arXiv subjects

A. G. Green

Publications and source records attributed to A. G. Green.

34 records · Page 2Linked to original sources

arXiv2010

Magnetic analogue of the LOFF phase in Sr3Ru2O7

The phase diagram of Sr3Ru2O7 contains a metamagnetic transition that bifurcates to enclose an anomalous phase with intriguing properties - a large resistivity with anisotropy that breaks the crystal-lattice symmetry. We propose that this is a magnetic analogue of the spatially inhomogeneous superconducting Fulde-Ferrell-Larkin-Ovchinnikov state. Based on a microscopic theory of Stoner magnetism we derive a Ginzburg-Landau expansion where the magnetisation transverse to the applied field can become spatially inhomogeneous. We show that this reproduces the observed phase diagram of Sr3Ru2O7.

cond-mat.str-el↗

arXiv2009

Inhomogeneous phase formation on the border of itinerant ferromagnetism

A variety of analytical techniques suggest that quantum fluctuations lead to a fundamental instability of the Fermi liquid that drives ferromagnetic transitions first order at low temperatures. We present both analytical and numerical evidence that, driven by the same quantum fluctuations, this first order transition is pre-empted by the formation of an inhomogeneous magnetic phase. This occurs in a manner that is closely analogous to the formation of the inhomogeneous superconducting Fulde-Ferrel-Larkin-Ovchinnikov state. We derive these results from a field theoretical approach supplemented with numerical Quantum Monte Carlo simulations. Our analytical approach represents a considerable simplification over diagrammatic methods, makes contact with older analyses of the unitarity limit, and enables a simple physical picture to emerge.

cond-mat.str-el↗

arXiv2009

Inhomogeneous magnetic phases: a LOFF-like phase in Sr3Ru2O7

The phase diagram of Sr3Ru2O7 contains a metamagnetic transition that bifurcates to enclose an anomalous phase with intriguing properties - a large resistivity with anisotropy that breaks the crystal-lattice symmetry. We propose that this is a magnetic analogue of the spatially inhomogeneous superconducting Fulde-Ferrel-Larkin-Ovchinnikov state. We show - through a Ginzburg-Landau expansion where the magnetisation transverse to the applied field can become spatially inhomogeneous - that a Stoner model with electronic band dispersion can reproduce this phase diagram and transport behaviour.

cond-mat.str-el↗

arXiv2009

Magnetothermoelectric Response near Quantum Critical Points

Following on from our previous work [Phys. Rev. Lett. 98, 166801 (2007)] we examine the finite temperature magnetothermoelectric response in the vicinity of a quantum critical point (QCP). We begin with general scaling considerations relevant to an arbitrary QCP, either with or without Lorentz invariance, and in arbitrary dimension. In view of the broad connections to high temperature superconductivity, and cold atomic gases, we focus on the quantum critical fluctuations of the relativistic Landau--Ginzburg theory. This paradigmatic model arises in many contexts, and describes the (particle-hole symmetric) superfluid--Mott insulator quantum phase transition in the Bose--Hubbard model. The application of a magnetic field opens up a wide range of physical observables, and we present a detailed overview of the charge and thermal transport and thermodynamic response. We combine several different approaches including the epsilon expansion and associated Quantum Boltzmann Equation (QBE), entropy drift, and arguments based on Lorentz invariance. The results differ markedly from the zero field case, and we include an extended discussion of the finite thermal conductivity which emerges in the presence of a magnetic field. We derive an integral equation that governs its response and explore the crossover upon changing the magnetic field. This equation may be interpreted as a projection equation in the low field limit, and clearly highlights the important role of collision invariants (or zero modes) in the hydrodynamic regime. Using an epsilon expansion around three-dimensions, our analytic and numerical results interpolate between our previously published value and the exact limit of two-dimensional relativistic magnetohydrodynamics.

cond-mat.str-el↗

arXiv2008

Universal non-linear conductivity near to an itinerant-electron ferromagnetic quantum critical point

We study the conductivity in itinerant-electron systems near to a magnetic quantum critical point. We show that, for a class of geometries, the universal power-law dependence of resistivity upon temperature may be reflected in a universal non-linear conductivity; when a strong electric field is applied, the resulting current has a universal power-law dependence upon the applied electric field. For a system with thermal equilibrium current proportional to $T^α$ and dynamical exponent $z$, we find a non-linear resistivity proportional to $E^{\frac{z-1}{z(1+α)-1}}$.

cond-mat.str-el↗

arXiv2006

Magnetothermoelectric Response at a Superfluid--Mott Insulator Transition

We investigate the finite temperature magnetothermoelectric response in the vicinity of a superfluid--Mott insulator quantum phase transition. We focus on the particle-hole symmetric transitions of the Bose--Hubbard model, and combine Lorentz invariance arguments with quantum Boltzmann calculations. By means of an epsilon expansion, we find that a non-vanishing thermoelectric tensor and a finite thermal conductivity are supported in this quantum critical regime. We comment on the singular Nernst effect in this problem.

cond-mat.str-el↗

arXiv2006

Current fluctuations near to the 2D superconductor-insulator quantum critical point

Systems near to quantum critical points show universal scaling in their response functions. We consider whether this scaling is reflected in their fluctuations; namely in current-noise. Naive scaling predicts low-temperature Johnson noise crossing over to noise power $\propto E^{z/(z+1)}$ at strong electric fields. We study this crossover in the metallic state at the 2d z=1 superconductor/insulator quantum critical point. Using a Boltzmann-Langevin approach within a 1/N-expansion, we show that the current noise obeys a scaling form $S_j=T Φ[T/T_{eff}(E)]$ with $T_{eff} \propto \sqrt{E}$. We recover Johnson noise in thermal equilibrium and $S_j \propto \sqrt{E}$ at strong electric fields. The suppression from free carrier shot noise is due to strong correlations at the critical point. We discuss its interpretation in terms of a diverging carrier charge $\propto 1/\sqrt{E}$ or as out-of-equilibrium Johnson noise with effective temperature $\propto \sqrt{E}$.

