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Spera M, Scarfato A, Pásztor Á, Giannini E, Bowler DR, Renner C. Insight into the Charge Density Wave Gap from Contrast Inversion in Topographic STM Images. Phys Rev Lett 2020; 125:267603. [PMID: 33449793 DOI: 10.1103/physrevlett.125.267603] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/29/2020] [Revised: 09/23/2020] [Accepted: 12/01/2020] [Indexed: 06/12/2023]
Abstract
Charge density waves (CDWs) are understood in great detail in one dimension, but they remain largely enigmatic in two-dimensional systems. In particular, numerous aspects of the associated energy gap and the formation mechanism are not fully understood. Two long-standing riddles are the amplitude and position of the CDW gap with respect to the Fermi level (E_{F}) and the frequent absence of CDW contrast inversion (CI) between opposite bias scanning tunneling microscopy (STM) images. Here, we find compelling evidence that these two issues are intimately related. Combining density functional theory and STM to analyze the CDW pattern and modulation amplitude in 1T-TiSe_{2}, we find that CI takes place at an unexpected negative sample bias because the CDW gap opens away from E_{F}, deep inside the valence band. This bias becomes increasingly negative as the CDW gap shifts to higher binding energy with electron doping. This study shows the importance of CI in STM images to identify periodic modulations with a CDW and to gain valuable insight into the CDW gap, whose measurement is notoriously controversial.
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Affiliation(s)
- M Spera
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
| | - A Scarfato
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
| | - Á Pásztor
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
| | - E Giannini
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
| | - D R Bowler
- London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
| | - Ch Renner
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
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Pásztor Á, Scarfato A, Renner C. Note: Mechanical in situ exfoliation of van der Waals materials. Rev Sci Instrum 2017; 88:076104. [PMID: 28764490 DOI: 10.1063/1.4993738] [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: 06/07/2023]
Abstract
Exfoliation, namely, the peeling of layered materials down to a single unit-cell thin foil, opens promising avenues to fabricate novel electronic materials. New properties and original functionalities emerge in the single and few layer configurations of a number of layered compounds, in particular in transition metal dichalcogenides. However, many of these thin exfoliated materials are very sensitive to ambient conditions impeding the exploration of this new and fascinating parameter space. Here we describe a method of mechanical exfoliation in ultra-high vacuum (UHV). This technique is easily adaptable to any UHV system and allows preparing and studying air sensitive nanoflakes in situ. We present the basic design and proof-of-concept scanning tunneling microscopy imaging of VSe2 nanoflakes.
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Affiliation(s)
- Á Pásztor
- DQMP, Université de Genève, 24 Quai Ernest Ansermet, CH-1211 Geneva, Switzerland
| | - A Scarfato
- DQMP, Université de Genève, 24 Quai Ernest Ansermet, CH-1211 Geneva, Switzerland
| | - Ch Renner
- DQMP, Université de Genève, 24 Quai Ernest Ansermet, CH-1211 Geneva, Switzerland
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Novello AM, Spera M, Scarfato A, Ubaldini A, Giannini E, Bowler DR, Renner C. Stripe and Short Range Order in the Charge Density Wave of 1T-Cu_{x}TiSe_{2}. Phys Rev Lett 2017; 118:017002. [PMID: 28106462 DOI: 10.1103/physrevlett.118.017002] [Citation(s) in RCA: 8] [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: 09/22/2016] [Indexed: 06/06/2023]
Abstract
We study the impact of Cu intercalation on the charge density wave (CDW) in 1T-Cu_{x}TiSe_{2} by scanning tunneling microscopy and spectroscopy. Cu atoms, identified through density functional theory modeling, are found to intercalate randomly on the octahedral site in the van der Waals gap and to dope delocalized electrons near the Fermi level. While the CDW modulation period does not depend on Cu content, we observe the formation of charge stripe domains at low Cu content (x<0.02) and a breaking up of the commensurate order into 2×2 domains at higher Cu content. The latter shrink with increasing Cu concentration and tend to be phase shifted. These findings invalidate a proposed excitonic pairing as the primary CDW formation mechanism in this material.
