Phase Diagrams and Competing Orders in Iron-based

Phase Diagrams and Competing Orders in Ironbased Superconductors
Mini-Workshop, KIT Campus Nord, Karlsruhe
Tuesday, October 11th 2011
PROGRAM
10:00 – 10:30
µSR and Infrared Spectroscopy Studies of the Coexistence of
Magnetism and Superconductivity in Fe-arsenides and selenides
C. Bernhard, University of Fribourg
10:30 – 10:50
Optical Properties of Iron-based Superconductors
A. Charnukha, MPI-FKF Stuttgart
10:50 – 11:10
Proximity of Iron-Pnictide Superconductors to a Quantum Critical
Point
C. Ortix, IFW Dresden
11:10 – 11:30
Interplay between Superconductivity and Magnetism in KxFe2-ySe2
Y. Su, FZ Jülich
11:30 – 11:50
Itinerant Magnetic Excitations in Rb0.8Fe1.6Se2
G. Friemel, MPI-FKF Stuttgart
11:50 – 12:10
Competing Multi-Orbital Nematic and Superconductive Orders in
Bad-Metallic Iron Arsenides
M. S. Laad, RWTH Aachen
12:10 – 13:30
LUNCH
13:30 – 14:00
Anisotropic In-Plane Resistivity in the Nematic Phase of the Iron
Pnictides
J. Schmalian, KIT – TKM Karlsruhe
14:00 – 14:20
Interplay
of
Antiferromagnetism,
Superconductivity in EuFe2(As1-xPx)2
P. Gegenwart, University of Göttingen
14:20 – 14:40
Interplay of 4f Magnetism and Superconductivity in CeFe(As,P)O
M. Nicklas, MPI-CPS Dresden
14:40 – 15:00
Phase Diagram and Thermal-Expansion of BaFe2(As1-xPx)2
A. Böhmer, KIT-IFP Karlsruhe
Ferromagnetism
and
15:00 – 15:20
Arsenic Free Pnictide Superconductors containing Bi and Sb
synthesized by Molecular Beam Epitaxy
R. Retzlaff, TU Darmstadt
15:20 – 15:50
COFFEE BREAK
15:50 – 16:10
From
Density-Functional
Theory
Renormalization Group: Competing
Superconductors
C. Platt, University of Würzburg
16:10 – 16:30
Pressure Study of Superconductivity and Antiferromagnetism in
Rb0.8Fe1.6Se2
S. Medvedev, University of Mainz
16:30 – 16:50
Phase Separation in Superconducting and Antiferromagnetic
Rb0.8Fe1.6Se2 probed by Mössbauer Spectroscopy
V. Ksenofontov, University of Mainz
16:50 – 17:10
Probing Unconventional
Quasiparticle Interference
C. Hess, IFW Dresden
17:10 – 18:30
POSTER SESSION
to
the
Functional
Orders in Fe-based
Superconductivity
in
LiFeAs
by
POSTER CONTRIBUTIONS
Asynchronous Optical Sampling: from Electron-Phonon Coupling to Heat Diffusion in
Cuprates
I. Avigo, University of Duisburg-Essen
C-axis Transport of Pnictide Superconductors
C. Steiner, University of Erlangen-Nürnberg
Magnetic Properties of Novel Cd-doped Iron Oxo-Selenides
S. Landsgesell, Helmholtz-Zentrum Berlin
Tunnelling Spectroscopy (SI-STM) Study of Fe(Se,Te)
S. White, MPI-FKF Stuttgart
Gutzwiller Theory of Band Magnetism in LaOFeAs
T. Schickling, University of Marburg
Uniaxial versus Hydrostatic Pressure Effects on the 122 Iron-Pnictide Family
M. Tomic, University of Frankfurt
Synthesis, Growth and Characterization of Fe-based Superconductors
A. A. Haghighirad, University of Frankfurt
Multiband Effects and Interband Coupling in Fe-based Superconductors
L. Ortenzi, MPI-FKF Stuttgart
Exploring the Phase Diagram of EuFe2(As1-xPx)2
S. Zapf, University of Stuttgart
Spurious Superconductivity in BaFe2As2 and KFe2Se2-type Single Crystals
T. Wolf, KIT-IFP Karlsruhe
Doping and Impurities in 122 Iron-Pnictide Superconductors
F. Hardy, KIT-IFP Karlsruhe
Electronic Structure of Single-Crystalline Sr(Fe1-xCox)2As2 probed by X-ray Absorption
Spectroscopy
M. Merz, KIT-IFP Karlsruhe
Microwave Absorption Study of Pinning Effects in the CaFe2-xCoxAs2 and Ba(Fe1xCox)2As2 Single Crystals
N. Panarina, IFW Dresden
Electronic Transport Properties of LiFeAs in Comparison with Transition-Metal Doped
LiFeAs
D. Bombor, IFW Dresden
Low Temperature Specific-Heat and Thermal-Expansion Measurements of Co-doped
Ba122 Single Crystals in High Magnetic Fields
P. Burger, KIT-IFP Karlsruhe
High-Resolution Thermal Expansion of Ba(Fe1-xCox)2As2: Evidence for Quantum
Criticality
C. Meingast, KIT-IFP Karlsruhe
Small-q phonon-mediated unconventional superconductivity in the iron pnictides
P. Kotetes, KIT-TFP Karlsruhe
Contrasting P- and F- doping of the CeFeAsO System by Means of µSR and Mössbauer
Spectroscopy
P. Materne, TU Dresden
Superconductivity, Magnetic Order, and Smearing of the Magnetic Phase Transition in
