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D. Shahar

Publications and source records attributed to D. Shahar.

At least 37 records · Page 2Linked to original sources
arXiv2004

Superconductivity-Related Insulating Behavior

We present the results of an experimental study of superconducting, disordered, thin-films of amorphous Indium Oxide. These films can be driven from the superconducting phase to a reentrant insulating state by the application of a perpendicular magnetic field ($B$). We find that the high-$B$ insulator exhibits activated transport with a characteristic temperature, $T_I$. $T_I$ has a maximum value ($T_{I}^p$) that is close to the superconducting transition temperature ($T_c$) at $B$ = 0, suggesting a possible relation between the conduction mechanisms in the superconducting and insulating phases. $T_{I}^p$ and $T_c$ display opposite dependences on the disorder strength.

cond-mat.supr-con↗
arXiv2003

Near-Perfect Correlation of the Resistance Components of Mesoscopic Samples at the Quantum Hall Regime

We study the four-terminal resistance fluctuations of mesoscopic samples near the transition between the $ν=2$ and the $ν=1$ quantum Hall states. We observe near-perfect correlations between the fluctuations of the longitudinal and Hall components of the resistance. These correlated fluctuations appear in a magnetic-field range for which the two-terminal resistance of the samples is quantized. We discuss these findings in light of edge-state transport models of the quantum Hall effect. We also show that our results lead to an ambiguity in the determination of the width of quantum Hall transitions.

cond-mat.mes-hall↗
arXiv2002

The quantized Hall effect in the presence of resistance fluctuations

We present an experimental study of mesoscopic, two-dimensional electronic systems at high magnetic fields. Our samples, prepared from a low-mobility InGaAs/InAlAs wafer, exhibit reproducible, sample specific, resistance fluctuations. Focusing on the lowest Landau level we find that, while the diagonal resistivity displays strong fluctuations, the Hall resistivity is free of fluctuations and remains quantized at its $ν=1$ value, $h/e^{2}$. This is true also in the insulating phase that terminates the quantum Hall series. These results extend the validity of the semicircle law of conductivity in the quantum Hall effect to the mesoscopic regime.

cond-mat.mes-hall↗
arXiv1999

Phase diagram of the integer quantum Hall effect in p-type Germanium

We experimentally study the phase diagram of the integer quantized Hall effect, as a function of density and magnetic field. We used a two dimensional hole system confined in a Ge/SiGe quantum well, where all energy levels are resolved, because the Zeeman splitting is comparable to the cyclotron energy. At low fields and close to the quantum Hall liquid-to-insulator transition, we observe the floating up of the lowest energy level, but NO FLOATING of any higher levels, rather a merging of these levels into the insulating state. For a given filling factor, only direct transitions between the insulating phase and higher quantum Hall liquids are observed as a function of density. Finally, we observe a peak in the critical resistivity around filling factor one.

cond-mat.mes-hall↗
arXiv1999

From the magnetic-field-driven transitions to the zero-field transition in two-dimensions

For more than a decade it was widely accepted that two-dimensional electrons are insulating at zero temperature and at zero magnetic-field. Experimentally it was demonstrated that, when placed in a strong perpendicular magnetic field, the insulating phase turns into a quantum-Hall state. While this transition was in accordance with existing theoretical models (KLZ), the density driven metal-insulator transition at zero magnetic-field, recently observed in high-quality two-dimensional systems, was unforeseen and, despite considerable amount of effort, its origins are still unknown. In order to improve our understanding of the zero magnetic-field transition, we conducted a study of the insulator to quantum-Hall transition in low-density, two-dimensional, hole system in GaAs that exhibits the zero magnetic-field metal-insulator transition. We found that, in the low field insulating phase, upon increasing the carrier density towards the metal-insulator transition, the critical magnetic-field of the insulator to quantum-Hall transition decreases and converges to the zero magnetic-field metal-insulator transition. This implies a common origin for both the finite magnetic-field and the zero magnetic-field transitions.

cond-mat↗
arXiv1998

Semicircle: An exact relation in the Integer and Fractional Quantum Hall Effect

We present experimental results on the quantized Hall insulator in two dimensions. This insulator, with vanishing conductivities, is characterized by the quantization (within experimental accuracy) of the Hall resistance in units of the quantum unit of resistance, h/e^2. The measurements were performed in a two dimensional hole system, confined in a Ge/SiGe quantum well, when the magnetic field is increased above the nu=1 quantum Hall state. This quantization leads to a nearly perfect semi-circle relation for the diagonal and Hall conductivities. Similar results are obtained with a higher mobility n-type modulation doped GaAs/AlGaAs sample, when the magnetic field is increased above the nu=1/3 fractional quantum Hall state.

cond-mat.mes-hall↗
arXiv1998

The Quantized Hall Insulator: A New Insulator in Two-Dimensions

Quite generally, an insulator is theoretically defined by a vanishing conductivity tensor at the absolute zero of temperature. In classical insulators, such as band insulators, vanishing conductivities lead to diverging resistivities. In other insulators, in particular when a high magnetic field (B) is added, it is possible that while the magneto-resistance diverges, the Hall resistance remains finite, which is known as a Hall insulator. In this letter we demonstrate experimentally the existence of another, more exotic, insulator. This insulator, which terminates the quantum Hall effect series in a two-dimensional electron system, is characterized by a Hall resistance which is approximately quantized in the quantum unit of resistance h/e^2. This insulator is termed a quantized Hall insulator. In addition we show that for the same sample, the insulating state preceding the QHE series, at low-B, is of the HI kind.

