Transcript Document 1149720

Breakdown of the Kondo effect at an
antiferromagnetic instability
F. Steglich
MPI for Chemical Physics of Solids, 01187 Dresden, Germany
Outline
• HF quantum critical points (QCPs)
• Kondo breakdown QCP in YbRh2Si2
• Superconductivity in YbRh2Si2?
Collaboration
MPI CPfS:
M. Brando, S. Ernst, C. Geibel,
S. Hartmann, S. Kirchner, C. Krellner,
S. Lausberg, H. Pfau, L. Steinke,
O. Stockert, S. Wirth
TU Braunschweig:
G. Zwicknagl
U. Goettingen:
P. Gegenwart,
TU Vienna:
J. Custers, S. Paschen
U. Cambridge:
S. Friedemann, F.M. Grosche
Rice U.:
Q. Si
Rutgers U.:
P. Coleman
Zhejiang U.:
H.Q. Yuan
Two types of QCPs
[P. Gegenwart, Q. Si & F.S., Nature Phys. 4, 186 (2008)]
SDW QCP
(Hertz, Millis, Moriya…)
exemplary material:
Kondo breakdown at AF QCP
(Si et al., Coleman et al., 2001)
CeCu2Si2
YbRh2Si2
TK ≈ 15 K
TK ≈ 30 K
cf. O. Stockert‘s talk
Emerging local Kondo screening and spatial coherence in
the HF metal YbRh2Si2
[S. Ernst et al., Nature 474, 362 (2011), cf. S. Wirth‘s talk]
Hierarchy of energy scales from STS
J = 7/2
single-ion TK
CEF splitting
TK,high  100 K
17, 25 & 43 meV
TK,low  30 K
[INS, O. Stockert
et al., Physica B
378, 157 (’06)]
[TEP, U. Koehler et al.,
Phys. Rev. B 77,
104412 (’08)]
Kondo-lattice temperature TL  TK,low
YbRh2Si2: T – B phase diagram
[J. Custers et al., Nature 424, 524 (2003);
T. Westerkamp, Dissertation, TU Dresden (2008)]
Seff = 1/2
Crossed-field Hall-effect results
[S. Friedemann et al., PNAS 107, 14547 (2010)]
RH(B2) = lim ρH(B1, B2)/B1
B1 → 0
solid lines:
RH ( B2 ) 

RH


RH
 mB2  RH0
1  ( B2 / B0 )
p
 mB2
Fermi surface collapse
[S. Friedemann et al., PNAS 107, 14547 (2010)]
Crossover position T*(B)
Crossover width
0.3
0.3
FWHM(T)
YbRh2Si2
FWHM (T)
0.2
0.2
T (K)
TLFL
0.1
TN
YbRh2Si2
crossfield
singlefield
RH(B2)
~ (B )
R
H
1
0.1
0.2
B2 (T)
0.3
singlefield
RH(B2)
RH(B1)
~
magnetoresistivity
(B2)
sample 1
sample 2
0.1
magnetoresistivity
(B2)
0.0
0.0
sample 1
sample 2
0.0
0.0
crossfield
0.4
FWHM ~ T
0.1
T (K)
0.2
0.3
ω/T scaling (Q. Si, S. Kirchner)
1.0
(P. Gegenwart et al., Science 315, 969 (2007))

T*(B) agrees with data from ρ, λ, M
0

(RH-RH )/(RH -RH )
T0
0.5
T
0.0
0.0
0.5
1.0
B/ B0
1.5
2.0
2.5
Specific heat of YbRh2Si2
under hydrostatic pressure p ≤ 1.4 GPa
[R. Borth, M. Nicklas, unpublished]
TN anomaly in C(T)/T in Yb(Rh1-xCox)2Si2
[C. Krellner et al., Phys. Rev. Lett. 102, 196402 (2009);
Phys. Stat. Solidi B 247, 734 (2010)]
C±(t) = A± α-1│t│- α + b + E · t;
t = (T - TN)/TN;
+: t > 0;
-: t < 0
x = 0.38
α = 0.38
α = -0.12
δTN/TN = 5 ·10-3
Specific heat (B = 0)
[J. Custers et al., Nature 424, 524 (2003)]
ΔC = C – Cph – CQ
= γT+βT3
γ ≈ 1.7 J/K2mole
Magnon contribution Cm (~κm) = β T3