Field Induced Metal Insulator Research Articles

Dielectric permittivity and conductivity spectra of La 0.5Ca 0.5MnO 3 thin films presenting electric field induced metal–insulator transition

Electric behavior of polycrystalline La 0.5Ca 0.5MnO 3 thin films, pulsed laser deposited on a (100) silicon substrate, is reported and discussed. An electrically induced metal-insulating transition around 150 K is found, which is voltage and thickness dependent. At low temperatures, the film conductivity is non-Ohmic and moderate electric fields lead to resistivity switching towards metaestable low-resistive states. Impedance spectroscopy measurements were also performed in order to determine the film dielectric permittivity and conductivity and to estimate the characteristic metal–semiconductor interface parameters. The obtained results show that the fraction of ferromagnetic metallic regions does not change when a voltage is applied, and that the mechanism responsible for the low temperature metal insulator transition and the conduction behavior is the appearance of connective paths due to the enhanced mobility of carriers activated by the electric field.

Magnetic and electric transport properties of polycrystalline manganite Pr 0.6Na 0.4MnO 3

The magnetic and electric transport properties of the polycrystalline manganite Pr 0.6Na 0.4MnO 3 were reported. In zero field, the sample shows an insulator behavior. Under applied fields higher than 10 kOe, a field-induced insulator–metal transition occurs, and at the same time the colossal magnetoresistance (CMR) effect is also observed. At the temperature of 5 K, the metamagnetic transition from charge ordering/antiferromagnetic (CO/AFM) to ferromagnetic (FM) state is observed. With increasing temperature, the metastable FM phases can transform back to the CO state, accompanied by a high field hysteresis. The CMR effect of the sample can be understood by a percolative regime and is related to the CO–FM transition under a large magnetic field.

Zener tunneling in one-dimensional organic semiconductors

Tunneling effect in one-dimensional organic semiconductors in the presence of an external electric field is studied within the framework of a tight-binding model and a nonadiabatic dynamical method. It is found that under a high electric field, electrons can transit from the valence band (VB) to the conduction band (CB), which is demonstrated to be Zener tunneling in organic semiconductors. The results also indicate a field-induced insulator–metal transition accompanied by the vanishing of the energy gap. It is found that, after the field is turned off, the Peierls phase cannot be recovered.

Electrical transport and magnetic properties of Nd1−xNaxMnO3 manganites

Abstract The magnetic and electrical properties of Nd 1− x Na x MnO 3 ( x = 0, 0.1, 0.167, 0.25) are studied. A negative net magnetization is observed in the sample with x = 0. The other three samples show a ferromagnetic (FM)-insulator behavior, and intrinsic inhomogeneous magnetic characteristics are observed. Under a field of 130 kOe, the sample with x = 0.1 exhibits a sizable magnetoresistance (MR). For the samples with x = 0.167 and 0.25, there are charge ordering (CO) transitions near 250 K. The colossal magnetoresistance (CMR) effect can be understood under a percolative regime and is related to the CO-FM transition in a larger magnetic field. When the applied field is more than 30 kOe, a field-induced insulator–metal and a following metal–insulator transition at a lower temperature are observed, accompanied a thermal hysteresis.

Correlation between degree of crystallinity and transition field in electric or magnetic field-induced insulator–metal transition of Pr 0.5Ca 0.5MnO 3 thin films

Degree of crystallinity (DOC) effect onto electric and magnetic field-induced insulator–metal (IM) transition of Pr 0.5Ca 0.5MnO 3 (PCMO) thin films was studied. The films with various DOC were prepared by a pulsed laser deposition method, varying the substrate materials (single crystalline MgO or amorphous SiO 2) and the deposition temperature (600 or 750 ∘C). Planer-type junctions were fabricated with these films for the observation of IM transition. In electric field-induced IM transition, there was a tendency that the films with lower DOC show lower transition field. In the magnetic field, on the other hand, the film of higher DOC showed an IM transition under lower field, in contrast to the case of the electric field.

X-ray Diffraction Study of Electric Field-Induced Metal–Insulator Transition of Vanadium Dioxide Film on Sapphire Substrate

An X-ray diffraction study of the electric field-induced metal–insulator transition (MIT) of a vanadium dioxide (VO2) film on a sapphire (001) substrate, which exhibited MIT at a temperature of approximately around 68 °C with a three-order change in resistivity, was performed. A simple device fabricated using a VO2 film having aluminum electrodes as ohmic contacts with a gap separation of 1.0 mm showed current jump at an applied voltage of 48 V. The X-ray diffraction of an in situ electrically biased VO2 film in transition state showed a crystalline structure change from monoclinic VO2 to tetragonal VO2 when compliance current was higher than a certain value. We found that this macroscopic crystalline structure change is not induced by electric field in the case of the lowest compliance current, indicating the significance of the current for fast and durable operation of VO2-based devices.

