Abstract We apply the femtosecond optical pump-probe spectroscopy to study the relaxation dynamics in photoexcited Co-doped Ba(Fe 1−x Co x ) 2 As 2 single crystals in the underdoped spin-density wave (SDW) state region of the x − T phase diagram. Underdoped SDW samples with Co-0 % and Co-2.5 % show a bottleneck in the relaxation as a consequence of the partial charge gap opening in the orthorhombic SDW phase, similar to previous results in other SDW iron-pnictides. Moreover, the charge gap magnitude decreases with increasing doping. The sample with Co-5.1 % displays both a SDW ordering and superconductivity at low T . We were able to observe a 2-fold anisotropy in our samples, existing up to ∼200 K, without any applied uniaxial stress. We associate the 2-fold symmetry breaking in nominally tetragonal phase with nematic ordering of the Fe d orbitals

Ian R Fisher

J.-H Chu

Juin-Haw Chu

L Stojchevska

T Mertelj

· D Mihailovic

· I R Fisher

· T Mertelj

CiteSeerX

J Supercond Nov Magn (2013) 26:2593–2596
DOI 10.1007/s10948-013-2141-4
O R I G I NA L PA P E R
Dynamics of Photoexcited Carriers in Ba(Fe1−xCox)2As2 Single
Crystals with Spin-Density-Wave Ordering
L. Stojchevska · T. Mertelj · Juin-Haw Chu · Ian
R. Fisher · D. Mihailovic
Received: 2 December 2012 / Accepted: 14 February 2013 / Published online: 16 March 2013
© Springer Science+Business Media New York 2013
Abstract We apply the femtosecond optical pump-probe
spectroscopy to study the relaxation dynamics in photoex-
cited Co-doped Ba(Fe1−xCox)2As2 single crystals in the
underdoped spin-density wave (SDW) state region of the
x − T phase diagram. Underdoped SDW samples with Co-
0 % and Co-2.5 % show a bottleneck in the relaxation as
a consequence of the partial charge gap opening in the or-
thorhombic SDW phase, similar to previous results in other
SDW iron-pnictides. Moreover, the charge gap magnitude
decreases with increasing doping. The sample with Co-
5.1 % displays both a SDW ordering and superconductivity
at low T . We were able to observe a 2-fold anisotropy in our
samples, existing up to ∼200 K, without any applied uni-
axial stress. We associate the 2-fold symmetry breaking in
nominally tetragonal phase with nematic ordering of the Fe
d orbitals.
Keywords Femtosecond optical spectroscopy · Iron-based
pnictides · Spin-density-wave systems · Carrier relaxation
dynamics
L. Stojchevska () · T. Mertelj · D. Mihailovic
Complex Matter Department, Jozef Stefan Institute, Jamova 39,
1000 Ljubljana, Slovenia
e-mail: ljupka.stojchevska@ijs.si
J.-H. Chu · I.R. Fisher
Geballe Laboratory for Advanced Materials and Department of
Applied Physics, Stanford University, Stanford, CA 94305, USA
J.-H. Chu · I.R. Fisher
SLAC National Accelerator Laboratory, Stanford Institute for
Materials and Energy Sciences, 2575 Sand Hill Road, Menlo
Park, CA 94025, USA
1 Introduction
Discovery of the superconductivity in the iron-based pnic-
tides [1–3] set up plenty of new challenges for the re-
searchers worldwide. Even though there have been signifi-
cant experimental and theoretical attempts to reveal the puz-
zles related to their properties, we still have not been able to
answer all open questions. In addition, different experimen-
tal techniques [4–12] have revealed some unusual proper-
ties of the normal state, such as pseudogap and normal-state
electronic nematic fluctuations. In the cuprates, the pseudo-
gap is omnipresent [13–17], whereas in the iron-based pnic-
tides its omnipresence is still questionable [18, 19]. Simi-
larly as in cuprates, understanding the unusual normal state
is important for explaining the mechanism leading to the su-
perconductivity in iron-based pnictides.
It has been shown that the femtosecond optical spec-
troscopy is a powerful tool not only for real-time studies of
the dynamics of photoinduced processes and determination
of the life-time of the excited state, but also for separation of
different processes since it can distinguish between different
excitations by their different relaxation dynamics.
In this paper we present a femtosecond optical spec-
troscopy study of the quasi-particle relaxation dynamics in
Ba(Fe1−xCox )2As2 for the orthorhombic underdoped SDW
