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Unconventional free charge in the correlated semimetal Nd2Ir2O7

Nature Physics volume 16, pages 1194–1198 (2020)Cite this article

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Abstract

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.

References

  1. Wan, X., Turner, A. M., Vishwanath, A. & Savrasov, S. Y. Topological semimetal and Fermi-arc surface states in the electronic structure of pyrochlore iridates. Phys. Rev. B 83, 205101 (2011).

    Article  ADS  Google Scholar 

  2. Tian, Z. et al. Field-induced quantum metal–insulator transition in the pyrochlore iridate Nd2Ir2O7. Nat. Phys. 12, 134–138 (2016).

    Article  Google Scholar 

  3. Armitage, N. P., Mele, E. J. & Vishwanath, A. Weyl and Dirac semimetals in three-dimensional solids. Rev. Mod. Phys. 90, 015001 (2018).

    Article  ADS  MathSciNet  Google Scholar 

  4. Ohtsuki, T. et al. Strain-induced spontaneous Hall effect in an epitaxial thin film of a Luttinger semimetal. Proc. Natl Acad. Sci. USA 116, 8803–8808 (2019).

    Article  ADS  Google Scholar 

  5. Pesin, D. & Balents, L. Mott physics and band topology in materials with strong spin–orbit interaction. Nat. Phys. 6, 376–381 (2010).

    Article  Google Scholar 

  6. Moon, E.-G., Xu, C., Kim, Y. B. & Balents, L. Non-Fermi-liquid and topological states with strong spin–orbit coupling. Phys. Rev. Lett. 111, 206401 (2013).

    Article  ADS  Google Scholar 

  7. Morimoto, T. & Nagaosa, N. Weyl Mott insulator. Sci. Rep. 6, 19853 (2016).

    Article  ADS  Google Scholar 

  8. Dzero, M., Sun, K., Galitski, V. & Coleman, P. Topological Kondo insulators. Phys. Rev. Lett. 104, 106408 (2010).

    Article  ADS  Google Scholar 

  9. Kim, D. J., Xia, J. & Fisk, Z. Topological surface state in the Kondo insulator samarium hexaboride. Nat. Mater. 13, 466–470 (2014).

    Article  ADS  Google Scholar 

  10. Nakatsuji, S., Kiyohara, N. & Higo, T. Large anomalous Hall effect in a non-collinear antiferromagnet at room temperature. Nature 527, 212–215 (2015).

    Article  ADS  Google Scholar 

  11. Kuroda, K. et al. Evidence for magnetic Weyl fermions in a correlated metal. Nat. Mater. 16, 1090–1095 (2017).

    Article  ADS  Google Scholar 

  12. Lai, H.-H., Grefe, S. E., Paschen, S. & Si, Q. Weyl–Kondo semimetal in heavy-fermion systems. Proc. Natl Acad. Sci. USA 115, 93–97 (2018).

    Article  ADS  Google Scholar 

  13. Xu, Y., Yue, C., Weng, H. & Dai, X. Heavy Weyl fermion state in CeRu4Sn6. Phys. Rev. X 7, 011027 (2017).

    Google Scholar 

  14. Kondo, T. et al. Quadratic Fermi node in a 3D strongly correlated semimetal. Nat. Commun. 6, 10042 (2015).

    Article  ADS  Google Scholar 

  15. Cheng, B. et al. Dielectric anomalies and interactions in the three-dimensional quadratic band touching Luttinger semimetal Pr2Ir2O7. Nat. Commun. 8, 2097 (2017).

    Article  ADS  Google Scholar 

  16. Tomiyasu, K. et al. Emergence of magnetic long-range order in frustrated pyrochlore Nd2Ir2O7 with metal–insulator transition. J. Phys. Soc. Jpn 81, 034709 (2012).

    Article  ADS  Google Scholar 

  17. Guo, H. et al. Magnetic order in the pyrochlore iridate Nd2Ir2O7 probed by muon spin relaxation. Phys. Rev. B 88, 060411 (2013).

    Article  ADS  Google Scholar 

  18. Sagayama, H. et al. Determination of long-range all-in-all-out ordering of Ir4+ moments in a pyrochlore iridate Eu2Ir2O7 by resonant X-ray diffraction. Phys. Rev. B 87, 100403 (2013).

    Article  ADS  Google Scholar 

  19. Ma, E. Y. et al. Mobile metallic domain walls in an all-in-all-out magnetic insulator. Science 350, 538–541 (2015).

    Article  ADS  Google Scholar 

  20. Donnerer, C. et al. All-in-all-out magnetic order and propagating spin waves in Sm2Ir2O7. Phys. Rev. Lett. 117, 037201 (2016).

    Article  ADS  Google Scholar 

  21. Chun, S. H. et al. Magnetic excitations across the metal–insulator transition in the pyrochlore iridate Eu2Ir2O7. Phys. Rev. Lett. 120, 177203 (2018).

    Article  ADS  Google Scholar 

  22. Go, A., Witczak-Krempa, W., Jeon, G. S., Park, K. & Kim, Y. B. Correlation effects on 3D topological phases: from bulk to boundary. Phys. Rev. Lett. 109, 066401 (2012).

    Article  ADS  Google Scholar 

  23. Witczak-Krempa, W. & Kim, Y. B. Topological and magnetic phases of interacting electrons in the pyrochlore iridates. Phys. Rev. B 85, 045124 (2012).

