Quasi Two-dimensional System Research Articles

Ratchet effect induced by linearly polarized near- and mid-infrared radiation in InAs nanowires patterned quasi two-dimensional electron system

In this letter, we studied the polarization dependent current excited by near- and mid-infrared radiations in InAs nanowires patterned quasi two-dimensional structures. We observed a current with a predominated direction along the wires at normal incidence of mid-infrared radiation and attributed it to the ratchet effect. Photogalvanic effect induced by obliquely incident excitation is also observed. Under near-infrared excitation, however, the normal incident ratchet effect becomes insignificant and the linear photogalvanic effect at oblique incidence dominantly contributes to the electric current.

Giant Nernst-Ettingshausen Oscillations in Semiclassically Strong Magnetic Fields

We consider the Nernst-Ettingshausen (NE) effect in the presence of semiclassically strong magnetic fields for a quasi-two-dimensional system with a parabolic or linear dispersion of carriers. We show that the occurring giant oscillations of the NE coefficient are coherent with the recent experimental observation in graphene, graphite, and bismuth. In the 2D case we find the exact shape of these oscillations and show that their magnitude decreases (increases) with enhancement of the Fermi energy for Dirac fermions (normal carriers). With a crossover to the 3D spectrum the phase of the oscillations shifts, their amplitude decreases, and the peaks become asymmetric.

Snapshots of cooperative atomic motions in the optical suppression of charge density waves

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale. The study of the interplay among these various degrees of freedom in strongly coupled electron-lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials, with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems. Numerous time-resolved experiments have been performed on CDWs, probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred. Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction to study the quasi two-dimensional CDW system 1T-TaS(2). Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of ∼0.1 Å, is suppressed by about 20% on a timescale (∼250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron-phonon energy transfer (∼350 femtoseconds) and are followed by fast recovery of the CDW (∼4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems.

Isotopic Fractionation by Transverse Dispersion: Flow-through Microcosms and Reactive Transport Modeling Study

Flow-through experiments were carried out to investigate the role of transverse dispersion on the isotopic behavior of an organic compound during conservative and bioreactive transport in a homogeneous porous medium. Ethylbenzene was selected as model contaminant and a mixture of labeled (perdeuterated) and light isotopologues was continuously injected in a quasi two-dimensional flow-through system. We observed a significant fractionation of ethylbenzene isotopologues during conservative transport at steady state. This effect was particularly pronounced at the plume fringe and contrasted with the common assumption that physical processes only provide a negligible contribution to isotope fractionation. Under the experimental steady state conditions, transverse hydrodynamic dispersion was the only process that could have caused the observed fractionation. Therefore, the measured isotope ratios at the outlet ports were interpreted with different parameterizations of the transverse dispersion coefficient. A nonlinear compound-specific parameterization showed the best agreement with the experimental data. Successively, bioreactive experiments were performed in two subsequent stages: a first oxic phase, involving a single strain of ethylbenzene degraders and a second phase with aerobic and anaerobic (i.e., ethylbenzene oxidation coupled to nitrate reduction) degradation. Significant fractionation through biodegradation occurred exclusively due to the metabolic activity of the anaerobic degraders. We performed analytical and numerical reactive transport simulations of the different experimental phases which confirmed that both the effects of physical processes (diffusion and dispersion) and microbially mediated reactions have to be considered to match the observed isotopic fractionation behavior.

Field-angle dependence of interlayer off-diagonal magnetoresistance in quasi two-dimensional layered conductors

We studied field-angle effects on the interlayer off-diagonal element of the magnetoresistance in a quasi two-dimensional (Q2D) system. We numerically calculated the angle dependence of the off-diagonal resistance ρ xz in the framework of the semiclassical transport theory when the current parallel to the interlayer z-direction was applied and the magnetic field was rotated from the z-direction to the intra-layer x-direction. The calculated ρ xz showed the oscillatory structure analogous to the angle dependent magnetoresistance oscillation (AMRO). This effect which we call “the off-diagonal AMRO” can be explained as the Fermi surface topological effect in the same way as AMRO. The off-diagonal AMRO was experimentally verified in an organic conductor α-(BEDT-TTF) 2NH 4Hg(SCN) 4.

