Molecular and Materials Science

Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning: Mott insulators are materials that should conduct electricity under conventional band theories but are actually insulators due to electron–electron interactions. They have applications in thin-film magnetic heterostructures and high-temperature superconductivity. Published in Nature Communications (2016), the work presents µSR results from the CMMS combined with X-ray, neutron and µSR data from other laboratories in a study of metal-insulator transitions of prototypical Mott insulators. The composition of RENiO₃ with various fractions of rare earths RE=La, Pr, Ni and Sm, provides a tuning parameter in the temperature-composition phase diagram, controlling a phase transition between an antiferromagnetic Mott insulating state and a paramagnetic metallic state, terminating in a quantum critical point at T=0. Being a sensitive probe of magnetism, µSR is the ideal tool to measure the magnetic field distribution, the magnetic volume fraction and excitations from the ordered magnetic ground state. This experiment produced evidence that the quantum phase transition (QPT) in RENiO3 is first order: the magnetically ordered volume fraction decreases continuously to zero at the QPT, while the ordered magnetic moment retains essentially its full value until it abruptly vanishes at the QPT. These results unambiguously demonstrate that the QPT in these materials proceeds in a distinctly first-order fashion. Studies of antiferromagnetic Mott insulators, as well as emergent quantum phenomena in other kinds of materials, are useful to elucidate both the system-specific and more universal roles of first-order behaviour in quantum phase evolution.    

bNMR Investigation of the Depth-Dependent Magnetic Properties of an Antiferromagnetic Surface Hard drives use disks made of magnetic material to store information, and an electromagnet in the read/write head writes information to the disk by magnetizing small sections of the disk. Increasing the information on a hard drive requires shrinking the size of the magnetic sections and this means the near-surface regions are increasingly important. The prototypical antiferromagnet α-Fe₂O₃ has a first-order transition known as the Morin transition at 260 K, where the orientation of antiferromagnetic order with respect to the crystal lattice undergoes an abrupt change. In this work the static spin orientation and dynamic spin correlations within nanometers from the surface of a single crystal was studied via the nuclear spin polarization of implanted ⁸Li ions and detected via bNMR spectroscopy. As reported in Physical Review Letters (2016), the experiment found that the Morin transition temperature was independent of depth from 1 to 100 nm from the free (110) surface but the fluctuations of the electronic spins are faster near the crystal surface and decay into the bulk over a characteristic length of 11 nm.  The results suggest the magnetic order parameter undergoes a continuous gradient rather than a phase separation of bulk vs. surface magnetism. Whereas previous studies made use of nanoparticles to achieve sufficient near-surface volume fraction to extract a signal, bNMR spectroscopy allowed a depth-resolved characterisation of the magnetic order parameter into a macroscopic single crystal of α-Fe₂O₃, differentiating free-surface and finite-size effects on magnetic order.

A new, tunable quantum spin liquid offers next-generation technology promise: Research at CMMS has demonstrated that it's possible to tweak a magnetic materials' overall electron structure in a new way to create novel states that could have numerous technological applications, including for improved magnetic information storage. As reported in Physical Review Letters (2018), using TRIUMF's µSR probe it was discovered that a material's magnetic ground state can be tuned by changing its charge order. A quantum spin liquid (QSL) is a strange ground state of a magnetic material with long-range entanglement and emergent excitations. The usual ingredients required to make a QSL are small spins and competing interactions that create geometric frustration. CMMS added an additional ingredient to generate QSL physics: charge order. The materials Li₂In_1-xSc_xMo₃O₈ have an asymmetric lattice and the electrons are dilute, with only a third of the sites occupied. As the ratio of scandium (Sc) to indium (In) atoms in the material is altered, the lattice becomes more symmetric and the electrons form a particular “plaquette” charge order which generates more frustration and induces a new, tunable QSL ground state.  

bNMR reveals nanoscale surface details in topological insulators: Wolfgang Pauli said, “God made the bulk; surfaces were invented by the devil”, a recognition of the fact that theories and experimental measurements of near-surface properties are very difficult. Surfaces may be difficult to study, but it is where much interesting physics arise. Topological insulators (TI) are materials where the bulk is an insulator but whose surface contains conducting states, which means that electrons can only move along the surface of the material. Topologically protected states could act as a source of spin-polarized electrons with properties relevant to spintronics applications including quantum computing. As published in the Proceedings of the National Academy of Sciences (2015) researchers at CMMS used bNMR spectroscopy as a nano-scale depth-resolved probe of magnetism and conductivity within about 10 nm of the free surface of (Bi,Sb)2Te3. This depth-dependent study of electronic and magnetic properties of TI epitaxial layers using implanted, spbin-polarized ⁸Li⁺ ions reveals differences in the band structure between the near-surface and deeper into the bulk material.