the Co doped SrFe2-xCoxAs2 Iron-Pnictide System under High Pressure
H. Maeter, TU Dresden
Phase Diagrams and Competing Orders in
Iron-based Superconductors
Mini-Workshop, KIT Campus Nord, Karlsruhe
Tuesday, October 11th 2011
Abstracts
Microwave Absorption study of pinning effects in the
CaFe2-xCoxAs2 and Ba(Fe1-xCox)2As2 single crystals
N. R. Beysengulov1, N. Yu. Panarina1, Yu. I. Talanov1, T. S. Shaposhnikova1, E.
Vavilova1, L. Harnagea2, S. Singh2, S. Wurmehl2, C. Hess2, G. Friemel2, R.
Klingeler2, V. Kataev2, B. Büchner2
1
2
Zavoisky Physical-Technical Institute, Kazan 420029, Russian Federation
Leibniz Institute for Solid State and Materials Research IFW Dresden, D-01171 Dresden, Germany
Microwave absorption (MWA) study was performed on CaFe2-xCoxAs2 and Ba(Fe1-xCox)2As2 single
crystals grown by different methods. The position of irreversibility lines on the phase diagram of
magnetic fields and temperatures determined from MWA hysteresis loops, as well as estimates of the
critical current density strongly depend on Co doping level. The analysis of the obtained data suggests
that nonsuperconducting (magnetic) inclusions present at certain Co doping serve as additional pinning
centers, and in such a way contribute to pinning in the system. Comparison of the critical parameters of
different high-temperature superconductors allows to estimate their potential in practical applications.
Electronic transport properties of LiFeAs in comparison with
transition metal doped LiFeAs
Dirk Bombor, Anne Bachmann, Luminita Harnagea, Saicharan Aswartham,
Claudia Nacke, Sabine Wurmehl, Christian Hess, Bernd Buechner
IFW-Dresden, Institute for Solid State and Materials Research, D-01171 Dresden, Germany
Electronic transport properties of the unconventional 111-superconductors LiFeAs in comparison with
transition metal doped LiFeAs have been studied. Unlike in other iron arsenide superconductors the
stoichiometric LiFeAs doesn't show any nesting of the Fermi surface and therefore exhibits no spin
density wave but even the undoped compound becomes superconducting below 18K. We studied the
magnetoresistance and extracted the Hc2(T) phase diagram. In contrast to other iron arsenide
superconductiors we find that doping by substitution of iron with Co, Ni, Cr or Rh as well as Lideficiency suppresses superconductivity. We also discuss unusual behaviour of magnetoresistance and
Hall resistivity in these compounds.
Optical Properties of Iron-Based Superconductors
A. Charnukha and A.V. Boris
Max Planck Institute for Solid State Research,
Heisenbergstrasse 1, D-70569 Stuttgart, Germany
We show that the far-infrared conductivity of hole-doped Ba0.68K0.32Fe2As2 is accurately described
by a strong-coupling model in the clean limit [1], with parameters those are quantitatively consistent
with thermodynamic data [2]. This means that elastic scattering by impurities has essentially no
influence on the optical spectra. The same model in the dirty limit also accurately describes the optical
conductivity of electron-doped BaFe1.85Co0.15As2 iron arsenides, which turned out to be dominated by
impurity scattering. The difference between electron- and hole-doped compounds is a consequence of
the doping mechanism, which involves substitution inside and out of the iron arsenide planes,
respectively. Our data and analysis allows a universal understanding of the optical properties of this
entire family of materials.
As an unusual complement to these results we observe a superconductivity-induced suppression of
an absorption band at an energy of 2.5 eV, two orders of magnitude above the superconducting gap
energy [3]. Based on density-functional calculations, this band is assigned to transitions from As-p to
Fe-d orbitals crossing the Fermi surface. This anomaly is explained as a consequence of nonconservation of the total number of unoccupied states involved in the corresponding optical transitions
due to the opening of the superconducting gaps and redistribution of the occupation of the different
bands below Tc, which can potentially enhance superconductivity in iron - pnictides.
