Bulletin of the American Physical Society
2024 APS March Meeting
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session HH05: V: Superconductivity: Theory and ExperimentVirtual Only

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Sponsoring Units: DCMP Chair: Ekta Bhatia, NYCREATES and SUNY; Yang Zhang, University of Tennessee Room: Virtual Room 05 
Wednesday, March 6, 2024 11:30AM  11:42AM 
HH05.00001: Competing charge density waves in the kagome lattice Xianxin Wu, RuiQing Fu, Jiangping Hu, Ziqiang Wang, Sen Zhou The exotic charge density wave order with time reversal symmetry breaking in kagome superconductors has attracted tremendous attention. However, its origin still remains elusive and its microscopic model realization has proven challenging. In this work, we investigate the competing electronic order in the kagome model with nearestneighbor (NN) and next NN Coulomb repulsion at the van Hove filling. The sublattice texture on the Fermi surface is found to play a crucial role in determining the electronic instabilities. For weak interactions, we find the NN interaction favors a 2x2 charge bond order, while the next NN interaction promotes a 2x2 imaginary bond order, i.e. loop current order. When interactions are strong, a CDW order emerges, characterized by rotational symmetry breaking. Upon slight doping, superconductivity arises from these charge fluctuations, revealing three different pairings. Our work offers a microscopic realization of loop current order in the kagome lattice, and we also discuss potential implications for the vanadiumbased kagome superconductors AV_{3}Sb_{5}. 
Wednesday, March 6, 2024 11:42AM  11:54AM 
HH05.00002: An electronic origin of charge order in infinitelayer nickelates Hanghui Chen, Yifeng Yang, Guangming Zhang, Hongquan Liu A charge order (CO) with a wavevector q~(1/3,0,0) is observed in infinitelayer nickelates. Here we use firstprinciples calculations to demonstrate a chargetransferdriven CO mechanism in infinitelayer nickelates, which leads to a characteristic Ni^{1+}Ni^{2+}Ni^{1+} stripe state. For every three Ni atoms, due to the presence of nearFermilevel conduction bands, Hubbard interaction on Nid orbitals transfers electrons on one Ni atom to conduction bands and leaves electrons on the other two Ni atoms to become more localized. We further derive a lowenergy effective model to elucidate that the CO state arises from a delicate competition between Hubbard interaction on Nid orbitals and charge transfer energy between Nid orbitals and conduction bands. With physically reasonable parameters, q=(1/3,0,0) CO state is more stable than uniform paramagnetic state and usual checkerboard antiferromagnetic state. Our work highlights the multiband nature of infinitelayer nickelates, which leads to some distinctive correlated properties that are not found in cuprates. 
Wednesday, March 6, 2024 11:54AM  12:06PM 
HH05.00003: Advances and challenges in the singlecrystal synthesis of infinitelayer and RuddlesdenPopper phase nickelate single crystals Pascal Puphal, Vignesh Sundaramurthy, Valentin L Zimmermann, YuMi Wu, Eren Suyolcu, Masahiko Isobe, Bjorn Wehinger, Bernhard Keimer, Matthias Hepting High Tc superconductivity remains one of the main focuses in condensed matter research, in which, rareearth nickel oxides have emerged as a new class with potential for unconventional superconducting behavior. Here, two types of structures have drawn particular interest: The first are nickelates with the infinitelayer crystal structure, such as Nd_{0.8}Sr_{0.2}NiO_{2}, with superconducting transition temperatures up to 20 K. The second type was found in RuddlesdenPopper phase nickelates, such as La_{3}Ni_{2}O_{7}, which under hydrostatic pressure realize a remarkably high Tc of 80 K. Many details of these systems are not yet fully understood especially as for the first type only thin films show superconductivity up to now. In my talk I will show the challenges of optimal doping perovskite single crystals, and show the advancements we have done in obtaining optimal reduced crystals as well as the effects of intercalation on these single crystals. Furthermore, I present our findings on La_{3}Ni_{2}O_{7}, where sizable single crystals can be readily grown using the optical floating zone method at 15 bar oxygen pressure. Here, we recently observed that these crystals exhibit multiple crystallographic phases and a pronounced sensitivity to oxygen stoichiometry, affecting their physical properties and fascinating pressure dependence as will be highlighted in the talk. 
