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Bera A, Singh A, Gupta SN, Glazyrin K, Muthu DVS, Waghmare UV, Sood AK. Pressure-induced isostructural electronic topological transitions in 2H-MoTe 2: x-ray diffraction and first-principles study. J Phys Condens Matter 2021; 33:065402. [PMID: 33169727 DOI: 10.1088/1361-648x/abaeac] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Synchrotron x-ray diffraction measurements on powder 2H-MoTe2 (P63/mmc) up to ∼46 GPa have been performed along with first-principles based density functional theoretical analysis to probe the isostructural transition in low pressure regime and two electronic topological transitions (ETT) of Lifshitz-type in high pressure regime. The low pressure isostructural transition at ∼7 GPa is associated with the lattice parameter ratio c/a anomaly and the change in the compressibility of individual layers. The pressure dependence of the volume by linearizing the Birch-Murnaghan equation of state as a function of Eulerian strain shows a clear change of the bulk modulus at the ETT pressure of ∼20 GPa. The minimum of c/a ratio around 32 GPa is associated with the change in topology of electron pockets marked as second ETT of Lifshitz-type. We do not observe any structural transition up to the maximum applied pressure of ∼46 GPa under quasi-hydrostatic condition.
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Affiliation(s)
- Achintya Bera
- Department of Physics, Indian Institute of Science, Bangalore 560 012, India
| | - Anjali Singh
- Center for Study of Science, Technology and Policy, Bangalore-560094, India
| | | | - K Glazyrin
- Photon Science, Deutsches Elektronen-Synchrotron, Notkestrasse 85, 22607 Hamburg, Germany
| | - D V S Muthu
- Department of Physics, Indian Institute of Science, Bangalore 560 012, India
| | - U V Waghmare
- Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560 064, India
| | - A K Sood
- Department of Physics, Indian Institute of Science, Bangalore 560 012, India
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2
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Das S, Debnath K, Chakraborty B, Singh A, Grover S, Muthu DVS, Waghmare UV, Sood AK. Symmetry induced phonon renormalization in few layers of 2H-MoTe 2 transistors: Raman and first-principles studies. Nanotechnology 2021; 32:045202. [PMID: 33036010 DOI: 10.1088/1361-6528/abbfd6] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Understanding of electron-phonon coupling (EPC) in two-dimensional (2D) materials manifesting as phonon renormalization is essential to their possible applications in nanoelectronics. Here we report in situ Raman measurements of electrochemically top-gated 2, 3 and 7 layered 2H-MoTe2 channel based field-effect transistors. While the [Formula: see text] and B2g phonon modes exhibit frequency softening and linewidth broadening with hole doping concentration (p) up to ∼2.3 × 1013/cm2, A1g shows relatively small frequency hardening and linewidth sharpening. The dependence of frequency renormalization of the [Formula: see text] mode on the number of layers in these 2D crystals confirms that hole doping occurs primarily in the top two layers, in agreement with recent predictions. We present first-principles density functional theory analysis of bilayer MoTe2 that qualitatively captures our observations, and explain that a relatively stronger coupling of holes with [Formula: see text] or B2g modes as compared with the A1g mode originates from the in-plane orbital character and symmetry of the states at valence band maximum. The contrast between the manifestation of EPC in monolayer MoS2 and those observed here in a few-layered MoTe2 demonstrates the role of the symmetry of phonons and electronic states in determining the EPC in these isostructural systems.
