Magnetic Field Induced Metal Insulator Research Articles

Quantum linear magnetoresistance and magnetic-field-induced metal–insulator transition in the Weyl semimetal candidate HgSe

In ultrapure single HgSe crystal, a large linear magnetoresistance with a record magnitude of 15 000% in mercury chalcogenides was discovered in a quantum limit at the magnetic field of 12 T in the temperature range of 20–40 K. The effect was described in terms of Abrikosov's theory of quantum linear magnetoresistance, thus providing its first experimental confirmation. In the temperature range of 0.4–4.2 K, a magnetic field-induced metal–insulator transition appeared in the ultrapure HgSe. Estimates show that the critical magnetic field of this transition of 6.8 T satisfies the criterion for stabilization of a condensed Wigner phase in the quantum limit.

Large magneto-thermoelectric effect on the verge of metal–insulator and topological transitionsin pyrochlore iridates

We investigate the Seebeck and Nernst effects of pyrochlore iridium oxides (Nd1−xPrx)2Ir2O7 across the thermally induced and magnetic field-induced metal–insulator and topological transitions. Nd2Ir2O7 exhibits the salient temperature dependence of the Seebeck coefficient accompanied by the enhancement of the Nernst effect in the vicinity of the thermal magnetic transitions. Moreover, the Seebeck coefficient shows a remarkable magnetic field hysteresis with the differential magnitude reaching as large as 110 μV/K, as the conductive magnetic domain walls are generated/annihilated by the external field. For x = 0.5, the Nernst signal increases rapidly across the field-induced metal–insulator transitions, exceeding the values reported in existing ferromagnetic oxides. These findings indicate that the thermoelectric effects increase significantly near the topological electronic phase transitions in strongly correlated systems, providing a new guideline for thermoelectric material design.

Grain-Size-Induced Collapse of Variable Range Hopping and Promotion of Ferromagnetism in Manganite La0.5Ca0.5MnO3

Among transition metal oxides, manganites have attracted significant attention because of colossal magnetoresistance (CMR)—a magnetic field-induced metal–insulator transition close to the Curie temperature. CMR is closely related to the ferromagnetic (FM) metallic phase which strongly competes with the antiferromagnetic (AFM) charge ordered (CO) phase, where conducting electrons localize and create a long range order giving rise to insulator-like behavior. One of the major open questions in manganites is the exact origin of this insulating behavior. Here we report a dc resistivity and magnetization study on manganite La1−xCaxMnO3 ceramic samples with different grain size, at the very boundary between CO/AFM insulating and FM metallic phases x=0.5. Clear signatures of variable range hopping (VRH) are discerned in resistivity, implying the disorder-induced (Anderson) localization of conducting electrons. A significant increase of disorder associated with the reduction in grain size, however, pushes the system in the opposite direction from the Anderson localization scenario, resulting in a drastic decrease of resistivity, collapse of the VRH, suppression of the CO/AFM phase and growth of an FM contribution. These contradictory results are interpreted within the standard core-shell model and recent theories of Anderson localization of interacting particles.

Fermi surface topology and large magnetoresistance in the topological semimetal candidate PrBi

We report a detailed magnetotransport study on single crystals of PrBi. The presence of $f$-electrons in this material raises the prospect of realizing a strongly correlated version of topological semimetals. PrBi shows a magnetic field induced metal insulator transition below $T \sim 20$ K and a very large magnetoresistance ($\approx 4.4 \times 10^4~$) at low temperatures ($T= 2$ K). We have also probed the Fermi surface topology by de Haas van Alphen (dHvA) and Shubnikov de Haas (SdH) quantum oscillation measurements complimented with density functional theory (DFT) calculations of the band structure and the Fermi surface. Angle dependence of the SdH oscillations have been carried out to probe the possible signature of surface Dirac fermions. We find three frequencies corresponding to one electron ($\alpha$) and two hole ($\beta$ and $\gamma$) pockets in experiments, consistent with DFT calculations. The angular dependence of these frequencies is not consistent with a two dimensional Fermi surface suggesting that the transport is dominated by bulk bands. Although the transport properties of this material originate from the bulk bands, the high mobility and small effective mass are comparable to other compounds in this series proposed as topologically nontrivial.

Magnetic field induced metal–insulator transition in single nodal ring topological semimetals

Our theoretical investigation indicates that an applied magnetic field can open a gap between the conduction and valence bands of a nodal line semimetal (NLSM), though it is a kind of gapless material in the absence of a magnetic field. The emerging bandgap depends sensitively on the strength and orientation of the magnetic field which implies a tunable and large anisotropy of magnetoresistance in such kinds of topological materials. Following such a theoretical finding, we predict that in some candidates of NSLMs with a single nodal ring, such as the materials of CaP3 family, a transition between metallic and insulating states driven by a magnetic field is possibly observed experimentally. Consequently, a magnetic field can be viewed as a novel mechanism for metal–insulator transition of solid materials, in additional to the well-known conventional ones such as the Anderson and Mott transitions.

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.

Picosecond dynamics of the spin-lattice relaxation inLa0.7Ca0.2MnO3:Magnetic-field dependence

We have employed ultrafast optical pump-probe measurements at 1.5 eV to study the picosecond dynamics of the magnetic-field-induced metal-insulator (MI) transition in ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ thin films in fields up to 7 T. We show that field induced modification of the photoinduced dynamic spectral weight transfer (DSWT) is related to changes in the magnetic entropy. The magnetic-field-induced increase of the ferromagnetic-paramagnetic transition temperature is evident in the spin-lattice relaxation time and amplitude scaling of the DSWT.

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.

Magneto-quantum oscillations in the Hall constant of three-dimensional metallic semiconductors

Magneto-quantum oscillations of the Hall constant have been observed in metallically doped three-dimensional semiconductors in a magnetic field far below the magnetic-field-induced metal-insulator (MI) transition. Especially around the magnetic field where all the electrons enter the lowest Landau level, a strong increase of the Hall constant is observed. This phenomenon is explained in terms of a field-induced MI transition in the tails of the spin-split Landau levels which can be understood in analogy to the well known MI transition in metallic semiconductors.

Magnetic-field-induced localization in narrow-gap semiconductors Hg1-xCdxTe and InSb.

Magnetotransport measurements on n-type ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te (x\ensuremath{\simeq}0.2) and InSb and far-infrared magnetooptical spectroscopy in ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te are reported. The transport data at magnetic fields below and near the magnetic-field-induced metal-insulator (M-I) transition indicate clear similarity of ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te and InSb. At fields well above the M-I transition and at low temperatures, the magnetotransport coefficients for ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te show anomalously weak dependences on field and temperature. This is attributed to shorting of the bulk by a conducting surface layer. Below the M-I transition field, an anomalous Hall effect is observed in both ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te and InSb. We interpret this effect within a model in which the M-I transition takes place in the donor impurity band. The impurity cyclotron resonance observed in ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te provides conclusive evidence for donor-bound electrons in this semiconductor and further confirms its similarity to InSb. The cyclotron-resonance data are in agreement with theoretical predictions for hydrogenic donors in a strong magnetic field. These observations provide strong evidence against the Wigner crystallization of electrons in ${\mathrm{Hg}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Cd}}_{\mathrm{x}}$Te.