cond-mat.mes-hall↗
arXiv1998

Microwave Resonance and Carrier-Carrier Interaction in Two-Dimensional Hole Systems at High Magnetic Field

Microwave frequency conductivity, Re($σ_{xx}$), of high quality two-dimensional hole systems (2DHS) in a large perpendicular magnetic field (B) is measured with the carrier density ($n_s$) of the 2DHS controlled by a backgate bias. The high B insulating phase of the 2DHS exhibits a microwave resonance that remains well-defined, but shifts to higher peak frequency ($f_{pk}$) as $n_s$ is reduced. Over a wide range of $n_s, f_{pk} \propto n_s^{-1/2}$ is observed for the two samples we measured. The data clearly indicate that both carrier-carrier interactions and disorder are indispensable in determining the dynamics of the insulator. The $n_s$ dependence of $f_{pk}$ is consistent with a weakly pinned Wigner crystal in which domain size increases with $n_s$, due to larger carrier-carrier interaction.

cond-mat.mes-hall↗
arXiv1998

Wigner crystallization and metal-insulator transition of two-dimensional holes in GaAs/AlGaAs at B=0

We report the transport properties of a low disorder two-dimensional hole system (2DHS) in the GaAs/AlGaAs heterostructure, which has an unprecedentedly high peak mobility of $7\times 10^5cm^2/Vs$, with hole density of $4.8\times 10^9 cm^{-2}<p<3.72\times 10^{10}cm^{-2}$ in the temperature range of $50mK<T<1.3K$. From their T, p, and electric field dependences, we find that the metal-insulator transition in zero magnetic field in this exceptionally clean 2DHS occurs at $r_s=35.1\pm0.9$, which is in good agreement with the critical $r_s$ for Wigner crystallization ${r_s}^c=37\pm 5$, predicted by Tanatar and Ceperley for an ideally clean 2D system.

cond-mat↗
arXiv1998

Novel Properties of The Apparent Metal-Insulator Transition in Two-Dimensional Systems

The low-temperature conductivity of low-density, high-mobility, two-dimensional hole systems in GaAs was studied. We explicitly show that the metal-insulator transition, observed in these systems, is characterized by a well-defined critical density, p_0c. We also observe that the low-temperature conductivity of these systems depends linearly on the hole density, over a wide density range. The high-density linear conductivity extrapolates to zero at a density close to the critical density.

cond-mat.mes-hall↗
arXiv1998

Universality at integer quantum Hall transitions

We report in this paper results of experimental and theoretical studies of transitions between different integer quantum Hall phases, as well as transition between the insulating phase and quantum Hall phases at high magnetic fields. We focus mainly on universal properties of the transitions. We demonstrate that properly defined conductivity tensor is universal at the transitions. We also present numerical results of a non-interacting electron model, which suggest that the Thouless conductance is universal at integer quantum Hall transitions, just like the conductivity tensor. Finite temperature and system size effects near the transition point are also studied.

cond-mat.mes-hall↗
arXiv1997

The Metallic-Like Conductivity of a Two-Dimensional Hole System

We report on a zero magnetic field transport study of a two-dimensional, variable-density, hole system in GaAs. As the density is varied we observe, for the first time in GaAs-based materials, a crossover from an insulating behavior at low-density, to a metallic-like behavior at high-density, where the metallic behavior is characterized by a large drop in the resistivity as the temperature is lowered. These results are in agreement with recent experiments on Si-based two-dimensional systems by Kravchenko et al. and others. We show that, in the metallic region, the resistivity is dominated by an exponential temperature-dependence with a characteristic temperature which is proportional to the hole density, and appear to reach a constant value at lower temperatures.

cond-mat.mes-hall↗
arXiv1997

Symmetry in the insulator - quantum Hall - insulator transitions observed in a Ge/SiGe quantum well

We examine the magnetic field driven insulator-quantum Hall-insulator transitions of the two dimensional hole gas in a Ge/SiGe quantum well. We observe direct transitions between low and high magnetic field insulators and the $ν=1$ quantum Hall state. With increasing magnetic field, the transitions from insulating to quantum Hall and quantum Hall to insulating are very similar with respect to their transport properties. We address the temperature dependence around the transitions and show that the characteristic energy scale for the high field transition is larger.

cond-mat.mes-hall↗
arXiv1997

A New Transport Regime in the Quantum Hall Effect

This paper describes an experimental identification and characterization of a new low temperature transport regime near the quantum Hall-to-insulator transition. In this regime, a wide range of transport data are compactly described by a simple phenomenological form which, on the one hand, is inconsistent with either quantum Hall or insulating behavior and, on the other hand, is also clearly at odds with a quantum-critical, or scaling, description. We are unable to determine whether this new regime represents a clearly defined state or is a consequence of finite temperature and sample-size measurements.

cond-mat.mes-hall↗
arXiv1996

A different view of the quantum Hall plateau-to-plateau transitions

We demonstrate experimentally that the transitions between adjacent integer quantum Hall (QH) states are equivalent to a QH-to-insulator transition occurring in the top Landau level, in the presence of an inert background of the other completely filled Landau levels, each contributing a single unit of quantum conductance, $e^{2}/h$, to the total Hall conductance of the system.

cond-mat.mes-hall↗
arXiv1996

Duality Near Quantum Hall Transitions

A recent experiment by Shahar et al, on the phase transitions between quantum Hall states and the insulator, found that the current-voltage characteristics in the two phases are related by symmetry. It was suggested in this work that this is evidence for charge-flux duality near quantum Hall transitions. Here we provide details of this analysis. (Appearances notwithstanding, this is a theoretical paper.)

cond-mat.mes-hall↗