We also report the complex dielectric function of a Rb2Fe4Se5 superconductor with Tc = 32 K in
the spectral range from 1 meV to 6.5 eV [4]. In cooperation with DE1762/1-1 project led by Dr.
Joachim Deisenhofer, we have demonstrated that Rb2Fe4Se5 displays a clear metallic response in the
THz spectral range below 100 K with pl = 100 meV, which is partially suppressed in the
superconducting state. Such a small charge carrier density suggests that the optical conductivity of
Rb2Fe4Se5 represents an effective-medium response of two separate phases dominated by the magnetic
semiconducting phase.
This project is supported by the DFG grant BO 3537/1-1 within SPP 1458.
[1] A. Charnukha, O.V. Dolgov, A.A. Golubov, Y. Matiks, D.L. Sun, C.T. Lin, B. Keimer, and A.V. Boris,
Preprint: arXiv:1103.0938 (2011).
[2] P. Popovich, A.V. Boris, O.V. Dolgov, A.A. Golubov, D.L. Sun, C.T. Lin, R.K. Kremer, and B. Keimer, Phys. Rev.
Lett. 105, 027003 (2010).
[3] A. Charnukha, P. Popovich, Y. Matiks, D.L. Sun, C.T. Lin, A.N. Yaresko, B. Keimer, and A.V. Boris, Nature
Communications 2, 219 (2011).
[4] A. Charnukha, J. Deisenhofer, D. Pröpper, M. Schmidt, Z. Wang, Y. Goncharov, A. N. Yaresko, V. Tsurkan, B. Keimer,
A. Loidl, and A. V. Boris, Preprint: arXiv:1108.5698 (2011).
Itinerant magnetic excitations in Rb0.8Fe1.6Se2
Gerd Friemel and Dmytro Inosov
Max Planck Institute for Solid State Research,
Heisenbergstrasse 1, D-70569 Stuttgart, Germany
Recently, a new family of iron-based superconductors, comprising KxFe2-ySe2, CsxFe2-ySe2, RbxFe2ySe2 , was discovered, reaching a Tc as high as 32 K [1]. The chemical structure is similar to the 122
iron pnictide compounds except that the composition requires an iron deficiency. The vacancies form a
√5x√5 superstructure below 580 K [2]. In addition, an exceptionally large ordered moment of 3.3μB
was observed, rendering a coexistence of magnetism and superconductivity unlikely [3]. Recent
investigation by STM, TEM and x-ray diffraction reported a separation in insulating phase with the
√5x√5-superstructure and a presumably superconducting phase with √2x√2-superstructure [4]. In
addition, ARPES measurements on superconducting samples show the absence of hole-pockets at the
Г-point and large electron pockets at the M point. The Fermi surface resembles strongly overdoped 122
iron pnictide, suggesting a different driving mechanism of superconductivity in the selenide
compounds. Here, we present inelastic neutron scattering on a superconducting Rb0.8Fe1.6Se2-sample,
where we found a strong normal state response around at Q = (π,0.5π) and E = 14 meV, which
becomes enhanced upon entering the superconducting state [6]. The observation of such a resonant
mode favors an unconventional symmetry for the superconducting order parameter in this new family.
Furthermore, we present the fine structure of the resonant intensity in Q-space, which has two
dimensional character and anisotropic in-plane shape. The center of the peak lies at an incommensurate
wave vector which implies a nesting scenario as the origin of the enhanced imaginary part of the
susceptibility. We support this interpretation by LDA-calculations of the bare Lindhard susceptibility,
which can reproduce the in-plane Q-structure of the resonance.
[1] J. Guo et al., Phys. Rev. B 82, 180520 (2010).
[2] A. Ricci et al., Supercond. Sci. Technol. 24, 082002 (2011).
[3] V. Yu. Pomjakushin et al., Phys. Rev. B 83, 144410 (2011); F. Ye et al., arXiv 1102.2882, (unpublished); W. Bao et al.,
arXiv:1102.0830 (unpublished).
[4] Y. J. Song et al., EPL 95, 37007 (2011). Y. J. Yan et al., arXiv: 1104.4941, (2011). A. Ricci et al., Phys. Rev. B 84,
060511 (2011). P. Cai et al., arXiv 1108.2798, (2011). W. Li et al., arXiv 1108.0069, (2011).
[5] T. Qian et al., Phys. Rev. Lett. 106, 187001 (2011).Lin Zhao et al., Phys. Rev. B 83, 140508 (2011). Y. Zhang et al., Nat
Mater 10, 273–277 (2011).
