Session P47: Superconducting Materials: Growth, Structure, and Properties

Live

Realization of superconductivity in group IV semiconductors is anticipated to be a key in the scalability of the qubit devices. Such materials are promising candidates for fault-tolerant hybrid semiconductor-superconductor quantum systems due to their high purity and ease of processing. In this study, superconducting Ge is realized via ion implantation of Ga, followed by activation annealing. For all annealing temperatures, transport measurements showed an abrupt normal-superconductor transition at 2.5–3.5 K, with residual resistances < 50 mΩ at 20 mK. Typical values for critical magnetic fields were about 0.6 to 0.8 T, corresponding to coherence lengths of 20 to 25 nm. Microscopy measurements revealed the presence of a 20nm thick polycrystalline Ga-rich Ge layer near the top surface. To localize the superconducting region, implantation energy was reduced. The resulting superconducting layer showed no grain structure in micrometer scale, but Raman measurements confirmed its nanocrystalline nature. By reducing the implantation energy and the annealing temperatures, coherence length only decreased to ~18 nm. To further characterize the superconducting Ge films, strategies for their integration into all-Ge Josephson junctions and transistors will be discussed.   

Live

We demonstrate a-axis YBa₂Cu₃O_7-x/PrBa₂Cu₃O_7-x/YBa₂Cu₃O_7-x trilayers grown on (100) LaAlO₃ substrates with improved interface smoothness. [1] The trilayers are synthesized by ozone-assisted molecular-beam epitaxy. The thickness of the PrBa₂Cu₃O_7-x layer is held constant at 8 nm and the thickness of the YBa₂Cu₃O_7-x layers is varied from 24 nm to 100 nm. X-ray diffraction measurements show all trilayers to have >95% a-axis content. The rms roughness of the thinnest trilayer is <0.7 nm and this roughness increases with the thickness of the YBa₂Cu₃O_7-x layers. The thickness of the YBa₂Cu₃O_7-x layers also affects the transport properties: while all samples exhibit an onset of the superconducting transition at and above 85 K, the thinner samples show wider transition widths, ΔT_c. High-resolution scanning transmission electron microscopy reveals coherent and chemically sharp interfaces, and that growth begins with a cubic (Y,Ba)CuO_3-x perovskite phase that transforms into a-axis oriented YBa₂Cu₃O_7-x as the substrate temperature is ramped up.

[1] Y. E. Suyolcu, J. Sun, B. H. Goodge, J. Park, J. Schubert, L. F. Kourkoutis, and D. G. Schlom, arXiv:2010.12624 (2020)   

Live

Nanoscale electronic inhomogeneity is a well-known aspect of perovskite-related quantum materials. Measurements of superconducting (SC) fluctuations above the bulk SC transition temperature in cuprates and other oxide superconductors revealed a universal inhomogeneity-dominated regime indicative of underlying structural inhomogeneity [1]. We studied several cuprate superconductors using neutron and X-ray diffuse scattering and found short-range-correlated structural distortions over wide doping and temperature ranges. Using structure modelling and the 3D-ΔPDF method [2], we reveal the real-space structure associated with these distortions and find atomic shifts perpendicular to the CuO₂ planes, with a characteristic length scale similar to the SC correlation length. Along with a recent phenomenological model of the cuprates [3], these insights pave the way toward a comprehensive understanding of cuprate superconductivity.

[1] D. Pelc et al., Nat. Commun. 9, 4327 (2018); G. Yu et al., Phys. Rev. B. 99, 214502 (2019); D. Pelc et al., Nat. Commun. 10, 2729 (2019)
[2] M. J. Krogstad et al., Nat. Mater. 19, 63 (2019)
[3] D. Pelc et al., Sci. Adv. 5, eaau4538 (2019)   

Live

Previous μSR work on polycrystalline samples showed that LaNiGa₂ breaks time-reversal symmetry upon entering the superconducting state. Follow up work suggested that this rare property occurs because of spin-triplet interband Cooper pairing amongst “nearly degenerate” Fermi surfaces. All previous results have been obtained on polycrystalline samples due to the lack of a single crystal synthesis technique. Here we report on the single crystal synthesis of LaNiGa₂, and updated physical properties. Subsequent electronic structure calculations and angle-resolved photoemission spectroscopy data are discussed in the context of the proposed scenario for spin-triplet superconductivity.   

