Quantum and classical criticality in a dimerized quantum antiferromagnet

Abstract

A quantum critical point (QCP) is a singularity in the phase diagram arising due to quantum mechanical fluctuations. The exotic properties of some of the most enigmatic physical systems, including unconventional metals and superconductors, quantum magnets, and ultracold atomic condensates, have been related to the importance of the critical quantum and thermal fluctuations near such a point. However, direct and continuous control of these fluctuations has been difficult to realize, and complete thermodynamic and spectroscopic information is required to disentangle the effects of quantum and classical physics around a QCP. Here we achieve this control in a high-pressure, high-resolution neutron scattering experiment on the quantum dimer material TlCuCl 3 . By measuring the magnetic excitation spectrum across the entire quantum critical phase diagram, we illustrate the similarities between quantum and thermal melting of magnetic order. We prove the critical nature of the unconventional longitudinal ("Higgs") mode of the ordered phase by damping it thermally. We demonstrate the development of two types of criticality, quantum and classical, and use their static and dynamic scaling properties to conclude that quantum and thermal fluctuations can behave largely independently near a QCP.In "classical" isotropic antiferromagnets, the excitations of the ordered phase are gapless spin waves emerging on the spontaneous breaking of a continuous symmetry [1]. The classical phase transition, occurring at the critical temperature T N , is driven by thermal fluctuations. In quantum antiferromagnets, quantum fluctuations suppress long-range order, and can destroy it completely even at zero temperature [2]. The ordered and disordered phases are separated by a quantum critical point (QCP), where quantum fluctuations restore the broken symmetry and all excitations become gapped, giving them characteristics fundamentally different from the Goldstone modes on the other side of the QCP (Fig. 1). At finite temperatures around a QCP, the combined effects of quantum and thermal fluctuations bring about a regime where the characteristic energy scale of spin excitations is the temperature itself, and this quantum critical (QC) regime has many special properties [3].Physical systems do not often allow the free tuning of a quantum fluctuation parameter through a QCP. The QC regime has been studied in some detail in heavy-fermion [14], and the green points depict the temperature and pressure values studied. Full details of this panel are presented in Fig. 4(c). The centre (E, T ) panel shows neutron intensity data collected from T = 1.8 K to 12.7 K at p = 1.75 kbar, where TN = 5.8 K. The rightmost (E, T ) panel shows the corresponding data at p = 3.6 kbar, where TN = 9.2 K. The data in both (E, T ) panels display a clear softening of the magnetic excitations at TN (p). The bottom (p, E) panel indicates the softening of the excitations, measured at T = 1.8 K, across the QPT [19]. T1, T2 and L denote the three gapped triplet excitations of th...

Spin dynamics in pressure-induced magnetically ordered phases in(C4H12N2)Cu2Cl<

“…9 This paper is concerned with the S = 1/2 quasi-two-dimensional gapped quantum antiferromagnet piperazinium hexachlorodicuprate [(C 4 H 12 N 2 )Cu 2 Cl 6 , hereafter PHCC]. PHCC crystallizes in the triclinic space group P1 with lattice parameters 10 a = 7.984(4)Å, b = 7.054(4)Å, c = 6.104(3)Å, α = 111.23 (8) • , β = 99.95 (9) • , γ = 81.26 (7) • . The spin-1/2 Cu 2+ ions are connected by a complex layered network (see Fig.…”

Higgs mode and its decay in a two-dimensional antiferromagnet

“…It is at the ordering wavevector where the stability of the Higgs mode critically depends on the dimensionality of the system. In three dimensions, earlier INS studies on a dimerized quantum magnet have established a well-defined Higgs mode 4 , which was then used to study its critical behaviour across a QCP 23,24 . In sharp contrast, our in-planepolarized spectrum measured at q = (π,π) shows only one clear peak for the T mode at approximately 14 meV, followed by a broad magnetic intensity distribution in the energy range 20-50 meV, which is, however, well above the detection limit (Fig.…”

Localized Higgs modes of superfluid Bose gases in optical lattices: A Gutzwiller mean-field study

“…Recently, this type of collective mode has attracted much attention thanks to experimental observations in various condensed-matter and quantum-gas systems, such as superconductors NbSe 2 [3][4][5] and Nb 1−x Ti x N [6][7][8][9], quantum antiferromagnets TlCuCl 3 [10,11] and KCuCl 3 [12], charge-density-wave materials K 0.3 MoO 3 [13,14] and TbTe 3 [15,16], superfluid 3 He B-phase [17,18], and superfluid Bose gases in optical lattices [19,20].…”