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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
Coworkers
P. Carretta, M. Moroni
University of PAVIA, Italy
R. De Renzi, P.Bonfà, S. Bordignon, G. Allodi
Univ. of PARMA, Italy
F. Hammerath, B. Büchner, G. Prando,
R. Kappenberger, S. J. Singh, S. Wurmehl
IFW Dresden, Germany
M. Moroni
Univ. Pavia
F. Hammerath R. Kappenberger
IFW Dresden
IFW Dresden
Sanna, SIF2015, 24th September 2015
T. Shiroka,
ETH, Zürich, Switzerland
Y. Kobayashi, M. Sato,
Nagoya University, Japan
G. Lamura, M. Putti
CNR-SPIN and Univ. of GENOVA, Italy
P. Carretta
Univ. Pavia
R. De Renzi
Univ. Parma
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
FO
REFeAsO:

e- charge doping
RE = La
Luetkens et al, Nature Materials (2009)
RE = Sm
Sanna et al, PRB 80, 052503 (2009)
LaFeAsO1-xFx
SmFeAsO1-xFx
SDW
Sanna, SIF2015, 24th September 2015
Fe/Mn substitution in optimally F-doped RE1111
SmFeAsO1-zFz
Hammerat PRB 92, 020505(R) (2015)
Sanna PRB (2009)
Sm
La0.8Y0.2
La
Mn is a poison for SC in RE=La !
Less than 0.2%of Mn is enough to kill SC
Process heavily affected by chemical pressure
Sanna, SIF2015, 24th September 2015
Fe/Mn substitution in optimally F-doped RE1111
LaFe1-xMnxAsO0.89F0.11
x=0-0.002  ≈ isoelectronic conditions
Mn
Magnetic
SC
Sato et al., JPSJ 79, 014710 (2010)
Resistivity is rapidly increased

Charge localization by Mn
Hammerath et al. PRB 89, 134503 (2014)
Mn quickly restores
static magnetism
Sanna, SIF2015, 24th September 2015
Evidence for a QCP
LaFe1-xMnxAsO0.89F0.11
Magnetic correlation length
x=0-0.002  ≈ isoelectronic conditions
Magnetic
SC
Hammerath et al. PRB 89, 134503 (2014)
2D AFM spin fluctuations @ QCP
LaFeAsO0.89F0.11
very close to a QCP!
Sanna, SIF2015, 24th September 2015
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unpublished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 ?
For stripe collinear order
magnetism enhanced for
smaller RE
Q=(p/a, 0)
Giovannetti Nat. Commun. 2, 398 (2011)
Orthomagnetic order
induced by Mn ?
Gastiasoro & Andersen,
PRL 113, 067002 (2014)
Sanna, SIF2015, 24th September 2015
Nanoscopic
coexistence
Q=(p/a, p/a)
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, 020505(R) (2015)
Sanna, SIF2015, 24th September 2015
Nanoscopic
coexistence
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 ??
PRB 89, 134503 (2014)
PRB 92, 020505(R) (2015)
Sanna, SIF2015, 24th September 2015