Noncentrosymmetric Crystals Research Articles

Growth, polarized Raman and fluorescence properties of TbCa4O(BO3)3 single crystal

A single crystal of TbCa4O(BO3)3 (TbCOB) with diameter of 35mm and length of 80mm has been successfully grown using the Czochralski (Cz) method. The polarized Raman spectra were studied to analyze the vibrational structure of TbCOB, especially the modes from [BO3]3− triangles that are responsible for the nonlinear optical generation. The fluorescence properties of the X-cut, Y-cut, and Z-cut non-centrosymmetric TbCOB crystals were measured for the first time, and both f–d and f–f transitions were identified according to the energy level diagram. Based on the non-radiative energy transfer process, TbCOB crystal shows the characteristic emission of Tb3+ corresponding to 5D4→7F6,5,4,3 transitions with the green light of 542nm (5D4→7F5) being the most prominent group. Additionally, the lifetime decay indicates single exponential character for the excited wavelength at 378nm, following the result of τ=2.07ms at room temperature. The long fluorescence lifetime and good spectra properties of TbCOB single crystal indicate its potential optical prospects.

Point-group sensitive orientation mapping of non-centrosymmetric crystals

We demonstrate polarity-sensitive orientation mapping of non-centrosymmetric phases by Electron Backscatter Diffraction (EBSD). The method overcomes the restrictions of kinematic orientation determination by EBSD, which is limited to the centro-symmetric Laue-groups according to Friedel's rule. Using polycrystalline GaP as an example, we apply a quantitative pattern matching approach based on simulations using the dynamical theory of electron diffraction. This procedure results in a distinct assignment of the local orientation according to the non-centrosymmetric point group of the crystal structure under investigation.

Elastico-mechanoluminescence and crystal-structure relationships in persistent luminescent materials and II–VI semiconductor phosphors

Elastico-mechanoluminescence (EML) has recently attracted the attention of a large number of researchers because of its potential in different types of mechano-optical devices. For understanding the mechanism of EML the relationships between elastico-mechanoluminescence (EML) and crystal-structure of a large number of persistent luminescent materials and II–VI semiconductor phosphors known to date are investigated. It is found that, although most of the non-centrosymmetric crystals exhibit EML, certain non-centrosymmetric crystals do not show EML. Whereas, many centrosymmetric crystals do not exhibit EML, certain centrosymmetric crystals exhibit EML. Piezoelectric ZnS:Cu,Cl single crystals do not show EML, but piezoelectric ZnS:Cu,Cl microcrystalline phosphors show very intense EML. Piezoelectric single crystals of undoped ZnS do not show EML. It seems that EML is related to local piezoelectrification near the impurities in crystals where piezoelectric constant is high. Suitable piezoelectric field near the local piezoelectric region and stable charge carriers in traps are required for appearance of EML. The EML of persistent luminescent materials and II–VI semiconductor phosphors can be understood on the basis of piezoelectrically-induced trap-depth reduction model of EML. Using suitable dopants both in non-centrosymmetric and centrosymmetric crystals intense elastico-mechanoluminescent materials emitting desired colours can be tailored, which may find applications in several mechano-optical devices.

Computer Simulation of Transition Regimes of Solitons in Stimulated Raman Scattering with Excitation of Polar Optical Phonons

The system of nonlinear equations modeling the process of nonstationary stimulated Raman scattering (SRS) in noncentrosymmetric crystals for the waves on laser, Stokes, polariton, and phonon frequencies is investigated by using the numerical methods. The general case for amplitudes of waves that resulted in doubling of the number of equations is considered. It is shown that the application of the methods of finite differences to the computer simulation of transition regimes is completely consistent with the analytical results found for the asymptotical solutions in form of solitons. The obtained results also indicate that the laser pulses of Gaussian shape appearing at the boundary of nonlinear medium tend to become solitons of Lorentzian shape. It was also found that the formation of solitons occurs when the vibrations of optical phonons and that of electromagnetic wave were either in or out of phase. It is shown that all electromagnetic waves entering the medium with different speeds become solitons having the same speed. In the second part of the paper we considered the computer simulation of soliton stability with respect to small (weak) perturbations of all interacting waves. In the present paper we considered the case of evolution of those disturbances in the vicinity of peaks of solitons. The numerical analysis showed that in wide range of parameters the solitons were stable.