Live

The tunneling density of states spectra from superconducting Nb/Al bilayer based normal-insulator-superconductor (N/I/S) junctions are presented as a function of the Al thickness, revealing a larger increase in the zero-bias conductance than can be explained by the change in the superconducting transition temperature alone. Nb/Al bilayers are widely used in fields such as quantum information, thermometry, and bolometry. We fabricated Nb/Al/AlOx/normal metal tunnel junctions with different Al thin film thickness and measured the tunnel junction properties. The shape of the tunneling density of states and the magnitude of the sub-gap conductance from the tunneling spectra is found to change significantly at different Al thicknesses and as a function of temperature. We accurately model the data using the Blonder-Tinkham-Klapwijk (BTK) theory and discuss trends in BTK barrier strength, the superconducting energy gap, and broadening as the Al film thickness and temperature are changed.   

Live

A recent experiment tracked the direct band gap and vibron frequency of hydrogen via infrared measurements up to ~425 GPa [1]. Above this pressure, the direct gap has a discontinuous drop to below ~0.1 eV. The authors interpreted this as a structural phase transition from the C2/c-24 molecular phase to another molecular phase. In our ab initio study of pressure dependent vibron frequency and direct band gap, we find that the experimental data is consistent with the C2/c-24 phase up to 425 GPa. We also find that the pressure induced changes in the band structure of the C2/c-24 phase lead to a discontinuous drop of the direct band gap, which can explain the observed drop without a structural transition [2]. We also investigate the superconducting properties of the C2/c-24 phase and compare it with the other candidate phases.

[1] P. Loubeyre, F. Occelli, P. Dumas, Synchrotron Infrared Spectroscopic Evidence of the Probable Transition to Metal Hydrogen, Nature 577, 7792 (2020).
[2] M. Dogan, S. Oh, M. L. Cohen, Observed Metallization of Hydrogen Interpreted as a Band Structure Effect, J. Phys.: Condens. Matter 33, 03LT01 (2020).   

Live

The properties of quantum materials are commonly tuned using experimental variables such as pressure, magnetic field and doping. Here we explore a different approach: irreversible, plastic deformation of single crystals. We show that compressive plastic deformation of SrTiO₃ (STO) induces low-dimensional superconductivity significantly above the superconducting transition temperature (T_c) of undeformed samples. We furthermore present evidence for unusual normal-state transport behaviour that suggests superconducting correlations at temperatures two orders of magnitude above the bulk T_c. The superconductivity enhancement is correlated with the appearance of self-organized dislocation structures, revealed by diffuse neutron and X-ray scattering. These results suggest that T_c in STO is strongly influenced by the local strain around dislocations, consistent with a theory of superconductivity enhanced by soft polar fluctuations. More broadly, our results demonstrate the promise of plastic deformation and dislocation engineering as tools to manipulate electronic properties of quantum materials [1].

[1] S. Hameed, D. Pelc et al., arXiv:2005.00514 (2020)   

Live

Recently, RhPb₂ was reported to be as a candidate of topological superconductor from the result of band calculation [1]. However, there has been so far no attempt to test it experimentally. According to the Rh-Pb binary phase diagram [2] the intermetallic compound RhPb₂ grows through peritectic reaction incongruently from the melt between 320^○C and 640^○C. We have tried to grow the single crystal by using the vertical pulling mechanism in a mirror furnace. It is clearly found that many single crystallites with a thin, long plate-like shape grow from the melt of RhPb₄. The preliminary X-ray diffraction analysis showed that the structure is similar to the β-PdBi₂ with the tetragonal symmetry with lattice parameters of a=3.40 Å and c=12.82 Å. In order to study the superconducting topological nature of RhPb₂, we perform the resistivity, the critical currents, the critical fields H_c1 and H_c2 measurements.

[1] J. F. Zhang et al., Phys. Rev. B 99 (2019) 045110.
[2]“Phase Equilibria, Crystallographic and Thermodynamic Data of Binary Alloy, Pb-Rh (Lead-Rhodium)", Landolt-Börnstein -Group IV Physical Chemistry 5I (1998).   

Live

Iron telluride, the non-superconducting parent compound of the Fe chalcogenide materials, exhibits a peculiar (π, 0) magnetic order not seen in other iron-based superconductors, which show (π,π) magnetic order. Theoretical predictions indicate that other magnetic phases can be stabilized in Fe_1+xTe through tuning of the coupling strength between magnetic moments at next-nearest neighbor Fe sites.
Using low temperature scanning tunneling microscopy of strained samples, we have studied, at the atomic scale, the impact of small lattice distortions on the magnetic order of Fe_1+xTe (x~0.06). Straining the material along the [110] direction, we have observed a new ordered phase that is characterized by a (π, π) order. I will discuss the origin of this new phase, and show measurements of its electronic and magnetic properties.   

