The spin-1/2 triangular-lattice antiferromagnet is a central model in frustrated quantum magnetism: it is the first two-dimensional magnet proposed to host a SU(2) symmetric resonating valence-bond ground-state and its fractionalized magnetic excitations. Although it is now accepted that the model, at least in its simplest Heisenberg form, orders magnetically, it remains intimately associated with the concepts of quantum spin-liquid and exotic magnetic excitations. In the last few years, advances in materials discovery, crystal growth, neutron spectroscopy and theory have fueled a lively triangular-lattice antiferromagnet “renaissance”. In this talk, I will describe recent neutron scattering investigations on two realizations of this model: the transition metal compound Ba3CoSb2O9 and the rare-earth system YbMgGaO4. Experimental results elucidate the role of quantum fluctuations, spin-orbit coupling, chemical disorder, and non-linear effects in generating anomalous spin dynamics in these materials. This project is supported by the NSF under grant DMR-1750186.

Host: Ribhu Kaul

One of the promising routes towards creating novel topological states and excitations is to combine superconductivity with quantum Hall (QH) effect. However, signatures of superconductivity in the QH regime remain scarce, and a superconducting current through a QH weak link has so far eluded experimental observation. Here, we explore high mobility graphene/boron nitride heterostructures contacted by type II superconducting electrodes that could withstand  magnetic fields of a few Tesla. At low magnetic fields, our devices demonstrate the Fraunhoffer pattern and Fabri-Perot oscillations, confirming their uniformity and ballisticity. At fields of 1-2 Tesla, when Landau quantization is fully developed, regions of superconductivity can be observed on top of the conventional QH fan diagram. The measured supercurrent is very small, on a few nA scale, and periodic in magnetic field. I discuss possible mechanisms that could mediate supercurrent along the QH edge states.

Recent discovery of topological electronic states has shed light on the role of strong spin-orbit coupling in condensed mater. When the coupling is combined with electron correlation effect, their combination leads to a new class of topological states accompanying exotic magnetism. The 5d transition metal iridates have received considerable attention as the candidates that harness Kitaev quantum spin liquid, Wyle semimetal or Axion insulator phases. For the first part of this talk, I discuss resonant inelastic x-ray scattering (RIXS) investigation of a honeycomb Na2IrO3. The observation of diffuse magnetic scattering points to presence of short-range magnetic orders resulting from competition of bond-directional magnetic anisotropies. This validates the novel route to realize Kitaev spin liquid and evidences the proximity to the spin liquid phase. In the second part, I present RIXS study of magnetic excitation in a pyrochlore Eu2Ir2O7. Its metal-insulator transition driven by all-in-all-out (AIAO) magnetic order is regarded as possible realization of topological Weyl semimetal. We observe gradual softening of the magnon excitations in the entire Brillouin zone while warming, whose temperature evolution shows a direct relationship with the metal-insulator transition. This result suggests substantial change of magnetic exchange due to the varying electronic structure, and thereby classifies intermediate electron correlation strength: a requisite for realizing Weyl semimetal state.

The existence of strong spin-orbit coupling has brought the iridates to the forefront of materials research, whereas strong electronic correlation has proven to produce a plethora of novel properties within the manganites. The physical properties of interfaces between such materials were investigated by synthesizing a series of artificial superlattices consisting of the 5d paramagnetic metal SrIrO₃ and 3d antiferromagnetic insulators AMnO₃, where A = Sr or La. Through our experimental investigations by x-ray diffraction, SQUID magnetometry, dc-transport, x-ray circular dichroism, and polarized neutron reflectometry measurements, both novel magnetic and transport properties were observed, which drastically differ from those of the constituent materials and are highly sensitive to the degree of dimensional confinement within the superlattices. Here I will present these results and discuss the implications of these intriguing magnetic and electronic properties.

The energy crisis and critical need of advanced materials for engineering, pharmaceutical, and medical applications are calling for the development of new materials to response quickly and efficiently to such demand. For such imperative request, the fundamental understanding of materials, the origin of the properties in correlation with structure could be the main pathway toward the successful design and discovery. The principle of homologous series has been successful in creating new materials with control of specific module in the structure, to improve the electronic, magnetic and optical properties [1, 2]. For example, we can create and manipulate cooperatively in within the same crystal lattice of complex transition metal chalcogenide, ferromagnetism and semiconductivity, two properties difficult to combine in a conventional inorganic compound. However, ferromagnetic semiconductors are very attractive and might result in new physical phenomena and novel applications such as spintronic that has the particularities to use both the charge and the spin of electron to process and store information [3, 4]. In this talk, we will discuss my recent work using that principle of homologous series to develop a model of high Curie temperature ferromagnetic semiconductors using the complex metal chalcogenides. We will mainly focus on my recent discovery of both FeSb2Se4 [5] a p-type and FeBi2Se4 [6] n-type ferromagnetic semiconducting materials with a Curie temperature of 450K [5, 6] that can be tuned through doping with Sn or In.

_{Reference: [1] R. J. Cava, J.Am.Ceram.Soc. 83 [1] 5-28(2000) [2] A. Mrotzek and M. G. Kanatzidis, Accoun. Chem. Re. 36(2) 111-119 (2003) [3] H.Ohno et al. Appl. Phys. Lett. 69, 363-365 (1996) [4] M. N. Leuenberger et al. Nature, 410, 789-793 (2001) [5] H. Djieutedjeu et al. Angew. Chem. Int. Ed. 49, 9977-9981 (2010) [6] K. G. S. Ranmohotti et al. J. Am. Chem. Soc. 137, 691-698 [7] H. Djieutedjeu et al. Manuscript in progress.}

Small-molecule organic semiconductors form the basis for the emerging field of organic optoelectronics. In order to better understand the intrinsic photo-physical and transport phenomena in this important class of materials, it is necessary to study samples of very high structural order and chemical purity. Such materials exist in the form of molecular single crystals that can be used for fabrication of high-performance prototype devices, such as field-effect transistors, photo-conductors and photo-voltaic cells, in which intrinsic properties of organic semiconductors can be investigated without parasitic effects of disorder (see, e.g., [1,2,3]). This talk will overview some of the main achievements in the area of organic single-crystal devices, present resent progress and discuss novel methods of surface functionalization that result in an extremely low-noise charge transport regime at the surface of molecular crystals, leading to an observation of unprecedentedly clean and quiet (low-noise) Hall effect [4].  In addition, very interesting non-linear effects in photoconductivity originated from long-range exciton diffusion and multi-particle interactions will be discussed [5].     

References:

1. V. Podzorov, MRS Bulletin themed Issue: “Organic Single Crystals: Addressing fundamentals

   of organic electronics” introductory paper, MRS Bulletin 38, 15-24 (Jan. 2013).

2. V. Podzorov et al., "Hall effect in the accumulation layers on the surface of organic

    semiconductors", Phys. Rev. Lett. 95, 226601 (2005).

3. H. Najafov, B. Lee, Q. Zhou, L. C. Feldman, V. Podzorov, "Observation of long-range exciton

    diffusion in highly ordered organic semiconductors", Nature Mater. 9, 938 (2010).

4.  B. Lee, Y. Chen, D. Fu, H. T. Yi, K. Czelen, H. Najafov, V. Podzorov, “Trap healing and

     ultra low-noise Hall effect at the surface of organic semiconductors”, Nature Mater. 12, 1125

     (2013).

5. P. Irkhin, H. Najafov, V. Podzorov, submitted (2014).