Metal-insulator Transition Research Articles

Unravelling the crossover amongst band and various hopping conduction mechanisms in Mo doped ZnO thin films owing to carrier localization at defects

Understanding the carrier transport mechanism of transparent conducting oxides is crucial for its application in optoelectronic devices. Although, electrical transport mechanisms of various trivalent cation doped ZnO films have been reported adequately, a substantial study is needed to understand the transport mechanism in ZnO doped with higher valence (>+3) dopants. Unlike Al, In and Ga doped ZnO where metal-insulator transition is commonly observed, in this study, the crossover amongst band and various hopping mechanisms has been observed in the temperature range of 300–10 K in Mo doped ZnO films deposited at various substrate temperatures. All the films show electrons as the majority charge carriers and notably, the film deposited at 573 K shows the highest conductivity value of 167.22 (Ω.cm)−1. In-depth analyses of the conduction mechanism reveal grain boundary scattering dominated band conduction at and not far below the room temperature, which is shifted towards nearest neighbor hopping (NNH), and then to Mott variable range hopping (VRH) type of conductions as the temperature is decreased further. Unfamiliarly, a modified Efros-Shklovskii (ES) VRH type of conduction mechanism is shown to prevail in the lowest measured temperature range (∼30–10 K). Unprecedentedly, it is demonstrated here that the activation energies for various conduction mechanisms are higher at lower vacuum condition (∼10−1 mbar) as compared to those measured at higher vacuum condition (∼10−3 mbar) implying an added role of the surface traps in the charge transport process. The temperature dependent conductivity and X-ray photoelectron spectroscopy (XPS) studies proclaim that the film deposited at 573 K possess fewer defects related disorder for carrier localization as compared to the films deposited at 523 K and 698 K.

Dynamical quantum phase transitions in Stark quantum spin chains

We investigate the nonequilibrium dynamics of one-dimension spin models in the presence of a uniform force. The linear potential induces delocalization-localization transition in the free particles model which is known as the Wannier-Stark effect. We study dynamical quantum phase transition (DQPT) due to sudden global quenches across a quantum critical point when the system undergoes a localization-delocalization transition. In this regard, we consider the XX and XXZ spin chains and explore two types of quenches with and without ramping through the delocalization-localization point. The XX model was mapped to the free fermion particles, so both analytical and numerical results were provided. Results unveil that the dynamical signature of localization-delocalization transition can be characterized by the nonanalyticities in dynamical free energy (corresponds to the zero points in the Loschmit echo). We also explore the interaction effects considering XXZ spin chains, using the time-dependent extension of the numerical DMRG technique. Our results show that depending on the anisotropic parameter $\Delta\lessgtr1.0$, if both the initial and post-quench Hamiltonian are in the same phase or not, DQPTs may happen. Moreover, the interrelation between DQPTs with different correlation measures such as the equilibrium order parameters or entanglement entropy production of the system remains unclear. We provide more analyses on the feature of DQPTs, in both types of quenches, by connecting them to the average local magnetization, entanglement entropy production, and the Schmidt gap.

Technical Committee 13 Report 2020–2022 [MTT-S Society News

The mission of the Microwave Control Techniques Technical Committee (TC-13) is to promote research in functional materials alongside their application in RF components, such as circulators, phase shifters, limiters, and RF filters. A wide range of materials, such as piezoelectric, ferroelectric, ferromagnetic, and phase-change (e.g., metal–insulator transition switches) materials, is within the scope of TC-13’s research, targeting emerging RF applications, such as 6G communications, radar sensing, and military communications. The committee’s interests span across modeling, design, fabrication, and characterization of these materials and their corresponding RF components. Controllability of the material properties under external stimuli is also an important area of research to enable tunability and miniaturization of key antenna and RF front-end components.

Metal-insulator crossover in monolayer MoS2

We report on transport measurements in monolayer MoS2 devices, close to the bottom of the conduction band edge. These devices were annealed in situ before electrical measurements. This allows us to obtain good ohmic contacts at low temperatures, and to measure precisely the conductivity and mobility via four-probe measurements. The measured effective mobility up to μ eff = 180 cm2 V−1 s−1 is among the largest obtained in CVD-grown MoS2 monolayer devices. These measurements show that electronic transport is of the insulating type for σ ≤ 1.4e 2/h and n ≤ 1.7 × 1012 cm−2, and a crossover to a metallic regime is observed above those values. In the insulating regime, thermally activated transport dominates at high temperature (T > 120 K). At lower temperatures, conductivity is driven by Efros–Schklovkii variable range hopping in all measured devices, with a universal and constant hopping prefactor, that is a clear indication that hopping is not phonon-mediated. At higher carrier density, and high temperature, the conductivity is well modeled by the Boltzmann equation for a non-interacting Fermi gas, taking into account both phonon and impurity scatterings. Finally, even if this apparent metal-insulator transition can be explained by phonon-related phenomena at high temperature, the possibility of a genuine 2D MIT cannot be ruled out, as we can observe a clear power-law diverging localization length close to the transition, and a one-parameter scaling can be realized.

