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# 75As nuclear magnetic resonance study of antiferromagnetic fluctuations in the normal state of LiFeAs

30 Apr 2010-Physical Review B (AMER PHYSICAL SOC)-Vol. 81, Iss: 14, pp 140511-1-140511-4

Abstract: We present a detailed study of $^{75}\text{A}\text{s}$ nuclear magnetic resonance Knight shift and spin-lattice relaxation rate in the normal state of stoichiometric polycrystalline LiFeAs. Our analysis of the Korringa relation suggests that LiFeAs exhibits strong antiferromagnetic fluctuations, if transferred hyperfine coupling is a dominant interaction between $^{75}\text{A}\text{s}$ nuclei and Fe electronic spins, whereas for an on-site hyperfine coupling scenario, these are weaker, but still present to account for our experimental observations. Density-functional calculations of electric field gradient correctly reproduce the experimental values for both $^{75}\text{A}\text{s}$ and $^{7}\text{L}\text{i}$ sites.

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As-75 nuclear magnetic resonance study of antiferromagnetic
fluctuations in the normal state of LiFeAs
Citation for published version:
Jeglic, P, Potocnik, A, Klanjsek, M, Bobnar, M, Jagodic, M, Koch, K, Rosner, H, Margadonna, S, Lv, B,
Guloy, AM & Arcon, D 2010, 'As-75 nuclear magnetic resonance study of antiferromagnetic fluctuations in
the normal state of LiFeAs', Physical review B: Condensed matter and materials physics, vol. 81, no. 14,
140511, pp. -. https://doi.org/10.1103/PhysRevB.81.140511
Digital Object Identifier (DOI):
10.1103/PhysRevB.81.140511
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75
As nuclear magnetic resonance study of antiferromagnetic ﬂuctuations
in the normal state of LiFeAs
P. Jeglič,
1
A. Potočnik,
1
M. Klanjšek,
1
M. Bobnar,
1
M. Jagodič,
2
K. Koch,
3
H. Rosner,
3
4
B. Lv,
5
A. M. Guloy,
5
and D. Arčon
1,6
1
Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
2
Institute of Mathematics, Physics and Mechanics, Jadranska 19, 1000 Ljubljana, Slovenia
3
Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany
4
School of Chemistry, University of Edinburgh, West Mains Road, EH9 3JJ Edinburgh, United Kingdom
5
Department of Chemistry and TCSUH, University of Houston, Houston, Texas 77204-5002, USA
6
Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia
Received 16 December 2009; published 30 April 2010
We present a detailed study of
75
As nuclear magnetic resonance Knight shift and spin-lattice relaxation rate
in the normal state of stoichiometric polycrystalline LiFeAs. Our analysis of the Korringa relation suggests that
LiFeAs exhibits strong antiferromagnetic ﬂuctuations, if transferred hyperﬁne coupling is a dominant interac-
tion between
75
As nuclei and Fe electronic spins, whereas for an on-site hyperﬁne coupling scenario, these are
weaker, but still present to account for our experimental observations. Density-functional calculations of
electric ﬁeld gradient correctly reproduce the experimental values for both
75
As and
7
Li sites.
DOI: 10.1103/PhysRevB.81.140511 PACS numbers: 74.70.b, 76.30.v, 76.60.k
Following the discovery of superconductivity in
LaFeAsO
1−x
F
x
,
1
nuclear magnetic resonance NMR pro-
vided one of the earliest evidences for unconventional pair-
ing in the superconducting SC state,
24
multigap
superconductivity,
57
pseudogap PG behavior in the normal
state,
2,3,8
and antiferromagnetic AFM ordering of Fe
2+
spins in the undoped parent compounds of Fe-As
superconductors.
3,9,10
Although the SC pairing mechanism is
still under debate, it is commonly believed that AFM
ﬂuctuations play an important role in promotion of
high-temperature superconductivity in this family. This
is indicated by the presence of the AFM phase next to
the SC ground state in the phase diagrams of REFeAsO Ref.
11兲共“1111,” RE=rare earth and AFe
2
As
2
Ref. 12
“122,” A=alkaline-earth metal compounds.
Recently, LiFeAs, the so-called “111” member of the
Fe-As superconductors, has been reported
13
to undergo a
transition to the SC state at T
c
doping and apparent AFM ordering or accompanying struc-
tural phase transition. Its structure is a simpliﬁed analog of
the 1111 or “122” members: FeAs layers comprised of
edge-sharing FeAs
4
tetrahedra are separated by double layers
of Li ions. However, the tetrahedra are deformed and the
Fe-Fe distance is considerably shorter compared to other
Fe-As superconductors. Moreover, T
c
linearly decreases with
applied pressure, similarly as in overdoped K
x
Sr
