Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R35: 2D Materials - Superconductivity and Charge Density Waves III |
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Sponsoring Units: DMP Chair: Shiwei Wu, Fudan Univ Room: LACC 409B |
Thursday, March 8, 2018 8:00AM - 8:12AM |
R35.00001: Competition of strong charge and spin fluctuations in monolayer NbS2 Malte Roesner, Erik G. C. P. van Loon, Gunnar Schönhoff, Mikhail Katsnelson, Tim Wehling The interplay Coulomb and electron-phonon interactions with thermal and quantum fluctuations facilitates rich phase diagrams, especially in two-dimensional electron systems. Layered transition metal dichalcogenides hosting charge, excitonic, spin and superconducting order form an epitomic material class in this respect. However, despite intensive research a realistic understanding of the competition between these quantum phases has remained essentially unresolved. |
Thursday, March 8, 2018 8:12AM - 8:24AM |
R35.00002: Emergence of charge density waves and a pseudogap in single-layer TiTe2 Peng Chen, Woei Wu Pai, Yang-hao Chan, A Takayama, Caizhi Xu, A. Karn, S. Hasegawa, Mei-Yin Chou, Sung-Kwan Mo, Alexei Fedorov, Tai-Chang Chiang Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe2 from the single-layer to the bulk limit. Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 × 2) charge density wave order in single-layer TiTe2 with a transition temperature of 92 ± 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in two- and multi-layer TiTe2, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation. |
Thursday, March 8, 2018 8:24AM - 8:36AM |
R35.00003: The Effect of Copper Doping on Exciton Condensation in 1T-TiSe2 Measured with meV-resolution EELS Melinda Rak, Samantha Rubeck, Matteo Mitrano, Ali Husain, Anshul Kogar, Sean Vig, Goran Karapetrov, Emilia Morosan, Peter Abbamonte The physical mechanism behind the charge density wave (CDW) in 1T-TiSe2 has been a subject of ongoing debate. Using momentum-resolved electron energy-loss spectroscopy (M-EELS), we observed a soft electronic collective mode in TiSe2 at the CDW transition temperature, TCDW = 185 K, demonstrating that the CDW results from exciton condensation. As TiSe2 is doped with copper to form CuxTiSe2, TCDW steadily decreases until a doping of x ~ 0.04, above which TCDW remains approximately constant and a superconducting dome emerges. At low doping values (x ≤ 0.02), the energy of the plasmon at q = 0 decreases as temperature decreases, but for high doping values (x = 0.07, 0.109) the plasmon energy increases as temperature decreases. We also find that for x ~ 0.004, the electronic mode softens partially, but not completely, at TCDW. For x > 0.01, the electronic mode no longer softens at TCDW, indicating that copper doping suppresses the exciton condensate in TiSe2 and that the CDW in CuxTiSe2 becomes a conventional, structural Peierls transition. |
Thursday, March 8, 2018 8:36AM - 8:48AM |
R35.00004: Local Probe Studies of Electronic Structure in Monolayer TaSe2 Yi Chen, Hsin-Zon Tsai, Wei Ruan, Hyejin Ryu, Shujie Tang, Oliver Albertini, Caihong Jia, Salman Kahn, Franklin Liou, Zahid Hussain, Zhi-Xun Shen, Sung-Kwan Mo, Amy Liu, Michael Crommie Metallic transition metal dichalcogenides (TMDs) are ideal platforms for exploring collective electronic phenomena such as charge density wave (CDW) and superconductivity. Thinning down TMDs to the monolayer limit may help clarify the role of interlayer coupling effect and dimensionality in these intriguing phases. Here we present an electronic structure study of single-layer TaSe2 by means of scanning tunneling microscopy/spectroscopy, angle-resolved photoemission spectroscopy and first-principle calculation. It provides new insight into the interplay among charge order, dimensionality and electron-electron interaction in this model CDW system. |
Thursday, March 8, 2018 8:48AM - 9:00AM |
R35.00005: Probing the Charge Density Wave State in Bulk to Monolayer 2H-TaSe2 by Raman Spectroscopy Heather Hill, Sugata Chowdhury, Jeffrey Simpson, Albert Rigosi, David Newell, Helmuth Berger, Francesca Tavazza, Angela Hight Walker Tantalum diselenide (TaSe2) is a metallic, layered transition metal dichalcogenide that can be cleaved into 2D layers. Bulk 2H-TaSe2 undergoes a structural phase transition to an incommensurate charge density wave state (CDW) at 122 K and a commensurate CDW state at 90 K. The CDW amplitude modes are Raman-active and well understood. Additionally, there are previously unanalyzed Raman peaks associated with the CDW that can be observed. We report the temperature and polarization dependence of all CDW modes and the undistorted-state Raman modes of TaSe2 for 633 nm, 515 nm, and 476 nm excitation. The intensity, frequency, and width of these peaks are monitored for samples from bulk down to monolayer thickness. We also track the transition temperature of the incommensurate and commensurate CDW states as a function of thickness. Finally, our DFT calculations provide insight into the nature and behavior of the CDW Raman modes. |
