Holon-doublon Pair Research Articles

Magnetically bound nature of a holon-doublon pair in two-dimensional photoexcited Mott insulators

We revisit the holon-doublon binding problem in two-dimensional (2D) photoexcited Mott insulators. Low-energy photoexcited states in Mott insulators are described as a pair state of a doublon and a holon. The most basic question is whether its bound state is formed in the lowest-energy state, and negative and positive responses have been discussed in the past. In this study we begin with the 2D Hubbard model, and transform it into the first effective model, which is based on the $t/U$ expansion, with $U$ and $t$ being the Hubbard $U$ and the electron hopping energy, respectively. We find that quantitative reliability is assured for $U/t\ensuremath{\gtrsim}10$. Furthermore, we transform it into a second effective model that selects essential states in the low-energy region. In both effective models we distinguish two magnetic terms, namely, the spin-exchange interaction and the three-site transfer, and parametrize the two terms with the parameters ${J}_{\mathrm{ex}}$ and ${J}_{3\mathrm{site}}$. By changing the parameters apart from the restriction given by the Hubbard model, any positive ${J}_{\mathrm{ex}}$ value with ${J}_{3\mathrm{site}}=0$ yields a finite amount of binding, whereas a finite value of ${J}_{3\mathrm{site}}$ suppresses the binding significantly, still leaving the Hubbard case of $U=10t$ in the vicinity of the bound-unbound boundary.

Probing the phase transition in VO2 using few-cycle 1.8 μm pulses

We observe a nearly instantaneous triggering of the phase transition in ${\mathrm{VO}}_{2}$ using transient, time-resolved absorption techniques from few-cycle, infrared (1.8 $\ensuremath{\mu}\mathrm{m}$) laser pulses. The results are in agreement with the Mott-Hubbard insulator model, characterized by electronic holon-doublon pair creation that initiates the insulator-to-metal phase transition within the material. The spectral resolution provided by this technique can be exploited to measure the chirp of the probe pulses. Effects from probing above and below the band gap of the material are also discussed.

Current reversals and metastable states in the infinite Bose-Hubbard chain with local particle loss

We present an algorithm which combines the quantum trajectory approach to open quantum systems with a density-matrix renormalization group scheme for infinite one-dimensional lattice systems. We apply this method to investigate the long-time dynamics in the Bose-Hubbard model with local particle loss starting from a Mott-insulating initial state with one boson per site. While the short-time dynamics can be described even quantitatively by an equation of motion (EOM) approach at the mean-field level, many-body interactions lead to unexpected effects at intermediate and long times: local particle currents far away from the dissipative site start to reverse direction ultimately leading to a metastable state with a total particle current pointing away from the lossy site. An alternative EOM approach based on an effective fermion model shows that the reversal of currents can be understood qualitatively by the creation of holon-doublon pairs at the edge of the region of reduced particle density. The doublons are then able to escape while the holes move towards the dissipative site, a process reminiscent---in a loose sense---of Hawking radiation.

Photoinduced absorptions inside the Mott gap in the two-dimensional extended Hubbard model

We theoretically investigate pump-probe optical responses in the two-dimensional extended Hubbard model describing cuprates by using a time-dependent Lanczos method. At half filling, pumping generates photoinduced absorptions inside the Mott gap. A part of low-energy absorptions is attributed to the independent propagation of photoinduced holons and doublons. The spectral weight just below the Mott gap increases with decreasing the on-site Coulomb interaction $U$. We find that the next-nearest-neighbor Coulomb interaction $V_1$ enhances this $U$ dependence, indicating the presence of biexcitonic contributions formed by two holon-doublon pairs. Photo-pumping in hole-doped systems also induces spectral weights below remnant Mott-gap excitations, being consistent with recent experiments. The induced weights are less sensitive to $V_1$ and may be related to the formation of a biexcitonic state in the presence of hole carriers.

Charge relaxation and recombination in photo-excited Mott insulators

Recent femtosecond pump-probe experiments on Mott insulators reveal charge recombination, which is in picosecond range, i.e., much faster than in clean bandgap semiconductors although excitation gaps in Mott insulators are even larger. The charge response in photo-excited insulators can be generally divided in femtosecond transient relaxation of charge excitations, which are holons and doublons, and a second slower, but still very fast, holon–doublon (HD) recombination. We present a theory of the recombination rate of the excited HD pairs, based on the two-dimensional (2D) model relevant for cuprates, which shows that such fast processes can be explained even quantitatively with the multi-magnon emission. We show that the condition for the exponential decay as observed in the experiment is the existence of the exciton, i.e., the bound HD pair. Its recombination rate is exponentially dependent on the charge gap and on the magnon energy, while the ultrafast process can be traced back to strong charge-spin coupling. We comment also fast recombination times in the one-dimensional (1D) Mott insulators, as e.g., organic salts. The recombination rate in the latter cases can be explained with the stronger coupling with phonon excitations.