1−x
Fe
2
As
2
,
although the charge count of −1 per FeAs unit would rather
compare LiFeAs to undoped SrFe
2
As
2
.
14
LiFeAs is also
claimed to be a weakly to moderately,
15
or moderately to
strongly
16
correlated system. These conﬂicting results raise
an important question about the signiﬁcance of AFM ﬂuctua-
tions and the placement of LiFeAs in the general Fe-As su-
perconductor phase diagram.
Here we employ
75
As NMR to quantitatively account for
the extent of spin correlations in the normal state of LiFeAs
and compare it to a typical “122” member. We ﬁnd that the
spin-lattice relaxation rate T
1
−1
is enhanced, compared to the
values calculated for the noninteracting electron scenario.
The quantitative comparison with cuprates and organic
superconductors
17
indicates that AFM correlations may also
play an important role in the LiFeAs superconductor.
Stoichiometric polycrystalline LiFeAs was synthesized
from high-temperature reactions as described in detail in Ref.
13. For magnetic resonance experiments the LiFeAs sample
was sealed into the quartz tube under vacuum to avoid con-
tamination with moisture during the measurements. To check
the quality of our polycrystalline LiFeAs samples, we per-
formed electron paramagnetic resonance measurements in
the vicinity of SC transition. A nonresonant microwave ab-
sorption effect
18
occurs sharply below 21 K Fig. 1a, dem-
onstrating the onset of SC state at T
c
20 K in agreement
with Ref. 13 and demonstrating the high quality of our
sample.
75
As I=3/ 2 NMR frequency-swept spectra were
measured in a magnetic ﬁeld of 9.4 T with a two-pulse se-
quence
echo, a pulse length
=5
s, inter-
pulse delay
=100
s, and repetition time 100 ms at room
temperature. The reference frequency of
75
As
=68.484 MHz was determined from a NaAsF
6
standard. The
75
As T
1
−1
was measured with inversion-recovery technique.
The band-structure calculations were performed within the
local-density approximation LDA, as described in detail in
Refs. 10 and 19. As basis set Li/2s2p3d +3s3p,
Fe3s3p / 4s4p3d+5s5p, and As3s3p / 4s4p3d +5s5p were
chosen for semicore/ valence+polarization states. A well-
converged k mesh with 1183 k points in the irreducible part
of the Brillouin zone was used. The structural parameters
were taken from Ref. 13. The calculated V
zz
component of
the electric ﬁeld gradient EFG tensor is converted into the
Q
using the
relation
Q
=3eV
zz
Q/ 2hI2I −1兲兴 with the quadrupole mo-
ment Q and nuclear spin I given in Table I.
Representative
75
As NMR spectra of the central
1
2
1
2
and the satellite
3
2
1
2
transitions for the
polycrystalline LiFeAs sample are shown in Fig. 1b for
PHYSICAL REVIEW B 81, 140511R兲共2010
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temperatures between room temperature and T
c
. Over the
entire temperature range the line shape remains characteristic
for an axially symmetric EFG tensor, in accordance with the
75
As site symmetry 4mm, indicating the absence of a struc-
tural phase transition, as encountered in the undoped 1111
and “122” members of the Fe-As superconductors family.
Analysis of the splitting between both singularities belonging
to the satellite transitions reveals only a moderate tempera-
ture dependence of
Q
, which monotonically decreases from
21.35 MHz at room temperature reaching 20.87 MHz at low
temperatures inset to Fig. 1c. There is no indication of
AFM ordering down to T
c
, which would be seen as an abrupt
broadening of the NMR line shape due to the appearance of
internal magnetic ﬁelds.
3,9,10
In Fig. 2a we show the
7
Li I =3/ 2 NMR spectrum
measured at 300 K. Contrary to the
75
As resonance the shift
of the
7
Li NMR line is small and negative Fig. 2a. How-
ever, the value of 615 ppm cannot be attributed to the
pure orbital shift typical values are an order of magnitude
smaller, which may indicate an incomplete charge transfer
from the Li layer to the FeAs layer. From the
7
Li NMR
linewidth
90 kHz, we conclude that
7
Li has a very
small
Q
. In order to extract
7
Li
Q
we performed an echo-
decay measurement. The
7
Li echo amplitude clearly shows
characteristic quadrupole oscillations as a function of inter-
pulse delay
in the two-pulse
echo experiment
Fig. 2b.
21
Oscillations with the period t
Q
=59
s yield
Q
=2/ t
Q
34 kHz. The
7
Li I =3/ 2 NMR line-shape simu-
lation taking into account the quadrupole splitting
Q
=34 kHz and the magnetic anisotropy of 1605 ppm
both obeying axial symmetry in accordance with the
7
Li site
symmetry ﬁts the experimental NMR spectrum very well
