Anomalous magnetic scattering in the paramagnetic state of some heavy rare-earth compounds

1. Anomalous magnetic scattering in the paramagnetic state of some heavy rare-earth compounds E.V. Sampathkumaran T. I. F. R., Mumbai, India • RCuAs2 • R7Rh3

2. Normal behavior in heavy rare-earths

3. For Ce Kondo systems

4. Gd2PdSi3: Polycrystals & Single crystal Mallik et al Europhys Lett. 1998; PRB 1999

5. Comparison of 3 classes of Gd compounds with respect to “magnetic precursor effects: Effect seen in as well as in C Effect not seen in , but seen in C Effect not seen in as well as in C

6. Is this resistivity minimum anomaly unique to Gd? • Can this be seen for other “normal” R? PRL 2003 JPCM (Letters, IOP select) 2004 Europhys. Lett. 2005 Solid State Commun. 2006 Collaborators: Kausik Sengupta, Sudhindra Rayaprol and Kartik Iyer

7. As Be As Ce Ge Cu As Be Ca Ce Ge As Be Ge Derived from CaBe2Ge2-structure, but Be (or Cu) layers vacant in one half of the unit-cell. CaBe2Ge2 RCuAs2 (ZrCuSi2 type) Crystal structure Space group P4/nmm

8. Magnetic susceptibity & Heat-capacity TN= 4.1 K 2.5 K 13 K 9 K 8 K 8 K 4 K

9. RCuAs2  Lanthanide contraction Not obeying de Gennes scaling

10. Prominent Minimum above TN, not only for Gd, but also for R= Sm, Tb, Dy! Insensitive to applications of H RCuAs2 Electrical resistivity:

11. No Variable-range hopping No Coulomb gap No activation-type No Kondo No weak-localisation No Kondo, even in 4f-part Difficult to understand within hitherto known concepts

12. Metamagnetism & positiveMR (in some cases) below TN prove antiferromagnetism

13. MR = Magnetoresistance in metals Magnetic field dependent change in the electrical resistance: • ωcτ = λ/rc = Hσ(T,0) / ne • ωc Ʈ>> 1 conductionelectrons complete many orbits before successive scattering • ωc Ʈ << 1 scattering processes dominate • For Cu at H= 30 Tesla and at T= 273 K, • ωc Ʈ = 0.14

14. Typical MR expected in the paramagnetic state & near room temperature

15. R7Rh3 - Crystal Structure • Hexagonal P63mc structure • 3-crystallographically inequivalent sites for R

16. R7Rh3 - Electrical Transport Tsutuoka et al J. Alloys Comp, 1998; J. Phys. Soc. Japan, 2001. At high T, • Lighter rare earths show dr/dT >0 • Heavier rare earths show dr/dT < 0 • Lanthanide contraction playing some role (?)

17. TN TC ? Dy7Rh3 • antiferromagnetic, TN ~ 59 K • spontaneous magnetization below TC ~ 34 K

18. dρ/dT < 0 above 150 K • broad peak in ρ in the paramagnetic state • upturn in ρ below TN due to superzone-gap Dy7Rh3 - Resistivity • Magnetic gap suppressed at very high fields, large magneto resistance results K. Sengupta, et al.,J. Phys.: Condens. Matter 16 (2004) L495-L498 (IOP Select)

19. TN ~ 59 K Dy7Rh3 – MR in paramagnetic state • In paramagnetic states, MR varies as H2 • MR is large in the paramagnetic state

20. Dy7Rh3 – MR vs H & M vs H • T < TN, steeper changes in MR due to metamagnetic transition • Nature of plots suggests that magnetic state below & above 30 K are different Tsutuoka et al Physica B 294-295 (2001) 199

21. Gd7Rh3 • Ordering aniferromagnetically ~ 140 K

22. Gd7Rh3 - Resistivity • Large suppression of ρ by H above TN • Due to possible influence of H on magnetic gap, large magnetoresistance MR below TN • dρ/dT < 0 above 150 K • upturn in ρ below TN due to superzone-gap K. Sengupta, et al., Europhysics Letters 69 454-460 (2005)

23. Gd7Rh3– MR, MH above TN TN ~ 140 • MR scales with M2 • spin-disorder responsible for paramagnetic MR anomalies

24. Gd7Rh3: MR and MH below TN

25. Tb7Rh3

27. Summary on Gd7Rh3,Tb7Rh3 andDy7Rh3 • Exhibit interesting temperature dependence of ρ • MR is quite large in the magnetically ordered state as well as paramagnetic state --- Magnetic in origin. Anomalous spin-scattering effects! MR is T-dependent  T-dependent spin-disorder contribution? • Can be classified as metallic GMR systems, also near room temperature? “Paramagnetic GMR”

28. Main Conclusion • Do we understand transport and magnetotransport behavior relatively simple systems, e.g., “normal” heavy rare-earths?

