Session Q27: Superconductivity:Including Nitrides&Halides

Ion-beam Assisted Sputtering of Titanium Nitride Thin Films

Titanium nitride is a material of interest for many superconducting devices such as nanowire

microwave resonators and photon detectors. Thus, controlling the growth of TiN thin films with

tunable and desirable properties is of high importance [1–3]. Polakovic et al. [4] have shown

that for niobium nitride, ion beam assisted sputtering (IBAS) reduces nitrogen sensitivity during

deposition in tandem with an increase in nominal critical temperature. We have deposited thin

films of titanium nitride by both the conventional method of DC reactive magnetron sputtering

and IBAS to compare their superconducting critical temperature Tc as a function of thickness,

sheet resistance, and nitrogen flow rate. Electrical and structural characterizations were performed

by four-wire electric transport and X-ray diffraction measurements. Compared to the conventional

method of reactive sputtering, we have demonstrated a 10% increase in nominal critical temperature

and 33% reduced sensitivity to nitrogen flow for bulk 300 nm films using the IBAS technique without

noticeable variation in the lattice structure. In addition, the non-IBAS films thickness dependence

on Tc is consistent with associated weak localization or quantum size effects, while the IBAS films

show some deviations away from these models.

References:

[1] M. R. Vissers et al., Applied Physics Letters 97,232509 (2010).

[2] J. Gao et al., Applied Physics Letters 92, 152505 (2008).

[3] Y. L. et al., Materials Science and Engineering: A 445-446, 223 (2007).

[4] T. Polakovic et al., APL Materials 6, 76107 (2018).

    

Discovery of superconductivity in β-Nb2N and its electronic properties

Since the discovery of relatively high Tc in δ-NbN in 1941, the Nb-N system has been studied extensively. However, during the last 80 years, superconductivity of β-Nb₂N has not been observed due to its relatively low Tc and the difficulty in making a high quality crystal of this phase. We present the growth conditions used to make phase-pure β-Nb₂N using Molecular Beam Epitaxy (MBE), its electronic and structural properties and its superconducting transition at about 0.5K. β-Nb₂N, due to its hexagonal symmetry, is symmetry-matched to Wurtzite AlN and coincidentally they are also lattice matched. This allows us to grow highest quality Josephson Junctions (JJs) where the barrier and the superconductor belong to same family of materials (Nitrides in our case) and are symmetry and lattice matched. Devices were fabricated out of β-Nb₂N/AlN/β-Nb₂N JJ heterostructure and their properties will be discussed.   

Metallic and superconducting properties of transition-metal-nitride structural phases: an ab initio study

The recent epitaxial integration of metallic and superconducting Nb_xN crystalline phases into the IIIA-nitride semiconductor family opens new directions for integrated low-temperature electronics. Furthermore, the possibility of engineered hybrid quantum systems based on these materials is spurred by the recent demonstration of concurrent superconductivity and the quantum Hall effect in a single epitaxial device. A challenge for superconducting devices in this platform comes from the incompatibility of the hexagonal GaN crystal structure with the cubic NbN phase that has been the focus of existing experimental work, which leads to twin domains that may hamper device design. Here we use first-principles theory to explore the structural polytypes of transition-metal nitrides, focused on their metallic and superconducting properties, to gather insights into the microscopic physics at play. We discuss this in the context of current experimental capabilities, weighing desired properties with the structural symmetry-dictated domains towards the goal of domain- and defect-free epitaxial devices.   

Properties of epitaxial superconductor/semiconductor heterostuctures

The possibility of creating lattice-matched heterostructures of superconductors and semiconductors olds the promise of combining the best of both electronic phases of matter. In this work, we report the physical properties of epitaxial niobium nitride/gallium nitride superconductor/semiconductor heterojunctions. We measure the energy band offsets by transport and capacitance measurements, as well as the individual bandstructures by photoelectron spectroscopy. By determining the band offsets, we identify the required semiconductor composition that is required to form a transparent barrier. We also discuss the injection of supercurrent into the semiconductor and its transport in the semiconductor.   

