The Origin of the Giant Hall Effect in Metal-Insulator Composites

Near the metal-insulator transition, the Hall coefficient R of metal-insulator composites (M-I composite) can be up to 104 times larger than that in the pure metal called Giant Hall effect. Applying the physical model for alloys with phase separation developed in [1] [2], we conclude that the Giant Hall effect is caused by an electron transfer away from the metallic phase to the insulating phase occupying surface states. These surface states are the reason for the granular structure typical for M-I composites. This electron transfer can be described by [1] [2], provided that long-range diffusion does not happen during film production (n is the electron density in the phase A. uA and uB are the volume fractions of the phase A (metallic phase) and phase B (insulator phase). β is a measure for the average potential difference between the phases A and B). A formula for calculation of R of composites is derived and applied to experimental data of granular Cu1-y(SiO2)y and Ni1-y(SiO2)y films.

Manipulating metal-insulator transitions of VO_(2) films via embedding Ag nanonet arrays

Manipulating metal-insulator transitions in strongly correlated materials is of great importance in condensed matter physics,with implications for both fundamental science and technology.Vanadium dioxide(VO_(2)),as an ideal model system,is metallic at high temperatures and shown a typical metal-insulator structural phase transition at 341 K from rutile structure to monoclinic structure.This behavior has been absorbed tons of attention for years.However,how to control this phase transition is still challenging and little studied.Here we demonstrated that to control the Ag nanonet arrays(NAs)in monoclinic VO_(2)(M)could be effective to adjust this metal-insulator transition.With the increase of Ag NAs volume fraction by reducing the template spheres size,the transition temperature(Tc)decreased from 68°C to 51°C.The mechanism of Tc decrease was revealed as:the carrier density increases through the increase of Ag NAs volume fraction,and more free electrons injected into the VO_(2)films induced greater absorption energy at the internal nanometal-semiconductor junction.These results supply a new strategy to control the metal-insulator transitions in VO_(2),which must be instructive for the other strongly correlated materials and important for applications.

Well Dispersed SnO2 Nanoclusters Preparation and Modulation of Metal-Insulator Transition Induced by Ionic Liquid

Tin dioxide (SnO2) has attracted broad interest due to its particular gas-sensor property. Nano- or atom-scale SnO2 material has always been the aim in order to ultimately improve the sensitivity. However, until now, it remains difficult to synthesize SnO2 nanoclusters by using traditional methods. In the present work, we have achieved the preparation of SnO2 nanoclusters by using the cluster beam deposition technique. The obtained nanoclusters were well characterized by high resolution transmission electron microscope HR-TEM. Results indicated the formation of the well-dispersed SnO2 nanoclusters with uniform size distribution (5-7 nm). Furthermore, an obvious metal insulator transition was observed by gating with ionic liquid. Combined with theory calculation, the corresponding mechanism was systematically analyzed from oxygen vacancy induced electron doping.

Monolithic integration of an AlGaN/GaN metal-insulator field-effect transistor with an ultra-low voltage-drop diode for self-protection

In this paper, we present a monolithic integration of a self-protected AlGaN/GaN metal-insulator field-effect transistor （MISFET）. An integrated field-controlled diode on the drain side of the AlGaN/GaN MISFET features a self- protected function for a reverse bias. This diode takes advantage of the recessed-barrier enhancement-mode technique to realize an ultra-low voltage drop and a low turn-ON voltage. In the smart monolithic integration, this integrated diode can block a reverse bias （〉 70 V/μm） and suppress the leakage current （〈 5 × 10-11 A/mm）. Compared with conventional monolithic integration, the numerical results show that the MISET integrated with a field-controlled diode leads to a good performance for smart power integration. And the power loss is lower than 50% in conduction without forward current degeneration.

Tunable metal-insulator transition in LaTiO3/CaVO3 superlattices:A theoretical study

As one of intriguing physical results of electronic reconstruction,the metal-insulator transition plays an important role in exploring new electronic devices.In this study,the density functional theory is employed to investigate the metal-insulator transition in(LaTiO3)m/(CaVO3)n superlattices.Herein,three kinds of physical avenues,i.e.,stacking orientation,epitaxial strain,and thickness periods,are used to tune the metal-insulator transition.Our calculations find that the[001]-and[110]-oriented(LaTiO3)1/(CaVO3)1 superlattices on SrTiO3 substrate are insulating,while[111]-oriented case is metallic.Such metallic behavior in[111]orientation can also be modulated by epitaxial strain.Besides the structural orientation and strain effect,the highly probable metal-insulator transition is presented in(LaTiO3)m/(CaVO3)n superlattices with increasing thickness.In addition,several interesting physical phenomena have also been revealed,such as selective charge transfer,charge ordering,and orbital ordering.

