Coupling Among Carriers and Phonons in Femtosecond Laser Pulses

Subscriber access provided by Bibliothèque de l'Université Paris-Sud

Article

Coupling Among Carriers and Phonons in Femtosecond Laser Pulses-excited SrRuO3 – A Promising Candidate for Optomechanical and Optoelectronic Applications Kang Wang, Bingbing Zhang, Weimei Xie, Shenghua Liu, Xu Wei, Ziyuan Cai, Mingqiang Gu, Ye Tao, Tieying Yang, Chunfeng Zhang, Hong-Ling Cai, Fengming Zhang, and XiaoShan Wu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00728 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Nano Materials

Coupling Among Carriers and Phonons in Femtosecond Laser Pulses-excited SrRuO3 – A Promising Candidate for Optomechanical and Optoelectronic Applications

Kang Wang,† Bingbing Zhang,‡ Weimei Xie,† Shenghua Liu,† Xu Wei,‡ Ziyuan Cai,† Mingqiang Gu,§ Ye Tao,‡ Tieying Yang,∥ Chunfeng Zhang,† Hongling Cai,† Fengming Zhang,† and Xiaoshan Wu, †

†Collaborative

Innovation Center of Advanced Microstructures, Laboratory of Solid State

Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China. ‡Beijing

Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Sciences, Beijing 100049, China. §Department

of Materials Science and Engineering, Northwestern University, Evanston, Illinois

60208, United States. ∥Shanghai

Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China.

ACS Paragon Plus Environment

1

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

ABSTRACT Large reversible lattice strain that manipulates coupled degrees of freedom can be induced by optics in SrRuO3, suggesting it has great potential in optomechanical and optoelectronic devices. Photoexcitation and energy transfer via electron-phonon scattering have been studied, but recent studies inferred that more processes that remain to be identified contribute to the light-induced deformation. Here, by combining ultrafast X-ray diffraction and ultrafast optical reflectivity experiments, we image the excitation and relaxation characteristics of SrRuO3 films. Upon photoexcitation, photo-carriers redistribute both in space and energy, and then relax with fast direct recombination, phonon-assisted relaxation and phonon-phonon scattering dominating on a longer time scale. The phonon-assisted relaxation process, together with the thermal effect resulting from the electron-phonon scattering on sub-picosecond time scales, contributes to the photo-induced strain in SrRuO3. Phonon and electron effects on relaxations of SrRuO3 are studied, including the bottleneck effects of phonons on the coupled carrier and phonon relaxation and the scaled relationship of the fast carrier relaxation with the electron correlation. Our results suggest optomechanical and optoelectronic manipulations of SrRuO3 by electronic pumping, providing avenues for designs of applications with high performance.

KEYWORDS SrRuO3, photo-induced strain, pump-probe techniques, dynamics, electron-phonon scattering, optomechanical and optoelectronic applications


2

Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC FIGURE


3


Page 4 of 27

INTRODUCTION In complex oxides, novel physical properties manipulated by the lattice strain, where charge, spin, orbital, and lattice degrees of freedom are coupled together and electron-phonon interactions are important, have been extensively explored. SrRuO3 is a typical material that has attracted much attention because of its strong electron correlation and interesting properties, such as itinerant ferromagnetism,1 orbital ordering2 and non-Fermi liquid behavior (bad-metal behavior) at high temperature.3 In the last few years, a broad range of studies have proposed and demonstrated that the electromagnetic properties of SrRuO3 respond greatly to lattice deformation,4, 5 leading to novel physics including half-metal behavior6 and insulating ferromagnetic states.7 The variable electromagnetic properties are attributed to the competition between spin fluctuation and itinerant electronic states. These phenomena, together with the strain-manipulated enhancement of optical current8, electrocatalytic activity,9 etc., lead the strain manipulation to be one of the attractive topics in recent years, and suggest SrRuO3 a great potential in applications such as spintronic devices and chemical catalysis. Besides controlling strain by changing growing conditions, light illumination can reversibly induce lattice deformation of a single system, shedding lights on the future design of optomechanical and optoelectronic devices.8, 10 Photo-induced strain above 1% has been observed in SrRuO3 films with the optical excitation intensity above 60 W cm-2.10 Such a large photo-induced strain compared with that observed in other materials such as non-polar semiconductors including germanium and silicon,11 is due to the large absorption coefficient ( ~2.0 × 105 cm ―1 at the incident wavelength of 532 nm) in SrRuO3.8 Ultrafast X-ray diffraction (UXRD) measurements revealed that photoexcited SrRuO3 starts to deform at the SrRuO3 surface and SrRuO3/substrate interface.12 The strain wave then propagates to the substrate with a