29. Unusual transport behavior of CeCuAs2 • Kausik Sengupta, S. Rayaprol (TIFR) (, , M, C, MR down to 1.8 K) • Y. Uwatoko, T. Nakano, N. Fujiwara, M. Abliz, M. Hedo (ISSP) ( down to 45 mK range, high pressure, NMR) • T. Ekino, R.A. Ribeiro (Hiroshima) (Tunneling) • T. Takabatake, K. Shigetoh (Hiroshima) (Thermopower) • A. Chainani & coworkers (ISSP, Spring8) (Photoemission) • Th. Doert, J.P.F. Jemetio (Dresden) (Samples)

30. Lattice constants for RCuAs2 Brylak et al,JSSC 1995 Ce follows lanthanide contraction.  Ce is essentially 3+

31. Magnetic susceptibility • eff = 2.68 B (Ce is trivalent) • p = - 50 K (above 150 K) • No evidence for magnetic ordering • Kondo lattice

32. Negative for Ce only! Electrical resistivity

33. No activated behavior Kondo behavior above 30 K Thermopower Electrical resistivity Small thermopower values, like in trivalent Ce-based Kondo lattices. But not resembling pseudo-gapped systems like Ce3Bi4Pt3

34. No T2 dependence and no activated behavior; • But T-0.6dependence and the exponent H-independent Low-temperature resistivity NFL-like, but different from other NFL-systems!

35. No evidence for long-range magnetic ordering • Large  value  Heavy-Fermion • There is no rise in C/T at very low T, but a drop below 2 K; • A loss of density of states? • A pseudo-gap? Heat capacity

36. Tunneling Symmetric shoulders (±150 and ± 500 meV) w.r.t bias 2 pseudo-gaps, unlike in other Kondo semi-conductors?

37. 3d-4f Resonant photoemission Photon Energy (eV)

38. Change in the sign of T-coefficient at high P. Increase in TK and/or pseudo-gap closure? A weak upturn persists below 5 K at very high P also! Is it an evidence for two psuedo-gap? High pressure resistivity

39. Resistivity in applied field A sudden change in slope around 14K!

40. 63Cu NMR No unusual broadening down to 0.6 K  Non-magnetic In fact, there is an anomaly (narrowing) of T*W below 14 K! Is there any significance of this temperature?

41. Negative sign of KS! Gradual increase of KS with decreasing T down to 0.6 K 63Cu NMR (60 MHz) KS- linear relationship breaks down in the entire T-range! Relaxation rate drops below about 2 K Artifact of pseudo-gap?

42. Summary • CeCuAs2 is a new Kondo lattice, without magnetic order • Negative T-coefficient of  in the T-range 45 mK – 300 K; Large ! Kondo above 30 K • Behaves like a NFL, but with a low exponent (close to -0.6) and H-independent! • C/T drops below 2 K. Pseudo-gap at low temperatures? Tunneling & Photoemission: Pseudo-gap = Kondo semiconductor in the trivalent limit!

43. Kondo screening n  number of conduction band channels for Kondo screening s  local spin n= 2s Traditional Kondo effect (Fermi liquid) n<2s Underscreened n>2s Overscreened  Non-Fermi liquid (Nozieres 1983)

44. Vladar, Zawadowski PRB, 1983 Interaction between structural two-level systems (TLS) and the conduction electron electrons may also lead to “Overscreened (2-channel)”-Kondo-like situation Signatures: Resistivity varies T-0.5 with a H-independent exponent at low temperatures, with a logT dependence at higher temperatures. Its realisation: Claimed in ThAsSe, Cichorek et al (PRL2005) = Similar to CeCuAs2

45. ABSTRACT

47. Magnetic susceptibity & Heat-capacity TN= 4.1 K 2.5 K 13 K 9 K 8 K 8 K 4 K

48. Metamagnetism & positiveMR (in some cases) below TN prove antiferromagnetism

49. Gd2PdSi3: Polycrystals & Single crystal Mallik et al Europhys Lett. 1998; PRB 1999

50. Comparison of 3 classes of Gd compounds with respect to “magnetic precursor effects: Effect seen in as well as in C Effect not seen in , but seen in C Effect not seen in as well as in C