Spin frustration and orbital physics in vanadates

Lead Research Organisation: Royal Holloway University of London
Department Name: Physics


The study of spin frustration and orbital physics is one of the most exciting frontiers of contemporary research in condensed matter physics. The discovery of experimental realisations of model spin-orbital systems in vanadate compounds, coupled with recent developments of time-of-flight and polarised neutron, and resonant x-ray scattering techniques at central facilities, gives unprecedented opportunities to make major contributions in this field. However, in order to take maximum advantage the provision of single-crystal samples is mandatory. The growth of single-crystal vanadates is in its infancy, but we propose to employ a PDRA who has already produced some of the largest single crystals of these compounds using the floating-zone technique. We also propose to collaborate with a leading European materials discovery laboratory employing the complementary flux growth technique, and with condensed matter theorists working in this area. First, we have discovered frustrated ferromagnetism in square-lattices with significant cross-bond exchange, in Pb2VO(PO4)2 and related materials. By chemical substitution we will be able to realise a quantum disordered phase. Entirely new excitations are predicted in the spin liquid phase, and we propose to image this behaviour very directly in inelastic neutron scattering experiments. Secondly, the perovskite orthovanadates RVO3 exhibit novel magnetic phenomena including magnetisation reversal for LaVO3, a magnetic memory effect for GdVO3, and a staircase-like magnetisation for PrVO3. Our measurements will determine the orbital ordering and orbital excitations, and will tell us about the relative importance of the quantum fluctuations of quasi-one-dimensional orbital character, versus Jahn-Teller distortions. Finally, the AV2O4 spinels have a highly frustrated pyrochlore geometry for the V ions, and for divalent A = Zn, Mg, Cd and Mn, the orbital degree of freedom comes into play. We shall compare the orbital ground states determined using resonant x-ray scattering with theoretical predictions, and measure the complex exchange interactions using inelastic neutron scattering from large single crystals.One of the primary aims of the project is to train PhD students not only in the complementary use of neutrons and x-rays in an exciting and important area of science, but also with the synthesis know-how to produce materials allowing the fullest use to be made of world-leading neutron and synchrotron facilities.


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Description We identified the nature of the spin-orbital ground state and magnetic excitations in lutetium vanadate. In particular, we distinguished between models based on orbital-Peierls dimerization, taken as a signature of quantum effects in orbitals, and Jahn-Teller distortions, in favor of the latter. In order to solve this long-standing puzzle, polarized neutron beams were employed as a prerequisite in order to solve details of the magnetic structure, which allowed quantitative intensity analysis of extended magnetic-excitation data sets. The results of this detailed study enabled us to draw definite conclusions about the classical versus quantum behaviour of orbitals in this system and to discard the previous claims about quantum effects dominating the orbital physics of lutetium vanadate and similar systems.

The so-called J1-J2 model on a square lattice exhibits a rich variety of different forms of magnetic order that depend sensitively on the ratio of exchange constants J2/J1. We used bulk magnetometry and polarized neutron scattering to determine J1 and J2 unambiguously for two materials in a new family of vanadium phosphates, and we found that they have ferromagnetic J1. The ordered moment in the collinear antiferromagnetic ground state is reduced, and the diffuse magnetic scattering is enhanced, as the predicted bond-nematic region of the phase diagram is approached.
Exploitation Route Our work has solved a very important problem in orbital physics, and showed that quantum effects are much less plausible for orbitals than for spins. This is a key question in the study of strongly correlated electron systems, and is of profound interest to a much wider community of physicists, chemists and materials scientists.

The J1-J2 model on a square lattice is one of the most studied models in frustrated magnetism, and there is a vast literature of theoretical predictions. We have found a rare clear-cut experimental realization of this system. Furthermore, it is in an exotic region of the phase diagram. It will, therefore, be of huge interest to researchers working on strongly correlated physics.
Sectors Education,Electronics

Description Understanding of strongly correlated electron systems is one of the global challenges of Physics. Theoretical predictions can be tested more directly in magnets than any other system. Interest in the results extends way beyond the discipline of condensed matter physics, since the results map on to other areas of science, such as elementary particle physics. Ultimately, understanding how degrees of freedom, such as spins and orbitals, interact will enable the design of a new generation of quantum electronic devices.
First Year Of Impact 2014
Sector Education,Electronics
Impact Types Cultural