Metal-Insulator Transition of Peierls Type in Quasi-One-Dimensional Crystals of TTT₂I₃

We investigate the metal-insulator transition in quasi-one-dimensional organic crystals of tetrathiotetracene-iodide, TTT2I3, in the 2D model. A crystal physical model is applied which takes into account two the most important hole-phonon interaction mechanisms. One is similar to that of deformation potential and the other is of polaron type. The scattering on defects is also considered and it is crucial for the explanation of the transition. The phonon polarization operator and the renormalized phonon spectrum are calculated in the random phase approximation for different temperatures applying the method of Green functions. We show that the transition is of Peierls type. The effect of lattice distortion on the dispersion of renormalized acoustic phonons is analyzed.

Carrier-Induced Magnetic Solitons and Metal-Insulator Transition in Diluted Magnetic Semiconductors Ga_1-xMn_xAs

We discuss hole-induced magnetic solitons and metal-insulator transition of transport properties in diluted magnetic semiconductors Ga1-xMnxAs from the standpoint of a field theoretical formulation, and analyze experimental data of transport properties, using the supersymmetry sigma formula and the effective Lagrangian of diffusion model.

Metal-Insulator Transition and Superconductivity in Y_(1-x)Pr(Ce)_xBa_2Cu_3O_7

To interpret the metal-insulator transition and depression of Tc induced by Pr-and Ce-doping in YBa_2Cu_3O_7, we propose a model of mixed local hole states which describe a strong admixture of 4f^1 state with states of 4f^2 plus a hole in the CuO_2 planes for Y_(1-x)-Pr_x-0_7 and 4f^0 state with states of 4f^1 plus a hole in Y_(1-x)Ce_x-O_7. Our model resolves the controversy between the magnetic and spectroscopic measurements. As a natural consequence, most of the experimental results on Y_(1-x)Pr_x-O_7 can be explained and certain properties of Y_(1-x)Ce_x-O_7 are predicted. The critical doping density of Pr will take the value of x_c ≈0.5.

Phase Transition Behaviour of VO2 Nanorods

The metal-insulator transition （MIT） of VO2 （M） nanorods was studied. It was found that there were two MITs in the differential scanning calorimetry （DSC） curves of the VO2（M） nanorods, one situated at low temperature from -3 ℃ to 19 ℃ and the other was at high temperature of 65-74℃. The low temperature MIT was always accompanied with VO2（B） nanorods, and the high temperature MIT existed singly only in pure VO2（M） nanorods. The mechanisms of these two MITs were analyzed and discussed.

Hidden metal-insulator transition in manganites synthesized via a controllable oxidation

Oxygen usually plays crucial roles in tuning the phase structures and functionalities of complex oxides such as high temperature superconductivity, colossal magnetoresistance, catalysis, etc. Effective and considerable control of the oxygen content in those functional oxides could be highly desired. Here, using perovskite manganite(La0.5Sr0.5)MnO3 as a paradigm, we develop a new pathway to synthesize the epitaxial thin films assisted by an in-situ chemical process, where the oxygen content can be precisely controlled by varying oxidative activity tuned by the atmospheric temperature(Tatm)during the growth. A hidden metal-insulator transition(MIT)emerges due to the phase competition, which is never shown in the phase diagram of this classic manganite. The oxygenmediated interaction between Mn ions together with the change of carrier density might be responsible for this emerging phase, which is compatible with the results of firstprinciple calculations. This work demonstrates that, apart from traditional cation doping, a precise modulation of anion(O2-, S2-, etc.) may provide a new strategy to control phase structures and functionalities of epitaxial compound thin films.

Structure and Transport Behaviors of Nanograin La_(0.8)Sr_(0.2)Mn_(1-x)Al_xO_3

The influence of aluminum doping at Mn-site in nanograin compound La0.8Sr0.2MnO3 was investigated based on X-ray diffraction, scanning electron microscope and resistivity measurement, in the light of structure and transport properties. The results showed that Al doping was favorable to the globurizing of powders and grain size uniformity, however, depressed the particles growth. The resistivity of system increased rapidly and the metal-insulator transition temperature (TIM) and room temperature magnetoresistance decreased as the aluminum concentration increased. In the T>TIM region, the current carriers were moving in variable range transition mode. The resistivity of La0.8Sr0.2Mn1-xAlxO3 for x=0.05 and 0.1 satisfied metal model in the T<TIM region. The characteristics of the transport behavior for aluminum doping were analyzed in terms of destroying the double exchange channel of Mn3+-O-Mn4+, distortion of the cell lattice and change of powder particles size and shape.

