Antiferromagnetic Spin-spin Interactions Research Articles

Many-body effects on second-order phase transitions in spinor Bose-Einstein condensates and breathing dynamics

We unravel the correlation effects of the second-order quantum phase transitions emerging on the ground state of a harmonically trapped spin-1 Bose gas, upon varying the involved Zeeman terms, as well as its breathing dynamics triggered by quenching the trapping frequency. It is found that the boundaries of the associated magnetic phases are altered in the presence of interparticle correlations for both ferromagnetic and anti-ferromagnetic spin-spin interactions, an effect which becomes more prominent in the few-body scenario. Most importantly, we unveil a correlation-induced shrinking of the anti-ferromagnetic and broken-axisymmetry phases implying that ground states with bosons polarized in a single spin-component are favored. Turning to the dynamical response of the spinor gas it is shown that its breathing frequency is independent of the system parameters while correlations lead to the formation of filamentary patterns in the one-body density of the participating components. The number of filaments is larger for increasing spin-independent interaction strengths or for smaller particle numbers. Each filament maintains its coherence and exhibits an anti-correlated behavior while distinct filaments show significant losses of coherence and are two-body correlated. Interestingly, we demonstrate that for an initial broken-axisymmetry phase an enhanced spin-flip dynamics takes place which can be tuned either via the linear Zeeman term or the quench amplitude.

Synthesis, crystal structure, and magnetic properties of oxynitride perovskites SrMn0.2M0.8O2.6N0.4 (M = Nb, Ta).

Complex perovskites SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 were synthesized; these are rare examples of octahedral Mn2+ in oxynitride perovskites. Joint Rietveld refinement of neutron and X-ray data revealed that SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 had orthorhombic symmetries, in contrast with those of analogous perovskites SrM'0.2M0.8O3-xNx (M' = Li, Na, Mg; M = Nb, Ta), which are all tetragonal. Both SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 exhibited paramagnetic behavior with effective magnetic moments of 5.60μB and 5.94μB, respectively, consistent with a high-spin Mn2+ (d5, S = 5/2) state. The Weiss constants were -24.7 K for SrMn0.2Nb0.8O2.6N0.4 and -15.4 K for SrMn0.2Ta0.8O2.6N0.4, indicating the presence of weak antiferromagnetic spin-spin interactions. The band gaps of SrMn0.2Nb0.8O2.6N0.4 and SrMn0.2Ta0.8O2.6N0.4 were determined to be 1.75 eV and 2.2 eV, respectively, suggesting that the Mn 3d electrons were essentially localized.

Ionothermal Synthesis, Structures, and Magnetism of Three New Open Framework Iron Halide-Phosphates.

A set of different open framework iron phosphates have been synthesized ionothermally using a task-specific ionic liquid, 1-butyl-4-methylpyridinium hexafluorophosphate, that acts in the synthesis as the reaction medium and mineralizer: (NH4)2Fe2(HPO4)(PO4)Cl2F (1) and K2Fe2(HPO4)(PO4)Cl2F (2) exhibit similar composition and closely related structural features. Both structures consist of {Fe2(HPO4)(PO4)Cl2F}2- macroanions and charge balancing ammonium or potassium cations. Their open framework structure contains layers and chains of corner-linked {Fe(1)O2Cl4} and {Fe(2)F2O4} octahedra, respectively, interconnected by PO4 tetrahedra forming 10-ring channels. KFe(PO3F)F2 (3) is built up by {Fe[(PO3F)4/3F2/2]}{Fe(PO3F)2/3F2/2F2} layers separated by K+ cations. Chains of alternating {FeF2O4} and {FeO2F4} octahedra, which are linear for 1 but undulated for 2, are linked to each other via corner-sharing {PO3F} tetrahedra with the fluorine pointing into the interlayer space. The compounds were characterized by means of single crystal and powder X-ray diffraction, infrared spectroscopy, and magnetic measurements. 1 reveals a strong ground state spin anisotropy with a spin 5/2 state and a magnetic moment of 5.3 μB/Fe3+. Specific heat and magnetic data unveil three magnetic transitions at 95, 50, and 3.6 K. Compound 2 has a very similar crystal structure as compared to 1 but exhibits a different magnetic behavior: a slightly lower magnetic moment of 4.7 μB/Fe3+ and a magnetic transition to a canted antiferromagnetic state below 90 K. Compound 3 exhibits typical paramagnetic behavior close to room-temperature (5.71 μB/Fe3+). There are no clear indications for a phase transition down to 2 K despite strong antiferromagnetic spin-spin interactions; only a magnetic anomaly appears at 50 K in the zero-field cooled data.

