Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H31: Superconductivity in Layered Chalcogenides 
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Sponsoring Units: DCMP Chair: Lev Gasparov, University of North Florida Room: LACC 407 
Tuesday, March 6, 2018 2:30PM  2:42PM 
H31.00001: Pressureinduced superconductivity in semimetallic 1TTiTe_{2} and its persistence nature upon decompression Sukanta Karmakar , Utpal Dutta , Pallavi S. Malavi , Subodha Sahoo 1TTiTe_{2} belongs to the family of layered transition metal dichalcogenide compounds that provide important playground revealing exotic ground state electronic orders by tuning the Fermi surface topology and many body effects, either purely by lowering temperature or carrier doping or external pressure [1,2]. A large Te spinorbit interaction and enhanced pd hybridization near Fermi level make its Fermi surfaces more susceptible to external parameters. 
Tuesday, March 6, 2018 2:42PM  2:54PM 
H31.00002: Using controlled disorder to probe the interplay between charge order and superconductivity in 2HNbSe_{2} Kyuil Cho , Serafim Teknowijoyo , Makariy Tanatar , Ruslan Prozorov , Marcin Konczykowski , Vivek Mishra , Peter Hirschfeld The interplay between superconductivity and charge density wave (CDW) in single crystals of 2HNbSe_{2} was probed by tuning the amount of point  like disorder induced by 2.5 MeV electron irradiation. With increasing dose, T_{c} shows $nontrivial$ behavior, first increasing from 7.25 K to 7.45 K and then decreasing to 7.3 K, whereas T_{CDW} monotonically decreases from 33 K and becomes unresolvable below 27 K. At this dose, T_{c} has a sharp drop by 0.3 K. The low temperature variation of the London penetration depth shows robust exponential behavior suggesting nodeless order parameter before and after irradiation, and overall trend is consistent with multiband superconductivity with two effective superconducting gaps. Our result supports the scenario of complex interplay of CDW and superconductivity,  competition for the density of states, but possible softening of the phon modes by long  range CDW. 
Tuesday, March 6, 2018 2:54PM  3:06PM 
H31.00003: Inducing strong superconductivity in WTe_{2} by proximity effect Ce Huang , Awadhesh Narayan , Enze Zhang , Faxian Xiu The superconducting states inheriting nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximityinduced superconductivity in typeII Weyl semimetal WTe2. When the WTe2 thickness (t_(WTe_2 )) reaches 21 nm, the superconducting transition occurs around the critical temperature (T_c) of NbSe2 with a gap amplitude (Δ_p) of 0.38 meV and an unexpected ultralong proximity length. Our theoretical calculations, based on the Bogoliubovde Gennes equations in the clean limit, predict that the induced superconducting gap is a sizable fraction of the NbSe2 superconducting one when t_(WTe_2 ) is less than 30 nm, and then decreases quickly as t_(WTe_2 ) increases. This agrees qualitatively well with the experiments. Such observations demonstrate a new direction in the search for superconducting phases in topological semimetals. 
Tuesday, March 6, 2018 3:06PM  3:18PM 
H31.00004: Uniaxial strain dependence of superconductivity in 2HNbSe_2 Jedrzej Wieteska , Benjamin Foutty , Benjamin Mazel , Lyuwen Fu , Zhengqian Cheng , Shuang Jia , Chris Marianetti , Abhay Narayan Uniaxial strain breaks crystal symmetries and couples to anisotropic electronic phenomena in solids. Such anisotropic electronic phenomena are commonly observed in unconventional superconductors such as the pnictides [1,2] and strontium ruthenate [3,4]. While spectacular effects on superconducting order have been observed in these compounds, the interaction between electronic anisotropies and superconductivity remains to be clarified. In this talk we will present data on the application of uniaxial strain to a conventional superconductor 2HNbSe_2, which also hosts a charge density wave. We find a linear coupling of superconducting Tc to the anisotropic strain in line with previous work [5]. However, we also find a surprisingly large quadratic coupling of Tc to uniaxial strain, indicating the importance of uniaxial order or fluctuations in this material. We will discuss theoretical scenarios for how this electronic anisotropy and its coupling to superconductivity can be explained. 
