Showing papers on "Mott transition published in 2014"

Light-cone effect and supersonic correlations in one- and two-dimensional bosonic superfluids

Abstract: We study the spreading of density-density correlations in Bose-Hubbard models after a quench of the interaction strength, using time-dependent variational Monte Carlo simulations. It gives access to unprecedented long propagation times and to dimensions higher than one. In both one and two dimensions, we find ballistic light-cone spreading of correlations and extract accurate values of the light-cone velocity in the superfluid regime. We show that the spreading of correlations is generally supersonic, with a light-cone propagating faster than sound modes but slower than the maximum group velocity of density excitations, except at the Mott transition, where all the characteristic velocities are equal. Further, we show that in two dimensions the correlation spreading is highly anisotropic and presents nontrivial interference effects.

Correlation effects in (111) bilayers of perovskite transition-metal oxides

Abstract: We investigate the correlation-induced Mott, magnetic, and topological phase transitions in artificial (111) bilayers of perovskite transition-metal oxides $\mathrm{La}\mathrm{Au}{\mathrm{O}}_{3}$ and $\mathrm{Sr}\mathrm{Ir}{\mathrm{O}}_{3}$ for which the previous density-functional theory calculations predicted topological insulating states Using the dynamical-mean-field theory with realistic band structures and Coulomb interactions, $\mathrm{La}\mathrm{Au}{\mathrm{O}}_{3}$ bilayer is shown to be far away from a Mott insulating regime, and a topological-insulating state is robust On the other hand, $\mathrm{Sr}\mathrm{Ir}{\mathrm{O}}_{3}$ bilayer is on the verge of an orbital-selective topological Mott transition and turns to a trivial insulator by an antiferromagnetic ordering Oxide bilayers thus provide a novel class of topological materials for which the interplay between the spin-orbit coupling and electron-electron interactions is a fundamental ingredient

Characterization of a topological Mott insulator in one dimension

Abstract: We investigate properties of a topological Mott insulator in one dimension by examining the bulk topological invariant and the entanglement spectrum of a correlated electron model. We clarify how gapless edge states in a noninteracting topological band insulator evolve into spinon edge states in a topological Mott insulator. Furthermore, we propose a topological Mott transition, which is a new type of topological phase transition which has never been observed in free fermion systems. This unconventional transition occurs in spin liquid phases in the Mott insulator and is accompanied by zeros of the single-electron Green's function and a gap closing in the spin excitation spectrum.

First-Order Insulator-to-Metal Mott Transition in the Paramagnetic 3D System GaTa 4 Se 8

Abstract: The nature of the Mott transition in the absence of any symmetry breaking remains a matter of debate. We study the correlation-driven insulator-to-metal transition in the prototypical 3D Mott system GaTa(4)Se(8), as a function of temperature and applied pressure. We report novel experiments on single crystals, which demonstrate that the transition is of first order and follows from the coexistence of two states, one insulating and one metallic, that we toggle with a small bias current. We provide support for our findings by contrasting the experimental data with calculations that combine local density approximation with dynamical mean-field theory, which are in very good agreement.

Emergence of a Novel Pseudogap Metallic State in a Disordered 2D Mott Insulator

Abstract: We explore the nature of the phases and an unexpected disorder-driven Mott insulator to metal transition in a single crystal of the layered dichalcogenide $1\mathrm{T}\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ that is disordered without changing the carrier concentration by Cu intercalation. Angle resolved photoemission spectroscopy measurements reveal that increasing disorder introduces delocalized states within the Mott gap that lead to a finite conductivity, challenging conventional wisdom. Our results not only provide the first experimental realization of a disorder-induced metallic state but in addition also reveal that the metal is a non-Fermi liquid with a pseudogap with a suppressed density of states that persists at finite temperatures. Detailed theoretical analysis of the two-dimensional disordered Hubbard model shows that the novel metal is generated by the interplay of strong interaction and disorder.

