Non-Adiabatic Geometric Phase in a Dispersive Interaction System

We investigate the geometric phase and dynamic phase of a two-level fermionic system with dispersive interaction, driven by a quantized bosonic field which is simultaneously subjected to parametric amplification. It is found that the geometric phase is induced by a counterpart of the Stark shift. This effect is due to distinct shifts in the field frequency induced by interaction between different states （｜e〉 and ｜g〉 ） and cavity field, and a simple geometric interpretation of this phenomenon is given, which is helpful to understand the natural origin of the geometric phase.     

Study of doping effect,phase separation and heterojunction in CMR manganites

Mixed-valence manganites have attracted considerable research focus in recent years not only because of the potential application of colossal magnetoresistance(CMR) in magnetic devices,but also because of many intriguing physical properties observed in these materials.Doping elements at A-site can alter the filling of 3d Mn band and the tolerance factor.Therefore the hole-and electron-doped CMR manganites exhibit a rich phase diagram.In addition,more theoretical and experimental results suggest that phase separation is a critical factor for the understanding of CMR phenomena.Recently,there is an increasing interest in the fabrication and investigation on manganite-based heterojunction,which demonstrated excellent rectifying property,large MR,and photovoltaic effect.     

Colossal magnetoresistance in manganites and related prototype devices＊

We review colossal magnetoresistance in single phase manganites, as related to the field sensitive spin-charge interactions and phase separation; the rectifying property and negative/positive magnetoresistance in manganite/Nb：SrTio3 p-n junctions in relation to the special interface electronic structure; magnetoelectric coupling in manganite/ferroelectric structures that takes advantage of strain, carrier density, and magnetic field sensitivity; tunneling magnetoresistance in tunnel junctions with dielectric, ferroelectric, and organic semiconductor spacers using the fully spin polarized nature of manganites; and the effect of particle size on magnetic properties in manganite nanoparticles.     

Pressure-Induced Phase Transition of Ruthenium Diboride

Based on the first-principles calculations, we firstly predict that RuB2 undergoes a phase transition from the orthorhombic phase to the hexagonal phase with a volume collapse of 1% when the applied pressure is 15. 7 GPa. The values of calculated elastic moduli indicate that RuB2 and RuN2 are low compressibility materials. Based on the calculated electronic density of states and valence charge density distribution, the bonding nature of RuB2 is examined to obtain a deeper insight into the physical origin of the mechanical properties. The metallieity and high elastic moduli of RuB2 and FuN2 suggest that they axe potential hard conductors.     

Spatial confinement tuning of quenched disorder effects and enhanced magnetoresistance in manganite nanowires

Complex oxides have rich functionalities and advantages for future technologies.In many systems,quenched disorder often holds the key to determine their physical properties,and these properties can be further tuned by chemical doping.However,understanding the role of quenched disorder is complicated because chemical doping simultaneously alters other physical variables such as local lattice distortions and electronic and magnetic environments.Here,we show that spatial confinement is an effective approach to tuning the level of quenched disorder in a complex-oxide system while leaving other physical variables largely undisturbed.Through the confinement of a manganite system down to quasi-one-dimensional nanowires,we observed that the nature of its metal-insulator phase transition exhibits a crossover from a discontinuous to a continuous characteristic,in close accordance with quenched disorder theories.We argue that the quenched disorder,finite size,and surface effects all contribute to our experimental observations.Noticeably,with reduced nanowire width,the magnetoresistance shows substantial enhancement at low temperatures.Our findings offer new insight into experimentally tuning the quenched disorder effect to achieve novel functionalities at reduced dimensions.     

First Principles Study on Electronic Structure of PbFe0.5Nb0.5O3

The full potential linearized augmented plane wave (FLAPW) method is used to study the crystal structure and electronic structure properties of PbFeo.5 Nbo.5O3 (PFN). The optimized crystal structure, density of states, band structure and electron density distribution have been obtained to understand the ferroelectric behaviour of PFN.The analysis result of the density of states shows there is an obvious change of Nbd states in the paraelectric-to-ferroelectric phase transition. The polarization result shows that the contribution to ferroelectricity of Nb atoms is larger than that of Fe atoms. In ferroelectric phase there is a hybridization of Fed-Op and Nbd-Opin ferroelectric PFN. This is consistent with the result of the electronic band structure. This hybridization is responsible for the tendency to its ferroelectricity.     

