Ferroelectricity in a diatomic Ising chain as investigated by the elastic Ising model

An elastic Ising model for a one-dimensional diatomic spin chain is proposed to explain the ferroelectricity induced by the collinear magnetic order with a low-excited energy state. A statistical theory based on this model is developed to calculate the electrical and magnetic properties of Ca₃CoMnO₆, a typical quasi-one-dimensional diatomic spin chain system. The calculated ferroelectric polarization and dielectric susceptibility show a good agreement with recently reported data on Ca₃Co_2-xMn_xO₆ (x ≈0.96) (Phys. Rev. Lett. 100 047601 (2008)), although the predicted magnetic susceptibility does not coincide well with experiment. We also address the rationality and deficiency of this model by including a first-order correction which improves the consistency between the model and experiment.     

Synthesis and characterization of Ca2Sn1-xCexO4 with blue luminescence originating from Ce^4＋ charge transfer transition

Ce^4＋-doped Ca2SnO4 with a one-dimensional structure, which emits bright blue light, is prepared by using a solid-state reaction method. The x-ray diffraction results show that the Ce^4＋ ions doped in Ca2SnO4 occupy the Sn^4＋ sites. The excitation and emission spectra of Ca2Sn1-xCexO4 appear to have broad bands with peaks at - 268nm and -442nm, respectively. A long excited-state lifetime （-83μs） for the emission from Ca2Sn1-xCexO4 suggests that the luminescence originates from a ligand-to-metal Ce^4＋ charge transfer （CT）. The luminescent properties of Ca2Snl_xCexO4 have been compared with those of Sr2CeO4, which is the only material reported so far to show Ce^4＋ CT luminescence. More interestingly, it is observed that the emission intensity of Ca2Sn1-xCexO4 with a small doping concentration （x - 0.03） is comparable to that of Sr2CeO4 in which the concentration of active centre is 100%.     

Effect of carbon nanotubes addition on thermoelectric properties of Ca3Co4O9 ceramics

Increasing the phonon scattering center by adding nanoparticles to thermoelectric materials is an effective method of regulating the thermal conductivity. In this study, a series of Ca$_{3}$Co$_{4}$O$_{9}/x$ wt.% CNTs ($x=0$, 3, 5, 7, 10) polycrystalline ceramic thermoelectric materials by adding carbon nanotubes (CNTs) were prepared with sol-gel method and cold-pressing sintering technology. The results of x-ray diffraction and field emission scanning electron microscopy show that the materials have a single-phase structure with high orientation and sheet like microstructure. The effect of adding carbon nanotubes to the thermoelectric properties of Ca$_{3}$Co$_{4}$O$_{9}$ was systematically measured. The test results of thermoelectric properties show that the addition of carbon nanotubes reduces the electrical conductivity and Seebeck coefficient of the material. Nevertheless, the thermal conductivity of the samples with carbon nanotubes addition is lower than that of the samples without carbon nanotubes. At 625 K, the thermal conductivity of Ca$_{3}$Co$_{4}$O$_{9}$/10 wt.% CNTs sample is reduced to 0.408 W$\cdot$m$^{-1}\cdot$K$^{-1}$, which is about 73% lower than that of the original sample. When the three parameters are coupled, the figure of merit of Ca$_{3}$Co$_{4}$O$_{9}$/3 wt.% CNTs sample reaches 0.052, which is 29% higher than that of the original sample. This shows that an appropriate amount of carbon nanotubes addition can reduce the thermal conductivity of Ca$_{3}$Co$_{4}$O$_{9}$ ceramic samples and improve their thermoelectric properties.     

