Topological excitonic superfluids in three dimensions

Abstract

We study the equilibrium and non-equilibrium properties of topological dipolar intersurface exciton condensates within time-reversal invariant topological insulators in three spatial dimensions without a magnetic field. We elucidate that, in order to correctly identify the proper pairing symmetry within the condensate order parameter, the full three-dimensional Hamiltonian must be considered. As a corollary, we demonstrate that only particles with similar chirality play a significant role in condensate formation. Furthermore, we find that the intersurface exciton condensation is not suppressed by the interconnection of surfaces in three-dimensional topological insulators as the intersurface polarizability vanishes in the condensed phase. This eliminates the surface current flow leaving only intersurface current flow through the bulk. We conclude by illustrating how the excitonic superfluidity may be identified through an examination of the terminal currents above and below the condensate critical current.Dipolar excitonic superfluidity (DES) has appeared in a veritable manifold of systems including microcavities 1-3 , cold atom systems 4-8 and semiconductor quantum wells 9-14 . Within condensed matter, emergent materials offer the possibility of finding new DESs. To this end, spatially segregated monolayers of graphene have been both theoretically 15-17 and experimentally 18 explored for signatures of excitonic superfluidity. While signs of interlayer correlation are experimentally observed, additional fermionic degrees of freedom, or flavors, screen the strength of the interlayer interaction 19 making the observation of DES in graphene multilayers challenging.The advent of time-reversal invariant topological insulators (TI) 20;21 has brought renewed interest in finding DES in condensed matter systems. In sufficiently thin TI films, it has been proposed that spatially segregated surface electrons and holes may bind into a topological dipolar intersurface exciton superfluid (TDIES). To this point, existing approaches to TDIES have considered strictly two-dimensional Dirac surface states separated by an insulating spacer 22-26 . Yet the existence of a TDIES in three-dimensions is not a foregone conclusion based on two-dimensional surface state analysis. The most obvious drawback being that in a 3D TI, each of the surfaces is interconnected and there exists no obvious mechanism to segregate the electron and hole layers, as in other proposed systems.In this Letter, we theoretically study the equilibrium and non-equilibrium properties of TDIES in 3D TI and show that a stable TDIES may be formed. We link this stability of TDIES in 3D TI to the fact that intersurface polarizability vanishes in the TDIES phase forbidding quasiparticle recombination via single particle mechanisms. Further, we find that in order to obtain the proper form of the condensate order parameter, the full 3D Hamiltonian must be used. We propose that the TDIES phase may be observed via examination of the FIG. 1: (a): Schematic of topological insula...

Chalcogenide Thermoelectrics Empowered by an Unconventional Bonding Mechanism

“…Furthermore, these results indicate that also electron-phonon coupling contributes to the positive temperature coefficients of the bandgap. [85][86][87] A similar scenario is also observed in PbSe and PbS using the same methods, whose L-and Σ-bands converge at 900 and 1000 K, respectively. [86] The temperature of band convergence can also be estimated by the peak position in the measurement of Hall coefficient with temperature.…”

“…[83] Alternatively, some reports claim that the effect of thermal disorder on the Pb sublattice in Pb chalcogenides is responsible for the positive temperature coefficient for the bandgap. [84,85] Indeed, ab initio molecular dynamics calculations demonstrate that the bandgap increase due only to lattice expansion is much smaller than the experimentally observed results for PbTe (Figure 7c,d). Furthermore, these results indicate that also electron-phonon coupling contributes to the positive temperature coefficients of the bandgap.…”

Effect of intrinsic defects on the thermal conductivity of PbTe from classical molecular dynamics simulations

“…The same potential was also utilized to investigate the effect of twin boundaries on the thermal transport in PbTe [24]. In [25], a combination of classical and ab initio MD was used to determine the effect of the strong anharmonicity of vibrations in PbTe on the electronic structure of the material and therefore on its thermoelectric figure of merit. The anomalous lattice dynamics observed in neutron diffraction experiments [26] was also interpreted as the result of a very large anharmonicity in short-range interactions [7,27].…”

Single interface effects dominate in exciton-condensate/normal-barrier/exciton-condensate (EC/N/EC) structures of long-barrier

“…In GaAs bilayers under strong magnetic field, the zero-bias tunneling anomaly due to coherent interlayer tunneling [6][7][8][9][10], the quantum Hall drag [6,[11][12][13][14] , and the counterflow supercurrents [6,10,13,[15][16][17][18][19][20], have each attracted intense research efforts while the new excitements are continued to be reported until to date [21][22][23][24][25][26][27] . The quest for indirect exciton condensates has recently extended to cover novel materials 3 , especially the topological materials [28][29][30][31][32] and the graphene based materials [33][34][35][36][37][38][39][40][41][42][43][44][45][46].The latter is particularly promising because the electron-hole separation can be reduced to atomic length scale in such systems to greatly ...…”