Fractionalized excitations in the spin liquid state of a kagomé lattice antiferromagnet (1307.5047v1)

Abstract: New physics can emerge in magnetic materials where quantum fluctuations are enhanced due to reduced dimensionality and strong frustration. One long sought example is the resonating-valence-bond (RVB) state, where atomic magnetic moments are strongly correlated but do not order or freeze even in the limit of T -> 0. The RVB ground state does not break conventional symmetries, such as lattice translation or spin-rotation. The realization of such a quantum spin liquid in two-dimensions would represent a new state of matter. It is believed that spin liquid physics plays a role in the phenomenon of high-Tc superconductivity, and the topological properties of the spin liquid state may have applications in the field of quantum information. We present neutron scattering measurements of the spin excitations on single crystal samples of the spin-1/2 kagom\'{e} lattice antiferromagnet ZnCu3(OD)6Cl2 (also called herbertsmithite). Our observation of a spinon continuum in a two-dimensional magnet is remarkable first. The results serve as a key fingerprint of the quantum spin liquid state in herbertsmithite.

• The paper demonstrates a spinon continuum in herbertsmithite using inelastic neutron scattering, confirming a quantum spin liquid state on the kagome lattice.
• It identifies broadened hexagonal rings in reciprocal space that indicate strong short-range resonating-valence-bond correlations and quantum fluctuations.
• Results reveal that magnetic excitations extend beyond nearest neighbors, challenging traditional models and prompting further theoretical and experimental studies.

Fractionalized Excitations in the Spin Liquid State of a Kagome Lattice Antiferromagnet

The paper presents compelling results from an investigation of fractionalized excitations in the spin liquid state of a kagome lattice antiferromagnet, particularly focusing on the compound herbertsmithite (ZnCu₃(OD)₆Cl₂). Utilizing inelastic neutron scattering, the paper identifies the presence of a spinon continuum, underscoring the realization of a quantum spin liquid (QSL) state in a two-dimensional kagome lattice.

Key Observations and Results

The central tenet of the investigation is the detection of deconfined spinons, characteristic excitations of a QSL, in herbertsmithite. These spinons, being $S=\frac{1}{2}$ excitations, emerge from the fractionalization of conventional spin-wave excitations. The results affirm theoretical predictions of resonating-valence-bond (RVB) states, elucidating correlations between atomic magnetic moments that neither order nor freeze, even as temperature approaches zero.

1. Spinon Continuum: The recorded neutron scattering data reveal a diffuse scattering pattern persisting over a broad range of energy transfers (0.25 meV to 11 meV), indicating a spinon continuum rather than discrete spin-wave modes typical of ordered magnet phases. The absence of a spin-gap down to 0.25 meV is a notable feature, challenging the expectations from valence bond crystals or gapped spin liquid theories.
2. Reciprocal Space Analysis: The scattering intensity does not peak at any particular point in reciprocal space, diverging from typical antiferromagnetic correlations observed in systems like the square lattice antiferromagnet La₂CuO₄. Instead, the data show broadened hexagonal rings in reciprocal space, suggesting significant short-range RVB states with considerable quantum fluctuations.
3. Magnetic Correlations: Energy-integrated dynamic structure factors suggest that while nearest-neighbor singlet formations are prevalent, correlations extend beyond the nearest neighbors. This observation is consistent with models proposing spin liquid states but also indicates complexities beyond simple RVB models.

Implications and Future Directions

The findings not only support the presence of a QSL state in herbertsmithite but also open new avenues for probing the quantum entanglement and topological properties inherent in such systems. The discovery emphasizes the potential of QSL states in applications related to quantum information and highlights the intricate balance between geometric frustration and quantum fluctuations in stabilizing such enigmatic states.

Future developments could involve:

• Theoretical Refinements: Modifying existing Heisenberg models to closely align with the empirical spin dynamics of herbertsmithite.
• Experimental Enhancements: Further neutron scattering experiments at even lower energy transfers to check the existence and nature of a minimal spin-gap.
• Material Innovations: Synthesis of kagome lattice compounds with reduced impurities to minimize external influences on low-energy excitations.

This research provides a robust platform for experimental tests of theoretical frameworks regarding spin liquid states on kagome lattices. Understanding the intricate dynamics of fractionalized excitations in QSLs could pave the way for breakthroughs in condensed matter physics and quantum computing.