Luminescence properties and relaxation processes of strongly correlated organic radical TTTA crystals and molecules

Photoluminescence of strongly correlated organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystals was measured at room temperature in order to elucidate the relaxation process in the excited state, which is responsible for an initiation of the photoinduced magnetic phase transition in this material. The electronic structure and luminescence properties of TTTA molecule were also investigated to clarify the relaxation process. We found that a luminescence band lies at 1.8 eV in the high-temperature (HT) phase crystal having almost same characteristics as that in TTTA molecule, suggesting that the intramolecular lattice distortion plays an important role for the relaxation process. On the other hand, a broad luminescence band appears at 1.4 eV with a large Stokes shift in the low-temperature (LT) phase crystal. The large Stokes shift observed shows that rearrangement of the dimerized TTTA molecules toward the evenly spaced radicals takes place due to a large intermolecular lattice distortion. This molecular rearrangement in the excited state initiates the photoinduced magnetic phase transition from the LT to HT phases in this material.     

Photoinduced phase transition in an organic radical crystal with room-temperature optical and magnetic bistability

A phase control by photoirradiation is successfully achieved in a spin-Peierls system of the organic radical crystal, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which exhibits optical and magnetic bistability around room temperature with a large hysteresis loop. A nanosecond laser pulse is found to induce a transition from a diamagnetic low-temperature phase to a paramagnetic high-temperature phase both inside (296 K) and outside (11 K) the hysteresis loop. Comparison of the excitation energy dependence between transition efficiency and photoconductivity suggests that the photoinduced transition is driven by suppression of the spin-Peierls instability by the accumulation of photocarriers.     

Real-space observation of nanoscale inhomogeneities and fluctuations in a phase transition of a surface quasi-one-dimensional system: In/Si(111)

We report direct visualizations of the fluctuation and condensation phenomena in a phase transition of a one-dimensional (1D) In/Si(111) system using scanning tunneling microscopy. The high-temperature (HT) and low-temperature (LT) phases are found to coexist on the nanometer scale near Tc. Above Tc, 1D LT-phase stripes fluctuate in the HT phase and coalesce into 2D islands with decreasing temperature. They condense to make the LT phase below Tc. Small areas of the HT phase also exist below Tc. The observed temperature-dependent evolution of the nanoscale inhomogeneities is consistent with the theoretical predictions for a second-order phase transition.     

Theory for Difference Between Photoinduced Phase and Thermally Excited Phase

A possible difference between the photoinduced phase and the thermally excited one is studied by using a two-dimensional extended Peierls-Hubbard model, which includes a strong electron-phonon coupling and a on-site interelectron repulsion, as well as an anharmonic lattice potential. Because of this anharmonicity, the system undergoes a first order phase transition from an insulating CDW state to a metallic one at a high temperature. Although some sign of an SDW order is expected to appear due to this repulsion, it is always hidden in any equilibrium phase of the present system. In fact, it is hidden, not only in the CDW ground state, but also in this metallic one, since the high temperature itself destroys the SDW order, far before the CDW-metal transition occurs, while a photo-excitation at low enough temperature is shown to generate a local metastable SDW domain. Therefore, to observe the presence of such Coulomb interaction and the resultant broken symmetry, a nonequilibrium photoinduced phase is shown to be most straightforward. Thus, the photoinduce phase transition can make an interaction appear as a broken symmetry only in this phase, even though this interaction is almost completely hidden in all the equilibrium phases from low temperature to high ones.     

New findings in phase transitions of spinel Li1.07Mn1.93O4-delta studied by transmission electron microscopy

Phase transitions and the related phase identifications at different temperatures for spinel Li(1.07)Mn(1.93)O(4-delta) crystals have been studied by transmission electron microscopy. The preliminary results clearly show the existence of at least two types of low-temperature phase: monoclinic and orthorhombic. Oxygen deficiency may raise the phase-transition temperature for the high-temperature (HT) cubic to low-temperature (LT) tetragonal, monoclinic, or orthorhombic phases. When the oxygen deficiency delta is very close to zero, the transition temperature is below room temperature (RT). Therefore, only the HT cubic spinel is observed at RT. When delta=0.182, the transition temperature is higher than RT, so the structures of the LT phases can be studied at RT. This study reveals the structural relationships between the LT phase and the HT phase. These relationships can be summarized as follows: (1) Two orthogonal cubic [440](Cubic)(*) and [440](Cubic)(*) reciprocal vectors are divided into two parts by {220} and {220} super reflections, respectively. This situation resembles the structures caused by correlated tilting and/or distortion of the octahedra in perovskites. (2) One of the cubic <440>Cubic(*) reciprocal vectors is divided into three parts by weak super-reflections. This situation resembles the modulated structure caused by the charge- and orbital-ordering in the perovskite La(1/3)Ca(2/3)MnO(3) of the space group Pnma. (3) One of the cubic <440>(Cubic)(*) reciprocal vectors, e.g. [440](Cubic)(*), is divided into three parts by stronger super-reflections, at the same time some strong reflections of the cubic spinel, e.g. (111)(Cubic)=(320)(Ortho)/2 disappear, indicating that there is a phase transition containing atomic movement. Items (2) and (3) are new findings in the present work.     

