Chuangtian Chen

New nonlinear-optical crystal: LiB 3 O 5

The boron–oxygen compound LiB3O5 is recognized as a new nonlinear-optical crystal. This follows theoretical calculations of the second-harmonic generation (SHG) coefficients using the anionic group theory and the complete neglect of differential overlap approximation to obtain the localized wave functions of component groups. An optically perfect single crystal with space group Pna21, grown at the Fujian Institute of Research on the Structure of Matter by the high-temperature flux method, is found to be transparent from 160 nm to 2.6 μm. It has a SHG coefficient comparable with that of β-BaB2O4 as well as two other outstanding advantages: a high damage threshold of 25 GW/cm2 (at 1.064 μm, 0.1 nsec) and a wide acceptance angle of 25 mrad for θ ≠ 90° and 95 mrad for θ = 90° with a 6-mm-long crystal.

CsB3O5: A new nonlinear optical crystal

On the basis of the anionic group theory, a new nonlinear optical crystal cesium triborate, (CsB3O5, CBO) was discovered. Single crystals of CBO in centimeter size were grown from a stoichiometric melt. The transparent range of CBO covers from 170 to 3000 nm. The preliminary measurement of its second harmonic generation coefficient shows that its d14 is ∼0.468×d11 (β−BaB2O4). Its measured damage threshold is as high as 26 GW/cm2 at 1.053 μm, 1.0 ns.

Phase diagram and electronic indication of high-temperature superconductivity at 65 K in single-layer FeSe films

Superconductivity in the cuprate superconductors and the Fe-based superconductors is realized by doping the parent compound with charge carriers, or by application of high pressure, to suppress the antiferromagnetic state. Such a rich phase diagram is important in understanding superconductivity mechanism and other physics in the Cuand Fe-based high temperature superconductors. In this paper, we report a phase diagram in the single-layer FeSe films grown on SrTiO3 substrate by an annealing procedure to tune the charge carrier concentration over a wide range. A dramatic change of the band structure and Fermi surface is observed, with two distinct phases identified that are competing during the annealing process. Superconductivity with a record high transition temperature (Tc) at 65±5 K is realized by optimizing the annealing process. The wide tunability of the system across different phases, and its high-Tc, make the single-layer FeSe film ideal not only to investigate the superconductivity physics and mechanism, but also to study novel quantum phenomena and for potential applications. 1 ar X iv :1 20 7. 68 23 v1 [ co nd -m at .s up rco n] 3 0 Ju l 2 01 2 In high temperature cuprate superconductors, superconductivity is realized by doping the parent Mott insulator with charge carriers to suppress the antiferromagnetic state[1]. In the process, the physical property experiences a dramatic change from antiferromagnetic insulator, to a superconductor and eventually to a non-superconducting normal metal. In the superconducting region, the transition temperature Tc can be tuned by the carrier concentration, initially going up with the increasing doping, reaching a maximum at an optimal doping, and then going down with further doping[1]. Such a rich evolution with doping not only provides a handle to tune the physical properties in a dramatic way, but also provides clues and constraints in understanding the origin of the high-Tc superconductivity. The same is true for the Fe-based superconductors where superconductivity is achieved by doping the parent magnetic compounds which are nevertheless metallic[2, 3]. Again, the superconducting transition temperature can be tuned over a wide doping range with an maximum Tc at the optimal doping. Understanding such a rich evolution is also a prerequisite in understanding the origin of high temperature superconductivity in the Fe-based superconductors. The latest discovery of high temperature superconductivity signature in the single-layer FeSe films[4, 5] is significant in a couple of respects. First, it may exhibit a high Tc that breaks the Tc record (∼55 K) in the Fe-based superconductors kept so far since 2008[6– 11]. Second, the discovery of such a high-Tc in the single-layer FeSe film is surprising when considering that its bulk counterpart has a Tc only at 8 K[9] although it can be enhanced to 36.7 K under high pressure[12]. Third, it provides an ideal system to investigate the origin of high temperature superconductivity. On the one hand, this system consists of a single-layer FeSe film that has a simple crystal structure and strictly two-dimensionality; its simple electronic structure may provide key insights on the high Tc superconductivity mechanism in the Fe-based compounds[5]. On the other hand, the unique properties of this system may involve the interface between the single-layer FeSe film and the SrTiO3 substrate that provides an opportunity to investigate the role of interface in generating high-Tc superconductivity[4]. Like in cuprates and other Fe-based superconductors, it is important to explore whether one can tune the single-layer FeSe system to vary its physical properties and superconductivity by changing the charge carrier concentration. In this paper, we report a wide range tunability of the electronic structure and physical properties that is realized in the single-The recent discovery of possible high-temperature superconductivity in single-layer FeSe films has generated significant experimental and theoretical interest. In both the cuprate and the iron-based high-temperature superconductors, superconductivity is induced by doping charge carriers into the parent compound to suppress the antiferromagnetic state. It is therefore important to establish whether the superconductivity observed in the single-layer sheets of FeSe--the essential building blocks of the Fe-based superconductors--is realized by undergoing a similar transition. Here we report the phase diagram for an FeSe monolayer grown on a SrTiO3 substrate, by tuning the charge carrier concentration over a wide range through an extensive annealing procedure. We identify two distinct phases that compete during the annealing process: the electronic structure of the phase at low doping (N phase) bears a clear resemblance to the antiferromagnetic parent compound of the Fe-based superconductors, whereas the superconducting phase (S phase) emerges with the increase in doping and the suppression of the N phase. By optimizing the carrier concentration, we observe strong indications of superconductivity with a transition temperature of 65±5 K. The wide tunability of the system across different phases makes the FeSe monolayer ideal for investigating not only the physics of superconductivity, but also for studying novel quantum phenomena more generally.

