Apparent Metal-insulator Transition Research Articles

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.

A novel conductivity mechanism of highly disordered carbon systems based on an investigation of graph zeta function

In the previous report (Matsutani and Suzuki, 2000 [21]), by proposing the mechanism under which electric conductivity is caused by the activational hopping conduction with the Wigner surmise of the level statistics, the temperature-dependent of electronic conductivity of a highly disordered carbon system was evaluated including apparent metal–insulator transition. Since the system consists of small pieces of graphite, it was assumed that the reason why the level statistics appears is due to the behavior of the quantum chaos in each granular graphite. In this article, we revise the assumption and show another origin of the Wigner surmise, which is more natural for the carbon system based on a recent investigation of graph zeta function in graph theory. Our method can be applied to the statistical treatment of the electronic properties of the randomized molecular system in general.

Possible Magnetic Background of the Metal-Insulator Transition in Two-Dimensional Correlated Electron System

Whether the apparent metal-insulator transition in two-dimensional (2D) correlated electron system is a true quantum phase transition or is a crossover phenomena—this question is in the core of ongoing debates. I present here a novel scenario of this phenomenon, based on experimental finding of the two-phase state in the correlated 2D system. The transport features in the suggested picture are the finite temperature phenomena and a consequence of the magnetic phase transition; the latter manifests in the sign change of the spin magnetization-per-electron. Physically, the magnetic transition means changing the tendency of the two-phase system to either paramagnetic Fermi liquid state (high density), or to the disordered ferromagnet (low density).

An apparent metal-insulator transition in high-mobility two-dimensional InAs heterostructures

We report on the first experimental observation of an apparent metal insulator transition in a 2D electron gas confined in an InAs quantum well. At high densities we find that the carrier mobility is limited by background charged impurities and the temperature dependence of the resistivity shows a metallic behavior with resistivity increasing with increasing temperature. At low densities we find an insulating behavior below a critical density of $n_{c} = 5 \times 10^{10}$ cm$^{-2}$ with the resistivity decreasing with increasing temperature. We analyze this transition using a percolation model arising from the failure of screening in random background charged impurities. We also examine the percolation transition experimentally by introducing remote ionized impurities at the surface. Using a bias during cool-down, we modify the screening charge at the surface which strongly affects the critical density. Our study shows that transition from a metallic to an insulating phase in our system is due to percolation transition.

Quantum-Classical Crossover and Apparent Metal-Insulator Transition in a Weakly Interacting 2D Fermi Liquid

We report the observation of an apparent parallel magnetic-field-induced metal-insulator transition in a high-mobility two-dimensional electron gas for which spin and localization physics most likely play no major role. The high-mobility metallic phase at low field is consistent with the established Fermi liquid transport theory including phonon scattering, whereas the phase at higher field shows a large insulatinglike negative temperature dependence at resistances much smaller than the quantum of resistance h/e(2). We argue that this observation is a direct manifestation of a quantum-classical crossover arising predominantly from the magneto-orbital coupling between the finite width of the two-dimensional electron gas and the in-plane magnetic field.

Indications for a line of continuous phase transitions at finite temperatures connected with the apparent metal–insulator transition in 2d disordered systems

In a recent experiment, Lai et al. [Phys. Rev. B 75 (2007) 033314] studied the apparent metal–insulator transition (MIT) of a Si quantum well structure. Tuning the charge carrier concentration n, they measured the conductivity σ ( T , n ) for a very dense set of n values. They observed linear T dependences of σ around the Fermi temperature and found that the corresponding T → 0 extrapolation σ 0 ( n ) exhibits a sharp bend just at the MIT. Reconsidering the data by Lai et al., it is shown here that this sharp bend is related to a peculiarity of σ ( T = const . , n ) , which is clearly detectable in the whole T range up to 4 K, the highest measuring temperature in that work. It may indicate a sharp continuous phase transition between the regions of apparent metallic and activated conduction to be present at finite temperature. This interpretation is confirmed by a scaling analysis without fit, which illuminates similarities to previous experiments and provides understanding of the shape of the peculiarity and of sharp peaks found in d log 10 σ / d n ( n ) . Simultaneously, the scaling analysis uncovers a strange feature of the apparent metallic state.

Indications for sharp continuous phase transitions at finite temperatures connected with the apparent metal-insulator transition in two-dimensional disordered systems

In a recent experiment, Lai et al. [Phys. Rev. B 75, 033314 (2007)] studied the apparent metal-insulator transition (MIT) of a Si quantum-well structure tuning the charge carrier concentration $n$. They observed linear temperature dependences of the conductivity $\ensuremath{\sigma}(T,n)$ around the Fermi temperature and found that the corresponding $T\ensuremath{\rightarrow}0$ extrapolation ${\ensuremath{\sigma}}_{0}(n)$ exhibits a sharp bend just at the MIT. Here, reconsidering the data published by Lai et al., it is shown that this sharp bend is related to a peculiarity of $\ensuremath{\sigma}(T=\text{const},n)$ clearly detectable in the whole $T$ range up to 4 K, which is the highest measuring temperature in that work. Since this peculiarity seems not to be smoothed out with increasing $T$, it may indicate a sharp continuous phase transition between the regions of apparent metallic and activated conduction to be present at finite temperature. Hints from the literature of such a behavior are discussed. Finally, a scaling analysis illuminates similarities to previous experiments and provides understanding of the shape of the peculiarity and of sharp peaks found in $\text{d}\text{ }{\text{log}}_{10}\text{ }\ensuremath{\sigma}/\text{d}n(n)$.

