Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session G25: Superconductivity:Low Dimensional-II |
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Sponsoring Units: DCMP Room: Room 217/218 |
Tuesday, March 7, 2023 11:30AM - 11:42AM |
G25.00001: Doping asymmetry in the three-band model for cuprate ladders Jeong-Pil Song, Sumitendra Mazumdar, R. Torsten Clay The validity of the two-dimensional one-band Hubbard model to the high temperature cuprate superconductivity remains controversial. The Hubbard ladder lattices are one of the models to precisely examine the difference between one- and multi-band models. While the ground state of the doped one-band ladder is known to be a Luther-Emery superconducting phase, recent results of the hole-doped three-band model found the rapid suppression of the superconducting phase for realistic cuprate parameters. Here we present density matrix renormalization group calculations for the electron-doped cuprate ladders over a wide range of doping levels. For electron doping we find the Luther-Emery superconductivity with quasi-long-range order. Our numerical results appear to be in agreement with recent studies on t-t'-J model that found strong pairing for positive t' (electron doping), but suppression of pairing for negative t' (hole doping). Our results demonstrate that the generic three-band model cannot be reduced to the effective Zhang-Rice model for hole doping. |
Tuesday, March 7, 2023 11:42AM - 11:54AM |
G25.00002: Spin-excitations in the superconducting states of K2Mo3As3 and K2Cr3As3 and indications of spin-triplet superconductivity Keith M Taddei, BING-HUA LEI, Michael A Susner, Hui-Fei Zhai, Thomas J Bullard, Liurukara D Sanjeewa, Qiang Zheng, Athena S Sefat, Songxue Chi, Clarina dela Cruz, Bing Lv, David J Singh In the race to find new topological materials, superconductivity has found renewed interest as a potential host of the Majorana Fermion. This exotic solution to the relativistic wave-equation is predicted to arise as a quasiparticle in superconductors with either spin-triplet pairing or with topological Dirac-like dispersions in their band structure. Recently, a new family of quasi-1D superconductors A1,2M3As3 (with A= Alkali metal and M = Cr or Mo) was discovered which is proposed to exhibit both of these rather rare properties. Here, we report the results of neutron scattering studies together with first principles calculations which reveal the presence of spin-excitations in both K2Cr3As2 and K2Mo3As3 and identify them as likely resulting form Fermi surface nesting between two quasi-one-dimensional sheets of the Fermi surface. Intriguingly, while these fluctuations gap below the superconducting transition in K2Mo3As3, they do not in K2Cr3As3. Considering the features of the Fermi surfaces the relative Fermi velocities and previous work on these materials we use these observations to strengthen the case for K2Cr3As3 as a spin-triplet superconductor. |
Tuesday, March 7, 2023 11:54AM - 12:06PM |
G25.00003: Quasiparticle focusing of Yu-Shiba-Rusinov states in two-dimensional singlet superconductors Mateo Uldemolins, Andrej Mesaros, Pascal SIMON A magnetic impurity on a superconducting substrate induces in-gap Yu-Shiba-Rusinov (YSR) bound states, whose intricate spatial structure crucially influences the possibilities of engineering collective impurity states (e.g., Shiba chains, lattices...) and topological superconductivity. The spatial structure is complex as it blends together the information about substrate band structure, pairing function and impurity coupling. We use a generalized saddle-point approximation to deal with a point-like magnetic impurity in a gapped two-dimensional superconductor and we unveil a simple analytical relationship between, on the one hand, the real-space anisotropy of decay and oscillations of YSR states, and on the other hand, the momentum-space anisotropy of the Fermi surface, Fermi velocity, and pairing function. We reveal explicitly a non-trivial quasiparticle focusing effect in real-space that appears both in a power-law prefactor, as for impurity states in normal metals, but also in the exponential decay factor due to the superconducting gap. Our analytical approximation is quantitatively accurate against tight-binding calculations in lattice models. We study models relevant for transition metal dichalcogenides and also discuss the six-fold anisotropy of YSR state observed in scanning tunneling spectroscopy experiments of magnetic impurities on NbSe2. |
Tuesday, March 7, 2023 12:06PM - 12:18PM |
