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We study the disorder effect on the transport properties in the HgTe/CdTe semiconductor quantum wells. We confirm that at a moderate disorder strength, the initially un-quantized two terminal conductance becomes quantized, and the system makes a transition to the novel topological Anderson insulator (TAI). Conductances calculated for the stripe and cylinder samples reveal the topological feature of TAI and supports the idea that the helical edge states may cause the anomalous quantized plateaus. The influence of disorder is studied by calculating the distributions of local currents. Base on the above-mentioned picture, the phenomena induced by disorder in the quantum spin Hall region and TAI region are directly explained. Our study of the local current configurations shed further light on the mechanism of the anomalous plateau.

We demonstrate that the radiation induced "zero-resistance state" observed in a two-dimensional electron gas is a result of the non-trivial structure of the density of states of the systems and the photon assisted transport. A toy model of a structureless quantum tunneling junction where the system has oscillatory density of states catches most of the important features of the experiments. We present a generalized Kubo-Greenwood conductivity formula for the photon assisted transport in a general system, and show essentially the same nature of the transport anomaly in a uniform system. The recent discovery of the "zero-resistance state" in a twodimensional (2D) electron gas (2DEG) presents a surprise to the physics community [1,2,3,4,5,6]. In these experiments [1,2,3], the magneto-resistance of a 2DEG under the influence of a microwave radiation exhibits strong oscillations vs. magnetic field. Unlike the well known Shubnikov-de Hass oscillation, the period of such oscillation is determined by the frequency of the microwave radiation, and the resistance shows minima near ω/ω c = n + 1/4, where ω is the frequency of the microwave radiation and ω c is the cyclotron frequency of electron in the magnetic field. When the microwave radiation is strong enough, the "zero-resistance states" are observed around the resistance minima. Durst et al. proposed a theory [4] which successfully explains the period and the phase of the magneto-resistance oscillation and also yields the negative resistance at the positions where the "zero-resistance state" was observed in the experiments. Andreev et al. [5] pointed out that such negative resistance state is essential to understanding the "zero-resistance state" , because the negative resistance is unstable in nature and could be interpreted as the "zero-resistance" by the measurement techniques employed in those experiments. A similar conclusion is also reached in Ref. 6. In essence, the existence of the negative resistance state is crucial in the current stage of theoretical understanding of the phenomenon.In this Letter, we show that such negative resistance state is the result of the non-trivial structure of the density of states of the 2DEG system and the photon assisted transport. The similar effect of photon assisted transport could be observed in other systems. A generalized Kubo-Greenwood formula is presented to provide a formal theory for such phenomena.To demonstrate our point in a clear and simple way, we first consider the transport through a quantum tunneling junction. Such a toy model catches most of the qualitative feature of the 2DEG experiments [1,2,3]. At the same time, the simplicity of the model provides us a clear view to the origin of the transport anomaly. Then we will present a generalized KuboGreenwood formula to calculate the conductivity of a general system under the influence of radiation, and provide a natural explanation of the success of the simple toy model.The structure of the toy model is shown in Fig. 1. We define the two systems across the junction...

We study transport properties of a two-dimensional electron system with Rashba spin-orbit coupling in a perpendicular magnetic field. The spin-orbit coupling competes with Zeeman splitting to introduce additional degeneracies between different Landau levels at certain magnetic fields. This degeneracy, if occurring at the Fermi level, gives rise to a resonant spin Hall conductance, whose height is divergent as 1/T and whose weight is divergent as -ln(T at low temperatures. The Hall conductance is unaffected by the Rashba coupling.

We study the heat generation in a nano-device with an electric current passing through the device.For the first time, a general formula for the heat generation is derived by using the nonequilibrium Keldysh Green functions. This formula can be applied in both the linear and nonlinear transport regions, for time-dependent systems, and with multi-terminal devices. The formula is also valid when the nano-device contains various interactions. As an application of the formula, the heat generation of a lead-quantum dot-lead system is investigated. The dc and ac biases are studied in detail. We find several interesting behaviors that are unique to nanostructures, revealing significant difference from heat generation in macroscopic systems.