Laser Induced Semiconductor-Metal Transition in a Quantum Well

The possibility of Semiconductor-Metal Transition under the influence of intense Laser field in a quasi-two dimensional semiconducting system like Al(x)Ga(1-x)As/GaAs Quantum Well in the finite barrier model has been investigated by showing an abrupt change in diamagnetic susceptibility of donor at critical concentration, which can be controlled by the amplitude of Laser field, using variational principle.

Out-of-plane thermopower of strongly correlated layered systems: An application toBi2(Sr,La)2CaCu2O8+δ

We calculate the out-of-plane thermopower in a quasi-two dimensional system, and argue that this quantity is an effective probe of the asymmetry of the single-particle spectral function. We find that the temperature and doping dependence of the out-of-plane thermopower in Bi_2(Sr,La)_2CaCu_2O_{8+\delta} single crystals is broadly consistent with the behavior of the spectral function determined from ARPES and tunneling experiments. We also investigate the relationship between out-of-plane thermopower and entropy in a quasi-two dimensional material. We present experimental evidence that at moderate temperatures, there is a qualitative correspondence between the out-of-plane thermopower in Bi_2(Sr,La)_2CaCu_2O_{8+\delta}, and the entropy obtained from specific heat measurements. Finally, we argue that the derivative of the entropy with respect to particle number may be the more appropriate quantity to compare with the thermopower, rather than the entropy per particle.

Localized Distributions of Quasi-Two-Dimensional Electronic States near Defects Artificially Created at Graphite Surfaces in Magnetic Fields

We measured the local density of states of a quasi two-dimensional electron system (2DES) near defects, artificially created by Ar-ion sputtering, on surfaces of highly oriented pyrolytic graphite (HOPG) with scanning tunneling spectroscopy (STS) in high magnetic fields. At valley energies of the Landau level spectrum, we found two typical localized distributions of the 2DES depending on the defects. These are new types of distributions which are not observed in the previous STS work at the HOPG surface near a point defect [Y. Niimi, Phys. Rev. Lett. 97, 236804 (2006).10.1103/PhysRevLett.97.236804]. With increasing energy, we observed gradual transformation from the localized distributions to the extended ones as expected for the integer quantum Hall state. We show that the defect potential depth is responsible for the two localized distributions from comparison with theoretical calculations.

Nonlinear magnetotransport in a high-mobility quasi-two-dimensional electron system

The influence of a dc electric current I dc on the low-temperature magnetotransport of high-mobility electrons in a GaAs double quantum well with two occupied size-quantization levels has been studied. The oscillations of the resistance ρ xx , which are periodic in the inverse magnetic field, have been shown to appear in the quasitwo-dimensional system under consideration at a temperature of T = 4.2 K in magnetic fields B > 0.1 T; the oscillations are caused by isoenergetic resonance transitions of the electrons between the Landau levels of different subbands. The inversion of the oscillations with an increase in I dc has been discovered. It has been found that the observed effect is due to the electron transport in a nonlinear regime.

STM/STS measurements of quasi two-dimensional electronic states near artificially created defects in magnetic fields

We observed the local density of states (LDOS) of a quasi two-dimensional electron system near defects, artificially created by Ar-ion sputtering, on a surface of highly oriented pyrolytic graphite in magnetic fields with ultra-low temperature scanning tunneling spectroscopy. In differential tunnel conductance images taken at the valley energies in between the adjacent Landau levels, two typical localized distributions of the LDOS were observed depending on the defects. We reveal that the defect potential depth is responsible for the two types of the spatial distributions of LDOS.

EXPLORATION AND SEARCH ON NEW HIGH TC SUPERCONDUCTIVE MATERIALS

On the basis of Fan and Malozovsky theory, which had discussed the non-phonon mechanism of the layered Fermi liquid superconductivity, and of the experimental studies and theoretical analyses obtained by Zhang et al. as well as other scientists, we have analyzed and discussed the extrinsic and intrinsic factors of affecting the transition temperature Tc of cuprate superconductors. We also will present some possible approaches to explore new high Tc superconductive materials, and pay more attention to the non-traditional phonon mechanism, low dimensional or quasi two-dimensional system, and hexagonal structure materials, etc.