Polarization-dependent optical absorption of MoS₂ for refractive index sensing.

As a noncentrosymmetric crystal with spin-polarized band structure, MoS2 nanomaterials have attracts increasing attention in many areas such as lithium ion batteries, flexible electronic devices, photoluminescence and valleytronics. The investigation of MoS2 is mainly focused on the electronics and spintronics instead of optics, which restrict its applications as key elements of photonics. In this work, we demonstrate the first observation of the polarization-dependent optical absorption of the MoS2 thin film, which is integrated onto an optical waveguide device. With this feature, a novel optical sensor combining MoS2 thin-film and a microfluidic structure has been constituted to achieve the sensitive monitoring of refractive index. Our work indicates the MoS2 thin film as a complementary material to graphene for the optical polarizer in the visible light range, and explores a new application direction of MoS2 nanomaterials for the construction of photonic circuits.

Hydrothermal growth of lanthanide borosilicates: a useful approach to new acentric crystals including a derivative of cappelenite.

The reaction of barium borosilicate glasses in hydrothermal fluids with NaOH mineralizer at ca. 600-650 °C in the presence of various sources of Eu(3+) ions leads to a new series of noncentrosymmetric single crystals characterized by single-crystal X-ray diffraction. Under three slightly different reaction modifications, three new Eu(3+)-containing noncentrosymmetric single crystals were isolated and characterized. Using a barium borosolicate glass with Eu(2)O(3) added to the reaction mixture led to NaBaEuSi(3)O(9) (1), which crystallizes in the noncentrosymmetric orthorhombic space group P2(1)2(1)2(1) with unit cell parameters a = 5.6373(11) Å, b = 11.483(2) Å, and c = 12.535(2) Å and consists of Eu(3+) ions linked by Si(3)O(9)(6-) rings. Using a similar glass feedstock with Eu(3+) included in the starting glass composition led to the compound NaBa(3)Eu(3)Si(6)O(20) (2), which was determined in the noncentrosymmetric polar space group Ama2 with unit cell parameters a = 14.777(3) Å, b = 23.926(5) Å, and c = 5.5533(11) Å. It is constructed of individual islands of Si(4)O(13)(10-) and lone Si(2)O(7)(6-) building fragments. Finally, use of the Eu-containing glass along with additional Eu(2)O(3) led to the compound BaEu(6)(Si(3)B(6)O(24))(OH)2 (3), which crystallizes in the polar space group P6mm with a = 10.8074(15) Å and c = 4.7296(9) Å. It is related to the rare and poorly understood mineral cappelenite Ba(Y,RE)(6)(Si(3)B(6)O(24))F(2) and contains six-membered tetrahedral borate rings linked by silicate tetrahedra. The high percentage of noncentrosymmetric crystals obtained from of a simple borosilicate glass starting material, along with various straightforward chemical additives, suggests strongly that many more noncentrosymmetric crystals can be isolated from this general system.

Growth, thermal, mechanical, structural and optical properties of organic NLO crystals of novel cis-2,6-bis(2-chlorophenyl)-3,3-dimethylpiperidin-4-one

An organic NLO material viz., cis-2,6-bis(2-chlorophenyl)-3,3-dimethylpiperidin-4-one (2C3DMPO), has been synthesized and a slow evaporation technique was applied to produce a single crystal. X-ray diffraction study on the single crystal 2C3DMPO reveals a non-centro symmetric crystal, possessing a monoclinic space group P21 and prefers to adopt a chair conformation. The crystal has been characterised using UV, FT-IR and NMR spectral studies. Solubility study and mechanical study using micro hardness methods have also been carried out. Furthermore, the thermal stability of the crystal was established by TG/DTA. The second harmonic conversion efficiency of the crystal was determined using the Kurtz and Perry powder technique and the activity observed was 3.83 times greater than that of KDP.