Tuesday, March 6, 2018 3:18PM  3:30PM 
H31.00005: Evolution of Higgs and Leggett modes with pressure in NbSe_{2} ZiYu Cao , Kai Zhang , XiaoJia Chen , XiaoJun Yang , ZhuAn Xu , Alexander Goncharov The coexistence of the charge density wave and superconducting phases has been a subject of much recent interest. 2HNbSe_{2} is a model superconductor with a charge density wave. By using Raman scattering technique, we show how the competition between the charge density wave and superconductivity in 2HNbSe_{2} can be tuned by temperature and pressure. We also report the experimental observation of a Higgs mode together with an additional Leggett mode and their evolution with pressure. The implication of these collective excitation modes to the understanding of unconventional superconductivity is discussed. 
Tuesday, March 6, 2018 3:30PM  3:42PM 
H31.00006: Superconductivity in SSubstituted TiSe2 Gilberto De La Pena , Sangjun Lee , Kannan Lu , Anshul Kogar , Xiaolan Sun , Yizhi Fang , Peter Abbamonte , Greg MacDougall We have observed superconductivity in resistivity measurements of sulfurdoped TiSe_(2x)S_x. We studied the SC phase boundary as a function of Ssubstitution using Xray scattering to characterize the charge density wave (CDW) phase in these samples as a function of doping and temperature. A charge density wave peak persists in the highest doped sample studied, at x = 1.09. Our temperature vs. doping phase diagram shows that the SC dome, located at around x = 0.10, is well within the boundary of the charge ordered phase, indicating that the dome is not associated with an amplitude quantum critical point. 
Tuesday, March 6, 2018 3:42PM  3:54PM 
H31.00007: GateAccessible Superconductivity and Helical Modes in Monolayer WTe_{2} Valla Fatemi , Sanfeng Wu , Yuan Cao , Quinn Gibson , Kenji Watanabe , Takashi Taniguchi , Robert Cava , Pablo JarilloHerrero I report on our transport measurements of electrostatically gated monolayer tungsten ditelluride (WTe_{2}), where we observe both superconductivity and helical edge modes in the same material. I will briefly introduce our experiments in the undoped regime which indicate that monolayer WTe_{2} is a quantum spin Hall insulator with edge conduction dominating up to 100K. In the doped regime, the same material is a superconductor at recordlow induced 2D carrier densities, with critical temperatures as high as 1 Kelvin. Unique among 2D electronic systems, monolayer WTe_{2} is fully tunable from a gapped topological insulator state to an intrinsic superconducting state using only solid state electrostatic gates. New routes for topological quantum devices are now possible, in which electrostatically accessible superconductivity and helical modes can be naturally interfaced within the same crystal. 
Tuesday, March 6, 2018 3:54PM  4:06PM 
H31.00008: SpinTriplet Ising Pairing Phase of Gated MoS_{2} Wenyu He , Noah F. Q. Yuan , Kam Tuen Law Motivated by recent tunneling experiments on superconducting gated MoS_{2}, we propose a spintriplet Ising pairing phase of the system, which is a triplet nodal superconductor characterized by the V shape density of states in the low energy. In the triplet Ising pairing phase, the calculated bulk density of states agree well with the experimental results. In the armchair edge, the nodal phase supports flat bands with a large number of zero energy modes and they would result in a large zerobias conductance peak in the tunneling experiments. Moreover, the inplane upper critical field in the triplet Ising pairing phase is systematically studied and the experimental data are consistently explained. 
Tuesday, March 6, 2018 4:06PM  4:18PM 
H31.00009: Spin triplet superconducting pairing in doped MoS_{2} Guang Yang , tianxing ma , Eduardo V. Castro , HaiQing Lin Searching for triplet superconductivity has been pursued intensively in a broad field of material science and quantum information for decades, nevertheless, these novel states remain rare. Within a simplified effective 3orbitals model, we reveal a spin triplet pairing in doped MoS_{2} by employing the finite temperature determinant quantum MonteCarlo approach. In a wide filling region of <n>=0.8~1.0, the fnwave pairing dominates over other symmetries, and as the filling is lower than <n>=0.8, the fwave tends to be dominated. The pairing susceptibility strongly increase as the onsite Coulomb interaction increases while it is insensitively to the spinorbit coupling. The observed dependences of the pairing susceptibility on electron fillings and spinorbit couplings are illustrated with the changes of Fermi surface topologies. 
