Short-range Antiferromagnetic Correlations Research Articles

Link between antiferromagnetism and superconductivity probed by nuclear spin relaxation in organic conductors

The interdependence of antiferromagnetism and superconductivity in the Bechgaard salts series of organic conductors is examined in the light of the anomalous temperature dependence of the nuclear spin-lattice relaxation rate. We apply the renormalization-group approach to the electron gas model to show that the crossover from antiferromagnetism to superconductivity along with the anomalous nuclear relaxation rate of the Bechgaard salts can be well described within a unified microscopic framework. For sizable nesting deviations of the Fermi surface, scaling theory reveals how pairing correlations enhance short-range antiferromagnetic correlations via magnetic Umklapp scattering over a large part of the metallic phase that precedes superconductivity. These enhanced magnetic correlations are responsible for the Curie-Weiss behavior observed in the NMR relaxation rate.

Structural properties and magnetic interactions in martensitic Ni-Mn-Sb alloys

We study the structural and magnetic properties of Heusler based Ni50Mn50−x Sb x in the composition range 5.0 ≤ x ≤ 25.0. The cubic phase is preserved in the range 17.0 ≤ x ≤ 25.0, and the presence of martensitic transformations are found in alloys with x ≤ 15.0. In a critical composition range 12.5 ≤ x ≤ 10.0, the magnetic coupling in both austenitic and martensitic phases is paramagnetic, however, the presence of antiferromagnetic short-range correlations is observed in both phases. We discuss the consequences of these correlations on the behavior of the magnetic moment with respect to electronic properties.

ChemInform Abstract: Magnetism in Quasicrystals

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Magnetic Field Dependence of Magnetic Clusters in the Random Magnet Fe65(Ni0.78Mn0.22)35

A neutron scattering study on a concentrated spin-glass alloy Fe65(Ni0:78Mn0:22)35 has been performed under magnetic field. The amplitude of the magnetic diffuse scattering pattern arising from short-range ferromagnetic correlations is markedly reduced by increasing magnetic field. On the other hand, the effect of external field on the diffuse scattering pattern arising from antiferromagnetic short-range correlations is small. These results show that the regions of ferromagnetic and antiferromagnetic shortrange correlations (ferromagnetic and antiferromagnetic clusters) coexist separately in this random magnetic material.

Pseudogap and Short-Range Antiferromagnetic Correlation Controlled Fermi Surface in Underdoped Cuprates: From Fermi Arc to Electron Pocket

Motivated by recent quantum oscillation observations in the underdoped high-temperature superconductors, the effect of the short-range antiferromagnetic correlations on the electronic properties of the short-ranged d-density wave state is investigated. At intermediate d-density wave correlations, the cross section of the energy dispersion at the Fermi energy consists of hole pockets and an electron pocket. It is argued that the electron pocket feature is smeared out by the effect of the short-range antiferromagnetic correlation, which could be the reason why any electron pockets have never been observed in the angle resolved photoemission experiments. The Hall resistance is calculated for the system with the electron band and the hole band using a finite temperature formula assuming Dirac fermion spectrum for the latter. It is argued that the scattering effect in the hole band should be anomalously large or much suppressed by superconductivity correlations and/or vortex contributions to explain the recent quantum oscillation observations.

Pseudogap ground state in high-temperature superconductors

By re-examining recently published photoemission data we demonstrate that the apparent Fermi surface of the pseudogap ground state is a disconnected arc centered on the nodes. This scenario is also consistent with results from Raman spectroscopy and NMR. We also show that it is consistent with doping-dependent entropy data and discuss the origins of this unusual electronic ``arc-liquid'' state. We show that the pseudogap closes, and the arcs become connected, when the antiferromagnetic (AF) correlation length falls to the size of the lattice parameter. These results are consistent with a picture of Fermi-surface reconstruction due to short-range AF correlations.

