Session E09: Novel and Less-common Superconductors

Superconductivity in Chevrel phases

Chevrel phases is a class of interesting materials discovered more than 40 years ago including more than 100 examples.
They are ternary compounds of general stoichiometry M_xMo₆X₈ (X = chalcogen and M=metal) and many of them show interesting superconducting properties with critical temperatures as high as 15K and high critical magnetic fields (arount 50 Tesla).
We present the results of the prediction of the superconducting phase in Chevrel compound, PbMo₆S_8, within the SuperConducting Density Functional Theory (SCDFT), a first-principles theory of the superconducting phase which does not rely on any semi-empirical parameter.
We succesfully predict the critical temperature of PbMo₆S₈ at ambient conditions highlighting the role of phase instability, electron-phonon coupling with different intra-molecular and inter-molecular phonon modes and the fundamental role of repulsive electron-electron interaction, treated from first-principles. The evolution of the superconducting critical temperature as a function of the external pressure results in excellent agreement with available experiments.
Despite the interesting physical and chemical properties of Chevrel phases their fundamental explanation was still lacking: SCDFT opens way to systematic investigation of this large class of compounds.   

New magnetic phases and the absence of superconductivity in doped Hematophanite, Pb4Fe3O8Cl

We searched for new magnetic and superconductivity phases in the hematophanite system [1], Pb₄Fe₃O₈Cl, which has structural similarities to some cuprates [2]. To enhance the potential number of subphases we synthesized inhomogeneous samples via powder metallurgy and explored electron and hole doping of both Pb, with Ga and Bi, and Fe, with Mn and Co. We used magnetic field modulated microwave spectroscopy [3], as well as vibrating sample magnetometry, to identify magnetic phase transitions as a function of temperature. Crystallographic phase information is obtained via x-ray diffraction and Rietveld refinement. We observe that in both the Ga and Co doped systems we induce multiple magnetic phase transitions that correspond to various subphases present in our samples. Although many magnetic phases were created in these samples there is no indication of the presence of a new superconducting phase above 4 K.
1. R.C. Rouse, Am. Mineral. 56, 625 (1971).
2. A.C. Mclaughlin and J.P. Attfield, The Synthesis, Structure and Physical Properties of the Layered Ruthenocuprates RuSr₂GdCu₂O₈ and Pb₂Sr₂Cu₂RuO₈Cl (Springer, Berlin, Heidelberg, 2002).
3. J.G. Ramírez, A.C. Basaran, J. de la Venta, J. Pereiro, and I.K. Schuller, Rep. Prog. Phys. 77, 093902 (2014).   

Discovery of a new strong-coupling superconductor Y7Ru4InGe12 by indium flux

Compounds containing rare earth, transition metal and germanium elements can exhibit interesting phenomena such as charge density wave, complex magnetism and superconductivity. Using indium flux makes it possible to synthesis new compounds under a relative low temperature. Here we discovered a new superconductor Y₇Ru₄InGe₁₂ with T_c ~ 5.8 K grown from the indium flux, which is confirmed by resistivity, magnetization and specific heat measurements. It is a type-II superconductor and its upper critical fields are estimated to be 5.4 and 2.7 T along and perpendicular to the c axis, respectively. The estimated mean free path along the c axis is ~ 29 Å, much smaller than the superconducting coherence length ~ 172 Å, putting it in a dirty limit regime. The specific heat jump of this superconductor ΔC/γ_eT_c ≈ 2.4 is much larger than the BCS theoretical value 1.43, pointing to a strong-coupling scenario. A new polymorphism of LuRuGe and several trials to synthesize isostructural compounds of Y₇Ru₄InGe₁₂ by replacing the rare earth elements will also be presented in this talk.   

Superconducting SrSnP with Strong Sn-P Antibonding Interaction: Is the Sn Atom Single or Mixed Valent?

The large single crystals of SrSnP were prepared using Sn self-flux method. SrSnP crystallizes in the CaGaN structure type with space group P4/nmm (S.G.129, Pearson symbol tP6). A combination of magnetic susceptibility, resistivity, and heat capacity measurements confirms the bulk superconductivity with T_c = 2.3(1) K in SrSnP. According to the X-ray photoelectron spectroscopy (XPS) measurement, the assignments of Sr²⁺ and P^3- are consistent with the chemical valence electron balance principles. Moreover, it is highly likely that Sn atom has only one unusual oxidation state. First-principles calculations indicate the bands around Fermi level are hybridized among Sr-d, Sn-p, and P-p orbitals. The strong Sn-P and Sr-P interactions pose as keys to stabilize the crystallographic structure and induce the superconductivity, respectively. The physics-based electronic and phononic calculations are consistent with the molecular viewpoint. After including the spin-orbit coupling (SOC) into the calculation, the band degeneracies at G-point in the first Brillouin zone (BZ) split into two bands, which yield to the van Hove singularities around Fermi level.   

