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

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

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


Outline
Outline Haldane spin-1 chain

  • Introduction to pure Y2BaNiO5

  • Site impurities (Mg2+↔ Ni2+)

  • Bond impurities (Ca2+↔ Y3+)

  • Conclusions and outlook


k Haldane spin-1 chain i

kf

Q

SPINS cold neutron spectrometer at NCNR


MAPS Spectrometer at ISIS in UK Haldane spin-1 chain


Haldane gap in y 2 banio 5
Haldane gap in Y Haldane spin-1 chain 2BaNiO5

60

hw(meV)

40

20

0

1.5

0

2

0.5

1

q (p)

O2-

MAPS (ISIS)

Ni

1-cosq

Y

S(qw)~d(w-e(q))

e(q)

Ba

T. Ito, K. Oka and H. Takagi


Inter chain coupling and anisotropy
Inter-chain coupling and anisotropy Haldane spin-1 chain

Chain

I(q,w) (1/meV)

Dint. (cts per 15 min.)

DaDbDc

f

I(q,w) (1/meV)

Dintensity (coutns per 15 min.)

Q


Facts on y 2 banio 5
Facts on Y Haldane spin-1 chain 2BaNiO5

Intra-chain exchange

Anisotropy

Inter-chain exchange


Outline1
Outline Haldane spin-1 chain

  • Introduction to pure Y2BaNiO5

  • Site impurities (Mg2+↔ Ni2+)

  • Bond impurities (Ca2+↔ Y3+)

  • Conclusions and outlook


Cutting chains in Y Haldane spin-1 chain 2BaNi1-xMgxO5

  • Mg2+on Ni2+ sites finite length chains

Ni2+

3

Mg2+

Mg2+

Pure

Kojima et al. (1995)

O2-

Y3+


Haldane gap with impurities
Haldane gap with impurities Haldane spin-1 chain

Kenzelmann et al. PRL (2003)


An ensemble of finite length chains
An ensemble of finite length chains Haldane spin-1 chain

8%

P(D)/max(P(D))

8%

P(L)

4%

4%

2%

2%

Chain length L

Probability spin in isolated segment of length L

Gap from chain of length L

Ensemble averaged scattering


Haldane gap with impurities1
Haldane gap with impurities Haldane spin-1 chain

Kenzelmann et al. PRL (2003)


Esr with neutrons
ESR with neutrons Haldane spin-1 chain

Kenzelmann et al. PRL (2003)


Minimal model for chain end spins
Minimal model for chain end spins Haldane spin-1 chain

L is even

Affleck, Kennedy, Lieb, and Tasaki PRL (1987)

L is odd

C. D. Batista et al., PRB(1999)

Singlet-triplet

Single ion anisotropy

Zeeman


Chain end composite spin
Chain-end composite spin Haldane spin-1 chain

Kenzelmann et al. PRL (2003)


Form factor for chain end spin
Form factor for chain-end spin Haldane spin-1 chain

Kenzelmann et al. PRL (2003)


Outline2
Outline Haldane spin-1 chain

  • Introduction to pure Y2BaNiO5

  • Site impurities (Mg2+↔ Ni2+)

  • Bond impurities (Ca2+↔ Y3+)

  • Conclusions and outlook


Hole doping Y Haldane spin-1 chain 2-xCaxBaNiO5

  • Ca2+ on Y3+ sites mobile bond defects

FM

Ca2+

Ca2+

Y3+

Pure

Kojima et al. (1995)

Ni

O2-


Transport in ca doped y 2 banio 5
Transport in Ca doped Y Haldane spin-1 chain 2BaNiO5

1D conductivity, no Charge ordering

T. Ito et al. PRL (2002)


Gap modes in 4% Ca-doped Y Haldane spin-1 chain 2BaNiO5


Does Haldane spin-1 chain d q vary with calcium concentration?

dq not strongly dependent on x

single impurity effect

Xu et al., Science 289, 419 (2000)


Bond Impurities in a spin-1 chain: Y Haldane spin-1 chain 2-xCaxBaNiO5

FM

Ni

Ca2+

Y3+

O


Form factor for fm coupled chain end spins
Form-factor for FM-coupled chain-end spins Haldane spin-1 chain

A symmetric AFM droplet

Ensemble of independent

randomly truncated AFM droplets


Gap modes in 4% Ca-doped Y Haldane spin-1 chain 2BaNiO5

Excited state ?

Quasi-elastic

From deg. GS


Minimal model of spin polaron
Minimal model of spin polaron Haldane spin-1 chain


Intra polaron dynamics in ca doped y 2 banio 5
Intra polaron dynamics in Ca-doped Y Haldane spin-1 chain 2BaNiO5

Clean gap

Intra quartet

?

Pure

Normalized Intensity (Arb)

4% Ca

0

0

20

15

5

25

10


Conclusions
Conclusions: Haldane spin-1 chain

  • 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


Outlook
Outlook Haldane spin-1 chain

  • 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/~broholm/MACS


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