Application of metamaterial engineering to ultrathin TiN films: insulator to superconductor

It is well known that decreasing the film thickness of superconducting thin films commonly leads to a decrease of the superconducting critical temperature, T_c. Ultrathin TiN films are insulating for the film thickness of 4.5 nm and below.  However, using ultrathin TiN layers with thickness below 4.5 nm in hyperbolic metamaterial TiN/dielectric multilayers restores superconductivity.  The electronic transport and anisotropy of optical properties of TiN/dielectric multilayers were studied. Enhancement of the T_c and the critical field in these metamaterials was found. The role of dielectric material and variation of thicknesses of superconductor and dielectric will be discussed.   

Strain induced multigpap superconductivity in electrene Mo2N.

Superconductivity in low dimensional materials and 2-D electrenes are topics of intense interest. Using Density functional theory (DFT), density functional perturbation theory DFTPT, and maximally localized Wannier functions associated with Migdal-Eliasberg approach, it is shown that the 2-D electrine Mo₂N presents superconductivity at ambient pressure with transition temperature T_c = 24.7 K. This study also shows how biaxial strain affects superconductivity in a monolayer of Mo₂N. Results indicate that 2-D Mo₂N presents strong electron-phonon coupling with large anisotropy in the superconducting energy gap. The effect of biaxial strain shows interesting behavior evidencing the emergency of multigap superconductivity in this material. T_c = 24.7 K, is  record high transition temperature for this class of material at ambient pressure.

    

High Field Probe of Superconducting Coupling in High-Tc Hydrides

High magnetic fields enable direct probes of the superconducting order and vortex matter in high temperature superconductors. A combination of diamond anvil cells and pulse magnets allows us to controllably tune and probe candidate systems at the extremes of pressure-field-temperature parameter envelope. We will discuss the effective enhancement of electron-phonon coupling and renormalization of Fermi velocity in the vicinity of the observed lattice instability. Work at NHMFL-LANL was performed under the auspices of the NSF, DoE, and State of Florida.   

Theoretical Investigation of High-Tc ternary superhydrides using evolutionary algorithm coupled with ab intio calculation

Recent theoretical reports on high-Tc hydride superconductors from the admixture of two binary hydrides have been successfully verified to be consistent with the experimental results. However, this is a very recent phenomenon, which needs a highly careful selection of two suitable binaries from the pool of existing binary superhydrides. Our successful theoretical prediction using evolutionary algorithm coupled with density functional theory of several high-Tc compressed superhydrides such as R¯3m-YCeH20 (122K at 300GPa)[1], R¯3m -LaCeH20 (116K at 250GPa)[1], P¯6m2-YCeH18 (173K at 150GPa)[1], Cmmm-LaYH12 (140K at 200GPa)[2], Cmmm-LaY3H24 (145K at 180GPa)[2], P4/mmm-YMgH8 (125K at 300GPa)[3], Fd¯3m -YMgH12 (190K at 200GPa)[3] is a step forward in this field. Herein, first, we perform convex-hull analysis to figure out the thermodynamically stable structure out of several thousand structures generated using evolutionary algorithm, secondly from the electron-phonon calculation, we confirm their dynamical stability followed by the prediction of their Tc employing Allen-Dynes-modified McMillan formula with Eliashberg function. Interestingly, we observed that ternary superhydrides could also show higher Tc than their parent binaries.   

