Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session B47: One and Two Dimensional Superconductivity1Live

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Sponsoring Units: DCMP Chair: Michael Susner, Air Force Research Lab  WPAFB 
Monday, March 15, 2021 11:30AM  11:42AM Live 
B47.00001: Crossover of highenergy spin fluctuations from collective triplon excitations to incoherent gapped magnetic modes in the
cuprate ladders of Sr_{14x}Ca_{x}Cu_{24}O_{41 } Jinu Thomas, Yi Tseng, Wenliang Zhang, Eugenio Paris, Pascal Puphal, Rabindranath Bag, Gouchu Deng, Teguh Citra Asmara, Vladimir N. Strocov, Surjeet Singh, Ekaterina Pomjakushina, Umesh Kumar, Alberto Nocera, Henrik M Ronnow, Steven S. Johnston, Thorsten Schmitt We studied the magnetic excitations in the quasionedimensional (q1D) selfholedoped ladder subsystem of the hybrid chainladder material Sr_{14x}Ca_{x}Cu_{24}O_{41} (SCCO) using Cu L_{3}edge resonant inelastic Xray scattering (RIXS). We track the evolution of the spectra from collective twotriplon (2T) excitations to weaklydispersive gapped magnetic modes as a function of Ca content. We compare the experiment to DMRG calculations of the RIXS response and find evidence of charge localization as Ca content increases. This result is supported by polarizationdependent RIXS measurements, where we disentangle the spinconserving ΔS=0 scattering from the predominant ΔS=1 signal. We discuss the implications of the changed spin fluctuations for the interplay between Ca content and superconductivity. 
Monday, March 15, 2021 11:42AM  11:54AM 
B47.00002: Unconventional superconductivity mediated by spin fluctuations in singlelayer NbSe_{2} Wen Wan, Paul Dreher, Rishav Harsh, Francisco Guinea, Miguel Ugeda Van der Waals materials provide an ideal platform to explore superconductivity in the presence of strong electronic correlations, which are detrimental of the conventional phononmediated Cooper pairing in the BCSEliashberg theory and, simultaneously, promote magnetic fluctuations. Despite recent progress in understanding superconductivity in layered materials, the glue pairing mechanism remains largely unexplored in the singlelayer limit, where electronelectron interactions are dramatically enhanced. Here we report experimental evidence of unconventional Cooper pairing mediated by magnetic excitations in singlelayer NbSe_{2}. Our highresolution tunneling spectroscopic measurements reveal a characteristic spin resonance excitation in the density of states that emerges from the quasiparticle coupling to a collective bosonic mode. This resonance, observed along with higher harmonics, gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values, which sets an unambiguous link to the superconducting state. Furthermore, we find clear anticorrelation between the energy of the spin resonance and its harmonics and the local superconducting gap, which invokes a pairing of electronic origin associated with spin fluctuations. 
Monday, March 15, 2021 11:54AM  12:06PM Live 
B47.00003: Ferroelectric Exchange Bias Affects Interfacial Electronic States Gal Tuvia, Yiftach Frenkel, Prasanna Rout, Itai Silber, Beena Kalisky, Yoram Dagan In polar oxide interfaces phenomena such as superconductivity, magnetism, 1D conductivity, and quantum Hall states can emerge at the polar discontinuity. Combining controllable ferroelectricity at such interfaces can affect the superconducting properties and sheds light on the mutual effects between the polar oxide and the ferroelectric oxide. Here, the interface between the polar oxide LaAlO_{3} and the ferroelectric Ca substituted SrTiO_{3} is studied by means of electrical transport combined with local imaging of the current flow with the use of scanning a superconducting quantum interference device (SQUID). Anomalous behavior of the interface resistivity is observed at low temperatures. The scanning SQUID maps of the current flow suggest that this behavior originates from an intrinsic bias induced by the polar LaAlO_{3} layer. Such intrinsic bias combined with ferroelectricity can constrain the possible structural domain tiling near the interface. The use of this intrinsic bias is recommended as a method of controlling and tuning the initial state of ferroelectric materials by the design of the polar structure. The hysteretic dependence of the normal and the superconducting state properties on gate voltage can be utilized in multifaceted controllable memory devices. 
