Fractionalized excitations in the spin-liquid state of a kagome-lattice antiferromagnet

Abstract

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.

A two-dimensional spin liquid in quantum kagome ice

“…Previous efforts have focused largely on SU(2) Hamiltonians on kagome or triangular lattice materials 6,7 . In contrast, we propose to concentrate the search on the quantum spin ice pyrochlore materials, subject to an external field along the 18 not considered in equation (1), our study highlights the importance of the of J ±± term, which is present for instance in Yb 2 Ti 2 O 7 (ref.…”

“…Some, such as emergent gauge structures and fractional charges, are implicated in a wide range of future technologies like high-temperature superconductivity 1,2 and topological quantum computing 3 . It is therefore remarkable that, despite extensive examination of the basic theoretical ingredients required to promote a 2D QSL in microscopic models 4,5 , the state remains elusive, with only a few experimental candidates existing today 6,7 .…”

Evidence for a spinon Fermi surface in a triangular-lattice quantum-spin-liquid candidate

“…4a, b as well as the E scans at a few given momentum points (Γ, M, K) in Fig. 4c.The broad continuum is an immediate consequence and strong evidence of spinon excitations in QSLs 1,7,12 . This differs from magnon-like excitations that would peak strongly at specific momenta in the reciprocal space, with or without static magnetic order 23,24 .…”

“…A QSL, as currently understood, does not fit into Landau's conventional paradigm of symmetry breaking phases 1,2,6,7 , and is arXiv:1607.02615v2 [cond-mat.str-el] 31 Jul 2017 2 instead an exotic state of matter characterized by spinon excitations and emergent gauge structures [1][2][3]6 . The search for QSLs in models and materials [8][9][10][11][12] has been partly facilitated by the Oshikawa-Hastings-Lieb-Schultz-Mattis (OHLSM) theorem that may hint at the possibility of QSLs in Mott insulators with odd electron fillings and a global U(1) spin rotational symmetry [13][14][15] .…”

Quantum Magnetic Properties in Perovskite with Anderson Localized Artificial Spin‐1/2

“…Most importantly, the high temperature excitations are gapless and occupy a significant portion of the Q – E space. The broad continuous tail in both energy and the momentum space is reminiscent of the quasi‐continuum spectrum, observed in geometrically frustrated lattice of spin‐1/2 9, 26. The quasi‐continuum excitation at high temperature complements the ac dynamic susceptibility measurements in x > 0.15 (Figure 3, and Figure S2, Supporting Information).…”