| 1. |
- Akhavan Attar, Ali, et al.
(author)
-
High Strength-Elongation Balance in Warm Accumulative Roll Bonded AA1050 Sheets
- 2022
-
In: Metals and Materials International. - : Springer. - 1598-9623 .- 2005-4149. ; 28, s. 346-360
-
Journal article (peer-reviewed)abstract
- Several studies had been performed on accumulative roll bonding (ARB) for AA1050; however, most of them were conducted at room temperature. Here, the ARB process was performed on AA1050 plates through nine cycles at elevated temperature. An innovation introduced a new parameter (UTS×El.ε) to compare the strength-elongation balance between the present study and previous works. Also, as another parameter, the toughness was compared. Comparing these parameters with previous works showed that the considered samples in the present study performed 14 to 63% better than the other samples, so they were more industrially favorable in terms of mechanical behavior and performance. ARB process at elevated temperature may slightly lead to grain growth compared to room/cryogenic temperature, but creates a better elongation, which ultimately leads to a better balance of the strength-elongation parameter. The results showed that the effect of inter-cycle heating was found significant on microstructural evolution and mechanical behavior. Upon five cycles of the process, the grain size was decreased from 35 to 1.8 μm. The yield strength and ultimate strength increased up to 305% and 94%, respectively. Microhardness test showed that warm ARB reduces inhomogeneity factor in the thickness after 3 cycles. Fractography by SEM showed that the sample failed through shear ductile rupture and that the dimples became smaller, more elongated, and shallower onto the failure surface as the number of ARB cycles increased. In short, the warm process is preferred to the cold process to achieve better mechanical performance and toughness.
|
|
| 2. |
- Akhavan Attar, Ali, et al.
(author)
-
Improving the fracture toughness of multi-layered commercial pure aluminum via warm accumulative roll bonding
- 2021
-
In: The International Journal of Advanced Manufacturing Technology. - : Springer. - 0268-3768 .- 1433-3015. ; 116, s. 3603-3617
-
Journal article (peer-reviewed)abstract
- In this study, the fracture toughness of the multi-layered commercial pure aluminum samples (AA1050) prepared by warm accumulative roll bonding (WARB) was investigated for the first time. Based on the ASTM E561 standard, the R-curve method was utilized to measure the plane stress fracture toughness. Compact tension (CT) samples were prepared from the sheets that were processed by different ARB cycles. Mechanical properties, microstructure, and fracture surfaces of the CT samples were studied by uniaxial tensile test, electron backscatter diffraction (EBSD), and scanning electron microscopy (SEM), respectively. By increasing the number of WARB cycles, fracture toughness increased; after five cycles, 78% enhancement was observed compared to the pre-processed state. A correlation was seen between the fracture toughness variations and ultimate tensile strength (UTS). WARB enhanced UTS up to 95%, while the grain size showed a reduction from 35 to 1.8 μm. Measured fracture toughness values were compared with the room temperature ARB outcomes, and the effective parameters were analyzed. Fractography results indicated that the presence of tiny cliffs and furrows and hollow under fatigue loading zones and shear ductile rupture in the Quasi-static tensile loading zone.
|
|
| 3. |
- Alavi, Ali, et al.
(author)
-
Dual Mode IR Position and State Transfer for Tangible Tabletops
- 2011
-
In: Proceedings of the ACM International Conference on Interactive Tabletops and Surfaces. - New York, NY, USA : ACM. ; 1:1, s. 278-279
-
Conference paper (peer-reviewed)abstract
- This paper presents a method for tracking multiple active tangible devices on tabletops. Most tangible devices for tabletops use infrared to send information about their position, orientation, and state. The method we propose can be realized as a tabletop system using a low-cost camera to detect position and a low-cost infrared (IR) receiver to detect the state of each device. Since two different receivers (camera and IR-receiver) are used simultaneously we call the method dual mode. Using this method, it is possible to use devices with a large variation of states simultaneously on a tabletop, thus having more interactive devices on the surface.
|
|
| 4. |
- Alavi, Ali, et al.
