1 FM chains were observed, showing the presence of significant magnetic anisotropy. The high-field magnetization and electron spin resonance (ESR) are reported for the two-dimensional triangular-lattice antiferromagnet Cu 2 (OH) 3 HCO 2. The compound has an antiferromagnetic (AFM) order at T N = 4.9 K with the Curie-Weiss temperature θ CW = − 3.8 K. The high-field magnetization up to 50 T exhibits a metamagnetic transition at 1.7 T followed by a half-saturated ferromagnetic (FM)-like magnetization process, showing the magnetic decoupling between the Cu1 FM chain and the Cu2 AFM chain. The unsaturated magnetization at 50 T serves as an evidence of much larger AFM exchange in the Cu2 chain. The temperature-dependent ESR spectra are in line with the development of magnetic correlations and AFM ordering. The observed ESR modes below T N originate from the AFM resonances of the antiferromagnetically coupled Cu1 FM chains. The zero-field gap of 111 GHz and the zero-frequency field of 2.4 T suggest the presence of significant magnetic anisotropy. [ABSTRACT FROM AUTHOR]

I theoretically propose an emergence of nonlinear nonreciprocal transport in centrosymmetric magnets with topological spin textures. By focusing on the stacking degree of freedom of the skyrmion crystals on a layered triangular lattice, I find that the ABC-stacking structure of the skyrmion crystals gives rise to out-of-plane nonreciprocal transport without the relativistic spin–orbit coupling. I show that such nonreciprocal transport is caused by an asymmetric band modulation due to the stacking structure, where the scalar chirality across the layers is a key ingredient. I also show an effective spin model to stabilize the ABC-stacking skyrmion structure. The present results indicate that the stacking structure of the skyrmion crystal becomes a source of further intriguing transport properties. • Stacking degree of freedom in the skyrmion crystals affects nonreciprocal transport. • The out-of-plane nonreciprocal transport is induced without the spin–orbit coupling. • The results can be applied to centrosymmetric magnets with noncoplanar spin textures. [ABSTRACT FROM AUTHOR]

We apply the coupled cluster method to high orders of approximation and exact diagonalizations to study the ground-state properties of the triangular-lattice spin- s Heisenberg antiferromagnet. We calculate the fundamental ground-state quantities, namely, the energy e 0 , the sublattice magnetization M sub , the in-plane spin stiffness ρ s and the in-plane magnetic susceptibility χ for spin quantum numbers s = 1 / 2 , 1 , … , s max , where s max = 9 / 2 for e 0 and M sub , s max = 4 for ρ s and s max = 3 for χ . We use the data for s ≥ 3 / 2 to estimate the leading quantum corrections to the classical values of e 0 , M sub , ρ s , and χ . In addition, we study the magnetization process, the width of the 1/3 plateau as well as the sublattice magnetizations in the plateau state as a function of the spin quantum number s . [ABSTRACT FROM AUTHOR]

We investigate the interaction conditions to stabilize topological spin textures in centrosymmetric magnets without an external magnetic field. By performing the simulated annealing for an effective spin model with the momentum-resolved interaction on a two-dimensional triangular lattice, we find that the competing interactions between the finite- q channel and the uniform (q = 0) channel lead to a zero-field skyrmion crystal with the skyrmion number of ± 1. Moreover, we obtain a variety of topological spin textures, such as a meron crystal, vortex crystal, and other multiple- Q states with and without a scalar chirality. We especially show that the bond-dependent anisotropic exchange interaction originating from the relativistic spin–orbit coupling plays an important role in inducing such topological spin textures, while the single-ion anisotropy does not. • Skyrmion crystals are stabilized at zero field in centrosymmetric triangular magnets. • The bond-dependent interaction is important for the zero-field skyrmion crystals. • Meron and vortex crystals are also stabilized depending on the interactions. [ABSTRACT FROM AUTHOR]

We studied some magnetic behaviors of the Blume-Capel (BC) model in a site diluted triangular lattice by means of the effective-field theory (EFT) with correlations. The effects of the exchange interaction ( J ), crystal field ( D ), concentration ( p ) and temperature ( T ) on the magnetic properties of the spin-1 BC model in a triangular lattice, such as magnetization, susceptibility, phase diagram and hysteresis behaviors, are investigated in detail. The phase diagrams of the system are presented in two different planes. The tricritical point as well as the tetracritical and critical end special points are found to depend on the physical parameters of the system. Moreover, when the hysteresis behaviors of the system are examined, single and double hysteresis loops are observed for various values of the physical parameters. We show that the hysteresis loops have different coercive field points in which the susceptibility makes a peak at these points. [ABSTRACT FROM AUTHOR]