Charge recombination in undoped cuprates

We theoretically analyse the process of charge recombination in the planar Mott-Hubbard insulators with the aim to explain short picosecond-range lifetime of photoexcited carriers, experimentally studied via pump-probe experiments on the undoped cuprates. The recombination mechanism consists of two essential ingredients: the formation of a metastable s-type bound holon-doublon pair, i.e. the Mott exciton, and the decay of such an excitonic state via the multimagnon emission. In spite of the large gap that requires many bosons to be emitted, latter process is fast due to large exchange scale and strong charge-spin coupling in planar systems. As the starting microscopic model we consider the single-band Hubbard model, and then more realistic three-band model for cuprates, both leading to the same minimal one. The decay rate of the exciton is evaluated numerically via the Fermi golden rule, having consistency also with the direct time-evolution calculation. The decay rate reveals exponential dependence on the ratio of the Mott-Hubbard gap and the exchange coupling - the result qualitatively reproduced also within a toy exciton-boson model.

Correlation Strength Crossover of Auger Recombination in One-Dimensional Mott Insulators

We theoretically investigate the relaxation of photogenerated charge carriers in one-dimensional Mott insulators. We adopt the Pariser–Parr–Pople model, and numerically calculate the time development of the nonequilibrium state excited by a weak light pulse. We investigate the dependence of the dynamics on the Coulomb correlation strength in the case where the binding effect between photogenerated opposite charges is significant. In the strong-correlation region (\(U \gg W\)), where U is the onsite Coulomb interaction energy and W is the bandwidth, the Auger recombination where two bound holon–doublon pairs decay into one unbound holon–doublon pair dominates the decay dynamics. From the strong- to intermediate (\(U \simeq W\))-correlation regions, the Auger recombination is always dominant, but the charge carriers involved in the process cannot be described by a holon and a doublon in the intermediate-correlation region. In the crossover region between these two regions, the Auger coefficient is strongly ...

Pressure-Dependent Relaxation in the Photoexcited Mott InsulatorET–F2TCNQ: Influence of Hopping and Correlations on Quasiparticle Recombination Rates

We measure the ultrafast recombination of photoexcited quasiparticles (holon-doublon pairs) in the one dimensional Mott insulator ET-F(2)TCNQ as a function of external pressure, which is used to tune the electronic structure. At each pressure value, we first fit the static optical properties and extract the electronic bandwidth t and the intersite correlation energy V. We then measure the recombination times as a function of pressure, and we correlate them with the corresponding microscopic parameters. We find that the recombination times scale differently than for metals and semiconductors. A fit to our data based on the time-dependent extended Hubbard Hamiltonian suggests that the competition between local recombination and delocalization of the Mott-Hubbard exciton dictates the efficiency of the recombination.

Ultrafast Charge Recombination in a Photoexcited Mott-Hubbard Insulator

We present a calculation of the recombination rate of the excited holon-doublon pairs based on the two-dimensional model relevant for undoped cuprates, which shows that fast processes, observed in pump-probe experiments on Mott-Hubbard insulators in the picosecond range, can be explained even quantitatively with the multimagnon emission. The precondition is the existence of the Mott-Hubbard bound exciton of the s-type. We find that its decay is exponentially dependent on the Mott-Hubbard gap and on the magnon energy, with a small prefactor, which can be traced back to strong correlations and consequently large exciton-magnon coupling.

Nonequilibrium dynamics of multiorbital correlated electron system under time‐dependent electric fields

AbstractTo clarify a key role of orbital degrees of freedom in the response of the many‐body electron state of correlated electrons to an external field, we investigate the real‐time dynamics in an eg‐orbital Hubbard model under applied electric fields by exploiting numerical techniques such as a time‐dependent density‐matrix renormalization group (DMRG) method. The ground state without applying an electric field is found to be an antiferromagnetic/ferro‐orbital state. After we switch on an oscillating electric field, it gives rise to the creation of holon–doublon pairs when the frequency of the applied electric field exceeds a charge excitation gap. We argue that a pair‐hopping process yields a dynamical deformation of the orbital configuration.