4

Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2𝐷

characteristic time 𝑇 = 𝑣SRO (13.9 ps with 𝐷 = 44 nm) determined by both film thickness 𝐷 and longitudinal sound velocity 𝑣SRO = 6312 m s ―1 in SrRuO3. In addition other strain manipulations, such as mode-selective coherent phonons in SrRuO3/SrTiO3 superlattices,13 and monochromatic sub-terahertz acoustic phonon wave packets created by a train of laser pulses,14, 15

have also be observed. These new phonon modes that are not present in SrRuO3 provide

ground tests to study non-linear phonon-phonon interactions. Since most studies of SrRuO3 systems focus on phonon behaviors as mentioned above, electronic originations of the photoinduced strain and relaxations of electrons and phonons after photoexcitation remain elusive. It was believed that the photo-induced strain originates from the initial electron excitation and subsequent electron-phonon scattering processes on sub-picosecond time scales.16, 17 The energy transfer via the scattering induces the increasing phonon temperature and thermal expansion. However, recent Raman studies on SrRuO3 revealed that the thermally induced strain is only a small part of the photo-induced strain.10 More phonon-assisted processes may occur and lead to the light-induced deformation. Non-thermal originations of the photo-induced strain have been observed in other materials such as ferroelectric oxides including BiFeO3 and LiNiO3.18-20 In ferroelectric oxides, the bulk photo-voltaic effect contributes to the lattice strain through the converse piezoelectric effect,18-20 but both effects only exist in crystals with noncentrosymmetric structures.20 SrRuO3 is centrosymmetric, and crystallize in an orthorhombic symmetry at room temperature, as presented in the insert in Figure 1. In SrRuO3, photostriction resulting from interactions between the lattice and photoexcited carriers was supposed to cause the lattice strain.10 However, it is different from the electron-phonon scattering occurring on subpicosecond time scales leading to the thermal strain, and the corresponding physics remain


5


Page 6 of 27

elusive. The exploration of photoexcitation of SrRuO3 and subsequent relaxations of electrons and phonons is of vital importance. In the present, we image the photoexcitation and relaxations of electrons and phonons in SrRuO3 films via two complementary methods that are UXRD and ultrafast optical reflectivity (UR) measurements. In addition to the thermal effect, the trap of carriers in high-energy bands and subsequent phonon-assisted relaxation process contribute to the photo-induced deformation. The bottleneck effects of phonons on the coupled carrier and phonon relaxation are proposed and demonstrated by temperature dependent measurements. The fast carrier relaxation supports a scaled behavior depending on the electron correlation. The scaled relationship for SrRuO3 shows a divergence from that derived from the Fermi-liquid model, which may give a hint that phonon effect occurs in the bad-metal region above the Curie temperature of SrRuO3. Revealing electron and phonon dynamics in femtosecond laser pulses-excited SrRuO3 may prompt designs of efficient applications of SrRuO3 in optomechanical and optoelectronic devices through the electronic pumping.

EXPERIMENTAL SECTION Sample preparation. High quality SrRuO3 films were epitaxially grown by pulsed laser deposition (PLD) on (001) SrTiO3 substrates with TiO2 termination, as schematically presented in Figure 1.21 The sample preparation procedure is similar to that reported in our previous works on SrTiO3/SrRuO3 superlattices.7, 22 Briefly, a KrF excimer laser (CompexPro205F, Coherent) with 248-nm ultraviolet radiation and a repetition rate of 4 Hz was used for sample growth. The estimated laser energy density focused on the stoichiometric SrRuO3 ceramic target was ~2.0 J cm-2. During sample growth, the substrate temperature was 750 °C and the oxygen pressure was


6

Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05 mbar. After deposition, we annealed samples in situ at 750 °C under the oxygen pressure of 0.5 mbar for 30 min and then cooled down samples naturally to room temperature. Structural characterization. The structural characterization of SrRuO3 was performed at the beamline BL14B1 of Shanghai Synchrotron Radiation Facility (SSRF) and beamline 1W1A of Beijing Synchrotron Radiation Facility (BSRF) at room temperature. The clear Laue oscillation observed in the X-ray diffraction (XRD) pattern as presented in Figure 2 infers high𝜆

quality of SrRuO3 films. The film thickness, 𝐷 = 44 nm, was calculated through 𝐷 = Δ𝜃cos 𝜃, where 𝜆, 𝜃 and Δ𝜃 are the X-ay wavelength, Bragg angle and the angle difference between the two adjacent Laue oscillation peaks, respectively. SrRuO3 with such a large thickness in our studies present bulk properties because the epitaxial strain is fully relaxed. Ultrafast X-ray diffraction and ultrafast optical reflectivity experiments. UXRD experiments were performed through a laser pump and X-ray probe technique, as schematically presented in Figure 3a, at the 1W2B wiggler beamline of BSRF. Laser pulses with the pulse duration of 300 fs and with the wavelength of 515 nm/343 nm were derived by frequency doubling/tripling the output of a Ti:sapphire laser system. A single bunch from the X-ray bunch train was extracted by a pixel-array X-ray detector (Pilatus 100k), which was synchronized with the laser pulse by an electronically adjustable time delay. Incident X-ray pulses with the photon energy of 8.024 keV were focused on a 72-μm (FWHM) spot diameter. The probe area was obtained by a polycarpellary X-ray lens. Repetition rates of 26, 155 and 210 KHz were used in experiments individually. The increasing frequency promises efficient measurements, but also leads to large thermal accumulations. In our studies, we use a one-dimensional thermal diffusion model to eliminate the effects of the unexpected thermal accumulations. Details of the thermal


7


Page 8 of 27

diffusion model are discussed below and in Supporting Information Note I. More details of the UXRD equipment have been reported in our previous works.23 Time-resolved optical reflectivity changes ∆𝑅/𝑅 =