Local Density of States Study of a Spin-Orbit-Coupling Induced Mott Insulator Sr 2 IrO 4

Abstract: We present scanning tunneling microscopy and spectroscopy experiments on the novel ${J}_{\mathrm{eff}}=1/2$ Mott insulator ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$. Local density of states (LDOS) measurements show an intrinsic insulating gap of 620 meV that is asymmetric about the Fermi level and is larger than previously reported values. The size of this gap suggests that ${\mathrm{Sr}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ is likely a Mott rather than Slater insulator. In addition, we found a small number of native defects which create in-gap spectral weight. Atomically resolved LDOS measurements on and off the defects show that this energy gap is quite fragile. Together the extended nature of the 5$d$ electrons and poor screening of defects help explain the elusive nature of this gap.

Monte Carlo simulation of the tight-binding model of graphene with partially screened Coulomb interactions

Abstract: We report on hybrid Monte Carlo simulations of the tight-binding model with long-range Coulomb interactions for the electronic properties of graphene. We investigate the spontaneous breaking of sublattice symmetry corresponding to a transition from the semimetal to an antiferromagnetic insulating phase. Our short-range interactions thereby include the partial screening due to electrons in higher energy states from ab initio calculations based on the constrained random phase approximation [T. O. Wehling et al., Phys. Rev. Lett. 106, 236805 (2011)]. In contrast to a similar previous Monte Carlo study [M. V. Ulybyshev et al., Phys. Rev. Lett. 111, 056801 (2013)], we also include a phenomenological model that describes the transition to the unscreened bare Coulomb interactions of graphene at half-filling in the long-wavelength limit. Our results show, however, that the critical coupling for the antiferromagnetic Mott transition is largely insensitive to the strength of these long-range Coulomb tails. They hence confirm the prediction that suspended graphene remains in the semimetal phase when a realistic static screening of the Coulomb interactions is included.

High-temperature Mott transition in wide-band-gap semiconductor quantum wells

Abstract: The crossover from an exciton gas to an electron-hole plasma is studied in a GaN/(Al,Ga)N single quantum well by means of combined time-resolved and continuous-wave photoluminescence measurements. The two-dimensional Mott transition is found to be of continuous type and to be accompanied by a characteristic modification of the quantum well emission spectrum. Beyond the critical density, the latter is strongly influenced by band-gap renormalization and Fermi filling of continuum states. Owing to the large binding energy of excitons in III-nitride heterostructures, their injection-induced dissociation could be tracked over a wide range of temperatures, i.e., from 4 to 150K. Various criteria defining the Mott transition are examined, which, however, do not lead to any clear trend with rising temperature: the critical carrier density remains invariant around 1012cm−2.

Magnetic properties and Mott transition in the Hubbard model on the anisotropic triangular lattice

Abstract: Magnetic phase diagram and Mott transition are studied in the Hubbard model on the anisotropic triangular lattice at zero temperature and half-filling by the variational cluster approximation, taking into account Neel, 120$^\circ$ Neel, and collinear orderings. Paramagnetic insulator (spin liquid) is realized above the metallic phase around the isotropic point. In general, this spin liquid state, continuously connected with the metallic state, changes to a magnetic state as the on-site Coulomb repulsion $U$ increases, but it persists up to large $U$ limit in a small window between 120$^\circ$ Neel and collinear phases. For very large $U$ another spin liquid state, separated from the metallic state by magnetic states, emerges around a narrow region where both Neel and 120$^\circ$ Neel orderings are highly suppressed due to the frustration and anisotropy. Implications for the $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ are discussed. As for the Mott transition, the structure of the self-energy in the spectral representation is studied in detail. As $U$ increases around the Mott transition point, single dispersion evolves in the spectral weights of the self-energy which splits the non-interacting band into the upper and lower Hubbard bands.

Self-energy and vertex functions from hybridization-expansion continuous-time quantum Monte Carlo for impurity models with retarded interaction

Abstract: Optimized measurements for the susceptibility, self-energy, as well as three-leg and four-leg vertex functions are introduced for the continuous-time hybridization-expansion quantum Monte Carlo solver for the impurity model in the presence of a retarded interaction. The self-energy and vertex functions are computed from impurity averages which involve time integrals over the retarded interaction. They can be evaluated efficiently within the segment representation. These quantities are computed within dynamical mean-field theory in the presence of plasmonic screening. In the antiadiabatic regime, the self-energy is strongly renormalized but retains features of the low energy scale set by the screened interaction. An explicit expression for its high-frequency behavior is provided. Across the screening driven and interaction driven metal-insulator transitions, the vertex functions are found to exhibit similar structural changes, which are hence identified as generic features of the Mott transition.