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Affiliation(s)
- Subhadip Das
- Department of Physics, Indian Institute of Science, Bangalore 560012, India
| | - Koyendrila Debnath
- Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India
| | | | - Anjali Singh
- Center for Study of Science, Technology & Policy (CSTEP), Bangalore 560094, India
| | - Shivani Grover
- Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India
| | - D V S Muthu
- Department of Physics, Indian Institute of Science, Bangalore 560012, India
| | - U V Waghmare
- Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India
| | - A K Sood
- Department of Physics, Indian Institute of Science, Bangalore 560012, India
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3
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Das S, Prasad S, Chakraborty B, Jariwala B, Shradha S, Muthu DVS, Bhattacharya A, Waghmare UV, Sood AK. Doping controlled Fano resonance in bilayer 1T'-ReS 2: Raman experiments and first-principles theoretical analysis. Nanoscale 2021; 13:1248-1256. [PMID: 33404576 DOI: 10.1039/d0nr06583h] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
In the bilayer ReS2 channel of a field-effect transistor (FET), we demonstrate using Raman spectroscopy that electron doping (n) results in softening of frequency and broadening of linewidth for the in-plane vibrational modes, leaving the out-of-plane vibrational modes unaffected. The largest change is observed for the in-plane Raman mode at ∼151 cm-1, which also shows doping induced Fano resonance with the Fano parameter 1/q = -0.17 at a doping concentration of ∼3.7 × 1013 cm-2. A quantitative understanding of our results is provided by first-principles density functional theory (DFT), showing that the electron-phonon coupling (EPC) of in-plane modes is stronger than that of out-of-plane modes, and its variation with doping is independent of the layer stacking. The origin of large EPC is traced to 1T to 1T' structural phase transition of ReS2 involving in-plane displacement of atoms whose instability is driven by the nested Fermi surface of the 1T structure. Results are compared with those of the isostructural trilayer ReSe2.
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Affiliation(s)
- Subhadip Das
- Department of Physics, Indian Institute of Science, Bangalore 560012, India.
| | - Suchitra Prasad
- Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India
| | | | - Bhakti Jariwala
- Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India
| | - Sai Shradha
- Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India
| | - D V S Muthu
- Department of Physics, Indian Institute of Science, Bangalore 560012, India.
| | - Arnab Bhattacharya
- Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai 400005, India
| | - U V Waghmare
- Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India
| | - A K Sood
- Department of Physics, Indian Institute of Science, Bangalore 560012, India.
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4
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Bera A, Singh A, Muthu DVS, Waghmare UV, Sood AK. Pressure-dependent semiconductor to semimetal and Lifshitz transitions in 2H-MoTe 2: Raman and first-principles studies. J Phys Condens Matter 2017; 29:105403. [PMID: 28008869 DOI: 10.1088/1361-648x/aa55a1] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
High pressure Raman spectroscopy of bulk 2H-MoTe2 up to ∼29 GPa is shown to reveal two phase transitions (at ∼6 and 16.5 GPa), which are analyzed using first-principles density functional theoretical calculations. The transition at 6 GPa is marked by changes in the pressure coefficients of A 1g and [Formula: see text] Raman mode frequencies as well as in their relative intensity. Our calculations show that this is an isostructural semiconductor to a semimetal transition. The transition at ∼16.5 GPa is identified with the changes in linewidths of the Raman modes as well as in the pressure coefficients of their frequencies. Our theoretical analysis clearly shows that the structure remains the same up to 30 GPa. However, the topology of the Fermi-surface evolves as a function of pressure, and abrupt appearance of electron and hole pockets at [Formula: see text] GPa marks a Lifshitz transition.
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Affiliation(s)
- Achintya Bera
- Department of Physics, Indian Institute of Science, Bangalore 560 012, India
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5
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Bera A, Pal K, Muthu DVS, Waghmare UV, Sood AK. Pressure-induced phase transition in Bi2Se3 at 3 GPa: electronic topological transition or not? J Phys Condens Matter 2016; 28:105401. [PMID: 26881905 DOI: 10.1088/0953-8984/28/10/105401] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
In recent years, a low pressure transition around P3 GPa exhibited by the A2B3-type 3D topological insulators is attributed to an electronic topological transition (ETT) for which there is no direct evidence either from theory or experiments. We address this phase transition and other transitions at higher pressure in bismuth selenide (Bi2Se3) using Raman spectroscopy at pressure up to 26.2 GPa. We see clear Raman signatures of an isostructural phase transition at P2.4 GPa followed by structural transitions at ∼ 10 GPa and 16 GPa. First-principles calculations reveal anomalously sharp changes in the structural parameters like the internal angle of the rhombohedral unit cell with a minimum in the c/a ratio near P3 GPa. While our calculations reveal the associated anomalies in vibrational frequencies and electronic bandgap, the calculated Z2 invariant and Dirac conical surface electronic structure remain unchanged, showing that there is no change in the electronic topology at the lowest pressure transition.
