Strongly correlated electron phenomena in filled skutterudites in - PowerPoint PPT Presentation

Strongly correlated electron phenomena in filled skutterudites in filled skutterudites M. Brian Maple University of California, San Diego Workshop on Properties and Applications of Thermoelectric Materials, Hvar, September, 2008

Crystal structure of the filled skutterudites • Filled skutterudites: MT X • Filled skutterudites: MT 4 X 12 (M = alkali metal, alkaline earth, rare earth, actinide; (M = alkali metal alkaline earth rare earth actinide; T = Fe, Ru, Os; X = P, As, Sb) (T = Pt, X = Ge; Dresden, Vienna) • Binary skutterudites: TX 3 (T = Co, Rh, Ir; X = P, As, Sb) Prototype CoAs 3 : Discovered in Skutterud, Norway P t t C A Di d i Sk tt d N • Cubic structure, T h symmetry • T cations (Os) – sc sublattice • T cations (Os) sc sublattice • X anions (Sb) – distorted corner sharing octahedra centered by T cation • M cations (Pr) – bcc sublattice “Fill” atomic cages formed by X anions bcc structure ( Im-3 ) a = 9 3068 Å a = 9.3068 Å PrOs 4 Sb 12 W. Jeitschko & D. J. Braun, 77

Crystal structure of the filled skutterudites After T. Yanagisawa (08)

Filled skutterudites • Many striking properties of filled skutterudites associated with M “filler ions” • Many striking properties of filled skutterudites associated with M filler ions and the X atomic cages within which they reside – “Tunneling” and “rattling” of M “filler ions” in oversize X atomic cages � “Rattling” of M “filler ions” can scatter phonons and reduce lattice contribution to the thermal conductivity κ l � Increases thermoelectric figure of merit g ZT = S 2 /( κ e + κ l ) ρ ZT ~ 1.2 at T ~ 800 K (e.g., CeFe 4-x Co x Sb 12 ) – Localized f-electron states of Ln and Ac ions hybridize with ligand states of 12 n.n. X ions � Large negative intra-atomic exchange interaction ⇒ “Kondo” physics g g g p y � Admixture of f n and f n-1 configurations ⇒ “valence fluctuation” physics � Can produce large value of S and, in turn, ZT • Interplay between charge, spin, orbital, lattice degrees of freedom – Competing interactions — readily “tuned” – “Knobs”: Chemical substitution pressure magnetic field Knobs : Chemical substitution, pressure, magnetic field – Wide variety of strongly correlated electron phenomena

Thermoelectric figure of merit of novel materials After Tritt et al. 08

Filled skutterudites: correlated electron phenomena • Conventional (BCS) superconductivity: e.g., LaFe 4 P 12 , PrRu 4 Sb 12 , PrRu 4 As 12 • Unconventional superconductivity (spin triplet?): e.g., PrOs 4 Sb 12 • Heavy fermion behavior: e.g., PrFe 4 P 12 , PrOs 4 Sb 12 , PrOs 4 As 12 • Non-Fermi liquid behavior (QCP): e.g., CeRu 4 Sb 12 , CeRu 4 As 12 , PrFe 4 Sb 12 • Ferromagnetic order (local moment): e.g., PrFe 4 As 12 , NdFe 4 Sb 12 , NdOs 4 Sb 12 • Ferromagnetic order (itinerant): e.g., LaFe 4 As 12 , LiFe 4 Sb 12 , NaFe 4 Sb 12 • Antiferromagnetic order: e.g., PrOs 4 As 12 • Spin fluctuations: e.g., BaFe 4 Sb 12 , CaFe 4 Sb 12 , SrFe 4 Sb 12 Spin fluctuations: e.g., BaFe 4 Sb 12 , CaFe 4 Sb 12 , SrFe 4 Sb 12 • Antiferroquadrupolar order: e.g., PrFe 4 P 12 , PrOs 4 Sb 12 • Hybridization gap semiconductivity (Kondo insulator behavior): e g • Hybridization gap semiconductivity (Kondo insulator behavior): e.g., CeFe 4 P 12 , CeFe P CeOs 4 Sb 12 , CeOs 4 As 12 • Metal-insulator transitions: e.g., PrRu 4 P 12 g 4 12