Alkali cation controlled tuning of the metal-insulator transition in vanadium hollandites

Alkali cation controlled tuning of the metal-insulator transition in vanadium hollandites

First-principles design of ferromagnetic monolayer MnO2 at the complex interface

Rapidly increasing interest in low-dimensional materials is driven by the emerging requirement to develop nanoscale solid-state devices with novel functional properties that are not available in three-dimensional bulk phases. Among the well-known low-dimensional systems, complex transition metal oxide interface holds promise for broad applications in electronic and spintronics devices. Herein, intriguing metal-insulator and ferromagnetic-antiferromagnetic transitions are achieved in monolayer MnO2 that is sandwiched into SrTiO3-based heterointerface systems through interface engineering. By using first-principles calculations, we modeled three types of SrTiO3-based heterointerface systems with different interface terminations and performed a comparative study on the spin-dependent magnetic and electronic properties that are established in the confined MnO2 monolayer. First-principles study predicts that metal-insulator transition and magnetic transition in the monolayer MnO2 are independent on the thickness of capping layers. Moreover, 100% spin-polarized two-dimensional electron gases accompanied by robust room temperature magnetism are uncovered in the monolayer MnO2. Not only is the buried MnO2 monolayer a new interface phase of fundamental physical interest, but it is also a promising candidate material for nanoscale spintronics applications. Our study suggests interface engineering at complex oxide interfaces is an alternative approach to designing high-performance two-dimensional materials.

Interface Effects of Strain-Energy Potentials on Phase Transition Characteristics of VO2 Thin-Films.

Metal-insulator-transition (MIT) of VO2 has attracted strong attention as a potential phenomenon to be utilized in nanostructured devices. Dynamics of MIT phase transition determines the feasibility of VO2 material properties in various applications, for example, photonic components, sensors, MEMS actuators, and neuromorphic computing. However, conventional interface strain model predicts the MIT effect accurately for bulk, but fairly for the thin films, and thus, a new model is needed. It was found that the VO2 thin film-substrate interface plays a crucial role in determining transition dynamics properties. In VO2 thin films on different substrates, coexistence of insulator-state polymorph phases, dislocations, and a few unit cell reconstruction layer form an interface structure minimizing strain energy by the increase of structural complexity. As a consequence, MIT temperature and hysteresis of structure increased as the transition enthalpy of the interface increased. Thus, the process does not obey the conventional Clausius-Clapeyron law anymore. A new model is proposed for residual strain energy potentials by implementing a modified Cauchy strain. Experimental results confirm that the MIT effect in constrained VO2 thin films is induced through the Peierls mechanism. The developed model provides tools for strain engineering in the atomic scale for crystal potential distortion effects in nanotechnology, such as topological quantum devices.

Facile Fabrication of High-Performance Thermochromic VO2-Based Films on Si for Application in Phase-Change Devices.

This work reports on an alternative and advantageous procedure to attain VO2-based thermochromic coatings on silicon substrates. It involves the sputtering of vanadium thin films at glancing angles and their subsequent fast annealing in an air atmosphere. By adjusting thickness and porosity of films as well as the thermal treatment parameters, high VO2(M) yields were achieved for 100, 200, and 300 nm thick layers treated at 475 and 550 °C for reaction times below 120 s. Comprehensive structural and compositional characterization by Raman spectroscopy, X-ray diffraction, and scanning-transmission electron microscopies combined with analytical techniques such as electron energy-loss spectroscopy bring to the fore the successful synthesis of VO2(M) + V2O3/V6O13/V2O5 mixtures. Likewise, a 200 nm thick coating consisting exclusively of VO2(M) is also achieved. Conversely, the functional characterization of these samples is addressed by variable temperature spectral reflectance and resistivity measurements. The best results are obtained for the VO2/Si sample with changes in reflectance of 30-65% in the near-infrared at temperatures between 25 and 110 °C. Similarly, it is also proven that the achieved mixtures of vanadium oxides can be advantageous for certain optical applications in specific infrared windows. Finally, the features of the different structural, optical, and electrical hysteresis loops associated with the metal-insulator transition of the VO2/Si sample are disclosed and compared. These remarkable thermochromic performances hereby accomplished highlight the suitability of these VO2-based coatings for applications in a wide range of optical, optoelectronic, and/or electronic smart devices.

