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Area of Science:

  • Condensed Matter Physics
  • Quantum Magnetism
  • Materials Science

Background:

  • Cerium (Ce) pyrochlores are a class of materials exhibiting complex magnetic behaviors due to the strong spin-orbit coupling of Ce ions.
  • Ce_{2}Hf_{2}O_{7} is a specific pyrochlore compound whose magnetic properties are of interest for understanding exotic quantum states.
  • Previous studies on related compounds like Ce_{2}Zr_{2}O_{7} and Ce_{2}Sn_{2}O_{7} have shown Schottky-like anomalies in their heat capacity.

Purpose of the Study:

  • To investigate the low-temperature magnetic heat capacity of a single crystal of Ce_{2}Hf_{2}O_{7}.
  • To characterize the ground state properties and identify potential magnetic ordering or quantum spin liquid behavior.
  • To compare the findings with theoretical predictions and experimental results from sister Ce pyrochlore compounds.

Main Methods:

  • Magnetic heat capacity (C_{P}) measurements were performed on a high-quality single crystal of Ce_{2}Hf_{2}O_{7} down to 0.02 K.
  • Analysis of the heat capacity data to identify characteristic peaks and entropy contributions.
  • Comparison of experimental results with theoretical models, including the XYZ Hamiltonian and NLC calculations.
  • Diffuse magnetic neutron scattering was used to probe magnetic correlations at low temperatures.

Main Results:

  • A two-peaked structure was observed in the heat capacity: a Schottky-like peak at T_{1}∼0.065 K and a sharper peak at T_{2}∼0.025 K.
  • The ground state appears to possess gapped excitations, with the entropy at T=0 K consistent with a pseudospin-1/2 doublet.
  • The sharp peak at T_{2} suggests a crossover from a classical spin liquid to a quantum spin liquid (QSL).
  • Diffuse neutron scattering data resembles that of Ce_{2}Zr_{2}O_{7}, a known π-flux quantum spin ice (QSI).

Conclusions:

  • Ce_{2}Hf_{2}O_{7} exhibits a distinct ground state characterized by gapped excitations, potentially a quantum spin liquid.
  • The observed features suggest a classical spin liquid regime above T_{2} and a zero-entropy quantum ground state below T_{2}.
  • Weak interactions beyond the nearest-neighbor XYZ Hamiltonian are likely relevant below 0.25 K.
  • The material shows characteristics suggestive of a quantum spin ice (QSI) or related quantum spin liquid state.