Spin-orbital quantum liquid on the honeycomb lattice

In addition to low-energy spin fluctuations, which distinguish them from band insulators, Mott insulators often possess orbital degrees of freedom when crystal-field levels are partially filled. While in most situations spins and orbitals develop long-range order, the possibility for the ground state to be a quantum liquid opens new perspectives. In this paper, we provide clear evidence that the SU(4) symmetric Kugel-Khomskii model on the honeycomb lattice is a quantum spin-orbital liquid. The absence of any form of symmetry breaking - lattice or SU(N) - is supported by a combination of semiclassical and numerical approaches: flavor-wave theory, tensor network algorithm, and exact diagonalizations. In addition, all properties revealed by these methods are very accurately accounted for by a projected variational wave-function based on the \pi-flux state of fermions on the honeycomb lattice at 1/4-filling. In that state, correlations are algebraic because of the presence of a Dirac point at the Fermi level, suggesting that the symmetric Kugel-Khomskii model on the honeycomb lattice is an algebraic quantum spin-orbital liquid. This model provides a good starting point to understand the recently discovered spin-orbital liquid behavior of Ba_3CuSb_2O_9. The present results also suggest to choose optical lattices with honeycomb geometry in the search for quantum liquids in ultra-cold four-color fermionic atoms.Comment: 10 pages, 7 figure

Systematic errors in Gaussian Quantum Monte Carlo and a systematic study of the symmetry projection method

Gaussian Quantum Monte Carlo (GQMC) is a stochastic phase space method for fermions with positive weights. In the example of the Hubbard model close to half filling it fails to reproduce all the symmetries of the ground state leading to systematic errors at low temperatures. In a previous work [Phys. Rev. B {\bf 72}, 224518 (2005)] we proposed to restore the broken symmetries by projecting the density matrix obtained from the simulation onto the ground state symmetry sector. For ground state properties, the accuracy of this method depends on a {\it large overlap} between the GQMC and exact density matrices. Thus, the method is not rigorously exact. We present the limits of the approach by a systematic study of the method for 2 and 3 leg Hubbard ladders for different fillings and on-site repulsion strengths. We show several indications that the systematic errors stem from non-vanishing boundary terms in the partial integration step in the derivation of the Fokker-Planck equation. Checking for spiking trajectories and slow decaying probability distributions provides an important test of the reliability of the results. Possible solutions to avoid boundary terms are discussed. Furthermore we compare results obtained from two different sampling methods: Reconfiguration of walkers and the Metropolis algorithm.Comment: 11 pages, 14 figures, revised version, new titl

Systematic errors in Gaussian Quantum Monte Carlo and a systematic study of the symmetry projection method

Gaussian Quantum Monte Carlo (GQMC) is a stochastic phase space method for fermions with positive weights. In the example of the Hubbard model close to half filling it fails to reproduce all the symmetries of the ground state leading to systematic errors at low temperatures. In a previous work [Phys. Rev. B {\bf 72}, 224518 (2005)] we proposed to restore the broken symmetries by projecting the density matrix obtained from the simulation onto the ground state symmetry sector. For ground state properties, the accuracy of this method depends on a {\it large overlap} between the GQMC and exact density matrices. Thus, the method is not rigorously exact. We present the limits of the approach by a systematic study of the method for 2 and 3 leg Hubbard ladders for different fillings and on-site repulsion strengths. We show several indications that the systematic errors stem from non-vanishing boundary terms in the partial integration step in the derivation of the Fokker-Planck equation. Checking for spiking trajectories and slow decaying probability distributions provides an important test of the reliability of the results. Possible solutions to avoid boundary terms are discussed. Furthermore we compare results obtained from two different sampling methods: Reconfiguration of walkers and the Metropolis algorithm.Comment: 11 pages, 14 figures, revised version, new titl

Prenatal intuitive coparenting behaviors

Micro-analytic research on intuitive parenting behaviors has shed light on the temporal dynamics of parent and child interactions. Observations have shown that parents possess remarkable implicit communicative abilities allowing them to adapt to the clues infants give and therefore stimulate the development of many of the infants' abilities, such as communication skills. This work focused on observing intuitive parenting behaviors that were synchronized and coordinated between the parents. We call them prenatal intuitive coparenting behaviors and used an observation task - the Prenatal Lausanne Trilogue Play procedure - to observe them. For this task, the parents role-play their first encounter with their future baby, represented by a doll. Two cases from a study on pregnancy after assisted reproductive technology are provided to illustrate how these behaviors manifest themselves. The observations from the first case suggest that expectant parents can offer the baby a coparental framework, whereas the observations from the second case show that opportunities for episodes of prenatal intuitive coparenting can be missed due to certain relationship dynamics.These kinds of observations deepen our knowledge of the prenatal emergence of the coparenting relationship and allow us to hone our strategies for intervening during pregnancy with couples who experience coparenting difficulties. Furthermore, these observations provide a novel and complementary perspective on prenatal intuitive parenting and coparenting behaviors

Effectual entrepreneurship and the social enterprise: an examination of the fit between the principles of effectuation and the sphere of the social enterprise as a form of new venture