Josephson vortices and intrinsic Josephson junctions in the layered iron-based superconductor Ca10(Pt3As8)((Fe0.9Pt0.1)2As2)5

Modulated electronic state due to the layered crystal structures brings about moderate anisotropy of superconductivity in the iron-based superconductors and thus Abrikosov vortices are expected in the mixed state. However, based on the angular and temperature dependent transport measurements in iron-based superconductor Ca$_{10}$(Pt$_3$As$_8$)((Fe$_{0.9}$Pt$_{0.1}$)$_2$As$_2$)$_5$ with $T_{\rm c} \simeq 12$ K, we find clear evidences of a crossover from Abrikosov vortices to Josephson vortices at a crossover temperature $T^{\star} \simeq 7 $ K, when the applied magnetic field is parallel to the superconducting FeAs layers, i.e., the angle between the magnetic field and the FeAs layers $\theta = 0^\circ$. This crossover to Josephson vortices is demonstrated by an abnormal decrease (increase) of the critical current (flux-flow resistance) below $T^{\star}$, in contrast to the increase (decrease) of the critical current (flux-flow resistance) above $T^{\star}$ expected for Abrikosov vortices. Furthermore, when $\theta$ is larger than $0.5^\circ$, the flux-flow resistance and critical current have no anomalous behaviors across $T^{\star}$. These anomalous behaviors can be understood in terms of the distinct transition from the well-pinned Abrikosov vortices to the weakly-pinned Josephson vortices upon cooling, when the coherent length perpendicular to the FeAs layers $\xi_\bot$ becomes shorter than half of the interlayer distance $d/2$. These experimental findings indicate the existence of intrinsic Josephson junctions below $T^{\star}$ and thus quasi-two-dimensional superconductivity in Ca$_{10}$(Pt$_3$As$_8$)((Fe$_{0.9}$Pt$_{0.1}$)$_2$As$_2$)$_5$, similar to those in the cuprate superconductors.     

Magnetocrystalline anisotropy and dynamic spin reorientation of half-doped Nd_(0.5)Pr_(0.5)FeO_3 single crystal

The single crystals of Nd_(0.5)Pr_(0.5)FeO_3 were successfully grown by optical floating zone method.Room temperature x-ray diffraction and Laue photograph declared the homogeneity and high quality of the crystal.The significant magnetic anisotropy and multiple magnetic transitions illustrate the complex magnetic structure.At high temperatures(T 66 K),it shows the typical characteristics of Γ_4(G_x,A_y,F_z) state.With the decrease of the temperature,it undergoes a first-order spin reorientation transition from Γ_4(G_x,A_y,F_z) to Γ_2(F_x,C_y,G_z) state in the temperature window of 45-66 K under an applied magnetic field of 0.01 T.As the temperature drops to ～17 K,a new magnetic interaction mechanism works,which results in a further enhancement of magnetization.The T-H phase diagram of Nd_(0.5)Pr_(0.5)FeO_3 single crystal was finally constructed.     

Exact solution on the magnetism and ferroelectricity in the Ising spin chain

In this paper, we explore the magnetoelectric coupling in Ca₃CoMnO₆-type compound with the consideration of interaction between spins. Under the linear approximation of nearest-neighbor spin interaction with respect to the ion displacement, both the Hamiltonian and the partition function of the system can be simplified as the summation of two independent items, one is linear harmonic oscillator relevant to the lattice vibration, and the other is only relevant to the spin. We obtain the magnetic and ferroelectric quantities of Ca₃CoMnO₆-type rigorously on the basis of the transfer-matrix method, qualitatively exhibiting the corresponding curves taken in the presence of temperature for zero magnetic field and various magnetic fields, respectively, and our calculation results are basically consistent with the behaviors in the experiment. We find that the magnetic susceptibility in the absence of magnetic field takes on the features of ferromagnetic Ising-like behavior. Moreover, the influence of different next-nearest-neighbor exchange interaction on the magnetic susceptibility and relative dielectric constant is given as well, exhibiting the corresponding complex response of the magnetic susceptibility based on the up-up-down-down spin structure.     