NaSr3Be3B3O9F4: a promising deep-ultraviolet nonlinear optical material resulting from the cooperative alignment of the [Be3B3O12F](10-) anionic group.

The demand for deep-ultraviolet (deep-UV) coherent light sources (l< 200 nm) has become increasingly urgent because they have important applications in semiconductor photolithography, laser micromachining, modern scientific instruments (super-high-resolution and angle-resolved photoemission spectrometer, for example) and so forth. To date, the most effective method to generate deep-UV coherent light with solid-state lasers is through cascaded frequency conversion, in particular multiharmonics, using deep-UV nonlinear optical (NLO) crystals. Therefore, the discovery of suitable deep-UV NLO crystals is of great importance. In the past decades, the anionic group theory, which reveals that the overall nonlinearity of a crystal is the geometrical superposition of the microscopic second-order susceptibility tensors of the NLO-active anionic groups, has been very successful in developing borate NLO crystals. Several important NLO crystals have been discovered, including b-BaB2O4 (BBO), [4] LiB3O5 (LBO), [5] CsB3O5 (CBO), CsLiB6O10 (CLBO), [7, 8] and YCa4O(BO3)3 (YCOB), which have been widely used in NLO optics. However, they cannot be used to generate deep-UV coherent light (l< 200 nm) by multiharmonic generation owing to some inherent shortcomings. Thus, the search for new NLO materials, particularly for deep-UVapplications, has attracted considerable attention. A deep-UV NLO material must have a very short absorption edge, and in this respect, beryllium borates are attractive as they are supposed to possess very large energy gap. It is also well known that the incorporation of fluorine can effectively cause the UV absorption edge of a crystal to blue-shift, so our group has made great efforts to search for new deep-UV NLO fluorine beryllium borate crystals. After more than ten years of intensive research in our group, the KBe2BO3F2 [16–18] (KBBF) crystal became the first practically usable deep-UV NLO crystal used to generate coherent 177.3 and 193 nm light. The excellent NLO properties of KBBF crystals are mainly determined by the (Be2BO3F2)1 layer made up of trigonal-planar [BO3] units and the tetrahedral [BeO3F] units. This deep-UV coherent light material has been used as a photon source in modern instruments and revealed many novel scientific phenomena which could not be observed by traditional techniques, as shown in the study of superconductor CeRu2 [19] and Bi2Sr2CaCu2O8+d. [20] Unfortunately, the KBBF crystal is very difficult to grow in thickness because of its strong layering tendency, which severely limits the coherent output power. Therefore, there is great demand for new types of fluorine beryllium borates which have deepUV transmission, moderate birefringence, and relatively large second harmonic generation (SHG) coefficients, and at the same time overcome the crystal-growth problems found in the KBBF crystal. Alkali-metal and alkaline-earth-metal cations are favorable for the transmission of UV light because there are no d–d electron or f–f electron transitions in this spectral region. As shown in numerous explorations, the size and charge of cations have great influence on the macroscopic packing of anions, which in turn determines the overall NLO properties in a crystal. 