Intertwined electronic and magnetic structure of the van-der-Waals antiferromagnet Fe2P2S6

Many unusual and promising properties have been reported recently for the transition metal trichalcogenides of the type MPS3 (M = V, Mn, Fe, Ni ...), such as maintaining magnetic order to the atomically thin limit, ultra-sharp many-body excitons, metal-insulator transitions and, especially for Fe2P2S6, giant linear dichroism among others. Here we conduct a detailed investigation of the electronic structure of Fe2P2S6 using angle-resolved photoemission spectroscopy, q-dependent electron energy loss spectroscopy, optical spectroscopies and density functional theory. Fe2P2S6 is a Mott insulator with a gap of Egap ≈ 1.4 eV and zigzag antiferromagnetism below TN = 119 K. The low energy excitations are dominated by Fe 3d states. Large and sign-changing linear dichroism is observed. We provide a microscopic mechanism explaining key properties of the linear dichroism based on the correlated character of the electronic structure, thereby elucidating the nature of the spin-charge coupling in Fe2P2S6 and related materials.

Flexible Terahertz Metamaterials Absorber based on VO2

Terahertz (THz) metamaterials have attracted great attention due to their widely application potential in smart THz devices; however, most of them are fabricated on rigid substrate and thus limit the exploration of flexible THz electronics. In this paper, a flexible THz metamaterial absorber (MMA) incorporated with phase change material vanadium dioxide (VO2) is proposed. The simulation results indicate that two absorption peaks at around 0.24 THz (marked as A) and 0.46 THz (marked as B) can be observed by designing a I-shaped metamaterial combined with split ring structure. The strong absorption over 92% at 0.24 THz is bending-insensitive, but the absorption at 0.46 THz is bending-sensitive, across the bending angle in the range of 0–50 degrees. Moreover, dynamic modulation of the absorption can be achieved across the insulator-metal phase transition of VO2. Particularly, the absorption of the A-peak can be tuned from 99.4% to 46.9%, while the absorption of the B-peak can be tuned from 39.6% to 99.3%. This work would provide significance for the design of flexible THz smart devices.

Variation in the electronic, mechanical, and structural properties among the polymorphs of bismuth ferrite: a first-principles approach

Bismuth ferrite has been under intense research for many years as it can exhibit first- and second-order transitions where all the phases have distinct properties encapsulating various exciting phenomena. This work reports a computational study of bismuth ferrite and its varied phases using density functional theory with the implementation of Hubbard correction for increased accuracy. The proposed method is validated through Linear Response Theory using Quantum ESPRESSO. The phase transition and the mechanical properties are explored by calculating elastic tensors for different polymorphs. A negative Poisson's ratio for the tetragonal phase supporting its growth in compressive environments is predicted. The electronic properties of different phases of bismuth ferrite are explored, which helps in understanding properties such as charge transfer excitation, metal–insulator transition, ferroelectric nature based on lone pair charges and orbital hybridization. The phonon modes of different phases are also investigated.Graphical abstract

Density-tuned effective metal-insulator transitions in two-dimensional semiconductor layers: Anderson localization or Wigner crystallization

Electrons (or holes) confined in 2D semiconductor layers have served as model systems for studying disorder and interaction effects for almost 50 years. In particular, strong disorder drives the metallic 2D carriers into a strongly localized Anderson insulator (AI) at low densities whereas pristine 2D electrons in the presence of no (or little) disorder should solidify into a Wigner crystal at low carrier densities. Since the disorder in 2D semiconductors is mostly Coulomb disorder arising from random charged impurities, the applicable physics is complex as the carriers interact with each other as well as with the random charged impurities through the same long-range Coulomb coupling. By critically theoretically analyzing the experimental transport data in depth using a realistic transport theory to calculate the low-temperature 2D resistivity as a function of carrier density in 11 different experimental samples covering 9 different materials, we establish, utilizing the Ioffe-Regel-Mott criterion for strong localization, a direct connection between the critical localization density for the 2D metal-insulator transition (MIT) and the sample mobility deep into the metallic state, which for clean samples could lead to a localization density low enough to make the transition appear to be a Wigner crystallization. We believe that the insulating phase is always an effective Coulomb disorder-induced localized AI, which may have short-range WC-like correlations at low carrier densities. Our theoretically calculated disorder-driven critical MIT density agrees with experimental findings in all 2D samples, even for the ultra-clean samples. In particular, the extrapolated critical density for the 2D MIT seems to vanish when the high-density mobility goes to infinity, indicating that transport probes a disorder-localized insulating ground state independent of how low the carrier density might be.