Electron spin resonance study of the Ba-doping manganite Nd0.5Sr0.5MnO3

We have performed magnetization measurements and electron spin resonance （ESR） on polycrystalline manganites of Nd0.5Sr0.5-xBaxMnO3 （x = 0.1）. Phase separation and phase transitions are observed from the susceptibility and the ESR spectra data. Between 260 K （～ Tc） and 185 K （～ TN）, the system coexists of the paramagnetic phase and the ferromagnetic （FM） phase. Between 185 K and 140 K, the system coexists of the FM phase and the antiferromagnetic （AFM） phase. These results indicate that the system has a very complex magnetic state due to the origin of the instability stemming from manganite Nd0.5Sr0.4Ba0.1MnO3 by partially substituting the larger Ba^2＋ ions for the smaller Sr^2＋ ions.     

The magnetic and dielectric properties of multiferroic Sr- substituted Zn2-Y hexagonal ferrites

The magnetic and dielectric properties of Sr-substituted Zn2-Y hexagonal ferrites （Ba2-x SrxZn2Fe12O22, 1.0 〈 x ≤ 1.5） are studied in this paper. Sr substitution will lead to the variation of cation occupation, which influences both the magnetic and electric properties. As Sr content x rises from 1.0 to 1.5, magnetic hysteresis loop gets wider gradually and the permeability drops rapidly due to the transformation from ferrimagnetic to antiferromagnetic phase. Moreover, permittivity rises with increasing Sr content. Under a certain external magnetic field, the phase transition of helical spin structure of Ba0.5Srl.5Zn2Fe12O22 at about 295 K seems to open a possibility for the room-temperature ferroelectricity induced by magnetic field. But its low resistivity prevents the observation of ferroelectric and magnetoelectric properties at room-temperature.     

Structure of continuous matrix product operator for transverse field Ising model: An analytic and numerical study

We study the structure of the continuous matrix product operator (cMPO)^[1] for the transverse field Ising model (TFIM). We prove TFIM's cMPO is solvable and has the form $T=\rm{e}^{-\frac{1}{2}\hat{H}_{\rm F}}$. $\hat{H}_{\rm F}$ is a non-local free fermionic Hamiltonian on a ring with circumference $\beta$, whose ground state is gapped and non-degenerate even at the critical point. The full spectrum of $\hat{H}_{\rm F}$ is determined analytically. At the critical point, our results verify the state-operator-correspondence^[2] in the conformal field theory (CFT). We also design a numerical algorithm based on Bloch state ansatz to calculate the low-lying excited states of general (Hermitian) cMPO. Our numerical calculations coincide with the analytic results of TFIM. In the end, we give a short discussion about the entanglement entropy of cMPO's ground state.     

Kinetic Ising model in a time-dependent oscillating external magnetic field: effective-field theory

Recently,Shi et al.[2008 Phys.Lett.A 372 5922] have studied the dynamical response of the kinetic Ising model in the presence of a sinusoidal oscillating field and presented the dynamic phase diagrams by using an effective-field theory(EFT) and a mean-field theory(MFT).The MFT results are in conflict with those of the earlier work of Tom’e and de Oliveira,[1990 Phys.Rev.A 41 4251].We calculate the dynamic phase diagrams and find that our results are similar to those of the earlier work of Tom’e and de Oliveira;hence the dynamic phase diagrams calculated by Shi et al.are incomplete within both theories,except the low values of frequencies for the MFT calculation.We also investigate the influence of external field frequency(ω) and static external field amplitude(h0) for both MFT and EFT calculations.We find that the behaviour of the system strongly depends on the values of ω and h0.     