Thermodynamic and magnetocaloric properties of geometrically frustrated Ising spin clusters of selected shapes and sizes are studied by exact enumeration. In the ground state the magnetization and the entropy show step-wise variations with an applied magnetic field. The number of steps, their widths and heights depend on the cluster shape and size. While the character of the magnetization plateau heights is always increasing, the entropy is not necessarily decreasing function of the field, as one would expect. For selected clusters showing some interesting ground-state properties, the calculations are extended to finite temperatures by exact enumeration of densities of states in the energy-magnetization space. In zero field the focus is laid on a peculiar behavior of some thermodynamic quantities, such as the entropy, the specific heat and the magnetic susceptibility. In finite fields various thermodynamic functions are studied in the temperature-field parameter plane and particular attention is paid to the cases showing an enhanced magnetocaloric effect. The exact results on the finite clusters are compared with the thermodynamic limit behavior obtained from Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

We theoretically report an instability toward a skyrmion crystal in centrosymmetric magnets under hexagonal and trigonal single-ion anisotropy. The results are obtained for a minimal spin model with a crystal-dependent single-ion anisotropy on a triangular lattice by performing simulated annealing. By constructing magnetic phase diagrams in the presence of six different types of single-ion anisotropy while changing the amplitudes of external magnetic field and single-ion anisotropy in a systematic way, we find that the hexagonal and trigonal single-ion anisotropy becomes a source of the skyrmion crystal depending on the magnetic-field direction. We show that the skyrmion crystal is stabilized by the uniaxial-type or trigonal-type single-ion anisotropy under the out-of-plane magnetic field, while it is stabilized by the hexagonal-type inplane single-ion anisotropy under the inplane magnetic field. We also find various multiple- Q spin states depending on the types of the single-ion anisotropy. • Skyrmion crystals are stabilized in centrosymmetric triangular magnets under single-ion anisotropy. • The uniaxial-type or trigonal single-ion anisotropy in an out-of-plane magnetic field induces the skyrmion crystals. • The hexagonal-type inplane single-ion anisotropy in an inplane magnetic field induces the skyrmion crystal. [ABSTRACT FROM AUTHOR]

By introducing a substantial amount of non-magnetic impurities into an antiferromagnetic 2 d Potts model on a triangular lattice it is that the impurities in spin systems described by this model result in the change of a first-order to a second-order phase transition. The systems with linear sizes L × L=N, L =18−48 are considered. Investigations are performed using the standard Metropolis algorithm along with Monte-Carlo single-cluster Wolff algorithm. [ABSTRACT FROM AUTHOR]

Abstract: We carried out a comparative study of the A=Ca, Sr, Ba compounds of the α-ACr2O4 series, a family of layered chromites, which can be classified as S=3/2 quasi-2D triangular Heisenberg antiferromagnets (2DTHAF). The sizeable distortion, with respect to the perfect triangle lattice, of the spin layer topology can be progressively decreased by increasing the size of A2+, which leads to an increase of both the average distance between nearest-neighbouring Cr3+ and of the interplane spacing. The evolution with A2+ of the antiferromagnetic transition T N , the intraplane coupling J, and the interplane coupling J′ has been determined on the basis of magnetization and heat capacity measurements and is discussed in the framework of the standard theoretical models describing quasi 2DTHAF. [Copyright &y& Elsevier]

Abstract: We employ an effective-field theory with correlations in order to study the phase diagram and ground-state magnetizations of a selectively diluted Ising antiferromagnet on triangular and honeycomb lattices. Dilution of different sublattices with generally unequal probabilities results in a rather intricate phase diagram in the sublattice dilution parameters space. In the case of the frustrated triangular lattice antiferromagnet the selective dilution affects the degree of frustration which can lead to some peculiar phenomena, such as reentrant behavior of long-range order or unsaturated sublattice magnetizations at zero temperature. The selectively diluted Ising antiferromagnet on the honeycomb lattice is obtained as a special case when one sublattice of the triangular lattice is completely removed by dilution. [Copyright &y& Elsevier]

Abstract: During the course of tuning anisotropy from Ising type to zero, the variation of magnetization (M) steps against magnetic field (h) is investigated in a triangular antiferromagnetic Heisenberg model using Monte Carlo techniques. It is revealed that the anisotropy is an essential key to induce the temperature-dependent stepwise M(h) curve observed in frustrated magnetic system, and it can be employed to modulate this steplike magnetic behavior effectively. When the anisotropy is strengthened, a ground state transition occurs from the homogeneous 120° triangular structure to the collinear partially disordered antiferromagnetic state. No M step is detected in the system without anisotropy. But if the anisotropy is nonzero, the M 0/3 step (where M 0 is the saturated M) will emerge on M(h) curve, which is due to an h-induced quasi-collinear ferrimagnetic state. This M 0/3 step can be extended by increasing the anisotropy. When the M 0/3 plateau dominates the h-range broad enough, the equidistant metastable substeps, which originates from the disorders frozen in the frustrated collinear spin structure, appear to be superposed on the M 0/3 plateau. Thus the system with a strong anisotropy presents the whole temperature evolution of stepwise M(h) curve in quantitative agreement with the experiments of Ca3Co2O6. [Copyright &y& Elsevier]

