inhomogeneous level splitting in pr x bi 2-x ru 2 o 7 collin broholm and joost van duijn department...
DESCRIPTION
3/12/04PSI seminar3 Outline Introduction Bulk properties of 4d n and 5d n pyrochlores Spin correlations on TM sites Crystal field excitations on RE sites Level splitting in Pr x Bi 2-x Ru 2 O 7 Ground state doublet spin dynamics Model of inhomogeneous level splitting Discussion and Conclusions Possible relevance for other non-Kramers doublet systemsTRANSCRIPT
Inhomogeneous Level Splitting in PrxBi2-xRu2O7
Collin Broholm and Joost van DuijnDepartment of Physics and AstronomyJohns Hopkins University
3/12/04 PSI seminar 2
CollaboratorsRuthenium pyrochloresK. H. Kim Rutgers N. Hur RutgersD. Adroja ISISQ. Huang NISTS.-W. Cheong RutgersT. G. Perring ISISIridium pyrochloresSatoru Nakatsuji Kyoto Yo Machida KyotoYoshiteru Maeno KyotoToshiro Sakakibara ISSPTakashi TayamaISSP
3/12/04 PSI seminar 3
Outline Introduction
Bulk properties of 4dn and 5dn pyrochlores Spin correlations on TM sites Crystal field excitations on RE sites
Level splitting in PrxBi2-xRu2O7 Ground state doublet spin dynamics Model of inhomogeneous level splitting
Discussion and Conclusions Possible relevance for other non-Kramers
doublet systems
3/12/04 PSI seminar 4
A and B sites on vertices of corner-sharing tetrahedra
A3+ site is trivalent RE with 8-fold O2- coordination
B4+ site is tetravalent TM with 6-fold O2- coordination
Can have both A3+ and B4+ magnetism
The pyrochlore structure A2B2O7
cstSJ
Jij
ji
2
21
SS
Nearest neighbor exchange selects a manifold of states characterized byZero-spin tetrahedra
3/12/04 PSI seminar 5
B=Ru site magnetism of Y2Ru2O7
□ Neutron Diffraction reveals long range magnetic order □ ZFC/FC hysteresis suggests some form of disorder that produces glassy canted AFM
Taira et al. (2000)Taira et al. (1999)
3/12/04 PSI seminar 6
“Strong Coupling” Transition in Y2Ru2O7
T=90 KT=1.5 K
3/12/04 PSI seminar 7
PM and AFM Spin Fluctuations
□ Phase Transition pushes significant spectral weight into “resonance”□ The Q-dependence of scattering reflects the form factor for AFM cluster degrees of freedom
1.5 K
90 K
1.5 K
90 K
3/12/04 PSI seminar 8
2 Magnetic Ions 2 Phase Transitions
N. Taira et al. (2003)
Er2Ru2O7 M. Ito et al. (2001) Pr3+
Yb3+
R2Ru2O7
3/12/04 PSI seminar 9
Metal Insulator Transition Bi-doping increase
bandwidth causing Mott Hubbard MIT
Magnet order is found only in the insulating state
Electronic DOS at EF is enhanced close to the MIT
Yoshii and Sato (1999)
3/12/04 PSI seminar 10
A Highly Entropic Metallic State
K. H. Kim et al.
3/12/04 PSI seminar 11
Praseodymium Crystal Field levels:
10 meV
50 meV
85 meV
105 meV
116 meV
0
4 4 1GS
200 K
5 K Pr1.2Bi0.8Ru2O7
Five transitions from GS implies it is doublet:
3/12/04 PSI seminar 12
Fluctuations in metallic Pr1.2Bi0.8Ru2O7
3/12/04 PSI seminar 13
Inhomogeneous Level Splitting
Broad Spectrum unchanged Upon heating to Bk T
Wave vector dependence Follows Pr form factor2
Neutron Scattering measures the level splitting spectrum
3/12/04 PSI seminar 14
Inhomogeneous two level system The level distribution function: Singlet-singlet susceptibility:
Sample averaged susceptibility
Fluctuation-dissipation theorem yields
2
/ 2
11B
ege e Z
2
/ 20
1( ) ( ) (| |)1B
ed ge e Z
/ 22
1 1 e e Z
S
3/12/04 PSI seminar 15
Data Collapse Confirms “quenched broadening”
3/12/04 PSI seminar 16
Specific heat of ()-split doublet C
(T)/T
(J/m
ole-
f.u./K
2 )
C(T
)/T (J
/mol
e-f.u
./K2 )
2
20
exp,
4 1 expzH
B z
z
HdC T H k H dH
2 22z B zH g H
22
0
exp
1 expBC T k d
3/12/04 PSI seminar 17
Susceptibility of ()-split doublet
2/ 2
0
121B
eT g de e Z
For gapless spectrum low T limiting form is ~lnT
3/12/04 PSI seminar 18
Clues to origins of level splitting T-independent distribution function
⇒ Static not dynamic phenomenon Continuous not discrete spectrum
⇒ Large rare defect or density wave producing distribution of environments
Same distribution describes all x<1.2⇒ Distribution may not come from Bi doping
Magnetic field enters in quadrature⇒ Electrostatic not magnetostatic
inhomogeneity
3/12/04 PSI seminar 19
Where else might this occur? Elements that can have non-Kramers
doublet ground states: Pr3+ , Pm3+, Sm2+, Eu3+, Tb3+, Ho3+, Tm3+, Yb2+, and U4+
Nominally cubic and stoichiometric systems may have clandestine level splitting disorder
Doping effects may be controlled by the induced level splitting
3/12/04 PSI seminar 20
Other Materials to Reconsider Tb2Ti2O7: A non ordering magnet with
mysterious low energy mode Ho2Ti2O7: Long range ordered with unusual
thermodynamics YbBiPt: Ultra heavy fermion system with
mysterious low E mode (Robinson et al (1995).
LiHoxY1-xF4 quantum spin glass. Strain induced level splitting adds an effective transverse field
TbxY2-xTi2O7: Dilution has added effect of neutralizing Tb through level splitting
3/12/04 PSI seminar 21
Metal insulator transition in R2Ir2O7 Yanagishima and Maeno (2001)
Nakatsuji et al.
3/12/04 PSI seminar 22
Conclusions Ir and Ru pyrochlores offer MIT transitions
with frustrated magnetism Y2Ru2O7 : “strong coupling” transition
Spectral weight pushed to finite E resonance Q-dependence unaffected by ordering
PrxBi2-xRu2O7 : static inhomogeneous distribution of level splittings Neutrons: “T-independent” broad spectrum C(T)/T: broadened Schottky anomaly (T): possible logarithmic divergence at low T
Watch out for level splitting in non-Kramers ions, which are “not really magnets”