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Spin–charge conversion is a spin–orbit coupling phenomena where electrical currents can generate transverse spin currents and vice versa. It is one of the central topics in spintronics and widely applied to manipulate the spin a...
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Spin–charge conversion is a spin–orbit coupling phenomena where electrical currents can generate transverse spin currents and vice versa. It is one of the central topics in spintronics and widely applied to manipulate the spin and charge degrees of freedom in materials. Previous research on spin–charge conversion was mainly carried out by transport measurements, which lies in the (quasi)equilibrium and DC/low-frequency limit. The recent development of THz emission spectroscopy applied to this field provides additional insights into the dynamics of the spin–charge conversion process, i.e., its ultrafast timescales. Here, the underlying physics and the latest progress of THz studies on spintronics are introduced. The technical details and some features of this technique are summarized, including spin current generation, signal detection, and data analysis. Finally, some possible developments are discussed as well as future research and applications.
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Hybrid inorganic-organic perovskites have recently attracted much interest because of both rich fundamental sciences and potential applications such as the primary energy-harvesting material in solar cells. However, an understandi...
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Hybrid inorganic-organic perovskites have recently attracted much interest because of both rich fundamental sciences and potential applications such as the primary energy-harvesting material in solar cells. However, an understanding of electronic and optical properties, particularly the complex dielectric function, of these materials is still lacking. Here, we report on the electronic and optical properties of selective perovskites using temperature-dependent spectroscopic ellipsometry, x-ray absorption spectroscopy supported by first-principles calculations. Surprisingly, the perovskite FA 0.85 Cs 0.15 PbI 2.9 Br 0.1 has a very high density of low-energy excitons that increases with increasing temperature even at room temperature, which is not seen in any other material. This is found to be due to the strong, unscreened electron-electron and partially screened electron-hole interactions, which then tightly connect low- and high-energy bands caused by doping.
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Lead halide perovskites have many suitable features as materials for solar cells and a better understanding of the underlying physics will help engineer improved performance values. Here, the authors use broadband transient absorp...
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Lead halide perovskites have many suitable features as materials for solar cells and a better understanding of the underlying physics will help engineer improved performance values. Here, the authors use broadband transient absorption spectroscopy to probe the excited state dynamics of mixed-halide perovskites and the role of excitons and carriers.
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By combining polarized pump-probe spectroscopic and Laue x-ray diffraction measurements, we have observed nonequivalent transient optical responses with the probe beam polarized along the x and v axes in overdoped NaFe_(1-x)CO_x A...
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By combining polarized pump-probe spectroscopic and Laue x-ray diffraction measurements, we have observed nonequivalent transient optical responses with the probe beam polarized along the x and v axes in overdoped NaFe_(1-x)CO_x As superconductors. Such transient anisotropic behavior has been uncovered in the tetragonal phase with the doping level and temperature range far from the borders of static nematic phases. The measured transient anisotropy can be well explained as a result of nematic fluctuation driven by an orbital order with energy splitting of the d_(xz)- and d_(yz)-dominant bands. In addition, the doping level dependence and the pressure effect of the crossover temperature show significant differences between the transient nematic fluctuation and static nematic phase, implying spin and orbital orders may play different roles in static and transient nematic behaviors.
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The temperature dependence of the relaxation dynamics in the bilayer thin film heterostructure composed of multiferroic BiFeO_3 (BFO) and superconducting YBa_2Cu_3O_7 (YBCO) grown on a (001) SrTiO_3 substrate is studied by a time-...
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The temperature dependence of the relaxation dynamics in the bilayer thin film heterostructure composed of multiferroic BiFeO_3 (BFO) and superconducting YBa_2Cu_3O_7 (YBCO) grown on a (001) SrTiO_3 substrate is studied by a time-resolved pump-probe technique, and compared with that of pure YBCO thin film grown under the same growth conditions. The superconductivity of YBCO is found to be retained in the heterostructure. We observe a speeding up of the YBCO recombination dynamics in the superconducting state of the heterostructure, and attribute it to the presence of weak ferromagnetism at the BFO/YBCO interface as observed in magnetization data. An extension of the Rothwarf-Taylor model is used to fit the ultrafast dynamics of BFO/YBCO, that models an increased quasiparticle occupation of the ferromagnetic interfacial layer in the superconducting state of YBCO.
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Since tantalum arsenide (TaAs) has been experimentally verified as a Weyl semimetal, intensive research has been devoted to study of the unique properties of the material. Despite this, the ultrafast dynamics of TaAs is still not ...