[6] J. T. Park et al., arXiv 1107.1703, (2011)
Synthesis, Growth and Characterization of Fe-based
Superconductors
A. Haghighirad, M.de Souza, S. Knöner, M. Kuhnt, A. Adamski, F. Freund, M.
Lang, and Wolf Assmus
Physikalisches Institut, Goethe-Universität, Max-von-Laue-Str. 1, D-60438, Frankfurt am Main,
Germany
The discovery of high-temperature superconductivity in the ZrCuSiAs type rare-earth oxypnictide
LaFeAsO (F-doped) with Tc of 26 K [1] has created strong interest in the exploration of iron-based
superconductors [2,3].
Sofar, five different structure classes of Fe-based-superconductors have been found [2]. Irrespective of
the structure-type, the common feature in all Fe-based-superconductors, is the presence of layered
structure based on a plane layer of Fe atoms joined by tetrahedrally coordinated pnictogen (P, As) or
chalcogen (S, Se and Te) anions. The latter are arranged in a stacked sequence separated by alkali,
alkaline-earth or rare-earth and oxygen/fluorine layers.
Due to toxicity and high vapor pressure of arsenic, phosphorous and selenium elements and the high
reactivity of rare-earth, alkali and alkaline-earth metals, the synthesis of these compounds is more
difficult than that of cuprates. In order to understand the intrinsic properties of the Fe-basedsuperconductors, systematic investigations of, sizable, high quality single crystals are indispensable.
There are several ways to grow crystals of Fe-based superconductors. The AFe2As2 (A = Ba, Ca, Sr and
Eu) compounds can be grown in single crystalline form by high-temperature solution growth method
using either a Fe-As self-flux or a Sn flux [4]. However, the incorporation of the flux results in some
cases to the decrease of the structural/magnetic transition in the latter compounds. The growth of large
single crystals of the 1111-system has been proven to be difficult. High pressure growth has been more
successful in growing larger crystals from salt flux and it has been shown to be more effective for Fdoping [5].
Despite the structural simplicity of FeSe among the Fe-based-compounds, crystal growth of this
compound is very challenging taking into account the different polymorphs and the extreme sensitivity
of superconductivity to small deviations depending on the elemental composition, synthesis process
and synthesis conditions of temperature or pressure [6].
In this contribution I would like to emphasize mainly the synthesis and crystal growth activity of
FeAs-based 11-, and 1111-type compounds in Frankfurt.
[1] Y. Kamihara, T. Wanatabe, M. Hirano, and h. Hossono, J. Am. Chem. Soc. 130, 3296 (2008)
[2] J. Paglione and R. L. Greene, Nature Phys. 6, 645 (2010)
[3] D. C. Johnston, Adv. Phys. 59, 803 (2010)
[4] M. D. Lumsden and A. D. Christianson, J. Phys. Condens. Matter 22, 203203 (2010)
[5] J. Karpinski, et al. 469, Physica C, 370 (2009)
[6] Y. Mizuguchi and Y. Takano, J. Phys. Soc. Jpn. 79, 102001-1 (2010)
Probing unconventional superconductivity in LiFeAs by
quasiparticle interference
C. Hess, T. Hänke, S. Sykora, M. Scheffler, D. Baumann, R. Schlegel, L. Harnagea,
S. Wurmehl, M. Daghofer, J. van den Brink, B. Büchner
IFW-Dresden, Institute for Solid State Research, D-01171 Dresden, Germany
A crucial step in revealing the nature of unconventional superconductivity is to investigate the
symmetry of the superconducting order parameter. Scanning tunneling spectroscopy has proven a
powerful technique to probe this symmetry by measuring the quasiparticle interference (QPI) which
sensitively depends on the superconducting pairing mechanism. A particularly well suited material to
apply this technique is the stoichiometric superconductor LiFeAs as it features clean, charge neutral
cleaved surfaces without surface states and a relatively high Tc~18K. Our data reveal that in LiFeAs
the quasiparticle scattering is governed by a van-Hove singularity at the center of the Brillouin zone
which is in stark contrast with other pnictide superconductors where nesting is crucial for both
scattering and s+--superconductivity. Indeed, within a minimal model and using the most elementary
order parameters, calculations of the QPI suggest a dominating role of the hole-like bands for the
quasiparticle scattering. Our theoretical findings do not support the elementary singlet pairing
symmetries s++, s+-, and d-wave. This brings to mind that the superconducting pairing mechanism in
LiFeAs is based on an unusual pairing symmetry such as an elementary p-wave (which provides
optimal agreement between the experimental data and QPI simulations) or a more complex order
parameter (e.g. s+id-wave symmetry).
