Structure and dynamics of spin polarons induced by doping a Haldane spin-1 chain

1
Structure and dynamics of spin polarons induced
by doping a Haldane spin-1 chain
Collin Broholm Johns Hopkins University and NIST
Center for Neutron Research
M. Kenzelmann JHU K. Oka Tsukuba T. G.
Perring ISIS H. Takagi ISSP M. E. Bisher
NEC M. M. J. Treacy NEC R. Paul NIST
Ying Chen LANL Guangyong Xu BNL G.
Aeppli UCL J. F. DiTusa LSU I. A.
Zaliznyak BNL C. D. Frost ISIS T. Ito Tsukuba
Xu et al., Science 289, 419 (2000) Kenzelmann et
al., PRL 90, 087202 (2003)
Supported by the National Science Foundation
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Outline

• Introduction to pure Y2BaNiO5
• Site impurities (Mg2? Ni2)
• Bond impurities (Ca2? Y3)
• Conclusions and outlook

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SPINS cold neutron spectrometer at NCNR
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MAPS Spectrometer at ISIS in UK
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Haldane gap in Y2BaNiO5
O2-
MAPS (ISIS)
Ni
1-cosq
Y
S(qw) d(w-e(q))
e(q)
Ba
T. Ito, K. Oka and H. Takagi
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Inter-chain coupling and anisotropy
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Facts on Y2BaNiO5
Intra-chain exchange
Anisotropy
Inter-chain exchange
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Outline

• Introduction to pure Y2BaNiO5
• Site impurities (Mg2? Ni2)
• Bond impurities (Ca2? Y3)
• Conclusions and outlook

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Cutting chains in Y2BaNi1-xMgxO5
O2-
Y3
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Haldane gap with impurities
Kenzelmann et al. PRL (2003)
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An ensemble of finite length chains
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Haldane gap with impurities
Kenzelmann et al. PRL (2003)
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ESR with neutrons
Kenzelmann et al. PRL (2003)
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Minimal model for chain end spins
Affleck, Kennedy, Lieb, and Tasaki PRL (1987)
L is odd
C. D. Batista et al., PRB (1999)
Singlet-triplet
Single ion anisotropy
Zeeman
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Chain-end composite spin
Kenzelmann et al. PRL (2003)
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Form factor for chain-end spin
Kenzelmann et al. PRL (2003)
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Outline

• Introduction to pure Y2BaNiO5
• Site impurities (Mg2? Ni2)
• Bond impurities (Ca2? Y3)
• Conclusions and outlook

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Hole doping Y2-xCaxBaNiO5
Ni
O2-
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Transport in Ca doped Y2BaNiO5
1D conductivity, no Charge ordering
T. Ito et al. PRL (2002)
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Gap modes in 4 Ca-doped Y2BaNiO5
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Does d q vary with calcium concentration?
dq not strongly dependent on x
single impurity effect
Xu et al., Science 289, 419 (2000)
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Bond Impurities in a spin-1 chain Y2-xCaxBaNiO5
Ni
Ca2
Y3
O
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Form-factor for FM-coupled chain-end spins
A symmetric AFM droplet
Ensemble of independent randomly truncated AFM
droplets
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Gap modes in 4 Ca-doped Y2BaNiO5
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Minimal model of spin polaron
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Intra polaron dynamics in Ca-doped Y2BaNiO5
Pure
Normalized Intensity (Arb)
4 Ca
0
0
5
15
20
25
10
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Conclusions

• Experimental observation of increase in Haldane
  gap with decreasing chain length
• Dilute impurities in the Haldane spin chain
  create sub-gap composite spin degrees of freedom.
• Edge states have an AFM wave function that
  extends into the bulk over distances of order the
  Haldane length.
• Holes in Y2-xCaxBaNiO5 are surrounded by AFM spin
  polaron with central phase shift of p
• Low energy spin polaron excitations could be
• oxygen spin flip between aligned chain-end spins
• Or anisotropy-split quartet

Viewgraphs and publications at http//www.pha.jhu.
edu/broholm/homepage
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Outlook

• Determine spin polaron level assignment in field
• What is localization length for spin polaron?
• Spin polaron structure in frustrated magnets
• Spin polarons in high TC superconductors?
• Magnetism of interacting spin polarons

Design by T. D. Pike http//www.pha.jhu.edu/broho
lm/MACS