Thursday, March 8, 2018 9:00AM - 9:12AM |
R35.00006: Rearrangement of the Cation (Ta) of 2H-TaSe2 in the Charge Density Wave Phase (CDW): A DFT Study Sugata Chowdhury, Heather Hill, Jeffrey Simpson, Francesca Tavazza, Angela Hight Walker Tantalum diselenide (TaSe2), a layered transition-metal dichalcogenide (TMD), is an electronically interesting material because it exhibits several phase transitions with decreasing temperature. As we lower the temperature, the metallic ground state changes first to an incommensurate charge density wave (ICDW) state at ≈ 122 K, then to a commensurate charge density wave (CCDW) state at ≈ 90 K. Using first-principles calculations (DFT), we present the structural changes, band dispersions at the Fermi level and optical properties of bulk 2H-TaSe2 as a function of electronic temperature. We show that CDW phase appears due to the in-plane displacement of Ta atoms, which leads to the formation of well-defined Ta stripes in CDW phases. Our calculations also reveal new Raman modes in the CDW phases, due to displacement of the cation (Ta) as well as to the strong electron-phonon coupling. Comparison with experiments validate our findings as they, too, observed that all these new Raman modes that are very sensitive to temperature and are not observed in the metallic phase at room-temperature. |
Thursday, March 8, 2018 9:12AM - 9:24AM |
R35.00007: Mono-, Bi-, and Tri-layer Charge Density Waves in 1T-TaS2: Direct observation without a substrate Satoshi Tanda, Zheng Liu, Kazu Suenaga, Keiji Nakatsugawa Charge-density-waves (CDW), which occur in low-dimensional systems, have a macroscopic wave function similar to superfluids and superconductors. Kosterlitz–Thouless (KT) transition is observed in superfluids and superconductors, but the presence of KT transition in ultra-thin CDW systems has been an open problem. |
Thursday, March 8, 2018 9:24AM - 9:36AM |
R35.00008: Realization of a Hidden Structural Phase in 2D TaS2 Crystals Masaro Yoshida, Jianting Ye, Yijin Zhang, Yasuhiko Imai, Shigeru Kimura, Akihiko Fujiwara, Terukazu Nishizaki, Norio Kobayashi, Masaki Nakano, Yoshihiro Iwasa Recently, structural phase or polymorph of 2D material is attracting growing interest as a controlling parameter to functionalize their physical properties. Various polymorphs can exist in transition metal dichalcogenides (TMDs), and polymorphism has significant impacts on electronic states and properties. Here, we report the discovery of an unprecedented polymorph of a TMD 2D material [1]. By mechanical exfoliation, we made thin flakes from a single crystal of 2Ha-type tantalum disulfide (TaS2), a metallic TMD with a charge-density-wave (CDW) phase. We fabricated microdevices for the flakes, and performed microbeam X-ray diffraction measurements and electrical transport measurements. Both measurement results indicate that some flakes possess a polymorph different from any one known in TaS2 bulk crystals. Moreover, the flakes with the unique polymorph displayed the dramatically enhanced CDW ordering temperature. The present results suggest the potential existence of diverse structural and electronic phases accessible only in 2D materials. [1. M. Yoshida et al. Nano Lett. 17, 5567 (2017)]. |
Thursday, March 8, 2018 9:36AM - 9:48AM |
R35.00009: Substrate Suppression of Charge-Density-Wave Phase in Monolayer 1H-TaS2 on Au Heraclitos Lefcochilos-Fogelquist, Oliver Albertini, Amy Liu Recent STM/LEED measurements of monolayer 1H-TaS2 grown on Au(111) found no charge-density wave (CDW) phase, even well below the bulk CDW transition temperature. Additionally, ARPES data suggested that the monolayer becomes n-doped by the substrate. Density-functional calculations (DFT) have found that the freestanding monolayer has a 3x3 CDW instability similar to that of the bulk, but the instability is progressively removed with electron doping. We present DFT calculations of monolayer 1H-TaS2 on Au(111) which indicate that the CDW instability is suppressed primarily by S-Au interactions beyond simple doping. Calculations find lateral and out-of-plane distortions of the TaS2 monolayer with a 7x7 periodicity dictated by the commensurate interface with Au rather than by intrinsic instabilities of the monolayer. Simulated STM images of this 7x7 distortion are consistent with experimental STM Moiré patterns. In contrast, we find the intrinsic 3x3 CDW instability to be robust in monolayer 1H-TaS2 on a graphene substrate. |
Thursday, March 8, 2018 9:48AM - 10:00AM |
R35.00010: Photocurrent imaging of charge density wave transitions in ultrathin 1T-TaS2 Tarun Patel, Bowen Yang, Hyun Ho Kim, Yu Liu, Wenjian Lu, Y.P. Sun, Adam Tsen We use scanning photocurrent microscopy to image the nearly commensurate/commensurate charge density wave transition in 1T-TaS2. Ultrathin flakes are fabricated by mechanical exfoliation and protected from oxidation with h-BN capping in an inert atmosphere. We study the effect of dimensionality and in-plane current on the first-order phase transition with spatial resolution within the diffraction limit. |
