Atomic, Molecular, and Optical Physics

Diffraction of entangled photon pairs by ultrasonic waves

Lü-bi Deng

In this paper, we have presented and established a new theoretical formulation of photon optics based on photon path and Feynman path integral idea. We have used Feynman path integral approach to discuss Fraunhofer, Fresnel diffraction of entangled photon pairs by ultrasonic wave and obtained below results: i) quantum state and probability distribution of entangled photon pairs by Fraunhofer and Fresnel ultrasonic diffraction, ii) oblique incidence Raman-Nath and Bragg diffraction conditions, and iii) total correlation state and its probability distribution. We have also obtained quantum state and probability distribution of diffraction of single photon by ultrasonic wave. Our calculation results are in agreement with the experiment results.

Simulating cyclotron-Bloch dynamics of a charged particle in a 2D lattice by means of cold atoms in driven quasi 1D optical lattices

Andrey R. Kolovsky

Quantum dynamics of a charged particle in a two-dimensional (2D) lattice subject to magnetic and electric fields is a rather complicated interplay between cyclotron oscillations (the case of vanishing electric field) and Bloch oscillations (zero magnetic field), details of which has not yet been completely understood. In the present work we suggest to study this problem by using cold atoms in optical lattices. We introduce a one-dimensional (1D) model which can be easily realized in laboratory experiments with quasi 1D optical lattices and show that this model captures many features of the cyclotron-Bloch dynamics of the quantum particle in 2D square lattices.

Determination of the 5d6p ³F4–5d^{2 3}F transition probabilities of Ba I

Jia-qi Zhong, Geng-hua Yu, Jin Wang, Ming-sheng Zhan*

Whether the transitions between 6s5d 3D and 5d6p ³F can be used for laser cooling of barium heavily depends upon the transition probabilities of 5d6p ³F–5d^{2 3}F. Taking the transition 6s5d ³D3–5d6p ³F4 as a scale, the leakage rate of 5d6p ³F4–5d^{2 3}F was used to evaluate the transition probabilities. 706 nm laser pulses with different durations were applied to a barium atomic beam for 6s5d ³D3–5d^{2 3}F4 optical pumping, and the remaining percentages in 6s5d ³D3 were measured. After exponential fitting, the transition probability of 5d6p ³F4–5d^{2 3}F3,4 was determined to be 2.1(4)×104 s⁻¹, which is in agreement with theoretical calculations using the scaled Thomas–Fermi–Dirac method.

Theory of superfluidity and drag force in the one-dimensional Bose gas

Alexander Yu. Cherny, Jean-S´ebastien Caux, and Joachim Brand*

The one-dimensional Bose gas is an unusual superfluid. In contrast to higher spatial dimensions, the existence of non-classical rotational inertia is not directly linked to the dissipationless motion of infinitesimal impurities. Recently, experimental tests with ultracold atoms have begun and quantitative predictions for the drag force experienced by moving obstacles have become available. This topical review discusses the drag force obtained from linear response theory in relation to Landau’s criterion of superfluidity. Based upon improved analytical and numerical understanding of the dynamical structure factor, results for different obstacle potentials are obtained, including single impurities, optical lattices and random potentials generated from speckle patterns. The dynamical breakdown of superfluidity in random potentials is discussed in relation to Anderson localization and the predicted superfluid-insulator transition in these systems.

Electronic and optical properties of semiconductor and graphene quantum dots

W. D. Sheng*, M. Korkusinski, A. D. Güclü, M. Zielinski, P. Potasz, E. Kadantsev, O. Voznyy, and P. Hawrylak*

Our recent works on the electronic and optical properties of semiconductor and graphene quantum dots are reviewed. For strained self-assembled InAs quantum dots on GaAs or InP substrate atomic positions and strain distribution are described using valence-force field approach and continuous elasticity theory. The strain is coupled with the effective mass, k · p, effective bond-orbital and atomistic tight-binding models for the description of the conduction and valence band states. The single-particle states are used as input to the calculation of optical properties, with electronelectron interactions included via onfiguration interaction (CI) method. This methodology is used to describe multiexciton complexes in quantum dot lasers, and in particular the hidden symmetry as the underlying principle of multiexciton energy levels, manipulating emission from biexcitons for entangled photon pairs, and optical control and detection of electron spins using gates. The self-assembled quantum dots are compared with graphene quantum dots, one carbon atom thick  nanostructures. It is shown that control of size, shape and character of the edge of graphene dots allows simultaneously to manipulate electronic, optical, and magnetic properties in a single material system.

