Undoped Crystals Research Articles

Self-diffusion of constituent elements in nominally undoped LaAlO3 single crystals

Self-diffusion of constituent elements in nominally undoped LaAlO3 single crystals

High pressure induced superconductivity and chirality-neutral Fermi surface in SrSi2

In this study, we investigate the electronic structure and topological properties of the compound SrSi2 using angle-resolved photoemission spectroscopy (ARPES), electrical transport measurements, and calculations. In contrast to recent theoretical predictions that SrSi2 is a Weyl semimetal with robust Weyl nodes and Fermi arcs, our ARPES measurements on undoped and Ca-doped SrSi2 single crystals show no evidence of the predicted Weyl fermions or Fermi arcs at ambient pressure. Instead, ARPES and transport data show that SrSi2 is a narrow-gap semiconductor at ambient conditions. Our hybrid functional calculations also find a small band gap, in agreement with experiments. However, by applying external pressure, we induce a topological phase transition where the electronic bands overlap and Weyl nodes appear, as confirmed by transport measurements and calculations. Interestingly, we also observe a pressure-induced superconducting transition above 20 GPa. Our results establish SrSi2 as a platform to study the interplay between topological states and superconductivity driven by external pressure. Importantly, our results challenge the previously predicted intrinsic Weyl semimetallic nature of SrSi2 and highlight the need for combined experimental and advanced computational approaches to correctly describe the complex electronic structures in topological materials. Published by the American Physical Society 2024

Bifunctional Design of Ferroelectric-Order and Band-Engineering in Cu:KTN Crystal for Extended Self-Powered Photoelectric Response.

Photoelectric conversion in ferroelectric crystals can support many important applications in modern on-chip technology, but suffering from two problems, low responsive current and narrow responsive range. Especially, wide-gap ferroelectric oxides are only active at short-wavelength ultraviolet region with weak photocurrent at nanoampere levels. Here, a bifunctional design strategy of ferroelectric-order and electronic-band to improve the photocurrent and extend the responsive range simultaneously, is proposed. In a Cu-doped KTa1- xNbxO3 (KTN) perovskite crystal, a conductive channel is constructed by "head-to-head" ferroelectric domains, associated with the emergence of micrometer-scale supercells. In addition, the introduction of Cu+ ion can induce defect levels, thus extending the responsive range beyond the inherent absorption of pure KTN. Through rational device optimization, a record self-powered responsivity of 5.23 mA W-1 is realized in Cu:KTN photodetector, which is two orders of magnitude higher than undoped KTN crystal. The temperature-dependent light diffraction and photocurrent show that the ferroelectric-order is dominated in this photoresponse behavior. Moreover, Cu:KTN detector is active in the broadband range from 390 to 1030 nm, covering ultraviolet, visible, and near-infrared regions. This work provides an effective method for the design of next-generation self-powered photodetectors with ultrahigh responsivity and ultrawide responsive range.

Ultrahigh piezoelectric properties and high transparency relaxor ferroelectric single crystals obtained by the combination of rare-earth doping and AC-poling

High-performance relaxor ferroelectric single crystals, such as Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT), play a crucial role in the applications of medical ultrasonic imaging due to their high piezoelectric properties. In this study, we successfully grew a 2-in. Eu-doped PMN–PT single crystal whose composition variation and electrical properties were systematically investigated. It was found that the PT content of the crystal boule increased gradually along the growth direction, while the change of Eu3+ content was opposite. The property results show that Eu-doped PMN–PT single crystals exhibit a remarkable piezoelectric coefficient of 3300 pC N−1 at DC-poling while maintaining the rhombohedral phase to a tetragonal phase transition temperature (TRT) at 88 °C and the coercive field (EC) at 2.7 kV cm−1, higher to the undoped single crystals. Through the study of the local structure of Eu-PMN–PT, it illustrated that the introduction of Eu3+ increased the standard deviation of an A sublattice from 3.0% to 4.8%. The dopant simultaneously enhanced a tetragonal phase ratio and local structural heterogeneity, resulting in improved piezoelectric properties and temperature stability of the crystal. We further treated the doped crystals with AC-poling to obtain ultrahigh piezoelectric performance (d33 > 4000 pC N−1) and high transparency. This work provides a new approach to achieve a relaxor ferroelectric single crystal with high performance and transparency with good temperature stability, which could be the material of choice for optoelectronic devices in the future.

