Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A14: 2D Materials (Metals, Superconductors, and Correlated Materials) -- CDWsFocus
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Sponsoring Units: DMP DCOMP Chair: Daniel Rhodes, Columbia Univ Room: BCEC 153C |
Monday, March 4, 2019 8:00AM - 8:12AM |
A14.00001: Orbital- and kz-selective hybridisation of Se 4p and Ti 3d states at the CDW transition of TiSe2 Matthew Watson, Oliver J Clark, Federico Mazzola, Igor Marković, Veronika Sunko, Timur Kim, Kai Rossnagel, Philip King We revisit the enduring problem of the 2×2×2 charge density wave (CDW) order in TiSe2, utilising photon energy-dependent angle-resolved photoemission spectroscopy (ARPES) to probe the full three-dimensional high- and low-temperature electronic structure. Our measurements demonstrate how a mismatch of dimensionality between the 3D conduction bands and the quasi-2D valence bands in this system leads to a hybridisation that is strongly kz-dependent. This 3D momentum-selective coupling shifts the strongly hybridized states well away from the Fermi level, providing the energy gain required to form the CDW. However, we show how additional ``passenger'' states remain, and dominate the low energy physics in the ordered state. In particular we find that a branch of the conduction band with 3dz2 character remains essentially unhybridised in the ordered phase, forming a coherent and ungapped electron-like Fermi surface. We conclude by making a comparison to the 2x2 CDW of monolayer TiSe2, where kz-selective effects are absent, but orbital-selective hybridisation persists. [1] Watson et al., arXiv:1808.07141 (2018). |
Monday, March 4, 2019 8:12AM - 8:24AM |
A14.00002: Pairing induced by CDW fluctuations: the case of superconductivity in TiSe2 Chuan Chen, Antonio Helio Castro Neto, Vitor Pereira We theoretically studied the modes and spectrum of fluctuations of the CDW in TiSe2 using a model that describes its CDW state as a result of the intrinsic excitonic instability in TiSe2. The charge carriers are electrons within the CDW-renormalized conduction band (CDW quasiparticles). A calculation of the effective action for the CDW fluctuations shows the presence of gapless phase modes and gapped amplitude modes. Upon integrating out these fluctuations, we find that the amplitude fluctuations induce attractive pairing between the CDW quasiparticles resulting in s-wave superconductivity. A subsequent mean-field phase diagram of both CDW and superconducting critical temperatures as a function of doping reveals a dome-shaped superconducting phase coexisting with CDW in the near-commensurate regime, in agreement with the experimental phase diagram. |
Monday, March 4, 2019 8:24AM - 8:36AM |
A14.00003: Electronic properties and charge density wave transition in single-layer VSe2 Kien Nguyen-Cong, Paula Coelho Neto, Matthias Batzill, Ivan Oleynik Single-layer VSe2 has been recently attracted attention due to experimental observations of ferromagnetism and charge density wave (CDW) transition. There are controversies from both theory and experiment concerning ferromagnetism in both bulk and single layer VSe2. In addition, CDW transition in VSe2 is not well understood. In this work, structural, electronic, magnetic and CDW properties of this system are investigated using first-principle calculations. The calculated electronic structure is compared with recent APPRES measurements and conclusions concerning its magnetic state are made. The calculated phonon spectra are used in investigation of CDW transition mechanism. Crystal structure of the CDW state is determined using the evolutionary crystal structure prediction combined with lattice dynamics. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A14.00004: Unique gap structure and symmetry of the charge density wave in single-layer VSe2 Peng Chen, Woei Wu Pai, Yang-hao Chan, Vidya Madhavan, Mei-Yin Chou, Sung-Kwan Mo, Alexei V Fedorov, Tai-Chang Chiang
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Monday, March 4, 2019 8:48AM - 9:00AM |
A14.00005: Metal-Insulator Transition and High Temperature Charge Density Waves in monolayer VSe2 Duvjir Ganbat, Byoung Ki Choi, Iksu Jang, Soren Ulstrup, Soonmin Kang, Ly Trinh, Sanghwa Kim, Young Hwan Choi, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Je-Guen Park, Raman Sankar, Ki-Seok Kim, Jungdae Kim, Young Jun Chang Although emergent phenomena driven by electronic reconstructions in oxide heterostructures have been intensively discussed, such interface-driven phenomena in shaping the electronic properties has not been well established in van der Waals heterointerfaces. By diminishing the material thickness and forming a heterointerface, we observed two types of charge-ordering transitions in monolayer VSe2 on graphene substrates. Comprehensive combination of ARPES, STM, and renormalization group analysis enable us to reveal the low-dimensionality and the heterointerface play important roles in enhancing charge density wave temperature to 350 K contrasted to the 105 K in bulk VSe2 and in driving the unexpected metal-insulator transition at 135 K in the family of monolayered transition metal dichalcogenides. We will discuss implications of our observations in comparison with the similar results reported by two other groups. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A14.00006: General and specific features of the charge density waves in transition metal dichalcogenides Kapila Wijayaratne, Junjing Zhao, Christos Malliakas, Duck Young Chung, Genda Gu, Mercouri Kanatzidis, Jasper Van Wezel, Utpal Chatterjee We report a series of elaborate Angle-Resolved Photoemission Spectroscopy (ARPES) studies on the charge density wave (CDW) phase in transition metal dichalcogenides (TMD) 2H-TaS2, TaSe2 and NbSe2, analyzed above/ below the CDW transition temperature (TCDW). Generally; i) On Fermi surface, CDW energy gap appears only in the vicinity of some symmetry points, while other momentum points are ungapped. ii) Gap is asymmetric in particles and holes. iii) Gap existed above TCDW, beyond the CDW phase, (pseudogap). iv) Conventional Fermi surface nesting model is incapable of describing the CDWs in TMDs. v) phonon related band renormalizations were observed In 2H-TaS. Compound-specific features, due to different orbital orientations were; i) Size variation of some Fermi pockets are related to TCDW while other pockets were similar among compounds. ii) About a given symmetry point, in 2H-NbSe2 the gap size varied upon momentum in contrast to 2H-TaS2, where this variation was insignificant. A tight binding model that includes orbital selectivity and electron-phonon coupling can explain both general and specific features and might be extendable to a broad set of two-dimensional materials with CDW order, including cuprates. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A14.00007: Ion intercalation tuned quantum transport, charge density waves, and superconductivity in atomically thin crystals of 2H-TaSe2 Yueshen Wu, Chao-Sheng Lian, Hui Xing, Chao Yao, Duan Wenhui, Jinyu Liu, Zhiqiang Mao, Ying Liu Quantum transport in an electronic system dealing with the quantum interference of diffusive electrons is influenced strongly by spin-orbital and electron-phonon interactions. Two-dimensional (2D) crystals of transition metal chalcogenides (TMCs) were found recently to show interesting behavior in quantum transport, which can be tuned, for example, by electric field effects. However, ion intercalation has not been used to tune quantum transport involving weak localization or weak antilocalization in 2D TMCs. We carried out magneto electrical transport measurements on atomically thin crystals of 2H-TaSe2 intercalated by Li ions controlled by ionic gating. Gate voltage and temperature tuned crossovers from weak antilocalization to weak localization were observed. Meanwhile, ion intercalation was found to suppress charge density waves and enhance superconductivity at the same time. Band structure and phonon spectrum calculations show that the electronic density of states and the number of acoustic phonons available for scattering are reduced. Our observations suggest that the ion intercalation leads to an enhancement of the electron-phonon interaction and a simultaneous reduction in spin-orbital scattering. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A14.00008: Atomic-scale visualization of the low temperature charge density wave phase in 1T’-TaTe2 Ismail El Baggari, Nikhil Sivadas, Gregory Stiehl, Daniel Ralph, Craig J Fennie, Lena F Kourkoutis Many transition-metal dichalcogenides, such as TaS2 and NbSe2, exhibit charge density wave (CDW) instabilities which, by coupling to the lattice, induce complex periodic lattice displacement patterns. Tellurium-based TMDs further exhibit large electronic and structural anisotropy and significant interlayer correlations. Here, we use cryogenic scanning transmission electron microscopy (STEM) to probe in real space the high temperature (HT) and low temperature (LT) CDW phases in metallic 1T’-TaTe2. We visualize Ta trimer states at room temperature and observe below the CDW transition temperature (Tc ~ 170 K) the formation of a superstructure arising from complex, periodic intensity modulations in STEM data. The modulations reflect longitudinal periodic lattice displacements that stack in a staggered fashion between the layers. Cross-sectional imaging, density functional theory calculations, and multislice image simulations further elucidate the nature of the HT and LT states. These atomically-resolved measurements reveal the complex role of lattice degrees of freedom in CDW transitions. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A14.00009: First principles description of Charge Density Waves in single-layer TiSe2 and TiTe2. Bogdan Guster, Roberto Robles, Miguel Pruneda, Enric Canadell, Pablo Ordejon We present some of our recent work on the understanding of the appearance of Charge Density Waves (CDW) in single-layer 2D transition metal dichalcogenides by means of first principles electronic structure calculations. This is done in connection with recent experimental studies that have been able to demonstrate the presence of CDS in several 2D single-layer materials like TiSe2 and TiTe2. The evolution of the CDW with external electrostatic doping, which has been achieved experimentally using field effect transistor setups, will be analyzed for the case of TiSe2 [1]. For TiTe2, we focus on the recently observed CDW in the single layer, which is not present in the bulk material [2]. