Spectroscopic scanning tunneling microscopy insights into Fe-based superconductors

8.4 MB·English

by  Jennifer E. Hoffman

Spectroscopic STM insights into Fe-based superconductors
Jennifer E. Hoffman1
1Department of Physics, Harvard University, Cambridge, MA 02138, U. S. A.
InthefirstthreeyearssincethediscoveryofFe-basedhighT superconductors,scanningtunneling
c
microscopy (STM) and spectroscopy have shed light on three important questions. First, STM
has demonstrated the complexity of the pairing symmetry in Fe-based materials. Phase-sensitive
quasiparticle interference (QPI) imaging and low temperature spectroscopy have shown that the
pairing OP varies from nodal to nodeless s± within a single family, FeTe Se . Second, STM has
1−x x
imaged C4 → C2 symmetry breaking in the electronic states of both parent and superconducting
materials. Asalocalprobe,STMisinastrongpositiontounderstandtheinteractionsbetweenthese
2 broken symmetry states and superconductivity. Finally, STM has been used to image the vortex
1 state, giving insights into the technical problem of vortex pinning, and the fundamental problem
0 ofthecompetingstatesintroducedwhensuperconductivityislocallyquenchedbyamagneticfield.
2 Here we give a pedagogical introduction to STM and QPI imaging, discuss the specific challenges
associatedwithextractingbulkpropertiesfromthestudyofsurfaces,andreportonprogressmade
n
in understanding Fe-based superconductors using STM techniques.
a
J
PACSnumbers: 68.37.Ef,74.25.Jb
6

]
n CONTENTS I. INTRODUCTION
o
c
I. Introduction 1 The 2008 discovery of high-T superconductivity in
- c
r Fe-based materials marked an exciting turning point in
p
u II. Scanning Tunneling Microscopy 2 the study of unconventional superconductivity[1]. Until
s A. Tunneling Current 2 then, cuprates were an anomalous island, hosting a zoo
. of confusing properties, with little outside perspective
t B. Topography 3
a to determine which of these properties deserved deeper
m C. Spectroscopy 3
theoretical and experimental attention. Furthermore,
D. QPI 4
- the cuprates’ brittle material properties, gross electronic
d
anisotropy,andpoornormalstateelectricalconductivity
n III. Fe-based Superconductor Overview 6
made them challenging to incorporate into widespread
o
c technology. The Fe-based superconductors provided a
IV. Surface Considerations 7
[ foil for comparison and a fresh start for all involved in
A. 1111 Materials 7 the cuprate quagmire. Furthermore, their relative mal-
1
v B. 122 Materials 8 leability, isotropy, and metallic normal state led to high
0 C. 11 Materials 15 hopes for useful applications. These considerations have
8 D. 111 Materials 16 launched a whirlwind of research. Three years into this
3 E. Other distant cousins 16 new exploration, we review their properties, focusing on
1
scanning tunneling microscopy (STM) experiments.
.
1 V. Superconducting Order Parameter 16 STM has proven to be an ideal tool to study cor-
0 A. Gap Symmetry 17 related electron materials. These materials are prone
2
B. Gap magnitude 19 to nanoscale inhomogeneities whose effects may broaden
1
spectral features or transitions measured by bulk tech-
: C. Two Gaps 20
v niques. STM has been applied with great success to
D. Gap Inhomogeneity 22
i cuprate superconductors, addressing pairing symmetry,
X
gap inhomogeneity, dopant placement, vortex pinning,
r VI. Parent Compound 22
a and competing phases [2]. In recent years, STM has
A. C4 → C2 symmetry breaking 22
also been used to gain insight into momentum space, via
B. Pseudogap 26
quasiparticle interference (QPI) imaging. QPI imaging
can even provide a phase-sensitive determination of the
VII. Vortex State 28 superconducting order parameter (OP).
A. Vortex Pinning 28 An early review of STM of Fe-based superconductors
B. Vortex Core States 28 was written by Yin et al[3]. More recent, comprehen-
sive reviews of the thousands of papers to date on Fe-
VIII. Conclusions & Future 30 basedsuperconductorshavebeenwrittenbyJohnston[4]
and Stewart[5]. These longer reviews include short sum-
References 31 maries of STM results. Here we give a more thorough
2
ε
z feedback
I. eV< ε
-x +x
+y eV ε
F
I (tip)
Iset II. 0< ε< eV
ε
F 0
(sample)
II
meas
III. ε< 0
e-
V
sample
sample vacuum tip
DOS barrier DOS
FIG. 1. Schematic of a scanning tunneling microscope. A
voltageisappliedbetweentheconductingsamplesurfaceand FIG. 2. Schematic of tip-sample tunneling. Energy is along
asharpmetallictip,leadingtoameasurabletunnelingcurrent the vertical axis, and DOS of the sample and tip are shown
whose magnitude decays exponentially with tip-sample sepa- along the horizontal axes. Filled states are shown in green.
ration. Intopographicmode,themeasuredtunnelingcurrent In this case, a positive bias voltage V has been applied to
I iscomparedtothesetpointcurrentI ,andthediffer- the sample, which effectively lowers its Fermi level by energy
meas set
ence(theerrorsignal)isfedbacktothezpiezotocontrolthe eV with respect to the Fermi level of the tip. This allows for
tip height. filled states from the tip (right) to tunnel into empty states
inthesample(left). Thetunnelingcurrentismeasuredbyan
external circuit.
review including a pedagogical introduction to STM, a
discussion of surface configurations, and several signifi- A. Tunneling Current
cant new results.
