Quantum spin liquids (QSLs) are promising new and exciting phases in condensed matter, where exotic quantum states of matter could be realized. They were proposed theoretically in 1973 by physicist Philip Anderson to exist in a frustrated triangular lattice. Because spin-liquid states could make topological quantum computation possible, and because they may also help in the understanding of high temperature superconductivity, they are attracting a lot of research attention.
As explained in the video of Impact Award winner Andrej Zorko, the magnets in quantum spin liquids differ from conventional magnets because the electronic spins do not stabilise and form a long-range magnetic order. This is the case even at zero temperature. In traditional magnetic materials, the spins arrange with long range ordered below a critical temperature.
QSL behaviour has been observed mainly in geometrically frustrated transition metal based oxide based spin systems because of the competing exchange interactions. These competing interactions prevent a long range magnetically ordered ground state being formed even at zero temperature. This means the frustrated spins can form the quantum entangled ground state crucial to a QSL, giving fractionalised excitations.
In strongly correlated metallic magnets (SCMM), known as heavy fermions, a phase transition takes place at T = 0 K. This phenomenon is known as a zero-temperature quantum phase transition. In this scenario, quantum fluctuations dominate over thermal fluctuations. This prevents a metallic spin-liquid state being formed in SCMMs.
In this study, published in Phys Rev B, the international research group, led by ISIS' Devashibhai Adroja, were able to identify the quantum critical spin liquid phase in a Ce-based SCMM. They were able to do this by using muon spin rotation on MuSR and neutron scattering on Merlin, Mari, Osiris and GEM, along with bulk measurements in the laboratory.
Their MuSR measurements at ISIS and J-PARC confirmed the presence of a non-magnetic ground state down to a few mK. They also saw that the temperature and field dependence of the relaxation rate exhibits the same type of behaviour as seen in the oxide based QSL. The absence of magnetic ordering was confirmed by using neutron diffraction on Osiris.
The group measured the low energy spin dynamics using the IN6 beamline at the ILL, and saw Energy by Temperature (E/T) scaling, with the scaling exponent dependant on spin and charge fluctuations. This was confirmed using the Merlin spectrometer at ISIS. Expanding their investigation to study a higher electron doped sample on GEM revealed a magnetic structure that indicates weakening of the magnetic frustration.
This discovery of a quantum critical spin liquid phase in an SCMM opens up a new way to investigate quantum critical phase transitions and will be important in theoretical development to understand the quantum critical spin liquid phase present near the quantum phase transition in SCMM.
The full paper can be found online at DOI: 10.1103/PhysRevB.106.064436