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The Goldstone theorem mandates that a spontaneous symmetry breaking entails the emergence of gap(mass)less excitations. In the case where a rotational invariance of a system of spin magnetic moments is broken by an antiferromagnetic order, these are well-known transverse spin waves. The interaction of such Goldstone magnons with the Higgs amplitude mode of the order parameter is usually discarded, even though glimpses of Higgs physics have recently been reported in a quantum magnet, a topological insulator, and ferroelectric and disordered superconductor systems. The Goldstone-Higgs interactions could be expected to grow in importance near a quantum critical point (QCP), where the symmetry-breaking order is weak, and its amplitude fluctuations are significant. Here we report an electron spin resonance (ESR) study of a nearly one-dimensional spin-1/2 chain system, Sr$_2$CuO$_3$, which presents exactly such a case. The ESR spectra at $T > T_N$, in the disordered Luttinger-spin-liquid phase with unconfined spinons reveal ideal Heisenberg-chain behavior with only very small, field-independent linewidth, $sim 1/T$. In the ordered state, below $T_N$, we identify antiferromagnetic resonance (AFMR) modes, which are well described by pseudo-Goldstone magnons in the model of a collinear biaxial antiferromagnet with two gaps, $Delta_1 = 23.0$ GHz and $Delta_2 = 13.3$ GHz. Additionally, we observe a major resonant response of special nature, which we attribute to magnon interaction with the Higgs amplitude mode in a weakly ordered antiferromagnet. Its unusual field dependence indicates the presence of a quantum phase transition at $mu_0 H simeq 9.4$ T.
The low energy magnetic excitation spectrum of the Heisenberg antiferromagnetic $S = 1/2$ chain system Sr$_2$CuO$_3$ with Ni- and Ca-impurities is studied by neutron spectroscopy. In all cases, a defect-induced spectral pseudogap is observed and show
We investigate the effect of disorder on the heat transport properties of the $S=tfrac{1}{2}$ Heisenberg chain compound Sr$_2$CuO$_3$ upon chemically substituting Sr by increasing concentrations of Ca. As Ca occupies sites outside but near the Cu-O-C
The electron spin resonance spectrum of a quasi 1D S=1/2 antiferromagnet K2CuSO4Br2 was found to demonstrate an energy gap and a doublet of resonance lines in a wide temperature range between the Curie--Weiss and Ne`{e}l temperatures. This type of ma
Strong spin-orbital coupling (SOC) was found previously to lead to dramatic effects in quantum materials, such as those found in topological insulators. It was shown theoretically that local noncentrosymmetricity resulting from the rotation of RuO$_6
We studied the magnetic properties of YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_{x}$] ($x$ = 0.33 and 0.45), where Cu$^{2+}$ ions form two-dimensional kagome layers. There is no magnetic order down to 50 mK while the Curie-Weiss temperature is in the ord