Quantum magnetism and quantum phase transitions

Abstract:

The magnetic excitations of the quasi-2D spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice Cs2CuCl4 are explored throughout the 2D Brillouin zone using inelastic neutron scattering. In the spin liquid phase above the transition to magnetic order extended excitation continua are observed, characteristic of fractionalization of S=1 spin waves into pairs of deconfined S=1/2 spinons and the hallmark of a resonating-valence-bond (RVB) state. The weak inter-layer couplings stabilize incommensurate spiral order at low temperatures and in this phase sharp magnons carrying a small part of the total scattering weight are observed at low energies below the continuum lower boundary. Linear spin-wave theory including one- and two-magnon processes can describe the sharp magnon excitation, but not the dominant continuum scattering, which instead is well described by a parameterized two-spinon cross-section. Those results suggest a cross-over in the nature of excitations from S=1 spin waves at low energies to S=1/2 spinons at medium to high energies, which could be understood if Cs2CuCl4 was in the close proximity of a transition between a fractional spin liquid and a magnetically-ordered phase.

Abstract:

Cs2CuCl4 is a 2D frustrated quantum magnet that has recently been shown to display a very unusual quantum spin liquid state. The excitations are not spin-1 magnons as observed in other un-frustrated 2D quantum magnets but instead are spinons carrying a fractional spin of 1.2, as predicted by a resonating-valence-bond picture. Here we use high magnetic fields to induce a transition from this fractional quantum spin-liquid phase to the fully-polarized phase where spins are ferromagnetically aligned along the field. In this field-induced phase all quantum fluctuations are quenched by the external field and the system is expected to behave like a classical magnet with spin-1 magnon excitations. Measurements are made in fields up to 12 T and temperatures below 0.2 K. Ferromagnetic saturation is observed at the critical field Bc = 8.44(1)T ∥ a. Above Bc the measured excitations lineshapes show well-defined, almost resolution-limited peaks as expected for spin-1 magnons and are gapped throughout the zone. The gap to the lowest energy excitation decreases linearly upon decreasing field and closes at Bc, below which the system orders into a cone phase. This transition provides an opportunity to study how ordered phases arise from the condensation of excitations.

Abstract:

Quasi-elastic and inelastic neutron scattering studies of the quasi-one-dimensional S = 2 antiferromagnet McCl3(C10D8N2) are reported. The quasi-elastic measurements exhibit a broad peak at Q ≈ 0.69 Å^-1, which is consistent with short-range antiferromagnetic coupling between neighboring Mn³⁺ ions. Inelastic experiments, at 150 mK and Q = 0.70 Å^-1, reveal decreased magnetic scattering at energies less than 0.2 meV when compared to similar studies at 20 K. These results provide microscopic evidence for the presence of a Haldane gap and are consistent with the bulk magnetization measurements of Granroth et al.

Abstract:

The electronic properties of heavy fermion alloys are dominated by spin fluctuations which are expected to become critical when tuned by pressure to a quantum critical point (QCP), entering a magnetic ordered state. Apart from the onset of exotic superconductivity, unexpected "normal conducting" behavior is found close to the QCP, which does not seem only to escape the conventional view of metals (Fermi liquids) but also the "conventional view" of an antiferromagnetic quantum phase transition in these f-metals. So far only few compounds have been investigated by neutron scattering to directly reveal the critical fluctuations spectrum. In CeCu59Au01 the fluctuations develop an unusual energy dependence, characterized by an exponent α = 0.75, which persist over the entire Brillouin zone, provoking an unexpected local non Fermi liquid behavior. The same unusual exponent derived from E/T scaling determines the H/T scaling of the uniform magnetization. Recent neutron scattering data in magnetic fields further confirm this picture of nearly free local magnetic moments (modified by α) emerging at the antiferromagnetic QCP in this strongly correlated electron system.