On the magnetic structure of PrMn2O5: a neutron diffraction study

The long-range magnetic ordering of PrMn(2)O(5) has been studied on polycrystalline samples from neutron diffraction and specific heat measurements. The onset of antiferromagnetic ordering is observed at T(N) ≈ 25 K. In the temperature interval 18 K < T < 25 K the magnetic structure is defined by the propagation vector k(1) = (1/2,0,0). Below 18 K, some additional magnetic satellites appear in the NPD patterns, which are indexed with k(2) = (0,0,1/2). Therefore, below 18 K the magnetic structure consists of two independent magnetic domains, defined by the propagation vectors k(1) and k(2). The magnetic structure of the k(1)-domain is given by the basis vectors (C(x),0,0) and (C(x)',0,0) for Mn(4h) and Mn(4f), respectively. In the k(2)-domain, the magnetic structure is defined by the basis vectors (0,0,G(z)) and (F(x)',G(y)',0) for Mn(4h) and Mn(4f), respectively. At T = 1.5 K, for the magnetic phase associated with k(1), the magnetic moments of the Mn atoms at the 4h and 4f sites are 1.82(7) and 1.81(6) μ(B), respectively; for the magnetic phase associated with k(2), the magnetic moments for the Mn(4h) and Mn(4f) atoms are 0.59(5) and 2.62(5) μ(B), respectively.     

Spacer layer thickness fluctuation scattering in a modulation-doped Al_xGa_1-xAs/GaAs/Al_xGa_1-xAs quantum well

We theoretically study the influence of spacer layer thickness fluctuation(SLTF) on the mobility of a twodimensional electron gas(2DEG) in the modulation-doped Al x Ga 1 x As/GaAs/Al x Ga 1 x As quantum well.The dependence of the mobility limited by SLTF scattering on spacer layer thickness and donor density are obtained.The results show that SLTF scattering is an important scattering mechanism for the quantum well structure with a thick well layer.     

Infrared spectroscopy of the charge ordering transition of Na0.5CoO2

We report infrared reflectivity study of charge ordering in a Na0.5CoO2 single crystal. In comparison with x=0.7 and 0.85 compounds, we found that the effective carrier density increases systematically with decreasing Na contents. The charge ordering transition only affects the optical spectra below 1000 cm(-1). A hump near 800 cm(-1) develops below 100 K, which is accompanied by the appearance of new lattice modes as well as the strong antiresonance feature of phonon spectra. These observations signify a polaronic characteristic of charge carriers. Below T(co), an optical gap develops at the magnitude of 2Delta approximately 3.5k(B)T(co) (T相似文献   

Magnetotransport properties of two-dimensional electron gas in AlGaN/AlN/GaN heterostructures

Magnetotransport measurements are carried out on the AlGaN/AlN/GaN in an SiC heterostructure, which demonstrates the existence of the high-quality two-dimensional electron gas (2DGE) at the AlN/GaN interface. While the carrier concentration reaches 1.32 × 10¹³ cm ^{- 2} and stays relatively unchanged with the decreasing temperature, the mobility of the 2DEG increases to 1.21 × 10⁴ cm²/(V·s) at 2 K. The Shubnikov—de Haas (SdH) oscillations are observed in a magnetic field as low as 2.5 T at 2 K. By the measurements and the analyses of the temperature-dependent SdH oscillations, the effective mass of the 2DEG is determined. The ratio of the transport lifetime to the quantum scattering time is 9 in our sample, indicating that small-angle scattering is predominant.     

Nonlinear optical studies of the transient coherence in the Quantum Hall system

We review recent investigations of the femtosecond nonlinear optical response of the two-dimensional electron gas (2DEG) in a strong magnetic field. We probe the Quantum Hall (QH) regime for filling factors ν∼1. Our focus is on the transient coherence induced via optical excitation and on its time evolution during early femtosecond timescales. We simultaneously study the interband and intraband coherence in this system by using a nonlinear spectroscopic technique, transient three-pulse four wave mixing optical spectroscopy, and a many-body theory. We observe striking differences in the temporal and spectral profile of the nonlinear optical signal between a modulation doped quantum well system (with the 2DEG) and a similar undoped quantum well (without a 2DEG). We attribute these qualitative differences to Coulomb correlations between the photoexcited electron-hole pairs and the 2DEG. We show, in particular, that intraband many-particle coherences assisted by the inter-Landau-level magnetoplasmon excitations of the 2DEG dominate the femtosecond nonlinear optical response. The most striking effect of these exciton-magnetoplasmon coherences is a large off-resonant four-wave-mixing signal in the case of very low photoexcited carrier densities, not observed in the undoped system, with strong temporal oscillations and unusually symmetric temporal profile.     