Electrodynamics of Abrikosov vortices: The field theoretical formulation

Electrodynamic phenomena related to vortices in superconductors have been studied since their prediction by Abrikosov, and seem to hold no fundamental mysteries. However, most of the effects are treated separately, with no guiding principle. We demonstrate that the relativistic vortex worldsheet in spacetime is the object that naturally conveys all electric and magnetic information, for which we obtain simple and concise equations. Breaking Lorentz invariance leads to down-to-earth Abrikosov vortices, and special limits of these equations include for instance dynamic Meissner screening and the AC Josephson relation. On a deeper level, we explore the electrodynamics of two-form sources in the absence of electric monopoles, in which the electromagnetic field strength itself acquires the characteristics of a gauge field. This novel framework leaves room for unexpected surprises.

Topological aspects of novel insulating states
Yong-Shi Wu

In recent years, the theoretical prediction of the quantum spin Hall effect opened a new chapter in science on the control and manipulation of spin or spin current with the electric (rather than magnetic) field. This effect is essentially due to the spin-orbit interaction, which often leads to non-trivial topological nature and exotic properties of the electronic states in condensed matter systems. Among others, novel insulating states, the so-called topological insulators, are the most dramatic example. In this article, we review the mathematical structural insight into the nontrivial topological nature of these novel electronic states, and explain how the insight has led to the discovery of exotic physical properties as well as the search for realistic systems.

Studies on the electronic structures of three-dimensional topological insulators by angle resolved photoemission spectroscopy

Yulin Chen
Three-dimensional topological insulators represent a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. The unusual surface states of topological insulators rise from the nontrivial topology of their electronic structures as a result of strong spin-orbital coupling. In this review, we will briefly introduce the concept of topological insulators and the experimental method that can directly probe their unique electronic structure: angle resolved photoemission spectroscopy (ARPES). A few examples are then presented to demonstrate the unusual electronic structures of different families of topological insulators and the properties of the topological surface states. Finally, we will briefly discuss the future development of topological quantum materials and the some recent advances in photoemission spectroscopy.

(3+1)-TQFTS and topological insulators

Kevin Walker, Zhenghan Wang*

Levin-Wen models are microscopic spin models for topological phases of matter in (2+1)-dimension. We introduce a generalization of such models to (3 + 1)-dimension based on ribbon fusion categories. We discuss the ground states degeneracy on 3-manifolds and statistics of excitations which include both point particles and defect lines. Potential connection with recently proposed fraction topological insulators is described.

Zhiyong Wang, Peng Wei, Jing Shi*

Bi₂Se₃ has been predicted to be a three-dimensional (3D) topological insulator (TI) with Dirac fermions residing on the two-dimensional (2D) surface. Unique transport properties such as high carrier mobility due to the suppressed backscattering are expected for the Dirac fermions. In order to eliminate the contribution of the bulk carriers, therefore, to place the Fermi level in the band gap of Bi₂Se₃, we first introduce various amounts of Ca dopants into the crystal to realize the bulk insulating state. Then by avoiding uncontrolled heating and electron beam irradiation in the nanofabrication process, we maintain the insulating state in thin devices. By sweeping the gate voltage, we have observed a conductivity maximum that is expected for the Dirac fermions in the band gap of 3D TIs.

Topological insulator nanostructures: from materials synthesis to device applications

Hui Li, Wen-hui Dang, Hai-lin Peng*, Zhong-fan Liu

Nanostructured topological insulator materials such as ultrathin films, nanoplates, nanowires, and nanoribbons are attracting much attention for fundamental research as well as potential applications in low-energy dissipation electronics, spintronics, thermoelectrics, magnetoelectrics, and quantum computing due to their extremely large surface-to-volume ratios and exotic metallic edge/surface states. Layered Bi₂Se₃ and Bi₂Te₃ serve as reference topological insulator materials with a large nontrivial bulk gap up to 0.3 eV (equivalent to 3600 K) and simple single-Dirac-cone surface states. In this mini-review, we present an overview of recent advances in nanostructured topological insulator Bi₂Se₃ and Bi₃Te₃ from the viewpoints of controlled synthesis and physical properties. We summarize our recent achievements in the vapor-phase synthesis and structural characterization of nanostructured topological insulator Bi₂Se₃ and Bi₂Te₃, such as nanoribbons and ultrathin nanoplates. We also demonstrate the evolution of Raman spectra with the number of few-layer topological insulators, as well as the transport measurements that have succeeded in accessing the surface conductance and surface state manipulations in the device of topological insulator nanostructures.