Fig. 2a.
Next we compare the experimental values of quadrupole
splittings for
75
As and
7
Li with those obtained from the
band-structure calculations. As usually encountered in Fe-As
superconductors, the displacement of As site along the z axis
has a huge inﬂuence on the EFG at the As site, see
Fig. 2c. Experimental
Q
matches the calculated one for
z =z z
exp
=0, where z
exp
=0.2635 is the experimental As z
position
13
see Table I for details. The minimum in energy
with respect to the As z position predicts the displacement of
As by almost z=0.3 Å marked by the black arrow in Fig.
2c. The corresponding
Q
0 fails to correctly reproduce
the measured
75
As
Q
. This is in line with ﬁndings in the
“122” compounds
22
but in striking contrast to studies of the
1111 compounds, where the calculated and measured
Q
’s
agree well for the optimized As z position.
10,19
Calculated
EFG at the Li site is much smaller, less dependent on the As
z position, and does not reach the value
Q
=0 in the covered
interval of z inset to Fig. 2c. This can be understood by
different bonding situations: whereas Fe and As build a
polyanionic sublattice formed by covalent bonds, Li only has
a slightly ﬁlled 2p shell. As such, the EFG at the Li site does
not provide such a stringent test for the quantity z, in con-
trast to the EFG at the As site. Anyway, the measured
7
Li
Q
TABLE I. Comparison between calculated and experimental
Q
s for
75
As and
7
Li sites. Quadrupole moments Q are taken from
Ref. 20.
Site I
Q
fm
2
V
zz
calc
V/ m
2
Q
calc
MHz
Q
exp
MHz
75
As 3/2 31.4 −5.82 10
21
−22.1 21.35
7
Li 3/2 −4.01 0.1110
21
0.054 0.034
FIG. 1. Color online. a Microwave absorption near the SC
transition at low magnetic ﬁeld indicating T
c
20 K. Arrows show
different ﬁeld sweep directions. Sharp peaks at around 1700 G
originate from a dielectric resonator. b
75
As NMR spectrum at 300
and 20 K for a chosen orientation of LiFeAs polycrystalline sample.
A comparison with simulated powder spectrum with
Q
=21.35 MHz and K
iso
=0.32% demonstrates that the sample con-
tains at least few tens of grains. Inset shows the experimental
Q
as
a function of temperature. c Temperature dependence of the high-
frequency singularity of the
75
As NMR central transition.
µ
µ
FIG. 2. Color online兲共a Experimental solid red line and cal-
culated dotted black line
7
Li NMR spectra at 300 K and magnetic
ﬁeld 4.7 T
ref
LiCl=77.7247 MHz of LiFeAs polycrystalline
sample. b
7
Li echo amplitude as a function of interpulse delay
measured at 300 K see text for details. c The calculated V
zz
at
the As green diamonds,Lired circles, and Fe blue squares
sites as a function of z see text for details, together with experi-
mental data for Li black circle and As black diamond. The mini-
mum in energy regarding the As z position is marked by the black
arrow. The inset shows the Li values on a smaller scale.
JEGLIČ et al. PHYSICAL REVIEW B 81, 140511R兲共2010
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compares relatively well to the range of calculated values.
We now focus on the role of AFM correlations in LiFeAs.
We begin with the determination of the spin part of the
75
As
NMR Knight shift from the temperature dependence of the
high-frequency singularity of the
75
As central transition
Fig. 1c. The position of this singularity is given by
=
0
1+K
iso
+3
Q
2
/ 16
0
, where
0
is the
75
As Larmor fre-
quency and K
iso
=K
orb
+K
s
represents an isotropic
75
As shift.
The latter has two contributions, the orbital part K
orb
and the
spin part K
s
. Taking into account the slight temperature
variation in
Q
inset of Fig. 1b we can extract the precise
temperature dependence of K
iso
. For the
75
As orbital
contribution we assume K
orb
K
s
T 0=0 inset of Fig. 3a in accordance with the spin-
singlet Cooper pairing.
4
We ﬁnd that K
s
is strongly reduced
with decreasing temperature and changes from K
s
=0.16% to
K
s
=0.055% between room temperature and T
c
=15 K at 9.4
T Fig. 3a. Such suppression of K
s
is reminiscent of the
PG behavior observed in many Fe-As superconductors.
2,3,8
Because it has been reported for a wide range of x in
BaFe
1−x
Co
x
2
As
2
,
23
the observation of the PG-like behavior
is not yet conclusive about the positioning of LiFeAs in the
Fe-As superconductor phase diagram.
We obtain complementary information from the tempera-
ture dependence of
75
As spin-lattice relaxation rate T
1
−1
Fig. 3b. The nuclear magnetization recovery curves fol-
low Mt M
0
0.1 expt/ T
1
+0.9 exp−6t / T
1

