准一维材料蓝青铜电荷密度波动力学的掺杂效应研究

对包括纯的、铷掺杂、钨掺杂的准一维电荷密度波材料蓝青铜系列单晶样品的电阻率温度行为、非线性输运行为进行了比较研究.实验发现,与铷掺杂相比,钨掺杂对样品的电输运特性和派尔斯相变的影响更加剧烈.特别地,对于纯样品和铷掺杂样品,电荷密度波的解钉扎阈值在100 K附近表现出极大值行为;而对于钨掺杂样品,阈值行为是单调的.针对以上结果,从蓝青铜在100 K附件可能存在的非公度-公度转变以及电荷密度波与样品中杂质缺陷和正常载流子间的相互作用角度进行了讨论.     

掺杂蓝青铜的低频电压噪声研究

通过对掺杂蓝青铜的低频宽带噪声(BBN)的研究,发现在掺杂样品中杂质和缺陷能大大的加强低频电压噪声,并且噪声的幅度、波形又强烈地与单晶在冷却过程中的初始电流状态有关,从而揭示出电荷密度波的宽带噪声可能来自于样品中宏观缺陷的微观实质。 关键词：     

Phase transitions and incommensurability in the layered semiconductor TlInS2--an NMR study

We report on the first NMR study of powder and single crystal samples of thallium indium sulfide, TlInS(2). The crystal under study is a pure single-layer TlInS(2) polytype. Our findings show that transformation from the high temperature paraelectric phase to the low temperature ferroelectric phase occurs via an incommensurate phase that exists in the temperature range from T(c) = 192 K to T(i) = 205 K. On approaching the phase transition at T(i) from above, the crystal exhibits a soft mode behavior. A discrepancy in the literature data on the phase transitions in TlInS(2) is discussed and ascribed to polytypism of the TlInS(2) crystals.     

Light figure studies of optical anisotropy induced by nanoscale spatial modulation in TlInS2

The incommensurate material, TlInS₂, with layer structure was studied by light figure spectroscopy techniques in spectral range 400-700 nm in temperature interval 80-350 K. Evolution of light figures with temperature in phase transition region was reliably established. The optic angle of TlInS₂ was found to be sample-dependent with essentially two types of critical behavior with temperature; peak-like behavior that was ascribed to a pure incommensurate phase and peak-free behavior that was attributed to the influence of the domains or defects. In latter case the samples showed symmetry forbidden rotation of optic plane with temperature. As a whole, the obtained results allow for a conclusion that incommensurate spatial modulation with correlation length at the nanoscale was contributing to biaxial anisotropy of TlInS₂ to a level of 10⁻³ versus 10⁻⁴ for basic lattice.     

Quantum spin liquid in frustrated one-dimensional LiCuSbO4

A quantum magnet, LiCuSbO4, with chains of edge-sharing spin-1/2 CuO6 octahedra is reported. While short-range order is observed for T<10 K, no zero-field phase transition or spin freezing occurs down to 100 mK. Specific heat indicates a distinct high-field phase near the 12 T saturation field. Neutron scattering shows incommensurate spin correlations with q=(0.47±0.01)π/a and places an upper limit of 70 μeV on any spin gap. Exact diagonalization of 16-spin easy-plane spin-1/2 chains with competing ferro- and antiferromagnetic interactions (J1=-75 K, J2=34 K) accounts for the T>2 K data.     

Sign and amplitude of charge density waves in 1T-TaS2 and TaSe2

The effect of charge density waves has been observed in X-ray photoelectron spectra of 1T-TaS₂ and TaSe₂ as a perturbation of the core-electron binding energies. In the commensurate CDW range the Ta 4f levels show a marked splitting; in the quasi-commensurate and incommensurate states they show broadening of magnitude comparable to this splitting.     