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Affiliation(s)
- Achintya Bera
- Department of Physics, Indian Institute of Science, Bangalore 560 012, India
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Bhattacharjee S, Gupta K, Jung N, Yoo SJ, Waghmare UV, Lee SC. Site preference of NH3-adsorption on Co, Pt and CoPt surfaces: the role of charge transfer, magnetism and strain. Phys Chem Chem Phys 2015; 17:9335-40. [DOI: 10.1039/c4cp05557h] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Oxidation of Co at the surface poses a major problem in the cyclable use of CoPt, a cost-effective catalyst for proton exchange membrane fuel cells.
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Affiliation(s)
- S. Bhattacharjee
- Indo-Korea Science and Technology Center (IKST)
- Bangalore
- India
- Theoretical Science Unit & Sheikh Saqr Laboratory
- Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR)
| | - K. Gupta
- Indo-Korea Science and Technology Center (IKST)
- Bangalore
- India
- Theoretical Science Unit & Sheikh Saqr Laboratory
- Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR)
| | - N. Jung
- Fuel Cell Research Center
- Korea Institute of Science
- Korea
| | - S. J. Yoo
- Fuel Cell Research Center
- Korea Institute of Science
- Korea
| | - U. V. Waghmare
- Theoretical Science Unit & Sheikh Saqr Laboratory
- Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR)
- Bangalore
- India
| | - S. C. Lee
- Indo-Korea Science and Technology Center (IKST)
- Bangalore
- India
- Electronic Materials Research Center
- Korea Institute of Science
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7
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Devidas TR, Chandra Shekar NV, Sundar CS, Chithaiah P, Sorb YA, Bhadram VS, Chandrabhas N, Pal K, Waghmare UV, Rao CNR. Pressure-induced structural changes and insulator-metal transition in layered bismuth triiodide, BiI3: a combined experimental and theoretical study. J Phys Condens Matter 2014; 26:275502. [PMID: 24934819 DOI: 10.1088/0953-8984/26/27/275502] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/10/2023]
Abstract
Noting that BiI3 and the well-known topological insulator (TI) Bi2Se3 have the same high symmetry parent structures, and that it is desirable to find a wide-band gap TI, we determine here the effects of pressure on the structure, phonons and electronic properties of rhombohedral BiI3. We report a pressure-induced insulator-metal transition near 1.5 GPa, using high pressure electrical resistivity and Raman measurements. X-ray diffraction studies, as a function of pressure, reveal a structural peculiarity of the BiI3 crystal, with a drastic drop in c/a ratio at 1.5 GPa, and a structural phase transition from rhombohedral to monoclinic structure at 8.8 GPa. Interestingly, the metallic phase, at relatively low pressures, exhibits minimal resistivity at low temperatures, similar to that in Bi2Se3. We corroborate these findings with first-principles calculations and suggest that the drop in the resistivity of BiI3 in the 1-3 GPa range of pressure arises possibly from the appearance of an intermediate crystal phase with a lower band-gap and hexagonal crystal structure. Calculated Born effective charges reveal the presence of metallic states in the structural vicinity of rhombohedral BiI3. Changes in the topology of the electronic bands of BiI3 with pressure, and a sharp decrease in the c/a ratio below 2 GPa, are shown to give rise to changes in the slope of phonon frequencies near that pressure.
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Affiliation(s)
- T R Devidas
- Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India
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8
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Srirangarajan A, Datta A, Gandi AN, Ramamurty U, Waghmare UV. Universal binding energy relation for cleaved and structurally relaxed surfaces. J Phys Condens Matter 2014; 26:055006. [PMID: 24356124 DOI: 10.1088/0953-8984/26/5/055006] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The universal binding energy relation (UBER), derived earlier to describe the cohesion between two rigid atomic planes, does not accurately capture the cohesive properties when the cleaved surfaces are allowed to relax. We suggest a modified functional form of UBER that is analytical and at the same time accurately models the properties of surfaces relaxed during cleavage. We demonstrate the generality as well as the validity of this modified UBER through first-principles density functional theory calculations of cleavage in a number of crystal systems. Our results show that the total energies of all the relaxed surfaces lie on a single (universal) energy surface, that is given by the proposed functional form which contains an additional length-scale associated with structural relaxation. This functional form could be used in modelling the cohesive zones in crack growth simulation studies. We find that the cohesive law (stress-displacement relation) differs significantly in the case where cracked surfaces are allowed to relax, with lower peak stresses occurring at higher displacements.
