Pressure-tuned quantum criticality in the large-D antiferromagnet DTN.

Strongly correlated spin systems can be driven to quantum critical points via various routes. In particular, gapped quantum antiferromagnets can undergo phase transitions into a magnetically ordered state with applied pressure or magnetic field, acting as tuning parameters. These transitions are characterized by z = 1 or z = 2 dynamical critical exponents, determined by the linear and quadratic low-energy dispersion of spin excitations, respectively. Employing high-frequency susceptibility and ultrasound techniques, we demonstrate that the tetragonal easy-plane quantum antiferromagnet NiCl2 ⋅ 4SC(NH2)2 (aka DTN) undergoes a spin-gap closure transition at about 4.2 kbar, resulting in a pressure-induced magnetic ordering. The studies are complemented by high-pressure-electron spin-resonance measurements confirming the proposed scenario. Powder neutron diffraction measurements revealed that no lattice distortion occurs at this pressure and the high spin symmetry is preserved, establishing DTN as a perfect platform to investigate z = 1 quantum critical phenomena. The experimental observations are supported by DMRG calculations, allowing us to quantitatively describe the pressure-driven evolution of critical fields and spin-Hamiltonian parameters in DTN.

Electron Spin Resonance of the Interacting Spinon Liquid.

We report experimental verification of the recently predicted collective modes of spinons, stabilized by backscattering interaction, in a model quantum spin chain material. We exploit the unique geometry of uniform Dzyaloshinskii-Moriya interactions in K_{2}CuSO_{4}Br_{2} to measure the interaction-induced splitting between the two components of the electron spin resonance (ESR) response doublet. From that we directly determine the magnitude of the "marginally irrelevant" backscattering interaction between spinons for the first time.

Miniature capacitive Faraday force magnetometer for magnetization measurements at low temperatures and high magnetic fields.

A Faraday force magnetometer is presented for measurements of magnetization at temperatures down to 100 mK and in magnetic fields up to 14 T. The specimen is mounted on a flexible cantilever forming a force-sensing capacitor in combination with a fixed back plate. Two different cantilever designs are presented. A torsion resistant cantilever allows us to measure the magnetization of highly anisotropic single crystal samples. Measurements of the metal organic quantum magnets (C5H12N)2CuBr4 (BPCB) and NiCl2 · 4 SC(NH2)2 (DTN) demonstrate the device's capabilities. Routinely, a specimen's magnetic moment is measured with a resolution better than 10-7 A m2 (10-4 emu). The device is miniaturized to fit in almost any cryostat.

Magnetic-Field-Induced Bound States in Spin-1/2 Ladders.

Motivated by the recently observed intriguing mode splittings in a magnetic field with inelastic neutron scattering in the spin ladder compound (C_{5}H_{12}N)_{2}CuBr_{4} (BPCB), we investigate the nature of the spin ladder excitations using a density matrix renormalization group and analytical arguments. Starting from the fully frustrated ladder, for which we derive the low-energy spectrum, we show that bound states are generically present close to k=0 in the dynamical structure factor of spin ladders above H_{c1}, and that they are characterized by a field-independent binding energy and an intensity that grows with H-H_{c1}. These predictions are shown to explain quantitatively the split modes observed in BPCB.