The extended Hubbard model in the atomic limit, which is equivalent to lattice S = 1 / 2 fermionic gas, is considered on the triangular lattice. The model includes onsite Hubbard U interaction and both nearest-neighbor (W 1) and next-nearest-neighbor (W 2) density–density intersite interactions. The variational approach treating the U term exactly and the W l terms in the mean-field approximation is used to investigate thermodynamics of the model and to find its finite temperature (T > 0) phase diagrams (as a function of particle concentration) for W 1 > 0 and W 2 < 0. Two different types of charge-order (i.e., DCO and TCO phases) within 3 × 3 unit cells as well as the nonordered (NO) phase occur on the diagram. Moreover, several kinds of phase-separated (PS) states (NO/DCO, DCO/DCO, DCO/TCO, and TCO/TCO) are found to be stable for fixed concentration. Attractive W 2 < 0 stabilizes PS states at T = 0 and it extends the regions of their occurrence at T > 0. The evolution of the diagrams with increasing of | W 2 | / W 1 is investigated. It is found that some of the PS states are stable only at T > 0. Two different critical values of | W 2 | / W 1 are determined for the PS states, in which two ordered phases of the same type (i.e., two domains of the DCO or TCO phase) coexist. • Atomic limit of the extended Hubbard model on the triangular lattice is analyzed. • Phase diagrams of the lattice S = 1 / 2 fermionic gas model are found. • The effects of next-nearest-neighbor attractive interaction are investigated. • The diagrams have complex structure with different multicritical points. • The stability regions of various phase separated states are found. [ABSTRACT FROM AUTHOR]

Abstract: Spin nematic order is investigated for a spin-1 bilinear-biquadratic model on triangular lattice. We studied an antiferro nematic order with three-sublattice structure, and particularly the effects of quantum fluctuation at zero temperature. We have calculated several magnetic quantities and discussed the dependence of the coupling parameters in order to explain many of characteristic features of recently discovered spin liquid state in the triangular magnet, NiGaS. [Copyright &y& Elsevier]

Abstract: Nonmagnetic impurity effects on the spin disordered state in the triangular antiferromagnet were studied through magnetic and thermal measurements for . The robust feature of the -dependent specific heat and its scaling behavior with the Weiss temperature, for all substitution up to 30%, indicate the existence of a coherent Nambu–Goldstone mode. The absence of either conventional magnetic order or bulk spin freezing suggests a novel symmetry breaking of the ground state. [Copyright &y& Elsevier]

Abstract: We report intensive studies on the cobalt oxide superconductors via 59Co NMR and NQR measurements. In the sample with and , we find that the spin–lattice relaxation rate shows a variation below and down to very low temperatures, which indicates the presence of line nodes in the superconducting (SC) gap function. We have also studied the spin susceptibility below via the Knight shift measurement. The spin part of the Knight shift decreases below in both the - and -directions indicating singlet spin state of the Cooper pairs. Our results strongly suggest that the superconducting state is a d-wave state. [Copyright &y& Elsevier]

Abstract: The Hubbard model at quarter filling defined on the one-dimensional (1D) triangular lattice (i.e., three chains coupled with interchain zigzag bonds) is studied by the density-matrix renormalization group method in order to clarify the symmetry of the Cooper-pair wave function of our proposed spin-triplet superconductivity induced by the ring-exchange mechanism. We calculate the rotational symmetry of the pair correlation function and show that the pairing occurs predominantly in the spin-triplet f-wave channel. Possible experimental relevance is discussed. [Copyright &y& Elsevier]

Mössbauer and magnetization have been measured for the samples Ca3(Co1-xMx)2O6, M=57Fe and Zn, with triangular lattice consisting of 1-D ferromagnetic chains. For the sample of x=0.01 (M=Fe), a broad splitting Mössbauer spectrum appears around 40 K and changes to the one consisting of three sextets below 30 K, which can be attributable to the slow relaxation of 57Fe3+ spins embedded in the 1-D chains of Co with strong anisotropy. A comparison of the spectra between x=0.01 (M=Fe) and 0.05 (Fe) samples reveals that increase of Fe-impurity enhances Fe3+ spin fluctuation. It is concluded that the substitution of Fe-impurity cuts the ferromagnetically ordered linear chains and, consequently, the Ising nature of Co3+ spins is much weakened, which provides a new relaxation path for Fe-spin fluctuation. [Copyright &y& Elsevier]

Abstract: We have investigated the magnetic properties of one-dimensional (1D) compounds (, Sr, Ba) by susceptibility measurements. The transition to a partially disordered antiferromagnetic state, which takes place at 25K for (), was not observed down to 2K for the –5 samples, except that a precursory behavior was found for below 3.2K. The absence of a long-range order in the –5 samples can be understood by accounting for the increase in the distance between the 1D chains, which are characteristic to the present compounds. At high temperatures, we observed a change in the slope of the temperature dependence of the inverse susceptibility, which we attribute to the appearance of a short-range 1D ferromagnetic order along the chain. [Copyright &y& Elsevier]

Abstract: In order to elucidate the x–s–T phase diagram (s: the Co valence and T: temperature) of the sodium cobalt oxyhydrate, Na x (H3O) z CoO2·yH2O, a magnetically ordered phase (M phase) in the system is investigated by measuring magnetization curves of samples with three chemical compositions. The M phase is located at the range of 0.33