Unusual magnetic excitations in the weakly ordered spin-$\frac{1}{2}$ chain antiferromagnet ${\mathrm{Sr}}_{2}{\mathrm{CuO}}_{3}$: Possible evidence for Goldstone magnon coupled with the amplitude mode

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Unusual magnetic excitations in the weakly ordered spin- $\frac{1}{2}$ chain antiferromagnet ${Sr}_{2} {CuO}_{3}$ : Possible evidence for Goldstone magnon coupled with the amplitude mode

E. G. Sergeicheva, S. S. Sosin, L. A. Prozorova, G. D. Gu, and I. A. Zaliznyak

Phys. Rev. B 95, 020411(R) – Published 18 January 2017

Abstract

We report on an electron spin resonance (ESR) study of a nearly one-dimensional (1D) spin- $\frac{1}{2}$ chain antiferromagnet, ${Sr}_{2} {CuO}_{3}$ , with extremely weak magnetic ordering. The ESR spectra at $T > T_{N}$ , in the disordered Luttinger-spin-liquid phase, reveal nearly ideal Heisenberg-chain behavior with only a very small, field-independent linewidth, $\sim 1 / T$ . In the ordered state, below $T_{N}$ , we identify field-dependent antiferromagnetic resonance modes, which are well described by pseudo-Goldstone magnons in the model of a collinear biaxial antiferromagnet. Additionally, we observe a major resonant mode with unusual and strongly anisotropic properties, which is not anticipated by the conventional theory of Goldstone spin waves. We propose that this unexpected magnetic excitation can be attributed to a field-independent magnon mode renormalized due to its interaction with the high-energy amplitude (Higgs) mode in the regime of weak spontaneous symmetry breaking.

Received 24 June 2016
Revised 10 August 2016

DOI:https://doi.org/10.1103/PhysRevB.95.020411

Physics Subject Headings (PhySH)

Antiferromagnetism Fractionalization Magnetic order Magnons Spin waves Spinon

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

E. G. Sergeicheva¹, S. S. Sosin^1,*, L. A. Prozorova¹, G. D. Gu², and I. A. Zaliznyak^2,†

¹P. Kapitza Institute for Physical Problems, Moscow 119334, Russia
²CMPMSD, Brookhaven National Laboratory, Upton, New York 11973, USA

^*Also at High School of Economics, Moscow 101000, Russia; sosin@kapitza.ras.ru
^†zaliznyak@bnl.gov

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Vol. 95, Iss. 2 — 1 January 2017

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Figure 1
Temperature dependence of magnetic resonance spectra measured at $ν = 27.0$ GHz (left panel) and $ν = 139.3$ GHz (right panel) for the magnetic field $H ∥ c$ axis of the sample. The signals are normalized to a unit level at maximum and consecutively shifted by +0.07, from bottom to top. Red and black curves correspond to ordered and spin-liquid phases, respectively. Arrows mark different resonance peaks discussed in the text.
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Figure 2
(a) The resonance field shift, (b) the linewidth (HWHM), and (c) the integral intensity of the principal ESR line as a function of temperature, measured at three excitation frequencies: 27.0 GHz ( $\circ$ and $•$ are the two spectral components below $T_{N}$ ), 79.0 GHz ( $▵$ ), and 139.3 GHz ( $□$ ), for $H ∥ c$ . Solid lines in (b) and (c) show $α + β / T$ and $C / T$ fits, respectively, for the width of the main line and the intensity of the residual paramagnetic component associated with the defects. (d) The decomposition of a typical low-frequency resonance line in the ordered phase into Lorentzian (AFM1, PM) and Gaussian (S1, S2) spectral components. (e) The square of the AFMR gap, $Δ_{1}^{2} (T) / ν^{2}$ , obtained from the temperature dependence of the AFM1 resonance field measured at $ν = 27.0$ GHz ( $□$ ) and the intensity of the $(0, 0.5, 0.5)$ neutron magnetic Bragg peak ( $▿$ ) vs temperature.
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Figure 3
The resonance absorption spectra measured for the $H ∥ b$ axis at frequency $ν = 73.6$ GHz on decreasing temperature from above $T_{N}$ to 1.3 K (upper panel), and the low-temperature records made at various excitation frequencies for $H ∥ b$ (middle panel) and $H ∥ c$ (lower panel). Arrows mark absorption maxima corresponding to different resonance modes discussed in the text.
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Figure 4
Frequency-field diagrams of the magnetic resonance spectra measured at $T = 1.3$ K for the three principal directions of the applied field with respect to the crystal axes, $H ∥ a, b$ (top), and $H ∥ c$ (bottom). The two AFMR modes are shown with the open symbols, and the solid lines are the theoretical calculations [18, 19, 39] for the biaxial collinear antiferromagnet. Solid circles show the low-temperature AM mode; the dashed lines are linear fits discussed in the text. The dashed-dotted lines show the paramagnetic resonance position for the $g$ factors obtained from high-temperature measurements. The drawing in the lower panel illustrates a mechanism of dynamical coupling of the amplitude mode and the transverse Goldstone magnon corresponding to oscillations of the order parameter in the plane perpendicular to the applied field.
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Physical Review B

covering condensed matter and materials physics

Unusual magnetic excitations in the weakly ordered spin- $\frac{1}{2}$ chain antiferromagnet ${Sr}_{2} {CuO}_{3}$ : Possible evidence for Goldstone magnon coupled with the amplitude mode

E. G. Sergeicheva, S. S. Sosin, L. A. Prozorova, G. D. Gu, and I. A. Zaliznyak

Phys. Rev. B 95, 020411(R) – Published 18 January 2017

Abstract

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Physical Review B

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Unusual magnetic excitations in the weakly ordered spin-12 chain antiferromagnet Sr2CuO3: Possible evidence for Goldstone magnon coupled with the amplitude mode

E. G. Sergeicheva, S. S. Sosin, L. A. Prozorova, G. D. Gu, and I. A. Zaliznyak

Phys. Rev. B 95, 020411(R) – Published 18 January 2017

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Unusual magnetic excitations in the weakly ordered spin- $\frac{1}{2}$ chain antiferromagnet ${Sr}_{2} {CuO}_{3}$ : Possible evidence for Goldstone magnon coupled with the amplitude mode