Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L35: 2D Materials -Superconductivity and Charge Density Waves IIFocus Session
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Sponsoring Units: DMP Chair: Jun Zhu, Penn State Room: LACC 409B |
Wednesday, March 7, 2018 11:15AM - 11:51AM |
L35.00001: Ising superconductivity and quantum metal in the two-dimensional transition metal dichalcogenides TaS$_2$ and NbSe$_2$ Invited Speaker: Benjamin Hunt Atomically-thin transition metal dichalcogenides (TMDs) are of significant contemporary interest, particularly for the strong ``Ising'' spin-orbit coupling (SOC) that results from the broken inversion symmetry in the individual atomic layers. NbSe$_2$, a classic layered superconductor, is the first metallic TMD to have exhibited superconductivity down to this monolayer (1L) limit, and we have recently shown that 1L TaS$_2$, with stronger SOC, also exhibits superconductivity. The atomic-scale thickness of the TMD crystals implies that we can explore the regime where quenching of superconductivity is entirely due to paramagnetic effects. We study this regime, where superconductivity can survive well above 30 T in-plane, and find a strong enhancement of the upper critical field relative to the Pauli paramagnetic limit for both 1L and multilayer samples. I discuss the implications of our results for various spin-orbit coupling effects, local inversion-symmetry breaking in even-layer samples, spin-triplet pairing, and possible topological and other exotic phases of superconductivity. I will also discuss our recent experiments on bilayer and few-layer NbSe$_2$ and TaS$_2$ and our observation of an anomalous metallic phase in the zero-temperature limit induced by a small perpendicular magnetic field. This quantum metal phase has only been observed in highly-disordered thin film superconductors and its observation in a crystalline superconductor, along with a distinct magnetic field scaling, forces reexamination of the diagram of possible electronic phases in two dimensions at zero temperature. |
Wednesday, March 7, 2018 11:51AM - 12:03PM |
L35.00002: Microscopic Theory of Superconductivity in NbSe_2 Monolayer in Presence of a Magnetic Field Daniel Shaffer, Jian Kang, Fiona Burnell, Rafael Fernandes Monolayers of NbSe_2, a 2D transition metal dichalcogenide, have recently been found to be superconducting below a critical temperature of 3K. This superconducting state survives even in the presence of in-plane magnetic fields of up to 35 T, far above the Pauli limit, indicating strong out-of-plane spin-orbit coupling (SOC). This remarkable observation led to interesting theoretical proposals of Ising superconductivity and topological superconductivity in this and also in other similar 2D dichalcogenides. Here, we introduce a microscopic low-energy model for NbSe_2 monolayer that includes the Ising SOC, the Rashba SOC, an external in-plane magnetic field, and all possible low-energy electronic interactions. By performing a parquet renormalization group calculation, we investigate the superconducting instabilities of the system that arise due to purely repulsive interactions. We discuss the possible emergence of topological superconductivity and finite-momentum pairing, and compare our results with experiments. |
Wednesday, March 7, 2018 12:03PM - 12:15PM |
L35.00003: Gate Tunable Superconductivity in monolayer β-MoTe2 Daniel Rhodes, Younghun Jung, Abhinandan Antony, Bumho Kim, Cory Dean, Abhay Narayan, James Hone Due to the emergence of mechanically exfoliated, highly crystalline superconductors, research into superconducting-insulator transitions and quantum phase transitions in the low disorder limit has become more accessible. This allows for the exploration of novel layered superconductors in the few layer limit. Materials such as bilayer NbSe2 exhibit a Bose-metal state, while others like WTe2 can have 3 distinct phases, modified by the application of a gate voltage. |
Wednesday, March 7, 2018 12:15PM - 12:27PM |
