Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J2: Focus Session: Beyond Graphene - Strongly Correlated Phenomena |
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Sponsoring Units: DMP Chair: Kin Fai Mak, Pennsylvania State University Room: 001B |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J2.00001: Control of Metastable Charge Density Wave Phases in Ultrathin 1T-TaS$_{2}$ Adam Tsen, Robert Hovden, Dennis Wang, Young Duck Kim, Yu Liu, Wenjian Lu, Yuping Sun, James Hone, Lena Fitting Kourkoutis, Philip Kim, Abhay Pasupathy Among the most intriguing aspects of reduced dimensionality in condensed matter systems is the enhancement of various correlation effects (electron-electron, electron-phonon, etc.). In quasi-2D metallic chalcogenides, they lead to electronic instabilities that give rise to a wealth of exotic ground states such as charge density waves (CDWs), spin density waves, and superconductivity. 1T-TaS$_{2}$ is a unique layered material which exhibits a number of different CDW states as well as a Mott phase at low temperatures. Although its electronic structure is largely two dimensional, the CDWs are stabilized by an out-of-plane stacking. By combining low temperature transmission electron microscopy with electrical transport measurements, we investigate how the various CDW phases in 1T-TaS$_{2}$ change as it approaches the physical 2D limit. We find that in well-controlled samples, the lock-in transition from a nearly commensurate CDW to a fully commensurate CDW gradually disappears with reduced thickness as both phases become increasingly metastable. I will discuss the physical reasons underlying this behavior as well as demonstrate how to manipulate this phase transition in few-layer samples by application of an in-plane electric field. [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J2.00002: Tantalum Disulfide Ionic Field-Effect Transistors Yijun Yu, Fangyuan Yang, Xiu Fang Lu, Ya Jun Yan, Y. H. Cho, Liguo Ma, Xiaohai Niu, Sejoong Kim, Yong-Woo Son, Donglai Feng, Shiyan Li, Sang-Wook Cheong, Xian Hui Chen, Yuanbo Zhang The ability to tune material properties using gate electric field is at the heart of the modern electronic technology. Electrolyte gating has recently emerged as an important technique to reach extremely high surface charge carrier concentration in a variety of materials through the formation of electric double layer (EDL) at the sample surface. Here we demonstrate a new mechanism of electrolyte gating that modulates the volumetric carrier density by gate-controlled intercalation in layered materials. We fabricate field-effect transistors (referred to as ionic field-effect transistor, iFET) based on transition metal dichalcogenides 1T-TaS$_2$ and 2H-TaS$_2$. The unprecedented large doping induces dramatic changes in the transport properties of the sample, including CDW phase transitions, superconductivity and metal-to-insulator transitions. The controllable and reversible intercalation of different ion spices into layered materials opens up new possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit. [Preview Abstract] |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J2.00003: Scanning Tunneling Microscopy and Spectroscopy of the commensurate charge density wave phase of 1T-TaS$_{2}$ Adina Luican-Mayer, Andrew DiLullo, Yang Li, Saw Wai-Hla The 1T polymorph of TaS$_{2}$, 1T-TaS$_{2}$, has one of the richest phase diagrams among the transition metal dichalcogenides: It is metallic at higher temperatures; it has four temperature-dependent charge density wave (CDW) phases with different structures; at low temperatures it shows Mott insulator behavior and under pressure and doping it becomes superconducting. In this talk we focus on the low temperature commensurate charge density wave phase. Using scanning tunneling microscopy and spectroscopy, we explore the spatial variation of the electronic properties of the commensurate CDW phase at the atomic level. The role that defects play in the formation of this phase will also be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:42PM |
J2.00004: Gate-tunable phase transitions in thin flakes of 1T-TaS$_{2}$ Invited Speaker: Yuanbo Zhang The competition, coexistence and cooperation of various collectively ordered electronic states in two-dimensional (2D) systems has been breeding ground for novel states of matter. Controllable manipulation of the various phases through gate-tunable charge doping may lead to new device paradigm for future material science and technology. We develop a new doping method that utilizes a gate electric field to drive lithium ions in and out of the layered 1T-TaS$_{2}$, and introduces high doping levels in each atomic layer. This is realized in a device, referred to as ionic field-effect transistor (iFET), that controls the electronic properties of a layered material (1T-TaS$_{2}$ in our case) by gate-controlled intercalation. In this talk, we explore previously inaccessible parameter space in 1T-TaS$_{2}$ based iFETs following two pathways: i) reducing the dimensionality of 1T-TaS$_{2}$ by thinning it down to a few atomic layers, and ii) doping it by gate-controlled intercalation. Our complete phase diagram reveals the importance of dimensionality and gate-controlled ionic doping in layered atomic crystals, and provides fresh insights into the relation between superconducting