    Article  ADS  Google Scholar 

  24. Witczak-Krempa, W., Go, A. & Kim, Y. B. Pyrochlore electrons under pressure, heat, and field: shedding light on the iridates. Phys. Rev. B 87, 155101 (2013).

    Article  ADS  Google Scholar 

  25. Shinaoka, H., Hoshino, S., Troyer, M. & Werner, P. Phase diagram of pyrochlore iridates: all-in-all-out magnetic ordering and non-Fermi-liquid properties. Phys. Rev. Lett. 115, 156401 (2015).

    Article  ADS  Google Scholar 

  26. Ueda, K. et al. Magnetic-field induced multiple topological phases in pyrochlore iridates with Mott criticality. Nat. Commun. 8, 15515 (2017).

    Article  ADS  Google Scholar 

  27. Ueda, K. et al. Variation of charge dynamics in the course of metal–insulator transition for pyrochlore-type Nd2Ir2O7. Phys. Rev. Lett. 109, 136402 (2012).

    Article  ADS  Google Scholar 

  28. Ueda, K., Fujioka, J. & Tokura, Y. Variation of optical conductivity spectra in the course of bandwidth-controlled metal–insulator transitions in pyrochlore iridates. Phys. Rev. B 93, 245120 (2016).

    Article  ADS  Google Scholar 

  29. Sushkov, A. B. et al. Optical evidence for a Weyl semimetal state in pyrochlore Eu2Ir2O7. Phys. Rev. B 92, 241108 (2015).

    Article  ADS  Google Scholar 

  30. Machida, Y., Nakatsuji, S., Onoda, S., Tayama, T. & Sakakibara, T. Time-reversal symmetry breaking and spontaneous Hall effect without magnetic dipole order. Nature 463, 210–213 (2010).

    Article  ADS  Google Scholar 

  31. Tabert, C. J. & Carbotte, J. P. Optical conductivity of Weyl semimetals and signatures of the gapped semimetal phase transition. Phys. Rev. B 93, 085442 (2016).

    Article  ADS  Google Scholar 

  32. Tabert, C. J., Carbotte, J. P. & Nicol, E. J. Optical and transport properties in three-dimensional Dirac and Weyl semimetals. Phys. Rev. B 93, 085426 (2016).

    Article  ADS  Google Scholar 

  33. Lide, D. R. CRC Handbook of Chemistry and Physics 71st edn (CRC Press, 1990).

  34. Hosur, P., Parameswaran, S. A. & Vishwanath, A. Charge transport in Weyl semimetals. Phys. Rev. Lett. 108, 046602 (2012).

    Article  ADS  Google Scholar 

  35. Zaanen, J. Planckian dissipation, minimal viscosity and the transport in cuprate strange metals. SciPost Phys. 6, 061 (2019).

    Article  ADS  MathSciNet  Google Scholar 

  36. Watahiki, M. et al. Crystalline electric field study in the pyrochlore Nd2Ir2O7 with metal–insulator transition. J. Phys. Conf. Ser. 320, 012080 (2011).

    Article  Google Scholar 

  37. Phillips, P. Mottness. Ann. Phys. 321, 1634–1650 (2006).

    Article  ADS  Google Scholar 

  38. Wang, R., Go, A. & Millis, A. Weyl rings and enhanced susceptibilities in pyrochlore iridates: k⋅p analysis of cluster dynamical mean-field theory results. Phys. Rev. B 96, 195158 (2017).

    Article  ADS  Google Scholar 

  39. Grüner, G. The dynamics of charge-density waves. Rev. Mod. Phys. 60, 1129–1181 (1988).

    Article  ADS  Google Scholar 

  40. Ishikawa, J. J., O’Farrell, E. C. T. & Nakatsuji, S. Continuous transition between antiferromagnetic insulator and paramagnetic metal in the pyrochlore iridate Eu2Ir2O7. Phys. Rev. B 85, 245109 (2012).

    Article  ADS  Google Scholar 

  41. Chen, G. & Hermele, M. Magnetic orders and topological phases from f–d exchange in pyrochlore iridates. Phys. Rev. B 86, 235129 (2012).

    Article  ADS  Google Scholar 

  42. Ruminy, M. et al. First-principles calculation and experimental investigation of lattice dynamics in the rare-earth pyrochlores R2Ti2O7 (R = Tb, Dy, Ho). Phys. Rev. B 93, 214308 (2016).

  43. Optics, Transport, Specific Heat and Magnetic Susceptibility of the Correlated Semimetal Nd2Ir2O7 (Yareta, 2020); https://doi.org/10.26037/yareta:3k2uscc2bjbqdisikigdjcx2lm

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Acknowledgements

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

  1. Department of Quantum Matter Physics, University of Geneva, Geneva, Switzerland

    K. Wang, C. W. Rischau, N. Bachar, B. Michon, J. Teyssier & D. van der Marel

  2. Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, Fribourg, Switzerland

    B. Xu

  3. Institute for Solid State Physics, The University of Tokyo, Kashiwa, Japan

    Y. Qiu, T. Ohtsuki & S. Nakatsuji

  4. The Institute for Quantum Matter and the Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD, USA

    Bing Cheng, N. P. Armitage & S. Nakatsuji

  5. CREST, Japan Science and Technology Agency, Kawaguchi, Saitama, Japan

    S. Nakatsuji

  6. Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan

    S. Nakatsuji

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Contributions

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=∣r∣2, 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). https://doi.org/10.1038/s41567-020-0955-0

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