Novel photoinduced phase transitions in transition metal oxides and diluted magnetic semiconductors
© Mizokawa; licensee Springer. 2012
Received: 17 July 2012
Accepted: 3 October 2012
Published: 23 October 2012
Some transition metal oxides have frustrated electronic states under multiphase competition due to strongly correlated d electrons with spin, charge, and orbital degrees of freedom and exhibit drastic responses to external stimuli such as optical excitation. Here, we present photoemission studies on Pr0.55(Ca1 − ySr y )0.45MnO3 (y = 0.25), SrTiO3, and Ti1 − xCo x O2 (x = 0.05, 0.10) under laser illumination and discuss electronic structural changes induced by optical excitation in these strongly correlated oxides. We discuss the novel photoinduced phase transitions in these transition metal oxides and diluted magnetic semiconductors on the basis of polaronic pictures such as orbital, ferromagnetic, and ferroelectric polarons.
In metal oxides and diluted magnetic semiconductors containing transition metal ions, correlated transition metal d electrons with spin, charge, and orbital degrees of freedom provide surprisingly rich electric and magnetic properties. In particular, electron–electron and electron-lattice interactions for the d electrons produce frustrated electronic states under multiphase competition which exhibit drastic responses to external stimuli such as optical excitation. Various photoinduced phase transitions have been reported in transition metal oxides such as Pr0.55(Ca1 − ySry)0.45MnO3 and in diluted magnetic semiconductors including In1 − xMn x As. In Pr0.55(Ca1 − ySr y )0.45MnO3, the optically excited electrons locally destroy the spin-charge-orbital ordering, followed by rapid expanding of the disordered state for a persistent phase transition. On the other hand, in the diluted magnetic semiconductors such as In1 − xMn x As, the optically excited carriers globally align the transition metal spins to induce ferromagnetism. Photoemission spectroscopy is one of the powerful techniques to observe the electronic structural changes induced by the optical excitation as demonstrated in various transition metal compounds such as spinel-type CuIr2S4 and perovskite-type Cs2Au2Br6. Here, we discuss photoinduced electronic states at the surfaces of Pr0.55(Ca1 − ySr y )0.45MnO3 (y = 0.25), SrTiO3, and Ti1 − xCo x O2 (x = 0.05, 0.10) on the basis of polaronic pictures.
The ultraviolet photoemission spectroscopy (UPS) measurements were performed using a SES-100 analyzer (VG Scienta, Tokyo, Japan) with the He I line (hν = 21.2 eV) as a vacuum ultraviolet source. The energy resolution was set to 30 meV. The X-ray photoemission spectroscopy (XPS) measurements were performed using a JPS-9200 analyzer (JEOL Ltd., Akishima, Tokyo, Japan) with monochromatized AlKα (hν = 1486.6 eV) as an X-ray source. The energy resolution was set to 600 meV. Base pressures of the photoemission chambers were in the 1 × 10−7 Pa range. For the optical excitation, a Nd:YAG laser was used which released 10 to 30 pulse shots per second. The second and third harmonic waves (wavelengths of 532 and 355 nm) were produced and introduced into the photoemission chamber through a quartz window. The laser output per pulse was set to 0.1 to 2.0 mJ/pulse. In order to obtain a clean surface of SrTiO3 which is suitable for the observation of the photoinduced effect, single crystals of SrTiO3 should be annealed at 870 K for 20 min under oxygen pressure ranging from 3 × 10−4 to 7 × 10−4 Pa. If the crystal was annealed at 870 K with PO2 lower than 3 × 10−4 Pa, the crystal surface was reduced too much, and the giant photoconductivity at 20 K was lost. The samples were sandwiched by the copper plates which were grounded. The entire sample surface and the edge of the copper plate were irradiated by the laser light to establish the conducting path. High-quality thin-film samples of perovskite-type Pr0.55(Ca1 − ySr y )0.45MnO3 and anatase-type Ti1 − xCo x O2 − δ have been provided by the Miyano group, University of Tokyo and Kawasaki group, Univesity of Tokyo. The details of the Pr0.55(Ca1 − ySr y )0.45MnO3 thin film are reported in, and those of the Ti1 − xCo x O2 − δ thin film is reported in. We measured the anatase-type Ti1 − xCo x O2 − δ thin films (thickness of 40 nm) which typically show electron mobility with two orders of magnitude higher than the rutile films. Samples of n = 1.1 × 1019 and 4.5 × 1018 cm−3, and n = 8.7 × 1019 and 3.5 × 1019 cm−3 were prepared for x = 0.05 and 0.10, respectively, where n is the carrier concentration and is controlled by the amount of the oxygen deficiency δ. Ferromagnetism was clearly observed above room temperature in all samples except the x = 0.05 and n = 4.5 × 1018 cm−3 sample.
