Session W61: Superconductivity: Low Dimensional Systems-II

Properties of Cu-intercalated TaSe2

TaSe2 belongs to a group of materials named transition metal dichalcogenides, exhibiting various interesting spin and transport properties. The layered nature of TaSe2 allows for intercalation, which introduces elements such as electrons, holes, or magnetic moments to the system. The superconductivity of TaSe2 is enhanced by nickel intercalation, the origin of which is not well-understood. To contrast with magnetic intercalation, Cu-intercalated samples are synthesized. The experimental synthesis methods and findings on Cu-intercalated samples will be discussed in the context of recent evidence of enhanced superconductivity with Ni-intercalation of this material. 

Reentrant orbital effect against superconductivity in the quasi-two-dimensional superconductor NbS2

We derive integral equation for superconducting gap, which takes into account the quantum nature of electron motion in a

parallel magnetic field in a quasi-two-dimensional (Q2D) superconductor in the presence of a non-zero perpendicular field

component. By comparison of our theoretical results with the recent experimental data obtained on the NbS₂, we show that the

orbital effect against superconductivity  partially destroys superconductivity in the so-called Ginzburg-Landau area of this

Q2D conductor, as expected. Nevertheless, at relatively high magnetic fields, H > 15 T, the orbital effect

starts to improve the Fulde-Ferrell-Larkin-Ovchinnikov phase in the NbS₂, due to the quantum nature of electron motion in a

parallel magnetic field. In our opinion, this is the most clear demonstration that the orbital effect against superconductivity

in a parallel magnetic field has a reentrant nature.   

Direct measurements of diamagnetism in few-layer van der Waals superconductors

Thermodynamic properties of few-layer van der Waals superconductors are not well characterized, due to the small volume of the exfoliated devices. Here, we report direct measurement of the diamagnetism of few-layer NbSe2 and ionic gated MoS2 devices. By utilizing a scanning SQUID susceptometer, we report the temperature, magnetic field, and frequency dependence of the susceptibility of the superconductor. We observe signatures suggestive of a broadened vortex binding unbinding transition, and discuss the relation of this transition to the Berenzinskii-Kosterlitz-Thouless (BKT) transition. We report efforts to extract superconducting parameters from the temperature dependence of the magnetic response.   

Superconducting properties of purely distorted transition metal dichalcogenides

The distorted phases (such as 2M) of transition metal dichalcogenides (TMDCs) exhibit exotic physical properties, such as superconductivity, topological energy gap, and Majorana bound states [1-2]. However, because the distorted phases are metastable and they usually appear in domains within their thermodynamically stable counterpart, transport experiments of these phases have been limited. Here we present a TMDC with a purely distorted phase that features unconventional superconductivity with an extremely anisotropic upper critical field. Remarkably, the critical field along the in-plane direction violates the well-established Pauli limit. We find that this superconductor shows a surprisingly large critical current density, which is two orders of magnitude higher than that in typical two-dimensional superconductors. Our work provides a promising route to understand the nature of superconductivity in distorted TMDCs.   

Van der Waals 2D cuprate superconductors – potential device building blocks for quantum applications

Quantum computers (QC) will provide a revolutionary tool for solving upmost complex problems that cannot be tackled efficiently by silicon-based technologies. Current Qbits, the constituents of QC, are based on conventional superconductors. These materials pose serious operating constraints as they work only close to absolute 0 K. To overcome this issue, 2-dimensional van der Waals (vdW) encapsulated high temperature superconductor (HTSC) stacks could be used as building blocks for future Qbits, thanks to recent advancements in material nanofabrication. First, a detailed understanding of their structure and electronic dynamics is required.

In this work, we present low T resonant soft X-ray studies on ultrathin Bi₂Sr₂CaCu₂O_8+y (BSCCO) vdW heterostructures - promising candidates for large scale Qbits. BSCCO crystals are characterized by two incommensurate lattice modulations (ILM), which can be exploited to witness the relation between structure and electrons in low dimensions. For the first time, we report of ILMs and structural [004] peak maps taken across the SC transition temperature (Tc ≈ 60 K). We believe this sheds light onto the electronic interplay as we find the intensity of ILMs depends on the SC state of the flake while [004] seems not to.   

