Strong Antiferromagnetic Correlations Research Articles

The temperature evolution of the magnetic correlations in pure and diluted spin ice Ho2−xYxTi2O7

Diffuse polarized neutron scattering studies have been carried out on single crystals of pyrochlore spin ice Ho2−xYxTi2O7 (x=0, 0.3, and 1) to investigate the effects of doping and anisotropy on spin correlations in the system. The crystals were aligned with the (1−10) orientation coincident with the direction of neutron polarization. For all the samples studied the spin flip (SF) diffuse scattering (i.e. the in-plane component) reveals that the spin correlations can be described using a nearest-neighbour spin ice model (NNSM) at higher temperatures (T=3.6K) and a dipolar spin ice model (DSM) as the temperature is reduced (T=30mK). In the non-spin flip (NSF) channel (i.e. the out-of-plane component), the signature of strong antiferromagnetic correlations is observed for all the samples at the same temperature as the dipolar spin ice behaviour appears in the SF channel. Our studies show that the non-magnetic dopant Y does not significantly alter SF or NSF scattering for the spin ice state, even when Y doping is as high as 50%. In this paper, we focus on the experimental results of the highly doped spin ice HoYTi2O7 and compare our results with pure spin ice Ho2Ti2O7. The crossover from a dipolar to a nearest-neighbour spin ice behaviour and the doping insensitivity in spin ices are briefly discussed.

Scanning tunneling spectroscopic evidence of crossover transition in the two-impurity Kondo problem

To explore the crossover transition in the two-impurity Kondo problem, we calculate the differential conductance (d I / d V) corresponding to scanning tunneling spectroscopy (STS) measurements of a magnetic dimer adsorbed on a metal surface covered by a decoupling layer. With the aid of the numerical renormalization group (NRG) technique, we find that the peak structure of the d I / d V spectra near the Fermi level changes gradually as a function of the adatom separation and the coupling between the adatom localized spins and the metal surface conduction band. When the coupling becomes small, the peak disappears and, instead, a dip structure appears near the Fermi level. This dip structure is the manifestation of the strong antiferromagnetic correlation between the localized spins. We conclude that the gradual change of the d I / d V structure from a peak structure to a dip structure is an evidence of the crossover transition in the two-impurity Kondo problem.

Paramagnetic spin correlations inCaFe2As2single crystals

Magnetic correlations in the paramagnetic phase of CaFe2As2 (T_N=172 K) have been examined by means of inelastic neutron scattering from 180 K (~ 1.05 T_N) up to 300 K (~1.8 T_N). Despite the first-order nature of the magnetic ordering, strong but short-range antiferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasi-elastic scattering centered at the wavevector Q_{AFM} of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer (E> 60 meV). The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c-direction corresponding to nearly two-dimensional fluctuations in the (ab) plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt doped superconducting BaFe2As2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-range anisotropic spin correlations with overdamped spin dynamics.

Optical conductivity of spin/lattice polarons in underdoped copper oxides

Optical and spectral properties of carriers in the presence of strong antiferromagnetic correlations and interacting with optical phonon modes are analyzed using Dynamical Mean Field Theory. We interpret the mid-infrared band in σ( ω) in term of mixed spin lattice polaronic excitations which arise from the stabilization of the lattice polaron due to the antiferromagnetic correlations. We compare our results with experimental data in NCCO showing that the doping and temperature dependences of the optical conductivity in this compound is naturally reproduced within a spin/lattice polaronic model.

Non local theory of excitations applied to the Hubbard model

We propose a nonlocal theory of single-particle excitations. It is based on an off-diagonal effective medium and the projection operator method for treating the retarded Green function. The theory determines the nonlocal effective medium matrix elements by requiring that they are consistent with those of the self-energy of the Green function. This arrows for a description of long-range intersite correlations with high resolution in momentum space. Numerical study for the half-filled Hubbard model on the simple cubic lattice demonstrates that the theory is applicable to the strong correlation regime as well as the intermediate regime of Coulomb interaction strength. Furthermore the results show that nonlocal excitations cause sub-bands in the strong Coulomb interaction regime due to strong antiferromagnetic correlations, decrease the quasi-particle peak on the Fermi level with increasing Coulomb interaction, and shift the critical Coulomb interaction UC2 for the divergence of effective mass towards higher energies at least by a factor of two as compared with that in the single-site approximation.