Results and discussion
Pr0.55(Ca1 − ySry)0.45MnO3 and SrTiO3 have the perovskite structure. In Pr0.55(Ca1 − ySr y )0.45MnO3, Mn4+(d3, S = 3/2) and Mn3+(d4, S = 2) configurations are mixed, and spin-charge-orbital orderings are expected. On the other hand, SrTiO3 with Ti4+(d0) is basically a wide-gap semiconductor. Pr0.55(Ca1 − ySr y )0.45MnO3 exhibits competition between the ferromagnetic metallic (FM) state and the antiferromagnetic charge-orbital ordered insulating (COOI) state. The boundary between the FM state and the COOI state is located at y = 0.25 in the electronic phase diagram of Pr0.55(Ca1 − ySr y )0.45MnO3. At the boundary with the phase competition, Takubo et al. have discovered a photoinduced transition from the COOI state to the FM state using resistivity measurement under laser illumination. In this context, the perovskite-type Mn oxide is one of the most important systems to study their electronic structural changes by the photoexcitation. On the other hand, SrTiO3 exhibits quantum paraelectricity and giant photoconductivity at low temperatures[7–10]. In the quantum paraelectric state, the dielectric constant of SrTiO3 dramatically increases with decreasing temperature although SrTiO3 does not reach ferroelectric transition even at the lowest temperature. The quantum paraelectric state is very fragile against external perturbations such as uniaxial stress, the electric field, and the isotope substitution[7–10]. As for the giant photoconductivity, the ultraviolet light irradiation induces Ti 3d carriers with high mobility and provides a dramatic increase of carrier density at low temperatures[11, 12]. In addition, green luminescence with a long lifetime up to several milliseconds is observed under the ultraviolet irradiation at low temperatures. In an infrared absorption study by Okamura et al., a broad absorption band appears under ultraviolet light irradiation, indicating that the electronic structure of SrTiO3 is strongly modified by the ultraviolet light irradiation.
The photoinduced spectral weight in the bandgap is consistent with the absorption band under ultraviolet light irradiation. The spectral weight in the bandgap indicates that most of the photoexcited carriers are trapped in the in-gap states. However, the tail of the in-gap states reaches the Fermi level and is probably responsible for the giant photoconductivity. As shown in the inset of Figure2, the spectral weight at the Fermi level is increased under the ultraviolet illumination. Therefore, one can speculate that the photoinduced free carriers at the Fermi level and the electrons trapped within the bandgap coexist in SrTiO3 under the ultraviolet illumination. As for the electrons trapped in the bandgap, the electrons with higher binding energies are expected to be more localized due to the local lattice distortion of the TiO6 octahedron. This is because the more localized electrons tend to be more stabilized due to the strong electron-lattice interaction. Also, the more localized electrons with higher binding energies are expected to couple with the photocreated O 2p holes more strongly due to the Coulomb attractive force when the localized electrons are close to the localized O 2p holes.
The Ti 3d yz, zx, and xy electrons are stabilized due to the lattice distortions which induce the dipole moments along the x-, y-, and z-axes and due to the excitonic coupling with the O 2p x, y, and z holes. For example, under the distortion with the dipole moments along the z-axis, the Ti 3d xy electrons and the O 2p z holes are stabilized. The photoinduced in-gap states made up from the Ti 3d xy orbital are expected to induce such lattice distortion and dipole moment. Therefore, the enhancement of the dielectric constant under the ultraviolet illumination in SrTiO3 is probably related to the formation of the in-gap states with Ti 3d xy orbital polarization. This situation, which is schematically shown in Figure2, can be viewed as a kind of ferroelectric polaron where the photoinduced Ti 3d electron is surrounded by the dipole moments of the distorted TiO6 octahedra. The ferroelectric polaron is consistent with most of the experimental results on the quantum paraelectric phase of SrTiO3.
