The quantum spin-liquid compound (C4H12N2)Cu2Cl6 is studied by µSR under hydrostatic pressures up to 23.6 kbar. At low temperatures, pressure-induced incommensurate magnetic order is detected beyond a quantum critical point at Pc ∼ 4.3 kbar. An additional phase transition to a different ordered phase is observed at P1 ∼ 13.4 kbar. The data indicate that the high-pressure phase may be a commensurate one. The established (P −T ) phase diagram reveals the corresponding pressure-induced multicritical point at P1, T1 = 2.0 K.Traditionally, magnetic insulators have been the most important prototype systems for testing concepts and theories of phase transitions, universality and scaling [1,2]. They owe this to their well-defined short-range interactions, a broad range of interaction topologies and dimensionalities, and their amenability to numerical modeling. With a more recent interest in quantum phase transitions [3,4], magnetic insulators have become the prototypes of choice to study quantum critical points (QCPs). Realizations of such important QCPs as BoseEinstein condensation (BEC) [5], deconfinement in one dimension [6,7], and the Ising model in a transverse field [8] have been found in quantum magnets in applied magnetic fields. Magnetic BEC, for example, occurs in gapped quantum antiferromagnets (AFMs) with a spin singlet ground state, when an external field drives the energy gap to zero by virtue of Zeeman effect. The result in spontaneous long-range magnetic order in the perpendicular direction, and thus a breaking of SO(2) symmetry [5]. At the QCP, the soft mode has a parabolic dispersion, so that the dynamical critical exponent is z = 2. By now, this transition has been extensively studied experimentally and theoretically [5].A qualitatively different type of soft mode transition in gapped quantum AFMs may occur if the spin gap is driven to zero by varying the ratio of exchange constants. The resulting spontaneous long-range magnetic order breaks SO(3) symmetry, and the spectrum is expected to be linear at the QCP (z = 1). In practice, the only way to continuously tweak the exchange interactions is by applying external pressure. Closing the spin gap with pressure in quantum Heisenberg AFMs has been attempted in experiments [9][10][11]. However, only one good realization of pressure-induced ordering in such systems has been found to date, namely, that in TlCuCl 3 [12][13][14]. Further studies of this QCP brought fascinating new insights [15], particularly the observation of a longitudinal mode, which is a magnetic analog of the celebrated Higgs boson [4]. In the present work, we report the observation of pressure-induced ordering in the S = 1/2 frustrated gapped quantum AFM (C 4 H 12 N 2 )Cu 2 Cl 6 (abbreviated PHCC), and use muon spin rotation (µSR) experiments to map out the P − T phase diagram. We show that the pressure-driven transition leads to an incommensurate magnetic order. At still higher pressures, we detect an additional transition and multicritical point. The indication is that these are an inc...

We report an inelastic neutron scattering study of magnetic excitations in a quantum paramagnet driven into a magnetically ordered state by chemical substitution, namely Ni(Cl1−xBrx)2·4SC(NH2)2 with x = 0.21(2). The measured spectrum is well accounted for by the generalized spin wave theory (GSWT) approach [M. Matsumoto and M. Koga, J. Phys. Soc. Jap. 76, 073709 (2007)]. This analysis allows us to determine the effective Hamiltonian parameters for a direct comparison with those in the previously studied parent compound and "underdoped" system. The issue of magnon lifetimes due to structural disorder is also addressed. * povarovk@phys.ethz.ch †

We report on the spontaneous appearance of antiferromagnetic order in a model gapped quantum paramagnet Ni(Cl1−xBrx)2·4SC(NH2)2 induced by a change in bromine concentration x. This transition is qualitatively similar to a z = 1 magnetic quantum critical point. However, the observed critical scaling of thermodynamic and magnetic properties has rather unusual critical exponents.

is a quasi-one-dimensional frustrated S = 1/2 multiferroic that displays magnetic fieldtunable ferroelectricity with no long-range magnetic order. To interrogate this intriguing behavior we employed dielectric measurements, neutron diffraction, and muon spin spectroscopy to study powder samples at mK temperatures and tesla magnetic fields. Subtle signatures in the experimental data allow the identification of different dielectric and magnetic regimes, none of which conclusively suggest the existence of magnetic long-range order. The results are broadly consistent with the system hosting a gapped nonmagnetic ground state in zero field, that undergoes a magnetic field-induced transition at ∼2 T into a phase with a static, disordered spin arrangement. We discuss our results in the context of previous work on this system, and conclude that the origin of a magnetic field-dependent electric polarization with no long-range magnetic order could call for a new paradigm of magnetically driven ferroelectricity.

Citation for published item:wnnigD eF nd w¤ ollerD tF F nd hedeD wF nd r¤ uvonenD hF nd vnsterD F nd ioD pF nd illimsD FgF nd quguhiD F nd uhsnovD F nd worenzoniD iF nd heludevD eF @PHITA 9i'et of disorder on pressureEindued z a I mgneti quntum phse trnsitionF9D hysil review fFD WR @IRAF pF IRRRIVF Further information on publisher's website: eprinted with permission from the emerin hysil oietyX wnnigD eF nd w¤ ollerD tF F nd hedeD wF nd r¤ uvonenD hF nd vnsterD F nd ioD p nd illimsD FgF nd quguhiD F nd uhsnovD F nd worenzoniD iF nd heludevD eF @PHITA 9i'et of disorder on pressureEindued z a I mgneti quntum phse trnsitionF9D hysil review fFD WR @IRAF pF IRRRIV PHIT y the emerin hysil oietyF eders my viewD rowseD ndGor downlod mteril for temporry opying purposes onlyD provided these uses re for nonommeril personl purposesF ixept s provided y lwD this mteril my not e further reproduedD distriutedD trnsmittedD modi(edD dptedD performedD displyedD pulishedD or sold in whole or prtD without prior written permission from the emerin hysil oietyF Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Pressure-induced ordering close to a z = 1 quantum-critical point is studied in the presence of bond disorder in the quantum spin system (C 4 H 12 N 2 )Cu 2 (Cl 1−x Br x ) 6 (PHCX) by means of muon-spin rotation and relaxation. As for the pure system (C 4 H 12 N 2 )Cu 2 Cl 6 , pressure allows PHCX with small levels of disorder (x 7.5%) to be driven through a quantum-critical point separating a low-pressure quantum paramagnetic phase from magnetic order at high pressures. However, the pressure-induced ordered state is highly inhomogeneous for disorder concentrations x > 1%. This behavior might be related to the formation of a quantum Griffiths phase above a critical disorder concentration 7.5% < x c < 15%. Br substitution increases the critical pressure and suppresses critical temperatures and ordered moment sizes.