WestartwithanintroductiontoSTMinsectionII,in- When a positive voltage V is applied to the sample,
cluding a pedagogical explanation of QPI. In section III with respect to a grounded tip, the Fermi level of the
we give a brief overview of Fe-based superconductivity. sample is lowered with respect to that of the tip. Elec-
In section IV, we discuss the crystal structure and sur- trons will flow primarily from the filled states of the tip
face characterization of the several families of Fe-based into the empty states of the sample, as illustrated in fig-
superconductors. In section V we report on measure- ure 2. The elastic tunneling current from the tip to the
mentsofthesuperconductingOP,focusingonphasesen- sample, for states of energy ε with respect to the Fermi
sitive measurements via QPI imaging. In section VI we level ε ≡0 of the sample, is given by
F
discuss the parent compound and competing electronic
orders. InsectionVII,wediscussthevortexstate. Insec-
2π
tion VIII, we conclude with suggestions for future STM I (ε)=−2e· |M|2·
t→s (cid:126)
experimentsthatcouldshedadditionallightonthesema-
ρ (ε−eV)f(ε−eV) · ρ (ε)[1−f(ε)] (1)
terials. t s
(cid:124) (cid:123)(cid:122) (cid:125) (cid:124) (cid:123)(cid:122) (cid:125)
#filledtipstates #emptysamplestates
fortunnelingfrom fortunnelingto
where there is a factor of 2 for spin, −e is the electron
II. SCANNING TUNNELING MICROSCOPY charge,|M|2 isthematrixelementforthetunnelingbar-
rier, ρ (ε) is the density of states (DOS) of the sample,
s
ρ (ε) is the DOS of the tip, and f(ε) is the Fermi distri-
A scanning tunneling microscope consists of a sharp t
metallictipwhichisrasteredseveral˚Angstromsabovean bution,
electricallyconductingsamplesurface[6]. Thepositionof
the tip can be varied with sub-˚A precision by means of 1
a piezoelectric scanner. When a voltage V is applied be- f(ε)= . (2)
1+eε/kBT
tween tip and sample, a current will flow. This current
can be measured as a function of (x,y) location and as Though the dominant flow of electrons for positive
a function of V. The microscope is illustrated schemat- sample voltage V will be from tip to sample, there will
ically in figure 1. There are several excellent textbooks also be a smaller flow of electrons from sample to tip,
about STM[7–9]. given by
3
The matrix element |M|2 can be calculated for an
s-wave tip[11]. The tunneling probability through the
2π
I (ε)=−2e· |M|2· vacuum barrier is approximated by |M|2 = e−2γ with
s→t (cid:126) γ = d(cid:112)2mϕ/(cid:126)2, where m is the mass of the electron, d
ρ (ε)f(ε) · ρ (ε−eV)[1−f(ε−eV)]. (3)
s t isthewidthofthebarrier(tip-sampleseparation),andϕ
(cid:124) (cid:123)(cid:122) (cid:125) (cid:124) (cid:123)(cid:122) (cid:125)
is the height of the barrier, which is some mixture of the
#filledsamplestates #emptytipstates
fortunnelingfrom fortunnelingto workfunctionsofthetipandsample,typically∼3−5eV
for metals (see table I). In summary, the tunneling cur-
Summing these counter-propagating currents, and inte-
rent is fairly well approximated by
grating over all energies ε, gives a net tunneling current
from tip to sample
4πe −d(cid:113)8mϕ (cid:90) eV
4πe(cid:90) ∞ It→s ≈− (cid:126) e (cid:126)2 ρt(0) 0 ρs(ε)dε. (8)
I =− |M|2ρ (ε)ρ (ε−eV)·
t→s (cid:126) s t
−∞
[f(ε−eV)−f(ε)]dε. (4)
Element ϕ(eV) Element ϕ(eV) Element ϕ(eV)
This expression can be simplified at low temperature, Se 5.9 W 4.32-5.22 As 3.75
wheretheFermifunctioncutsoffverysharplyatε . (For Pt 5.12-5.93 Te 4.95 Ca 2.87
F
example, at T=4.2K, the cutoff width is ∼2k T = 0.72 Ir 5.42-5.76 Fe 4.67-4.81 Sr 2.59
B
meV, which can be compared to a typical superconduct- Au 5.31-5.47 Bi 4.34 Ba 2.52
inggapinFe-basedsuperconductorsof∼5meV.)Inthe Co 5.0 Pb 4.25 K 2.29
approximationofaperfectlyabruptcutoff,theintegrand
isnegligibleexceptintherange0<ε<eV. (Likewise,a TABLE I. Work functions of several relevant elements.
Ranges correspond to different crystalline orientations[12].
negative bias voltage −|V| applied to the sample, would
give an integration range of −|eV| < ε < 0.) The tun-
neling current for positive V is therefore
4πe(cid:90) eV B. Topography
I ≈− |M|2ρ (ε)ρ (ε−eV)dε. (5)
t→s (cid:126) s t
0
In topographic mode (illustrated in figure 1), the tip-
In reality, equation 5 will be modified by an apparent sample bias voltage is fixed, and a feedback loop holds
smearing of energy features with width ∼ 4kBT (com- the tunneling current constant by varying the tip height
posed of ∼ 2kBT for the sample and ∼ 2kBT for the while it is scanned over the sample surface. Therefore,
tip). the tip follows a contour of constant integrated DOS
A second simplification comes from the choice of a tip (DOS). In the case of a homogeneous metal, the contour
materialwithafeaturelessDOSnearεF. Typicalchoices of constant DOS corresponds to the geometric topogra-
are W, Pt, or PtIr. If the tip DOS is flat in the energy phy of the sample surface. However, if the local DOS
range of interest, then ρt(ε + eV) can be treated as a varies spatially, the resulting image contains a mixture
constant and taken outside the integral, giving of DOS and true topographic information. When the
tip-sample bias voltage can be set far from the energy
rangeofspatiallyinhomogeneousstates,thecontribution
4πe (cid:90) eV
I ≈− ρ (0) |M|2ρ (ε)dε. (6) ofthegeometrydominatesthetopographicimage,asde-
t→s (cid:126) t s
sired. The recorded tip height is linear in the geometric
0
topography, but logarithmic in the integrated DOS.
ThefinalsimplificationisduetoBardeen’sdemonstra-
tion that under several realistic assumptions, the matrix
elementfortunnelingwillbevirtuallyindependentofthe
C. Spectroscopy
energydifferencebetweenthetwosidesofthebarrier[10].
In particular, the matrix element will remain unchanged
even if one side transitions from the normal state to the In addition to revealing the geometrical surface struc-
superconducting state. To a reasonable approximation, tureofasample,STMcanalsomeasurethesampleDOS
the matrix element can therefore be taken outside the as a function of energy, up to several eV from the Fermi
integral, giving level in both occupied and unoccupied states (where the
upperboundisseteitherbycatastrophicsurfacedestruc-
tion at high fields, or by breakdown of the tunneling ap-
4πe (cid:90) eV proximation). Thisisaccomplishedbysweepingthebias
I ≈− |M|2ρ (0) ρ (ε)dε. (7)
t→s (cid:126) t s voltage V and measuring the tunneling current I while
0
4
maintaining constant tip-sample separation d. By differ- ky ky
entiating I(V), the conductance dI/dV is found to be (a) (b) q
proportional to the sample DOS.