Interfacial Thermocapillary Vortical Flow for Microfluidic Mixing

We present a method for the mixing of fluids in a quasi two-dimensional system with low Reynolds number by means of generating a vortical flow. A two-dimensional cavitation bubble is induced in liquid-expanded phase by locally heating a Langmuir monolayer at the air/liquid interface with an IR laser. The laser-induced cavitation bubble works as a microfluidic pump and generates a thermocapillary flow around the pump. As a result, the surrounding liquid-expanded phase flows in one direction. Perturbing the thermocapillary flow with solid folds that are created by compression and reexpansion of the monolayer induces the vortical flow behind the folds. Applying the equation of creeping flow, we find a torque halfway from the center causing the vortical flow. The vorticity created in this way stretches the liquid-expanded and gaseous phase in the azimuthal direction and at the same time thins both phases in the radial direction. If the vortical flow could be maintained long enough to reach a radial thinning that would allow the interdiffusion of surfactants at the surface, then this technique would open a route for the effective two-dimensional microfluidic mixing at low Reynolds numbers.

Collective behavior of interwell excitons laterally confined in GaAs/AlGaAs double quantum wells

The luminescence of interwell excitons laterally confined by long range potential fluctuations and with the use of inhomogeneous electric field in n-i-n GaAs/AlGaAs heterostructures double quantum wells has been investigated under variation of excitation power and temperature. Above mobility threshold very narrow interwell exciton line has been observed and its intensity decrease is linearly dependent on temperature growth. The observed phenomena, which were critical to exciton density and temperature, are attributed to the Bose-condensation in laterally confined quasi-two dimensional system of interwell excitons. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Anisotropy in Magnetic and Transport Properties of GdCrSb3.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Anisotropy in magnetic and transport properties of GdCrSb 3

We report the first measurements of anisotropy in magnetic susceptibility, magnetization, and electrical resistivity using single crystals of GdTSb 3 (T = Cr, V). GdTSb 3 is a quasi-two dimensional system with orthorhombic crystal structure (space group: Pbcm). Unlike the other light rare earth chromium antimonides (R = Ce–Nd, Sm), in which two magnetic transitions are observed, GdCrSb 3 undergoes a single ferrimagnetic transition at T C=86 K, which is evident in both the magnetic susceptibility and electrical resistivity. Within the b–c plane, GdCrSb 3 is found to have metallic behavior from 5 to 300 K, but it is found to have insulating behavior (d ρ a /d T<0) along the stacking axis. GdVSb 3 is found to have a Néel transition at 5 K due to the localized Gd ions, but no ferromagnetic transition.

Relaxor ferroelectric behavior and collective modes in theπ−dcorrelated anomalous metalλ−(BEDT−TSF)2FeCl4

