We have performed thermodynamic and neutron scattering measurements on the S=1/2 kagomé lattice antiferromagnet ZnCu3(OH)6Cl2. The susceptibility indicates a Curie-Weiss temperature of theta CW approximately = -300 K; however, no magnetic order is observed down to 50 mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1 meV. The specific heat at low-T follows a power law temperature dependence. These results suggest that an unusual spin liquid state with essentially gapless excitations is realized in this kagomé lattice system.

The collective behaviour of interacting magnetic moments can be strongly influenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. We report a study of the spin chirality on a two-dimensional geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large single-crystal samples of KFe3(OH)6(SO4)2, an ideal Kagomé lattice antiferromagnet. Combined thermodynamic and neutron scattering measurements reveal that the phase transition to the ordered ground-state is unusual. At low temperatures, application of a magnetic field induces a transition between states with different non-trivial spin-textures.

Neutron scattering measurements were performed to investigate magnetic excitations in a single-crystal sample of the ternary iron arsenide BaFe 2 As 2 , a parent compound of a recently discovered family of Fe-based superconductors. In the ordered state, we observe low energy spin-wave excitations with a gap energy ⌬ = 9.8͑4͒ meV. The in-plane spin-wave velocity v ab and out-of-plane spin-wave velocity v c measured at 12 meV are 280͑150͒ and 57͑7͒ meV Å, respectively. At high energy, we observe anisotropic scattering centered at the antiferromagnetic wave vectors. This scattering indicates two-dimensional spin dynamics, which possibly exist inside the Stoner continuum. At T N = 136͑1͒ K, the gap closes and quasielastic scattering is observed above T N , indicative of short-range spin fluctuations. In the paramagnetic state, the scattering intensity along the L direction becomes "rodlike," characteristic of uncorrelated out-of-plane spins, attesting to the twodimensionality of the system.

Crumpled sheets have a surprisingly large resistance to further compression. We have studied the crumpling of thin sheets of Mylar under different loading conditions. When placed under a fixed compressive force, the size of a crumpled material decreases logarithmically in time for periods up to three weeks. We also find hysteretic behavior when measuring the compression as a function of applied force. By using a pretreating protocol, we control this hysteresis and find reproducible scaling behavior for the size of the crumpled material as a function of the applied force.

The spin-1 2 kagome lattice antiferromagnet herbertsmithite, ZnCu3(OH)6Cl2, is a candidate material for a quantum spin liquid ground state. We show that the magnetic response of this material displays an unusual scaling relation in both the bulk ac susceptibility and the low energy dynamic susceptibility as measured by inelastic neutron scattering. The quantity χT α with α ≃ 0.66 can be expressed as a universal function of H/T or ω/T . This scaling is discussed in relation to similar behavior seen in systems influenced by disorder or by the proximity to a quantum critical point.PACS numbers: 75.40. Gb, 75.50.Ee, 78.70.Nx A continuing challenge in the field of frustrated magnetism is the search for candidate materials which display quantum disordered ground states in two dimensions. In recent years, a great deal of attention has been given to the spin-1 2 nearest-neighbor Heisenberg antiferromagnet on the kagome lattice, consisting of corner sharing triangles. Given the high frustration of the lattice and the strength of quantum fluctuations arising from spin-1 2 moments, this system is a very promising candidate to display novel magnetic ground states, including the "resonating valence bond" (RVB) state proposed by Anderson [1]. A theoretical and numerical consensus has developed that the ground state of this system is not magnetically ordered [2][3][4][5][6][7][8], although the exact ground state is still a matter of some debate. Experimental studies of this system have long been hampered by a lack of suitable materials displaying this motif.The mineral herbertsmithite [9,10], ZnCu 3 (OH) 6 Cl 2 , is believed to be an excellent realization of a spin-1 2 kagome lattice antiferromagnet. The material consists of kagome lattice planes of spin-1 2 Cu 2+ ions. The superexchange interaction between nearest-neighbor spins leads to an antiferromagnetic coupling of J = 17±1 meV. Extensive measurements on powder samples of herbertsmithite have found no evidence of long range magnetic order or spin freezing to temperatures of roughly 50 mK [11][12][13]. Magnetic excitations are effectively gapless, with a Curie-like susceptibility at low temperatures. The magnetic kagome planes are separated by layers of nonmagnetic Zn 2+ ions; however, it has been suggested that there could be some site disorder between the Cu and Zn ions [14,15]. This possible site disorder, with ≈ 5% of the magnetic Cu ions residing on out-of-plane sites, as well as the presence of a Dzyaloshinskii-Moriya (DM) interaction [16], would likely influence the low energy magnetic response.In this Letter we report a dynamic scaling analysis of the susceptibility of herbertsmithite as measured in both the bulk ac susceptibility and the low energy dynamic susceptibility measured by inelastic neutron scattering. In particular, we find that the quantity χT α can be expressed as a universal function in which the energy or field scale is set only by the temperature. This type of scaling behavior, when measured in quantum antiferromagnets [17] and heavy-fermion me...