22] Herein, we utilize both alkali-metal and alkaline-earth-metal cations. Different charge/size combinations of mixed cations may have different influences on the packing of anions, so it is more likely to isolate new phases with interesting stoichiometries, structures, and properties. To date, no fluorine beryllium borates with mixed cations have been reported. Guided by this idea, we successfully obtained a new alkali-metal/alkaline-earth-metal fluorine beryllium borate NaSr3Be3B3O9F4, which contains the novel anionic group [Be3B3O12F] 10 as the basic building unit. Furthermore, the arrangement of these [Be3B3O12F] 10 groups is very favorable for generating large a NLO response and moderate birefringence and especially for avoiding the layering tendency during bulk crystal growth. Herein, we report the synthesis, crystal growth, structure, linear and nonlinear optical properties, thermal behavior, and electronic structure of NaSr3Be3B3O9F. These results indicate that the NaSr3Be3B3O9F4 crystal may be a promising NLO material in the deep-UV range. NaSr3Be3B3O9F4 [23] crystallizes in the noncentrosymmetric trigonal space group R3m. The crystal structure is depicted in Figure 1a. In the asymmetric unit, Sr, Na, Be, B each occupy one crystallographically unique position, and there are two unique F and O positions. The B atom is coordinated to three O atoms to form a planar BO3 unit with B O bond lengths [*] H. Huang, J. Yao, Z. Lin, X. Wang, R. He, W. Yao, N. Zhai, Prof. C. Chen Center for Crystal Research and Development Technical Institute of Physics and Chemistry Chinese Academy of Sciences, Beijing 100190 (China) E-mail: cct@mail.ipc.ac.cn

The anionic group theory of the non-linear optical effect and its applications in the development of new high-quality NLO crystals in the borate series

Abstract Starting from a general quantum-mechanical perturbation theory on the nonlinear optical (NLO) effect in crystals, this review gives a systematic presentation of the basic concepts and calculation methods of the ‘anionic group theory for the NLO effect of crystals’ and a brief discussion of the approximations involved. Calculations have been made for the second harmonic generation (SHG) coefficients of a few typical NLO crystals. Comparisions between these theoretical values and the experimental values made both on powdered crystals and on single crystals suffice to show the feasibility of the theoretical treatment and calculation methods. On this basis, borate ions of various structure types are classified and systematic calculations are carried out for the NLO susceptibilities of some typical borate crystals with good prospects of applications in opto-electronics. Through these calculations, a series of structural criteria serving as useful guidelines for searching and developing new NLO crystal...

Computer‐Assisted Search for Nonlinear Optical Crystals

Based on anionic group theory, a computer-assisted material design system (CAMDS) has been developed. This method has proved to be a highly efficient means of discovering nonlinear optical crystals. In this method, important optical properties of the target compounds (borates, for example), such as the dij coefficients, refractive indices, and energy bandgap, are calculated so that a prior evaluation can be made before experiments. The results have given a meaningful guide to ensuing experiments, which have led to our discoveries of KBBF (KBe2BO3F2) and SBBO (Sr2Be2B2O7) in the past several years, followed by other members of the SBBO family in recent years. On the other hand, this system can also be used to evaluate the dij coefficients of the borate nonlinear optical (NLO) crystals discovered recently whose dij coefficients have not been determined experimentally.

