arXiv subjects

A. J. Schofield

Publications and source records attributed to A. J. Schofield.

At least 37 records · Page 2Linked to original sources

arXiv2008

The precursor state to superconductivity in CeIrIn${_5}$: Unusual scaling of magnetotransport

We present an analysis of the normal-state Hall effect and magnetoresistance in the heavy fermion superconductor CeIrIn${_5}$. It is demonstrated that the modified Kohler's scaling--which relates the magnetoresistance to the Hall angle--breaks down prior to the onset of superconductivity due to the presence of a precursor state to superconductivity in this system. A model-independent, single-parameter scaling of the Hall angle governed solely by this precursor state is observed. Neither the Hall coefficient nor the resistivity exhibit this scaling implying that this precursor state preferentially influences the Hall channel.

cond-mat.str-el↗

arXiv2008

Symmetry-breaking Fermi surface deformations from central interactions in two dimensions

We present a mean field theory of the Pomeranchuk instability in two dimensions, starting from a generic central interaction potential described in terms of a few microscopic parameters. For a significant range of parameters, the instability is found to be pre-empted by a first-order quantum phase transition. We provide the ground state phase diagram in terms of our generic parameters.

cond-mat.str-el↗

arXiv2008

Pomeranchuk instability: symmetry breaking and experimental signatures

We discuss the emergence of symmetry-breaking {\it via} the Pomeranchuk instability from interactions that respect the underlying point-group symmetry. We use a variational mean-field theory to consider a 2D continuum and a square lattice. We describe two experimental signatures: a symmetry-breaking pattern of Friedel oscillations around an impurity; and a structural transition.

cond-mat.str-el↗

arXiv2007

Probing e-e interactions in a periodic array of GaAs quantum wires

We present the results of non-linear tunnelling spectroscopy between an array of independent quantum wires and an adjacent two-dimensional electron gas (2DEG) in a double-quantum-well structure. The two layers are separately contacted using a surface-gate scheme, and the wires are all very regular, with dimensions chosen carefully so that there is minimal modulation of the 2DEG by the gates defining the wires. We have mapped the dispersion spectrum of the 1D wires down to the depletion of the last 1D subband by measuring the conductance \emph{G} as a function of the in-plane magnetic field \emph{B}, the interlayer bias $V_{\rm dc}$ and the wire gate voltage. There is a strong suppression of tunnelling at zero bias, with temperature and dc-bias dependences consistent with power laws, as expected for a Tomonaga-Luttinger Liquid caused by electron-electron interactions in the wires. In addition, the current peaks fit the free-electron model quite well, but with just one 1D subband there is extra structure that may indicate interactions.

cond-mat.mes-hall↗

arXiv2006

Pomeranchuk and Topological Fermi Surface Instabilities from Central Interactions

We address at the mean field level the emergence of a Pomeranchuk instability in a uniform Fermi liquid with \emph{central} particle-particle interactions. We find that Pomeranchuk instabilities with all symmetries except $l=1$ can take place if the interaction is repulsive and has a finite range $r_{0}$ of the order of the inter-particle distance. We demonstrate this by solving the mean field equations analytically for an explicit model interaction, as well as numerical results for more realistic potentials. We find in addition to the Pomeranchuk instability other, subtler phase transitions in which the Fermi surface changes topology without rotational symmetry-breaking. We argue that such interaction-driven topological transitions may be as generic to such systems as the Pomeranchuk instability.

cond-mat.str-el↗

arXiv2005

Breakdown of weak-field magnetotransport at a metallic quantum critical point

We show how the collapse of an energy scale in a quantum critical metal can lead to physics beyond the weak-field limit usually used to compute transport quantities. For a density-wave transition we show that the presence of a finite magnetic field at the critical point leads to discontinuities in the transport coefficients as temperature tends to zero. The origin of these discontinuities lies in the breakdown of the weak field Jones-Zener expansion which has previously been used to argue that magneto-transport coefficients are continuous at simple quantum critical points. The presence of potential scattering and magnetic breakdown rounds the discontinuities over a window determined by tau Delta < 1 where Delta is the order parameter and tau is the quasiparticle elastic lifetime.

