2014-2015

Academic year 2014 - 2015

13 May Andrey Bogdanov, ITMO University and Ioffe Institute, St Petersburg, Russia

"Temperature-tunable semiconductor metamaterials"

We provide a brief review of hyperbolic metamaterials. Then we will discuss a novel class of temperature-tunable semiconductor metamaterials that exhibit negative refraction in the terahertz spectral range. These metamaterials are based on doped semiconductor superlattices with ultrathin barriers of about 1 nm thickness. Due to the tunnel transparency of the barriers, layers of the superlattice cannot be considered as isolated and, therefore, the classical homogenization approach is inapplicable. We develop a theory of quantum homogenization which is based on the Kubo formula for conductivity. The proposed approach takes into account the wave functions of the carriers, their distribution function and energy spectrum. We show that the components of the dielectric tensor of the semiconductor metamaterial can be efficiently manipulated by external temperature and a topological transition from the dielectric to hyperbolic regime of metamaterial can be observed at room temperature. Using a GaAs/AlGaAs superlattice slab as an example, we provide a numerical simulation of an experiment which shows that the topological transition can be observed in the reflection spectrum from the slab.

16 February Albert Gallemí, Universitat de Barcelona

"Engineering semifluxon states in Bose-Hubbard trimers"

We consider a Bose-Hubbard trimer, i.e. an ultracold Bose gas populating three quantum states. The latter can be either different sites of a triple-well potential or three internal states of the atoms. The bosons can tunnel between different states with variable tunnelling strength between two of them. This allows us to study; i) different geometrical configurations, i.e. from a closed triangle to three aligned wells and ii) a triangular configuration with a π-phase by setting one of the tunnellings negative. By solving the corresponding three-site Bose-Hubbard Hamiltonian we obtain the ground state of the system as a function of the trap topology. We characterise the different ground states by means of the coherence and entanglement properties. For small repulsive interactions, fragmented condensates are found for the π-phase case. These are found to be robust against small variations of the tunnelling in the small interaction regime. A low-energy effective many-body Hamiltonian restricted to the degenerate manifold provides a compelling description of the π-phase degeneration, and explains the low-energy spectrum as excitations of discrete semifluxon states.

13 October Hector Ochoa, Instituto de Ciencia de Materiales de Madrid (CSIC)

"Spintronics and weak (anti) localization in two-dimensional transition metal dichalcogenides"

Bulk transition metal dichalcogenides can be exfoliated down to a single layer. These novel two-dimensional semiconductors (MoS2, WSe2, etc) have attracted the attention of the community due to their potential application in electronics and optoelectronics. In this talk, I will review the consequences on spin transport of the large spin-orbit coupling provided by the heavy transition metal atoms and the absence of a center of inversion in their crystal structure [1], with special emphasis on the effect of corrugations [2]. I will discuss the possibility of engineering a quantum spin Hall effect by applying strains or considering superlattice potentials [3]. Finally, I will describe the different regimes of quantum transport depending on the carrier concentrations, the magnitude of the spin-orbit splitting of the bands, and the type of disorder, leading to crossovers between the orthogonal, double-unitary and symplectic ensembles [4].

[1] HO and R.Roldán, "Spin-orbit mediated spin relaxation in monolayer MoS2", Phys. Rev. B 87, 245421 (2013)

[2] HO, F. Guinea, and V. I. Fal’ko, "Spin-lattice relaxation in two-dimensional hexagonal crystals", Phys. Rev. B 88, 195417, (2013)

[3] M. A. Cazalilla, HO, and F. Guinea, "Quantum spin Hall effect in two-dimensional crystals of transition metal dichalcogenides", Phys. Rev. Lett. 113, 077201 (2014)

[4] HO, F. Finocchiaro, F. Guinea, and V. I. Fal’ko, "Spin-valley relaxation and quantum transport regimes in two-dimensional transition metal dichalcogenides", arXiv:1408.2121

25 September Yoshitaka Tanimura, Department of Chemistry, Kyoto University, Japan

"A resonant tunneling diode and a quantum ratchet described by a Caldeira-Leggett model" (note: this seminar will take place at 11:00).

Quantum dissipative dynamics is investigated on a rigorous basis for the first time. We employ a Caldeira-Leggett Hamiltonian with an effective potential calculated self-consistently, accounting for the electron distribution. The heat bath can be characterized by the spectral distribution function, parameterised by the correlation time of the noise and the electron-phonon coupling strength. With this Hamiltonian, we can derive reduced hierarchy equations of motion (HEOM) in real-time for dynamics [1,2] and in imaginary-time for equilibrium distribution [3], which can deal with non-Markovian and non-perturbative system-bath interactions at finite temperature without approximation.

First we apply the HEOM approach to solve the resonant tunneling problem numerically rigorously. Hysteresis and both single and double plateau-like behavior are observed in the negative differential resistance (NDR) region. We find two distinct types of current oscillations, with large and small oscillation amplitudes, respectively, in some parts of the plateau in the NDR region.[4] Then we apply the HEOM to the quantum and classical ratchet problem. We found that the ratchet current decreases when tunneling effects play a dominant role, because the effect of ratchet potential becomes weaker for stronger tunneling case.

[1] Y. Tanimura and R. Kubo, J. Phys. Soc. Jpn. 58, 101 (1989).

[2] Y. Tanimura J. Phys. Soc. Jpn. 75, 082001 (2006).

[3] Y. Tanimura, J. Chem. Phys. 141, 044114 (2014).

[4] A. Sakurai and Y. Tanimura, J. Phys. Soc. Jpn. 82, 033707 (2013); NJP 16, 015002 (2014).

[5] A. Kato and Y. Tanimura, J. Phys. Chem. B 117, 13132 (2013).