Role of nitrogen in the mobility drop of electrons in modulation-doped GaAsN/AlGaAs heterostructures

We report transport properties of a 2 dimension electron gas (2DEG) in molecular beam epitaxy-grown GaAs_1−xN_x/AlGaAs modulation-doped heterostructures. Quantum oscillations in far infrared cyclotron resonance prove the efficient electron transfer and formation of the 2DEG. The 2DEG mobility strongly depends on the N concentration in the channel layer. It shows a drastic decrease as compared to N-free samples, even for the smallest amount of N (0.02%). For this N composition, the electron effective mass was found to be 0.073m₀. Reduced growth temperature (450 °C) was found to improve the mobility of N-containing channels. Examination of transport properties from 4 to 300 K and cyclotron resonance experiments give evidence of the presence of ionised impurity-like scattering centres in GaAsN.     

Alloy disorder scattering limited mobility of two-dimensional electron gas in the quaternary AlInGaN/GaN heterojunctions

Nitride heterojunction field effect transistors (HFETs) with quaternary AlInGaN barrier layers have achieved remarkable successes in recent years based on highly improved mobility of the two-dimensional electron gases (2DEGs) and greatly changed AlInGaN compositions. To investigate the influence of the AlInGaN composition on the 2DEG mobility, the quaternary alloy disorder (ADO) scattering to 2DEGs in AlInGaN/GaN heterojunctions is modeled using virtual crystal approximation. The calculated mobility as a function of AlInGaN alloy composition is shown to be a triangular-scarf-like curved surface for both cases of fixed thickness of AlInGaN layer and fixed 2DEG density. Though the two mobility surfaces are quite different in shape, both of them manifest the smooth transition of the strength of ADO scattering from quaternary AlInGaN to ternary AlGaN or AlInN. Some useful principles to estimate the mobility change with the Al(In,Ga)N composition in Al(In,Ga)N/GaN heterojunctions with a fixed 2DEG density are given. The comparison between some highest Hall mobility data reported for Al_xGa_1−xN/GaN heterojunctions (x=0.06~0.2) at very low temperature (0.3~13 K) and the calculated 2DEG mobility considering ADO scattering and interface roughness scattering verifies the influence of ADO scattering. Moreover, the room temperature Hall mobility data of Al(In,Ga)N/AlN/GaN heterojunctions with ADO scattering eliminated are summarized from literatures. The data show continuous dependence on Hall electron density but independence of the Al(In,Ga)N composition, which also supports our theoretical results. The feasibility of quaternary AlInGaN barrier layer in high conductivity nitride HFET structures is demonstrated.     

Magnetotransport properties of a magnetically modulated two-dimensional electron gas with the spin-orbit interaction

We study the electrical transport properties of a two-dimensional electron gas (2DEG) with the Rashba spin-orbit interaction in the presence of a constant perpendicular magnetic field (B(0)( ?z) which is weakly modulated by B1 = B1 cos(qx) ?z, where B(1) ? B(0) and q = 2π/a with a the modulation period. We obtain the analytical expressions of the diffusive conductivities for spin-up and spin-down electrons. The conductivities for spin-up and spin-down electrons oscillate with different frequencies and produce beating patterns in the amplitude of the Weiss and Shubnikov-de Haas oscillations. We show that the Rashba strength can be determined by analyzing the beating pattern in the Weiss oscillation. We find a simple equation which determines the Rashba spin-orbit interaction strength if the number of Weiss oscillations between any two successive nodes is known from the experiment. We compare our results with the electrically modulated 2DEG with the Rashba interaction. For completeness, we also study the beating pattern formation in the collisional and the Hall conductivities.     