Magnetic properties of novel Cd-doped iron oxo-selenides
Sven Landsgesell
Helmholtz-Zentrum Berlin fuer Materialien und Energie GmbH
The cuprate oxides appear to teach us that what we need for high-TC superconductivity is an S=1/2
antiferromagnetic Mott insulator ground state where the transition metal resides in a square planar
geometry. The newly discovered iron pnictide superconductors violate this notion as the parent’s phase
ground state is metallic and the Fe ion resides in a tetrahedrally coordinated site.
Recent work has shown that the oxy-selinide compound Fe2La2O3Se2 provides for a similar local
environment of the Fe-ion to that found in the Fe-superconductors but is an antiferromagnetic Mott
insulator that undergoes a structural transition at TN=110K.
We hole-doped different rare earth iron oxo-selenides with Cd for the rare earth in analogy to the doped
iron pnictide superconductors and we find that the resulting solid solution follows the expected Vagads
Law. For these samples we find that increasing amount of Cd substitution results in higher values of
TN that is opposite to what is known for doped iron pnictide compounds. Our bulk magnetisation
measurements results indicate that the antiferromagnetic ordering increases steadily to 120K. However,
it may possibly indicate the onset of a new magnetic order as well, something that in the focus of
scheduled neutron experiments.
Superconductivity, magnetic order, and smearing of the
magnetic phase transition in the cobalt doped SrFe2−xCoxAs2 iron
pnictide system under high pressure
H. Maeter,1 H.-H. Klauss,1 H. Luetkens,2 R. Khasanov,2 A. Amato,2 M. Nicklas,3
M. Kumar,3 E. Lengyel,3 A. Jesche,3 . Leithe-Jasper,3 H. Rosner,3 W. Schnelle,3
and C. Geibel3
1
2
Institute for Solid State Physics, TU Dresden, D–01069 Dresden, Germany
Laboratory for Muon–Spin Spectroscopy, Paul Scherrer Institut, CH–5232 Villigen PSI, Switzerland
3
Max Planck Institute for Chemical Physics of Solids, D–01187 Dresden, Germany
We have investigated the magnetic and superconducting properties of the electron doped iron pnictide
High TC system SrFe2−xCoxAs2 for 0 ≤ x ≤ 0. 4 by means of muon spin relaxation (μ SR), Moessbauer
spectroscopy, resistivity and ac susceptibility measurements under ambient and high pressure. Our
investigations of SrFe2−xCoxAs2 under ambient pressure show that electronic phase coexistence occurs
at the phase boundary between antiferromagnetic order and superconductivity. Cobalt doping as well as
the application of high pressures cause a reduction of TN . This magnetic transition has been
investigated with μ SR under high pressure for x = 0. 1, 0. 15. We present a new model for the
interpretation of the pressure dependence of the μ SR data. This model allows an analysis of disorder
effects that lead to a smearing of the magnetic phase transition. The emergence of superconductivity
under pressure is studied by means of resistivity and ac susceptibility.
Interplay of 4f magnetism and superconductivity in
CeFe(As,P)O
K. Mydeen, A. Jesche, C. Geibel, and M. Nicklas
Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany.
We have studied the temperature-pressure phase diagram CeFeAsO in the pressure range up to 8 GPa.
The iron ordering temperature TN,Fe is suppressed on increasing pressure but levels off at about 25 K in
the pressure range above 4 GPa. With further increasing pressure we loose the signature for iron
ordering. The observed behavior is similar to the observations in the substitution series CeFe(As,P)O,
but in sharp contrast to the findings in other iron-pnictide superconductors. At low temperatures the
ordering temperature of the local 4f moments of the Ce is quite insensitive to pressure. However, we
find clear evidence for a change of the type of the magnetic order from antiferromagnetism to
ferromagnetism in the same pressure region where TN,Fe is leveling off. We will discuss the connection
between the iron ordering and the ordering of the 4f moments of the cerium. Furthermore, we will
elucidate the possible occurrence of superconductivity in CeFe(As,P)O.
Arsenic free pnictide superconductors containing Bi and Sb
synthesized by molecular beam epitaxy
R. Retzlaff, A. Buckow, J. Kurian, and L. Alff
Institute of Materials Science, Technische Universität Darmstadt, Petersenstr. 23, 64287 Darmstadt,
Germany
Since the discovery of the iron based pnictide superconductors a large variety of new compounds has
been found . Currently, the highest transition temperatures is at 55 K. The majority of studies is based
on As as pnictogen. Arsenic is a potent poisons and, thus, it is not very likely to be used in future largescale applications. Furthermore, there is no clear theoretical picture how the phase diagrams are
changed when As is replaced by other pnictogens such as Bi and Sb. We are using molecular beam
epitaxy as synthesis method sui generis which even allows the formation of thermodynamically
unstable compounds and allows scanning a phase diagram. We have grown epitaxial thin films of
LaNiBi2 and LaNiSb2 on MgO substrates from elemental sources in a custom designed UHV chamber.
