Journal of Magnetism and Magnetic Materials 272–276 (2004) 920–921 Field-induced quantum phase transition in the quasi 1D XY-like antiferromagnet Cs 2 CoCl 4 C.J. Mukherjee a, *, R. Coldea a , D.A. Tennant a,b , M. Koza c , M. Enderle c , K. Habicht d , P. Smeibidl d , Z. Tylczynski e a Clarendon Laboratory, University of Oxford Parks Rd, Oxford OX1 3PU, UK b ISIS, Rutherford Appleton Laboratory, Didcot, Oxon OX11 0QX, UK c Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France d Hahn-Meitner Institut, BENSC, Berlin D-14109, Germany e Institute of Physics, Adam Mickiewicz University, Umultowska 85, Poznan 61-614, Poland Abstract We explore the magnetic excitations as a function of applied field in the quasi 1D s ¼ 1 2 XY -like magnet Cs 2 CoCl 4 : A transition from spin–flop antiferromagnetic order to a quantum paramagnet occurs at the critical field B C ¼ 2:15 T and fields higher than 2:5 T fully align the spins along the field. In the spin–flop phase gapped scattering continua dominate, identified with the creation of pairs of quantum domain walls. In the saturated phase sharp peaks are observed, as expected for magnons in a ferromagnetically ordered ground state. r 2003 Elsevier B.V. All rights reserved. PACS: 75.10.Jm; 75.10.Pq; 75.40.Gb Keywords: Cs 2 CoCl 4 ; Quantum phase transition; XXZ spin chain 1. Introduction Research into quantum phase transitions and critical behaviour is receiving considerable current theoretical and experimental interest [1]. Here we explore a field driven quantum phase transition between antiferromag- netic spin–flop ordering and a quantum paramagnet in Cs 2 CoCl 4 [2]. This is well described with the minimal model of a 1D spin- 1 2 XXZ chain in a transverse field, H ¼ X i J ðS x i S x iþ1 þ S y i S y iþ1 þ DS z i S z iþ1 Þþ gmB x S x i ð1Þ with anisotropy D ¼ 0:25 and J ¼ 0:23 meV: The low exchange allows the field-temperature phase diagram to be fully characterized by neutron diffraction (see Fig. 1). The spin–flop phase at low field has a small ferromag- netically aligned moment along the field and antiferro- magnetic-order perpendicular. The antiferromagnetic order is suppressed at the critical field B C ¼ 2:15 T; and the spins become fully aligned above 2:5T: The intermediate region 2:15 ToBo2:5 T is a spin liquid with a finite fluctuating spin moment. 2. Experimental details We explored the magnetic excitations across the phase diagram using the IN6 time-of-flight spectrometer at the ILL in France. A base temperature of 70 mK was provided by a dilution insert and magnetic fields up to 2:5 T by a vertical cryomagnet. A 9:0 g single crystal of the orthorhombic Cs 2 CoCl 4 was mounted in the ðb; cÞ horizontal scattering plane with chains lying along b and fields applied along the a-axis. The sample was aligned so that the data collected in the detector bank covered an entire 1D Brillouin zone. Incident neutrons of 2:35 meV gave an energy resolution of 0:05 meV ARTICLE IN PRESS *Corresponding author. Tel.: +44-1865-272-367; fax: +44- 1865-272-400. E-mail address: c.mukherjee1@physics.ox.ac.uk (C.J. Mukherjee). 0304-8853/$-see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1115