Insulator-insulator Transition Research Articles

Observation of an exotic insulator to insulator transition upon electron doping the Mott insulator CeMnAsO

A promising route to discover exotic electronic states in correlated electron systems is to vary the hole or electron doping away from a Mott insulating state. Important examples include quantum criticality and high-temperature superconductivity in cuprates. Here, we report the surprising discovery of a quantum insulating state upon electron doping the Mott insulator CeMnAsO, which emerges below a distinct critical transition temperature, TII. The insulator-insulator transition is accompanied by a significant reduction in electron mobility as well as a colossal Seebeck effect and slow dynamics due to decoupling of the electrons from the lattice phonons. The origin of the transition is tentatively interpreted in terms of many-body localization, which has not been observed previously in a solid-state material.

Effects of Ga doping on the phase transitions of V2O3

${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ undergoes a first-order metal-insulator transition accompanied by a magnetostructural transition at ${T}_{\mathrm{MI}}\ensuremath{\sim}160\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Here, we report a comprehensive study on gallium-doped (4%) polycrystalline ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ by employing various experimental techniques such as synchrotron x-ray diffraction, thermal expansion, Raman spectroscopy, DC magnetization, time-of-flight neutron diffraction, heat capacity, and electrical transport. Our studies show several interesting features related to the structural, magnetic, and electronic phase transitions in Ga-doped sample. Intriguingly, the nonmagnetic doping in ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ enhances the N\'eel temperature by 25 K $({T}_{\mathrm{N}}=185\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. Further, we find the decoupling of the structural transition from the magnetic and insulator-insulator transition $({T}_{\mathrm{II}}\ensuremath{\sim}185\phantom{\rule{0.16em}{0ex}}\mathrm{K})$. Raman spectroscopic studies reveal that the structural phase transition begins at ${T}^{*} (=195\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, which is slightly above ${T}_{\mathrm{N}}$ or ${T}_{\mathrm{II}}$, as evidenced from the softening of ${E}_{g}$ mode and the splitting of ${A}_{1g}$ mode. Additionally, the rhombohedral and monoclinic phases coexist in a broad temperature range, and a complete phase transformation to monoclinic phase occurs at 127 K.

Atomic and electronic structure of doped Si(111)(23×23)R30∘ -Sn interfaces

The hole doped Si(111)(2root3x2root3)R30(degrees)-Sn interface exhibits a symmetry-breaking insulator-insulator transition below 100 K that appears to be triggered by electron tunneling into the empty surface-state bands. No such transition is seen in electron-doped systems. To elucidate the nature and driving force of this phenomenon, the structure of the interface must be resolved. Here we report on an extensive experimental and theoretical study, including scanning tunneling microscopy and spectroscopy (STM/STS), dynamical low-energy electron diffraction (LEED) analysis, and density functional theory (DFT) calculations, to elucidate the structure of this interface. We consider six different structure models, three of which have been proposed before, and conclude that only two of them can account for the majority of experimental data. One of them is the model according to Tornevik et al. [C. Tornevik et al., Phys. Rev. B 44, 13144 (1991)] with a total Sn coverage of 14/12 monolayers (ML). The other is the 'revised trimer model' with a total Sn coverage of 13/12 ML, introduced in this work. These two models are very difficult to discriminate on the basis of DFT or LEED alone, but STS data clearly point toward the T\"ornevik model as the most viable candidate among the models considered here. The STS data also provide additional insights regarding the electron-injection-driven phase transformation as well as the critical role of valence band holes in this process. Similar processes may occur at other metal/semiconductor interfaces, provided they are non-metallic and can be doped. This could open up a new pathway toward the creation of novel surface phases with potentially very interesting and desirable electronic properties.

Raman study of high temperature insulator-insulator transition in Ba2Co9O14

The insulator-insulator transition, at Tt = 570 K, in layered cobalt oxide Ba2Co9O14 was investigated using Raman scattering technique. High temperature (300–800 K) measurements have evidenced no structural transition occurring at Tt. The obtained results are rather consistent with low to high spin-state transition of Co3+ ions in the Co3O12 octahedral trimer. More precisely, only one cobalt ion located in the central octahedron of the trimer undergoes this transition.