Theoretical study of anisotropy in orbital and antiferromagnetic spin orderings in CMR manganites

We report here a tight binding model for doped rare earth manganites consisting of Kubo-Ohata type double exchange (DE) interaction between the eg and t2g band electron spins of same site. A Heisenberg type antiferromagnetic spin-spin interaction is considered among the core band electrons. Both nearest and next nearest neighbouring interactions are taken in the Heisenberg coupling and kinetic energy interaction. The presence of DE interaction induces antiferromagnetism (AFM) in eg band. The Jahn-Teller (JT) distortion leads to anisotropy in the manganite systems. We have considered JT distortion as an extra mechanism. The model Hamiltonian is solved using Zubarev's Green's function technique. The interplay between temperature dependent lattice strain and transverse spin fluctuations is studied. We have studied the in-plane anisotropy observed in the AFM spin fluctuation and its effect on the lattice strain.

Dramatic enhancement of spin-spin coupling and quenching of magnetic dimensionality in compressed silver difluoride.

Meta-GGA calculations of the ambient and high-pressure polymorphs of silver difluoride indicate that the compression-induced structural changes lead to a 3.5-fold increase in the strength of antiferromagnetic spin-spin interactions resulting in coupling constant values higher than those found for record-holding oxocuprates(ii).

Fluorenyl Based Macrocyclic Polyradicaloids.

Synthesis of stable open-shell polyradicaloids including control of intramolecular spin-spin interactions is a challenging topic in organic chemistry and materials science. Herein, we report the synthesis and physical characterization of two series of fluorenyl based macrocyclic polyradicaloids. In one series (FR-MCn, n = 4-6), the fluorenyl radicals are directly linked at 3,6-positions; whereas in the other series (MC-FnAn, n = 3-5), an additional ethynylene moiety is inserted between the neighboring fluorenyl units. To access stable macrocyclic polyradicaloids, three synthetic methods were developed. All of these stable macrocycles can be purified by normal silica gel column chromatography under ambient conditions. In all cases, moderate polyradical characters were calculated by restricted active space spin-flip method due to the moderate intramolecular antiferromagnetic spin-spin interactions. The excitation energies from the low-spin ground state to the lowest high-spin excited state were evaluated by superconducting quantum interference device measurements. Their physical properties were also compared with the respective linear fluorenyl radical oligomers (FR-n, n = 3-6). It is found that the geometry, i.e., the distortional angle and spacer (w or w/o ethynylene) between the neighboring fluorenyl units, has significant effect on their polyradical character, excitation energy, one-photon absorption, two-photon absorption and electrochemical properties. In addition, the macrocyclic tetramers FR-MC4 and MC-F4A4 showed global antiaromatic character due to cyclic π-conjugation with 36 and 44 π-electrons, respectively.

Ambient Stable Radical Cations, Diradicaloid π-Dimeric Dications, Closed-Shell Dications, and Diradical Dications of Methylthio-Capped Rylenes.

Radical cations and dications of π-conjugated systems play vital roles in organic electronic devices, organic conductors, and conducting polymers. Their structures, charge and spin distribution, and mechanism of charge transport are of great interest. In this article, radical cations and dications of a series of newly synthesized methylthio-capped rylenes were synthesized and isolated. Their ground-state structures, physical properties, and solid-state packing were systematically investigated by various experimental methods, such as X-ray crystallographic analysis, UV/Vis/NIR absorption spectroscopy, (spectro-)electrochemistry, nuclear magnetic resonance spectroscopy, electron spin resonance spectroscopy, superconducting quantum interference device, and Raman spectroscopy, assisted by DFT calculations. It was found that all the charged species show an exceptional stability under ambient air and light conditions due to the efficient spin and charge delocalization over the whole rylene backbone. The dication of hexarylene turned out to have an unusual open-shell singlet rather than closed-shell ground state, thus it can be described as a diradical dication. Dimerization was observed for the radical cations and even the dications in crystals due to the strong intermolecular antiferromagnetic spin-spin interaction and π-π interaction, which result in unique magnetic properties. Such intermolecular association was also observed in solution.