Magnetic resonance in quantum spin chains

The present understanding of quantum spin chains is reviewed from the magnetic resonance point of view. This includes both the ideal one-dimensional properties in the spin sector as well as the complex interplay with orbital, charge, and lattice degrees of freedom which govern the ground state. In copper-phosphates we observe an extremely extended paramagnetic regime governed by strong antiferromagnetic correlations with record values of the ratio kBTN/J < 6×10−4, which compares the ordering temperature of a Neel state to the magnitude of the exchange J between neighbouring spins. A detailed quantitative discussion of NMR and ESR relaxation within this paramagnetic regime elucidates the relevant exchange interactions in typical bonding geometries of most common quantum-spin-chain systems like KCuF3, CuGeO3, NaxV2O5, and LiCuVO4. Concerning the ground state, paramount topics of modern solid-state physics arise among these examples as there are multiferroicity, charge order, metal-insulator transition, and spin dimerization as well as phase separation.

Impact of High Pressure and High Temperature on Annealed Oxygen-deficient CoSrO3−δ Perovskites: Structural and Magnetization Studies

In this brief article, structural, magnetization studies of oxygen deficient perovskite cobalt oxides CoSrO3−δ synthesized in two different ways are reported. The structural refinements (JANA2006) of X-ray powder diffraction data indicate that the sample prepared under ambient pressure (stage-1) shows a hexagonal structure with P63/mmc (194) as a possible space group. The stage-1 sample subsequently sintered at 1450 °C for 1–2 h under high-pressure 6 GPa conditions (stage-2) crystallizes in tetragonal symmetry with I4/mcm (140) space group having a=5.444(7) A and c=7.68(2) A. While the stage-1 sample exhibits a paramagnetic nature in the magnetic susceptibility, M(T), measurements, interestingly the sample annealed under high-pressure conditions shows ferromagnetism in the magnetic susceptibility, M(T) and field dependence magnetization, M(H) measurements. The high-pressure annealed sample shows hysteresis opening with a quite large coercive field of 7.3 kOe at 1.8 K. The temperature dependence of the inverse molar susceptibility curves exhibits linear behavior in the high-temperature regime and could be fitted to the Curie–Weiss expression, χ(T)=C/(T−θW). The experimental values of θW and peff obtained from the linear region of the inverse molar magnetic susceptibility curves are found to be: −210.7(5) K and 2.38(2) μB/Co for stage-1 and 260.2(7) K and 1.87(3) μB/Co for stage-2 samples, respectively. A negative sign of θW indicates rather strong antiferromagnetic correlations in the stage-1 sample. Apart from these results, the structural parameters reported by various groups for the strontium-based perovskite cobalt oxides are also presented in the form of literature collections.

Antiferromagnetically driven electronic correlations in iron pnictides and cuprates

The iron pnictides and the cuprates represent two families of materials, where strong antiferromagnetic correlation drives three other distinct ordering tendencies: (1) superconducting pairing, (2) Fermi-surface distortion, and (3) orbital-current order. We propose that (1)--(3) and the antiferromagnetic correlation are the hallmarks of a class of strongly correlated materials to which the cuprates and pnictides belong. In this paper, we present the results of the functional renormalization-group studies to support the above claim. In addition, we show that as a function of the interlayer hopping parameter, the double-layer Hubbard model nicely interpolates between the cuprate and the iron pnictide physics. Finally, as a check, we will present the renormalization-group study of a ladder version of the iron pnictide and compare the results to those of the two-dimensional model.

Turning a Nickelate Fermi Surface into a Cupratelike One through Heterostructuring

Using the local density approximation and its combination with dynamical mean-field theory, we show that electronic correlations induce a single-sheet, cupratelike Fermi surface for hole-doped 1/1 LaNiO3/LaAlO3 heterostructures, even though both eg orbitals contribute to it. The Ni 3d3z(2)-1} orbital plays the role of the axial Cu 4s-like orbital in the cuprates. These two results indicate that "orbital engineering" by means of heterostructuring should be possible. As we also find strong antiferromagnetic correlations, the low-energy electronic and spin excitations in nickelate heterostructures resemble those of high-temperature cuprate superconductors.