(cid:40) (cid:41) q k2 k1
dI 4πe −d(cid:113)8mϕ d (cid:90) eV
dV =− (cid:126) e (cid:126)2 ρt(0)dV ρs(ε)dε k2 k1 kx kx
0
4πe2 −d(cid:113)8mϕ
=− (cid:126) e (cid:126)2 ρt(0)ρs(eV) (9)
To reduce noise, a lock-in technique is typically used: a
(cid:22)
samgea,llanAdCtmheodreuslualttiaonntitsusnunmelminegdcwuirtrhenttheisDdCemboidasulvaoteltd- (c) (cid:2)(cid:3) (cid:5)(cid:6) =cos( (cid:14)(cid:4)(cid:15)(cid:8)(cid:14)(cid:8) ∙(cid:18)(cid:6)) |(cid:2)(cid:3) (cid:5)(cid:6) (cid:21) =(cid:23)(cid:22)+(cid:23)(cid:22)cos((cid:25)(cid:6)∙(cid:5)(cid:6))
to yield dI/dV. The dI/dV spectrum is a good mea-
|(cid:2) (cid:5)(cid:6) |,
sure of the sample DOS up to an overall constant which (cid:3)(cid:4) (cid:5)(cid:6) (cid:5)(cid:6)
depends on the tip-sample bias voltage and current set- |(cid:2)(cid:3)(cid:8) (cid:5)(cid:6) | |(cid:2) (cid:5)(cid:6) (cid:21)(cid:22)=(cid:23)−(cid:23)cos((cid:25)(cid:6)∙(cid:5)(cid:6))
points. If these dI/dV spectra are recorded at a dense (cid:2)(cid:3) (cid:5)(cid:6) =sin((cid:14)(cid:4)(cid:15)(cid:8)(cid:14)(cid:8) ∙(cid:18)(cid:6)) (cid:3) (cid:22) (cid:22)
array of locations in real space, spatial variation in the (cid:5)(cid:6)
sample DOS can be extracted. (cid:5)(cid:6) (cid:5)(cid:6)
FIG. 3. Schematic of QPI. (a-b) BZs for two hypotheti-
calmaterialsshowthatlargeJDOScanariseintwodifferent
D. QPI
ways. The thin blue lines represent CCEs. (a) Nested re-
gions,whichwillgivelargeJDOS,areemphasizedbythicker
When materials are inhomogeneous and electrons bluelines. (b)Regionsofshallowdispersion,wheretheCCEs
are highly correlated, a full understanding of their are far apart, will also give large JDOS. Examples of these
properties requires knowledge of both the real space and so-called‘hotspots’arenotedwithbluedots. Inboth(a)and
momentum space behavior of quasiparticles. QPI (QPI) (b),pairsofeigenstates(cid:126)k1 and(cid:126)k2 arenestedwithwavevector
imaging provides some of this complementary knowl- (cid:126)q=(cid:126)k1−(cid:126)k2. (c)Schematicshowinghowtwodifferentspatially
edge. When quasiparticles scatter off defects or other homogeneousBlochstates(left)canmixtoformmodulations
structureswithinthecrystal,energy-dependentstanding of the wavefunction (middle) and the LDOS (right). For
superconducting Bogoliubov quasiparticles, the filled states
waves form. (The special case of standing waves at
(bottomright)andemptystates(topright)oftheLDOSare
the Fermi energy are known as Friedel oscillations[13].)
spatially out of phase with each other[28].
The resulting interference patterns in the quasiparticle
DOS can be imaged with scanning tunneling microscopy
and spectroscopy[14–17]. The Fourier transform of the
real-space interference patterns highlights the dominant
Substitution of a Bloch wavefunction into equation 10
sets of quasiparticle momenta[18–23]. This combination
shows that LDOS(E,(cid:126)r) does not carry any directly ob-
of imaging and analysis is often called Fourier-transform servable spatial modulation at wavevector(cid:126)k.
scanning tunneling spectroscopy (FT-STS). FT-STS is
Spatial structures such as impurities, crystal defects,
a powerful technique because it simultaneously yields
magnetic vortices, or static or fluctuating spatial orders
energy-dependent real-space and momentum-space in-
cause elastic scattering which mixes eigenstates of dif-
formation on the quasiparticle wavefunctions, scattering
ferent (cid:126)k but the same ε((cid:126)k). A full treatment of this
processes, and coherence factors. This information can
mixing was carried out using a Green’s function ap-
be used to distinguish between candidate superconduct-
proachforanormalmetalinthepresenceofaweaklocal
ing OPs[24–27].
potential[30]. A simpler picture can be explained as fol-
lows: elastic scattering mixes states that are located on
Metals In an ideal metal, the Landau quasiparticle
thesamequasiparticlecontourofconstantenergy(CCE)
eigenstates are Bloch wavefunctions characterized by
wavevector (cid:126)k and energy ε. Their dispersion relation, in(cid:126)k-space. For example, the blue lines in figures 3a and
ε((cid:126)k), can be measured with momentum resolved tech- b show CCEs for two different hypothetical materials.
When scattering mixes states k(cid:126) and k(cid:126) , the result is a
niquessuchasangleresolvedphotoemissionspectroscopy 1 2
standing wave in the quasiparticle wavefunction ψ of
(ARPES)[29]. By contrast, real space imaging tech- k
niques, such as STM, cannot directly measure ε((cid:126)k). The wavevector (k(cid:126)1 −k(cid:126)2)/2. Since LDOS is proportional to
|ψ |2,theLDOSwillcontainaninterferencepatternwith
local DOS, LDOS(E,(cid:126)r), is related to the (cid:126)k-space eigen- k
wavevector (cid:126)q = k(cid:126) −k(cid:126) , as sketched in figure 3c. LDOS
states ψ ((cid:126)r) by 1 2
k modulations of wavelength λ=2π/q can be observed by
STM as spatial modulations of the differential tunneling
LDOS(E,(cid:126)r)∝(cid:88)|ψ ((cid:126)r)|2δ(E−ε((cid:126)k)) (10) conductance dI/dV.
k
Although the full Green’s function treatment includes
k
5
additionalterms,theexperimentallyobservedamplitude 1.0
(a) (b)
of these modulations can be simply understood from (cid:10)(cid:15) (cid:10)(cid:14) +1
Fermi’s golden rule: 0.5
(cid:8) (cid:10)(cid:16) (cid:10)(cid:13)
(cid:7)
(cid:6)
w(i→f)∝ 2(cid:126)π|V((cid:126)q)|2ni(Ei,(cid:126)ki)nf(Ef,(cid:126)kf) (11) (cid:1)(cid:3)(cid:4)/(cid:9) 0 (cid:10)(cid:17) (cid:10)(cid:12) (c)
-0.5
where E = E for elastic scattering, (cid:126)q =(cid:126)k −(cid:126)k , V((cid:126)q)
i f f i (cid:10) -1
is the Fourier component of the scattering potential at (cid:11)
-1.0
wavevector(cid:126)q,andni andnf arethedensityoftheinitial -1.0 -0.5 0 0.5 1.0
and final states. (cid:1)(cid:2)(cid:3)(cid:4)/(cid:6)(cid:7)(cid:8)
It is apparent from equation 11 that a particular 0.2 2.0 Low High Dec. Inc.
