Metal-insulator Phase Transition Research Articles

Transmittance in a thin aluminum layer at nanosecond pulsed laser ablation

AbstractSince the beginning of the 1960s and up to the present time, laser ablation from metal surfaces under the action of short light pulses has been the topic of many researches. One of the first objects in the early ablation experiments presented thin metal films evaporated by radiation, which were used in lasers with nanosecond pulses as optical gates for Q-switching in resonators or for decoupling in amplifiers. Bleaching of the gates based on metal layers with a time constant τt ≈ 10−8 s observed in a number of experiments, was usually considered as a result of a simple evaporation of matter. We analyze the data of the experiments with a mylar tape [≈0.05 µm aluminum (Al) layer on ≈20 µm lavsan substrate] used as a gate for optical isolation in one of the first Nd: glass laser facilities with a power of ≈1 GW. That gate was irradiated by pulses of Q-switched oscillators: pulse duration 10−7–10−8 s, intensity 107–108 W/cm2. A jump in the transmittance of the expanding Al layer was registered (from ≈0.1 to 50% at τt ≈ 10−9 s). The present study shows that one should consider the metal–insulator phase transition in a superheated liquid metal layer as the mechanism of the fast (up to 10–10–10–11 s) increase in the transmittance of the Al film gate at nanosecond-pulsed laser ablation.

Influence of oxygen flow rate on metal–insulator transition of vanadium oxide thin films grown by RF magnetron sputtering

High-quality vanadium oxide (VO2) films have been fabricated on Si (111) substrates by radio frequency (RF) magnetron sputtering deposition method. The sheet resistance of VO2 has a significant change (close to 5 orders of magnitude) in the process of the metal–insulator phase transition (MIT). The field emission-scanning electron microscope (FE-SEM) results show the grain size of VO2 thin films is larger with the increase of oxygen flow. The X-ray diffraction (XRD) results indicate the thin films fabricated at different oxygen flow rates grow along the (011) crystalline orientation. As the oxygen flow rate increases from 3 sccm to 6 sccm, the phase transition temperature of the films reduces from 341 to 320 K, the width of the thermal hysteresis loop decreases from 32 to 9 K. The thin films fabricated in the condition of 5 sccm have a high temperature coefficient of resistance (TCR) −3.455%/K with a small resistivity of 2.795 ρ/Ω cm.

Exact out-of-time-ordered correlation functions for an interacting lattice fermion model

Exact solutions for local equilibrium and nonequilibrium out-of-time-ordered correlation (OTOC) functions are obtained for a lattice fermion model with on-site interactions, namely the Falicov-Kimball (FK) model, in the large dimensional and thermodynamic limit. Our approach is based on the nonequilibrium dynamical mean-field theory generalized to an extended Kadanoff-Baym contour. We find that the density-density OTOC is most enhanced at intermediate coupling around the metal-insulator phase transition. In the high-temperature limit, the OTOC remains nontrivially finite and interaction-dependent, even though dynamical charge correlations probed by an ordinary response function are completely suppressed. We propose an experiment to measure OTOCs of fermionic lattice systems including the FK and Hubbard models in ultracold atomic systems.

Эффективное поле и фазовый переход металл—диэлектрик в модели Хаббарда

Эффективное поле и фазовый переход металл—диэлектрик в модели Хаббарда

Optimization of VO2 nanowire polymer composite thermochromic films by optical simulation

Thermochromic films with high efficiency, transparency, and flexibility are highly desirable for energy-efficient smart window films. Vanadium oxide (VO2)-nanoparticle-embedded flexible polymer composite films are the most promising thermochromic films because of the sharp phase transition of insulating to metallic phases of VO2 at 68 °C with visible transparency and a large change in transmittance at near-infrared wavelengths before and after the metal–insulator phase transition. This paper describes the simulation of high-efficiency thermochromic polymer composite films embedded with VO2 nanoparticles of various sizes to investigate the optimum VO2 nanowire size and length.

The thermal metal-insulator phase transition in (EDO-TTF)2PF6

ABSTRACTThe thermal metal-insulator phase transition in the π-stacked (EDO-TTF)2PF6 charge transfer salt is of the Peierls type. It is related to geometrical reorganisations and charge ordering phenomena. We report that dimerising displacements are involved in the mechanism of this transition. By using periodic quantum chemical calculations, we find a double well potential in which dimerisation and charge localisation become manifest. By analysing the nuclear wavefunctions we discuss the mechanism of the phase transition in terms of thermal fluctuations.