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Affiliation(s)
- Aarti Srirangarajan
- Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India
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9
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Tamilarasan S, Sarma D, Bhattacharjee S, Waghmare UV, Natarajan S, Gopalakrishnan J. Exploring the Color of Transition Metal Ions in Irregular Coordination Geometries: New Colored Inorganic Oxides Based on the Spiroffite Structure, Zn2–xMxTe3O8 (M = Co, Ni, Cu). Inorg Chem 2013; 52:5757-63. [DOI: 10.1021/ic302557j] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- S Tamilarasan
- Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India
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10
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Bera A, Pal K, Muthu DVS, Sen S, Guptasarma P, Waghmare UV, Sood AK. Sharp Raman anomalies and broken adiabaticity at a pressure induced transition from band to topological insulator in Sb2Se3. Phys Rev Lett 2013; 110:107401. [PMID: 23521295 DOI: 10.1103/physrevlett.110.107401] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/26/2012] [Indexed: 06/01/2023]
Abstract
The nontrivial electronic topology of a topological insulator is thus far known to display signatures in a robust metallic state at the surface. Here, we establish vibrational anomalies in Raman spectra of the bulk that signify changes in electronic topology: an E(g)(2) phonon softens unusually and its linewidth exhibits an asymmetric peak at the pressure induced electronic topological transition (ETT) in Sb(2)Se(3) crystal. Our first-principles calculations confirm the electronic transition from band to topological insulating state with reversal of parity of electronic bands passing through a metallic state at the ETT, but do not capture the phonon anomalies which involve breakdown of adiabatic approximation due to strongly coupled dynamics of phonons and electrons. Treating this within a four-band model of topological insulators, we elucidate how nonadiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator.
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Affiliation(s)
- Achintya Bera
- Department of Physics, Indian Institute of Science, Bangalore 560012, India
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11
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Choudhury D, Mandal P, Mathieu R, Hazarika A, Rajan S, Sundaresan A, Waghmare UV, Knut R, Karis O, Nordblad P, Sarma DD. Near-room-temperature colossal magnetodielectricity and multiglass properties in partially disordered La2NiMnO6. Phys Rev Lett 2012; 108:127201. [PMID: 22540618 DOI: 10.1103/physrevlett.108.127201] [Citation(s) in RCA: 69] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/07/2011] [Revised: 01/12/2012] [Indexed: 05/31/2023]
Abstract
We report magnetic, dielectric, and magnetodielectric responses of the pure monoclinic bulk phase of partially disordered La2NiMnO6, exhibiting a spectrum of unusual properties and establish that this compound is an intrinsically multiglass system with a large magnetodielectric coupling (8%-20%) over a wide range of temperatures (150-300 K). Specifically, our results establish a unique way to obtain colossal magnetodielectricity, independent of any striction effects, by engineering the asymmetric hopping contribution to the dielectric constant via the tuning of the relative-spin orientations between neighboring magnetic ions in a transition-metal oxide system. We discuss the role of antisite (Ni-Mn) disorder in emergence of these unusual properties.
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Affiliation(s)
- D Choudhury
- Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India
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12
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Kumar P, Kumar A, Saha S, Muthu DVS, Prakash J, Waghmare UV, Ganguli AK, Sood AK. Temperature-dependent Raman study of a CeFeAsO(0.9)F(0.1) superconductor: crystal field excitations, phonons and their coupling. J Phys Condens Matter 2010; 22:255402. [PMID: 21393798 DOI: 10.1088/0953-8984/22/25/255402] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
We report temperature-dependent Raman spectra of CeFeAsO(0.9)F(0.1) from 4 to 300 K in the spectral range of 60-1800 cm(-1) and interpret them using estimates of phonon frequencies obtained from first-principles density functional calculations. We find evidence for strong coupling between the phonons and crystal field excitations; in particular the Ce(3 + ) crystal field excitation at 432 cm(-1) couples strongly with the E(g) oxygen vibration at 389 cm(-1). Below the superconducting transition temperature, the phonon mode near 280 cm(-1) shows softening, signaling its coupling with the superconducting gap. The ratio of the superconducting gap to T(c), thus estimated to be ~10, suggests CeFeAsO(0.9)F(0.1) to be a strong coupling superconductor. In addition, two high frequency modes observed at 1342 and 1600 cm(-1) are attributed to electronic Raman scattering from the (x(2)-y(2)) to xz /yz d-orbitals of Fe.
