Physics of a strong-leg quantum spin ladder

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In the last decades, low-dimensional electronic and magnetic systems, and their extraordinary quantum-mechanical properties, have attracted a lot of experimental and theoretical interest. This thesis is dedicated to the experimental study of the organometallic material (C7H10N)2CuBr4 (DIMPY), discovered in 2007. Due to its crystal structure, the copper ions and their electronic spins build a one-dimensional coupling network, a so-called antiferromagnetic Heisenberg spin ladder in the strong-leg coupling regime. The clean onedimensionality of this system and the comparatively small interaction strength between copper spins enable to strongly influence the magnetic properties at low temperatures or applied magnetic fields and to drive it through a series of quantum phase transitions. In fact, it is possible to access the full temperature-field phase diagram of this material with conventionally available low temperatures in the millikelvin-range and magnetic fields up to a few dozen of Tesla. In this thesis, the static and dynamic magnetic properties of DIMPY were studied with a series of inelastic neutron scattering experiments and magneto-thermodynamic measurements, throughout its phase diagram. The following topics were covered: 1.) Large and deuterated single crystals with masses up to several grams were grown and characterized in our laboratories at ETH Zurich. 2.) Inelastic neutron scattering experiments were performed at zero field and at lowest temperatures, by applying both the triple-axis and time-of-flight neutron scattering techniques. The thereby obtained excitation spectrum was studied in combination with numerical Density Matrix Renormalization Group (DMRG) calculations, enabling for example to determine the exchange constants of the 1D Hamiltonian. 3.) The field-dependence of the dynamical structure factor was measured with inelastic neutron scattering experiments. At a critical magnetic field of μ0H ' 2.6 T, the quantum phase transition from the low-field gapped spin liquid to a gapless Tomonaga-Luttinger spin liquid (TLSL) state was observed, along with the associated spectral changes. In particular, the transformation of the sharp and gapped excitation modes at low fields into intercalated, highly structured, gapped and gapless continua above the critical field was studied. We were able to confirm so far experimentally untested quantum field theoretical predictions concerning several spectral features. Furthermore, numerical DMRG calculations enabled to reproduce the measured spectra, in quantitative agreement. In addition to a recent study of the opposite and complementary strong-rung spin ladder material BPCB, addressing the lowest-energy continuum only, we reported the first observation of the complete spectrum in the field-induced TLSL regime of a strong-leg spin ladder. 4.) The finite-temperature properties at zero field were studied using a series of inelastic neutron scattering experiments. The magnon excitation was observed to be subject to a