the whole temperature range. Below 40 K we detect a slight
enhancement in T
1
T
−1
followed by a sharp decrease below
T
c
. However, since T
1
T
−1
does not follow the PG-like be-
havior seen in K
s
, we conclude that AFM ﬂuctuations are
present already above 40 K, which is the reason for almost
temperature-independent T
1
T
−1
above T
c
. Enhancement
and divergent behavior of T
1
T
−1
due to the slowing down
of AFM ﬂuctuations has been reported for underdoped “122”
superconductors.
23
With increasing doping the AFM ﬂuctua-
tions become less pronounced and T
1
T
−1
shows PG behav-
ior in the overdoped regime. Our results suggest that LiFeAs
is somewhere in between these two limits with properties
analogous to those of optimally doped Fe-As superconduct-
ors. It seems that this can explain the relatively high T
c
, its
decrease with the applied pressure and the absence of AFM
ordering.
In order to quantitatively verify the presence of AFM
ﬂuctuations in the normal state of LiFeAs, we turn to the
analysis of the Korringa relation for
75
As,
T
1
TK
s
2
=
4
k
B
e
2
n
2
, 1
where
e
and
n
are the electron and nuclear gyromagnetic
ratios, respectively. The phenomenological parameter
,
called the Korringa factor, characterizes the extent of spin
correlations.
24
In case
75
As couples to the noninteracting
Fe 3d electrons i.e., Fermi gas via the on-site Fermi contact
interaction, the Korringa factor is
=
0
=1. Strong ferro-
magnetic ﬂuctuations increase the value of
while AFM
ﬂuctuations decrease it. However, it has been recently pro-
posed for the Fe-As superconductors
4
that the
75
As nuclei are
coupled to the localized Fe electronic spins via the isotropic
transferred hyperﬁne coupling.
25,26
According to Millis
et al.
25
this renormalizes the noninteracting
0
value.
Namely, T
1
−1
due to the q-dependent spin ﬂuctuations is ob-
tained from Moriya’s expression
1
T
1
T
q
Aq兲兩
2
q,
n
n
, 2
where
q ,
n
is the imaginary part of the electron spin
susceptibility at the wave vector q and at the nuclear Larmor
frequency
n
. In case
75
As nucleus is coupled to the local-
ized Fe electronic spins via isotropic transferred hyperﬁne
coupling, we have Aq兲兩
2
cos
2
q
x
a
2
cos
2
q
y
a
2
, where a
is the
distance between two neighboring Fe
2+
spins. For noninter-
acting spins,
q ,
n
has no strong singularities in the q
space, and can be taken out of the summation integrals in
Eq. 2. Compared to the on-site scenario, we get an extra
factor 兰兰dq
x
dq
y
/ 兰兰dq
x
dq
y
cos
2
q
x
a
2
cos
2
q
y
a
2
=4, which renor-
malizes the noninteracting
0
value to
0
=4. From here we
proceed as usual: in case
4 ferromagnetic ﬂuctuations are
predicted, whereas AFM ﬂuctuations should lead to
4.
For instance, in cuprates,
25
a prototypical example of a sys-
tem where AFM ﬂuctuations are important,
is reduced by a
factor of 15, compared to the noninteracting electron sce-
nario with transferred hyperﬁne coupling. A similar factor is
found in some organic superconductors.
17
The experimentally extracted Korringa factor
for
75
As
in LiFeAs is displayed in Fig. 3c. It amounts to 0.7 at
room temperature and then monotonically reduces to 0.1
approaching T
c
. We stress that the absolute values of
de-
pend on our choice of K
orb
. For K
orb
=0.13% and
K
orb
=0.17% the low-temperature value of
changes to 0.17
and 0.03, respectively. Regardless of this uncertainty, the
analysis above demonstrates the enhancement of T
1
−1
at low
temperatures with respect to noninteracting electron limits in
both scenaria considered above, and demonstrates the
strength of AFM ﬂuctuations in LiFeAs. For comparison we
FIG. 3. Color online Temperature dependence of the
75
As
NMR: a spin part of Knight shift, b兲共T
1
T
−1
, and c Korringa
factor
above T
c
, measured for LiFeAs green circles and
SrFe
2
As
2
red squares. Horizontal dashed lines indicate expected
values for
in case of noninteracting electrons for on-site
0
and
transferred coupling
0