Electric Field Quench in AdS/CFT

Abstract: An electric field quench, a suddenly applied electric field, can induce nontrivial dynamics in confining systems which may lead to thermalization as well as a deconfinement transition. In order to analyze this nonequilibrium transitions, we use the AdS/CFT correspondence for $$ \mathcal{N}=2 $$ supersymmetric QCD that has a confining meson sector. We find that the electric field quench causes the deconfinement transition even when the magnitude of the applied electric field is smaller than the critical value for the static case (which is the QCD Schwinger limit for quark-antiquark pair creation). The time dependence is crucial for this phenomenon, and the gravity dual explains it as an oscillation of a D-brane in the bulk AdS spacetime. Interestingly, the deconfinement time takes only discrete values as a function of the magnitude of the electric field. We advocate that the new deconfinement phenomenon is analogous to the exciton Mott transition.

Phase transition in a single VO 2 nano-crystal: potential femtosecond tunable opto-electronic nano-gating

Abstract: The first-order semiconductor–metal Mott transition in single nano-crystal of VO2 has been observed using scanning tunneling spectroscopy. The variation of the band gap Eg with an external thermal stimulus on a single VO2 nano-crystal in the temperature range of 293.5–361.0 K is reported for the first time. The corresponding tuneable I–V characteristics versus temperature could be applied in thermally or optically tunable electronic nano-gating in the femtosecond regime in view of the ultrafast dynamic in VO2.

Mott metal-insulator transition induced by utilizing a glasslike structural ordering in low-dimensional molecular conductors

Abstract: We utilize a glasslike structural transition in order to induce a Mott metal-insulator transition in the quasi-two-dimensional organic charge-transfer salt $\ensuremath{\kappa}\ensuremath{-}{\text{(BEDT-TTF)}}_{2}\mathrm{Cu}[\mathrm{N}{\text{(CN)}}_{2}\mathrm{Br}]$. In this material, the terminal ethylene groups of the BEDT-TTF molecules can adopt two different structural orientations within the crystal structure, namely eclipsed (E) and staggered (S) with the relative orientation of the outer C--C bonds being parallel and canted, respectively. These two conformations are thermally disordered at room temperature and undergo a glasslike ordering transition at ${T}_{g}\ensuremath{\sim}75$ K. When cooling through ${T}_{g}$, a small fraction that depends on the cooling rate remains frozen in the S configuration, which is of slightly higher energy, corresponding to a controllable degree of structural disorder. We demonstrate that, when thermally coupled to a low-temperature heat bath, a pulsed heating current through the sample causes a very fast relaxation with cooling rates at ${T}_{g}$ of the order of several $1000 \mathrm{K}/\mathrm{min}$. The freezing of the structural degrees of freedom causes a decrease of the electronic bandwidth $W$ with increasing cooling rate, and hence a Mott metal-insulator transition as the system crosses the critical ratio ${(W/U)}_{c}$ of bandwidth to on-site Coulomb repulsion $U$. Due to the glassy character of the transition, the effect is persistent below ${T}_{g}$ and can be reversibly repeated by melting the frozen configuration upon warming above ${T}_{g}$. Both by exploiting the characteristics of slowly changing relaxation times close to this temperature and by controlling the heating power, the materials can be fine-tuned across the Mott transition. A simple model allows for an estimate of the energy difference between the E and S state as well as the accompanying degree of frozen disorder in the population of the two orientations.

Direct Observation of the Bandwidth Control Mott Transition in the NiS 2-x Se x Multiband System

Abstract: The bulk electronic structure of ${\mathrm{NiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ has been studied across the bandwidth-control Mott transition (BCMT) using soft x-ray angle-resolved photoemission spectroscopy. We show that Se doping does not alter the Fermi surface volume. When approaching the insulating phase with decreasing Se concentration, we observed that the Fermi velocity continuously decreases. Meanwhile, the weight of the coherent quasiparticle, which sits on a large incoherent spectrum, continuously decreases and is transferred to higher binding energies, until it suddenly disappears across the Mott transition. In the insulating phase, there is still finite spectral weight at the Fermi energy, but it is incoherent and dispersionless due to strong correlations. Our results provide a direct observation of the distinct characters of BCMT in a multiband non-half-filled system.