wavevector (cid:126)q can dominate the QPI at energy E, if (d) 0 T (e) 0 T (f) 11T –0T
the CCE contains a large joint DOS (JDOS) of (cid:126)k-pairs (cid:10)(cid:15)(cid:10)(cid:14) (cid:10)(cid:13)(cid:10)(cid:12) (cid:10)(cid:15)(cid:10)(cid:14) (cid:10)(cid:13)(cid:10)(cid:12)
connected by that (cid:126)q. Large JDOS for a particular (cid:126)q can (cid:10)(cid:16) (cid:10)(cid:11) (cid:10)(cid:16) (cid:10)(cid:11)
arise from nested regions where two equal-energy con- (cid:10)(cid:17) (cid:10)(cid:17)
tours are roughly parallel and separated by (cid:126)q, as shown
in figure 3a, or from large flat regions in k-space, where
DOS(E)∝1/|(cid:53) (E)| is large, i.e. the CCE are farther 10 nm
k
apart,asshowninfigure3b. Theserelativelyflatregions
of k-space are often called ‘hotspots’. (The extreme case FIG. 4. Determination of the cuprate d-wave OP via QPI
of shallow dispersion is a van Hove singularity (vHS). imaging. (a) Schematic of the cuprate BZ. The normal state
A vHS has large JDOS with all other (cid:126)k-states, thus FS is shown in red. As the material becomes superconduct-
ing, anodalgapopens, resultinginthebluecontoursofcon-
it will give rise to a FT-STS image which mimics the
stant energy (CCEs). The DOS is largest in the flat regions
original band structure[31].) The simple JDOS picture
at the end of the banana-shaped CCEs. The QPI signal is
can be experimentally verified for a given material by dominated by the 7 (cid:126)q ’s connecting this octet of high-DOS
j
a careful comparison between the autocorrelation of regions. (b-c)Occupationfactorsu andv forad-wavegap,
k k
the ARPES-derived band structure (i.e. the JDOS), mappedinthesameBZasin(a). (d)Z((cid:126)r,E =4.4meV)im-
and the Fourier transform of the STM-derived QPI age of Ca2−xNaxCuO2Cl2 at zero field and T=1.6K (setup:
modulations[32–35]. Vsample = −100mV;Iset = 100pA). (e) FT-STS image: 8-
fold-symmetrizedFouriertransformof(d),showingthe7(cid:126)q ’s
j
expected from the connections between the 8 ‘hotspots’ in
Superconductors The Bogoliubov quasiparticles in a
(a). (f) The difference between high-field and zero-field QPI:
superconductorarealsoBlochstates,butwithdispersion
Z((cid:126)q,E,B = 11 T)−Z((cid:126)q,E,B = 0 T) for E=4.4 meV. This
figure is taken from Ref. 26.
(cid:113)
E ((cid:126)k)=± ε((cid:126)k)2+|∆((cid:126)k)|2 (12)
±
where|∆((cid:126)k)|isthe(cid:126)k-dependentmagnitudeofthesuper- Therefore, in a superconductor, a large JDOS will result
conducting energy gap at the Fermi surface (FS). (The in large QPI signal only when the additional constraint
FS is the CCE for ε((cid:126)k) = 0 in the normal state.) Elas- is satisfied that the coherence factor is non-zero.
ticscatteringofBogoliubovquasiparticlescanalsoresult Hanaguri[28] has also pointed out that for supercon-
in conductance modulations, which have been treated in ductors,theQPIdispersionsqj(E)areparticle-holesym-
detail by several authors[30, 36–42]. In a superconduc- metric, i.e. qj(E) = qj(−E) where qj represents a dom-
tor there is an additional complication from coherence inant scattering wavevector as exemplified in figure 3a
factors[36, 38, 40–42]. In the simple picture, Fermi’s and b, and E is measured with respect to εF. However,
golden rule is augmented as foragivenqj,thereal-spaceconductancemodulationsat
+E and −E are spatially exactly out of phase with each
other, as shown in the right panel of figure 3c. The QPI
2π
w(i→f)∝ |u u∗ ±v v∗ |2|V((cid:126)q)|2n (E ,(cid:126)k )n (E ,(cid:126)k ) signal can therefore be enhanced relative to the back-
(cid:126) ki kf ki kf i i i f f f
ground by computing the ratio map
(13)
where the plus sign is for magnetic scatterers, the minus dI((cid:126)r,E)
signisfornon-magneticscatterers,and|u |2and|v |2are Z((cid:126)r,E)≡ dV . (14)
k k dI((cid:126)r,−E)
the probabilities that a pair of states with wavevectors dV
±(cid:126)k isemptyorfilled,respectively[43]. TheseBogoliubov The ratio also eliminates possible spurious effects from
coefficients uk and vk are given by the tunneling matrix element[28].
(cid:115)
(cid:18) (cid:19)
∆(k) 1 ε(k) (cid:112) Example: Cuprate Superconductors Although the
u = 1+ ; v = 1−|u |2
k |∆(k)| 2 E(k) k k pairing symmetry of cuprate superconductors has long
6
been known[44, 45], a recent STM study on supercon- (a) (b) (c) (d)
ducting Ca2−xNaxCuO2Cl2 (x ∼0.14 and Tc ∼28K) O/F As Fe
demonstratedtheuseofQPIimagingasaphase-sensitive Fe
As
probe of a superconducting OP[26]. Here we will walk
throughthisexampleaspreparationtobetterunderstand Fe
thedeterminationoftheFeSeOPusingQPIimagingdis- As
cussed later in section V. Fe
In optimally doped cuprates, there is a single Cu d
band crossing the Fermi level, resulting in a single large
hole pocket centered at the M point of the single-CuO -
2 FIG.5. Structuresof4familiesofFe-basedsuperconductors.
plaquette Brillouin zone (BZ). The normal state FS is
(a)1111(specifically,LaFeAsO F ),fromRef.47. (b)122
shown schematically as a red line in figure 4a. Below 1−x x
(specifically, BaFe As ), from Ref. 48. (c) 111 (specifically,
2 2
T , a gap opens with four nodal points, resulting in the
c LiFeAs),fromRef.49. (d)11(specifically,FeTe),fromRef.50
banana-shaped CCEs shown schematically in blue in fig-
ure 4a. The JDOS is therefore dominated by the 7 in-
equivalent q-vectors connecting the octet of flat regions
at the ends of the ‘bananas’[21, 36, 46]. The measured diagrams representative of three different families of Fe-
FT-STS image in figure 4e indeed shows all 7 of these based superconductors are shown in figure 6.