Electronic and magnetic properties of single-layer and double-layer VX_2(X=Cl,Br) under biaxial stress

First-principles calculations and Monte Carlo simulations reveal that single-layer and double-layer VX_2(X = Cl, Br)can be tuned from antiferromagnetic(AFM) semiconductors to ferromagnetic(FM) state when biaxial tensile stress is applied. Their ground states are all T phase. The biaxial tensile stress at the phase transition point of the double-layer VX_2 is larger than that of the single-layer VX_2. The direct band gaps can be also manipulated by biaxial tensile stress as they increases with increasing tensile stress to a critical point and then decreases. The Néel temperature(TN) of double-layer VX_2 are higher than that of single-layer. As the stress increases, the TNof all materials tend to increase. The magnetic moment increases with the increase of biaxial tensile stress, and which become insensitive to stress after the phase transition points.Our research provides a method to control the electronic and magnetic properties of VX_2 by stress, and the single-layer and double-layer VX_2 may have potential applications in nano spintronic devices.     

Function of large-volume high-pressure apparatus at SECUF

Pressure allows the precise tuning of a fundamental parameter, the interatomic distance, which controls the electronic structure and virtually all interatomic interactions that determine material properties. Hence, pressure tuning is an effective tool in the search for new materials with enhanced properties. To realize pressure tuning on matter, large-volume press(LVP) apparatuses have been widely used not only to synthesize novel materials but also to implement the in situ measurement of physical properties. Herein, we introduce the LVP apparatuses, including belt-type, cubic anvil, and 6–8 type multi-anvil, that will be constructed at the Synergetic Extreme Condition User Facility(SECUF) at Jilin University.Typically, cell volumes of 1000 mm~3 can be obtained at 20 GPa in a belt-type apparatus that is significantly larger than that obtained in a 6–8 type multi-anvil apparatus at the same pressure. Furthermore, the in situ measurement of physical properties, including thermological, electrical, and mechanical behaviors, is coupled to these LVP apparatuses. Some typical results of both synthetic experiments and in situ measurements obtained from the LVP apparatuses are also reviewed.     

Graphene's photonic and optoelectronic properties——A review

Due to its remarkable electrical and optical properties,graphene continues to receive more and more attention from researchers around the world.An excellent advantage of graphene is the possibility of controlling its charge density,and consequently,the management of its conductivity and dielectric constant,among other parameters.It is noteworthy that the control of these properties enables the obtaining of new optical/electronic devices,which would not exist based on conventional materials.However,to work in this area of science,it is necessary to have a thorough knowledge regarding the electrical/optical properties of graphene.In this review paper,we show these graphene properties very well detailed.     

Critical fluctuations upon photoinduced phase transition in manganite strips

Light,acting as an external stimulus to induce various intriguing phenomena ranging from photovoltaics to photoinduced catalysis,exerts prominent effects in strongly correlated systems.It would be of particular interest to investigate photon-induced emerging phenomena in spatially confined strongly correlated systems,which are important for applications of these materials in future electronic devices.Colossal magnetoresistive manganites materials offer an ideal platform for such study due to their sensitivity to photo-excitation.Here,we fabricated 900 nm wide La_(0.325)Pr_(0.3)Ca_(0.375)Mn O_3strips,whose width is comparable to the size of the electronic phase separation(EPS)domains in this system.We observed the photoinduced critical fluctuations in the strips,where abrupt resistivity jumps occurred upon photoinduced phase transition depending sensitively on the light intensity.Based on the microscopic views of the EPS domains under photoexcitation,we conclude that such photo-induced resistivity fluctuations originate from the photoinduced phase fluctuations of individual EPS domains when their size becomes comparable to the strip width.     

Instability of magnetism and conductivity in CMR manganites: role of Mn-site doping and thermal cycling

The magnetic and transport properties of the manganites with the perovskite structure are mainly characterized by a competition between ferromagnetism and antiferromagnetism, and between a metallic like and an insulating behavior. Charge and orbital ordering, and phase separation play a prominent role in the appearance of such properties, since they can be modified in a spectacular manner by external factors, making the different physical properties metastable. There we describe two effects that deeply modify those properties, the doping of Mn sites and the thermal cycling under a magnetic field.The doping of Mn sites by various magnetic cations—Cr, Co, Ni, Ru, Rh, Ir—destroys charge/orbital-ordering and induces ferromagnetism and metallicity in the antiferromagnetic matrix of the manganites. The magnetic phase diagram of the systems Ln_1−xCa_xMnO₃ is considerably modified by such doping. The metastability of the magnetic states is explained in terms of models based on the electronic structure of the doping elements in connection with a possible valence fluctuation.The thermal cycling is also a spectacular effect, observed in chromium doped manganites. For instance, an increase of resistivity by several orders of magnitude can be observed by thermal cycling under a magnetic field, whereas no effect is obtained in the corresponding undoped material. This behavior is interpreted in terms of strains induced charge localization, at the interface between ferromagnetic/antiferromagnetic domains in the antiferromagnetic matrix.     