(author)
-
Multi-State Device Tracking for Tangible Tabletops
- 2011
-
In: Larsson, Thomas ; Kjelldahl, Lars ; Jää-Aro, Kai-Mikael (eds): Proceedings of SIGRAD 2011. Evaluations of Graphics and Visualization — Efficiency, Usefulness, Accessibility, Usability, November 17–18, 2011, KTH, Stockholm, Sweden. - 1650-3686. ; 1:1, s. 114-
-
Conference paper (peer-reviewed)abstract
- On tangible tabletops, Tangible User Interfaces (TUIs) can signalize their identity, position, orientation, and state by active infrared light. This provides rich interaction capabilities in complex, dynamic scenarios. If TUIs have to transfer additional high-resolution information, many bits are required for each update. This has a negative impact on the overall update rate of the system. In the first part of this paper, we present an in-house map application where interaction with time-dependent contour lines may benefit from high-resolution TUI states. Prototypical TUI concepts such as slider, ruler, and dials further motivate the benefit of high-resolution tracking. In the second part of the paper, we depart from a device tracking overview and then show how tangible devices for tabletops typically use infrared (IR) emitters and a camera to send information about their position, orientation, and state. Since transferring many additional information bits via a normal camera-based tabletop system is not feasible anymore, we introduce next a new system setup that still offers a sufficiently high update rate for a smooth interaction. The new method can be realized as a tabletop system using a low-cost camera detecting position, combined with a low-cost infrared receiver detecting the state of each device. Since both kinds of sensors are used simultaneously we call the method “dual mode.” This method combines a camera-based tracking with the possibility to transfer an almost unlimited amount of states for each device.
|
|
| 5. |
- Barucha-Dobrautz, Werner, 1987, et al.
(author)
-
Toward Real Chemical Accuracy on Current Quantum Hardware Through the Transcorrelated Method
- 2024
-
In: Journal of Chemical Theory and Computation. - 1549-9626 .- 1549-9618. ; 20:10, s. 4146-4160
-
Journal article (peer-reviewed)abstract
- Quantum computing is emerging as a new computational paradigm with the potential to transform several research fields including quantum chemistry. However, current hardware limitations (including limited coherence times, gate infidelities, and connectivity) hamper the implementation of most quantum algorithms and call for more noise-resilient solutions. We propose an explicitly correlated Ansatz based on the transcorrelated (TC) approach to target these major roadblocks directly. This method transfers, without any approximation, correlations from the wave function directly into the Hamiltonian, thus reducing the resources needed to achieve accurate results with noisy quantum devices. We show that the TC approach allows for shallower circuits and improves the convergence toward the complete basis set limit, providing energies within chemical accuracy to experiment with smaller basis sets and, thus, fewer qubits. We demonstrate our method by computing bond lengths, dissociation energies, and vibrational frequencies close to experimental results for the hydrogen dimer and lithium hydride using two and four qubits, respectively. To demonstrate our approach’s current and near-term potential, we perform hardware experiments, where our results confirm that the TC method paves the way toward accurate quantum chemistry calculations already on today’s quantum hardware.
|
|
| 6. |
- Dobrautz, Werner, 1987, et al.
(author)
-
Combined unitary and symmetric group approach applied to low-dimensional Heisenberg spin systems
- 2022
-
In: Physical Review B. - 2469-9969 .- 2469-9950. ; 105:19
-
Journal article (peer-reviewed)abstract
- A novel combined unitary and symmetric group approach is used to study the spin-1/2 Heisenberg model and related Fermionic systems in a total spin-adapted representation, using a linearly-parameterised Ansatz for the many-body wave function. We show that a more compact ground-state wave function representation-indicated by a larger leading ground-state coefficient-is obtained when combining the symmetric group S-n, in the form of permutations of the underlying lattice site ordering, with the cumulative spin coupling based on the unitary group U(n). In one-dimensional systems the observed compression of the wave function is reminiscent of block-spin renormalization group approaches, and allows us to study larger lattices (here taken up to 80 sites) with the spin-adapted full configuration interaction quantum Monte Carlo method, which benefits from the sparsity of the Hamiltonian matrix and the corresponding sampled eigenstates that emerge from the reordering. We find that in an optimal lattice ordering the configuration state function with highest weight already captures with high accuracy the spin-spin correlation function of the exact ground-state wave function. This feature is found for more general lattice models, such as the Hubbard model, and ab initio quantum chemical models, exemplified by one-dimensional hydrogen chains. We also provide numerical evidence that the optimal lattice ordering for the unitary group approach is not generally equivalent to the optimal ordering obtained for methods based on matrix-product states, such as the density-matrix renormalization group approach.
|
|
| 7. |
- Dobrautz, Werner, 1987, et al.