Streaky RHEED patterns during deposition indicate the epitaxial growth and Laue oscillations in the
radial XRD measurements reveal the high crystallinity of the thin films. First results reveal a Tonset C of
about 3.5K with a transition width T of 0.3K for LaNiBi2, while the LaNiSb2 shows no
superconducting transition down to 1.5 K. Recently, the bulk synthesis of superconducting LaNi0:65Bi2
with TC = 4K has been reported by Mizoguchi [1]. Further studies are on the way investigating the
substitution at the transition metal site.
[1] H. Mizoguchi, S. Matsuishi, M. Hirano, M. Tachibana, E. Takayama-Muromachi, H. Kawaji, and H. Hosono, Phys. Rev.
lett. 106, 057002 (2011)
Proximity of iron pnictide superconductors to a quantum
tricritical point
Carmine Ortix and Jeroen van den Brink
Institute for Solid State Research, IFW Dresden
In several materials, unconventional superconductivity appears nearby a quantum phase transition
where long-range magnetic order vanishes as a function of a control parameter like charge doping,
pressure or magnetic field. The nature of the quantum phase transition is of key relevance, because
continuous transitions are expected to favour superconductivity, due to strong fluctuations.
Discontinuous transitions, on the other hand, are not expected to have a similar role. We will discuss
the nature of the magnetic quantum phase transition, which occurs as a function of doping, in the ironbased superconductor LaFeAsO1-xFx by making use of constrained density functional calculations that
provide ab initio coefficients for a Landau order parameter analysis. The outcome is intriguing, as this
material turns out to be remarkably close to a quantum tricritical point, where the transition changes
from continuous to discontinuous, and several susceptibilities diverge simultaneously. We discuss the
consequences for superconductivity and the phase diagram [1]
[1] G. Giovannetti, C. Ortix, M. Marsman, M. Capone, J. van den Brink, J. Lorenzana, Nature Comm. 2, 398 (2011).
Contrasting P and F doping of the CeFeAsO system by means of
µSR and Mössbauer spectroscopy
T. Dellmann1, H. Maeter1, P. Materne1, J. Spehling1, H. Luetkens2, R. Khasanov2,
A. Amato2, A. A. Gusev3, K. V. Lamonova3, D. A. Chervinskii3, R. Klingeler4, C.
Hess4, G. Behr4, C. Hess4, B. Büchner4, A. Jesche5, C. Krellner5, C. Geibel5
and H.–H. Klauss1
1
2
Institute for Solid State Physics, TU Dresden, Dresden, Germany
Laboratory for Muon–Spin Spectroscopy, Paul Scherrer Institute, Villigen PSI, Switzerland
3
A. A. Galkin Donetsk Phystech NASU, Donetsk, Ukraine
4
Leibniz Institute for Solid State and Materials Research Dresden, Dresden, Germany
5
Max Planck Institute for Chemical Physics of Solids, Dresden, Germany
CeFeAsO offers several doping routes that lead to the suppression of magnetic order and eventually
give rise to superconductivity. Charge doping can be achieved by replacing oxygen with fluor.
Isovalent (“chemical pressure”) doping can be achieved by replacing arsenic with phosphorous. On the
phosphorous rich side, CeFePO is a heavy fermion metal with a tendency towards spin glass
magnetism below 700mK. Upon replacing P with As ferromagnetic long range order of the Ce
moments develops with transition temperatures as high as 8K. On the As rich side, P doping causes a
reduction of the Néel temperature of the Fe antiferromagnetic order. It is not fully suppressed before
superconductivity emerges in a narrow doping range. The charge doping also leads to a suppression of
the Fe magnetic order. However, after sufficient suppression of magnetic order, superconductivity is
found that is stabilized by doping and fully suppresses the Fe and Ce magnetic order. On the border
between magnetism and superconductivity phase separation is found. With the combination of muon
spin relaxation and Mössbauer spectroscopy we gain a detailed picture of the different ordered phases
on a local scale.
Anisotropic in-plane resistivity in the nematic phase of the iron
pnictides
J. Schmalian
Karlsruhe Institute of Technolgy, Institute for Theoretical Condensed Matter Physics, Karlsruhe
We show that the interference between scattering by impurities and by critical spin fluctuations gives
rise to anisotropic transport in the Ising-nematic state of the iron pnictides. The effect is closely related
to the non-Fermi liquid behavior of the resistivity near an antiferromagnetic quantum critical point. Our
theory not only explains the observed sign of the resistivity anisotropy in electron doped systems, but
also predicts a sign change of the anisotropy upon sufficient hole doping. Furthermore, our model
naturally addresses the changes upon sample annealing and alkaline-earth substitution.