复杂氧化物中电子相分离的量子调控

复杂氧化物可以呈现出高温超导、庞磁阻以及多铁效应等诸多新奇的物理现象.这类材料中的电荷/自旋/轨道和晶格自由度之间的强耦合相互作用,可以导致多种相互竞争且能量非常接近的电子态的空间共存,这就是电子相分离现象.如果可以将材料的空间尺寸缩小到电子相分离的特征长度,其物理性质甚至电子关联作用本身都会发生根本的变化,从而有可能实现复杂氧化物中的量子调控.本文综述了我们课题组在过去几年中针对复杂氧化物中电子相分离的量子调控取得的进展,内容包括：发现了锰氧化物边缘电子态,通过氧化物微纳加工技术,实现了量子态空间分布的调控,提高了庞磁阻锰氧化物的临界温度;研究了当材料空间尺度小于其电子相分离特征尺度时电子相分离的表现,确定了在电子相分离消失以后体系的磁结构;通过超晶格生长技术调控了材料中的掺杂有序度,对锰氧化物中大尺度的电子相分离的物理机理从实验上给出了解释.     

稀土掺杂锰氧化物庞磁电阻效应

过去十多年来，具有庞磁电阻效应的稀土掺杂锰氧化物成为了凝聚态物理研究的重要领域。锰氧化物的载流子自旋极化率高，且在居里温度附近表现出很大的磁电阻效应，因此在自旋电子学中有潜在的应用前景。另一方面，锰氧化物是典型的强关联电子体系，它对目前有关强关联体系的认识提出了很大挑战。本文综述了锰氧化物的各种性质及其物理原因。全文首先概述了锰氧化物的庞磁电阻效应及其晶格和电子结构，简单介绍了其他一些庞磁电阻材料；随后综述了锰氧化物的电荷／轨道有序相及其输运性质；在第四部分简单介绍了锰氧化物中庞磁电阻效应的机制；最后讨论了锰氧化物的一些可能的应用，如低场磁电阻效应、磁隧道结、磁p-n结以及全钙钛矿的场效应管和自旋极化电子注入装置等。     

稀土掺杂锰氧化物庞磁电阻效应

过去十多年来,具有庞磁电阻效应的稀土掺杂锰氧化物成为了凝聚态物理研究的重要领域。锰氧化物的载流子自旋极化率高,且在居里温度附近表现出很大的磁电阻效应,因此在自旋电子学中有潜在的应用前景。另一方面,锰氧化物是典型的强关联电子体系,它对目前有关强关联体系的认识提出了很大挑战。本文综述了锰氧化物的各种性质及其物理原因。全文首先概述了锰氧化物的庞磁电阻效应及其晶格和电子结构,简单介绍了其他一些庞磁电阻材料;随后综述了锰氧化物的电荷/轨道有序相及其输运性质;在第四部分简单介绍了锰氧化物中庞磁电阻效应的机制;最后讨论了锰氧化物的一些可能的应用,如低场磁电阻效应、磁隧道结、磁p＿n结以及全钙钛矿的场效应管和自旋极化电子注入装置等。     

Mixed-valence manganites

Mixed-valence manganese oxides (R₁-_χA_χ)MnO₃ (R=rare-earth cation, A=alkali or alkaline earth cation), with a structure similar to that of perovskite CaTiO₃, exhibit a rich variety of crystallographic, electronic and magnetic phases. Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin films has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. This review gives an account of the literature on mixed-valence manganites, placing new results in the context of established knowledge of these materials, and other magnetic semiconductors. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, the role of cation disorder and the Jahn-Teller effect. Sample preparation, and the properties of related ferromagnetic oxides are also discussed.     