Thursday, March 8, 2018 10:00AM - 10:12AM |
R35.00011: Raman Signatures of Charge Density Waves in 2H-TaS2 Jaydeep Joshi, Heather Hill, Christos Malliakas, Utpal Chatterjee, Angela Hight Walker, Patrick Vora Many group V transition metal dichalcogenides (TMDs) exhibit quantum phase transitions from a metallic state to a charge density wave (CDW) state at low temperatures. Strong electron-phonon interactions drive this transition, and therefore knowledge of the phonon modes in the CDW state is important for understanding this phenomenon. 2H-TaS2 is known to undergo a CDW transition at ~75 K based on electrical and magnetic measurements, however studies of this transition using optical methods, especially Raman, are limited.1 We perform a comprehensive study of the metal-to-CDW transition in bulk 2H-TaS2 capped with hexagonal boron nitride (h-BN) by combining temperature, polarization, and excitation-dependent Raman spectroscopy. Using quasi-resonant excitation, we observe three optical modes associated with the CDW phase and their symmetries, of which the feature at 65 cm-1 is previously unreported. Temperature-dependent studies confirm the correlation of the mode intensities to the CDW transition and we estimate the strength of the electron-phonon coupling parameter by extracting the CDW mode frequencies and their linewidths. These results lay the foundation for future explorations of layer-dependent CDW physics in 2H-TaS2. |
Thursday, March 8, 2018 10:12AM - 10:24AM |
R35.00012: Locally Resolved Electronic Properties of the Commensurate Charge Density Wave Phase of 1T-TaS2 Adina Luican-Mayer, Yuan Zhang, Andrew DiLullo, Yang Li, Saw Hla Among the metallic TMDCs, 1T-TaS2 has a rich phase diagram due to an intriguing interplay of electron-electron / electron-phonon interactions and dimensionality. We use scanning tunnelling microscopy and spectroscopy at low temperatures to investigate local variations in the electronic properties of the commensurate charge density wave (CCDW) phase. In this talk, we discuss high-resolution tunnelling spectra that show remarkably distinct electronic states among the Ta atoms comprising the Star-of-David configuration. We reveal electronic states that are localized at the position of the CDW maxima and discuss their consequences. |
Thursday, March 8, 2018 10:24AM - 10:36AM |
R35.00013: Charge Density Wave Order Driven By Momentum Dependent Electron-Phonon Coupling In 2H-TaS2 Kapila Wijayaratne, Junjing Zhao, Christos Malliakas, Duck Young Chung, Mercouri Kanatzidis, Utpal Chatterjee Charge density waves (CDW) are exhibited by many low dimensional layered materials. However, when it comes to incommensurate CDW formation in transition metal dichalcogenides (TMD), the underlying microscopic mechanism is in controversy. Angle-resolved photoemission spectroscopy (ARPES) measurements on 2H-TaS2 show that the CDW vector is not compatible with the pertinent Fermi surface nesting vectors, ruling out the textbook picture of Peierls like Fermi surface nesting as a candidate mechanism. Pronounced many-body renormalizations were observed in the electronic dispersion, which are manifested by the presence of multiple slope changes (`kinks’). The temperature independence of the kink energies implies that a strong electron-phonon coupling is present which was further observed to be momentum-dependent. The similar results gained on 2H-TaSe2 and 2H- NbSe2 rationalize the possible universality of the momentum-anisotropic strong electron-phonon coupling of incommensurate CDW order in 2H polytype of TMDs. |
Thursday, March 8, 2018 10:36AM - 10:48AM |
R35.00014: X-Ray Absorption Near-Edge Spectroscopy of Structurally Tunable Charge Density Wave Systems Nathan Turner, Jason Leicht, James Brozik, Dimosthenis Sokaras, Thomas Kroll, Matthew Marcus, Sirine Fakra, Susan Dexheimer Mixed-valence halide-bridged transition metal linear chain (MX) complexes have proven to be excellent model systems for investigating a range of phenomena inherent to correlated electron systems owing to their structural tunability. In these materials, the nature of the ground electronic state is determined by competing electron-electron and electron-phonon interactions, which can be systematically tuned by varying the metal ion (M) and the halide ion (X) that comprise the linear chain structure. Platinum-halide materials exhibit a charge density wave ground state with fractional charge states on alternating Pt ions in the chain, together with a Peierls distortion with alternating Pt-halide bond lengths. We have carried out oriented Pt LIII XANES measurements on a series of PtX materials with X = Cl, Br, and I, in which the halide ion controls the amplitude of the charge density wave. FEFF9 modeling of the XANES spectra and LDOS shows systematic changes in orbital occupancy, reflecting variation of the amplitude of the charge density wave with coupling strength. |
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