Exploring the phase diagram of EuFe2(As1-xPx)2
Sina Zapf1, Felix Klingert1, Dan Wu1, Lapo Bogani1, Hirale S. Jeevan2,
Philipp Gegenwart2 and Martin Dressel1
1
2
Universität Stuttgart, 1. Physikalisches Institut, D-70550 Stuttgart, Germany
Universität Göttingen, I. Physikalisches Institut, D-37077 Göttingen, Germany
EuFe2As2 is an exceptional member of the 122 iron pnictides, since in addition to the spin-densitywave (SDW) ordering in the FeAs layers below TSDW = 190K, magnetic order of the localized Eu2+ ions
can be observed below TN = 19K [1]. Eu2+ possesses a large magnetic moment leading to a so called
“A-type” antiferromagnetic order at low temperatures, meaning that the Eu2+ moments align
ferromagnetically (FM) along the a-axis and antiferromagnetically (AFM) along the c-axis. The
interplay of the Eu2+ magnetic order with the SDW as well as with the superconducting phase in
pressurized systems is drawing increasing attention. It is still under debate, which kind of Eu2+ order
coexists with superconductivity [2, 3].
We have studied a series of EuFe2(As1-xPx)2
samples (x = 0, 0.05, 0.12, 0.25 and 0.35) and
present a scheme of the Eu2+ spin alignment,
reconciling different existing phase diagrams [2, 3].
A canting of the Eu2+ spins leads to a ferromagnetic
signal when measuring susceptibility along the cdirection. This canting becomes stronger with
pressure, until superconductivity sets in [3]. With
susceptibility measurements, we are able to
distinguish between this canting and the interlayer
coupling: At low temperatures, the interlayer
coupling is antiferromagnetic. Reducing the
interlayer distance by chemical pressure, the
interlayer coupling turns ferromagnetic and
superconductivity is suppressed. Superconductivity
in the ab-plane coexists with antiferromagnetic
interlayer coupling, but a ferromagnetic Eu2+ spin
component along the c-direction.
[1] Y. Xiao et al., Phys. Rev. B 80, 174424 (2009)
[2] H. S. Jeevan et al., Rev. B 83, 054511 (2011)
[3] I. Nowik et al., Phys.: Condens. Matter 23, 065701 (2011)
[4] S. Zapf et al.. arXiv:1103.2446 (2011)
From density-functional theory to the functional
renormalization group: Competing orders in Fe-based
superconductors
Ch. Platt1, R. Thomale2, W. Hanke1
1
Institute for Theoretical Physics, University of Wuerzburg
2
Physics Department, Stanford University
A combined density functional theory and functional renormalization group method is introduced [1]
which takes into account orbital-dependent interaction parameters to derive the effective low-energy
theory of weakly to intermediately correlated Fermi systems. As an application, the competing
fluctuations in LiFeAs are investigated, which is the main representative of the 111 class of iron
pnictides displaying no magnetic order, but superconductivity, for the parent compound. The
superconducting order parameter is found to be of s+/- type driven by collinear antiferromagnetic
fluctuations. They eventually exceed the ferromagnetic fluctuations stemming from the small hole
pocket at the Gamma point, as the system flows to low energies.
As a different manifestation of superconducting order in the pnictides, the superconducting phase in the
hole-doped KxBa1-xFe2As2 compound is explored [2] and it is shown that the system develops a
nodeless s+- order parameter in the moderately doped regime and a nodal d-wave state at strong hole
doping.
The observed competition of s-wave and d-wave pairing is a general feature of the multipocket Fermi
surface in iron-based superconductors and may lead to a time-reversal-symmetry breaking (s+id)pairing state [3].
[1] Platt, Thomale, and Hanke, arXiv 1103.2101
[2] Thomale, Platt, Hanke, Hu, Bernevig, Phys. Rev. Lett. 107, 117001 (2011)
[3] Platt, Thomale, Honerkamp, Zhang, Hanke, arXiv 1106.5964
Competing Multi-Orbital Nematic and Superconductive Orders
in Bad-Metallic Iron Arsenides
Mukul S. Laad1 and Luis Craco2
1
2
Institute for Theoretical Physics, RWTH Aachen
Physical Chemistry, Technical University Dresden
Fe arsenides are the latest example of a growing class of correlated systems showing the defining
characteristics of novel quantum matter:
(i) proximity to correlation induced Mottness with orbital and magnetic order
(ii) bad-metallicity and non-Landau Fermi liquid, or quantum critical behavior
(iii) competition between unconventional ordered states directly from (ii) without intervening "normal"
Fermi liquid behavior.