Synthesis, X-ray structure, spectroscopic, electrochemical properties and DFT calculation of a bridged dinuclear copper(II) complex

A bridged dinuclear Schiff base-type copper(II) complex, [(Cu2(L2)2CH3COO)]2[Cu(NO3)4]·2CH3COOCH2CH3 (HL2=1-(2-{[(E)-3,5-dibromo-2-hydroxybenzylidene]amino}phenyl)ethanone oxime), has been synthesized via complexation of Cu(II) nitrate trihydrate with HL1 (2-(3,5-dibromo-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazoline-3-oxide, H is the deprotonatable hydrogen). During the reaction, the C–N bond in the HL1 is converted into the CN–OH group in the ligand HL2. The Cu(II) complex was analyzed using IR and UV–Vis spectroscopy, elemental analysis and single-crystal X-ray crystallography. The spectroscopic data of the Cu(II) complex was compared with the ligand HL1. In the crystal, each complex links with four other molecules into an infinite 2-D supramolecular structure through intermolecular hydrogen bonding and π⋯π stacking interactions. Moreover, the electrochemical properties of HL1 and Cu(II) complex have been studied by cyclic voltammetry. The powder EPR spectrun of the dinuclear Cu(II) complex indicated that the existence of an antiferromagnetic spin-spin interaction between the magnetically nonequivalent two Cu(II) ions. The geometries and electronic properties of both the ligand HL1 and the Cu(II) complex were studied by DFT calculation.

Static and dynamic properties of interacting spin-1 bosons in an optical lattice

We study the physics of interacting spin-$1$ bosons in an optical lattice using a variational Gutzwiller technique. We compute the mean-field ground state wave-function and discuss the evolution of the condensate, spin, nematic, and singlet order parameters across the superfluid-Mott transition. We then extend the Gutzwiller method to derive the equations governing the dynamics of low energy excitations in the lattice. Linearizing these equations, we compute the excitation spectra in the superfluid and Mott phases for both ferromagnetic and antiferromagnetic spin-spin interactions. In the superfluid phase, we recover the known excitation spectrum obtained from Bogoliubov theory. In the nematic Mott phase, we obtain gapped, quadratically dispersing particle and hole-like collective modes, whereas in the singlet Mott phase, we obtain a non-dispersive gapped mode, corresponding to the breaking of a singlet pair. For the ferromagnetic Mott insulator, the Gutzwiller mean-field theory only yields particle-hole like modes but no Goldstone mode associated with long range spin order. To overcome this limitation, we supplement the Gutzwiller theory with a Schwinger boson mean-field theory which captures super-exchange driven fluctuations. In addition to the gapped particle-hole-like modes, we obtain a gapless quadratically dispersing ferromagnetic spin-wave Goldstone mode. We discuss the evolution of the singlet gap, particle-hole gap, and the effective mass of the ferromagnetic Goldstone mode as the superfluid-Mott phase boundary is approached from the insulating side. We discuss the relevance and validity of Gutzwiller mean-field theories to spinful systems, and potential extensions of this framework to include more exotic physics which appears in the presence of spin-orbit coupling or artificial gauge fields.

Critical phenomena in one dimension from a Bethe ansatz perspective

This article briefly reviews recent theoretical developments in quantum critical phenomena in one-dimensional (1D) integrable quantum gases of cold atoms. We present a discussion on quantum phase transitions, universal thermodynamics, scaling functions and correlations for a few prototypical exactly solved models, such as the Lieb–Liniger Bose gas, the spin-1 Bose gas with antiferromagnetic spin-spin interaction, the two-component interacting Fermi gas as well as spin-3/2 Fermi gases. We demonstrate that their corresponding Bethe ansatz solutions provide a precise way to understand quantum many-body physics, such as quantum criticality, Luttinger liquids (LLs), the Wilson ratio, Tan's Contact, etc. These theoretical developments give rise to a physical perspective using integrability for uncovering experimentally testable phenomena in systems of interacting bosonic and fermonic ultracold atoms confined to 1D.

Finite temperature phase diagram of spin-1/2 bosons in two-dimensional optical lattice

We study a two-species bosonic Hubbard model on a two-dimensional square lattice by means of quantum Monte Carlo simulations and focus on finite temperature effects. We show in two different cases, ferro- and antiferromagnetic spin-spin interactions, that the phase diagram is composed of solid Mott phases, liquid phases and superfluid phases. In the antiferromagnetic case, the superfluid (SF) is polarized while the Mott insulator (MI) and normal Bose liquid (NBL) phases are not. On the other hand, in the ferromagnetic case, none of the phases is polarized. The superfluid-liquid transition is of the Berezinsky-Kosterlitz-Thouless type whereas the solid-liquid passage is a crossover.