Optical Conductivity Measurement of a Dimer Mott-Insulator to Charge-Order Phase Transition in a Two-Dimensional Quarter-Filled Organic Salt Compound

We report a novel insulator-insulator transition arising from the internal charge degrees of freedom in the two-dimensional quarter-filled organic salt β-(meso-DMBEDT-TTF)2PF6. The optical conductivity spectra above Tc=70 K display a prominent feature of the dimer Mott insulator, characterized by a substantial growth of a dimer peak near 0.6 eV with decreasing temperature. The dimer peak growth is rapidly quenched as soon as a peak of the charge order appears below Tc, indicating a competition between the two insulating phases. Our infrared imaging spectroscopy has further revealed a spatially competitive electronic phase far below Tc, suggesting a nature of quantum phase transition driven by material-parameter variations.

Nanomechanical actuation from phase transitions in individual VO2 micro-beams

The mechanical response due to structural phase transitions in individual VO2 nanowires was quantitatively investigated in situ in a transmission electron microscope. The electron beam was employed to locally heat the nanowire, and stress-strain was measured in situ using a microfabricated push-to-pull device. Tensile loading was found to increase the metal-insulator transition temperature and decrease the insulator-insulator transition temperature, consistent with the phase diagram of VO2. These phase transitions resulted in an axial mechanical response of the VO2 nanowires, an effect that can potentially be used to actuate nanostructures or gauge the local temperature change induced by electron beam irradiation.

Cover Picture: Observation of insulating-insulating monoclinic structural transition in macro-sized VO2 single crystals (Phys. Status Solidi RRL 3/2011)

To clarify the origin of metal–insulator transition (MIT) in VO2, it is of crucial importance to under-stand the interplay among monoclinic M1, monoclinic M2, and rutile R phases. The Letter by Bongjin Simon Mun et al. (pp. 107–109) reports the unusual insula-tor–insulator structural phase transition (SPT) of VO2 single crystals, which involves monoclinic M1 and M2 phase. The VO2 crystals exhibit an extremely abrupt MIT at 67.8 °C and an insulator–insulator transition (IIT) at ∼49 °C. Using synchrotron-based X-ray microdiffraction, it is found that the IIT in this VO2 crystal is related to a SPT between the M2 and M1 phases while the MIT occurs with a SPT of M1 and R phase. Also, a stable M2 phase is found at room temperature without any presence of external stress, which has not been reported previously. The authors believe that further investigations on this intriguing system will not only enrich the fundamental understanding of the system, but also will be exploited in future technologies.

Observation of insulating–insulating monoclinic structural transition in macro‐sized VO2 single crystals

AbstractVO2 single crystals with unprecedented quality, exhibiting a first‐order metal–insulator transition (MIT) at 67.8 °C and an insulator –insulator transition (IIT) at ∼49 °C, are grown using a self‐flux evaporation method. Using synchrotron‐based X‐ray microdiffraction analysis, it is shown that the IIT is related to a structural phase transition (SPT) from the monoclinic M2 phase to the M1 phase upon heating while the MIT occurs together with a SPT of M1 to the rutile R phase. All previous reports have shown that VO2 exists in the M1 phase at room temperature in contrast to the M2 phase observed in this work. We suggest that internal strain inside single crystal VO2 may generate the previously unobserved IIT and the unusual room temperature structure. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlations in a band insulator

We study a model of a covalent band insulator with on-site Coulomb repulsion at half filling using dynamical mean-field theory. Upon increasing the interaction strength the system undergoes a discontinuous transition from a correlated band insulator to a Mott insulator with hysteretic behavior at low temperatures. Increasing the temperature in the band insulator close to the insulator-insulator transition we find a crossover to a Mott insulator at elevated temperatures. Remarkably, correlations decrease the energy gap in the correlated band insulator. The gap renormalization can be traced to the low-frequency behavior of the self-energy, analogously to the quasiparticle renormalization in a Fermi liquid. While the uncorrelated band insulator is characterized by a single gap for both charge and spin excitations, the spin gap is smaller than the charge gap in the correlated system.

Critical properties of the band-insulator-to-Mott-insulator transition in the strong-coupling limit of the ionic Hubbard model

We investigate the neutral-to-ionic insulator-insulator transition in one-dimensional materials by treating a strong-coupling effective model based on the ionic Hubbard model using the density-matrix renormalization group and finite-size scaling. The effective model, formulated in a spin-one representation, contains a single parameter. We carry out an extensive finite-size scaling analysis of the relevant gaps and susceptibilities to characterize the two zero-temperature transitions. We find that the transition from the ionic band-insulating phase to an intermediate spontaneously dimerized phase is Ising, and the transition from the dimerized phase to the Mott-insulating phase is Kosterlitz-Thouless, in agreement with the field-theory-based predictions.