| 1. |
- Chen, Si, et al.
(författare)
-
Unveiling Temperature-Induced Structural Domains and Movement of Oxygen Vacancies in SrTiO3 with Graphene
- 2020
-
Ingår i: ACS Applied Materials and Interfaces. - : AMER CHEMICAL SOC. - 1944-8244 .- 1944-8252. ; 12:47, s. 52915-52921
-
Tidskriftsartikel (refereegranskat)abstract
- Heterointerfaces coupling complex oxides exhibit coexisting functional properties such as magnetism, superconductivity, and ferroelectricity, often absent in their individual constituent. SrTiO3 (STO), a canonical band insulator, is an active constituent of such heterointerfaces. Temperature-, strain-, or mechanical stress-induced ferroelastic transition leads to the formation of narrow domains and domain walls in STO. Such ferroelastic domain walls have been studied using imaging or transport techniques and, often, the findings are influenced by the choice and interaction of the electrodes with STO. In this work, we use graphene as a unique platform to unveil the movement of oxygen vacancies and ferroelastic domain walls near the STO surface by studying the temperature and gate bias dependence of charge transport in graphene. By sweeping the back gate voltage, we observe antihysteresis in graphene typically observed in conventional ferroelectric oxides. Interestingly, we find features in antihysteresis that are related to the movement of domain walls and of oxygen vacancies in STO. We ascertain this by analyzing the time dependence of the graphene square resistance at different temperatures and gate bias. Density functional calculations estimate the surface polarization and formation energies of layer-dependent oxygen vacancies in STO. This corroborates quantitatively with the activation energies determined from the temperature dependence of the graphene square resistance. Introduction of a hexagonal boron nitride (hBN) layer, of varying thicknesses, between graphene and STO leads to a gradual disappearance of the observed features, implying the influence of the domain walls onto the potential landscape in graphene.
|
|
| 2. |
- Chen, Xin, 1992-, et al.
(författare)
-
Room temperature two-dimensional antiferromagnetic Weyl semimetal CrO with giant spin-splitting and spin-momentum locked transport
-
Tidskriftsartikel (refereegranskat)abstract
- Giant spin-splitting was recently predicted in collinear antiferromagnetic materials with a specific class of magnetic space group. In this work, we have predicted a two-dimensional (2D) antiferromagnetic Weyl semimetal (WS), CrO with large spin-split band structure, spin-momentum locked transport properties and high Néel temperature. It has two pairs of spin-polarized Weyl points at the Fermi level. By manipulating the position of the Weyl points with strain, four different antiferromagnetic spintronic states can be achieved: WSs with two spin-polarized transport channels (STCs), WSs with single STC, semiconductors with two STCs, and semiconductors with single STC. Based on these properties, a new avenue in spintronics with 2D collinear antiferromagnets is proposed.
|
|
| 3. |
- Chen, Xin, 1992-
(författare)
-
Theoretical Investigations of Two-Dimensional Materials : Studies on Electronic, Magnetic, Mechanical, and Thermal Properties
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Two-dimensional (2D) materials have been paid enormous attention since the first realization of graphene in 2004, in connection to high-speed flexible electronics, 2D magnetism, optoelectronics, and so on. Apart from graphene, many new 2D materials with special properties have been predicted and synthesized. For the understanding of several interesting phenomena and prediction of new 2D materials, materials-specific density functional theory (DFT) plays a very important role.In this thesis, based on first-principles calculations, structural, magnetic, electronic, mechanical, and thermal transport properties of two kinds of 2D systems are investigated.The first kind of 2D materials is based on the synthesized material or the predicted structure with ultralow energy. These materials were functionalized by adsorbing transition metal atoms or oxygen atoms, which makes a significant difference in the properties. A part of the thesis covers the study of the self-assembly process of 3d transition metal hexamers on graphene with different defects. Interestingly, it is found that the easy axis of magnetization can be tuned between in-plane and out-of-plane directions in the presence of an external electric field. The second subsection is the oxygen functionalized form of 2D honeycomb and zigzag dumbbell silicene. Interestingly, both the structures are Dirac semimetal.The other kind of 2D materials discussed in this thesis are new materials which were never reported before. Starting from a global structure search, we predicted several structures with ultrahigh stability and novel properties. One work is about a new allotrope of graphene, namely PAI-graphene. It is a new structural motif, which is energetically very close to graphene with interesting properties. PAI-graphene is a semimetal with distorted Dirac cones. By applying tensile strain, three different topological phases can be achieved. The second subsection is the work about new 2D structural forms of A2B (A=Cu, Ag, Au, and B=S, Se). Our obtained square-A2B (s-A2B) structures are energetically more favored than all the reported 2D structures for A2B. s-A2B structures are direct bandgap semiconductors with high carrier mobilities. All the s-A2B structures have unusually low lattice thermal conductivities. Moreover, s-A2B monolayers have ultra-low Young’s moduli and in-plane negative Poisson’s ratios. The third work is about the phase transition in s-A2B monolayers. We proposed two new s-A2B structure, s(I)- and s(II)-Au2Te. S(I)-Au2Te is an auxetic direct-gap semiconductor, while s(II)-Au2Te is a topological insulator. By applying strain or using thermal means, we can achieve a structural phase transition between the two phases.
