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We report the observation of mid-infrared room-temperature electroluminescence from a p-i-n Ge/Ge0.922Sn0.078/Ge double heterostructure diode. The device structure is grown using low-temperature molecular beam epitaxy. Emission spectra under various injection current densities in the range of 318 A/cm2–490 A/cm2 show two distinct profiles peaked at 0.545 eV (2.275 μm) and 0.573 eV (2.164 μm), corresponding to indirect and direct bandgaps of the Ge0.922Sn0.078 active layer, respectively. This work represents a step forward towards the goal of an efficient direct-bandgap GeSn light-emitting device on a Si substrate by incorporating higher Sn content of 7.8% in a diode structure that operates at lower current densities.

Magnetic-field-induced phase transitions were studied with a two-dimensional electron Al-GaAs/GaAs system. The temperature-driven flow diagram shows the features of the Γ(2) modular symmetry, which includes distorted flowlines and shiftted critical point. The deviation of the critical conductivities is attributed to a small but resolved spin splitting, which reduces the symmetry in Landau quantization. [B. P. Dolan, Phys. Rev. B 62, 10278.] Universal scaling is found under the reduction of the modular symmetry. It is also shown that the Hall conductivity could still be governed by the scaling law when the semicircle law and the scaling on the longitudinal conductivity are invalid. *corresponding author:yhchang@phys.ntu.edu.tw PACS numbers: 1 Magnetic-field-induced phase transitions in two-dimensional electron systems (2DESs) have been an active research topic since the discovery of the quantum Hall effect. [1-18] The law of corresponding states proposed by Kivelson, Lee, and Zhang (KLZ) [2], which was based on the effective field Maxwell-Chern-Simon theory, provides a powerful method for classifying quantum Hall states and the transitions between them. According to the law of corresponding states, all the magnetic-field-induced phase transitions are of an equivalent class. In the integer quantum Hall effect (IQHE), the equivalence is established by the Landau-level addition transformation [1,2]. Magnetic-field-induced phase transitions are believed to be good examples of quantum phase transitions. [1,19] Universal properties suchas the reflection symmetry [3], universality of critical conductivities [1,4], and the universal scaling with same critical exponent [5,6] are expected and in addition, it is also expected that the temperature-driven flow lines [1,7,8] are governed by the semicircle law.Because of the existence of the law of corresponding states, the phase diagram of the QHE has a symmetry equivalent to the Γ 0 (2) symmetry group, which is a subgroup of the modular group. [8][9][10][20][21][22][23][24] The universal properties mentioned above can be taken as the manifestations of the Γ 0 (2) modular symmetry. [8-10] However, this symmetry relies on the assumption that all the Landau bands are equally spaced in energy, a condition satisfied

Insulator-quantum Hall conductor transitions at low magnetic field B were studied with a gated GaAs-AlGaAs heterostructure. A low field disorder-magnetic field phase diagram was constructed based on the experimental results. This phase diagram shows no floating up of the extended state and allows transitions from the insulating state directly to any Landau level states. The critical filling factor can change from 16 to 6 as the disorder in the sample increases. By inspecting the raw data from this and the other samples and analyzing the scaling behaviors near the transition points, we found that the observed transition has the properties of a genuine phase transition.

Magneto-transport measurements are performed on the two-dimensional electron system (2DES) in an AlGaAs/GaAs heterostructure. By increasing the magnetic field perpendicular to the 2DES, magnetoresistivity oscillations due to Landau quantisation can be identified just near the direct insulator-quantum Hall (I-QH) transition. However, different mobilities are obtained from the oscillations and transition point. Our study shows that the direct I-QH transition does not always correspond to the onset of strong localisation. * ctliang@phys.ntu.edu.tw and ochiai@faculty.chiba-u.jp PACS numbers: 72.15.Rn, 71.70.Di, The insulator to quantum Hall (I-QH) transition in a two-dimensional electron system (2DES) at low perpendicular magnetic fields B has attracted much attention [1, 2,3,4,5,6,7,8,9,10,11]. Theoretically, the direct I-QH transition from the insulator to an integer QH state of ν = 1 is forbidden in an infinite, non-interacting 2DES with arbitrary amount of disorder, where ν is the Landau level filling factor [1, 2,3]. In such a system, the only allowed state at B = 0 is the insulating one, and the 2DES undergoes the I-QH transition to enter the ν = 1 QH state [12,13]. Realistically, however, only systems of finite sizes are available, and the effects of the electron-electron (e-e) interaction are significant in some 2DESs [4,5,14,15,16,17,18]. As a result, the 2DESs may experience the direct I-QH transition from the low-field insulator to QH states of higher filling factors [2,3,4,8,16,17,18]. Such a transition can be related to the zero-field metal-insulator transition, to which e-e interaction cannot be ignored [4]. Given that most 2DESs show metallic behavior at B=0, the investigation of the direct I-QH transition at low B should be conducted in low-mobility 2DESs [1,12].