The Magnetic Phase Diagram of (Sr,Ca)2(Ru,Ti)O4 Revealed by SR

Jeremy P. Carlojeremy.carlo@nrc.gc.ca

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