expected dominant (cid:126)q ’s.
j
The dominant superconducting OP in cuprates is d- 200(a)SmFeAsO F (b)
1-x x 160 LaFeAsO F
awgaovnea,lws,itahstdheenogtaepd∆bky cthhaenwgihnigtesiagnndaglornayg tshheadBinZgdiin- T (K) TPetMra. 112400 1-x x
figure4a. Thesignchangein∆k resultsinsign-changing 100 TPeMtra. 100 Tetra. (cid:1)(cid:2)from XRD
Bogoliubov coefficients u and v , shown in figure 4b- 80 SDW (cid:1)(cid:2)from (cid:3)(cid:1)
c. The 7 (cid:126)qj’s therefore hkave diffekrent coherence factors OArFthFoe. 4600 (cid:1)(cid:1)(cid:6)(cid:4)ffrroomm (cid:5)(cid:5)SSRR
|wuhkiicuh∗kft±ypvekiovfk∗fs|c2a.ttTehrienqgudeostmioinnaitse,sw,iatnhdintahgeriveefonresawmhpilceh, 00.00 0.0A2FSm0.04 0.06SC0.08 0.2 2000 0.04Supe0r.c0o8ndu0c.t1iv2ity 0.16 0.20
Nominal F content(cid:7) Nominal F content(cid:7)
signapplieswithinthecoherencefactor? Forweakscalar
150
potential scattering, the coherence factor is suppressed (c) (d)
Ba(Fe Co)As
for (cid:126)q ’s that preserve the sign of ∆ , namely q , q , and 1-x x2 2
j k 1 4 y = 0.01
qe∆n5.c,eFnfoaarcmtsoeclraytitsqesr,uinqpgp,rfeqrsos,meqdm.foaWrgn(cid:126)qhjee’tsnictahimamtpacughrnaitenitegisce,fittheheledsciiogshnaepor--f T (K)100 (cid:1)(cid:2) (cid:1)(cid:2) Tet Torth
k 2 3 6 7 Orth
plied, the pinned vortices serve as an additional form of 50 Orth
gap-sign-preserving scattering[41], thus suppressing q , AFM (cid:1)
2 (cid:6)
q , q , q while preserving or enhancing q , q , and q . SC
3 6 7 1 4 5 0
This effect is observed experimentally in figure 4f. The 0 0.020.040.060.080.100.12
Co content(cid:7)(from WDS)
phase of the superconducting gap in cuprates is thus re-
vealed by the evolution of the QPI upon application of FIG. 6. Phase diagrams of several Fe-based super-
the magnetic field. conductors, representative of the 1111, 122, and 11 fam-
ilies. (a) SmFeAsO F [51]; (b) LaFeAsO F [52];
1−x x 1−x x
(c) Ba(Fe Co ) As [53]; (d) FeTe Se [54]. The or-
1−x x 2 2 1−x x
thorhombictransitionmarkedin(d)comesfromx-raydiffrac-
III. FE-BASED SUPERCONDUCTOR
tion measurements on a powder sample with stoichiometry
OVERVIEW
Fe Se [55]. In each diagram, stars mark the phase space
1.01
locations of STM experiments to be discussed later.
The common feature of all known Fe-based supercon-
ductorsisanFeXsquarelattice,whereXmaybeapnic-
togen (As or P), or a chalcogen (Te, Se, or S). Between There are a number of points of confusion when con-
these FeX layers there may be an additional oxide layer sidering the real space unit cell or the BZ (BZ) of these
(‘1111’ materials), ionic layer (‘122’ or ‘111’ materials), materials. The common FeX layer in all of these materi-
more complex intermediaries (e.g., ‘42622’ materials), or als (X=As,P,Se,Te,S) should be more accurately written
nothing (‘11’ materials). Representative crystal struc- as Fe X , because half of the X lie above the Fe plane,
2 2
tures are shown in figure 5. while half lie below, as can be seen in figure 5. In the
Fe-based superconductors, like cuprates, exhibit a high temperature state, the material is tetragonal, with
dome-shaped phase diagram with an antiferromagnetic aunitcelltwiceaslarge(dashedsquareinfigure7a)and
(AF) parent compound. Additionally, they undergo a BZhalfaslarge(figure7d)asmightbenaivelyexpected
structural phase transition which is often but not always from the Fe square sublattice (solid square in figure 7a,
simultaneous with the magnetic phase transition. Phase and BZ in figure 7c). In the underdoped region of the
7
phase diagram, the material undergoes a tetragonal to (a) (b)
orthorhombicphasetransitionasthetemperatureislow-
b
O
ered. Then the unit cell doubles again (figure 7b), and
the BZ halves again (figure 7e). However, the difference a
a O
T
between the new a and b axes in the orthorhombic
O O a
state is typically < 1% (e.g. see tables A8 and A9 in Fe-Fe
(c)M Y M (d) (e)2(cid:2) 2(cid:2)
Ref. 4) which is below the calibration resolution of the electron β1, β2 α1, α2, γ (cid:5)(cid:4) (cid:3)(cid:4)
paOiezaondscbaOntuaxbeessctaynpnicoatllbyeudsiesdtinbgyuiSshTeMd.bTyhSeTreMforuenltehses 2(cid:2) Γ α1α2 Xβ1 eΓh YX M
a twin boundary between orthorhombic domains can be (cid:3) M
found[56, 57]. (cid:7)(cid:8)(cid:9)(cid:7)(cid:8) hole γ 2(cid:2) 2(cid:2) hole
2(cid:2) β2 (cid:3)(cid:6) (cid:3)(cid:6) electron
There is an additional source of confusion regarding (cid:3)(cid:7)(cid:8)(cid:9)(cid:7)(cid:8)
the 122 materials. The 1111, 111, and 11 materials have
FIG. 7. Real space and momentum space unit cell of Fe-
only a single FeX layer per unit cell. However, the 122
basedsuperconductors. (a)Fe X lattice,wherethereddots
materials have a double layer along the c-axis, so their 2 2
representFeatoms,andthebluefilled(open)circlesrepresent
unit cell is technically body-centered tetragonal, with a
X=As,P,Se,Te,orSatomsabove(below)theFeplane. The
BZ that is rotated by 45◦ from the tetragonal BZ of the
solidsquareshowsaunitcellwithonlyoneFe;becauseofthe
other Fe-based families. In this review, since STM im- two inequivalet X sites above and below the Fe plane, this
ages only a single layer at a time, we will ignore this is not the true unit cell of the crystal. The dashed square
complicationandwewilltalkaboutthe122unitcelland shows the unit cell of the tetragonal structure typically seen
BZ in the same tetragonal/orthorhombic notation as is at high temperatures. (b) The a and b axes of the low T
used for the other Fe-based superconductors. orthorhombic unit cell (black rectangle) typically differ by
<1%; the difference is exaggerated in this figure for visual
Fe-based superconductors are more complicated than clarity. (c) Unfolded BZ of the unit cell containing only a
their cuprate cousins because all five Fe d-bands cross single Fe atom. Band structure calculations typically predict
theFS(FS),incontrasttothesingleCudbandcrossing as many as three inequivalent hole FSs (blue), centered at
the FS in cuprates. The schematic bands are shown in the Γ point, and 2 inequivalent electron FSs (red), centered
at the M points. Variations in structure and charge doping
the unfolded BZ in Fig 7c. Local density approximation
canreducethenumberofholeFSs,insomecasesleavingonly
(LDA) band structure calculation showed that Fe-based
a single α surface at the Γ point. (d) Calculated LDA FSs
materialsaresemi-metalswithholebandsattheΓpoint
in the tetragonal BZ of Ba(Fe Co ) As [59]. Note that
and electron bands at the M point, in the tetragonal BZ 0.9 0.1 2 2
in this doubled real space unit cell and halved BZ, all hole
(figure 7d)[58]. The materials may be electron-doped or
FSs from (c) have now been folded into the central Γ point,
hole-doped, butoveralargerangeofdopingthereisstill while the two electron FSs have been folded onto the same
usually significant nesting between the hole and electron M point. (e)CalculatedLDAFSintheorthorhombicBZfor
FSs. CaFe As in the AF state[60]. The real space unit cell has
2 2
doubled again, and the BZ has halved again with respect to
(d).