Giant bulk photovoltaic effect in thin ferroelectricBaTiO3films

The voltage generated in a noncentrosymmetric crystal due to the bulk photovoltaic effect (BPE) can greatly exceed the energy gap, however, the light energy conversion efficiency is extremely low. Here we show that the BPE is remarkably enhanced in the case of thin films. The measurements of the BPE in heteroepitaxial single domain ferroelectric ${\mathrm{BaTiO}}_{3}$ thin films reveal the enhancement of both photoinduced electric field and conversion efficiencies of the BPE by more than 4 orders of magnitude. Besides the fundamental aspect, our results indicate the potential for the use of the BPE in photovoltaic applications.

Electric Octupole Order in Bilayer Ruthenate Sr3Ru2O7

Although the odd-parity multipole order barely occurs in crystals with high symmetry, it can be formed in locally noncentrosymmetric crystals. We illustrate the odd-parity electric octupole order generated in a bilayer structure. When the local electric quadrupole is alternatively stacked between layers, it is regarded as an electric octupole order from the viewpoint of symmetry. We show that the $p_y s_x + p_x s_y$ spin nematic order is induced by the spin-orbit coupling in the electric octupole state, and it results from the spontaneous inversion symmetry breaking leading to the $D_{2d}$ point group symmetry. We investigate the possible realization of the electric octupole order in the bilayer ruthenate Sr$_3$Ru$_2$O$_7$, assuming a local electric quadrupole arising from the Pomeranchuk instability and/or the orbital order. Effects of the lattice distortion and magnetic field are also clarified, and the nature of the "electronic nematic state" of Sr$_3$Ru$_2$O$_7$ is discussed. It is proposed that the asymmetric band structure is a signature of the electric octupole order in Sr$_3$Ru$_2$O$_7$. The odd-parity multipole order in other strongly correlated electron systems is discussed.

Determination of Chirality of the Chiral Space Groups with Two-fold Screw Axis

Crystals are classified according to their symmetry operations into 230 space groups. A two-fold screw axis, 21, a symmetry operation, is frequently found in the crystal, especially non-centrosymmetric organic crystals. The two-fold screw axis shows a chiral character when molecules in the crystal tilt against the screw axis as shown in Figure. The crystals belonging to a chiral space group with two-fold screw axis such as a P21or P212121exhibit two different types with opposite chirality. However, although the two crystals are not identical in a molecular arrangement, we cannot distinguish them with the present notation of space group. Recently, the idea of determining the handedness of two-fold screw axis have been successfully proposed using hierarchical interpretation of the crystals.(I. Hisaki, T. Sasaki et al., 2012) Nevertheless, the issue on a notation to distinguish chiral crystals with two-fold screw axis still remains unsettled as far as we know. Therefore, we attempt to propose a novel notation for the crystals belonging to chiral space groups with two-fold screw axis. We focus on the relationship between the absolute structure and optical activity of the crystals. We have selected alanine crystals, which belong to P212121, as model crystals to discuss the notation. We have determined the absolute structure of the alanine crystals by X-ray diffraction and have measured their optical activity with Generalized High Accuracy Universal Polarimeter (G-HAUP).(M. Tanaka, N. Nakamura et al., 2012) G-HAUP is an apparatus that can measure simultaneously the linear birefringence, linear dichroism, circular birefringence, i.e., optical activity, and circular dichroism in any solid material. These experimental results have successfully correlated the absolute structure to the optical activity of the alanine crystals.

Towards User Operations of SONICC at GM/CA@APS

Second order nonlinear optical imaging of chiral crystals (SONICC), based on femtosecond laser scanning microscopy, has been implemented at GM/CA@APS undulator beamline 23ID-B for rapid protein crystal localization and centering. The technique is based on infrared laser light impinging on non-centrosymmetric crystals of proteins, which selectively may yield a frequency-doubled, visible signal generated by the anharmonic response of the electron cloud of the protein in response to the laser field. One aim of this method is to locate small crystals grown in opaque crystallization media for centering in X-ray beams of only a few microns or less in cross-section [1]. The optical system implemented at the beamline includes `trans' and `epi' detection of Second Harmonic Generation (SHG) signals [2]. In addition, scanning visible laser light across the sample and detecting two-photon excited UV fluorescence (TPE-UVF) provides complementary contrast based on the native fluorescence of proteins. An update on progress towards offering a user-friendly system to users will be provided. Different factors that influence imaging signals and the practice of successfully locating and accurately positioning a crystal via SONICC will be discussed.

