Journal of the Physical Society of Japan

Neutron Scattering Study of Magnetic Excitations in the SpinS=1One-Dimensional Heisenberg AntiferromagnetY2BaNiO5

Takehiro Sakaguchi, Kazuhisa Kakurai, Tetsuya R. Yokoo, Jun Akimitsu

Published Sep 15, 1996• 71 citations

Abstract

Magnetic excitations of the ideal one-dimensional, spin S =1, antiferromagnet Y 2 BaNiO 5 are studied by means of inelastic neutron scattering on single crystal sample. Our experimental results at low temperature indicate that Y 2 BaNiO 5 can be regarded as the best isotropic Haldane-system sofar studied with the intrachain exchange energy J / k B =-24.1 meV and the averaged gap Δ( T =7 K)=8.5 meV. Y 2 BaNiO 5 exhibits a good magnetic one-dimensionality with the inter-/intrachain exchange ratio of | J ′ / J |≤10 -4 and has only very small easy-axis and in-plane single ion anisotropy of | D / J |=0.03 and | E / J |∼0.01. In addition the temperature dependence of the Haldane gap Δ( T ) and its damping rate was determined.

DOI: 10.1143/jpsj.65.3025

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Journal of the Physical Society of Japan

Neutron Scattering Study of Magnetic Excitations in the SpinS=1One-Dimensional Heisenberg AntiferromagnetY2BaNiO5

Takehiro Sakaguchi, Kazuhisa Kakurai, Tetsuya R. Yokoo, Jun Akimitsu

Published Sep 15, 1996• 71 citations

Publisher

Abstract

DOI: 10.1143/jpsj.65.3025

Related papers

Y2 BaNi O5 : A nearly ideal realization of the S=1 Heisenberg chain with antiferromagnetic interactions

Guangyong Xu, J. F. DiTusa, T. Ito +4 more

We report an inelastic-neutron-scattering experiment on single crystals of the one-dimensional spin-one antiferromagnet ${\mathrm{Y}}_{2}$ BaNi ${\mathrm{O}}_{5}$ . The data show that this compound is a nearly ideal material for studying the Haldane conjecture in the Heisenberg limit. In particular, the Haldane gap at $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}=\ensuremath{\pi}$ is almost isotropic, taking on the values of 7.5(1), 8.6(1), and 9.6(1) meV for polarizations parallel to the three principal orthorhombic axes respectively. Interchain coupling along the edges of the orthorhombic unit cell is found to be very weak ($\frac{{J}^{\ensuremath{'}}}{J}<~5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$). Finally, we show that defects in our sample at the 1% level cause visible broadening of the excitation at $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}=\ensuremath{\pi}$.We report an inelastic-neutron-scattering experiment on single crystals of the one-dimensional spin-one antiferromagnet ${\mathrm{Y}}_{2}$BaNi${\mathrm{O}}_{5}$. The data show that this compound is a nearly ideal material for studying the Haldane conjecture in the Heisenberg limit. In particular, the Haldane gap at $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}=\ensuremath{\pi}$ is almost isotropic, taking on the values of 7.5(1), 8.6(1), and 9.6(1) meV for polarizations parallel to the three principal orthorhombic axes respectively. Interchain coupling along the edges of the orthorhombic unit cell is found to be very weak ($\frac{{J}^{\ensuremath{'}}}{J}<~5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$). Finally, we show that defects in our sample at the 1% level cause visible broadening of the excitation at $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}=\ensuremath{\pi}$.