Quantum phase transition between orbital-selective Mott states in Hund's metals

Abstract: We report a quantum phase transition between orbital-selective Mott states, with different localized orbitals, in a Hund's metals model. Using the density matrix renormalization group, the phase diagram is constructed varying the electronic density and Hubbard $U$, at robust Hund's coupling. We demonstrate that this transition is preempted by charge fluctuations and the emergence of free spinless fermions, as opposed to the magnetically driven Mott transition. The Luttinger correlation exponent is shown to have a universal value in the strong-coupling phase, whereas it is interaction dependent at intermediate couplings. At weak coupling we find a second transition from a normal metal to the intermediate-coupling phase.

Origins of lasing emission in a resonance-controlled ZnO random laser

Abstract: We investigate the origins of lasing emission in a scatterer-resonance-controlled random laser made of ZnO nanopowder over a wide temperature range (20–300 K). At higher temperatures ( K), the lasing emission appears around exciton recombination energies and the lasing threshold carrier density is comparable to the Mott density, indicating that the resonance-controlled random laser is going toward showing excitonic lasing; at lower temperatures, random lasing is caused by usual electron–hole plasma recombination because of the threshold carrier density being much larger than the Mott density.

Fractionalized charge excitations in a spin liquid on partially filled pyrochlore lattices.

Abstract: We study the Mott transition from a metal to cluster Mott insulators in the $1/4$- and $1/8$-filled pyrochlore lattice systems. It is shown that such Mott transitions can arise due to charge localization in clusters or in tetrahedron units, driven by the nearest-neighbor repulsive interaction. The resulting cluster Mott insulator is a quantum spin liquid with a spinon Fermi surface, but at the same time a novel fractionalized charge liquid with charge excitations carrying half the electron charge. There exist two emergent U(1) gauge fields or ``photons'' that mediate interactions between spinons and charge excitations, and between fractionalized charge excitations themselves, respectively. In particular, it is suggested that the emergent photons associated with the fractionalized charge excitations can be measured in x-ray scattering experiments. Various other experimental signatures of the exotic cluster Mott insulator are discussed in light of candidate materials with partially filled bands on the pyrochlore lattice.

Doublon-holon binding, Mott transition, and fractionalized antiferromagnet in the Hubbard model

Abstract: We argue that the binding between doubly occupied (doublon) and empty (holon) sites governs incoherent excitations and plays a key role in the Mott transition in strongly correlated Mott-Hubbard systems. We construct a new saddle-point solution with doublon-holon binding in the Kotliar-Ruckenstein slave-boson functional integral formulation of the Hubbard model. On a half-filled honeycomb lattice and square lattice, the ground state is found to exhibit a continuous transition from a paramagnetic semimetal/metal to an antiferromagnetic-ordered Slater insulator with coherent quasiparticles at U-cl, followed by a Mott transition into an electron-fractionalized AF* phase without coherent excitations at U-c2. Such a phase structure appears to be generic for bipartite lattices without frustration. We show that doublon-holon binding unites the three important ideas of strong correlation: coherent quasiparticles, incoherent Hubbard bands, and deconfined Mott insulator.

Spectral properties near the Mott transition in the two-dimensional Hubbard model with next-nearest-neighbor hopping

Abstract: The single-particle spectral properties near the Mott transition in the two-dimensional Hubbard model with next-nearest-neighbor hopping are investigated by using cluster perturbation theory Complicated spectral features of this model are simply interpreted, by considering how the next-nearest-neighbor hopping shifts the spectral weights of the two-dimensional Hubbard model Various anomalous features observed in hole-doped and electron-doped cuprate high-temperature superconductors are explained in a unified manner as properties near the Mott transition in a two-dimensional system whose spectral weights are shifted by next-nearest-neighbor hopping

Exciton–Mott Physics in Two-Dimensional Electron–Hole Systems: Phase Diagram and Single-Particle Spectra

Abstract: Exciton Mott physics in two-dimensional electron–hole (e–h) systems is studied in the quasiequilibrium, which is the crossovers or phase transitions between the insulating exciton gas and the metallic e–h plasma. By developing a self-consistent screened T-matrix approximation, we succeed in obtaining the “global” phase diagram on the plane of the e–h density and the temperature as a contour plot of the exciton ionization ratio. The detailed features of the exciton–Mott crossover at high temperature are figured out beyond the conventionally used concept of the Mott density. At low temperature, we find not only the region unstable toward the inhomogeneity but the pure Mott transition point characterized by the discontinuity in the ionization ratio. The single particle spectra also exhibit interesting features reflecting the excitonic correlations.