Growth, thermophysical and dielectric properties of undoped and Al-doped CsLiB6O10 crystals

Undoped and Al-doped CsLiB6O10 (CLBO) crystals have been grown by top-seeded solution growth (TSSG) method using Cs2O–Li2O–MoO3 flux system. The thermophysical properties of undoped and Al-doped CLBO were systematically studied, including thermal expansion, specific heat, thermal diffusion and thermal conductivity. The measured thermal expansion coefficients of undoped and Al-doped CLBO in the temperature range of 30–500 °C are αa=1.81×10-5/°C, αc=-2.98×10-5/°C and αa=1.10×10-5/°C, αc=-2.56×10-5/°C, respectively, displaying much weaker anisotropy than those of widely used nonlinear optical crystals, such as β-BaB2O4 and CsB3O5. CLBO exhibits specific heat of 0.86–1.03 Jg-1K-1 in the temperature range from 30 to 500 °C. The thermal conductivities of undoped and Al-doped CLBO at 30 °C are respectively calculated to be κa=1.73 Wm-1K-1, κc=1.69 Wm-1K-1 and κa=1.74 Wm-1K-1, κc=1.49 Wm-1K-1, which are larger than those of β-BaB2O4. Dielectric properties were investigated in the temperature range of 20–400 °C within the frequency range of 100 Hz–1 MHz. The measured dielectric constants of undoped and Al-doped CLBO at 1 MHz and 25 °C are εa=5.27, εc=5.92 and εa=4.96 , εc=5.49, respectively. Additionally, undoped and Al-doped CLBO exhibit significantly low dielectric losses (0.005–0.016 at 1 MHz and 25 °C) and superior dielectric temperature stability (20–300 °C). These results indicate that undoped and Al-doped CLBO possess excellent thermophysical performance and could be two promising dielectric material candidates for high frequency device applications.

Generation of a coherent longitudinal optical phonon plasmon coupled mode in an n-type InSb single crystal in the absence and/or strong suppression of the photo-Dember effect confirmed with the use of terahertz time-domain spectroscopy

We explored the feasibility of generating a coherent longitudinal optical (LO) phonon–plasmon coupled mode in an n-type InSb single crystal with the condition that the photo-Dember effect is strongly suppressed and/or eliminated. We systematically performed terahertz time-domain spectroscopic measurements of the n-type InSb single crystal and undoped InSb crystal. The terahertz electromagnetic wave originating from the photo-Dember effect was extinguished in the n-type InSb single crystal, while the clear signal from the photo-Dember effect was detected in the undoped InSb single crystal. Meanwhile we detected the clear signal of the terahertz wave from the coherent LO-phonon–plasmon coupled mode in the n-type InSb single crystal, indicating that the photo-Dember effect is not dominant in the generation of the coherent LO-phonon–plasmon coupled mode although the photo-Dember effect has been attributed to the dominant generation mechanism of the coherent LO-phonon–plasmon coupled mode so far. We explain the present phenomenon, taking account of the fact that the impulsive stimulated Raman scattering process has a possibility of generating coherent LO-phonon–plasmon coupled modes in light-absorbing solids [Nakamura et al., Phys. Rev. B 99, 180301(R) (2019)].

Thermal conductivity in solid solutions of lithium niobate tantalate single crystals from 300 K up to 1300 K

Lithium niobate tantalate (LiNb1−xTaxO3, LNT) solid solutions offer exciting new possibilities for applications ranging from optics, piezotronics, and electronics beyond the capabilities of the widely used singular compounds of lithium niobate (LiNbO3, LN) or lithium tantalate (LiTaO3, LT). Crystal growth of homogeneous LNT single crystals by the Czochralski method is still challenging. One key aspect of homogeneous growth is the accurate knowledge of thermal conductivity through the crystal boule during the growth, which is central to control the crystal growth. Therefore, the temperature dependent thermal conductivity of pure LN, LT, and LNT solid solutions, as well as of selected doped LN and LT crystals (Mg, Zn) was investigated across the temperature range from 300 to 1300 K. The results that span across the whole composition range can directly be applied for optimizing growth conditions of both LNT solid solutions as well as doped and undoped LN and LT crystals.