Dynamics of doublon-holon pairs in Hubbard two-leg ladders

The dynamics of holon-doublon pairs is studied in Hubbard two-leg ladders using the time-dependent density matrix renormalization group method. We find that the geometry of the two-leg ladder, which is qualitatively different from a one-dimensional chain due to the presence of a spin gap, strongly affects the propagation of a doublon-holon pair. Two distinct regimes are identified. For weak interleg coupling, the results are qualitatively similar to the case of the propagation previously reported in Hubbard chains, with only a renormalization of parameters. More interesting is the case of strong interleg coupling where substantial differences arise, particularly regarding the double occupancy and properties of the excitations such as the doublon speed. Our results suggest a connection between the presence of a spin gap and qualitative changes in the doublon speed, indicating a weak coupling between the doublon and the magnetic excitations.

Dielectric Breakdown in Spin-Polarized Mott Insulator

Nonlinear response of a Mott insulator to external electric field, corresponding to dielectric breakdown phenomenon, is studied within of a one-dimensional half-filled Hubbard model. It is shown that in the limit of nearly spin-polarized insulator the decay rate of the ground state into excited holon-doublon pair can be evaluated numerically as well to high accuracy analytically. Results show that the threshold field depends on the charge gap as F(th)∝Δ(3/2). Numerical results on small systems indicate on the persistence of a similar mechanism for the breakdown for decreasing magnetization down to unpolarized system.

Charge Excitations in Two-Leg Ladders: A tDMRG Approach

We study the dynamics of holon–doublon pairs in two-leg Hubbard ladders with the time-dependent Density Matrix Renormalization-Group approach. Benchmark results show that the Krylov algorithm is well suited to calculate the time dependence of observables in these systems. Furthermore, we show that the dynamics of the holon–doublon depend strongly on the coupling asymmetry within the ladder, indicating that the ladder geometry plays a role in the decay of these elementary charge excitations.

Quantum interference between charge excitation paths in a solid-state Mott insulator

Ultrafast spectroscopy reveals the many-body effects behind the metallization of a one-dimensional Mott insulator. Unlike in ultracold gases, these femtosecond excitation studies of quantum dynamics occur at room temperature. Competition between electron localization and delocalization in Mott insulators underpins the physics of strongly correlated electron systems. Photoexcitation, which redistributes charge, can control this many-body process on the ultrafast timescale1,2. So far, time-resolved studies have been carried out in solids in which other degrees of freedom, such as lattice, spin or orbital excitations3,4,5, dominate. However, the underlying quantum dynamics of ‘bare’ electronic excitations has remained out of reach. Quantum many-body dynamics are observed only in the controlled environment of optical lattices6,7 where the dynamics are slower and lattice excitations are absent. By using nearly single-cycle near-infrared pulses, we have measured coherent electronic excitations in the organic salt ET-F2TCNQ, a prototypical one-dimensional Mott insulator. After photoexcitation, a new resonance appears, which oscillates at 25 THz. Time-dependent simulations of the Mott–Hubbard Hamiltonian reproduce the oscillations, showing that electronic delocalization occurs through quantum interference between bound and ionized holon–doublon pairs.

Real-time dynamics of particle-hole excitations in Mott insulator-metal junctions

Charge excitations in Mott insulators (MIs) are distinct from their band-insulator counterparts and they can provide a mechanism for energy harvesting in solar cells based on strongly correlated electronic materials. In this paper, we study the real-time dynamics of holon-doublon pairs in a MI connected to metallic leads using the time-dependent density matrix renormalization group method. The transfer of charge across the MI-metal interface is controlled by both the electron-electron interaction strength within the MI and the voltage difference between the leads. We find an overall enhancement of the charge transfer as compared to the case of a (noninteracting) band insulator-metal interface with a similar band gap. Moreover, the propagation of holon-doublon excitations within the MI dynamically changes the spin-spin correlations, introducing time-dependent phase shifts in the spin structure factor.

Charge-spin-orbital dynamics of one-dimensional two-orbital Hubbard model

We study the real-time evolution of a charge-excited state in a one-dimensional eg-orbital degenerate Hubbard model, by a time-dependent density-matrix renormalization group method. Considering a chain along the z direction, electrons hop between adjacent 3z2−r2 orbitals, while x2−y2 orbitals are localized. For the charge-excited state, a holon-doublon pair is introduced into the ground state at quarter filling. At initial time, there is no electron in a holon site, while a pair of electrons occupies 3z2−r2 orbital in a doublon site. As the time evolves, the holon motion is governed by the nearest-neighbor hopping, but the electron pair can transfer between 3z2−r2 orbital and x2−y2 orbital through the pair hopping in addition to the nearest-neighbor hopping. Thus holon and doublon propagate at different speed due to the pair hopping that is characteristic of multi-orbital systems.