Tuesday, March 6, 2018 4:18PM  4:30PM 
H31.00010: Evidences of nodeless multigap superconductivity in 2HPd_{x}TaSe_{2 }from thermal transport Chanhee Kim , Dilip Bhoi , ByungGu Jeon , Byeong Min , Woohyun Nam , Kee Kim 2HTaSe_{2} is one of unique transition metal dichalcogenides which exhibits an incommensurate charge density wave (ICDW) state at ≈ 122 K followed by a commensurate charge density wave (CCDW) state at ≈ 90 K and superconductivity at T_{C} ~ 0.14 K. Recently, it was found that upon systematic intercalation of Pd ions into 2HTaSe_{2}, the CCDW order is destabilized more rapidly than ICDW to indicate a hidden quantum phase transition point at x ~ 0.090.10. Moreover, T_{C} shows a dramatic enhancement up to 3.3 K at x = 0.08, ~24 times of T_{C} in 2HTaSe_{2} [1]. In this work, we measured the inplane thermal conductivity (k) of a 2HPd_{x}TaSe_{2} single crystals with T_{c} ~ 3.13 K down to 100 mK. In zero magnetic field, the absence of the residual linear term in κ_{0}/T at T → 0, supports the nodeless superconducting gap feature. Moreover, magnetic field dependence of the normalized residual linear term (κ_{0}(H)/T)/(κ_{N}/T)) exhibits similar behavior as observed in the multiband superconductor like NbSe_{2 }[2]. These results strongly support the presence of multiple nodeless superconducting gaps in 2HPd_{x}TaSe_{2}. 
Tuesday, March 6, 2018 4:30PM  4:42PM 
H31.00011: Quantum Metal behavior in ultraclean 2HNbSe2 devices Avishai Benyamini , Evan Telford , Da Wang , Dante Kennes , Andrew Millis , Abhay Pasupathy , James Hone , Cory Dean Low temperature measurements of fewlayer 2HNbSe2 in the 2D limit at a finite magnetic field exhibit a metallic behavior instead of a superconductor to insulator transition. The physical nature of this phase, known as a quantum metal, is still under debate. In these 2D systems disorder and defects may play a role in understanding the underlying mechanisms, NbSe2 is especially sensitive as it is airsensitive. Using a novel fabrication technique based on encapsulation with hBN prepatterned with metal via contacts, we are able to simultaneously protect the NbSe2 layer and make electrical contact in a single, lithographyfree, step. With this technique we fabricate robust and stable devices of high quality with an array of parallel contact to study NbSe2 nonlocally. We show that at zero magnetic field we can stabilize a normalsuperconducting boundary by sourcing a local current enabling the study of quasiparticle excitations of the superconductor and Andreev processes. At a finite magnetic field our nonlocal measurement shed light into the nature of the metallic phase by examining the powerlaw behavior of the resistance with magnetic field. We further compare our results to TimeDependentGinzburgLandau simulations. 
Tuesday, March 6, 2018 4:42PM  4:54PM 
H31.00012: Disorder induced multifractal superconductivity in monolayer transition metal dichalcogenides Xiao Xiao , Shuaihua Ji , Xi Chen , Kam Tuen Law It is commonly believed that disorders have weak or detrimental effects on superconductivity. About one decade ago, it was proposed that disorder can enhance superconductivity by driving the electron wavefunctions to the socalled multifractal regime. However, no examples of multifractal superconductors have been identified. In this work, using a realistic model for monolayer NbSe_{2}, we calculated the superconducting pairing gap selfconsistently and show that the pairing gap can be enhanced by several times at the intermediate disorder regime. Importantly, the enhancement of superconductivity can be traced to the strengthening of electronelectron correlation due to the multifractal wavefunctions. We suggest that monolayer NbSe_{2} gives the first example of the long sought after multifractal superconductors. 
Tuesday, March 6, 2018 4:54PM  5:06PM 
H31.00013: Loop currents from nonunitary chiral dwave superconductivity Philip Brydon , David Abergel , Daniel Agterberg , Victor Yakovenko

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