In-plane magnetic-field-induced metal-insulator transition in(311)AGaAstwo-dimensional hole systems probed by thermopower

We report thermopower measurements in dilute (311)A GaAs two-dimensional holes subjected to an in-plane magnetic field B that drives the system through a metal-insulator transition (MIT). The diffusion thermopower S-d decreases linearly with temperature at low B for both low-mobility [01 (1) over bar] and high-mobility [(2) over bar 33] directions, as expected for metallic systems. At high B, in the insulating phase, S-d changes sign along [01 (1) over bar], while S-d drops to zero along [(2) over bar 33]. This behavior suggests that the system does not undergo any ground-state modification but, rather, that the apparent MIT transition is accompanied by a dramatic change in the dominant scattering mechanisms.

Intermediate phase at the metal-insulator boundary in a magnetically doped two-dimensional electron system

A magnetotransport study in magnetically doped (Cd,Mn)Te 2D quantum wells reveals an apparent metal-insulator transition as well as an anomalous intermediate phase just on its metallic side. This phase is characterized by colossal magnetoresistance-like phenomena, which are assigned to the phase separation of the electron fluid and the associated emergence of ferromagnetic bubbles.

Scaling analysis of an apparent metal–insulator transition in a Fe 3O 4/Nb:SrTiO 3 bilayer

Magnetite films were deposited on Nb-doped SrTiO 3 substrates by pulsed laser deposition. The current–voltage characteristics were measured perpendicular to the interface. Strongly nonlinear, asymmetric current–voltage curves were observed. The resistance shows a crossover from insulating to metallic behavior for increasing voltages. The data can be neatly scaled using a scaling approach based on a quantum critical transition theory. This shows that a scaling analysis is clearly insufficient to conclude on the existence of a bulk-like metal/superconductor–insulator transition.

Evidence of spatial inhomogeneity near the onset of magnetically induced insulating state in superconducting thin films

Non-monotonic differential resistance (dV/dI) is observed in magnetically induced insulating films which exhibit apparent superconductor-metal-insulator transitions in the low temperature limit; at low bias currents the nonlinear transport is insulator-like while at high bias currents it is characteristic of metallic phase. The non-monotonic dV/dI may be evidence that the insulating state consists of metallic domains connected by point contacts (insulating gaps), implying that spatial inhomogeneities play a dominant role in determining the nature of the apparent metal-insulator transition.

Thermodynamic Density of States of Two-Dimensional GaAs Systems near the Apparent Metal-Insulator Transition

We perform combined resistivity and compressibility studies of two-dimensional hole and electron systems which show the apparent metal-insulator transition--a crossover in the sign of deltaR/deltaT with changing density. No thermodynamic anomalies have been detected in the crossover region. Instead, despite a tenfold difference in r(s), the compressibility of both electrons and holes is well described by the theory of nonlinear screening of the random potential. We show that the resistivity exhibits a scaling behavior near the percolation threshold found from analysis of the compressibility. Notably, the percolation transition occurs at a much lower density than the crossover.

Interaction effects on magneto-oscillations in a two-dimensional electron gas

Motivated by recent experiments, we study the interaction corrections to the damping of magnetooscillations in a two-dimensional electron gas (2DEG). We identify leading contributions to the interaction-induced damping which are induced by corrections to the effective mass and quantum scattering time. The damping factor is calculated for Coulomb and short-range interaction in the whole range of temperatures, from the ballistic to the diffusive regime. It is shown that the dominant effect is that of the renormalization of the effective electron mass due to the interplay of the interaction and impurity scattering. The results are relevant to the analysis of experiments on magnetooscillations (in particular, for extracting the value of the effective mass) and are expected to be useful for understanding the physics of a high-mobility 2DEG near the apparent metal-insulator transition.

Quenched disorder effects in electron transport in Si inversion layers in the dilute regime

In order to reveal the effects of disorder in the vicinity of the apparent metal-insulator transition in 2D, we studied electron transport in the same Si device after cooling it down to 4 K at different fixed values of the gate voltage Vcool. Different Vcool did not significantly modify either the momentum relaxation rate or the strength of electron-electron interactions. However, temperature dependences of the resistance and the magnetoresistance in parallel magnetic fields in the vicinity of the 2D metal-insulator transition carry a strong imprint of the quenched disorder determined by Vcool. This demonstrates that the observed transition between the metallic and insulating regimes, besides the universal effects of electron-electron interaction, depends on the sample-specific localized states (disorder). We report on evidence for a weak exchange of electrons between the reservoirs of extended and resonant localized states that occur at low densities. The strong cool-down dependent variations of ρ(T), we believe, are evidence for a developing spatially inhomogeneous state in the critical regime.

