Semiconducting field-effect transistors are the workhorses of the modern electronics era. Recently, application of the field-effect approach to compounds other than semiconductors has created opportunities to electrostatically modulate types of correlated electron behaviour—including high-temperature superconductivity and colossal magnetoresistance—and potentially tune the phase transitions in such systems. Here we provide an overview of the achievements in this field and discuss the opportunities brought by the field-effect approach.

Electric field effect in correlated oxide systems

The electrical and optical measurements, in combination with density functional theory calculations, show distinct layer-dependent semiconductor-to-semimetal evolution of 2D layered PtSe2 . The high room-temperature electron mobility and near-infrared photo-response, together with much better air-stability, make PtSe2 a versatile electronic 2D layered material.

High‐Electron‐Mobility and Air‐Stable 2D Layered PtSe2 FETs

In recent years, quantum phase transitions have attracted the interest of both theorists and experimentalists in condensed matter physics. These transitions, which are accessed at zero temperature by variation of a non-thermal control parameter, can influence the behaviour of electronic systems over a wide range of the phase diagram. Quantum phase transitions occur as a result of competing ground state phases. The cuprate superconductors which can be tuned from a Mott insulating to a d-wave superconducting phase by carrier doping are a paradigmatic example. This review introduces important concepts of phase transitions and discusses the interplay of quantum and classical fluctuations near criticality. The main part of the article is devoted to bulk quantum phase transitions in condensed matter systems. Several classes of transitions will be briefly reviewed, pointing out, e.g., conceptual differences between ordering transitions in metallic and insulating systems. An interesting separate class of transitions is boundary phase transitions where only degrees of freedom of a subsystem become critical; this will be illustrated in a few examples. The article is aimed at bridging the gap between high-level theoretical presentations and research papers specialized in certain classes of materials. It will give an overview on a variety of different quantum transitions, critically discuss open theoretical questions, and frequently make contact with recent experiments in condensed matter physics.

https://iopscience.iop.org/article/10.1088/0034-4885/66/12/R01/pdf

Quantum phase transitions

The interplay between strong Coulomb interactions and randomness has been a long-standing problem in condensed matter physics. According to the scaling theory of localization in two-dimensional systems of non-interacting or weakly interacting electrons, the ever-present randomness causes the resistance to rise as the temperature is decreased, leading to an insulating ground state. However, new evidence has emerged within the past decade indicating a transition from the insulating to metallic phase in two-dimensional systems of strongly interacting electrons. We review earlier experiments that demonstrate the unexpected presence of a metallic phase in two dimensions, and present an overview of recent experiments with emphasis on the anomalous magnetic properties that have been observed in the vicinity of the transition.

https://iopscience.iop.org/article/10.1088/0034-4885/67/1/R01/pdf

Metal insulator transition in two-dimensional electron systems

The interplay between strong Coulomb interactions and randomness has been a long-standing problem in condensed matter physics. According to the scaling theory of localization, in two-dimensional systems of noninteracting or weakly interacting electrons, the ever-present randomness causes the resistance to rise as the temperature is decreased, leading to an insulating ground state. However, new evidence has emerged within the past decade indicating a transition from insulating to metallic phase in two-dimensional systems of strongly interacting electrons. We review earlier experiments that demonstrate the unexpected presence of a metallic phase in two dimensions, and present an overview of recent experiments with emphasis on the anomalous magnetic properties that have been observed in the vicinity of the transition.