phase and various other charge-ordered phases. [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J2.00005: New STM Tip-induced Phases in 1T-TaS$_2$ Liguo Ma, Yijun Yu, Xiu Fang Lu, Ya Jun Yan, Y.H. Cho, Sang-Wook Cheong, Xian Hui Chen, Yuanbo Zhang Transition metal dichalcogenide 1T-TaS$_2$ is a layered material featuring a unique set of charge density wave (CDW) phases. The close proximity of the CDW phases in energy makes the material prone to external perturbation, and the intricate electron-phonon and electron-electron interactions often lead to electronic/structural phase transitions in 1T-TaS$_2$. Here we report a new phase transition from the insulating commensurate CDW (also known as a Mott state) to a new metallic CDW state that is induced in 1T-TaS$_2$ by voltage pulses from an STM tip at low temperatures. We study the topographic and spectroscopic properties of the metallic CDW phase in detail with STM and Scanning Tunneling Spectroscopy (STS). [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J2.00006: Controlling non-equilibrium CDW states in 1T-TaS$_{2}$ nano-thick crystals Masaro Yoshida, Yijin Zhang, Ryuji Suzuki, Jianting Ye, Yoshihiro Iwasa Two-dimensional (2D) crystals provide an ideal platform for exotic electronic band structures in mono- or multi-layer forms. The thinning to nanoscale may also affect collective phenomena in interacting electron systems and can lead to unconventional states that are dramatically different from those in bulk. In this presentation, we report the systematic control of charge-density-wave (CDW) transitions by changing thickness and cooling rate in nano-thick crystals of 1T-type tantalum disulfide (1T-TaS$_{2}$). First, we discovered a new super-cooled nearly-commensurate CDW state, which shows metallic behavior at low temperatures. Furthermore, we achieved current-induced switching between various CDW states. The glassy behavior and non-linear response, possibly due to the reduced dimensionality, manifest the emergent complex nature of correlated electrons in 2D crystals with nanometer thickness. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J2.00007: Effects of Dimensionality on the Charge-Density Wave Phases of Transition-Metal Dichalcogenides Danilo Romero, Jeffrey Simpson, Helmuth Berger, Angela Hight-Walker We investigate the effects of dimensionality on the electronic properties of the transition-metal dichalcogenides \textit{2H}-TaSe$_{\mathrm{2}}$, \textit{1T}-TaSe$_{\mathrm{2}}$, and \textit{1T}-TaS$_{\mathrm{2}}$. In bulk, these materials exhibit various commensurate and incommensurate charge-density wave (CDW) phase transitions that are currently attracting interest for possible CDW field-effect device applications. We explore the evolution of the CDW groundstate properties as the materials approach a few monolayers, achieved via mechanical exfoliation of bulk single-crystals. Raman spectroscopy of \textit{2H}-TaSe$_{\mathrm{2}}$ and \textit{1T}-TaSe$_{\mathrm{2}}$, and \textit{1T}-TaS$_{\mathrm{2}}$, carried out over a wide-range of temperatures, was used as probe of the change in the lattice dynamics from the bulk to the single-layer phases of these materials. The effect of dimensionality on the CDW transition temperatures will be presented. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J2.00008: Ultrafast Two-Pulse Photocurrent Correlation Measurements of Single Atomic Layer MoS2 Photodetectors Haining Wang, Changjian Zhang, Wei-min Chan, Okan Koksal, Sandip Tiwari, Farhan Rana We present, for the first time, results from ultrafast measurements of carrier transport and carrier dynamics in monolayer MoS2 photodetectors by time resolved two-pulse photocurrent correlation technique [1]. The photocurrent transient contains information on the recombination dynamics and transport physics of the photoexcited carriers and excitons. The measured photocurrent correlation data shows two distinct decay time constants: one fast around 5 ps and one much slower around 100 ps. The observed dynamics are largely independent of temperature (10K to 300K) and pump fluence (1 to 16 $\mu$J/cm$^2$). The fast decay is attributed to the fast recombination of the photoexcited carriers rather than to the transport and extraction of the photoexcited carriers from the device. The decay time scales, the temperature and the pump fluence dependence of the observed dynamics are in good agreement with defect-assisted carrier recombination model via Auger scattering. The observed time scales also agree well with our all-optical pump-probe studies. The strong Coulomb interactions and resulting strong electron-hole correlations in monolayer MoS2, make carrier and exciton capture by defects the dominant carrier recombination mechanism.\\[4pt] [1] M. W. Graham, P. L. McEuen, Nat. Phys. 9, 103 (2013) [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J2.00009: Proximity-induced superconductivity in transition metal dichalcogenides Driss M. Badiane, Christopher Triola, E. Rossi In this work we study the proximity induced superconductivity in a monolayer of the transition metal MoS$_2$ placed on top of a superconducting substrate. We investigate the symmetries of the proximity-induced superconducting pairing amplitude and we find that superconducting substrates with spin-orbit coupling can induce odd-frequency pairing in the MoS$_2$ monolayer. We discuss the relevant experimental signatures of the proximity-induced superconducting phase in the MoS$_2$ monolayer. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J2.00010: In search of single layer superconductivity in semiconducting transition metal dichalcogenides Efren Navarro-Moratalla, Yafang Yang, Hugh Churchill, Pablo Jarillo-Herrero Charge carrier density control is a keystone in the study of 2D semiconductors. The use of ionic liquids as gate electrodes gives rise to the formation of high capacitance electrical double layers (EDLs) that permit exploring very high carrier density regimes ($n \quad \approx $ 10$^{\mathrm{15}}$ cm$^{\mathrm{-2}})$, opening the door for the study of field-induced correlated states, such as ferromagnetism or superconductivity. Though pioneering works on transition metal dichalcogenides have provided with proof of the use of EDLs for the induction of superconductivity in bulk crystals or in the surface of thick flakes, no reports of single layer superconductivity have been put forward. We take advantage of crystal growth techniques, the EDL approach, the wide range of metal dichalcogenides and the van der Waals stacking to fabricate ultraflat samples that will permit exploring the high carrier density regime in search for switchable single layer superconductivity. The use of a liquid gate opens the possibility of studying the effect that strain or even the presence of molecular species may have in the superconducting state. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J2.00011: Electronic and magnetic properties of NbSe$_{2}$ monolayer doped vacancy and transition metal atoms Priyanka Manchanda, David Sellmyer, Ralph Skomski Two-dimensional transition-metal dichalcogenides (2D TMDs) have attracted much attention recently due to potential applications including optoelectronic devices. Atomically thin layers of materials such as MoS$_{2}$, WS$_{2}$, NbS$_{2}$, NbSe$_{2}$, TaTe$_{2}$ can easily be synthesized by exfoliation techniques and exhibit variety electronic phases such as metal, semiconductor, superconductor depending on the choice of metal. Most of the TMDs are nonmagnetic and various techniques have been proposed to induce or modulate magnetic properties that are essential for nanoelectronic device applications. We use DFT calculations to analyze the effect of strain, hydrogen adsorption, and doping. Emphasis is on the magnetic properties of NbSe$_{2}$ monolayers containing vacancies and 3$d$ transition metal atoms. We find that magnetism can be induced by vacancy creation and transition metal-substitution in NbSe$_{2}$, with effects similar to strain and hydrogen adsorption. The moment mainly arises from the localized nonbonding 3d electrons of the transition-metal atoms. Our findings contribute to the ongoing search ``for-better-than-graphene'' thin-film materials for novel electronic devices. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J2.00012: Superconductivity Series of Ion-gated Transition Metal Dichalcogenides Wu Shi, Jianting Ye, Yijing Zhang, Ryuji Suzuki, Masaro Yoshida, Naoko Inoue, Yu Saito, Yoshihiro Iwasa Semiconducting transition metal dichalcogenides (TMDs) have attracted considerable interest as typical two-dimensional (2D) materials. By mechanical cleavage, atomically flat and chemically stable thin flakes of TMDs can be readily obtained from bulk crystals. Recently, coupling with high efficient ionic media, TMD thin flakes have exhibited extraordinary electronic and opto-valleytronic properties in the form of electrical double layer transistors (EDLTs). The introduction of high-density carriers have also induced metal-insulator transition and superconductivity in MoS2, revealing an enhanced Tc and a dome-like phase diagram that are inaccessible through conventional chemically doping. In this work, we report the discovery of a superconductivity series based on a further exploration of other available semiconducting TMDs (MoSe2, MoTe2, WS2, WSe2) by using different ionic media. The present results not only complement important superconducting properties in TMDs, but also suggest a close correlation between transistor operation and the possibility of inducing superconductivity, providing general guidelines for the optimization of charge accumulation and the induction of superconductivity in other material series via ionic gating. [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J2.00013: Synthesis and Characterization of $\beta$ Nickel Niobate Timothy Munsie, Anna Millington, Hanna Dabkowska, Jim Britten, Graeme Luke Members of the niobate family (ANb$_2$O$_6$, A=Ni, Co, Fe, Mn) are known to crystallize in the columbite structure with zig-zag chains of the metallic and typically magnetic cation, giving rise to a quasi-one-dimensional magnetic system. In our attempts to synthesize NiNb$_2$O$_6$ in its columbite structure, we discovered a previously unreported allotrope, $\beta$-NiNb$_2$O$_6$, with a completely different crystalline structure, magnetic environment and magnetic properties. This talk will discuss the difficulties with respect to synthesis via the optical floating zone (OFZ) technique, the results of our structural refinement utilizing single crystal x-ray diffraction, and both magnetic and transport measurements of this materials physical properties. [Preview Abstract] |
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