Evidence for a spinon Fermi surface in a triangular lattice quantum spin liquid candidate (1607.02615v2)

Abstract: A quantum spin liquid is an exotic quantum state of matter in which spins are highly entangled and remain disordered down to zero temperature. Such a state of matter is potentially relevant to high-temperature superconductivity and quantum-information applications, and experimental identification of a quantum spin liquid state is of fundamental importance for our understanding of quantum matter. Theoretical studies have proposed various quantum-spin-liquid ground states, most of which are characterized by exotic spin excitations with fractional quantum numbers (termed `spinon'). Here, we report neutron scattering measurements that reveal broad spin excitations covering a wide region of the Brillouin zone in a triangular antiferromagnet YbMgGaO4. The observed diffusive spin excitation persists at the lowest measured energy and shows a clear upper excitation edge, which is consistent with the particle-hole excitation of a spinon Fermi surface. Our results therefore point to a QSL state with a spinon Fermi surface in YbMgGaO4 that has a perfect spin-1/2 triangular lattice as in the original proposal of quantum spin liquids.

• The paper demonstrates that neutron scattering reveals broad, diffuse spin excitations indicative of a spinon Fermi surface in YbMgGaO4.
• Detailed analysis shows a V-shaped boundary in the energy spectrum, suggesting a high density of low-energy spinon states.
• The absence of magnetic ordering down to 30 mK confirms the QSL state and highlights YbMgGaO4 as a promising candidate for quantum spin liquid research.

Evidence for a Spinon Fermi Surface in a Triangular Lattice Quantum Spin Liquid Candidate

The paper "Evidence for a spinon Fermi surface in a triangular lattice quantum spin liquid candidate" investigates the experimental signatures of a quantum spin liquid (QSL) state in the triangular lattice antiferromagnet YbMgGaO$_4$. This paper employs neutron scattering techniques to reveal the presence of broad spin excitations, which provide critical evidence supporting a spinon Fermi surface in this material. The authors assert that this is consistent with the particle-hole excitations typically associated with a spinon Fermi surface, offering new insights into the understanding of QSLs and their potential applications.

Key Findings and Methodology

1. Quantum Spin Liquid Perspective: The paper identifies YbMgGaO$_4$ as a candidate for realizing the elusive QSL state proposed by Anderson in the context of triangular lattice Heisenberg antiferromagnets. Unlike traditional magnetic ordering, QSLs do not conform to Landau's symmetry-breaking paradigm and are instead characterized by fractional excitations known as spinons.
2. Experimental Observations: Utilizing inelastic neutron scattering (INS), the authors detect broad, diffusive magnetic excitations throughout the Brillouin zone at low temperatures. These excitations lack the sharpness associated with magnon peaks, typical of ordered magnetic systems, indicating the potential realization of a QSL with a spinon Fermi surface.
3. Characteristics of Spin Excitations: The paper details how these excitations are consistent with the presence of a spinon Fermi surface. The paper finds that the spectral intensity near the $\Gamma$ point decreases with increasing energy, identifying a V-shaped boundary in the excitation spectrum. This suggests a high density of low-energy spinon states, unparalleled by gapped QSLs or Dirac QSLs, and entirely consistent with a spinon Fermi surface scenario.
4. Material Properties: YbMgGaO$_4$ demonstrates no magnetic ordering or symmetry breaking down to 30 mK, despite strong spin-orbit coupling (SOC) and a significant exchange energy scale of 4 K. This positions it as an intriguing system not directly susceptible to the constraints proposed by the Oshikawa-Hastings-Lieb-Schultz-Mattis (OHLSM) theorem.
5. Theoretical Implications: The researchers propose a minimal mean-field spinon Hamiltonian to describe this state. Their model, featuring a uniform spinon hopping on a triangular lattice, aligns with a spinon Fermi surface characterized by particle-hole pair excitations. This provides a comprehensive explanation for the observed neutron scattering spectra.

Numerical and Empirical Analysis

The results exhibit a clear upper excitation edge and a continuum extending to relatively low energies. The consistent absence of ordered magnetic states down to low temperatures, together with the neutron scattering data, supports the QSL designation. The findings carry potential implications for understanding high-temperature superconductivity and emergent phenomena in complex quantum systems.

Future Directions and Theoretical Implications

The elucidation of a spinon Fermi surface in YbMgGaO$_4$ not only enhances the understanding of QSLs but also suggests avenues for further research. Investigations can be expanded to other materials with similar structural and magnetic profiles to explore the universality of these findings. Additionally, the continued refinement of theoretical models to include anisotropic interactions and SOC effects could yield further insights into the exotic properties of these quantum states.

Ultimately, the research presented in this paper contributes significantly to the ongoing efforts to characterize and utilize QSLs in next-generation quantum technologies. By advancing both experimental and theoretical frameworks, this paper helps bridge existing gaps in the understanding of quantum spin systems and their broader applications.

Overall, this paper serves as a pivotal contribution to the characterization of spinon excitations, advancing the broader understanding of quantum spin liquids and their potential technological applications.