Superconductivity and Sn-Mössbauer spectra of the antiperovskite oxide Sr3-xSnO

We recently reported bulk superconductivity in the antiperovskite oxide Sr_3-xSnO, with a possibility of topological nature. We investigate the evolution of superconducting properties with the nominal Sr deficiency x₀. In all superconducting samples, two superconducting phases with T_c ≈ 5 K and 1 K appear concurrently that originate from Sr_3-xSnO. Mössbauer spectroscopy reveals an unusual Sn^4- ionic state in both stoichiometric and superconductive samples. In addition to the main absorption peak, we observe a satellite peak attributable to the Sn sites with nearest-neighbor Sr deficiencies. The intensities of these two peaks exhibit a rather different temperature dependence, possibly reflecting different environment of the lattice vibrations. These results clarifying the Sr deficiency dependence of the normal and superconducting properties of the antiperovskite oxide Sr_3-xSnO will promote future work on this class of materials.   

Palladium Dependent Superconductivity in the Nb2PdxSe5 System

Single crystals of Nb₂Pd_xSe₅ across the range 0.68<x<0.95 were grown with the objective of studying the superconducting properties as a function of Pd stoichiometry. Electrical conductivity measurements down to 0.4K indicate a Pd-concentration dependent dome of superconductivity in this system. Here we discuss the crystal growth method, structural characterization, and the evolution of the superconducting properties within the accessible range of Pd stoichiometries.   

Highly reproducible superconductivity in K-doped triphenylbismuthine and tris(2/4-methylphenyl)bismuthine

To search for new organic superconductors, we perform a systematic study on the magnetic and electrical properties of K-doped triphenylbismuthine and tris(2/4-methylphenyl)bismuthine, which are successfully synthesized by a two-step method - ultrasound treatment and low temperature annealing. The combination of dc and ac magnetic measurements show that one hundred percent of synthesized samples exhibit superconductivity at 3.5 K and/or 7.2 K for triphenylbismuthine, and at 3.6 K for tris(2/4-methylphenyl)bismuthine. The observed superconductivity is strongly supported by the resistivity measurements. Both calculated electronic structures and measured Raman spectra indicate that superconductivity is realized by transferring electron from potassium to carbon atom. Our study opens an encouraging window for the search of organic superconductors in organometallic molecules.
  

Structures and charge transfer in metal-doped aromatic hydrocarbons

Organic based compounds were suggested as candidates for high temperature or room temperature superconductors since the electrons can interact with much higher excitation energy than the phonon energy in these materials. Recently, the discovery of superconductivity in aromatic hydrocarbons seemingly supports the above idea suggested by Little and Ginzburg. Chen et al reported that the Tc exceeds 120 K in K-doped p-terphenyl. This result clearly shows that organic compounds are potential room-temperature superconductors. However, the structural characteristics of metal-doped aromatic hydrocarbons and the charge transfer from metal to organic molecule have always been puzzled. Here we report these research results by employing the first-principles calculations. In this work, we will present the atomic positions in metal (K, Al, Ga, and In)-doped aromatic hydrocarbons and the distribution and the number of transferred charge, as well analyze the relation between them and electronic structures. The results is meaningful to understand this superconductivity in this kind of materials.   

Signatures of two-gap superconductivity in epitaxial La(PtxSi1-x)2 films

Symmetry plays an important role in superconductivity. In noncentrosymmetric superconductors, the pairing state might become a mixture of spin-singlet and spin-triplet components due to the anti-symmetric spin-orbit coupling (ASOC), which is “protected” by the lack of an inversion center. This mixing can further lead to two-gap physics. We investigated the transport properties of various La(Pt_xSi_1-x)₂ thin films grown by molecular beam epitaxy (MBE), where the inversion symmetry and spin-orbit coupling strength are tunable by varying the Pt concentration x. For certain x, magnetoresistance shows a two-step transition at finite temperatures below T_c, and the upper critical field is well described by a two-band model. Furthermore, the current-voltage characteristics, along with the differential resistance measurements, reveal a two-gap structure in magnetic fields up to 1 T. These observations indicate the multi-gap superconductivity in La(Pt_xSi_1-x)₂ films.   

Phase separation at the dimer-superconductor transition in Ir1−xRhxTe2

The detailed evolution of the local atomic structure across the (x, T) phase diagram of Ir_1−xRh_xTe₂ superconductor (0≤x≤0.3, 10K≤T≤300K) is obtained from high-quality x-ray total scattering data using the atomic pair distribution function method. The observed hysteretic thermal structural phase transition from a trigonal to a triclinic dimer phase for low Rh content emphasizes the intimate connection between the lattice and electronic properties. For superconducting samples away from the dimer/superconductor boundary, structural transition is absent and the local structure remains trigonal down to 10 K. In the narrow range of compositions close to the boundary structural phase separation is observed, suggestive of weak first-order character of the Rh-doping induced dimer-superconductor quantum phase transition. Samples from this narrow range show weak anomalies in electronic transport and magnetization, hallmarks of the dimer phase, as well as superconductivity albeit with incomplete diamagnetic screening. The results imply competition of the dimer and superconducting orders.
[1] R. Yu et al., Phys. Rev. B 98, 134506 (2018).
[2] ] R. Yu et al., Phys. Rev. B 97, 174515 (2018).   