Influence of magnetism on phonons inCaFe2As2as seen via inelastic x-ray scattering

In the iron pnictides, the strong sensitivity of the iron magnetic moment to the arsenic position suggests a significant relationship between phonons and magnetism. We measured the phonon dispersion of several branches in the high temperature tetragonal phase of CaFe2As2 using inelastic x-ray scattering on single-crystal samples. These measurements were compared to ab initio calculations of the phonons. Spin polarized calculations imposing the antiferromagnetic order present in the low temperature orthorhombic phase dramatically improve agreement between theory and experiment. This is discussed in terms of the strong antiferromagnetic correlations that are known to persist in the tetragonal phase.

Strong antiferromagnetic correlation effects on the momentum distribution function of theHubbard model

Strong antiferromagnetic correlations can radically change the behavior of an otherwiseparamagnetic system, inducing anomalous effects in the single particle properties anddriving the behavior of spin–spin response functions. In order to investigate such physics,we have studied the momentum distribution function of the two-dimensional Hubbardmodel at low doping, low temperatures and high values of the on-site Coulombrepulsion. The interpretation of the results has greatly benefited from the parallelanalysis of the filling, temperature and interaction dependences of the spin–spincorrelation function, the antiferromagnetic correlation length and the pole of thespin–spin propagator. On reducing doping or temperature and on increasing theinteraction strength, the correlations become stronger and stronger and the Fermisurface develops hole and electron pockets and shows a dual nature (small–large).

Anisotropy in the Electrical Resistivity and Susceptibility of SuperconductingBaFe2As2Single Crystals

We report that sizable single crystals of BaFe2As2 have been grown with the self-flux method. Measurements and anisotropy of intrinsic transport and magnetic properties from high-quality single crystal are first presented. The resistivity anisotropy (rho{c}/rho{ab}) is as large as 150 and independent of temperature. In contrast to the susceptibility behavior observed in polycrystalline samples, no Curie-Weiss behavior is observed, and a linear-T dependent susceptibility occurs from the spin-density-wave transition temperature, (T{s}), to 700 K. This result suggests that strong antiferromagnetic correlations are present well above T{s}. A twofold symmetry of susceptibility in the ab plane indicates a stripelike spin structure as observed by neutron scattering. The resistivity minimum is strongly dependent on the magnetic field, suggesting that the upturn of the resistivity at low temperatures should be related to spin fluctuation.

Local Environments and Lithium Adsorption on the Iron Oxyhydroxides Lepidocrocite (γ-FeOOH) and Goethite (α-FeOOH): A 2H and 7Li Solid-State MAS NMR Study

2H and 7Li MAS NMR spectroscopy techniques were applied to study the local surface and bulk environments of iron oxyhydroxide lepidocrocite (gamma-FeOOH). 2H variable-temperature (VT) MAS NMR experiments were performed, showing the presence of short-range, strong antiferromagnetic correlations, even at temperatures above the Néel temperature, T(N), 77 K. The formation of a Li+ inner-sphere complex on the surface of lepidocrocite was confirmed by the observation of a signal with a large 7Li hyperfine shift in the 7Li MAS NMR spectrum. The effect of pH and relative humidity (RH) on the concentrations of Li+ inner- and outer-sphere complexes was then explored, the concentration of the inner sphere complex increasing rapidly above the point of zero charge and with decreasing RH. Possible local environments of the adsorbed Li+ were identified by comparison with other layer-structured iron oxides such as gamma-LiFeO2 and o-LiFeO2. Li+ positions of Li+-sorbed and exchanged goethite were reanalyzed on the basis of the correlations between Li hyperfine shifts and Li local structures, and two different binding sites were proposed, the second binding site only becoming available at higher pH.

Spin, charge, and electronic excitations in cuprate superconductors: Role of the long-range hoppings

The single-particle spectral functions of the t– t′– t″– J model are exactly calculated for a full range of the Brillouin zone by introducing the twisted boundary conditions for a 20-site cluster. By using the spectral functions, we calculate the optical conductivity σ( ω). In the hole-doped system, σ( ω) shows a good agreement with the exact ones based on the linear response theory. On the contrary, σ( ω) in the electron-doped system disagree with the exact ones. This is due to the presence of strong antiferromagnetic correlations induced by the long-range hopping t′ and t″.

