Quantum criticality in a dissipative (2+1)-dimensional XY model of circulating currents in high-Tc cuprates

We present large-scale Monte Carlo results for the dynamical critical exponent z and the spatio-temporal two-point correlation function of a (2+1)-dimensional quantum XY model with bond dissipation, proposed to describe a quantum critical point in high-Tc cuprates near optimal doping. The phase variables of the model, originating with a parametrization of circulating currents within the CuO_2 unit cells in cuprates, are compact, {\theta_{r,\tau}} \in [-\pi,\pi>. The dynamical critical exponent is found to be z \approx 1, and the spatio-temporal correlation functions are explicitly demonstrated to be isotropic in space-imaginary time. The model thus has a fluctuation spectrum where momentum and frequency enter on equal footing, rather than having the essentially momentum-independent marginal Fermi liquid-like fluctuation spectrum previously reported for the same model.Comment: 5 pages, 2 figures. Accepted for publication in Phys. Rev. B Rapid Communication

Criticality of compact and noncompact quantum dissipative Z4Z_4 models in (1+1)(1+1) dimensions

Using large-scale Monte Carlo computations, we study two versions of a $(1+1)D$ $Z_4$-symmetric model with Ohmic bond dissipation. In one of these versions, the variables are restricted to the interval $[0,2\pi>$, while the domain is unrestricted in the other version. The compact model features a completely ordered phase with a broken $Z_4$ symmetry and a disordered phase, separated by a critical line. The noncompact model features three phases. In addition to the two phases exhibited by the compact model, there is also an intermediate phase with isotropic quasi-long-range order. We calculate the dynamical critical exponent $z$ along the critical lines of both models to see if the compactness of the variable is relevant to the critical scaling between space and imaginary time. There appears to be no difference between the two models in that respect, and we find $z\approx1$ for the single phase transition in the compact model as well as for both transitions in the noncompact model

Monte Carlo simulations of dissipative quantum Ising models

The dynamical critical exponent $z$ is a fundamental quantity in characterizing quantum criticality, and it is well known that the presence of dissipation in a quantum model has significant impact on the value of $z$. Studying quantum Ising spin models using Monte Carlo methods, we estimate the dynamical critical exponent $z$ and the correlation length exponent $\nu$ for different forms of dissipation. For a two-dimensional quantum Ising model with Ohmic site dissipation, we find $z \approx 2$ as for the corresponding one-dimensional case, whereas for a one-dimensional quantum Ising model with Ohmic bond dissipation we obtain the estimate $z \approx 1$.Comment: 9 pages, 8 figures. Submitted to Physical Review

Quantum criticality in spin chains with non-ohmic dissipation

We investigate the critical behavior of a spin chain coupled to bosonic baths characterized by a spectral density proportional to $\omega^s$, with $s>1$. Varying $s$ changes the effective dimension $d_\text{eff} = d + z$ of the system, where $z$ is the dynamical critical exponent and the number of spatial dimensions $d$ is set to one. We consider two extreme cases of clock models, namely Ising-like and U(1)-symmetric ones, and find the critical exponents using Monte Carlo methods. The dynamical critical exponent and the anomalous scaling dimension $\eta$ are independent of the order parameter symmetry for all values of $s$. The dynamical critical exponent varies continuously from $z \approx 2$ for $s=1$ to $z=1$ for $s=2$, and the anomalous scaling dimension evolves correspondingly from $\eta \gtrsim 0$ to $\eta = 1/4$. The latter exponent values are readily understood from the effective dimensionality of the system being $d_\text{eff} \approx 3$ for $s=1$, while for $s=2$ the anomalous dimension takes the well-known exact value for the 2D Ising and XY models, since then $d_{\rm{eff}}=2$. A noteworthy feature is, however, that $z$ approaches unity and $\eta$ approaches 1/4 for values of $s < 2$, while naive scaling would predict the dissipation to become irrelevant for $s=2$. Instead, we find that $z=1,\eta=1/4$ for $s \approx 1.75$ for both Ising-like and U(1) order parameter symmetry. These results lead us to conjecture that for all site-dissipative $Z_q$ chains, these two exponents are related by the scaling relation $z = \text{max} {(2-\eta)/s, 1}$. We also connect our results to quantum criticality in nondissipative spin chains with long-range spatial interactions.Comment: 8 pages, 6 figure

We Are Not You - But We Are Part of What You Are - And What You Will Be, Address to the Council of the AICPA, May 8, 1973 - Colorado Springs, Colorado

https://egrove.olemiss.edu/aicpa_assoc/1965/thumbnail.jp

Reconstructing the phylogeny and characterizing the patterns of molecular evolution of the tetraploid freshwater suckers (Cypriniformes: Catostomidae)