shows the behavior of K
s
below T
c
=15 K vertical dashed line at
9.4 T.
75
As NUCLEAR MAGNETIC RESONANCE STUDY OF PHYSICAL REVIEW B 81, 140511R兲共2010
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values for SrFe
2
As
2
Ref. 27 to Fig. 3. In this case,
is systematically larger by a factor of 1.6 compared to
LiFeAs, and above 250 K
is larger than
0
. In case of the
hyperﬁne transferred coupling scenario, the experimental
should be compared to
0
rather than to
0
. Then, the en-
hancement of T
1
−1
in LiFeAs for a factor as large as 4020
at low temperatures suggests strong AFM ﬂuctuations, as
recently predicted by quantum chemical calculations.
16
How-
ever, our LDA calculations, which correctly predict
Q
for
both
75
As and
7
Li sites without taking into account strong
electronic correlations, speak against well-deﬁned localized
moments at the Fe sites as assumed in the transferred hyper-
ﬁne coupling scenario. In this case, the correct reference
valid for the on-site coupling is
0
=1 and the enhancement
of T
1
−1
in LiFeAs is reduced to a factor of 10 5 speaking for
weaker AFM ﬂuctuations. It is not clear at the moment how
strongly
is enhanced since cross terms between different
bands in the LiFeAs multiband structure can inﬂuence T
1
−1
values,
28
while they do not affect NMR Knight shifts, so that
we cannot unambiguously discriminate between the on-site
Fermi contact and the transferred coupling mechanisms. The
ambiguity in the analysis above opens three important issues,
which will have to be addressed in future studies: i is the
coupling of
75
As to itinerant electrons in LiFeAs really on-
site while it is transferred in “122” members? ii If this is
the case, is it related to structural differences of the FeAs
layer between the two families? And, iii should LiFeAs
really be treated as a strongly correlated system?
In summary, NMR and band-structure investigations were
employed to investigate the normal-state properties of the
LiFeAs superconductor. The presence of a PG in the uniform
spin susceptibility measured by the
75
As Knight shift is over-
shadowed by AFM ﬂuctuations in the T
1
−1
measurements. Al-
though the precise determination of the strength of AFM
ﬂuctuations should be a subject of further investigations, we
believe that LiFeAs is the simplest Fe-As superconductor
where correlation effects might be important and should be
considered in future studies.
We acknowledge stimulating discussions with P. Pre-
lovšek, I. Sega, and D. Mihailović. This work was supported
in part by the Slovenian Research Agency. A.M.G. and B.L.
acknowledge the NSF Grant No. CHE-0616805 and the
R. A. Welch Foundation Grant No. E-1297 for support.
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Journal ArticleDOI
G. R. Stewart1Institutions (1)
Abstract: Kamihara and coworkers' report of superconductivity at ${T}_{c}=26\text{ }\text{ }\mathrm{K}$ in fluorine-doped LaFeAsO inspired a worldwide effort to understand the nature of the superconductivity in this new class of compounds. These iron pnictide and chalcogenide (FePn/Ch) superconductors have Fe electrons at the Fermi surface, plus an unusual Fermiology that can change rapidly with doping, which lead to normal and superconducting state properties very different from those in standard electron-phonon coupled conventional'' superconductors. Clearly, superconductivity and magnetism or magnetic fluctuations are intimately related in the FePn/Ch, and even coexist in some. Open questions, including the superconducting nodal structure in a number of compounds, abound and are often dependent on improved sample quality for their solution. With ${T}_{c}$ values up to 56 K, the six distinct Fe-containing superconducting structures exhibit complex but often comparable behaviors. The search for correlations and explanations in this fascinating field of research would benefit from an organization of the large, seemingly disparate data set. This review provides an overview, using numerous references, with a focus on the materials and their superconductivity.

