Session W57: Superconductivity: TMC & Other SC materials -IV

Anisotropic superconducting properties of Pd3Bi2Se2 single crystals

The Parkerite-type transition-metal chalcogenides have been known for their intriguing properties such as charge density wave, superconductivity, and intercalation reactions. Here, we report electrical transport down to 1.8 K and tunnel diode oscillator (TDO) measurements down to 450 mK on high quality single crystalline Pd₃Bi₂Se₂ adopting the Parkerite structure. The temperature dependence of the electrical resistivity shows typical metallic behavior. Superconductivity is observed with an onset temperature of ~0.82 K via the TDO measurements. By measuring the temperature dependence of the TDO resonant frequency in various applied fields, the phase diagrams are constructed for both in-plane and out of plane directions. The ratio of the upper critical field H_c2(in-plane)/H_c2(out of plane) is found to be 1.75, indicating a weak anisotropic nature of the superconductivity in Pd₃Bi₂Se₂. This low value of the anisotropy is unexpected considering the layered monoclinic structure of Pd₃Bi₂Se₂ suggesting that the bonding along the out of plane direction is reasonably strong in this material.   

Enhanced superconductivity, new polymorphic phase, and rich phase diagram in the PdSe2-xTex system

We report a combined experimental and theoretical study of the PdSe_2−xTe_x system.  With increasing Te fraction, structural evolutions, first from an orthorhombic phase (space group Pbca) to a new monoclinic phase (space group C2/c) and then to a trigonal phase (space group P−3m1), are observed accompanied with clearly distinct electrical transport behaviors. This new monoclinic phase (C2/c) belongs to a very rare Verbeekite polymorphism and is discovered within a narrow range of Te composition (0.3≤x≤0.8). Interestingly, in the trigonal phase region, superconductivity with significantly enhanced critical temperature is observed within a narrow range of Te content (1.0≤x≤1.2) in comparison with that of 1T-PdTe₂ phase. The superconductivity has been unambiguously confirmed by the magnetization, resistivity, and specific heat measurements. The detailed electrical transport analysis and electronic calculations across the rich phase diagram will be presented. The unusual superconductivity properties, the new polymorphic structure, and the rich phase diagram in this system could further stimulate interest to explore the new superconducting mechanism and new types of polymorphs in the large family of transition metal dichalcogenides.

    

Role of magnetic and nonmagnetic impurities in unconventional behavior of Ising superconductors

Apart from their signature giant anistotropy of the critical field, Ising superconductors (IS) exhibit other rather unexpected properties: (1) in tunneling through a ferromagnetic barrier, NbSe₂ IS demonstrates anticorrelaton between the tunneling gap value and gap broadening, internal hysteretic behavior that sets 2 K below T_c, spontaneous breaking of the C₆ symmetry (interpreted originally as"nematic superconductivity") and (2) NbSe₂ IS shows nonmonotonic behavior of Tc and Δ as a function of Se⇒S (initially interpreted as "multifractal superconductivity") and Nb⇒Mo doping . We show, utilizing results of first principle and model calculations, that all these effects are consequence of highly nontrivial interaction of IS with magnetic and nonmagnetic impurity. In this talk we will discuss (1), while (2) will be presented in the following talk by D. Wickramaratne. Finally, we will briefly discuss some nontrivial predictions/suggestions for new experiments that follow from the presented theory.   

Ising superconductivity in NbSe2 alloys

Monolayer NbSe₂ and NbS₂ are isostructural two-dimensional materials that exhibit contrasting superconducting properties. Monolayer NbSe₂ is an Ising superconductor,while there are no reports of superconductivity in monolayer NbS₂.Recent experiments on NbSe₂ alloyed with sulfur have shown T_c varies non-monotonically with sulfur content.This was interpreted as a manifestation of fractal superconductivity. In this talk we will show that the nonmonotonic dependence of T_c is insufficient evidence for fractal superconductivity. Using first-principles calculations we highlight how this non-monotonic change in T_c can arise due to magnetic point defects such as chalcogen vacancies, which we expect to be present in appreciable concentrations, along with changes in the density of states and proximity to magnetism as a function of alloying.b We use these insights to show how alloying on the Nb site with molybdenum may also lead to a non-monotonic change of T_c in NbSe₂.