(author)
-
Performance of a one-parameter correlation factor for transcorrelation: Study on a series of second row atomic and molecular systems
- 2022
-
In: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 156:23
-
Journal article (peer-reviewed)abstract
- In this work, we investigate the performance of a recently proposed transcorrelated (TC) approach based on a single-parameter correlation factor [E. Giner, J. Chem. Phys. 154, 084119 (2021)] for systems involving more than two electrons. The benefit of such an approach relies on its simplicity as efficient numerical-analytical schemes can be set up to compute the two-and three-body integrals occurring in the effective TC Hamiltonian. To obtain accurate ground state energies within a given basis set, the present TC scheme is coupled to the recently proposed TC-full configuration interaction quantum Monte Carlo method [Cohen et al., J. Chem. Phys. 151, 061101 (2019)]. We report ground state total energies on the Li-Ne series, together with their first cations, computed with increasingly large basis sets and compare to more elaborate correlation factors involving electron-electron-nucleus coordinates. Numerical results on the Li-Ne ionization potentials show that the use of the single-parameter correlation factor brings on average only a slightly lower accuracy (1.2 mH) in a triple-zeta quality basis set with respect to a more sophisticated correlation factor. However, already using a quadruple-zeta quality basis set yields results within chemical accuracy to complete basis set limit results when using this novel single-parameter correlation factor. Calculations on the H2O, CH2, and FH molecules show that a similar precision can be obtained within a triple-zeta quality basis set for the atomization energies of molecular systems.
|
|
| 8. |
- Eriksson, Olof, et al.
(author)
-
On the use of [F-18]DOPA as an imaging biomarker for transplanted islet mass
- 2014
-
In: Annals of Nuclear Medicine. - : Springer Science and Business Media LLC. - 0914-7187 .- 1864-6433. ; 28:1, s. 47-52
-
Journal article (peer-reviewed)abstract
- AimIslet transplantation is being developed as a potential cure for patients with type 1 diabetes. There is a need for non-invasive imaging techniques for the quantification of transplanted islets, as current transplantation sites are associated with a substantial loss of islet viability. The dopaminergic metabolic pathway is present in the islets; therefore, we propose Fluorine-18 labeled l-3,4-dihydroxyphenylalanine ([F-18]DOPA) as a biomarker for transplanted islet mass.MethodsThe expression of enzymes involved in the dopaminergic metabolic pathway was investigated in both native and transplanted human islets. The specific uptake of [F-18]DOPA in islets and immortalized beta cells was studied in vitro by selective blocking of dopa decarboxylase (DDC). Initial in vivo PET imaging of viable subcutaneous human islets was performed using [F-18]DOPA.ResultsDDC and vesicular monoamine transporter 2 are co-localized with insulin in the native human pancreas, and the expression is retained after transplantation. Islet uptake of the [F-18]DOPA could be modulated by inhibiting DDC, indicating that the uptake followed the normal dopaminergic metabolic pathway. In vivo imaging revealed [F-18]DOPA uptake at the site of the functional islet graft. Based on the in vitro and in vivo results presented in this study, we propose to further validate [F-18]DOPA-PET as a sensitive imaging modality for imaging extrahepatically transplanted islets.
|
|
| 9. |
- Galván, Ignacio Fdez., et al.
(author)
-
OpenMolcas : From Source Code to Insight
- 2019
-
In: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 15:11, s. 5925-5964
-
Journal article (peer-reviewed)abstract
- In this Article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics, and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the Article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism, and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with postcalculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory, and new electronic and muonic basis sets.
|
|
| 10. |
- Haupt, J. Philip, et al.