cond-mat.str-el↗

arXiv2005

Transport anomalies in a simplified model for a heavy electron quantum critical point

We discuss the transport anomalies associated with the development of heavy electrons out of a neutral spin fluid using the large-N treatment of the Kondo-Heisenberg lattice model. At the phase transition in this model the spin excitations suddenly acquire charge. The Higgs process by which this takes place causes the constraint gauge field to loosely ``lock'' together with the external, electromagnetic gauge field. From this perspective, the heavy fermion phase is a Meissner phase in which the field representing the difference between the electromagnetic and constraint gauge field, is excluded from the material. We show that at the transition into the heavy fermion phase, both the linear and the Hall conductivity jump together. However, the Drude weight of the heavy electron fluid does not jump at the quantum critical point, but instead grows linearly with the distance from the quantum critical point, forming a kind of ``gossamer'' Fermi-liquid.

cond-mat.str-el↗

arXiv2005

Quantum criticality

As we mark the centenary of Albert Einstein's seminal contribution to both quantum mechanics and special relativity, we approach another anniversary--that of Einstein's foundation of the quantum theory of solids. But 100 years on, the same experimental measurement that puzzled Einstein and his contemporaries is forcing us to question our understanding of how quantum matter transforms at ultra-low temperatures.

cond-mat.str-el↗

arXiv2005

c-axis transport in highly anisotropic metals: role of small polarons

We show in a simple model of interlayer hopping of single electrons, that transport along the weakly coupled c-axis of quasi-two-dimensional metals does not always probe only the in-plane electron properties. In our model where there is a strong coupling between electrons and a bosonic mode that propagates in the $c$ direction only, we find a broad maximum in the c-axis resistivity at a temperature near the characteristic energy of the bosonic mode, while no corresponding feature appears in the $ab$ plane transport. At temperatures far from this bosonic energy scale, the c-axis resistivity does track the in-plane electron scattering rate. We demonstrate a reasonable fit of our theory to the apparent metallic to non-metallic crossover in the c-axis resistivity of the layered ruthenate $\rm Sr_2 Ru O_4$.

cond-mat.str-el↗

arXiv2004

Phase diagram and quasiparticle properties of the Hubbard model within cluster two-site DMFT

We present a cluster dynamical mean-field treatment of the Hubbard model on a square lattice to study the evolution of magnetism and quasiparticle properties as the electron filling and interaction strength are varied. Our approach for solving the dynamical mean-field equations is an extension of Potthoff's "two-site" method [Phys. Rev. B. 64, 165114 (2001)] where the self-consistent bath is represented by a highly restricted set of states. As well as the expected antiferromagnetism close to half filling, we observe distortions of the Fermi surface. The proximity of a van Hove point and the incipient antiferromagnetism lead to the evolution from an electron-like Fermi surface away from the Mott transition, to a hole-like one near half-filling. Our results also show a gap opening anisotropically around the Fermi surface close to the Mott transition (reminiscent of the pseudogap phenomenon seen in the cuprate high-Tc superconductors). This leaves Fermi arcs which are closed into pockets by lines with very small quasiparticle residue.

cond-mat.str-el↗

arXiv2003

Momentum Resolved tunneling in a Luttinger Liquid

We consider momentum resolved tunneling between a Luttinger liquid and a two dimensional electron gas as a function of transverse magnetic field. We include the effects of an anomalous exponent and Zeeman splitting on both the Luttinger liquid and the two dimensional electron gas. We show that there are six dispersing features that should be observed in magneto-tunneling, in contrast with the four features that would be seen in a non-interacting one dimensional electron gas. The strength of these features varies with the anomalous exponent, being most pronounced when $γ_ρ=0$. We argue that this measurement provides an important experimental signature of spin-charge separation.

cond-mat.str-el↗

arXiv2002

Small polarons and c-axis transport in highly anisotropic metals

Motivated by the anomalous c-axis transport properties of the quasi two-dimensional metal, $\rm Sr_2 Ru O_4$, and related compounds, we have studied the interlayer hopping of single electrons that are coupled strongly to c-axis bosons. We find a c-axis resistivity that reflects the in-plane electronic scattering in the low and very high temperature limits (relative to the characteristic temperature of the boson $T_{\rm boson}$). For temperatures near the $T_{\rm boson}$, a broad maximum in the resistivity can appear for sufficiently strong electron-boson coupling. This feature may account for the observed ``metallic to non-metallic crossover'' seen in these layered oxides, where the boson may be a phonon.

