Abstract-Quantized photonic spin Hall effect in graphene

Liang Cai, Mengxia Liu, Shizhen Chen, Yachao Liu, Weixing Shu, Hailu Luo, and Shuangchun Wen

https://journals.aps.org/pra/abstract/10.1103/PhysRevA.95.013809

We examine the photonic spin Hall effect (SHE) in a graphene-substrate system with the presence of an external magnetic field. In the quantum Hall regime, we demonstrate that the in-plane and transverse spin-dependent splittings in the photonic SHE exhibit different quantized behaviors. The quantized SHE can be described as a consequence of a quantized geometric phase (Berry phase), which corresponds to the quantized spin-orbit interaction. Furthermore, an experimental scheme based on quantum weak value amplification is proposed to detect the quantized SHE in the terahertz frequency regime. By incorporating the quantum weak measurement techniques, the quantized photonic SHE holds great promise for detecting quantized Hall conductivity and the Berry phase. These results may bridge the gap between the electronic SHE and photonic SHE in graphene.

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Abstract-SUB-CYCLE MEASUREMENT OF INTENSITY CORRELATIONS IN THE TERAHERTZ RANGE

Ileana-Cristina Benea-Chelmus, Giacomo Scalari, Mattias Beck, Jerome Faist

http://www.mathpubs.com/detail/1512.02198v1/Sub-cycle-measurement-of-intensity-correlations-in-the-Terahertz-range

The Terahertz frequency range bears intriguing opportunities, beyond very advanced applications in spectroscopy and matter control. Peculiar quantum phenomena are predicted to lead to light emission by non-trivial mechanisms. Typically, such emission mechanisms are unraveled by temporal correlation measurements of photon arrival times, as demonstrated in their pioneering work by Hanbury Brown and Twiss. So far, the Terahertz range misses an experimental implementation of such technique with very good temporal properties and high sensitivity. In this paper, we propose a room-temperature scheme to measure photon correlations at THz frequencies based on electro-optic sampling. The temporal resolution of 146 fs is faster than one cycle of oscillation and the sensitivity is so far limited to ~1500 photons. With this technique, we measure the photon statistics of a THz quantum cascade laser. The proposed measurement scheme allows, in principle, the measurement of ultrahigh bandwidth photons and paves the way towards THz quantum optics.

Abstract-Terahertz dynamics of a topologically protected state: Quantum Hall effect plateaus near the cyclotron resonance of a two-dimensional electron gas

A. V. Stier, C. T. Ellis, J. Kwon, H. Xing, H. Zhang, D. Eason, G. Strasser, T. Morimoto, H. Aoki, H. Zeng, B. D. McCombe, and J. Cerne

https://journals.aps.org/prl/accepted/7407eY6bA1211624e5cb9524f017aaf11454ad9dc

We measure the Hall conductivity of a two-dimensional electron gas formed at a GaAs/AlGaAs heterojunction in the terahertz regime close to the cyclotron resonance frequency using highly sensitive Faraday rotation measurements. The sample is electrically gated, allowing the electron density to be changed continuously by more than a factor of three. We observe clear plateau- and step-like features in the Faraday rotation angle vs. electron density and magnetic field (Landau-level filling factor) even at fields/frequencies very close to cyclotron resonance absorption. These features are the high frequency manifestation of quantum Hall plateaus - a signature of topologically protected edge states. We observe both odd and even filling factor plateaus and explore the temperature dependence of these plateaus. Although dynamical scaling theory begins to break down in the frequency region of our measurements, we find good agreement with theory.

Titus Neupert receives Swiss Physical Society Award in General Physics for "Fractional quantum Hall states at zero magnetic field"

SPS Award in General Physics, sponsored by ABB

Titus Neupert is awarded with the SPS 2013 Prize in General Physics for his pioneering PhD work, especially for his theoretical discovery of "Fractional quantum Hall states at zero magnetic field".
The integer and fractional quantum Hall effects were experimentally detected in 1981 and 1982, respectively, at cryogenic temperatures. The discovery of graphene in 2005 established that the integer quantum Hall effect could be achieved at room temperature. Theorists predicted that the integer quantum Hall effect was one out of many examples of a larger family of semiconducting states supporting quantized susceptibilities in materials called topological band insulators. Titus Neupert gave the first quantitative answer to the question whether strong interactions could drive a fractional topological insulator in very much the same way as interactions drive a fractional quantum Hall insulator. One of the most remarkable prediction made by the award winner is that, by taking advantage of materials with strong spin-orbit coupling, it might become possible to achieve a fractional quantum Hall effect that is robust at room temperature and this without the use of any laboratory magnetic field.