Live

Recently, two-dimensional (2D) materials, including transition metal dichalcogenides (TMDs) have received considerable attention due to their emergent physical properties, which in turn have potential to revolutionize many fields in both fundamental science and technological applications. Especially, PdTe₂ recently attracted much attention now due to its phase coexistence of type-II Dirac semimetal and type-I superconductivity. Here we report an enhancement of superconducting transition temperature in PdTe₂. The enhanced superconductivity has been unambiguously confirmed by the magnetization, resistivity and specific heat measurements, which shows type-II superconductivity with large anisotropy and non-bulk superconductivity nature with volume fraction 20% estimated from magnetic and heat capacity measurements. Enhanced superconductivity in PdTe₂ expands the family of superconducting transition metal dichalcogenides and thus provides additional insights for understanding superconductivity and topological physics in the 1T-PdTe₂ system   

Live

Unusual superconducting properties of the noncentrosymmetric compound BeAu have recently attracted a considerable amount of attention. While type-II superconductivity was initially reported in this material [1], it was later shown that the compound is in fact a type-I superconductor [2]. In-depth muon spin rotation, relaxation, and resonance (μSR) experiments have revealed that the time reversal symmetry is preserved in BeAu [2]. Furthermore, precise measurements of the thermodynamic critical field via μSR technique have indicated that its evolution as a function of temperature cannot be described within a single-gap scenario [3]. Instead, the data can be successfully fitted using a self-consistent two-gap approach. Additionally, a joint analysis of specific heat and μSR data has indicated that the superconductivity of BeAu is non-BCS-like [4]. This work illustrates that unconventional superconductivity of BeAu could only be revealed by using a joint analysis of several thermodynamic quantities.

[1] A. Amon et al., Physical Review B 97, 014501 (2018)
[2] J. Beare et al., Physical Review B 99, 134510 (2019)
[3] R. Khasanov et al., Physical Review Research 2, 023142 (2020)
[4] R. Khasanov et al., Physical Review B 102, 014514 (2020)   

Live

Understanding the origin of superconductivity is one of the greatest challenges in condensed matter physics. An outstanding question remains as to whether superconductivity is related to nematic electronic or magnetic correlations or to structural disorder. In this respect, superconducting Cuprates exhibit many imperfections at their crystallographic sites in addition to a heavily tunable oxygen vacancy framework supporting many short-range oxygen ordering schemes. In this talk, I will discuss the structural properties of five ErBa₂Cu₃O_6+x single crystals with oxygen contents tuned to produce T_C’s = 0, 22, 55, 75 and 92 K. Systematic three-dimensional reciprocal space mapping was performed at sector 6 at the Advanced Photon Source at temperatures between 30 and 300 K. We observe modulated diffuse scattering intensities arising from short-range local correlations between the Ba and Cu layers. The modulated intensities weaken upon increased doping until they disappear for the 92 K crystal. Our data provide no evidence for charge density waves which, if present, must be much weaker than those commonly observed in YBCO, LSCO or other related Cuprates.   

Live

We performed epitaxial growth of (110)-oriented YBa₂Cu₃O₇ / (110)-oriented PrBa₂(Cu_0.8Ga_0.2)₃O₇ heterostructure using pulsed laser based thin film deposition technique for the nanofabrication of Superconductor (S) / Insulator (I) / Superconductor (S) tunneling Josephson junction device which may operate with low cost and simple liquid nitrogen based cryogenic system and may provide high I_cR_n product (with I_c being the junction critical current and R_n the normal resistance). X-ray diffraction pattern (XRD) analysis, atomic force microscopy (AFM), electrical transport, and optical characterization studies were performed to check the orientation and thickness, surface morphology, critical temperature (T_C), electrical resistivity, proximity effect, and optical properties of the heterostructure. Here, we present experimental results on structural, electrical transport, and optial properties of the heterostructure.   

Live

Recently discovered heavy-fermion superconductivity in UTe₂ with the transition temperature T_c~1.6 K attracts much attention. The superconducting state of UTe2 closely resembles that of ferromagnetic superconductors. However, the normal state of UTe₂ is, uniquely, paramagnetic. UTe₂ is hosting two independent field-induced superconducting phases with a reentrant behavior at 45 T and persisting up to 65 T. Most likely spin-triplet superconductivity is realized in UTe₂. Variation of T_c and the order parameter, single or multiple components, has been reported, which is likely to depend on the quality of the single crystals. We would like to discuss the impact of growth techniques and treatments of single crystals on the superconducting state in UTe₂ .