Temperature dependence of the second threshold field for rubidium blue bronze Rb0.3MoO3

We have investigated the threshold properties of Rubidium blue bronze Rb_0.3MoO₃ under high dc electric field in a large temperature range 20-150 K. The second threshold fields have been observed at temperature up to 102.4 K, and have quasi-linear relationships with temperatures 20-45 K and 55-100 K, respectively. A novel crossover platform has been found firstly in the temperature dependence of the second threshold field E_T2 at about 45-55 K. The results indicate that the dynamical behavior of the second threshold effects may originate from different mechanisms. We suggest that the highly conducting state at 20-45 K and 55-100 K result from the undamped sliding motion of rigid CDW and current inhomogeneity, respectively.     

Magnetic structure of TbNiAl4

Magnetisation and specific heat measurements, in the range 2 K to room temperature, demonstrate that three magnetic phases exist for the intermetallic compound TbNiAl₄. Powder neutron diffraction, also carried out over a wide temperature range, establishes that the intermediate magnetic phase is incommensurate, and confirms that the lowest temperature phase has a linear antiferromagnetic structure with a (0 1 0) propagation vector. The respective transition (Néel) temperatures, in zero applied magnetic field are 34.0 and 28.0 K.     

Temperature-dependent attenuation of surface acoustic waves in proton-exchanged 128°-rotated Y-cut LiNbO3

3 has been experimentally studied in the frequency range 100 to 460 MHz and in the temperature range 90 to 300 K. At room temperature, the proton exchange leads to the considerable enhancement of acoustic attenuation as compared to the pure samples. Annealing in air, in general, reduces the attenuation. However, an anomalous enhancement of the attenuation at several frequencies for particular conditions of annealing is observed. When the temperature is reduced, the attenuation decreases practically to zero in as-exchanged samples. In the annealed ones, the attenuation attains a minimum in the vicinity of 160 K, and begins to grow at lower temperatures. A sharp peak in the attenuation is observed at 210 K. Several different physical mechanisms including acousto-protonic interaction and structural phase transition seem to be responsible for the observed acoustic attenuation behaviour. Received: 17 July 1996/Accepted: 2 December 1996     

Mean field phase diagram of the discrete Frenkel-Kontorova model

The mean field phase diagram of the discrete Frenkel-Kontorova model (harmonic chain of particles exposed to a sinusoidal potential) is studied using a fixed irrational value for the ratio of atoms and available sites as a boundary condition. Independent variables are the temperature and the height of the sinusoidal potential. The phase diagram contains two different solid phases, namely an intrinsically pinned incommensurate and an unpinned incommensurate phase. The position of the phase boundary and the nature of the phases are determined. We also investigate the influence of an electric field and the resulting pinning-unpinning transition for fields smaller than the threshold field.     

Metamagnetic phase transitions induced by static and pulsed fields in (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics

It has been found that the magnetic susceptibility of (Sm_0.5Gd_0.5)_0.55Sr_0.45MnO₃ ceramic samples in zero external magnetic field exhibits a sharp peak near the temperature of 48.5 K with a small temperature hysteresis that does not depend on the frequency of measurements and is characteristic of the phase transition to an antiferromagnetic state with a long-range charge orbital ordering, which is accompanied by an increase in the magnetic susceptibility with a decrease in the temperature. The magnetization isotherms in static and pulsed magnetic fields at temperatures below 60 K demonstrate the occurrence of an irreversible metamagnetic transition to a homogeneous ferromagnetic state with a critical transition field independent of the measurement temperature, which, apparently, is associated with the destruction of the insulating state with a long-range charge ordering. In the temperature range 60 K ?? T ?? 150 K, the ceramic samples undergo a magnetic-field-induced reversible phase transition to the ferromagnetic state, which is similar to the metamagnetic transition in the low-temperature phase and is caused by the destruction of local charge/orbital correlations. With an increase in the temperature, the critical transition fields increase almost linearly and the field hysteresis disappears. Near the critical fields of magnetic phase transitions, small ultra-narrow magnetization steps have been revealed in pulsed fields with a high rate of change in the magnetic field of ??400 kOe/??s.     

