The Magnetic Phase Diagram of (Sr,Ca) 2 (Ru,Ti)O 4 Revealed by SR Jeremy P. Carlo...
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Transcript of The Magnetic Phase Diagram of (Sr,Ca) 2 (Ru,Ti)O 4 Revealed by SR Jeremy P. Carlo...
The Magnetic Phase Diagram of (Sr,Ca)2(Ru,Ti)O4 Revealed by SR
Jeremy P. [email protected]
June 2, 2010
Columbia University
Canadian Neutron Beam Centre,National Research Council
Outline• Overview
– Correlated electron materials• Magnetic order• Superconductors
• The SR method– Local probe of magnetism
• (Sr,Ca)2RuO4 & Sr2(Ru,Ti)O4– Superconductivity– Magnetic Phase Diagram
Overview
• Relation between magnetic order & superconductivity– BCS: Cooper pairs: electron-phonon interaction– High-Tc: magnetic fluctuations more important
– “Canonical” cuprate phase diagram:
– Parent compound: AF
– Magnetic order close toSC dome
Overview• Ongoing questions:
• Behavior of different families of unconventional SCs?Cuprates Heavy fermion SCs Organic SCs
Sr2RuO4 Fe pnictides etc.• How do magnetism / magnetic fluctuations relate?• “Normal” state behavior, M-I / structural links?
• Holy Grail:• What is the comprehensive theory
of unconventional superconductivity?
• Present Study
The SR method• Production of muons
– Protons extracted from cyclotron/synchrotron– p + low Z production target → + + stuff– + → + +
– parity violation: beam is spin polarized– separate out positrons, etc.– collimate / steer beam to sample
Polarized muon sources: TRIUMF, Vancouver BC
PSI, Switzerland
ISIS, UK (pulsed)
KEK, Japan (pulsed)
• Muon beam – Positive muons +
– Can rotate polarization
– Insert muons one at a time
– Come to rest• Interstitial sites• Near anions• Along bonds
Continuous-beam SR
Decay Asymmetry
Muon spin
at decay
= E / Emax normalized e+ energy
Detection:+ → e+ + + e
e+
e+ detector U
sample
e+ detector D
detector time
D 2.5
incoming muon counter
e+
e+ detector U
sample
e+ detector D
detector time
D 2.5
U 1.7
incoming muon counter
e+
e+ detector U
sample
e+ detector D
detector time
D 2.5
U 1.7
D 1.2
incoming muon counter
e+
e+ detector U
sample
e+ detector D
detector time
D 2.5
U 1.7
D 1.2
D 9.0
incoming muon counter
+ 106-107 more…
Representsmuons in auniform field
135.5 MHz/T
Histograms for opposing counters
asy(t) = A0 Gz(t) (+ baseline)
Total asymmetry ~0.2-0.3
Muon spin polarization function
Field configurations• ZF-SR:
– sees: field due to nearby moments– Spontaneous ordering?
• Precession• Rapid relaxation
vs. out-of-plane doping
vs. in-plane doping
T-dependence (in-plane doping)
T-dependence (out-of-plane doping)
NbO6
La
(CuCl)LaNb2O7
[CuCl]+
Example
Field configurations
• LF-SR:– sees: skewed
local field distribution
– Static order– Decoupling if Happl ~ Bint
– Dynamic order– No decoupling– Drift of “1/3 tail”
H initial muon spin
Example
Field configurations• wTF-SR:
– Calibration of baseline (), total asymmetry (A0)
– sees: – (mostly) applied field (paramagnetic state), – appl. + internal fields (ordered state)
H initial muon spin
Determine ordered, PM fractions
Example
Field configurations
• (strong) TF-SR:– Order induced by applied field
– Metamagnetism, etc.