𝑅(𝑡) ― 𝑅0 𝑅0

were measured through a

table-top system constructed in-house. 𝑅(𝑡) is the optical reflectivity of SrRuO3 measured at the delay 𝑡, and 𝑅0 is the optical reflectivity measured without photoexcitation. SrRuO3 films were excited by 400-nm laser pulses with a pulse duration of 200 fs. We used 800-nm laser pulses to probe the time-resolved optical reflectivity changes of SrRuO3 films. The energy-dependence of normalized intensities of pump and probe optics is presented in Figure 3b. The photon energy (wavelength) of the probe optical pulses is exactly in resonance to the Ru t2g intra-band excitation,24 suggesting that ∆𝑅/𝑅 spectrum reflects variations of the electronic states in Ru t2g bands. Calculations of band structures of SrRuO3. We performed first-principles calculations through the density functional theory (DFT) approach implemented in the Vienna ab initio simulation package (VASP), to calculate band structures of SrRuO3.25 The projector-augmented wave (PAW) potentials were used to model the ion cores. A plane-wave basis set with a cut-off energy of 500 eV was employed to model the valence electrons through the Perdewe-BurkeeErnzerhof (PBE) functional.26 To correct the on-site Coulomb interaction of the Ru 4d orbitals in the electronic structure calculations, the generalized gradient approximation (GGA)+U method was employed with 𝑈 = 2.5 eV and 𝐽 = 0.4 eV. The Brillouin-zone integrations were performed using a Gaussian smearing of SIGMA = 0.1 eV.

RESULTS AND DISCUSSIONS


8

Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Photo-induced strain in SrRuO3. The generation of photo-induced strain that is defined as 𝑆(𝑡) =

𝑐(𝑡) ― 𝑐0 𝑐0

is evidenced by the shift of the Bragg angle after photoexcitation, as illustrated

in Figure 4a. 𝑐(𝑡) is the out-of-plane lattice parameter of SrRuO3 measured at the delay time 𝑡, and 𝑐0 is the lattice parameter measured when the pump laser was off. Photoexcitation of SrRuO3 by laser pulses with the energy fluence of 1.5 mJ cm-2 gives rise to the lattice strain of about 0.05%. The strain in SrRuO3 after photoexcitation is smaller than photo-induced strain observed in other studies even by one order of magnitude,10 because of the smaller energy fluence 𝐹p of the pump laser we exert. When we increase the pump fluence, the photo-induced strain increases (Figure 4b). The linear dependence of the photo-induced strain on 𝐹p indicates that no Auger process which would lead to nonlinear features occurs in SrRuO3. The thermal effect resulting from electron-phonon scattering processes was believed to cause the lattice strain. However, the thermal strain (𝑆q < 0.038%) we calculate, as proceeding in Supporting Information Note I, through the energy (∆𝐸e ― ph) transferred from excited carriers to phonons, is smaller than the photo-induced strain (0.05%) observed in Figure 4. This suggests that more phonon-assisted processes that remain to be identified may cause the light-induced deformation of SrRuO3. Coupled carrier and phonon dynamics in SrRuO3. We measured both the timedependent lattice strain and optical reflectivity changes ∆𝑅/𝑅, to explore photoexcitation of SrRuO3 and subsequent relaxations of electrons and phonons. The photo-induced strain shows a long relaxation process on nanosecond time scales (Figure 5a), and ∆𝑅/𝑅 shows complete relaxation in one nanosecond (Figure 5b). In the time-dependent lattice strain, the thermal component 𝑆q(𝑡) can be simulated from a one-dimensional thermal diffusion model (see


9


Page 10 of 27

Supporting Information for simulation details) and be deducted from the photo-induced strain in SrRuO3. This eliminates the effects of the unexpected strain at negative delay time which is a result of the incomplete thermal relaxation between two sequential laser pump pulses (Figure 5a). The remaining portion of the lattice strain 𝑆c(𝑡) = 𝑆(𝑡) ― 𝑆q(𝑡) describes carriers-related phonon relaxations in SrRuO3. Biexponential functions respectively presented in eq 1 and eq 2, with the assumption of 𝜏c2 = 𝜏l1, well fit 𝑆c(𝑡) and ∆𝑅/𝑅, as illustrated in Figure 5. This suggests that multiple carrier and phonon dynamics occur, and carrier and phonon relaxations are coupled on the time scale of 𝜏c2 (𝜏l1). Fitting results yield 𝜏l2 = 7.5 ± 1.8 ns, 𝜏c2 = 𝜏l1 = 0.8 ± 0.1 ns and 𝜏c1 = 52 ± 35 ps. Error bars are derived from the data fitting. 𝑆c(𝑡) = 𝑎l1e ―𝑡/𝜏l1 + 𝑎l2e ―𝑡/𝜏l2

(1)

∆𝑅/𝑅 = 𝑎c1e ―𝑡/𝜏c1 + 𝑎c2e ―𝑡/𝜏c2

(2)

The slow relaxation process (𝜏l2) observed in 𝑆c(𝑡) corresponds to the phonon-phonon scattering process. Even thermal dissipation has been excluded, the observation of the slow relaxation is due to the discrepancy between the ideal model to calculate 𝑆q(𝑡) and the practical situation. In the one-dimensional thermal diffusion model, we assume the insulating boundary condition at the SrRuO3 surface and the ideal boundary condition at the SrRuO3/SrTiO3 interface. The relaxation time of 𝜏l2 and the thermal conductivity of SrRuO3 are in the trend that the relaxation time of the thermal strain decreases when increasing the thermal conductivity as observed in diverse materials, as discussed in Supporting Information Note I. The relaxation process on the time scale of 𝜏c1 observed in ∆𝑅/𝑅 is absent in the timeresolved strain profile. This confirms that the relaxation (𝜏c1) relates to carrier dynamics without phonon absorption or emission. Photoexcitation of SrRuO3 induces inter-band transitions of