The present thesis aims to determine whether the decision-making processes that lead to the launch of social enterprises are consistent with frameworks embraced by commercial ventures. The underlying assumption is that a social enterprise is but one type of new venture. Its specificity lies in the fact that its creation is often the result of an entrepreneur’s desire to address an existing social problem (as opposed to new market creation through the design of a new product or service). However, the decision-making process - as defined within the effectuation logic - and the evolution of the venture are hypothesized to be similar to that of other forms of new venture. A longitudinal, qualitative case study method was employed. This case study led to the formulation of a conceptual framework and seven propositions from which to assess the decision-making processes of social entrepreneurs and their impact on the sustainability of the firm. The current research indicates that when entrepreneurs eschew one or more of the principles of effectuation, the likelihood of success is impacted. Findings of the present thesis have a number of important implications for the field of social entrepreneurship, as well as for public policy makers and organizations that support the development of social ventures. On the practical side, findings from this thesis appear to indicate that social enterprises function along the same decision-making principles as other forms of commercial new ventures, helping practitioners build successful enterprises by adding to their toolbox. Thinking along effectual lines helps entrepreneurs to do consciously what they might otherwise have done subconsciously. In addition, providing a framework within which to assess potential indicators of sustainability helps investors in social enterprises make informed choices. On the theoretical side, this thesis contributes to theory on social entrepreneurs and the launch of social enterprises, while also addressing claims that social enterprises cannot be viewed through the same lens as for-profit enterprises. Indeed, findings indicate that effectuation is a useful tool for assessing the decision-making principles of social entrepreneurs and that the various elements that compose effectuation logic are important in determining the sustainability of a social enterprise. As such, this thesis extends the use of the effectuation framework from commercial entrepreneurship to social entrepreneurship. To conclude, the primary contribution to the field is the manner in which this study has examined the launch of social enterprises in a new way. The present findings are important, as social enterprises are a growing phenomenon around the world. Researchers, policy makers, consultants, and practitioners are therefore advised to consider the importance of committed stakeholders and partners in the success and, therefore, sustainability of social enterprises

High-resolution geophysical surveying at the Springfield Fault, New Zealand

To trace the active Springfield Fault (South Island, New Zealand) and map its character at shallow depths on a terrace where it exhibits no surface expression, we recorded 3-D georadar data across an approximately rectangular 110 x 40 m survey area. In addition, we carried out multi-electrode geoelectric measurements along a 198 m long profile that crossed the georadar survey area. Although the georadar depth penetration was limited to only ~5 m, the processed images revealed the presence of a prominent reflecting horizon disrupted by three main discontinuities. Semi-continuous subhorizontal reflection patterns were interpreted to represent sedimentary units within the fluvial deposits, whereas three detected discontinuities were interpreted as fault traces with small near-vertical offsets (~0.4 m). This interpretation was supported by vertical and lateral changes visible on the final inverted resistivity model indicating lithological boundaries and fault branches

Magnetization of SrCu2(BO3)2 in ultrahigh magnetic fields up to 118 T

The magnetization process of the orthogonal-dimer antiferromagnet SrCu2(BO3)2 is investigated in high magnetic fields of up to 118 T. A 1/2 plateau is clearly observed in the field range 84 to 108 T in addition to 1/8, 1/4 and 1/3 plateaux at lower fields. Using a combination of state-of-the-art numerical simulations, the main features of the high-field magnetization, a 1/2 plateau of width 24 T, a 1/3 plateau of width 34 T, and no 2/5 plateau, are shown to agree quantitatively with the Shastry-Sutherland model if the ratio of inter- to intra-dimer exchange interactions J'/J=0.63. It is further predicted that the intermediate phase between the 1/3 and 1/2 plateau is not uniform but consists of a 1/3 supersolid followed by a 2/5 supersolid and possibly a domain-wall phase, with a reentrance into the 1/3 supersolid above the 1/2 plateau.Comment: 5 pages + 10 pages supplemental materia

Quantum Monte Carlo simulations in the trimer basis:First-order transitions and thermal critical points in frustrated trilayer magnets

The phase diagrams of highly frustrated quantum spin systems can exhibit first-order quantum phase transitions and thermal critical points even in the absence of any long-ranged magnetic order. However, all unbiased numerical techniques for investigating frustrated quantum magnets face significant challenges, and for generic quantum Monte Carlo methods the challenge is the sign problem. Here we report on a general quantum Monte Carlo approach with a loop-update scheme that operates in any basis, and we show that, with an appropriate choice of basis, it allows us to study a frustrated model of coupled spin-1/2 trimers: simulations of the trilayer Heisenberg antiferromagnet in the spin-trimer basis are sign-problem-free when the intertrimer couplings are fully frustrated. This model features a first-order quantum phase transition, from which a line of first-order transitions emerges at finite temperatures and terminates in a thermal critical point. The trimer unit cell hosts an internal degree of freedom that can be controlled to induce an extensive entropy jump at the quantum transition, which alters the shape of the first-order line. We explore the consequences for the thermal properties in the vicinity of the critical point, which include profound changes in the lines of maxima defined by the specific heat. Our findings reveal trimer quantum magnets as fundamental systems capturing in full the complex thermal physics of the strongly frustrated regime.Comment: 27 pages, 10 figures, Resubmission to SciPos