A. 1111 Materials
IV. SURFACE CONSIDERATIONS
Although LaFePO was found to superconduct with
Likethecuprates,theFe-basedsuperconductorshavea T ∼5K in 2006[61], the Fe-based superconductors first
c
layeredstructure. Consequently,high-qualitysinglecrys- rocketed to worldwide attention in February 2008 with
talscanbemechanicallycleavedtoobtainatomicallyflat the discovery of superconductivity up to T =26K in
c
andcleansurfacessuitableforcharacterizationbysurface LaFeAsO F [1]. These so-called ‘1111’ materials have
1−x x
sensitive probes such as STM (STM) and spectroscopy in common iron-pnictogen planes, separated by and ion-
(STS).Witheffort,samplesassmallas∼ 100µmcanbe ically bound to oxide planes. Because of the relatively
cleaved and imaged (although typically 1mm is a more high T , and the immediate superficial similarities be-
c
convenient size). Just because a surface is clean and tween LaFeAsO F and the cuprates (e.g. the lay-
1−x x
atomically flat does not necessarily mean it is a window ered structure, dome-shaped phase diagram, and sur-
into the bulk properties of the material. Surfaces may prising presence of Fe suggesting a possible spin-based
have different carrier concentration than the bulk, and superconducting mechanism), the reaction was immedi-
may exhibit electronic or structural reconstructions due ate. Research labs around the world swiftly turned their
to the reduced coordination number. In this section, we attention to the new materials. Within a few months,
will discuss the surface characteristics for the different RFeAsO had been formulated with almost a dozen rare
families of Fe-based superconductors. earth elements (R) [62–66], raising T up to 56K[67].
c
8
Large single crystals proved challenging to grow, so the introduction of F, the 2-dimensional surface FS was
studiesofintrinsicanisotropyorinhomogeneityremained seen to develop a large superconducting gap, likely via
out of reach. A few early, painstaking experiments man- theproximityeffectfromthebulksuperconductingstate.
aged to isolate single crystals as large as ∼ 100 µm for Thesurfacestatesuperconductinggapmayhavedifferent
transport[68] or ARPES[69]. However, early STM ex- magnitude and additional symmetry components, which
periments on polycrystalline samples[70, 71] likely suf- may confuse surface investigations of the bulk supercon-
fered from degraded surfaces. The ‘as-grown’ surface of ductivity.
a ∼100 µm single crystal of SmFeAsO F (with nom- Although large single crystals of LaFePO have been
1−x x
inal x=0.2, and measured T =45K, denoted by a red (cid:63) available since 2008, there have been no STM studies to
c
infigure6a)couldnotbeimagedwithatomicresolution, date, likely because of its low T , and the surface chal-
c
but did show a reproducible V-shaped gap of the ap- lenges associated with all 1111 materials.
proximate expected bulk value, that disappeared around
the bulk superconducting T [72]. SmFeAsO F spec-
c 1−x x
traareshowninfigures15aand21a,andwillbediscussed B. 122 Materials
further in section VB.
In late 2009, a breakthrough allowed the growth of In June 2008, a second family of Fe-based supercon-
large single crystals of the ‘1111’ materials using NaAs ductors was discovered, including the same FeAs plane,
flux at ambient pressure[73]. Their growth method has but only a single layer of intervening A (alkaline earth
since been replicated by other groups[74]. The first metal) ions separating the FeAs layers, giving formula
STM study on a cleaved single crystal ‘1111’ material AFe As [83]. Particular members of this ‘122’ family
2 2
LaFeAsO1−xFx (both parent compound, and nominal are referred to as Ba122, Sr122, Ca122, etc.
composition x=0.1, denoted by red (cid:63)’s in figure 6b) ap-
The 122 parent compounds become superconducting
pearedayearlater[75]. Zhouetal wereabletocleaveand
upon the introduction of hole or electron dopants, or
image atomically flat FeAs and LaO planes (figures 19,
the application of chemical or physical pressure. For ex-
a and b), neither of which showed any structural recon-
ample, BaFe As becomes a hole-doped superconductor
2 2
struction of the surface. upon replacement of Ba2+ by K+ [83], or an electron-
Although ‘1111’ single crystals may cleave beautifully, dopedsuperconductoruponreplacementofFe2+byCo3+
surface-sensitive studies are expected to be problematic or Ni4+ [84, 85], or a pressure-induced superconductor
due to the ionic nature of the interlayer bonding, which uponreplacementofAsbytheisovalentbutsmalleratom
isexpectedtoresultinapolarcatastropheatthecleaved P[86],orapplicationofphysicalpressure[87]. Theemer-
surface[76]. The surface electronic structure is likely to genceofsuperconductivitythroughchemicalsubstitution
bedifferentfromthatofthebulk. Inthebestcase,there directly into the superconducting layer[84–86] is in stark
is just a rigid band shift which affects measurements of contrastwiththecuprates,wherethesubstitutionofim-
carrier density and Fermi level [77]. In addition, atomic purity elements for even a small percentage of the Cu
reconstruction, adsorption of charged contaminants, and atoms can destroy superconductivity [88].
