Infinite randomness and quantum Griffiths effects in a classical system: the randomly layered Heisenberg magnet

We investigate the phase transition in a three-dimensional classical Heisenberg magnet with planar defects, i.e., disorder perfectly correlated in two dimensions. By applying a strong-disorder renormalization group, we show that the critical point has exotic infinite-randomness character. It is accompanied by strong power-law Griffiths singularities. We compute various thermodynamic observables paying particular attention to finite-size effects relevant for an experimental verification of our theory. We also study the critical dynamics within a Langevin equation approach and find it extremely slow. At the critical point, the autocorrelation function decays only logarithmically with time while it follows a nonuniversal power-law in the Griffiths phase.Comment: 10 pages, 2 eps figures included, final version as published

Winners and Losers from Enacting the Financial Modernization Statute

Previous studies of the announcement effects of relaxing administrative and legislative restraints show that signal events leading up to the enactment of the Financial Services Modernization Act (FSMA) increased the prices of several classes of financial-institution stocks. An unsettled question is whether the gains observed for these stocks arise mainly from projected increases in efficiency or from reductions in customer or competitor bargaining power. This paper documents that the value increase came at the expense of customers and competitors. The stock prices of credit-constrained customers declined during FSMA event windows and experienced significant increases in beta in the wake of its enactment. These findings reinforce evidence in the literature on bank mergers that large-bank consolidation is adversely affecting access to credit for capital-constrained firms.

Fractional quantum Hall states in two-dimensional electron systems with anisotropic interactions

We study the anisotropic effect of the Coulomb interaction on a 1/3-filling fractional quantum Hall system by using an exact diagonalization method on small systems in torus geometry. For weak anisotropy the system remains to be an incompressible quantum liquid, although anisotropy manifests itself in density correlation functions and excitation spectra. When the strength of anisotropy increases, we find the system develops a Hall-smectic-like phase with a one-dimensional charge density wave order and is unstable towards the one-dimensional crystal in the strong anisotropy limit. In all three phases of the Laughlin liquid, Hall-smectic-like, and crystal phases the ground state of the anisotropic Coulomb system can be well described by a family of model wave functions generated by an anisotropic projection Hamiltonian. We discuss the relevance of the results to the geometrical description of fractional quantum Hall states proposed by Haldane [ Phys. Rev. Lett. 107 116801 (2011)].Comment: 8 pages, 8 figure

Emerging criticality in the disordered three-color Ashkin-Teller model

We study the effects of quenched disorder on the first-order phase transition in the two-dimensional three-color Ashkin-Teller model by means of large-scale Monte Carlo simulations. We demonstrate that the first-order phase transition is rounded by the disorder and turns into a continuous one. Using a careful finite-size-scaling analysis, we provide strong evidence for the emerging critical behavior of the disordered Ashkin-Teller model to be in the clean two-dimensional Ising universality class, accompanied by universal logarithmic corrections. This agrees with perturbative renormalization-group predictions by Cardy. As a byproduct, we also provide support for the strong-universality scenario for the critical behavior of the two-dimensional disordered Ising model. We discuss consequences of our results for the classification of disordered phase transitions as well as generalizations to other systems.Comment: 18 pages, 18 eps figures included, final version as publishe

Strong-randomness infinite-coupling phase in a random quantum spin chain

We study the ground-state phase diagram of the Ashkin-Teller random quantum spin chain by means of a generalization of the strong-disorder renormalization group. In addition to the conventional paramagnetic and ferromagnetic (Baxter) phases, we find a partially ordered phase characterized by strong randomness and infinite coupling between the colors. This unusual phase acts, at the same time, as a Griffiths phase for two distinct quantum phase transitions both of which are of infinite-randomness type. We also investigate the quantum multi-critical point that separates the two-phase and three-phase regions; and we discuss generalizations of our results to higher dimensions and other systems.Comment: 9 pages, 6 eps figures, final version as publishe

Promoting fair and equitable research partnerships to respond to global challenges

This report presents the findings from a programme of strategic research funded by UKRI through the GCRF. The research sought to elicit a 'partners' perspective' on participation in UKRI-funded research by generating data from three groups of partner: i) academics based in the global South; ii) civi society practitioners based in the global South; and iii) international NGOs and research capacity providers based in the UK. Drawing on this data, the report identifies eight principles for understanding and improving fair and equitable research collaboration which form the basis of a series of targeted learning modules for 6 groups of stakeholders: UK-based research funders; UK-based academics; research brokers and capacity providers; international NGOs; academics based in the global South; and civil society practitioners based in the global South

Fermi-Bose crossover using engineered disorder potential

We present the first instance of a disorder tuned Fermi-Bose crossover that could be realized in superconducting systems. More specifically, harnessing a non perturbative numerical technique we analyze the ground state behavior of a two-dimensional attractive Hubbard model subjected to spin selective disorder potential. In particular, using spectroscopic properties we provide unambiguous evidence of the change in the Fermi surface topology as a function of the disorder, establishing incontrovertibly a Fermi-Bose crossover. Interplay of strong correlations and strong disorder brings out the spin selectivity in the properties of this system giving rise to spin selective "Bose metal/insulator" phase. We propose an experimental set-up where such disorder tuned Fermi-Bose crossover could be observed in two-dimensional electron gas formed at oxide interface. Finally, we speculate on the possible implication of such spin-selective disorder on unraveling signatures of Bose-Fermi cross-over on doped iron chalcogenide superconductors.Comment: 8 pages, 6 figure