First-principles prediction of a rising star of solar energy material: SrTcO3.

SrTcO3 as a new star of solar energy material is investigated in terms of its band gap evolution with biaxial strain from first-principles calculations. Compared to the theoretical equilibrium lattice constant a(b) of bulk SrTcO3, a set of lattice constants with a deviation of -8.75% to +3.35% are considered to include the strain effect. Since the in-plane lattice constant of SrTcO3 is larger than that of the commonly used substrate SrTiO3(STO)/La0.3Sr0.7Al0.35Ta0.35O9 (LSAT)/NdGaO3(NGO)/LaAlO3(LAO), we mainly focus on the modulation of compressive strain. It is found that the band gap decreases with increasing compressive/tensile strain. When the compressive strain reaches 8.75%, the band gap drops to zero and an insulator-metal phase transition appears. Particularly, upon a compressive strain of 1.3%/2.2%/2.4%/4.1%, which can be realized by growing SrTcO3 on substrate STO/LSAT/NGO/LAO, the band gap becomes 1.56/1.47/1.43/1.12 eV, which falls in the range for efficient solar cell materials. Our work suggests that SrTcO3 is a good candidate for a new solar energy material.

Current induced polycrystalline-to-crystalline transformation in vanadium dioxide nanowires.

Vanadium dioxide (VO2) exhibits a reversible insulator-metal phase transition that is of significant interest in energy-efficient nanoelectronic and nanophotonic devices. In these applications, crystalline materials are usually preferred for their superior electrical transport characteristics as well as spatial homogeneity and low surface roughness over the device area for reduced scattering. Here, we show applied electrical currents can induce a permanent reconfiguration of polycrystalline VO2 nanowires into crystalline nanowires, resulting in a dramatically reduced hysteresis across the phase transition and reduced resistivity. Low currents below 3 mA were sufficient to cause the local temperature in the VO2 to reach about 1780 K to activate the irreversible polycrystalline-to-crystalline transformation. The crystallinity was confirmed by electron microscopy and diffraction analyses. This simple yet localized post-processing of insulator-metal phase transition materials may enable new methods of studying and fabricating nanoscale structures and devices formed from these materials.

Lattice vibrations and electrical transport in (Bi1−xInx)2Se3 films

We present Raman, terahertz transmission, and transport measurements on (Bi1−xInx)2Se3 films to study the evolution of phonon modes and resistivity with an increasing indium content across the metal-insulator phase transition. The frequencies of two Raman-active modes E g 2 and A 1 g 2 as well as an infrared-active mode Eu increase with an increasing indium content due to the smaller atomic weight of indium compared to bismuth. Terahertz data are fitted by a Drude-Lorentz model. Drude scattering rates increase from 47 to 75 cm−1 with an increasing indium content from 0% to 16% due to stronger impurity scattering. The carrier concentration drops significantly for x = 24%. The temperature dependence of the resistivity switches from metallic at x = 16% to insulating at x = 24%, indicating a metal-insulator transition in between.

First-principles investigation on solar radiation shielding performance of rutile VO2 filters for smart windows

Vanadium dioxide (VO2) undergoing reversible metal-insulator phase transition could allow for the formation of an efficient thermochromic material for smart windows. However, solar radiation shielding performance is determined by transparent rutile VO2 filters, and the puzzling metal-insulator transition mechanism makes it challenging to explain the origin of the coexistence of strong near infrared absorption with high optical transparency. The band structure, the density of states, and the optical properties of rutile VO2 were calculated using the first-principles calculations. The calculated results of the structural and optical properties are in good agreement with the previously reported experimental findings. The calculated dielectric functions, electron energy-loss function and solar radiation shielding performance of the rutile VO2 filters indicate that rutile VO2 is a promising near-infrared absorption/reflectance material with the near-infrared radiation insulating abilities and a visible light transmittance. These properties arise from plasma oscillation and a collective oscillation (volume plasmons) of carrier electrons.

Evolution of Insulator-Metal Phase Transitions in Epitaxial Tungsten Oxide Films during Electrolyte-Gating.