Monday, March 15, 2021 12:06PM  12:18PM Not Participating 
B47.00004: Quantum transport in inplane hybrid InSbAl nanowire networks Di Xu, Roy Op het Veld, Vanessa Schaller, Jason Jung, Qingzhen Wang, Michiel De Moor, Bart Hesselmann, Kiefer Vermeulen, Jouri Bommer, Joon Sue Lee, Mihir Pendharkar, Chris J Palmstrom, Erik P. A. M. Bakkers, Leo Kouwenhoven, Hao Zhang Strong spinorbit semiconductor nanowires with induced superconductivity are predicted to host Majorana zero modes (MZMs). Exchange (braiding) operations of MZMs form the logical gates on topological qubits and require a network of nanowires. Here, we demonstrate essential quantum transport properties on our novel inplane selectivearea growth InSbAl platform which allows complex semiconductorsuperconductor nanowire networks. The quantum transport phenomena include phasecoherent transport up to 5 harmonics of AharonovBohm oscillations with a phase coherence length of ~10 μm. Tunneling spectroscopy on inplane hybrid InSb/Al nanowires demonstrates a hard superconducting gap, accompanied by distinct oscillations of Andreev bound states while increasing the magnetic field. A Cooper pair island integrated in the nanowire network shows 2eperiodic Coulomb oscillations. The results confirm the high quality of the InSb nanowire networks, holding great promise for this platform for scalable topological networks. 
Monday, March 15, 2021 12:18PM  12:30PM Live 
B47.00005: Nontopological zero bias peaks in fullshell nanowires induced by flux tunable Andreev states Marco Valentini, Fernando Peñaranda, Andrea Hofmann, Matthias Brauns, Robert Hauschild, Peter Krogstrup, Pablo SanJose, Elsa Prada, Ramon Aguado, Georgios Katsaros A semiconducting nanowire core fully wrapped by a superconducting shell has been proposed as an alternative geometry for obtaining Majorana modes without the need of fine tuning the chemical potential or an external magnetic field [1]. While this robustness seems to avoid interpretation ambiguities in terms of nontopological Andreev bound states, we here demonstrate that the appearance of subgap states is actually governed by the junction region in tunneling spectroscopy measurements, not the fullshell nanowire itself [2]. Short tunneling regions never show subgap states, while longer junctions always do. This can be understood in terms of quantum dots forming in the junction and hosting Andreev levels in the YuShibaRusinov regime. Their intricate magneticfield dependence, both through the Zeeman and the LittleParks effects, may result in robust zerobias peaks, a feature that could be easily misinterpreted as originating from Majoranas, but is unrelated to topology. 
Monday, March 15, 2021 12:30PM  12:42PM Live 
B47.00006: Absence of LutherEmery Phase in the threeband model for cuprate ladders R. Torsten Clay, JeongPil Song, Sumit Mazumdar Ladders are one system where predictions of stronglycorrelated models of superconductivity (SC) can be checked rigorously. Under weak doping the spingapped state in the undoped twoleg single band Hubbard model evolves into a LutherEmery liquid with gapless charge and gapped spin modes. Density Matrix Renormalization Group (DMRG) calculations show quasilong range order of superconducting pairpair correlations with distance decay slower than 1/r. The presence of SC near a spin gapped state has strongly influenced many theories of correlated SC in two dimensions which claim a proximity of SC and spin gap. However, the single band ladder theory does not seem applicable to real cuprate ladder superconductors, which undergo a onetotwo dimensional crossover. The ladder picture of SC has not been tested within the more realistic threeband model for the cuprates. We argue that due to the presence of the oxygen orbitals, the effective mass of pairs is expected to be far larger within the threeband ladder, suppressing pairing correlations [1]. DMRG calculations on a threeband cuprate ladder confirm these ideas, finding a decay with distance of pairpair correlations indistinguishable from that of free fermions. 