Ti1 − xCo x O2 − δ has been attracting great interest since the discovery of the carrier-induced ferromagnetism at room temperature. The carrier-induced ferromagnetism has been supported by various experiments including the anomalous Hall resistivity, magnetic circular dichroism, and photoemission spectroscopy[18–21]. In this mechanism, the ferromagnetism is driven by the exchange interaction between the Co spins and the n-type carriers in the Ti 3d band. Since Ti1 − xCo x O2 − δ shows carrier-induced ferromagnetism, it is expected that photoinduced carriers by ultraviolet illumination cause a magnetic transition similar to In1 − xMn x As. In this context, the photoemission study of Ti1 − xCo x O2 − δ under ultraviolet illumination is very interesting and useful. In the previous photoemission study on rutile-type Ti1 − xCo x O2 − δ thin films, the core-level peak position under ultraviolet illumination was shifted to a lower binding energy with increasing Co content x. The energy shift was attributed to the chemical potential shift due to the increase of exchange splitting of the conduction band with increasing Co content. Here, we discuss anatase-type Ti1 − xCo x O2 − δ thin films for various carrier concentrations n and Co contents x and their electronic structural change induced by the ultraviolet illumination.
Interestingly, the shifted and sharpened O 1s spectra are persistent even after switching off the ultraviolet illumination. Before the ultraviolet illumination, there are no carriers in the depletion layer due to the band bending, and the Co spins in the depletion layer are disordered. Under the ultraviolet illumination, the band bending is removed by the photoinduced Ti 3d carriers, and the Co spins in the depletion layer are aligned by the carriers which are injected into the depletion region. The Ti 3d carriers are expected to form ferromagnetic polarons as illustrated in Figure3. Such ferromagnetic polarons are locally very stable, and consequently, the shifted and sharpened O 1s spectra are persistent even after switching off the ultraviolet illumination. Specifically, due to the strong interaction between the carriers and the Co spins, the ferromagnetic state and the insulating paramagnetic state almost degenerate in energy, and the small amount of photoinduced carriers can cause the drastic change from the insulating and paramagnetic surface with the depletion layer to the metallic and ferromagnetic surface without the depletion layer.
The unusual electronic states induced by laser illumination in SrTiO3, Pr0.55(Ca1 − ySr y )0.45MnO3 (y = 0.25) and Ti1 − xCo x O2 − δ (x = 0.05, 0.10) have been discussed on the basis of various polaronic pictures. In Pr0.55(Ca1 − ySry)0.45MnO3 (y = 0.25), the visible light irradiation can cause the metal-to-insulator and insulator-to-metal transitions in which Mn 3d orbital polarons play important roles. SrTiO3 shows the in-gap state under the ultraviolet illumination which is assigned to the localized electronic state with polarized lattice distortion. This state can be interpreted as a ferroelectric polaron state. In Ti1 − xCo x O2 − δ thin films, the band bending at the surface region is removed by the ultraviolet illumination, indicating the possibility of the photoinduced ferromagnetism in the surface region at room temperature. The persistent change can be explained by the stability of the ferromagnetic polaron in the diluted magnetic semiconductor. In the future, various photoinduced phase transitions at surfaces or interfaces of transition metal oxides should be explored more systematically using spectroscopic and theoretical approaches.
TM is an associate professor at Graduate School of Frontier Sciences, University of Tokyo.
The author would like to thank Y. Fujiwara, N. Takaiwa, N. Yamashita, K. Takubo, J.-Y. Son, J. W. Quilty, N. Takubo, K. Miyano, Y. Yamada, H. Toyosaki, T. Fukumura, M. Kawasaki, M. Takesada, T. Ishikawa, S. Koshihara, M. Itoh, and E. Hanamura for the continuous collaborations on photoemission studies of photoinduced phase transitions. The author would like to thank the financial support from Toray Science Foundation.
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