    

Multi-band s++ superconductivity in V3Si determined from the response to a controlled disorder

Superfluid density, ρ(T), of a high-quality V₃Si single crystal shows fully-gapped isotropic BCS superconductivity with an additional distinct feature of two almost decoupled superconducting gaps. However, this ρ(T) can be obtained if these two order parameters have different (s_±) or the same (s₊₊) sign. To investigate, the sample was irradiated at 20 K by 2.5 MeV electrons three times, repeating the measurements between the irradiation runs. A large total dose of 6×10¹⁹ cm^-2 was accumulated, after which the superconducting transition temperature, T_c, decreased from 16.4 K in pristine state to 14.7 K. This substantial suppression is impossible for a single isotropic gap, yet it is not large enough for a sign-changing s_±  pairing state. The electronic band structure calculations show how five bands crossing the Fermi level can naturally support two effective gaps, not dissimilar from the iron pnictides. The two-gap self-consistent theories for both, ρ(T) and △T_c, describe the data very well. Thus, the experimental results and theoretical analysis provide strong support for s₊₊ superconductivity with two unequal gaps, △₁ =2.5 meV and △₂ =1.4 meV, and a very weak inter-band coupling in the V₃Si superconductor.   

Crossover of Ising- to Rashba-Type Superconductivity in Epitaxial Bi2Se3/Monolayer NbSe2 Heterostructures

When two different materials are brought together, the resultant interface between them often shows unexpected quantum phenomena. For example, the interface between a topological insulator (TI) and an s-wave superconductor can host an unusual form of superconductivity known as topological superconductivity. In this work, we grew the Bi₂Se₃/monolayer NbSe₂ heterostructures with different Bi₂Se₃ thicknesses using molecular beam epitaxy. We found that the gapless Dirac surface states are formed when the Bi₂Se₃ thickness is greater than 3 quintuple layers (QLs). Moreover, we observed the Rashba-type bulk conduction bands for the Bi₂Se₃ thickness greater than 2 QLs. Our observations are well interpreted by the first-principles calculations. By performing magneto-transport measurements, we found that the in-plane upper critical magnetic field of the superconductivity in Bi₂Se₃/monolayer NbSe₂ heterostructures is greatly suppressed when the Rashba bands emerge, indicating the occurrence of a crossover from Ising- to Rashba-type pairings. Our success in the synthesis of Bi₂Se₃/monolayer NbSe₂ heterostructures and the demonstration of the crossover from Ising- to Rashba-type superconductivity in these heterostructures provide more impetus for exploring the topological superconductivity in TI/superconductor heterostructures.   

Superconductivity mediated by polar modes in doped ferroelectrics

The occurrence of superconductivity in doped SrTiO₃ and related materials at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades.  We present experimental results showing the pressure dependence of the superconducting transition temperature, T_c, that sheds light on the nature of this interaction.  We find that T_c increases dramatically when the energy gap of ferroelectric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached, in a way reminiscent of behaviour observed in magnetic counterparts.  However, in contrast to the latter, the coupling of itinerant electrons to the critical modes in ferroelectrics is predicted to be small.  We present a superconductivity model that shows quantitative agreement with experiment without the use of adjustable parameters which involves electron pairing via the virtual exchange of longitudinal hybrid-polar-modes.  The model accounts for the domes of superconductivity observed as function of carrier density and pressure and the enhancement of T_c close to quantum criticality even in the absence of a direct coupling to the transverse critical modes.   

Enhanced superconductivity in 2M-WS2 through doping

A new member of transition metal dichalcogenide (TMD) family, 2M-WS₂, has been recently discovered and shown to display superconductivity with a critical temperature (T_c) of 8.8 K, the highest T_c among superconducting TMDs at ambient pressure. Using first-principles calculations combined with the anisotropic Migdal-Eliashberg formalism, we explore how the superconducting properties of 2M-WS₂ can be enhanced through doping. We find that compared to the parent compound the electron-phonon coupling can be increased by p-type doping, leading to an enhancement of the T_c up to 50%.   

Suppression of electron-phonon mediated superconductivity by strong spin fluctuation effects in the 2D Ising superconductor NbSe2

Two-dimensional materials particularly transition metal dichalcogenides (TMDs) are promising candidates for atomically thin electronic applications, such as spintronic, valleytronic, and quantum hall devices. A later addition to the landscape pertains to the discovery of an unusual superconducting state in monolayers of NbSe₂ dubbed ‘Ising superconductivity’. It is generally believed that the superconductivity of monolayer NbSe₂ is dominated primarily by electron-phonon effects. However, comparison with the experiment indicates that conventional first-principles calculation of the electron-phonon coupling severely overestimates the critical temperature and superconducting gap. Since it was recently found that this system is close to a magnetic instability, and given that the first principles Eliashberg theory has been very successful in the prediction of the superconducting temperature in phonon superconductors, it is natural to assume that superconductivity in NbSe2 is partially suppressed by spin fluctuations. In our work, we include the latter on the level of Berk-Schrieffer's theory. It appears crucially important to include retardation effects accounting for the different energy scales of phonons and paramagnons. Both momentum and frequency dependence of spin-fluctuations were extracted from ab initio calculations, along with the full electron-phonon momentum-resolved Eliashberg function. After solving the full band and momentum-resolved Eliashberg equations, we extracted the momentum and temperature-dependent order parameter, as well as the critical temperature. We then investigated the average order parameter averaged over each Fermi surface, as well as over the all electronic state, and after comparison with the tunneling data, discuss, as to what level the calculated anisotropy survives in real samples.   