Monday, March 15, 2021 12:42PM  12:54PM Live 
B47.00007: Suppression of superconductivity in the threeband Hubbard model for cuprate ladders JeongPil Song, Sumitendra Mazumdar, R. Torsten Clay We present Density Matrix Renormalization Group calculations of the spin gap and pairing correlations in the Hubbard ladder model on a copper oxide lattice with multiband topology. The large spin gaps for the undoped ladder are in close agreement with the previous DMRG studies of the singleband ladder model. Dopants equally reside both on the rung and leg oxygen sites, and the extrapolated spin gap considerably decreases with increasing doping concentration. The extrapolated pairpair correlations decrease more sharply than their singleband counterparts. Our quantitative results on the realistic sets of parameters for the cuprates show that the powerlaw decay of the correlation function is nearly indistinguishable from that of free fermions. Our DMRG studies suggest that the weaklydoped singleband Hubbard model is not adequate for both ladder and layered superconducting materials. These results also raise disturbing questions about the applicability of many of the existing theories of superconductivity in cuprates, which assume gapped spin liquid states proximate to superconductivity. 
Monday, March 15, 2021 12:54PM  1:06PM Live 
B47.00008: Quantitative assessment of the role of spin fluctuations in the 2D Ising superconductor NbSe_{2} Suvadip Das, Igor Mazin, Elena R Margine The recent discovery of a strong spinorbit effect in superconducting NbSe_{2} monolayers, dubbed Ising superconductivity, has attracted a lot of attention. It was recently found that this material is close to magnetism. Thus it was proposed that spin fluctuations may play an important role in determining the superconducting order parameter (OP), in particular its parity. In our previous paper, we predicted spin fluctuations to be ferromagnetic, however, these calculations were utilizing uncontrollable approximations. In this talk, we report more accurate DFT calculations of the spin fluctuation spectrum, by calculating energies of spin spirals stabilized by artificially enhanced Hubbard interaction. From these calculation, the fully renormalized qdependent DFT spin susceptibility can be extracted by inverting the RPA formula. It appears that the structure of spin fluctuations in the momentum space is more complicated than previously thought of. In the second part, we report stateoftheart DFT calculations of the momentumresolved electronphonon coupling function α^{2}F(k, k', ω). Together, the two parts provide a complete framework for addressing quantitatively the structure of the Ising OP in NbSe_{2}. 
Monday, March 15, 2021 1:06PM  1:18PM Live 
B47.00009: Zero energy modes to midgap states in one dimensional charge conserving superconductors  Exact results Parameshwar Pasnoori, Natan Andrei, Patrick Azaria We consider one dimensional charge conserving superconductors where intrinsic attractive pairing interactions between the electrons gives rise to superconductivity. Using Bethe ansatz we show that in such systems with open geometry, when appropriate boundary conditions are applied, the system can exhibit topological phase characterized by the existence of zero energy bound states localized at the edges. For certain boundary conditions these bound states gives rise to degeneracy in the ground state structure which results in the existence of fractionalized 1/4 spins at the edges. We show that by tuning the boundary parameters appropriately, these zero energy modes can be turned into bound states who's energy is within the dynamically generated mass gap in the bulk, and hence are called midgap states. We provide physical realization of such systems. 
Monday, March 15, 2021 1:18PM  1:30PM Live 
B47.00010: Integration of twodimensional transitionmetal dichalcogenide superconductors into 2D3D hybrid superconducting resonators Michael Sinko, Olivia Lanes, Sergio de la Barrera, Michael Jonathan Hatridge, Benjamin Matthew Hunt Atomicallythin twodimensional (2D) transitionmetal dichalcogenide superconductors enable uniform, flat and clean van der Waals tunneling interfaces, as well as increased kinetic inductance due to the atomicallythin geometry, and resilience to large inplane magnetic fields, motivating their integration into conventional superconducting circuits. We created 2D3D Josephson junction contacts with R=0 and critical currents between 0.15uA128uA. We have embedded these 2D3D contacts in RF tank circuits (Q > 4000) operating between 2.55 GHz to measure the kinetic inductance of the TMD superconductor NbSe_{2}, which is extracted by observing a shift in frequency from a fully conventional control sample. The 2D3D hybrid resonators are subsequently measured using standard DC transport techniques to extract junction critical currents and subtract Josephson inductance from the RF results. Our work lays the foundation for the analysis of TMD nanodevices in superconducting circuits. 
