二维３态Potts量子系统的保真度与序参量

二维无限正方格子上的量子3态Potts模型是发生一级相变还是二级相变?通过运用无限纠缠投影对态 (iPEPS) 算法,在进行数值模拟时任意选取初态,能得到二维无限正方格子上的3态Potts模型的三个不同的简并基态波函数,这些简并的情况是由自发对称性破缺引起的.首先,揭示了在二维系统中自发对称性破缺引起的相变可以运用单点基态保真度的分叉来研究,也反映了在二维系统中约化保真度同样有一个分叉行为;再者,还提出了二维系统的普适序参量以及多分量的复数局域序参量的行为来尝试研究二维3态Potts模型,共同确定系统发生的量子相变的临界点及其类型.即基于iPEPS算法,从单点基态保真度、约化保真度、普适序参量以及局域序参量的角度,来研究3态Potts模型的量子相变,其为一级相变.     

二维3态Potts量子系统的保真度与序参量

二维无限正方格子上的量子3态Potts模型是发生一级相变还是二级相变?通过运用无限纠缠投影对态(i PEPS)算法,在进行数值模拟时任意选取初态,能得到二维无限正方格子上的3态Potts模型的三个不同的简并基态波函数,这些简并的情况是由自发对称性破缺引起的.首先,揭示了在二维系统中自发对称性破缺引起的相变可以运用单点基态保真度的分叉来研究,也反映了在二维系统中约化保真度同样有一个分叉行为;再者,还提出了二维系统的普适序参量以及多分量的复数局域序参量的行为来尝试研究二维3态Potts模型,共同确定系统发生的量子相变的临界点及其类型.即基于i PEPS算法,从单点基态保真度、约化保真度、普适序参量以及局域序参量的角度,来研究3态Potts模型的量子相变,其为一级相变.     

用晶格气体模型模拟表面吸附状态过程中序参量的计算

主要讨论了在用晶格气体模型模拟气体分子在固体表面吸附状况时所涉及到的一个重要物理参量———序参量的定义以及其表达式的推导和论证。首先 ,运用简单的逻辑推理和代数运算对表面吸附状态的物理描述进行代数化 ,也即是用具体的代数式值来表达整个表面分子的吸附状态 ;然后再将这些代数式进行组合 ,构成一个可以表达不同表面吸附状态的函数 ,也即是将讨论的序参量 ;最后 ,将所得的理论公式与实际应用相结合便得到了可以直接引用的序参量表达式。根据需要 ,在这里主要讨论了P(2× 2 ) ,P(2× 1)和P(3× 3)三种有序结构的序参量     

Orientational order parameters of nematogenic trans-4-propyl cyclohexyl-4 (trans-4-alkyl cyclohexyl) benzoates

The refractive indices and densities of three members of trans-4-propyl cyclohexyl-4 (trans-4-alkyl cyclohexyl) benzoates are reported as functions of temperature in the nematic and isotropic phases. The principal molecular polarizabilities and order parameters (S) have been evaluated using the anisotropic internal field model (Neugebauer’s approach). Also order parameters have been estimated by studying the dipole-dipole splittings in the wide line proton magnetic resonance (PMR) spectra. The results obtained are in close agreement with those obtained from refractive index measurements.     

Mesomorphic properties and orientational order parameter of 4-pentyl cyclohexyl-4-(4-propyl cyclohexyl) benzoate from X-ray diffraction studies

The mesogen 4-pentyI cyclohexyl-4-(4-propyl cyclohexyl) benzoate, molecular formula C₂₇H₄₂O₂, has been investigated at different temperatures using X-ray and texture studies. For X-ray studies an arrangement for taking X-ray photographs at different temperatures has been designed and fabricated. In addition to the nematic phase the mesogen exhibits two smectic phases. On the basis of microscopic investigation and X-ray studies the two smectic phases exhibited have been identified as the SmE and SmB phases. From X-ray patterns, the average intermolecular distances and apparent molecular length/layer thickness for the different mesomorphic phases have been determined as a function of temperature. Orientational order parameters (P2) and (P4) of the sample ordered by a magnetic field have been determined in the SmB phase.     

Pt插层对铁磁/反铁磁界面交换耦合的影响

在铁磁层(FM)/反铁磁层(FeMn)耦合体系中插入Pt 插层或对靠近FM/FeMn界面处的FeMn掺杂Pt元素,研究了体系的交换偏置场 H_ex及矫顽力H_c随Pt插层深度 d_Pt与Pt掺杂层厚度t_PtFeMn的变化关系. 实验结果表明,引入Pt插层后NiFe/FeMn(d_Pt)/Pt/FeMn体系的未补偿磁矩(UCS)的数量得到很大的提高,从而对H_ex与H_c 起到增强的作用; 同时, 从实验结果可以推测FeMn层内部UCS的分布深度约为1.3 nm. 另外,对靠近FM/FeMn界面处的FeMn掺杂Pt元素,发现掺入Pt元素后体系的H_ex 得到有效增强, 这是因为掺入Pt元素后体系UCS的数量也得到很大的提高.     