10

Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electrons from O 2p to Ru 4d t2g states, and ∆𝑅/𝑅 reflects variations of electronic states in Ru 4d t2g bands (Figure 2b).9, 24 These suggest that the fast relaxation (𝜏c1) is assigned to the direct electron-hole recombination process. Most interestingly, we observe the coupling between carrier and phonon relaxations on the time scale of 𝜏c2 (𝜏l1). In eq 1 and eq 2, we ignore the difference in 𝜏c2 and 𝜏l1 resulting from different photon energy 𝐸ph and energy fluence 𝐹p of pump lasers we employed in UXRD and UR experiments. This ignorance is based on the fact that the relaxation of SrRuO3 is almost independent of 𝐸ph and 𝐹p as presented in Supporting Information Note III. A two-temperature model (TTM) is used to describe time evolutions of the increasing temperature of two subsystems (conduction electrons with an increasing temperature ∆𝑇e and lattice with an increasing temperature ∆𝑇i) in SrRuO3. 𝑑(∆𝑇e,1 + ∆𝑇e,2) 𝐶e = ― 𝐺1(∆𝑇e,1 ― ∆𝑇i,1) ― 𝐺2(∆𝑇e,2 ― ∆𝑇i,2) + 𝑃(𝑡) 𝑑𝑡 𝑑(∆𝑇i,1 + ∆𝑇i,2)

𝐶i

𝑑𝑡

𝑑2(∆𝑇i,1 + ∆𝑇i,2)

= 𝜅i

𝑑𝑧2

+ 𝐺1(∆𝑇e,1 ― ∆𝑇i,2) + 𝐺2(∆𝑇e,2 ― ∆𝑇i,2)

(3)

(4)

In eq 3 and eq 4, 𝐶e and 𝐶i are specific heats of electrons and lattice, respectively. 𝜅i is the heat conductivity of lattice, and 𝑃(𝑡) is the power of pump lasers. In above equations, in addition to the electron-phonon scattering occurring on sub-picosecond time scales, which is described by electron-phonon coupling constant 𝐺1, we also take into considerations electronphonon scattering on the time scale of 𝜏c2. The strength of the scattering occurring on the longer time scale is denoted by 𝐺2. We assume that the electron temperature is homogeneous, and that the heat transport is one-dimensional because of the large ration between laser spot diameter and


11


Page 12 of 27

film thickness. The lattice strain 𝛼(∆𝑇i,1 + ∆𝑇i,2) (𝛼 = 10 ―5 K ―1 is the linear thermal expansion coefficient of SrRuO3) calculated from eq 3 and eq 4 (see Supporting Information Note IV for calculation details) well reproduces the experimental results as presented in Figure S3. The data fitting yields 𝐺2 = 3.0 × 1014 W K ―1 m ―3 that is smaller than 𝐺1 by two orders of magnitude. This confirms that carrier and phonon relaxations are coupled on a longer time scale of 𝜏c2 (𝜏l1). We look into the band structure of SrRuO3 and electron transitions after photoexcitation (Figure 5c). In our studies, photoexcitation of SrRuO3 with the photon energy varying from 2.41 to 3.62 eV mainly induces the inter-band transition (process i in Figure 5d) of electrons from O 2p to Ru t2g states.24 Similar with that observed in other materials, including metals16, 17 and semiconductors,27 photoexcited carriers in high-energy states rapidly relax via energy transfer to phonons on the sub-picosecond time scale. This intra-band relaxation induces the increasing phonon temperature and hence the thermal expansion. However, photoexcited carriers are in non-equilibrium states in reciprocal space, as shown in Figure 5c, some of which rapidly recombine on the timescale of 𝜏c1 (process iii in Figure 5d), while the remainder are trapped and momentum-forbidden for the direct recombination (black arrows in Figure 5c and process iv in Figure 5d). This gives rise to the phonon-assisted carrier relaxation pathway that also contributes to the photo-induced strain in SrRuO3. The carrier’s trapping on a longer time scale is confirmed by the observation of the relaxation process with the relaxation time 𝜏c2 in ∆𝑅/𝑅, since the optical reflectivity 𝑅 reflects the electronic states in Ru t2g energy bands. The coupled carrier and phonon dynamics in SrRuO3 is different from the dynamics in materials with direct band gaps where direct recombination might be dominate.28 Phonon effects on photo-induced strain and relaxations of SrRuO3. The initial relaxation of SrRuO3 after photoexcitation is dictated by direct carrier recombination, while the