– Vortex lattice in Type-II SC• Rlx √<B2> 1/2 ns /m*
• = penetration depth• ns /m* = superfluid density
• Polyxtal samples: distribution broadened ~ Gaussian
=> Gaussian rlx => 1/
=> sf. density
H initial muon spin
J. E. Sonier, 1998 & 2007
Srn+1RunO3n+1• Ruddlesden-Popper series
– n=: SrRuO3 (113)• perovskite structure• Ferromagnetic, Tc 165K
– n=3: Sr4Ru3O10 (4-3-10)• multi-layered structure• FM, Tc 105K
– n=2: Sr3Ru2O7 (327)• quantum metamagnetism• FM, AF fluctuations• mag. ordering w/ Mn
– n=1: Sr2RuO4 (214)• Unconventional SC Tc 1.5K • Spin-triplet pairing, p-wave• isostructural to LBCO, LSCO
RuO66
Sr
(Sr,Ca)RuO3 = ‘113’• n=• 3-D structure• Ca/Sr substitution
– SrxCa1-xRuO3
– isoelectronic doping– FM suppressed x 0.25 – Phase separation, QPT
Sr2RuO4 = ‘214’
• n=1• SC state (Maeno et al. 1994)• Tc up to 1.5 K• NMR: Spin-triplet pairing• TRSB – (Luke et al. 1996)
distinguish between p-wave states• Incommensurate spin fluctuations
q ≈ (0.6/a, 0.6/a, 0)• Normal state: 2-D Fermi liquid
• Doping:– “Out-of-plane:” Ca on Sr site: SrxCa2-xRuO4 – “In-plane:” Ti on Ru site: Sr2Ru1-yTiyO4
• Small doping on either site suppresses SC
Maeno et al. 1994
Fermi surface:
MacKenzie & Maeno, 2003
Luke et al. 1996
Ca2RuO4
– AF insulator, moment 1.3B
• Competition between A- and B- type ordering• TN 110-150K
• Ca doping induces Mott transition• Decreased bandwidth• Increased on-site Coulomb repulsion• → Increased U/W• Ru-Ru in-plane
dist > Sr2RuO4
• RuO6 flattening, tilting
Ca2-xSrxRuO4M-I transition near x=0.2 (I-II)Near x=0.5: (II-III)
Sharp increase in susceptibility Correlations more FM-ish
Low susc @ higher x
Old Picture: Ordering at low x onlyAntiferro. near x=0Susc. peak near x=0.5Paramagnetic at higher xSC at x=2
SR:Rapid relaxation observed 0.2 ≤ x ≤ 1.6Peaks near x 0.5, 1.5
Ordered ground state throughout!
Nakatsuji & Maeno, 2000.
Susceptibility @ 2K:
Nakatsuji & Maeno, 2003.
Sr2Ru1-yTiyO4
• y=0: SC Sr2RuO4
• <0.2% Ti doping suppresses Tc
• >2.5% doping induces magnetic ground state
• neutrons: Braden et al. (2002)– Incommensurate AF in y=0.09
• q (0.3, 0.3, qz)
• SR: rapid relaxation with increasing y.
from MacKenzie et al. 2003
ExperimentsSamples
(Ca2-xSrx)2RuO4 x = 0.0, 0.2, 0.3, 0.5, 0.57, 0.65, 0.9, 1.0, 1.4, 1.5, 1.6, 1.8, 1.95
Sr2(Ru1-yTiy)O4 y = 0.01, 0.03, 0.05, 0.09
single xtals from Kyoto U. (Maeno et al. or Tsukuba (Yoshida et al.
ZF- & LF-SR: M20 (LAMPF) and/or M15 (DR)DC Susceptibility: ZFC, FC, H ~ 50-100 G
He gas-flow cryo1.7K < T < 300K
Dilution fridge15mK < T < 10K
Ca2RuO4
• SR spectra:• Sum of 2 frequencies
Ca2RuO4 ZF-SR
ZF-SR Temperature Scans(Ca,Sr) system
ZF-SR Temperature Scans(Ru,Ti) system
dynamic + static d as
“root-exponential” “Lorentzian Kubo-Toyabe”
Uemura “spin glass” function (Uemura, 1985):
aass = a = a √√QQ
dd = 4a = 4a22(1-Q)/(1-Q)/
Edwards-Anderson order parameter
Field width
Fluctuation rate
• ZF Relaxation vs. Temp: Magnetic ordering!
Define: Rlx = sqrt ( d2 + as
2 )
Fit to: Rlx(T) = R [ 1 – (T/To)]
all
Ti only
Ca only
zoom
• LF @ base temp: decoupling → static order
Static ordering at base temp!
Fit to tanh(H/Ho)
• LF temp scans: map out dynamics
• Comparison of ZF & LF field estimates
R [ 1 – (T/To) ]
Adapted from Braden et al. (2002)Neutrons: BradenMuons: present study
DC Susceptibility
Curie-Weiss:
moreAF
Old view:
New View:
Summary: (Sr,Ca)2(Ru,Ti)O4 Past:Sr2RuO4 p-wave SC
Tc 1.5K, TRSB
magnetic fluctuations
Sr2Ru1-yTiyO4
y 0.002 suppresses SCneutrons: incommensurate AF y = 0.09
Ca2RuO4 AF insulator
TN 100-150K
Sr2-xCaxRuO4
M-I transition x 0.2susceptibility peak x 0.5
New: Sr2-xCaxRuO4
muons : magnetic order over almost entire range
x = 0: commensurate AF, gone by x = 0.2
peaks x 0.5 (FM-ish?), 1.5 (more AF)incommensurate AF / SDW ?need long-range magnetic probe!
Sr2Ru1-yTiyO4
muons: rapid relaxation y ≥ 0.03 susc: large negative w → AF