Evidence Against Superconductivity in Flux Trapping Experiments on Hydrides Under High Pressure

It has recently been reported that hydrogen-rich materials under high-pressure trap magnetic flux [1], a tell-tale signature of superconductivity. We argue that the evidence in that paper does not support superconductivity, because the trapped moment under ZFC was found to be linear in field, rather than quadratic as expected [2]. Furthermore those results are incompatible with the magnetization measurements on the same samples reported in Ref. [3], under the assumption that they originate in superconductivity. Namely, they imply that the samples can trap flux even when the flux does not penetrate the sample, and that supercurrents that trap magnetic flux are much larger than those that shield magnetic flux, which is unphysical. Instead, we propose that the magnetic signatures observed in both experiments result from localized magnetic moments in the sample and its environment rather than electric supercurrents. Together with other experimental evidence analyzed earlier [4], this indicates that these materials are not superconductors.

 

[1] V. S. Minkov et al, arXiv:2206.14108 (2022).

[2] J. E. Hirsch and F. Marsiglio, J Supercond Nov Magn 35, 3141–3145 (2022).

[3] V. S. Minkov et al, Nat Commun 13, 3194 (2022).

[4] J. E. Hirsch and F. Marsiglio, MRE 7, 058401 (2022).

    

Higher hydride formation in lanthanides through mechanical milling

Superhydride compounds exhibit high temperature superconductivity due to the presence of high metal-to-hydrogen ratio, but only occur at very high formation pressures. Here we discuss how modifications of lanthanide precursors using mechanical ball milling prior to synthesis in a high-pressure diamond anvil cell can lead to a hydride phase with hydrogen content above that expected from the equilibrium phase diagram and improve hydrogen reactivity. Lanthanide precursors were subjected to a cryogenic ball milling technique and then exposed to a source of hydrogen in a diamond anvil cell. X-ray diffraction at HPCAT was used to explore the phase transformations of the metal hydride phase. We found an increase in hydrogen stoichiometry with mechanical ball milling relative to as received forms of lanthanide powders.

    

Optimal alloying in hydrides: Reaching room-temperature superconductivity in LaH10

Doping represents one of the most promising avenues for optimizing superconductors, such as reaching record-breaking critical temperatures of hydride superconductors. In this work, we perform an thorough and extensive search for substitutional dopants in LaH₁₀, looking for elements that enhance its electronic structure (especially the density of states at the Fermi level). In total, 70 elements were investigated as possible substitutions of La-sites at the doping ratio of 12.5% under high pressure. By using our systemetical and efficient screening protocal, we found Ca as the best candidate dopants, which shift the van Hove singularity and increase the electronic DOS at the Fermi level. With harmonic-level phonon calculations and performin first-principles calculation of T_c, Ca-doped LaH₁₀ shows T_c which is 15% higher than the one of LaH₁₀. It provides a promising route to reach the room-temperature superconductivity in pressurized hydrides by doping. Meanwhile, this process of screening based on structure relaxation and electronic structure calculation can be generalized to apply to other clathrate hydride systems to reach higher T_c by optimal doping.   

High-throughput search and discovery of near-room temperature superconductors under extreme pressures

Since the discovery of superconductivity in solid mercury, countless scientists have been searching for a material whose Tc exceeds room temperature. Despite intense research, only very recently observations of superconductivity at 200K in pressed H₃S and 250-260 K in LaH₁₀ at pressures near 200 GPa gave some hope that superconductivity at room temperature may be possible soon. However, the experimental study of these materials under extreme pressures is challenging, and the equilibrium structure of materials at these pressures is usually very different than those under ambient conditions. Hence, as a result, first-principles-based computational searches have become extremely important in predicting new materials and guiding high-pressure experimental measurements.

We have developed a super-efficient, fast, high-throughput method for searching high-T_c hydride superconductors. We introduce new "metrics" strongly correlated to electron-phonon solid coupling and Tc, but it is much quicker to calculate them. We have searched more than 100,000 binary hydride and metal-borates superconductors using our new method and discovered many new high-Tc systems. Our work will not only significantly accelerate the discovery of new high-Tc conventional superconductors but also give a detailed understanding of important factors that yield superconductivity near room temperature at extreme pressures.