12

Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


phonon-assisted carrier dynamics and phonon-phonon scattering dominate at longer timescales. This indicates that the relaxation of SrRuO3 is slowed down when phonons are involved. We performed UXRD experiments at different temperatures, to study the effects of phonons on the structural and carrier dynamics of SrRuO3. Since the Debye temperature ΘD of SrRuO3 is about 525 K,29 the number of thermally activated phonons depends greatly on the temperature when the temperature is far below ΘD. Figure 6a shows the photo-induced strain after 600-ps excitation of SrRuO3 recorded from 130 to 280 K. The photo-induced strain increases with increasing temperature. We attribute the large deformation at high temperature both to the increasing number of photoexcited carriers as inferred from the enhanced optical conductivity 𝜎 of SrRuO3,24 and to the enhanced electron-phonon scattering as discussed in Supporting Information Note V. Figure 6b shows relaxations of photo-induced strain in SrRuO3 measured at 120 K and 300 K. Obviously, the induced strain at low temperature relaxes within a shorter time while a long-lived process is observed at high temperature, especially for phonon-assisted carrier dynamics (𝜏l1). The slower relaxation at elevated temperature is because that photoexcited carriers transfer more energy to lattice via enhanced electron-phonon scattering on the time scale of 𝜏l1. In contrast to the temperature dependent 𝜏l1, a smaller 𝜏l2 is found at the higher temperature (Figure 6b), because of the increasing thermal conductivity with rising temperature,30 since 𝜏l2 relates to the thermal dissipation as discussed previously. Our results suggest that there is a bottleneck effect of phonons on the coupled carrier and phonon relaxation process. Scaled relevance of the fast carrier relaxation and electron correlation. The direct recombination in SrRuO3 is considerably slower than the fast carrier relaxation in noble


13


Page 14 of 27

metals,16, 17 whose characteristic times vary from the sub-picosecond to picoseconds. The differences in fast carrier relaxation in these materials may be attributed to the disparity of electron correlation. Figure 7 shows a general pattern for different materials that the carrier relaxation (𝜏d, 𝜏c1 mentioned above) is slower as the mean free path (𝑙MFP) decreases.16, 17, 31-37 The characteristic time 𝜏d (𝜏c1 mentioned above) as a function of the 𝑙MFP is well fitted by 𝜏𝑑/𝜏c1 = (𝑙

𝑎

MFP/𝑙SRO)

𝑏

where 𝑙SRO is the mean free path of electrons in SrRuO3. The data fitting yields

dimensionless parameters 𝑎 = 23.5 ± 2.4 and 𝑏 = 0.83 ± 0.10 (Figure 7). We note that above equation is similar to the form of an equation derived from the Fermi-liquid model which gives 𝑚2𝑒 𝑣𝐹

the free electron relaxation time 𝜏𝑑 ∝ 𝑛2𝑙

𝑒 MFP

1

∝ 𝑙MFP,38 but the scaling number 𝑏 shows a slight

difference. Diverse effective mass 𝑚e, Fermi velocity 𝑣F and carrier density 𝑛e in different materials31, 36, 37 may lead to a variation of the scaling number, but only have comparatively small effects on the divergence. The Fermi liquid assumption is applicable for metals like Au and Ag; however, this assumption is invalid for SrRuO3 at high temperature because phonons may be important. This may give a hint of the phonon effect of SrRuO3 on the bad-metal behavior in the high temperature region, which gives rise to the divergence of scaling number b.

CONCLUSIONS To conclude, time-resolved strain and carrier dynamics of SrRuO3 are revealed by UXRD and UR measurements. Relaxations linked with direct recombination, phonon-assisted carriers relaxation and phonon-phonon scattering are identified. The phonon-assisted relaxation process, together with the thermal effect, leads to the photo-induced strain in SrRuO3. The bottleneck effect of phonons on the coupled carrier and phonon relaxation is revealed by temperature-


14

Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dependent UXRD measurements. We observe a slow carrier relaxation pathway with enhanced electron correlation. This also explains the temperature dependence of the carrier relaxation observed in noble metals.16 With increasing temperature, electron interplay increases and the carrier relaxation is slowed down. This is different from the temperature dependent 𝜏l1 where the phonon is the predominant factor. These results clarify the detailed origination of photo-induced strain in SrRuO3, and indicate that the study of ultrafast carrier and phonon dynamics may provide a new avenue to understand the phonon effect occurring in the bad-metal region above Curie temperature. From this aspect, more intriguing dynamics may be present in SrRuO3 that remain to be explored. For example, when temperature is below the Curie temperature 𝑇C ≈ 150 K, magnetostriction effect has been observed;39 however, magnetic or phonon effects on diverse dynamics remain unclear. This requires studies of a variety of SrRuO3 systems, which may also give a hint on much more interesting physics. Phonon and electron effects on relaxation processes provide a guidance to achieve strain manipulation by light via modulating phonon and electronic states, which may be applied in future optomechanical systems. The large characteristic time of the carrier’s separation compared with that observed in conventional metals also infers that SrRuO3 has a great potential in applications of optoelectronic devices. Via band manipulations such as fabricating materials with the indirect band gap,8 the relaxation time may be greatly enhanced which may also give rise to an enhancement of the photo current.

ASSOCIATED CONTENT Supporting Information.