Reversible metal-insulator transition in SrIrO3 ultrathin layers by field effect control of inversion symmetry breaking

SrIrO3 is a correlated semimetal with narrow t2g d-bands of strong mixed orbital character resulting from the interplay of the spin-orbit interaction due to heavy iridium atoms and the band folding induced by the lattice structure. In ultrathin layers, inversion symmetry breaking, occurring naturally due to the presence of the substrate, opens new orbital hopping channels, which in presence of spin-orbit interaction causes deep modifications in the electronic structure. Here, we show that in SrIrO3 ultrathin films the effect of inversion symmetry breaking on the band structure can be externally manipulated in a field effect experiment. We further prove that the electric field toggles the system reversibly between a metallic and an insulating state with canted antiferromagnetism and an emergent anomalous Hall effect. This is achieved through the spin-orbit driven coupling of the electric field generated in an ionic liquid gate to the electronic structure, where the electric field controls the band structure rather than the usual band filling, thereby enabling electrical control of the effective role of electron correlations. The externally tunable antiferromagnetic insulator, rooted in the strong spin-orbit interaction of iridium, may inspire interesting applications in spintronics.

Single crystal growth and electronic structure of Rh-doped Sr3Ir2O7

Ruddlesden-Popper iridate Sr3Ir2O7 is a spin–orbit coupled Mott insulator. Hole doped Sr3Ir2O7 provides an ideal platform to study the exotic quantum phenomena that occur near the metal–insulator transition (MIT) region. Rh substitution of Ir is an effective method to induce hole doping into Sr3Ir2O7. However, the highest doping level reported in Sr3(Ir1−x Rh x )2O7 single crystals was only around 3%, which is far from the MIT region. In this paper, we report the successful growth of single crystals of Sr3(Ir1−x Rh x )2O7 with a doping level of ~ 9%. The samples have been fully characterized, demonstrating the high quality of the single crystals. Transport measurements have been carried out, confirming the tendency of MIT in these samples. The electronic structure has also been examined by angle-resolved photoemission spectroscopy (ARPES) measurements. Our results establish a platform to investigate the heavily hole doped Sr3Ir2O7 compound, which also provide new insights into the MIT with hole doping in this material system.

Continuous Metal-Insulator Transition and Scale Laws in Metallic n-Type InP

We have studied the transport properties in metallic n-type InP semiconductor. We show that the dependence on temperature of metallic electrical conductivity obey to the law 𝜎 = 𝜎(𝑇 = 0) + 𝑚𝑇1/2. We highlight the absence of a minimum electrical conductivity 𝜎𝑚𝑖𝑛 proposed by Mott at the metal-insulator transition. We show that the conductivity at temperature T = 0 K, 𝜎(𝑇 = 0) , follows a scaling law as a function of the effective parallel and perpendicular Bohr radii 𝑎∥ and 𝑎⊥.

Transmittance correlated real-time resistivity modulation and insulator-metal transition of electrochromic WO3 thin films

A quantitative relationship between optical and electrical properties of crystalline WO3 films during Li + insertion has been established in this work. Detailly, the exponential relationship between transmittance and resistivity of crystalline WO3 thin films in visible and near-infrared wavebands is determined, and the real-time characterization of the resistivity of WO3 thin films during electrochromic process is realized. The experimental results show the resistivity of WO3 film can be regulated in the range of 0.078–0.98 Ω m with rapid response (∼6 s) when applied ±1 V voltage for 30 s. By analyzing the fitted resistivity-transmittance curve of WO3 thin film, a high-efficiency optical modulation of WO3 film in near-infrared 1000 nm can be achieved. Combining the IMT phenomenon during Li+ insertion, when x of LixWO3 changes from 0.08 to 0.13, the WO3 film is in a transition state from the insulating state to the metallic state with the tetragonal crystalline phase. The symmetry structural evolution of crystalline WO3 thin films during the Li + insertion is further demonstrated by transmission electron microscopy (TEM). These results contribute to the development of WO3 based optoelectronic bifunctional electrochromic devices.