Spin fluctuations in a magnetically frustrated metal LiV(2)O(4).

Inelastic neutron scattering is used to characterize spin fluctuations in the d-electron heavy fermion spinel LiV(2)O(4). The spin-relaxation rate, gamma(Q), for Q = 0.6 A(-1) is 1.4(2) meV at low temperatures and increases linearly with temperature at a rate of 0.46(8)k(B). There is antiferromagnetic short-range order at low temperatures with a characteristic wave vector Q(c) = 0.64(2) A(-1) and a correlation length of 6(1) A. While warming shifts intensity towards lower Q, the staggered susceptibility peaks at a finite wave vector for T < 80 K. The data are compared with conventional heavy fermion systems, geometrically frustrated insulating magnets, and recent theories for LiV(2)O(4).     

调制掺杂GaAs/AlGaAs 2DEG材料持久光电导及子带电子特性研究

在掺Si的GaAs/AlGaAs二维电子气(2DEG)结构中，得到μ_2K=1.78×10^{6cm2/(V·s)的高迁移率.在低温(2K)和高磁场(6T)的条件下，对样品进行红 光辐照，观察到持久光电导(PPC)效应，电子浓度在光照后显著增加.通过整数量子霍尔效应 (IQHE)和Shubnikov-de Haas (SdH)振荡的测量，研究了2DEG的子带电子特性.样品在低温光 照后2DEG中第一子带和第二子带的电子浓度同时随电子总浓度的增加而增加；而且电子迁移 率也明显提高.同时，通过整数霍尔平台的宽度对光照前后电子的量子寿命变短现象作了理 论分析. 关键词： 二维电子气 量子霍尔效应 SdH振荡 持久光电导效应}     

Scattering mechanisms in (Al, Ga)As/GaAs 2DEG structures

We have prepared a large number of high mobility two-dimensional electron gas (2DEG) structures, with undoped spacer thicknesses ranging from 9 to 3200Å. For samples with 400Å of (Al, Ga)As Si-doped at 1.3×10¹⁸ cm⁻³, there is a peak in the 4K mobility at spacers of 400–800Å, with a maximum value of 2×10⁶ cm² V⁻¹ s⁻¹. Increasing the thickness of the doped (Al, Ga)As to 500Å produced an increase in mobility to 3×10⁶ cm² V⁻¹ s⁻¹ for a 400Å space sample. We have compared these results with published analyses of scattering processes in 2DEG structures, and conclude that a combination of ionised impurity and acoustic phonon scattering gives a qualitative explanation of the behaviour, but that the experimental mobility values are generally higher than those predicted theoretically.     

Oscillating magnetocaloric effect in quantum nanoribbons

We investigate the oscillating magnetocaloric effect on a diamagnetic nanoribbon, using the model of a quasi-one-dimensional electron gas (Q1DEG) made with a parabolic confinement potential. We obtained analytical expressions for the thermodynamic potential and for the entropy change. The entropy change exhibits the same dependence on field and temperature observed for other diamagnetic systems. The period of the field-oscillating pattern is ~0.1 mT and the temperature of maximum entropy change is ~0.1 K with an applied field of the order of 1 T. An interesting feature of the results is the dependence of the oscillations with the strength of the confinement potential, as well as the possibility to provide a relationship among this last with nanoribbon width. In the limit of null confinement potential our expressions match those for the 2D diamagnetic system.     

One-dimensional instability in BaVS3

The 3d(1) system BaVS3 undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at T(MI)=70 K is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a 2k(F)=c(*)/2 charge-density wave (CDW) instability of the d(z(2)) electron gas. We discuss the formation below T(MI) of an unconventional CDW state involving the condensation of the other V4+ 3d(1) electrons of the quasidegenerate e(t(2g)) orbitals. This study stresses the role of the orbital degrees of freedom in the physics of BaVS3 and reveals the inadequacy of current first principle band calculations to describe its electronic ground state.     