15


Page 16 of 27

Supplementary descriptions and results of simulation details for the thermally-induced strain; Ultrafast X-ray diffraction and ultrafast optical reflectivity measurements; Relaxations of SrRuO3 depending on energy fluence and photon energy of pump laser pulses; The twotemperature model and electron-phonon scattering process. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions K.W., B.Z., X.W., Z.C. and Y.T. performed the ultrafast x-ray diffraction experiments. W.X. prepared the sample. K.W., S.L. and C.Z. performed the time-resolved optical reflectivity measurements. K.W., W.X., T.Y., H.C., F.Z. and X.W. performed the high-resolution x-ray diffraction and transport experiment. M. G. conducted the first-principles calculations. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the beamline BL14b1 of Shanghai Synchrotron Radiation Facility (SSRF), beamline 1W1A and 1W2B of Beijing Synchrotron Radiation Facility (BSRF) for providing the beam time. We acknowledge the financial support by the National Natural Science Foundation of China through Grant No. 11874200, and by the Major Research Plan through Grant No. 2017YFA0303202.


16

Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES (1) Koster, G.; Klein, L.; Siemons, W.; Rijnders, G.; Dodge, J. S.; Eom, C.-B.; Blank, D. H. A.; Beasley, M. R. Structure, physical properties, and applications of SrRuO3 thin films. Rev. Mod. Phys. 2012, 84, (1), 253-298. (2) Jeng, H.-T.; Lin, S.-H.; Hsue, C.-S. Orbital Ordering and Jahn-Teller Distortion in Perovskite Ruthenate SrRuO3. Phys. Rev. Lett. 2006, 97, (6), 067002. (3) Kaur, P.; Sharma, K. K.; Pandit, R.; Choudhary, R. J.; Kumar, R. Structural, electrical, and magnetic properties of SrRuO3 thin films. Appl. Phys. Lett. 2014, 104, (8), 081608. (4) Ishigami, K.; Yoshimatsu, K.; Toyota, D.; Takizawa, M.; Yoshida, T.; Shibata, G.; Harano, T.; Takahashi, Y.; Kadono, T.; Verma, V. Thickness-dependent magnetic properties and strain-induced orbital magnetic moment in SrRuO3 thin films. Phys. Rev. B 2015, 92, (6), 064402. (5) Wang, C.; Chen, C.; Chang, C.-H.; Tsai, H.-S.; Pandey, P.; Xu, C.; Böttger, R.; Chen, D.; Zeng, Y.; Gao, X. Defect-induced exchange bias in a single SrRuO3 layer. ACS Appl. Mater. Interfaces 2018, 10, (27472). (6) Verissimo-Alves, M.; García-Fernández, P.; Bilc, D. I.; Ghosez, P.; Junquera, J. Highly confined spin-polarized two-dimensional electron gas in SrTiO3/SrRuO3 superlattices. Phys. Rev. Lett. 2012, 108, (10), 107003. (7) Gu, M.; Wang, K.; Wang, Y.; Xie, Q.; Cai, H.; Zhang, G.-P.; Wu, X. Enhancement of orbital ordering and spin polarization by controlling the dimensionality of the octahedra network. npj Quantum Mater. 2016, 1, 16011. (8) Liu, H.-J.; Wang, J.-C.; Cho, D.-Y.; Ho, K.-T.; Lin, J.-C.; Huang, B.-C.; Fang, Y.-W.; Zhu, Y.-M.; Zhan, Q.; Xie, L. Giant photoresponse in quantized SrRuO3 monolayer at oxide interfaces. ACS Photonics 2018, 5, (3), 1041-1049. (9) Lee, S. A.; Oh, S.; Hwang, J.-Y.; Choi, M.; Youn, C.; Kim, J. W.; Chang, S. H.; Woo, S.; Bae, J.-S.; Park, S. Enhanced electrocatalytic activity via phase transitions in strongly correlated SrRuO3 thin films. Energy Environ. Sci. 2017, 10, (4), 924-930. (10) Wei, T.-C.; Wang, H.-P.; Liu, H.-J.; Tsai, D.-S.; Ke, J.-J.; Wu, C.-L.; Yin, Y.-P.; Zhan, Q.; Lin, G.-R.; Chu, Y.-H.; He, J.-H. Photostriction of strontium ruthenate. Nat. Commun. 2017, 8, 15018-15018. (11) Peng, C.-Y.; Huang, C.-F.; Fu, Y.-C.; Yang, Y.-H.; Lai, C.-Y.; Chang, S.-T.; Liu, C. Comprehensive study of the Raman shifts of strained silicon and germanium. J. Appl. Phys. 2009, 105, (8), 083537. (12) Schick, D.; Herzog, M.; Bojahr, A.; Leitenberger, W.; Hertwig, A.; Shayduk, R.; Bargheer, M. Ultrafast lattice response of photoexcited thin films studied by X-ray diffraction. Struct. Dynam. 2014, 1, (6), 064501. (13) Bojahr, A.; Schick, D.; Maerten, L.; Herzog, M.; Vrejoiu, I.; von Korff Schmising, C.; Milne, C.; Johnson, S. L.; Bargheer, M. Comparing the oscillation phase in optical pump-probe spectra to ultrafast x-ray diffraction in the metal-dielectric SrRuO3/SrTiO3 superlattice. Phys. Rev. B 2012, 85, (22), 224302. (14) Herzog, M.; Bojahr, A.; Goldshteyn, J.; Leitenberger, W.; Vrejoiu, I.; Khakhulin, D.; Wulff, M.; Shayduk, R.; Gaal, P.; Bargheer, M. Detecting optically synthesized quasimonochromatic sub-terahertz phonon wavepackets by ultrafast x-ray diffraction. Appl. Phys. Lett. 2012, 100, (9), 094101.


