Sign switching of dimer correlations in SrCu2(BO3)2 under hydrostatic pressure

Magnetic and vibrational excitations in SrCu2(BO3)2 are studied using Raman spectroscopy at hydrostatic pressures up to 34 kbar and temperatures down to 2.6 K. The frequency of a particular optical phonon, the so-called pantograph mode, shows a very strong anomalous temperature dependence below about 40 K. We link the magnitude of the effect to the magnetic exchange energy on the dimer bonds in the Sutherland-Shastry spin lattice in this material. The corresponding dimer spin correlations are quantitatively estimated and found to be strongly pressure dependent. At around P2∼22 kbar they switch from antiferromagnetic to being predominantly ferromagnetic.ISSN:2643-156

Dynamics and field-induced order in the layered spin S = 1/2 dimer system (C5H6N2F)2CuCl4

International audienceThe quasi-two-dimensional Heisenberg spin S = 1/2 dimer system bis(2-amino-5-fluoro-pyridinium) tetrachlorocuprate(II) is studied by means of inelastic neutron scattering, calorimetry and nuclear magnetic resonance (NMR) experiments. In the absence of an applied magnetic field we find dispersive triplet excitations with a spin gap of Delta=1.112(15) meV and a bandwidth of 0.715(15) meV within the layers and 0.116(15) meV between the layers, respectively. In an applied magnetic field of 8.5 T the spin gap is closed and we find a field induced antiferromagnetically ordered phase

Critical dielectric susceptibility at a magnetic BEC quantum critical point

Magnetic-field-induced phase transitions are investigated in the frustrated gapped quantum paramagnet Rb2Cu2Mo3O12 through dielectric and calorimetric measurements on single-crystal samples. It is clarified that the previously reported dielectric anomaly at 8 K in powder samples is not due to a chiral spin liquid state as has been suggested, but rather to a tiny amount of a ferroelectric impurity phase. Two field-induced quantum phase transitions between paraelectric and paramagnetic and ferroelectric and magnetically ordered states are clearly observed. It is shown that the electric polarization is a secondary order parameter at the lower-field (gap closure) quantum critical point but a primary one at the saturation transition. Having clearly identified the magnetic Bose-Einstein condensation (BEC) nature of the latter, we use the dielectric channel to directly measure the critical divergence of BEC susceptibility. The observed power-law behavior is in very good agreement with theoretical expectations for three-dimensional BEC. Finally, dielectric data reveal magnetic presaturation phases in this compound that may feature exotic order with unconventional broken symmetries.ISSN:2643-156

LT Scaling in Depleted Quantum Spin Ladders

Using a combination of neutron scattering, calorimetry, quantum Monte Carlo simulations, and analytic results we uncover confinement effects in depleted, partially magnetized quantum spin ladders. We show that introducing nonmagnetic impurities into magnetized spin ladders leads to the emergence of a new characteristic length L in the otherwise scale-free Tomonaga-Luttinger liquid (serving as the effective low energy model). This results in universal LT scaling of staggered susceptibilities. Comparison of simulation results with experimental phase diagrams of prototypical spin ladder compounds bis(2,3-dimethylpyridinium)tetrabromocuprate(II) (DIMPY) and bis(piperidinium)tetrabromocuprate(II) (BPCB) yields excellent agreement.ISSN:0031-9007ISSN:1079-711

Presaturation phase with no dipolar order in a quantum ferro-antiferromagnet

Magnetization, magnetocaloric, calorimetric, neutron- and x-ray-diffraction, and inelastic-neutron-scattering measurements are performed on single crystals of BaCdVO(PO4)2. The low-temperature crystal structure is found to be of a lower symmetry than previously assumed. The result is a more complicated model spin Hamiltonian, which we infer from measurements of the spin wave dispersion spectrum. The main finding is a spin state which emerges in high magnetic fields after antiferromagnetic order is terminated at Hc1≃4.0T. It is a distinct thermodynamic phase with a well-defined phase boundary at Hc2≃6.5T and is clearly separate from the fully saturated phase, yet it shows no conventional (dipolar) magnetic long-range order. We argue that it is fully consistent with the expectations for a quantum bond-nematic state.ISSN:2643-156