electronic reconstruction might occur at the surface in Because the 122 materials were successfully grown as
order to compensate for the charge imbalance [78]. In large single crystals soon after their discovery [89], they
fact, detailed calculations predict that several new sur- were the first target of serious STM study among Fe-
face bands appear in cleaved LaFeAsO1−xFx, on both based superconductors. Many papers have reported on
La-terminated and As-terminated surfaces[79]. Zhou et their surface properties, yet the structure of the cleaved
al found strong evidence for an electronic surface state surface remains controversial.
on the LaO plane: the 2-dimensional electronic standing The 122 materials are expected to cleave with FeAs
waves were so pronounced that the atomic lattice was layers intact, but A ions may end up on either of the
almost completely concealed (see figure 19b). two cleaved FeAs surfaces. If the A ions do not divide
Theexistenceofdramaticallydifferentsurfacestatesin evenly, each resulting surface will be polar, just as in the
1111 materials is further supported by ARPES measure- 1111 family. If the A ions do divide evenly, the resulting
ments of the enclosed FS area on the cleaved surfaces half-A surfaces will be nonpolar, and possibly ordered
of LaFePO [80] and LaFeAsO [81]. In both cases, the into one of several superstructures. Atomically resolved
ARPES-measured FS area falls short of the bulk elec- images of cleaved 122 surfaces typically show 1/2 of the
tron count that would be expected from Luttinger’s the- atoms which would be expected for a complete As or
orem. An even more dramatic discrepancy is found in Ba/Sr/Ca layer. The STM groups who have achieved
another ARPES experiment which sees the surface of these images are sharply divided into two camps: those
NdFeAsO F to be hole-doped, although the bulk is who believe that the observed structures result from a
0.9 0.1
knowntobeelectron-doped[69]. Afollow-upARPES[82] 1/2-layer of Ba/Sr/Ca, vs. those who believe that the
study found an additional large hole-like, Γ-centered FS observed structures result from a reconstruction of a
onLaFeAsO,whichwasdeterminedtobeasurfacestate complete As layer, i.e. all As atoms are present but 1/2
by its lack of k dispersion, and by comparison with the- of these atoms are ‘invisible’ to STM. In the following
z
ory. When the LaFeAsO was electron-doped through sections, we will present the arguments of both camps.
9
(a) (b) (c)
Ba/Sr/Ca Surface The first atomically resolved STM
images of a Fe-based superconductor were reported by
Boyer et al on hole-doped Sr K Fe As (T =32K),
1−x x 2 2 c
cleaved at T = 10K and imaged at T = 5.3K[90]. The
low temperature cleave exposed a flat surface with aver-
age atomic spacing ∼4˚A as would be expected for either 1nm 1nm 1nm
acompleteSr/KorAslayer. Theseimagesalsoshoweda (d) (e) (f)
2×1stripereconstructionat45◦totheorthorhombiclat-
tice. Patches of missing atoms showed windows through
toanunderlyingsquarelattice, alsowithatomicspacing
∼4˚A. Figure 8a shows that the underlying square lattice
is laterally shifted by half a unit cell from the topmost
2×1 surface, and lies beneath it by 2.8(4)˚A, close to the 5nm 5nm 5nm
expected 2˚A vertical distance between Sr and As layers
in the bulk. The observed atomic separation of ∼4˚A in
FIG.8. Varioustopographicimagesofcleaved122surfaces,
bothstripedregionandsquarelatticeregionarenotcon- presumedtobeBa/Sr-terminated. (a-c)5×5nm2 imagesof
sistentwiththeFe-Fedistance. Boyerconcludedthatthe cold-cleaved surfaces. (a) Sr K Fe As (T =32K) cleaved
1−x x 2 2 c
upperstripedregionwasareconstructionofthecomplete at∼10Kandimagedat5.3K.Thisisbelievedtobeanearly-
Sr/Klayer,whiletheunderlyingsquarelatticepatchwas complete Sr layer, with a glimpse through to the As layer
the bare As layer. below in one region[90] (setup: Vsample = −100mV;Iset =
200pA). (b) Ba(Fe Co ) As (T =25K) cleaved at ∼25K
Yin et al reported atomically resolved images of 0.9 0.1 2 2 c
andimagedat∼6K[91]Thissurfaceisbelievedtoshowevery
optimally electron-doped Ba(Fe Co ) As (nominal
1−x x 2 2 other row of Ba, with the intervening As just barely visi-
x=0.1, and measured T =25.3K) which showed a 2×1
c ble beneath[3] (setup: V = −20mV;I = 40pA). (c)
reconstruction[91] (see figure 8b). They found no step sample set
Ba(Fe Co ) As cleaved at 120K, imaged at 5K with
0.915 0.085 2 2
edges, but occasional 1/2-period lateral shifts. Upon in-
very small junction resistance to allow atomic resolution[94]
versetransformingthefourbrightestspotsintheFourier (setup: V =20mV;I =2nA). (d-f) 20×20 nm2 im-
sample set
transform of this topography, a pattern emerged which ages of warm-cleaved surfaces, showing greater disorder. (d)
suggested that the surface consisted of half of the Ba SrFe As , cleaved at room temperature, imaged at 40K[92].
2 2
atoms, arranged into stripes of single atom width, allow- (e) Ba(Fe0.93Co0.07)2As2 cleaved at room temperature, im-
ing a glimpse between stripes to the As layer beneath aged at 4.2K[95] (junction resistance RJ ∼ 0.75GΩ). (f)
[3]. A Ba half-layer would imply a nonpolar surface of Ba(Fe0.93Co0.07)2As2 cleaved at room temperature, imaged
at 4.2K[95] (R ∼0.75GΩ).
bulk-like carrier concentration, which is consistent with J
theobservationofaubiquitoussuperconductinggap,and
otherexpectedfeaturesofbulksuperconductivitysuchas
magnetic vortices[91].