Magnetic anisotropy of few-layer superconducting NbSe2

We present a unifying picture of the magnetic in-plane anisotropies of two-dimensional superconductors based on transition metal dichalcogenides. The symmetry considerations are first applied to constrain the form of the conductivity tensor. We hence conclude that the two-fold periodicity of transport distinct from the planar Hall related contributions requires a tensor perturbation. At the same time, the six-fold periodic variation of the critical field results from the Rashba spin-orbit coupling on a hexagonal lattice. We have considered the effect of a weak tensor perturbation on the critical field, gap function, and magneto-conductivity. The latter is studied using the time-dependent Ginzburg-Landau phenomenology. The common origin of the two-fold anisotropy in transport and thermodynamics properties is identified. The scheme constructed here is applied to describe the existing theoretical scenarios from a unified point of view. This allows us to single out the differences and similarities between the suggested approaches.   

Spin-valley locking in the chiral superconductor 4Hb-TaS2

Van der Waals materials offer unprecedented control of electronic properties via stacking of different types of two-dimensional materials. A fascinating frontier, largely unexplored, is the stacking of strongly correlated phases of matter. We study 4Hb-TaS₂, which naturally realizes an alternating stacking of 1T-TaS₂ and 1H-TaS₂. The former is a well-known Mott insulator, which has recently been proposed to host a gapless spin-liquid ground state. The latter is a superconductor known to also host a competing charge density wave state.  4Hb-TaS₂ is a superconductor with Tc=2.7K that was recently reported to break time reversal symmetry.

Using spin and angle-resolved photoemission spectroscopy, we find a significant charge transfer from the 1T-TaS₂ to the 1H-TaS₂ layers in good agreement with DFT calculations.

In addition, we find that the spin of quasiparticles in 4????- ??????₂ is locked to the valley degree-of-freedom leading to a non-trivial spin-texture of the Fermi-surface.   

Ising superconductivity in bulk (LaSe)1.14(NbSe2)m=1,2 misfit compounds

Extreme in-plane upper critical magnetic fields B_c2//ab violating the Pauli limit have been observed in the (LaSe)_1.14(NbSe₂) and (LaSe)_1.14(NbSe₂)₂ single crystals with T_c = 1.23 K and 5.7 K, respectively.  The crystals show a 2D-3D transition at the temperatures slightly below T_c with an upturn in the temperature dependence of B_c2//ab, a huge temperature dependent superconducting anisotropy and a cusp-like behavior of the angular dependence of B_c2.[1] Both misfits are characterized by a strong charge transfer from LaSe to NbSe₂. (LaSe)_1.14(NbSe₂)₂ is electronically equivalent to highly doped NbSe₂ monolayers.[2] Then, the strong upper critical field can be attributed to the Ising coupling recently discovered in atomically thin transition metal dichalcogenides with strong spin-orbit coupling and lack of inversion symmetry.[3] A similar behavior is found in (LaSe)_1.14(NbSe₂), where the charge transfer is even larger, almost filling the NbSe₂ conduction band. Here, the larger spin splitting at the top of the band leads to a stronger Ising behavior. Yet, the topmost layer exhibits entirely different electronic properties. Both misfits behave as a stack of almost decoupled superconducting atomic layers proving that Ising superconductivity can also exist in bulk materials.   

Hidden magnetic memory and spontaneous superconducting vortices in alternating stacking compound 4Hb-TaS2

Van der Waals heterostructures provide a unique opportunity to examine proximity effects between materials with vastly different ground states. 4Hb-TaS2 naturally realizes this opportunity as its structure is an alternate stacking of two lattice structures, 1T and 1H, a candidate spin liquid and a superconductor. In my talk, I will show scanning superconducting quantum interference device (SQUID) data, mapping the magnetic landscape of this compound. The data reveal hidden magnetic memory in the normal state which affects the superconducting state. Our results indicate an unconventional interplay between time reversal symmetry breaking and superconductivity in this compound.    

