www.sciencemag.org/cgi/content/full/331/6014/189/DC1

Supporting Online Material for

Light-Induced Superconductivity in a Stripe-Ordered Cuprate D. Fausti,* R. I. Tobey, N. Dean, S. Kaiser, A. Dienst, M. C. Hoffmann, S. Pyon, T.

Takayama, H. Takagi, A. Cavalleri*

*To whom correspondence should be addressed. E-mail: andrea.cavalleri@mpsd.cfel.de (A.C.); daniele.fausti@mpsd.cfel.de (D.F.)

Published 14 January 2011, Science 331, 189 (2011)

DOI: 10.1126/science.1197294

This PDF file includes:

Materials and Methods Figs. S1 to S3

SUPPORTING ONLINE MATERIAL

Materials and Methods

Sample. Single crystals of La1.675Eu0.2Sr0.125CuO4 were grown using a traveling solvent

floating zone technique, confirmed to be non-superconducting above 5 K by resistivity

and magnetization measurements. Hard X-ray diffraction and Hall coefficient

measurements evidence the appearance of the static stripe ordering below 80 K.

Equilibrium broadband optical constants of LESCO1/8. The equilibrium optical

constants for LESCO1/8 were calculated by Kramers Kroening transformations of

broadband reflectivities, in and out of plane (see figure S1a). Outside of the measured

range the in-plane reflectivity (black dots) was extrapolated at low energy by a Drude

model and by a quadratically decreasing reflectivity on the high-energy side. The out-of-

plane reflectivity was approximated at low frequency by a parabolic function, joining the

FTIR measurements (red) with time domain THz spectroscopy (blue).

Fig S1 (a) Static reflectivity of LESCO1/8, measured with a combination of FTIR and Time domain THz spectroscopy. (b) Extinction coefficient for light polarized in (black) and orthogonal to the planes (red).

Note that a correction to the THz measurements has been introduced to take into account

the different incidence angles, in the FTIR measurements (normal incidence) and the time

domain THz measurements (45 degree angle of incidence). Fig.1(b) reports the extinction

coefficient α, for two polarizations. The extinction depth (1/α) of the 16-µm pump pulses

is at least 50 times smaller than that of the probing THz field.

Transient THz response (LESCO1/8): Femtosecond pulses of 35-fs duration at 800-nm

wavelength were generated by an amplified Ti:Sapphire laser, delivering ~2mJ energy at

1KHz repetition rate. THz radiation was generated by optical rectification in a ZnTe

crystal. The THz pulses reflected by the sample surface were measured through Electro

Optic Sampling (EOS) of the THz fields with 800nm pulses. All the experiments were

performed in a vacuum chamber containing THz generation, delivery to the sample and

pick up optics. This avoided the use of windows in the cryostat, and maximized the

available pump and probe flux. Mid-IR pump pulses of 2µJ energy were generated by

different frequency generation (DFG) between the signal and the idler from an optical

parametric amplifier OPA. The spectrum of the NIR pulses generated was measured with

a linear interferometer equipped with a Mercury Cadmium Tellurite detector. The

absolute equilibrium THz reflectivity was obtained by comparing the measured field

reflectance of the sample with that of a gold film deposited on the sample surface.

To derive the changes in the conductivity of the photo-excited layer ( ) from the

changes in the reflected field ΔE/E, we considered a thin photo-excited layer and a semi-

infinite unperturbed bulk sample beneath. Making use of a thin film approximation the

perturbed layer as following:

    (S1)

where is the frequency dependent pump-induced changes in conductivity at

time delay , is the vacuum permittivity, is the thickness of the layer, and is

the complex refractive index of the unperturbed material. Two main contributions to the

transient conductivity are found, a flat negative response and a 1/ω dependent

contribution at low frequency. See figure M2. The flat contribution can be fitted by a

photo-induced shift of an oscillator at high energy. The c-axis low optical constants are

!

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dominated by the presence of phonon modes at about 250cm-1. Photo-excitation of the

600cm-1 mode will result in a strong perturbation of the lower energy modes and in a

frequency independent shift of the conductivity at frequencies below 100cm-1.

Fig S2: The transient imaginary part of the conductivity for different time delays. The transient response exhibits two responses: a negative flat background and a 1/ω positive component at low frequency.  

Photo-susceptibility. We note that the amplitude of the experimental signal scales as

ln(F), where F is the fluence of the pump. This is evidenced by the linear dependence of

the signal in the semi-log plot of figure S3. This is for two reasons.

First, in the plane parallel to the optical surface of the sample, the fluence profile of the

excitation beam has a Gaussian dependence on the radial coordinate r, scaling as

€

F(x) = F0e−r 2 /σ 2

(where r is the distance from the centre of the beam and σ is its width).

Above a fluence value Fsat the optical constants reach a saturation value characteristic of

the superconducting phase and remain unchanged if the fluence is increased further. This

implies that for F>Fsat , one can define a radius rsat such that the reflectivity change ΔR =

ΔRsat for all r<rsat. This leads to a signal scaling ΔR(F) that is proportional to σ 2ln(F/Fsat),

as discussed in J.M. Liu Opt. Lett. 7, 192 (1982). We note that σ is a function of

wavelength, since each set of measurements is obtained after a different optimization of

the optical parametric amplifier. Thus the slope of the logarithmic growth varies for

different excitation wavelengths.

Second, in direction perpendicular to the surface, the pump fluence scales like

€

F(x) = F0e−αz, implying that for F>Fsat one can define a depth zsat such that the reflectivity

change ΔR = ΔRsat for all z<zsat. This leads to a signal scaling ΔR(F) that is again

proportional to ln(F/Fsat).

For the purposes of this paper, we define an operational photo-susceptibility given by

1/Fsat, which gives a quantitative estimate of the fluence at which the sample is locally

turned superconducting. In figure S3, we plot the logarithmic scaling of the signal for

different pump wavelengths, and 1/Fsat(λ), where λ is the pump wavelength in microns. A

clear resonance near 15 microns is observed.

Fig S3: Amplitude of the JPR signal as a function of fluence (left graph). The zero-amplitude-signal intercept gives the threshold. The inverse threshold is plotted as a function of wavelength, providing an operational description of the photo-susceptibility.