Signature of magnetic phase separation in the ground state ofPr1−xCaxMnO3

Neutron scattering has been used to investigate the evolution of long- and short-range charge-ordered (CO), ferromagnetic, and antiferromagnetic (AF) correlations in single crystals of Pr(1-x)Ca(x)MnO(3). The existence and population of spin clusters as reflected by short-range correlations are found to drastically depend on the doping and temperature. Concentrated spin clusters coexist with long-range canted AF order in a wide temperature range in the x=0.3 while clusters do not appear in the x=0.4 crystal. In contrast, both CO and AF order parameters in the x=0.35 crystal show a precipitous decrease below similar to 35 K where spin clusters form. These results provide direct evidence of magnetic phase separation and demonstrate a critical doping x(c) (close to x=0.35) that divides the inhomogeneous from homogeneous CO ground state.

Magnetic short-range correlations and quantum critical scattering in the non-Fermi liquid regime ofURu2−xRexSi2(x=0.2–0.6)

The spin dynamics of uranium ions in the non-Fermi liquid compounds ${\text{URu}}_{2\ensuremath{-}x}{\text{Re}}_{x}{\text{Si}}_{2}$, for $x=0.2$ to 0.6, have been investigated using inelastic neutron scattering. The wave vector $(q)$ dependence of the magnetic scattering provides evidence of short-range antiferromagnetic correlations at low temperatures for $x=0.2,0.25$, but the scattering is nearly $q$ independent at $x=0.35,0.6$. The magnetic response, $\overline{S}(\ensuremath{\omega})$, obtained from the $q$-independent part of neutron scattering, varies as ${\ensuremath{\omega}}^{\ensuremath{-}\ensuremath{\alpha}}$ with a composition-dependent exponent $\ensuremath{\alpha}=0.2--0.5$. The dynamic magnetic susceptibility ${\ensuremath{\chi}}^{\ensuremath{''}}(q,\ensuremath{\omega})$ of the $q$-independent part exhibits $\ensuremath{\omega}/T$ scaling for the energy transfer $\ensuremath{\hbar}\ensuremath{\omega}$ between 3.5 and 17 meV in the temperature $(T)$ range of 5--300 K at all the compositions. This scaling, which indicates local quantum criticality, breaks down in the $q$ range, $0.6--1.1\text{ }{\text{\AA{}}}^{\ensuremath{-}1}$ at $x=0.2$ and 0.25, that is dominated by short-range antiferromagnetic correlations. The appearance of power laws in the magnetic response measured by inelastic neutron scattering over a wide Re doping region indicates a disorder driven non-Fermi liquid mechanism for the low-temperature physical properties in these compounds.

Magnetism in quasicrystals

The present understanding of magnetic properties of quasicrystals (QCs) is reviewed. In AlMn and AlPdMn QCs, only a few Mn atoms, with concentrations from a few per cent down to 10−4 carry a localized magnetic moment. These moments are coupled via RKKY interactions, leading to a canonical spin-glass ordering at low temperature. They also exhibit a Kondo effect and AlPdMn QCs turned out to be a model system for studying experimentally the competition between Kondo and RKKY interactions in the absence of long-range magnetic ordering. Besides, the presence of localized moments has very unusual consequences for the electronic transport in AlPdMn QCs. In RMgZn and RMgCd QCs (R is a magnetic rare earth), with R concentration around 9%, a spin-glass like freezing is observed. However the short-range antiferromagnetic correlations detected above the freezing temperature are unusual and are certainly influenced by the quasiperiodicity.

Magnetic behavior of isolated Mo impurities in antiferromagnetic Cr metal

We report experimental and theoretical studies on the magnetic moment and hyperfine field of isolated Mo impurities in an antiferromagnetic Cr host. The magnetic response of 92,94Mo, measured by means of the time differential perturbed angular distribution (TDPAD) technique, show the saturation hyperfine field B hf ( 0 ) to be − 4.7 T . In addition, we find evidence of short range antiferromagnetic spin correlation above T N , revealed from the observation of large B hf values at temperatures far above T N . Our ab initio calculations, based on the generalized gradient approximation (GGA) of density functional theory (DFT), yield an induced magnetic moment of 0.45 μ B for the Mo impurity in Cr. The magnitude of the calculated hyperfine field B hf cal = − 3.35 T closely agrees with the experimental value. The results presented in this work would be helpful for a common understanding of hyperfine fields of impurity atoms in ferro- and antiferromagnetic materials.