|
|
| 4. |
- Chen, Xin, 1992-, et al.
(författare)
-
Two-dimensional oxygen functionalized honeycomb and zigzag dumbbell silicene with robust Dirac cones
- 2021
-
Ingår i: New Journal of Physics. - : Institute of Physics Publishing (IOPP). - 1367-2630. ; 23:2
-
Tidskriftsartikel (refereegranskat)abstract
- Dumbbell-like structures are recently found to be energetically favored in group IV two-dimensional (2D) materials, exhibiting rich physics and many interesting properties. In this paper, using first-principles calculations, we have investigated the oxidized form of the hexagonal honeycomb (ODB-h) and zigzag dumbbell silicene (ODB-z). We confirm that both oxidization processes are energetically favorable, and their phonon spectra further demonstrate the dynamic stability. Contrary to the pristine dumbbell silicene structures (PDB-h and PDB-z silicene), these oxidized products ODB-h and ODB-z silicene are both semimetals with Dirac cones at the Fermi level. The Dirac cones of ODB-h and ODB-z silicene are at the K point and between Y and Gamma points respectively, possessing high Fermi velocities of 3.1 x 10(5) m s(-1) (ODB-h) and 2.9-3.4 x 10(5) m s(-1) (ODB-z). The origin of the Dirac cones is further explained by tight-binding models. The semimetallic properties of ODB-h and ODB-z are sensitive to compression due to the self-absorption effect, but quite robust against the tensile strain. These outstanding properties make oxidized dumbbell silicene a promising material for quantum computing and high-speed electronic devices.
|
|
| 5. |
|
|
| 6. |
- Husain, Sajid, et al.
(författare)
-
Large Damping-Like Spin–Orbit Torque in a 2D Conductive 1T-TaS2 Monolayer
- 2020
-
Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 20:9, s. 6372-6380
-
Tidskriftsartikel (refereegranskat)abstract
- A damping-like spin-orbit torque (SOT) is a prerequisite for ultralow-power spin logic devices. Here, we report on the damping-like SOT in just one monolayer of the conducting transition-metal dichalcogenide (TMD) TaS2 interfaced with a NiFe (Py) ferromagnetic layer. The charge-spin conversion efficiency is found to be 0.25 +/- 0.03 in TaS2(0.88)/Py(7), and the spin Hall conductivity (14.9 x 10(s) h/2e Omega(-1) m(-1) is found to be superior to values reported for other TMDs. We also observed sizable field-like torque in this heterostructure. The origin of this large damping-like SOT can be found in the interfacial properties of the TaS2/Py heterostructure, and the experimental findings are complemented by the results from density functional theory calculations. It is envisioned that the interplay between interfacial spinorbit coupling and crystal symmetry yielding large damping-like SOT. The dominance of damping-like torque demonstrated in our study provides a promising path for designing the next-generation conducting TMD-based low-powered quantum memory devices.
|
|
| 7. |
- Li, Linyang, et al.