Spin-Disorder and Non-Degenerate Energy States in Geometrically Frustrated Materials

We have studied chalcogenide Ni <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , showing geometrical frustration effect. M-H curves at 4.2 K reveal that the disordered spins cannot be rotated completely along the direction of high external field of 5 T, since these spins are strongly constrained in the triangular lattice. H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> increases with increasing Fe concentration, being consistent with the enhanced antiferromagnetic (AFM) spin-spin interactions and the suppressed spin-fluctuation due to the increase in freezing temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> . The specific heat (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> /T) measurement do not show any phase transformation between 2 and 160 K and there is no clear indication of gap in the temperature dependent C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P</sub> curve between 0 and 7 T, because the atomic short-range ordering of the strained spin in the geometrically frustrated triangular lattice exists at low temperature. Though typical geometrically frustrated magnet shows degeneracy, Mossbauer analysis at 4.2 K shows that 5D of 3d orbit in the samples studied here is splitted into <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2g</sub> and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2g</sub> , and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2g</sub> is further splitted into singlet and doublet by large electric quadrupole splitting. This suggests that Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ionic state has no absolute degenerate energy states in all samples due to Jahn-Teller effect.

Greenberger-Horne-Zeilinger states in a quantum dot molecule

We present a microscopic theory of a lateral quantum dot molecule in a radial magnetic field with a Greenberger-Horne-Zeilinger (GHZ) maximally entangled three particle ground state. The quantum dot molecule consists of three quantum dots with one electron spin each forming a central equilateral triangle. The antiferromagnetic spin-spin interaction is changed to the ferromagnetic interaction by additional doubly occupied quantum dots, one dot near each side of a triangle. The magnetic field is provided by micromagnets. The interaction among the electrons is described within an extended Hubbard Hamiltonian which is solved by using exact diagonalization techniques. The set of parameters is established for which the ground state of the molecule in a radial magnetic field is well approximated by a GHZ state.

Quantum chemical interpretation of different types of exchange magnetic interactions in dinuclear copper azomethine chelates

B3LYP/LANL2DZ calculations of two series of isomeric dinuclear copper chelates based on the tridentate dibasic salicylaldimines from aromatic and heterocyclic amines were performed. According to calculations, the complexes with N,N-or S,S-donor atoms in the exchange fragment are mainly characterized by ferromagnetic exchange, whereas the formation of O, O-bridging fragments leads to antiferromagnetic spin-spin interaction.

Ferromagnetic properties ofp-(Cd,Mn)Te quantum wells: Interpretation of magneto-optical measurements by Monte Carlo simulations

In order to single out dominant phenomena that account for carrier-controlled magnetism in $p{\text{-Cd}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{Te}$ quantum wells we have carried out magneto-optical measurements and Monte Carlo simulations of time-dependent magnetization. The experimental results show that magnetization relaxation is faster than 20 ns in the paramagnetic state. Decreasing temperature below the Curie temperature ${T}_{C}$ results in an increase of the relaxation time but to less than $10\text{ }\ensuremath{\mu}\text{s}$. This fast relaxation may explain why the spontaneous spin splitting of electronic states is not accompanied by the presence of nonzero macroscopic magnetization below ${T}_{C}$. Our Monte Carlo results reproduce the relative change of the relaxation time on decreasing temperature. At the same time, the numerical calculations demonstrate that antiferromagnetic spin-spin interactions, which compete with the hole-mediated long-range ferromagnetic coupling, play an important role in magnetization relaxation of the system. We find, in particular, that magnetization dynamics is largely accelerated by the presence of antiferromagnetic couplings to the Mn spins located outside the region, where the holes reside. This suggests that macroscopic spontaneous magnetization should be observable if the thickness of the layer containing localized spins will be smaller than the extension of the hole wave function. Furthermore, we study how a spin-independent part of the Mn potential affects ${T}_{C}$. Our findings show that the alloy disorder potential tends to reduce ${T}_{C}$, the effect being particularly strong for the attractive potential that leads to hole localization.

Novel CT Complexes Derived from 4-Alkylamino-TEMPO and 2,4,6,8-Tetracyanoazulene

Novel CT complexes derived from 4-alkylamino-TEMPO radical and 2,4,6,8-tetracyanoazulene have been prepared. Curie-Weiss behavior has been observed in the magnetic susceptibility of each complex with antiferromagnetic spin-spin interactions in the low temperature region on one hand and rather high conductivity has been found in the electrical conductivity of each complex with semiconducting behavior on the other hand.

