Pseudogap phase: Key to Unlocking High-Tc Superconductivity
Pseudogap phase
A New Theory Suggests Geometric Orthogonal Metals
New research may uncover the microscopic origins of the pseudogap phase in high-temperature superconductors, a major advance in condensed matter physics. These complicated materials feature stripe and AFM phases. Henning Schlömer, Annabelle Bohrdt, and Fabian Grusdt from Regensburg University, LMU München, and MCQST conducted the study. High-temperature superconductivity mysteries include the hole-doped cuprate pseudogap phase. Its remarkable features, discovered decades ago, may explain how certain materials become superconducting at high temperatures. Despite much research, its presence and relationship to other observable phases like antiferromagnetic and stripe orders remain unexplained. Quantum oscillations and transport measurements (Hall, optical conductivity, magnetoresistance) show Fermi-liquid-like quasiparticles forming a small Fermi surface, but photoemission experiments show no coherent fermionic quasiparticles. Luttinger's theorem predicts a carrier density of 1+δ, however this small Fermi surface has a carrier density of δ (hole doping), challenging the usual view. Previous speculative explanations for the pseudogap included the formation of new fractionalised Fermi liquid (FL) and orthogonal metal (OM) phases and linkages to symmetry-breaking orders like pairing or striping. Experimental and numerical studies have recently supported the complex relationship between stripe order and the pseudogap phase, despite FL and OM phases' topological nature being compatible with a small Fermi surface without symmetry breaking, a feature observed experimentally. The new theory proposes a credible alternative, the Geometric Orthogonal Metal (GOM). This approach emphasises “hidden order”. This framework has an ordered antiferromagnetic backdrop due to material spins. Over this background, charge variations stabilise fluctuating domain barriers. These domain barriers disrupt and hide space's long-range antiferromagnetic order, providing the impression that investigations only involve short-range AFM correlations. Importantly, these background spins' reference frame preserves magnetic order. The proliferation of altering domain walls may be the pseudogap phase's core. A remarkable outcome of this hidden order is the appearance of fermionic quasiparticles as magnetic polarons. In the ground state, these magnetic polarons actively interact with Z2 topological excitations of a domain-wall string-net condensate. This topological order is significant because it explains how a small Fermi surface develops with short-range magnetism, supporting several experimental observations. GOM, an orthogonal metal, has fermionic quasiparticles that are “gauge-charged,” or orthogonal to electrons. Transport and quantum oscillation measurements detect them, which is consistent with cuprate experiments, even if their orthogonality prevents them from emerging coherently in ground-state Angle-Resolved Photoemission Spectroscopy. This seems to violate Luttinger's theorem because Oshikawa's flux insertion approach absorbs momentum with topological excitations.
The GOM framework clarifies the pseudogap transition. The researchers believe the hidden order disappears at a certain doping value. This loss drives a shift to a typical Fermi liquid at a “hidden quantum critical point” (hQCP), which exhibits quantum critical transport properties. The original lattice model's short-ranged correlations on both sides of the transition hide the hQCP from divergent correlation lengths. Charge fluctuations that hamper the effective spin system in “squeezed space” at higher doping levels may induce this shift. GOM provides a profound unifying perspective and explains the pseudogap. The antiferromagnetic, stripe, and pseudogap phases have a tight link, suggesting that they all originated from the same origin where the fluctuating domain walls “hidden” the spin background's SU(2) symmetry, which abruptly shatters in the pseudogap phase. This theory also proposes that superconductivity mechanisms may be unified across material classes, such as heavy fermion compounds, where magnetic fluctuations are seen as the pairing mechanism, and electron- and hole-doped cuprates. Unlike prior “fluctuating stripe” theories that focused on unidirectional stripes and competing orders, the GOM imagines isotropically fluctuating string-net structures to explain the pseudogap without quantum critical fluctuations of conflicting orders. The authors support their theoretical predictions with ultracold-atom simulators. These advanced experimental platforms might directly test the GOM theory and measure nonlocal correlations by examining spin- and charge-resolved snapshots for hidden order. Recent advances in Fermi-Hubbard system cryogenic temperatures allow such investigations, which considerably improves our understanding of the pseudogap's microscopic origins and its important significance in high-temperature superconductivity. This new GOM scenario's sophisticated idea of fluctuating domain barriers mediating concealed antiferromagnetic provides a robust and comprehensive framework that could assist clarify several inconsistent data in cuprite superconductors and probably beyond. It is a huge step towards understanding one of the most difficult physics problems.