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Affiliation(s)
- A M Novello
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
| | - M Spera
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
| | - A Scarfato
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
| | - A Ubaldini
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
| | - E Giannini
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
| | - D R Bowler
- London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
| | - Ch Renner
- Department of Quantum Matter Physics, University of Geneva, 24 Quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland
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Hildebrand B, Didiot C, Novello AM, Monney G, Scarfato A, Ubaldini A, Berger H, Bowler DR, Renner C, Aebi P. Doping nature of native defects in 1T-TiSe2. Phys Rev Lett 2014; 112:197001. [PMID: 24877961 DOI: 10.1103/physrevlett.112.197001] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/19/2014] [Indexed: 06/03/2023]
Abstract
The transition-metal dichalcogenide 1T-TiSe2 is a quasi-two-dimensional layered material with a charge density wave (CDW) transition temperature of T(CDW) ≈ 200 K. Self-doping effects for crystals grown at different temperatures introduce structural defects, modify the temperature-dependent resistivity, and strongly perturbate the CDW phase. Here, we study the structural and doping nature of such native defects combining scanning tunneling microscopy or spectroscopy and ab initio calculations. The dominant native single atom dopants we identify in our single crystals are intercalated Ti atoms, Se vacancies, and Se substitutions by residual iodine and oxygen.
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Affiliation(s)
- B Hildebrand
- Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, CH-1700 Fribourg, Switzerland
| | - C Didiot
- Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, CH-1700 Fribourg, Switzerland
| | - A M Novello
- Département de Physique de la Matière Condensée, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
| | - G Monney
- Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, CH-1700 Fribourg, Switzerland
| | - A Scarfato
- Département de Physique de la Matière Condensée, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
| | - A Ubaldini
- Département de Physique de la Matière Condensée, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
| | - H Berger
- Institut de Génie Atomique, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
| | - D R Bowler
- London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
| | - C Renner
- Département de Physique de la Matière Condensée, University of Geneva, 24 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
| | - P Aebi
- Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, CH-1700 Fribourg, Switzerland
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Giubileo F, Piano S, Scarfato A, Bobba F, Di Bartolomeo A, Cucolo AM. A tunneling spectroscopy study of the pairing symmetry in the electron-doped Pr(1-x)LaCe(x)CuO(4-y). J Phys Condens Matter 2010; 22:045702. [PMID: 21386321 DOI: 10.1088/0953-8984/22/4/045702] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [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 have performed scanning tunneling spectroscopy and point contact spectroscopy measurements on the electron-doped superconductor Pr(1-x)LaCe(x)CuO(4-y) (x = 0.12, T(c) is approximately equal 25 K). We address the question of the symmetry of the order parameter and of the amplitude of the energy gap. We compare three possible scenarios, i.e. isotropic s-wave, 'anisotropic' s-wave, and d-wave. Evidence for a d-wave symmetry of the order parameter is given. From the temperature evolution of the dI/dV versus V characteristics we extract a BCS-like temperature dependence of the superconducting energy gap Δ. Despite the variety of measured spectra we give a consistent explanation for the whole set of data, indicating Δ = (3.6 ± 0.2) meV and a ratio 2Δ/K(B)T(C) is approximately equal 3.5 ±0.2. In particular, point contact characteristics showing gap-like features at higher voltages have been interpreted by considering the formation of an intergrain Josephson junction in series with the point contact junction. Further confirmation of the correctness of the model is given by the behavior of the critical current of the intergrain Josephson junction versus temperature which follows the Ambegaokar-Baratoff behavior.
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Affiliation(s)
- F Giubileo
- CNR-INFM Laboratorio Regionale SUPERMAT, and Dipartimento di Fisica E.R. Caianiello, Università di Salerno, Baronissi (SA), Italy.
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Piano S, De Santis A, Bobba F, Giubileo F, Longobardi M, Di Bartolomeo A, Polichetti M, Scarfato A, Zola D, Vecchione A, Cucolo AM. Structural, electrical and magnetic characterization of artificial ferromagnetic/superconducting (La(0.7)Ca(0.3)MnO(3)/YBa(2)Cu(3)O(7-x)) heterostructures. J Phys Condens Matter 2009; 21:254205. [PMID: 21828429 DOI: 10.1088/0953-8984/21/25/254205] [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: 05/31/2023]
Abstract
The fabrication and characterization of superconducting and ferromagnetic heterostructures is an open field due to the fundamental interest in the physics of the coexistence of these two competing orders and their possible applications in the spintronics industry. In this paper we present structural, electrical and magnetic characterization for the single La(0.7)Ca(0.3)MnO(3) (LCMO) thin layer, La(0.7)Ca(0.3)MnO(3)/YBa(2)Cu(3)O(7-x) (LCMO/YBCO) bilayers and the LCMO/YBCO/LCMO trilayers. In particular, we show a detailed magnetic characterization of the LCMO thin films by means of low temperature magnetic force microscopy. We discuss the different dynamics of the magnetic domains observed, depending on the substrate induced strain and on the film thickness.
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Affiliation(s)
- S Piano
- Physics Department 'E R Caianiello', University of Salerno, Via S. Allende, 84081 Baronissi (SA), Italy. Laboratorio Regionale SUPERMAT, CNR-INFM, Via S. Allende, 84081 Baronissi (SA), Italy. School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK
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