352 research outputs found
Hubbard model on triangular N -leg cylinders: Chiral and nonchiral spin liquids
No full textThe existence of a gapped chiral spin liquid has been recently suggested in the vicinity of the metal-insulator transition of the Hubbard model on the triangular lattice, by intensive density-matrix renormalization group (DMRG) simulations [A. Szasz, J. Motruk, M. P. Zaletel, and J. E. Moore, Phys. Rev. X 10, 021042 (2020)10.1103/PhysRevX.10.021042]. Here, we report the results obtained within the variational Monte Carlo technique based upon Jastrow-Slater wave functions, implemented with backflow correlations. As in DMRG calculations, we consider N-leg cylinders. For N=4 and in the presence of a next-nearest-neighbor hopping, a chiral spin liquid emerges between the metal and the insulator with magnetic quasi-long-range order. Within our approach, the chiral state is gapped and breaks the reflection symmetry. By contrast, for both N=5 and 6, the chiral spin liquid is not the state with the lowest variational energy: in the former case, a nematic spin liquid is found in the entire insulating regime, while for the less frustrated case with N=6 the results are very similar to that obtained on two-dimensional clusters [L. F. Tocchio, A. Montorsi, and F. Becca, Phys. Rev. B 102, 115150 (2020)2469-995010.1103/PhysRevB.102.115150], with an antiferromagnetic phase close to the metal-insulator transition and a nematic spin liquid in the strong-coupling regime
Hubbard model on triangular N-leg cylinders: Chiral and nonchiral spin liquids
No full textThe existence of a gapped chiral spin liquid has been recently suggested in the vicinity of the metal-insulator transition of the Hubbard model on the triangular lattice, by intensive density-matrix renormalization group (DMRG) simulations [A. Szasz, J. Motruk, M. P. Zaletel, and J. E. Moore, Phys. Rev. X 021042 (2020)]. Here, we report the results obtained within the variational Monte Carlo technique based upon Jastrow-Slater wave functions, implemented with backflow correlations. As in DMRG calculations, we consider N-leg cylinders. For N = 4 and in the presence of a next-nearest-neighbor hopping, a chiral spin liquid emerges between the metal and the insulator with magnetic quasi-long-range order. Within our approach, the chiral state is gapped and breaks the reflection symmetry. By contrast, for both N = 5 and 6, the chiral spin liquid is not the state with the lowest variational energy: in the former case, a nematic spin liquid is found in the entire insulating regime, while for the less frustrated case with N = 6 the results are very similar to that obtained on two-dimensional clusters [L. F. Tocchio, A. Montorsi, and F. Becca, Phys. Rev. B 102, 115150 (2020)], with an antiferromagnetic phase close to the metal-insulator transition and a nematic spin liquid in the strong-coupling regime
Eco-Political Film Histories: Land Cinema in Japan and beyond - with Becca Voelcker
No full textIn this lecture, author Dr Becca Voelcker will discuss Japan's centrality in a genre she calls 'land cinema', offering examples ranging from downtown Tokyo to rural rice paddies in Yamagata. Diving into little-known archives to explore films that resonate across geographies, Becca's research spans Japan, Mali, Colombia, the USA, Orkney, the Navajo Nation and more, to map a global history of eco-political filmmaking. Made half a century ago, land cinema films resonate with our own era of social and environmental crises, offering audiences alternatives for the future. This talk coincides with the publication of Becca's first book, Land Cinema in an Age of Extraction, by the University of California Press.
Dr Becca Voelcker is a writer and historian of art, film, and visual culture. Much of her work considers land and senses of place. She is a Lecturer at Goldsmiths, University of London and earned her PhD at Harvard University. In 2024 Becca was named a BBC New Generation Thinker. Her book, Land Cinema in an Age of Extraction (University of California Press, 2025) is accompanied by a BBC Radio programme and a curated film season at the Barbican (5-26 November 2025). Originally from Wales, Becca has lived and researched in Japan for extended periods since 2013 when she first moved to Tokyo as a Daiwa Anglo-Japanese Foundation Scholar. www.beccavoelcker.co
Quantum quenches in one-dimensional gapless systems
No full textWe present a comparison between the bosonization results for quantum quenches and exact diagonalizations in microscopic models of interacting spinless fermions in a one-dimensional lattice. The numerical analysis of the long-time averages shows that density-density correlations at small momenta tend to a non-zero limit, mimicking a thermal behavior. These results are at variance with the bosonization approach, which predicts the presence of long-wavelength critical properties in the long-time evolution. By contrast, the numerical results for finite momenta suggest that the singularities at 2k F in the density-density correlations and at k F in the momentum distribution are preserved during the time evolution. The presence of an interaction term that breaks integrability flattens out all singularities, suggesting that the time evolution of one-dimensional lattice models after a quantum quench may differ from that of the Luttinger model
Incommensurate charge-density-wave instability in the extended three-band Hubbard model
No full textThe infinite-U three-band Hubbard model is considered in order to describe the CuO2 planes of the high-temperature superconducting cuprates. The charge instabilities are investigated when the model is extended with a nearest-neighbor repulsion between holes an copper d and oxygen p orbitals and in the presence of a long-range Coulombic repulsion. It is found that a first-order valence instability line ending with a critical point is present as in the previously investigated model without long-range forces. However, the dominant critical Instability is the formation of incommensurate charge-density waves, which always occur before the valence-instability critical point is reached, An effective singular attraction arises in the proximity of the charge-density wave instability, accounting for both a strong pairing mechanism and for the anomalous normal-state properties
Spectral signatures of fractionalization in the frustrated Heisenberg model on the square lattice
No full textWe employ a variational Monte Carlo approach to efficiently obtain the dynamical structure factor for the spin-1/2 J(1)-J(2) Heisenberg model on the square lattice. Upon increasing the frustrating ratio J(2)/J(1), the ground state undergoes a continuous transition from a Ned antiferromagnet to a Z(2) gapless spin liquid. We identify the characteristic spectral features in both phases and highlight the existence of a broad continuum of excitations in the proximity of the spin-liquid phase. The magnon branch, which dominates the spectrum of the unfrustrated Heisenberg model, gradually loses its spectral weight, thus releasing nearly deconfined spinons, whose signatures are visible even in the magnetically ordered state. Our results provide an important example on how magnons fractionalize into deconfined spinons across a quantum critical point
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
Extracting the Mott gap from energy measurements in trapped atomic gases
No full textWe show that the measure of the so-called release energy, which is an experimentally accessible quantity, makes it possible to assess the value of the Mott gap in the presence of the unavoidable confinement potential in the actual experimental setup. Indeed, the curve of the release energy as a function of the total number of particles shows kinks that are directly related to the existence of excitation gaps. Calculations are presented within the Gutzwiller approach, but the final results go beyond this simple approximation and represent a genuine feature of the real system. In the case of harmonic confinement, the Mott gaps may be renormalized with respect to the uniform case. On the other hand, in the case of the recently proposed off-diagonal confinement, our results show good agreement with the homogeneous case
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
Peierls-like transition induced by frustration in a two dimensional antiferromagnet
No full textWe show that the introduction of frustration into the spin-1/2 two-dimensional (2D) antiferromagnetic Heisenberg model on a square lattice via a next-nearest-neighbor exchange interaction can lead to a Peierls-like transition, from a tetragonal to an orthorhombic phase, when the spins are coupled to adiabatic phonons. The two different orthorhombic ground states define an Ising order parameter, which is expected to lead to a finite temperature transition. Implications for Li(2)VOSiO(4) , the first realization of that model, will be discussed
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