G25.00004: Quantum geometric effects on BCS-BEC crossover in a non-s wave superconductor: Application to magic-angle twisted multilayer Graphene Zhiqiang Wang, Ke Wang, Qijin Chen, Kathryn Levin Recent experiments on magic-angle twisted multilayer Graphene (MATMG) suggest that the superconductivity observed is associated with both an ultrashort coherence length and superconducting gap nodes. The former hints that BCS-BEC crossover could play a key role. The latter requires a deeper understanding of BCS-BEC crossover for a non-s wave superconductor. Experience with $d$-wave pairing in the cuprates shows that pair localization arising from the extended size of the pairs can lead to destruction of the superconducting state at stronger coupling. In MATMG, pair localization is well known to be problematic for an altogether different reason: the underlying normal state energy band structure involves flat energy bands. Importantly, in these systems quantum geometry associated with multi-orbital physics has been shown to be vital for stabilizing two-dimensional s-wave superconductivity. This leads us to present a theory of non-s wave BCS-BEC crossover with both nontrivial quantum geometry and flat energy bands. Our work clarifies the role of quantum geometry on both the Cooper pair mass and condensation temperature for this nodal superconductor across the BCS-BEC spectrum. Related experimental consequences for MATMG are discussed. |
Tuesday, March 7, 2023 12:18PM - 12:30PM |
G25.00005: Pauli limit violation, finite-momentum pairing, superconducting diode effect in moire Ising superconductors Yingming Xie, Noah F.Q. Yuan, Kam Tuen Law In this work, we propose a new type of noncentrosymmetric superconductor in moir'e materials, in which the Ising spin-orbital coupling is much larger than the moir'e band bandwidth. We call such kind of Ising superconductor as a moir'e Ising superconductor. Using a realistic continuum model of twisted homobilayer transition metal dichalcogenides, we show that moir'e Ising superconductors possess the following novel properties: (i) The spin-singlet paring and spin-triplet pairing are guaranteed to be equally mixing; (ii) The in-plane critical magnetic field shows a violation of Pauli limit as the conventional Ising superconductor. But the suppression of the superconductivity is caused by orbital effects, while the Zeeman effects are neglectable; (iii) The orbital effects of in-plane magnetic fields can further drive finite momentum pairings. In particular, due to the noncentrosymmetric behaviour and finite interlayer tunneling, we find a $2q_B$-Fulde–Ferrell (FF) pairing ($2q_B=eBd$ is a momentum shift caused by the magnetic field $B$, $d$ denotes the layer separation) is more favourable, in which the cooper pairs at two layers both carry $2q_B$ momentum; (iv) Remarkably, we further find that the $2q_B$-Fulde–Ferrell pairing would result in a gaint ($Delta j_csim 50\%$) and gate-tunable superconducting diode effect. Our theory establishes the concept of moir'e Ising superconductors and motives the studies of superconducting moir'e materials with strong Ising spin-orbital coupling. |
Tuesday, March 7, 2023 12:30PM - 12:42PM |
G25.00006: Superconducting pairing correlations in the 2D Hubbard model with next nearest hoping. Hao Xu, Chia-Min Chung, Mingpu Qin, Ulrich J Schollwoeck, Steven R White, Shiwei Zhang We study the superconducting pairing correlations in the ground state of the two-dimensional doped Hubbard models --- with nearest hoping t and next nearest hoping t’. Following the previous work on the pure Hubbard model [1], we again employ two computational methods, auxiliary field quantum Monte Carlo (AFQMC) and density matrix renormalization group (DMRG). These methods complement each other, and provide a powerful combination to tackle this challenging problem, where the results are sensitive to numerical details and finite size effects. We deploy our latest algorithmic advances, including a more robust procedure for self consistent constraint in AFQMC and the use of twist average boundary conditions. As a result, we achieve much higher accuracy than previously possible. We present detailed and systematic studies of the superconducting order parameter in the ground state for a number of parameter sets (t', doping), and discuss its interplay with magnetic and charge orders. |
Tuesday, March 7, 2023 12:42PM - 12:54PM |