Filled skutterudite arsenide & antimonide single crystals NdOs 4 Sb 12 NdOs 4 Sb 12 (UCSD) “cubes” and “rectangular parallelepipeds” PrOs As PrOs 4 As 12 (ILTSR, Wroclaw) (ILTSR Wroclaw) “truncated octahra” 5 mm Zygmunt Henkie PrOs 4 As 12 “Skutterudite” Skutterudite (CoAs 3 ) National Museum Washington D C Washington, D.C. 1 mm

This talk • Correlated electron behavior in La, Ce, and Pr-based filled skutterudite Correlated electron behavior in La Ce and Pr based filled skutterudite compounds (particularly, arsenides and antimonides) • La and Ce-based filled skutturudite compounds p • Pr-based filled skutterudite compounds – Brief review of heavy fermion (HF) behavior & unconventional y ( ) superconductivity (SC) in PrOs 4 Sb 12 – Experiments on pseudoternary systems Pr(Os 1-x Ru x ) 4 Sb 12 & P & Pr 1-x Nd x Os 4 Sb 12 Nd O Sb � Insight into HF state & unconventional SC in PrOs 4 Sb 12 – HF behavior and antiferromagnetic (AFM) ground state in PrOs 4 As 12 HF b h i d tif ti (AFM) d t t i P O A – Comparison with correlated electron ground states of other PrT 4 X 12 compounds compounds • Overview — progress report on very active research area on worldwide scale (Europe, Japan, North America, . . ) • More questions than answers, at this juncture!

Acknowledgements University of California, San Diego Kobe University E D Bauer E. D. Bauer LANL LANL H. Harima H Harima R. E. Baumbach University of Waterloo, Canada N. P. Butch U. Maryland R. W. Hill N. A. Frederick IPA. Inc. S. Rahimi J. R. Jeffries LLNL Université de Sherbrooke Université de Sherbrooke P.-C. Ho CSU, Fresno S. Li S. K. Kim ISU, Ames L. Taillefer J. Paglione U. Maryland T. A. Sayles SOM, UCSD University of Tennessee & Oak Ridge National L. Shu L. Shu Laboratory Laboratory B. J. Taylor T. Barnes T. Yanagisawa Hokkaido U Songxue Chi W. M. Yuhasz ISU, Ames Pengcheng Dai V. S. Zapf NHMFL, LANL F. Ye I Institute of Low Temperature & Structure tit t f L T t & St t National Institute of Standards & Technology Research, Polish Academy of Sciences, Wroclaw H. J. Kang T. Cichorek J. W. Lynn Z. Henkie Rutherford Appleton Laboratory A. Pietraszko A. Pietraszko R. Bewley R. Wawryk Lawrence Livermore National Laboratory Niigata University M. Fluss T. Goto S. K. McCall Y. Nemoto M W M Elf M. W. McElfresh h H. Watanabe U. Toronto Los Alamos National Laboratory C. S. Turel J. B. Betts J. T. Wei P. A. Goddard MPI Ch MPI Chemical Physics, Dresden i l Ph i D d A L A. Lacerda d J. Singleton A.C. Mota Research supported by US DOE and NSF F. Steglich

La and Ce-based filled skutterudites

Correlated electron ground states in La filled skutterudites •Most La filled skutterudites are superconducting LaT 4 X 12 – Highest T c ’s for T = Ru, X = As – LaRu 4 As 12 : T c = 10.3 K T • Exceptions E ti – LaFe 4 As 12 � Weak FM ( θ C = 5 2 K) Weak FM ( θ C 5.2 K) – LaFe 4 Sb 12 � Spin fluctuations (T sf ~ 50 K) X X sf •Reflects tendency of Fe to form local moments in LnFe 4 X 12 – Often for X = Sb Oft f X Sb – Occasionally for X = As – Rarely for X = P Rarely for X P