Anomalous Fermi-liquid phase in metallic skyrmion crystals

In non-centrosymmetric crystals such as MnSi, magnetic order can take the form of a skyrmion crystal (SkX) . In this phase, conduction electrons coupled to the local magnetic moments acquire a Berry's phase, leading to an emergent electromagnetism. Motivated by experimental reports of a non-Fermi liquid phase in MnSi, in which resistivity is observed to scale as $\Delta \rho \sim T^{3/2}$, here we examine the effect of coupling phonons of an incommensurate SkX to electrons. Despite the formal similarity to a system consisting of a Fermi surface coupled to an electromagnetic field, the Berry phase fluctuations do not lead to non-Fermi liquid behavior. Instead, we propose a different mechanism in which electrons scatter off columnar fluctuation in a three-dimensional SkX. When the effects of lattice induced anisotropy are neglected, these fluctuations are ultra-soft and induce an `anomalous Fermi liquid' in which Landau quasiparticles survive but with an anomalous $\Delta \rho(T)\sim T^{7/4}$ resistivity perpendicular to the columns, and a Fermi liquid resistivity along them.

Valley-dependent spin polarization in bulk MoS2 with broken inversion symmetry.

The valley degree of freedom of electrons is attracting growing interest as a carrier of information in various materials, including graphene, diamond and monolayer transition-metal dichalcogenides. The monolayer transition-metal dichalcogenides are semiconducting and are unique due to the coupling between the spin and valley degrees of freedom originating from the relativistic spin-orbit interaction. Here, we report the direct observation of valley-dependent out-of-plane spin polarization in an archetypal transition-metal dichalcogenide--MoS2--using spin- and angle-resolved photoemission spectroscopy. The result is in fair agreement with a first-principles theoretical prediction. This was made possible by choosing a 3R polytype crystal, which has a non-centrosymmetric structure, rather than the conventional centrosymmetric 2H form. We also confirm robust valley polarization in the 3R form by means of circularly polarized photoluminescence spectroscopy. Non-centrosymmetric transition-metal dichalcogenide crystals may provide a firm basis for the development of magnetic and electric manipulation of spin/valley degrees of freedom.

Neutron optics of noncentrosymmetric crystals: New possibility of searches for the neutron electric dipole moment and CP-violating forces

New neutron-optics effects, which were predicted and discovered quite recently in noncen- trosymmetric crystals and which can be used to study the fundamental properties of the neutron, are discussed. In particular, strong electric fields of strength up to 10 8 or 10 9 V/cm may act in such crystals on the neutron, and this provides new possibilities for measuring the neutron electric dipole moment (EDM) by the crystal-diffraction method. It also becomes possible to perform searches for pseudomagnetic forces acting on neutrons and violating CP invariance. For the range of such forces that satisfies the condition λA < 10 −5 cm, the best constraints on the product of the scalar and pseudoscalar coupling constants have already been obtained for the interaction induced by the exchange of a light pseudoscalar (axion-like) particle. The present-day status and prospects of neutron-optics crystal-diffraction experiments aimed at searches for the neutron EDM and pseudomagnetic forces are considered.

Terahertz and Raman spectra of non-centrosymmetrical organic molecular crystals

The terahertz waves have been applied to the spectroscopy of molecular crystals. The group based on hybrid organic–inorganic crystals like triglycine zinc chloride, triglycine sulphate, triglycine selenate, diglycine hydrobromide, diglycine hydrochloride, diglycine lithium nitrate, glycine lithium nitrate has been considered. The dispersion relation phonon–polariton have been shown between 10cm−1 and 80cm−1 for diglycine lithium nitrate, triglycine zinc chloride, and diglycine hydrobromide.

Temperature dependence of flexoelectricity in BaTiO3 and SrTiO3 perovskite nanostructures

Abstract Piezoelectricity is a property of non-centrosymmetric crystals. In most typically used ferroelectrics, this property is lost as the temperature is increased beyond the Curie point thus strongly reducing the availability of efficient materials that can be used for high temperature energy harvesting. Flexoelectricity, as can be shown from simple symmetry arguments, is a universal and linear electromechanical coupling that dictates the development of polarization upon application of inhomogeneous strains. The implications of this phenomenon become amplified at the nanoscale. In this communication, we develop a molecular dynamics approach predicated on a specially tailored interatomic force-field to extract the temperature dependence of flexoelectricity. Surprisingly, we find that it, at least for Barium Titanate and Strontium Titanate nano structures, increases with temperature. Apart from cataloging this interesting observation for the future use in high temperature energy harvesting, we also examine the physical mechanisms that lead to the observed temperature dependence.

