Nematicity and quantum paramagnetism in FeSe (1501.00844v2)

Abstract: In common with other iron-based high temperature superconductors, FeSe exhibits a transition to a ``nematic'' phase below 90Kelvin in which the crystal rotation symmetry is spontaneously broken. However, the absence of strong low-frequency magnetic fluctuations near or above the transition has been interpreted as implying the primacy of orbital ordering. In contrast, we establish that quantum fluctuations of spin-1 local moments with strongly frustrated exchange interactions can lead to a nematic quantum paramagnetic phase consistent with the observations in FeSe. We show that this phase is a fundamental expression of the existence of a Berry's phase associated with the topological defects of a N\'eel antiferromagnet, in a manner analogous to that which gives rise to valence bond crystal order for spin 1/2 systems. We present an exactly solvable model realizing the nematic quantum paramagnetic phase, discuss its relation with the spin-1 $J_1-J_2$ model, and construct a field theory of the Landau-forbidden transition between the N\'eel state and this nematic quantum paramagnet.

Nematicity and Quantum Paramagnetism in FeSe: An Analytical Perspective

The paper focuses on the complex interplay between nematicity and quantum paramagnetism in iron-based high-temperature superconductor FeSe. The authors explore the emergence of nematic phases, a characteristic of many iron-based superconductors wherein the crystal's rotational symmetry is spontaneously broken without concurrent magnetic order. Unlike the prototypical iron-based superconductors exhibiting significant low-frequency magnetic fluctuations, FeSe presents an enigmatic case due to its apparent lack of such fluctuations at the nematic transition temperature (T_nem) of approximately 90 Kelvin. This observation has led to speculations about the dominance of orbital ordering in driving this phase transition. Contradicting such perspectives, the authors here propose a novel explanation based on spin-1/2 systems, particularly emphasizing the role of frustrated magnetism mediated by spin fluctuations.

Key assertions made in the paper revolve around the potential for strong quantum fluctuations of spin-1 local moments to induce a nematic quantum paramagnetic phase in FeSe. The authors establish a plausible theoretical framework, suggesting that these quantum fluctuations, when coupled with frustrated exchange interactions, enable the system to attain such a phase state. They draw parallels with valence bond crystal orders in spin-1/2 systems, highlighting the significance of topological defects characterized by Berry's phase effects. An exactly solvable model is presented to corroborate their hypothesis on the nematic quantum paramagnet phase, revealing its conceptual connection to the spin-1 J_1-J_2 model and addressing the Landau-forbidden transition between Néel and nematic quantum paramagnetic phases.

The paper illustrates how Fe$^{2+}$ ions on a square lattice, underpinned by a spin-1 model, can stabilize a nematic quantum paramagnetic phase through short-range frustrated interactions, challenging the notion that nematicity necessitates orbital orderings. Numerical results from the density matrix renormalization group are invoked, alongside small lattice exact diagonalization for various models, to substantiate the connection to the $J_1$-$J_2$ model, ultimately reinforcing the proposal that local moment interactions alone might suffice to induce the nematic phase observed in FeSe.

The implications of such a framework are manifold, advancing our understanding of frustrated quantum magnets and their potential relevance to FeSe's behavior. The absence of an enhancement in low-frequency magnetic fluctuations at T_nem supports the notion of magnetism-driven nematicity. This finding could significantly affect our conceptualization of superconductivity mechanisms in iron-based materials, tipping the focus from mere electronic or lattice considerations to nuanced spin dynamic interactions.

Future research building on this theoretical candidature should aim to tackle the effects of itinerant carriers, given FeSe's advent in electronic and hole-like Fermi pockets. Furthermore, experimental strategies, perhaps integrating advanced neutron scattering and Raman scattering techniques, would be vital in discerning the subtleties of spin dynamics and nematic order fluctuations inherent in FeSe and similar iron-based superconductors. Through such investigations, new regimes of magnetic behavior in condensed matter systems might be uncovered, contributing to a profound grasp of unconventional superconductors' physics.