Spin polarization effect for Co2 molecule

The density functional theory (DFT)(b3p86) of Gaussian 03 has been used to optimize the structure of the Co$_{2}$ molecule, a transition metal element molecule. The result shows that the ground state for the Co$_{2}$ molecule is a 7-multiple state, indicating a spin polarization effect in the Co$_{2}$ molecule. Meanwhile, we have not found any spin pollution because the wavefunction of the ground state is not mingled with wavefunctions of higher-energy states. So for the ground state of Co$_{2}$ molecule to be a 7-multiple state is the indicative of spin polarization effect of the Co$_{2}$ molecule, that is, there exist 6 parallel spin electrons in a Co$_{2}$ molecule. The number of non-conjugated electrons is the greatest. These electrons occupy different spacial orbitals so that the energy of the Co$_{2}$ molecule is minimized. It can be concluded that the effect of parallel spin in the Co$_{2}$ molecule is larger than the effect of the conjugated molecule, which is obviously related to the effect of electron d delocalization. In addition, the Murrell--Sorbie potential functions with the parameters for the ground state and the other states of the Co$_{2}$ molecule are derived. The dissociation energy $De$ for the ground state of Co$_{2}$ molecule is 4.0489eV, equilibrium bond length $R_{\rm e}$ is 0.2061~nm, and vibration frequency $\omega _\e $ is 378.13~cm$^{ - 1}$. Its diatomic molecule force constants $f_2$, $f_3$, and $f_4$ are 2.4824~aJ$\cdot$nm$^{ - 2}$, -7.3451~aJ$\cdot$nm$^{ - 3}$, and 11.2222~aJ$\cdot$nm$^{ - 4 }$respectively(1~aJ=$10^{-18}$~J). The other spectroscopic data for the ground state of Co$_{2}$ molecule $\omega_{\e}\chi _{\e}$, $B_{\e}$, and $\alpha_{\e}$ are 0.7202~cm$^{-1}$, 0.1347~cm$^{-1 }$, and 2.9120$\times $ 10$^{-1}$~cm$^{-1}$ respectively. And $\omega_{\e}\chi _{\e}$ is the non-syntonic part of frequency, $B_{\e}$ is the rotational constant, $\alpha_{\e}$ is revised constant of rotational constant for non-rigid part of Co$_2$ molecule.     

Stochastic resonance and nonequilibrium dynamic phase transition of Ising spin system driven by a joint external field

The dynamic response and stochastic resonance of a kinetic Ising spin system （ISS） subject to the joint action of an external field of weak sinusoidal modulation and stochastic white-nolse are studied by solving the mean-field equation of motion based on Glauber dynamics. The periodically driven stochastic ISS shows that the characteristic stochastic resonance as well as nonequilibrium dynamic phase transition （NDPT） occurs when the frequency ω and amplitude h0 of driving field, the temperature t of the system and noise intensity D are all specifically in accordance with each other in quantity. There exist in the system two typical dynamic phases, referred to as dynamic disordered paramagnetic and ordered ferromagnetic phases respectively, corresponding to a zero- and a unit-dynamic order parameter. The NDPT boundary surface of the system which separates the dynamic paramagnetic phase from the dynamic ferromagnetic phase in the 3D parameter space of ho-t-D is also investigated. An interesting dynamical ferromagnetic phase with an intermediate order parameter of 0.66 is revealed for the first time in the ISS subject to the perturbation of a joint determinant and stochastic field. The intermediate order dynamical ferromagnetic phase is dynamically metastable in nature and owns a peculiar characteristic in its stability as well as the response to external driving field as compared with a fully order dynamic ferromagnetic phase.     

Magnetism and giant magnetocaloric effect in rare-earth-based compounds R_3BWO_9(R = Gd,Dy, Ho)

The magnetism and magnetocaloric effect(MCE) of rare-earth-based tungstate compounds R_3 BWO_9(R=Gd,Dy,Ho) have been studied by magnetic susceptibility,isothermal magnetization,and specific heat measurements.No obvious long-range magnetic ordering can be found down to 2 K.The Curie-Weiss fitting and magnetic susceptibilities under different applied fields reveal the existence of weak short-range antiferromagnetic couplings at low temperature in these systems.The calculations of isothermal magnetization exhibit a giant MCE with the maximum changes of magnetic entropy being 54.80 J/kg-K at 2 K for Gd_3 BWO_9,28.5 J/kg-K at 6 K for Dy_3 BWO_9,and 29.76 J/kg-K at 4 K for Ho_3 BWO_9,respectively,under a field change of 0-7 T.Especially for Gd_3 BWO_9,the maximum value of magnetic entropy change(-ΔS_M~(max)) and adiabatic temperature change(-ΔT_(ad)~(max)) are 36.75 J/kg·K and 5.56 K for a low field change of 0-3 T,indicating a promising application for low temperature magnetic refrigeration.     

