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
  • Introduction to pure Y2BaNiO5
  • Site impurities (Mg2+↔ Ni2+)
  • Bond impurities (Ca2+↔ Y3+)
  • Conclusions and outlook
slide3

ki

kf

Q

SPINS cold neutron spectrometer at NCNR

haldane gap in y 2 banio 5
Haldane gap in Y2BaNiO5

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

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 Y2BaNiO5

Intra-chain exchange

Anisotropy

Inter-chain exchange

outline1
Outline
  • Introduction to pure Y2BaNiO5
  • Site impurities (Mg2+↔ Ni2+)
  • Bond impurities (Ca2+↔ Y3+)
  • Conclusions and outlook
slide9

Cutting chains in Y2BaNi1-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

Kenzelmann et al. PRL (2003)

an ensemble of finite length chains
An ensemble of finite length chains

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

Kenzelmann et al. PRL (2003)

esr with neutrons
ESR with neutrons

Kenzelmann et al. PRL (2003)

minimal model for chain end spins
Minimal model for chain end spins

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

Kenzelmann et al. PRL (2003)

form factor for chain end spin
Form factor for chain-end spin

Kenzelmann et al. PRL (2003)

outline2
Outline
  • Introduction to pure Y2BaNiO5
  • Site impurities (Mg2+↔ Ni2+)
  • Bond impurities (Ca2+↔ Y3+)
  • Conclusions and outlook
slide18

Hole doping Y2-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 Y2BaNiO5

1D conductivity, no Charge ordering

T. Ito et al. PRL (2002)

slide21

Does d q vary with calcium concentration?

dq not strongly dependent on x

single impurity effect

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

form factor for fm coupled chain end spins
Form-factor for FM-coupled chain-end spins

A symmetric AFM droplet

Ensemble of independent

randomly truncated AFM droplets

slide24

Gap modes in 4% Ca-doped Y2BaNiO5

Excited state ?

Quasi-elastic

From deg. GS

intra polaron dynamics in ca doped y 2 banio 5
Intra polaron dynamics in Ca-doped Y2BaNiO5

Clean gap

Intra quartet

?

Pure

Normalized Intensity (Arb)

4% Ca

0

0

20

15

5

25

10

conclusions
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

outlook
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/~broholm/MACS

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