An interface between an oxide and an electrolyte gives rise to various processes as exemplified by electrostatic charge accumulation/depletion and electrochemical reactions such as intercalation/decalation under electric field. Here we directly compare typical device operations of those in electric double layer transistor geometry by adopting A-site vacant perovskite WO3 epitaxial thin films as a channel material and two different electrolytes as gating agent. In situ measurements of X-ray diffraction and channel resistance performed during the gating revealed that in both the cases WO3 thin film reaches a new metallic state through multiple phase transitions, accompanied by the change in out-of-plane lattice constant. Electrons are electrostatically accumulated from the interface side with an ionic liquid, while alkaline metal ions are more uniformly intercalated into the film with a polymer electrolyte. We systematically demonstrate this difference in the electrostatic and electrochemical processes, by comparing doped carrier density, lattice deformation behavior, and time constant of the phase transitions.

Magnetoresistance Effect in La1.5Sr0.5NiO4-Doped La0.7Ca0.3MnO3 Nanocomposites

Our experimental results point out that most La1.5Sr0.5NiO4 nanoparticles were distributed at the grain boundaries and on the surfaces of the La0.7Ca0.3MnO3 nanoparticles. The nanocomposite samples exhibit a ferromagnetic-paramagnetic and a metal-insulator phase transitions at TC and TMI, respectively. With increasing La1.5Sr0.5NiO4-doped content, TC value is almost unchanged while TMI value decreases from 251 K for an undoped sample to 65 K for 20 % La1.5Sr0.5NiO4-doped one. Particularly, the La1.5Sr0.5NiO4-doped samples higher than 20 % exhibit the insulating properties in the whole temperature range. Magnetoresistance effect at low field (H= 3 kOe) of all the samples is observed. In order to explore the nature of their magnetoresistance effect at low field according to temperature and magnetic field change, we analyzed carefully the obtained data based on the phenomenological model related to the spinpolarized transport of conduction electrons at grain boundaries. With this, the temperature dependences of magnetoresistances (including the intrinsic magnetoresistances and spinpolarized transport magnetoresistances) can be described well by an expression of the Curie-Weiss law-like behavior, a + b/(c + T). We also have been able to observe the percolation threshold in this system for 25 % of La1.5Sr0.5NiO4 component, and around this critical point a great increase of magnetoresistances has been detected.

Nonlinear-optical study of magnetoelectric interactions in multilayer structures

ABSTRACTTemperature dependences of electric field-induced and magnetic field-induced optical second harmonic generation (SHG) were used to study La0.7Ca0.25MnO3/BaTiO3 (LCMO/BTO) superlattice. Ferroic properties of the superlattice are reveal by magnetic field-induced and electric field-induced SHG hysteresis loops (“magnetic” and “electric” SHG loops). However, direct modifications of the SHG hysteresis loops by the other field (electric field for the “magnetic loop” and vice versa) were not found. Strong modifications of the “electric” SHG hysteresis loops occur at temperatures close to the paramagnetic-ferromagnetic and the insulator-metal phase transitions in LCMO. In this way, the magnetoelectric interaction in an LCMO/BTO superlattice was established indirectly.

The control of magnetism near metal-to-insulator transitions of VO2 nano-belts

The magnetic properties of paramagnetic/weakly ferromagnetic films are strongly affected by the proximity to materials that undergo a metal to insulator phase transition. Here, we show that under the deposition conditions associated with structural changes near the metal-insulator phase transition of VO2 produces magnetoelastic anisotropy. We observe intrinsic paramagnetic centres (PM-C) both at the near film surface and bulk/deep PM-C that are affected by the metal-insulator phase transition in VO2. We study the evolution of vibrational modes of VO2 thin films in the vicinity of the phase transition by using temperature controlled-Raman microscopy.

Charge Mediated Reversible Metal-Insulator Transition in Monolayer MoTe2 and WxMo1-xTe2 Alloy.

Metal-insulator transitions in low-dimensional materials under ambient conditions are rare and worth pursuing due to their intriguing physics and rich device applications. Monolayer MoTe2 and WTe2 are distinguished from other TMDs by the existence of an exceptional semimetallic distorted octahedral structure (T') with a quite small energy difference from the semiconducting H phase. In the process of transition metal alloying, an equal stability point of the H and the T' phase is observed in the formation energy diagram of monolayer WxMo1-xTe2. This thermodynamically driven phase transition enables a controlled synthesis of the desired phase (H or T') of monolayer WxMo1-xTe2 using a growth method such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). Furthermore, charge mediation, as a more feasible method, is found to make the T' phase more stable than the H phase and induce a phase transition from the H phase (semiconducting) to the T' phase (semimetallic) in monolayer WxMo1-xTe2 alloy. This suggests that a dynamic metal-insulator phase transition can be induced, which can be exploited for rich phase transition applications in two-dimensional nanoelectronics.

