Electron-doped%20Cuprates

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Electron-doped
Cuprates
R. L. Greene
University of Maryland Center for
Superconductivity Research
Collaborators
Yoram Dagan---UMd
Amlan Biswas-------UMd/UFl Hamza
Balci---------UMd Mumtaz Qazilbash--UMd
Patrick Fournier----Sherbrooke Girsh
Blumberg----Lucent, Bell Labs
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OUTLINE

• Background on Cuprates
• phase diagram
• Normal State Properties
• evidence for a QCP under the SC dome
• pseudogap issues
• nature of the ground state
• SC state
• pairing symmetry as a function of doping

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Phase diagram of electron- and hole-doped cuprates
(La,Pr,Nd)2-xCexCuO4-y
La2-xSrxCuO4-y
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Theoretical Phase Diagrams

• Phase Ordering

• Quantum Critical

TMF
T? max
Quantum critical region
T
T
Pseudogap
Ordered (pseudogap)
Disordered (Fermi liquid)
Tc
x
QCP
x
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Transport evidence for a Quantum Phase Transition
and QCP
For details see Dagan et al., Cond-mat/0310475
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Résistivité
? T2
? T
YBCO J.M. Harris et al., Phys.Rev. B 46, 14293
(92).
LSCO B. Batlogg et al., J. Low Temp. Phys. 95,
23 (94).
PCCO nos travaux sur couches minces...
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Resistivity vs doping
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? ?0 AT?
x0.17
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? ?0 AT?
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ab-plane resistivity for Pr2-xCexCuO4 films with
HgtHc2
Fournier et al., PRL 81,4720 (98)
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? T2
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Hall vs H
X0.17
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Tmin(K)
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ARPES Nd2-xCexCuO4
N.P. Armitage et al. PRL 88 257001 (2002).
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• Linear in T resistivity is expected above a 2D
  AFM to paramagnetic
• metal QCP. Other powers of T would be expected
  at nearby
• dopings because of fitting the data over a low
  temperature
• Fermi Liquid T2 regime and the quantum critical
  regime at higher
• temperature. This is exactly the behavior we
  find!
• The lowest temperature Hall Coefficient shows a
  kink at the same
• doping (within error) as the linear in T
  resistivity. This is strong
• confirming evidence of the QCP.
• ARPES shows a drastic change in the Fermi surface
  near the
• same doping.However, the ARPES doping
  resolution is not as
• good as our transport data.
• The doping dependence of the low energy pseudogap
  ( to be shown in
• a few slides) is consistent with the QCP
  scenerio from transport.

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Pseudogap
High Energy (100mev)----- optics, Raman,
ARPES Low Energy (5meV) --------tunneling,
Raman
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Conductivity
Y. Onose et al. PRL 87 217001 (2001).
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Break junction between PCCO (x0.15) and Ag
H0, Superconducting state
H9 T ( c-axis), Normal state
2?SC
Normal state gap (NSG)
SC gap clearly seen

• Not residual SC at interface
• Not effect of tunnel barrier

Biswas et al., PRB 64, 104519 (2001)
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Grain Boundary Tunneling
Alff et al., Nature 422, 698 (2003)
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Nature of the Ground State
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Magnetism
Kang et al., Nature 423, 522 (2003)
Antiferromagnetic Order as the Competing Ground
State in electron-doped Nd1.85Ce0.15CuO4
They find TN37K for HgtHc2
AF order different than for undoped parent NCO
Sonier et al., PRL 91, 147002 (2003)
Antiferromagnetic order in the vortex cores
of Pr1.85Ce0.15CuO4
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Violation of Wiedemann-Franz law
R.W. Hill et al. NATURE 414 711 (2001).
Pr1.85Ce0.15CuO4 Crystal
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NMR in PLCCO
Zheng et al., PRL 90, 197005 (2003)
Fermi Liquid GS
No pseudogap
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Pairing Symmetry of Superconducting State
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For n-doped cuprates
Disagreements even for optimally doped compounds