二维钾、钠紫青铜单晶A0.9Mo6O17(A=K,Na)的热电势特性

在８０—２８０Ｋ范围内对准二维钾、钠紫青铜单晶的热电势与温度的关系进行了研究，发现：（１）对钾紫青铜，在１０５Ｋ附近热电势有符号变化，并且在高温正常金属相，热电势Ｓ具有较小的负值，可以用经验关系Ｓ＝ＡＴ＋Ｂ来描述，自由电子为支配载流子；而在Ｔ＜１０５Ｋ的低温ＣＤＷ态，Ｓ为正值，可表示为Ｓ＝Ａ′Ｔ＋Ｂ′／Ｔ的形式，载流子的声子曳引热电势占支配作用．（２）对钠紫青铜，在Ｔ＞８０Ｋ范围内Ｓ与Ｔ具有很好的线性关系，没有观察到Ｐｅｉｅｒｌｓ相变，电子为支配载流子．对造成两者差异的根源，也作了初步探讨 关键词：     

Structure and orientational ordering of nitrogen molecules physisorbed on graphite

The structure and orientational ordering of nitrogen molecules physisorbed on graphite have been studied by low-energy diffraction (LEED). A two-sublattice in-plane herringbone structure with glide lines along two perpendicular directions is inferred from LEED patterns at T < 30 K from the monolayer where the molecular centers have the commensurate $\text{(}\text{3}\text{×}\text{3}\text{)}$ 30° structure. The orientational order-disorder transition of this commensurate phase was examined by superlattice spot intensity and angular profile measurements for 20 < T < 38 K. A rapid drop in superlattice intensity is observed near 27 K. The persistence of some intensity to 38 K. is suggestive of residual short-range orientational ordering and perhaps finite size or heterogeneity effects. For increasing coverage at T = 15 K, there is first a transition to a previously unobserved uniaxial incommensurate phase and then a transition to an apparently triangular incommensurate phase. The orientational superlattice spots are clearly present in the uniaxial phase, but are much weaker in the triangular incommensurate phase. At 31 < T < 35 K, an apparently triangular incommensurate phase with no detectable orientational superlattice spots is observed. The lattice constant versus equilibrium vapor pressure curve has been determined in the latter case assuming a continuous transition. The lattice constants of the incommensurate phases are used to place limits on the extent of possible phase-coexistence regions between the commensurate, uniaxial incommensurate, and triangular incommensurate phases. The LEED patterns from the bilayer at T = 15 K indicate a double-period superlattice structure of the triangular incommensurate phase which does not have the glide line symmetries of the commensurate monolayer. Some effects of heterogeneity on these phase transitions are discussed. A phase diagram for 10 < T < 40 K is proposed.     

Polar nanodomains and giant converse magnetoelectric effect in charge-ordered Fe2OBO3

The magnetoelectric coupling and polar nanodomains in the charge-ordered Fe2OBO3 have been extensively studied from room temperature down to 100?K. In?situ TEM investigations demonstrate that the charge-ordering transition characterized by an incommensurate modulation could evidently result in remarkable polar nanodomains at low temperatures. This kind of nanodomain could play a critical role in triggering a high dielectric constant and notable dielectric dispersion as observed in Fe2OBO3. Moreover, measurements of the magnetoelectric coupling under electrical field demonstrate the existence of giant electrically induced changes in magnetization around the magnetic transition.     

Phase transitions and metastable states in TlGaSe2

The results of measurements of the dielectric constant of TlGaSe₂ in temperature range of successive phase transitions are presented. An anomaly in the temperature dependence of the real part of dielectric constant in TlGaSe₂ has been observed at about 242?K in addition to anomalies at 115, 108, and also near 65?K as reported in previous publications. The presence of temperature hysteresis effects in temperature interval between 115 and 242?K allowed making a conclusion about possible existence of an incommensurate phase in the mentioned temperature range. A model of succession of the structural phase transitions in TlGaSe₂ has been suggested.     

