1. R. L. Greene Electron-doped Cuprates 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

2. 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

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

4. Theoretical Phase Diagrams
Phase Ordering
Quantum Critical
TMF
Tθ max
Quantum critical region T T
Pseudogap
Ordered
(pseudogap)
Disordered
(Fermi liquid) Tc x
QCP x

5. Transport evidence for a Quantum Phase Transition and QCP
For details see Dagan et al., Cond-mat/

6. Résistivité :
 ~ T2
 ~ T
YBCO : J.M. Harris et al., Phys.Rev. B 46, (92).
LSCO : B. Batlogg et al., J. Low Temp. Phys. 95, 23 (94).
PCCO : nos travaux sur couches minces...

7. Resistivity vs doping

8.  = 0 +AT
x=0.17

9.  = 0 + AT

10. ab-plane resistivity for Pr2-xCexCuO4 films with H>Hc2
Fournier et al., PRL 81,4720 (98)

11.  ~ T2

12. Hall vs H
X=0.17

17. Tmin(K)

18. ARPES Nd2-xCexCuO4
N.P. Armitage et al.
PRL (2002).

19. 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.

20. Pseudogap High Energy (~100mev)----- optics, Raman, ARPES
Low Energy (~5meV) tunneling, Raman

21. Conductivity
Y. Onose et al.
PRL (2001).

22. “Break junction” between PCCO (x=0.15) and Ag
H=0, Superconducting state
H=9 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, (2001)

23. Grain Boundary Tunneling
Alff et al., Nature 422, 698 (2003)

24. Nature of the Ground State

25. Magnetism
Kang et al., Nature 423, 522 (2003)
Antiferromagnetic Order as the Competing Ground State
in electron-doped Nd1.85Ce0.15CuO4
They find TN=37K for H>Hc2
AF order different than for undoped parent NCO
Sonier et al., PRL 91, (2003)
Antiferromagnetic order in the vortex cores of
Pr1.85Ce0.15CuO4

26. Violation of Wiedemann-Franz law
R.W. Hill et al.
NATURE (2001).
Pr1.85Ce0.15CuO4
Crystal

27. NMR in PLCCO Fermi Liquid GS No pseudogap
Zheng et al., PRL 90, (2003)
Fermi Liquid GS
No pseudogap

28. Pairing Symmetry of Superconducting State

29. Disagreements even for optimally doped compounds
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, (2002)
Specific heat
Balci et al., PRB 66, (2002)
d-wave
Penetration depth
Alff et al., PRL 83, 2644 (1999); Kim et al., PRL 91, (2003);
Skinta et al., Phys. Rev. Lett. 88, (2002)
Tunneling spectroscopy Kashiwaya et al., PRB 57, 8680 (1998) ; Alff et al., PRB 58, (1998).
s-wave

30. To distinguish between d-wave and s-wave by tunneling spectroscopy
d-wave (110) direction
Blonder, Tinkham and Klapwijk (BTK)
Phys. Rev. B 25, 4515 (1982)
Tanaka and Kashiwaya
Phys. Rev. Lett. 74, 3451 (1995) N S
Z = 0  barrierless contact between N and S
Z  1  tunneling limit
Barrier strength Z

31. Evidence for a transition from d-wave to s-wave
pairing symmetry in Pr2–xCexCuO4
Tunneling conductance G=dI/dV vs. voltage V
A. Biswas, P. Fournier, M. M. Qazilbash, V. N. Smolyaninova, H. Balci,
and R. L. Greene, Phys. Rev. Lett. 88, (2002)
Underdoped sample with x=0.13
Zero-bias peak characteristic for d-wave
Overdoped sampe with x=0.17
Double peak characteristic for s-wave

32. 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, Pis’ma 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)

33. 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)

34. Comparison with different theoretical predictions
Balci et al. PRB 66, (02)

36. s-wave theory:
C(H)  nTH/Hc2 slope 2.5*2/4=1.3 mJ/mole KT

37. 2D excitation in optimally doped NCCO crystal
Tc=22 K
G. Blumberg et al, PRL 88, (2002)

38. Raman spectra of overdoped NCCO crystal (Tc = 14K)

39. Isotropic gap in overdoped NCCO crystal

40. d- to s-wave transition: l-2(T)
J.A. Skinta et al.
PRL (2002);
PRL (2002).
l-2(T): T2 -> Exp(-D/T)

41. ARPES Nd2-xCexCuO4
N.P. Armitage et al.
PRL (2002).

42. Theoretical explanation of a symmetry change in n-doped cuprates+

43. 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:

44. - - + + + + - -
d-wave symmetry for hole doped

45. electron-doped case at low doping+ + + + - -

46. electron-doped in the high doping regime+ + + + - -

47. Doping dependence

48. 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