A. J. Schofield

Probing Spin-Charge Separation in a Tomonaga-Luttinger Liquid

Electron Breakdown An electron possesses charge and spin. In general, these properties are confined to the electron. However, in strongly interacting many-body electronic systems, such as one-dimensional wires, it has long been theorized that the charge and spin should separate. There have been tantalizing glimpses of this separation experimentally, but questions remain. Jompol et al. (p. 597) looked at the tunneling current between an array of one-dimensional wires and a two-dimensional electron gas and argue that the results reveal a clear signature of spin-charge separation. Electronic spin and charge respond differently during tunneling between low-dimensional electron systems. In a one-dimensional (1D) system of interacting electrons, excitations of spin and charge travel at different speeds, according to the theory of a Tomonaga-Luttinger liquid (TLL) at low energies. However, the clear observation of this spin-charge separation is an ongoing challenge experimentally. We have fabricated an electrostatically gated 1D system in which we observe spin-charge separation and also the predicted power-law suppression of tunneling into the 1D system. The spin-charge separation persists even beyond the low-energy regime where the TLL approximation should hold. TLL effects should therefore also be important in similar, but shorter, electrostatically gated wires, where interaction effects are being studied extensively worldwide.

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 magnetization 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.

Metamagnetic Quantum Criticality in Metals

We present a renormalization group treatment of metamagnetic quantum criticality in metals. We show that for clean systems the universality class is that of the overdamped, conserving (dynamical exponent z = 3) Ising type. We obtain detailed results for the field and temperature dependence of physical quantities including the differential susceptibility, resistivity, and specific heat. Our results are shown to be in quantitative agreement with data on Sr3Ru2O7 except very near to the critical point itself.

Non-Fermi liquids

Our present understanding of how the interactions between electrons affect the metallic state has, for forty years, rested on the foundations of Landaus Fermi-liquid theory. It provides the basis for understanding metals in terms of weakly interacting electron (-like) particles. Recent years have seen the discovery of metals which appear to fall outside this framework-perhaps most notably in the normal state of the high temperature cuprate superconductors. While the theory for understanding the cuprate metals remains controversial, there are a number of clear examples where we do believe we understand the new underlying theoretical concepts. In this article I illustrate four such routes towards forming a non-Fermi liquid metal and illustrate, where possible, how these have been realized in a number of materials. The proximity to a quantum phase transition and reduced effective dimensionality can both play important roles.

Ferromagnetic Superconductivity Driven by Changing Fermi Surface Topology

We introduce a simple but powerful zero temperature Stoner model to explain the unusual phase dia-gram 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 magnetization. The twin-peak DOS may be derived from a tight-binding, quasi-one-dimensional band structure, inspired by previous band-structure calculations.

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 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 interparticle 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.

How should we interpret the two transport relaxation times in the cuprates

We observe that the appearance of two transport relaxation times in the various transport coefficients of cuprate metals may be understood in terms of scattering processes that discriminate between currents that are even, or odd under the charge-conjugation operator. We develop a transport equation that illustrates these ideas and discuss its experimental and theoretical consequences.

Quasilinear magnetoresistance in an almost two-dimensional 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.

High-field study of normal-state magnetotransport in Tl2Ba2CuO6+δ

We present a study of in-plane normal state magneto-transport in single crystal Tl-2201 in 60T pulsed magnetic fields. In optimally doped samples (Tc ~ 80K) the weak-magnetic-field regime extends to fields as high as 60T, but in overdoped samples (Tc ~ 30K) we are able to leave the weak field regime, as shown by the behavior of both the magnetoresistance and the Hall resistance. Data from samples of both dopings provide constraints on the class of model necessary to describe normal state transport in the cuprates.

Ca3Ru2O7: Density Wave Formation and Quantum Oscillations in the Hall Resistivity

We describe transport measurements in single-crystal, high-purity Ca 3 Ru 2 O 7 . The observation of a large linear magnetoresistance together with low frequency quantum oscillations is shown to be consistent with a small volume Fermi surface incompletely gapped by density wave formation. This complements previous ARPES experiments. The quantum oscillations are more pronounced in the Hall signal than in the longitudinal resistivity. This unusual observation is also explained by the peculiar electronic structure in this material. We remark on the similarity between our observations in Ca 3 Ru 2 O 7 and the observations of quantum oscillations in the underdoped cuprate superconductors.