The observation of vanishing electrical resistance in condensed matter has led to the discovery of new phenomena such as, for example, superconductivity, where a zero-resistance state can be detected in a metal below a transition temperature T(c) (ref. 1). More recently, quantum Hall effects were discovered from investigations of zero-resistance states at low temperatures and high magnetic fields in two-dimensional electron systems (2DESs). In quantum Hall systems and superconductors, zero-resistance states often coincide with the appearance of a gap in the energy spectrum. Here we report the observation of zero-resistance states and energy gaps in a surprising setting: ultrahigh-mobility GaAs/AlGaAs heterostructures that contain a 2DES exhibit vanishing diagonal resistance without Hall resistance quantization at low temperatures and low magnetic fields when the specimen is subjected to electromagnetic wave excitation. Zero-resistance-states occur about magnetic fields B = 4/5 Bf and B = 4/9 Bf, where Bf = 2pifm*/e,m* is the electron mass, e is the electron charge, and f is the electromagnetic-wave frequency. Activated transport measurements on the resistance minima also indicate an energy gap at the Fermi level. The results suggest an unexpected radiation-induced, electronic-state-transition in the GaAs/AlGaAs 2DES.

We examine the radiation induced modification of the Hall effect in high mobility GaAs/AlGaAs devices that exhibit vanishing resistance under microwave excitation. The modification in the Hall effect upon irradiation is characterized by (a) a small reduction in the slope of the Hall resistance curve with respect to the dark value, (b) a periodic reduction in the magnitude of the Hall resistance, Rxy, that correlates with an increase in the diagonal resistance, Rxx, and (c) a Hall resistance correction that disappears as the diagonal resistance vanishes.

We examine the phase and the period of the radiation-induced oscillatory-magnetoresistance in GaAs/AlGaAs devices utilizing in-situ magnetic field calibration by Electron Spin Resonance of DiPhenyl-Picryl-Hydrazal (DPPH). The results confirm a f -independent 1/4 cycle phase shift with respect to the hf = jhωc condition for j ≥ 1, and they also suggest a small (≈ 2%) reduction in the effective mass ratio, m * /m, with respect to the standard value for GaAs/AlGaAs devices.

We report the detection of novel zero-resistance states induced by electromagnetic wave excitation in ultra high mobility GaAs/AlGaAs heterostructure devices, at low magnetic fields, B, in the large filling factor limit. Vanishing resistance is observed in the vicinity of B = [4/(4j + 1)]B f , where* /e, where m * is the effective mass, e is the charge, and f is the microwave frequency. The dependence of the effect is reported as a function of f, the temperature, and the power.

We report the observation of inverse-magnetic-field-periodic, radiation-induced magnetoresistance oscillations in GaAs/AlGaAs heterostructures prepared in W. Wegscheider's group, compare their characteristics with similar oscillations in V. Umansky's material, and describe the lineshape variation vs. the radiation power, P , in the two systems. We find that the radiation-induced oscillatory ∆Rxx, in both materials, can be described by ∆Rxx = −Aexp(−λ/B)sin(2πF/B), where A is the amplitude, λ is the damping parameter, and F is the oscillation frequency. Both λ and F turn out to be insensitive to P . On the other hand, A grows nonlinearly with P .

In the quasi two-dimensional GaAs/AlGaAs system, we investigate the effect of rotating in-situ the electric field of linearly polarized microwaves relative to the current, on the microwave-radiationinduced magneto-resistance oscillations. We find that the frequency and the phase of the photoexcited magneto-resistance oscillations are insensitive to the polarization. On the other hand, the amplitude of the resistance oscillations are remarkably responsive to the relative orientation between the microwave antenna and the current-axis in the specimen. The results suggest a striking linear polarization sensitivity in the radiation-induced magnetoresistance oscillations.

Large changes in the electrical resistance induced by the application of a small magnetic field are potentially useful for device-applications. Such Giant Magneto-Resistance (GMR) effects also provide new insights into the physical phenomena involved in the associated electronic transport. This study examines a “bell-shape” negative GMR that grows in magnitude with decreasing temperatures in mm-wide devices fabricated from the high-mobility GaAs/AlGaAs 2-Dimensional Electron System (2DES). Experiments show that the span of this magnetoresistance on the magnetic-field-axis increases with decreasing device width, W, while there is no concurrent Hall resistance, Rxy, correction. A multi-conduction model, including negative diagonal-conductivity, and non-vanishing off-diagonal conductivity, reproduces experimental observations. The results suggest that a size effect in the mm-wide 2DES with mm-scale electron mean-free-paths is responsible for the observed “non-ohmic” size-dependent negative GMR.

Electronic carriers in graphene show a high carrier mobility at room temperature. Thus, this system is widely viewed as a potential future charge-based high-speed electronic material to complement–or replace–silicon. At the same time, the spin properties of graphene have suggested improved capability for spin-based electronics or spintronics and spin-based quantum computing. As a result, the detection, characterization and transport of spin have become topics of interest in graphene. Here we report a microwave photo-excited transport study of monolayer and trilayer graphene that reveals an unexpectedly strong microwave-induced electrical response and dual microwave-induced resonances in the dc resistance. The results suggest the resistive detection of spin resonance, and provide a measurement of the g-factor, the spin relaxation time and the sub-lattice degeneracy splitting at zero magnetic field.