arXiv subjects

M. W. Long

Publications and source records attributed to M. W. Long.

12 recordsLinked to original sources

arXiv2025

A permutation-based power series representation of the Baker-Campbell-Hausdorff formula

The Baker-Campbell-Hausdorff formula was recently resummed exactly in one variable, and left as a power series in the other (Moodie and Long 2021 J. Phys. A: Math. Theor. 54 015208). The coefficients of the power series were provided as a sum of products of three hyperbolic functions that are analogous to the familiar commutator expansion. We find a new form of the power series coefficients that is a linear combination of just one of the hyperbolic functions. This linear combination can be understood through elementary permutations of the arguments of the hyperbolic function. We use generating functions and hyperbolic identities to relate the representations. The permutation representation is radically different to previously known structures in the Baker-Campbell-Hausdorff formula and naturally supersedes the previous power series representation for future use, in our opinion. It also allows us to prove a set of intriguing marching identities that are useful in physical applications and were discovered in the initial work.

math-ph↗

arXiv2025

An explicit formula for perturbation theory at any order with infinitely many perturbations

We provide a systematic formula, in terms of integer partitions, that generates perturbation theory explicitly at an arbitrary order. Our approach naturally includes an infinite number of perturbations and uses a single matrix equation that contains the information for both the eigenvalue and eigenvector corrections. The formula reduces to the standard case of one perturbation in the appropriate limit. This formulation streamlines the derivations that are traditionally tedious in perturbation theory, facilitating high-order calculations.

cond-mat.str-el↗

arXiv2019

A Rigorous Demonstration of Superconductivity in a Repulsive Hubbard Model

We have rigorously shown that a strong Hubbard repulsion can cause superconductivity. The model, which has a particular set of local symmetries, manifests the phase diagram of many unconventional superconductors; anti-ferromagnetism paramagnetism, superconductivity, and even ferromagnetism are all observed. The key technique in the analysis is an exact non-linear fermion transformation, which preserves the correlated motion of electrons while removing the strong interactions. Using resolvent formalism, it is exactly shown that two holes at the Mott point bind to form a localised Cooper pair. As interactions are now weak, we then use BCS mean field theory to calculate the energy, excess pairing, and superconducting gap. These results are compared to exact diagonalisation of finite sized systems and show good agreement. At the Mott point the system is an anti-ferromagnet, and a superconducting phase quickly appears upon doping, and then vanishes.

cond-mat.supr-con↗

arXiv2019

Transition temperature scaling in weakly coupled two-dimensional Ising models

We investigate the proposal that for weakly coupled two-dimensional magnets the transition temperature scales with a critical exponent which is equivalent to that of the susceptibility in the underlying two-dimensional model, $ γ$. Employing the exact diagonalization of transfer matrices we can determine the critical temperature for Ising models accurately and then fit to approximate this critical exponent. We find an additional logarithm is required to predict the transition temperature, stemming from the fact that the heat capacity exponent $ α$ tends to zero for this Ising model, complicating the elementary prediction. We believe that the excitations of the transfer matrix correspond to thermalized topological excitations of the model and find that even the simplest model exhibits significant changes of behavior for the most relevant of these excitations as the temperature is varied.

cond-mat.stat-mech↗

arXiv2015

Fermi Surface Reconstruction and Quantum Oscillations in Underdoped YBa$_2$Cu$_3$O$_{7-x}$ Modeled in a Single Bilayer with Mirror Symmetry Broken by Charge Density Waves

Hole-doped high-temperature cuprate superconductors below optimum doping have small electron-like Fermi surfaces occupying a small fraction of the Brillouin zone. There is strong evidence that this is linked to charge density wave (CDW) order, which reconstructs the large hole-like Fermi surfaces predicted by band structure calculations . Recent experiments have revealed the structure of the two CDW components in the benchmark bilayer material YBa$_2$Cu$_3$O$_{7-x}$ in high field where quantum oscillation (QO) measurements are performed. We have combined these results with a tight-binding description of the bands in an isolated bilayer to give a minimal model revealing the essential physics of the situation. Here we show that this approach, combined with the effects of spin-orbit interactions and the pseudogap, gives a good qualitative description of the multiple frequencies seen in the QO observations in this material. Magnetic breakdown through weak CDW splitting of the bands will lead to a field-dependence of the QO spectrum and to the observed fourfold symmetry of the results in tilted fields.

cond-mat.supr-con↗

arXiv2011

The Magnetic Structure of $Er_{2}Ti_{2}O_{7}$

We employ the previously published neutron scattering experiments\cite{1,2,3} to suggest that Er$_2$Ti$_2$O$_7$ has a broken-symmetry multiple-{\bf q} state with tetragonal magnetic symmetry\cite{4}. The ordered moments do not appear to lie close to the crystal-field anticipated directions\cite{5} and we suggest that the low energy gapless mode, visible in specific-heat measurements\cite{6}, is not of the usual transverse isotropic Goldstone-mode type, but is longitudinal and is associated with the internal transfer of magnetism between distinct magnetic Bragg spots.