Analysis of the order parameter for uniform nearest particle system

The uniform nearest particle system (UNPS) is studied, which is a continuoustime Markov process with state space . The rigorous upper bound ^(mf) = ( – 1)/ for the order parameter ₂, is given by the correlation identity and the FKG inequality. Then an improvement of this bound ^(mf) is shown in a similar fashion; C( – 1)/|log( – 1) for >1. Recently, Mountford proved that the critical value _c=1. Combining his result and our improved bound implies that if the critical exponent exists, it is strictly greater than the mean-field value 1 in the weak sense.     

Forced ferromagnetic ordering in systems with magnetic ions of two types

The distribution of magnetization in domain walls of ferromagnetic (FM) crystals with two order parameters interacting with each other—F_1z and F_2z, is considered. In one case the interaction is represented as (F_1zF_2z) and in another—(F_1z²F_2z²). It is shown that in the former case the magnetization distribution has space amplitude modulation, and in latter case appears space frequency modulation. Changes of the paramagnetic-ferromagnetic phase transition temperature in view of interaction of two FM subsystems have been studied.     

Numerical study of exchange bias in antiferromagnetic/ferromagnetic core/shell nanoparticles with non-magnetic defects

We perform Monte Carlo simulations for an antiferromagnetic/ferromagnetic core/shell nanoparticle with a doubly inverted structure. We investigate the dependence of the exchange bias field and coercivity on the magnetic dilution of the shell-interface and shell part. It is demonstrated that exchange bias and coercivity can exhibit monotonic or non-monotonic behavior depending on the location of the non-magnetic components. Also, temperature dependence of the exchange bias and coercivity of the system are studied for a particular defect concentration value. Our results provide an alternative way for tunning the magnetic properties of doubly inverted nanoparticles.     

Absence of self-averaging of the order parameter in the Sherrington-Kirkpatrick model

We prove that if _N is the Sherrington-Kirkpatrick (SK) Hamiltonian and the quantity converges in the variance to a nonrandom limit asN, then the mean free energy of the model converges to the expression obtained by SK. Since this expression is known not to be correct in the low-temperature region, our result implies the non-self-averaging of the order parameter of the SK model. This fact is an important ingredient of the Parisi theory, which is widely believed to be exact. We also prove that the variance of the free energy of the SK model converges to zero asN, i.e., the free energy has the self-averaging property.See the Remarks after the proof of Theorem 1 on the validity of our results for more general distributions ofJ _ij .     

Transport studies of Kondo behavior,ferromagnetic and antiferromagnetic order in U1−xThxSb

We report electrical resistivity, Hall effect and magnetization measurements in the system U_1?xTh_xSb for magnetic fields up to 100 kOe. In U_0.14Th_0.86Sb a Kondo-like behavior of the resistivity is detected and the interaction J_df between the conduction d electrons and the uranium f electrons is found to be about ?0.2 eV. The dilution of USb by ThSb leads to large modifications of the electrical transport properties, reflecting the change from antiferromagnetism to ferromagnetism and simultaneously a decrease of the ordered magnetic moment per uranium atom occurs. A simple model is presented which accounts for this decrease assuming that all the conduction electrons added by thorium are polarized antiparallel to the remaining uranium f electrons due to the negative J_df. The Kondo temperature is used to estimate the band width and the binding energy of the 5f state.     

Aggregation of ferromagnetic and paramagnetic atoms at edges of graphenes and graphite

In this work we report that when ferromagnetic metals (Fe,Co and Ni) are thermally evaporated onto n-layer graphenes and graphite,a metal nanowire and adjacent nanogaps can be found along the edges regardless of its zigzag or armchair structure.Similar features can also be observed for paramagnetic metals,such as Mn,Al and Pd.Meanwhile,metal nanowires and adjacent nanogaps cannot be found for diamagnetic metals (Au and Ag).An external magnetic field during the evaporation of metals can make these unique features disappear for ferromagnetic and paramagnetic metal;and the morphologies of diamagnetic metal do not change after the application of an external magnetic field.We discuss the possible reasons for these novel and interesting results,which include possible one-dimensional ferromagnets along the edge and edge-related binding energy.     

Carrier-induced ferromagnetic order in the narrow gap III–V magnetic alloy semiconductor (In,Mn)Sb

Preparation and physical properties of p- and n-InMnSb epitaxial films with Mn contents up to 10% were studied with the aim of seeking phenomena induced by the spin exchange interaction between carrier and Mn spins. For p-type samples with Mn_eff=4.5×10²⁰ and p=1.1×10²⁰ cm⁻³, carrier-induced ferromagnetic order with a Curie temperature of 20 K was observed. The sign of the anomalous Hall coefficient is found to be negative. Tellurium-doped n-type samples (n=8.6×10¹⁸ cm⁻³) with net Mn contents of 10% are found to be paramagnetic.     

