Coherent acoustic phonons in YBa2Cu3O7/La1/3Ca2/3MnO3 superlatticesWei Li, Bin He, Chunfeng Zhang, Shenghua Liu, Xiaoran Liu, S. Middey, J. Chakhalian, Xiaoyong Wang, andMin Xiao Citation: Applied Physics Letters 108, 132601 (2016); doi: 10.1063/1.4945333 View online: http://dx.doi.org/10.1063/1.4945333 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/108/13?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Persistent photoconductivity in oxygen deficient YBa2Cu3O7−δ/La2/3Ca1/3MnO3−x superlattices grown bypulsed laser deposition Appl. Phys. Lett. 103, 122603 (2013); 10.1063/1.4821746 Thermoelectric properties of YBa2Cu3O7−δ–La2/3Ca1/3MnO3 superlattices Appl. Phys. Lett. 101, 131603 (2012); 10.1063/1.4754707 Antiferromagnetism at the YBa 2 Cu 3 O 7 / La 2/3 Ca 1/3 MnO 3 interface Appl. Phys. Lett. 84, 3927 (2004); 10.1063/1.1741038 Superconductivity depression in ultrathin YBa 2 Cu 3 O 7−δ layers in La 0.7 Ca 0.3 MnO 3 / YBa 2 Cu 3 O 7−δsuperlattices Appl. Phys. Lett. 81, 4568 (2002); 10.1063/1.1526463 Magnetism and superconductivity in La 0.7 Ca 0.3 MnO 3 /YBa 2 Cu 3 O 7−δ superlattices J. Appl. Phys. 89, 8026 (2001); 10.1063/1.1370994

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Coherent acoustic phonons in YBa2Cu3O7/La1/3Ca2/3MnO3 superlattices

Wei Li,1,2 Bin He,1 Chunfeng Zhang,1,a) Shenghua Liu,1 Xiaoran Liu,3 S. Middey,3

J. Chakhalian,3 Xiaoyong Wang,1 and Min Xiao1,3,b)

1National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093,China2Institute of Signal Processing and Transmission, Nanjing University of Posts and Telecommunications,Nanjing 210003, Jiangsu, China3Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA

(Received 27 January 2016; accepted 22 March 2016; published online 30 March 2016)

We investigate photo-induced coherent acoustic phonons in complex oxide superlattices consisting

of high-Tc superconductor YBa2Cu3O7�x and ferromagnetic manganite La1/3Ca2/3MnO3 epitaxial

layers with broadband pump-probe spectroscopy. Two oscillatory components have been observed

in time-resolved differential reflectivity spectra. Based on the analysis, the slow oscillation mode

with a frequency sensitive to the probe wavelength is ascribed to the stimulated Brillouin scattering

due to the photon reflection by propagating train of coherent phonons. The fast oscillation mode

with a probe-wavelength-insensitive frequency is attributed to the Bragg oscillations caused by

specular phonon reflections at oxide interfaces or the electron-coupling induced modulation due to

free carrier absorption in the metallic superlattices. Our findings suggest that oxide superlattice is

an ideal system to tailor the coherent behaviors of acoustic phonons and to manipulate the thermal

and acoustic properties. VC 2016 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4945333]

Phonons are major excitations involved in heat and sound

transportation in nanostructures. By controlling phonons with

artificial periodic structures, a remarkable progress has been

made to alter thermal and acoustic properties in materials.1–11

For example, the elastic periodic structures, such as phononic

crystals12,13 (analogues of the electronic and photonic crys-

tals), can modify the phonon dispersion and form a thermal

bandgap,1 which is critical for enhanced functionality of ther-

mal and acoustic devices.14–18 Additionally, coherent phonon

behavior is a recent active subject which has been intensively

investigated in phononic crystals, resulting in a number of fas-

cinating phenomena of prominent applied significance, includ-

ing coherent phonon scattering,4 coherent thermal transport,6

and coherent phonon amplification,7 to name a few.

In a simplified view, the propagation of coherent pho-

nons in a periodic structure modulates the dielectric constant

dynamically, which can be monitored by time-resolved

pump-probe spectroscopy.7,19–24 Ultrafast optical excitation

of solids suddenly launches the forces that can excite the

coherent coupled electron and lattice oscillations. With the

propagation of coherent acoustic phonons into the sample or

the generation of coherent optical phonons with in-phase

atomic vibrations, the oscillatory behavior may be captured

in time domain by monitoring the optical reflectivity of a

probe beam.25 These oscillations originating from either light-

wave interference or phonon-wave interference can provide

wealth of insightful information on the coherent lattice

dynamic.25 Periodic structures of two-dimensional ultrathin

layers, known as superlattices, is an ideal playground to study

the physics driven by coherent phonons.4,6,7,15,26–31 Towards

this end, coherent phonons in superlattices have been exten-

sively investigated in semiconductor heterojunctions7,21,32–37

by ultrafast pump-probe spectroscopy. In contrast, their

dynamic behaviors in superlattices of oxide compounds with

strongly correlated electrons have been rarely explored.