We study the localization properties in the transition from a two-dimensional electron gas at zero magnetic field into an integer quantum Hall (QH) liquid. By carrying out a direct calculation of the localization length for a finite size sample using a transfer matrix technique, we systematically investigate the field and disorder dependences of the metal-insulator transition in the weak field QH regime. We obtain a different phase diagram from the one conjectured in previous theoretical studies. In particular, we find that: (1) the extended state energy Ec for each Landau level (LL) is always linear in magnetic field; (2) for a given Landau level and disorder configuration there exists a critical magnetic field Bc below which the extended state disappears; (3) the lower LLs are more robust to the metal-insulator transition with smaller Bc. We attribute the above results to strong LL coupling effect. Experimental implications of our work are discussed.PACS numbers: 71.30.+h, 73.20.Jc, 73.40.Hm It is very important to understand the localization properties in the transition from two-dimensional electron gas (at zero magnetic field) into an integer quantum Hall liquid [1]. According to the scaling theory of localization [2] all electrons in a two-dimensional system are localized in the absence of magnetic field. When the twodimensional electron system is subject to a strong perpendicular magnetic field, the energy spectrum becomes a series of impurity broadened Landau levels. Extended state appears in the center of each Landau band, while states at other energies are localized. This gives rise to the integer quantum Hall effect. The interesting issue is to understand the evolution of the extended states in the weak field regime as the magnetic field goes to zero where all extended states disappear.There could be two scenarios for the fate of the extended states as B → 0. The first one was proposed by Kivelson, Lee, and Zhang [3] in their global phase diagram of the quantum Hall effect. According to this phase diagram, in a strongly disordered quantum Hall system, the extended states stay with the center of the Landau bands at strong magnetic field, but float up in energy at small magnetic field and go to infinity as B → 0. This phase diagram is consistent with the semiclassical argument put forth by Khmelnitskii [4] and Laughlin [5].In this letter, we are proposing an alternative scenario for the behavior of the extended states at weak magnetic field limit. In our picture, each extended state is simply destroyed by strong disorder at a critical magnetic field instead of floating up in energy. By carrying out a direct calculation of the localization length for a finite size sample using a transfer matrix technique, we systematically investigate the field and disorder dependence of the metal-insulator transition in the weak field quantum Hall regime. We find that: (1) the extended state energy E c for each Landau level (LL) is always linear in magnetic field; (2) for a given Landau level and disorder configurat...

Topological lasers are immune to imperfections and disorder. They have been recently demonstrated based on many kinds of robust edge states, which are mostly at the microscale. The realization of 2D on-chip topological nanolasers with a small footprint, a low threshold and high energy efficiency has yet to be explored. Here, we report the first experimental demonstration of a topological nanolaser with high performance in a 2D photonic crystal slab. A topological nanocavity is formed utilizing the Wannier-type 0D corner state. Lasing behaviour with a low threshold of approximately 1 µW and a high spontaneous emission coupling factor of 0.25 is observed with quantum dots as the active material. Such performance is much better than that of topological edge lasers and comparable to that of conventional photonic crystal nanolasers. Our experimental demonstration of a low-threshold topological nanolaser will be of great significance to the development of topological nanophotonic circuitry for the manipulation of photons in classical and quantum regimes.

In 1860s, Gustav Kirchhoff proposed his famous law of thermal radiation, setting a fundamental contradiction between the infrared reflection and thermal radiation. Here, for the first time an ultrathin plasmonic metasurface is proposed to simultaneously produce ultralow specular reflection and infrared emission across a broad spectrum and wide incident angle range by combining the low emission nature of metal and the photonic spin-orbit interaction in spatially inhomogeneous structures. As a proof-of-concept, a phase gradient metasurface composed of sub-wavelength metal gratings is designed and experimentally characterized in the infrared atmosphere window of 8-14 µm, demonstrating an ultralow specular reflectivity and infrared emissivity below 0.1. Furthermore, it is demonstrated that infrared illusion could be generated by the metasurface, enabling not only invisibility for thermal and laser detection, but also multifunctionalities for potential applications. This technology is also scalable across a wide range of electromagnetic spectrum and provides a feasible alternative for surface coating.