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Affiliation(s)
- Pradeep Kumar
- Department of Physics, Indian Institute of Science, Bangalore, India
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13
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Gupta A, Kumar A, Hegde MS, Waghmare UV. Structure of Ce1−xSnxO2 and its relation to oxygen storage property from first-principles analysis. J Chem Phys 2010; 132:194702. [PMID: 20499979 DOI: 10.1063/1.3425662] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Affiliation(s)
- Asha Gupta
- Materials Research Centre, Indian Institute of Science, Bangalore 560012, India
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14
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Deepak J, Pradeep T, Waghmare UV. Interaction of small gold clusters with carbon nanotube bundles: formation of gold atomic chains. J Phys Condens Matter 2010; 22:125301. [PMID: 21389485 DOI: 10.1088/0953-8984/22/12/125301] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
We use first-principles density functional theory to simulate the interaction of bundles of semiconducting (10, 0) and metallic (6, 6) carbon nanotubes (CNTs) with small gold clusters (Au(n), n = 3, 5) inserted in their interstitial spaces. We find that gold clusters spontaneously evolve to form atomic chains along the axis of nanotubes and induce weak metallicity in the semiconducting nanotubes through charge transfer. We further show that a similar structural evolution of Pt(3) clusters occurs in the interstitial spaces of a (10, 0) CNT bundle. Our calculations show that these structural changes, along with interesting changes in the electronic structure, occur at moderate pressures that are readily achievable in a laboratory, and should be relevant to devices that make use of gold-nanotube contacts.
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Affiliation(s)
- J Deepak
- Indian Institute of Technology Madras, Chennai 600 036, India
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15
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16
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Das A, Pisana S, Chakraborty B, Piscanec S, Saha SK, Waghmare UV, Novoselov KS, Krishnamurthy HR, Geim AK, Ferrari AC, Sood AK. Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor. Nat Nanotechnol 2008; 3:210-5. [PMID: 18654505 DOI: 10.1038/nnano.2008.67] [Citation(s) in RCA: 1313] [Impact Index Per Article: 82.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/30/2007] [Accepted: 02/28/2008] [Indexed: 05/20/2023]
Abstract
The recent discovery of graphene has led to many advances in two-dimensional physics and devices. The graphene devices fabricated so far have relied on SiO(2) back gating. Electrochemical top gating is widely used for polymer transistors, and has also been successfully applied to carbon nanotubes. Here we demonstrate a top-gated graphene transistor that is able to reach doping levels of up to 5x1013 cm-2, which is much higher than those previously reported. Such high doping levels are possible because the nanometre-thick Debye layer in the solid polymer electrolyte gate provides a much higher gate capacitance than the commonly used SiO(2) back gate, which is usually about 300 nm thick. In situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, but the 2D peak shows a different response to holes and electrons. The ratio of the intensities of the G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor the doping.
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Affiliation(s)
- A Das
- Department of Physics, Indian Institute of Science, Bangalore 560012, India
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17
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Raidongia K, Jagadeesan D, Upadhyay-Kahaly M, Waghmare UV, Pati SK, Eswaramoorthy M, Rao CNR. Synthesis, structure and properties of homogeneous BC4N nanotubes. ACTA ACUST UNITED AC 2008. [DOI: 10.1039/b712472d] [Citation(s) in RCA: 72] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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18
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Tinte S, Burton BP, Cockayne E, Waghmare UV. Origin of the relaxor state in Pb(B{x}B{1-x}{'})O(3) perovskites. Phys Rev Lett 2006; 97:137601. [PMID: 17026074 DOI: 10.1103/physrevlett.97.137601] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/10/2006] [Indexed: 05/12/2023]
Abstract
Molecular dynamics simulations of first-principles-based effective Hamiltonians for Pb(Sc{1/2}Nb{1/2})O(3) under hydrostatic pressure and for Pb(Mg{1/3}Nb{2/3})O(3) at ambient pressure show clear evidence of a relaxor state in both systems. The Burns temperature is identified as the temperature below which dynamic nanoscale polar clusters form, pinned to regions of quenched chemical short-range order. The effect of pressure in Pb(Sc{1/2}Nb{1/2})O(3) demonstrates that the stability of the relaxor state depends on a delicate balance between the energetics that stabilize normal ferroelectricity and the average strength of random local fields which promote the relaxor state.