Evolution of magnetic structure driven by synthetic spin-orbit coupling in a two-component Bose-Hubbard model

Abstract: We study the evolution of magnetic structures driven by a synthetic spin-orbit coupling in a one-dimensional two-component Bose-Hubbard model. In addition to the Mott insulator-superfluid transition, in the Mott insulator phases we found a transition from a gapped ferromagnetic phase to a gapless chiral phase by increasing the strength of the spin-orbit coupling. Further increasing the spin-orbit coupling drives a transition from the gapless chiral phase to a gapped antiferromagnetic phase. These magnetic structures persist in superfluid phases. In particular, in the chiral Mott insulator and chiral superfluid phases, incommensurability is observed in characteristic correlation functions. These unconventional Mott insulator phase and superfluid phase demonstrate the different effects arising from the competition between the kinetic energy and the spin-orbit coupling.

Phase separation in doped Mott insulators

Abstract: Motivated by the commonplace observation of Mott insulators away from integer filling, we construct a simple thermodynamic argument for phase separation in first-order doping-driven Mott transitions. We show how to compute the critical dopings required to drive the Mott transition using electronic structure calculations for the titanate family of perovskites, finding good agreement with experiment. The theory predicts the transition is percolative and should exhibit Coulomb frustration.

Mott transition in cuprate high-temperature superconductors

Abstract: In this study, we investigate the metal-insulator transition of charge transfer type in high-temperature cuprates. We first show that we must introduce a new band parameter in the three-band model to reproduce the Fermi surface of high-temperature cuprates such as BSCCO, YBCO and Hg1201. We present a new wave function of a Mott insulator based on the improved Gutzwiller function, and show that there is a transition from a metal to a charge-transfer insulator for such parameters by using the variational Monte Carlo method. This transition occurs when the level difference between d and p orbitals reaches a critical value . The energy gain , measured from the limit of large , is proportional to for (\Delta_{dp})_c$ SRC=http://ej.iop.org/images/0295-5075/107/2/27004/epl16387ieqn7.gif/>: . We obtain using the realistic band parameters.

Transient carrier dynamics in a Mott insulator with antiferromagnetic order

Abstract: We study transient dynamics of hole carriers injected into a Mott insulator with antiferromagnetic long-range order. This ``dynamical hole doping'' contrasts with chemical hole doping. The theoretical framework for the transient carrier dynamics is presented based on the two-dimensional $t$-$J$ model. The time dependencies of the optical conductivity spectra, as well as the one-particle excitation spectra, are calculated based on the Keldysh Green's function formalism at zero temperature combined with the self-consistent Born approximation. In the early stage after dynamical hole doping, the Drude component appears, and then incoherent components originating from hole-magnon scattering start to grow. Fast oscillatory behavior owing to coherent magnon and slow relaxation dynamics are confirmed in the spectra. The time profiles are interpreted as doped bare holes being dressed by magnon clouds and relaxed into spin polaron quasiparticle states. The characteristic relaxation times for Drude and incoherent peaks strongly depend on the momentum of the dynamically doped hole and the exchange constant. Implications for recent pump-probe experiments are discussed.

Surface charge sensing by altering the phase transition in VO2

Abstract: Detection of surface charges has various applications in medicine, electronics, biotechnology, etc. The source of surface charge induction may range from simple charge-polarized molecules like water to complicated proteins. It was recently discovered that surface charge accumulation can alter the temperature at which VO2 undergoes a Mott transition. Here, we deposited polar molecules onto the surface of two-terminal thin-film VO2 lateral devices and monitored the joule-heating-driven Mott transition, or conductance switching. We observed that the power required to induce the conductance switching reduced upon treatment with polar molecules and, using in-situ blackbody-emission direct measurement of local temperature, we show that this reduction in power was accompanied by reduction in the Mott transition temperature. Further evidence suggested that this effect has specificity to the nature of the species used to induce surface charges. Using x-ray absorption spectroscopy, we also show that there is no detectable change in oxidation state of vanadium or structural phase in the bulk of the 40 nm VO2 thin-film even as the phase transition temperature is reduced by up to 20 K by the polar molecules. The ability to alter the phase transition parameters by depositing polar molecules suggests a potential application in sensing surface charges of different origins and this set of results also highlights interesting aspects of the phase transition in VO2.

