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Magnetic Neutron Scattering Quantum Singlet Ground State in the Spin-Peierls System CuGeO 3 Singlet...
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Transcript of Magnetic Neutron Scattering Quantum Singlet Ground State in the Spin-Peierls System CuGeO 3 Singlet...
• Magnetic Neutron Scattering
• Quantum Singlet Ground State in the Spin-Peierls System CuGeO3
• Singlet Ground State and Triplet Excited States in the Shastry-Sutherland System SrCu2(BO3)2
• Spin Polarons in SrCu(2-x)Mg(x)(BO3)2
B. D. Gaulin
Neutron Scattering Studies of Tough Quantum Magnetism Problems
Collaborators• S. Haravifard• S.R. Dunsiger 1 • A.J. Berlinsky McMaster University• K.C. Rule2
• J.P. Castellan3 • H.A. Dabkowska
• J. Bonca J. Stefan Institute• S. El Shawish University of Ljubljana, Slovenia
• M.T.F. Telling ISIS Pulsed Neutron Facility, UK• T.G. Perring
• Y. Qiu NIST• J.R.D. Copley• S.H. Lee
• Z. Yamani CNBC, NRC, Chalk River
now at : TU Munich1, HMI Berlin2, Argonne National Lab3
C. G. Shull et al, 1951 Magnetic Structure of MnO
Paramagnet Ferromagnet Antiferromagnet T>TC T<TC T<TN
d-electrons: 10 levels to fill
4f 14 levels 5f
Magnetism = Net Angular Momentum
Neutrons are s=1/2 , neutral particlesEn (meV) = 81.8/so A neutrons have En=20.5 meV
Neutrons scatter from nuclei and magnetic moments in solids
Neutron Scattering Cross Section:
d2/d dE´ = ( r0)2/(2πħ) k´/k N{1/2 g Fd()}2
magnetic form factor
× ( – ) l exp(i∙l)
only measure S ┴ κ
× ∫ <exp(-i∙u0))exp(i∙ul(t))>
× <S0(0) Sl
(t)> exp(-i t) dt scattering ~ S2 so s=1/2 is the hardest case
eg orbitals
t2g orbitals
eg orbitals
t2g orbitals
3d5 : Mn2+
eg orbitals
t2g orbitals
3d9 : Cu2+
Sr2+ substitutes for La3+:- pulls electrons out of CuO2 planes
• Magnetic Mott insulator • Strange metal• D wave superconductor• Conventional metal
All while doping Sr for La at the 15% level or less!
Transport, structure, and magnetism are all closely connected
State-of-the-artneutron scattering
techniques
Large and Pristine Single Crystals grown by
Floating Zone Image Furnace
DC susceptibility shows singletground state
– no phase transition in SrCu2(BO3)2
DC susceptibility shows singletground state
– Spin-Peierls phase transition in CuGeO3
CuGeO3
1 lll J SSH
)'( 122122 lllll JJ SSSSH
Quasi-1D S=1/2 AF – structural phase transition to dimerized singlet state
Spin-Peierls Phase Transition
S=1/2 spin-Peierls chain: Collective Singlet
Introduce magnetic vacancies: S=1/2 degrees of freedom occur within odd length chain segments
or
J0totS
1totS
2
2
Singlet
Triplet
SrCu2(BO3)2
• Quasi-2D Cu-BO3 planes• Sr ions between planes• Mott insulator
S=1/2 moments at Cu2+
sites arranged in orthogonaldimers on square lattice
• Shastry-Sutherland model
nn nnn
jiji SSJSSJH
Case I J>>J’ : Ground State – Isolated singlet dimer on nn (diagonal) bonds
Case II J<<J’ : Ground State – Ordered Neel antiferromagnetic
SrCu2(BO3)2
| > | > J’/J = xcrit ~ 0.69
DC susceptibility shows singletground state
– no phase transition in SrCu2(BO3)2
DC susceptibility shows singletground state
– Spin-Peierls phase transition in CuGeO3
3 Bands of Singlet-Tripletexcitations directly
observable with neutron scattering
~ 3 meV
~ 4.85 meV
~ 9.5 meV
Temperature dependence of 1-triplet excitation and 2-triplet excitation are identical and follow the
complement of the dc-susceptibility (1-).
Time of Flight Neutron Scattering Data
B.D. Gaulin et al,
PRL 93, 267202 (2004)
SrCu2(BO3)2 enters collective singlet state at low temperature:
Large magnetic fields drive triplets to zero energy; producing steps in the magnetization.
G.A. Jorge et al. PRB, 71, 092403, 2005.
Hard core Boson models for interacting triplets (Miyahara and Ueda, J. Phys. CM 15, R327, 2003)
Plateaus appear at commensurate filling of lattice with triplets
Calculated dynamical spin structure factor using the zero temperature Lanczos method.
Comparison to TheoryCalculations by S. El Shawish & J. Bonča
Variational Calculation(El Shawish and Bonca, PRB 74, 174420, 2006)
Starting point for spin defectaround impurity
GS 1st ES
Starting point not an eigenfunction of H
Conclusions:
Neutrons scatter from magnetic moments and nuclei in solids
New neutron scattering infrastructure leads to New Science Remarkable new opportunities with new time-of-flight neutron infrastructure at SNS, JSNS, 2TS@ISIS
Quasi-1D Spin-Peierls singlet ground state in CuGeO3 occurs via phase transition.
Quasi-2D Shastry-Sutherland singlet ground state system SrCu2(BO3)2 shows triplet, n-triplet excited states. Bose condensation of triplets in presence of magnetic field.
Weak substitution of Cu2+ with Mg2+ in SrCu(2-x)Mg(x)(BO3)2 gives rise to in-gap states – Spin Polarons.