G25.00007: Quantum Monte Carlo Study of Superconductivity in a Composite Bilayer System Yutan Zhang, Richard T Scalettar, Thomas A Maier, Steven S Johnston The proposition that the superconducting transition temperature Tc can be enhanced in a composite system by coupling a superconductor with a large pairing scale but strong phase fluctuations to a metal remains an interesting topic to date. To validate this prediction, we use the Determinant Quantum Monte Carlo(DQMC) method to study the composite bilayer system of a negative-U Hubbard layer coupled with a metal(U=0) layer on a square lattice. In the regime of U~W where W is the bandwidth of the metal layer, a previous study with the Dynamical Cluster Approximation(DCA) shows that Tc is enhanced for intermediate inter-layer hopping t⊥ because of an increase in the superfluid stiffness despite a reduction in the pairing interaction. However, the DCA result depends sensitively on the cluster size for small t⊥, makes estimations of Tc less precise. The DQMC method is sign problem free for the model, and provides an accurate measurement of physical quantities on finite clusters. By calculating the superfluid stiffness and the pairing correlations, we estimate Tc and compare with the DCA result. |
Tuesday, March 7, 2023 12:54PM - 1:06PM |
G25.00008: The Heat Escape time deduced from the generation of hot spot in NbTi Filament Khalil Harrabi, Abdelkarim Mekki, Hocine Bahlouli, Milorad V Milosevic The Heat Escape time deduced from the generation of hot spot in NbTi Filament |
Tuesday, March 7, 2023 1:06PM - 1:18PM |
G25.00009: Tunable phase transition and superconductivity in the Weyl semimetal Mo1-xWxTe2 Liangzi Deng, Rabin Dahal, Zheng Wu, Melissa J Gooch, Roy Arrieta, Jakoah Brgoch, Ching-Wu Chu Weyl semimetals research has been slower than desired for many years due to the lack of experimentally feasible candidate materials. While transition-metal chalcogenides have been theoretically predicted, new Weyl semimetals are being experimentally discovered. One example is WTe2, which has recently attracted significant interest due to its very large, non-saturating magnetoresistance up to 60 T and pressure-induced superconductivity. From this discovery, a tunable Weyl state in MoxW1-xTe2 was then proposed. Our group systematically investigated MoxW1-xTe2 single crystals in terms of their doping-dependent magnetotransport properties, phase transitions, and high-pressure effects. A phase diagram under pressure up to 43 GPa was constructed. With an increase in pressure, the magnetoresistance of Mo0.5W0.5Te2 is suppressed, its Hall coefficient changes sign, and superconductivity emerges, suggesting a significant reconstruction of the Fermi surface. The Tc-P phase diagram shows dome-shaped superconducting behavior in Mo0.5W0.5Te2. Theoretical analysis will also be discussed. |
Tuesday, March 7, 2023 1:18PM - 1:30PM |
G25.00010: High-order coupling between CDW and PDW in transition-metal dichalcogenide Ge He, Xiaolong Liu, Qiangqiang Gu, Joseph Paul Carroll, Seamus Davis Intertwined states of quantum matter are at the focus of modern physics. Among them, charge density wave (CDW) and Cooper-pair density wave (PDW), that originate from modulated chemical potential and pairing potential in real space respectively, are two pearls on the crown. Recently, the PDW state was observed in NbSe2, where a global δφ≅±2π/3 phase difference between the PDW and CDW states was disclosed, leaving the interaction between them to be unsolved [1]. To clarify this issue, we carefully measured PDW and CDW states in defect-free regions by atomic-resolution scanned Josephson-tunneling microscopy. We detected a 2π/3 phase sum of the three components of the CDW order simultaneously accompanied with a 2π/3 phase difference between each CDW and PDW component. The linear coupling in the Ginzburg-Landau-Wilson model among CDW, PDW and s-wave superconducting states seems unable to explain these data. Hence, we explore a high order coupling term with the form of ΔS*ΔPiΔC-jΔC-k(i, j, k = Q1, Q2, Q3 and i≠j≠k), in pursuit of the phase difference issue. This is a new route to investigating the interactions between novel ordered states in transition-metal dichalcogenides as well as other quantum matter. |
Tuesday, March 7, 2023 1:30PM - 1:42PM |
