1,720,978 research outputs found
Metallic and insulating stripes and their relation with superconductivity in the doped Hubbard model
Full text linkThe dualism between superconductivity and charge/spin modulations (the so-called stripes) dominates the phase diagram of many strongly-correlated systems. A prominent example is given by the Hubbard model, where these phases compete and possibly co-exist in a wide regime of electron dopings for both weak and strong couplings. Here, we investigate this antagonism within a variational approach that is based upon Jastrow-Slater wave functions, including backflow correlations, which can be treated within a quantum Monte Carlo procedure. We focus on clusters having a ladder geometry with M legs (with M ranging from 2 to 10) and a relatively large number of rungs, thus al-lowing us a detailed analysis in terms of the stripe length. We find that stripe orde rwith periodicity λ=8 in the charge and 2λ=16 in the spin can be stabilized at doping δ=1/8. Here, there are no sizable superconducting correlations and the ground state has an insulating character. A similar situation, with λ=6, appears at δ=1/6. Instead,for smaller values of dopings, stripes can be still stabilized, but they are weakly metallic at δ=1/12and metallic with strong superconducting correlations at δ=1/10, as well as for intermediate (incommensurate) dopings. Remarkably, we observe that spin modulation plays a major role in stripe formation, since it is crucial to obtain a stable striped state upon optimization. The relevance of our calculations for previous density-matrix renormalization group results and for the two-dimensional case is also discussed
Hidden Mott transition and large-U superconductivity in the two-dimensional Hubbard model
Full text linkWe consider the one-band Hubbard model on the square lattice by using variational and Green's function Monte Carlo methods, where the variational states contain Jastrow and backflow correlations on top of an uncorrelated wave function that includes BCS pairing and magnetic order. At half-filling, where the ground state is antiferromagnetically ordered for any value of the on-site interaction U, we can identify a hidden critical point UMott, above which a finite BCS pairing is stabilized in the wave function. The existence of this point is reminiscent of the Mott transition in the paramagnetic sector and determines a separation between a Slater insulator (at small values of U), where magnetism induces a potential energy gain, and a Mott insulator (at large values of U), where magnetic correlations drive a kinetic energy gain. Most importantly, the existence of UMott has crucial consequences when doping the system: We observe a tendency for phase separation into hole-rich and hole-poor regions only when doping the Slater insulator, while the system is uniform by doping the Mott insulator. Superconducting correlations are clearly observed above UMott, leading to the characteristic dome structure in doping. Furthermore, we show that the energy gain due to the presence of a finite BCS pairing above UMott shifts from the potential to the kinetic sector by increasing the value of the Coulomb repulsion
Emergent lattices with geometrical frustration in doped extended Hubbard models
No full textSpontaneous charge ordering occurring in correlated systems may be considered as a possible route to generate effective lattice structures with unconventional couplings. For this purpose we investigate the phase diagram of doped extended Hubbard models on two lattices: (i) the honeycomb lattice with on-site U and nearest-neighbor V Coulomb interactions at 3/4 filling (n=3/2) and (ii) the triangular lattice with on-site U, nearest-neighbor V, and next-nearest-neighbor V' Coulomb interactions at 3/8 filling (n=3/4). We consider various approaches including mean-field approximations, perturbation theory, and variational Monte Carlo. For the honeycomb case (i), charge order induces an effective triangular lattice at large values of U/t and V/t, where t is the nearest-neighbor hopping integral. The nearest-neighbor spin exchange interactions on this effective triangular lattice are antiferromagnetic in most of the phase diagram, while they become ferromagnetic when U is much larger than V. At U/t∼(V/t)^3, ferromagnetic and antiferromagnetic exchange interactions nearly cancel out, leading to a system with four-spin ring-exchange interactions. On the other hand, for the triangular case (ii) at large U and finite V', we find no charge order for small V, an effective kagome lattice for intermediate V, and one-dimensional charge order for large V. These results indicate that Coulomb interactions induce [case (i)] or enhance [case(ii)] emergent geometrical frustration of the spin degrees of freedom in the system, by forming charge order
Spin-liquid and magnetic phases in the anisotropic triangular lattice: The case of kappa-(ET)(2)X
No full textThe two-dimensional Hubbard model on the anisotropic triangular lattice, with two different hopping amplitudes t and t' , is relevant to describe the low-energy physics of kappa-(ET)_2X, a family of organic salts. The ground-state properties of this model are studied by using Monte Carlo techniques, on the basis of a recent definition of backflow correlations for strongly correlated lattice systems. The results show that there is no magnetic order for reasonably large values of the electron-electron interaction U and frustrating ratio t'/ t= 0.85, suitable to describe the nonmagnetic compound with X = Cu_2(CN)_3 . On the contrary, Néel order takes place for weaker frustrations, i.e., t'/t~0.4–0.6, suitable for materials with X = Cu_2(SCN)_2 , Cu[N(CN)_2]Cl, or
Cu[N(CN)_2]Br.The two-dimensional Hubbard model on the anisotropic triangular lattice, with two different hopping amplitudes t and t('), is relevant to describe the low-energy physics of kappa-(ET)(2)X, a family of organic salts. The ground-state properties of this model are studied by using Monte Carlo techniques, on the basis of a recent definition of backflow correlations for strongly correlated lattice systems. The results show that there is no magnetic order for reasonably large values of the electron-electron interaction U and frustrating ratio t(')/t=0.85, suitable to describe the nonmagnetic compound with X=Cu-2(CN)(3). On the contrary, Neacuteel order takes place for weaker frustrations, i.e., t(')/t similar to 0.4-0.6, suitable for materials with X=Cu-2(SCN)(2), Cu[N(CN)(2)]Cl, or Cu[N(CN)(2)]Br
