Quantum magnetism and quantum phase transitions

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.

Abstract:

We propose a method for measuring spin Hamiltonians and apply it to the spin- 1/2 Heisenberg antiferromagnet Cs2CuCl4, which shows a 2D fractionalized resonating valence bond state at low fields. By applying strong fields we fully align the spin moment of Cs2CuCl4, transforming it into an effective ferromagnet. In this phase the excitations are conventional magnons and their dispersion relation measured using neutron scattering give the exchange couplings directly, which are found to form an anisotropic triangular lattice with small Dzyaloshinskii-Moriya terms. Using the field to control the excitations we observe Bose condensation of magnons into an ordered ground state.

Abstract:

We explore the effects of noncommuting applied fields on the ground-state ordering of the quasi-one-dimensional spin-1/2 XY-like antiferromagnet Cs2CoCl4 using single-crystal neutron diffraction. In zero field interchain couplings cause long-range order below T_N=217(5) mK with chains ordered antiferromagnetically along their length and moments confined to the (b,c) plane. Magnetic fields applied at an angle to the XY planes are found to initially stabilize the order by promoting a spin-flop phase with an increased perpendicular antiferromagnetic moment. In higher fields the antiferromagnetic order becomes unstable and a transition occurs to a phase with no long-range order in the (b,c) plane, proposed to be a spin liquid phase that arises when the quantum fluctuations induced by the noncommuting field become strong enough to overcome ordering tendencies. Magnetization measurements confirm that saturation occurs at much higher fields and that the proposed spin-liquid state exists in the region 2.10 < H_SL < 2.52 T || a. The observed phase diagram is discussed in terms of known results on XY-like chains in coexisting longitudinal and transverse fields.