Field-induced paramagnons at the metamagnetic transition of Ca1.8Sr0.2RuO4

The magnetic excitations in Ca1.8Sr0.2RuO4 were studied across the metamagnetic transition and as a function of temperature using inelastic neutron scattering. At low temperature and low magnetic field the magnetic response is dominated by a complex superposition of incommensurate antiferromagnetic fluctuations. Upon increasing the magnetic field across the metamagnetic transition, paramagnon and finally well-defined magnon scattering is induced, partially suppressing the incommensurate signals. The high-field phase in Ca1.8Sr0.2RuO4, therefore, has to be considered as an intrinsically ferromagnetic state stabilized by the magnetic field.     

Quasiperiodic noise voltage and metastability in the charge density wave conductivity of K0.30MoO3

Slow relaxation phenomena as well as quasiperiodic noise have been studied in the non linear regime of conductivity which takes place above a well defined threshold electric field in the semiconducting incommensurate phase of the quasi one-dimensional blue bronze K_0.30MoO₃. The noise frequencies are found to be proportional to the excess current attributed to the charge density wave (CDW). In some temperature range, the CDW current is found to decrease logarithmically vs time. These results indicate the presence of metastable states related to domains and domain walls.     

Comparative study of ESR spectra in incommensurate antiferromagnets

The electron spin resonance is studied for noncollinear low-dimensional antiferromagnets RbMnBr₃ and RbFe(MoO₄)₂ in a wide range of frequencies and fields. Both compounds have incommensurate spin structures appearing due to a low-symmetry distortion of an ideal hexagonal crystal lattice. Magnetic field applied in the spin plane induces a first-order transition into the commensurate phase. The low-energy resonance branch corresponding to a uniform oscillation of the spin system in the easy plane is observed in the two compounds in both incommensurate and commensurate phases, with a dramatic change of the spectra taking place near the transition field. The resonance spectrum of a nearly commensurate spin structure with long-wave modulations is analyzed in clean and dirty limits in the framework of a hydrodynamic approach. The resonance branch with steep field dependence in the incommensurate state is attributed to the acoustic mode with the gap resulted from pinning of local domain walls (discommensurations) on defects of the crystal structure.     

Mössbauer studies of spatial spin-modulated structure and hyperfine interactions in multiferroic Bi<Superscript>57</Superscript>Fe<Subscript>0.10</Subscript>Fe<Subscript>0.85</Subscript>Cr<Subscript>0.05</Subscript>O<Subscript>3</Subscript>

Results of Mössbauer investigations on ⁵⁷Fe nuclei in multiferroic material Bi⁵⁷Fe_0.10Fe_0.85Cr_0.05O₃ in the temperature range from 5.2 to 300 K are presented. Bulk rhombohedral samples were obtained by solidstate synthesis at high pressure. Mössbauer spectra were analyzed using the model of spatial incommensurate spin-modulated structure of the cycloidal type. Information on the influence of substituting Cr cations for Fe cations on hyperfine spectral parameters was obtained: the shift and quadrupolar shift of a Mössbauer line, and isotropic and anisotropic contributions into the hyperfine magnetic field. The anharmonicity parameter m of the spatial spin-modulated structure increases almost 1.7 times at 5.2 K when BiFeO₃ is doped with chromium. The data on m were used for calculation of the uniaxial magnetic anisotropy constants and their temperature dependences for pure and chromium-doped BiFeO₃.     

Metal-insulator transition investigation in V2O3 by nuclear magnetic resonance and relaxation

The high temperature metal-insulator transition in pure V₂O₃ has been investigated by nuclear magnetic resonance and relaxation. The relaxation rate in the range 160–320K was found to satisfy the Korringa relaxation and at temperature above 550K to be constant as expected for a paramagnetic insulator. In the intermediate temperature range the high field resonance line shape showed broadening which we interpret as due to the coexistence of a metallic and an insulating phase.