We have investigated the microwave response at 44.5 GHz with respect to temperature T and external magnetic field H in $\ensuremath{\lambda}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TSF}{)}_{2}{\mathrm{FeCl}}_{4}$ forming a quasi two-dimensional electronic system with $\ensuremath{\pi}\ensuremath{-}d$ correlations. At 8.3 K $[{=T}_{\mathrm{MI}},$ the metal-insulator (MI) transition temperature] $&lt;T&lt;70\mathrm{K}$ ${(=T}_{\mathrm{FM}}),$ the microwave dielectric constant along the c axis, ${\ensuremath{\epsilon}}_{1}^{c},$ takes positive large values amounting to 1000--2000. Furthermore, the microwave conductivity ${\ensuremath{\sigma}}_{1}^{c}$ starts to deviate resistively from the dc conductivity ${\ensuremath{\sigma}}_{\mathrm{dc}}^{c},$ and the difference between ${\ensuremath{\sigma}}_{1}^{c}$ and ${\ensuremath{\sigma}}_{\mathrm{dc}}^{c}$ reaches about two orders of magnitude just above ${T}_{\mathrm{MI}}.$ The present results are consistent with the previous results at 16.3 GHz, and consequently the appearance of the anomalous metallic state is confirmed. An anomalous microwave response has also been observed in the ${a}^{*}$ and ${b}^{*}$ directions, and there exist large anisotropies depending sensitively on the orientations. The broad maximum of ${\ensuremath{\epsilon}}_{1}^{c}$ around 30 K is reminiscent not of a usual ferroelectric transition, but of relaxor ferroelectric behaviors. It is expected that dielectric domains or stripes with less metallic conductions emerge inhomogeneously in the $\ensuremath{\pi}$ electronic systems. Above ${T}_{\mathrm{FM}},$ where microwave anomalies are not present, the interplane and intraplane microwave conductivities hold anisotropies ${\ensuremath{\sigma}}_{1}^{c}/{\ensuremath{\sigma}}_{1}^{{b}^{*}}\ensuremath{\approx}{10}^{3}$ and ${\ensuremath{\sigma}}_{1}^{c}/{\ensuremath{\sigma}}_{1}^{{a}^{*}}\ensuremath{\approx}10.$ In the antiferromagnetic insulating state, ${\ensuremath{\sigma}}_{1}^{c}$ becomes much conductive in comparison with ${\ensuremath{\sigma}}_{\mathrm{dc}}^{c}.$ Together with low frequency data, ${\ensuremath{\epsilon}}_{1}^{c}$ is found to exhibit a large frequency dispersion. The microwave response is not attributed to single particle excitations, but to some collective mode excitations associated with charge degrees of freedom. The $H\ensuremath{-}T$ phase diagram of the MI transition determined by the present microwave measurements is independent of the orientations of H, and coincides well with the phase diagram obtained by the dc magnetoresistivity and magnetization. Spin waves for the hard axis are observed as an absorption peak in the width change for the microwave magnetic field applied parallel to both H and ${a}^{*}.$

Quantum computing with electrons floating on liquid helium

Electrons on a helium surface form a quasi two-dimensional system which displays the highest mobility reached in condensed matter physics. We propose to use this system as a set of interacting quantum bits. We will briefly describe the system and discuss how the qubits can be addressed and manipulated. The working frequency of the proposed quantum computer is ~ 1GHz. Careful analysis shows that the relaxation rate can be at least 5 orders of magnitude smaller, for low temperatures.

Semiconductor–metal transition in a quasi two-dimensional system

The possibility of semiconductor–metal transition in a quantum well of the GaAs/Ga 1− x Al x As system is investigated within the effective mass approximation. The vanishing of donor ionization energy as a function of well width and donor concentration shows that no transition is possible below a well width of 60 Å. Results are discussed in the light of existing literature.

The ground states of the BETS Family: effects of localized spins

Quasi-two dimensional electronic system of organic BETS compounds is studied theoretically on the basis of mean-field calculation for the Coulomb interaction, U, together with the investigation of the effect of spin-spin interaction, J, between BETS and high Fe spin on anions. New classification on two different types of molecular arrangement, λ- and K-type, is introduced through a single parameter of band overlap, where the former with a smaller band overlap shows a Mott-like insulating state, while the latter favors a nobel spin density wave(SDW)-like state. In both states, the spin amplitude is increased by the effect of J. The value of U on BETS sites relatively smaller than that of ET is expected to be close to the metal-insulator boundary, which makes the system's ground state nature sensitive even to the effect of small value of J as well as to the difference in the band structure, which explains the various ground states of this family within the same scheme, in accordance with the experiments.

Low Dimensional Triangular Magnetic Systems in Transition Metal Intercalates of Misfit Layer Compounds

Low dimensional triangle magnetism was studied for transition metal intercalates of incommensurate misfit layer compounds (RS)x[M0.33(NbS2)2] (R = La, Ce, M = Fe, Mn). Magnetization measurements for the single crystals of the Fe intercalated compounds (RS)x[Fe0.33(NbS2)2] show the successive magnetic transition at 14 K and 22 K. The strong frustration appears between two transition temperatures as quasi two-dimensional antiferro-magnetic triangular Ising spin system. In contrast, the Mn compounds (RS)x[Mn0.33(NbS2)2] has very low transition temperature of 3.0 K due to the nature of quasi two-dimensional Heisenberg spin system.