cond-mat.str-el↗

arXiv2005

Phase Bifurcation and Quantum Fluctuations in Sr3Ru2O7

The bilayer ruthenate Sr_3Ru_2O_7 has been cited as a textbook example of itinerant metamagnetic quantum criticality. However, recent studies of the ultra-pure system have revealed striking anomalies in magnetism and transport in the vicinity of the quantum critical point. Drawing on new experimental data, we propose that the anomalous phase behavior can be accommodated by a mechanism which relies on the effects of strong magnetoelastic coupling. We argue that the phenomenological theory identified here is more generic and raises the possibility of quantum tricritical behaviour.

cond-mat.str-el↗

arXiv2005

Non-linear quantum critical transport and the Schwinger Mechanism

Scaling arguments imply that quantum critical points exhibit universal non-linear responses to external probes. We investigate the origins of such non-linearities in transport, which is especially problematic since the system is necessarily driven far from equilibrium. We argue that for a wide class of systems the new ingredient that enters is the Schwinger mechanism--the production of carriers from the vacuum by the applied field-- which is then balanced against a scattering rate which is itself set by the field. We show by explicit computation how this works for the case of the symmetric superfluid-Mott insulator transition of bosons.

cond-mat.supr-con↗

arXiv2002

Schrodinger equation with a spatially and temporally random potential: Effects of cross-phase modulation in optical communication

We model the effects of cross-phase modulation in frequency (or wavelength) division multiplexed optical communications systems, using a Schrödinger equation with a spatially and temporally random potential. Green's functions for the propagation of light in this system are calculated using Feynman path-integral and diagrammatic techniques. This propagation leads to a non-Gaussian joint distribution of the input and output optical fields. We use these results to determine the amplitude and timing jitter of a signal pulse and to estimate the system capacity in analog communication.

physics.optics↗

arXiv2002

Dissipative Transport in Quantum Hall Ferromagnets by Spinwave Scattering

We report on a study of the effect upon electrical transport of spinwave scattering from charged quasiparticles in nu=1 quantum Hall ferromagnets (QHFs), including both Heisenberg (single layer) and easy-plane (bilayer) cases. We derive a quantum Langevin equation to describe the resulting diffusive motion of the charged particle and use this to calculate the contribution to low temperature conductivity from a density of charged particles. This conductivity has a power law dependence upon temperature. The contribution is small at low temperatures increasing to a large value at relatively modest temperatures. We comment upon high temperature transport and upon the contribution of scattering to the width of the zero bias peak in inter-layer tunneling conductance.

cond-mat.mes-hall↗

arXiv2000

Quantum Critical Dynamics of the Skyrmion Lattice

Near to filling fraction $ν=1$, the quantum Hall ferromagnet contains multiple Skyrmion spin excitations. This multi-Skyrmion system has a tremendously rich quantum-critical structure. This is simplified when Skyrmions are pinned by disorder. We calculate the nuclear relaxation rate in this case and compare the result with experiment. We discus how such measurements may be used to further probe the quantum-critical structure of the multi-Skyrmion system.

cond-mat↗

arXiv1998

Localization of spinwaves in the quantum Hall ferromagnet

The quantum Hall ferromagnet at filling fraction $ν= 1$ has some unusual properties due to the remarkable identity between the topological density of a spin distortion and the associated electrical charge density. We investigate the localization of spinwaves by coupling to a scalar disorder potential via their topological density. A low energy description of the system in terms of a non-linear sigma model of unitary supermatrices is derived. All states of this model are localized in two dimensions. A possible experimental signature of these effects in photo-luminescence is suggested.

cond-mat.mes-hall↗

arXiv1998

Disorder and the quantum Hall ferromagnet

The distinguishing feature of the quantum Hall ferromagnet is the identity between the electrical and topological charge densities of a spin distortion. In addition to the wealth of physics associated with Skyrmionic excitations of the quantum Hall ferromagnet, this identification permits a rather curious coupling of spinwaves to the disorder potential. A wavepacket of spinwaves has an associated, oscillating dipole charge distribution, due to the non-linear form of the topological density. We investigate the way in which this coupling modifies the conductivity and temperature dependence of magnetization of the quantum Hall ferromagnet.

cond-mat.mes-hall↗

arXiv1996

Skyrmions in the quantum Hall effect at finite Zeeman coupling

Using a self consistent approximation for the spin distribution of Skyrmions in the quantum Hall effect, we obtain an effective action for the Skyrmion coordinates. The energy functional is minimised for a periodic distribution of Skyrmions with an underlying hexagonal symetry. We calculate the phonon spectrum of this lattice and find that near the classical minimum, breathing modes of the Skyrmions are strongly suppressed. The resulting equal sized Skyrmions interact via a residual Coulomb potential. Neglecting coupling between phonons and spin waves due to dipole and higher order Coulomb interactions, the Skyrmion crystal has a phonon spectrum identical to that of an electronic Wigner crystal. The transition to a liquid of equal sized Skyrmions is discussed using the theory of dislocation melting and a comparison is made between the predictions of this theory and the results of a recent experiment.

cond-mat↗

arXiv1995

Skyrmion Liquid in SU(2)-invariant Quantum Hall Systems

We report on a theory of the Skyrmion states which occur in quantum Hall regime near certain filling factors. It is shown that in the limit of zero Zeeman splitting in a realistic temperature range the Skyrmion plasma is a liquid described by the effective model of massive two-dimensional Dirac fermions.

cond-mat↗