Fractional quantum Hall states at zero magnetic field
A central theme of condensed matter physics is to classify and understand phases of matter. The Landau theory of symmetry breaking has been the long-standing paradigm for this classification: Two phases are distinct if they have different symmetries. In recent years, the study of topological phases showed that a second paradigm must be considered on equal footing: Two phases are distinct if they have different topological character, even if they share the same symmetries. Topological properties cannot be changed smoothly, thus endowing a topological state with a natural universality and protection against perturbations.
Topological phases are understood and classified in the limit of small electron–electron interactions. The opposite limit, in contrast, is at the frontier of current research. Strong electron–electron interactions can be responsible for the emergence of correlated topological states with excitations that have a fraction of the electron's charge, so-called fractional topological insulators (FTIs). The first example of an FTI that is well studied both experimentally and theoretically is the fractional quantum Hall effect of electrons in partially filled Landau levels. Recently, we discovered another type of FTIs, the fractional Chern insulator [1]. These states arise in lattice models in two spatial dimensions, if a nearly dispersionless band with a nonzero Chern number is partially filled with repulsively interacting electrons. Fractional Chern insulators share many universal and topological properties with the fractional quantum Hall effect in Landau levels, where the role of the strong magnetic field is replaced by time-reversal symmetry breaking electronic hopping integrals on the lattice. Comparing and contrasting the fractional Chern insulators with the fractional quantum Hall effect allows us to better understand what are the core ingredients for a fractional topological state to emerge.
In a combination of numerical and analytical work, we have studied several aspects of FTIs in two spatial dimensions. For example, we found that if a topological insulator, as is realized in HgTe quantum wells, has a sufficiently small bandwidth, repulsive electron–electron interactions can favor a spontaneous breaking of time-reversal symmetry along with the formation of an anomalous quantum Hall effect or a fractional Chern insulator state [2].
[1] T. Neupert, L. Santos, C. Chamon, and C. Mudry, Phys. Rev. Lett. 106, 236804 (2011).
[2] T. Neupert, L. Santos, S. Ryu, C. Chamon, and C. Mudry, Phys. Rev. B 84, 165107 (2011).

Abstract-Terahertz out-of-plane resonances due to spin-orbit coupling

K. Morawetz

http://arxiv-web3.library.cornell.edu/abs/1310.4405

A microscopic kinetic theory is developed which allows to investigate non-Abelian SU(2) systems interacting with meanfields and spin-orbit coupling under magnetic fields in one, two, and three dimensions. The coupled kinetic equations for the scalar and spin components are presented and linearized with respect to an external electric field. The dynamical classical and quantum Hall effect are described in this way as well as the anomalous Hall effect where a new symmetric dynamical contribution to the conductivity is presented. The coupled density and spin response functions to an electric field are derived including arbitrary magnetic fields. The magnetic field induces a staircase structure at frequencies of the Landau levels. It is found that for linear Dresselhaus and Rashba spin-orbit coupling a dynamical out-of-plane spin response appears at these Landau level frequencies establishing terahertz resonances.

Abstract-Quantum Hall effect in exfoliated graphene affected by charged impurities: metrological measurements

J. Guignard, D. Leprat, D. C. Glattli, F. Schopfer, W. Poirier
http://arxiv.org/abs/1110.4884

Metrological investigations of the quantum Hall effect (QHE) completed by transport measurements at low magnetic field are carried out in a-few-$\mu\mathrm{m}$-wide Hall bars made of monolayer (ML) or bilayer (BL) exfoliated graphene transferred on $\textrm{Si/SiO}_{2}$ substrate. From the charge carrier density dependence of the conductivity and from the measurement of the quantum corrections at low magnetic field, we deduce that transport properties in these devices are mainly governed by the Coulomb interaction of carriers with a large concentration of charged impurities. In the QHE regime, at high magnetic field and low temperature ($T<1.3 \textrm{K}$), the Hall resistance is measured by comparison with a GaAs based quantum resistance standard using a cryogenic current comparator. In the low dissipation limit, it is found quantized within 5 parts in $10^{7}$ (one standard deviation, $1 \sigma$) at the expected rational fractions of the von Klitzing constant, respectively $R_{\mathrm{K}}/2$ and $R_{\mathrm{K}}/4$ in the ML and BL devices. These results constitute the most accurate QHE quantization tests to date in monolayer and bilayer exfoliated graphene. It turns out that a main limitation to the quantization accuracy, which is found well above the $10^{-9}$ accuracy usually achieved in GaAs, is the low value of the QHE breakdown current being no more than $1 \mu\mathrm{A}$. The current dependence of the longitudinal conductivity investigated in the BL Hall bar shows that dissipation occurs through quasi-elastic inter-Landau level scattering, assisted by large local electric fields. We propose that charged impurities are responsible for an enhancement of such inter-Landau level transition rate and cause small breakdown currents.