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A14.00010: Spectroscopic signatures of many-body renormalizations in 1T-TiSe2 Utpal Chatterjee, Nandini Trivedi, Kyungmin Lee, Goran Karapetrov, Junjing Zhao We have investigated the many-body renormalization of the single-particle excitations in 1T-TiSe2by employing high resolution angle resolved photoemission spectroscopy (ARPES) measurements. The energy distribution curves of the ARPES data reveal an intrinsic peak-dip-hump feature. The electronic dispersion extracted from the momentum distribution curves highlights, for the first time, multiple kink structures. These are canonical signatures of coupling between the electronic degrees of freedom and bosonic modes in the system. Theoretical modeling of electrons coupled to an Einstein mode provides insight into the peak-dip-hump features observed in the data. From a self- energy analysis of our ARPES data, we find a bosonic mode at 26 meV that correlates with ab-initio phonon-dispersion calculations and observation of Raman active shear (Eg) mode in Raman scattering experiments. The direct observation of band-renormalization due to such electron-boson coupling suggests that such modes could be important for driving charge density wave (CDW) and superconductivity in 1T-TiSe2 like in many other transition metal dichalcogenides. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A14.00011: Charge density wave order in a transition metal dichalcogenide probed by second harmonic generation Bryan Fichera, Anshul Kogar, Linda Ye, Joseph Checkelsky, Nuh Gedik
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Monday, March 4, 2019 10:12AM - 10:24AM |
A14.00012: Dynamical slowing down in an ultrafast photo-induced phase transition Alfred Zong, Anshul Kogar, Pavel E Dolgirev, Emre Ergecen, Mehmet B Yilmaz, Ya-Qing Bie, Timm Rohwer, I-Cheng Tung, Joshua Straquadine, Philip Walmsley, Xiaozhe Shen, Jie Yang, Renkai Li, Suji Park, Matthias C Hoffmann, Benjamin K Ofori-Okai, Michael E Kozina, Haidan Wen, Xijie Wang, Ian R Fisher, Pablo Jarillo-Herrero, Nuh Gedik Complex systems, which consist of a large number of interacting constituents, often exhibit universal behavior near a phase transition. A slowdown of certain dynamical observables is one such recurring feature found in a vast array of contexts. This phenomenon, known as critical slowing down, is well studied mostly in thermodynamic phase transitions. However, it is less understood in highly nonequilibrium settings, where the time it takes to traverse the phase boundary becomes comparable to the timescale of dynamical fluctuations. Using transient optical spectroscopy and femtosecond electron diffraction, we studied a photo-induced transition of a model charge-density-wave (CDW) compound, LaTe3. We observed that it takes the longest time to suppress the order parameter at the threshold photoexcitation density, where the CDW transiently vanishes. This finding can be quantitatively captured by generalizing the time-dependent Landau theory to a system far from equilibrium. The experimental observation and theoretical understanding of dynamical slowing down may offer insight into other general principles behind nonequilibrium phase transitions in many-body systems. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A14.00013: Superconductivity in Weyl semimetal MoxW1-xTe2 driven by high pressure Rabin Dahal, Liangzi Deng, Narayan Poudel, Melissa Gooch, Zheng Wu, Ching-Wu Chu Tungsten ditelluride and molybdenum ditelluride have attracted tremendous attention due to recent discoveries of the Weyl semimetal state, large unsaturated magnetoresistance, and superconductivity in these materials. MoxW1-xTe2 is reported to be a Weyl semimetal with x = 0.25 and Weyl semimetal states are also predicted to exist in other doping. We carried out systematic high pressure measurements on MoxW1-xTe2 single crystals with doping via homemade BeCu clamp cells. High-quality single crystals of MoxW1-xTe2, with x = 0.25, 0.60, 0.90, were grown by chemical vapor transport method which were then characterized by X-ray diffraction and energy dispersive X-ray spectroscopy. In Mo0.25W0.75Te2, a structural transition was observed under pressure 12.3 kbar with no signature of superconductivity at temperature down to 1.3 K and under pressure up to 17 kbar. In Mo0.60W0.40Te2 and Mo0.90W0.10Te2, structural transitions were observed at ambient pressure. Superconductivity appears in Mo0.60W0.40Te2 with an onset Tc of 1.7 K under 8.4 kbar and in Mo0.90W0.10Te2 with an onset Tc of 1.4 K under 3.9 kbar. Tc continuously increases as pressure increases up to 17 kbar, the highest pressure we applied. Measurements under higher pressure range using diamond anvil cells are in progress. |
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