Towards a Holographic Bose-Hubbard Model

Abstract: We present a holographic construction of the large-N Bose-Hubbard model. The model is based on Maxwell fields coupled to charged scalar fields on the AdS2 hard wall. We realize the lobe-shaped phase structure of the Bose-Hubbard model and find that the model admits Mott insulator ground states in the limit of large Coulomb repulsion. In the Mott insulator phases, the bosons are localized on each site. At zero hopping we find that the transitions between Mott insulating phases with different fillings correspond to first order level-crossing phase transitions. At finite hopping we find a holographic phase transition between the Mott phase and a non-homogeneous phase. We then analyze the perturbations of fields around both the Mott insulator phase and inhomogeneous phase. We find almost zero modes in the non-homogeneous phase.

Metallicity in fullerides

Abstract: Metallic salts formed from fullerenes became popular because of their superconducting properties with a relatively high transition temperature, and were initially regarded as conventional metals and superconductors. Recently, owing to improved synthetic methods and a renewed interest in the study of their physical properties, many of them were found to exhibit exotic metallic and superconducting phases. In this paper, we summarize earlier results on unconventional metallic fulleride phases as well as the newly discovered expanded fulleride superconductors. The proximity of the Mott transition, a typical solid-state effect, results in molecular crystals, where molecular spectroscopic methods prove very successful. We concentrate on infrared and optical spectroscopy which is very well suited to follow metallicity and phase transitions in this class of substances.

Mott transition and antiferromagnetism of cold fermions in the decorated honeycomb lattice

Abstract: We investigate two-component ultracold fermions loaded in a decorated honeycomb lattice described by the Hubbard model with repulsive interactions and nearest-neighbor hopping. The phase transitions are studied by combining the cellular dynamical mean-field theory with the continuous-time quantum Monte Carlo method. For weak interactions, the quadratic band crossing point is broken to a linear band crossing point and the system becomes a semimetal. With increasing interaction, the system undergoes a first-order phase transition to an antiferromagnetic Mott insulator at low temperatures. Below the critical temperature, due to the charge nematic fluctuation, a nematic metal forms between the semimetal and the antiferromagnetic Mott insulator. The effects of lattice anisotropy are also addressed. Furthermore, we discuss how to detect these phases in real experiments.

Spectroscopic signatures of the Mott transition on the anisotropic triangular lattice

Abstract: We use a new Monte Carlo method to study the Mott transition on the anisotropic-triangular-lattice Hubbard model. Our real space approach accurately treats the emergence of non-trivial magnetic correlation in this frustrated structure with reducing temperature. For hopping anisotropy typical of the two-dimensional organic salts, the coupling of electrons to self-generated magnetic moments leads to a pseudogap, huge d.c. resistivity, and non-Drude optical response over a wide temperature window. In addition to these generic signatures of a "bad metal", the spatial correlation among the magnetic moments leads to pronounced momentum dependence of quasiparticle extinction and pseudogap formation on the Fermi surface as the Mott transition is approached.

Biexcitonic molecules survive excitons at the Mott transition

Abstract: When the carrier density is increased in a semiconductor, according to the predictions of Sir Nevil Mott, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma. This crossover, usually referred to as the Mott transition, is driven by the mutual effects of phase-space filling and Coulomb screening because of the presence of other charges nearby. It drastically affects the optical and electrical characteristics of semiconductors and may, for example, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. Usually, the possible existence of excitonic molecules (or biexcitons) is neglected in the understanding of the Mott transition because the biexciton is supposed to be less robust against screening effects. Here, against common beliefs, we observe that the biexciton in a GaN quantum well is more stable towards the Mott transition than the exciton.