Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y35: 2D Materials - Superconductivity and Ferroelectricity |
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Sponsoring Units: DMP Chair: Amy Liu, Georgetown Univ Room: LACC 409B |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y35.00001: Thermoelectric Effect in Ultrathin FeSe Sunao Shimizu, Shiogai Junichi, Nayuta Takemori, Shiro Sakai, Hiroaki Ikeda, Ryotaro Arita, Tsutomu Nojima, Atsushi Tsukazaki, Yoshihiro Iwasa The enhancement of the superconducting transition temperature Tc in monolayer FeSe/SrTiO3 has been a central issue in 2D materials research in the last few years. However, the experimental investigation of monolayer FeSe has been limited to STM, ARPES, and electrical resistivity measurements due to the sensitivity to the air. Here we report the thermoelectric measurements in ultrathin films of FeSe through the in-situ electrochemical etching technique. The thickness of the FeSe films is controllable in ionic liquid media and can be reduced to monolayer [J. Shiogai et al., Nature Phys. 12, 42 (2016)]. With thinning the thin film with 18 nm thickness, Tc was enhanced up to ~40 K, and the anomalous enhancement of the Seebeck coefficient was observed. The thickness and the temperature dependence of the thermoelectric properties in FeSe will be discussed in detail. |
Friday, March 9, 2018 11:27AM - 11:39AM |
Y35.00002: Suppression of high temperature superconductivity in single layer FeSe nanoribbons Zhuozhi Ge, Chenhui Yan, Michael Weinert, Lian Li At reduced dimensions, there exists additional characteristic length scales below which superconductivity can be suppressed. The determination of those critical parameters is of fundamental importance in understanding Cooper pairing mechanisms, and the application of these materials in nanoscale superconducting devices. Here, single layer FeSe nanoribbons of well-defined widths from a few to tens of nanometers are prepared by molecular beam epitaxy on SrTiO3(001) substrates. Scanning tunneling spectroscopy reveals a superconducting gap of up to 20 meV, and a critical width of 7.2 nm below which superconductivity is suppressed. Those findings and possible mechanisms for this suppression will be discussed at the meeting. |
Friday, March 9, 2018 11:39AM - 11:51AM |
Y35.00003: A universal trend in Hall coefficient and superconducting transition temperature in FeSe electric double layer transistors Shiogai Junichi, Tomoki Miyakawa, Takayuki Harada, Tsutomu Nojima, Atsushi Tsukazaki Since the discovery of high-Tc superconductivity in monolayer FeSe on SrTiO3 [1], tremendous efforts have been addressed onto exploration of electronic origin for the high-Tc superconductivity using in-situ spectroscopic measurements. We have recently demonstrated ex-situ electrical transport measurements of the high-Tc FeSe with detailed thickness dependences using electrochemical etching technique in electric double layer transistor (EDLT) configuration [2]. In this talk, we will present a systematic thickness dependence of superconducting properties of FeSe-EDLT deposited on various oxide substrates. We found the universal relationship between the Hall coefficient and Tc, pointing out the importance of charge balance between electron and hole concentrations for emergence of high-Tc at 40 K [3]. We will also report on thickness dependences of the upper critical magnetic field and critical current density, both of which sharply increase towards thin film limit. The large values of the critical parameters clearly indicate the robustness of the high-Tc state in FeSe ultrathin films [1] Q. Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012). [2] J. Shiogai et al., Nature Phys. 12, 42 (2016). [3] J. Shiogai et al., Phys. Rev. B 95, 115101 (2017). |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y35.00004: Topological superconductivity in chromium dioxide heterostructures Xu Dou, Kangjun Seo, Bruno Uchoa Heterostructures of chromium dioxide and titanium dioxide were recently predicted to support a spin polarized 2D Dirac electronic phase. Due to the presence of spin-orbit coupling, the system opens a gap, forming a Chern insulator. In this work, we investigate the development of intrinsic triplet superconductivity in this system. Based on crystal symmetries of the heterostructure, we show that attractive interactions can produce p+ip superconductivity. Performing calculations on the lattice at the mean field level, we find that the system has a sequence of topological phase transitions as a function of the chemical potential, which can be tuned with a gate voltage. Among several different phases, we identity a chiral topological phase having a single Majorana mode at the edge. We calculate the spectroscopic signatures for the edge modes in this system. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y35.00005: Transport Properties of Thin Layers of Bi2Sr2CaCu2O8+δ Yijun Yu, Liguo Ma, Peng Cai, Cun Ye, Ruidan Zhong, Jian Shen, Genda Gu, Xianhui Chen, Yuanbo Zhang Many of the high temperature superconductors have layered atomic structures. In Bi2Sr2CaCu2O8+δ (BSCCO), van der Waals interaction between adjacent bismuth-oxygen layers makes the crystal easily cleavable. Few-layer BSCCO is thus an ideal system for investigating high temperature superconductivity in the two-dimensional limit. By fabricating samples in an inert atmosphere, we are able to obtain thin layers of exfoliated BSCCO samples and to probe the evolution of superconductivity as the dimenionality is reduced. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y35.00006: Visualizing the electronic structure of thin layers of Bi2Sr2CaCu2O8+delta Liguo Ma, Yijun Yu, Peng Cai, Cun Ye, Ruidan Zhong, Jian Shen, Genda Gu, Xianhui Chen, Yuanbo Zhang The role of dimensionality in high Tc superconductivity is an interesting issue: after all, many of the high Tc superconductor have layered atomic structures, and yet the link between the high Tc superconductivity and the two-dimensional nature of the crystal structure remains elusive. Here, we fabricate few-layer Bi2Sr2CaCu2O8+delta samples, and use scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to investigate the electronic structure, such as superconducting gap, pseudo gap and charge order, in Bi2Sr2CaCu2O8+delta in the 2D limit. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y35.00007: Two-Dimensional Superconductivity in Bilayers of BaBiO3 and BaPbO3 German Hammerl, Betina Meir, Stefan Gorol, Thilo Kopp Lead-doped barium bismuthate is intriguing on account of its relatively high critical temperature |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y35.00008: Room-Temperature Ferroelectricity in Two-Dimensional Material CuInP2S6: Synthesis and Devices Pai-Ying Liao, Mengwei Si, Gang Qiu, Peide (Peter) Ye Copper indium thiophosphate (CuInP2S6) is a novel two-dimensional (2D) material with single crystal layered structure and shows decent stability in air. Furthermore, it performs clear switching from para-electricity to ferro-electricity with transition temperature around 320 K. Here, air-stable yellow powder of CuInP2S6 with single crystal embedded inside was synthesized. Raman spectroscopy and scanning electronic microscope (SEM) results confirm the high quality of our CuInP2S6 samples. Room-temperature ferro-electricity of 2D CuInP2S6 is studied by polarization-voltage measurement directly on a fabricated metal/CuInP2S6/metal capacitor. The ferroelectric capacitors were fabricated by mechanical exfoliation of thin layered films of CuInP2S6 onto Ni/SiO2/p++ Si substrate and deposition of metal electrodes on top of the flakes. The thickness dependent and temperature dependent characteristics of the ferroelectric CuInP2S6 devices are systemically studied. The realization of 2D ferroelectric films opens the way to study full 2D negative capacitance field-effect transistors (NC-FETs) or ferro-electric transistors (FE-FETs). |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y35.00009: Ferroelectricity and dipole locking in 2D In2Se3 crystal Jun Xiao, Hanyu Zhu, YING WANG, Wei Feng, Yunxia Hu, Arvind Dasgupta, Yimo Han, Yuan Wang, David Muller, Lane W. Martin, PingAn Hu, Xiang Zhang Ferroelectricity in ultrathin crystal has been long believed to exhibit rich phase competition physics with quantum confinement and enhanced quasiparticle interactions. Its realization is also critical to scale down memory and develop versatile ferroelectric devices with large electrical and mechanical tunablity1. However, its own depolarizing electrostatic field prevents the existence of out-of-plane ferroelectricity at two-dimensional (2D) limit. Here we report the discovery of out-of-plane 2D ferroelectricity in atomically thin In2Se3 crystal. We experimentally found that in-plane lattice asymmetry and out-of-plane polarization is strictly locked, a new mechanism to stabilize the polar order. Such unique locking enables robust 2D ferroelectricity at ambient conditions and results in a very high transition temperature (~700K). In addition, it also enables electrical manipulation of atomic lattice anisotropy, which is the key to 2D spintronics and valleytronics. This discovery is potentially important to the atomically thin sensors and actuators. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y35.00010: Coexistence of Antiferroelectric and Ferroelectric Phases in Ultrathin SnTe Films Kai Chang, Thaneshwor Kaloni, Qi-Kun Xue, Xi Chen, Shuaihua Ji, Salvador Barraza-Lopez, Stuart S Parkin Two-dimensional (2D) SnTe films with a thickness of as little as 2 atomic layers (AL) have recently been shown to be ferroelectric with in-plane polarization. Remarkably, they exhibit transition temperatures (TC) that reach above room temperature, much higher than that of bulk SnTe. The mystery of such a dramatic enhancement of TC in 2D SnTe was not understood. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy and ab initio calculations, we have unveiled the underlying mechanism of the TC enhancement, which relies on the formation of a van der Waals structural phase with antiferroelectric inter-layer coupling in the few-AL thick SnTe films. This newly discovered structure has an “AB” bilayer stacking sequence distinct from the “AA” stacking of the bulk phase. In the AB phase, the 4n − 2 AL (n = 1, 2, 3…) thick films are found to be ferroelectric, while the 4n AL thick films are antiferroelectric. In the AB phase films thicker than 4 AL, significant lattice distortions persist to above 400 K. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y35.00011: Structural, electronic, ferroelectric, and topological properties of SnTe from atomic layer to bulk Thaneshwor Kaloni, Kai Chang, Qi-Kun Xue, Xi Chen, Shuaihua Ji, Stuart S Parkin, Salvador Barraza-Lopez In this talk, motivated from the recent experiment [1] and theory [2,3,4], I will discuss the structural parameters, elastic energy barrier, electronic structure, and electric polarization for 2 to 40 atomic layers (ALs) of SnTe thin films, as obtained from DFT calculations. At 40 ALs, SnTe films transit from semiconductor to a topological phase, with an in-plane electric polarization vanishing. The AB phase found to be stable up to 6 atomic layers and AA phase takes over thereafter. In this talk, I will also present simulated and experimental STM images of 2 AL SnTe films with/without defects. |
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