G25.00011: Realization of a structural phase transition in few-layer WS2 Sabin Gautam, Joseph McBride, Piumi I Samarawickrama, Zhuangen Fu, John Ackerman, Brian Leonard, Jifa Tian In recent years, two-dimensional (2D) tungsten disulfide (WS2) with different crystal structures has attracted significant attention due to its interesting physical properties. For instance, 2H-phase WS2 is a semiconductor with layer thickness-dependent band structures, while the metallic 2M-phase WS2 is a topological superconductor candidate. However, no progress has been made in realizing structural phase transitions between 2H and 2M phases of WS2 thus far. In this work, we report the structural phase transition from 2M to 2H in WS2 atomic layers by controlling the sample temperature using either a laser or thermal heating. We first characterized the treated 2M WS2 thin flakes using Raman spectroscopy and observed the characteristic spectrum of 2H WS2. Furthermore, we confirmed such a phase transition through the photoluminescence (PL) measurements. No PL signal has been obtained in the pristine 2M WS2 thin layer, whereas a clear PL peak ~ 1.9 eV can be detected after the thermal treatment, consistent with the semiconducting nature of 2H WS2. Our results may open a way of locally engineering the phase and physical properties of 2D transition metal dichalcogenides for functional device applications. |
Tuesday, March 7, 2023 1:42PM - 1:54PM |
G25.00012: Manipulating superconductivity of 2M WS2 by strain Piumi I Samarawickrama, jifa Tian, Xingtao Liu, Joseph McBride, Wenyong Wang, Gary Cheng, Brian Leonard Strain engineering has been widely used to modulate the physics properties of materials as well as to enhance device performance in the last few decades. In recent years, laser shocking technique, being an effective way to induce strain, has been adopted in the field of two dimensional (2D) materials down to the monolayer limit. In this work, we report the strain effect on superconductivity of 2M WS2 few layers. The strain was exerted on the few-layer WS2 samples by exposing them under a nanosecond laser. The type and magnitude of the strain are characterized by Raman spectroscopy. The electrical transport properties of the 2M WS2 samples before and after straining were systematically characterized using electrical transport measurements. We observed a drastic change of the corresponding transport properties of the strained WS2 samples compared with the unstrained ones. For example, the critical temperature and field of a 15 nm thick WS2 device without applying strain were 8.42 K and 2.79 T. But for an 18 nm thick 2M WS2 device after straining these two values decrease down to 7.8 K and 2.79 T, respectively. Correspondingly, the strain effect on the pairing symmetry of superconducting WS2 will also be revealed. Therefore, our work may shed light on understanding and manipulating the unconventional superconductivity in low-dimensional systems. |
Tuesday, March 7, 2023 1:54PM - 2:06PM |
G25.00013: Spin-fluctuation mediated superconductivity in monolayer NbSe2 Prachi Sharma, Turan Birol, Rafael M Fernandes The Ising spin-orbit coupling (SOC) generated by the lack of inversion symmetry in monolayer NbSe2 generally causes an admixture between the spin-singlet and spin-triplet components of the superconducting gap function. The resulting Ising superconducting state has attracted significant attention, as it has been proposed to display a wide range of unusual properties, including topologically nontrivial ones. Recent experiments suggest that monolayer NbSe2 has a subleading unconventional pairing state in close proximity to the leading s-wave instability. However, the microscopic mechanism responsible for this subleading state remains under debate. Here, motivated by density functional theory results demonstrating that monolayer NbSe2 is near to a ferromagnetic instability. We investigate unconventional pairing in NbSe2 mediated by ferromagnetic fluctuations. Focusing first on the Γ pocket, we show that upon increasing the Ising SOC, the pairing state changes from a (p+d) symmetry to an (s+f) symmetry. We also solve the coupled gap equations between the pockets at Γ and K/K' points, discussing the impact of the ferromagnetic correlation length in promoting a robust pairing state. |
Tuesday, March 7, 2023 2:06PM - 2:18PM |
G25.00014: Subgap states in superconducting van der Waals heterostructures Paritosh Karnatak, Zarina Mingazheva, Kenji Watanabe, Takashi Taniguchi, Helmuth Berger, László Forró, Christian Schonenberger Superconductivity in van der Waals materials, such as NbSe2 and TaS2, is fundamentally novel due to the effects of dimensionality, crystal symmetries, and strong spin-orbit coupling. Large spin-orbit coupling and the crystal symmetry in these materials leads to effects such as the survival of superconductivity up to tens of Teslas of applied in-plane magnetic field. |
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