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Electronic and magnetic properties of single-layerMPX3metal phosphorous trichalcogenides
Chittari, Bheema Lingam,Park, Youngju,Lee, Dongkyu,Han, Moonsup,MacDonald, Allan H.,Hwang, Euyheon,Jung, Jeil American Physical Society 2016 Physical Review B Vol.94 No.18
<P>We survey the electronic structure and magnetic properties of two-dimensional (2D) MPX3 (M = V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and X = S, Se, Te) transition-metal chalcogenophosphates to shed light on their potential role as single-layer van der Waals materials that possess magnetic order. Our ab initio calculations predict that most of these single-layer materials are antiferromagnetic semiconductors. The band gaps of the antiferromagnetic states decrease as the atomic number of the chalcogen atom increases (from S to Se to Te), leading in some cases to half-metallic ferromagnetic states or to nonmagnetic metallic states. We find that the competition between antiferromagnetic and ferromagnetic states can be substantially influenced by gating and by strain engineering. The sensitive interdependence we find between magnetic, structural, and electronic properties establishes the potential of this 2D materials class for applications in spintronics.</P>
Accurate Gap Determination in Monolayer and Bilayer Graphene/<i>h</i>-BN Moiré Superlattices
Kim, Hakseong,Leconte, Nicolas,Chittari, Bheema L.,Watanabe, Kenji,Taniguchi, Takashi,MacDonald, Allan H.,Jung, Jeil,Jung, Suyong American Chemical Society 2018 Nano letters Vol.18 No.12
<P>High mobility single and few-layer graphene sheets are in many ways attractive as nanoelectronic circuit hosts but lack energy gaps, which are essential to the operation of field-effect transistors. One of the methods used to create gaps in the spectrum of graphene systems is to form long period moiré patterns by aligning the graphene and hexagonal boron nitride (<I>h</I>-BN) substrate lattices. Here, we use planar tunneling devices with thin <I>h</I>-BN barriers to obtain direct and accurate tunneling spectroscopy measurements of the energy gaps in single-layer and bilayer graphene-<I>h</I>-BN superlattice structures at charge neutrality (first Dirac point) and at integer moiré band occupancies (second Dirac point, SDP) as a function of external electric and magnetic fields and the interface twist angle. In single-layer graphene, we find, in agreement with previous work, that gaps are formed at neutrality and at the hole-doped SDP, but not at the electron-doped SDP. Both primary and secondary gaps can be determined accurately by extrapolating Landau fan patterns to a zero magnetic field and are as large as ≈17 meV for devices in near-perfect alignment. For bilayer graphene, we find that gaps occur only at charge neutrality where they can be modified by an external electric field.</P> [FIG OMISSION]</BR>
Evidence of a gate-tunable Mott insulator in a trilayer graphene moiré superlattice
Chen, Guorui,Jiang, Lili,Wu, Shuang,Lyu, Bosai,Li, Hongyuan,Chittari, Bheema Lingam,Watanabe, Kenji,Taniguchi, Takashi,Shi, Zhiwen,Jung, Jeil,Zhang, Yuanbo,Wang, Feng NATURE PUBLISHING GROUP 2019 NATURE PHYSICS Vol.15 No.3
Magnetic ground state of the multiferroic hexagonal LuFeO3
Suresh, Pittala,Vijaya Laxmi, K.,Bera, A. K.,Yusuf, S. M.,Chittari, Bheema Lingam,Jung, Jeil,Anil Kumar, P. S. American Physical Society 2018 Physical Review B Vol.97 No.18
<P>The structural, electric, and magnetic properties of bulk hexagonal LuFeO3 are investigated. Single phase hexagonal LuFeO3 has been successfully stabilized in the bulk form without any doping by sol-gel method. The hexagonal crystal structure with P6(3)cm space group has been confirmed by x-ray-diffraction, neutron-diffraction, and Raman spectroscopy study at room temperature. Neutron diffraction confirms the hexagonal phase of LuFeO3 persists down to 6 K. Further, the x-ray photoelectron spectroscopy established the 3+ oxidation state of Fe ions. The temperature-dependent magnetic dc susceptibility, specific heat, and neutron-diffraction studies confirm an antiferromagnetic ordering below the Neel temperature (T-N) similar to 130 K. Analysis of magnetic neutron-diffraction patterns reveals an in-plane (ab-plane) 120 degrees antiferromagnetic structure, characterized by a propagation vector k = (0 0 0) with an ordered moment of 2.84 mu(B)/Fe3+ at 6 K. The 120 degrees antifferomagnetic ordering is further confirmed by spin-orbit coupling density functional theory calculations. The on-site coulomb interaction (U) and Hund's parameter (J(H)) on Fe atoms reproduced the neutron-diffraction Gamma(1) spin pattern among the Fe atoms. P-E loop measurements at room temperature confirm an intrinsic ferroelectricity of the sample with remnant polarization P-r similar to 0.18 mu(C) cm(2). A clear anomaly in the dielectric data is observed at similar to T-N revealing the presence of magnetoelectric coupling. A change in the lattice constants at T-N has also been found, indicating the presence of a strong magnetoelastic coupling. Thus a coupling between lattice, electric, and magnetic degrees of freedom is established in bulk hexagonal LuFeO3.</P>
Dynamic band-structure tuning of graphene moiré superlattices with pressure
Yankowitz, Matthew,Jung, Jeil,Laksono, Evan,Leconte, Nicolas,Chittari, Bheema L.,Watanabe, K.,Taniguchi, T.,Adam, Shaffique,Graf, David,Dean, Cory R. Nature Publishing Group UK 2018 Nature Vol.557 No.7705
<P>Heterostructures can be assembled from atomically thin materials by combining a wide range of available van der Waals crystals, providing exciting possibilities for designer electronics'. In many cases, beyond simply realizing new material combinations, interlayer interactions lead to emergent electronic properties that are fundamentally distinct from those of the constituent layers'. A critical parameter in these structures is the interlayer coupling strength, but this is often not easy to determine and is typically considered to be a fixed property of the system. Here we demonstrate that we can controllably tune the interlayer separation in van der Waals heterostructures using hydrostatic pressure, providing a dynamic way to modify their electronic properties. In devices in which graphene is encapsulated in boron nitride and aligned with one of the encapsulating layers, we observe that increasing pressure produces a superlinear increase in the moire-superlattice-induced bandgap nearly doubling within the studied range together with an increase in the capacitive gate coupling to the active channel by as much as 25 per cent. Comparison to theoretical modelling highlights the role of atomic-scale structural deformations and how this can be altered with pressure. Our results demonstrate that combining hydrostatic pressure with controlled rotational order provides opportunities for dynamic band-structure engineering in van der Waals heterostructures.</P>