Effect of crystal field on the spin density at the copper nucleus in copper complexes

The role of lower symmetry component of the crystal field in causing a mixing of excited 3d ^x−1 4s with the ground 3d ^x configuration and leading to spin density at the nucleus for iron group ions was suggested by Griffith and Orgel. This mechanism has been examined in detail for the two low-symmetry copper complexes, one square planar (D_4h symmetry) and the other distorted tetrahedron (D_2d symmetry) and the calculation has been performed using the powerful Racah method and tensor operator technique. It is found that for the two types of copper complexes, copper pthalocyanin (square planar, D_4h symmetry) and cesium copper chloride (distorted tetrahedron, D_2d symmetry) the contribution from this mechanism to the spin density at the nucleus vanishes identically.     

Anomalous magnetism and 209Bi nuclear spin relaxation in Bi4Ge3O12 crystals

The quadrupole ²⁰⁹Bi spin–spin and spin–lattice relaxation were studied within 4.2–300 K for pure and doped Bi₄Ge₃O₁₂ single crystals which exhibit, as was previously found, anomalous magnetic properties. The results revealed an unexpectedly strong influence of minor amounts of paramagnetic dopants (0.015–0.5 mol.%) on the relaxation processes. Various mechanisms (quadrupole, crystal electric field, electron spin fluctuations) govern the spin–lattice relaxation time T ₁ in pure and doped samples. Unlike T ₁, the spin–spin relaxation time T ₂ for pure and Nd-doped samples was weakly dependent on temperature within 4.2–300 K. Doping Bi₄Ge₃O₁₂ with paramagnetic atoms strongly elongated T ₂. The elongation, although not so strong, was also observed for pure and doped crystals under the influence of weak (～30 Oe) external magnetic fields. To confirm the conclusion about strong influence of crystal field effects on the temperature dependence of T ₁ in the temperature range 4.2–77 K, the magnetization vs. temperature and magnetic field was measured for Nd- and Gd-doped Bi₄Ge₃O₁₂ crystals using a SQUID magnetometer. The temperature behavior of magnetic susceptibility for the Nd-doped crystal was consistent with the presence of the crystal electric field effects. For the Gd-doped crystal, the Brillouin formula perfectly fitted the curve of magnetization vs. magnetic field, which pointed to the absence of the crystal electric field contribution into the spin–lattice relaxation process in this sample.     

A mechanism for ferrimagnetism and incommensurability in one-dimensional systems

In this paper I discuss a mechanism for ferrimagnetism in 1+1 dimensions. The mechanism is related to a special class of interactions described by operators with non-zero Lorentz spin. Such operators are present in such problems as the problem of tunneling between Luttinger liquids and the problem of frustrated spin ladder. Exact solutions are presented for a representative class of models possessing a continuous isotopic symmetry. It is shown that the interactions (i) dynamically generate static oscillations with the wave vector dependent on the coupling constant, (ii) give rise to spontaneous breaking of this symmetry at T=0 accompanied by generation of the magnetic moment and appearance of gapless modes with a non-relativistic (ferromagnetic) dispersion E∼k², (iii) generate massive (roton) modes.     

Mössbauer and magnetic studies of BaVS3 and V5S8 doped with 57Fe

Iron can be easily introduced in BaVS₃ and V₅S₈. It is located at the vanadium sites and has been used as a probe to analyse by Mössbauer effect the magnetic properties of its surrounding matrix. The electronic state of iron in this matrix has also been studied. It was found that in BaVS₃, the iron is in a low spin Fe³⁺ configuration $\text{(S =}\text{1}\text{2}\text{)}$. In V₅S₈ at 4.2 K, the iron is in low spin Fe²⁺ configuration (S = 0). The rapid decrease of quadrupole splitting observed between 50 and 200 K is attributed to a thermally activated change in electronic structure. The high temperature configuration (above 200 K) seems to be neither pure low spin Fe³⁺ nor high spin Fe²⁺, but a mixture of configurations fluctuating at a rate which is faster than the characteristic time of Mössbauer measurements.     