(author)
-
Optimizing Jastrow factors for the transcorrelated method
- 2023
-
In: Journal of Chemical Physics. - 1089-7690 .- 0021-9606. ; 158:22
-
Journal article (peer-reviewed)abstract
- We investigate the optimization of flexible tailored real-space Jastrow factors for use in the transcorrelated (TC) method in combination with highly accurate quantum chemistry methods, such as initiator full configuration interaction quantum Monte Carlo (FCIQMC). Jastrow factors obtained by minimizing the variance of the TC reference energy are found to yield better, more consistent results than those obtained by minimizing the variational energy. We compute all-electron atomization energies for the challenging first-row molecules C2, CN, N2, and O2 and find that the TC method yields chemically accurate results using only the cc-pVTZ basis set, roughly matching the accuracy of non-TC calculations with the much larger cc-pV5Z basis set. We also investigate an approximation in which pure three-body excitations are neglected from the TC-FCIQMC dynamics, saving storage and computational costs, and show that it affects relative energies negligibly. Our results demonstrate that the combination of tailored real-space Jastrow factors with the multi-configurational TC-FCIQMC method provides a route to obtaining chemical accuracy using modest basis sets, obviating the need for basis-set extrapolation and composite techniques.
|
|
| 11. |
- Jazayeri, S.B., et al.
(author)
-
Situation of medical sciences in 50 top countries from 1996 to 2010- based on quality and quantity of publications
- 2012
-
In: Acta Medica Iranica. - 1735-9694 .- 0044-6025. ; 50:4, s. 273-278
-
Journal article (peer-reviewed)abstract
- In research outputs both quality and quantity are of critical value and evaluation of both aspects is required for best evaluation. Several studies have worked on single or two-dimensional methods that provide the assessment of quality, quantity or both simultaneously in different branches of sciences, but none of them have played a role in a particular area of medicine. This study has been conducted to compare countries worldwide in the field of medical researches. Measuring both quality and quantity of researches is performed separately. In order to evaluate countries from both aspects of quantity and quality of research outputs, a modified form of the citation per publication (CPPm) and publication per population (PPPm) were used to make these indices comparable through different years and nations by normalizing the values according to the world average standards. When countries are ranked by CPPm, Iceland ranks the 1st with the score of 1.98, Faroe Islands and Gambia rank the 2nd and the 3rd with scores of 1.84 and 1.63, respectively. In PPPm Switzerland, Sweden and Denmark ranked the top three with scores of 13.34, 11.67 and 10.32, respectively. Iran ranked 71 in CPPm and 141 in PPPm. Ranking countries makes it possible to identify countries which have performed better in research outcomes by means of quality and quantity; thus, reforming policies can be taken into action to lower the waste of money, higher quality and quantity of outputs while providing helpful tip(s) for institutions to improve.
|
|
| 12. |
- Kunz, Andreas, et al.
(author)
-
Multistatus Interaktionsgeräte für interaktive Tische
- 2013
-
In: icom. - 1618-162X. ; 12:1, s. 39-44
-
Journal article (peer-reviewed)abstract
- This article introduces a procedure and a prototype that allow using multiple interactive devices on an interactive tabletop without reducing the system‘s overall responsiveness. Furthermore, the active interaction devices could transfer their individual state to the system using a multi-digit bit word. This allows entering additional information with a high resolution.
|
|
| 13. |
- Kunz, Andreas, et al.
