Coherent Kondo State in a Dense Kondo Substance :

Coherent Kondo State in a Dense
Kondo Substance : CexLa1-xCu6
A.Sumiyama et al.,
J.Phys.Soc.Jpn.55(1986)
Shimizu-group
Katsuya TOKUOKA
Contents



Introduction
■ Dilute Kondo effect
■ Coherent Kondo effect
Experimental data about CexLa1-xCu6
Summary
The simple metallic resistivity
potassium
102 R / R290K
The resistivity decreases
monotonically as the
temperature decreases.
Impurity scattering
Residual
resistivity
（残留抵抗）
T (K)
C.Kittel Introduction to Solid State Physics
Kondo effect
Discovery of the resistivity
minimum in dilute magnetic
alloys(希薄磁性合金).(1930’s)
The electrical resistivity increases
steeply as the temperature decreases
In the low temperature region.
de Haas et al., Physica 1 (1934) 1115
Jun Kondo has derived the logarithmic increase
of the resistivity in the low temperature region. (1964)
http://www.aist.go.jp/aist_j/information/emeritus_advisor/index.html
th
→ 40 anniversary in 2004 !
Dilute Kondo effect(希薄系近藤効果)
T > TK
T < TK
Spin singlet (スピン一重項)
S=0
de Haas et al., Physica 1 (1934) 1115
But this Kondo effect does not appear if the metal contains a large
amount of magnetic impurities.
TK : 近藤温度
A new type Kondo effect
In Ce compounds, although it contains a large
amount of magnetic impurities (>10%), Kondo
effect appeared.
Temperature dependence of resistivity in CeCu6
Y.Ōnuki et al., J.Phys.Soc.Jpn. 53 (1984) 1210-1213
f - electron
La3+ Ce3+ Pr3+ …
0
1
2
Closed shell 5s25p6
|r R(r)|2
Lanthanoides
4f-electrons
r (a.u.)
http://www.phy.saitama-u.ac.jp/~saso/saso.html
f - electron is localized.
Coherent (Dense) Kondo effect
(高濃度近藤効果)
In Ce compounds, the Kondo effect appears.
T > TK
T < TK
f-localized (局在)
Dilute
f-itinerant (遍歴 )
Y.Ōnuki et al., J.Phys.Soc.Jpn. 53 (1984) 1210-1213
Heavy fermion state
(重い電子状態)
Coherent Kondo substance
CeCu6
• Coherent Kondo substance
• Pauli paramagnet (パウリ常磁性)
(no magnetic ordering)
• Large specific heat coefficient
(電子比熱係数)
γ= 1.6 J / mole·K2
The effective mass (有効質量)
is 1000 times or more as large
as the free electron mass !
Crystal structure of CeCu6
Y.Onuki et al., J.Phys.Soc.Jpn. 54 (1985)
1964-1974
Motivation
LaCu6 is a normal metal which has the same structure.
(no f – electrons)
To clarify the process from the incoherent Kondo
state to the coherent Kondo state, we can
investigate the compound : CexLa1-xCu6 (x=0-1).
Dilute (incoherent)
Ce density
x:0
Dense(Coherent)
1
Experiment
Single crystals of CexLa1-xCu6 were grown by the
Czochralski pulling method.
a large single crystal !
Measurements
・electrical resistivity
from 300 K~ 18 mK
・magnetoresistance (磁気抵抗効果)
Temperature dependence of electrical
resistivity
Electrical resistivity increases
as temperature increases in the
low temperature region.
A maximum appeared around
5-15 K for an x value of 0.731.0.
Temperature dependence of
electrical resistivity in CexLa1-xCu6
f-electrons are itinerant in
the low temperature region.
Magnetic resistivity
The normal metal
ρ( T ) = ρ0 + ρe-e(T)
(Matthiessen’s rule)
The magnetic substance
ρ( T ) = ρ0 + ρe-e(T) + ρm(T)
ρm= ρCexLa1-xCu6 - ρLaCu6
(magnetic resistivity)
Magnetic resistivity
Dilute
x = 0.094 – 0.50
Coherent
x = 0.73 – 1.0
Consecutive change
from dilute to coherent
Temperature dependence of magnetic
resistivity in CexLa1-xCu6
Magnetoresistance
A effective method to investigate
heavy fermion system
In normal metal
Cyclotron motion
→ The resistivity increases.
Ｉ
H
c
a
b
Heavy fermion system
magnetization
→ The resistivity decreases due to magnetic ordering.
Magnetic field dependence of
resistivity
Magnetoresistance at 1.5 K
Dilute Kondo system
ρhas a Maximum around 20 kOe.
Because of the suppression of the
coherent Kondo effect, the resistivity
increases.
Itinerant
localized
Magnetic field dependence of
resistivity in CeCu6
The coherent Kondo effect is
suppressed by applied magnetic
field.
Summary

Dilute Kondo effect appears for x = 0.096 – 0.50.
Coherent Kondo effect appears for x = 0.73 – 1.0.

Consecutive change from dilute Kondo state to
coherent Kondo state occurs.

The coherent Kondo effect is suppressed by
applying a magnetic field.

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