The Bose condensation of 2D dipolar excitons in quantum wells is numerically studied by the diffusion Monte Carlo simulation method. The correlation, microscopic, thermodynamic, and spectral characteristics are calculated. It is shown that, in structures of coupled quantum wells, in which low-temperature features of exciton luminescence have presently been observed, dipolar excitons form a strongly correlated system. Their Bose condensation can experimentally be achieved much easily than for ideal or weakly correlated excitons.
Strongly correlated quantum particles in lattice potentials are the building blocks for a large variety of quantum insulators, for instance Mott phases and density waves breaking the lattice symmetry. Such collective states are accessible to bosonic and fermionic systems. To expand further the spectrum of accessible quantum matter phases, mixing both species is theoretically appealing, since density order then competes with phase separation. Here we manipulate such Bose-Fermi mixture by confining neutral (boson-like) and charged (fermion-like) dipolar excitons in an artificial square lattice o…
Two-dimensional (2D) moire materials provide a new solid-state platform with unprecedented controllability for studies of correlated quantum phenomena. To date, experimental studies have focused on the correlated electronic states; the correlated bosonic states in moire materials have remained practically unexplored. Here, we report a correlated dipolar excitonic insulator, a charge insulating state driven by the formation of excitons, in a Coulomb-coupled WSe2 monolayer and WSe2/WS2 moire bilayer at total hole doping density equal to the moire density. The system is a Mott insulator when all …
Bose condensation has shaped our understanding of macroscopic quantum phenomena, having been realized in superconductors, atomic gases, and liquid helium. Excitons are bosons that have been predicted to condense into either a superfluid or an insulating electronic crystal. Using the recently developed momentum-resolved electron energy-loss spectroscopy (M-EELS), we study electronic collective modes in the transition metal dichalcogenide semimetal, 1T-TiSe$_2$. Near the phase transition temperature, T$_c$ = 190 K, the energy of the electronic mode falls to zero at nonzero momentum, indicating d…
Excitonic condensation of strongly correlated electrons: the case of Pr$_{0.5}$Ca$_{0.5}$CoO$_3$
我们使用动态平均场模型计算和LDA+U材料特定计算相结合的方法研究了（Pr $_{1-y}$ R $_y$）$_x$ Ca $_{1-x}$ CoO $_3$（R=Nd，Sm，Eu，Gd，Tb，Y）系列化合物（PCCO）的低温相变.转变，标志着由一个尖锐的峰值在比热，导致直流电阻率的指数增加和磁化率的下降，但尚未确定序参量。我们表明，凝聚的自旋三重态，原子大小的激子提供了一个一致的解释所观察到的物理。特别是，它解释了交换分裂的Pr网站和同时Pr价跃迁。PCCO中的激子凝聚是在自旋态转变附近的某些系统中预期的一般行为的示例。
The Hubbard model constitutes one of the most celebrated theoretical frameworks of condensed-matter physics. It describes strongly correlated phases of interacting quantum particles confined in lattice potentials. For bosons, the Hubbard Hamiltonian has been deeply scrutinised for short-range on-site interactions. However, accessing longer-range couplings has remained elusive experimentally. This marks the frontier towards the extended Bose-Hubbard Hamiltonian that allows insulating ordered phases at fractional lattice fillings. Here we implement this Hamiltonian by confining semiconductor dip…
Extensive theoretical and experimental investigation has been conducted on fermion pair condensation and exciton condensation as distinct classes of Bose-Einstein-like condensation. In this work, the existence of a fermion-exciton condensate---a single quantum state in which character of both fermion pair and exciton condensates coexist---is established computationally in the low-particle-number ($N$) limit and theoretically in the large-$N$ thermodynamic limit. The trade-off between the fermion pair and excitonic character of the fermion-exciton condensate is shown to be elliptic in nature. T…
Comment on: "Auger decay, Spin-exchange, and their connection to Bose-Einstein condensation of excitons in Cu_2O " by G.M. Kavoulakis, A. Mysyrowicz (cond-mat/0001438)
Exciton condensation indicating the spontaneous formation of electron-hole pair can cause the phase transition from a semimetal to an excitonic insulator by gap opening at the Fermi surface. While the idea of this excitonic insulator has been proposed for decades, current theoretical approaches can only provide qualitative descriptions, and a quantitative predicting tool is still missing. To shed insight on this problem, we developed an $ab~initio$ method based on the finite-temperature density functional theory and many-body perturbation theory to compute the exciton condensation critical beh…
The Bose-Einstein condensation of excitons into a single quantum state is known as exciton condensation. Exciton condensation, which potentially supports the frictionless flow of energy, has recently been realized in graphene bilayers and van der Waals heterostructures. Here we show that exciton condensates can be predicted from a combination of reduced density matrix theory and cumulant theory. We show that exciton condensation occurs if and only if there exists a large eigenvalue in the cumulant part of the particle-hole reduced density matrix. In the thermodynamic limit we show that the lar…
Tunneling spectroscopy reveals evidence for interlayer electron-hole correlations in quantum Hall bilayer two-dimensional electron systems at layer separations near, but above, the transition to the incompressible exciton condensate at total Landau level filling $ν_T=1$. These correlations are manifested by a non-linear suppression of the Coulomb pseudogap which inhibits low energy interlayer tunneling in weakly-coupled bilayers. The pseudogap suppression is strongest at $ν_T=1$ and grows rapidly as the critical layer separation for exciton condensation is approached from above.
Quadrupolar and dipolar phases of excitons in transition-metal dichalcogenide trilayer heterostructures
The Racah Institute of Physics, The Hebrew University of Jerusalem（拉卡物理研究所，耶路撒冷希伯来大学）；Department of Physics, Technion（理工学院物理系）；The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem（分子动力学弗里茨·哈伯研究中心，耶路撒冷希伯来大学）
An exciton is an electron-hole pair bound by attractive Coulomb interaction. Short-lived excitons have been detected by a variety of experimental probes in numerous contexts. An excitonic insulator, a collective state of such excitons, has been more elusive. Here, thanks to Nernst measurements in pulsed magnetic fields, we show that in graphite there is a critical temperature (T = 9.2 K) and a critical magnetic field (B = 47 T) for Bose-Einstein condensation of excitons. At this critical field, hole and electron Landau sub-bands simultaneously cross the Fermi level and allow exciton formation.…
At a large displacement field, in rhomboedral and Bernal-stacked graphene a normal paramagnetic state transitions to a correlated state. Recent experiments showed that such systems have several phase transitions as a function of the carrier density. The phase adjacent to a paramagnetic state has anomalously high resistance and reduced degeneracy of the Fermi sea. We show that both phenomena can be explained through a concept of partial intervalley exciton condensation: a fraction of particles condenses into excitons, and another forms an intervalley coherent Fermi liquid. The exciton part of t…
The Bose-Einstein condensation of excitons continues to garner immense attention as a prototypical example for observing emergent properties from many-body quantum effects. In particular, Titanium Diselenide (TiSe$_2$) is a promising candidate for realizing exciton condensation and was experimentally observed only very recently. Surprisingly, the condensate was experimentally characterized by a soft plasmon mode that only exists near the transition temperature, $T_c$, of the charge density wave (CDW). Here, we characterize and analyze the experimental spectra using linear-response time-depende…
Trimer superfluidity of antiparallel dipolar excitons in a bilayer heterostructure
Racah Institute of Physics, The Hebrew University of Jerusalem（拉卡物理研究所，耶路撒冷希伯来大学）；The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem（弗里茨·哈伯分子动力学研究中心，耶路撒冷希伯来大学）