ordering region of the phase diagram [20]. The aim of the
experiments presented here [24] is systematic study of the
temperature dependence of the optical reflectivity transients
as a function of Co doping, for x = 0 %, 2.5 % and Co-
5.1 %.
2 Experimental Details
The experiments were performed using standard pump-
probe technique as described in detail elsewhere [21]. The
2594 J Supercond Nov Magn (2013) 26:2593–2596
Fig. 1 The photoinduced
reflectivity R/R transients at
13 µJ/cm2 pump fluence as a
function of temperature at
Co-0 % (a, b) and Co-2.5 % (c,
d) doping. The left column
corresponds to the P + and the
right one to the P − polarization.
Full circled line represents the
TSDW, where the full triangled
line refers to Ts (obtained from
the phase diagram [20])
samples were excited with 50 fs laser pulses generated from
a 250-kHz Ti:sapphire regenerative amplifier seeded with
Ti:sapphire oscillator. The pump photons were at 3.1 eV
photon energy and for the probe we had 1.55 eV. Pump and
probe beams were perpendicularly polarized to each other
and oriented relatively to the crystals in a way to obtain max-
imum/minimum response amplitude at low temperatures.
Single crystals of Ba(Fe1−xCox )2As2 with Co doping
x = 0 %, 2.5 % and 5.1 % were grown from a self-flux and
characterized as described elsewhere [9, 12, 20]. Samples
with Co-0 % and Co-2.5 % belong to the underdoped region
of the phase diagram with SDW ordering, where Co-5.1 %
sample displays both a SDW and superconductivity at low
T . Before mounting the crystals in a liquid-He flow cryostat,
they were glued onto a copper holder and cleaved by a razor
which resulted in a terrace like surface with a width of a few
µm and step height of a few nm.
3 Results and Discussion
In Fig. 1 we plot the temperature dependence of the pho-
toinduced reflectivity transients for underdoped Co-0 % and
Co-2.5 % samples. One of the most astonishing feature of
our experimental results is the observation of the 2-fold ro-
tational anisotropy in nominally tetragonal phase at T <
200 K. For easier following of the presented experimental
results, we denote the polarization according to the low-T
minimum and maximum R/R peak as P − and P +, re-
spectively. Even though we were not able to determine the
precise origin of the 2-fold rotational anisotropy, we believe
that it might appear as a consequence of the anisotropic com-
ponent of the surface strain induced by the laser heating in
the presence of the unidirectional terraces on the surface of
the cleaved crystals. Since the increase of the temperature in
samples during the experiments is of the order of a few K
and there is not any external magnetic field to order spins,
we believe that the 2-fold symmetry breaking is related with
nematic fluctuations or ordering of the Fe d orbitals.
3.1 Temperature Dependence
The undoped Ba(Fe1−xCox )2As2 samples exhibit similar
response as in undoped SrFe2As2 [22] and SmFeAsO [23].
The initial relaxation dynamics in Co-0 % can be fitted
well with a single exponential decay in both polarization,
whereas in the Co-2.5 % and Co-5.1 % samples a multi-
component relaxation is observed (see Fig. 2). To determine
T -dependence of the R/R transients, we apply three expo-
nentially decaying component fit [25] and we denote com-
ponents with A, B, and C. Component A with relaxation
time τA ∼ 0.2 ps is visible below 150 K and it exists in all
samples, except in Co-0 %. It is associated with the initial re-
laxation of the optically excited electrons towards the Fermi
energy or the inter-band momentum scattering between the
states near the Fermi energy at different parts if the Fermi
surface. The second component B is present in all samples
and its relaxation time τB shows a critical slowing down at
Ts . At the end, by increasing the temperature above Ts com-
ponent C becomes visible. Although it has the longest re-
laxation time, its behavior is similar to component B, both
belonging to a single process with a non-exponential decay
dynamics.
J Supercond Nov Magn (2013) 26:2593–2596 2595
Fig. 2 Raw T -dependence of
R/R transients in Co-5.1 %
sample, taken at 13 µJ/cm2
pump fluence. (a) corresponds
to the P + and (b) to the P −