Kapellasite: a kagome quantum spin liquid with competing interactions

Magnetic susceptibility, NMR, muon spin relaxation, and inelastic neutron scattering measurements show that kapellasite, Cu_{3}Zn(OH)_{6}Cl_{2}, a geometrically frustrated spin-1/2 kagome antiferromagnet polymorphic with herbertsmithite, is a gapless spin liquid showing unusual dynamic short-range correlations of noncoplanar cuboc2 type which persist down to 20?mK. The Hamiltonian is determined from a fit of a high-temperature series expansion to bulk susceptibility data and possesses competing exchange interactions. The magnetic specific heat calculated from these exchange couplings is in good agreement with experiment. The temperature dependence of the magnetic structure factor and the muon relaxation rate are calculated in a Schwinger-boson approach and compared to experimental results.     

Stability of small Ni-Ti bimetallic clusters studied by density functional theory

The low-energy structures and the electronic and the magnetic properties of small NiNiNi$_{m}$Ti$_{n}$ clusters, lowest-energy structure, electronic and magnetic propertiesProject supported by the National Natural Science Foundation of China (Grant No.~10874039), and the Natural Science Foundation of Hebei Province of China (Grant Nos.~A2009000246 and 2009000243).3120A, 3640B, 2110K7/3/2009 12:00:00 AMThe low-energy structures and the electronic and the magnetic properties of small NiNiNi$_{m}$Ti$_{n}$ clusters, lowest-energy structure, electronic and magnetic propertiesProject supported by the National Natural Science Foundation of China (Grant No.~10874039), and the Natural Science Foundation of Hebei Province of China (Grant Nos.~A2009000246 and 2009000243).3120A, 3640B, 2110K7/3/2009 12:00:00 AMThe low-energy structures and the electronic and the magnetic properties of small Ni$_{n}$Ti$_{n}$ ($n=1$--$6$) and Ni$_{m}$Ti$_{n}$ ($1 \le n \le 4$, $1 \le m \le 4$, $n \ne m$) clusters are investigated by performing all-electron calculations based on density functional theory. Ground states and several isomers near the ground states are determined for these clusters. The results indicate that the growth of small Ni$_{m}$Ti$_{n}$ clusters prefers to form rich Ti--Ni and Ti--Ti bonds. When the percentage of titanium atoms is significantly greater than that of nickel atoms, the nickel atoms are most frequently found above the surface; in contrast, the titanium atoms prefer the bridging sites. A M\"{u}lliken spin population analysis indicates that the total spin of titanium-nickel clusters is not always zero.http://cpb.iphy.ac.cn/CN/10.1088/1674-1056/19/4/043102https://cpb.iphy.ac.cn/CN/article/downloadArticleFile.do?attachType=PDF&id=111644NimTin;clusters;lowest-energy;structure;electronic;and;magnetic;propertiesThe low-energy structures and the electronic and the magnetic properties of small Nin Tin(n = 1-6) and Ni m Ti n(1 ≤ n ≤ 4,1 ≤ m ≤ 4,n ≠ m) clusters are investigated by performing all-electron calculations based on density functional theory.Ground states and several isomers near the ground states are determined for these clusters.The results indicate that the growth of small Ni m Ti n clusters prefers to form rich Ti-Ni and Ti-Ti bonds.When the percentage of titanium atoms is significantly greater than that of nickel atoms,the nickel atoms are most frequently found above the surface;in contrast,the titanium atoms prefer the bridging sites.A Mu¨lliken spin population analysis indicates that the total spin of titanium-nickel clusters is not always zero.     