(author)
-
Tangible tabletops for emergency response: an exploratory study
- 2013
-
In: Proceedings of the International Conference on Multimedia, Interaction, Design and Innovation (MIDI '13). - 9781450323031 ; , s. 10-
-
Conference paper (peer-reviewed)abstract
- Effective handling of location-based data is important to emergency response management (ERM). Expert team members co-located around maps typically discuss events while drawing freeform areas or while using physical placeholders representing incidents. Key ERM functions are filtering data, selecting information recipients, searching datasets, drawing time-dependent freeform areas, and zooming in on one region while leaving others unchanged. Under time pressure the mouse and keyboard could be insufficient; intuitive graspable solutions, such as tangible user interfaces (TUIs), may be better suited for ERM. We present CoTracker, a tangible tabletop system with expected potential for ERM teamwork. On an interactive map expert team members can discuss an operational picture using TUIs like bricks, frames, and pens. With the participation of domain experts for cognitive walk-through studies, we examined how generic and specialized TUIs can support ERM-related functions. We present some insights into the design of ERM-focused tangible tabletops.
|
|
| 14. |
- Le, Khanh Duy, 1989, et al.
(author)
-
Digimetaplan: Supporting facilitated brainstorming for distributed business teams
- 2019
-
In: ACM International Conference Proceeding Series. - New York, NY, USA : ACM.
-
Conference paper (peer-reviewed)abstract
- While facilitated brainstorming is a proven ideation method for professional teams, distributed teams are currently not able to enjoy its benefits. As workers are shifting towards collaborating in distributed settings, understanding how interactive systems can support facilitated brainstorming is becoming necessary to ensure that distributed teams remain creative. To address this challenge we designed, implemented, and evaluated DigiMetaplan—an interactive surface-based system for distributed facilitated brainstorming for co-located and remote users. The design of DigiMetaplan was inspired by a widely-used facilitated brainstorming method called Metaplan, where the brainstorming process of a group is coordinated by a facilitator. We evaluated the usability of DigiMetaplan with five hybrid teams consisting of a co-located facilitator and two team members connected with one remote participant. Results showed that the features used in DigiMetaplan on interactive surfaces effectively supported teams in performing facilitated collaborative brainstorming in partially distributed settings. We contribute knowledge on how a brainstorming environment translated into a multi-surface distributed system affects facilitated collaboration.
|
|
| 15. |
- Le, Khanh Duy, 1989, et al.
(author)
-
Immersive environment for distributed creative collaboration
- 2017
-
In: Proceedings of the ACM Symposium on Virtual Reality Software and Technology, VRST. - New York, NY, USA : ACM. - 9781450355483 ; Part F131944
-
Conference paper (peer-reviewed)abstract
- While videoconferencing has been available for years, tools for distributed creative collaboration such as brainstorming have not yet reached professional use. Unlike regular conferencing, brainstorming relies on information exchange across multiple channels in shared task and communication spaces. If a participant joins a facilitated brainstorming session remotely through his/her mobile device, perceiving these spaces is challenging. By proposing an immersive environment for the remote participant, this tech note addresses how to provide him/her a stronger engagement. Offered as a client application for the remote participant's tablet device, it enables him/her to see all channels and select which one to interact with. Our proof-of-concept client allows us to examine how this immersive environment for distributed creative collaboration can provide the remote participant with an increased engagement.
|
|
| 16. |
- Manni, Giovanni Li, et al.
(author)
-
The OpenMolcas Web : A Community-Driven Approach to Advancing Computational Chemistry
- 2023
-
In: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 19:20, s. 6933-6991
-
Journal article (peer-reviewed)abstract
- The developments of the open-source OpenMolcas chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations.
|
|
| 17. |
- Matuozzo, Daniela, et al.
(author)
-
Rare predicted loss-of-function variants of type I IFN immunity genes are associated with life-threatening COVID-19.