Superconducting phase diagram in a model for tetragonal and cubic systems with strong antiferromagnetic correlations

We calculate the superconducting phase diagram as a function of temperature and $z$-axis anisotropy in a model for tetragonal and cubic systems having strong antiferromagnetic fluctuations. The formal basis for our calculations is the fluctuation exchange approximation applied to the single-band Hubbard model near half-filling. For nearly cubic lattices, two superconducting phase transitions are observed as a function of temperature with the low-temperature state having the time-reversal symmetry-breaking form ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}\ifmmode\pm\else\textpm\fi{}i{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$. With increasing tetragonal distortion, the time-reversal symmetry-breaking phase is suppressed, giving way to only ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ or ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ single-component phases. Based on these results, we propose that a time-reversal symmetry-breaking superconducting state may be found in cubic systems with pairing driven by antiferromagnetic fluctuations.

String excitations of a hole in a quantum antiferromagnet and photoelectron spectroscopy

The aim of the present paper is twofold. The first goal is to show that high resolution angle-resolved photoelectron spectra from cuprates indicate the presence of string-like excitations of the quasihole excitation in a quantum antiferromagnet. In order to compare with the experimental intensity plots, we calculate the spectral function of the t-J and the t-t'-t"-J models within the self-consistent Born approximation for widely accepted values of the parameters of these models. The main features of the high resolution photoelectron spectra are in general agreement with the results based on the above models and can be understood by considering not only the lowest energy quasiparticle peak but, also, the higher energy string-like excitations of the hole. The second goal of the present paper is to make the case that a proper analysis of both the numerical results obtained from these models and of the experimental results, suggests a theoretical picture for the internal structure of the hole-quasiparticle and a string-exchange pairing mechanism due to strong antiferromagnetic correlations among the background spins. We find that, using a simple model in which the Hilbert space is restricted to states of only unbroken strings attached to the holes and in which the holes are connected with unbroken strings, we can provide a good quantitative description of the most accurate numerical results. We find that the holes experience an effective interaction due to a string-exchange mechanism which is characterized by a rather large string tension and this can provide pairing energy scales much larger that those suggested by spin-fluctuation mediated pairing models. In addition, it is argued that such string-exchange interaction tends to bind holes in a bound state with the $d_{x^2-y^2}$ symmetry.

Non-Fermi liquid behavior in the 2D Hubbard model within COM(SCBA)

The two-dimensional Hubbard model is studied within the composite operator method (COM) with the electronic self-energy computed in the self-consistent Born approximation (SCBA). The COM describes interacting electrons in terms of the composite elementary excitations appearing in the system owing to the strong correlations; the residual interactions among these excitations are treated within the SCBA. The behavior of the momentum distribution function n ( k ) is analyzed for various values of filling. In contrast with the ordinary Fermi-liquid behavior found at high doping, non-Fermi-liquid features in n ( k ) are reported for low doping. These anomalous features are explained by looking at the strong antiferromagnetic correlations developing in the system in proximity of half-filling.

Phase separation in the Hubbard model using the dynamical cluster approximation

Phase separation in the Hubbard model is investigated with the dynamical cluster approximation. We find that it is present in the paramagnetic solution for values of filling smaller than 1 and at finite temperature when a positive next-nearest-neighbor hopping is considered. The phase-separated region is characterized by a mixture of a strongly correlated metallic and Mott insulating phases. Our results indicate that phase separation is driven by the formation of doped regions with strong antiferromagnetic correlations and low kinetic energy.

Phase Diagram of the Two-Channel Kondo Lattice Model in One Dimension

Employing the density matrix renormalization group method and strong-coupling perturbation theory, we study the phase diagram of the SU(2)xSU(2) Kondo lattice model in one dimension. We show that, at quarter filling, the system can exist in two phases depending on the coupling strength. The weak-coupling phase is dominated by RKKY exchange correlations, while the strong-coupling phase is characterized by strong antiferromagnetic correlations of the channel degree of freedom. These two phases are separated by a quantum critical point. For conduction-band fillings of less than one-quarter, we find a paramagnetic metallic phase at weak coupling and a ferromagnetic phase at moderate to strong coupling.

Roles of the strong spin–phonon coupling in the geometrically frustrated pyrochlore Y [formula omitted]Ru [formula omitted]O [formula omitted

We investigated temperature-dependent infrared-active transverse phonon modes of insulating pyrochlore Y 2 Ru 2 O 7 , which shows a spin-glass-like transition around 80 K. We found quite abnormal temperature-dependent behaviors for some phonon modes, which should be contributed by a strong antiferromagnetic correlation, not by a structural distortion. Through quantitative analysis, we suggested that the strong spin–phonon coupling effects should play an important role in the intriguing magnetic state of this geometrically frustrated compound.