In this letter, we report on coherent acoustic phonons in

ultra-thin complex oxide superlattices composed of epitaxial

high-Tc cuprate YBa2Cu3O7�x (YBCO) and ferromagnetic

manganite La1/3Ca2/3MnO3 (LCMO) layers. By applying

ultrafast pump-probe spectroscopy, we have identified two os-

cillatory modes in time domain: namely, the slow mode with

a clear frequency dependence on the probe wavelength which

is assigned to the stimulated Brillouin scattering by the propa-

gating coherent acoustic phonons; and the fast mode without a

dependence on the probe wavelength which is ascribed to the

photon Bragg reflections at the YBCO/LCMO interfaces or

different free carrier absorption in the metallic layers. These

attributions have been further confirmed by performing addi-

tional measurements on the superlattices with different thick-

nesses of YBCO layer.

The superlattices used in this study were grown in a

layer-by-layer mode on atomically flat single crystal SrTiO3

(001) substrates by laser MBE.38 Two superlattices with three

repeats of YBCO/LCMO heterostructures have been experi-

mentally studied. The thicknesses of individual LCMO layers

were �10 nm (26 unit cells) in both samples while the thick-

nesses of YBCO layers in the two samples are �10 nm (9 unit

cells) and �5 nm (5 unit cells), or (YBCO)9/(LCMO)26 and

(YBCO)5/(LCMO)26, respectively. In addition, for the refer-

ence purpose, the single layer films of YBCO and LCMO of

the same thickness of �50 nm have been grown under same

growth condition. We discuss the measurements performed at

room temperature to uncover the effect of periodic structure

on the coherent phonon behavior. Time-resolved pump-probe

measurements were performed with femtosecond pulses from

a kHz Ti:sapphire regenerative amplifier (90 fs, Libra,

Coherent, Inc.) as briefly described in the inset of Fig. 1(a).

a)cfzhang@nju.edu.cnb)mxiao@uark.edu

0003-6951/2016/108(13)/132601/5/$30.00 VC 2016 AIP Publishing LLC108, 132601-1

APPLIED PHYSICS LETTERS 108, 132601 (2016)

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23:52:30

The fundamental output at 800 nm was used as the pump beam

which excited normally onto the sample. An optical parametric

amplifier system (Opera Sola, Coherent, Inc.) pumped by the

regenerative amplifier is employed as the probe beam for

wavelength-dependent measurements. The incident angle of

the probe beam is �15� so that the scattering angle is �5�

inside the sample with a refractive index of �2.5. In our

experiments, the pump fluence was set at �500 lJ/cm2 while

the probe pulse was about two orders of magnitude weaker.

The absorption lengths at a wavelength of 800 nm are

�100 nm in both YBCO and LCMO, so that the excitation

penetrates through the superlattice samples. The energy

absorbed by the superlattice sample can induce a transient tem-

perature increase of <20 K. The diameter of pump laser spot

was set to be �1 mm, which is orders of magnitude larger than

the sample thickness.

Figure 1(a) shows a typical pump-probe trace recorded

from the (YBCO)9/(LCMO)26 superlattice sample probed at

480 nm. As seen, following an abrupt change in reflectivity,

the recovery dynamics is manifested as a multi-exponential

decay entangled with periodic oscillations. The exponential

decay dynamics is attributed to the dynamics of photo-induced

quasiparticles.39 Here, we mainly concern the oscillatory

components after subtracting off the exponential decay contri-

bution (inset, Fig. 1(b)). As clearly seen, the oscillatory signal

consists of two different modes: a fast oscillation that damps

strongly at the early stage (<20 ps) and a slow oscillation that

persists to a longer time (>300 ps). To quantify the oscillation

frequencies, we performed a Fourier transformation. The

Fourier spectrum shown in Fig. 1(b) indicates the presence of

the two modes at 81 and 236 GHz that are well separated in

the frequency domain.

Next we discuss the possible underlying mechanisms for

the two distinct oscillatory modes. First we note that for the

light-absorbing ultrathin film grown on a bulk substrate, the

absorbing layer may act as a transducer where a pulse of coher-

ent acoustic phonons is generated by the pump beam.40–42

After that, the strain pulse (coherent acoustic phonons) gener-

ated in the film may propagate into the substrate, thus inducing

changes in reflectivity of the probe pulse. This effect has been

frequently discussed in the scheme of stimulated Brillouin scat-

tering where the probe beam reflected by the propagating strain

interferes with the light reflected at the surface, resulting in

oscillations in the pump-probe traces.22,40,42 In this process, the

oscillation frequency f depends on the probe wavelength (k)

as19,22,40–42

f ¼ tq

2p¼ 2n kð Þt

kcos h; (1)

where nðkÞ, t, q and h are the wavelength-dependent refrac-

tive index, the propagation velocity of the strain pulse, the

acoustic wave vector, and the scattering angle, respectively.