Insulator-to-metal-like transition in thin films of a biological metal-organic framework

Temperature-induced insulator-to-metal transitions (IMTs) where the electrical resistivity can be altered by over tens of orders of magnitude are most often accompanied by structural phase transition in the system. Here, we demonstrate an insulator-to-metal-like transition (IMLT) at 333 K in thin films of a biological metal-organic framework (bio-MOF) which was generated upon an extended coordination of the cystine (dimer of amino acid cysteine) ligand with cupric ion (spin-1/2 system) – without appreciable change in the structure. Bio-MOFs are crystalline porous solids and a subclass of conventional MOFs where physiological functionalities of bio-molecular ligands along with the structural diversity can primarily be utilized for various biomedical applications. MOFs are usually electrical insulators (so as our expectation with bio-MOFs) and can be bestowed with reasonable electrical conductivity by the design. This discovery of electronically driven IMLT opens new opportunities for bio-MOFs, to emerge as strongly correlated reticular materials with thin film device functionalities.

Disordered impenetrable two-component fermions in one dimension

We study the one-dimensional Hubbard model for two-component fermions with infinitely strong on-site repulsion ($t\ensuremath{-}0$ model) in the presence of disorder. Our analytical treatment demonstrates that the type of disorder drastically changes the nature of the emerging phases. The case of spin-independent disorder can be treated as a single-particle problem with Anderson localization. On the contrary, recent numerical findings show that spin-dependent disorder, which can be realized as a random magnetic field, leads to the many-body localization-delocalization transition. We find an explicit analytic expression for the matrix elements of the random magnetic field between the eigenstates of the $t\ensuremath{-}0$ model with potential disorder on a finite lattice. Analysis of the matrix elements supports the existence of the many-body localization-delocalization transition in this system and provides an extended physical picture of the random magnetic field.

Investigation of nanoscale core-shell structure of silicon microcrystal doped by boron and nickel: Properties and application

The article is devoted to characteristics of charge carrier transfer in silicon microcrystals modified with a transition metal admixture with an unfilled 3d+ local magnetic moment envelope and doped with boron to concentrations that correspond to the metal-dielectric transition. At cryogenic temperatures, the magnetoresistance of microcrystals was examined under the influence of magnetic fields up to 14 T. The outcomes of investigations into the magneto-transport characteristics of crystals were thoroughly examined. It was discovered that the transport of charge carriers for silicon microcrystals at low temperatures relies on hopping polarization conductivity. Based on the results of the Si < B, Ni > crystal magnetization study, the concentration of magnetic centers was determined, which is 4 × 1017см−3. It is suggested to employ silicon microcrystals in magnetic field sensors that operate on the magnetoresistive principle.

Magneto transport behavior of La0.67Pb0.33MnO3manganite system with various additives

The nanostructured manganites show low field magnetoresistance, where grain boundary effects influence the phenomenon. The current investigation aims to comprehend the impact of the grain boundaries on the magnetic and magneto transport characteristics of La0.67Pb0.33MnO3 manganite with 1% of additives ZrO2, Bi2O3, and Li2CO3. Two step synthesis processes was adopted to prepare the pure and additive samples. The samples were characterized with XRD, XPS and FESEM to establish the structure, phase purity, oxidation states, elemental analysis and morphology respectively. The prepared samples were formed in single-phase rhombohedral structure with the enhanced crystallite and grain size with additives when compared to the pure sample. The observed magnetic transition temperature, are well above room temperature (TC = 353 K) and a soft ferromagnetic nature with 70% of ferromagnetic fraction is noticed at T = 300 K for all the studied samples. The metal insulator transition temperature (TP) remains invariant for ZrO2, Bi2O3 additives, while it reduces for Li2CO3 additive. It is observed that 1% of additives are acting as conduction barriers at the grain boundaries as a consequence there is an increase in resistivity. The contribution to magnetoresistance is seen as intrinsic and extrinsic in pure and additive materials exhibiting high percentage both at low and room temperature. All the sample shows an appreciable temperature coefficient of resistance percentage at their respective transition temperatures which can be made useful for bolometric applications. Conduction mechanisms in the ferromagnetic and paramagnetic phases were examined with the electron-magnon scattering and small polaron hopping model respectively.