Excitonic condensate has been long-sought within bulk indirect-gap semiconductors, quantum wells, and 2D material layers, all tried as carrying media. Here we propose intrinsically stable 2D semiconductor heterostructures with doubly-indirect overlapping bands as optimal platforms for excitonic condensation. After screening hundreds of 2D materials, we identify candidates where spontaneous excitonic condensation mediated by purely electronic interaction should occur, and hetero-pairs Sb2Te2Se/BiTeCl, Hf2N2I2/Zr2N2Cl2, and LiAlTe2/BiTeI emerge promising. Unlike monolayers, where excitonic condensation is hampered by Peierls instability, or other bilayers, where doping by applied voltage is required, rendering them essentially non-equilibrium systems, the chemically-specific heterostructures predicted here are lattice-matched, show no detrimental electronic instability, and display broken type-III gap, thus offering optimal carrier density without any gate voltages, in true-equilibrium. Predicted materials can be used to access different parts of electron-hole phase diagram, including BEC-BCS crossover, enabling tantalizing applications in superfluid transport, Josephson-like tunneling, and dissipationless charge counterflow.
The optical spectra of vertically stacked MoSe$_2$/WSe$_2$ heterostructures contain additional 'interlayer' excitonic peaks that are absent in the individual monolayer materials and exhibit a significant spatial charge separation in out-of-plane direction. Extending on a previous study, we used a many-body perturbation theory approach to simulate and analyse the excitonic spectra of MoSe$_2$/WSe$_2$ heterobilayers with three stacking orders, considering both momentum-direct and momentum-indirect excitons. We find that the small oscillator strengths and corresponding optical responses of the in…
Superfluidity in e-h bilayers in graphene and GaAs has been predicted many times but not observed. A key problem is how to treat the screening of the Coulomb interaction for pairing. Different mean-field theories give dramatically different conclusions, and we test them against diffusion Monte-Carlo calculations. We get excellent agreement with the mean-field theory that uses screening in the superfluid state, but large discrepancies with the others. The theory predicts no superfluidity in existing devices and gives pointers for new devices to generate superfluidity.
We predict the formation and superfluidity of polaritons in an optical microcavity formed by excitons in gapped graphene embedded there and microcavity photons. The Rabi splitting related to the creation of an exciton in a graphene layer in the presence of the band gap is obtained. The analysis of collective excitations as well as the sound velocity is presented. We show that the superfluid density and temperature of the Kosterlitz-Thouless phase transition are decreasing functions of the energy gap.
Moiré patterns of transition metal dichalcogenide (TMD) heterobilayers have proven to be an ideal platform to host unusual correlated electronic phases, emerging magnetism, and correlated exciton physics. While the existence of novel moiré excitonic states is established through optical measurements, the microscopic nature of these states is still poorly understood, often relying on empirically fit models. Here, combining large-scale first-principles GW-BSE calculations and micro-reflection spectroscopy, we identify the nature of the exciton resonances in WSe$_2$/WS$_2$ moiré superlattices, di…
Van der Waals heterostructures of 2D materials provide a powerful approach towards engineering various quantum phases of matters. Examples include topological matters such as quantum spin Hall (QSH) insulator, and correlated matters such as exciton superfluid. It can be of great interest to realize these vastly different quantum matters on a common platform, however, their distinct origins tend to restrict them to material systems of incompatible characters. Here we show that heterobilayers of two-dimensional valley semiconductors can be tuned through interlayer bias between an exciton superfl…
Moiré Phonons and Emergent Exciton-Phonon Coupling in a Moiré Heterobilayer
University of California, Berkeley（加州大学伯克利分校）；Kavli Energy NanoScience Institute（卡弗里能源纳米科学研究所）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Indian Institute of Science（印度科学学院）；Newcastle University（纽卡斯尔大学）；National Institute for Materials Science（国立材料研究所）；University of Texas at Austin（德克萨斯大学奥斯汀分校）；Northeastern University（东北大学）
Due to their unique two-dimensional nature, charge carriers in semiconducting transition metal dichalcogenides (TMDs) exhibit strong unscreened Coulomb interactions and sensitivity to defects and impurities. The versatility of van der Waals layer stacking allows spatially separating electrons and holes between different TMD layers with staggered band structure, yielding interlayer few-body excitonic complexes whose nature is still debated. Here we combine quantum Monte Carlo calculations with spectrally and temporally resolved photoluminescence measurements on a top- and bottom-gated MoSe2/WSe…