Luminescence and scintillation properties of TlCdCl3:Sb crystals

An ideal scintillator for X- and gamma-ray detection should have a high light yield and a high effective atomic number (Zeff). In this study, we developed undoped TlCdCl3 crystals and TlCdCl3:Sb (Sb: 0.5, 1.0 and 1.5 mol%) crystals with high Zeff (TlCdCl3:68.7), as candidate scintillators. The crystals were grown using the self-seeding solidification method, and their photoluminescence (PL) and scintillation properties were investigated. For the undoped TlCdCl3 crystals, emission peaks were observed at 460 nm and 485 nm in the PL and X-ray-induced radioluminescence (XRL) spectra, respectively. For the TlCdCl3:Sb crystals, the PL spectra showed emission peaks at 480, 480 and 500 nm, while the XRL spectra exhibited peaks at approximately 510 nm. The emission bands of the TlCdCl3:Sb crystals were shifted to longer wavelengths than those of the undoped TlCdCl3 crystals. The scintillation decay time constants for the undoped TlCdCl3 crystals were 51 and 2613 ns, whereas for the TlCdCl3:Sb crystals, they were in the range of 65–81 and 4550–7350 ns. These results suggest that the incorporation of Sb3+ ions induces long components of several thousand nanoseconds. The redshift and appearance of these long components indicate that Sb3+ ions act as new luminescence centers in the undoped TlCdCl3 crystal. The scintillation light yield of the TlCdCl3:Sb 1.0 mol% crystal was measured at 10,300 photons/MeV. The doping of Sb3+ ions into the TlCdCl3 lattice improved the scintillation light yield by up to four times compared to the undoped TlCdCl3 crystal.

Influence of swift heavy ions irradiation on optical and luminescence properties of Y3Al5O12 single crystals

Optical and luminescence properties of Y3Al5O12 (YAG) single crystals preliminary irradiated by swift heavy ions were studied. Swift heavy Xe ions with fluences ranging from 6·1010 to 2·1012 ions/cm2 were utilized for the irradiation of nominally undoped YAG single crystals. A stable strong induced absorption observed in the 200–600 nm spectral range correlates with the irradiation fluence. It is suggested that several centers are responsible for this induced absorption in YAG single crystals and their possible origin (F-type centers) is proposed and discussed. The swift heavy ions irradiation strongly modifies the luminescence properties of YAG, namely, the excitonic emission at liquid helium temperature is drastically suppressed in heavily irradiated crystals.

Optical, scintillation, and dosimetric properties of undoped and Mn-doped MgGa2O4 single crystals

Optical, scintillation, and thermally and optically stimulated luminescence (TSL and OSL) properties of the undoped and Mn-doped MgGa2O4 single crystals were investigated. All the crystals showed a luminescence peak at 500 nm caused by the 3d–3d transitions of Mn2+. The 0.03 %-Mn-doped crystal exhibited the highest OSL intensity, whereas the 0.3 %-Mn-doped crystal exhibited the highest photoluminescence quantum yield and scintillation intensity. There was an inverse relationship between the scintillation and OSL intensity in the 0.03–0.3 % Mn-doped crystals. The results indicated that the 0.03 % Mn-doped crystal exhibits the highest efficiency during carrier trapping. In the OSL dose response functions, the lower detection limit of the 0.03 % Mn-doped crystal was 0.01 mGy, which was comparable with that of a commercial product.

Neutron detection properties of Ti-doped LiGaO2 single crystalline scintillator

Photoluminescence (PL) and scintillation properties of undoped and Ti-doped LiGaO2 single crystals were investigated. In both PL and scintillation, the Ti-doped LiGaO2 crystals showed luminescence due to charge transfer between Ti4+–O2- while the undoped LiGaO2 crystal showed luminescence due to oxygen vacancies. Under thermal neutron irradiation, all the LiGaO2 crystals exhibited a clear thermal neutron peak, and the highest light yield was observed from the 3% Ti-doped LiGaO2 crystal. The light yield of the 3% Ti-doped LiGaO2 crystal was ten times higher than that of the undoped LiGaO2 crystal and comparable with that of a commercial neutron scintillator GS20. In addition, the Ti-doped LiGaO2 crystals showed high α/γ ratios up to about 0.7.