cond-mat.str-el↗

arXiv2002

Ferromagnetic superconductivity driven by changing Fermi surface topology

We introduce a simple but powerful zero temperature Stoner model to explain the unusual phase diagram of the ferromagnetic superconductor, UGe2. Triplet superconductivity is driven in the ferromagnetic phase by tuning the majority spin Fermi level through one of two peaks in the paramagnetic density of states (DOS). Each peak is associated with a metamagnetic jump in magnetisation. The twin peak DOS may be derived from a tight-binding, quasi-one-dimensional bandstructure, inspired by previous bandstructure calculations.

cond-mat.str-el↗

arXiv2002

Small-angle scattering in a marginal Fermi-liquid

We study the magnetotransport properties of a model of small-angle scattering in a marginal Fermi liquid. Such a model has been proposed by Varma and Abrahams [Phys. Rev. Lett. 86, 4652 (2001)] to account for the anomalous temperature dependence of in-plane magnetotransport properties of the high-Tc cuprates. We study the resistivity, Hall angle and magnetoresistance using both analytical and numerical techniques. We find that small-angle scattering only generates a new temperature dependence for the Hall angle near particle-hole symmetric Fermi surfaces where the conventional Hall term vanishes. The magnetoresistance always shows Kohler's rule behavior.

cond-mat.str-el↗

arXiv2001

Metamagnetic Quantum Criticality

A renormalization group treatment of metamagnetic quantum criticality in metals is presented. In clean systems the universality class is found to be of the overdamped, conserving (dynamical exponent z=3) Ising type. Detailed results are obtained for the field and temperature dependence of physical quantities including the differential susceptibility, resistivity and specific heat near the transition. An application of the theory is made to Sr3Ru2O7, which appears to exhibit a metamagnetic critical end-point at a very low temperature and a field of order 5-7T.

cond-mat.str-el↗

arXiv2001

Non-Fermi liquid behaviour and superconductivity in the dissipative model

We study an exactly-solvable model which shows a zero-temperature transition from a non-Fermi liquid to a Fermi liquid as a function of particle density. The quantum critical point separating these two states is not associated with the usual ordering transition of a bosonic degree of freedom. We analyze the properties of this critical point in terms of an effective density of states, and we use this concept to generalize this transition to a class of related quantum critical points. We study the superconducting instability of the low-density (non-Fermi liquid) regime and we show that it has an enhanced pair susceptibility relative to the Fermi liquid. We comment on the applicability of our results to low-carrier density superconductors.

cond-mat.str-el↗

arXiv2000

Quasi-linear magnetoresistance in an almost 2D band structure

We present a theoretical study of the orbital magnetoresistance in a unixial anisotropic metal within the relaxation-time approximation. The appearance of a new dimensionless scale, delta=4t_perp/epsilon_F, allows the possibility of a new region at intermediate fields where the magnetoresistance is linear in applied magnetic field for currents flowing along the unixial direction. (Here, t_perp characterizes the bandwidth along the unixial direction.) In the limit of large anisotropy (small delta), corresponding to a quasi-two-dimensional metal made up of weakly coupled layers, we obtain an analytic expression for the magnetoresistance valid for all magnetic fields. We test our analytic results numerically and we compare our expressions with the c-axis magnetoresistance of Sr_2RuO_4.

cond-mat.str-el↗

arXiv2000

Metamagnetism and critical fluctuations in high quality single crystals of the bilayer ruthenate Sr3Ru2O7

We report the results of low temperature transport, specific heat and magnetisation measurements on high quality single crystals of the bilayer perovskite Sr3Ru2O7, which is a close relative of the unconventional superconductor Sr2RuO4. Metamagnetism is observed, and transport and thermodynamic evidence for associated critical fluctuations is presented. These relatively unusual fluctuations might be pictured as variations in the Fermi surface topography itself. No equivalent behaviour has been observed in the metallic state of Sr2RuO4.

cond-mat.str-el↗