arXiv subjects

K. Ryan

Publications and source records attributed to K. Ryan.

4 recordsLinked to original sources

arXiv2025

Dimerization in $O(n)$-invariant quantum spin chains

We establish dimerization in $O(n)$-invariant quantum spin chains with large enough $n$, in a much larger region of the phase diagram than previous results. This entails identifying two distinct infinite volume ground states which are gapped, are translations by one unit of each other, and which both have exponentially decaying correlations. We show these states are the limits of finite-volume, finite temperature states as the graph tends to $\mathbb{Z}$ and temperature tends to zero in any order. Our method relies on a probabilistic representation of the quantum system in terms of random loops, and an adaptation of a method developed for loop $O(n)$ models on the hexagonal lattice by Duminil-Copin, Peled, Samotij and Spinka.

math-ph↗

arXiv2018

Anisotropic superconductivity and magnetism in single-crystal RbEuFe$_4$As$_4$

We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe$_4$As$_4$ using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu-moments of $T_m$ = 15 K and a superconducting transition temperature of $T_c$ = 36.8 K. The superconducting phase diagram is characterized by high upper critical field slopes of -70 kG/K and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of $\Gamma$ = 1.7. Ginzburg-Landau parameters of $\kappa_c \sim 67$ and $\kappa_{ab} \sim 108$ indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu-magnetism is quasi-two dimensional as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and $<$ 0.04 K. A consequence of the quasi-2D nature of the Eu-magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a cusp-like anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 kG and 4 kG.

cond-mat.supr-con↗

arXiv2017

An increase in $T_c$ under hydrostatic pressure in the superconducting doped topological insulator Nb$_{0.25}$Bi$_2$Se$_3$

We report an unexpected positive hydrostatic pressure derivative of the superconducting transition temperature in the doped topological insulator \NBS via $dc$ SQUID magnetometry in pressures up to 0.6 GPa. This result is contrary to reports on the homologues \CBS and \SBS where smooth suppression of $T_c$ is observed. Our results are consistent with recent Ginzburg-Landau theory predictions of a pressure-induced enhancement of $T_c$ in the nematic multicomponent $E_u$ state proposed to explain observations of rotational symmetry breaking in doped Bi$_2$Se$_3$ superconductors.

cond-mat.supr-con↗

arXiv2016

Encoding of low-quality DNA profiles as genotype probability matrices for improved profile comparisons, relatedness evaluation and database searches

Many DNA profiles recovered from crime scene samples are of a quality that does not allow them to be searched against, nor entered into, databases. We propose a method for the comparison of profiles arising from two DNA samples, one or both of which can have multiple donors and be affected by low DNA template or degraded DNA. We compute likelihood ratios to evaluate the hypothesis that the two samples have a common DNA donor, and hypotheses specifying the relatedness of two donors. Our method uses a probability distribution for the genotype of the donor of interest in each sample. This distribution can be obtained from a statistical model, or we can exploit the ability of trained human experts to assess genotype probabilities, thus extracting much information that would be discarded by standard interpretation rules. Our method is compatible with established methods in simple settings, but is more widely applicable and can make better use of information than many current methods for the analysis of mixed-source, low-template DNA profiles. It can accommodate uncertainty arising from relatedness instead of or in addition to uncertainty arising from noisy genotyping. We describe a computer program GPMDNA, available under an open source license, to calculate LRs using the method presented in this paper.

stat.ME↗