Challenging experiments for high-Tc superconductors
Kazuyoshi Yamada
WPI, Tohoku University

Final goal of the high-Tc research is to make clear answers tothe following questions
What is the mechanism ?
How to lift up Tc ?
My task here is to propose challenging experiments towards the final goal
“Fish story（まゆつば話）” will be acceptable and better than no proposalThree fundamental issues related with the mechanism
1) How do Mott insulators become
metals by dilute carrier doping ?

doping-induced metal-insulatortransition2) How do the doped metals becomesuperconductors in undedoped
region?

pseudogap state３) How do the superconductors
degrade by overdoping?

doping dependence of pairing
interaction

Search and study metallic
antiferromagnets in dilute doped
cupratesCompare with insulating
antiferromagnets induced bycarrier localization
polarization analysis of spin fluctuaion

Search for dynamical vortexesin pseudogap statepolarized neutron small angle scattering

Determine orbital character of
doped carriers in overdoped
regionWhat happens in the pseudogap (PG) state ?
(I) magnetic order below T*
induced by spontaneous circular current?

Yasuoka et al., (1989)

pseudogapstateSC
B. Fauqué et al., PRL96 (2006)

(II) Superconducting paring fluctuation below T* ?

diamagnetism above Tc

Nernst and diamagnetic signal above Tc
Lu Li et al., cond-mat 0611731

Hzy
In vortex liquid phase, vortex flow(velocity v ) occurs under magnetic
fields towards cooler region:v -->Ey-ΔTxInduced electric field E=Bxv
Nernst signal

eN=Ey/|ΔT|
Tc=86.5K

How to obtain direct evidence of vortex-like signal and its spatialcorrelation above Tc ?
Proposed experiment

Polarized small angle neutron scattering

T onset : onset temperature of Nernst signal?

?

Small angle neutron scattering?

vortex lattice(crystal)

vortex powder(powder like)vortex liquid(liquid like)

Lu Li et al., cond-mat 0611731

dynamical vortex?

A fish story ?

How to merge two phenomena ?

Proposed experiment
Study magnetic field dependence of the magnetic order-----> Collapse into the vortex-like state ????
(I) Magnetic order ?

(II) Superconducting fluctuation ?

?
Magnetic field

1) How do Mott insulators become metals by dilute carriers
doping ?

doping-induced metal-insulator transition

Origin of material dependence of phase diagram(a professional version of fish story)

Material dependence of phase
diagram
How to obtain unified picture ?
LSCOYBCO

spin glass

Sanna et al., PRL 93(‘04)

Mukuda et al. PRL 96(2006) and unpublished data

Sushkov cond-mat 2008

Nernst signal is also observed below Xc (lower criticalconcentration of SC)

3D-AFM

Metallic properties already appear below Xc in LSCO?

Spin glassXc~0.055
Lu Li et al., cond-mat 0611731

Metallic transport already appears by dilute doping at high temperature
Carrier localization and hopping transport occur at low temperature

Variable range hopping conduction

range of carrier hopping
range of carrier localization

Incommensurate spin density modulation
precipitates in AFM order by dilute holedoping
Magnetic Bragg peakSpin glass

3D-AFM

M.Matsuda et al. PRB65(‘02)

Proposed experiment
Precise measurement of spin excitation as a function of temperature
Carrier localization effect at low temperature andmetallic character at high temperature ?

What happened by stronger carrier localization ?

Carrier localization is enhanced by impurity
La2-xSrxCu1-zZnzO4Sr(x)=0.017

Hücker et al.
PRB59,(1999)

3D-AFM order

Strong carrier localization stabilizes 3D-AFM

spin-glass

TN / TN max

1

LCO (Zn)
LSCO(Ni)0.8
0.6

dilution

0.4
0.2

Sr-doping
SC00

0.05

Li-substitution
Sr+Zn

0.15

0.2

0.25

doping/substitution
Sr+Ni

Hiraka et al., JPSJ(2007)

0.1

0.3

LSCO

spin glass

AFM region expands bycarrier localizationYBCO

AFM region expands bycarrier delocalization

How to obtain unified picture of phase diagram ?
TN / TN max

1

LCO (Zn)
LSCO(Ni)0.8
0.6

dilution

0.4
0.2

Sr-doping00

Li-substitution
Sr+Zn

0.05

0.1

0.15

0.2

0.25

doping/substitution
Sr+Ni

0.3

TN=300~400K
XSC=0.05~0.3

YBCO

LSCO

temperature

AFMM?
SCAFMM?
SC
commensurate AFMI

SC
~0.1

~0.3

~0.1 (Ni-substitution)~0.3

carrier
localization

~0.17

~0.05

SG~0.05site-dilution

carrierdelocalization

Multilayer

carrier doping

AFM Insulator <----- Spin-glass phase -----> AFM Metalstrong

carrier localization

weakNSF(T-mode)

Proposed experimentSF(L-mode)

Pz

Detect metallic nature in spin fluctuations

Search for amplitude fluctuation of spins

MQ

L-modeCr (Spin density wave)
T-modeS. Wakimoto et al., unpublisheddata

Y. Endoh et al., JPSJ(‘97)

T. Fukuda et al., JPSJ(‘96)

Summary(1) Pseudogap state
Direct evidence of superconducting fluctuation ?
Relation with magnetic order ?
---> polarized neutron small angle scattering
magnetic order under magnetic field(2) Metallic transition by dilute doping
In LSCO metallic signature is seen even at dilute doping---> precise temperature dependence of spin excitation in doped AFM
AFMI <----> spin-glass phase <---> AFMM
Clarify the relation between AFMM and spin-glass phases?
---> polarization analysis of spin fluctuation