Magnetism in PrIr2Si2: A single crystal study

We present the results of magnetization, susceptibility and specific-heat measurements of the high-temperature (HT) and low-temperature (LT) phases of PrIr₂Si₂ performed on single-crystalline samples. The HT and LT phases adopt the tetragonal CaBe₂Ge₂-type and ThCr₂Si₂-type structure, respectively. We have found no magnetic phase transition for the HT phase at temperatures down to 2 K. On the other hand, the LT phase apparently orders antiferromagnetically (AF) at 45.5 K and undergoes a transition to another AF phase at T_t=23.7 K. Complexity of the magnetic phase diagram is amplified by two metamagnetic transitions induced by magnetic field applied along the c-axis at temperatures below T_t. The results will be discussed with respect to other polymorphic compounds PrNi₂As₂ and UCo₂Ge₂.     

The double-phase transition of ErFe4Ge2. An XRPD study

High-quality powder XRD data of the compound ErFe₄Ge₂ collected in the ESRF beam line BM16, are presented for the entire magnetically ordered regime (T_N=44 K). The data analysis reveals the occurrence of a double symmetry breaking at the magnetic transition. This experiment has allowed us to distinguish between structural and magnetic satellites, both present in the neutron patterns, and to demonstrate the interdependence of structural and magnetic transitions. The high-temperature (HT) phase disproportionates by a first-order transition into two distinct phases: P4₂/mnm (T_c, T_N=44 K)→Cmmm (majority LT phase)+Pnnm (minority IT Phase) which coexist in proportions varying with temperature down to 4 K. The phase diagram comprises three temperature regions: (a) the HT range with T>T_N for the tetragonal P4₂/mnm phase; (b) the IT (intermediate temperature) range, 20 K<T<T_N, where the two phases coexist in strongly variable proportions and the Pnnm phase reaches its highest concentration (≈31%) around 30 K and (c) the LT (low temperature) range, 1.5–20 K, where the Cmmm phase is dominating (up to 95%). We suggests that this phenomenon is the result of competing magneto-elastic mechanisms involving the Er crystal field anisotropy, the Er–Er, Er–Fe and the Fe–Fe exchange interactions and their coupling with the lattice strains.     

磁力显微镜对相分离锰氧化物薄膜中反铁磁绝缘相融化的成像

用自制磁力显微镜研究了一个受各向异性应变的锰氧化物薄膜中的相分离以及由磁场导致的从反铁磁绝缘相到铁磁金属相的转变．磁力显微镜图片显示,在0 T这两种竞争的相就已经共存,且两种相 的畴呈非常明显的各向异性的条状分布,这可以定性解释输运上的各向异性．在2.1 T 以上,反铁磁绝缘相逐渐转变为铁磁金属相,并在3.2 T时结束．当去掉磁场时铁磁金属相能够保持．     

Thermal phase transition of RbMnFe(CN)6 observed by X-ray emission and absorption spectroscopy

The thermal phase transition of RbMnFe(CN)₆ has been observed by Mn and Fe 3p-1s X-ray emission spectroscopy (XES) and 1s X-ray absorption spectroscopy (XAS). The thermal variations of the spin states and the valences of Mn and Fe were determined to be Mn²⁺(S=5/2)-NC-Fe³⁺(S=1/2) for the high-temperature (HT) phase and Mn³⁺(S=2)-NC-Fe²⁺(S=0) for the low-temperature (LT) phase. These transitions are thus caused by charge transfer between Mn and Fe. The temperature dependences of Mn and Fe 3p-1s XES and 1s XAS were observed as the composition of the spectra of the HT and LT phases. The ratios of the HT component in each spectrum show good agreement with the thermal transition curves observed with magnetic susceptibility measurements.     