Monday, March 15, 2021 1:30PM  1:42PM Live 
B47.00011: Fluctuation conductivity above upper critical field in ultrathin niobium films Marta Cieplak, Iryna Zaytseva, Aleksander Abaloszew, Bruno C Camargo Fluctuating Cooper pairs surviving above upper critical field in two dimensional superconductors are predicted to introduce negative contribution to conductivity in the low temperature limit [1]. Here we report on the observation of such negative contribution in ultrathin Nb films [2]. The films, of thickness d ranging from 1.2 nm to 20 nm, undergo a transition from amorphous to polycrystalline structure at d about 3.3 nm. The superconductormetal transition induced by magnetic field is observed, with the critical field approximately constant or decreasing as a powerlaw with the film conductance in polycrystalline or amorphous films, respectively. The scaling analysis indicates distinct scaling exponents in these two types of films. Negative contribution of the SC fluctuations to conductivity, well described by theoretical predictions, is particularly pronounced in amorphous films. Our observations suggest the development of local inhomogeneities in the amorphous films, in the form of proximitycoupled superconducting islands. [1] A. A. Varlamov, et al., Rev. Mod. Phys. 90, 015009 (2018). [2] I. Zaytseva et al., Sci. Rep. 10, 19062 (2020) 
Monday, March 15, 2021 1:42PM  1:54PM 
B47.00012: Magnetoresistance of the superconducting and 'stripe' phases at EuO/KTO (111) interfaces. Changjiang Liu, Xianjing Zhou, Deshun Hong, Brandon Fisher, John Pearson, J Samuel Jiang, Dafei Jin, Anand Bhattacharya The twodimensional electron gas formed at interfaces of KTaO_{3} (111) becomes superconducting at temperatures as high as 2.2 K for a range of carrier densities (see C. Liu et al., arXiv:2004.07416). In EuO/KTO (111) samples, we also find evidence for a highly anisotropic phase in transport measurements. In particular, for temperatures in between the normal and superconducting states, the resistance along the [112] direction is lower than in the [110] direction, being several times lower in the most anisotropic samples. We will present detailed measurements of magnetoresistance in these samples over a range of temperatures. Our measurements support the existence of a state where the superconductivity is spatially modulated in a 'stripe' like pattern, oriented along the [112] direction. We will present measurements for both inplane and outofplane magnetic fields, and discuss a simple physical model based on Josephson coupling between stripes that explains some of our observations. 
Monday, March 15, 2021 1:54PM  2:06PM Live 
B47.00013: A representation of the 12 by 12 spectral function of the Hubbard model with variational Monte Carlo Maxime Charlebois, Masatoshi Imada A method to calculate the onebody Green's function for ground states of correlated electron materials is formulated by extending the variational Monte Carlo method. The new method have been recently published in Ref. [1]. We benchmark against the exact diagonalization (ED) for the oneand twodimensional Hubbard models of 16 site lattices, which proves high accuracy of the method. The application of the method to largersized Hubbard model on the square lattice correctly reproduces the Mott insulating behaviour at halffilling and gap structures of dwave superconducting state in the hole doped Hubbard model, evidencing a wide applicability to strongly correlated electron systems. 
Monday, March 15, 2021 2:06PM  2:18PM Live 
B47.00014: BerezinskiiKosterlitzThouless transitions in spintriplet superconductor Suk Bum Chung, Se Kwon Kim As the spintriplet superconductivity arises from the condensation of spinful Cooper pairs, spin ordering is an essential feature. Hence the possibility arises for the BerezinskiiKosterlitzThouless (BKT) phase transitions in not only the charge sector but also the spin sector in a 2d easyplane spintriplet superconductivity. However, it turns out that there are actually three possible BKT transitions, involving the unbinding of, respectively, the conventional vortices, the merons and the halfquantum vortices with vorticity in both the charge and the spin current. We show how all the transitions can be characterized by the relation between the voltage drop and the spinpolarized current bias and each phase by its characteristic Josephson coupling. 
Monday, March 15, 2021 2:18PM  2:30PM Live 
B47.00015: Tunable Flux Vortices in 2D Dirac Superconductors Sina Zeytinoglu, Atac Imamoglu, Sebastian Huber The nontrivial geometry encoded in the Quantum Mechanical wavefunction has important consequences 
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