1,721,091 research outputs found
Electronic properties driven by strong correlation
Full text linkThe fascinating subject of superconductivity was opened over a century ago by Onnes [1], but until quite recently it was strictly a low-temperature phenomenon. The discovery of the cuprate superconductors [2] in a family of transition metal oxides, with transition temperatures up to Tc ~ 100K, has generated tremendous excitement for two main reasons. First, from a practical point of view, these compounds open a new temperature realm for superconducting devices which may have interesting commercial applications, and these potential benefits have attracted extraordinary attention from the whole scientific community. The second reason, relevant to those in a more abstract field, is the interest in the microscopic mechanism driving superconductivity
Quantum phase transition in coupled spin ladders
No full textThe ground state of an array of coupled, spin-1/2 antiferromagnetic ladders is studied using spin-wave theory, exact diagonalization (up to 36 sites), and quantum Monte Carlo techniques (up to 256 sites). Our results clearly indicate the occurrence of a zero-temperature phase transition between a Neel ordered and a nonmagnetic phase at a finite value of the interladder coupling (a(c)similar or equal to0.3). This transition is marked by remarkable changes in the structure of the excitation spectrum
Spin-phonon interactions on the kagome lattice: Dirac spin liquid versus valence-bond solids
Full text linkWe investigate the impact of the spin-phonon coupling on the S=1/2 Heisenberg
model on the kagome lattice. For the pure spin model, there is increasing
evidence that the low-energy properties can be correctly described by a Dirac
spin liquid, in which spinons with a conical dispersion are coupled to emergent
gauge fields. Within this scenario, the ground-state wave function is well
approximated by a Gutzwiller-projected fermionic state [Y. Ran, M. Hermele,
P.A. Lee, and X.-G. Wen, Phys. Rev. Lett. 98, 117205 (2007)]. However, the
existence of U(1) gauge fields may naturally lead to instabilities when small
perturbations are included. Since phonons are ubiquitous in real materials,
they may play a relevant role in the determination of the actual physical
properties of the kagome antiferromagnet. We perform a step forward in this
direction, including phonon degrees of freedom (at the quantum level) and
applying a variational approach based upon Gutzwiller-projected fermionic
Ans\"atze. Our results suggest that the Dirac spin liquid is stable for small
spin-phonon couplings, while valence-bond solids are obtained at large
couplings. Even though different distortions can be induced by the spin-phonon
interaction, the general aspect is that the energy is lowered by maximizing the
density of perfect hexagons in the dimerization pattern
Absence of static stripes in the two-dimensional t-J model by an accurate and systematic quantum Monte Carlo approach
No full textWe examine the two-dimensional t−J model by using variational approach combined with well established
quantum Monte Carlo techniques [S. Sorella et al., Phys. Rev. Lett. 88, 117002 (2002)] that are used to improve
systematically the accuracy of the variational ansatz. Contrary to recent density-matrix renormalization group
and projected entangled-pair state calculations [P. Corboz et al., Phys. Rev. B 84, 041108(R) (2011)], a uniform
phase is found for J /t = 0.4, even when the calculation is biased with an ansatz that explicitly contains stripe
order. Moreover, in the small hole doping regime, that is, δ 0.1, our results support the coexistence of
antiferromagnetism and superconductivity
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
Quantum Monte Carlo approaches for correlated systems
No full textOver the past several decades, computational approaches to studying strongly-interacting systems have become increasingly varied and sophisticated. This book provides a comprehensive introduction to state-of-the-art quantum Monte Carlo techniques relevant for applications in correlated systems. Providing a clear overview of variational wave functions, and featuring a detailed presentation of stochastic samplings including Markov chains and Langevin dynamics, which are developed into a discussion of Monte Carlo methods. The variational technique is described, from foundations to a detailed description of its algorithms. Further topics discussed include optimisation techniques, real-time dynamics and projection methods, including Green's function, reptation and auxiliary-field Monte Carlo, from basic definitions to advanced algorithms for efficient codes, and the book concludes with recent developments on the continuum space. Quantum Monte Carlo Approaches for Correlated Systems provides an extensive reference for students and researchers working in condensed matter theory or those interested in advanced numerical methods for electronic simulation
Ground-state properties of the disordered Hubbard model in two dimensions
No full textWe study the interplay between electron correlation and disorder in the two-dimensional Hubbard model at half filling by means of a variational wave function that can interpolate between Anderson and Mott insulators. We give a detailed description of our improved variational state and explain how the physics of the Anderson-Mott transition can be inferred from equal-time correlations functions, which can be easily computed within the variational Monte Carlo scheme. The ground-state phase diagram is worked out in both the paramagnetic and the magnetic sector. Whereas in the former a direct second-order Anderson-Mott transition is obtained, when magnetism is allowed variationally, we find evidence for the formation of local magnetic moments that order before the Mott transition. Although the localization length increases before the Mott transition, we have no evidence for the stabilization of a true metallic phase. The effect of a frustrating next-nearest-neighbor hopping t' is also studied in some detail. In particular, we show that t' has two primary effects. The first one is the narrowing of the stability region of the magnetic Anderson insulator, also leading to a first-order magnetic transition. The second and most important effect of a frustrating hopping term is the development of a "glassy" phase at strong couplings, where many paramagnetic states, with disordered local moments, may be stabilized
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
Dynamical Structure Factor of the J1−J2 Heisenberg Model on the Triangular Lattice: Magnons, Spinons, and Gauge Fields
Full text linkUnderstanding the nature of the excitation spectrum in quantum spin liquids is of fundamental importance, in particular for the experimental detection of candidate materials. However, current theoretical and numerical techniques have limited capabilities, especially in obtaining the dynamical structure factor, which gives a crucial characterization of the ultimate nature of the quantum state and may be directly assessed by inelastic neutron scattering. In this work, we investigate the low-energy properties of the S=1/2 Heisenberg model on the triangular lattice, including both nearest-neighborJ1 and next-nearest-neighbor J2 superexchanges, by a dynamical variational Monte Carlo approach that allows accurate results on spin models. For J2=0, our calculations are compatible with the existence of a well-defined magnon in the whole Brillouin zone, with gapless excitations at K points (i.e., at the corners of the Brillouin zone). The strong renormalization of the magnon branch (also including rotonlike minima around the M points, i.e., midpoints of the border zone) is described by our Gutzwiller-projected state, where Abrikosov fermions are subject to a nontrivial magnetic π flux threading half of the triangular plaquettes. When increasing the frustrating ratio J2/J1, we detect a progressive softening of the magnon branch at M, which eventually becomes gapless within the spin-liquid phase. This feature is captured by the band structure of the unprojected wave function (with two Dirac points for each spin component). In addition, we observe an intense signal at low energies around the K points, which cannot be understood within the unprojected picture and emerges only when the Gutzwiller projection is considered, suggesting the relevance of gauge fields for the low-energy physics of spin liquids
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