Magnetic field induced metal–insulator transition in semiconductors

Metal–insulator transition in doped semiconductors is investigated in the presence of intense magnetic fields. Using Thomas–Fermi screening function and a screened coulomb potential in the Hamiltonian, a two-parameter varitional calculation leads to metallization. The correlation effect among the electrons is considered through an effectivemass that depends on the spatial seperation between impurities. Results are provided for a many valley semiconductor (Si) and two single valley semiconductors (GaAs and CdTe). Our results show that in a magnetic field the critical concentration at which metallization occurs increases. The correlation effects bring out one order change in the critical concentration values, in intense magnetic fields.

Magnetoresistance and magnetic field induced metal–insulator transition in intercalated amorphous carbon

In this work, we discuss the preparation of iodine and sulfur incorporated amorphous carbon (a-C) films by vapor phase pyrolysis technique. Iodine incorporated a-C films depict an unusual resistivity behavior at low temperatures, where a clear positive temperature coefficient (PTCR) is witnessed, which also depicts a pyrolysis temperature and iodine incorporation induced metal–insulator (M–I) transition. Magnetic field induced M–I transition is also observed in iodinated samples. Magnetoresistance measurements performed on these films from 1.3–10 K in the presence of magnetic field (0–7 T) account for a positive magnetoresistance irrespective of the regimes they fall in. Universal scaling of magnetoconductance observed in intercalated a-C system depicts the dominance of electron–electron interaction.

Magnetic field induced metal–insulator transitions in p-SiGe

Low density modulation doped p-SiGe, where the holes lie in a strained SiGe quantum well, frequently exhibits anomalous insulating behaviour between filling factors ν=2 and 1. There is also anomalous metallic behavior with a metal–insulator transition between the two. It is shown that in these samples exchange effects are sufficiently large to induce the paramagnetic–ferromagnetic phase transition predicted by Giuliani and Quinn in 1985, also that the metallic and insulating behavior is associated with the coincidence of two Landau levels of opposite spin. A model calculation shows that while a ferromagnetic polarization may occur at integer filling factors screening suppresses it for non-integer filling factors. It is argued the Landau levels then stick-together and allow a spin-density instability to form. Because of the strong spin–orbit coupling in p-SiGe the transport properties of this state differ from those of other systems where a similar quantum Hall ferromagnet probably forms.

Metastable states in phase separated electron doped Sm 0.15Ca 0.85MnO 3

In order to study the mechanism behind the phase separation scenario in the Sm 0.15Ca 0.85MnO 3 compound, magnetization and resistivity measurements have been carried out in pulsed magnetic fields up to 50 T at temperatures 4.2 K< T<200 K. It is found that external magnetic field causes a collapse of a C-type AFM ( P2 1/ m) phase resulting in field-induced insulator–metal transition, which is irreversible below T 1=75 K. In zero field the content of a G-type phase in the mixed C–G state can vary from 10 to 17% at T=10 K. A set of metastable states with different volume ratios of G-type to C-type phases is observed below T 1 depending on the history of the sample. The obtained results indicate that the phase separation plays a dominant role for the electric and the magnetic properties of this material.

Magnetic and electronic transport properties of YbxCa1−xMnO3 compounds

The polycrystalline YbxCa1−xMnO3 (x=0.1,0.2) were studied by x-ray powder diffraction, magnetic, and electrical resistivity measurements. The YbxCa1−xMnO3 crystallizes in an orthogonally distorted perovskite structure, and shows the ferromagnetic ordering with TC more than 110 K. However, the field dependence of magnetization and electrical resistivity exhibits very complicated behavior. A magnetic field induced insulator–metal transition has been found in the Yb0.2Ca0.8MnO3 compound. In addition, the large asymmetry in magnetization and resistivity hysteresis loops has been observed in this compound at 10 K, which might be due to the charge ordering and magnetocrystalline anisotropy.

Conduction electron spin resonance evidence for internal field in graphite

We report conduction electron spin resonance measurements performed on highly oriented pyrolitic graphite samples between 10 and 300 K using S ( ν=4 GHz), X ( ν=9.4 GHz), and Q ( ν=34.4 GHz) microwave bands for the external DC-magnetic field applied parallel ( H∥ c) and perpendicular ( H⊥ c) to the sample hexagonal c-axis. The results obtained in the H∥ c geometry are interpreted in terms of the presence of an effective internal ferromagnetic-like field, H eff int( T, H), that increases as the temperature decreases and the applied DC-magnetic field increases. We associate the occurrence of the H eff int( T, H) with the field-induced metal–insulator transition in graphite and discuss its origin in the light of relevant theoretical models.

