Normal behavior in heavy rare-earths Electrical resistivity minimum - PowerPoint PPT Presentation

E. V. Sampatkumaran Tata Institute of Fundamental Research Mumbai 400005, India Unusual electrical and magnetoresistance anomalies in the paramagnetic state of NORMAL rare-earth compounds • RCuAs 2 (R= Normal R, also Ce) • R 7 Rh 3 � (R= Heavy R)

At Hvar meeting, 2002: “Do we understand electron correlation effects in Gd compounds?” Normal behavior in heavy rare-earths

Electrical resistivity minimum in Gd 2 PdSi 3 (also in single crystals), T N = 23 K

� • Is this resistivity (transport) anomaly unique to Gd? Can this be seen for other “normal” R?

� Unexpected anomalies in RCuAs 2 1 . � Phys. Rev. Lett. 91 (2003) 036603. 2. � Phys. Rev. B. 70 (2004) 064406. 3 . � Physica B 348 (2004) 465. 4. � SCES2004, Physica B, 2005. Heavy rare-earths: Kausik Sengupta, S. Rayaprol (TIFR), Th. Doert, J.P.F. Jemetio (Dresden) (Samples)

Crystal structure Space group P4/nmm Be As Ge Be Ce Ca As Ge Cu Be As Ge Ce CaBe 2 Ge 2 As Derived from CaBe 2 Ge 2 -structure, but Be (or Cu) layers vacant in one RCuAs 2 (ZrCuSi 2 type) half of the unit-cell.

� � � � � RCuAs 2 Electrical resistivity: Prominent minimum above T N, not only for � Gd, but also for R= Sm, Tb, Dy! Insensitive to applications of H in some cases

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

Increasing spin-disorder contribution before the onset of long-range magnetic order in some rare-earth compounds?

Typical MR expected in the paramagnetic state & near room temperature

R 7 Rh 3 - Crystal Structure • Hexagonal P6 3 mc structure • 3-crystallographically inequivalent sites for R

Large Magnetoresistance Anomalies in the Paramagnetic State of R 7 Rh 3 (R= rare earth) K. Sengupta, S. Rayaprol and E. V. Sampathkumaran JPCM (Letters) Eur. Phys. Lett. 2004-2005

R 7 Rh 3 - Electrical Transport Tsutuoka et al J. Alloys Comp, 1998; J. Phys. Soc. Japan, 2001. At high T, • Lighter rare earths show d r/ dT >0 • Heavier rare earths show d r/ dT < 0 • Lanthanide contraction playing some role (?)

Dy 7 Rh 3 T N T C ? • antiferromagnetic, T N ~ 59 K • spontaneous magnetization below T C ~ 34 K

Dy 7 Rh 3 - Resistivity • d ρ /dT < 0 above 150 K • broad peak in ρ in the paramagnetic state • upturn in ρ below T N due to superzone-gap • 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)

Dy 7 Rh 3 – MR in paramagnetic state T N ~ 59 K • In paramagnetic states, MR varies as H 2 • MR is large in the paramagnetic state

Dy 7 Rh 3 – MR vs H & M vs H Tsutuoka et al Physica B 294-295 (2001) 199 • T < T N , steeper changes in MR due to metamagnetic transition • Nature of plots suggests that magnetic state below & above 30 K are different

Gd 7 Rh 3 • Ordering aniferromagnetically ~ 140 K

Gd 7 Rh 3 - Resistivity • d ρ /dT < 0 above 150 K • upturn in ρ below T N due to superzone-gap • Large suppression of ρ by H above T N • Due to possible influence of H on magnetic gap, large magnetoresistance MR below T N K. Sengupta, et al., Europhysics Letters 69 454-460 (2005)

Gd 7 Rh 3 – MR, MH above T N T N ~ 140 MR scales with M 2 èspin-disorder responsible for paramagnetic MR anomalies

Gd 7 Rh 3 : MR and MH below T N

� Summary on Dy 7 Rh 3 & Gd 7 Rh 3 • 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 à T-dependent spin-disorder contribution? • Can be classified as metallic GMR systems, also near room temperature?

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

� � Unusual transport behavior of CeCuAs 2 • Kausik Sengupta, S. Rayaprol (TIFR) ( ρ , χ , M, C, MR down to 1.8 K) • Y. Uwatoko, T. Nakano, N. Fujiwara, M. Abliz, M. Hedo (ISSP) ( ρ 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)

Lattice constants for RCuAs 2 Brylak et al,JSSC 1995 Ce follows lanthanide contraction. è Ce is essentially 3+

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

Electrical resistivity Negative for Ce only!

Electrical Thermopower resistivity No activated behavior Small thermopower values, like in trivalent Ce-based Kondo lattices. But not resembling pseudo-gapped systems like Ce 3 Bi 4 Pt 3 Kondo behavior above 30 K

Low-temperature resistivity • No T 2 dependence and no activated behavior; • But T -0.6 dependence and the exponent H- independent • Overscreened Kondo: structural TLS? NFL-like, but different from other NFL-systems!

Heat capacity No evidence for long-range magnetic ordering Large γ 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?

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

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

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

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