Intense optical second harmonic generation from centrosymmetric nanocrystalline para-nitroaniline

We demonstrate an approach that uses normally centrosymmetric, but highly polarizable organic molecules of para-nitroaniline (p-NA) as very efficient second harmonic generator (SHG) for near infra-red light. The approach is based on an effective manipulation of the nanocrystalline size and their assembly into a highly orientated mesocrystalline structures. The resulting mesocrystalline form of p-NA consists of multiple nanocrystals with common orientation, providing a set of surfaces with highly aligned molecules forming a head-to-tail polar arrangement. The second harmonic generation efficiency from this arrangement of mesocrystalline nanostructures is comparable to that from the conventional non-centrosymmetric organic crystals. The experimental results indicate that a very strong second-order nonlinear response can be obtained from small D-π-A organic molecules with elevated molecular hyperpolarizabilities even if these molecules tend to crystallize in centrosymmetric structures.

Generation of terahertz radiation in cubic non-centrosymmetric crystals

The conditions of parametric radiation generation on polaritons in cubic noncentrosymmetric crystals are studied. The possibility of such generation is theoretically justified. The polariton radiation frequencies are calculated for GaP, ZnSe, ZnTe, and GaAs crystals. The obtained generation frequencies are compared to the experimental results on Raman scattering on polaritons. The block diagram of the terahertz radiation generator operation using a GaP crystal and a pulsed laser with high peak power at low energy of laser pump pulses is presented. The lasing frequency shift is analyzed depending on the scattering geometry. The coefficient of exciting radiation conversion to the terahertz range is determined.

X-ray spectroscopy study of the electronic structure of non-centrosymmetric Ag2CdSnS4 single crystal

We report on studies of the electronic structure of Ag2CdSnS4 single crystal grown by the horizontal gradient freeze technique. For the crystal under consideration, X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated surfaces have been measured. The present X-ray photoelectron spectroscopy (XPS) results indicate that Ag2CdSnS4 single crystal surface is very rigid with respect to Ar+ ion-irradiation. In particular, Ar+ bombardment of the single crystal surface does not induce any significant changes of values of the binding energies of core-level electrons as well as the energy distribution of electronic states within the valence-band region. The Ar+ bombardment does not change the elemental stoichiometry of the Ag2CdSnS4 surface. For the Ag2CdSnS4 compound, the X-ray emission bands representing the energy distribution of the valence Ag d, Cd d and S p states were recorded and compared on a common energy scale with the XPS valence-band spectrum. Results of such a comparison indicate that the S 3p states contribute predominantly in the upper and central portions of the valence band of the Ag2CdSnS4 single crystal. In addition, our data reveal that the Ag 4d and Cd 4d states contribute mainly in the central portion and at the bottom of the valence band, respectively.

Ginzburg–Landau Description of Twin Boundaries in Noncentrosymmetric Superconductors

We study theoretically a model for twin boundaries in superconductors with Rashba spin-orbit coupling, which can be relevant to both three-dimensional noncentrosymmetric tetragonal crystals and two-dimensional gated superconductors such as the LaAlO$_3$/SrTiO$_3$ interface. In both cases, the broken inversion symmetry allows for a coexistence of singlet and triplet pairing. Within the framework of a Ginzburg-Landau theory, we identify two $\mathbb{Z}_2$ symmetries that are broken via two consecutive second order phase transitions as the temperature is lowered. We show that a time-reversal symmetry breaking superconducting state nucleates near the twin boundary if singlet and triplet pairing amplitudes are of comparable magnitude. As a consequence, the tendency towards ferromagnetic order is locally increased along with the emergence of spontaneous supercurrents parallel to the twin boundary. Spin currents, which are present in the form of Andreev bound states, are found enhanced in the time-reversal broken phases.