Superconducting Fluctuations and Paraconductivity in Ultrathin a-Pb Films near Superconductor-Insulator Transition

The Aslamazov-Larkin (AL) equation describes the extra conductance above T_c of a superconductor due to superconducting fluctuations. While agreement with AL model has been found in some conventional 2D superconductors, its applicability in ultrathin limit near the superconductor-insulator transition (SIT) has not been ascertained. Here we report a study of superconducting fluctuation and paraconductivity in ultrathin 2D amorphous Pb films near SIT. The films were incrementally deposited in a dilution refrigerator. Electrical measurements were performed in situ at each thickness, resulting in a series of R(T) across the SIT. Paramagnetic impurities (Cr) were then deposited, driving the film back to SIT and yielding another set of R(T). The two sets of G(T) were fitted to AL equation with T_c and magnitude of the paraconductivity as fitting parameters. In both cases, as T_c decreases below ~2 K, the paraconductivity increases precipitously, reaching more than an order of magnitude higher than the prediction of AL model. The significant enhancement of superconducting fluctuations near SITs suggests pronounced emergent electronic inhomogeneities in the uniform 2D amorphous films.   

Observation of Quantum Griffith’s singularity and anomalous metal in LaScO3/SrTiO3 heterostructure.

Two-dimensional (2D) disordered superconducting systems in the presence of strong quenched disorder can exhibit the phenomenon called Quantum Griffith's singularity. In this phase, the system cannot maintain long-range order but instead form locally ordered phases called rare regions. Sometimes, the formation of these rare regions changes the critical behavior of superconductor-insulator-transition (SIT) to an Infinite randomness quantum critical point. In our work, we show experimentally that for the quasi-2DEG at the LaScO3/SrTiO3 heterostructures interface, the superconductor-to-insulator phase transition is consistent with quantum Griffith's singularity and is associated with an infinite randomness quantum critical point. The emergence of the anomalous metallic phase in the low-temperature regime cuts off the corresponding Griffith's singularity associated with SIT, and the system deviates from the activated dynamical scaling. These deviations direct us to explore the infinite randomness quantum critical point when the system is in the anomalous metallic state.    

Improving the Crystal Quality of Niobium on Sapphire Thin Films in 2D Transmon Fabrication via UHV Annealing

Superconducting metallic thin films such as Nb are utilized in the fabrication of 2D superconducting qubits, and the crystalline quality of the films is believed to be among the possible factors affecting coherence. Conventional deposition methods include DC sputtering and high-power impulse magnetron sputtering (HiPIMS), resulting in textured Nb (110) thin films.  While these textured Nb (110) thin films are suitable for superconducting transmons, the coherence times of the resulting devices are believed to be limited by material defects, thus motivating efforts to realize higher quality Nb thin films. Here, we present ultra-high vacuum (UHV) annealed Nb thin films, previously deposited by HiPIMS, at base pressures of ~10^-10 Torr and temperatures up to 1000°C. The samples were deposited both on α-Al₂O₃ (006) and (110) substrates to make use of the epitaxial relationships with Nb (110). Specular and off-specular X-ray diffraction measurements suggest improvements in crystal quality following UHV annealing. Specifically, we show that the as-grown samples have a degree of epitaxy that is further enhanced by UHV annealing. Finally, the impact of the improved Nb crystal quality on superconducting properties is probed using cryogenic charge transport measurements.   

Critical current non-reciprocity in quasi-one-dimensional systems

The recent observations of the superconducting diode effect pose the challenge to fully understand the necessary ingredients for non-reciprocal phenomena in superconductors. In this theoretical work, we focus on the non-reciprocity of  the critical current in a quasi-one-dimensional superconductor and analyze the roles of symmetry and topology on the superconducting diode effect. We define the critical current as the value of the supercurrent at which the quasiparticle excitation gap closes (depairing). Once the critical current is exceeded,  the quasiparticles can exchange energy with the superconducting condensate, giving rise to dissipation. Our minimal model can be microscopically derived as a  low-energy limit of a Rashba spin-orbit coupled superconductor in a Zeeman  field. Within the proposed model, we explore the nature of the non-reciprocal effects of the critical current both analytically and numerically. Our results quantify how system parameters  such as spin-orbit coupling and chemical potential affect the strength of the superconducting diode effect. Our theory provides a complementary description to the Ginzburg-Landau theory of the effect.