To check whether such a picture is indeed responsible for

the oscillation modes observed here, we have performed

experiments with different probe wavelengths (Fig. 2(a)). As

seen, the two oscillatory components show different depend-

ences on the probe wavelength. Specifically, the frequency

of early stage fast oscillation is almost independent of the

probe wavelength, whereas the frequency of the slow mode

decreases significantly with increasing probe wavelength. In

addition, as shown in Fig. 2(b), the frequency of slow mode

is linearly dependent on nðkÞ=k (i.e., the wave vector) and

can be well fitted to Equation (1), suggesting that the slow

component is indeed relevant to the stimulated Brillouin

scattering. To further confirm such assignments, we have

performed additional control measurements on the reference

FIG. 1. (a) Pump-probe trace recorded from the [(YBCO)9/(LCMO)26]� 3

superlattice sample with probe wavelength at 480 nm. The inset shows a dia-

gram of pump-probe spectroscopy. (b) Fourier transformation of the oscilla-

tion component shows two oscillating modes with frequencies at 81 GHz

and 236 GHz, respectively. The inset shows the oscillatory component

obtained by subtracting off the multi-exponential decay component.

FIG. 2. (a) The oscillatory component

probed at different wavelengths. (b)

Frequencies of the two oscillatory modes

are plotted as a function of the wave

vector. The slow oscillation frequency

can be well produced by Equation (1)

(green squares), while the fast oscillation

frequency is nearly independent of the

probe wavelength (blue circles).

132601-2 Li et al. Appl. Phys. Lett. 108, 132601 (2016)

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samples of pure YBCO and LCMO films, where the effects

from the YBCO/LCMO interface are absent. After subtract-

ing off the multi-exponential decay components, the experi-

mental data shown in Fig. 3 reveal that the pump-probe

signals from all three samples exhibit slow oscillations with

the same frequency. The results immediately imply that

the primary origin of the slow component is the propagation

of coherent phonons in SrTiO3 substrates that are initially

launched in the transducer of superlattice film (see Fig.

3(b)).41 From the slope in Fig. 2(b), the sound velocity

can be evaluated to be �7950 m/s, which is consistent with

the acoustic velocity in SrTiO3 from literature.40 The mean

free path for the slow mode in SrTiO3 probed at 480 nm is

�1.6 lm.

The fast oscillation mode, however, is more intriguing

since it is independent of the probe wavelength and is only

observed in the superlattice sample (see Fig. 3). To elucidate

this, we also note that in correlated oxide materials with

complex phase diagrams besides coherent acoustic phonons,

the oscillatory signals in pump-probe traces may also be gen-

erated by some collective modes such as coherent optical

phonons or magnetic excitations (i.e., magnons). First, we

point out that the fast oscillation mode is unlikely to be con-

nected with optical phonons since the observed oscillation

frequency (236 GHz) is quite low and the signal is independ-

ent of the polarization of incident beams. At the same time,

the magnetic origin of the signal is also unlikely since the

measurement is performed at room temperature, which is

well above the Neel temperature. Finally, we attempt to

understand this mode by considering the specular reflection

of acoustic phonons at the oxide interface. Toward this, con-

sidering the well-defined sharp interfaces for the superlattice

structure,43 the pump-induced phonon pulse may be reflected

in a coherent manner by the interfaces that modulates the

light reflectivity.1,5 Previously, phononic reflection at the

interface between superlattice and buffer layer has been

observed with echo signals in differential reflectivity in

Bi2Te3/Sb2Te3 superlattice.44 In our samples, the phonon

reflections at the interfaces inside the superlattices may cause

acoustic wave interference. Such an effect can be analyzed

with a model considering the phonon Bragg reflection.45 At

quasi-normal incidence, the Bragg condition requires 2D=kA

to be an integer, where D is the period of superlattice and kA

is the phonon wavelength. Thus, the center frequency (f ) for

the mini Brillouin zone can be expressed as45

f ¼ tef f

D: (2)

Here, tef f is the effective sound velocity in the superlattice

that can be approximately given by45

D

tef f¼ dY

tYþ dL

tL; (3)

where dY , dL, tY , and tL are the layer thicknesses of and

propagation velocities in the YBCO and LCMO layers,

respectively. This model predicts that the oscillation fre-

quency is sensitive to the layer thickness but independent of

the probe wavelength.

To check the plausibility of this scenario as the origin

of the fast oscillation mode, we compare the experimental

data recorded from the two superlattice samples (YBCO)9/

(LCMO)26 and (YBCO)5/(LCMO)26, with the only differ-

ence in the thickness of YBCO layer. Figure 4(a) shows the

obtained pump-probe traces recorded from the two samples.