Bogoliubov Fermi surfaces due to out-of-plane magnetic fields in monolayer superconducting NbSe$_2$

We use a low-energy model to investigate the effect of an out-of-plane magnetic field on the superconducting properties of monolayer NbSe$_2$.  We find that, even when the singlet-channel interactions dominate, such that the zero-field superconducting state is fully gapped, there is a threshold Zeeman field beyond which the quasiparticle spectrum is no longer fully gapped, giving rise to Bogoliubov Fermi surfaces.  We establish this by self-consistently solving the gap equations for mixed singlet and triplet pairing on the $\Gamma$ pocket, in the presence of both Ising spin-orbit coupling and Zeeman field.  We find that the resulting evolution of the density of states with magnetic field qualitatively agrees with the scanning tunneling microscopy data in this material.   

Phonon-induced rotation of the electronic nematic director in superconducting doped Bi2Se3

Nematic superconductivity has been directly observed in the doped topological insulator A_xBi₂Se₃, with A={Cu, Nb, Sr}, as manifested by a two-fold symmetric in-plane critical field H_c2. The orientation of the elliptically-shaped in-plane H_c2 is set by the electronic nematic director, which is parametrized by an angle α. Since the nematic order parameter follows the Z₃-Potts model, the angle α naturally aligns with one of the three high-symmetry directions of the trigonal lattice of Bi₂Se₃. We find that the inclusion of acoustic phonons changes this phenomenology in a fundamental way in trigonal systems, but not in hexagonal ones. Trigonal systems have an additional elastic constant c₁₄, stemming from the fact that the in-plane and out-of-plane shear strain doublets belong to the same irreducible representation. We show that, when c₁₄ is large enough, the contribution to the nematic action coming from the coupling to the acoustic phonons makes the nematic director unlock from the high-symmetry directions. We discuss the implication of this rotation of the nematic director in H_c2 measurements, as well as the applicability of these results to doped Bi₂Se₃.   

Investigation of superconducting properties of AuAgTe4 under pressure

Materials containing gold have been seldom explored since gold is a very inert element forming relatively few compounds. In this work, we investigated the mineral sylvanite, AuAgTe₄, under compression using first-principles calculations and found a structural phase transition from monoclinic P2/c to P2/m structure around 5 GPa (in agreement with X-ray diffraction and Raman results). We further examined the superconducting properties of the P2/m phase using the EPW package and identified that the main contribution to the electron-phonon coupling arises from the low-energy phonon modes.   

Investigation of pressure-driven superconductivity in TlInTe2

A dome-shaped curve has been typically observed in many superconductors under compression. However, it has recently been found that the Zintl compound TlInTe₂ exhibits a V-shaped phase diagram of superconducting critical temperature (T_c) upon increase in pressure (P). In this work, we have explored the superconducting properties of TlInTe₂ using the Migdal-Eliashberg formalism to shed light on this T_c-P behavior.   

Charge Density Wave Proximity Effect Induced in Graphene on 1T-TaS2

Recent experiments have shown that the charge density wave (CDW) lattice of 1T-TaS₂ can be observed within a graphene layer placed on top using scanning tunneling microscopy and spectroscopy (STM/STS)^1-3. This observation suggests the possibility of an unexplored proximity-induced CDW within graphene when placed in contact with 1T-TaS₂. The nature of such a novel CDW proximity effect is shown to involve short-range Coulomb and exchange interactions between itinerant Dirac electrons in graphene and localized electrons in 1T-TaS₂. We further explore the CDW proximity effect by probing the local electronic properties of graphene both on top of and in lateral contact with 1T-TaS₂ using STM and STS. Finally, we organize our results by relating the CDW proximity effect with well-known contact proximity effects such as the spin-orbit, magnetic, and superconducting proximity effects.   

Nematic, chiral, and topological superconductivity in transition metal dichalcogenides

We introduce and study a realistic model for superconductivity in twisted bilayer WSe2, where electron pairing arises from spin-valley fluctuations in the weak-coupling regime. Our model comprises both the full continuum model moiré bandstructure and a short-ranged repulsive interaction that accounts for the Coulomb interaction projected onto the localized Wannier orbitals. By calculating the spin-valley susceptibility, we identify a Fermi surface nesting feature near half-filling of the top-most moiré band, which induces significantly enhanced spin-valley fluctuations. We then analyze the dominant Kohn-Luttinger pairing instabilities due to these spin-valley fluctuations and show that the leading instability can induce nematic, chiral and topological superconductivity. As our findings are asymptotically exact for small interaction strengths, they provide a viable starting point for future studies of superconductivity in twisted transition metal dichalcogenide bilayers.