Magnetism in (Ca,Sr) 2RuO 4 observed by 119Sn-Mössbauer spectroscopy

We report on the magnetic behavior observed by 119Sn-Mössbauer spectroscopy in Ca 2− x Sr x RuO 4 samples ( x=0.09, 0.2, and 2.0). The 119Sn-Mössbauer probe substitutes Ru 4+ at the RuO 2-layers as Sn 4+. Mössbauer spectra at 4.2 K show the presence of magnetic correlations due to transferred hyperfine field ( B hf) at the Sn(Ru)-sites for two samples ( x=0.09 and 0.20), while it was not observed for Sr 2RuO 4 ( x=2.0). The B hf softly decreases with x going from 0.09 to 0.2, indicating remaining short-range antiferromagnetic correlations at x=0.2 concentration, where a first structural transition has been reported. In both samples, the transferred B hf shows a canted magnetic structure.

Origin of shadow bands in single-layered Bi 2Sr 2CuO 6+ δ studied by high-resolution angle-resolved photoemission spectroscopy

We report systematic high-resolution angle-resolved photoemission results on single-layered Pb-substituted Bi 2Sr 2CuO 6+ δ . We found that the intensity of shadow bands is invariable with respect to temperature and doping. This result rules out the possibility of short-range antiferromagnetic correlation and supports the structural origin of shadow bands.

Magnetization of undoped 2-leg [formula omitted] spin ladders in La4Sr10Cu24O41

Magnetization data of single crystalline La4Sr10Cu24O41 are presented. In this compound, doped spin chains and undoped spin ladders are realized. The magnetization, at low temperatures, is governed by the chain subsystem with a finite interchain coupling which leads to short range antiferromagnetic spin correlations. At higher temperatures, the response of the chains can be estimated in terms of a Curie–Weiss law. For the ladders, we apply the low temperature approximation for a S=1/2 2-leg spin ladder.

Superconductivity and a Mott Transition in a Hubbard Model on an Anisotropic Triangular Lattice

A half-filled-band Hubbard model on an anisotropic triangular lattice ( t in two bond directions and t ' in the other) is studied using an optimization variational Monte Carlo method, to consider a Mott transition and superconductivity arising in κ-(BEDT-TTF) 2 X. Adopting wave functions with doublon–holon binding factors, we reveal that a first-order Mott (conductor-to-nonmagnetic insulator) transition takes place at U = U c , approximately of the bandwidth, for a wide t '/ t range. This transition is not directly connected to magnetism. Robust d -wave superconductivity appears in a restricted parameter range: immediately below U c and the moderate strength of frustration (\(0.4 {\lesssim} t'/t {\lesssim} 0.7\)), where short-range antiferromagnetic correlation sufficiently develops but does not come to a long-range order. The relevance to experiments is also discussed.

Doping evolution of the chemical potential, spin-correlation gap, and charge dynamics ofNd2−xCexCuO4

We report an optical reflectivity study on ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ over a broad doping range $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.20$. The study reveals a systematic shift of the chemical potential with doping. A pronounced peak structure specific to the electron-doped cuprate is observed in the optical scattering rate based on the extended Drude model analysis. The energy scale of the peak could be very different from that of the peak in conductivity spectrum. We clarify that the ``hot spots'' gap probed in photoemission experiments is more closely related to the sharp suppression feature in scattering rate than in conductivity spectra. The gap is associated with the short-range antiferromagnetic correlation, and disappears in a manner of fading away with increasing doping and temperature. In the heavily overdoped region, a dominant ${\ensuremath{\omega}}^{2}$-dependence of the scattering rate is identified up to very high energy.