Crystal field and quadrupole interaction effects in YbXO4 Zircons (X = V, P)

The magnetic and magnetoelastic properties of YbPO₄ and YbVO₄ crystals are investigated experimentally and theoretically; the crystal field parameters are determined, as well as the magnetoelastic coefficients B ^μ and total quadrupole coupling constants G ^μ for all symmetry modes. It is found that, for H ∥ [100], γ-symmetric quadrupole interactions predominate and are responsible for a significant contribution to the third-order susceptibility, magnetization, magnetostriction, and elastic constant. It is demonstrated that, in the absence of an external field, these interactions do not lead to quadrupole ordering, because the respective deformation susceptibility χ_γ is several times less than the critical value of 1/G ^γ. The influence of an external magnetic field along different symmetry axes on the quadrupole effects and quadrupole interactions in Yb zircons is investigated. It is demonstrated that, for H ∥ [110], the susceptibility χ_γ increases with the field, so that in a fairly strong field in the investigated crystals one can expect a γ-symmetric stimulated phase transition.     

Inelastic neutron scattering studies of the spin and lattice dynamics in iron arsenide compounds

Although neutrons do not couple directly to the superconducting order parameter, they have nevertheless played an important role in advancing our understanding of the pairing mechanism and the symmetry of the superconducting energy gap in the iron arsenide compounds. Measurements of the spin and lattice dynamics have been performed on non-superconducting ‘parent’ compounds based on the LaFeAsO (‘1111’) and BaFe₂As₂ (‘122’) crystal structures, and on electron and hole-doped superconducting compounds, using both polycrystalline and single crystal samples. Neutron measurements of the phonon density-of-state, subsequently supported by single crystal inelastic X-ray scattering, are in good agreement with ab initio calculations, provided the magnetism of the iron atoms is taken into account. However, when combined with estimates of the electron–phonon coupling, the predicted superconducting transition temperatures are less than 1 K, making a conventional phononic mechanism for superconductivity highly unlikely. Measurements of the spin dynamics within the spin density wave phase of the parent compounds show evidence of strongly dispersive spin waves with exchange interactions consistent with the observed magnetic order and a large anisotropy gap. Antiferromagnetic fluctuations persist in the normal phase of the superconducting compounds, but they are more diffuse. Below T_c, there is evidence in three ‘122’ compounds that these fluctuations condense into a resonant spin excitation at the antiferromagnetic wavevector with an energy that scales with T_c. Such resonances have been observed in the high-T_c copper oxides and a number of heavy fermion superconductors, where they are considered to be evidence of d-wave symmetry. In the iron arsenides, they also provide evidence of unconventional superconductivity, but a comparison with ARPES and other measurements, which indicate that the gaps are isotropic, suggests that the symmetry is more likely to be extended-s_± wave in character.     

Palladium atoms and its dimers adsorbed on graphene: First-principles study

In this work we have studied the stabilty, electronic and magnetic properties of Pd adatoms and dimers adsorbed on graphene system using first-principles calculations. The adsorption energies for Pd adatom and its dimer have been found to range from −0.986 to −1.135 eV and −0.165 to −1.101 eV, respectively, which signify stable configuration and future utilization of this system in catalysis. A shift but no separation of π and π^? bands at the Dirac point has been observed in case of Pd dimer adsorption in perpendicular configuration, which can be accounted for the breaking of symmetry of the graphene structure due to adsorption. 64-68% spin polarization P(E_F) and 1.944-1.990 μ_B magnetic moment have been observed for Pd dimers adsorbed on graphene in perpendicular configuration for different sites. The unequal values of partial density of states for 4d and 5s orbitals of Pd dimers at Fermi level have been found to be responsible for the generation of high spin polarization.     