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Affiliation(s)
- Silvia Tinte
- Ceramics Division, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8520, USA
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Abstract
Laser evaporation of MoS(2) nanoflakes gives negatively charged magic number clusters of compositions Mo(13)S(25) and Mo(13)S(28), which are shown to have closed-cage structures. The clusters are stable and do not show fragmentation in the post-source decay analysis even at the highest laser powers. Computations suggest that Mo(13)S(25) has a central cavity with a diameter of 4.5 A. The nanosheets of MoS(2) could curl upon laser irradiation, explaining the cluster formation.
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20
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Sluiter MHF, Kawazoe Y, Sharma P, Inoue A, Raju AR, Rout C, Waghmare UV. First principles based design and experimental evidence for a ZnO-based ferromagnet at room temperature. Phys Rev Lett 2005; 94:187204. [PMID: 15904408 DOI: 10.1103/physrevlett.94.187204] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/26/2004] [Indexed: 05/02/2023]
Abstract
The introduction of ferromagnetic order in ZnO results in a transparent piezoelectric ferromagnet and further expands its already wide range of applications into the emerging field of spintronics. Through an analysis of density functional calculations we determine the nature of magnetic interactions for transition metals doped ZnO and develop a physical picture based on hybridization, superexchange, and double exchange that captures chemical trends. We identify a crucial role of defects in the observed weak and preparation sensitive ferromagnetism in ZnO:Mn and ZnO:Co. We predict and explain co-doping of Li and Zn interstitials to both yield ferromagnetism in ZnO:Co, in contrast with earlier insights, and verify it experimentally.
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Affiliation(s)
- Marcel H F Sluiter
- Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
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Abstract
Computer simulations, using the Stillinger-Weber potential, have previously been employed to demonstrate a liquid-liquid transition in supercooled silicon near 1060 K. From calculations of electronic structure using an empirical psuedopotential, we show that silicon undergoes an associated metal to semimetal transition with a resistivity jump of roughly 1 order of magnitude. We show that the electronic states near the Fermi energy become localized in the low temperature phase, and that changes in electronic structure between the two phases arise from a change in atomic structure, and not from a change in density.
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Affiliation(s)
- S S Ashwin
- Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur PO, Bangalore 560 064, India.
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Wang J, Neaton JB, Zheng H, Nagarajan V, Ogale SB, Liu B, Viehland D, Vaithyanathan V, Schlom DG, Waghmare UV, Spaldin NA, Rabe KM, Wuttig M, Ramesh R. Epitaxial BiFeO3 multiferroic thin film heterostructures. Science 2003; 299:1719-22. [PMID: 12637741 DOI: 10.1126/science.1080615] [Citation(s) in RCA: 962] [Impact Index Per Article: 45.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Enhancement of polarization and related properties in heteroepitaxially constrained thin films of the ferroelectromagnet, BiFeO3, is reported. Structure analysis indicates that the crystal structure of film is monoclinic in contrast to bulk, which is rhombohedral. The films display a room-temperature spontaneous polarization (50 to 60 microcoulombs per square centimeter) almost an order of magnitude higher than that of the bulk (6.1 microcoulombs per square centimeter). The observed enhancement is corroborated by first-principles calculations and found to originate from a high sensitivity of the polarization to small changes in lattice parameters. The films also exhibit enhanced thickness-dependent magnetism compared with the bulk. These enhanced and combined functional responses in thin film form present an opportunity to create and implement thin film devices that actively couple the magnetic and ferroelectric order parameters.
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Affiliation(s)
- J Wang
- Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA
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Chen M, Waghmare UV, Friend CM, Kaxiras E. A density functional study of clean and hydrogen-covered α-MoO3(010): Electronic structure and surface relaxation. J Chem Phys 1998. [DOI: 10.1063/1.477252] [Citation(s) in RCA: 146] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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Rabe KM, Waghmare UV. Localized basis for effective lattice Hamiltonians: Lattice Wannier functions. Phys Rev B Condens Matter 1995; 52:13236-13246. [PMID: 9980513 DOI: 10.1103/physrevb.52.13236] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 04/12/2023]
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