The experimental realization of quantum spin liquids is a long-sought goal in physics, as they represent new states of matter. Quantum spin liquids cannot be described by the broken symmetries associated with conventional ground states. In fact, the interacting magnetic moments in these systems do not order, but are highly entangled with one another over long ranges. Spin liquids have a prominent role in theories describing high-transition-temperature superconductors, and the topological properties of these states may have applications in quantum information. A key feature of spin liquids is that they support exotic spin excitations carrying fractional quantum numbers. However, detailed measurements of these 'fractionalized excitations' have been lacking. Here we report neutron scattering measurements on single-crystal samples of the spin-1/2 kagome-lattice antiferromagnet ZnCu(3)(OD)(6)Cl(2) (also called herbertsmithite), which provide striking evidence for this characteristic feature of spin liquids. At low temperatures, we find that the spin excitations form a continuum, in contrast to the conventional spin waves expected in ordered antiferromagnets. The observation of such a continuum is noteworthy because, so far, this signature of fractional spin excitations has been observed only in one-dimensional systems. The results also serve as a hallmark of the quantum spin-liquid state in herbertsmithite.

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.

There is great interest in finding materials possessing quasiparticles with topological properties. Such materials may have novel excitations that exist on their boundaries which are protected against disorder. We report experimental evidence that magnons in an insulating kagome ferromagnet can have a topological band structure. Our neutron scattering measurements further reveal that one of the bands is flat due to the unique geometry of the kagome lattice. Spin wave calculations show that the measured band structure follows from a simple Heisenberg Hamiltonian with a Dzyaloshinkii-Moriya interaction. This serves as the first realization of an effectively two-dimensional topological magnon insulator-a new class of magnetic material that should display both a magnon Hall effect and protected chiral edge modes. DOI: 10.1103/PhysRevLett.115.147201 PACS numbers: 75.30.Ds When quantum particles are confined to move in reduced dimensions, such as in planes, unexpectedly rich physics can emerge as a result of the geometry and interactions. The quantum Hall effect is a famous example, which results from placing a two-dimensional (2D) gas of electrons or quasiparticles in a large magnetic field [1]. Pioneering theoretical work by Haldane showed that some systems may inherently possess topological bands that allow them to exhibit quantum Hall physics without applied magnetic fields [2]. The discovery of materials in which strong spin-orbit coupling leads to topological bands, such as topological insulators, has led to a flurry of activity in condensed matter physics research [3,4]. Recently, theoretical studies have focused on 2D topological band structures that include flat bands due to the possibility of achieving fractional quantum hall physics in the absence of magnetic fields [5]. Flat bands (bands that are dispersionless in energy) hold unique interest because the interaction energy between particles may dominate the kinetic energy, leading to novel correlated electron states. A number of theoretical models for the fractional quantum Hall effect have been proposed based on flat topological bands [6][7][8]; however, these invariably require tuning of parameters, which is difficult to control in real materials.Topological band structures are not unique to systems with electronlike quasiparticles. It has been demonstrated that topological photon modes can be realized in experimental systems [9][10][11]. Possible realizations of topological bosonic systems that include flat bands have been proposed using dipolar molecules trapped in an optical lattice [12], and using photonic lattices [13] based on the interaction between photons and arrays of superconducting circuits [14], although experimental confirmation has yet to be demonstrated. In this Letter, we show that topological bands exist for another class of quasiparticles: magnons in an insulating ferromagnet. Our material serves as the first realization of an effectively 2D topological magnon insulator [15], an electrically insulating state in which the spin degre...

We demonstrate and characterize a high-flux beam source for cold, slow atoms or molecules. The desired species is vaporized using laser ablation, then cooled by thermalization in a cryogenic cell of buffer gas. The beam is formed by particles exiting a hole in the buffer gas cell. We characterize the properties of the beam (flux, forward velocity, temperature) for both an atom (Na) and a molecule (PbO) under varying buffer gas density, and discuss conditions for optimizing these beam parameters. Our source compares favorably to existing techniques of beam formation, for a variety of applications.

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...