Wednesday, March 6, 2024 12:06PM  12:18PM 
HH05.00004: Frustrationinduced Superconductivity in the tt' Hubbard Model Changkai Zhang, JhengWei Li, Jan von Delft The twodimensional (2D) Hubbard model is widely believed to capture key ingredients of highTc superconductivity in cuprate materials. However, compelling evidence remains elusive. In particular, various magnetic orders may emerge as strong competitors of superconducting orders. Here, we study the ground state properties of the doped 2D tt′ Hubbard model on a square lattice via the infinite Projected EntangledPair State (iPEPS) method with U(1) or SU(2) spin symmetry. The former is compatible with antiferromagnetic orders, while the latter forbids them. Therefore, we obtain by comparison a detailed understanding of the magnetic impact on superconductivity. Moreover, an additional t′ term accommodates the particlehole asymmetry, which facilitates studies on the discrepancies between electron and holedoped systems. We demonstrate that (i) a positive t′/t significantly amplifies the strength of superconducting orders; (ii) at sufficiently large doping levels, the tt′ Hubbard model favors a uniform state with superconducting orders instead of stripe states with charge and spin modulations; and (iii) the enhancement of magnetic frustration, by increasing either the strength of nextnearest neighbor interactions or the charge doping, impairs stripe orders and helps stabilize superconductivity. 
Wednesday, March 6, 2024 12:18PM  12:30PM 
HH05.00005: Critical magnetic fields H_{c}(x), H_{c}_{2}(x) and flux φ(x) vs superfluid density x in a superconductor SangBoo Nam By the BCS framework [Bardeen, Cooper and Schrieffer, Phys. Rev. 108, 1175 (1957); Nam, Phys. Rev. 156, 470 (1967)], with the notion of finite pairing interaction energy range [the cutoff energy] T_{d}=2zT_{c} [critical temperature], we present novel results, at zero temperature, H_{c}(x)= b(x)H_{c}(1), H_{c}(1) [BCS]= [4πN(0)]^{1/2}Δ(1), b(x)= 2Δ(x)/Δ(1)(1+x), H_{c}_{2}(x)= b^{2}(x)H_{c}_{2}(1), H_{c}_{2}(1) 2πξ^{2}(1) [GinzburgLandau coherence length square] =φ(1)[flux quantum]=[hc/2e], ξ(1)=ξ(x)b(x) = πξ_{BCS}/12^{½}, with the superfluid density x=ρ_{s}/ρ= tanh(G/2) [Nam, APSMAR22/A61.4 (2022)], G= 1/N(0)V =∫_{0}^{z} dy tanh(y)/y = Σ_{j }(2/w_{j}) tan^{1}(z/w_{j}), w_{j}=(π/2) j(odd integer) [Nam, Phys. Lett. A193, 111 (1994); (E) ibid A197, 458 (1995)], Δ(x)/2T_{c} =A(z)= z/sinhG, where N(0) and V are the density of states/spin at a reference energy and the BCS pairing interaction energy, respectively. A(∞) [BCS]= 0.88 and A(+0)=1. The upper value H_{c}_{2}(x) is to be 5.165 H_{c}_{2}(1). The different values of local ξ(x)= ξ(1)/b(x) may account for the fractional flux quanta reported by Iguchi et al., Science 380, 1244 (2023). Superconductors [x<1] are multiconnected [Nam, Phys. Lett. A198, 447 (1995)]. 
Wednesday, March 6, 2024 12:30PM  12:42PM 
HH05.00006: Stoichiometric control of 2D superconductivity and mobility at SrTiO_{3}based interfaces Gyanendra Singh, Roger Guzman, Guilhem Saiz, Wu Zhou, Jaume Gazquez, Jordi Fraxedas, Fereshteh Masoudinia, Dag Winkler, Tord Claeson, Nicolas Bergeal, Gervasi Herranz, Alexei Kalaboukhov SrTiO_{3}based quasitwodimensional electron gases (q2DEGs), that exhibit coexistence of gate tunable multiorbital 2D superconductivity and strong Rashba spinorbit coupling (RSOC), have ingredients to generate topological superconducting electronics for quantum application. However, a comparison between the electron mean free path and the superconducting coherence length reveals that superconductivity in these systems is in the dirty limit, which tends to suppress nonconventional pairing and therefore challenges these expectations. Here, in LaAlO_{3}/SrTiO_{3} interfaces, we show that precise control of the La/Al ratio during the epitaxial growth of LaAlO_{3}, monitored by XPS and STEMEELS measurements, allows us to finely tune the electron mobility, carrier density and the superconducting properties. Remarkably, our findings reveal electronic mobilities that surpass previous reports by up to 15fold, with mean free paths comparable to the superconducting coherence length, thereby approaching the clean limit. In addition, we also show that in our extremely clean q2DEGs, superconductivity can develop at low electron density, where electrons only populate lowenergy d_{xy} subbands. This is in sharp contrast to the conventional understanding of superconductivity in LaAlO_{3}/SrTiO_{3 }q2DEGs, which relies solely on Cooper pairing in the highenergy d_{xy}/d_{xz} subbands. What makes this even more surprising is that the density of states in the d_{xy} subbands is very low, which, according to the BCS theory, is considered detrimental to superconductivity. 