范德瓦尔斯磁性材料CrOCl变磁性相变的磁力显微镜研究

近年来,二维范德瓦尔斯磁性材料因为在自旋电子学的应用前景而吸引了广泛的关注.CrOCl是一种范德瓦尔斯磁性材料,理论预言其单层具有高达160 K的居里温度,因此吸引了广泛的关注。为了更好的理解这一材料的磁性,我们利用磁力显微镜研究了CrOCl变磁性相变中磁畴结构随磁场的变化。实验发现,在2K下CrOCl样品表面出现随磁场变化的方格条纹,给出了变磁性相比中反铁磁相和铁磁相竞争的图样,并通过二维快速傅里叶变换证实了CrOCl磁性的各向异性。我们的结果为后续研究CrOCl薄层的磁性提供了参考依据。     

A new quasi-one-dimensional molecular solid based on Ni(mnt)2 anion: Crystal structure and spin-gap transition

A new molecular solid, [1-(4′-bromo-2′-fluorobenzyl)-4-dimetylaminopyridinium]-bis(maleonitriledithiolato)nickel(III), (BrFBzPyN(CH₃)₂(Ni(mnt)₂)(1), has been prepared and characterized by elemental analyses, IR, ESI-MS spectra, single crystal X-ray diffraction and magnetic measurements. Compound 1 crystallizes in the orthorhombic space group Pnma, a=20.579(4) Å, b=7.078(1) Å, c=17.942(4) Å, α=β=γ=90°, V=2613.3(9) Å³, Z=4. The Ni(III) ions of 1 form a quasi-one-dimensional Zigzag magnetic chain within a Ni(mnt)₂⁻ column through Ni?S, S?S, Ni?Ni, or π?π interactions with an Ni?Ni distance of 4.227 Å. Magnetic susceptibility measurements in the temperature range 2-300 K show that 1 exhibits a spin-gap transition around 200 K, and antiferromagnetic interaction in the high-temperature phase (HT) and spin gap in the low-temperature phase (LT). The transition for 1 is second-order phase transition as determined by DSC analyses.     

A model of Photoinduced Regular-Dimerized Stack Transitions

We present a model of photoinduced regular-dimerized stack transitions and discuss theoretically the behavior of the organic molecular mixed-stack charge-transfer crystal, Tetrathiafulvalene-Chloranil (TTF-CA). We investigate both the photoinduced structural phase transitions and the thermal phase transition between the regular-stack neutral phase at high temperatures and the dimerized-stack ionic phase at low temperatures. Our model is based on a simple phenomenological insight that the molecules have three stable displacements in the crystal. The cooperative effect is accounted for by introducing harmonic coupling between the molecules. It is the first attempt to include not only the photo-excitation but also the temperature effect on an equal footing.     

Enhanced ferromagnetism in Mn-doped TiO2 films during the structural phase transition

The crystal structures and magnetic properties of Ti_0.9747Mn_0.0253O₂ films prepared by sol–gel dip coating have been investigated. Room temperature ferromagnetism was observed both in the films of pure anatase phase and of mixed anatase and rutile phases. For the first time, enhancement of the ferromagnetism was revealed as the phase transition from anatase to rutile occurs: from 0.7±0.01μ_B/Mn to 1.1±0.05μ_B/Mn. The possible mechanism for the observed magnetism enhancement is discussed.     

The magnetic phases of NdCu2

The magnetic phase diagram of a NdCu₂ single crystal is investigated by means of specific-heat measurements and neutron diffraction as a function of temperature and external magnetic field applied along the crystallographic b-direction. In the low temperature region we observe three commensurate phases AF1, F1, and F2. Their magnetic structures all consist of ferromagnetic (bc)-planes with different stacking sequences along the a-direction comprising 5, 3 and 8 chemical unit cells, respectively. Above 2.8 T the system is in a ferromagnetically aligned state (F3). Furthermore, there is an incommensurate phase AF3 between the low temperature commensurate phases and the paramagnetic state and, in a very narrow temperature region of only 0.2 K, an intermediate phase AF2 between AF1 and AF3. The Nd-moments are oriented along the b-direction in all phases.     

NDMAP: the best material to study the Haldane's prediction

As predicted by Haldane, spin, S=1 one-dimensional (1D) Heisenberg antiferromagnet (HAF) has an energy gap between the singlet ground state and first excited triplet. On application of magnetic field, the triplet state Zeeman splits and the energy of one of the triplet state becomes zero at a critical field, H_c. Above H_c the system recovers magnetism. Then, we expect that a quasi-1D HAF will show a magnetic long-range ordering (LRO) at low temperatures due to the inter-chain coupling. This field-induced LRO has not been observed before due to complication of the crystal structure in the materials studied so far and/or technical difficulty.From a heat capacity measurement on a single crystal of an S=1 quasi-Q1D HAF, Ni(C₅H₁₄N₂)₂N₃(PF₆), we found an anomaly at a temperature in finite fields indicating a field-induced phase transition. A magnetic LRO is confirmed by a neutron diffraction measurement on the same sample. The temperature versus magnetic field phase diagram of this compound is constructed and discussed.     

