michael gingras- frustrated magnetic pyrochlores- a rich playground for the study of exotic...
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Michel GingrasDepartment of Physics & Astronomy, University of Waterloo, Waterloo, Ontario, CanadaandCanadian Institute for Advanced Research/Quantum Materials Program
Frustrated Magnetic Pyrochlores – Frustrated Magnetic Pyrochlores – a Rich Playground for the Study of a Rich Playground for the Study of Exotic Collective Phenomena: Exotic Collective Phenomena:
the Case of Spin Ice the Case of Spin Ice
AlbaNova and Nordita Physics Colloqium, Stockholm, September 24, 2009
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Collaborators• Ali Tabei (WaterlooUChicago)• Pawel Stasiak (Waterloo)• Taras Yavors’kii (Waterloo)• Hamid Molavian (Waterloo) • Adrian del Maestro (HarvardUBC)• Jacob Cosman (UBC)• Francois Vernay (Waterloo PSI)• Matt Enjalran (USC)• Ying-Jer Kao (Nat. U. Taiwan,Tapei)• Jean-Yves Fortin (CNRS, Strasbourg)• Benjamin Canals (CNRS, Grenoble)• Steve Bramwell (U.Coll. London)• Tom Fennell (U. Coll. London)• Bruce Gaulin (McMaster)• John Greedan (McMaster)• Jason Gardner (NIST)• Rob Hill (Waterloo)• Rob Kiefl (UBC/TRIUMF)• Jan Kycia (Waterloo)• Jeff Quilliam (Waterloo)• Kate Ross (WaterlooMcMaster)• Linton Corruccini (UC Davis)• Oleg Petrenko (Warwick)
Theory
Experiment
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FundingOperating
Infrastructure
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Outline• PART I – Introduction: Magnetism, frustration and
frustrated magnetism– Magnetism– Frustration– Why study frustrated magnetism
• PART II – Three examples:– Spin ice phenomenology– “Monopoles” in spin ice– Emergence of frustration from quantum fluctuations
• Conclusion
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PART I
Introduction:
Geometric frustration and frustrated magnetism
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Frustration is ubiquitous incondensed matter physics –even in other correlated states of matter
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What is Frustration?Inability of a system to minimize its total classical ground state energy by minimizing the energy of all pairwise interactions, pair by pair.
Arises in:• molecular and ionic crystals (e.g. N2, H2 , KCN)• superconducting Josephson junctions in a B field• liquid crystalline materials (e.g. nematic/smectic)• proteins (e.g. protein folding problem)• random disordered magnets (e.g. spin glasses)• modulated nuclear matter in neutron stars (a.k.a. “nuclear pasta”)
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Néel state(two sublattices)
Louis Néel1904-2000; Physics Nobel Prize 1970
,
; 1z z zi j i
i j
H J S S S
The non-frustrated case first
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or ?
Despite frustration, Heisenberg and XY spins display a (three sublattices) Néel ordered state but Th and Qu fluctuations are enhanced
Frustrated plaquette / Ising spins
120º
O(N) spins can somewhatrelieve the frustration
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Not all frustrated condensedmatter systems are magnetic
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The Protein Folding ProblemThe Protein Folding Problem
is a frustration problem too … is a frustration problem too …
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The structure of hydrogen/protons in Ice
O
H
is a geometrical frustration problem too … is a geometrical frustration problem too …
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Ice Water
Covalent bond
Hydrogen bond
H
O
H
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Pressure-Temperature Phase Diagram of Water
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Hexagonal Ice
Crystal structure:
• O2- ordered; H+ disordered
• Bernal & Fowler Ice rules:
[1] 1 H on each O-O bond
[2] “2-H close, 2-H far”
• Lowest energy state not obtained.
• S(T0) ≠ 0.
• Hexagonal ice (Ih) is the most common form of ice.
O
H
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Molecular Rotation in Ice at 10oK, Free Energy of Formation and Entropy of Water W. Giauque and M. Ashley, Phys. Rev. 43, 81 (1933)
0 45.1 44.23 0.87S S R R R
1895-1982Chem. NobelPrize, 1949
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J. Am. Chem. Soc. 57, 2680 (1935) exp
0 0~ log(3/ 2) vs ~ 0.87S R S R
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• Somehow … ice is frustrated.