Magnetic correlations, geometrical frustration, and tunable disorder in arrays of superconducting rings

A superconducting ring, biased in an external flux ${\mathrm{\ensuremath{\Phi}}}_{0}$/2, can be in either of two energetically degenerate fluxoid states. In one state, the supercurrent flows in a clockwise direction with a resulting downward magnetic moment; the current in the other state flows in a counterclockwise direction and its moment points up. There is thus a strong analogy between such a ring and an Ising spin. Two nearby but electrically isolated rings can interact magnetically; this interaction favors an antiparallel alignment of moments and is thus analogous to an antiferromagnetic spin-spin interaction. Regular arrays of such rings may thus be expected to exhibit effects of lattice geometry and geometrical frustration. To study these issues, we have fabricated arrays containing up to 2.4 \ifmmode\times\else\texttimes\fi{}${10}^{5}$ aluminum rings, each approximately 1.6 \ensuremath{\mu}m across. We have used a sensitive superconducting quantum interference device-based magnetometer to probe the global magnetic properties of the arrays; local information about particular spin configurations was obtained using a high-resolution scanning Hall probe microscope. The magnetic measurements show that individual rings do indeed behave as Ising spins, showing a paramagnetic susceptibility which freezes out only a few milliKelvin below the critical temperature ${\mathrm{T}}_{\mathrm{c}}$. This illustrates that the ring dynamics is dominated by an energy barrier between the two states which rises rapidly as the temperature is lowered below ${\mathrm{T}}_{\mathrm{c}}$. The magnetic measurements also show a hysteretic field dependence of the susceptibility which can be quantitatively interpreted in terms of an antiferromagnetic interaction between the rings. To explore possible ordering of the spins, we have used the Hall microscope to directly image specific configurations of spins. We find significant antiferromagnetic nearest-neighbor correlations, but no evidence for any long-range ordering. We attribute this to a significant degree of disorder in the system related to small fluctuations in the areas of the aluminum rings. The effective disorder may be increased by working at higher fractions of ${\mathrm{\ensuremath{\Phi}}}_{0}$. The observed short-range correlations drop rapidly at these higher fractions.

Synthesis, crystal structure and magnetism of Sr 3Sb 2NiO 9—A ferrimagnetic perovskite

A new nickel(II) oxide, Sr 3Sb 2NiO 9, has been synthesised at 1200 °C. Rietveld fits to the powder synchrotron X-ray diffraction pattern show that this compound has orthorhombic symmetry [ a = 5.6237(1), b = 5.6442(1), c = 7.9574(1) A ̊ at 300 K ] and adopts a pseudo-double perovskite structure with Sb:Ni ratios of 40:60 and 93:7 at alternate cation sites. Powder neutron refinements demonstrate that space group symmetry is Pnnm between 1.5 and 300 K due to partly disordered rotations of the Sb/NiO 6 octahedra. Sr 3Sb 2NiO 9 behaves as a Curie-Weiss paramagnet at high temperatures with μ eff = 3.25(1) BM per Ni 2+ ion and ⊖ = − 119(1) K reflecting dominant antiferromagnetic spin-spin interactions, but a weak ferromagnetic component observed below the ordering temperature of 20 K suggests overall ferrimagnetic behaviour.

Motion of two holes in an interacting spin background.

An antiferromagnetic spin model (S=1/2) consisting of a chain of octahedra is considered. In a certain parameter regime of the Hamiltonian the exact ground state is a local resonating-valence-bond state. We start with the t-J Hamiltonian for this model describing two holes in the same octahedron, one in the base and the other at one of the vertex sites of the octahedron. The t-J Hamiltonian consists of three parts: ${\mathit{H}}_{\mathit{t}1}$, describing the motion of holes along the chain, ${\mathit{H}}_{\mathit{t}2}$, describing the motion of holes in the bases of the octahedra, and ${\mathit{H}}_{\mathit{J}}$, describing the antiferromagnetic spin-spin interaction. Some exact eigenstates of ${\mathit{H}}_{\mathit{t}2}$ and ${\mathit{H}}_{\mathit{J}}$ are constructed. Successive applications of ${\mathit{H}}_{\mathit{t}1}$ on some of these states show two types of localization for one of the holes. In one type of localization one of the holes is localized at its original location while the other hole moves. In the other type of localization, a hole, located in the base of an octahedron, cannot move away from the other hole sitting at a vertex site of the same octahedron. The localization in both cases is produced by the topology of the lattice on which the t-J model is defined.

Many Commensurate Phases of a One-Dimensional Systemof Hard-Core Particles with Ising Spin Variables

We introduce a new one-dimensional competitive system constructed of particles with Ising spin variables in a periodic substrate potentials, a hard-core type inter-particle interaction and a spin-spin interaction whose exchange energy depends linearly on the distance between adjacent particles. The hard-core interaction mediates an antiferromagnetic spin-spin interaction. Although the range of the inter-particle interaction is finite, long-period commensurate phases are caused. Many commensurate phases appear in the ground state phase diagram.