Collective properties of excitons in the presence of a two-dimensional electron gas

We have studied the collective properties of two-dimensional (2D) excitons immersed within a quantum well which contains 2D excitons and a two-dimensional electron gas (2DEG). We have also analyzed the excitations for a system of 2D dipole excitons with spatially separated electrons and holes in a pair of quantum wells (CQWs) when one of the wells contains a 2DEG. Calculations of the superfluid density and the Kosterlitz–Thouless (K–T) phase transition temperature for the 2DEG-exciton system in a quantum well have shown that the K–T transition temperature increase with increasing exciton density and that it might be possible to have fast long-range transport of excitons. The superfluid density and the K–T transition temperature for dipole excitons in CQWs in the presence of a 2DEG in one of the wells increases with increasing inter-well separation.     

DX-center energy calculation with quantitative mobility spectrum analysis in n-AlGaAs/GaAs structures with low Al content

Experimental Hall data that were carried out as a function of temperature (60–350 K) and magnetic field (0–1.4 T) were presented for Si-doped low Al content (x=0.14) n–Al_xGa_1−xAs/GaAs heterostructures that were grown by molecular beam epitaxy (MBE). A 2-dimensional electron gas (2DEG) conduction channel and a bulk conduction channel were founded after implementing quantitative mobility spectrum analysis (QMSA) on the magnetic field dependent Hall data. An important decrease in 2DEG carrier density was observed with increasing temperature. The relationship between the bulk carriers and 2DEG carriers was investigated with 1D self consistent Schrödinger–Poisson simulations. The decrement in the 2DEG carrier density was related to the DX-center carrier trapping. With the simulation data that are not included in the effects of DX-centers, 17 meV of effective barrier height between AlGaAs/GaAs layers was found for high temperatures (T>300 K). With the QMSA extracted values that are influenced by DX-centers, 166 meV of the DX-center activation energy value were founded at the same temperatures.     

Magnetic resonant mode in the low-energy spin-excitation spectrum of superconducting Rb2Fe4Se5 single crystals

We have studied the low-energy spin-excitation spectrum of the single-crystalline Rb(2)Fe(4)Se(5) superconductor (T(c)=32 K) by means of inelastic neutron scattering. In the superconducting state, we observe a magnetic resonant mode centered at an energy of ?ω(res)=14 meV and at the (0.5 0.25 0.5) wave vector (unfolded Fe-sublattice notation), which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors. Our finding suggests that the 245-iron selenides are unconventional superconductors with a sign-changing order parameter, in which bulk superconductivity coexists with the √5×√5 magnetic superstructure. The estimated ratios of ?ω(res)/k(B)T(c)≈5.1±0.4 and ?ω(res)/2Δ≈0.7±0.1, where Δ is the superconducting gap, indicate moderate pairing strength in this compound, similar to that in optimally doped 1111 and 122 pnictides.     

On interference effects in the elastodynamic Green's functions G33(x, omega) of Si and GaAs

In this paper, we analyze interference effects present in the elastodynamic Green's functions G33(x,omega) of the cubic crystals Si and GaAs, which are associated with folded portions of the wave surface of the slow transverse (ST) acoustic mode. G33(x,omega) represents the three dimensional extension of the amplitude distribution imaged in the transmission acoustic microscopy of these crystals. The intensity contrast for oscillations of a particular wave vector k in the interference pattern is determined essentially by the 3D Fourier transform of G33(x,omega)G33*(x,omega). According to the Fourier autocorrelation theorem, that transform is equivalent to the autocorrelation function of the corresponding distribution G(33)(k,omega) in k-space. We show that due to the linear mapping between k-space and the slowness vector s-space, the interference phenomena discussed here are related to geometrical features of the slowness surface of the ST mode. We present calculations of these effects based on the angular spectrum technique.     

Oscillations of the magnetoresistance of a two-dimensional electron gas under microwave irradiation: Influence of the irradiation frequency

We present measurements of the magnetoresistance of a two-dimensional electron gas (2DEG) under continuous microwave as a function of the irradiation frequency. In a previous work by Simovič et al. [Phys. Rev. B 71 (2005) 233303], the magnetoresistance under microwave was shown to be modulated by oscillations of large amplitude that are periodic with magnetic field, their period and phase depending strongly on the electron density. Here we show that the phase and the amplitude of the microwave-induced oscillations also depend on the frequency of irradiation and the sign of the magnetic field.