Solution Growth, Characterization and Applications of Monoisotropic Hexagonal Boron Nitride

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Date: Tuesday, December 10, 2019
Speaker: Dr. James Edgar
Description: The 2D electrically insulating material hexagonal boron nitride (hBN) is being developed for a broad range of applications including infrared chemical sensors, ultraviolet emitters, quantum communications, neutron detectors, membrane separation of isotopes, and as a substrate and dielectric for 2D electronics and optoelectronics.  These applications require hBN with a high degree of crystal perfection and controlled impurity concentrations. Such crystal are being grown a metal flux. Molten mixtures of nickel and chromium or iron and chromium were used to grow crystals over 1 cm2 in area and up to 200 µm thick. These crystals, produced in a simple, atmospheric pressure process, match the quality of the best hBN crystals reported in the literature.  They have narrow Raman peak widths (<8 cm-1) and room temperature phonon-assisted photoluminescence peaks above 5.7 eV. The properties of hBN are further enhanced by using isotopically pure boron sources, to produce monoisotopic hBN, made with either pure 10B or 11B isotopes. For example, the elimination of isotopic disorder in monoisotopic hBN increases its room temperature thermal conductivity by 40% and increase the phonon lifetimes by a factor of three compared to hBN with the natural (20% 10B and 80% 11B) distribution of boron isotopes. This seminar will describe both the process for growing hBN crystals and the characterization of their properties. 

Designer Electronic States in van der Waals Heterostructures

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Date: Tuesday, November 5, 2019
Speaker: Dr. Brian LeRoy
Description: The ability to create arbitrary stacking configurations of layered two-dimensional materials has opened the way to the creation of designer band structures. Twisted bilayer graphene and graphene on hexagonal boron nitride (hBN) are two of the simplest examples of such a van der Waals heterostructure where the electronic properties of the composite material can be fundamentally different from either individual material. These van der Waals heterostructures can be formed using a wide variety of layered materials including transition metal dichalcogenides, graphene and topological insulators. This talk will mostly focus on creating novel electronic states by controlling the twist angle and breaking inversion symmetry. The lattice mismatch and twist angle between layers in the heterostructure produces a moiré pattern which affects its electronic properties. For graphene on hBN, the moiré pattern creates a new set of superlattice Dirac points. In small twist angle bilayer graphene or transition metal dichalcogenides, the long-wavelength moiré pattern leads to the creation of flat bands and a wide range of correlated electronic states. In this talk, I will discuss our fabrication of these heterostructures and measurements using scanning probe microscopy.

Revealing fundamental parameters of 2D semiconductors via optical spectroscopy in (really) high magnetic fields

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Date: Friday, October 18, 2019
Speaker: Dr. Scott Crooker
Description: In semiconductor physics, many fundamental material properties relevant for optoelectronics (electron masses, dielectric parameters, etc) can be experimentally revealed via optical spectroscopy in sufficiently large magnetic fields. For the new family of monolayer transition-metal dichalcogenide (TMD) semiconductors such as MoS2 and WSe2, this magnetic field scale is substantial -- of order 100 tesla! - due to heavy carrier masses and huge exciton binding energies. Fortunately, modern pulsed magnets can achieve this scale. Using exfoliated monolayers affixed to single-mode optical fibers, we performed low-temperature magneto-absorption spectroscopy up to ~90T of all members of the monolayer TMD family. By following the diamagnetic shifts and valley Zeeman splittings of the exciton's 1s ground state and its excited 2s, 3s, ... ns Rydberg states, we determined exciton masses, radii, binding energies, dielectric properties, and free-particle bandgaps. These data provide essential ingredients for the rational design of optoelectronic van der Waals structures.