Transport properties of Bi₂Se₃ thin films tunable with back-gates

Yong-qing Li*, Ke-hui Wu, Li Lv, Jun-ren Shi, Xin-cheng Xie

We review progresses made in the study of transport properties of three-dimensional topological insulators with emphasis on the Bi₂Se₃ thin films in which the chemical potential can be tuned with back-gates. Experimental signatures for the effective gate-tuning include the gate-voltage dependence of the longitudinal resistivity, Hall resistivity, and the magnetoconductivity due to weak antilocalization. Latest data on the transport measurements down to milli-Kelvin temperatures will also be discussed in the context of the electron-electron interaction and the metal-insulator transition.

Pressure induced superconductivity in topological compounds

Jun-liang Zhang, Si-jia Zhang, Hong-ming Weng, Wang Zhang, Qing-qing Liu, Xian-cheng Wang, Ri-cheng Yu, Shoucheng Zhang, Xi Dai, Zhong Fang, Chang-Qing Jin*

We report successful observation of pressure induced superconductivity in topological compound Bi2Te3 single crystal induced via pressure. The combined high pressure structure investigations with first-principles calculations indicated that the superconductivity occurs at the ambient phase of topologically nontrivial. The results suggest topological superconductivity can be realized in parent state of Bi2Te3.

Quantum spin hall effect and superconducting proximity effect in inas/gasb quantum wells: Experimental aspects

Ivan Knez, Rui-Rui Du*

The quantum Hall effect refers to the quantization of charge Hall resistance (in units of h/e², where h is the Planck constant and e is the electronic charge), observed in a quantum well when an intense magnetic field is applied at low temperatures. The Quantum Spin Hall Effect (QSHE) is an emergent phenomenon recently discovered in semiconductors and other novel materials, where the intrinsic spin Hall conductance is quantized in the absence of any magnetic field. This will review the experimental progress in low temperature quantum transport studies in InAs/GaAs quantum wells, in which an inverted band structure can be engineered and fine-tuned by electrostatic gates. We describe experimental results showing energy gap and edge transport in this system. We will also describe preliminary results for superconducting proximity effect in Nb/InAs/GaSb junctions. The prospects for observing QSHE in this material and novel correlated properties, as predicted by recent theories, will be discussed. 

Studies on multiferroic materials in high magnetic fields

M. Tokunaga

In this article, studies on the magnetoelectric effects of multiferroic materials in high magnetic fields, particularly pulsed magnetic fields, are discussed and results for some representative materials are presented. In the discussions on representative materials, the relationship between the crystallographic symmetry and the linear magnetoelectric effect in Cr2O3 is introduced. Then drastic changes in polarization caused by magnetic transitions are discussed through a case study of manganites with a perovskite-type structure. In addition, high field studies on the magnetoelectric effects in BiFeO3, which is an exceptional multiferroic material, are presented and discussed in the framework of the Landau–Ginzburg theory.

Electronic Structure of YMn₂O₅ Studied by EELS and First-principles Calculations

The electronic structure of multiferroic YMn₂O₅ material has been studied by using the generalized gradient approximation (GGA). The results demonstrate that the oxygen 2p and manganese 3d orbitals are strongly hybridized. Considering the on-site Coulomb interaction U, we performed the GGA+U calculations for 0<U≤8eV, it is found that the increase of U could enlarge the band gap and, on the other hand, weaken the Mn-O hybridization. Experimental measurements of the electron energy-loss spectrometry (EELS) exhibit a rich variety of structural features in both O-K edge and Mn-L edges. Theoretical and experimental analysis on the O-K edge suggests that the on-site Coulomb interaction (U) in YMn₂O₅ could be less than 4 eV.  Certain electronic structural features of LaMn₂O₅ have been discussed in comparison with that of YMn₂O₅.