• Penetration depth
• Kokales et al., PRL 85, 3696 (2000), Prozorov et
  al., PRL 85, 3700 (2000)
• ARPES
• Armitage et al., PRL 86, 1126 (2001) Sato et
  al., Science 291, 1517 (2001)
• Tricrystal experiment
• Tsuei et al., PRL 85, 182 (2000)
• Raman scattering
• Blumberg et al., PRL 88, 107002 (2002)
• Specific heat
• Balci et al., PRB 66, 174510 (2002)

d-wave

• Penetration depth
• Alff et al., PRL 83, 2644 (1999) Kim et al.,
  PRL 91, 087001 (2003)
• Skinta et al., Phys. Rev. Lett. 88, 207003 (2002)
  
• Tunneling spectroscopy Kashiwaya et al., PRB
  57, 8680 (1998) Alff et al., PRB
  58, 11197 (1998).

s-wave
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To distinguish between d-wave and s-wave by
tunneling spectroscopy
s-wave
d-wave (110) direction
Blonder, Tinkham and Klapwijk (BTK) Phys. Rev. B
25, 4515 (1982)
Tanaka and Kashiwaya Phys. Rev. Lett. 74, 3451
(1995)
Z 0 ? barrierless contact between N and S Z ? 1
? tunneling limit
N
S
Barrier strength Z
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Evidence for a transition from d-wave to s-wave
pairing symmetry in Pr2xCexCuO4
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How to differentiate s- wave and d-wave

• Use the different field dependence
• of Cel in s-wave and d-wave.
• In the temp and field range of our experiment, if
• s-wave Þ CelµH
• d-wave Þ
• CelµH1/2 in the clean limit
• G. E. Volovik, Pisma Zh. ksp. Teor. Fiz. 58,
  457 (1993) JETP Lett. 58, 469 (1993).
• CelµH log(H) in the dirty limit
• C. Kubert and P. J. Hirschfeld, Solid State
  Commun. 105, 459 (1998)

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Temperature dependence
x 0.15

• Global fitting of the form C/T??T2 gives
• ?N 6.7 0.5 mJ/mole K2 (intercept of 10 T
  data)
• ?(0) 1.4 0.2 mJ/mole K2 (intercept of 0 T
  data)

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Comparison with different theoretical predictions
Balci et al. PRB 66, 174510 (02)
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s-wave theory C(H) ? ?nTH/Hc2? slope
?2.52/41.3 mJ/mole KT
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2D excitation in optimally doped NCCO crystal
Tc22 K
G. Blumberg et al, PRL 88, 107002 (2002)
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Raman spectra of overdoped NCCO crystal (Tc 14K)
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Isotropic gap in overdoped NCCO crystal
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d- to s-wave transition l-2(T)
J.A. Skinta et al. PRL 88 207003 (2002) PRL 88
207005 (2002).
l-2(T) T2 -gt Exp(-D/T)
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ARPES Nd2-xCexCuO4
N.P. Armitage et al. PRL 88 257001 (2002).
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Theoretical explanation of a symmetry change in
n-doped cuprates
-

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The antiferromagnetic spin fluctuations (ASF)
peaked at the wave vector Q (?,?) are
responsible for d-wave superconductivity The
interaction via ASF has the highest strength at
the so-called hot spots, the points on the Fermi
surface connected to each other by the vector Q.
the interaction via ASF is repulsive in the
singlet channel
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d-wave symmetry for hole doped
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electron-doped case at low doping
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electron-doped in the high doping regime
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Doping dependence
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SUMMARY

• QCP scenario seems to be valid in n-type
• Kink in RH and ? T1 at xc 0.165
• d- to s-wave symmetry change near xc
• Pseudogaps in n- and p-type are different
• Significance of subtle differences in n- and
  p-type properties not yet clear