Superfluidity of excitons and polaritons in novel two-dimensional materials

Abstract:

This talk reviews the theoretical studies of the Bose-Einstein condensation (BEC) and superfluidity of indirect excitons and microcavity polaritons in quasi-two-dimensional (quasi-2D) van der Waals nanomaterials such as transition metal dichalcogenide (TMDC) heterostructures and phosphorene. Indirect excitons are the Coulomb-bound pairs of electrons and holes confined to different parallel monolayers of a layered planar nanomaterial structure. The high-T superfluidity of the two-component weakly-interacting Bose gas of the A-type and B-type indirect excitons in the TMDC heterostructures is proposed [1,2]. The critical temperature and superfluid velocity of the indirect excitons in a bilayer phosphorene nanostructure is shown to be anisotropic, dependent strongly on the particular direction of the exciton propagation [3]. The spin Hall effect for polaritons (SHEP) in a TMDC monolayer embedded in a microcavity is predicted [4]. It is demonstrated that two counterpropagating laser beams incident on a TMDC monolayer can deflect a superfluid polariton flow due to the generation the effective gauge vector and scalar potentials [4]. The polaritons cloud is formed due to the coupling of excitons created in a TMDC layer and microcavity photons. It was demonstrated that the polariton flows in the same valley are splitting: the superfluid components of the Aand Bpolariton flows propagate in opposite directionsalong the counterpropagating beams, while the normal components of the flows slightly deflect in opposite directionsand propagate almost perpendicularly to the beams [4]. The components of polariton conductivity tensor were obtained for polaritons without Bose-Einstein condensation (BEC) and in the presence of BEC and superfluidity [4]. The possible experimental observation of SHEP is discussed. These results open up new avenues for the experimental realization of the exciton and polariton BEC and superfluidity phenomena as well as their practical applications in optoelectronics [5].

[1] O. L. Berman and R. Ya. Kezerashvili, Phys. Rev. B 93, 245410 (2016)

[2] O. L. Berman and R. Ya. Kezerashvili, Phys. Rev. B 96, 094502 (2017). 

[3] O. L. Berman, G. Gumbs, and R. Ya. Kezerashvili, Phys. Rev. B 96, 014505 (2017).

[4] O. L. Berman, R. Ya. Kezerashvili, and Yu. E. Lozovik, Phys. Rev. B 99, 085438 (2019).

[5] D. W. Snoke and J. Keeling, Physics Today 70, 54 (2017).

Unconventional thermal transport

David Broido
Department of Physics
Boston College

According to conventional theories of heat conduction in semiconductors and insulators, only crystals composed of strongly-bonded light elements can have high lattice thermal conductivity, k_L, and intrinsic thermal resistance comes only from lowest-order anharmonic three-phonon interactions. In this talk, I will discuss aspects of a new paradigm for achieving high k_L that we proposed, in which the vibrational properties are tailored to reduce the phase space for three-phonon scattering [1].  Our ab initio calculations predicted that one candidate material, cubic Boron Arsenide (BAs), indeed had ultrahigh three-phonon limited k_L comparable to that of the best heat conductor, diamond, and significantly higher than any other semiconductor [1]. In BAs, three-phonon scattering can become so weak that four-phonon scattering also plays an important role in limiting k_L [2, 3]. Such unconventional transport behavior has been confirmed in recent experiments [3-5].  It gives rise to anomalous non-monotonic pressure dependence of k_L [6]. I will review the challenging material constraints, which must be overcome in order to achieve the unconventional high k_L. An interesting case is that of group V transition metal carbides, NbC, TaC and VC.  These metals have ideal vibrational properties for the desired weak phonon-phonon scattering. But, their nested Fermi surfaces give rise to strong scattering between phonons and electrons, which results in an orders-of-magnitude lower k_L that is nearly temperature independent, contrary to the typical behavior.

[1] L. Lindsay, D. A. Broido, and T. L. Reinecke, Phys. Rev. Lett. 111, 025901 (2013).
[2] T. Feng, L. Lindsay, and X. Ruan, Phys. Rev. B, 96, 161201 (2017).
[3] F. Tian et al., Science 361, 582 (2018).
[4] J. S. Kang M. Li, H. Wu, H Nguyen, and Y. Hu., Science 361, 575 (2018).
[5] S. Li, Q. Zheng, Y. Lv, X. Liu, X. Wang, P. Y. Huang, D. G. Cahill, and B. Lv, Science 361, 579 (2018).
[6] N. K. Ravichandran and D. Broido, Nature Communications (2019).
[7] C. Li, N. K. Ravichandran, L. Lindsay, and D. Broido,Phys. Rev. Lett. 121, 175901 (2018).

Jun Zhu - Penn State

host: Phil Allen

Host — Phil Allen

host: Jin Wang

host: Phil Allen