As clearly seen in Fig. 4(a), the frequency of slow oscillation

mode remains the same while the fast mode shows a very

strong variation with YBCO thickness, indicating that this

fast mode is indeed of the intrinsic response from the peri-

odic structure. More specifically, the frequency of the fast

mode increases from 236 GHz in the (YBCO)9/(LCMO)26

superlattice to 351 GHz in the (YBCO)5/(LCMO)26 superlat-

tice. To highlight the difference, we plot the differential of

the signal in time domain in Fig. 4(b). The experimental data

can be quantitatively compared with the theoretical values

estimated from the model. By using literature values of the

sound velocity in YBCO46 and LCMO47 bulk materials, the

calculated frequencies from Equation (2) yield 257 GHz for

(YBCO)9/(LCMO)26 and 348 GHz for (YBCO)5/(LCMO)26,

respectively. Both values are in remarkable agreements with

the experimentally measured data. Moreover, the time con-

stants for the amplitude damping of the fast components are

FIG. 3. (a) Transient reflectivity signals of YBCO, LCMO, and SL films

probed at 800 nm are shown in a scale normalized to the signal at zero delay.

The slow oscillation components are shown in the insets. (b) Fourier trans-

formation indicates the frequencies of the slow component are the same in

the three samples.

FIG. 4. (a) Pump-probe signals recorded from two superlattices with different

thicknesses of YBCO layer. The signals are recorded with probe wavelength

of 400 nm. (b) The differential signal shows a clear structure dependence of

the fast oscillation component.

132601-3 Li et al. Appl. Phys. Lett. 108, 132601 (2016)

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23:52:30

�12 ps and �9 ps, respectively, in the two samples, giving

the mean free paths to be �57 nm and �48 nm, which are

close to the superlattice thicknesses. These facts imply that

the finite superlattice thickness is probably the primary limit

for the phonon propagation, which is also consistent with the

picture of phonon Bragg reflection. The phonons whose

wave vectors satisfy the Bragg condition will be reflected off

the superlattice and be unable to propagate into the substrate,

so that only few oscillations can be observed at the early

stage (Fig. 4(b)).

Although the scenario of phononic Bragg reflection can

well describe the fast modes, it is worth noting that the free

carrier absorption of metallic layers in our superlattice sam-

ples may also affect the coherent phonon dynamics. In litera-

ture, it has been shown that the electronic coupling induced

modulations give rise to the fast oscillations in metal-

dielectric SrRuO3/SiTiO3 superlattice samples as probed by

ultrafast x-ray diffraction.48–50 This effect may function in

YBCO/LCMO superlattice samples since both the metallic

layers absorb the pump light. The different energy absorbed

in the two metallic materials could possibly cause different

degrees of expansions and set the vibrational modes. The fre-

quency of such oscillations is also dependent on the periodic-

ity of the superlattice where Equation (2) is also applicable.

These results indicate that the coherent behaviors of acoustic

phonons can be effectively controlled in oxide superlattices

through superlattice periodicity.

In summary, we have found two oscillating modes

appearing in ultrafast pump-probe traces due to coherent

acoustic phonons in LCMO/YBCO superlattices. These two

modes show markedly different probe-wavelength dependen-

ces, which can be ascribed as due to the stimulated Brillouin

scattering linked to the light reflection by propagating strain

pulse and the phononic Bragg oscillations or free carrier

absorption, respectively. Our work suggests that artificial

oxide superlattice is a promising structure to manipulate the

coherent phonon scattering. This can be of great importance in

engineering thermal2,3 and acoustic properties for potential

applications such as coherent heat conductors,5,6 thermal

bandgap materials,1 and acoustic lasers.7,51 It is worth noting

that the dynamics of coherent phonons, e.g., the attenuation of

the fast mode, can serve as a metric to characterize the quality

of the oxide interfaces (i.e., shaper interface results in less

attenuation). Moreover, the tunable lattice dynamics in oxide

superlattice, as well as the long range transfer of electron-

phonon coupling in oxide superlattices observed in literature,52

may hold the key to uncover the roles played by electron-

phonon interactions for multiple and often coupled order

parameters (i.e., charge-density wave or pseudogap) of the rich

phase diagram in such strongly correlated materials.47,53,54

The work at Nanjing University was supported by the

National Basic Research Program of China (2013CB932903

and 2012CB921801, MOST) and the National Science

Foundation of China (91233103, 11574140, 11227406, and

11321063). J.C. and X.L. acknowledge the support by the

Department of Energy under Grant No. DE-SC0012375.

S.M. was funded by the DOD-ARO under Grant No. 0402-

17291. The authors acknowledge Dr. Xuewei Wu for his

technical assistance.

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