features of up to 20˚A width, which evolved smoothly
Hsieh et al reported atomically resolved images of
without step edges from the more commonly observed
the parent SrFe As , cleaved at room temperature and
2 2 8˚A stripes. In room temperature cleaves, they saw dis-
cooled to T =40K[92]. The surface (figure 8d) was only
ordered 8˚A stripes (figure 8e), similar to those seen by
partially ordered into a similar 2×1 reconstruction as √ √
Hsieh et al. They also sometimes saw a 5.5˚A 2× 2
shown by Boyer and Yin. Upon heating to 200K and
structure, with meandering antiphase boundaries (fig-
reimaging, the long range order of the 2×1 reconstruc-
ure 8f). In total, Massee observed at least 8 different
tion was lost. Upon recooling, the surface sometimes √ √
reordered, but sometimes remained disordered[93]. This surface structures[95], although 2×1 and 2× 2 were
observation suggests that the ordered 2×1 surface re- foundtobedominant[96]. These8surfacestructuresare
construction may be a metastable ordering of a mobile sketched in figure 9. It is hard to imagine that a single
half-Srlayer,resultingfromthelowtemperaturecleaving clean As layer would display so many different surfaces
process, easily destroyed upon heating, and not always structures.
reformed upon recooling. It is harder to imagine that a Zhanget al imagedBa(Fe Co ) As withsixdiffer-
1−x x 2 2
clean, complete As layer would disorder so extremely on ent nominal dopings: x=0, 0.04, 0.085, 0.10, 0.125, and
heating to 200K, and fail to reorder upon recooling. 0.16. The latter four were found to be superconducting
Massee et al showed a large number of topographies with Tc=9K, 25K, 22K, and 9K, respectively[94]. Sam-
of Ba(Fe Co ) As , cleaved either at room temper- pleswereimagedat5Kaftercleavingat120K(abovethe
0.93 0.07 2 2
ature or at T < 80K [95] (see figure 8, e and f). At 100K at which the extra surface superstructure diffrac-
low T, they often observed the same ∼8 ˚A stripes seen tionspotsstartedtodisappear,asdiscussedinthefollow-
previously[90, 91]. Like Yin, they observed occasional ingsection,butbelowthe200Katwhichthesurfacelong
half-periodstripeshifts,resultingina‘ribcage’structure range order has fully disappeared[92, 95]). Zhang found
√ √
(figure 9f). They also saw less regular, larger rodlike that a 2× 2 superstructure dominated at all dopings
10
(a) (b) (c) (d) maining Ba (figure 10a). Li et al hypothesized here that
half of the As atoms were invisible due to different spin
environments. Furthermore, they imaged domain walls,
as seen zigzagging across the bottom half of figure 10a.
Larger images showed that these domain walls always
2× 2 (1,0)APB (1,1)APB 2x1in 2× 2 formed closed loops. On either side of a domain wall,
the orthorhombic a and b axes maintained their orienta-
tion, indicating no orthorhombic twin boundaries. But
Å
9 the visible and invisible atoms did switch, suggesting an
3.
antiphase boundary of the the spin state. These domain
walls were similar to those seen by Massee, shown in fig-
(e) 2x1 (f) ribcage (g) rod (h)‘exotic’
ure 8f. (The prevalence of such antiphase boundaries
may explain the discrepancy between the large Fe spin
FIG. 9. Sketch of various possible partial-Ba surface
moments computed from density functional theory, and
configurations[95]. Large(small)circlesindicatethepresence
the small spin moments measured experimentally[101].)
(absence) of a Ba atom.
(a) (c)
0.5
(figure 8c). Theoretical simulations with a half-Ba ter-
minatinglayershowedastrongfeatureat-200meV,and 1nm 0Å
a gross energy asymmetry with larger empty-state DOS,
in good agreement with the spectra measured on all six
samples(seefigure20a). Zhangthereforeconcludedthat
√ √
this 2× 2 structure was a half-Ba surface. Note that 5nm 5 nm
the six dopings span the bulk orthorhombic to tetrago-
√ √ 4x 4x
naltransition(seefigure6c),sotheirubiquitous 2× 2 (b)
structure cannot be due solely to the bulk orthorhombic
transition. Inonlytwosamplesoutofmanystudied,they
saw patches of the 2×1 stripes.
The argument for the partial-Ba-terminated surface
was further supported by measurements of the work
function,ϕ[97]. Workfunctionsforrelevantelementsare
shownintableI,takenfromRef. 12. IndozensofBa122
√ √
samples studied by Massee et al, representing 2× 2,
2×1, and other surface organizations, the work function 10nm
was never found to be greater than 1.8 eV, and typically
found to be ∼1.5 eV. On Ca122, Massee also found a
FIG. 10. Topographic images of cleaved parent 122 sur-
slightly higher work function, ∼1.9 eV, suggesting that
faces, presumed to be As-terminated. Images are scaled
the surface termination was not As, which would be
to the same unit cell size. (a) BaFe As cleaved at 80K,
expected to show the same, much higher work function √ √ 2 2
showing a 2 × 2 surface reconstruction with a domain
in both Ba122 and Ca122.
boundary zigzagging across the bottom of the image[100]
(setup: V =23mV;I =200pA). (b) SrFe As show-
sample set 2 2
As Surface Nascimento et al presented STM images of ing 2×1 reconstruction on the left, merging smoothly into
√ √
BaFe As , cleaved at T=80K, revealing a 5.6˚A square 2× 2reconstructionontheright,withoutstepedges[102]
2 2 √ √
lattice[98]. This is the correct spacing for a 2× 2 or- (RJ =0.8GΩ). Theleftinsetat4×magnificationshowsthat
ganization of a half Ba layer. In contrast, the complete atverylowjunctionresistance(lowerleftstrip,RJ =10MΩ),
As layer would form a 3.8˚A, nearly square lattice. How- thestripeappearstobetwoatomswide. Therightinsetat4×
magnification shows that half of the presumed As atoms are
ever, in combination with low energy electron diffraction √ √
invisibleinthe 2× 2reconstructionregion(R =1.7GΩ).
(LEED) data (next section), Nascimento et al hypothe- J
(c)Non-superconductingCa(Fe Co ) As showinga2×
0.97 0.03 2 2
sizedthathalfoftheAswereinvisibleduetoastructural
1 surface reconstruction, with a domain boundary in the
reconstruction. Theypointedoutthatthesurfaceis1-6% lower left corner (setup: V = −50mV;I = 10pA),
sample set
orthogonal (in comparison to the 0.7% orthogonality of and 4× magnified inset in the upper right corner (setup:
the bulk), and that this surface orthorhombicity persists V =−5mV;I =100pA)[56].
sample set
across the superconducting dome[99].
In a second study by the same group[100], more im-
√ √
ages were presented of the parent Ba122 2× 2 sur- Niestemski et al studied SrFe2As2, cleaved both at
face, sporting some white blobs which could be moved room temperature and at T = 77K[102]. Like Massee,
√ √
with the STM tip, which were identified as sparse re- they observed a 2× 2 structure in some areas, which