Metal-organic decomposition growth of thin film metastable perovskite nickelates with kinetically improved quantum transitions

The multiple quantum transitions within d-band correlation oxides such as rare-earth nickelates(RENiO_(3))triggered by critical temperatures and/or hydrogenation opened up a new paradigm for correlated electronics applications,e.g.ocean electric field sensor,bio-sensor,and neuron synapse logical devices.Nevertheless,these applications are obstructed by the present ineffectiveness in the thin film growth of the metastable RENiO_(3)with flexibly adjustable rare-earth compositions and electronic structures.Herein,we demonstrate a metal-organic decompositions(MOD)approach that can effectively grow metastable RENiO_(3)covering a large variety of the rare-earth composition without introducing any vacuum process.Unlike the previous chemical growths for RENiO_(3)relying on strict interfacial coherency that limit the film thickness,the MOD growth using reactive isooctanoate percussors is tolerant to lattice defects and therefore achieves comparable film thickness to vacuum depositions.Further indicated by positron annihilation spectroscopy,the RENiO_(3)grown by MOD exhibit large amount of lattice defects that improves their hydrogen incorporation amount and electron transfers,as demonstrated by the resonant nuclear reaction analysis and near edge X-ray absorption fine structure analysis.This effectively enlarges the magnitude in the resistance regulations in particular for RENiO_(3)with lighter RE,shedding a light on the extrinsic regulation of the hydrogen induced quantum transitions for correlated oxides semiconductors kinetically via defect engineering.

Negative thermal expansion of Ca2RuO4 with oxygen vacancies

Oxygen vacancies have a profound effect on the magnetic,electronic,and transport properties of transition metal oxides but little is known about their effect on thermal expansion.Herein we report the effect of oxygen defects on the structure formation and thermal expansion properties of the layered perovskite Ca2RuO4(CRO).It is shown that the CRO containing excess oxygen crystallizes in a metallic L-CRO phase without structure transition from 100 K to 500 K and displays a normal thermal expansion behavior,whereas those with oxygen vacancies adopt at room temperature an insulating S-CRO phase and exhibit an enormous negative thermal expansion(NTE)from 100 K to about 360 K,from where they undergo a structure transition to a high temperature metallic L-CRO phase.Compared to the L-CRO containing excess oxygen,the S-CRO structure has increasingly large orthorhombic strain and distinctive in-plane distortion upon cooling.The in-plane distortion of the RuO6 octahedra reaches a maximum across 260 K and then relaxes monotonically,providing a structure evidence for the appearance of an antiferromagnetic orbital ordering in the paramagnetic phase and the A_g phonon mode suppression and phase flip across the same temperature found recently.Both the L-and S-CRO display an antiferromagnetic ordering at about 150-110 K,with ferromagnetic ordering components at lower temperature.The NTE in S-CRO is a result of a complex interplay among the spin,orbital,and lattice.

Emergent phenomena in manganites under spatial confinement

It is becoming increasingly clear that the exotic properties displayed by correlated electronic materials such as high- Tc superconductivity in cuprates, colossal magnetoresistance （CMR） in manganites, and heavy-fermion compounds are intimately related to the coexistence of competing nearly degenerate states which couple simultaneously active degrees of freedom---charge, lattice, orbital, and spin states. The striking phenomena associated with these materials are due in a large part to spatial electronic inhomogeneities, or electronic phase separation （EPS）. In many of these hard materials, the functionality is a result of the soft electronic component that leads to self-organization. In this paper, we review our recent work on a novel spatial confinement technique that has led to some fascinating new discoveries about the role of EPS in manganites. Using lithographic techniques to confine manganite thin films to length scales of the EPS domains that reside within them, it is possible to simultaneously probe EPS domains with different electronic states. This method allows for a much more complete view of the phases residing in a material and gives vital information on phase formation, movement, and fluctuation. Pushing this trend to its limit, we propose to control the formation process of the EPS using external local fields, which include magnetic exchange field, strain field, and electric field. We term the ability to pattern EPS ＂electronic nanofabrication.＂ This method allows us to control the global physical properties of the system at a very fundamental level, and greatly enhances the potential for realizing true oxide electronics.

Growth and Characteristics of n-VO2/p-GaN based Heterojunctions

n-VO2/p-GaN based oxide-nitride heterojunctions were realized by growing high quality VO2 films with precisely controlled thickness on p-GaN/sapphire substrates by oxide molecular beam epitaxy(O-MBE).The high crystalline quality of the n-VO2/p-GaN heterojunctions were confirmed by X-ray diffraction(XRD)and scanning electron microscope(SEM)analysis.The phase transition characteristics of the as-grown n-VO2/p-GaN heterojunctions were systematically investigated by temperature-dependent resistivity and infrared transmittance measurements.The results indicated that an excellent reversible metal-to-insulator(MIT)transition is observed with an abrupt change in both resistivity and infrared transmittance(IR)at 330 K,which was lower than the 341 K for bulk single crystal VO2.Remarkably,the resistivity-temperature curve was well consistent with that obtained from the temperature dependent IR transmittance.Meanwhile,the current-voltage characteristics originated from the n-VO2/p-GaN interface were demonstrated both before and after MIT of VO2 overlayer,which were attributed to the p-n junction behavior and Schottky contact character,respectively.The design and modulation of the n-VO2/p-GaN based heterostructure devices will benefit significantly from these achievements.

