Novel Pauli-paramagnetic quantum phase in a Mott insulator.

Area of Science:

• Condensed Matter Physics
• Quantum Materials
• Magnetism

Background:

• Mott insulators exhibit electron localization due to strong Coulomb repulsion.
• Geometrical frustration in 2D systems can disrupt magnetic order, potentially forming quantum spin liquids.
• The magnetic ground states of quantum spin liquids have remained largely enigmatic.

Purpose of the Study:

• To investigate the magnetic ground state of the organic Mott insulator EtMe(3)Sb[Pd(dmit)(2)](2) with a 2D triangular lattice.
• To characterize the low-energy excitations and magnetic properties of this frustrated quantum system.
• To determine if the system exhibits characteristics of itinerant fermions or a magnetically gapless state.

Main Methods:

• Torque magnetometry measurements were performed down to 30 mK and up to 32 T.
• The study utilized deuteration to assess the robustness of observed phenomena.
• Analysis focused on residual paramagnetic susceptibility and low-energy spin excitations.

Main Results:

• The organic Mott insulator exhibits Pauli-paramagnetic-like low-energy excitations, characteristic of itinerant fermions.
• Distinct residual paramagnetic susceptibility, comparable to a 2D metal, indicates a magnetically gapless ground state.
• Results remain robust against deuteration, suggesting an extended 'quantum critical phase'.

Conclusions:

• The quantum spin liquid state in EtMe(3)Sb[Pd(dmit)(2)](2) behaves like a paramagnetic metal with a Fermi surface at low energies.
• This finding demonstrates a unique state where spin excitations mimic itinerant fermions despite frozen charge degrees of freedom.
• The study provides strong evidence for an extended quantum critical phase in frustrated Mott insulators.