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Condensed Matter Seminar

Our condensed matter seminars are held on Tuesdays at 3:30pm in Chemistry-Physics Building, Room 179, unless otherwise noted below. A number of the department colloquium may also be of interest.

Condensed Matter Seminar: Artificial Solids by Design: Harnessing Electronic and Magnetic Properties in Complex Inorganic Solid/Hybrid Materials

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.

Date:
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CP179

Condensed Matter Seminar: Organic Single-Crystal Devices: Addressing Intrinsic Properties of Molecular Semiconductors

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).

 

Date:
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Condensed Matter Seminar: Constraints on topological order in Mott insulators

The hunt for anyonic excitations in quantum magnets is frustrated by the absence of any order parameter that could be used to detect such phases. Consequently a very important ally is the Hastings-Oshikawa-Lieb-Schultz-Mattis theorem for 2D quantum magnets, which guarantees that a fully symmetric gapped Mott insulator must be topologically ordered, though is silent on which topological order is permitted. After introducing the HOLSM theorem,  I will explain a new line of argument that constrains which topological order is permitted in a symmetric gapped Mott insulator. For example, I'll show that a fully symmetric magnet with S = 1/2 per unit cell cannot be in the double-semion topological phase. An application of our result is to the Kagome lattice quantum antiferromagnet where recent numerical calculations of entanglement entropy indicate a ground state compatible with either toric code or double semion topological order. Our result rules out the latter possibility.
 
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Condensed Matter Seminar: Generating a Molecular Level Understanding of Organic Photovoltaics

 
Organic photovoltaics (OPVs) utilize strongly absorbing blends of pi-conjugated organic molecules to convert light into electrical power.  Efficient OPVs consist of two different molecules or polymers- one of which functions as an electron donating compound while the other acts as an electron accepting compound.  Charge transfer and separation take place at a molecular interface between these electron donating and accepting compounds, thus the nature of this interface plays a critical role in the photovoltaic performance of the device.  Using multiple analytical tools, including measurements of the charge-transfer state energy, combined with specifically designed polymers, we explore how molecular details at this interface impact the performance of OPVs.  In addition to the completed research presented here, I’ll also highlight the photoelectron spectroscopy, inverse photoelectron spectroscopy, device characterization, and research directions that my group will be working on here at UK.
Date:
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Condensed Matter Seminar: Quantum rotor in nanostructured superconductors

Despite its apparent simplicity, the idealized model of a particle constrained to move on a circle has intriguing dynamic properties and immediate experimental relevance. While a rotor is rather easy to set up classically, the quantum regime is harder to realize and investigate. Here we demonstrate that the quantum dynamics of quasiparticles in certain classes of nanostructured superconductors can be mapped onto a quantum rotor. Furthermore, we provide a straightforward experimental procedure to convert this nanoscale superconducting rotor into a regular or inverted quantum pendulum with tunable gravitational field, inertia, and drive. We detail how these novel states can be detected via scanning tunneling spectroscopy. The proposed experiments will provide insights into quantum dynamics and quantum chaology.



Reference: S.H. Lin, M. Milosevic, L. Covaci, F. Peeters, B. Janko, Nature Sci. Rep. 4, 4542 (2014).

Date:
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Condensed Matter Seminar: Mott Transitions of Correlated Fermions from SU(2) to SU(N)"

The encoding of SU(N) symmetric spin degrees of freedom in ultra-cold atom systems has recently brought the realization of antiferromagnetic order in optical lattices into close reach. We explore the quantum phases and phase transitions emerging in the most fundamental interacting models on the lattice, the SU(N)-symmetric Hubbard and Heisenberg model, by means of projective quantum Monte Carlo simulations from SU(2) to the large-N limit. We discuss the instabilities upon the introduction of local Coulomb repulsion and explicit SU(N)-symmetric Heisenberg-like spin exchange to different Fermi-surfaces. The extension to higher symmetries allows us to study the melting of phases as a function of correlations as well as symmetry.

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Spin-Orbit Tuned Ground States in Single-Crystal Iridates

The iridates have become fertile ground for studies of new physics driven by spin-orbit coupling (SOC) that is comparable to the on-site Coulomb and other relevant  interactions.  This unique circumstance creates a delicate balance between interactions that drives complex magnetic and dielectric behaviors and exotic states seldom or never seen in other materials. A profound manifestation of this competition is the novel Jeff = 1/2 Mott state that was observed in the layered iridates with tetravalent Ir4+(5d5) ions. On the other hand, very little attention has been drawn to iridates having pentavalent Ir5+(5d4) ions, primarily because the strong SOC limit is expected to impose a nonmagnetic singlet ground state (Jeff  = 0). In this talk, we review the underlying physical properties of the iridates including perovskites, honeycomb lattices and double perovskites with pentavalent Ir5+ ions, and report results of our recent studies that emphasize spin-orbit-tuned ground states stabilized by chemical doping, application of pressure and magnetic field. In addition, we address the urgent question that the Jeff states may not survive in the presence of strong non-cubic crystal fields and/or exchange interactions.

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