• That is, its microscopic Hamiltonian (even with restricted oxygen positions, and focusing only on the protons) must be frustrated.
• Perhaps surprisingly, after 75 years, such microscopic understanding of ice water in terms of frustration is still incomplete.
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• Following the discovery of high-temperature superconductivity in a research theme that developed in the late 1980s and early1990s, and which still commanding increasing attention within condensed matter physics, has been to search for novel strongly correlated classical and quantum mechanical low-temperature magnetic states.
• One highly successful approach has been the study of highly frustrated magnetic (HMF) systems.
• These are very interesting in that they do not admit a long-range ordered state at zero temperature even at the classical level.
• They are therefore extremely sensitive to small perturbations and quantum mechanical effects, hence their propensity to exhibit exotic behaviours.
Why Study Highly Frustrated Magnetic Systems?Why Study Highly Frustrated Magnetic Systems?
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kagome
hyperkagome
pyrochlore garnet
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kagome
hyperkagome
pyrochlore garnet
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Example: Kagome Lattice
120 degrees between spins
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Ground State of the Kagome Lattice Antiferromagnet
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kagome
hyperkagome
pyrochlore garnet
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kagomehyperkagome
pyrochlore garnet
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,
1, 2, 3, 4, 0
1, 2, 3, 4,
2~
2
0
i ji j
H J S S
JH S S S S E
S S S S
1 23 4
Collective Paramagnetism on the Pyrochlore Lattice
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• Lots and lots of phenomena have been observed in HFMs over the past twenty years
• In that context, the magnetic pyrochlore oxides of generic formula A2B2O7 are particularly interesting:
(A3+)2(B) 4+) 2(O2-)7
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What is possible in terms of A2B2O7 pyrochlores:
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What is possible in terms of A2B2O7 pyrochlores:
What is possible
Conditional possibility
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Pyrochlore oxides have been shown to exhibit a multitude of phenomena:
- conventional long range order: Gd2Sn2O7 - disorder-free spin glass freezing: Y2Mo2O7
- thermal and/or quantum order-by-disorder: Er2Ti2O7
- anomalous Hall effect: Nd2Mo2O7
- persistent T→0 spin dynamics seen in muSR: all of them- field-driven transitions of various kinds- spin ice phenomenology: (Ho,Dy)2(Ti,Sn)2O7
- spin liquid/collective paramagnetic behavior: Tb2Ti2O7
- exotic deconfined topological excitations in spin ices
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Pyrochlore oxides have been shown to exhibit a multitude of phenomena:
- conventional long range order: Gd2Sn2O7 - disorder-free spin glass freezing: Y2Mo2O7
- thermal and/or quantum order-by-disorder: Er2Ti2O7
- anomalous Hall effect: Nd2Mo2O7
- persistent T→0 spin dynamics seen in muSR: all of them- field-driven transitions of various kinds- spin ice phenomenology: (Ho,Dy)2(Ti,Sn)2O7
- spin liquid/collective paramagnetic behavior: Tb2Ti2O7
- exotic deconfined topological excitations in spin ices
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PART II: Three examples
• Three examples:
– Spin ice phenomenology
– Monopoles in spin ice
– Emergence of frustration from quantum fluctuations
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Lack of Order in a Ferromagnet
M. J. Harris, S. T. Bramwell,. F. McMorrow, T. Zeiske and K. W. Godfrey, Phys. Rev. Lett. 79, 2554 (1997)
- No order down to T~20mK despite CW ~ + 2K!- f CW/Tmin> 100
• Ho2Ti2O7
• CW ~ + 2K! (i.e. ferromagnetic interactions)
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111 Single Ion Anisotropy in pyrochlore oxides
Ho,Dy crystal electric field effect
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AF interaction
FM interaction
(111) Ising(Non-Frustrated)
Heisenberg(Frustrated Coop. PM/ Spin Liquid )
(111) Ising(Frustrated Spin Ice)
Heisenberg(Non-Frustrated)
2ˆ
i iD S z
D
iz
FM and AFMswap
frustration!
Local 111 Ising anisotropy
6 ground states for a tetrahedron
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• Tetrahedral proton coordination• Bernal-Fowler ice rules
Proton displacement: vector at mid-point
Ferromagnetically coupled Ising magnetic moments on pyrochlore lattice are ice-like!!