17


Page 18 of 27

(15) Bojahr, A.; Gohlke, M.; Leitenberger, W.; Pudell, J.; Reinhardt, M.; von Reppert, A.; Roessle, M.; Sander, M.; Gaal, P.; Bargheer, M. Second Harmonic Generation of Nanoscale Phonon Wave Packets. Phys. Rev. Lett. 2015, 115, (19), 195502. (16) Groeneveld, R. H.; Sprik, R.; Lagendijk, A. Femtosecond spectroscopy of electronelectron and electron-phonon energy relaxation in Ag and Au. Phys. Rev. B 1995, 51, (17), 11433. (17) Del Fatti, N.; Voisin, C.; Achermann, M.; Tzortzakis, S.; Christofilos, D.; Vallée, F. Nonequilibrium electron dynamics in noble metals. Phys. Rev. B 2000, 61, (24), 16956. (18) Daranciang, D.; Highland, M. J.; Wen, H.; Young, S. M.; Brandt, N. C.; Hwang, H. Y.; Vattilana, M.; Nicoul, M.; Quirin, F.; Goodfellow, J. Ultrafast photovoltaic response in ferroelectric nanolayers. Phys. Rev. Lett. 2012, 108, (8), 087601. (19) Wen, H.; Chen, P.; Cosgriff, M. P.; Walko, D. A.; Lee, J. H.; Adamo, C.; Schaller, R. D.; Ihlefeld, J. F.; Dufresne, E. M.; Schlom, D. G. Electronic origin of ultrafast photoinduced strain in BiFeO3. Phys. Rev. Lett. 2013, 110, (3), 037601. (20) Holtz, M.; Hauf, C.; Salvador, A.-A. H.; Costard, R.; Woerner, M.; Elsaesser, T. Shiftcurrent-induced strain waves in LiNbO3 mapped by femtosecond x-ray diffraction. Phys. Rev. B 2016, 94, (10), 104302. (21) Kawasaki, M.; Takahashi, K.; Maeda, T.; Tsuchiya, R. Atomic control of the SrTiO (3) crystal surface. Science 1994, 266, (5190), 1540. (22) Gu, M.; Xie, Q.; Shen, X.; Xie, R.; Wang, J.; Tang, G.; Wu, D.; Zhang, G.; Wu, X. Magnetic ordering and structural phase transitions in a strained ultrathin SrRuO3/SrTiO3 Superlattice. Phys. Rev. Lett. 2012, 109, (15), 157003. (23) Sun, D.-R.; Xu, G.-L.; Zhang, B.-B.; Du, X.-Y.; Wang, H.; Li, Q.-J.; Zhou, Y.-F.; Li, Z.J.; Zhang, Y.; He, J. Implementation of ultrafast X-ray diffraction at the 1W2B wiggler beamline of Beijing Synchrotron Radiation Facility. J. Synchrotron Radiat. 2016, 23, (3), 830-835. (24) Jeong, D.; Choi, H. C.; Kim, C. H.; Chang, S. H.; Sohn, C.; Park, H.; Kang, T.; Cho, D.Y.; Baek, S.; Eom, C. Temperature Evolution of Itinerant Ferromagnetism in SrRuO3 Probed by Optical Spectroscopy. Phys. Rev. Lett. 2013, 110, (24), 247202. (25) Anisimov, V. I.; Zaanen, J.; Anderson, O. K. Band theory and Mott insulators: Hubbard U instead of Stoner I. Phys. Rev. B 1991, 44, 943. (26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, (18), 3865. (27) Rose-Petruck, C.; Jimenez, R.; Guo, T.; Cavalleri, A.; Siders, C. W.; Rksi, F.; Squier, J. A.; Walker, B. C.; Wilson, K. R.; Barty, C. P. Picosecond–milliångström lattice dynamics measured by ultrafast X-ray diffraction. Nature 1999, 398, (6725), 310. (28) ’t Hooft, G.; Van Opdorp, C. Temperature dependence of interface recombination and radiative recombination in (Al, Ga) As heterostructures. Appl. Phys. Lett. 1983, 42, (9), 813-815. (29) Kiyama, T.; Yoshimura, K.; Kosuge, K.; Ikeda, Y.; Bando, Y. Invar effect of SrRuO3: Itinerant electron magnetism of Ru 4 d electrons. Phys. Rev. B 1996, 54, (2), R756. (30) Oh, D.-W.; Ravichandran, J.; Liang, C.-W.; Siemons, W.; Jalan, B.; Brooks, C. M.; Huijben, M.; Schlom, D. G.; Stemmer, S.; Martin, L. W. Thermal conductivity as a metric for the crystalline quality of SrTiO3 epitaxial layers. Appl. Phys. Lett. 2011, 98, (22), 221904. (31) Vancea, J.; Hoffmann, H.; Kastner, K. Mean free path and effective density of conduction electrons in polycrystalline metal films. Thin Solid Films 1984, 121, (3), 201-216. (32) Elsayed-Ali, H.; Norris, T.; Pessot, M.; Mourou, G. Time-resolved observation of electron-phonon relaxation in copper. Phys. Rev. Lett. 1987, 58, (12), 1212.