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Raman study of one-dimensional spin fluctuations in the spin-1 Heisenberg-chain compound Y2 BaNiO5 : Contrast with simple expectations

P. E. Sulewski, S‐W. Cheong

Spin fluctuations in the Haldane ground state should exhibit a quantum spin gap observable with Raman scattering. The Raman spectra of single-crystalline ${\mathrm{Y}}_{2}$ ${\mathrm{BaNiO}}_{5}$ , a linear spin-1 Heisenberg antiferromagnet, exhibit broad continuum scattering between \ensuremath{\sim}200 and \ensuremath{\sim}800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, in the energy range expected for magnetic excitations (\ensuremath{\sim}J=224 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). Numerous weak and broad features superimposed on this continuum scattering possibly result from quantization of the magnon continuum by local molecular fields from nearby chains. A strong and relatively narrow mode at \ensuremath{\sim}680 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, near 3J, unrelated to lattice vibrations, may also be of magnetic origin. All of these features are strongly resonant in the red (${\ensuremath{\lambda}}_{\mathit{L}}$=6328 \AA{}), disappearing for shorter excitation wavelengths. These spectra provide a view of the Haldane spin system excitations complementary to that given by inelastic neutron scattering. Modeling the excitations with Schwinger bosons or spin waves predicts a highly asymmetric scattering feature with a sharp onset near 2\ensuremath{\Delta}(\ensuremath{\sim}0.8J), gradually diminishing toward higher energies. The observed spectra disagree with this theoretical expectation, perhaps due to spin-phonon coupling which may provide an efficient decay channel for the magnon pairs.Spin fluctuations in the Haldane ground state should exhibit a quantum spin gap observable with Raman scattering. The Raman spectra of single-crystalline ${\mathrm{Y}}_{2}$${\mathrm{BaNiO}}_{5}$, a linear spin-1 Heisenberg antiferromagnet, exhibit broad continuum scattering between \ensuremath{\sim}200 and \ensuremath{\sim}800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, in the energy range expected for magnetic excitations (\ensuremath{\sim}J=224 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). Numerous weak and broad features superimposed on this continuum scattering possibly result from quantization of the magnon continuum by local molecular fields from nearby chains. A strong and relatively narrow mode at \ensuremath{\sim}680 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, near 3J, unrelated to lattice vibrations, may also be of magnetic origin. All of these features are strongly resonant in the red (${\ensuremath{\lambda}}_{\mathit{L}}$=6328 \AA{}), disappearing for shorter excitation wavelengths. These spectra provide a view of the Haldane spin system excitations complementary to that given by inelastic neutron scattering. Modeling the excitations with Schwinger bosons or spin waves predicts a highly asymmetric scattering feature with a sharp onset near 2\ensuremath{\Delta}(\ensuremath{\sim}0.8J), gradually diminishing toward higher energies. The observed spectra disagree with this theoretical expectation, perhaps due to spin-phonon coupling which may provide an efficient decay channel for the magnon pairs.

Physical review. B, Condensed matterFeb 1, 199510 citations

We report an inelastic-neutron-scattering experiment on single crystals of the one-dimensional spin-one antiferromagnet

{\mathrm{Y}}_{2}

BaNi

{\mathrm{O}}_{5}

. The data show that this compound is a nearly ideal material for studying the Haldane conjecture in the Heisenberg limit. In particular, the Haldane gap at $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}=\ensuremath{\pi}$ is almost isotropic, taking on the values of 7.5(1), 8.6(1), and 9.6(1) meV for polarizations parallel to the three principal orthorhombic axes respectively. Interchain coupling along the edges of the orthorhombic unit cell is found to be very weak ($\frac{{J}^{\ensuremath{'}}}{J}<~5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$). Finally, we show that defects in our sample at the 1% level cause visible broadening of the excitation at $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{q}=\ensuremath{\pi}$.

A full description of a series of spin-wave measurements in

{\mathrm{CsNiCl}}_{3}

in the three-dimensional (3D) and one-dimensional (1D) phases and their analysis to provide the first experimental evidence for the Haldane gap is presented. Neutron scattering experiments were performed on a single crystal of

{\mathrm{CsNiCl}}_{3}

with both (h,0,l) and (h,h,l) as the scattering plane. The order parameter in both 3D phases was measured and the spin-wave dispersion determined in the lower phase. The spin-wave spectrum calculated from a dynamic susceptibility method was compared with the experimental response in the lower 3D phase to obtain the following values for the exchange and anisotropy constants: J=0.345\ifmmode\pm\else\textpm\fi{}0.008 THz, J'=0.0060\ifmmode\pm\else\textpm\fi{}0.005 THz, and D=-0.0130\ifmmode\pm\else\textpm\fi{}0.0015 THz. These parameters confirm that