1,207 citations

### Cites background from "75As nuclear magnetic resonance stu..."

• ...The next 2D layered FePn superconductor discovered, LiFeAs, shows bulk superconductivity at Tc ¼ 18 K but has neither a magnetic nor a structural transition, although there are very strong magnetic fluctuations (Jeglic et al., 2010)....

[...]

Journal ArticleDOI
Abstract: The surprising discovery of high-temperature superconductivity in a material containing a strong magnet—iron—has led to thousands of publications. By placing all the data in context, it becomes clear what we know and where we are headed.

1,119 citations

Journal ArticleDOI
Abstract: The recently discovered Fe-pnictide and chalcogenide superconductors display low-temperature properties suggesting superconducting gap structures which appear to vary substantially from family to family, and even within families as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely 'sign-changing s-wave' (s?) symmetry. We review theoretical aspects, materials properties and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.Satisfactoriness has to be measured by a multitude of standards, of which some, for aught we know, may fail in any given case; and what is more satisfactory than any alternative in sight, may to the end be a sum of pluses and minuses, concerning which we can only trust that by ulterior corrections and improvements a maximum of the one and a minimum of the other may some day be approached.??????????????????????William James, Meaning of Truth

773 citations

Journal ArticleDOI
Abstract: The recently discovered Fe-pnictide and chalcogenide superconductors display low-temperature properties suggesting superconducting gap structures which appear to vary substantially from family to family, and even within families as a function of doping or pressure. We propose that this apparent nonuniversality can actually be understood by considering the predictions of spin fluctuation theory and accounting for the peculiar electronic structure of these systems, coupled with the likely 'sign-changing s-wave' (s\pm) symmetry. We review theoretical aspects, materials properties and experimental evidence relevant to this suggestion, and discuss which further measurements would be useful to settle these issues.

589 citations

Journal ArticleDOI
Peter Hirschfeld1Institutions (1)
Abstract: I review theoretical ideas and implications of experiments for the gap structure and symmetry of the Fe-based superconductors. Unlike any other class of unconventional superconductors, one has in these systems the possibility to tune the interactions by small changes in pressure, doping or disorder. Thus, measurements of order parameter evolution with these parameters should enable a deeper understanding of the underlying interactions. I briefly review the “standard paradigm” for s-wave pairing in these systems, and then focus on developments in the past several years which have challenged this picture. I further discuss the reasons for the apparent close competition between pairing in s- and d-wave channels, particularly in those systems where one type of Fermi surface pocket – hole or electron – is missing. Observation of a transition between s- and d-wave symmetry, possibly via a time reversal symmetry breaking “s + id” state, would provide an important confirmation of these ideas. Several proposals for detecting these novel phases are discussed, including the appearance of order parameter collective modes in Raman and optical conductivities. Transitions between two different types of s-wave states, involving various combinations of signs on Fermi surface pockets, can also proceed through a T -breaking “s + is” state. I discuss recent work that suggests pairing may take place away from the Fermi level over a surprisingly large energy range, as well as the effect of glide plane symmetry of the Fe-based systems on the superconductivity, including various exotic, time and translational invariance breaking pair states that have been proposed. Finally, I address disorder issues, and the various ways systematic introduction of disorder can (and cannot) be used to extract information on gap symmetry and structure.