We discuss the conditions under which the predicted (but not yet observed) zero-field interlayer excitonic condensation in double layer graphene has a critical temperature high enough to allow detection. Crucially, disorder arising from charged impurities and corrugation in the lattice structure --- invariably present in all real samples --- affects the formation of the condensate via the induced charge inhomogeneity. In the former case, we use a numerical Thomas-Fermi-Dirac theory to describe the local fluctuations in the electronic density in double layer graphene devices and estimate the ef…
Quantum spin Hall (QSH) insulator with large gap is highly desirable for potential spintronics application. Here we realize electrically tunable QSH insulator with large gap in van der Waals heterobilayer of monolayer transition metal dichalcogenide (TMD) and hexagonal BAs. When the type II band alignment gets inverted in an electric field, the hybridization by interlayer hopping between the spin-valley locked valence band edges in TMD and the BAs conduction band edges leads to a stacking-configuration dependent topological band inversion. In the non-interacting limit, the double spin degenera…
Valley selective hybridization and residual coupling of electronic states in commensurate van der Waals heterobilayers enable the control of the orbital character of interlayer excitons. We demonstrate electric field control of layer index, orbital character, lifetime and emission energy of indirect excitons in MoS2/WS2 heterobilayers embedded in an vdW field effect structure. Different excitonic dipoles normal to the layers are found to stem from bound electrons and holes located in different valleys of MoS2/WS2 with a valley selective degree of hybridization. For the energetically lowest emi…
In the first part of this letter we discuss electrodynamics of an excitonic condensate in a bilayer. We show that under certain conditions the system has a dominant energy scale and is described by the effective electrodynamics with "planar magnetic charges". In the second part of the paper we point out that a vortex liquid state in bilayer superconductors also possesses dipolar superfluid modes and establish equivalence mapping between this state and a dipolar excitonic condensate. We point out that a vortex liquid state in an N-layer superconductor possesses multiple topologically coupled di…
Ensembles of indirect or interlayer excitons (IXs) are intriguing systems to explore classical and quantum phases of interacting bosonic ensembles. IXs are composite bosons that feature enlarged lifetimes due to the reduced overlap of the electron-hole wave functions. We demonstrate electric Field control of indirect excitons in MoS2/WS2 hetero-bilayers embedded in a field effect structure with few-layer hexagonal boron nitrite as insulator and few-layer graphene as gate-electrodes. The different strength of the excitonic dipoles and a distinct temperature dependence identify the indirect exci…
We propose experiments to observe Bose-Einstein condensation (BEC) and superfluidity of quasi-two-dimensional (2D) spatially indirect magnetoexcitons in bilayer graphene. The magnetic field $B$ is assumed strong. The energy spectrum of collective excitations, the sound spectrum as well as the effective magnetic mass of magnetoexcitons are presented in the strong magnetic field regime. The superfluid density $n_S$ and the temperature of the Kosterlitz-Thouless phase transition $T_c$ are shown to be increasing functions of the excitonic density $n$ but decreasing functions of $B$ and the interla…
The theory of what happens to a superfluid in a random field, known as the ``dirty boson'' problem, directly relates to a real experimental system presently under study by several groups, namely excitons in coupled semiconductor quantum wells. We consider the case of bosons in two dimensions in a random field, when the random field can be large compared to the repulsive exciton-exciton interaction energy, but is small compared to the exciton binding energy. The interaction between excitons is taken into account in the ladder approximation. The coherent potential approximation allows us to deri…
The influence of a random field induced by impurities, boundary irregularities etc. on the superfluidity of a quasi-two-dimensional (2D) system of spatially indirect excitons in coupled quantum wells is studied. The interaction between excitons is taken into account in the ladder approximation. The random field is allowed to be large compared to the dipole-dipole repulsion between excitons. The coherent potential approximation (CPA) allows us to derive the exciton Green's function for a wide range of the random field, and the CPA results are used in the weak-scattering limit, which results in …
The collective properties of different quasiparticles in various graphene based structures in high magnetic field have been studied. We predict Bose-Einstein condensation (BEC) and superfluidity of 2D spatially indirect magnetoexcitons in two-layer graphene. The superfluid density and the temperature of the Kosterlitz-Thouless phase transition are shown to be increasing functions of the excitonic density but decreasing functions of magnetic field and the interlayer separation. The instability of the ground state of the interacting 2D indirect magnetoexcitons in a slab of superlattice with alte…