- 2023
-
In: Genome medicine. - 1756-994X. ; 15:1
-
Journal article (peer-reviewed)abstract
- We previously reported that impaired type I IFN activity, due to inborn errors of TLR3- and TLR7-dependent type I interferon (IFN) immunity or to autoantibodies against type I IFN, account for 15-20% of cases of life-threatening COVID-19 in unvaccinated patients. Therefore, the determinants of life-threatening COVID-19 remain to be identified in~80% of cases.We report here a genome-wide rare variant burden association analysis in 3269 unvaccinated patients with life-threatening COVID-19, and 1373 unvaccinated SARS-CoV-2-infected individuals without pneumonia. Among the 928 patients tested for autoantibodies against type I IFN, a quarter (234) were positive and were excluded.No gene reached genome-wide significance. Under a recessive model, the most significant gene with at-risk variants was TLR7, with an OR of 27.68 (95%CI 1.5-528.7, P=1.1×10-4) for biochemically loss-of-function (bLOF) variants. We replicated the enrichment in rare predicted LOF (pLOF) variants at 13 influenza susceptibility loci involved in TLR3-dependent type I IFN immunity (OR=3.70[95%CI 1.3-8.2], P=2.1×10-4). This enrichment was further strengthened by (1) adding the recently reported TYK2 and TLR7 COVID-19 loci, particularly under a recessive model (OR=19.65[95%CI 2.1-2635.4], P=3.4×10-3), and (2) considering as pLOF branchpoint variants with potentially strong impacts on splicing among the 15 loci (OR=4.40[9%CI 2.3-8.4], P=7.7×10-8). Finally, the patients with pLOF/bLOF variants at these 15 loci were significantly younger (mean age [SD]=43.3 [20.3] years) than the other patients (56.0 [17.3] years; P=1.68×10-5).Rare variants of TLR3- and TLR7-dependent type I IFN immunity genes can underlie life-threatening COVID-19, particularly with recessive inheritance, in patients under 60years old.
|
|
| 18. |
- Sokolov, Igor O., et al.
(author)
-
Orders of magnitude increased accuracy for quantum many-body problems on quantum computers via an exact transcorrelated method
- 2023
-
In: Physical Review Research. - 2643-1564. ; 5:2
-
Journal article (peer-reviewed)abstract
- Transcorrelated methods provide an efficient way of partially transferring the description of electronic correlations from the ground-state wave function directly into the underlying Hamiltonian. In particular, Dobrautz et al. [Phys. Rev. B 99, 075119 (2019)2469-995010.1103/PhysRevB.99.075119] have demonstrated that the use of momentum-space representation, combined with a nonunitary similarity transformation, results in a Hubbard Hamiltonian that possesses a significantly more "compact"ground-state wave function, dominated by a single Slater determinant. This compactness/single-reference character greatly facilitates electronic structure calculations. As a consequence, however, the Hamiltonian becomes non-Hermitian, posing problems for quantum algorithms based on the variational principle. We overcome these limitations with the Ansatz-based quantum imaginary-time evolution algorithm and apply the transcorrelated method in the context of digital quantum computing. We demonstrate that this approach enables up to four orders of magnitude more accurate and compact solutions in various instances of the Hubbard model at intermediate interaction strength (U/t=4), enabling the use of shallower quantum circuits for wave-function Ansätzes. In addition, we propose a more efficient implementation of the quantum imaginary-time evolution algorithm in quantum circuits that is tailored to non-Hermitian problems. To validate our approach, we perform hardware experiments on the ibmq_lima quantum computer. Our work paves the way for the use of exact transcorrelated methods for the simulations of ab initio systems on quantum computers.
|
|
| 19. |
- Yun, Sujun, et al.
(author)
-
Ferromagnetic domains in the large- U Hubbard model with a few holes: A full configuration interaction quantum Monte Carlo study
- 2023
-
In: Physical Review B. - 2469-9969 .- 2469-9950. ; 107:6
-
Journal article (peer-reviewed)abstract
- Two-dimensional Hubbard lattices with two or three holes are investigated as a function of U in the large-U limit. In the so-called Nagaoka limit (one-hole system at infinite U), it is known that the Hubbard model exhibits a ferromagnetic ground state. Here, by means of exact full configuration interaction quantum Monte Carlo simulations applied to periodic lattices up to 24 sites, we compute spin-spin correlation functions as a function of increasing U. The correlation functions clearly demonstrate the onset of ferromagnetic domains, centered on individual holes. The overall total spin of the wave functions remains the lowest possible (0 or 12, depending on the number of holes). The ferromagnetic domains appear at interaction strengths comparable to the critical interaction strengths of the Nagaoka transition in finite systems with strictly one hole. The existence of such ferromagnetic domains is the signature of Nagaoka physics in Hubbard systems with a small (but greater than 1) number of holes.
|
|