https://arxiv.org/pdf/cond-mat/0309140.pdf

Metal-insulator transition in two-dimensional electron systems

We have studied the temperature dependence of resistivity, \ensuremath{\rho}, for a two-dimensional electron system in silicon at low electron densities ${\mathit{n}}_{\mathit{s}}$\ensuremath{\sim}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, near the metal-insulator transition The resistivity was empirically found to scale with a single parameter ${\mathit{T}}_{0}$, which approaches zero at some critical electron density ${\mathit{n}}_{\mathit{c}}$ and increases as a power ${\mathit{T}}_{0}$\ensuremath{\propto}\ensuremath{\Vert}${\mathit{n}}_{\mathit{s}}$-${\mathit{n}}_{\mathit{c}}$${\mathrm{\ensuremath{\Vert}}}^{\mathrm{\ensuremath{\beta}}}$ with \ensuremath{\beta}=16\ifmmode\pm\else\textpm\fi{}01 both in metallic (${\mathit{n}}_{\mathit{s}}$g${\mathit{n}}_{\mathit{c}}$) and insulating (${\mathit{n}}_{\mathit{s}}$${\mathit{n}}_{\mathit{c}}$) regions This dependence was found to be sample independent We have also studied the diagonal resistivity at Landau-level filling factor \ensuremath{\nu}=3/2, where the system is known to be in a true metallic state at high magnetic field and in an insulating state at low magnetic field The temperature dependencies of resistivity at B=0 and \ensuremath{\nu}=3/2 were found to be identical These behaviors suggest a true metal-insulator transition in the two-dimensional electron system in silicon at B=0, in contrast with the well-known scaling theory

Scaling of an anomalous metal-insulator transition in a two-dimensional system in silicon at B =0

The nonlinear (electric field-dependent) resistivity of the 2D electron system in silicon exhibits scaling as a function of electric field and electron density in both the metallic and insulating phases, providing further evidence for a true metal-insulator transition in this 2D system at $B\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$. Comparison with the temperature scaling yields separate determinations of the correlation length exponent, $\ensuremath{\nu}\ensuremath{\approx}1.5$, and the dynamical exponent, $z\ensuremath{\approx}0.8$, close to the theoretical value $z\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$.

https://arxiv.org/pdf/cond-mat/9608101.pdf

Electric Field Scaling at a B=0 Metal-Insulator Transition in Two Dimensions.

We study the behavior of the extended states of a two-dimensional electron system in silicon in a magnetic field, $B$. Our results show that the extended states, corresponding to the centers of different Landau levels, merge with the lowest extended state as $B\ensuremath{\rightarrow}0$. Using our data, we construct an experimental-based ``disorder vs filling factor'' phase diagram for the integer quantum Hall effect (QHE). Generalizing this diagram to the case of the fractional QHE, we show that it is consistent with the recently observed direct transitions between insulator and fractional QHE at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2/5$, 2/7, and 2/9.

https://arxiv.org/pdf/cond-mat/9506046.pdf

Global Phase Diagram for the Quantum Hall Effect: An Experimental Picture

We have studied the resistivity of a two-dimensional electron system in silicon in the temperature range 200 mK T7.5 K at zero magnetic field at low electron densities, when the electron system is in the insulating regime. Our results show that at an intermediate temperature range, \ensuremath{\rho}=${\mathrm{\ensuremath{\rho}}}_{0}$exp[(${\mathit{T}}_{0}$/T${)}^{1/2}$] for at least four orders of magnitude up to 3\ifmmode\times\else\texttimes\fi{}${10}^{9}$ \ensuremath{\Omega}. This behavior is consistent with the existence of a Coulomb gap. Near the metal/insulator transition, the prefactor was found to be ${\mathrm{\ensuremath{\rho}}}_{0}$\ensuremath{\approxeq}h/${\mathit{e}}^{2}$, and resistivity scales with temperature. For very low electron densities ${\mathit{n}}_{\mathit{s}}$, the prefactor diminishes with diminishing ${\mathit{n}}_{\mathit{s}}$. A comparison with the theory shows that a specific set of conditions is necessary to observe the behavior of resistivity consistent with the existence of the Coulolmb gap.

https://arxiv.org/pdf/cond-mat/9506006.pdf

Whitney Mason

Papers

Scaling of an anomalous metal-insulator transition in a two-dimensional system in silicon at B =0

Electric Field Scaling at a B=0 Metal-Insulator Transition in Two Dimensions.

Global Phase Diagram for the Quantum Hall Effect: An Experimental Picture

Experimental evidence for a Coulomb gap in two dimensions

Experimental evidence of the Coulomb gap in a high-mobility 2D electron system in silicon