L35.00004: Promoting Unconventional p-wave Superconductivity in Transition Metal Dichalcogenides Junhua Zhang, Vivek Aji The hole-doped monolayer transition metal dichalcogenides have a pronounced spin-valley locking feature in which the low-energy physics is characterized by spin-oppositely-polarized Fermi pockets at opposite valleys separated in momentum space. This unique structure offers new possibilities for broken symmetry phases. Due to the d-character of the electrons, strong correlation effect allows the realization of a p-wave superconducting state consisting of two intra-pocket paired chiral p-wave condensates. In addition, each intra-pocket pairing order parameter has a spatially-modulated phase, akin to the FFLO states, associated with spontaneous supercurrent loops on the unit cell scale. Under time-reversal symmetry, the chiralities of the two condensates are opposite and the counter-propagating current loops of the two condensates cancel each other. We show that circularly-polarized photoexcitation exhibits chirality-dependent pair-breaking effect on the chiral p-wave states. This chirality-selective circular dichroism provides an experimental approach to promote one chiral condensate than the other. We then investigate the interesting properties of the superconducting state dominated by one FFLO-type topological condensate. |
Wednesday, March 7, 2018 12:27PM - 12:39PM |
L35.00005: Probing the Two-gap Superconductivity in a Few-layer NbSe2-graphene Heterojunction Tianyi Han, Junying Shen, Jiangxiazi Lin, Rolf Lortz, Ning Wang 2H-NbSe2, a layered transition metal dichalcogenide, offers an ideal platform for the study of superconductivity from a bulk system to two-dimensional limit. In our work, differential conductance spectroscopy was exploited to probe the superconductivity in a few-layer NbSe2-graphene heterojunction. Owing to the gate-tunable Fermi level of few-layer graphene, the sensitively gate-dependent differential conductance allowed us to investigate the superconducting gap structure of NbSe2 with continuously tuned junction transparency between tunneling regime and Andreev reflection limit. Features of two-gap superconductivity in NbSe2 were observed with a characteristic differential conductance of a central dip and two sets of coherence peaks when the Fermi level was finely tuned to the charge neutrality point of the junction. Through Blonder-Tinkham-Klapwijk fits, two gaps along with their temperature dependence were extracted. The gap to Tc ratio resulted from the fits and the temperature behavior of the two gaps were further analyzed and discussed, pointing to a weak to moderately strong coupling scenario for the few-layer NbSe2. |
Wednesday, March 7, 2018 12:39PM - 12:51PM |
L35.00006: Charge Density Waves in Single Layer 2H-NbSe2 Liwei Liu, Zhuozhi Ge, Chenhui Yan, Michael Weinert, Lian Li Bulk 2H-NbSe2 is metallic at room temperature, but exhibits (3×3) charge density wave (CDW) order below 30 K. Recent work has shown that this CDW modulation persists even at the single layer limit. In this work, single layer 2H-NbSe2 is grown by molecular beam epitaxy on epitaxial graphene/SiC(0001) substrate. In situ scanning tunneling microscopy (STM) shows layer-by-layer growth, resulting in van der Waals heterostructures of single layer 2H-NbSe2/graphene and bilayer 2H-NbSe2/graphene. The (3×3) CDW order is observed in both cases below 20 K, as well as a gap-like feature at the Fermi level as revealed by tunneling spectroscopy. This indicates that this CDW modulation is independent of layer thickness and stacking, though there are modifications of the conduction band for the bilayer heterostructures. These findings and their implication on the collective electronic states of single layer 2H-NbSe2 will be discussed at the meeting. |
Wednesday, March 7, 2018 12:51PM - 1:03PM |
L35.00007: Understanding the charge-density-wave in monolayer 1T-NbSe2 from first principles. Diego Pasquier, Oleg Yazyev Layered Transition Metal Dichalcogenides (TMDs) host a large variety of charge-density-waves/ periodic lattice distortions that are often associated with superconductivity. One of the most studied material in this family is NbSe2 in the 2H polytype. Recently, a monolayer of the 1T phase of NbSe2 has been successfully synthesized for the first time. The observed superlattice structure was suggested to be a commensurate Star-Of-David phase with √13x√13 periodicity and the insulating behavior ascribed to a possible Mott gap, reminiscent of 1T-TaS2. |
Wednesday, March 7, 2018 1:03PM - 1:15PM |
