Dc Magnetotransport Research Articles

Transport in split gate MOS quantum dot structures

We have fabricated symmetric 200 nm and asymmetric 100 by 200 nm quantum dots by the split gate technique within a MOSFET structure. DC electrical and magnetotransport measurements were performed at 4.2 K in a liquid-Helium cryostat. It is found that varying the electrochemical potential by changing the bias on a top gate leads to oscillations in the DC conductance through the dot resembling Coulomb blockade peaks, but when the depletion gate biases are swept, these peaks become more complex in nature, exhibiting crossing or anti-crossing behavior.

Phase coherence in mesoscopic systems at low temperatures

Recent dc magnetotransport experiments indicate that---contrary to common belief---the phase-coherence time remains finite for temperatures $\stackrel{\ensuremath{\rightarrow}}{T}0.$ While a first tentative explanation of this behavior was given in terms of electron-electron interactions, we show that in open systems phase-destructive phonon emission processes remain important even at zero temperature and may well be at the origin of the observed experimental facts.

Phenomenological interpretations of the ac Hall effect in the normal state ofYBa2Cu3O7

Ac and dc magnetotransport data in the normal state of YBa2Cu3O7 are analyzed within Fermi-liquid and non-Fermi-liquid models. In the Fermi-liquid analysis we use the Fermi surface deduced from band structure calculations and angular resolved photoemission experiments and assume that the electron relaxation rate varies over the Fermi surface. The non-Fermi-liquid models are the two-dimensional Luttinger liquid model and the charge-conjugation-symmetry model. We find that the existing experimental data can be adequately fitted by any of these models. This work provides a framework for the analysis of experiments that may discriminate among these models.

Magnetic-field-induced phase transition and sliding motion of charge-density waves in η-Mo4O11scrystals

Transverse magnetoresistance and Hall effect of the charge-density-wave (CDW) material \ensuremath{\eta}-${\mathrm{Mo}}_{4}$ ${\mathrm{O}}_{11}$ have been measured at 4.2 K (second CDW state) by dc and ac methods over the frequency range 50--500 kHz in pulsed magnetic fields up to 40 T. These quantities are both reversible and frequency independent for a low-field sweep up to near 10 T (=${\mathrm{H}}_{\mathrm{c}}$ ), beyond which an appreciable frequency-dependent hysteresis effect appears. The Cole-Cole plots of the real versus imaginary parts of the magnetoresistance at high magnetic fields show a monodispersion. The magnetic-field dependence of the real part of the ac Hall resistivity shows interesting behaviors, peaking near ${\mathrm{H}}_{\mathrm{c}}$ , followed by a decrease with H and a leveling off at high fields, while the imaginary components are very small and less frequency dependent. Using a multicarrier model consisting of the remaining and nested electron and hole bands, we have performed computer simulations for these dc and ac magnetotransport quantities, in satisfactory agreement with the observations. We have taken into account (1) the Zeeman effect for two types of the remaining hole and electron bands, (2) the CDW-gap narrowing of the nested electron and hole bands, (3) magnetic-field-induced CDW-to-normal phase transition in part of these nested bands, and (4) the magnetic-field-induced 'CDW oscillation' around some mean position of the CDW condensates (or thermal excitation of the CDW phasons over a pinning potential), according to the existing CDW model.

Magnetotransport in nearly antiferromagnetic Fermi liquids

Abstract The temperature dependence of the normal-state dc magnetotransport in high-temperature superconductors is investigated using the nearly anti-ferromagnetic Fermi-liquid description of planar quasiparticles. A direct numerical solution of the Boltzmann equation is obtained in a finite magnetic field. Highly anisotropic quasiparticle scattering is found at different regions of the Fermi surface, giving rise to the measured anomalous temperature dependences of the resistivity, the Hall coefficient and the orbital magnetoresistance.

Dc magnetotransport and Hall effect in multiply connected quantum-wire systems.

In the Landauer-B\"uttiker approach, the dc-transport properties of a system are expressed in terms of its scattering matrix. We propose a method to sum the multiple-scattering events for an arbitrary ensemble of interconnected scatterers, which leads to closed analytic expression for the matrix elements of the overall-scattering matrix. This approach is used to investigate the dc magnetotransport of a planar periodic lattice of interconnected quantum wires. The calculated conductance as well as the Hall voltage depend strongly on the Fermi wave vector and on the magnetic field. This behavior is a consequence of the interference between waves traveling along different scattering paths. It is closely related to the eigenvalue spectrum of electrons in a two-dimensional lattice in the presence of a perpendicular magnetic field, which is described by the Hofstadter butterfly. We present a detailed analysis of the observed features of the Hall voltage.

Far-infrared spectroscopy of one- and zero-dimensional electronic systems

We review the far infrared (FIR) response of one- and zero-dimensional electronic systems (0DES and 1DES), quantum wires and quantum dots, respectively, which have been realized by ultrafine mesa etching of modulation-doped AlGaAs/GaAs heterostructures. From dc magnetotransport measurements on quantum wires the formation of 1D subbands with typical energy separations of 1 to 3 meV was found in electron channels of 400 to 150 nm width. However, in the far infrared (FIR) response resonances at significantly higher frequencies were observed as compared to the single particle energy level separation. We will discuss that the optical response exhibits a very complex behaviour which is dominated by collective effects. In the case of a purely parabolic external potential the FIR response can be easily understood as the excitation of a rigid oscillation of the whole electron distribution. Especially in our quantum dot structures we are able to resolve additional excitation modes and a resonant anticrossing, probing the internal motion of the electrons within a single dot. We will discuss these excitations starting from two different models, namely from a classical one, which is based on plasmonic excitations in 2DES of finite size, and, in more details, from a quantum mechanical model, which treats atom-like systems with discrete energy levels including collective corrections.

Percolation at v=1 in the 2D electron system oF e −beam irradiated AlGaAs/GaAs heterostructures

We report on the low temperature AC and DC magneto transport properties of e −-beam irradiated AlGaAs/GaAs heterojunctions. This work has been motivated by earlier investigations of the cyclotron resonance (CR) which show a spectacular difference between the AC and DC mobility. While the DC transport in such irradiated samples reflects a very low mobility of the carriers, the CR linewidth is very small for high frequencies and at magnetic fields corresponding to the extreme quantum limit of single Landau level occupation, suggesting a high mobility of the carriers. We show that at low temperature the longitudinal resistance strongly increases when the system is in this quantum limit of an apparent high mobility CR. This discrepancy between AC and DC behaviour is attributed to a percolation of the 2D electron system.

DC and far-infrared experiments on one-dimensional multi-layered quantum wires

One-dimensional electronic systems (1DES) have been realized by ultrafine mesa etching of modulation doped GaAs/AlGaAs heterostructures and multi quantum well systems. The electronic properties of these single and multi-layered quantum wires have been characterized by dc magneto transport measurements. Typical energy separations of 1 to 3 meV for the 1D subbands were found in electron channels of 400 to 150 nm width. The 1D subband separation is not directly observed in the far infrared (FIR) response, rather, resonances at significantly higher frequencies are observed, indicating a dominant influence of collective effects. This gives the FIR resonances the character of plasmon modes which are localized by the microstructure. The plasmon character is in particular manifested by the observation of layer-coupled plasmon modes with n resonances in an n-layered quantum wire.