150 citations

##### References
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Journal ArticleDOI
TL;DR: It is reported that a layered iron-based compound LaOFeAs undergoes superconducting transition under doping with F- ions at the O2- site and exhibits a trapezoid shape dependence on the F- content.
Abstract: We report that a layered iron-based compound LaOFeAs undergoes superconducting transition under doping with F- ions at the O2- site. The transition temperature (Tc) exhibits a trapezoid shape dependence on the F- content, with the highest Tc of ∼26 K at ∼11 atom %.

6,260 citations

Journal ArticleDOI
TL;DR: A phenomenological model of a system of antiferromagnetically correlated spins is shown to give a good quantitative description of NMR, nuclear-quadrupole-resonance, and Knight-shift measurements on yttrium, planar copper, and planar oxygen sites in YBa{sub 2}Cu{sub 3}O{sub 7}.
Abstract: A phenomenological model of a system of antiferromagnetically correlated spins is shown to give a good quantitative description of NMR, nuclear-quadrupole-resonance, and Knight-shift measurements on yttrium, planar copper, and planar oxygen sites in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. The antiferromagnetic correlation length is estimated to be \ensuremath{\sim}2.5 lattice constants at T=100 K. The temperature dependence of the correlation length ceases at ${\mathit{T}}_{\mathit{x}}$\ensuremath{\simeq}100 K. The enhancement of the observed relaxation rates over what is expected for weakly interacting electrons is calculated and shown to be large. Extension of the calculation to other cuprate superconductors is discussed.

654 citations

Journal ArticleDOI
Joshua Tapp1, Zhongjia Tang1, Bing Lv1, Kalyan Sasmal1  +5 moreInstitutions (3)
Abstract: The synthesis and properties of LiFeAs, a high-${T}_{c}$ Fe-based superconducting stoichiometric compound, are reported. Single crystal x-ray studies reveal that it crystallizes in the tetragonal PbFCl type (P4/nmm) with $a=3.7914(7)\text{ }\text{\AA{}}$ and $c=6.364(2)\text{ }\text{\AA{}}$. Unlike the known isoelectronic undoped intrinsic FeAs compounds, LiFeAs does not show any spin-density wave behavior but exhibits superconductivity at ambient pressures without chemical doping. It exhibits a respectable transition temperature of ${T}_{c}=18\text{ }\text{K}$ with electronlike carriers and a very high critical field, ${\text{H}}_{c2}(0)g80\text{ }\text{T}$. LiFeAs appears to be the chemical equivalent of the infinite layered compound of the high-${T}_{c}$ cuprates.

628 citations

Journal ArticleDOI
Guy Deutscher1, K. A. Müller2Institutions (2)
TL;DR: The short coherence length of high-${\mathrm{T}$ oxides is shown to induce considerable weakening of the pair potential at surfaces and interfaces, and it is argued that this effect is responsible for the existence of internal Josephson junctions at twin boundaries.
Abstract: The short coherence length of high-${\mathrm{T}}_{\mathrm{c}}$ oxides is shown to induce considerable weakening of the pair potential at surfaces and interfaces. It is argued that this effect is responsible for the existence of internal Josephson junctions at twin boundaries, which are at the origin of the superconductive glassy state, as well as for gapless tunneling characteristics.

516 citations

Journal ArticleDOI
Hubertus Luetkens1, Hans-Henning Klauss2, M. Kraken3, F. J. Litterst3  +16 moreInstitutions (5)
Abstract: The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mossbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Neel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

280 citations

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