Metal-insulator transition and strong-coupling spin liquid in the t-t ' Hubbard model
Full text linkWe study the phase diagram of the frustrated t-t' Hubbard model on the square lattice by using a novel variational wave function. Taking the clue from the backflow correlations that have been introduced long-time ago by Feynman and Cohen and have been used for describing various interacting systems on the continuum (like liquid (3)He, the electron jellium, and metallic Hydrogen), we consider many-body correlations to construct a suitable approximation for the ground state of this correlated model on the lattice. In this way, a very accurate ansatz can be achieved both at weak and strong coupling. We present the evidence that an insulating and non-magnetic phase can be stabilized at strong coupling and sufficiently large frustrating ratio t'/t
Superconductivity in the Hubbard model: a hidden-order diagnostics from the Luther-Emery phase on ladders
Full text linkShort-range antiferromagnetic correlations are known to open a spin gap in the repulsive Hubbard model on ladders with
M
legs, when
M
is even. We show that the spin gap originates from the formation of correlated pairs of electrons with opposite spin, captured by the hidden ordering of a spin-parity operator. Since both spin gap and parity vanish in the two-dimensional limit, we introduce the fractional generalization of spin parity and prove that it remains finite in the thermodynamic limit. Our results are based upon variational wave functions and Monte Carlo calculations: performing a finite size-scaling analysis with growing
M
, we show that the doping region where the parity is finite coincides with the range in which superconductivity is observed in two spatial dimensions. Our observations support the idea that superconductivity emerges out of spin gapped phases on ladders, driven by a spin-pairing mechanism, in which the ordering is conveniently captured by the finiteness of the fractional spin-parity operator
Stripes in the extended Hubbard model: A Variational Monte Carlo analysis
Full text linkBy using variational quantum Monte Carlo techniques, we investigate the
instauration of stripes (i.e., charge and spin inhomogeneities) in the Hubbard
model on the square lattice at hole doping , with both nearest-
() and next-nearest-neighbor hopping (). Stripes with different
wavelengths (denoting the periodicity of the charge inhomogeneity)
and character (bond- or site-centered) are stabilized for sufficiently large
values of the electron-electron interaction . The general trend is that
increases going from negative to positive values of and
decreases by increasing . In particular, the stripe obtained
for and [L.F. Tocchio, A. Montorsi, and F. Becca, SciPost
Phys. {\bf 7}, 21 (2019)] shrinks to for . For
, the stripe with is found to be remarkably stable,
while for , stripes with wavelength and
are also obtained. In all these cases, pair-pair correlations are highly
suppressed with respect to the uniform state (obtained for large values of
), suggesting that striped states are not superconducting at
.Comment: 18 pages, 10 figures, submission to SciPos
Variational wave functions for the S=1/2 Heisenberg model on the anisotropic triangular lattice: Spin liquids and spiral orders
Full text linkBy using variational wave functions and quantum Monte Carlo techniques, we investigate the complete phase diagram of the Heisenberg model on the anisotropic triangular lattice, where two out of three bonds have superexchange couplings J and the third one has instead J'. This model interpolates between the square lattice and the isotropic triangular one, for J'/J <= 1, and between the isotropic triangular lattice and a set of decoupled chains, for J/J' <= 1. We consider all the fully symmetric spin liquids that can be constructed with the fermionic projective-symmetry group classification (Zhou and Wen, arXiv:cond-mat/0210662) and we compare them with the spiral magnetic orders that can be accommodated on finite clusters. Our results show that, for J'/J <= 1, the phase diagram is dominated by magnetic orderings, even though a spin- liquid state may be possible in a small parameter window, i.e., 0.7 less than or similar to J'/J <= 0.8. In contrast, for J/J' <= 1, a large spin-liquid region appears close to the limit of decoupled chains, i.e., for J/J' less than or similar to 0.6, while magnetically ordered phases with spiral order are stabilized close to the isotropic point
Spontaneous symmetry breaking in correlated wave functions
No full textWe show that Jastrow-Slater wave functions, in which a density-density Jastrow factor is applied onto an uncorrelated fermionic state, may possess long-range order even when all symmetries are preserved in the wave function. This fact is mainly related to the presence of a sufficiently strong Jastrow term (also including the case of full Gutzwiller projection, suitable for describing spin models). Selected examples are reported, including the spawning of Néel order and dimerization in spin systems, and the stabilization of charge and orbital order in itinerant electronic systems
Importance of anisotropy in the spin-liquid candidate Me 3EtSb[Pd(dmit)2]2
Full text linkOrganic charge-transfer salts based on the molecule Pd(dmit)2 display strong electronic correlations and geometrical frustration, leading to spin-liquid, valence bond solid, and superconducting states, among other interesting phases. The low-energy electronic degrees of freedom of these materials are often described by a single band model: a triangular lattice with a molecular orbital representing a Pd(dmit)2 dimer on each site. We use ab initio electronic structure calculations to construct and parametrize low-energy effective model Hamiltonians for a class of Me4-n Et nX[Pd(dmit)2]2 (X= As, P, N, Sb) salts and investigate how best to model these systems by using variational Monte Carlo simulations. Our findings suggest that the prevailing model of these systems as a t-t′ triangular lattice is incomplete and that a fully anisotropic triangular lattice description produces importantly different results, including a significant lowering of the critical U of the spin-liquid phase
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