The Catostomidae, colloquially known as the suckers, is a family of freshwater fish endemic to North America and Asia. This family is hypothesized to have evolved sometime before or during the Paleocene (56-66 Mya) from a single tetraploid ancestor, which is thought to be the product of a hybridization event between two closely related, diploid cypriniforms. Currently, there are 79 recognized, extant species, some of which are difficult to discriminate between in the field. Despite the numerous studies that have aimed to reconstruct the evolutionary history of this family, little consensus exists for the relationships of the subfamilies within the Catostomidae, with practically every combination of subfamilial relationships having been proposed in the past. Additionally, and of importance to our understanding of the evolution of the catostomids, little is still known about the consequences of whole genome duplication on molecular evolution, especially for polyploid animals. In this study, we sought to reconstruct the evolutionary history of the Catostomidae as well as characterize the patterns of molecular evolution of lineages within this family. Two nucleotide sequence, genome-scale data sets were generated with the aim to reconstruct the evolutionary history of the Catostomidae as well as characterize patterns of molecular evolution of their polyploid genomes. These data sets, an unphased data including one sequence for each taxon and a phased data with the number of sequences per taxon representative of their ploidy level, included 179 and 267 loci, respectively. From the reconstruction of the evolutionary history of the family, we recovered a topology which places Myxocyprinus asiaticus as the sister taxon to all other extant catostomids and Cycleptus elongatus as the sister taxon to an Ictiobinae + Catostominae clade. Additionally, we found that Catostomus was recovered as paraphyletic, with Deltistes luxatus, Chasmistes liorus, and Xyrauchen texanus forming strongly supported sister species relationships with species within Catostomus. In the second chapter, we found that the ictiobines, cycleptines, and myxocyprinines tended to have more polymorphic alleles than taxa within Catostominae. We also found that rates of molecular evolution were significantly greater within catostomine lineages than all other catostomid lineages

Dissipative quantum phase transitions and high-temperature superconductors

This thesis presents seven research papers on topics in condensed-matter theory. Five of the papers report on Monte Carlo studies of quantum phase transitions in various (d+1)-dimensional statistical mechanics models featuring Caldeira-Leggettlike dissipation. The principal motivation for these studies was to investigate a particular bond-dissipative (2+1)-dimensional XY model of circulating currents in cuprate high-temperature superconductors. It has been proposed that quantum critical fluctuations associated with a local quantum critical point described by this model can explain the marginal-Fermi-liquid behaviour of the normal state of these compounds. We present simulation results for this model for both compact and noncompact phase variables and show unambiguously that the quantum critical point in the compact case is not local. If the phases are taken to be noncompact variables, the model is also a model of resistively shunted Josephson junction arrays. The results in this case reveal a more complicated phase diagram, but we have not been able to establish critical behaviour consistent with the scenario of local quantum criticality. The study of extended quantum dissipative models is also motivated by the general effect on condensed-matter systems of the coupling to environmental degrees of freedom. Their influence on quantum critical phenomena is characterized by the dynamical critical exponent z, a measure of spatiotemporal anisotropy, the value of which can be estimated by naive scaling arguments. We confirm by numerical means that such scaling estimates give correct results to a good approximations (with a few reservations), irrespective of system dimensionality, order parameter symmetry, or whether the variables are compact or noncompact. Corrections to the naive scaling estimates have to be invoked for strongly super-Ohmic dissipation for d = 1 due to relatively large values of the anomalous scaling dimension η. The two last research papers are concerned with the superconducting pairing state of the recently discovered class of iron-based high-temperature superconductors. Here, we calculate possible signatures of the proposed s±-wave pairing state in conductance-spectroscopy and Josephson-effect experiments.PhD i fysikkPhD in Physic

Evaluating grid development strategies for a regional grid using dynamic line rating sensors

Traditionally, regional high voltage distribution grid planning relies on conservative line ratings and present value calculations to evaluate alternative grid development plans, as well as strict adherence to the N−1 criterion. However, it is now becoming possible to use dynamic line rating (DLR) sensors to operate the grid closer to its real capacity, as well as adapting probabilistic risk criteria. This work proposes a grid development methodology which compares conservative seasonal line ratings (SLR) to sensor-based DLR of regional grid overhead lines: First, the risk of insufficient reserves in the grid over the long-term planning horizon is quantified using time series power flow and contingency analyses with either SLR or DLR. Then, a risk acceptance threshold is used as a planning criterion to determine when grid investments should be made given a load development scenario. Finally, the strategies are evaluated by quantifying both the risks and the real option of using DLR as a measure to postpone investment decisions. The proposed methodology is demonstrated in a case study of a real regional distribution grid (132 kV) in Norway. The results demonstrate that in order to realize the option value of using DLR in grid planning, the grid company needs to reduce the risk margins used in the operation of the grid.Evaluating grid development strategies for a regional grid using dynamic line rating sensorsacceptedVersio

A Tale of Two Investors: Exploring Differences in Trading Behavior around Macroeconomic Announcements : A study of institutional and retail investors in the US market

We study whether the trading behaviour of institutional and retail investors differs on the days surrounding key macroeconomic announcements, and the impact of this difference on equity premiums earned. Through analysis of trading data from the 50 largest US companies between January 2017 and October 2022, we find a significant difference of 2.11 pp in order imbalances two days prior to announcements. Further, we find a significant difference of 2.06 pp in the equity premiums earned by institutions and retail investors on the day after announcements. We attribute these differences to the higher risk appetite of institutional investors and the slower reaction times and higher attention-sensitivity of retail investors.nhhma