Magnetism in (Sr,Ca)RuO4 and FeAs-based superconductors

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The Magnetic Phase Diagram of (Sr,Ca)2(Ru,Ti)O4
Revealed by mSR
Jeremy P. Carlo jeremy.carlo_at_nrc.gc.ca
Columbia University Canadian Neutron Beam
Centre, National Research Council
June 2, 2010
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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

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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 to SC dome

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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

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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)

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Continuous-beam ?SR

• Muon beam
• Positive muons ?
• Can rotate polarization
• Insert muons one at a time
• Come to rest
• Interstitial sites
• Near anions
• Along bonds

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Decay Asymmetry
Muon spin at decay
Detection ? ? e ??? ?e
e E / Emax normalized e energy
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e detector U
incoming muon counter
sample
e
m
detector
time
D 2.5
e detector D
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e detector U
incoming muon counter
sample
e
m
detector
time
D 2.5
e detector D
U 1.7
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e detector U
incoming muon counter
sample
e
m
detector
time
D 2.5
e detector D
U 1.7
D 1.2
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e detector U
incoming muon counter
sample
e
m
detector
time
D 2.5
e detector D
U 1.7
D 1.2
D 9.0
106-107 more
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Histograms for opposing counters
asy(t) A0 Gz(t) ( baseline)
a
Total asymmetry 0.2-0.3
Muon spin polarization function
135.5 MHz/T
Represents muons in a uniform field
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Field configurations

• ZF-?SR
• ?? sees field due to nearby moments
• Spontaneous ordering?
• Precession
• Rapid relaxation

T-dependence (in-plane doping)
vs. out-of-plane doping
T-dependence (out-of-plane doping)
vs. in-plane doping
Example
(CuCl)LaNb2O7
La
NbO6
CuCl
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Field configurations
Example

• 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
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Field configurations

• wTF-?SR
• Calibration of baseline (a), total asymmetry (A0)
• ?? sees
• (mostly) applied field (paramagnetic state),
• appl. internal fields (ordered state)

H ?? initial muon spin
Determine ordered, PM fractions
Example
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Field configurations

• (strong) TF-?SR
• Order induced by applied field
• Metamagnetism, etc.
• Vortex lattice in Type-II SC
• Rlx ? vlt?B2gt ? 1/?2 ? ns /m
• ? penetration depth
• ns /m superfluid density
• Polyxtal samples distribution broadened
  Gaussian
• gt Gaussian rlx
• gt 1/?2
• gt sf. density

H ?? initial muon spin
J. E. Sonier, 1998 2007
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Srn1RunO3n1
RuO66

• Ruddlesden-Popper series
• n? SrRuO3 (113)
• perovskite structure
• Ferromagnetic, Tc ? 165K
• n3 Sr4Ru3O10 (4-3-10)
• multi-layered structure
• FM, Tc ? 105K
• n2 Sr3Ru2O7 (327)
• quantum metamagnetism
• FM, AF fluctuations
• mag. ordering w/ Mn
• n1 Sr2RuO4 (214)
• Unconventional SC Tc ? 1.5K
• Spin-triplet pairing, p-wave
• isostructural to LBCO, LSCO

Sr
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(Sr,Ca)RuO3 113
Past Work

• n?
• 3-D structure
• Ca/Sr substitution
• SrxCa1-xRuO3
• isoelectronic doping
• FM suppressed x ? 0.25
• Phase separation, QPT

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Sr2RuO4 214
Maeno et al. 1994
MacKenzie Maeno, 2003
Fermi surface

• n1
• 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

Luke et al. 1996
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Ca2RuO4

• AF insulator, moment ?1.3?B
• 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 gt Sr2RuO4
• RuO6 flattening, tilting

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Ca2-xSrxRuO4
Susceptibility _at_ 2K
M-I transition near x0.2 (I-II) Near x0.5
(II-III) Sharp increase in susceptibility
Correlations more FM-ish Low susc _at_ higher
x Old Picture Ordering at low x
only Antiferro. near x0 Susc. peak near
x0.5 Paramagnetic at higher x SC at
x2 ?SR Rapid relaxation observed 0.2 x
1.6 Peaks near x ? 0.5, 1.5 Ordered ground state
throughout!
Nakatsuji Maeno, 2000.
Nakatsuji Maeno, 2003.
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Sr2Ru1-yTiyO4

• y0 SC Sr2RuO4
• lt0.2 Ti doping suppresses Tc
• gt2.5 doping induces magnetic ground state
• neutrons Braden et al. (2002)
• Incommensurate AF in y0.09
• q ? (0.3, 0.3, qz)
• ?SR rapid relaxation with increasing y.

from MacKenzie et al. 2003
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Experiments

• Samples
• (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-mSR M20 (LAMPF) and/or M15 (DR)
• DC Susceptibility ZFC, FC, H 50-100 G

Dilution fridge 15mK lt T lt 10K
He gas-flow cryo 1.7K lt T lt 300K
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Ca2RuO4 ZF-?SR
Ca2RuO4

• ?SR spectra
• Sum of 2 frequencies

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ZF-?SR Temperature Scans (Ca,Sr) system
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ZF-?SR Temperature Scans (Ru,Ti) system
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Edwards-Anderson order parameter
Uemura spin glass function (Uemura, 1985)
dynamic
static
?d
as root-exponential
Lorentzian Kubo-Toyabe

Field width
as a vQ
ld 4a2(1-Q)/n
Fluctuation rate
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• ZF Relaxation vs. Temp Magnetic ordering!

Define Rlx sqrt ( ?d2 as2 )
Fit to Rlx(T) R 1 (T/To)g
zoom
all
Ti only
Ca only
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• LF _at_ base temp decoupling ? static order

Fit to tanh(H/Ho)
Static ordering at base temp!
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• LF temp scans map out dynamics

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• Comparison of ZF LF field estimates

tanh(H/Ho)
R 1 (T/To)g
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Adapted from Braden et al. (2002) Neutrons
Braden Muons present study
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DC Susceptibility
Curie-Weiss
more AF
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Old view
New View
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Summary (Sr,Ca)2(Ru,Ti)O4

Past Sr2RuO4 p-wave SC Tc ? 1.5K, TRSB
magnetic fluctuations Sr2Ru1-yTiyO4 y ? 0.002
suppresses SC neutrons incommensurate AF y
0.09 Ca2RuO4 AF insulator TN ?
100-150K Sr2-xCaxRuO4 M-I transition x ?
0.2 susceptibility 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