Studies of Lower Dimensional Metals at Clark University.

Charles Agosta -Laboratory Head

We study anisotropic superconducting materials WOW! at the extremes of parameter space, including low Probe image temperature, high pressure, and high magnetic field to understand the relation of their chemical properties to correlated electron properties such as superconductivity, spin density waves, charge density waves, and the quantum Hall effect. Organic superconductors and heavy fermion materials are ideal experimental systems for this research because they are well understood chemically and because their low Fermi energies and lower dimensionality favor correlated electron ground states and novel types of superconductivity. At our laboratory we can increase the magnetic field much above the level necessary to quench the superconducting or other correlated electron state to observe quantum oscillations and probe the Fermi surface. In this way, we can explore the fundamental properties of the charge carriers and test current theories of correlated electrons.

Over the past three years we have developed an innovative apparatus based on a tunnel diode Probe image oscillator to study our materials in moderate to high magnetic fields. Our tunnel diode oscillator apparatus uses an rf signal to probe the penetration depth, resistance, or magnetization of a sample and works equally well in dc or pulsed magnetic fields. Recent innovations have allowed us to use our rf technique in diamond anvil and gas pressure cells. Using this apparatus, we have obtained results that show the existence of a new inhomogeneous superconducting state in a heavy fermion material and magnetic breakdown effects in a low-dimensional organic conductor. We also have determined the critical fields perpendicular and parallel to the conducting layers in highly anisotropic organic superconductors at ambient pressure and up to 6 kbar.

As we mentioned above, our major goal is to learn how the physical structure of a material affects its electronic structure, and how the electronic structure determines the ground state. In particular, the anisotropic nature of organic and heavy fermion superconductors allows us to eliminate the orbital destruction of superconductivity and to probe the spin coupling or Pauli paramagnetic limit. In this limit the critical magnetic fields are near both the Zeeman energy and Fermi energy. This juxtaposition of energy scales is within easy reach of our pulsed magnetic fields.

Probe image Our crystaline samples are supplied by collaborators at Indiana University, the Electrotechnical Laboratory in Japan, Argonne National Laboratory, Oxford University, Los Alamos National Laboratory, and the Institute for Chemical Physics in Russia. Our close collaborations with chemists allow us to make progress toward our ultimate goal of developing new electron systems with a particular set of electronic parameters. Our research is a focused and feasible step towards this goal because the low dimensional systems of interest lend themselves to the study of how electron correlations change with controlled changes in chemical structure.

Many of our experiments are carried out in our own pulsed magnetic field laboratory where we can produce magnetic fields over 50 tesla. We also have dc magnet that can reach 8 tesla. For particular experiments where lower temperatures or higher dc fields are needed we visit the NHMFL in Tallahassee or Los Alamos.

Below we outline some of our experiments and lead the adventurous physicist/websurfer to the more fun details of our work.


We study the properties of anisotropic superconductors and the mechanisms of superconductivity using RF penetration and transport measurements of a number of organic and heavy fermion materials. Organic conductors tend to be very anisotropic, and have a low carrier density which favors higher electron correlations and competing ground states. It is not uncommon for an organic conductor to have a metallic states, spin density wave states, and superconductivity all as a function of temperature and pressure. In fact, their temperature-pressure phase diagram is reminicent of the temperatrue-carrier doping phase diagram of the high temperature cuprates. In contrast to the high temperature cuprates, the organic conductors are very clean, and show very high quality quantum oscillations such as Shubnikov-de Haas and de Haas-van Alphen oscilations. From these measurements we can characterize the Fermi surface of the organic conductors and pull out such parameters as the Fermi energy, the effective electron mass, the mean free path, the Fermi velocity and the g factor. Our reseach capitilizes on two properties of the organic and heavy fermion suuperconductors, their anisotropy and long mean free path. The anisotropy allows us to apply a magnetic field parallel to the most conducting planes and minimize the effect of vorticies. Read More! In this orientation we can study the Cooper pairs more directly than in isotropic materials. The clean nature of the materials favors exotic superconducting states, such as the FFLO state. and allows us to measure high quality quantum oscillations to characterize the Fermi surface as described in the next paragraph.

Quantum Effects

Much of the original interest in organic conductors concerned their lower dimensional nature. TMTSF (quasi 1-D), as an example, has an extremely rich magnetic phase diagram containing metallic, superconducting and spin density wave states and a form of the quantum Hall effect. Because of the high quality quantum oscillations in thse materials there are many detailed studies that can be done of the electrons on the Fermi surface. As one example we have been carefully studying the magnetic breakdown effect. Magnetic breakdown occurs when the magnetic energy is great enough to cause qusiparticles to tunnel across a small gap in the Fermi surface. The tunneling not only changes the apparent shape of the Fermi surface, it also creates quantum interference at the tunneling points which makes for a interesting problem. Other compounds such as the (BEDO-TTF)2ReO4H2O or (BEDT-TTF) based conductors (quasi 2-D) have large quantum oscillations and low Fermi energies with make them interesting candidates for extreme quantum effects. These Fermi surface studies and search for quantum effects in organic conductors tend to demand very high magnetic fields and low temperatures.

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