Proton Position in Ice vs Spin Orientation in FM Ising Pyrochlore
S. T. Bramwell and M. J. Harris, J. Phys. Condens. Matter 10, L215 (1998)
O2-
H+
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Degeneracy of Ising Pyrochlore Ferromagnet
6N/4(6/16)N/4
Hence, at T=0, the entropy per spin is: 2
3ln
2
1B0 kS T
Same (Pauling) entropy at T=0 as water ice!!! hence the name “spin ice”
M. J. Harris, S. T. Bramwell, D. F. McMorrow, T. Zeiske and K. W. Godfrey, Phys. Rev. Lett. 79, 2554 (1997)
For N moments on the pyrochlore lattice with n.n.ferromagnetic Jij
exchange, there are
equivalent groundstates of the system!
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Real materials show manifestations of Pauling’s ground state entropy magnetic analogues of water ice
Ramirez et. al, “Zero Point Entropy in Spin Ice”, Nature 399, 333 (1999)
2
12 1
( )( ) ( )
T
T
C TS T S T dT
T
Difference is Pauling’s S0 !!
This means, the value of S(T=0 )Should have beeen set to S0 not 0
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Insulating Rare-Earth Magnetic Materials
• 4f orbitals are burried under 5s, 6s, 5d orbitals: exchange interactions Jij are “small”: θCW ~ 100 —101 K
• R.E.3+ can have large magnetic moments ~ 100 —101 B
• Magnetic dipolar interactions, D ~ 10-1 — 100 K ~ θCW
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Dipolar Ising Spin Ice Model
Moved from J=15/2 S=1/2 effective classical Ising spin
den Hertog and Gingras, Phys. Rev. Lett. 84, 3430 (2000)Bramwell and Gingras, Science 294, 1495 (2001)Yavors’kii et al. Phys. Rev. Lett. 101, 037204 (2008)
,
3 5
ˆ ˆ
ˆ ˆ ˆ ˆ3( )( )
ji
ji
zzi j i j
i j
zi j i ij j ij zi j
i jij ij
H z z
z z z r z r
rD
J
r
Not known
Known
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den Hertog and Gingras, Phys. Rev. Lett. 84, 3430 (2000).
Real materials show manifestations of Pauling’s ground state entropy magnetic analogues of water ice
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S(q) obtained from Monte Carlo results on a generalized dipolar spin ice model: Jij beyond n.n.
ExperimentMonte Carlo
Yavors’kii et al. Phys. Rev. Lett. 101, 037204 (2008)
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Some of what I have not talked about re: spin ice
• Spin ice in a field (topological {Kastelyn} transitions)
• Where do the ice rules really come from?• Magnetic monopole-like topological defects• Artificial ice, stuffed ice• Dynamics of spin ice • Structural and spin fluctuations in spin ice• How is the 3rd law of thermodynamic (S0≠0)
resolved?• How to put back quantum mechanics? –
Relevant to the Tb2Ti2O7 spin liquid material
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Some of what I have not talked about re: spin ice
• Spin ice in a field (topological {Kastelyn} transitions)
• Where do the ice rules really come from?• Magnetic monopole-like topological defects• Artificial ice, stuffed ice, other ice-like materials• Dynamics of spin ice • Structural and spin fluctuations in spin ice• How is the seemingly violation of the 3rd law of
thermodynamic (S0≠0) ultimately resolved?• How to put back quantum mechanics? –
Relevant to the Tb2Ti2O7 spin liquid material
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PART II: Three examples
• Three examples:
– Spin ice phenomenology
– Monopoles in spin ice
– Emergence of frustration from quantum fluctuations
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Singular correlations in spin ice state• The spin ice rule can be mapped in the long length scale
limit to a non-divergent field
• The excitations (spin flip) that break the ice rule create effective “magnetic charges”, or monopoles in that field.
• The system obeys a “magnetic Gauss’ Law” which relates the density of poles to the divergence of the field
0
monopoles
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Singular correlations in spin ice state• Because of the ice rule, hence the divergence free-
condition of the field • The spin-spin correlations in real-space decay as an
effective dipolar type
• Very different than the exponential decay of the spin-spin correlations in a thermally disordered paramagnet
• As a result, the Fourier transform, hence the neutron scattering, show singular behaviors, “pinch points”, at specific reciprocal lattice points.
