Na0'33V2O5

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HFI-NQI 2007
Muon Spin Relaxation (mSR) Studies of Quantum
Phase Transitions
Yasutomo J. Uemura, Columbia University
MnSi, (Sr,Ca)RuO3 Metallic heli / ferromag ?
Para
HTSC Cuprates Static Stripe ? Superconducting
Frustrated J1/J2 and Kagome Systems Insul. AF
? Spin Gap
Collaborators
T. Goko, I.M. Gat-Malureanu, J. Carlo Columbia
G.M. Luke, McMaster
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Ch. Pfleiderder, P. Boeni, Munchen TU MnSi
K. Yoshimura, Kyoto U (Sr,Ca)RuO3
S. Uchida, T. Ito, R.J. Birgeneau, G.D. Gu HTSC
H. Kageyama, Kyoto U Cu(Cl,Br)La(Nb,Ta)2O7
J1/J2 system
NSF Inter-American Materials Collaboration (CIAM)
E. Baggio-Saitovitch, CBPF (Rio de Janeiro)
J. Sereni, CAB (Bariloche, Argentina)
Y. Sushko, U. Kentucky
S. Julian, U. Toronto,
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MnSi
180 Angstrom period
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itinerant
localized
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In both cases,
Heisenberg Slater Stoner Doniach Moriya
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1979 PRL
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mSR in MnSi PRL 1978
HLF 700 G
1/T1 T/(T-Tc) 1/T1T c i.e. T1 (1 Tc/T)
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Support for the SCR theory of Moriya
mSR covers wide temperature region, complementary
to NMR
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ZF precession in ordered state frequency
scales with ordered moment
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T2K
Weak TF 100 G
Precession amplitude represents para- /
non-magnetic volume
T70K
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Magnetic order only below pC 15 kbar
Non-Fermi-liquid behavior with T1.5 resistivit
y
both p lt pC at T gtTC and p gt pC
Pfleiderer, Julian, Lonzarich PRB 1997
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Pfleiderer et al., PRB1997 Nature2004
neutron scatt.
Anomalous short-range correlations at p gt p
Transport /suscepti. weakly first order
transition above p12 kbar
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Drop of intensity below p
S. Brown et al., 29Si NMR in ZF
Suggests heterogeneity above p
Limited by pressure spread expected for
powderized NMR sample No information about spin
dynamics
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1998 - MuSR in MnSi under applied pressure
---- difficulty in large background from
cell and limited accessible
pressure
2004 Summer and November major progress with T.
Goko MuSR in Transeverse Field of 100
G. Single crystal specimen from
Pfleiderer (TU Munich)
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Single crystal
WTF 100 G
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TF 100 G
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p
pC
(kbar)
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MuSR in Weak Transverse Field
MnSi
Phase Separation between p and pC
No Static Magnetism above pC 14.6 kbar
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MuSR in Longitudinal Field 200 G
T1 measurements
Suppression of dinamic critical behavior with
increasing pressure
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8.3
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New Information from MuSR
p
pC

• Finite Static Volume Fraction between 1.2 1.5
  GPa
• ----- Spontaneous Phase Separation
• No static component above 1.5 GPa
• No dynamic critical behavior above p

Also Little temperature dependence of ordered
moment size (ZF-mSR frequency) --- confirming
NMR ---
? First-order QPT
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Neutron intensity S2 times Volume fraction
Volume average
Looks like second order
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Neutron energy resolution 50 m eV
Mn spin fluctuation rate n lt 1011 /s at 16 kbar
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Mn spin fluctuation rate n at 16 kbar ( pC lt p
) at T 2.5 K
MuSR n gt 1010 /s, if instantaneous
moment size 0.2 mB
Neutron n lt 1011 /s
1010 lt n lt 1011 /s

• Very slow but definitely dynamic fluctutions of
  the partial order correlations

? In para/non mag state, very slow soft mode
towards competing magnetic state
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(Sr,Ca)RuO3
Another system which shows crossover from
itinerant ferro to correlated paramagnet
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para
ferro
Ca con. x
Kiyama, Yoshimura (Kyoto) JPSJ 1999
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constant paramagnetic moment peff
decreasing and small ordered moment MS
? typical itinerant weak ferro
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Nature Physics 3 (January 2007) 29-35.
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HTSC Cuprates Cu/Zn substitution
TF-MuSR
Heterogeneity Swiss-cheese Zn (a)(b) Stripe
Island (c) Spontaneous (d)
s 1/l2 ns/m
STM
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ZF
Size 30 40 A
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HTSC La214
Spontaneous Phase Separation between Static
stripe spin islands (non-superconducting) and Supe
rconducting sea without static magnetism
Island size 30 40 A s.c. coherence length
Similar phase separation also found in Zn-doped
and overdoped HTSC
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Swiss Cheese
nn
Phase separation also in overdoped cuprates
paired
unpaired
ns/m
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BCS
Coexisting paired and unpaired charges in the
overdoped region
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ns/m
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Uemura et al. PRL 1991
BE-BCS
TKT
TB3.2 K
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Frustrated Spin Systems -- insulators --
quantum phase transtion between
antiferromagnetic and spin-singlet states
Square-lattice J1/J2 system Kagome lattice
system ---- S ½ Cu2
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square lattice J1-J2 model
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collinear
Neel
Spin liquid
no experimental example of spin-liquid state
collinear order
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(CuCl)LaNb2O7
Kageyama et al., JPSJ2005
(CuCl)LaNb2O7
La
NbO6
CuCl
Temperature(K)
Dc /kB 26.5 K
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Magnetic Structure of (CuBr)LaNb2O7
collinear (stripe) ordering
Oba et al., JPSJ2006
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(CuCl)LaNb2O7
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Zero-Field MuSR
(CuCl)LaNb2O7
Nuclear Dipolar Field

• No static magnetic order
• ? Spin Singlet Ground State
• _at_ 15 mK

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(CuCl)La(Nb,Ta)2O7
(Nb,Ta)O6
(Nb,Ta) solid solution ? No Randomness on the
exchange path
Ta doping suppresses the spin gap and promotes
magnetic order
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Phase separation sataic mag. vs. para/non mag.
(CuCl)La(Nb1-XTaX)2O7
(Nb,Ta)O6
1.0
Paramagnetic Asymmetry
0.5
0.0
0 5
10 15
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Temperature (K)
x0.4 TN 5 K
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High-field magnetization
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ZF-mSR
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Dependence of damping on domain radius
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Temperature dependent volume fraction
Paramagnetic Asymmetry
Temperature (K)
2
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Island volume fraction depends on Composition
(x) Temperature (T)
Moment size and configuration within an island
independent of T x
Highly disordered spin configuration
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P. Mendels et al. PRL 2007
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Low Energy mSR
surface
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Summary
Quantum magnetic phase boundaries
MnSi itinerant electron heli ? para
(Sr,Ca)RuO3 itinerant electron ferro ? para
HTSC static spin/charge stripe ? superconductor
Cu(Cl,Br)La(Nb,Ta)2O7 J1/J2 AF ? spin gap
Spontaneous phase separation
First-order transition both quantum thermal
Generic to all QPT ?