These have attracted intense attention in the last threre years. Based on extensive perusal of various
experiments, we propose a "sizable correlation" scenario for these systems, namely that, even though
metallic, their physical responses require approaches based on the ubiquitous itinerant-localised
"duality" characteristic of correlated quantum matter. This bears important implications for the
ordering instabilities (either magnetism or superconductivity), since the usual BCS picture cannot be
applied for systems where the normal state is not a Landau Fermi liquid. In this talk, I will discuss this
aspect in combination with our work that attempts to address (i)-(iii) above. In particular, I will discuss
how a multi-orbital correlation-based approach can achieve excellent quantitative accord with a variety
of one- and two-particle responses in the "normal" phases across members of Fe-based family. Armed
with these positive features, I will discuss a novel theoretical mechanism which suggests that an
unconventional orbital-nematic order competes with unconventional superconductivity, and shows how
both emergedirectly from the incoherent metal.
I will also emphasise the important role of lattice effects in this last context, and discuss possible
avenues toward "true first-principles" LDA+DMFT approaches including lattice effects,in future.
Gutzwiller theory of band magnetism in LaOFeAs
Tobias Schickling1, Florian Gebhard1, Jörg Bünemann2 , Lilia Boeri3,
Ole K. Andersen3, Werner Weber4
1
Fachbereich Physik, Philipps Universität, D-35037 Marburg, Germany
2
Institut für Physik, BTU Cottbus, D-03013 Cottbus, Germany
3
Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany
4
Fakultät Physik, TU Dortmund, D-44221 Dortmund, Germany
For the iron pnictide LaOFeAs we investigate multi-band Hubbard models which are assumed to
capture the relevant physics [1, 2]. In our calculations, we employ the Gutzwiller variational theory
which is a genuine many particle approach [3]. We will present results both on the paramagnetic and
antiferromagnetic phases of our model systems. These results show that a five band-model is not
adequate to capture the relevant physics in LaOFeAs [4]. However, our results for the eight band-model
which includes the arsenic 4p bands reproduce the experimental data, especially the small magnetic
moment, for a broad parameter regime [5].
[1] S. Graser, T.A. Maier, P.J. Hirschfeld and D.J. Scalapino. New J. Phys. 11, 025016 (2009)
[2] O. K. Andersen and L. Boeri. Annalen der Physik, 523:8-50 (2011)
[3] J. Bünemann, F. Gebhard and W. Weber. In A. Narlikar, editor, Frontiers in Magnetic Materials. Springer, Berlin, 2005.
[4] T. Schickling, F. Gebhard and J. Bünemann. PRL, 106:146402 (2011)
[5] T. Schickling, F. Gebhard, J. Bünemann, L. Boeri, O.K. Andersen and W. Weber. arXiv: 1109.0929
Tunnelling spectroscopy (SI-STM) study of Fe(Se,Te)
S.C. White1, U.R. Singh1, Y. Liu1, C.T. Lin1, V. Tsurkan2, J. Deisenhofer2, P. Wahl1
1
Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany
2
Universität Augsburg, Augsburg, Germany
We present a spectroscopic imaging scanning tunnelling microscopy (SI-STM) study of Fe1+yTe1−xSex.
This iron-based superconductor crystallizes non-stoichiometrically in a layered, tetragonal structure,
with excess Fe atoms occupying octahedral positions [1]. It has been suggested that the presence of
excess Fe atoms in this compound, representing local moments in the proximity of the superconducting
plane, could provide an opportunity for experimental investigation of the interplay between
superconductivity and pair breaking magnetic scattering [2]. The crystal structure of Fe(Se, Te)
provides a well defined cleavage plane between its chalcogenide layers, making it ideal for STM
investigation. SI-STM measurements have recently produced evidence that the order parameter of
Fe(Se,Te) could have s± symmetry [3]. In our study we investigate the local electronic structure of this
superconductor by spatially mapping the density of states on samples with bulk transition temperatures
of up to 14K [4]. We observe strong spectral features associated with atomic Fe defects, indicating a
significant local role of same in electronic interactions. We investigate further the spatial extent of a
possible local suppression of superconductivity [5] associated with at least two separate species of
atomic excess Fe.
[1]
[2]
[3]
[4]
[5]
P.L. Paulose, C.S. Yadav and K.M. Subhedar, EPL 90, 27011 (2010)
L. Zhang, D.J. Singh and M.H. Du, Phys. Rev. B, 79, 012506 (2009)
T. Hanaguri, S. Niitaka, K. Kuroki and H. Takagi, Science 328, 474 (2010)
B.C. Sales, A. S. Sefat, M. A. McGuire, R. Y. Jin, D. Mandrus and Y. Mozharivskyj, Phys. Rev. B 79, 094521
(2009)
T.J. Liu, X. Ke, B. Qian, J. Hu, D. Fobes, E. K. Vehstedt, H. Pham, J. H. Yang, M. H. Fang, L. Spinu, P. Schiffer,
Y. Liu, and Z. Q. Mao, Phys. Rev. B 80, 174509 (2009)