1. University of PAVIA, Italy
Tuning the quantum critical point by Mn impurities and chemical pressure in LaFeAsO0.89F0.11
Samuele Sanna
Physics Department
University of PAVIA, Italy
Sanna, SIF2015, 24th September 2015

2. Coworkers R. Kappenberger P. Carretta Univ. Pavia R. De Renzi
P. Carretta, M. Moroni
University of PAVIA, Italy
R. De Renzi, P.Bonfà, S. Bordignon, G. Allodi
Univ. of PARMA, Italy
T. Shiroka,
ETH, Zürich, Switzerland
Y. Kobayashi, M. Sato,
Nagoya University, Japan
F. Hammerath, B. Büchner, G. Prando,
R. Kappenberger, S. J. Singh, S. Wurmehl
IFW Dresden, Germany
G. Lamura, M. Putti
CNR-SPIN and Univ. of GENOVA, Italy
R. Kappenberger
IFW Dresden
P. Carretta
Univ. Pavia
R. De Renzi
Univ. Parma
F. Hammerath
IFW Dresden
M. Moroni
Univ. Pavia
Sanna, SIF2015, 24th September 2015

3. Tuning magnetism and superconductivity in 1111
REFeAsO (RE1111) RE
Charge doping, e.g. F  O
Hydrostatic pressure
Chemical pressure
Substitution Mn  Fe
Sanna, SIF2015, 24th September 2015

4. REFeAsO: F  O  e- charge doping
RE = La
RE = Sm
Luetkens et al, Nature Materials (2009)
Sanna et al, PRB 80, (2009)
LaFeAsO1-xFx
SmFeAsO1-xFx
SDW
Sanna, SIF2015, 24th September 2015

5. Fe/Mn substitution in optimally F-doped RE1111
SmFeAsO1-zFz
Hammerat PRB 92, (R) (2015)
Sanna PRB (2009) Sm
La0.8Y0.2 La
Mn is a poison for SC in RE=La !
Mn content
Less than 0.2%of Mn is enough to kill SC
Process heavily affected by chemical pressure
Sanna, SIF2015, 24th September 2015

6. Fe/Mn substitution in optimally F-doped RE1111
LaFe1-xMnxAsO0.89F0.11
x=  ≈ isoelectronic conditions Mn
Magnetic SC
Sato et al., JPSJ 79, (2010)
Resistivity is rapidly increased 
Charge localization by Mn
Hammerath et al. PRB 89, (2014)
Mn quickly restores static magnetism
Sanna, SIF2015, 24th September 2015

7. Magnetic correlation length 2D AFM spin fluctuations @ QCP
Evidence for a QCP
LaFe1-xMnxAsO0.89F0.11
x=  ≈ isoelectronic conditions
Magnetic correlation length
Magnetic SC
Hammerath et al. PRB 89, (2014)
2D AFM spin QCP
LaFeAsO0.89F0.11
very close to a QCP!
Sanna, SIF2015, 24th September 2015

8. Chemical pressure and Mn impurity effects
unpublished
Mn drives the system through a QCP
Chemical pressure moves the QCP away
Magnetic correlations depressed
by a smaller ion
Why ?
Q=(p/a, 0)
For stripe collinear order magnetism enhanced for smaller RE
Giovannetti Nat. Commun. 2, 398 (2011)
NOT HERE
Orthomagnetic order induced by Mn ?
Q=(p/a, p/a)
Gastiasoro & Andersen, PRL 113, (2014)
Nanoscopic coexistence
Sanna, SIF2015, 24th September 2015

9. Chemical pressure and Mn impurity effects
unpublished
Mn drives the system through a QCP
Chemical pressure moves the QCP away
Superconductivity suppressed by Mn
Why ?
1) Mn suppresses (p/a, 0) fluctuations which sustain superconductivity
2) Mn induces charge localization
3) Mn pins the stripes
Hammerat PRB 92, (R) (2015)
Nanoscopic coexistence
Sanna, SIF2015, 24th September 2015

10. MANY THANKS FOR YOUR ATTENTION !!
Main conclusions (and questions)
REFe1-xMnxAsO0.89F0.11
optimally F-doped
Mn is a poison for SC in La1111 (xc=0.002 !)
possible orthomagnetic order?
Chemical pressure moves away the QCP
why ??
MANY THANKS FOR YOUR ATTENTION !!
PRB 89, (2014)
PRB 92, (R) (2015)
Sanna, SIF2015, 24th September 2015