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Unconventional free charge in the correlated semimetal Nd2Ir2O7


Nd2Ir2O7 is a correlated semimetal with the pyrochlore structure, in which competing spin–orbit coupling and electron–electron interactions are believed to induce a time-reversal symmetry-broken Weyl semimetal phase characterized by pairs of topologically protected Dirac points at the Fermi energy1,2,3,4. However, the emergent properties in these materials are far from clear, and exotic new states of matter have been conjectured5,6,7. Here, we demonstrate optically that, at low temperatures, the free carrier spectral weight is proportional to T2, where T is the temperature, as expected for massless Dirac electrons. However, we do not observe the corresponding T3 term in the specific heat. That the system is not in a Fermi liquid state is further corroborated by the charge carrier scattering rate approaching critical damping and the progressive opening of a correlation-induced gap at low temperatures. These observations cannot be reconciled within the framework of band theory of electron-like quasiparticles and point towards the effective decoupling of the charge transport from the single particle sector.

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Fig. 1: Experimental transport and optical conductivity data of Nd2Ir2O7.
Fig. 2: Comparison and analysis of the free charge spectral weight, entropy and low-energy optical conductivity of Nd2Ir2O7.
Fig. 3: Power-law analysis of the interband transitions of Nd2Ir2O7.

Data availability

The datasets generated and analysed during the current study are available in ref. 43. These will be preserved for 10 years. All other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.


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D.v.d.M. acknowledges insightful discussions with D. Abanin and N. Nagaosa. This project was supported by the Swiss National Science Foundation (project no. 200020-179157). This work is partially supported by CREST (JPMJCR18T3), the Japan Science and Technology Agency (JST), by Grants-in-Aids for Scientific Research on Innovative Areas (15H05882 and 15H05883) from the Ministry of Education, Culture, Sports, Science and Technology of Japan and by Grants-in-Aid for Scientific Research (19H00650). Work at JHU was supported through the Institute for Quantum Matter, an EFRC funded by the US DOE, Office of BES (DE-SC0019331).

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Authors and Affiliations



K.W., B.X., C.W.R., N.B., B.M. and J.T. performed experiments. Y.Q., T.O., B.C. and S.N. prepared samples. K.W., N.B., B.M. and D.v.d.M. analysed data. N.P.A. and D.v.d.M. planned the project. K.W. and D.v.d.M. wrote the manuscript with input and comments from all other authors.

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Correspondence to D. van der Marel.

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Extended data

Extended Data Fig. 1 Reflectance spectra.

Solid curves: Near normal incidence reflectivity, R=r2, of Nd2Ir2O7 for selected temperatures. Dotted curves below 50 cm−1: Extrapolations using simultaneous Drude–Lorentz fitting to the reflectance spectra and the ellipsometric data beween 4000 and 18000 cm−1 of Extended Data Fig. 2. a, Between 30 and 300 K. b, Between 6 and 30 K.

Extended Data Fig. 2 Ellipsometric data between 4000 and 18000 cm−1.

a, Real and imaginary part of the dielectric function at room temperature measured using spectroscopic ellipsometry at θ = 65 degrees with the surface normal. b, Phase of the normal incidence reflection coefficient using the Fresnel equation \(| {\rm{r}}| {{\rm{e}}}^{{\rm{i}}\phi }=(1-\sqrt{\varepsilon })/(1+\sqrt{\varepsilon })\). c, Absolute square of the reflection coefficient.

Extended Data Fig. 3 Thermodynamic properties of the Nd 4f states.

Model calculation of the exchange potential at the Nd sites Δ(T), entropy per primitive cell sf(T), and specific heat per primitive cell cf(T) (see Methods). a,c,e,f, Δ0 =6.5 K and TN=37 K. b,d,g, Δ0 =15 K and TN =30 K.

Extended Data Fig. 4 Comparison of the contributions to the specific heat per primitive cell.

Experimental specific heat per primitive cell of Nd2Ir2O7. Red curve: Calculated phonon contribution (see Methods). Olive curve: Calculated contribution of the itinerant Ir 5d band electrons (see Methods). Brown curve: The contribution from the localized Nd 4f electrons (see Methods).

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Wang, K., Xu, B., Rischau, C.W. et al. Unconventional free charge in the correlated semimetal Nd2Ir2O7. Nat. Phys. 16, 1194–1198 (2020).

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