Wednesday, March 6, 2024 12:42PM  12:54PM 
HH05.00007: Nonlinear Optical Responses in Superconductors under Magnetic Field Hiroto Tanaka, Youichi Yanase, Hikaru Watanabe Noncentrosymmetric superconductors have been attracting attention as a material platform for exotic nonreciprocal responses and transports, which occur in the presence of Cooper pairs and inversion symmetry breaking. Nonlinear conductivity plays a key role in the nonreciprocal responses. The secondorder nonlinear response in superconductors was formulated based on the Bogoliubovde Gennes Hamiltonian [1,2], and the mechanism in timereversal (T) symmetric superconductors was clarified [3]. Although the T symmetry breaking further enriches the properties and mechanism of nonlinear responses, it remains unexplored in spite of potential playgrounds such as superconductors under magnetic fields. 
Wednesday, March 6, 2024 12:54PM  1:06PM 
HH05.00008: Sum rule of the optical activity in noncentrosymmetric superconductors Koki Shinada Optical responses in superconductors have provided important insights since the first observation of the superconducting gap in 1956 using farinfrared rays. For example, the optical spectrum has contributed to identifying the gap symmetry and the measurement of the superfluid density and the penetration length using the FerrellGloverTinkham (FGT) sum rule. 
Wednesday, March 6, 2024 1:06PM  1:18PM 
HH05.00009: Hightemperature superconducting photonic integrated circuits: controlling photoresponse of superconductor van der Waals plasmonic metasurfaces at optical telecommunication wavelengths Kaveh Delfanazari, Mingqi Zhang We report on a novel numerical modelling approach to exploit the optical properties of nanostructured metasurface arrays at the surface of hightemperature superconductors (HTS) BSCCO van der Waals. The optical response results in resonance wavelength from visible light to the infrared region between T=15K and 85K suggest that the hightemperature superconducting plasmonic photoresponse could be efficiently controlled by altering the thickness and width of BSCCO nanowire arrays on both quartz and sapphire substrates. 
Wednesday, March 6, 2024 1:18PM  1:30PM 
HH05.00010: Energyphase relation of a boundary mode in a charge density wave state Maksim Litskevich, Md. Shafayat Hossain, Songbo Zhang, ZiJia Cheng, Satya N Guin, Nitesh Kumar, Chandra Shekhar, Zhiwei Wang, Yongkai Li, Guoqing Chang, Guangming Cheng, JiaXin Yin, Qi Zhang, YuXiao Jiang, Nana Shumiya, Tyler A Cochran, Daniel Multer, Xian Yang, Xiaoxiong Liu, Nan Yao, Yugui Yao, Titus Neupert, Claudia Felser, Zahid Hasan Individually, charge order and topology have been widely studied in various condensed matter platforms. Nevertheless, the manifestation of the topology in charge ordered systems remains largely unexplored. Here, using scanning tunneling microscopy, we unveil the bulk and boundary phenomenology of the charge density wave (CDW) in a topological material. This material, considered a candidate for a Weyl semimetal, exhibits a phase transition to CDW. Below the CDW transition temperature we find an ingap boundary mode (edge state) at a monolayer step edge. Remarkably, the edge state exhibits modulations along the edge that match wavelength of the CDW projected onto the edge. The wavelength of the edge state modulations is energyindependent, while the phase of the edge state smoothly shifts by π as a function of energy when crossing the CDW energy gap. This results in a continuous connection of fully gapped valence and conduction band states, which are originally separated by π phase in the bulk, via the energyphase relation of the boundary mode. Such mechanism is similar to the topological spectral flow, where the gapless edge modes link the gapped bulk modes in energymomentum, while here, the connectivity occurs in energyphase. The existence of the edge state within insulating CDW gap along with its nontrivial energyphase relation hint to a topological nature of the CDW state. 
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