polarization. Full circled line
represents the TSDW, the full
triangled line refers to Ts and
full squared line applies to Tc
(obtained from the phase
diagram [20])
Table 1 Obtained charge gap magnitudes
x 2(0)/kBTs 2PG (K)
0 % 9 ± 2 –
2.5 % 7 ± 2 –
5.1 % 4 ± 2 800 ± 100
To perform the analysis of the temperature dependence
of the transients and therefore estimate the magnitude of the
charge gap we apply a bottleneck model (see Fig. 3). Since
the reflectivity transients display a characteristic shape re-
lated with the appearance of the bottleneck in the relaxation
dynamics below Ts [13]:
npe ∝ 1/
[(
1 + kBT
2(T )
)(
1 + gph
√
kBT
(T )
× exp
(
− kBT
(T )
))]
(1)
R = (γ0 + η(T 2))npe (2)
The magnitudes of the charge gap (its values are summa-
rized in Table 1) are consistent with previous results [17,
23] and decrease with increasing x as a consequence of a
decrease of the stability of the SDW state.
In the superconducting sample Co-5.1 % (Figs. 2 and 4)
we observe an additional component in the relaxation dy-
namics, visible below critical temperature Tc . Similarly to
the previous studies [21], by changing the laser pulse inten-
sity, the superconducting state is destroyed and the charac-
teristic superconducting signal saturates when a certain laser
fluence threshold FT is reached. The SC component can be
obtained by subtraction of the normal state R/R signal
above Tc from the transient measured at 5 K (Fig. 4).
Above Ts , tails in the T -dependent R/R transients are
visible which cannot be fitted with (2). They are related with
the presence of a bottleneck in the relaxation and therefore
Fig. 3 The photoinduced reflectivity R/R transients amplitudes as a
function of temperature at different dopings. The full and open symbols
correspond to the P + and P − polarization, respectively. Thin lines are
fits of Eq. (2) discussed in the text. Vertical lines indicate Ts , TSDW and
Tc , obtained from Ref. [20]
with a pseudogap at the Fermi energy. In order to estimate
the magnitude of the pseudogap, we fit the amplitudes of the
T -dependence of R/R signal with
R ∝ npe ∝
[
1 + gphexp
(
− kBT
PG(T )
)]−1
(3)
It is important to mention that estimated value of the
pseudogap magnitude is in good agreement with previous
studies [23] and has similar value as the spin pseudogap
magnitude obtained from the Knight shift [6].
4 Conclusions
We present detailed study of the relaxation dynamics in
Ba(Fe1−xCox )2As2 single crystals by using femtosecond
optical spectroscopy. In the SDW state we observe a bot-
2596 J Supercond Nov Magn (2013) 26:2593–2596
Fig. 4 Low temperature
photoinduced reflectivity R/R
transients at 3.9 µJ/cm2 pump
fluence in Co-5.1 % where
(a) corresponds to the P + and
(b) to the P − polarization
tleneck in the relaxation dynamics due to the opening of a
partial charge gap, similarly as in other iron-based pnictides.
Its magnitude 2(0)/kBTs decreases with increasing of Co-
doping, in agreement with a decrease of the orthorhom-
bic/SDW state stability. We associate observed 2-fold sym-
metry breaking with nematic fluctuations or ordering of the
Fe d orbitals. Breaking of the 4-fold symmetry appears due
to the anisotropic boundary which introduce an anisotropic
surface strain. Moreover, strain appears as a consequence of
the local crystal expansion due to the local thermal load. We
relate the anisotropy in nominally tetragonal phase of the
strain to the unidirectional terraces on the cleaved surface,
characterized with the atomic force microscope (AFM). Re-
garding the superconductivity existing in Co-5.1 % sample,
an additional SC component in the relaxation dynamics is
appearing, consistent with the previous related work. More-
over, the saturation of the amplitude of the SC signal for
pump fluences above FT is linked with non-thermal destruc-
tion of the SC state.
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Dynamics of Photoexcited Carriers in Ba(Fe 1−x Co x ) 2 As 2 Single Crystals with Spin-Density-Wave Ordering

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Dynamics of Photoexcited Carriers in Ba(Fe 1−x Co x ) 2 As 2 Single Crystals with Spin-Density-Wave Ordering

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