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CMP
Credit: Jack Hobhouse

Dr Dharmalingam Prabhakaran

Researcher

Research theme

  • Quantum materials

Sub department

  • Condensed Matter Physics

Research groups

  • Synthesis and crystal growth
dharmalingam.prabhakaran@physics.ox.ac.uk
Telephone: 01865 (2)72270,01865 (2)72351,01865 (2)72341
Clarendon Laboratory, room 177,377,373
  • About
  • Publications

Topological triplon modes and bound states in a Shastry–Sutherland magnet

Nature Physics Springer Nature 13:8 (2017) 736-741

Authors:

PA McClarty, F Krüger, T Guidi, SF Parker, K Refson, AW Parker, D Prabhakaran, Radu Coldea

Abstract:

The twin discoveries of the quantum Hall effect1, in the 1980s, and of topological band insulators2, in the 2000s, were landmarks in physics that enriched our view of the electronic properties of solids. In a nutshell, these discoveries have tau ght us that quantum mechanical wavefunctions in crystalline solids may carry nontrivial topological invariants which have ramifications for the observable physics. One of the side effects of the recent topological insulator revolution has been that such physics is much more widespread than was appreciated ten years ago. For example, while topological insulators were originally studied in the context of electron wavefunctions, recent work has initiated a hunt for topological insulators in bosonic systems: in photonic crystals3-6, in the vibrational modes of crystals7, and in the excitations of ordered magnets8. Using inelastic neutron scattering along with theoretical calculations, we demonstrate that, in a weak magnetic field, the dimerized quantum magnet SrCu 2 (BO 3 ) 2 is a bosonic topological insulator with topologically protected chiral edge modes of triplon excitations.
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Doping dependence of collective spin and orbital excitations in the Spin-1 quantum antiferromagnet La₂₋ₓSrₓNiO₄ oObserved by X rays.

Physical Review Letters American Physical Society 118:15 (2017) 156402

Authors:

Gilberto Fabbris, Derek Meyers, Lei Xu, Vamshi M Katukuri, Liviu Hozoi, Xuerong Liu, Z-Y Chen, Jun Okamoto, Thorsten Schmitt, Anne-Christine Uldry, Bernhard Delley, Gen D Gu, Dharmalingam Prabhakaran, Andrew Boothroyd, Jeroen van den Brink, DJ Huang, Mark PM Dean

Abstract:

We report the first empirical demonstration that resonant inelastic x-ray scattering (RIXS) is sensitive to collective magnetic excitations in S=1 systems by probing the Ni L₃ edge of La₂₋ₓSrₓNiO₄ (x=0, 0.33, 0.45). The magnetic excitation peak is asymmetric, indicating the presence of single and multi-spin-flip excitations. As the hole doping level is increased, the zone boundary magnon energy is suppressed at a much larger rate than that in hole doped cuprates. Based on the analysis of the orbital and charge excitations observed by RIXS, we argue that this difference is related to the orbital character of the doped holes in these two families. This work establishes RIXS as a probe of fundamental magnetic interactions in nickelates opening the way towards studies of heterostructures and ultrafast pump-probe experiments.
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Andreev bound states in superconductor/ferromagnet point contact Andreev reflection spectra

Physical Review B American Physical Society 95:9 (2017) 094516

Authors:

KA Yates, LAB Olde Olthof, ME Vickers, Dharmalingam Prabhakaran, M Egilmez, JWA Robinson, LF Cohen

Abstract:

As charge carriers traverse a single superconductor ferromagnet interface, they experience an additional spin-dependent phase angle that results in spin mixing and the formation of a bound state called the Andreev bound state. Here we explore whether point contact Andreev reflection can be used to detect the Andreev bound state and, within the limits of our experiment, we extract the resulting spin mixing angle. By examining spectra taken from La1.15Sr1.85Mn2O7-Pb junctions, together with a compilation of literature data on highly spin polarized systems, we suggest that the existence of the Andreev bound state would resolve a number of long standing controversies in the literature of Andreev reflection, as well as defining a route to quantify the strength of spin mixing at superconductor-ferromagnet interfaces. Intriguingly, we find that for high transparency junctions, the spin mixing angle appears to take a relatively narrow range of values across all the samples studied. The ferromagnets we have cho sen to study share a common property in terms of their spin arrangement, and our observations may point to the importance of this property in determining the spin mixing angle under these circumstances.
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Magnetic excitations of the charge stripe electrons below half doping in La2-xSrxNiO4 (x = 0.45, 0.4)

Physical Review B American Physical Society 95:6 (2017) 064403

Authors:

PG Freeman, K Hradil, RA Mole, P Cermak, Dharmalingam Prabhakaran

Abstract:

The low energy magnetic excitation spectrum of charge stripe ordered La2-xSrxNiO4, x=0.4 and x=0.45 samples, were studied by neutron scattering. Two excitation modes are observed in both materials, one from the ordered magnetic moments, and a second mode consistent with pseudo-one-dimensional antiferromagnetic excitations of the charge stripe electrons (q-1D). The dispersion of the q-1D excitation follows the same relation as in x=1/3 composition, with even spectral weight in the two counterpropagating branches of the x=0.4 sample, however in the x=0.45 sample only one dispersion branch has any measurable spectral weight. The evolution of the q-1D excitations on doping to the checkerboard charge ordered phase is discussed.
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Effect of doping and defects in pyrochlore compounds

ACTA CRYSTALLOGRAPHICA A-FOUNDATION AND ADVANCES 73 (2017) C1020-C1020

Authors:

Dharmalingam Prabhakaran, Sichen Wang, Richard Brearton, Andrew Boothroyd
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