Magnetic phase transitions in the two-dimensional frustrated quantum antiferromagnetCs2CuCl4

We report magnetization and specific heat measurements in the two-dimensional frustrated spin-$1∕2$ Heisenberg antiferromagnet ${\mathrm{Cs}}_{2}\mathrm{Cu}{\mathrm{Cl}}_{4}$ at temperatures down to $0.05\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and high magnetic fields up to $11.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ applied along $a$, $b$, and $c$ axes. The low-field susceptibility $\ensuremath{\chi}(T)\ensuremath{\simeq}M∕B$ shows a broad maximum around $2.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ characteristic of short-range antiferromagnetic correlations and the overall temperature dependence is well described by high temperature series expansion calculations for the partially frustrated triangular lattice with $J=4.46\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${J}^{\ensuremath{'}}∕J=1∕3$. At much lower temperatures $(\ensuremath{\leqslant}0.4\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and in in-plane field (along $b$ and $c$ axes) several new intermediate-field ordered phases are observed in between the low-field incommensurate spiral and the high-field saturated ferromagnetic state. The ground state energy extracted from the magnetization curve shows strong zero-point quantum fluctuations in the ground state at low and intermediate fields.

Asymmetric Tunneling Conductance in Doped Antiferromagnets

We calculate the tunneling conductance in the framework of the spin polaron model, which is an effective model for the t−J model, in the limit where at least short-range antiferromagnetic correlations exist. We show that both in the normal state and in the superconducting state the asymmetry of tunneling conductance is present. This fact is the implication of the particle–hole asymmetry of the spin polaron Hamiltonian in the limit of low density of spin polarons which are quasiparticles emerging in a hole doped antiferromagnet. Experimental evidence of analogous asymmetric tunneling conductance was found in the tunneling spectroscopy measurements of high Tc superconductors.

Ground states in a quasi-one-dimensional triangular Hubbard model

We study the ground state in a quasi-one-dimensional triangular Hubbard model using the exact numerical diagonalization, the constrained-path Monte Carlo, and the cluster perturbation theory. As the frustration hopping $t$ is small the system exhibits short-range antiferromagnetic correlation. As $t$ is greater than a critical point ${t}_{c}$, there is a transition from an antiferromagnetic state to a frustrated state, which is accompanied by a jump of local entanglement and an abrupt change of spin correlation. As $t$ is greater than another critical point ${t}_{{c}^{\ensuremath{'}}}$, the insulator-metal transition takes place. The successive order of two transitions depends on the on-site Coulomb interaction. The calculated spectra indicates that the spin-charge separation is destroyed by the frustration hopping. The frustrated state corresponds to the appearance of an additional dispersion band near the Fermi surface.

Synthesis, Structure, and Magnetic Properties of the Layered Copper(II) Oxide Na2Cu2TeO6.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Temperature dependence of spinon and holon excitations in one-dimensional Mott insulators

Motivated by the recent angle-resolved photoemission spectroscopy (ARPES) measurements on one-dimensional Mott insulators, SrCuO${}_{2}$ and Na${}_{0.96}$V${}_{2}$O${}_{5}$, we examine the single-particle spectral weight of the one-dimensional (1D) Hubbard model at half-filling. We are particularly interested in the temperature dependence of the spinon and holon excitations. For this reason, we have performed the dynamical density matrix renormalization group and determinantal quantum Monte Carlo (QMC) calculations for the single-particle spectral weight of the 1D Hubbard model. In the QMC data, the spinon and holon branches become observable at temperatures where the short-range antiferromagnetic correlations develop. At these temperatures, the spinon branch grows rapidly. In the light of the numerical results, we discuss the spinon and holon branches observed by the ARPES experiments on SrCuO${}_{2}$. These numerical results are also in agreement with the temperature dependence of the ARPES results on Na${}_{0.96}$V${}_{2}$O${}_{5}$.