L35.00008: Suppressed Charge Density Wave and Strain-Enhanced Superconductivity in Na-Intercalated Bilayer NbSe2 Chao-Sheng Lian, Chen Si, Jian Wu, Duan Wenhui Atomically thin NbSe2 is a metallic layered transition metal dichalcogenide (TMD) with coexisting superconducting and charge-density-wave (CDW) orders. Using first-principles calculations, we have systematically studied the lattice dynamics, electronic structure and electron-phonon coupling of Na-intercalated bilayer NbSe2. The results show that Na intercalation can effectively suppress the CDW instability of the bilayer NbSe2. In this process, a large electron doping from the intercalated Na contracts the Fermi surface of bilayer NbSe2, causing the reduction of electron-phonon coupling at qCDW. We further find that despite the disappearance of CDW, the superconductivity is still preserved in the NbSe2 intercalate with a predicted transition temperature Tc of 3 K. More interestingly, the biaxial compressive strain is found to largely increase electronic density of states at the Fermi level, soften phonon modes and improve the superconducting Tc of this system by more than 100% at a low strain level of 3%. The present work will stimulate future experimental studies of the synergistic effects of electron doping and strain on CDW and superconductivity in 2D metallic TMD materials. |
Wednesday, March 7, 2018 1:15PM - 1:27PM |
L35.00009: Plasmonic Superconductivity in Layered Materials Roelof Groenewald, Malte Rösner, Gunnar Schoenhoff, Jan Berges, Stephan Haas, Tim Wehling Due to a lack of screening in two dimensions the Coulomb interaction is generally enhanced and consequently plays a major role to understand many-body effects within layered materials. In the field of superconductivity it is usually introduced as an approximate, static, and adjustable parameter µ* which describes only effectively the Coulomb repulsion. |
Wednesday, March 7, 2018 1:27PM - 1:39PM |
L35.00010: Quantum phase transitions in highly crystalline 2D superconductors Yu Saito, Tsutomu Nojima, Yoshihiro Iwasa Recent technological advances of thin films fabrication, especially mechanical exfoliation, led to the successful preparation[N1] of highly-crystalline two-dimensional (2D) superconductors [1]; Examples include atomically thin NbSe2 and ion-gated 2D crystals, which exhibit intrinsic properties of 2D superconductors with minimal disorder. In this talk, we provide a unified picture of temperature versus magnetic field phase diagram in highly crystalline 2D superconductors containing quantum phase transitions. According to the magneto-transport measurements in 2D superconducting ZrNCl and MoS2, we found that the quantum metallic state (vortex liquid) [2] commonly observed at low magnetic fields is converted via the quantum Griffiths state to the weakly localized metal at high magnetic fields through a quantum phase transition with the diverging critical exponent. We discuss such evolution of quantum state is generic nature of highly crystalline 2D superconductors with weak pinning potentials. |
Wednesday, March 7, 2018 1:39PM - 1:51PM |
L35.00011: Dynamically-created Josephson Junctions in thin layers of NbSe2 Steven Tran, Albert Davydov, James Williams Niobium Diselenide (NbSe2) is a member of the transition metal dichalcogenides (TMDCs) family that displays unique superconducting properties due to broken in-plane mirror symmetry and multiply occupied bands. Here, we report on fabrication and low-temperature transport measurements of atomically-thin layers of NbSe2 in four terminal-configurations. We observe the formation of phase slip lines across our NbSe2 sheets which appear as non-zero resistances in the superconducting state. Additionally, we interact with these dynamically-created Josephson junctions through the application of RF radiation, observing results consistent with the AC Josephson effect. |
Wednesday, March 7, 2018 1:51PM - 2:03PM |
L35.00012: Phenomenological description of the coexistence between charge density waves, discommensurations and superconductivity in 1T-TiSe2 Chuan Chen, Lei Su, Antonio Helio Castro Neto, Vitor Pereira The underlying mechanisms for charge density wave (CCDW) and superconductivity (SC), as well as their interplay, in 1T-TiSe2 is still under debate. Recent experiments have suggested that, under electron doping, the commensurate CDW (CCDW) order becomes incommensurate (ICDW) and the SC order persists inside the discommensuration (DC) regions of the CDW. Motivated by the pioneering work of McMillan and others, we developed a phenomenological Ginzburg-Landau (GL) theory that describes the interplay between CCDW, ICDW and SC phases in this system through coupling of the superconducting order parameter to the CDW fluctuations. By numerically solving the saddle point (Euler-Lagrange) equations, we trace the phase diagram of the system, according to which: (i) upon doping, the CDW order transitions to incommensurate beyond a critical threshold, (ii) ICDW and SC order coexist in that regime, (iii) the SC order arises first within the CDW discommensurations, all of which are consistent with existing experimental results on 1T-TiSe2. |
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