Critical behaviour of the ferromagnetic Ising model on a triangular lattice

We use a new updated algorithm scheme to investigate the critical behaviour of the two-dimensional ferromagnetic Ising model on a triangular lattice with the nearest neighbour interactions. The transition is examined by generating accurate data for lattices with L= 8, 10, 12, 15, 20, 25, 30, 40 and 50. The updated spin algorithm we employ has the advantages of both a Metropolis algorithm and a single-update method. Our study indicates that the transition is continuous at T_c= 3.6403({2}). A convincing finite-size scaling analysis of the model yields υ=0.9995(21), β / υ = 0.12400({17}), γ / υ = 1.75223(22), γ '/υ=1.7555(22), α/υ= 0.00077(420) (scaling) and α / υ = 0.0010(42) (hyperscaling). The present scheme yields more accurate estimates for all the critical exponents than the Monte Carlo method, and our estimates are shown to be in excellent agreement with their predicted values.     

Growth and characterization of superconducting Ca1-xNaxFe2As2 single crystals by NaAs-flux method

High-quality superconducting Ca$_{1-x}$Na$_x$Fe$_2$As$_2$ single crystals have been successfully grown by the NaAs-flux method, with sodium doping level $x = 0.4$-0.64. The typical sizes of these crystals are more than 10 mm in $ab$-plane and $\sim 0.1$ mm along $c$-axis in thickness. X-ray diffraction, resistance and magnetization measurements are carried out to characterize the quality of these crystals. While no signature of magnetic phase transitions is detected in the normal state, bulk superconductivity is found for these samples, with a sharp transition at $T_{\rm c}$ ranging from 19.8 K ($x = 0.4$) to 34.8 K ($x = 0.64$). The doping dependences of the $c$-axis parameter and $T_{\rm c}$ are consistent with previous reports, suggesting a possible connection between the lattice parameters and superconductivity.     

Effects of charge on the structures and spin moments of Nil3 cluster

The structural stability and magnetic properties of the icosahedral Ni13, Ni13^＋1 and Ni13^-1 clusters have been obtained by utilizing all-electron density functional theory with the generalized gradient approximations for the exchange-correlation energy. The calculated results show that the ground states of neutral and charged clusters all favour a D3d structure, a distorted icosahedron, due to the Jahn-Teller effect. The radial distortions caused by doping one electron and by doping one hole are opposite to each other. Doping one electron will result in a 1/2 decrease and doping one hole will result in a 1/2 increase of the total spin. Both increasing interatomic spacing and decreasing coordination will lead to an enhancement of the spin magnetic moments for Nil3 clusters.     

Effect of Cu doping on the magnetic and electrical properties of n=2 Ruddlesden-Popper manganates

A series of polycrystalline Cu-doped n=2 Ruddlesden-Popper manganates La1.2Sr1.8CuzMn（2-x）O7 （x=0, 0.04, 0.13） were synthesized by the solid state reaction method. The effect of Cu doping on the magnetic and transport properties has been studied. It is found that Cu substitution for Mn greatly affects the magnetic and electrical properties of the parent phase La1.2Sr1.8Mn2O7. With the increase of Cu content, the system undergoes a transition from longrange ferromagnetic order to the spin glass state and further to an antiferromagnetic order. A little of Cu dopant can lead to the samples showing semiconductor or insulator behaviour in the whole observed temperature range while the parent phase has a metal-insulator transition. These samples show colossal magnetoresistance at low temperatures and the value of it decreases with increasing Cu content.     

Nanocrystallization behaviour of a ternary amorphous alloy during isothermal annealing： a Monte Carlo simulation

The nanocrystallization behaviour of Zr70Cu20Ni10 metallic glass during isothermal annealing is studied by employing a Monte Carlo simulation incorporating with a modified Ising model and a Q-state Potts model. Based on the simulated microstructure and differential scanning calorimetry curves, we find that the low crystal-amorphous interface energy of Ni plays an important role in the nanocrystallization of primary Zr2Ni. It is found that when T〈T1max （where T1max is the temperature with maximum nucleation rate）, the increase of temperature results in a larger growth rate and a much finer mierostrueture for the primary Zr2Ni, which accords with the microstructure evolution in ＂flash annealing＂. Finally, the Zr2Ni/Zr2Cu interface energy σG contributes to the pinning effect of the primary nano-sized Zr2Ni grains in the later formed normal Zr2Cu grains.