18

Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(33) Beaurepaire, E.; Merle, J.-C.; Daunois, A.; Bigot, J.-Y. Ultrafast spin dynamics in ferromagnetic nickel. Phys. Rev. Lett. 1996, 76, (22), 4250. (34) Zhang, W.; Brongersma, S. H.; Richard, O.; Brijs, B.; Palmans, R.; Froyen, L.; Maex, K. Influence of the electron mean free path on the resistivity of thin metal films. Microelectron. Eng. 2004, 76, (1), 146-152. (35) Li, T.; Luo, L.; Hupalo, M.; Zhang, J.; Tringides, M.; Schmalian, J.; Wang, J. Femtosecond population inversion and stimulated emission of dense Dirac fermions in graphene. Phys. Rev. Lett. 2012, 108, (16), 167401. (36) Palenskis, V. Drift mobility, diffusion coefficient of randomly moving charge carriers in metals and other materials with degenerated electron gas. World J. Condens. Matter Phys. 2013, 3, (01), 73. (37) Gall, D. Electron mean free path in elemental metals. J. Appl. Phys. 2016, 119, (8), 085101. (38) Gamaly, E. G.; Rode, A. V. Physics of ultra-short laser interaction with matter: From phonon excitation to ultimate transformations. Prog. Quant. Electron. 2013, 37, (5), 215-323. (39) Schmising, C. v. K.; Harpoeth, A.; Zhavoronkov, N.; Ansari, Z.; Aku-Leh, C.; Woerner, M.; Elsaesser, T.; Bargheer, M.; Schmidbauer, M.; Vrejoiu, I. Ultrafast magnetostriction and phonon-mediated stress in a photoexcited ferromagnet. Phys. Rev. B 2008, 78, (6), 060404.


19


Page 20 of 27

Figure 1. Pulsed laser deposition system that is used to deposit SrRuO3 films on SrTiO3 substrates with TiO2-termination. The insert shows the schematic view of the SrRuO3 unit cell with orthorhombic symmetry.


20

Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. High-resolution x-ray diffraction of a SrRuO3 film conducted at beamline BL14B1 of Shanghai Synchrotron Radiation Facility (SSRF). The X-ray wavelength is 1.293 Å.


21


Page 22 of 27

Figure 3. (a) Schematic diagram of the ultrafast X-ray diffraction (UXRD) configurations. Electrons (gray circle) are excited to a high-energy band by an ultrashort laser pulse, leaving mobile holes (white circle) in the low-energy band. The energy of photo excited electrons is then transferred to phonons by electron-phonon scattering, which induces lattice strain (red curve) in SrRuO3. (b) The energy-dependence of normalized intensities of pump and probe optics employed in the ultrafast optical reflectivity (UR) measurements. Blue dots are representatives of diverse optical transitions at different photon energies.


22

Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) X-ray diffraction patterns for a SrRuO3/SrTiO3 film before (black) /after (red) laser excitation. The pump energy fluence is about 1.5 mJ cm-2 and the photon energy (wavelength) is about 2.38 eV (515 nm). (b) Photo-induced strain relaxation as a linear function of pump fluence (𝐹p).


23


Page 24 of 27

Figure 5. Laser excitation and electron-phonon relaxation processes in SrRuO3 films. (a) Strain profile at different delay times derived from the time-resolved X-ray diffraction patterns shown in Supplementary Note I. The pump energy fluence is about 1.5 mJ cm-2 and the photon energy (wavelength) is about 2.38 eV (515 nm). The blue dotted line represents the biexponential fitting of the strain (𝑆c(𝑡)) excluding the thermal contributions (𝑆q(𝑡)). The black solid line is the sum of 𝑆c(𝑡) and 𝑆q(𝑡). (b) Ultrafast optical reflectivity changes as a function of delay time. The experimental data is fitted by a biexponential function (black line). (c) Band dispersions of SrRuO3 calculated from the first-principles calculations. Solid and dotted lines represent the majority and minority spin states, respectively. Ru t2g bands (red) are mainly located between -1 and 1 eV. O p states dominate in the bands (black) located below -2 eV. Photoexcitation in our studies leads to an inter-band transition from O p to Ru t2g states. Then excited carriers may be trapped (black arrows) because of the non-equilibrium states in reciprocal space. (d) Schematic diagram of the photoexcitation and relaxation processes. Processes (i-iv) represent (i)


24

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


photoexcitation of SrRuO3, (ii) electron-phonon scattering, (iii) direct carrier recombination, (iv) trap of carriers leading to the phonon-assisted carrier relaxation process.


25


Page 26 of 27

Figure 6. (a) Photo-induced strain in SrRuO3 as a function of temperature. Error bars are derived from multiple measurements. The gray solid line is a visual guide. (b) Time-resolved strain (scatters) and data fittings (shadow lines) at 300 (round) and 120 K (diamond). The inset shows characteristic times of 𝜏l1 and 𝜏l2 derived from numerical fittings. Error bars are from the data fitting.


26

Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. The relationship between the characteristic time 𝜏d of the fast carrier relaxation and the mean free path (𝑙MFP) in different materials. The line is fitted by 𝜏𝑑/𝜏c1 = (𝑙

𝑎

MFP/𝑙SRO)

𝑏

with

dimensionless parameters 𝑎 = 23.5 ± 2.4 and 𝑏 = 0.83 ± 0.10.


27

Coupling Among Carriers and Phonons in Femtosecond Laser Pulses

Recommend Documents