锰氧化物相竞争的理论研究

锰氧化物属于典型的强关联电子材料,具有包括庞磁电阻、电荷/轨道有序、电子相分离、多铁性等奇特的物理特性。这些现象涉及一系列凝聚态物理学基本问题,是近年来研究者一直关注的热点和难点。并且这些奇异的电磁性质也为开发量子调控器件提供了基本素材。虽然近20年来对锰氧化物的研究取得了丰硕成果,全世界的研究者仍在为理解并应用其特性作着孜孜不倦的努力。本综述将主要从理论角度,重点关注钙钛矿结构锰氧化物中多种相竞争和调制。由于有着多种竞争相互作用和多重量子自由度,锰氧化物有着丰富的相,这些相物理特性迥异,而自由能却可能相当接近。因此,自发或人为调制导致的相竞争是锰氧化物研究的一个核心问题,也是整个强关联物理领域中一个很有意义的课题。本综述将以电致电阻、多铁性和异质结界面处电子重组这三个具体实例,介绍如何采用蒙特卡罗模拟等方法研究其中的相竞争和调制。     

Effect of Jahn-Teller distortions on the structural and magnetic ordering in perovskite-type transition-metal oxides

The effect of local and cooperative distortions of oxygen octahedra containing Jahn-Teller ions on the elastic, electric, magnetic, and optical properties of lightly doped lanthanum strontium manganites and irondoped lithium niobate ferroelectric has been experimentally investigated. The formation of nano-and microscale clusters and domains around, respectively, Mn³⁺ and Fe²⁺ Jahn-Teller ions at phase transitions and in magnetic or electric fields were studied. A model is proposed to consistently explain the results of the investigations of Jahn-Teller ions in magnetic and/or electrically ordered materials.     

Nano-optical imaging and spectroscopy of order,phases, and domains in complex solids

The structure of our material world is characterized by a large hierarchy of length scales that determines material properties and functions. Increasing spatial resolution in optical imaging and spectroscopy has been a long standing desire, to provide access, in particular, to mesoscopic phenomena associated with phase separation, order, and intrinsic and extrinsic structural inhomogeneities. A general concept for the combination of optical spectroscopy with scanning probe microscopy emerged recently, extending the spatial resolution of optical imaging far beyond the diffraction limit. The optical antenna properties of a scanning probe tip and the local near-field coupling between its apex and a sample provide few-nanometer optical spatial resolution. With imaging mechanisms largely independent of wavelength, this concept is compatible with essentially any form of optical spectroscopy, including nonlinear and ultrafast techniques, over a wide frequency range from the terahertz to the extreme ultraviolet. The past 10 years have seen a rapid development of this nano-optical imaging technique, known as tip-enhanced or scattering-scanning near-field optical microscopy (s-SNOM). Its applicability has been demonstrated for the nano-scale investigation of a wide range of materials including biomolecular, polymer, plasmonic, semiconductor, and dielectric systems.

We provide a general review of the development, fundamental imaging mechanisms, and different implementations of s-SNOM, and discuss its potential for providing nanoscale spectroscopic including femtosecond spatio-temporal information. We discuss possible near-field spectroscopic implementations, with contrast based on the metallic infrared Drude response, nano-scale impedance, infrared and Raman vibrational spectroscopy, phonon Raman nano-crystallography, and nonlinear optics to identify nanoscale phase separation (PS), strain, and ferroic order. With regard to applications, we focus on correlated and low-dimensional materials as examples that benefit, in particular, from the unique applicability of s-SNOM under variable and cryogenic temperatures, nearly arbitrary atmospheric conditions, controlled sample strain, and large electric and magnetic fields and currents. For example, in transition metal oxides, topological insulators, and graphene, unusual electronic, optical, magnetic, or mechanical properties emerge, such as colossal magneto-resistance (CMR), metal–insulator transitions (MITs), high-T _C superconductivity, multiferroicity, and plasmon and phonon polaritons, with associated rich phase diagrams that are typically very sensitive to the above conditions. The interaction of charge, spin, orbital, and lattice degrees of freedom in correlated electron materials leads to frustration and degenerate ground states, with spatial PS over many orders of length scale. We discuss how the optical near-field response in s-SNOM allows for the systematic real space probing of multiple order parameters simultaneously under a wide range of internal and external stimuli (strain, magnetic field, photo-doping, etc.) by coupling directly to electronic, spin, phonon, optical, and polariton resonances in materials. In conclusion, we provide a perspective on the future extension of s-SNOM for multi-modal imaging with simultaneous nanometer spatial and femtosecond temporal resolution.