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With a rapidly growing family of vdW materials, the role of dielectric and metals have become more important than ever. In this talk, I will present challenges associated with the synthesis of atomically-precise three-dimensional (3D) perovskite nanomembranes followed by our group’s effort to address them. Using hybrid molecular beam epitaxy that employs a metal-organic precursor, titanium isopropoxide (TTIP), to supply both Ti and oxygen (without the need for additional oxygen), epitaxial SrTiO3 (STO) films were grown directly on a graphene layer transferred on to bulk STO substrate. Films were then successfully exfoliated and transferred onto other substrates. Using Raman spectroscopy and high-resolution X-ray diffraction, we show that the transferred STO membrane is single-crystalline and can be integrated with other vdW materials. I will also present sacrificial layer route to create oxide membranes resulting in room temperature dielectric constant of ~ 300. Finally, I will present...

In recent years, the experimental realization of magnetic long-range order in atomically thin 2D materials has shown a big potential in spintronic applications in ultrathin magnets due to the possibility of manipulation of magnetism by external fields, strain or proximity effects in van der Waals heterostructures. Specifically, the family of metallic magnets FenGeTe2 (n=3, 4, 5) has attracted a huge attention due to their high Curie temperatures and intriguing properties. In this talk, I will review the status of this research field, highlighting our own research by ab initio density functional theory, calculations of interatomic exchange interaction parameters and Monte Carlo simulations. A particular emphasis will be given on the systematic study of the electronic structure and magnetism of FenGeTe2 magnets along with some critical discussions on the importance of electron correlation with the aid of dynamical mean field theory, spin-orbit coupling and effects of transition metal dop...

Double perovskite oxides (DPOs) with two transition metal ions (A2BB′O6) offer a fascinating platform for exploring exotic physics and practical applications. Studying these DPOs as ultrathin epitaxial thin films on single crystalline substrates can add another dimension to engineering electronic, magnetic, and topological phenomena. Understanding the consequence of polarity mismatch between the substrate and the DPO would be the first step towards this broad goal. We investigate this by studying the interface between a prototypical insulating DPO Nd2NiMnO6 and a wide-band gap insulator SrTiO3. The interface is found to be insulating in nature. By combining several experimental techniques and density functional theory, we establish a site- selective charge compensation process that occurs explicitly at the Mn site of the film, leaving the Ni sites inert. We further demonstrate that such surprising selectivity, which cannot be explained by existing mechanisms of polarity compensation, i...

Observation of magnetic ordering in 2D layered materials at finite temperatures have drawn significant interest in recent years. Though Mermin-Wagner-Hohenburg theorem forbids long range ordering in 2D systems, anisotropy can lead to spin ordering at finite temperatures. One of such classes of 2D magnets explored recently is the transition metal phosphorus trisulphides (MPS3, M = Mn, Fe, and Ni) that hosts antiferromagnetic (AFM) ground state at low temperatures [1]. The AFM ground state exhibits different spin dimensionalities, (viz., n=1,2, and 3) due to the presence of an axial or planar anisotropy or in the absence of any anisotropic element [2] which may be described by the Ising (e.g., FePS3), XY (e.g.,
NiPS3), and Heisenberg (e.g., MnPS3) Hamiltonians, respectively [1]. Engineered heterostructures of magnetic layered materials with high spin-orbit coupled systems like topological materials has the potential to control the quantum interactions unravelling a variety of exotic phe...

Van der Waals (vdW) epitaxy is the growth mechanism of epitaxial layers on crystalline substrates, where the weak vdW forces govern epilayer-substrate interaction. This technique leads to heterointerfaces with negligible strain, despite large lattice mismatch and thermal expansion coefficients. With advances in the large area synthesis of 2D materials and heterostructures, it is important to comprehend the growth mechanism, prospects and challenges of vdW epitaxy to grow 2D materials on various crystalline substrates. In this talk, I will discuss the effects of various growth factors such as structure and symmetries of the substrate, growth temperature, interfacial strain and growth kinetics that ultimately controls the lateral coverage and thickness of the 2D/quasi-2D layers. Two specific cases of vdW epitaxial growth by chemical vapor deposition (CVD) will be discussed in detail: (1) quasi-2D system of Cr1+xTe2 and (2) monolayer WSe2, on crystalline substrates such as c-plane sapphir...

The fast-paced improvements in ultrafast radiation from X-ray, ultraviolet, visible, and infrared to terahertz frequencies is enabling simultaneous probing of electron, phonon, and spin dynamics on the ps-to-ns time scales, as well as sub-micrometer length scales. In this talk, I will present brief introduction to nonlinear optics, followed by examples of the discovery of new low symmetry phases with large property enhancements in decades old ferroelectrics using nonlinear optical microscopy. In a second example, I will show how an ultrafast laser pulse can create a complex polar supertextures with modulation periodicities of tens of nanometers in oxide thin film heterostructures that have built-in frustration by design.

Hydrodynamic flow of electrons in graphene has garnered significant attention over the past decade, emerging as a solid-state platform which can be used to probe the physics associated with relativistic plasma, black holes, and quantum gravity [1,2]. Particularly near the charge neutrality point, graphene is expected to behave like a “Dirac fluid” [3], with its shear viscosity per unit entropy density (η/s) reaching a universal holographic lower bound, ħ/4πk_B where ħ is the reduced Planck’s constant and kB is the Boltzmann’s constant. However, direct experimental evidence of this is still lacking. In this work [4], we have fabricated hBN-encapsulated ultraclean graphene devices with exceptionally high electron mobilities (~ 10^6 cm^2 V^-1 s^-1) and performed electrical and thermal transport from room temperature down to 20 K. We observed a giant violation of the Wiedemann-Franz Law near the charge neutrality point across a range of temperatures T >> T_F. We also computed t...

Recent experiments have revealed the possibility of achieving sizeable Coulomb interaction driven gapped phases in dual gated rhombohedral multilayer graphene devices where carrier densities and perpendicular electric fields can be simultaneously controlled. Of particular importance are the pentalayer and tetralayer devices whose spontaneous gaps of the order of a few tens of meV are attributable to layer pseudospin polarization of the states near the Dirac point in chiral 2DEG systems. The layer pseudospin polarization can take place in a variety of ways leading to different Hall conductivities depending on the signs of the mass terms for each one of the spin-valley flavors. By means of mean-field Hartree-Fock approach we examine and compare the self-consistent solutions corresponding to the different pseudospin magnetic phases. Addition of a spin-orbit coupling term or moire potentials are expected to sensitively alter the ground-state of the system, allowing for example to split the...