Layer-dependence study of two-dimensional ferromagnets: Fe3GeTe2 and Fe5Ge2Te2

We have investigated the electrical transport properties of nanodevices fabricated from exfoliated flakes of two-dimensional metallic ferromagnets Fe3GeTe2 (FGT) and Fe5Ge2Te2 (FG2T) down to below three layers in thickness. The per-layer anomalous Hall conductivity even in thick FGT and FG2T devices is found to be much smaller than ∼e2h, the approximate value calculated for thick undoped crystals. Moreover, we obtain a power-law scaling relation between the per-layer anomalous Hall and per-layer longitudinal conductivities with an exponent close to 1.6, which agrees with the universal value for poor ferromagnetic conductors. Both FGT and FG2T devices show clear layer-dependent Curie temperatures and layer-dependent perpendicular magnetic anisotropy, with FG2T dominating the former and FGT dominating the latter for all thicknesses. Despite their declining trend as the device thickness decreases, both Curie temperature and magnetic anisotropy retain a significant fraction of their bulk values (>60% and >80% of the bulk values, respectively, even in the thinnest FG2T device), indicating attractive potential for practical applications.

Analysis of defects dominating carrier recombination in CeO2 single crystal for photocatalytic applications

In photocatalytic water splitting and CO2 reduction, recently, cerium oxide (CeO2) has been widely investigated. Defects that can directly dominate carrier recombination are essential for the photocatalytic performance of CeO2. In this study, several photoluminescence (PL) peaks are observed on the (100) face of an undoped CeO2 single crystal, indicating the presence of defects. Moreover, we characterize carrier recombination using time-resolved photoluminescence (TR-PL) and microwave photoconductivity decay (μ-PCD) measurements. The temperature dependence of decay curves is the result of carrier trapping and emission at deep levels. These decay curves are observed separately using a 565 nm band-pass filter (BPF) based on the PL spectra. The trap energy level ( ET ) and majority carrier capture cross-section ( σT ) of each defect are also analyzed using rate equations to fit the experimental results. The temperature-dependent time constants are well reproduced by a recombination model using hole traps HTinfrared and HTvisible at ET of 0.76 and 0.55 eV from the conduction or valence band, with estimated σT of the order of 10−21 and 10−22 cm2 for without and with the BPF, respectively. These findings regarding the presence of multiple defects in a single crystal indicate the necessity to focus on defect control.

Semi-insulating [formula omitted]-Ga2O3 single crystals through vanadium doping: Growth, Optical and Terahertz characterization

We report the growth of electrically resistive, high quality vanadium-doped β-Gallium Oxide (β-Ga2O3) single crystals via the optical floating zone growth technique. The characteristic properties of such high resistive V-doped β-Ga2O3 are significantly different compared to the undoped, n-type V-doped and other acceptor-doped crystals. We study the optical absorption in the UV–visible and IR range, temperature-dependent photoluminescence, polarization-dependent Raman spectra and the terahertz transmission properties in the 0.2–2.6 THz range. The V-doped insulating Ga2O3 crystals can be used as an alternative insulating substrate for low leakage devices. Further, the strong anisotropy in terahertz transmission makes it an ideal material for optical applications in the THz region.

Dispersion of two-photon absorption and nonlinear refraction in β -Ga2O3 from 350 to 515 nm

We report the wavelength dependencies of the two-photon absorption coefficients β as well as the nonlinear refractive index n2 of undoped β-Ga2O3 single crystal in the spectral range 350–515 nm (0.51 < Ephoto/Eg < 0.75). Femtosecond Z-scan measurements show that n2 decreases monotonically toward the shortwave side, while the maximum value of n2 is 5.0 × 10−19 m2/W near half of the bandgap, Eg. Interestingly, a second upward trend in β was observed when the incident photon energy is larger than 0.6Eg, indicating the impact of additional inter-band transitions with higher energy. A higher-energy band-to-band transition around 6.0 eV was determined by both femtosecond nondegenerate two-photon absorption spectroscopy and ab initio calculations. Our results could provide guidance for designing nonlinear β-Ga2O3 photonic devices in the UV-visible spectral range.

Solution-grown millimeter-scale Mn-doped CsPbBr3/Cs4PbBr6 crystals with enhanced photoluminescence and stability for light-emitting applications.