Seeing is Believing: New sSNOM Tool for Nanoscale Optical Characterization

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Date: Tuesday, September 17, 2019
Speaker: Dr. Slava Rotkin
Description: The ability to investigate properties of materials by non-destructive optical tools is fundamentally limited by diffraction (Abbe’s limit). Since nanomaterials, by definition, may have properties modulated at the scale of a few nanometers, the ultimate methods of characterization are required, with good enough spatial resolution, sensitive to optical/electronic properties. Until recently, those have been limited to electron microscopy based.
This talk will present scattering-type scanning near-field optical microscopy (sSNOM) as a technique to reveal optical properties of regular (and twisted) 2DMs. A range of methods will be covered that allow to reach beyond simple imaging: for example, in twisted graphene, phonon-plasmon polariton coupling will be discussed in detail.

Intercultural Competence: From Theory to Practice

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Date: Tuesday, April 16, 2019
Speaker: Dr. Shakoor Ward
Description: The session facilitates the development of awareness, knowledge, and skills in relating and interacting with peers belonging to other social or cultural groups. Broadly, the session is designed to foster the cultural agility and intercultural competence of participants so that “shared spaces” are welcomed as opportunities for growth and self-development.

Quantum Anomalous Hall Effect in Magnetic Topological Insulator Thin Films

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Date: Tuesday, March 26, 2019
Speaker: Dr. Cui-Zu Chang
Description: The quantum anomalous Hall (QAH) effect can be considered as the quantum Hall (QH) effect without an external magnetic field, which can be realized by time-reversal symmetry breaking in a topologically non-trivial system [1, 2]. A QAH system carries spin-polarized dissipationless chiral edge transport channels without the need for external energy input, hence may have a huge impact on future electronic and spintronic device applications for ultralow-power consumption. The many decades quest for the experimental realization of QAH phenomenon became a possibility in 2006 with the discovery of topological insulators (TIs). In 2013, the QAH effect was observed in thin films of Cr-doped TI for the first time [3]. Two years later in a near-ideal system, V-doped TI, contrary to the negative prediction from first principle calculations [2], a high-precision QAH quantization with more robust magnetization and a perfectly dissipationless chiral current flow was demonstrated [4]. In this talk, I will introduce the route to the experimental observation of the QAH effect in aforementioned two systems [3, 4], and talk about our recent progress on QAH sandwich heterostructures from the axion insulator physics and the topological Hall effect [5,6].

[1] F. D. M. Haldane, Phys. Rev. Lett. 61, 2015 (1988).
[2] R. Yu et al, Science 329, 61 (2010).
[3] Cui-Zu Chang et al, Science 340, 167(2013).
[4] Cui-Zu Chang et al, Nat. Mater. 14, 473(2015).
[5] Di Xiao et al, Phys. Rev. Lett. 120, 056801 (2018).
[6] Jue Jiang et al, (2019, submitted).

Intrinsic Antiferromagnetic Topological Insulator MnBi2Te4 - Towards Realization of High-Temperature Quantum Anomalous Hall Insulator

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Date: Tuesday, February 26, 2019
Speaker: Dr. Seng Huat (Sam) Lee
Description: Quantum anomalous Hall (QAH) insulators hold great promise for applications in energy efficient electronics and quantum computation, since it supports spin-polarized dissipation-less chiral edge states. To date, the fully developed QAH state has only been realized in magnetically doped topological insulator (TI) [i.e. Cr/V doped (Bi,Sb)2Te3] films at a ‘critical temperature’ below ~ 1K, which severely constrains both the exploration of fundamental physics and meaningful technological applications based on this exotic phenomenon. Theory has shown that an intrinsic magnetic topological insulator MnBi2Te4 is an ideal platform to realize high-temperature QAH insulator and this prediction has attracted a great deal of interest. In this talk, I will discuss our studies on the spin scattering and non-collinear spin structure induced the intrinsic anomalous Hall effect probed in magnetotransport measurements.

Extreme Straintronics of Graphene

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Date: Tuesday, January 29, 2019
Speaker: Mr. Riju Banerjea
Description: Whereas traditional materials break under strain measuring a few percents, 2D materials like graphene can sustain extreme strain over 20%. Using a scanning tunneling microscope we have investigated atomic-scale mechanical and electrical properties of suspended graphene under extreme (>10%) strain conditions. We find periodic ripples and electronic structures consistent with alternating nanodomains of intense (~200 T) pseudomagnetic and electric fields.