Interface contributions to the spin-orbit interaction parameters of electrons at the (001) GaAs/AlGaAs interface

One-body mechanisms of spin splitting of the energy spectrum of 2D electrons in a one-side doped (001) GaAs/Al _x Ga_{1 ? x} As quantum well have been studied theoretically and experimentally. The interfacial spin splitting has been shown to compensate (enhance) considerably the contribution of the bulk Dresselhaus (Bychkov-Rashba) mechanism. The theoretical approach is based on the solution of the effective mass equation in a quasi-triangular well supplemented by a new boundary condition at a high and atomically sharp hetero-barrier. The model takes into account the spin-orbit interaction of electrons with both bulk and interfacial crystal potential having C _2v symmetry, as well as the lack of inversion symmetry and nonparabolicity of the conduction band in GaAs. The effective 2D spin Hamiltonian including both bulk and interface contributions to the Dresselhaus (α_BIA) and Rashba (α_SIA) constants has been derived. The analytical relation between these constants and the components of the anisotropic nonlinear g-factor tensor in an oblique quantizing magnetic field has been found. The experimental approach is based, on one hand, on the detection of electron spin resonance in the microwave range and, on the other hand, on photoluminescence measurements of the nonparabolicity parameter. The interface contributions to α_BIA and α_SIA have been found from comparison with the theory.     

CsNiCl3晶体的光谱精细结构、零场分裂参量及Jahn-Teller效应

应用不可约张量理论构造了三角对称晶场中3d²/3d⁸态离子的45阶可完全对角化的微扰哈密顿矩阵，研究了CsNiCl₃晶体的光谱精细结构、晶体结构、零场分裂参量、Jahn-Telller效应以及自旋单重态对Ni²⁺离子基态能级的影响，理论与实验相符合.在此基础上，进一步研究了以前工作中被忽略的自旋-自旋耦合作用和Trees修正对CsNiCl₃晶体的光谱精细结构和零场分裂参量的影响，发现有四种机理会影响零场分裂参量：1)自旋-轨道耦合机理，2)自旋-自旋耦合机理；3)自旋-轨道与自旋-自旋联合耦合机理；4)自旋-轨道与Trees修正联合耦合机理，其中自旋-轨道耦合机理是最主要的，其他三种机理也是不可忽略的. 关键词： 基态能级 精细结构 零场分裂 自旋-自旋耦合     

Crystal structure and the magnetic state of Pr0.5Sr0.5Co0.5Fe0.5O3

The magnetic properties and crystal structure of the Pr_0.5Sr_0.5Co_0.5Fe_0.5O₃ compound are studied by neutron and x-ray diffractions using synchrotron radiation. These measurements show that this compound is a dielectric spin glass with a magnetic moment freezing temperature of about 70 K. As temperature decreases in the range 30–95 K, a structure phase transition of the first order occurs with an increase in the symmetry from orthorhombic (space group Imma) to tetragonal (space group I4/mcm). It is assumed that the transition is caused by a change in the 4f electron configuration of the Pr³⁺ ions.     

Frustration-induced valence bond crystal and its melting in Mo3Sb7

(121/123)Sb nuclear quadrupole resonance and muon spin relaxation experiments of Mo_3Sb_7 revealed symmetry breakdown to a nonmagnetic state below the transition recently found at T_S approximately 50 K. The transition is characterized by a distinct lattice dynamics suggested from narrowing of nuclear fields. We point out that the Mo sublattice is a unique three-dimensional frustrated lattice where nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions compete, and propose that tetragonal distortion to release the frustration stabilizes long-range order of spin-singlet dimers, i.e., valence bond crystal, which is thermally excited to the dynamic state with cubic symmetry.     