Metal halide perovskites are particularly emerging for optoelectronic applications in light-emitting diodes, photodetectors, and solar cells due to their flourishing photophysical properties. However, the poor stability of three-dimensional (3D) lead halide perovskite nanocrystals (NCs) significantly hampers their optoelectronics and photovoltaics applications. Embedding 3D perovskites into zero-dimensional (0D) perovskite crystals and doping ions of appropriate elements into host lattices provide effective approaches to improve the stability and optical-electronic performance. In this study, millimeter-scale Mn-doped and undoped CsPbBr3/Cs4PbBr6 perovskite crystals were successfully fabricated by a one-step slow cooling method. We systematically investigated the effects of Mn2+ ion doping on the PL performance and stability of CsPbBr3/Cs4PbBr6 crystals. Compared with undoped crystals, the existence of Mn2+ ions not only blue-shifted the PL peak but also improved the luminescence performance and stability of the prepared millimeter-sized crystals. Moreover, doping Mn2+ ions can increase the proportion of radiative recombination at low temperature, which may be because Mn2+ ions can effectively accelerate the decay of a dark exciton by the magnetic mixing of bright and dark excitons. In addition, green LED devices with high efficiency packaged as-grown crystals are explored, which promises further application in display backlights.

Effects and mechanism of Ti, Cu, Y, La, Ce, Pr, Nd and Sm substitutions on the anti-disproportionation performance of ZrCo alloy

The intermetallic compound ZrCo is widely considered a good choice to substitute uranium in storing gaseous tritium. Whereas the storage capacity fading induced by the disproportionation reaction which happens during the hydriding and dehydriding processes becomes the main obstacle to overcome at the present stage. In this work, the effects and mechanism of Ti, Cu, Y, La, Ce, Pr, Nd and Sm substitutions on the anti-disproportionation performance of ZrCo alloy were carefully investigated by combining experiments and first-principles calculations. By the investigation of Zr1-xTixCo (x = 0, 0.05, 0.10, 0.15 and 0.20), it is confirmed that the increase of Zr–H (8e) bond length and decrease of 8e site size are beneficial to prevent the access of H into the 8e site of ZrCoH3, which helps the improvement of anti-disproportionation performance. It has shown that rare-earth elements are preferred to substitute Zr than Co atom in ZrCo and ZrCoH3 compounds. While alloying rare-earth elements may not contribute significantly to the enhancement of the anti-disproportionation property of ZrCo alloy. Additionally, the charge density distribution and polarized density of states of undoped and doped ZrCoH3 crystals were clarified.

Giant seebeck effect in an undoped single crystal of 2,5,8-triphenylbenzo[1,2-b:3,4-b':5,6-b'']trifuran

Abstract The application of 2,5,8-triphenylbenzo[1,2-b:3,4-bʹ:5,6-bʹʹ]trifuran (Ph3-BTF) as a thermoelectric generator is demonstrated. An undoped single crystal of Ph3-BTF exhibited a giant Seebeck effect (GSE) with a large negative Seebeck coefficient of up to −239 mV K−1 at a base temperature of 350 K. Ph3-BTF features a small reorganization energy and tight molecular packing in the single crystal (π–π stacking distance: 3.32 Å), which may contribute to the observed GSE.

Huge piezoelectricity and electro-optic effects in rhombohedral relaxor ferroelectric single crystals by Sm3+ doping

In recent years, relaxor ferroelectric single crystals have attracted people's attention and research because of their excellent piezoelectric properties at morphotropic phase boundary(MPB). The ternary system Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) crystals have higher Curie temperature and coercive field, which have the opportunity to be applied in high-temperature devices. The crystals of the rhombohedral phase have better stability, but their performance is not as good as that of the MPB component crystals. In this paper, the piezoelectric properties of rhombohedral PIN-PMN-28PT single crystals are improved by Sm3+ doping. The piezoelectric coefficient d33 after AC polarization along [001] direction can reach 3125pC/N, which is 60% higher than that of undoped crystals. Meanwhile, the crystal exhibits excellent optical performance after DC polarization along [110] direction. The crystal has an effective electro-optic coefficient of 386 pm/V at room temperature and a stable frequency-dependent characteristic, which is expected to be an ideal material for electro-optic devices.

Gamma-ray irradiation and the alterations in photoluminescence emissions of undoped and indium-doped ZnO single crystals

We investigate the effect of gamma-ray irradiation on the ultraviolet (UV) emissions of undoped and indium-doped zinc oxide (ZnO) single crystals. After irradiation, UV emission broadening is observed which is attributed to shallow defects induced in the strained crystal lattice. These defect states play a role in shortening the UV emission lifetimes, and the difference is found to be more pronounced in the doped ZnO crystal. The underlying phenomena responsible for the observed changes in the photoluminescence properties are discussed.