Angle-Resolved Photoemission Spectroscopy Study of Planar Nickelate Pr4Ni3O8

Planar nickelates share the similar crystal structure and 3d electron configuration with high T_C cuprate materials, and thus can be considered as a potential proxy for cuprate physics as well as a possible candidate for hosting high-temperature superconductivity. A previous study has shown that Pr₄Ni₃O₈, a metallic material in the planar nickelate family, has a pronounced orbital character of dx²-y²near the Fermi energy [1], which is a signature of the cuprate electronic structure. Here we present the first angle-resolved photoemission spectroscopy study of Pr₄Ni₃O₈. Consistent with the electron counting, planar nickelate Pr₄Ni₃O₈ shows a non-gapped Fermi surface that resembles the one in the heavily over-doped cuprate. A detailed comparison of the ARPES result shows more similarity in the electronic structure. Our observations demonstrate that planar nickelate Pr₄Ni₃O₈ material can be a potential test ground for the study of cuprate physics.


[1] Junjie Zhang et al. Large orbital polarization in a metallic square-planar nickelate. Nat. Phys.13, 864–869 (2017).
  

Isotope effect in the layered bismuth chalcogenide (BiCh2-based) superconductor

We have investigated the selenium isotope effect on T_c in the layered bismuth chalcogenide (BiCh₂-based) superconductor LaO_0.6F_0.4Bi(S,Se)₂ using ⁷⁶Se and ⁸⁰Se. For all examined samples, the Se concentration, which is linked to the superconducting properties, is successfully controlled within x =1.09–1.14 in LaO_0.6F_0.4BiS_2−xSe_x. From the transition temperature estimated form the magnetization and electrical resistivity measurements, we found that the changes in T_c between ⁷⁶Se and ⁸⁰Se were apparently smaller than that expected from phonon-mediated pairing mechanism. The estimated isotope effect exponent α_Se is close to zero (-0.04 ≦ α_Se ≦ +0.04) [1]. Our results may indicate that the pairing in LaO_0.6F_0.4Bi(S,Se)₂ is not mediated by phonons, which is consistent with recent theoretical and experimental studies suggesting that unconventional pairing mechanisms are essential for the BiCh₂-based superconductor. We will show the recent results on sulfur isotope effect on the BiS₂-based superconductors.

[1] K. Hoshi, Y. Goto, Y. Mizuguchi, Phys. Rev. B 97, 094509 (2018).   

MENDELIEV'S PERIODIC TABLE and SUPERCONDUCTIVITY:Here it is pointed out that,like chemical properties,elemental metals in the same column of the table and in one headed by a nonmagnetic metal typically have similar Tc's.

In the first and second columns of the periodic table, headed by lithium and beryllium, none of the 5 alkali metals or 5 alkaline earth metals
is a superconductor(see the table on p.356 of the 5th edition of Kittel).Of the many rare earth metals in the third column only lanthanum is a superconductor. In the fourth and fifth columns,headed by titanium and vanadium,all three metals in each column is a superconductor. In the fourth column Tc's vary between .37 K and .56 K. In the fifth column Tc's are an order of magnitude higher with niobium having the highest at 9.2 K. Skipping over the next few columns headed by magnetic metals we arrive at the column of the precious metals. None of the these ls a superconductor. Once again in the next column headed by zinc all three elemental metals are superconductors. .Mercury's Tc of 4.2K is higher than Zn(.8K) and Cd(.6K) perhaps due to it's low melting point(-40 C).This brings us to the last column headed by a metal in this case aluminum. All 4 elemental metals are superconductors. Ga(1.1K) has the lowest Tc while In(3.4K) has the highest. Of the 5 light elemental metals in the top two rows of the periodic table only aluminum is superconducting! Why?In time will it become possible to calculate its Tc accurately from first principles?   

Signature of Nematic Superconductivity in CuxBi2Se3: Ginzburg-Landau Theory with External Stress

Cu_xBi₂Se₃ displays superconductivity[1]. It garnerns considerable interest because Bi₂Se₃ is a topological insulator. Evidence of two-fold anisotropy suggests that the superconducting phase may be nematic[2, 3]. However, the upper critical field H_c2 also appears two-fold anisotropic[3], and this actually contradicts the nematic hypothesis[4]. A pre-existing "pinning field" could break the symmetry explicitly[4], but then this casts in doubt the interpretation of any two-fold anisotropy. We study the Ginzburg-Landau theory of a nematic order parameter coupled to external stress as a pinning field. We classify possible phase diagrams, and indicate experimental signatures that might distinguish, in a pre-existing pinning field, a true nematic phase from an isotropic phase.

1. Hor et. al. PRL 104, 057001
2. Matano et. al. Nat. Phys. 12, 852
3. Yonezawa et. al. Nat. Phys 13, 123
4. Venderbos et. al. PRB 94, 094522