Magnetic-field-induced metal-insulator quantum phase transition in CaFeAsF near the quantum limit

The quantum limit, where only the lowest Landau level is occupied by electrons, can be achieved under a high magnetic field when the Landau level splitting is comparable with the Fermi energy. The rather small Fermi pockets and Fermi energy in CaFeAsF reported recently make this compound a good candidate for investigating the electrical transport near the quantum limit.Here, we report high-field experiments up to 65 T on a single-crystalline CaFeAsF, which shows a metal-insulator quantum phase transition tuned by the out-of-plane magnetic field. The obtained critical exponent zν through the finite-size scaling analysis is very close to 4/3. This transition is closely associated with the evolution of electronic states approaching the quantum limit.The resistivity behaviors as a function of field and temperature were evaluated based on Adams-Holstein theory(A-H theory).Moreover, the in-plane component of the field, which does not affect the transport behavior in the classical region, suppressed the magnetoresistance near the quantum limit.

Effect of electrical contact on performance of WSe_(2) field effect transistors

Two-dimensional(2 D) transition metal dichalcogenides(TMDCs) such as tungsten diselenide(WSe_(2)) have spead many interesting physical properties, which may become ideal candidates to develop new generation electronic and optoelectronic devices. In order to reveal essential features of 2 D TMDCs, it is necessary to fabricate high-quality devices with reliable electrical contact. We systematically analyze the effect of graphene and metal contacts on performance of multilayered WSe_(2) field effect transistors(FETs). The temperature-dependent transport characteristics of both devices are tested.Only graphene-contacted WSe_(2) FETs are observed with the metal-insulator transition phenomenon which mainly attributes to the ultra-clean contact interface and lowered contact barrier. Further characterization on contact barrier demonstrates that graphene contact enables lower contact barrier with WSe_(2) than metal contact, since the Fermi level of graphene can be modulated by the gate bias to match the Fermi level of the channel material. We also analyze the carrier mobility of both devices under different temperatures, revealing that graphene contact can reduce the charge scattering of the device caused by ionized impurities and phonon vibrations in low and room temperature regions, respectively. This work is expected to provide reference for fabricating 2 D material devices with decent performances.

Emergent reversible giant electroresistance in spacially confined La_(0.325)Pr_(0.3)Ca_(0.375)MnO_3 wires

Micro-patterning is considered to be a promising way to analyze phase-separated manganites. We investigate resistance in micro-patterned La0.325Pr0.3Ca0.375MnO3 wires with width of 10 μm, which is comparable to the phase separation scale in this material. A reentrant of insulating state at the metal-insulator temperature Tp is observed and a giant resistance change of over 90% driven by electric field is achieved by suppression of this insulating state. This resistance change is mostly reversible, The I-V characteristics are measured in order to analyze the origin of the giant electroresistance and two possible explanations are proposed.

Influence of Doping on the Mott Metal—Insulator Transition in Infinite Dimensions

We have studied the effect of hole-doping on the established scenerio of the first-order Mott metal-insulator transition (MIT) at half-filling using dynamical mean-field theory and exact diagonalization technique. The Mott insulator state is changed into metallic state immediately as holes are doped into the system. The latter is expected to be Fermi liquid. The previously found unanalytical structure of MIT no longer exists for doping as small as 2 percent. We compare our results with that obtained from Gutzwiller approximation.

Significant control of metal-insulator transition temperature through catalytic excessive oxygen doping in high-performance vanadium dioxide nanobeam channel

The strategy of a reliable transition temperature control of vanadium dioxide(VO2)is reported.Rectangular VO2 nanobeams were synthesized by a thermal chemical vapor deposition(TCVD)system.The metal-insulator transition(MIT)temperature increases to above 380K when the TiO2 ratio of the source is 5 at.%,although the Ti source is not physically doped into VO2 nanobeams.The XPS spectra of the V 2p orbital reveal the excessive oxidation of V after the TCVD processes with a higher TiO2 ratio,indicating that the TiO2 precursor is important in the O-doping of the surface V O bonds when forming volatile Ti-O gas species.Thus,TiO2 reactants can be used as a VO2 surface chemical modifier to manipulate the MIT transition temperature and maintain a homogenous VO2 phase,which is useful for a Mott device application with a record on/off switching ratio>104 and Mott transition temperature>380 K.