氢化物气相外延生长高质量GaN膜生长参数优化研究

系统研究了低温成核层生长时间、高温生长时的V/Ⅲ 比以及生长温度对氢化物气相外延生长GaN膜晶体质量的影响. 研究发现合适的低温成核层为后续高温生长提供成核中心, 并能有效降低外延膜与衬底间的界面自由能, 促进成核岛的横向生长; 优化的V/Ⅲ比和最佳生长温度有利于降低晶体缺陷密度, 促进横向生长, 增强外延膜的二维生长. 利用扫描电子显微镜、原子力显微镜、高分辨X射线衍射、 低温光致发光谱和室温拉曼光谱对优化条件下生长的GaN外延膜进行了结构和光电特性表征. 测试结果表明, 膜表面平整光滑, 呈现二维生长模式表面形貌; (002)和(102)面摇摆曲线半高宽分别为317和343 arcsec; 低温光致发光谱中近带边发射峰为3.478 eV附近的中性施主束缚激子发射峰, 存在11 meV的蓝移, 半高宽为10 meV, 并且黄带发光强度很弱;常温拉曼光谱中E₂ (high) 峰发生1.1 cm^-1蓝移.结果表明, 优化条件下生长的GaN外延膜具有良好的晶体质量和光电特性, 但GaN 膜中存在压应力. 关键词： 氮化镓 氢化物气相外延 低温成核层     

Effect of pressure on magnetic phases of (Fe1-x Ni x )49Rh51

The effect of high pressure and magnetic field on the stability of magnetic phase transitions in (Fe_1?x Ni _x )₄₉Rh₅₁ has been studied. It has been shown that the increase in the Ni content suppresses the stability of the low-temperature antiferromagnetic phase and leads to its disappearance. Pressure on the other hand restores the stability to the antiferromagnetic order. These findings are explained using first-principles band-structure calculations.     

Low-temperature neutron diffraction study of the crystal and magnetic phase transitions in DyCrO4

The crystal and magnetic structures of DyCrO₄ were studied using neutron powder diffraction. Complete diffraction data at 3.6, 17, 27, and 40 K show that a crystal structural phase transition from tetragonal I4₁/amd to orthorhombic Imma symmetry is found to take place between 27 and 40 K. This transition does not involve a significant change in the unit cell volume. Strong ferromagnetic reflections are observed at 3.6 and 17 K, and can be fit well using the magnetic model of space group Im'ma', with the moments of both Dy³⁺ and Cr⁵⁺ ions aligning along the y-axis. Detailed temperature dependent magnetic intensities of 101/011 and 211/121 peaks reveal a Curie temperature of T_c=22.35(15) K.     

退火温度对高分子薄膜中掺杂小分子的结晶和光谱学响应特性的影响

研究了小分子材料苝(EPPTC)在高分子材料聚芴衍生物(F8BT)薄膜中的结晶特性随温度的变化规律,以及由此引起的两种材料构成的异质结中激发复合体荧光发射特性的变化。实验结果表明,退火温度的升高会加强小分子与高分子材料在固体薄膜中的相分离。而小分子相在析出过程中,会在分子间作用力诱导下发生π—π团聚而结晶。晶体的尺度在达到小分子材料相变温度之前基本上随温度升高而增大。这一过程将破坏异质结结构,减小小分子与高分子间的接触面积,从而降低激发复合体的形成及其荧光发射强度。同时,由高分子向小分子发生的能量转移过程被显著减弱,而小分子晶相的荧光发射成分提高。这对于调控有机半导体异质结结构,进而改善光伏器件性能具有重要意义。     

Phase stability in the LnCa2Mn2O7 (Ln=Pr, Nd, Sm and Gd) Ruddlesden-Popper series

The synthesis of the Ruddlesden-Popper series, LnCa₂Mn₂O₇, (Ln=Pr, Nd, Sm and Gd) is described and their structure and electronic properties investigated. The reduction in size of the A-site cation causes an increase in the distortion of their orthorhombic structures (space group Cmcm). All of these compounds form with a perovskite impurity, the amount of which increases on reduction of the cation size. The synthesis temperature also alters the amount of perovskite impurity in the phase, but only to a lower limit, implying the perovskite phase is intrinsic to the material and that a phase equilibrium exists between the layered Ruddlesden-Popper and perovskite phases, which is controlled by the cation size. The magnetic susceptibility show transitions characteristic of the perovskite phase, therefore little direct information can be obtained about the Ruddlesden-Popper phases, except that ferromagnetism is not observed in any of these materials.