{\mathrm{CsNiCl}}_{3}

in its disordered phase is a good approximation to a one-dimensional Heisenberg antiferromagnet. In the 1D phase the gap frequency for an isolated chain of

{\mathrm{Ni}}^{2+}

ions is found to be 0.32 THz close to the gap estimated in finite-chain calculations. The results constitute experimental support for the Haldane conjecture that the excitations of integer-spin chains, unlike half-integer--spin chains, exhibit an apparent anisotropy that arises not from the underlying isotropic Hamiltonian, but from many-body effects.

The spin correlations of the S=1 Heisenberg antiferromagnetic chains in

{\mathrm{AgVP}}_{2}

{\mathrm{S}}_{6}

have been studied by neutron scattering at temperatures of the order of T\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}2}$ J/

{\mathit{k}}_{\mathit{B}}

. On the polycrystalline samples, with a careful choice of experimental conditions, we have observed a Haldane gap of

{\mathit{E}}_{\mathit{g}}

=26 meV at the antiferromagnetic point at q=\ensuremath{\pi}. The excitation spectra are consistent with a mode that has a spin-wave velocity C=150 meV and a correlation length \ensuremath{\xi}\ensuremath{\approxeq}5.5, close to C/

{\mathit{E}}_{\mathit{g}}

. These results are in quantitative agreement with the predictions and numerical results on the Haldane state.

{\mathrm{C}}_{2}

{\mathrm{H}}_{8}

{\mathrm{N}}_{2}

{)}_{2}

{\mathrm{NO}}_{2}

{\mathrm{ClO}}_{4}

(NENP). We discuss the relationship between our results and the magnetic-susceptibility measurements as well as the inelastic-neutron-scattering data and on the Haldane conjecture. Values are proposed for the parameters D and J in the case of NENP.

7.5

times the intrachain exchange constant J. The calculations show that the excitations at the antiferromagnetic point

q=1

and at

q=0.5

remain resonant up to at least

T=2J,

confirming the recent experimental observation of resonant high-temperature domain wall excitations. The predicted first and second moments are in good agreement with experiment, except at temperatures where three-dimensional spin correlations are most important. The ratio of the structure factors at

q=1

and 0.5 is well predicted for the paramagnetic infinite-temperature limit. For

T<~2J,

however, we found that the experimentally observed intensity is considerably less than predicted. This suggests that domain wall excitations on different chains interact up to temperatures of the order of the spin band width.

Spin fluctuations in the Haldane ground state should exhibit a quantum spin gap observable with Raman scattering. The Raman spectra of single-crystalline

{\mathrm{Y}}_{2}

{\mathrm{BaNiO}}_{5}

, a linear spin-1 Heisenberg antiferromagnet, exhibit broad continuum scattering between \ensuremath{\sim}200 and \ensuremath{\sim}800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, in the energy range expected for magnetic excitations (\ensuremath{\sim}J=224 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). Numerous weak and broad features superimposed on this continuum scattering possibly result from quantization of the magnon continuum by local molecular fields from nearby chains. A strong and relatively narrow mode at \ensuremath{\sim}680 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, near 3J, unrelated to lattice vibrations, may also be of magnetic origin. All of these features are strongly resonant in the red (${\ensuremath{\lambda}}_{\mathit{L}}$=6328 \AA{}), disappearing for shorter excitation wavelengths. These spectra provide a view of the Haldane spin system excitations complementary to that given by inelastic neutron scattering. Modeling the excitations with Schwinger bosons or spin waves predicts a highly asymmetric scattering feature with a sharp onset near 2\ensuremath{\Delta}(\ensuremath{\sim}0.8J), gradually diminishing toward higher energies. The observed spectra disagree with this theoretical expectation, perhaps due to spin-phonon coupling which may provide an efficient decay channel for the magnon pairs.