Research ArticleCONDENSED MATTER PHYSICS

TaRh2B2 and NbRh2B2: Superconductors with a chiral noncentrosymmetric crystal structure

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Science Advances  04 May 2018:
Vol. 4, no. 5, eaar7969
DOI: 10.1126/sciadv.aar7969
  • Fig. 1 Crystal structure characterization of TaRh2B2 and NbRh2B2.

    (A) Room temperature pXRD pattern showing a LeBail fit for the new superconducting phases TaRh2B2 (top) and NbRh2B2 (bottom). The experimentally observed data are shown with red circles, the calculated pattern is shown with a black line, and the green vertical marks indicate the expected Bragg reflections for space group P31 (no. 144). Impurity peaks are marked with asterisks. (B) The crystal structure of chiral and noncentrosymmetric TaRh2B2 and isostructural NbRh2B2 viewed along the a direction, emphasizing the 31 screw axis (top) and along the c direction (bottom), emphasizing the Ta spirals and a single Rh honeycomb layer. Tantalum/niobium is shown in blue, rhodium is shown in pink, and boron is shown in green. arb. units, arbitrary units.

  • Fig. 2 Superconducting characterization of TaRh2B2 and NbRh2B2.

    (A and B) ZFC and FC temperature-dependent magnetic susceptibility χv(T) for TaRh2B2 (A) and NbRh2B2 (B) measured in an applied magnetic field of 1 mT. The insets show field-dependent volume magnetization (MV) measured at various temperatures below Tc. (C and D) Lower critical field Embedded Image versus temperature for TaRh2B2 (A) and NbRh2B2 (B). The data points were estimated from a difference between MV and Mfit as is shown in the insets. (E) Cp/T versus T plotted from 0 to 9 K for TaRh2B2 (green open squares) and NbRh2B2 (blue open circles) measured in zero applied field where the solid black lines outline the equal area construction shown with gray shading. This construction is used for estimation of Tc and the superconducting jump ΔC/Tc. Inset: Cp/T versus T2 measured in 9-T field (in the normal state) fitted to Cp/T = γ + βT2. (F) Temperature-dependent electronic specific heat Cel. for NbRh2B2 below 9 K. The solid curve through the data is a fit by a one-gap BCS model for superconductivity. Inset: Cel. − γ0T versus normalized temperature (Tc/T) for NbRh2B2 and TaRh2B2. (The lattice contribution was subtracted from the measured specific heat). emu, electromagnetic unit.

  • Fig. 3 Estimation of μ0Hc2(0) from resistivity data.

    Temperature-dependent electrical resistivity for polycrystalline (A) TaRh2B2 and polycrystalline (B) NbRh2B2 measured from 300 to 1.7 K with zero applied magnetic field. Inset: Plot of the dependence of the superconducting transition on applied magnetic field measured (A) from 1.7 to 7 K for the Ta variant in applied fields from 0 to 12 T and (B) from 1.7 to 9 K for the Nb variant in applied magnetic fields from 0 to 14 T in 0.5-T increments. The dashed black line shows 50% of the superconducting transition. (C) The Tc values at different applied fields were used to calculate μ0Hc2(0) to be 11.7(4) T for TaRh2B2 and 18.0(4) T for NbRh2B2.

  • Fig. 4 Electronic calculations for TaRh2B2 and NbRh2B2.

    Calculated BS for (A and B) TaRh2B2 and (D and E) NbRh2B2 both with (middle) and without (left) SOC plotted in the energy range from −2.0 to 2.0 eV. The total DOS calculated using the Wien2K with LDA-type pseudopotentials with SOC included (blue line) and without SOC (red line) for (C) TaRh2B2 and (F) NbRh2B2.

  • Table 1 Atomic coordinates and equivalent isotropic displacement parameters of TaRh2B2 at 293(2) K.

    Ueq is defined as one-third of the trace of the orthogonalized Uij tensor (Å2).

    AtomWyckoff
    position
    OccupancyxyzUeq
    Ta3a10.8481(2)0.8481(2)0.3131(2)0.0041(2)
    Rh13a10.6638(4)0.4824(4)0.0447(3)0.0058(2)
    Rh23a10.3324(4)0.8190(4)0.1072(2)0.0058(3)
    B13a10.627(7)0.550(8)0.549(3)0.012(5)
    B23a10.288(9)0.347(8)0.215(3)0.028(5)
  • Table 2 Observed superconductivity parameters of TaRh2B2 and NbRh2B2.
    ParameterUnitsTaRh2B2NbRh2B2
    TcK5.8(1)7.6(1)
    μ0Hc1(0)mT9.56(1)13.50(2)
    μ0Hc2(0)T11.7(4)18.0(4)
    μ0Hc(0)mT169(3)248(4)
    ξGLÅ53(1)43(1)
    λGLÅ2580(60)2190(70)
    κGL48(2)51(3)
    γmJ mol−1 K−25.8(1)8.8(2)
    ΔCTc1.56(4)1.60(3)
    μ0HPauliT10.7(2)14.1(2)
    λep0.624(5)0.688(4)
    N(EF)states eV−1 per fu1.52(3)2.21(5)
    ΘDK379(4)362(2)
    Δ0meV0.98(2)1.4(1)

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • table S1. Single-crystal crystallographic data for TaRh2B2.
    • table S2. Selected bond distances for TaRh2B2.

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    Other Supplementary Material for this manuscript includes the following:

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