Electronic origin of high-temperature superconductivity in single-layer FeSe superconductor

The recent discovery of high-temperature superconductivity in iron-based compounds has attracted much attention. How to further increase the superconducting transition temperature (T(c)) and how to understand the superconductivity mechanism are two prominent issues facing the current study of iron-based superconductors. The latest report of high-T(c) superconductivity in a single-layer FeSe is therefore both surprising and significant. Here we present investigations of the electronic structure and superconducting gap of the single-layer FeSe superconductor. Its Fermi surface is distinct from other iron-based superconductors, consisting only of electron-like pockets near the zone corner without indication of any Fermi surface around the zone centre. Nearly isotropic superconducting gap is observed in this strictly two-dimensional system. The temperature dependence of the superconducting gap gives a transition temperature T(c)~ 55 K. These results have established a clear case that such a simple electronic structure is compatible with high-T(c) superconductivity in iron-based superconductors.

The development of new NLO crystals in the borate series

It is well known that the inorganic borates exist in numerous structural types, and some crystals such as KB5 (KB 5 O 8 ·4H 2 O) and BBO(β-BaB 2 O 4 ) are excellent nonlinear optical (NLO) materials, particularly in the ultraviolet region. In this review, we are going to approach our topic from the anionic group theory, which we have begun to develop in the late 1960s. A systematic classification has been made of the borate series in terms of the structural types of the anionic groups. Calculations have been made for the linear and NLO properties for most of the important borate anionic groups, including second-order susceptibilities and absorption edges in the ultraviolet spectral region. We have therefore been led to the formulation of a series of structural criteria, which serve as useful guidelines for the screening, characterization, and development of certain borate crystals, such as BBO (β-BaB 2 O 4 , i.e., BaB 2 O 4 in its low-temperature modification) and LBO (LiB 3 O 5 ) as new materials. The characteristic features of BBO and LBO crystals, excellent NLO materials developed during the past few years in China at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, are also discussed.

Beryllium-free Li4Sr(BO3)(2) for deep-ultraviolet nonlinear optical applications

Nonlinear optical (NLO) materials are of great importance in laser science and technology, as they can expand the wavelength range provided by common laser sources. Few NLO materials, except KBe2BO3F2 (KBBF), can practically generate deep-ultraviolet coherent light by direct second-harmonic generation process, limited by the fundamental requirements on the structure-directing optical properties. However, KBBF suffers a strong layering tendency and high toxicity of the containing beryllium, which hinder the commercial availability of KBBF. Here we report a new beryllium-free borate, Li4Sr(BO3)2, which preserves the structural merits of KBBF, resulting in the desirable optical properties. Furthermore, Li4Sr(BO3)2 mitigates the layering tendency greatly and enhances the efficiency of second-harmonic generation by more than half that of KBBF. These results suggest that Li4Sr(BO3)2 is an attractive candidate for the next generation of deep-ultraviolet NLO materials. This beryllium-free borate represents a new research direction in the development of deep-ultraviolet NLO materials.

Deep-Ultraviolet Nonlinear Optical Materials: Na2Be4B4O11 and LiNa5Be12B12O33

Deep-UV coherent light generated by nonlinear optical (NLO) materials possesses highly important applications in photonic technologies. Beryllium borates comprising anionic planar layers have been shown to be the most promising deep UV NLO materials. Here, two novel NLO beryllium borates Na2Be4B4O11 and LiNa5Be12B12O33 have been developed through cationic structural engineering. The most closely arranged [Be2BO5]∞ planar layers, connected by the flexible [B2O5] groups, have been found in their structures. This structural regulation strategy successfully resulted in the largest second harmonic generation (SHG) effects in the layered beryllium borates, which is ~1.3 and 1.4 times that of KDP for Na2Be4B4O11 and LiNa5Be12B12O33, respectively. The deep-UV optical transmittance spectra based on single crystals indicated their short-wavelength cut-offs are down to ~170 nm. These results demonstrated that Na2Be4B4O11 and LiNa5Be12B12O33 possess very promising application as deep-UV NLO crystals.