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Since tantalum arsenide (TaAs) has been experimentally verified as a Weyl semimetal, intensive research has been devoted to study of the unique properties of the material. Despite this, the ultrafast dynamics of TaAs is still not very well understood. In this work, we study the relaxation dynamics in TaAs using transient reflection spectroscopy. From the transient reflection measurement, we observe either a single (fast) or a dual (last and slow) relaxation, depending on the probing wavelength. The additional relaxation channel is attributed to an asymmetric population of photoexciled electrons and holes.
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Two-dimensional (2D) hybrid perovskites exhibiting broadband light emission are attractive as down-converting phosphors in white light-emitting diodes (WLEDs). Despite active exploration of new members of this family of materials,...
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Two-dimensional (2D) hybrid perovskites exhibiting broadband light emission are attractive as down-converting phosphors in white light-emitting diodes (WLEDs). Despite active exploration of new members of this family of materials, fine-tuning of their emission through structural variation for realizing high color-rendering white light remains largely untapped. Here we report a series of (100)-oriented 2D perovskites whose structures are templated by the organic cations. By controlling the tilting of the inorganic octahedra, we were able to shift the broadband emission from blue to white. A photophysical study further suggests that the coexistence of self-trapped excitons and free excitons contributes to a double-peak broad emission, covering the entire visible spectrum. Using the broad-emitting perovskites as down-converting phosphors, we fabricated WLEDs with white-light emission having a correlated color temperature (CCT) of 6600 K and a high color rendering index (CRI, R _(a) ) of 86.
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The temperature dependence of relaxation dynamics of UX_3 (X = Al, Ga, Sn) compounds is studied using the time-resolved pump-probe technique in reflectance geometry. For UGa_3, our data are consistent with the formation of a spin ...
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The temperature dependence of relaxation dynamics of UX_3 (X = Al, Ga, Sn) compounds is studied using the time-resolved pump-probe technique in reflectance geometry. For UGa_3, our data are consistent with the formation of a spin density wave gap as evidenced from the quasidivergence of the relaxation time x near the Neel temperature T_N. ForUAl_3 and USn_3, the relaxation dynamics shows a change from single-exponential to two-exponential behavior below a particular temperature, suggestive of coherence formation of the 5/ electrons with the conduction band electrons. This particular temperature can be attributed to the spin fluctuation temperature T_(sf), a measure of the strength of Kondo coherence. Our T_(sf) is consistent with other data such as resistivity and susceptibility measurements. The temperature dependence of the relaxation amplitude and time of UAl_3 and USn_3 were also fitted by the Rothwarf-Taylor model. Our results show that ultrafast optical spectroscopy is sensitive to c-f Kondo hybridization in the f-electron systems.
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Time-resolved photoinduced reflectivity is measured in the spin-density-wave phase of the itinerant antiferromagnet UPtGa_5. Two relaxation components were seen: (a) a slow component whose amplitude appears below T_N, and relaxati...
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Time-resolved photoinduced reflectivity is measured in the spin-density-wave phase of the itinerant antiferromagnet UPtGa_5. Two relaxation components were seen: (a) a slow component whose amplitude appears below T_N, and relaxation time τ_(slow) exhibits an upturn near T_N, and (b) the fast component persists at all temperatures, with the relaxation time τ_(fast), also exhibiting an upturn near T_N. Comparing with pump-probe data on UNiGa_5, the differences are explained in the context of UPtGa_5 having A-type (rather than G-type) antiferromagnetism, resulting in partial Fermi surface nesting, partial gapping, and consequently finite density of states at the Fermi surface.
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Transparent conductive oxides (TCOs)-materials that have the twin desirable features of high optical transmission and electrical conductivity-play an increasingly significant role in the fields of photovoltaics and information tec...
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Transparent conductive oxides (TCOs)-materials that have the twin desirable features of high optical transmission and electrical conductivity-play an increasingly significant role in the fields of photovoltaics and information technology. As an excellent TCO, Ta-doped anatase TiO_2 shows great promise for a wide range of applications. Here, terahertz time-domain spectroscopy is used to study the complex optical conductivity δ (to) of the TCO-heavily Ta-doped TiO_2 thin films with different Ta-doping concentrations, in the frequency range of 0.3-2.7 THz and the temperature range of 10-300 K. Fitting the complex optical conductivity to a Drude-like behavior allows us to extract the temperature dependence of the effective mass, which suggests the existence of many-body large polarons. Moreover, the carrier scattering rate of Ta-doped TiO_2 with different carrier concentrations agrees with the interacting polaron gas theory. Our results suggest that with increasing electron density in TiO_2, the interaction between polarons is larger and electron-phonon coupling is smaller, which is beneficial for achieving high mobility and conductivity in TiO_2.
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