Adaptive lag synchronization and parameter identification of fractional order chaotic systems

This paper proposes a simple scheme for the lag synchronization and the parameter identification of fractional order chaotic systems based on the new stability theory. The lag synchronization is achieved and the unknown parameters are identified by using the adaptive lag laws. Moreover, the scheme is analytical and is simple to implement in practice. The well-known fractional order chaotic Lü system is used to illustrate the validity of this theoretic method.     

Stabilization of spiral wave and turbulence in the excitable media using parameter perturbation scheme

This paper proposes a scheme of parameter perturbation to suppress the stable rotating spiral wave, meandering spiral wave and turbulence in the excitable media, which is described by the modified Fitzhug-Nagumo （MFHN） model. The controllable parameter in the MFHN model is perturbed with a weak pulse and the pulse period is decided by the rotating period of the spiral wave approximatively. It is confirmed that the spiral wave and spiral turbulence can be suppressed greatly. Drift and instability of spiral wave can be observed in the numerical simulation tests before the whole media become homogeneous finally.     

铁基超导体能隙结构和机理的统一认识

铁基超导体作为除铜氧化物超导体之外的第二类高温超导体,自从被发现以来其超导电性的成因就备受关注。早期有人针对同时具有空穴型费米口袋和电子型费米口袋的铁砷基超导体提出了S^±配对模式,而且得到中子散射和隧道谱实验的初步支持。这一配对模型的基础是需要在布里渊区同时具有空穴型和电子型费米面。但是铁基超导体的费米面因材料而异,新发现的很多铁硒基超导体因缺乏空穴型费米面,对S^±电子配对模型来说是一个挑战。文章综述了过去几年来作者在这方面取得的工作进展。运用高精度的扫描隧道显微镜,在不同铁基超导体系中,针对超导能隙结构,或超导序参量的性质进行了仔细研究。首先作者在铁砷基超导体NaFe_1-xCo_xAs和Ba_1-xK_xFe ₂As₂中利用无磁性杂质态测量和隧道谱测量,发现S^±的明确证据。进一步,在只有电子型费米面的(Li_1−xFe_x)OHFeSe超导材料中发现两个各向异性的超导能隙,利用准粒子相干散射实验第一次把这两个超导能隙对应到由电子型费米面套叠或杂化后形成的内外两套费米面上。通过非磁性杂质诱导产生的能隙内杂质态和新型电子驻波相位敏感实验的探测,证明该材料中超导能隙符号也发生反转。因此作者的系列工作统一了有和没有空穴型费米面的铁基超导体的能隙形式,支持排斥势是导致电子配对和超导电性产生的关键因素。     

二维３态Potts量子系统的保真度与序参量

二维无限正方格子上的量子3态Potts模型是发生一级相变还是二级相变?通过运用无限纠缠投影对态算法(iPEPS),在进行数值模拟时任意选取初态,能得到二维无限正方格子上的3态Potts模型的三个不同的简并基态波函数,这些简并的情况是由自发对称性破缺引起的.首先,揭示了在二维系统中自发对称性破缺引起的相变可以运用单点基态保真度的分叉来研究,也反映了在二维系统中约化保真度同样有一个分叉行为;再者,还开创性提出了二维系统的普适序参量以及多分量的复数局域序参量的行为来尝试研究二维3态Potts模型,共同确定系统发生的量子相变的临界点及其类型.即基于iPEPS算法,从单点基态保真度、约化保真度、普适序参量以及局域序参量的角度,来研究3态Potts模型的量子相变,其为一级相变.     

Temperature dependence of ratio between dielectric anisotropy and order parameter in fluorinated nematic liquid crystals

Temperature dependence of ratio between dielectric anisotropy and order parameter of fluorinated nematic liquid crystal is investigated by using a semi-empirical molecular orbital package that can accurately calculate an angle between molecular dipole moment and long axis. We optimize the molecular conformations with three semi-empirical Hamiltonians AM1, PM3 and PM5, and then make a comparison between computational results and experimental measurements. It is shown that the results obtained from AM1 method are in good agreement with the measurements. The present study offers an applicable method to predict the dielectric properties of liquid crystal material.     

Temperature dependence of ratio between dielectric anisotropy and order parameter in fluorinated nematic liquid crystals

Temperature dependence of ratio between dielectric anisotropy and order parameter of fluorinated nematic liquid crystal is investigated by using a semi-empirical molecular orbital package that can accurately calculate an angle between molecular dipole moment and long axis. We optimize the molecular conformations with three semi-empirical Hamiltonians AM1, PM3 and PM5, and then make a comparison between computational results and experimental measurements. It is shown that the results obtained from AM1 method are in good agreement with the measurements. The present study offers an applicable method to predict the dielectric properties of liquid crystal material.