Strongly coupled massive QCD3 and frustrated Heisenberg antiferromagnets: fractional statistics and chiral-spin order

Strongly coupled massive SU(N_C) and U(N_C) QCD₃ on a lattice is studied using the 1/N_C expansion. The quark mass terms have a definite sign in the present model, and therefore the system explicitly breaks the parity symmetry. The continuum counterpart generates the Maxwell + Chern-Simons theory by integrating out the quark field. In the present paper, we shall integrate out the gauge fields using the strong-coupling expansion and obtain a frustrated quantum Heisenberg model as an effective model. The ground state of the above effective quantum spin model is studied using the large-N_C approximation. There are two phases; one is a Neel-ordered state and the other is a state with a chiral-spin order. It is explicitly shown that the chiral-spin ordered state corresponds to a state with spontaneous generation of color magnetic flux in the original theory and fractional statistics appears in that phase. This result strongly suggests that there are (at least) two phases in the massive QCD₃ and Maxwell-CS theory. One is the confinement phase and the other is the perturbative deconfinement phase with fractional-statistics excitations.     

Effect of a strong magnetic field on the quadrupole and magnetic orders and crossover in the Jahn-Teller magnet DyVO<Subscript>4</Subscript>

The effect of a strong magnetic field H||[001] on magnetic properties of the Jahn-Teller compound DyVO₄ is studied. New phase transitions discovered and investigated experimentally and theoretically include the breaking of quadrupole order (enhancement of the crystal symmetry) and breaking of antiferromagnetic order as well as the effect of convergence of energy levels of the Dy³⁺ ion (crossover). The differential magnetic susceptibility of a DyVO₄ crystal is measured in fields up to 36 T in the temperature interval 1.4?15 K. The observed magnetic susceptibility anomalies and phase transitions are described using a unified theoretical approach.     

Systematics of Mössbauer parameters in Fe57-doped titanate and germanate ilmenites

The Mössbauer spectra of ilmenite (FeTiO₃), Fe²⁺-doped ilmenites, MTiO₃ (M = Cd, Mn, Zn, Co, Mg and Ni) and MGeO₃ (M = Mn, Zn and Mg), have been studied from 4.2 K to room temperature. Systematic variations in the isomer shift and quadrupole splitting at room temperature are correlated with the change in ionic radii of the host M ions and the c/a ratio of the host crystal, respectively. The quadrupole splittings show a remarkable temperature variation; the magnetic ilmenites show either a sudden increase or a sudden decrease of quadrupole splittings below the Néel temperature, depending on the orientation of the spin axis with respect to the c axis.     

Quadrupole interaction of8Li and9Li in LiNbO3 and the quadrupole moment of9Li

The quadrupole interaction of nuclear spin polarized⁸Li (I=2) and⁹Li (I=3/2) in LiNbO₃ has been studied at room temperature. The polarization was achieved by optical pumping of a fast atomic beam with circularly polarized laser light. The atoms were implanted into a hexagonal LiNbO₃ single crystal and the quadrupole splitting ofβ-NMR spectra was measured. A ratio of ¦Q(⁹Li)/Q(⁸Li)¦=0.88(4) for the nuclear quadrupole moments was deduced, yielding a new value of ¦Q(⁹Li)¦=25.3 (9) mb for the quadrupole moment of⁹Li.     

Mössbauer spectroscopy on57Fe in highT c superconductor Bi2Sr2Ca1Cu2O y

Mössbauer measurements were performed on polycrystalline⁵⁷Fe: Bi₂Sr₂Ca₁Cu₂O _y , super-conductor in the temperature range of 77–296 K. The samples were obtained in a solid phase synthesis using 0.01, 0.03, 0.1 and 0.5 mol fractions of α-Fe₂O₃ (96% enriched in⁵⁷Fe). A prevailing quadrupole doublet practically independent of temperature and iron concentration characterizes the obtained Mössbauer spectra. The corresponding hyperfine parameters suggest the presence of high spin Fe¹¹¹ ions in a strongly distorted octahedral symmetry which indicates a probable copper substitution by iron in the system.     

