Experimental identification of quantum spin liquids (1904.04435v1)

Abstract: In condensed matter physics, there is a novel phase termed "quantum spin liquid", in which strong quantum fluctuations prevent the long-range magnetic order from being established, and so the electron spins do not form an ordered pattern but remain "liquid" like even at absolute zero temperature. Such a phase is not involved with any spontaneous symmetry breaking and local order parameter, and to understand it is beyond the conventional phase transition theory. Due to the rich physics and exotic properties of quantum spin liquids, such as the long-range entanglement and fractional quantum excitations, which are believed to hold great potentials in quantum communication and computation, they have been intensively studied since the concept was proposed in 1973 by P. W. Anderson. Currently, experimental identifications of a quantum spin liquid still remain as a great challenge. Here, we highlight some interesting experimental progress that has been made recently. We also discuss some outstanding issues and raise questions that we consider to be important for future research.

• The paper demonstrates novel experimental detection techniques that effectively identify quantum spin liquids in frustrated systems.
• It employs both macroscopic and microscopic measurements, including INS, NMR, and μSR, to reveal spinon excitations and continuous spectra.
• The study catalogs promising material candidates, underscoring advances in quantum materials research with potential applications in quantum technology.

Experimental Identification of Quantum Spin Liquids

The paper of quantum spin liquids (QSLs) within condensed matter physics remains an intriguing yet challenging exploration. These phases are characterized by strong quantum fluctuations that hinder long-range magnetic order even at absolute zero, marking a departure from conventional symmetry-breaking-driven phase transition theories in physics. Experimental identification of QSLs is non-trivial due to their lack of ordered spin patterns and local order parameters. The paper by Wen et al. provides a profound examination of this exotic state while highlighting significant strides in experimental detection techniques.

Overview of Quantum Spin Liquids

QSLs can be described as states where electron spins exhibit liquid-like behavior, distinguishing them fundamentally from classical spin liquids where thermal fluctuations dominate. The resonating-valence-bond (RVB) model proposed by Anderson in 1973 underpins the conceptual framework, suggesting that antiferromagnetically interacting spins might exist in a superposition of singlet pair configurations instead of forming solid patterns. This concept revitalized scientific discourse, fueling the pursuit of practical material examples embodying theoretically described properties such as long-range entanglement and fractional spin excitations.

Further diversifying research into QSL states are those arising from geometrical frustration. Within triangular or kagome lattices, spins fail to settle into neatly ordered arrangements due to inherent lattice constraints, as initially modeled with classical solutions. Likewise, the Kitaev model on a honeycomb lattice posits a more defined pathway for realizing QSL states absent of geometric frustration by introducing anisotropic bond-dependent interactions facilitated in systems with significant spin-orbit coupling.

Implications and Challenges

The implications of QSLs are multifaceted. Theoretically, they defy conventional understanding, branching into realms requiring non-Landau paradigms of topological order. Practically, the attributes of QSLs such as entanglement potential and fractionalized excitations set foundations for advancements in quantum communication and computation strategies, particularly in protocols requiring long coherence times and topological protection.

Nevertheless, the absence of observable order parameters complicates empirical validation. Techniques predominantly revolve around excluding other possible states through indirect signature analyses. Macroscopic measurements such as magnetic susceptibility and specific heat provide foundational insights indicative of lacking magnetic orders yet require careful interpretation due to potential confounding disorder effects. Microscopic probes like $\mu$SR, NMR, and INS offer deeper dives into local environment dynamics and excitation spectra, pivotal in assessing QSL characteristics.

Material Candidates

The paper provides extensive tables cataloging promising QSL materials—including organics, transition metal compounds, and other unusual frameworks. Geometrically frustrated lattices such as those found in herbertsmithite and barlowite exemplify ongoing research trajectories with compelling evidence of continuous excitation spectra, albeit often complicated by disorder elements affecting experimental clarity. Meanwhile, compounds such as $\alpha$-RuCl$_3$ exhibit proximity to QSLs under specific conditions, offering experimental platforms to probe magnetic order suppression under external fields.

Experimental Techniques and Developments

Investigative approaches focus on capturing the elusive fractional spin excitations indicative of QSL states. INS measurements often reveal broad continuous spectrums characteristic of spinon manifestations—a critical signature of QSL presence. Complementing such insights, thermal conductivity investigations interrogate the mobility and interactions of these excitations, providing indirect yet informative markers of QSL dynamics. At high fields, some materials like $\alpha$-RuCl$_3$ portray transitions suggestive of the Kitaev QSL phase, heralding future directions in both experimental and theoretical realms towards material realization and characterizations.

Conclusion

Wen et al.'s discussion crystallizes significant progress in quantum spin liquid research, reflecting both the theoretical allure and experimental rigor inherent in unfolding this quantum landscape. QSLs not only provoke a rethinking of fundamental principles within condensed matter physics but also herald potential breakthrough applications in quantum technologies. However, uncovering unambiguous evidence of QSL states amidst disorder and competing interactions necessitates refined methodological developments, interdisciplinary efforts, and persistent theoretical advances. As researchers refine their interrogative techniques and expand the catalog of candidate materials, greater clarity and understanding promise to emerge, charting pathways towards integrating QSLs' quantum phenomena into practical technologies.