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

Unusual electrical and magnetoresistance anomalies in the paramagnetic state of NORMAL rare-earth compounds

• RCuAs2 (R= Normal R, also Ce)
• R7Rh3

(R= Heavy R)

• E. V. Sampatkumaran

Tata Institute of Fundamental Research Mumbai 400005, India

At Hvar meeting, 2002: “Do we understand electron correlation effects in Gd compounds?”

Normal behavior in heavy rare-earths

Electrical resistivity minimum in Gd2PdSi3 (also in single crystals), TN= 23 K

• Is this resistivity (transport) anomaly

unique to Gd?

• Can this be seen for other “normal” R?

Unexpected anomalies in RCuAs2

• 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

CaBe2Ge2 Be Ca Ge Be Ge Be Ge

Space group P4/nmm

Ce Cu As As As Ce As RCuAs2 (ZrCuSi2 type)

Derived from CaBe2Ge2-structure, but Be (or Cu) layers vacant in one half of the unit-cell.

• Prominent minimum above TN, not only for
• Gd, but also for R= Sm, Tb, Dy!
• Insensitive to applications of H in some cases

RCuAs2

Electrical resistivity:

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

R7Rh3 - Crystal Structure

• Hexagonal P63mc structure
• 3-crystallographically inequivalent sites for R

Large Magnetoresistance Anomalies in the Paramagnetic State of R7Rh3 (R= rare earth)

• K. Sengupta, S. Rayaprol and E. V. Sampathkumaran

JPCM (Letters)

• Eur. Phys. Lett.

2004-2005

R7Rh3 - Electrical Transport

At high T,

• Lighter rare earths show dr/dT >0
• Heavier rare earths show dr/dT < 0
• Lanthanide contraction playing some role (?)

Tsutuoka et al

• J. Alloys Comp, 1998; J. Phys. Soc. Japan, 2001.

Dy7Rh3

• antiferromagnetic, TN ~ 59 K
• spontaneous magnetization below TC ~ 34 K

TN TC ?

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)

• dρ/dT < 0 above 150 K
• broad peak in ρ in the

paramagnetic state

• upturn in ρ below TN due to

superzone-gap

Dy7Rh3 – MR in paramagnetic state

• In paramagnetic states, MR varies as H2
• MR is large in the paramagnetic state

TN ~ 59 K

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

Gd7Rh3

• Ordering aniferromagnetically ~ 140 K

Gd7Rh3 - Resistivity

• Large suppression of

ρ by H above TN

• Due to possible influence of H on

magnetic gap, large magnetoresistance MR below TN

• K. Sengupta, et al.,

Europhysics Letters 69 454-460 (2005)

• dρ/dT < 0 above 150 K
• upturn in ρ below TN due to

superzone-gap

Gd7Rh3 – MR, MH above TN

TN ~ 140

MR scales with M2 èspin-disorder responsible for paramagnetic MR anomalies

Gd7Rh3: MR and MH below TN

Summary on Dy7Rh3 & Gd7Rh3

• 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?

• 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)

Unusual transport behavior of CeCuAs2

Lattice constants for RCuAs2

Ce follows lanthanide contraction. è Ce is essentially 3+ Brylak et al,JSSC 1995

Magnetic susceptibility

• µeff = 2.68 µB (Ce is trivalent)
• θp = - 50 K (above 150 K)
• No evidence for magnetic ordering
• Kondo lattice

Electrical resistivity

Negative for Ce only!

Electrical resistivity Thermopower

No activated behavior Kondo behavior above 30 K

Small thermopower values, like in trivalent Ce-based Kondo

• lattices. But not resembling

pseudo-gapped systems like Ce3Bi4Pt3

Low-temperature resistivity

• No T2 dependence and

no activated behavior;

• But T-0.6 dependence

and the exponent H- independent

• Overscreened Kondo:

structural TLS? NFL-like, but different from

• ther 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. 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?

Resistivity in applied field

A sudden change in slope around 14K!

Summary

• CeCuAs2 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?