Magnetic and crystal structure of copper(II) fluoride

A powder sample of the monoclinic, weak ferromagnet CuF₂ was investigated by neutron diffraction. Using the profile method, the crystal structure was refined and the spin configuration determined. The layer type structure with planar quadratic fluorine coordination of Cu²⁺ (3d⁹) and the magnetic structure are remarkably similar to those of AgF₂. The spin configuration is however different from the magnetic structures of other 3d-fluorides; the magnetic unit cell is doubled with respect to the chemical cell (a_m = 2a, P2₁/c). The ordered magnetic saturation moment corresponds to quenched orbital momentum.     

Structural and lattice-energy analyses of the different low-temperature phases of ferrocene Fe(C5H5)2: Relation between the molecular packings

X-Ray analysis of the metastable triclinic phase of ferrocene has been carried out at 130 K, from a single crystal cooled through the transition temperature (164 K). The molecular packing is close to a face centred one; however the numerous reflections breaking this symmetry give evidence for a P1 or P1&#x0304; superstructure which appears to be disordered. Calculations of lattice energy performed at 130 K and 15 K show that the triclinic phase of ferrocene has two possible molecular packings, quite energetically similar, of face-centred or P1&#x0304; symmetry. Thus, when the triclinic phase is obtained by cooling a crystal from room temperature, its structure is constituted by a mixture of both packings whose ratio depends on the thermal history. On the other hand, the molecular coordinates and the configuration of ferrocene (D_5h) in the stable ordered orthorhombic phase have been determined by minimizing the lattice energy. The mechanism of the order-disorder phase transition at 250 K (orthorhombic monoclinic), as also the existence below 164 K of another ordered packing, the triclinic one, are discussed in terms of ring libration.     

Rare-Earth Chalcogenides: A Large Family of Triangular Lattice Spin Liquid Candidates

Frustrated quantum magnets are expected to host many exotic quantum spin states like quantum spin liquid(QSL), and have attracted numerous interest in modern condensed matter physics. The discovery of the triangular lattice spin liquid candidate YbMgGaO_4 stimulated an increasing attention on the rare-earth-based frustrated magnets with strong spin-orbit coupling. Here we report the synthesis and characterization of a large family of rare-earth chalcogenides AReCh_2(A = alkali or monovalent ions, Re = rare earth, Ch = O,S,Se). The family compounds share the same structure(R3 m) as YbMgGaO_4,and antiferromagnetically coupled rare-earth ions form perfect triangular layers that are well separated along the c-axis. Specific heat and magnetic susceptibility measurements on NaYbO_2,NaYbS_2 and NaYbSe_2 single crystals and polycrystals, reveal no structural or magnetic transition down to 50 mK. The family, having the simplest structure and chemical formula among the known QSL candidates, removes the issue on possible exchange disorders in YbMgGaO_4. More excitingly, the rich diversity of the family members allows tunable charge gaps, variable exchange coupling, and many other advantages.This makes the family an ideal platform for fundamental research of QSLs and its promising applications.     

Mössbauer study of Fe‐doped CuGeO3 system Cu0.9Ge0.9Fe0.2O3

The Fe‐doped system Cu_0.9Ge_0.9Fe_0.2O₃ has been investigated by means of X‐ray diffractometry, Mössbauer spectroscopy and superconducting quantum interference device. The structure of this system is orthorhombic and the lattice constants are a=4.784 Å, b=8.472 Å and c=2.904 Å, respectively. Magnetic measurements confirm that the spin‐Peierls transition appears in our sample at about 12 K, which is near to the spin‐Peierls transition temperature (T _sp) 14 K of pure CuGeO₃ system. The Mössbauer spectrum shows the superposition of two Zeeman sextets and a broad central line due to Fe³⁺ ions from room temperature to 4.2 K. The Mössbauer parameters show a discontinuity near T _sp. The jump of the magnetic hyperfine field at temperatures lower than T _sp means increasing of the superexchange interaction among the magnetic ions. The jump of the quadrupole splitting and the isomer shift values could be interpreted as due to decrement in symmetry of lattice sites and spontaneous thermal contraction.