Thermochromic modulation of surface plasmon polaritons in vanadium dioxide nanocomposites.

We propose and implement a new concept for thermochromic plasmonic elements. It is based on vanadium dioxide (VO2) nanocrystals located in the near field of surface plasmon polaritons supported by an otherwise unstructured gold thin film. When the VO2 undergoes the metal-insulator phase transition, the coupling conditions for conversion of light into propagating surface plasmon polaritons change markedly. In particular, we realize thermochromic plasmonic grating couplers with substantial switching contrast as well as tunable plasmonic couplers in a Kretschmann configuration. The use of VO2 nanocrystals permits highly repetitive switching and room temperature operation. Simulations based on the actual dielectric function of our VO2 nanocrystals agree well with the experiment.

Four allotropes of semiconducting layered arsenic that switch into a topological insulator via an electric field: Computational study

We propose four different thermodynamically stable structural phases of arsenic monolayers based on ab-initio density functional theory calculations all of which undergo a topological phase transition on application of a perpendicular electric field. All the four arsenic monolayer allotropes have a wide band gap, varying from 1.21 eV to 3.0 eV (based on GW calculations), and in general they undergo a metal-insulator quantum phase transition on application of uniaxial in-layer strain. Additionally an increasing transverse electric field induces band-inversion at the {\Gamma} point in all four monolayer allotropes, leading to a nontrivial topological phase (insulating for three and metallic for one allotrope), characterized by the switching of the Z2 index, from 0 (before band inversion) to 1 (after band inversion). The topological phase tuned by the transverse electric field, should support spin-separated gapless edge states which should manifest in quantum spin Hall effect.

Relaxation oscillations in circuits containing sandwich switches based on vanadium dioxide

ABSTRACTElectrical switching with an S-shaped I–V characteristic is studied in metal/oxide/metal structures based on vanadium oxide thin films fabricated by electrochemical oxidation. This switching effect is associated with the metal–insulator phase transition in VO2, and the channels consisting of vanadium dioxide are formed in the initial anodic films during the process of electroforming. An expression for the relaxation oscillation frequency, as a function of the external circuit RC factor and I–V curve parameters, is obtained. It is shown that the experimental data on the frequency–capacitance dependence match well this theoretical relation. Applied aspects of the obtained results, particularly concerning oscillatory neural networks and frequency-output sensors, are discussed.

Modulating Photoluminescence of Monolayer Molybdenum Disulfide by Metal-Insulator Phase Transition in Active Substrates.

The atomic thickness and flatness allow properties of 2D semiconductors to be modulated with influence from the substrate. Reversible modulation of these properties requires an "active," reconfigurable substrate, i.e., a substrate with switchable functionalities that interacts strongly with the 2D overlayer. In this work, the photoluminescence (PL) of monolayer molybdenum disulfide (MoS2 ) is modulated by interfacing it with a phase transition material, vanadium dioxide (VO2 ). The MoS2 PL intensity is enhanced by a factor of up to three when the underlying VO2 undergoes the thermally driven phase transition from the insulating to metallic phase. A nonvolatile, reversible way to rewrite the PL pattern is also demonstrated. The enhancement effect is attributed to constructive optical interference when the VO2 turns metallic. This modulation method requires no chemical or mechanical processes, potentially finding applications in new switches and sensors.

Local Peltier-effect-induced reversible metal–insulator transition in VO2 nanowires

We report anomalous resistance leaps and drops in VO2 nanowires with operating current density and direction, showing reversible and nonvolatile switching. This event is associated with the metal–insulator phase transition (MIT) of local nanodomains with coexistence states of metallic and insulating phases induced by thermoelectric cooling and heating effects. Because the interface of metal and insulator domains has much different Peltier coefficient, it is possible that a significant Peltier effect would be a source of the local MIT. This operation can be realized by one-dimensional domain configuration in VO2 nanowires because one straight current path through the electronic domain-interface enables theoretical control of thermoelectric effects. This result will open a new method of reversible control of electronic states in correlated electron materials.