2
' 5
1 3( ) ( ') ~ ~
'u v uv
r r
r r rS r S r
r r r
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Pinch points
Contour plot of neutron scattering intensity
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Neutron scattering of Ho2Ti2O7 spin ice
Fennell et al., to appear in Science, Oct. 2009
002 point
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Fennell et al., to appear in Science, Oct. 2009
vicinity of 002 point
transverse scan through pinch point at 002
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Monopoles and Dirac strings
Castelnovo et al., Nature 451, 41 (2008).
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• Thermal excitations lead to deconfined monopoles• This is because the “Dirac string” of reversed magnetic moments• left behind is tensionless, thanks to the nature of the spin ice state
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Dirac strings in a magnetic field
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Dirac strings in a magnetic field
Morris et al., to appear in Science, Oct. 2009
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• Excitations in spin ices consists of deconfined monopole-like topological defects
• These have been recently observed and reported in 2 neutron scattering papers in Science and 1 muon spin relaxation paper in Nature.
• There is currently efforts in understanding the role of these objects on the spin dynamics in spin ice. 1theory paper addressing this issue was published in Nature Physics in April 2009.
• There are bound to be more interesting effects tied to monopoles and “Dirac strings” in spin ice to be reported over the next couple of years.
“Conclusion” about Monopoles in Spin ice
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PART II: Three examples
• Three examples:
– Spin ice phenomenology
– Monopoles in spin ice
– Emergence of frustration from quantum fluctuations
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Monte Carlo Phase Diagram of the Dipolar Spin Ice ModelMonte Carlo Phase Diagram of the Dipolar Spin Ice Model
Ho2Ti2O7
Jnn/Dnn - 0.22
Dy2Ti2O7
Jnn/Dnn - 0.52
Tb2Ti2O7
Jnn/Dnn - 1.1
PM
2nd OPT • LRO
• 1st OPT
910.D
J
nn
nn
C
TNo PT
C
T
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1/T
1 (
s-1)
Tf
Muon Spin Relaxation Study of Tb2Ti2O7
J. S. Gardner et al. Phys. Rev. Lett. 82, 1012 (1999)
θCW ~ -20 KTc < 20 mK
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Dy3+ (J=15/2)
15/2 + O(10-1)
~ 350 K
8 + O(10-1)
Ho3+ (J=8)
~ 280 K
mJ wavefunction decomposition
Tb3+ (J=6)(should be a nonmagnetic singlet, as Tm3+ !)
4 + g 1; g ~ O(10-1)
~ 20 K
5 + e2; e ~ O(10-1)
~ 80 K
~ 20 K
{-3,+6, -6
+3, -3,+6, -6
Gingras et al., Phys. Rev. B 61, 6496 (2000)
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Effective Hamiltonian MethodEffective Hamiltonian Method
P
P
P EE
R
00
effH PHP PHRHP
ex ex
e
eff
x dip dip ex dip dip
ex dip
( )
PH RH P
PH RH P PH RH P P
H
H
PH P
RH P
PH P
3 5,
J J (J )(J )J J ( 3 )i j i ij j ij
i ji j j i ij ij
R RH J D
R R
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= + + + +
+ + + +
N-body quantum effectsplay an important rolein the renormalizationof the Néel – spin-ice boundary.
+ + …
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• Perhaps Tb2Ti2O7 admits a truly novel/exotic quantum mechanical ground state!
• It is possible that the effective (field theory) describing this material is akin to 3+1 QED
• This would mean that this system may exhibit (i) emerging (gauge) “photon” (ii) fractionalized & deconfined excitations
• It is not clear at this time what is the role of the lattice vibration in this material on the magnetic phenomena observed
• More theoretical and experimental work is needed.
“Conclusion” about Tb2Ti2O7
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• Frustrated magnetic materials are presenting themselves as a rich source of new phenomena in condensed matter physics.
• Among them, spin ices have proven to be particularly interesting – I have discussed only a few topics about them.
• There is currently active worldwide research efforts in this field with networks in Japan and Europe.
• Other interesting phenomena are bound to be discovered in the next few years.
Conclusion
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The end
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