Two-dimensional metal-insulator transition and in-plane magnetoresistance in a high-mobility strained Si quantum well

The apparent metal-insulator transition is observed in a high quality two-dimensional electron system (2DES) in the strained Si quantum well of a Si/Si_{1-x}Ge_x heterostructure with mobility \mu=1.9 x 10^5 cm^2/Vs at density n=1.45 x 10^{11} cm^{-2}. The critical density, at which the thermal coefficient of low T resistivity changes sign, is ~ 0.32 x 10^{11} cm^{-2}, so far the lowest observed in the Si 2D systems. In-plane magnetoresistance study was carried out in the higher density range where the 2DES shows the metallic-like behavior. It is observed that the in-plane magnetoresistance first increases as ~ B_{ip}^2 and then saturates to a finite value \rho(B_c) for B_{ip} > B_c. The full spin-polarization field B_c decreases monotonically with n but appears to saturate to a finite value as n approaches zero. Furthermore, \rho(B_c)/\rho(0) ~ 1.8 for all the densities ranging from 0.35 x 10^{11} to 1.45 x 10^{11} cm^{-2} and, when plotted versus B_{ip}/B_c, collapses onto a single curve.

Coulomb drag near the metal-insulator transition in two dimensions

We studied the drag resistivity between dilute two-dimensional hole systems, near the apparent metal-insulator transition. We find the deviations from the ${T}^{2}$ dependence of the drag to be independent of layer spacing and correlated with the metalliclike behavior in the single-layer resistivity, suggesting they both arise from the same origin. In addition, layer-spacing dependence measurements suggest that while the screening properties of the system remain relatively independent of temperature, they weaken significantly as the carrier density is reduced. Finally, we demonstrate that the drag itself significantly enhances the metallic $T$ dependence in the single-layer resistivity.

Spin Polarization Dependence of the Coulomb Drag at Largers

We find that the temperature dependence of the drag resistivity between two dilute two-dimensional hole systems exhibits an unusual dependence upon spin polarization. Our main observation is that near the apparent metal-insulator transition, the temperature dependence of the drag, given by Talpha, weakens significantly with the application of a parallel magnetic field (B(||)), with alpha saturating at half its zero field value for B(||)>B(*), where B(*) is the polarization field.

Unexpected negative nonmonotonic magnetoresistance of the two-dimensional electrons in Si in a parallel magnetic field

We report observation of the unexpected negative and nonmonotonic magnetoresistance of 2D electrons in Si-MOSFET subjected to a varying in-plane magnetic field superimposed on a constant perpendicular field component. We show that this nonmonotonic magnetoresistance is irrelevant to the energy spectrum of mobile 2D electrons. We also observed variations of the density of mobile electrons with the in-plane field. We argue that both variations of the negative magnetoresistance and of the density of mobile electrons originate from the band of localized states. The latter coexist and interact with mobile electrons even at relatively high density, a factor of 1.5 higher than the critical density of the apparent metal-insulator transition.

Masking by weak localization of metallic behavior in a two-dimensional electron system in strong parallel magnetic fields

The rise and saturation, in a parallel magnetic field ${B}_{\ensuremath{\Vert}},$ of the resistivity of two-dimensional electron systems on the metallic side of the apparent metal-insulator transition (MIT) can be interpreted as the destruction of a metallic state. It is also compatible, however, with the reduction of screening of charged impurities, resulting from the breaking of the spin degeneracy, in a traditional Fermi liquid. We demonstrate, using a Si-MOSFET, that electrons in a strong magnetic field ${B}_{\ensuremath{\Vert}}$ parallel to the Si-${\mathrm{SiO}}_{2}$ interface may exhibit a metalliclike behavior, analogous to the effect observed at ${B}_{\ensuremath{\Vert}}=0,$ provided that weak-localization corrections are suppressed. Conventionally, this suppression is achieved by applying a perpendicular magnetic field, but it appears that this also happens when a strong electric field is applied between the source and drain. Both methods are used in this paper. At ${B}_{\ensuremath{\Vert}}=0,$ weak-localization corrections are also visible but the metalliclike contribution to the resistivity is greater. These results suggest that spin polarization may simply reduce the strength of mechanisms, such as screening, which contribute a metalliclike temperature dependence to the resistivity, relative to weak-localization corrections. The polarized system undergoes a transition from weak to strong localization at a lower critical resistivity and at a larger density than the unpolarized system.

Observation of the apparent metal–insulator transition of high-mobility two-dimensional electron system in a Si/Si1−xGex heterostructure

Field-effect transistors are fabricated from the Si/Si1−xGex heterostructures. The density of the two-dimensional electron system (2DES) in the strained Si quantum well can be controllably tuned from 2.13×1011 to 4.24×1011 cm−2. The temperature dependence of the resistivity of the 2DES was measured and the apparent metal–insulator transition was observed. Its main features are similar to those reported in other semiconductor-based two-dimensional systems.