(författare)
-
Monolayer 1T-LaN2 : Dirac spin-gapless semiconductor of p-state and Chern insulator with a high Chern number
- 2020
-
Ingår i: Applied Physics Letters. - : AMER INST PHYSICS. - 0003-6951 .- 1077-3118. ; 117:14
-
Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional transition-metal dinitrides have attracted considerable attention in recent years due to their rich magnetic properties. Here, we focus on rare-earth-metal elements and propose a monolayer of lanthanum dinitride with a 1T structural phase, 1T-LaN2. Using first-principles calculations, we systematically investigated the structure, stability, magnetism, and band structure of this material. It is a flexible and stable monolayer exhibiting a low lattice thermal conductivity, which is promising for future thermoelectric devices. The monolayer shows the ferromagnetic ground state with a spin-polarized band structure. Two linear spin-polarized bands cross at the Fermi level forming a Dirac point, which is formed by the p atomic orbitals of the N atoms, indicating that monolayer 1T-LaN2 is a Dirac spin-gapless semiconductor of p-state. When the spin-orbit coupling is taken into account, a large nontrivial indirect bandgap (86/354meV) can be opened at the Dirac point, and three chiral edge states are obtained, corresponding to a high Chern number of C=3, implying that monolayer 1T-LaN2 is a Chern insulator. Importantly, this kind of band structure is expected to occur in more monolayers of rare-earth-metal dinitride with a 1T structural phase.
|
|
| 8. |
- Pandey, Lalit, et al.
(författare)
-
Weak antilocalization and electron-electron interactions in topological insulator BixTey films deposited by sputtering on Si(100)
- 2022
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 6:4
-
Tidskriftsartikel (refereegranskat)abstract
- Topological insulators (TIs) which exhibit spin-momentum locking constitute an interesting class of quantum materials. In this paper, we present a systematic method to prepare BixTey films having tunable Bi/Te composition by employing a direct current (DC) cosputtering technique. Films of three different Te compositions, namely, the Te-deficient (BiTe), Te-stoichiometric (Bi2Te3), and Te-excessive (BiTe3) phases, have been investigated in detail by performing the measurements of their temperature-dependent resistivity and in-plane and out-of-plane magnetoresistance responses. Clear evidence of the presence of a weak antilocalization effect and electron-electron interaction are observed in all three films having different phases of Bi-Te system. Using the Hikami-Larkin-Nagaoka and Al'tshuler-Aronov models, the topological characteristics have been evidently distinguished in these three different phases of BixTey. The analysis of the fitting of the experimental magnetotransport data is performed to quantitatively determine the critical model parameters, viz., phase coherence length (l(phi)), surface state penetration depth (lambda), coherency factor (alpha), and dephasing parameter (p) through which Berry phase and dimensionality of transport channels can be estimated. In this paper, we reveal a systematic correlation between the composition of the BixTey films and their topological properties. The parameter kappa, slope of temperature dependence of conductivity correction, is studied at different magnetic fields, which suggested that the Testoichiometric sample shows better features of topological properties than the Te-deficient and Te-excessive samples. These experimental observations are supplemented by first-principles calculations. Additionally, we demonstrate that TIs can be grown by the sputtering technique as is desired for realizing industrial applications.
|
|
| 9. |
- Wang, Duo, et al.
(författare)
-
Unraveling complex magnetism in two-dimensional FeS2
- 2021
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 104:24
-
Tidskriftsartikel (refereegranskat)abstract
- Atomically thin two-dimensional (2D) magnets have given rise to emergent phenomena due to magnetic exchange and spin-orbit coupling showing great promise for realizing ultrathin device structures. In this paper, we critically examine the magnetic properties of 2D FeS2, a non van der Waals magnet, which has been recently claimed to exhibit room-temperature ferromagnetism in the (111) orientation. Our ab initio study based on collinear density functional theory has revealed the ground state as an antiferromagnetic one with an ordering temperature of around 100 K along with a signature of spin-phonon coupling, which may trigger a ferromagnetic coupling via strain. Moreover, our calculations based on spin spirals indicate the possibility of noncollinear magnetic structures, which is also supported by Monte Carlo simulations based on ab initio magnetic exchange parameters. This opens up an excellent possibility to manipulate magnetic structures by the application of directional strain.
|
|
| 10. |
|
|
