Dr Dharmalingam Prabhakaran

Abstract:

Nodal semimetals are a unique platform to explore topological signatures of the unusual band structure that can manifest by accumulating a nontrivial phase in quantum oscillations. Here we report a study of the de Haas–van Alphen oscillations of the candidate topological nodal line semimetal CaAgAs using torque measurements in magnetic fields up to 45 T. Our results are compared with calculations for a toroidal Fermi surface originating from the nodal ring. We find evidence of a nontrivial π phase shift only in one of the oscillatory frequencies. We interpret this as a Berry phase arising from the semiclassical electronic Landau orbit which links with the nodal ring when the magnetic field lies in the mirror (ab) plane. Furthermore, additional Berry phase accumulates while rotating the magnetic field for the second orbit in the same orientation which does not link with the nodal ring. These effects are expected in CaAgAs due to the lack of inversion symmetry. Our study experimentally demonstrates that CaAgAs is an ideal platform for exploring the physics of nodal line semimetals and our approach can be extended to other materials in which trivial and nontrivial oscillations are present.

Abstract:

We report neutron diffraction measurements of the magnetic structures in two pyrochlore iridates, Yb2Ir2O7 and Lu2Ir2O7. Both samples exhibit the all-in-All-out magnetic structure on the Ir4+ sites below TN≃150 K, with a low temperature moment of around 0.45μB/Ir. Below 2 K, the Yb moments in Yb2Ir2O7 begin to order ferromagnetically. However, even at 40 mK the ordered moment is only 0.57(2) μB/Yb, well below the saturated moment of the ground state doublet of Yb3+ (1.9μB/Yb), deduced from magnetization measurements and from a refined model of the crystal field environment, and also significantly smaller than the ordered moment of Yb in Yb2Ti2O7 (0.9μB/Yb). A mean-field analysis shows that the reduced moment on Yb is a consequence of enhanced phase competition caused by coupling to the all-in-All-out magnetic order on the Ir sublattice.

Abstract:

Quantum materials displaying intriguing magnetic and electronic properties could be key to the development of future technologies. However, it is poorly understood how the macroscopic behavior emerges in complex materials with strong electronic correlations. While measurements of the dynamics of excited electronic populations have been able to give some insight, they have largely neglected the intricate dynamics of quantum coherence. Here, we apply multidimensional coherent spectroscopy to a prototypical cuprate and report unprecedented coherent dynamics persisting for ~500 fs, originating directly from the quantum superposition of optically excited states separated by 20 to 60 meV. These results reveal that the states in this energy range are correlated with the optically excited states at ~1.5 eV and point to nontrivial interactions between quantum many-body states on the different energy scales. In revealing these dynamics and correlations, we demonstrate that multidimensional coherent spectroscopy can interrogate complex quantum materials in unprecedented ways.

Abstract:

We report an experimental study of the magnetic order and electronic structure and transport of the layered pnictide EuMnSb2, performed using neutron diffraction, angle-resolved photoemission spectroscopy (ARPES), and magnetotransport measurements. We find that the Eu and Mn sublattices display antiferromagnetic (AFM) order below T EuN = 21(1) K and T MnN = 350(2) K, respectively. The former can be described by an A-type AFM structure with the Eu spins aligned along the c axis (an in-plane direction), whereas the latter has a C-type AFM structure with Mn moments along the a -axis (perpendicular to the layers). The ARPES spectra reveal Dirac-like linearly dispersing bands near the Fermi energy. Furthermore, our magnetotransport measurements show strongly anisotropic magnetoresistance and indicate that the Eu sublattice is intimately coupled to conduction electron states near the Dirac point.