cond-mat.str-el↗

arXiv2010

The magnetic structure of Gd_2Ti_2O_7

We attempt to solve the magnetic structure of the gadolinium analogue of `spin-ice', using a mixture of experimental and theoretical assumptions. The eventual predictions are essentially consistent with both the Mossbauer and neutron measurements but are unrelated to previous proposals. We find two possible distinct states, one of which is coplanar and the other is fully three-dimensional. We predict that close to the initial transition the preferred state is coplanar but that at the lowest temperature the ground-state becomes fully three-dimensional. Unfortunately the energetics are consequently complicated. There is a dominant nearest-neighbour Heisenberg interaction but then a compromise solution for lifting the final degeneracy resulting from a competition between longer-range Heisenberg interactions and direct dipolar interactions on similar energy scales.

cond-mat.str-el↗

arXiv2008

Long-time asymptotics and conservation laws in integrable systems

One dimensional systems sometimes show pathologically slow decay of currents. This robustness can be traced to the fact that an integrable model is nearby in parameter space. In integrable models some part of the current can be conserved, explaining this slow decay. Unfortunately, although this conservation law is formally anticipated, in practice it has been difficult to find in concrete cases, such as the Heisenberg model. We investigate this issue both analytically and numerically and find that the appropriate conservation law can be a non-analytic combination of the known local conservation laws and hence is invisible to elementary assumptions.

cond-mat.str-el↗

arXiv2003

Finite Temperature Dynamical Correlations using the Microcanonical Ensemble and the Lanczos Algorithm

We show how to generalise the zero temperature Lanczos method for calculating dynamical correlation functions to finite temperatures. The key is the microcanonical ensemble, which allows us to replace the involved canonical ensemble with a single appropriately chosen state; in the thermodynamic limit it provides the same physics as the canonical ensemble but with the evaluation of a single expectation value. We can employ the same system sizes as for zero temperature, but whereas the statistical fluctuations present in small systems are prohibitive, the spectra of the largest system sizes are surprisingly smooth. We investigate, as a test case, the spin conductivity of the spin-1/2 anisotropic Heisenberg model and in particular we present a comparison of spectra obtained by the canonical and microcanonical ensemble methods.

cond-mat.str-el↗

arXiv2003

A pure Hubbard model with demonstrable pairing adjacent to the Mott-insulating phase

We introduce a Hubbard model on a particular class of geometries, and consider the effect of doping the highly spin-degenerate Mott-insulating state with a microscopic number of holes in the extreme strong-coupling limit. The geometry is quite general, with pairs of atomic sites at each superlattice vertex, and a highly frustrated inter-atomic connectivity: the one dimensional realization is a chain of edge-sharing tetrahedra. The sole model parameter is the ratio of intra-pair to inter-pair hopping matrix elements. If the intra-pair hopping is negligible then introducing a microscopic number of holes results in a ferromagnetic Nagaoka groundstate. Conversely, if the intra-pair hopping is comparable with the inter-pair hopping then the groundstate is low spin with short-ranged spin correlations. We exactly solve the correlated motion of a pair of holes in such a state and find that, in 1-d and 2-d, they form a bound pair on a length scale that increases with diminishing binding energy. This result is pertinent to the long-standing problem of hole motion in the copper-oxide planes of the high-temperature superconductors: we have rigorously shown that, on our frustrated geometry, the holes pair up and a short-ranged low-spin state is generated by hole motion alone.

cond-mat.str-el↗

arXiv1997

Single and Many Particle Correlation Functions and Uniform Phase Bases for Strongly Correlated Systems

The need for suitable many or infinite fermion correlation functions to describe some low dimensional strongly correlated systems is discussed. This is linked to the need for a correlated basis, in which the ground state may be postive definite, and in which single particle correlations may suffice. A particular trial basis is proposed, and applied to a certain quasi-1D model. The model is a strip of the 2D square lattice wrapped around a cylinder, and is related to the ladder geometries, but with periodic instead of open boundary conditions along the edges. Analysis involves a novel mean-field approach and exact diagonalisation. The model has a paramagnetic region and a Nagaoka ferromagnetic region. The proposed basis is well suited to the model, and single particle correlations in it have power law decay for the paramagnet, where the charge motion is qualitatively hard core bosonic. The mean field also leads to a BCS-type model with single particle long range order.

cond-mat.str-el↗

arXiv1994

Strong Coupling Descriptions of High Temperature Superconductors: Electronic Attraction from a Repulsive Potential

We consider the effect of the nearest neighbour copper-oxygen repulsion, V, when coupled to the charge transfer resonances Cu2+ to Cu3+ and Cu2+ to Cu+ in the high temperature cuprate superconductors. This is done by deriving effective low energy Hamiltonians correct to second order in the copper-oxygen hybridisation. Only hole doping is considered. When Cu2+ to Cu3+ fluctuations dominate we derive an effective one-band model of `Zhang-Rice' singlets with a nearest neighbour repulsion between these singlets. When Cu2+ to Cu+ fluctuations dominate we find rich and complex behaviour. For large V we show that clusters of charge are more stable than isolated charges. On the other hand, for small V the Hamiltonian contains both weak attractive and repulsive two body potentials. Calculations on clusters indicate that the attractive potentials have the same correlations as the more dominate `single particle' terms suggesting the possibility of `s' wave pairing.

cond-mat↗