Weak ferromagnetism in LaFe 4 As 12 P l Polycrystal – θ C ≈ 5 K t l 5 K θ S. Tatsuoka et al., JPSJ (08)

Weak ferromagnetism in LaFe 4 As 12 Single crystals – ILTSR, Wroclaw; UCSD; θ C ≈ 5 K

Weak ferromagnetism in LaFe 4 As 12 Polycrystal – θ C ≈ 5 K S. Tatsuoka et al., JPSJ (08)

Correlated electron ground states in Ce filled skutterudites CeT 4 X 12 T • Most are small gap semicon- ductors (“Hybridization gap semiconductors” or “Kondo semiconductors or Kondo insulators”) • First examples discovered in 1985 at UCSD: CeFe 4 P 12 , UFe 4 P 12 at UCSD: CeFe 4 P 12 , UFe 4 P 12 X X ∆ decreases with increasing a • A few exhibit NFL behavior – Suggests near QCP, gg , possibly associated with valence or M-I transition

Unit cell volume V a vs Ln ion for LnT 4 X 12 compounds Depression of V a for Ce: V Ce ≈ 3+: Kondo volume collapse V Ce > 3+: valence fluctuations Temporal admixture of 4f 0 and 4f 1 v Ce ≈ 3+ 3 T l d i t f 4f 0 d 4f 1 Hybridization (Hyb) of localized f- & conduction-electron states increases with decreasing V a v C > 3+ v Ce > 3+ F Fe Ru R O Os Hyb P V a Hyb V a As Sb D. J. Braun & W. Jeitschko 80

Electrical resistivity of Ce filled skutterudites Energy gap ∆ vs lattice constant a ∆ decreases with decreasing hybridization (increasing a) hybridization (increasing a) • “hybridization gap semiconductors”

Magnetic susceptibility of Ce filled skutterudites

Magnetoresistivity of CeOs 4 As 12 3 (eqn. 3) ρ -1 (T,H) = Σ A i exp(- ∆ i /k B T) i=1 Intrinsic gap: g p ∆ 1 = 73 K Donor, acceptor states in gap: ∆ 2 = 16 K ∆ 3 = 2.5 K H(T) a. - 0 b. - 0.3 c - 0 5 c. - 0.5 d. - 0.7 e. - 1.0 f. - 1.5 g. - 2.0 h h. - 3.0 3 0 i. - 5.0 j. - 7.0 k. - 9.0 Baumbach et al. 08

C(T) and χ (T) of CeOs 4 As 12 χ (T) = χ 0 + C imp /(T - θ ) • C(T)/T = γ + β T 2 χ 0 ≈ 1.1 x 10 -3 cm 3 /mol 1.1 x 10 cm /mol χ 0 θ = 270 K θ D = 270 K C imp = 3.12 x 10 -3 cm 3 /mol γ ≈ 12 mJ/mol K 2 (H = 0) θ = -3 K Wilson-Sommerfeld ratio: c imp ≈ 0.4% Ce 3+ ( µ eff = 2.54 µ B ) • R W = ( π 2 k B 2 /3 µ eff 2 )( χ 0 / γ ) ≈ 1.1 R. E. Baumbach et al., 08

Thermoelectric power of CeOs 4 As 12 R. E. Baumbach et al., 08

Non-Fermi liquid (NFL) behavior in CeRu 4 As 12 ρ ~ T 1.4 R. E. Baumbach et al., 07

Non-Fermi liquid (NFL) behavior in CeRu 4 As 12 NFL behavior NFL behavior ρ (T) ~ T 1.4 C(T)/T ~ lnT C(T)/T ~ -lnT χ (T) ~ -lnT FL behavior ρ (T) ~ T 2 C(T)/T ~ const χ (T) ~ const R. E. Baumbach et al., 07

Thermoelectric power of CeRu 4 As 12 R. E. Baumbach et al., 07

Pr-based filled skutterudites

PrOs 4 Sb 12