Field-driven phase transitions in a quasi-two-dimensional quantum antiferromagnet

• Published in: New journal of physics Vol. 9; no. 2; p. 31
• Main Authors: Stone, M B, Broholm, C, Reich, D H, Schiffer, P, Tchernyshyov, O, Vorderwisch, P, Harrison, N
• Format: Journal Article
• Language: English
• Published: United States IOP Publishing 16-02-2007
• Subjects: CONDENSATES; CRITICALITY; DEGREES OF FREEDOM; GROUND STATES; MAGNETIC FIELDS; MAGNETIC SUSCEPTIBILITY; MAGNETS; MAGNONS; NEUTRONS; PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; SCATTERING; SPECIFIC HEAT; SPIN; TRIPLETS
• ISSN: 1367-2630; 1367-2630
• DOI: 10.1088/1367-2630/9/2/031

We report magnetic susceptibility, specific heat, and neutron scattering measurements as a function of applied magnetic field and temperature to characterize the S = 1/2 quasi-two-dimensional (2D) frustrated magnet piperazinium hexachlorodicuprate (PHCC). The experiments reveal four distinct phases. At low temperatures and fields the material forms a quantum paramagnet with a 1 meV singlet triplet gap and a magnon bandwidth of 1.7 meV. The singlet state involves multiple spin pairs some of which have negative ground state bond energies. Increasing the field at low temperatures induces 3D long-range antiferromagnetic order at 7.5 Tesla through a continuous phase transition that can be described as magnon Bose-Einstein condensation. The phase transition to a fully polarized ferromagnetic state occurs at 37 Tesla. The ordered antiferromagnetic phase is surrounded by a renormalized classical region. The crossover to this phase from the quantum paramagnet is marked by a distinct anomaly in the magnetic susceptibility which coincides with closure of the finite temperature singlet-triplet pseudo gap. The phase boundary between the quantum paramagnet and the Bose-Einstein condensate features a finite temperature minimum at T = 0.2 K, which may be associated with coupling to nuclear spin or lattice degrees of freedom close to quantum criticality.