Evidence for internal field in graphite: a conduction electron spin-resonance study

We report on conduction electron spin-resonance measurements performed on highly oriented pyrolitic graphite samples between 10 and 300 K using S(ν=4 GHz), X(ν=9.4 GHz), and Q(ν=34.4 GHz) microwave bands for the external dc-magnetic field applied parallel ( H∥ c) and perpendicular ( H⊥ c) to the sample hexagonal c-axis. The results obtained in the H∥ c geometry are interpreted in terms of the presence of an effective internal ferromagnetic-like field, H int eff( T, H), that increases as the temperature decreases and the applied dc-magnetic field increases. We associate the occurrence of the H int eff( T, H) with the field-induced metal–insulator transition in graphite and discuss its origin in the light of relevant theoretical models.

Field-induced metal-insulator transition in a two-dimensional organic superconductor.

The quasi-two-dimensional organic superconductor beta"-(BEDT-TTF)2SF5CH2CF2SO3 (Tc approximately 4.4 K) shows very strong Shubnikov-de Haas (SdH) oscillations which are superimposed on a highly anomalous steady background magnetoresistance, Rb. Comparison with de Haas-van Alphen oscillations allows a reliable estimate of Rb which is crucial for the correct extraction of the SdH signal. At low temperatures and high magnetic fields insulating behavior evolves. The magnetoresistance data violate Kohler's rule, i.e., cannot be described within the framework of semiclassical transport theory, but converge onto a universal curve appropriate for dynamical scaling at a metal-insulator transition.

Localisation and interaction effects in metallic n-type InP

We present results on the magnetic field induced metal–insulator transition (MIT) in compensated InP. The experiments were carried out at low temperatures in the range 4.2–0.066 K and in magnetic fields up to 11 T. Not too close to the MIT the electrical conductivity σ was found to obey σ= σ( T=0)+ mT 1/2 where both the inelastic diffusion length L I∼( σ( T=0)) 3/2/ T 1/2 and the interaction length L T∼( σ( T=0)) 1/2/ T 1/2 contribute to a T 1/2 term in σ. It is convenient to develop a criterion to distinguish their contribution. It is based on the relation between the magnitude m and the zero temperature conductivity σ( T=0).

A novel transport model for the metal-to-insulator transition in quasi-three-dimensional wide quantum wells

We present a systematic study of the magnetic field induced metal–insulator transition in quasi-three-dimensional wide quantum wells by numerical calculations. Depending on carrier density and carrier mobility we observe both a change from metal to insulator-like temperature dependence as well as a magnetic field dependent transition between both regimes.

Magnetic field induced metal-to-insulator transition in wide quantum wells

We present a systematic experimental study of the magnetic field induced metal–insulator transition in high mobility quasi-3D wide quantum wells. Depending on carrier density and carrier mobility we observe both a metal-like and insulator-like temperature dependence as well as a magnetic field induced transition between both regimes.

Magnetic field–induced metal–insulator transitions in graphite and diluted magnetic semiconductors

Two topical subjects related with the effect of magnetic field on electrical conduction and the metal–insulator transition are discussed. The first topic is an electronic phase transition in graphite, which is interpreted as a manifestation of a nesting–type instability inherent to a one–dimensional narrow Landau sub–band. The second topic is spin–dependent tranport in III–V based diluted magnetic semiconductors; in particular, a large negative magnetoresistance observed in the vicinity of metal–non–metal transition.

Heat diffusivity of La1−xCaxMnO3 epitaxial layers

The Seebeck effect and the thermal diffusivity of La0.67Ca0.33MnO3−δ were investigated in the temperature range 35&amp;lt;T&amp;lt;300 K and in particular in the regime of the high magnetoresistance and maximum resistivity ρmax. We find a sign change of the thermoelectric power S(T) and a step in heat conductivity near ρmax which we attribute to a temperature-induced metal–insulator transition at about 80 K, suggesting that the large giant magnetoresistance found in these substances is due to a field-induced metal–insulator transition.

Origins of colossal magnetoresistance in perovskite-type manganese oxides (invited)

Phenomena of colossal magnetoresistance (MR) or magnetic field induced insulator–metal (I–M) transitions have been investigated for single crystals of perovskite-type manganese oxides with controlled carrier density and one-electron bandwidth. In addition to the canonical MR behavior near the Curie temperature, the first-order phase transition accompanying several orders of magnitude change in resistivity has been observed under an external magnetic field for many of the composition-controlled crystals as an intrinsic bulk phenomenon. It was proved by the systematic experimental investigations that the field-induced destruction of the charge-ordered state accompanying the lattice structural as well as metamagnetic transition is a major origin of such a colossal MR. Versatile MR phenomena and I–M phase diagrams in the T–H plane are presented with their interpretation.